ZAŠTITA MATERIJALA - sitzam.org.rssitzam.org.rs/zm/2012/ZASTITAMATERIJALA 1-2012.pdf · ZAŠTITA MATERIJALA G o d i n a LIII B e o g r a d, 2012. ... Uporedna svojstva uretan-alkidnih

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Glavni urednik – Editor in Chief Dr Milan M. Antonijević, University of Belgrade, Technical Faculty Bor, Serbia Pomoćnici urednika Dr Nebojša Nikolić, IHTM, Belgrade Dr Vladimir Panić, TMF, Belgrade Uređivački odbor - Editorial Board Dr Benedetto Bozzini, Italy Dr J. G. Gonzalez-Rodriguez, México Dr Heiner Jakob Gores, Germany Dr Tor Henning Hemmingsen, Norway Dr Dragica Jevtić, Serbia Dr S. Hadži Jordanov, R. Macedonia Dr K. F. Khaled, Egypt Dr Miomir G. Pavlović, Serbia Dr Andrzej Kowal, Poland Dr Ivan Krastev, Bulgaria Dr Časlav Lačnjevac, Serbia Dr Refik Zejnilović, Montenegro Dr Ronald Latanision, USA Dr Miodrag Maksimović, Serbia Dr Vesna Mišković-Stanković, Serbia Dr Milan Jaić, Serbia Dr S. Rajendran, India Dr A. S. Sarac, Turkey Dr Velu Saraswathu, India Dr Miodrag Stojanović, Serbia Dr Darko Vuksanović, Montenegro Dr Deana Wahyuningrum, Indonesia Izdavački savet – Publisher board Dr Dragoljub Dabić, predsednik, Serbia Mr Ivan Burić, Montenegro Zoran Ivljanin, Serbia Gordana Miljević, Serbia Bogdan Vujović, Serbia Mr Zoran Avramović, Serbia Momir Ilić, Serbia Mr Zorica Stanišić, Serbia Stevan Buinac, Slovenia

Dosadašnji glavni i odgovorni urednici Prof. dr Sreten Mladenović (1967–2001)

Prof. dr Miodrag Maksimović (2002 – 2005) Tehnički urednik – Technical editor Slavka Vukašinović Za izdavača – For publisher Prof. dr Časlav Lačnjevac, dipl. inž. predsednik Izdavač- Publisher INŽENJERSKO DRUŠTVO ZA KOROZIJU, Beograd, Kneza Miloša 9/I, tel/fax (011) 32-30-028, e-mail:[email protected], www.sitzam.org.rs

SADRŽAJ – CONTENT

Review paper M. Murkovic

Formation of carcinogenic substances during heating of foods...........................................................................3

Scientific papers A. Angamuthu, C. Thangavelu,S. Rajendran, T. Asokan

Sodium tungstate – Zn2+ as corrosion inhibitor for carbon steel ...........................................................................9

V. Panchal, A. Patel, N. Shah Inhibition of Al-Mg alloy in hydrochloric acid using schiff bases as corrosion inhibitors .........................................15

GY. Vastag∗, A. Shaban, I. Felhősi, E. Kálmán Study of inhibition properties of some thiazole derivatives against copper corrosion ......................................29

D. Radonjić, D. Vuksanović, J. Pješčić Istraživanje korozionih i elektrohemijskih karakteristika nemodifikovanih AlZnSnGaIn legura u rastvorima NaCl ...33

I. Mickova, D. Čamovska, Lj. Arsov Fotoelektrohemiska ispitivanja tankih filmova formiranih na Nb elektrodama u alakalnim rastvorima ......41

M. Jaić Uporedna svojstva uretan-alkidnih i vodorazredivih premaza za površinsku obradu drveta u eksterijeru ............45

B. Stojanović, B. Đukić, N. Stojanović, S. Smiljanić Korozija i zaštita rashladnog sistema.....................................51

D. V. Dimitrovski, Z. Lj. Bozinovski, K. T. Lisichkov, S. V. Kuvendziev

Arsenic removal through coagulation and flocculation from contaminated water in Macedonia ................................57

M Mihailović, A. Patarić, Z. Gulišija, Z. Janjušević, M. Sokić, Č. Lačnjevac

The possibility of bioactive coatings obtaining by electrophoretic deposition of HAp on the steel implants ......62

Stručni radovi V. Novaković, M. Gligorić, R. Grujić

Uticaj upotrebe komprimovanog vazduha pri izvođenju bunara na rezultate fizičko-hemijskih analiza vode..............67

S. Stankov Regulacija rada i nadgledanje stanica za katodnu zaštitu....73

Reklame...................................................................................83

EVROPSKA FEDERACIJA ZA KOROZIJU (EFC) SMATRA OVAJ ČASOPIS ZVANIČNOM PUBLIKACIJOM ZA OBJAVLJIVANJE IZVEŠTAJA I INFORMACIJA

EUROPEAN FEDERATION OF CORROSION (EFC) CONSIDERES THIS JOURNAL TO BE THE OFFICIAL PUBLICATION FOR PUBLISHING THE REPORTS AND INFORMATION

M. MURKOVIC FORMATION OF CARCINOGENIC SUBSTANCES DURING HEATING …

ZAŠTITA MATERIJALA 53 (2012) broj 1 3

MICHAEL MURKOVIC Review paper UDC:632.2/.3.099:614.876

Formation of carcinogenic substances during heating of foods

The formation of some selected carcinogens during cooking is associated with some certain types of foods. The heterocyclic amines are mainly found in meat and fish that are cooked at temperatures above 150°. Acrylamide is formed in foods containing asparagine and reducing sugars which are present in potatoes, cereals and similar products. The formation of acrylamide needs high temperatures similar to the HCAs which are occurring during baking, frying, and roasting. In contrast to the other two types of substances 5-hydroxymethyl-2-furfural is formed at lower temperatures and even during storage from carbohydrates in presence of amino acids or from fructose by direct dehydration. Since all these compounds are by-products of the aroma forming reactions it is difficult to find ways to reduce the content in foods without changing the characteristics of the food products. A proper selection of the raw materials and well controlled cooking/processing procedures in combination with a reduced uptake of some highly contaminated foods can give us a possibility to reduce the exposure. Key words: carcinogenic substances, contaminated foods, heating

INTRODUCTION

Since the 1970s carcinogenic compounds that are formed endogenously during heating of foods were identified and characterized in detail. In addition to the well known benzo[a]pyrene the heterocyclic aromatic amines from amino acid pyrolysates were identified (Trp-P-1, Trp-P-2, Glu-P-1, Glu-P-2, AαC, MeAαC) (Sugimura et al., 1977; Yamamoto et al., 1978; Yoshida et al., 1978). In a next step the compounds IQ and MeIQ were detected in broiled sardines (Kasai et al., 1980) and MeIQx in fried beef (Kasai et al., 1981). The precursors that are a prerequisite for the formation of quinolines and quinoxalines were identified by Jägerstad and coworkers (1984) being carbohydrates, amino acids, and creatinine. During the following years twenty of these compounds were identified and ten of them were shown to be carcinogenic. The formation of these compounds is linked to the presence of the precursors (carbohydrates, amino acids, creatine) and cooking temperatures of above 150 °C which are used at grilling, frying, broiling etc.

In 2002 the publication of the findings of a Swedish group showed that acrylamide is present in high amounts in foods. Especially heated products from potatoes and cereals contain rather high concentrations. The precursors for this product were

Author's address: Graz University of Technology, Institute of Biochemistry, Petersgasse 12/2, A-8010 Graz

Paper received: 20.08.2011.

identified as asparagine and sugars (Mottram et al., 2002). As it is well established that acrylamide is a toxic and carcinogenic compound a huge research effort was undertaken to evaluate the exposure and from these data the toxicological risk of acrylamide was derived. Based on the modes of action, Tardiff and co-workers (2010) a non-linear dose-response approach was applied for neurotoxicity and carcino-genicity. They concluded that the tolerable intakes of acrylamide should be set at 2.6 µg/kg BW/d and 16 µg/kg BW/d based on acrylamide and glycidamide, respectively, to avoid a cancer risk. This would be equivalent to 182 µg for a 70 kg human as a tolerable daily intake (TDI) for carcinogenic levels. The TDI for neurotoxicity from acrylamide was estimated to be 40 µg/kg BW/d which would be equivalent to 2,800 µg/d for a 70 kg human. The margins of exposure (MoE) were calculated for average acryl-amide consumers to be 300 and 500 based on acryl-amide and glycidamide, respectively; for cancer, the MoE for average acrylamide consumers was estima-ted to be 200 and 1200 based on acrylamide and glycidamide, respectively.

5-Hydroxymethyl-2-furfural (HMF) is a com-pound that is formed via different mechanisms from either glucose in presence of amino substituted com-pounds or from fructose directly via elimination of water. The concentrations can reach very high levels up to g/kg. The toxicity tests of HMF showed that this compound is safe but recent experiments with sulfotransferases showed that HMF can be activated and become mutagenic (Glatt, 1997; Durling et al., 2009).



Heterocyclic amines

N

N

CH3

CH3

NH2

H N

N

CH3

CH3

NH2

H N

NN

NH2

CH3

N

NN

NH2

CH3

CH3

N N

H

NH2

CH3

Trp-P-1 Trp-P-2 IQ MeIQ MeAαC

N

N

N NH2

CH3

N

N

N NH2

N

N NN

NH2

CH3H3C

N N

NCH3

NH2

N N

H

NH2

Glu-P-1 Glu-P-2 MeIQx PhIP AαC

Figure 1 - Structures of carcinogenic HAs

ANALYSIS OF HETEROCYCLIC AROMATIC AMINES

The analysis of these substances is a twofold challenge for the food chemist. The first challenge is the very low concentration in the heated meat and the second is the highly complex matrix that needs several clean-up steps before the analysis by HPLC can be carried out. During the last years the method developed by Gross (Gross and Grüter, 1992) became internationally accepted.

Extraction and clean up of HCAs: The first extraction with dichloromethane or ethyl acetate is done after saponification. Subsequently a two step solid phase extraction (SPE) is carried out which includes a strong cation exchanger (e.g. PRS) and subsequently a C18-material. A very interesting SPE material known as blue cotton can also be used. Blue cotton is a trisulfo copper phthalocyanin complex linked to cellulose. This planar system has a strong and selective interaction with the HAs (Hayatsu et al., 1983). Skog reviewed the published literature on the use of blue cotton for cleanup of HA extracts from heated meat and fish (Skog, 2004). Recently a method using solid-phase microextraction coupled with HPLC and UV detection was published (Cardenes et al., 2004). The authors could show that this technique being a well established clean up method in gas chromatography could also be used in HPLC analysis of polar compounds.

Chromatography and detection of HCAs: The chromatographic separation and detection is a crucial point in the analysis. In the recent years mass spectrometry especially MS-MS was shown to be the method of choice since the selectivity and sensitivity is excellent for complex matrices (e.g. Busquets et al., 2007). Another alternative that was published recently is the use of TLC combined with UV and fluorescence detection (Jautz et al., 2008).

OCCURRENCE OF HCAS IN FOODS Heterocyclic amines (HCA) are formed from

carbohydrates and amino acids at high cooking temperatures that are occurring during frying, grilling, and broiling. In the case of the polar HCAs creatinine is an additional precursor which limits the occurrence to food containing creatine which is mainly meat and fish. The concentration range of these compounds is in the low ng/g range but in some cases, especially in meat which is very well done the concentration of IFP can reach 25 ng/g. In the same sample 7-MeIgQx was found at levels of up to 11 ng/g and PhIP can even reach concentrations up to 300 ng/g (Ni et al., 2008).

In addition to earlier work a huge range of foods was investigated for the content of the HCAs. Sun et al. (2010) investigated mutton shashlik – a traditional and popular dish in China – contains typically low amounts of the carcinogenic HCAs being below 2 ng/g (AαC, PhIP). Tai and his group (2001) analyzed HCAs in fried fish fibre. Depending on the recipe (monosodium glutamate, ascorbic acid) the concen-trations of AαC and MeAαC were in the range of 1 to 10 ng/g. The addition of higher amounts of ascorbic acid resulted in a complete elimination of the two compounds. Oz (2011) analyzed Turkish meat balls by ultra fast liquid chromatography with UV detec-tion and found IQ, IQx, MeIQ, PhIP, AαC, MeAC in concentrations of up to 3.3 (IQx) in samples from different regions in Turkey. Barbecued sardines and Atlantic salmon were analysed by Costa and co-workers (2009). This group identified Glu-P-1, Trp-P-1, Trp-P-2, MeIQx, PhIP, AαC, and MeAαC in the different samples at concentrations of up to 13 ng/g. Especially the non-polar HCAs like PhIP, AαC, MeAαC, Trp-P-1, and Trp-P-2 were occurring at comparably high concentrations in some samples. Different species of trout were analyzed by the group of Oz (2007). In this publication extremely low concentrations of IQ and 4,8-DiMeIQx were reported being below 0.1 ng/g. In this case e.g. PhIP was not detected.



The efforts to mitigate the concentrations of the HCAs comprised the use of different marinades like fruit extracts (Cheng et al., 2007), green tea (Quelhas et al., 2010), garlic, onion, as well as lemon juice (Gibis, 2007), spice extracts (Damasius et al., 2011), red wine (Busquets et al., 2006; Melo et al., 2008), and beer (Melo et al., 2008).

INFLUENCE OF COOKING ON THE CONTENT OF HAs

All normally eaten meat products, which are derived from beef, pork or poultry, have to some degree mutagenic properties. The way of preparation influences the content of the HAs. The parameters with the highest influence are the cooking temperature and cooking time. It was shown several times that with increasing temperature the content of HAs increases also. In some of these experiments it was found that the mutagenic activity reaches a plateau at 200 - 250 °C. For example, the increase of the pan temperature of 50 °C during frying of meat leads to a doubling of the HA content between 200 and 300 °C (Nielsen et al., 1984). Although the content of the HAs can be reduced substantially with lower frying temperatures there is definitely no temperature limit where no HAs are formed. The influence of temperature on the formation of HAs in poultry meat was also shown. Having a low frying temperature of about 150 °C only low amounts of IQ, MeIQ, MeIQx, 4,8-DiMeIQx and PhIP are formed (Murkovic et al., 1997).

The HAs are not only found in the crust of the heated meat but also in the gravy or pan residue. When frying hamburgers about 23 % of the mutagenic activity is found in the gravy (Felton et al., 1981). If pork is sliced and fried for 10 min at 200 to 250 °C half of the mutagenic activity is found in the crust and gravy respectively (Murkovic et al., 1997). The analysis of 14 cooked meat dishes and the pan residues showed that up to a temperature of 150 °C the total content of HAs was below 1 ng/g. Temperatures of up to 175 °C led to a content of about 2 ng/g. The highest amounts of HAs were found in those foods that were heated at 200 to 225 °C (Skog et al., 1997). Other experiments showed that the mutagenic activity is predominantly formed in the pan residue when minced pork is heated to 200 °C for 5 - 25 min (Berg et al., 1990). If minced beef and poultry is heated to 220 °C for 10 min 20 - 40 % of the mutagenic activity is found in the pan residue (Knize et al., 1988). Janoszka and co-workers (2009) found similar concentrations in the gravy and in the meat and in some cases even lower concentrations in the gravy. In their manuscript concentrations were

reported being up to 10 ng/g with MeIQ showing the highest concentrations.

ACRYLAMIDE

The identification of acrylamide goes back to the year 2002 when a Swedish group published the finding of acrylamide in heated foods, especially in fried potato products like chips and crisps. Mottram and his co-workers published in 2002 that the main precursor is asparagine. The obviously simple mecha-nism of formation which includes a decarboxylation and ammonia elimination was evaluated in detail by the group of Yaylayan (Perez-Locas and Yaylayan, 2008). In these studies it was emphasised that a 5-oxazolidinone is an important intermediate in the formation reaction. This intermediate is a general intermediate which is similar in reactions of other amino acids.

ANALYSIS OF ACRYLAMIDE

Since acrylamide is a highly polar compound that is easily dissolved in water, this is used as extraction solvent. For purification of the extract normally a multimode solid phase extraction material is used. The purified extract is directly analysed by HPLC-MS/MS. The analysis by GC-MS is also possible either directly or after bromination. To increase the precision the use of a 13C3-acrylamide or D3-acrylamide as internal standard is recommended. For chromatography a Hypercarb(R) column can be used with water as eluent; other possibilities comprise hydrophilic end-capped reversed phase columns. A detailed overview and discussion of the analytical methods currently used is given by Eriksson (2005).

CONTENT OF ACRYLAMIDE IN FOODS AND DIETARY EXPOSURE

In 2011 the European Food Safety Agency (EFSA) published a report on the monitoring of acrylamide in foods during the recent years. Twenty three Member States and Norway submitted a total of 10,366 acrylamide results for the three-year period 2007 to 2009. In 2009, mean acrylamide levels ranged from 37 µg/kg for soft bread to 1504 µg/kg for substitute coffee, while the highest 95th percentile and maximum levels were reported for substitute coffee at 3,976 and potato crisps at 4,804 µg/kg, respectively. Based on the three years of information available it could be identified that acrylamide decreased in crackers, infant biscuits and gingerbread over the three years and increased in crisp bread and instant coffee. From this overview it was concluded that the mean acrylamide exposure in Europe ranges between 0.31 and 1.1 µg/kg BW/d for adults, between 0.43 and



1.4 µg/kg BW/d for adolescents (11-17 years), between 0.70 and 2.05 µg/kg BW/d for children (3-10 years) and between 1.2 and 2.4 µg/kg BW/d for toddlers (1-3 years). The major contributors to exposure for adults were fried potatoes (including French fries), coffee, and soft bread whereas for adolescents and children they were fried potatoes, soft bread and potato crisps or biscuits.

The mean dietary exposure to acrylamide for adults (>18 years) in Europe was estimated to range between 0.31 and 1.1µg/kg BW/d. High exposure at the 95th percentile varied from 0.58 to 2.3 µg/kg BW/d. These results are similar to the range reported in the latest JECFA acrylamide risk assessment report (FAO/WHO, 2010) in which mean and 95th – 97.5th percentile estimates ranged between 0.2 and 1 µg/kg BW/d and 0.6 to 1.8 µg/kg BW/d, respectively, for the general adult population. In this study fried potatoes (including French fries), soft bread and roasted coffee were identified as the major contributors to the overall adult acrylamide exposure. JECFA (FAO/WHO, 2010) identified French fries, potato crisps, bread and biscuits as the main contributors for the general adult population. (EFSA, 2011)

MITIGATION EFFORTS

Since the finding of acrylamide in foods a series of experiments were carried out to mitigate the formation of this potentially harmful substance. In a review Friedman and Levin summarized all published concepts to reduce the content of dietary acrylamide (2008). A mitigation could be achieved by several ways which comprise the proper selection of raw materials having optimized contents of the precursors, removing the precursors before processing, enzymatic hydrolysis of asparagine to aspartic acid using aspara-ginase, optimizing the process and storage conditions, adding components that are known to reduce the acrylamide content either by inhibiting the formation or enhancing further reactions of acrylamide, remo-ving acrylamide from the foods, and reducing the in vivo toxicity. The CIAA has published the acrylamide toolbox that should help the food producers to change their processes for obtaining acrylamide reduced products. The latest issue can be found at http://www. fooddrinkeurope.eu/uploads/publications_documents/Toolboxfinal260911.pdf.

Especially the use of asparaginase which was investigated in detail by the group of Ciesarova shows very promising applications in potatoes (Ciesarova et al., 2006). Similar applications were published by Hendriksen and co-workers in 2009.

HYDROXYMETHYLFURFURAL

5-Hydroxymethyl-2-furfural is a compound that was investigated for its toxicity – mainly because it was found in solutions for parenteral nutrition. The toxicity level which was established was estimated to an acute oral LD50 of 2.5 g/kg BW which is rather high (US EPA, 1992). This is a dose that is not easily reached with the uptake of foods. However, recently a possible activation metabolic pathway was identified during which the HMF becomes a highly reactive sulfuric acid ester and it was shown that this metabolite can react with the DNA (Glatt and Sommer, 2006; Glatt et al., 2011).

The legal limit for HMF in honey was set due to restricted processing conditions to 40 mg/kg. This limit is not based on toxicological reasons (EC Directive 74/409/EEC; UK Honey Regulations 2003). In the fair trade standards a quality grading system is used to produce honey with a HMF content as low as possible suggesting values of below 20 mg/kg (Fair-trade Standards 2005).

FORMATION OF HMF

The formation of HMF in the foods depends mainly on the presence of precursors (primarily glucose, fructose, amino acids) the temperature and the pH. Especially lower pH values – that means acidic foods – can form high amounts of HMF. It is not only the high processing temperature but also long storage times contribute to high HMF concentra-tions as it was shown for Madeira wines. Within the storage of 15 years the HMF content rises from very low concentrations up to 250 to 600 mg/L (Murkovic, unpublished). The contribution of HMF to the aroma of any food product is negligible which means that – if it is eliminated – the product quality would not change. However, there are currently no concepts available that would contribute to a mitigation of HMF.

ANALYSIS OF HMF

Similar to acrylamide HMF is water soluble and can be extracted with water. However, to reduce a solubilisation of proteins a mixture of methanol or acetonitril with water is used. Normally, the concentration of HMF is so high that the extract has to be diluted before analysis by RP-HPLC. A gradient of methanol and water is used for elution and due to the high concentrations and good absorption at 280 nm UV detection can be used. If the matrix is complex or the concentrations in the extract are low HMF can be derivatized with dinitro-phenyl-hydrazine which can be detected at 400 nm or with



good sensitivity by mass spectrometry (Murkovic, unpublished results; Jöbstl et al., 2010).

CONCENTRATION OF HMF IN FOODS AND DIETARY EXPOSURE

HMF is a substance that can occur in rather high concentrations. A selection of foods containing high amounts of HMF is given in Table 1. The concentrations are up to several grams per kg in some selected foods. Especially roasting of coffee and substitutes, heat processing and long storage can lead to extremely high concentrations. In a small study Husoy and co-workers (2008) showed that the intake of coffee, dried fruits, honey and alcoholic beverages contributed most to the exposure to HMF. Table 1 - Foods rich in HMF

Food HMF content (mg/kg) Reference

Coffee 100-1,900 a, b Coffee (instant) 90-4,100 a, c Chicory 200-22,500 a Malt 100-6,300 a Barley 100-1,200 a Jam 5.5-1,200 a, b Breakfast cereals 6.9-240 a Dried fruits 25-2,900 b, c Vinegar balsamic 190-3,400 a, d Sweet wines (Port, Sherry, Madeira) 10-650 d

a: extracted from Capuano and Fogliano, 2011; b: Murkovic and Pichler, 2006; c: Husoy et al, 2008; d: Murkovic unpublished

CONCLUSION Foods prepared with conventional cooking

methods contain HAs that are mutagenic and carcinogenic as well. These substances are found ubiquitous but in relevant amounts only in strongly heated meat and fish. Because of the well-established risk of cancer it is necessary to reduce the exposure to HAs. However, the precursors are present in all types of meat, which results in a low exposition of every human. Although it is impossible to prevent the HA formation completely a reduction of the uptake of these carcinogenic substances can be achieved by different ways: (1) intensive frying of meat and fish should be avoided, (2) if burnt food is served, these burnt parts should be removed and not eaten (3) the content of HAs is not so high if the food is prepared in the microwave oven (4) local overheating can be avoided if the meat is wrapped in aluminium foil (5) marinating and the use of antioxidant spices reduces the content of HAs (6) the consumption of meat and

fish should be reduced. If these points are taken into account when meat or fish is prepared the risk of cancer due to heterocyclic aromatic amines can be reduced. The selection of raw material is not only an issue for reducing the formation of acrylamide but also for exposure to HCAs.

All three types of compounds that are presented here can contribute to the dietary cancer risk. As these are side products of aroma forming reactions it is difficult to avoid the formation without changing the characteristics of the product. The reduction of the exposure can be achieved by changing the cooking methods and reduce the heating to avoid over cooking. In some cases it can also be recommended to reduce the uptake of certain foods.

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[41] Perez-Locas C., Yaylayan V. (2008) J. Agric. Food Chem. 56, 6069–6074.

[42] Quelhas I., Petisca C., Viegas O., Melo A., Pinho O. Ferreira I.M.P.L.V.O. (2010) Food Chem. 122, 98-104.

[43] Skog K. (2004) J. Chromatogr. B 802, 39-44. [44] Skog K., Augustsson K., Steineck G., Stenberg M.,

Jägerstad M. (1997) Food Chem. Toxicol. 35, 555-565.

[45] Sun L., Zhang F., Yong W., Chen Si., Yang M.-L., Ling Y., Chu X., Lin J.-M. (2010) Food Chem. 123, 647-652.

[46] Sugimura T, Nagao M., Kawachi T., Honda M., Yahagi T., Seino Y., Sato S., Matsukura N.

[47] Matsushima T., Shirai A., Sawamura M., Matsumoto H. (1977) Envirnm. Health Perspect. 98, 5-12.

[48] Tai C.-Y., Lee K.H., Chen B.H. (2001) Food Chem. 75, 309-316.

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[50] US EPA (1992) EPA/OTS. Doc. #88-920005429, Chicago, Illinois, USA.

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[52] K., Seino Y., Yahagi T., Nagao M., Sugimura T. (1978) Proc. Jpn. Acad., 54B, 248-250.

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IZVOD

FORMIRANJE KANCEROGENIH MATERIJA TOKOM ZAGREVANJA HRANE Formiranje neki izabranih kancerogena tokom kuvanja je povezano sa nekim vrstama hrane. Heterociklični amini se uglavnom nalaze u mesu i ribi koja se kuva na temperaturama iznad 150 stepeni. Akrilamidi formirani u hrani sadrže asparagin i smanjenje šećera koji su prisutni u krompiru, žitaricama i sličnim proizvodima. Za formiranje akrilamida potrebne su visoke temperature koje se dešavaju tokom pečenja i prženja. Za razliku od predhodna dva tipa supstanci, 5-hidroksimetil-2-furfural se formira na nižim temperaturama, čak i tokom čuvanja namirnica iz ugljenih hidrata u prisustvu amino kiselina, ili iz fruktoze direktno u toku dehidratacije. Pošto su sva ova jedinjenja nusproizvodi i formiraju se u toku reakcije, teško je da se pronađu načini da se smanji sadržaj u hrani bez promene karakteristika prehrambenih proizvoda. Pravilan izbor sirovina i dobro kontrolisano kuvanje/obrada namirnica, u kombinaciji sa smanjenjem uzimanja nekih visoko zagađenih namirnica, može da nam pruži mogućnost da se smanji izloženost uticaju štetnih sastojaka hrane. Kjluče reči: kancerogene materije, kontaminirana hrana, grejanje Rad primljen: 20.08.2011. Pregledni rad

A. ANGAMUTHU et al ... SODIUM TUNGSTATE – Zn2+ AS CORROSION INHIBITOR ...


A. ANGAMUTHU, C. THANGAVELU, Scientific paper S.RAJENDRAN, T. ASOKAN UDC:620.197.3:669.14

Sodium tungstate – Zn2+ as corrosion inhibitor for carbon steel

The inhibition efficiency of a sodium bisulphite (SBS) in combination with a bivalent cation like Zn2+ in controlling corrosion of carbon steel was investigated by weight – loss method. A formulation consisting of Zn2+, sodium tungstate, ST, and SBS can be used as a potent inhibitor to prevent the corrosion attack to carbon steel in neutral aqueous media. The ternary system ST (20ppm), Zn2+ (50ppm) and SBS (10ppm) is effective and has 95% I.E. The protective film consists of Zn(OH)2 and complexes of Fe2+ / Fe3+ and Zn2+ with SBS and ST. These complexes formation are confirmed by UV-Visible, FTIR and atomic force microscopic (AFM) spectra. The surface morphology of the protective film on the metal surface is also characterized by AFM. Key words:- Corrosion inhibition, carbon steel, sodium tungstate, sodium bisulphite, Zn2+, UV-visible and FTIR spectra, atomic force microscopy, mechanism.

1. INTRODUCTION

Due to its low cost, availability etc., carbon steel is considered as in wide area of practical applications such as water pipelines [1, 2], cooling water systems [3], boilers etc, due to its low cost, availability etc. However they are susceptible to different forms of corrosion inducted by chloride and so on. One of the most important methods in corrosion protection is to use inhibitors [4, 5], inhibitors should be of low toxicity and easily biodegradable in order to meet environmental protection requirements. Tungstate is an environmentally friendly inhibitor and has been extensively studied for its application in the protec-tion of iron, zinc and aluminium substrates in neutral, acidic and alkaline solutions. Normally it is not feasible to use tungstate alone as a corrosion inhibitor due to its low inhibition ability at low concentrations of tungstate and high cost. In most causes, the combinations of tungstate with co-inhibitors were described in the literature [6-10]. However previous literature has not reported the use of tungstate as the corrosion inhibitor for carbon steel with more than one co inhibitors. Phosphates in the form of polypho-sphates control the corrosion of ferrous metals [11]. They act as cathodic inhibitors [12, 13].

The objectives of the present study are as follows a) To study the synergistic inhibition of corrosion

of carbon steel using sodium tungstate and zinc ions (elevator 1)

b) To study the synergistic inhibition of carbon steel using sodium tungstate, zinc ions and inorganic additives. Here, the aim is to reduce the amount of

Author's address: Post Graduate and Research De-partment of Chemistry Periyar E.V.R. College (Autono-mous) Tiruchirappalli – 620 023, Tamil Nadu, India.

Paper received: 25. 10. 2011.

sodium tungstate as well as zinc ions by introducing inorganic additive as second elevator. In the present study, cheaply and abundantly available inorganic salt such as sodium bi sulphite used as inorganic additive.

c) To investigate the nature of the protective film formed on the metal surface by using FTIR spectro-scopy and atomic force microscopy. The mechanistic aspects of corrosion inhibition are studied in a holistic way, based on the results obtained from the classical weight-loss method and different surface examination techniques mentioned above and UV – visible absorption spectra of the solution.

2. EXPERIMENTAL

2.1. Preparation of carbon steel samples The composition of all carbon steel samples used

in this study was as follows: sulphur – 0.026 %, phosphorus – 0.06 %, manganese - 0.4 %, carbon – 0.1 % and the rest iron.

The carbon steel specimens of the dimensions 1.0 x 4.0 x 0.2 cm were polished to mirror finish and then degreased with acetones. They were used in the weight loss method and surface examination studies.

2.2. Weight - loss method Determination of surface area of the specimens

were determined with specimens and the length, breadth and the thickness of carbon steel specimens and the radius of the hole were determined with the help of a vernier calliper of high precision and the surface areas of the specimens were calculated.

2.3. Weighing the specimens before and after immersion All the weighing of the carbon steel specimens

before and after immersion were carried out using a ANAMED™ Electronic balance–AE 240 dual range



balance with readability of 0.01 mg in 40 g range and 0.1 mg in 200g range. This balance has repro-ducibility (standard deviation) of 0.02 mg in 40 g range and 0.1 mg in 200g and was supplied by instrument AG, CH-8606 Greifenesee, Switzerland.

2.4 Determination of corrosion inhibition efficiency The weighed specimens in triplicate were

suspended by means of glass hooks in 100 ml beakers containing 100 ml of different test solutions. After immersion, the samples were taken out, washed and weighed. From the change in weight of the specimens corrosion inhibition efficiency (I.E) was calculated using the equation.

I.E. = 100 [1- (W2 /W1)], in %

Where: W1 – corrosion rate in the absence of inhibitor, and W2 – corrosion rate in the presence of inhibitor.

2.5 UV- visible absorption spectra study The possibility of formation of Zn2+ - sodium

bisulphite complex, Zn2+- sodium tungstate complex, Fe3+ - sodium bisulphite complex, Fe3+- sodium tung-state complex in solutions was examined by mixing the respective solutions and recording their UV-vi-sible absorption spectra using Hitachi U-3400 spectrophotometer.

2.6 Surface examination study FTIR spectra After two days of immersion in corrosion media,

the samples were taken out of the test solutions and dried. The film formed on the surface was scratched carefully and it was thoroughly mixed so was to make it uniform throughout. FTIR spectrum of the powder (KBr pellet) was recorded using Perkin – Elmer 1600 FTIR spectrophotometer with a resolving power of 4 cm-1.

Surface morphology (AFM) Atomic force microscopy is a powerful technique

for the gathering of roughness statistics from a variety of surfaces (14). AFM is becoming an accepted method of roughness investigation (15-19)

All atomic force microscopy images were obtain-ned on a pico SPM 2100 AFM instrument operating in contact mode in air, the scan size of all the AFM images are 30 µm X 30µm areas at a scan rate of 2.4 lines per second.

3. RESULT AND DISCUSSION

3.1 Analysis of weight – loss method Corrosion inhibition efficiency (I.E) of carbon

steel in aqueous environment(100ppm Cl-) in the absence and presence of inhibitors obtained by weight

loss method are given in table 1 at pH = 7. It is ob-served that sodium tungstate (ST) has some efficiency in controlling corrosion of carbon steel. For 50 ppm - Zn2+ alone, inhibition efficiency is -25%. When 50 ppm Zn2+ is added to 5 ppm sodium tungstate, I.E. is 75%. So there is a synergistic effect between sodium tungstate and Zn2+. Upon addition of various con-centrations of sodium tungstate with 10 ppm so-diumbisulphite (SBS) in I.E is observed due to in-crease in rate of deposition of complex on metal surface. After reaching a high I.E, there is a decrease in I.E. due to desorption of formed complex from metal surface into the bulk of the solution.

3.2 Analysis of Influence of duration of immersion on the inhibition efficiency of sodium tungstate – Zn2+- sodium bisulphite system

It is observed that from Table - I as the duration of immersion decreased. This is due to the fact that the protective film formed on the metal surface , probably Fe2+/Fe3+ - active principle complex, is broken by the corrosive environment and the film is dissolved.

Table I - Corrosion inhibition efficiency of carbon steel in chloride aqueous environment (chloride = 100 ppm), in the presence and absence of inhibitor obtained by the weight – loss method

Inhibitor system: Sodium tungstate + Zn2+ + NaHSO3

Immersion period: I day, III day, V day, VII day; pH=7

I.E. % S.No

Concn. of ST (ppm)

Concn. of Zn2+

(ppm)

Concn. of SBS (ppm)

I day

III day

V day

VII day

1. 0 0 0 - - - -

2. 5 0 0 3 2 1 1

3. 0 50 0 -25 -30 -35 -33

4. 5 50 10 75 70 60 58

5. 10 50 10 80 75 65 55

6. 15 50 10 85 76 68 53

7. 20 50 10 95 79 70 50

8. 25 50 10 85 65 65 48

9. 30 50 10 80 60 63 45

10. 5 50 - 40 30 28 26

3.3. Analysis of UV – visible spectra It seems that there is an increase in absorbance

(Figures I andii ). This suggests that there will be the formation of complexes with carbon steel.



Figure i - UV – visible absorption spectrum of 25

ppm SBS +25 ppm ST +50ppm Zn2+

Figure (ii) - UV – visible spectrum of 25 ppm SBS +25ppm ST + 50ppm Zn2+ + 100ppm Fe2+

3.4 Analysis of FTIR spectra

The FTIR spectrum of pure sodium bisulphite is shown in figure iii. The bisulphite stretching frequ-ency of sodium bisulphite appears at 2877cm-1. The FTIR spectrum of pure sodium tungstate is shown in figure iv. The WO4

2- stretching frequency appears at 1639 cm-1. The FTIR spectrum (KBr Pellet) of the film formed an carbon metal surface after immersion in the aqueous solution containing 100ppm Cl- +25 ppm sodium bisulphite +25 ppm sodium tungstate +25 ppm Zn2+ is shown in figure V. The bisulphite stretching frequency of sodium bisulphite is shifter from 2877 cm-1 to 2862 cm-1. This suggest that bisul-phite is coordinative Fe3+ resulting in the formation of Fe3+- HSO3

- complex. The WO42- stretching frequ-

ency is shifted from 1639 cm-1 to 1635cm-1 due to the formation of Fe3+ - WO4

2- complex. The band at 1365 cm-1 due to Zn(OH)2 is shifted to 1363cm-1 formed on the cathodic sites of the metal surface.

cm-1 Figure (iii) - FTIR spectrum of pure SBS

cm-1 Figure-(iv) FTIR spectrum of pure ST

cm-1 Figure (v) - FTIR spectrum of film formed on the car-

bon steel surface after immersion in the aqu-eous environment containing 100 ppm Cl- +25 ppm SBS + 25 ppm ST + 25ppm Zn2+

3.5. Analysis of atomic force microscopy AFM is becoming an accepted method of rough-

ness investigation [20-24]. All atomic force micro-scopy images were obtained on a pico SPM 2100 AFM instrument operating in contact mode in air.



The scan size of all the AFM images are 5µm X 5µm areas at a scan rate of 2.4 lines per second.

Table II - AFM parameters for carbon steel surface immersed in inhibited and uninhibited environments

Samples RMS (Rq) Roughness

(nm)

Average

(Ra)

Roughness

(nm)

Maximum peak-to- Valley height

(P-v) (nm)

Polished metal

(control) 195 159 776

Carbon steel

immersed in Cl-(100ppm)+SBS (25ppm)+ST (25ppm)+Zn2+

(25ppm )

81.7 69.8 301.6

The two dimensional (2D), three dimensional

(3D) AFM morphologies and the AFM cross – secti-onal profile for polished carbon steel surface (refe-rence sample), and carbon steel immersed in aqueous solution containing 100 ppm (Cl-) + 25ppm (SBS) + 25ppm (ST) +25ppm (Zn2+), are shown as Figure vi (a,b), Figure vii (c,d) .

AFM image analysis was performed to obtain the average roughness, Ra (the average deviation of all points roughness profile from a mean line over the evaluation length), root-mean-square roughness, RRMS (the average of the measured height deviations taken within the evaluation length and measured from the mean line) and the maximum peak-to-valley (P-V) height values (largest single peak-to valley height in five adjoining sampling heights) [25]. RRMS is much more sensitive than Ra to large and small height deviations from the mean [26].

The Table II shows the various parameters obtain-ned for carbon steel immersed in different environ-ments.

The value of RRMS, Ra and P-V height for the polished carbon steel surface (reference sample) are 195nm, 159nm and 776nm respectively, which shows a more homogeneous surface, with some places in where the height is lower than the average depth. Figure vi (a), Figure vii (c) displays the uncorroded metal surface. The slight roughness observed on the polished carbon steel surface is due to atmospheric corrosion. The RRMS roughness, average roughness and P-V height values for the carbon steel immersed in aqueous solution containing 100ppm Cl-, 25ppm

(SBS),25ppm (ST), 25ppm (Zn2+) are reduced to 81.7nm, 69.8nm and 301.6nm.

These parameters confirmed that the surface appears smoother. The smoothness of the surface is due to the formation of a compact protective film of Fe2+ – SBS-ST complex and Zn(OH)2 on the metal surface and there by inhibiting the corrosion of carbon steel.

Figure vi (a) – 2D AFM images of polished carbon

steel surface (control)

Figure vi (b) -2D Carbon steel immersed in Cl-

(100ppm), SBS(25ppm), ST (25ppm), Zn2+ (25ppm).

Figure vii (c) – 3D AFM images of polished carbon

steelsurface (control).



Figure vii (d) – 3D Carbon steel immersed in Cl-

(100ppm), SBS (25ppm), ST (25ppm), Zn2+ (25ppm).

3.6. Corrosion process In order to explain the experimental results, the

following mechanism of corrosion inhibition is proposed. The mechanistic aspect of the inhibition of carbon steel in aqueous solution has 100ppm Cl- by ST – Zn2+ and SBS can be explained in terms of complexation.

i) Before immersion of carbon steel in aqueous environment, ST, Zn2+ and SBS, zinc ions form complexes, viz, Zn2+, ST and SBS ions.

ii) During the dissolution of iron, the pH incre-ases at the metal/electrolyte due to oxygen reduction. Thus Zn(OH)2 precipitate may take place at cathodic sites thus decreasing the rate of further oxygen reduction.

iii) Addition of SBS reduces metal dissolution; this may be due to adsorption and complex formation at the surface with the combined application of Zn2+

and SBS, the corresponding anodic and cathodic reactions of the metal can be generalized as follows. Zn2+ inhibited the local cathodic region and the local anodic region was inhibited by SBS.

iv) The Zn2+ – SBS complex diffuses from the bulk solution to the surface of the metal and is con-verted into a Fe- SBS complex, which is more stable than Zn2+ – SBS. The released Zn2+, causes Zn(OH)2 precipitation at the local cathodic sites. Thus the pro-tective film consists of an Fe- SBS complex and Zn(OH)2.

(v)The film formed on the metal surface of the carbon steel consists of oxides / hydroxides of iron and zinc. It is also likely to comprises of complexes of Fe2+/Fe3+ and Zn2+ with ST as well as with SBS.

(vi) The FTIR spectrum of the surface film suggests, the formation of these complexes and the presence of Zn(OH)2in the surface.

4. CONCLUSION

A formulation consisting of Zn2+ , ST and SBS can be used as a potential corrosion inhibitor of carbon steel in neutral. 1. SBS plays an excellent synergistic role in the ST

– Zn2+- SBS system. 2. The ternary system ST (20ppm) –Zn2+ (50ppm)-

SBS (10ppm) is effective and has 95% I.E. 3. Significant synergism was attained by the

combined application of ST – Zn2+-SBS. 4. This new inhibitor formulation is more

environment – friendly. 5. Both ST and SBS form stable complexes with

metal ions in the metal surface. 6. The protective film consists of Zn(OH)2 and com-

plexes of Fe2+ / Fe3+ and Zn2+ with SBS and ST. 7. UV-visible spectra confirm the formation of

these complexes. 8. The protective film formation was also confirmed

by atomic force microscopy study.

5. REFERENCES

[1] R.E.Melchers, R.Jeffery, Corrosion Reviews, 1(2005) 84.

[2] R.E.Melchers, R.Jeffery, Corrosion Reviews 6(2005) 297.

[3] G. Saha, N. Kurmaih, N.Hakerman, Journal of Physis chem., 59(1955)707.

[4] Q-J.Xu, Z. Lu, G.D. Zhou, Journal of East China University of Science and Technology , 2(29) ( 2003) 493.

[5] Q-J.Xu, Z. Lu, G.D. Zhou, ActaChemicaSinica, 59 (2001) 950.

[6] Z.Baiqing,L.Xia,X.Shujiao,Materials performance .47(9) (2008) 50.

[7] M.A.Amin, H.H.Hassan, O.A. Hazzazi, M.M. Qhatani, Journal of Applied Electrochemistry, (2008) I. [Article in press].

[8] M.Abdallah, A.Y.E1-Etre, M.G.Soliman, E.M. Mabrouk, Anti-Corrosion Methods and Materials, 53 (2) (2006) 118.

[9] S.M.A. Shibji., V.S.Saji, Corrosion Prevention and Contol, 50 (3) (2003) 136.

[10] J.M. Abd EI Kader, A.A. EI Warraky, A.M. Abd EI Aziz, British Corrosion Journal 33(2) (1998) 152.

[11] K.Madhavan. S.Muralidharan, S.K.V.Iyer, Bull. Elecrochem 17 ( 2001) 215.

[12] G.B. Hatch, Ind. Engg. Chem 44(1952) 1775. [13] K.S.Rajagopalan and K.Venu, Ind.J.Technol

6(1968) 239. [14] S. Rajendran, B.V.Apparao and N. Palaniswamy

Proc. 2nd Arabian corrosion conference, Kuwait, (1996),483.



[15] R.Vera, R.Schrebler, P.Cury, R.Del Rio and H.Romero, J. ApplElectrochem, 37 ( 2007) 519-525.

[16] Ph. Dumas, B. Butffakherddine, C. AmO.Vatel, E.Ands.R.Galindo and F. Salvan, Quantitative Microroughness analysis down to the nanometer scale, Europhys. Left 22 ( 1993) 717-722.

[17] J, M. Bennett, J. Jahannir, J.C. Podlesny. T, L. Baiter and D.T. Hobbs, Scanning force microscopy as a tool for studying optical surfaces, Appl. Opt., 43 (1995) 213 – 230.

[18] A.Duparre, N.Kasier, H.Truckenbrodi, M.Berger and A.Kohler, Microtography investigations of opticalsurfaces and thin films by light scattering, optical profilometry, and atomic force microscopy, Int, Symp.on optics,imaging and instrumentation, 1 1-16 Jul 1993. San Diego CA, Proc. SPIE 1995 (1993) 181-192.

[19] A. Duparre, N. Kasier and S,Jokobs,Morphology investigation by Atomic Force Microscopy of thin films and substances for excimer laser mirrors, Annual Symp.on Optical Materials for High Power Lasers. 27-29 October 1993, Boulder, CO, Proc.S.SPIE. 2114 (1993) 394.

[20] R.Vera, R.Schrebler, P.Cury, R.Del Rio and H.Romero, J ApplElectrochem, 37(2007) 519-525.

[21] N. Antony, H. B. Sherine, S. Rajendran, Investi-gation of the inhibiting effect of nano film by sodium meta silicate- Zn2+ system on the corrosion of carbon steel in neutral chloride solution, Zastita materijala, 51(2010) No 1, p.11-19

[22] Ph. Dumas, B.Butffakhreddine, C.Am. O.Vatel, E. Ands.R.Galindo and F. Salvan, Quantitative microroughness analysis down to the nanometer scale, Europhys. Lett. 22 (1993) 717-722.

[23] M.Aballah,A.S.Fouda,S.M.El-Ashrey, Zastita mate-rijala, 49 (2008) 9-23.

[24] J.M Bennett, J. Jahannir, J.C. Podlesny. T,L.Baiter and D,T. Hobbs, Scanning force microscopy as a tool for studying optical surfaces, Appl. Opt., 43 (1995) 213-230.

[25] A.Duparre, N.Kaiser, H.Truckenbrodi, M Berger and A. Kohler, Microtopography investigations of optical surfaces and thin films by light scattering, optical profilometry, and atomic force microscopy, Int,. Symp.on Optics, Imaging and instrumentation, l 1-16 Jul 1993. San Diego, CA, Proc. SPIE. 1995 (1993) 181-192.

[26] Duparre, N.Kaiser, and S.Jakobs, Morphology investigation by Atomic Force Microscopy of thin films and substances for excimer laser mirrors, Annual Symp. on Optical Materials for High Power Lasers. 27-29 October 1993, Boulder, CO. Proc. SPIE, 2114 (1993) 394.

[27] C. Amra C. Deumie D. Torricini P. Roche, R. Galindo. P. Dumas and F. Salvan, Overlapping of roughness spectra measured in microscopic (optical) and microscopic (AFM) bandwiths, Int. Symp. on Optical Interference Coatings. 6-10 June 1994. Grenoble, Proc. SPIE, 2253 (1994) 614 -630.

[28] T.R. Thomas “Rough Surfaces”, Longman New York,1982

ИЗВОД

НАТРИЈУМ ВОЛФРАМАТ–Zn2+ КАО КОРОЗИОНИ ИНХИБИТОР ЗА УГЉЕНИЧНИ ЧЕЛИК Ефикасност инхибиције натријум-бисулфитом (SBS) у комбинацији са двовалентним катјоном као што је Zn2+ у контроли корозије угљеничног челика истражена је методом губитка масе. Формулација која се састоји од Zn2+, натријум-волфрамата, ST, и SBS може да се користи као снажан инхибитор у спречавању корозије угљеничног челика у неутралној воденој средини. Тернарни систем ST (20 ppm), Zn2+ (50 ppm) и SBS (10 ppm) је ефикасан и има инхибирајућу ефикасност од 95%. Заштитни филм се састоји од Zn(OH)2 и комплекса Fe2+/Fe 3+ и Zn2+ са SBS и ST. Формирање ових комплекса је потврђено UV-Visible, FTIR и AFM спектрима. Морфо-логија заштитног филма на металној површини је окарактерисана применом микроскопије засноване на међуатомским силама (AFM). Кључне речи: Инхибиција корозије, угљенични челик, натријум-волфрамат, натријум-бисулфит, Zn2+, UV-Visible и FTIR спектри, микроскопија заснована на међуатомским силама, механизам. Рад примљен: 25. 10. 2011. Оригинални научни рад

V. PANCHAL et al ... INHIBITION OF Al-Mg ALLOY IN HYDROCHLORIC ACID …


VIDHI PANCHAL Scientific paper AESHA PATEL, NISHA SHAH UDC :620.197.3 :669.715

Inhibition of Al-Mg alloy in hydrochloric acid using schiff bases as corrosion inhibitors

The effect of some new synthesized Schiff bases namely o-Chloroaniline-N-(p-methoxy benzylidene) and p-Chloroaniline-N-(p-methoxy benzylidene) on the corrosion of Al-Mg alloy in 2 M HCl was investigated. The investigation involved weight loss, galvanostatic polarization and EIS measurements. The inhibition efficiency increased with increase in inhibitor concentration but decreased with increase in temperature. Schiff bases are found to adsorb on the metal surface according to the Langmuir isotherm. Thermodynamic parameters and activation energy were calculated to elaborate the mechanism of inhibition. The results of polarization method show that these Schiff bases act as mixed type, but the cathode is more preferentially polarized. Key words : Aluminium, EIS, Polarization, Weight loss, Acid inhibition

1. INTRODUCTION

The reaction and subsequent deterioration of metals when exposed to the environment encom-passes a fundamental principle of electrochemistry and metallurgy, known as corrosion. Corrosion is de-fined as the deterioration of the material, usually a metal, because of reaction with its environment and which requires the presence of an anode, a cathode, an electrolyte and an electrical circuit.

The aluminum and its alloys are important ma-terials due to their high technological value and wide range of industrial applications, especially in aero-space, household industries, and commonly used in marine applications as well. In addition, they are justified by low price, high electrical capacity and high energy density [1]. Many researchers were devo-ted to study the corrosion of aluminum in different aqueous solutions [2–7], and research into their ele-ctrochemical behavior and corrosion inhibition in wide variety of media [8–13].

Hydrochloric acid and sulphuric acid solutions are used for pickling of aluminium or for its chemical or electrochemical etching. It is very important to add corrosion inhibitors to decrease the rate of metal dissolution in such solutions. An important method of protecting materials against deterioration from corro-sion is by using inhibitors [14-16]. Acid inhibitors have many important roles in the industrial field as a component in pre-treatment composition, in cleaning

Author's address: Department of Chemistry, School of Sciences, Gujarat University, Ahmedabad-09, Gujarat, India


solution for industrial equipment and in acidization of oil wells. The inhibition of aluminium and its alloys in acidic solutions were extensively studied using organic compounds [17-24]. It was found that the organic compounds are effective corrosion inhibitors due to their ability to form an adsorbed protective film at the metal surface. The adsorption of the sur-factant on the metal surface can markedly change the corrosion resisting properties of the metal [25, 26]. So the study of the relations between the adsorption and corrosion inhibition is of great importance.

Some Schiff bases have recently reported as effective corrosion inhibitors for steel [27, 28], alu-minium [29], aluminium alloys [30, 31], and copper [32] in acidic media. Due to the presence of the > C = N – group, electronegative nitrogen, sulfur and/or oxygen atoms in the molecules, Schiff bases should be good corrosion inhibitors. The action of such inhibitors depends on the specific interaction between the functional groups and the metal surface. So it is very important to clarify the interactions between inhibitor molecules and metal surfaces in order to search new and efficient corrosion inhibitors [33-36].

In earlier work the inhibition of corrosion of zinc in sulphuric acid by Schiff bases of ethylenediamine [37], ortho-substituted aniline-N-salicylidenes [38], ortho-, meta-, and para-aminophenol-N-salicylidenes [39], meta-substituted aniline-N-salicylidenes [40] and Salicylidene-N-N’-morpholine [41] has been reported.

In the present work, the inhibitive effectiveness of o-Chloroaniline-N-(p-methoxy benzylidene) [o-CANPMB] and p-Chloroaniline-N-(p-methoxy ben-zylidene) [p-CANPMB] have been studied in re-tarding corrosion of Al-Mg alloy in 2.0 M HCl.



Weight loss method, galvanostatic polarization, ele-ctrochemical impedance spectroscopy (EIS) and quantum chemical calculation techniques were used to investigate the inhibiting influence of inhibitors.

2. EXPERIMENTAL 2.1. Synthesis of Schiff bases

The Schiff bases used in the present work were synthesized from equimolar amount of p-methoxy benzaldehyde and corresponding aniline (o- and p-chloro aniline) through a condensation reaction in ethanol media as per described by Shah et al. [42]. o-CANPMB and p-CANPMB are insoluble in water but soluble in ethanol. The compounds were characte-rized through its structure data [IR data (Figure 1)] and its purity was confirmed by thin-layer chro-matography (TLC). The chemical structure of the investigated compounds are given below:

N

Cl

OCH3

o-CANPMB

NCl

OCH3

p-CANPMB

2.2. Electrodes and electrolytes

Corrosion test were performed using coupons prepared from Al-Mg alloy. The chemical compo-sition (wt%) of the Al-Mg alloy sample is Mg(2.6%), Cu(0.1%), Si(0.6%), Fe(0.5%), Mn(0.5%), Cr(0.4%) and balance Al. Rectangular specimens of Al-Mg alloy of size 6 cm × 3 cm and thickness 0.087 cm with a small hole of about 2 mm diameter just near the upper end of the specimen were used for the determination of weight losses. The specimens were polished with ‘0’, ‘00’, ‘000’ and ‘0000’ grade Oakey emery paper. The specimens degreased by A. R. carbon tetrachloride (sulphur free).

For polarization and impedance measurements, metal coupons of circular design, diameter 2.802 cm with a handle 3 cm long and 0.5 cm wide and thickness 0.087 cm with a small hole of about 2 mm diameter just near the upper end of the specimen were used. The handle and the back of the coupon and of the auxiliary platinum electrode were coated with Perspex leaving only the circular portion of the specimen of apparent surface area 6.156 cm2 exposed to the solution.

The corrosive solution (2.0 M HCl) was prepared by dilution of analytical grade 37% HCl (NICE) with

double distilled water. The concentration range of employed inhibitor was 0.001% to 0.5% in 2.0 M HCl. The used all chemicals for preparation of Schiff bases were AR grade (MERCK).

2.3. Measurements Three methods namely weight loss method,

impedance spectroscopy, polarization study and quantum chemical calculation were used to determine the corrosion inhibition efficiencies of o-CANPMB and p-CANPMB. 2.3.1. Weight loss method

In weight loss method, the specimens were exposed to 2.0 M HCl solution containing controlled addition of o-CANPMB and p-CANPMB in the range of 0.001% to 0.5% inhibitor concentration. One specimen only was suspended by a glass hook in each beaker containing 230 ml of the test solution which was open to the air at 35° ± 0.5°C, to the same depth of about 1.5cm below the surface of the test solution. The experiments were repeated at different temperatures, ranging from 35° ± 0.5°C to 65° ± 0.5°C in the absence and presence of 0.5% inhibitor’s concentration after 30 minutes. 2.3.2. Electrochemical measurements

Electrochemical experiments were carried out using a standard electrochemical three-electrode cell. Al-Mg alloy was used as working electrode, platinum as counter electrode and saturated calomel electrode (SCE) as reference electrode. The test solution was contained in a H-type (80 ml in each limb) Pyrex glass cell with Luggin capillary as near to the electro-de surface as possible and a porous partition to sepa-rate the two compartments. The potential was measu-red against a saturated calomel electrode (SCE), in polarization study. The specimens were exposed to 2.0 M HCl solution containing controlled addition of 0.001%, 0.05% and 0.5% inhibitor concentration in polarization study.

The corrosion parameters such as corrosion potential (Ecorr), corrosion current density (Icorr) and Tafel plots were measured in polarization method. In this study, the current density was varied in the range of 2 × 10-4 to 3.25 × 10-2 A cm-2.

Electrochemical impedance measurements were carried out in the frequency range of 20 kHz – 0.1 Hz at the open circuit potential, after 30 minutes of im-mersion, by applying amplitude of 5 mV sine wave ac signal (AUTOLAB) in the range of 0.001% to 0.1% of inhibitor concentration. Double layer capacitance (Cdl) and charge transfer resistance (Rct) values were calculated from Nyquist plots as described by Hossei-ni [43]. EIS data were analyzed using frequency response analyzer (FRA) electrochemical setup.



o-chloroaniline-N-(p-methoxy benzylidene) [o-CANPMB]

aromatic ring :- 3130 nm, >C=N- :- 1642 nm, -C-OCH3 :- 1263 nm, -C-Cl :- 754 nm

m-chloroaniline-N-(p-methoxy benzylidene) [p-CANPMB]

aromatic ring :- 3165 nm, >C=N- :- 1634 nm, -C-OCH3 :- 1242 nm, -C-Cl :- 737 nm Figure 1 - IR Spectra of o-CANPMB and p-CANPMB

2.3.3 Quantum chemical calculations The quantum chemical calculation was performed

using Discovery studio 2.1 (Accelrys Inc.USA) program. MNDO semi-empirical method was employed to obtain the optimized geometry.

3. RESULTS AND DISCUSSION

3.1. Weight loss method 3.1.1. Effect of inhibitor concentration

The weight loss method of monitoring corrosion rate is useful because of its simple application and

reliability [44]. Weight loss of Al-Mg alloy was determined after 30 minute of immersion in 2.0 M HCl in the absence and presence of various concen-trations of o-CANPMB and p-CANPMB at 35°C. Table-1 shows that the calculated values of inhibition efficiency (%IE) and surface coverage (θ) for Al-Mg alloy dissolution in 2.0 M HCl in the absence and presence of inhibitors.

The inhibition efficiency (%IE) and surface cove-rage (θ) were calculated using following equations [45],



Inhibition efficiency (%IE) = u i

u

W WW−

X 100 (1)

Surface coverage (θ) = u i

u

W WW−

(2)

where Wu and Wi are the weight loss of Al-Mg alloy in 2.0 M HCl in the absence and presence of o-CANPMB and p-CANPMB.

Table 1 - Corrosion parameters for Al-Mg alloy in the presence and absence of different concentrations of o-CANPMB and p-CANPMB obtained from weight loss measurement at 35oC ± 0.5oC for exposure period of 30 minutes

Inhibitor Concent-

ration (% V/V)

Weight loss (mg

dm-2)

Surface coverage

(θ)

(%IE)

Blank - 1120 - -

0.001 617 0.4491 44.9

0.01 364 0.6750 67.5

0.05 221 0.8026 80.3

0.10 141 0.8741 87.4

o-CANPMB

0.50 19 0.9830 98.3

0.001 388 0.6535 65.4

0.01 263 0.7651 76.5

0.05 165 0.8526 85.3

0.10 96 0.9142 91.4

p-CANPMB

0.50 5 0.9955 99.6

It is clearly seen from the Table 1 that corrosion rates were reduced in the presence of o-CANPMB and p-CANPMB compared to their absence for Al-Mg alloy in 2.0 M HCl. Inhibition efficiency as given in Table-1 is found to increase with increase in con-centration of inhibitors. Inhibition efficiency incre-ases with inhibitor concentration to reach 99.6% for o-CANPMB and 98.3% for p-CANPMB at 0.5%, res-pectively. Increased inhibition efficiency with con-centration indicates that more o-CANPMB and p-CANPMB components are adsorbed on the metal surface at higher concentration leading to greater surface coverage.

This actually shows that the addition of inhibitors to the acid solution retards the corrosion rate of metal and that the extent of retardation is concentration depended. Both o-CANPMB and p-CANPMB inhibit the corrosion of Al-Mg alloy in 2.0M HCl in the range of concentrations studied but p-CANPMB is found to be more effective then o-CANPMB.

3.1.2. Adsorption behavior As known that organic inhibitors establish their

inhibition via the adsorption of the inhibitors mole-cules onto the metal surface. For organic inhibitors that have the ability to adsorb strongly on metal surface, thus impeding the dissolution reaction, the surface coverage (θ) can be evaluated as the inhi-bition efficiency. The relationship between the inhi-bition efficiency and bulk concentration of the inhi-bitor at constant temperature, which is known as isotherm, gives an insight into the adsorption process.

It is generally assumed that the adsorption of the inhibitor at the metal/solution interface is the first step in the mechanism of inhibition in aggressive media. Two main types of interaction can describe the adsorption of inhibition namely: 1) Physical adsor-ption, 2) Chemical adsorption. The proceeding of physical adsorption requires the presence of electri-cally charged metal surface and charged species in the bulk of the solution [46]. While chemical adsorption process involved charge sharing or charge transfer from the inhibitor molecules to the vacant p-orbital in Al surface [46].

log Cinh (mole/l)

log

(θ/1

-θ)

0.8

1.0

1.2

1.4

1.6

1.8

2.0

-2.5 -2.3 -2.1 -1.9 -1.7

Figure 2 - Langmuir isotherm for adsorption of Schiff

bases on the Al-Mg alloy metal surface The extent of corrosion inhibition depends on the

surface conditions and the mode of adsorption of the inhibitors [47], under the assumptions that the corrosion of the covered parts of the surface is equal to zero and that corrosion takes place only on the uncovered parts of the surface. The interaction between the inhibitor and the metal surface can be examined by the adsorption isotherm. The values of surface coverage, θ, for the different concentrations of the studied inhibitors have been used to explain the best adsorption isotherm to determine the adsorption process. The surface coverage (θ) values were calculated from Eq.(2) and shows in Table 1. The data obtained from weight loss method has been tested with several adsorption isotherms [Langmuir, Fruendlich, Temkin]. Langmuir adsorption isotherm was found to fit well with our experimental data. A straight line was obtained on plotting log (θ/1-θ) vs

o-CANPMB p-CANPMB



log Cinh (concentration of inhibitors) as shown in Figure 2, which suggested that the adsorption of the inhibitors used for 2.0 M HCl solution on metal follows Langmuir adsorption isotherm given by Eq.3.

Cinh =

K(1- ) θθ (3)

where Cinh is the inhibitor concentration, θ is the surface coverage values and K is the equilibrium constants of adsorption process. Thus, these results suggest that there are no interaction or repulsion forces between the adsorbed molecules. Langmuir isotherm assumed that the solid surface contains a fixed number of adsorption sites and each site holds

one adsorbed species [48] and the adsorption of organic molecule on the adsorbent is monolayer.

3.1.3. Effect of temperature The temperature could affect the interaction

between the metal surface and acidic media in the absence and presence of inhibitors. The effect of tem-perature on the inhibition efficiency in 2.0 M HCl containing 0.5% of o-CANPMB and p-CANPMB at temperature ranging 35°C to 65°C was obtained to calculate the thermodynamic parameters. The results are given in Table-2.

Table 2 - Effect of temperature on weight loss and inhibition efficiency for Al-Mg alloy in 2.0 M hydrochloric acid

Weight loss (mg dm-2) at temperature Inhibitor

Concentration (% V/V) 35o C 45o C 55o C 65o C

Blank - 1120 2399 4910 7608

o-CANPMB 0.5 19

(98.3%) 56

(97.7%) 133

(97.3%) 375

(95.1%)

p-CANPMB 0.5 5

(99.6%) 19

(99.2%) 80

(98.4%) 160

(97.9%)

At constant inhibitor concentration the inhibition efficiency decreases with increasing temperature. This is due to increased effect of temperature on the dissolution process of metal and partial desorption of the inhibitor from the metal surface. The decrease in inhibition efficiency shows that the film formed on the metal surface is less protective at higher temperature. 3.1.4 Thermodynamic parameters

In acidic solution the corrosion rate is related to temperature by Arrhenius equation [49],

ρ = A exp ﴾− aERT ﴿ (4)

where ρ is corrosion rate determined from the weight loss measurement, Ea is the apparent activation energy, A is the Arrhenius constant, R is the molar gas constant and T is the absolute temperature. The apparent activation energy was determined from the slopes of log ρ versus 1/T × 104 graph depicted in Figure 3.

The values of activation energies were calculated and given in Table-3. These values indicate that the presence of inhibitors increase the activation energy of the metal dissolution reaction. Inspection of Table-3 shows that the values of Ea determined in solutions containing o-CANPMB and p-CANPMB are higher

then that of in the absence of inhibitors (blank). The increase in Ea in the presence of inhibitor may be interpreted as physical adsorption that occurs in the first stage [50]. Szaues and Brandt [51], explained that the increase in activation energy can be attributed to an appreciable decrease in the adsorption of the inhibitor on the metal surface with increase in temperature. A corresponding increase in the corrosion rate occurs because of the greater area of metal that is consequently exposed to the acid environment [52].

The values of activation energies were calculated and given in Table-3. These values indicate that the presence of inhibitors increase the activation energy of the metal dissolution reaction. Inspection of Table-3 shows that the values of Ea determined in solutions containing o-CANPMB and p-CANPMB are higher then that of in the absence of inhibitors (blank). The increase in Ea in the presence of inhibitor may be interpreted as physical adsorption that occurs in the first stage [50]. Szaues and Brandt [51], explained that the increase in activation energy can be attributed to an appreciable decrease in the adsorption of the inhibitor on the metal surface with increase in temperature. A corresponding increase in the corro-sion rate occurs because of the greater area of metal that is consequently exposed to the acid environment [52].



0.0

0.6

1.2

1.8

2.4

3.0

3.6

4.2

4.8

29 30 31 32 33 34 35

T –1 × 104

Log ρ

(mg

dm–2

)

Figure 3 - Plotting log ρ vs. T–1 × 104 to calculate the activation energy of corrosion process in the presence

and absence of o-CANPMB and p-CANPMB Table 3 - Thermodynamic parameters and activation

energy for inhibitor adsorption in 2.0 M HCl for temperature range of 35°- 65°C at 0.5% Inhibitor Concentration.

Inhibitor Tempe-rature

Ea (kJ mol-1)

Qads (kJ mol-1)

∆Gads (kJ mol-1)

Blank - 59.8 - -

o-CANPMB 85.8 -30.5 -30.9

p-CANPMB 102.1 -46.4 -33.3

The higher values of Ea in the presence of o-

CANPMB and p-CANPMB compared to that in their absence and the decrease in the inhibition efficiency (%IE) with rise in temperature is interpreted as an indication of physisorption [53, 54].

If it is assumed that the inhibitor is adsorbed on the metal surface in the form of a monolayer film, covering at any instant a fraction, θ, of the metal surface in a uniform random manner, then the heat of adsorption (Qads) of the inhibitor can be calculated from the equation [55]:

(Qads) = 2.303R 2 1 1 2

2 1 2 1

log log1 1

T TT T

θ θθ θ

− − − −

(5)

where θ1 and θ2 are the values of surface coverage at temperature T1 and T2, respectively.

The values of the free energy of adsorption (∆Gads) were calculated from the following equation [55]:

log Cinh = log Blog1

−−θθ

(6)

where

log B = − 1.74 − [ 2.303∆Gads

RT ] (7)

The values of ∆Gads and Qads are shown in Table-3. The negative values of free energy of adsorption ∆Gads, as recorded in Table-3, indicate the sponta-neous adsorption of inhibitor molecules on metal surface. The large negative values of ∆Gads indicate the spontaneous adsorption of inhibitor molecules and are usually characteristics of strong interaction with the metal surface [56, 57].

Generally values of ∆Gads up to –20 kJ mol–1 are consistent with the electrostatic interaction between the charged molecule and the charged metal (physical adsorption), while those more negative then –40 kJ mol–1 involve charge sharing or transfer from inhibitor molecules to the metal surface to form a co-ordinate type of bend (chemical adsorption) [56]. In the present work, the calculated ∆Gads values are almost slightly less negative than –40 kJ mol–1 indicating that the adsorption of inhibitor molecule is merely physisorption or chemisorption but obeying a comprehensive adsorption (physical and chemical adsorption). It was observed, limited decrease in the absolute value of ∆Gads with an increase in the temperatures, indicating that the adsorption was somewhat unfavorable with increasing experimental temperature, indicating that physisorption has the major contribution while chemisorption has the minor contribution in the adsorption mechanism.

The negative values of the heat of adsorption (Qads) indicates that the adsorption process is exother-mic in nature. It also indicates that the degree of sur-face coverage decreased with rise in temperature, sup-porting the earlier proposed physisorption mechanism [58].

× Blank o-CANPMB p-CANPMB



3.2. Galvanostatic polarization Figure 4 show that the cathodic and anodic

polarization plots of Al-Mg alloy in 2.0 M HCl at 35°C in the absence and presence of different concentrations of o-CANPMB and p-CANPMB. Electrochemical parameters such as corrosion potential (Ecorr), cathodic and anodic slopes (bc and ba) and corrosion current density (Icorr) were extracted by Tafel extrapolating the anodic and cathodic lines

and are listed in Tabel 4. The inhibition efficiency (%IE) was calculated using the following equation [59, 60].

Inhibition Efficiency (%IE) = Iºcorr Icorr

Iºcorr −

X 100

(8)

where I°corr and Icorr are uninhibited and inhibited corrosion current density, respectively.

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-1200 -1100 -1000 -900 -800 -700 -600 -500

log

I (A

cm

-2)

E (mV vs SCE) (a)

log

I (A

cm

-2)

E (mV vs SCE)

-4

-3.5

-3

-2.5

-2

-1.5

-1

-1300 -1100 -900 -700 -500

(b)

Figure 4 - Anodic and cathodic polarization curves obtained for Al-Mg alloy metal at 35oC ± 0.5oC in 2.0 M HCl in various concentrations of studied Schiff bases (a) o-CANPMB, (b) p-CANPMB

From the results in Table-4 it can be observed that the values of corrosion current density of Al-Mg alloy in the inhibitor containing solutions were lower than those for the inhibitor free solution. The cor-rosion current densities at all inhibitor concentrations are decreased in the order o-CANPMB > p-CANPMB. This indicates on the more beneficial effect of inhibitors on corrosion inhibition of metal in

2.0 M HCl solution. From Figure 4, it is clear that both the cathodic and anodic reactions are inhibited and inhibition efficiency increased as the inhibitor concentration increased in acid media, but the cathode is more polarized. It is clear that the current density decreases with increasing of the concentration, this indicates that these compounds are adsorbed on the metal surface and hence inhibition occurs. The

♦ ♦ Blank 0.001%× × 0.05%

♦ ♦ Blank 0.001% × × 0.05%



polarization curves (Figure 4) show that these inhibitors have an effect on both, the cathodic and anodic slopes but the effect of inhibition is too small on anodic part so it can be ignored and inhibitor suppress only cathodic process at higher concentration, and indicate that the inhibitors act as adsorptive inhibitors (mainly cathodic), i.e., they retards the hydrogen evolution reaction via blocking the active reaction sites on the metal surface or even can screen the covered part of the electrode and therefore protect it from the action of the corrosion medium [61], which suggest that inhibitors are

powerfully inhibit the corrosion process of metal, and its ability as corrosion inhibitors are enhanced as its concentration is increased. The suppressed of cathodic process can be due to the covering of the surface with monolayer due to the adsorbed inhibitor molecules. Further inspection of Table-4 revels also that Ecorr values do not show any significantly change in the presence of various concentrations of the inhibitors suggesting that inhibitors can be classified as mixed type of inhibitors predominantly cathodic in 2 M HCl solution.

Table 4 - Electrochemical parameters of corrosion of Al-Mg alloy in the presence of different concentration of

o-CANPMB and p-CANPMB at 35oC ± 0.5oC and corresponding inhibition efficiencies obtained from polarization method.

Inhibitor Concentration (% W/V)

Ecorr

(mV) ba

(mV/dec) bc

(mV/dec) Icorr for

cathodic (A cm-2) (%IE)

Blank - -851 113 121 6.309 × 10-3 -

0.001 -854 134 146 2.454 x 10-3 61.1 o-CANPMB

0.05 -865 136 128 7.413 x 10-4 88.3

0.001 -893 75 108 1.202 x 10-3 80.9 p-CANPMB

0.05 -908 98 126 3.467 x 10-4 94.5

3.3 Electrochemical impedance spectroscopy (EIS) The corrosion behavior of Al-Mg alloy in acidic solution in the absence and presence of inhibitors was

investigated by the electrochemical impedance spectroscopy (EIS) at 35°C after 30 minute of immersion. The technique is based on the measurement of the impedance of the double layer at the metal/solution interface. Impedance diagrams are obtained for frequency range 20 kHz – 0.1 Hz at the open circuit potential for Al-Mg alloy in 2.0 M HCl in the presence and absence of inhibitors.

Table 5 - Impedance parameters and corresponding inhibition efficiency for the corrosion of Al-Mg alloy in the

2.0 M HCl.

Inhibitor Concentration (% W/V)

Rs (ohm)

Rct (ohm)

Cdl (µF) (%IE)

Blank - 1.036 2.89 109.7 -

0.001 1.015 5.29 103.0 45.2

0.01 0.861 8.98 96.97 67.7

0.05 0.838 15.20 88.61 80.9 o-CANPMB

0.10 0.940 24.42 78.35 88.1

0.001 0.880 8.70 106.6 66.7

0.01 0.923 12.67 98.21 77.1

0.05 0.854 21.39 92.54 86.4 p-CANPMB

0.10 0.813 37.35 84.71 92.2



(a)

(b)

Figure 5 - Impedance plot obtained at 35ºC in 2.0 M HCl in various concentrations of (a) o-CANPMB (b) p-CANPMB

The charge transfer resistance (Rct) values were calculated from the difference in impedance at lower and higher frequencies as suggested by Tsuru et. al. [62]. To obtain the double layer capacitance (Cdl) the frequency at which the imaginary component of the impedance is maximal (-Z”max) was found as represented as following equation,

Cdl = 1 Rctω (9)

Table-5 gives values of charge transfer resistance (Rct), double layer capacitance (Cdl) and solution

resistance (Rs) derived from Nyquist plots and in-hibition efficiency (%IE) calculated by the following equation

Inhibition Efficiency (%IE) = R Rº

Rct ct

ct−

X 100

(10) where Rct and R°ct are the charge transfer resistance in the HCl solution in the presence and absence of inhibitors, respectively. The corresponding Bode plots are show in Figure 6.

Table 6 - Quantum chemical parameters for used Schiff bases obtained from MNDO method.

Inhibitor EHomo (eV) ELumo (eV) ∆E (eV) µ (Debye) Surface area (°A) Inhibition efficiency (%IE)

o-CANPMB -8.612 -0.868 7.744 1.309 268.7 83.2

p-CANPMB -8.614 -0.951 7.663 3.355 270.0 88.7

+ + Blank 0.001% 0.01% 0.05%

× ×

+ + Blank 0.001% 0.01% 0.05%

× ×



(a)

(b)

Figure 6 - Bode plots obtained at 35ºC in 2.0 M HCl in various concentrations of (a) o-CANPMB (b) p-CANPMB

The impedance spectra for the Nyquist plots were

analyzed by fitting to the equivalent circuit model shown in Figure 7, which has been used previously to model the aluminium/acid interface. The circuit comprises a solution resistance Rs, in series with the parallel combination of the charge transfer resistance Rct and a constant phase element (CPE), replaced with a double layer Cdl [63, 64].

Figure 7 - The equivalent circuit model (Randle’s

model) used to fit the experimental result

+ + Blank 0.001% 0.01% 0.05%

× ×

+ + Blank 0.001% 0.01% 0.05%

× ×



Nyquist plots for Al-Mg alloy in 2.0 M HCl at various concentrations of inhibitors are presented in Figure 5. It is clear from these plots that impedance response of Al-Mg alloy in acid has significantly changed after the addition of inhibitor. As can be seen in Figure 5, Nyquist plots are depressed into the real axis and not perfect semicircles as expected from theory of EIS of assumed equivalent circuit and this is generally attributed to the inhomogeneity of the metal surface arising from surface roughness or interfacial phenomena [65, 66].

From Table-5. it is seen that Rct values increase and Cdl values decrease with increasing inhibitor concentration. The decrease in Cdl values can be attri-buted to a decrease in local dielectric constant and/or an increase in the thickness of electrical double layer. This suggests that inhibitor molecules inhibit the corrosion rate by adsorption at metal /solution interface [67].

3.4. Quantum chemical study To investigate the effect of molecular structure on

the inhibition mechanism and inhibition efficiency, some quantum chemical calculations were performed.

Quantum structure-activity relationship (QSAR) has been used to study the effect of molecular struc-ture on inhibition efficiency of used Schiff base com-pounds. However, satisfactory correlation has been recorded between the inhibition efficiency of other inhibitors and some quantum chemical parameters by other investigators [68-72].

The computation of some of the quantum chemi-cal parameters such as the energies of the molecular orbitals, EHomo (highest occupied molecular energy) and ELumo (lowest unoccupied molecular energy), ∆E = ELumo – EHomo (energy of the gap), µ (dipole mo-ment) as well as some other structural parameters have been obtained from MNDO semi empirical met-hod and are given in Table 6.

Average values of inhibition efficiency were obtained using three experimental methods (weight loss, polarization and EIS) used for QSAR modeling. The optimized MNDO geometry of inhibitors is shown in Figure 8. It could easily be found that the molecules are planer which results in the good interaction between inhibitors and coating surface.

A linear regression analysis was made of relation between %IE and EHomo, ELumo and µ values of the Schiff bases (o-CANPMB and p-CANPMB) follo-wing equations were obtained,

%IE = -23599.8 – 2750 EHomo (r2 = 1) (11)

%IE = 25.681 – 66.265 ELumo (r2 = 1) (12)

%IE = 79.681 + 2.688 µ (r2 = 1) (13)

(a)

(b)

Figure 8 - Optimized molecular structure of (a) o-CANPMB (b) p-CANPMB

EHomo is often associated with the electron

donating ability of a molecule. High values of EHomo are likely to indicate a tendency of the molecule to donate electrons to appropriate acceptors with low energy and empty molecular orbital. Therefore, the energy of the lowest unoccupied molecular orbital (ELumo) indicates the ability of the molecules to accept electrons. The lower values of ELumo, the more probable, it is the molecule would accept electrons.

The coefficient of ELumo in the Eq.12 is negative. This fact proves that the metal accept electrons from Schiff base compounds and feed back bonds form between metal and inhibitor molecules. Forming of feed back bond increases adsorption of Schiff base molecules on the metal surface and so increases the inhibition efficiencies of these in compounds. In the case of p-CANPMB ELumo is the lowest (-0.951) and the strong feed back bond form with respect to o-CANPMB. The forming of feed back bonds is the ability of the inhibitors to offer electrons, so the inhi-bition efficiency increases.

The negative sign of the coefficient of EHomo can be concluded that the adsorption of these Schiff bases on the metal surface has not chemical mechanism and it may be physical [52]. Physical adsorption results from electrostatic interaction between the charged centers of molecules and charged metal surface which results in a dipole interaction of molecule and metal surface, so the positive sign of the coefficient of µ



suggest that these Schiff bases can be adsorbed on metal surface by physical mechanism.

The separation energy (energy gap) ∆E is an important parameter as a function of reactivity of the inhibitor molecules towards the adsorption on metal-lic surface. As ∆E values decrease, the reactivity of the molecule increasing leading to increase the inhibition of the molecules [73]. For, p-CANPMB, the ∆E has the value 7.663 eV, which can facilitate its adsorption on the metal surface and according has higher inhibition efficiency.

3.5. Mechanism of Inhibition

Organic compounds containing one or more polar units, which may be regarded as the reaction centre/ centres for the adsorption process function as corrosion inhibitors. The adsorption bond strength is determined by the electron density on the atom (e.g., N, S, O, etc.) acting as the reaction centre and by the polarizability of the unit. Thus, organic corrosion inhibitors are adsorbed on the bulk metal, M, forming a charge transfer complex between their polar atoms and the metal:

M + RnX ↔ M : XRn (14)

The size, shape and orientation of the molecule, and the electron charge on the molecule determine the degree of adsorption and that way the effectiveness of the inhibitor. The inhibitors confer high degree of protection of Al-Mg alloy in hydrochloric acid when present in sufficient amount (0.5%) and function through adsorption on the metal surface following Langmuir adsorption isotherm. Galvanostatic polari-zation data have shown that the actions of the inhi-bitors are of mixed type with predominance on the cathodic regions.

The inhibitors contain tree parts through which they can get adsorbed on the metal : a benzaldehydic part having an aromatic ring containing methoxy group (–OCH3)in para-position, an iminic nitrogen (>C=N-) and aromatic ring with a chloro group (-Cl) in the ortho- and para- position. The first two are common to both the inhibitors studied and the compounds differ only by the position of the chloro group on the anilinic aromatic ring.

It is generally assumed that the adsorption of inhibitor at the metal/solution interface is the mecha-nism of inhibitor through electrostatic attraction bet-ween the charged molecules and charged metal. Inhibitors have been found to give an excellent inhi-bition due to the presence of the electron donating groups (such as –Cl, –OCH3) on the Schiff bases structure, which increases the electron density on the

nitrogen of the >C=N- group. Thus leads to be the string adsorption of inhibitors on the metal surface thereby resulting in high inhibition efficiency.

The free energy of adsorption (∆Gads) and heat of adsorption (Qads) are negative, which suggest that the adsorption process is spontaneous and exothermic, and values of ∆Gads (<-40 kJ mol−1) and Ea suggest that the adsorption is a physisorption type.

It is also possible that the compounds may form onium ions in acidic medium and move to the cathodic regions and then the adsorption will take place through the iminic nitrogen and also through the delocalized π-electrons of the benzene moiety. Then molecule will lie flat on the metal surface and further cover the adjoining positions of the surface. In p-CANPMB, the presence of –Cl group with +R (resonance), –I (inductive) effects will easily affect the electron density and activate of the aromatic ring.

4. CONCLUSION

The present study leads to the following the corrosion of Al-Mg alloy by o-CANPMB and p-CANPMB in 2.0 M HCl. 1. o-CANPMB and p-CANPMB have been found to

be good candidates of corrosion inhibitors for Al-Mg alloy in 2.0 M HCl medium. The inhibition efficiency of the studied inhibitors, increase with increasing inhibitor concentration and decreases with increasing temperature.

2. The inhibitor molecules adsorbed on the metal surface and tend to retard the rate of corrosion by reducing the number of available surface sites for corrosion.

3. The adsorption of both inhibitors investigated follows the Langmuir adsorption isotherm.

4. Thermodynamic parameters (∆Gads, Qads and Ea) show that the studied compounds are adsorbed on Al-Mg alloy surface by an exothermic, sponta-neous process and physical adsorption.

5. o-CANPMB and p-CANPMB inhibit both catho-dic and anodic reactions by adsorption but tremendous polarization effect on cathode and hence behave like mixed type inhibitors.

6. Obtained results about inhibition efficiencies from weight loss method, polarization study and EIS are in good agreement with each other.

Acknowledgement

The authors are grateful to Chemistry De-partment, School of Sciences, Gujarat University, Gujarat, for the laboratory facilities. Two of the author Vidhi Panchal and Aesha Patel are also thankful to UGC-BSR for Research fellowship.



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ИЗВОД

ИНХИБИЦИЈА Al-Mg ЛЕГУРЕ У ХЛОРОВОДОНИЧНОЈ КИСЕЛИНИ УПОТРЕБОМ ШИФОВЕ БАЗЕ КАО КОРОЗИОНИХ ИНХИБИТОРА Утицај неких ново синтетизованих Шифових база типа о-хлороанилин-N-(p-метокси бензилиден) и p -хлороанилин-N-(p-метокси бензилиден) на корозију Al-Mg легуре у 2 M HCl је био истражен. Истраживање је обухватило одређивање губитка тежине, галваностатска поларизациона мерења и мерење методом електрохемијске импедансне спектроскопије. Ефикасност инхибиције се повећавала са повећањем концентрације инхибитора али се смањивала са повећањем температуре. Нађено је да су се Шифове базе адсорбовале на металној површини према Ленгмировој изотерми. Термодинамички параметри и активациона енергија су били срачунати да елаборирају механизам инхибиције. Резултати поларизационих мерења показују да ове Шифове базе делују као мешовити тип, али катода је првенствено поларизована. Кључне речи: алуминијум, електрохемијска импедансна спектроскопија, поларизација, губитак тежине, инхибиција Рад примљен: 15. 10. 2011. Оригинални научни рад

GY. VASTAG et al ... STUDY OF INHIBITION PROPERTIES OF SOME THIAZOLE …


GY. VASTAG∗, A. SHABAN Scientific paper I. FELHŐSI, E. KÁLMÁN UDC:620.197.3:669.35

Study of inhibition properties of some thiazole derivatives against copper corrosion

The inhibitor efficiency of some thiazole derivatives against copper corrosion in acidic sulfate solution were investigated using polarization and electrochemical impedance spectroscopy (EIS) methods. Protection efficiency of four organic molecules: 5-benzylidene-2,4-dioxotetrahydro-1,3-thiazole (5-BDT), 5-(4′-isopropylbenzylidene)-2,4-dioxotetrahydro-1,3-thiazole (5-IPBDT), 5-(3′-thenylidene)-2,4-dioxotetrahydro-1,3-thiazole (5-TDT) and 5-(3′,4′-dimetoxybenzylidene)-2,4-dioxotetrahydro-1,3-thiazole (5-MBDT) were investigated on copper electrode in 0.1moldm–3 Na2SO4 solution at pH=2.95. Polarization measurements indicates that all investigated thiazole derivatives could reduces the cathodic reaction rate on copper electrode surface in 0.1 mol dm–3 acidic Na2SO4 solution. In investigation range concentration 0.01mmoldm-3 has the best inhibitor efficiency in case of all investigated thiazole. The best protection is obtained in presence of 5-IPBDT derivatives. EIS results showed that investigated thiazole derivatives formed a film on copper surface which was able to protect copper against corrosion in acidic media. 5-IPBDT derivative only formed closely packed inhibitor film, which is able to protect copper surface during the time. Key words: inhibitors, corrosion, copper, properties

INTRODUCTION

Copper and its alloys are used extensively in much kind of chemicals equipment. Copper normally does not displaced hydrogen from acid solutions and the presence of oxygen is essential for its dissolution [1,2].

Several organic molecules have been investigated as potential inhibitors for copper corrosion in acidic media. The thiazole derivatives are an interesting group of nitrogen and sulphor containing organic compounds which act as inhibitors in the dissolution of copper in acidic media. This kind of organic molecules can be adsorbed at the metal-solution interface as a result the metal reduces the corrosive attack in acidic media [3-5].

In this paper, protective effect of thiazole deriva-tives: 5-benzylidene-2,4-dioxotetrahydro-1,3-thiazole (5-BDT), 5-(4′-isopropylbenzylidene)-2,4-dioxotetra-hydro-1,3-thiazole (5-IPBDT), 5-(3′-thenylidene)-2,4-dioxotetrahydro-1,3-thiazole (5-TDT) and 5 - (3′,4′-dimetoxybenzylidene)-2,4-dioxotetrahydro -1,3- thia-zole (5-MBDT) on copper corrosion were testified in acidic Na2SO4 solution using polarization and electro-chemistry impedance specroscopy (EIS) techiques.

Author's address: HAS, Chemical Research Center, H-1525 Budapest, P. O Box 17, Hungary, ∗University of Novi Sad, Faculty of Natural Sciences and Mathematics, Trg D. Obradovića 3, 21000 Novi Sad, Serbia


EXPERIMENTAL

All the experiments were carried out with a polycrystalline copper (99.99%) electrode in the form of a cylinder (with an exposed area of 0.7 cm2) embedded in an epoxy resin. Before each experiment the electrode was wet-polished with SiC papers (grit sizes of 800 and 1200), the immersed in the solution. Saturated calomel electrode (SCE) was used as reference, and Pt as counter electrode. A computer-controlled potentiostat (Model Solartron ECI-1286) were applied for the electrochemical measurements. The polarization measurements were performed in room temperature (298K) at five different inhibitor concentration in range of 0.001 mmoldm-3- 0.01 mmoldm-3. The measurements were carried out when open circuit potential (OCP) was stabilized to 5 mv per 5 minute. The potential was scanned between OCP and 500 mV in both catodic and anodic directions at the scan rate of 10 mV min-1. The EIS measurements were performed at the open circuit potential at room temperature (298K) with Zahner electric IM 5d, after 30 minute relaxation time, in each hour during the 24 hour. The impedance measu-rements were carried out over a frequency range of 0.01-10 KHz, using 10 mV amplitude of sinusoidal voltage. The impedance spectra were analyzed using program Boukamp EQUIVCRT [6].

RESULTS AND DISCUSSION

Using polarization measurements, we were obta-ined that all investigated derivatives act as catodic



inhibitors against copper corrosion hindered the oxy-gen reduction in acidic solutions (Fig.1.). 5-IPBDT derivative, in addition to of inhibiting the reduction of oxygen, slows also down the dissolution of copper. The inhibition efficiency of investigated thiazole derivatives decrease in the following order: 5-IPBDT> 5-BDT > 5-MBDT > 5-TDT. In investiga-tion range concentration of 0.01mmoldm-3 has the best inhibitor efficiency in case of all investigated hiazole (Fig.2.).

-0.4 -0.2 0.0 0.2 0.4

-2

0

2

4

0,1 mol / dm3 Na2SO4

0,01 mmol / dm3 5-IPBDT 0,01 mmol / dm3 5-BDT 0,01 mmol / dm3 5-MBDT 0,01 mmol / dm3 5-TDT

log j / µ

A cm

-2

Potential / V(SCE)

Figure1 - Polarization curves for copper electrodes

without and with thiazole derivatives (c=0.01mmoldm-3)

0.000 0.002 0.004 0.006 0.008 0.0100

10

20

30

40

50

60

70

80

90

100

η / %

C / mmol dm-3

5-IPBDT 5-BDT 5-MBDT 5-TDT

Figure 2 - Concentration dependence of inhibitor

efficiency

Electrochemistry Impedance Spectros

Electrochemistry Impedance Spectroscopy measurements

Fig. 3a. and 3b. present Nyquist polts for copper electrodes in 0.1 moldm-3 Na2SO4 solution without and with 0.01 mmoldm-3 thiazole derivatives, after 30 minute and 24 hour. In all cases depressed semi-

circles were found, what was characteristic for copper in acidic sulfate solution [7-10].

0 2000 4000 6000 8000 10000

0

-1000

-2000

-3000

Na2SO45-BDT

5-TDT 5-MBDT

I / O

hm

R / Ohm

0 10000 20000 30000 40000 50000 60000 70000 80000

0

-5000

-10000

-15000

-20000 30 min

5-IPBDT

I / O

hm

R / Ohm

Figure 3a - Nyquist plots for copper electrodes

without and with thiazole derivatives (c=0.01mmoldm-3) after 30 minute

0 500 1000 1500 2000 2500 3000 3500 4000

0

-1000

Na2SO45-TDT5-BDT

5-MBDT

I / O

hm

R / Ohm

0 40000 80000 120000 160000 200000

0

-20000

-40000

-60000

-8000024h

5-IPBDT

I / O

hm

R / Ohm

Figure3b - Nyquist plots for copper electrodes

without and with thiazole derivatives (c=0.01mmoldm-3) after 24 hour

EIS data were analyzed using equivalent circuit,

given in Fig 4. In the equivalent circuits RΩ is the solution resistance, Rt the charge-transfer resistance, CPEdl represent the double-layer constant phase ele-ments, Ra the pseudo-resistance corresponding to the discharge of adsorbed species, and CPEa the pseudo-constant phase elements. Constant phase elements (CPE) are used to substitute for capacitors to fit the depressed semicircle more exactly. The impedance of a CPE is given by

njZ −= )(Y1

0CPE ω

Where Y0 is the magnitude of CPE and n an empirical exponent (0≤ n≤ 1), which value indicate the distri-bution of time constants caused by inhomogenities in the inhibitor film formed on copper surface [11-13].



R a

CPEdl R Ω CPE a

R t

Figure 4 - Electrical equivalent circuit for copper

electrode

Given model describe electrochemical process in both cases, in blank and inhibitor solution with two relaxation time constants. The first relaxation time constant describe the fast charge transfer process,

taking place in the high frequency range. The second constant in low frequency arc is arises from a charge transfer intermediate adsorption [8, 9]. This result indicated the corrosion process was controlled mainly by the second time constant in the low-frequency region.

The values of the elements of the equivalent circuit obtained by fitting are given in Table 1.

The total resistance of the copper/electrolyte interphase Rp, includes Rt and Ra values and a real value of the finite diffusion at ω→0 [14]. The total resistance of the system can be used as measure of the inhibiting efficiency.

Table 1 - Values of elements of the equivalent circuits in Fig 3a and fig 3b (after 30 min and 24 hour)

after 30 min after 24 hour

Rpol / Ω Cds / µFcm-2 n Rpol / Ω Cds / µFcm-2 n

Na2SO4 749 19,1 0,77 2840 14,3 0,89

5-IPBDT 74930 5,8 0,93 206200 3,8 0,96

5-BDT 2729 11,1 0,89 3930 13,2 0,86

5-MBDT 10410 10,4 0,92 2100 13,4 0,89

5-TDT 5120 12,4 0,88 2063 26,7 0,86

At the beginning, the effect of investigated thiazole derivatives in copper protection revealed the increases in the total resistance (Rp) and decrease of the double layer capacitance in relation to blank solu-tion. During the time, total resistance (Rp) increase in blank solution and in presence of 5-BDT and 5-IPBDT derivatives but in same time decrease in pre-sence of 5-MBDT and 5-TDT derivatives. That me-ans that the inhibitive efficiency of 5-MBDT and 5-TDT derivatives significantly decrease during the time.

The double layer capacitance during the time, decrease only in 5-IPBDT containing solution. Which indicates that in longer time only 5-IPBDT deriva-tives has protection efficience. This phenomenon can be explained that only this derivative can formed compact film on copper surface. Layer formed of ot-her derivatives during the time becomes porous and less protective.

CONCLUSION

Four thiazole derivatives were investigated as potential copper inhibitors in acidic 0.1 moldm–3 Na2SO4 solution. Using potentiostatic polarization measurements it was obtained that in investigation

range, concentration of 0.01mmoldm-3 has the best inhibitor efficiency in case of all investigated thia-zole. All derivatives acts as catodic inhibitors hin-dered the oxygen reduction. The inhibitor efficiency decrease in the following order: 5-IPBDT> 5-BDT > 5-MBDT > 5-TDT. Using electrochemical impedance spectroscopic measurements an empirical model of electrochemical cell with two relaxation time con-stants was obtained. The effect of the investigated thiazole derivatives in copper protection is revealed the increase in the total resistance and decrease of the double layer capacitance compared with blank solu-tion. Inhibitor film formed on copper surface was the highest quality in presence of 5-IPBDT derivatives. Only this derivative can protect copper against corrosion in acidic sulphate media during the time.

Acknowledgments

This work was supported by the Serbian Ministry of Science and Techology.

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[14] M.Metikoš-Huković, R. Babić and I. Paić, J. Appl. Electrochem. 30 (2000) 617

IZVOD

ISPITIVANJE INHIBITORSKIH SVOJSTVA DERIVATA TIAZOLA NA KOROZIJU BAKRA Ispitivana je inhibitorska efikasnost odabranih derivata tiazola u odnosu na koroziju bakra u kiseloj sredini snimanjem polarizacionih krivi i impedansnim merenjima. Određivana je zaštitna sposobnost sledećih organskim molekula: 5-benziliden-2,4-dioksotetrahidro-1,3-tiazol (5-BDT), 5-(4′-izopropil-benziliden)-2,4-dioksotetrahidro-1,3-tiazol (5-IPBDT), 5-(3′-teniliden)-2,4-dioksotetrahidro-1,3-tiazol (5-TDT) i 5-(3′,4′-dimetoksibenziliden)-2,4-dioksotetrahidro-1,3-tiazol (5-MBDT) u 0,1mol/dm–3 rast-voru Na2SO4 pri pH=2,95. Polarizaciona merenja ukazuju da svi ispitivani derivati tiazola smanjuju brzinu redukcije kiseonika, delujući kao katodni inhibitori korozije bakra kiselom u 0,1mol dm–3 rastvoru Na2SO4. U ispitivanom opsegu, koncentracija od 0,01 mmoldm-3 ima najveću inhibitorsku efikasnost kod svih ispitivanih derivata. Najbolja zaštita uočena je kod 5-IPBDT derivata. Merenja impedanse pokazuju da ispitivana jedinjenja formiraju film na površini bakarne elektrode koji je sposoban da štiti bakar od korozije u kiseloj sredini. 5-IPBDT derivat jedino formira dovoljno kompaktan film, koji može da štiti bakar od korozije u dužem vremenskom periodu. Ključne reči: inhibitori, korozija, bakar, svojstva Rad primljen: 17. 08. 2011. Originalni naučni rad

D. RADONJIĆ i ... ISTRAŽIVANJE KOROZIONIH I ELEKTROHEMIJSKIH KARAKTERISTIKA ...


DRAGAN RADONJIĆ Originalni naučni rad DARKO VUKSANOVIĆ UDC:620.197.3.5:669.715 JELENA PJEŠČIĆ

Istraživanje korozionih i elektrohemijskih karakteristika nemodifikovanih AlZnSnGaIn legura u rastvorima NaCl

U ovom radu su prikazani rezultati nemodifikovanih AlZnSnGaIn legura, dobijenih u laboratoriji za Livarstvo MTF-a Podgorica. Pomenuti uzorci su ispitivani na savremenoj PAR opremi u realnim rastvorima NaCl, bez prethodnog uvođenja azota ili drugih gasova. Savremenim metodama određene su vrijednosti: Rp, jcorr, ecorr, e(j=0), kao i katodne i anodne polarizacione krive za nemodifikovane AlZnSnGaIn legure. Istraživanja su rađena u rastvorima NaCl koncentracija od 5,1∗10-4 do 1,02 mol/dm3. Ključne reči: korozija, nemodifikovane legure, polarizacione krive

1. UVOD

Dosadašnja koroziona i elektrohemijska istraživa-nja ponašanja protektorskih legura aluminijuma ura-đena su na legurama modifikovanim različitim vrsta-ma modifikatora (AlTiB5, TiB5, B, Ti, Sr) (1-3). Istraživanja su utvrdila da modifikator ima izražen uticaj na strukturu, teksturu, veličinu zrna, homoge-nizaciju, izgled površine, vrstu i broj faza, kvalitet legiranja i dr. Karakteristike Al legura su uticale na koroziona i elektrohemijska ponašanja u ispitivanim rastvorima NaCl (5,1∗10-4÷1,02 mol/dm3). Na bazi velikog broja istraživanja na protektorskim legurama aluminijuma, projektovan je i ostvaren model i dobi-jeno je nekoliko tipova nemodifikovanih Al legura u laboratoriji za Livarstvo MTF-a Podgorica (4-6).

Hemijski sastava dobijenih nemodifikovanih AlZnSnGaIn legura određen je metodom bez razara-nja-emisionom spektroskopijom, izgled površine-mi-kroskopski i struktura na X-RAY difraktometru. Ko-roziona i elektrohemijska ispitivanja izvršena su na savremenoj PAR opremi u rastvorima NaCl, različitih koncentracija. Ovaj elektrolit predstavlja realni ra-stvor, jer nema uvođenja gasova prije korozionih i elektrohemijskih ispitivanja. Prije svih ispitivanja po-vršina uzoraka pripremljena je mehaničkim putem.

Cilj rada je dobijanje sopstvenih nemodifikovanih koroziono-aktivnih (protektorskih) Al legura meto-dom hlađenja vazduhom. Korozionim ispitivanjima dobijene su vrijednosti: polarizacionog otpora Rp, struje korozije jcorr, korozionog potencijala ecorr, e(j=0) i katodne i anodne potenciodinamičke pola-rizacione krive. Za sva koroziona i elektrohemijska istraživanja, korišćeni su sintetički rastvori hlorida koncentracija od 5,1∗10-4 do 1,02 mol/dm3.

2. EKSPERIMENTALNI DEO

Za eksperimentalna ispitivanja korišćena uzorci dvije nemodifikovane AlZnSnGaIn legure, koje su dobijene u laboratoriji za Livarstvo MTF-a Podgo-rica.

Kao sirovine za dobijanje legure korišćeni su: aluminijum 99,80% iz KAP-a, visokočisti cink, kalaj i indijum i tečni galijum.

Legura je dobijena u indukcionoj peći, uz primje-nu svih faza obrade. Hlađenje tečne legure je oba-vljeno u specijalnim sistemima za hlađenje vazdu-hom. Nemodifikovane Al legure su iskorišćene za izradu uzoraka debljine 5mm i prečnika 15mm.

Hemijski sastav Al legure određen je metodom bez razaranja-emisionom spektroskopijom.

Tabela 1 - Hemijski sastav nemodifikovanih AlZnSnGaIn legura (mas %) Legura Al Zn Sn Ga In Fe Si

1 93,56 5,46 0,32 0,5 0,05 0,05 0,03 2 92,95 6,15 0,22 0,5 0,1 0,04 0,03

Adresa autora: Metalurško-tehnološki fakultet,

Džordža Vašingtona bb, 81000 Podgorica, Crna Gora Rad primljen: 18. 09. 2011.



Mikrostrukture nemodifikovanih AlZnSnGaIn le-gura, snimljene su prije i poslije ispitivanja sa uveća-njima 100x i 400x.

Sintetički rastvori, koji su korišćeni za eksperi-mentalna ispitivanja pripremljeni su iz p.a. NaCl. Tabela 2 - Koncentracije NaCl korišćene u ispitiva-

njima Rastvor pH provodljivost S/m 1,02M 7,015 1,43 0,51M 7,027 1,12 0,051M 7,044 0,226

0,0051M 7,069 0,0324 0,00051M 7,079 0,0097

Za sva koroziona i elektrohemijska ispitivanja korišćena je oprema za koroziona istraživanja – sis-tem PAR 332, koji čine:

1. Potenciostat-galvanostat model 273 2. Diferencijalni elektrometar 3. Ćelija MK-047 za ubrzana ispitivanja 4. Standardna zasićena kalomel elektroda 5. Pomoćne elektrode-valjkasti elektrografit 6. Računar i štampač. Potenciostat-galvanostat služi za prenos podataka

sa računara, preko diferencijalnog elektrometra na ćeliju. Dobijeni podaci se zatim vraćaju iz ćelije na računar. Ćelija je staklena posuda, u kojoj su smješ-tene pomoćne elektrode, standardna kalomel elektro-da i radna elektroda sa uzorkom. Zasićena PAR-ova kalomel elektroda je smještena u Luginovoj kapilari, u kojoj se nalazi zasićen rastvor KCl. Pomoćna elek-troda, čiji je zadatak da zatvori strujno kola u ćeliji, je valjkasti elektrografit. Ova elektroda je vrlo otporna na agresivne sisteme, koji su korišćeni u ispitivanji-ma. Kao radne elektrode korišćene su nemodifiko-vane AlZnSnGaIn legure. Površina radne elektrode, u kojoj se nalazi uzorak iznosi 1 cm2, pa vrijednosti jačine struje predstavljaju gustinu struje korozije.

Ispitivanja su urađena na sobnoj temperaturi, bez uvođenja azota, vodonika ili kiseonika u rastvore hlorida, tako da dobijeni rezultati odgovaraju realnim sistemima.

3. REZULTATI EKSPERIMENTALNIH ISPITIVANJA

3.1. Mikrostrukturna ispitivanja

Na slici 1 prikazane su fotografije mikrostruk-turnih ispitivanja nemodifikovane AlZnSnGaIn legure 1 prije i poslije korozionih ispitivanja sa uvećanjem 100 x.

a) prije ispitivanja

b) poslije ispitivanja

Slika 1

a) prije ispitivanja

b) poslije ispitivanja

Slika 2



Na slici 2 su fotografije mikrostrukturnih ispitiva-nja za nemodifikovanu AlZnSnGaIn leguru 2 prije i poslije korozionih ispitivanja sa uvećanjem 100 x.

3.2. Metode i rezultati ispitivanja nemodifikovane AlZnSnGaIn legure 1 u rastvorima NaCl

U korozionim i elektrohemijskim ispitivanjima AlZnSnGaIn legure 1 korišćene su metode:

• praćenje promjene korozionog potencijala sa vremenom, ecorr=f(τ);

• metoda polarizacionog otpora, Rp;

• potenciodinamička metoda. U ovim ispitivanjima korišćeni su NaCl koncen-

tracija: 1,02M; 0,51M; 0,051M; 0,0051M; 0,00051M., dobijeni iz p.a. NaCl. Tabela 3 - Rezultati korozionih ispitivanja nemodifi-

kovane AlZnSnGaIn legure 1 dobijeni meto-dom ecorr=f(τ)

Koncentracija epoč (mV) ekon (mV) 1,02M -1427 -1492 0,51M -1420 -1448 0,051M -1360 -1329

0,0051M -1310 -1260 0,00051M -1265 -1201

Na slici 3. prikazan je eksperimentalno dobijen dijagram metodom ecorr=f(τ) za nemodifikovanu AlZnSnGaIn leguru 1.

Slika 3 - Dijagram promjene korozionog potencijala

sa vremenom za AlZnSnGaIn leguru 1 u 1,02M rastvoru NaCl na t=20oC

Na dijagramu na slici 3 se vidi da se korozioni potencijal u prvih 1400s pomjera prema pozitivnijim vrijednostima i to od početne od -1427mV do vrijed-

nosti -1400 mV, prema ZKE. Ove promjene se mogu objasniti rastvaranjem Al legure 1 u ovom rastvoru NaCl i stvaranjem Al2O3 i Al(OH)3. Nakon 1400s oksid i hidroksid nisu više zaštita za AlZnSnGaIn leguru u hloridima, jer je došlo do aktiviranja metalne površine i konstantnog pomjeranja potencijala prema negativnijim vrijednostima. Konačna vrijednost pri 3600s potencijal je iznosio -1492 mV, prema ZKE. Pomjeranje potencijala ka negativnijim vrijednostima objašnjava se odigravanjem reakcija:

Al Al3+ + 3e- (1)

Al + 3Cl- AlCl3 + 3e- (2)

Al3+ + 3Cl- AlCl3 (3)

Tabela 4 - Eksperimentalni rezultati dobijeni metodom Rp za nemodifikovanu AlZnSnGaIn leguru 1 u rastvorima NaCl

Konce-ntracija

ecorr (mV) Rp (kΩ) Jcorr

(µA/cm2) 1,02M -1429 0,3287 66,06 0,51M -1402 0,7691 28,23

0,051M -1352 2,3417 9,27 0,0051M -1329 6,09 3,57

0,00051M -1294 7,2 3,01

Na slici 4. prikazan je eksperimentalno dobijen dijagram metodom Rp.

Slika 4 - Polarizaciona kriva nemodifikovane

AlZnSnGaIn legure 1 u 1,02M NaCl na t=20oC

Na osnovu eksperimentalnih rezultata AlZnSnGaIn legure u 1,02 M rastvoru (slika 4), metodom Rp, određene su vrijednosti: Rp=0,3287 kΩ, jcorr=66,06 µA/cm2 i ecorr=-1429 mV, u odnosu za



ZKE. Eksperimentalne vrijednosti za AlZnSnGaIn leguru potvrđuju rezultate dobijene metodom ecorr=f(t). Tabela 5 - Rezultati korozionih istraživanja dobijenih

potenciodinamičkom metodom za nemodifiko-vanu AlZnSnGaIn leguru 1 u rastvorima NaCl Koncentracija e(j=0) (mV)

1,02M -1421,72

0,51M -1400,63

0,051M -1330,81

0,0051M -1289,56

0,00051M -1258,92

Na slici 5 prikazan je eksperimentalno dobijen dijagram potenciodinamičkom metodom za nemodifi-kovanu AlZnSnGaIn leguru 1.

Slika 5. -Potenciodinamička kriva za AlZnSnGaIn

leguru 1 u 1,02M NaCl na t=20oC

Sa dijagrama na slici 5 se vidi da u katodnoj oblasti ispitivanja u dijapazonu od 80 mV dolazi do brzog pada vrijednosti struje. Eksperimentom je određena vrijednost e(j=0)=-1421,72 mV. Nakon ove vrijednosti, u anodnom dijelu, takođe dolazi do brzog porasta gustine struje, što se objašnjava velikom brzinom korozije AlZnSnGaIn legure 1.

3.3. Metode i rezultati ispitivanja nemodifikovane AlZnSnGaIn legure 2 u rastvorima NaCl

U korozionim i elektrohemijskim ispitivanjima AlZnSnGaIn legure 2 korišćene su iste metode i ista serija rastvora kao i u ispitivanjima Al legure 1.

Na slici 6. prikazan je eksperimentalni dijagram AlZnSnGaIn legure 2 dobijen metodom ecorr=f(τ).

Tabela 6 - Vrijednosti početnog i konačnog koro-zionog potencijala AlZnSnGaIn legure 2 utvr-đene metodom ecorr=f(τ)

Koncentracija epoč (mV) ekon (mV) 1,02M -1358 -1502 0,51M -1336 -1492 0,051M -1297 -1461

0,0051M -1266 -1368 0,00051M -1262 -1300

Slika 6 - Dijagram promjene korozionog potencijala

u funkciji vremena za AlZnSnGaIn leguru 2 u 0,51M rastvoru NaCl na t=20oC

Sa dijagrama prikazanog na slici 6 vidi se pom-jeranje korozionog potencijala ka negativnijim vrijed-nostima u toku eksperimentalnog intervala od 3600s, od početne -1336mV do konačne vrijednosti korozio-nog potencijala -1492 mV, prema ZKE. Ovakva zavi-snost je posljedica aktiviranja površine AlZnSnGaIn legure 2 pri čemu se odigravaju reakcije (1), (2) i (3).

Tabela 7 - Eksperimentalni rezultati određeni meto-dom Rp za nemodifikovanu AlZnSnGaIn leguru 2 u rastvorima NaCl

Konce-ntracija ecorr (mV) Rp (kΩ) jcorr-

(µA/cm2) 1,02M -1468 0,3735 58,12 0,51M -1407 0,9434 23,01

0,051M -1312 1,1937 18,19 0,0051M -1302 1,2103 17,94

0,00051M -1267 3,0217 7,18 Na slici 7 prikazana je polarizaciona kriva

AlZnSnGaIn legure 2 u 0,51M NaCl.



Slika 7 - Polarizaciona kriva nemodifikovane


Na osnovu eksperimentalno utvrđene polarizacio-ne krive za AlZnSnGaIn leguru 2 u 0,51 M rastvoru (slika 7), određene su vrijednosti: Rp=0,9434 kΩ, jcorr =23,01 µA/cm2 i ecorr=-1407 mV, u odnosu za ZKE. Eksperimentalne vrijednosti za AlZnSnGaIn leguru 2 potvrđuju rezultate dobijene metodom promjene korozionog potencijala sa vremenom.

Tabela 8 - Rezultati istraživanja dobijenih potencio-dinamičkom metodom za nemodifikovanu AlZnSnGaIn leguru 2 u rastvorima NaCl

Koncentracija e(j=0) (mV)

1,02M -1457,58

0,51M -1405,95

0,051M -1220

0,0051M -1183,66

0,00051M -1111

Slika 8. prikazuje potenciodinamičke katodne i anodne polarizacione krive nemodifikovane AlZnSnGaIn legure 2.

Na osnovu dijagrama prikazanog na slici 8 može se uočiti da u katodnoj oblasti eksperimentalnih istra-živanja od 94 mV dolazi do brzog pada vrijednosti struje. Eksperimentalno je određena vrijednost e(j=0)=-1405,95 mV. Nakon ove vrijednosti, u anod-nom dijelu, ponovo dolazi do brzog porasta gustine struje, što se objašnjava velikom brzinom korozije AlZnSnGaIn legure 2.

Slika 8 - Potenciodinamička kriva nemodifikovane


4. ANALIZA EKSPERIMENTALNIH REZULTATA ISTRAŽIVANJA

4.1. Analiza rezultata nemodifikovane AlZnSnGaIn legure 1

Na osnovu eksperimentalnog ponašanja nemodifi-kovane AlZnSnGaIn legure 1 u hloridnim rastvorima, konstruisan je dijagram ekon=f(logCNaCl) koji je pri-kazan na slici 9.

1E-4 1E-3 0.01 0.1 1 10

-1500

-1450

-1400

-1350

-1300

-1250

-1200 ekon (mV)

e (m

V)

logCNaCl

Slika 9 - Zavisnost vrijednosti konačnog korozionog

potencijala od koncentracije NaCl za nemodifikovanu AlZnSnGaIn leguru 1

Sa dijagrama prikazanog na slici 9 može se utvrditi da povećanje sadržaja Cl- jona u rastvoru utiče na pomjeranje konačnog potencijala AlZnSnGaIn legure 1 ka negativnijim vrijednostima, što vodi porastu brzine korozije legure. Za očekivati



je da se na površini nemodifikovane AlZnSnGaIn legure 1 osim reakcije:

Al + 3Cl- AlCl3 + 3e- (4) dolazi i do rastvaranja Zn i Sn, koji se nalaze u obliku nekih faza, po reakcijama(7,8):

Zn + 2Cl- ZnCl2 + 2e- (5)

Sn + 2Cl- SnCl2 + 2e- (6)

Bilo bi veoma interesantno odrediti kvalitativni i kvantitativni sadržaj pojedinih faza koje su prisutne u leguri, kao i sadržaj jonskih vrsta u rastvoru.

Na bazi eksperimentalnih istraživanja prikazanih u tabeli 4. konstruisan je dijagram zavisnosti polari-zacionog otpora i gustine struje korozije od koncen-tracije NaCl za AlZnSnGaIn leguru 1 i prikazan je na slici 10.

1E-4 1E-3 0.01 0.1 1 10

0

2

4

6

8 Rp jcorr

logCNaCl

Rp

(kO

hms)

0

10

20

30

40

50

60

70

jcorr (uA/cm

2)

Slika 10 - Zavisnost vrijednosti Rp i jcorr od

koncentracije NaCl za nemodifikovanu AlZnSnGaIn leguru 1

Dijagram na slici 10 pokazuje sniženje vrijednosti polarizacionog otpora i povećanje vrijednosti gustine struje korozije za 22 puta sa porastom koncentracije NaCl od 2000 puta. Ove vrijednosti su pokazatelj povećanja brzine korozije AlZnSnGaIn legure 1 sa porastom koncentracije NaCl.

Na osnovu vrijednosti iz tabele 5 konstruisan je dijagram zavisnosti e(j=0) od koncentracije NaCl za nemodifikovanu AlZnSnGaIn leguru 1 koji je prika-zan na slici 11.

Iz pokazanog dijagrama sa slike 11 jasno se vidi pomjeranje potencijala legure 1 ka negativnijim vrijednostima za 162mV pri povećanju koncentracije NaCl za 2000 puta. Ovi podaci potvrđuju rezultate dobijene prethodnim metodama o povećanju brzine korozije AlZnSnGaIn legure 1 sa povećanjem kon-centracije NaCl.

1E-4 1E-3 0.01 0.1 1 10-1440

-1420

-1400

-1380

-1360

-1340

-1320

-1300

-1280

-1260

-1240

e (m

V)

logCNaCl

e(j=0)

Slika 11 - Zavisnost vrijednosti e(j=0) od koncen-

tracije NaCl za AlZnSnGaIn leguru 1

4.2. Analiza rezultata nemodifikovane AlZnSnGaIn legure 2

Na bazi eksperimentalnih rezultata nemodifikova-ne AlZnSnGaIn legure 2 u rastvorima NaCl, konstrui-san je dijagram ekon=f(logCNaCl) koji je prikazan na slici 12.

1E-4 1E-3 0.01 0.1 1 10-1550

-1500

-1450

-1400

-1350

-1300

-1250 ekon (mV)

e (m

V)

logCNaCl

Slika 12 - Zavisnost konačnog korozionog potencijala

od logaritma koncentracije NaCl za nemo-difikovanu AlZnSnGaIn leguru 2

Sa dijagrama na slici 12, vidi se da povećanje sa-držaja Cl- jona u rastvoru, utiče na pomjeranje ko-načnog potencijala AlZnSnGaIn legure 2 ka negativ-nijim vrijednostima, što utiče na porast brzine korozije. Prema tome, na površini nemodifikovane AlZnSnGaIn legure 2 odigravaju se reakcije (4), (5) i (6).

Na osnovu eksperimentalnih rezultata prikazanih u tabeli 7. konstruisan je dijagram zavisnosti polariza-cionog otpora i gustine struje korozije od koncen-tracije NaCl za AlZnSnGaIn leguru 2 i prikazan je na slici 13.



1E-4 1E-3 0.01 0.1 1 10

0

2

4 Rp jcorr

logCNaCl

Rp

(kO

hms)

0

10

20

30

40

50

60

jcorr (uA/cm

2)

Slika 13 - Zavisnost vrijednosti Rp i jcorr od logaritma

koncentracije NaCl za nemodifikovanu AlZnSnGaIn leguru 2

Slika 13 pokazuje sniženje vrijednosti polari-zacionog otpora i povećanje vrijednosti gustine struje korozije za 8 puta sa porastom koncentracije NaCl od 2000 puta. Ovakva zavisnost prati porast brzine ko-rozije AlZnSnGaIn legure 2 sa povećanjem koncen-tracije Cl- jona.

Na bazi eksperimentalnih podataka iz tabele 8 konstruisan je dijagram e(j=0)=f(logCNaCl) za nemodi-fikovanu AlZnSnGaIn leguru 2 koji je prikazan na slici 14.

1E-4 1E-3 0.01 0.1 1 10-1440

-1400

-1360

-1320

-1280

-1240

-1200

-1160

-1120

-1080

e (m

V)

logCNaCl

e(j=0)

Slika 14 - Zavisnost vrijednosti e(j=0) od logaritma

koncentracije NaCl za AlZnSnGaIn leguru 2

Iz dijagrama sa slike 14 vidi se pomjeranje potencijala legure 2 prema negativnijim vrijednostima za 364mV za porast koncentracije NaCl od 2000 puta. Ovi podaci potvrđuju rezultate dobijene prethodnim metodama o povećanju brzine korozije AlZnSnGaIn legure 2 sa porastom koncentracije hlorida(9).

5. ZAKLJUČAK

Nemodifikovane Al legure nemaju homogenu strukturu i veličinu zrna, što za posljedicu ima ne-ravnomjerno rastvaranje površine legura i pojavu pi-ting korozije u rastvorima hloridima, što je pokazano mikrostrukturnim ispitivanjima površine legura.

Rezultati korozionih i elektrohemijskih istraživa-nja na nemodifikovanim AlZnSnGaIn legurama u ras-tvorima NaCl, utvrdili su da sopstveno dobijene Al-legure imaju visoku korozionu aktivnost u hloridnim rastvorima svih ispitivanih koncentracija. Ova činje-nica potvrđena je jako negativnim vrijednostima ko-rozionog potencijala (-1500 mV u odnosu na ZKE) i malim vrijednostima polarizacionog otpora (0,33 kΩ).

Isti materijali se mogu uspješno koristiti kao žrtvovana anoda za zaštitu Fe i Fe-legura u vodenim rastvorima hlorida.

6. LITERATURA

[1] M.Pourbaix, Atlas of Electrochemical Equilibrium in aqueous solutions, Oxford, 1966.

[2] K.B.Pai, R.Raman, Sacrificial Aluminum Anodes, J.Electrochem. Soc. India, p.819, 1982.

[3] A.R.Despić, V.R.Parkhutik, Modern Aspects of Electrochemistry, Vol.20, Ed. by J.O, New York, 1989.

[4] P. Živković, Corrosion Investigation on Al and Al-alloys in Montenegro, XVIII Jugoslovenski Simpo-zijum o koroziji i zaštiti materijala 2000, str. 45.

[5] P. Živković, D. Radonjić, Istraživanje korozionih i elektrohemijskih karakteristika nemodifikovane AlZnSnGa legure i Al 99,80% u NaCl rastvorima, Zaštita materijala 43, 1 (2002) 1.

[6] Vuksanović Darko, Zivković Petar, Radonjić Dragan, Jordović Branka (2009) Investigation Of The Behaviour In Chloride Solution Of Aluminium Alloys As Materials For Protector Protections, Materiali in Tehnologije, vol. 43, no. 1, p. 49-53.

[7] J. Pješčić, D. Radonjić, D. Vuksanović, Lj. Sa-mardžić, Ispitivanje ponašanja protektorskih legura na bazi sistema Al-Zn u rastvorima hlorida različitih koncentracija, Zaštita materijala 51, 2 (2010) str. 94-98.

[8] J. Pješčić, D. Radonjić, D. Vuksanović, B. Jordović, LJ. Samardžić, Ispitivanje korozionih karakteristika legura na bazi Al-Zn namijenjenih za protektorsku zaštitu, Zaštita materijala, 52, 4 (2011) str. 270-275.

[9] M.M. Antonijević, V. Gardić, S.M. Milić, S.Č. Alagić, A.T.Stamenković, M.Jojić, Elektrohemijsko ponašanje Cu24Zn5Al legure u rastvoru boraksa u prisustvu 1-fenil-5-merkapto-tetrazola, Zaštita ma-terijala, 50, 1 (2009) str. 19-29.



ABSTRACT

INVESTIGATION OF CORROSION AND ELECTROCHEMICAL CHARACTERISTICS OF AlZnSnGaIn UNMODIFIED ALLOYS IN NaCl SOLUTIONS

This paper presents the results of unmodified AlZnSnGaIn alloys, obtained in the Laboratory for casting in Faculty of Metallurgy and Technology – Podgorica. The aforementioned samples were analyzed at the modem PAR equipment in real solutions of NaCl, without the introduction of other gases. Modern methods were determined: polarization resistance, corrosion current density, corrosion potential, e(j=0), and cathodic and anodic polarization curves for unmodified AlZnSnGaIn alloy. Investigations were carried out in solutions of sodium chloride concentration of 5,1·10-4 to 1,02 mol/dm3. Key words: corrosion, unmodified alloys, polarization curves Paper received: 18.09.2011. Scientific paper

I. MICKOVA et al ... FOTOELEKTROHEMISKA ISPITIVANJA TANKIH FILMOVA ...


IRENA MICKOVA Originalni naučni rad DRAGICA ČAMOVSKA UDC:620.181:621.3.032.2 LJUBOMIR ARSOV

Fotoelektrohemiska ispitivanja tankih filmova formiranih na Nb elektrodama u alakalnim rastvorima

Fotoelektrohemijska istraživanja tankih pasivnih i anodnih filmova izvodjena su na Nb elektrodi u rastvoru 2 M KOH. Pokazano je da formirani pasivni filmovi imaju poluprovodničke osobine čija širina zabranjene zone iznosi 3.26 eV. Sa povećanjem napona do 10 V povećava se debljina anodnih filmova a opada širina zabranjene zone. U području napona od 10 V do 30 V debljina anodnih filmova i dalje raste dok širina zabranjene zone poprima konstantnu vrednost od 3.06 eV. Ključne reči: fotosinteza, pasivni i anodni filmovi, zabranjene zone, poluprovodnici

UVOD

Tanki filmovi formirani termičkom ili anodnom oksidacijom na površini ventilskih metala imaju polu-provodničke osobine i mogu se ispitivati pomoću fotoelektrohemijskih metoda. Poznato je da polupro-vodničke elektrode efikasno absorbuju sunčanu svet-lost tako što se u njima oslobadjaju delokalizirani na-boji koji se mogu medjusobno separirati u zavisnosti od njihovog predznaka. Na taj način dolazi do pojave usmerenog toka električne struje što predstavlja kon-verzija sunčane energuje u električnu. Sadašnja saz-nanja nauke predlažu 3 metode pomoću kojih se svet-lost (sunčana energija) može pretvoriti u korisnu energiju:

1. Fotosinteza je prirodan proces pretvaranja sun-čane energije u hemisku, što omogućava održljiv ži-vot na planeti zemlji. Ugljen dioksid i voda hemiskim putem oslobadjaju šećer i kiseonik, a manji deo sunčane energije je skladiran u vidu hemiske energije proizvedene tokom fotosinteznog ciklusa. Obicno 3 – 5% sunčane energije je skladirana kao Gibsova slo-bodna energija u biljnom svetu, što je dovoljno za održavanje života na zemlji.

2. Fotovoltaične ćelije pretvaraju sunčanu u elek-tričnu energiju i sastoje se od 2 čvrste elektrode: metalne (M) i poluprovodničke (SC). Pod dejstvom sunčane svetlosti njihov medjusobni kontakt omogu-ćava direktan tok električne struje kroz spoljašnje kolo.

Adresa autora: University St. Cyril and Methodius, Faculty of Technology and Metallurgy, 1000 Skopje, Republic of Macedonia

Rad primljen: 15. 07. 2011.

3. Fotoelektrohemiske ćelije u kojima polupro-vodnička elektroda absorbuje svetlost, konverzija energije izvodi se preko hemiskih reakcija na granici kontakta poluprovodničke elektrode sa tečnom fazom. Ove ćelije mogu pretvoritu sunčanu energiju u hemisku, ili u električnu, ili u obe energije.

Poluprovodničke osobine tankih filmova formi-ranih termičkom ili anodnom oksidacijom na površi-nama ventilskih metala potiču od nestehiometriskog odnosa metala i kiseonika u metalnom oksidu i od prisutnostih raznih kontaminirajućih jona u elektrolit-nom rastvoru, koji su se inkorporirali u kristalnoj rešetci oksida u toku anodne polarizacije. U literaturi može se naći veći broj podataka u vezi sa istraži-vanjem klasičnih poluprovodnika kao što su: Si, Ga, As, InP i t.d., nego u vezi sa istraživanjem polu-provodničkih osobina pasivnih filmova [1-3].

U istraživanju poluprovodničkih osobina tankih oksidnih filmova najčesće korisćene metode su mere-nja fotoefekta i fotostruje u kombinaciji sa klasičnim elektrohemiskim metodama kao što su: potencio-statska, potenciodinamička, galvanostatska, impedan-sna spektrskopija, kapacitivna merenja itd. Foto-efekat, kao fenomen pojave dopunske struje tokom osvetljenja poluprovodničke elektrode, poznat je duži period vremena, medjutim fotoelektrohemija polupro-vodničkih elektroda beleži veći uspon poslednjih 30 godina u cilju proizvodnje novih materijala koji se koriste u solarnim ćelijama. Dosadašnja istraživanja u glavnom su bila usmerena na: strukturi dvojnog sloja, apsorpciju pojedinih jona na graničnoj površini me-talni oksid-elektrolit, distribuciju potencijala i naboja u prostornom sloju poluprovodnika, koncentraciju no-sioca naelektrisanja, uticaju različitih redoks procesa na širinu zabranjene zone, energiju Fermijevg nivoa itd. [2].



absorpcija

R

SC M

H2O H2O

e

O2 H2

SC M

eR

nCO2 + nH2O + hv (CH2O)n + nO2

fotovoltaik

fotoelektrohemijafotosinteza

Slika 1 - Pretvaranje sunčane svetlosti u korisnu energiju korisćenjem triju navedenih metoda

Dosadašnja fotoelektrohemiska istraživanja niobi-

juma pretežno u slabijim koncentracijama H2SO4, pokazala su postojenje n-tipa poluprovodnika. Medju-tim ne postoje sistematičnija istraživanja pasivnih filmova u alkalnim rastvorima, kao i vrednosti polu-provodničkih parametara u zavisnosti od debljine filmova [3].

U ovom radu dati su preliminarni podatci naših istraživanja pasivnih i anodnih oksidnih filmova nio-bijuma u alkalnim sredinama sa cijlem da se ukaže na komparativne razlike polu-provdničkih osobina dobi-venih u kiselim i alkalnim elektrolitima.

EKSPERIMENTALNI DEO

Kao radne elektroda bili su korisćeni Nb 99.8 % (Alfa Aesar, a Johnson Mathey Company) cilindrični diskovi sa dijametrom od 6.35 mm i debljinom od oko 3 mm. Elektrode su najpre bile mehanički sukce-sivno polirane sa abrazivnom hartijom gradacije 1000, 4000 i 5000, zatim fino polirtane sa dijamant-skim pastama do minimalne gradacije 0.1 µm i na kraju ultrazvučno čisćene u etanolu. Pre anodne oksi-dacije Nb uzorci su bili stavlajni u teflonskim nosači-ma u kojima je frontalna strana cilindra sa konstant-nom površinom bila u dodiru sa elektrolitnim rastvo-rima. Fotostrujna merenja izvođena su odmah na istim uzorcima postavljenim u teflonskim nosačima nakon anodne oksidacija. Za ova merenja korisćena je specijalno prilagodjena elektrohemiska ćelija sa odeljcima za: radnu (Nb), pomoćnu (Pt) i referentnu (Hg/HgO, 1 M KOH) elektrodu kao i fotoelektro-

hemiski sistem sastavljen od: potenciostata/galvano-stata, ksenon lampe, monohromatora, čopera, lock-in pojačivača i sistema za registriranje izlaznih signala. Posle svakog merenja elektrode su bile ponovo meha-nički polirane i čisćene za naredni eksperimenat.

REZULTATI I DISKUSIJA

Fotostrujna merenja izvodjena su na pasivnim i anodnim filmovima niobijuma dobivenih elektro-hemiskim putem u rastvorima 2 M KOH. Na slici 2 date su potenciodinamičke krive pomoću kojih se mo-že odrediti potencijalno područje stvaranja pasivnih filmova.

0.0

0.1

0.2

0.3

0.4

-1.5 -1.0 -0.5 0.0 0.5 1.0

2

1

E (V)

I (m

A)

Slika 2 - Ciklični voltamogrami Nb elektrode snmljeni

u 2 M KOH, 1 – prvi ciklus, 2 – drugi ciklus



U prvom polaznom ciklusu dolazi do aktivnog rastvaranja Nb površine prema reakciji:

−++→ e5NbNb 5

pri čemu se pojavljuje dobro izražen aktivacijski pik. Kritična struja ovog pika drastično raste povećanjem koncentracije rastvora KOH, dok se primarni pasi-vacioni potencijal pomera u katodnom pravcu /1/. U pasivnom području stvara se poluprovodnički Nb2O5 film čija debljina raste sa povećanjem anodnog potencijala prema jednačini:

−+ ++→+ e10H10ONbOH5Nb2 522

U prvom povratnom ciklusu anodna struja u celom mernom području ima niske vrednosti koje se približavaju 0, tako što elektrodna površina postaje potpuno pasivirana. U drugom polaznom i povratnom ciklusu pojavljuje se samo mala, skoro konstantna anodna struja čije vrednosti su bliske 0. Evidentno je da posle prvog ciklusa stvoreni pasivni film blokira sve moguće redoks reakcije na graničnoj površini Nb/pasivni film/elektrolit. U alkalnim rastvorima pa-sivno područje nalazi se u negativnijem potencijal-nom području nego u kiselim rastvorima /1,2/. Prema tome naša fotostrujna merenja u pasivnom području izvodjena su na potencijalu od 0.2 V i u anodnom području na 1.5 V.

Na slici 3. dati su fotostrujna merenja u zavisnosti od talasne dužine incidentne svetlosti. Kao što se vidi sa slike 3. fotostrujni maksimumi nalaze se na talasnoj dužini od oko 310 nm i oni su za oko 18 nm pomešteni ka višim talasnim dužinama u odnos na spektre snimljene u kiselim rastvorima /3/. U preseku linearnih delova fotostrujnh krivulja sa apscisnom osom slika 3. (b) otčitava se energija zabranjene zone od oko 3.26 eV, što je veoma blizu vrednosti od 3.25 eV dobivene u H2SO4 i teoretske vrednosti za Nb2O5 od 3.3 eV. Ovo nam pokazuje da nema bitnih razlika u polu-provodničkim osobinama pasivnih filmova formiranih u 1 M H2SO4 i 2M KOH.

Fotostrujni parametri za anodne oksidne filmove dobivene za napne do 30 V dati su u tabeli 1. Tabela 1 - Fotostrujni parametri anodnih oksidnih

filmova dobiveni na Nb elektrodi u 2 M KOH E (V) λ (nm) Iph (A) Ebg (eV) 5 309 27.2 3.15 10 310 25.0 3.06 15 311 23.5 3.06 20 311 22.0 3.06 25 312 20.5 3.06 30 312 19.0 3.06

0

5

10

15

20

25

30

200 300 400 500 600 700

λ (nm)

I ph

( µA) 1

2

(a)

0

2

4

6

8

10

12

1.5 2.5 3.5 4.5 5.5

h.ν (eV)

(I pc.

h ν)1/

2

(b)

Slika 3 - (a) Fotostrujni spektri Nb elektrode snim-ljeni u 2 M KOH, (b) Oodredjivanje širine zabranjene zone na osnopvu spektra sa Sl (a)

ZAKLJUČAK

Od izvedenih fotoelektrohemiskih merenja može se zaključiti da pasivni filmovi formirani na Nb elektrodi u 2M KOH imaju skoro iste polu-provodničke osobine kao i filmovi formirani u 1 M H2SO4. Kod viših napona polarizacije, sa povećanjem debljine anodnih filmova raste njihova provodljivost sve do početka pojave rušenje filma još za vreme njihovog formiranja na naponu od 10 V. Od 10 pa sve do 30 V na mestima razrušenog filma nadogradjuje se novi film koji održava približno konstantnu vrednost provodljivosti filma.

LITERATURA

[1] I.Mickova, A.Prusi, T.Grchev, Lj.Arsov, Portug. Electrochim.Acta 24 (2006) 377

[2] I. Mickova, A.Prusi, T,Grchev, Lj.Arsov, Croat. Chim.Acta, 79 (2006), 527

[3] I.Mickova, Macedonian J.Chem.and Chem.Eng., 23 (2010) 234



ABSTRACT

PHOTOELECTROCHEMICAL INVESTIGATIONS OF THIN FILMS FORMED ON Nb ELECTRODES IN ALKALINE SOLUTIONS Photo electrochemical investigations of thin passive and anodic films on Nb electrodes in 2 M KOH have been performed. It was shown that the formed passive films have semi-conducting properties with band gap energy of 3.26 eV. By increasing the film thickness of anodic films decrease the value of band gap energy and for films formed from 10 V to 30 V this energy has a constant value of 3.06 eV. Key words: Photosynthesis, the anodic passive films, band gap, semiconductors Paper received: 15.07.2011. Scientific paper

M. JAIĆ UPOREDNA SVOJSTVA URETAN-ALKIDNIH I VODORAZREDIVIH …


MILAN JAIĆ Originalni naučni rad UDC:620.197.6:667.289

Uporedna svojstva uretan-alkidnih i vodorazredivih premaza za površinsku obradu drveta u eksterijeru

Zaštita drveta u eksterijeru, kao što su okviri prozora i vrata, može se vršiti primenom različitih sistema premaza. Najviše u upotrebi su konvencionalni isparavajući premazi – na bazi rastvarača (alkidni, modifikovani alkidi itd.). Poslednjih godina, zbog normativno restriktivne emisije organskih rastvarača, trend razvoja premaza dovodi do smanjenja isparljivih organskih komponenti. Smanjuje se prisustvo organskih rastvarača ili zamenjuju ekološki bezbednijim rastvaračima, kao što su voda ili alifatični rastvarači. Premazi na bazi vode su u stalnom razvoju. Pored smanjene emisije rastvarača premazi na bazi vode imaju i mnoge druge prednosti, kao i neke nedostatke, u poređenju sa klasičnim premazima sa rastvaračima. U ovom radu se analiziraju i upoređuju karakteristike vodorazredivog premaza na bazi akrilne disperzije sa modofikovanim uretan-alkidnim premazom. Ključne reči: drvo, eksterijer, površinska zaštita, uretan-alkidni premazi, vodorazredivi premazi, UV zračenje, adhezija, upijanje vode, propustljivost vodene pare

1. UVOD

Jedna od najvažnijih karakteristika po kojima se drvo razlikuje, od metala sa jedne strane i mineralnih podloga sa druge strane, je njegova visoka poroznost.

Druga važna karakteristika drveta je izrazita ap-sorpcija i desorpcija vode, koje su posledica higro-skopnog karaktera organskih celuloznih elemenata drveta i visoke kapilarnosti, a uzrokuju bubrenje i skupljanje drveta. Ovakva higroskopna priroda drveta utiče na dimenzionalnu nestabilnost drveta kao pod-loge, koja može da dostigne i 2-3% [10].

Ukoliko sadržaj vlage u drvetu pređe limit od 20% stvaraju se uslovi za razvoj buđi i gljiva, čije su spore u vazduhu uvek prisutne. Truljenje koje izazi-vaju može u potpunosti da razori podlogu [5, 16].

U spoljnjim uslovima drvo je izloženo dejstvu UV zračenja, koje u kombinaciji sa vlagom i uvek prisutnim kiseonikom, uzrokuje degradaciju viših slojeva drvene podloge.

Na gore navedene karakteristike veoma utiču klimatski uslovi u kojima se vrši eksploatacija drveta.

Da bi drvo, koje prati promene spoljašnje sredine, izdržalo dejstvo svih ovih faktora, njegova površina mora biti zaštićena premaznim sredstvima. Za zaštitu

Adresa autora: Univerzitet u Beogradu, Šumarski fakultet, Odsek za preradu drveta, Katedra finalne prerade drveta, Beograd, Kneza Višeslava 1

Rad primljen: 30.09.2011.

se koriste različiti tipovi premaza (premazi koji suše oksidacijom na vazduhu, vodorazredive polimerne disperzije, elastični poliuretanski premazi, sintetički makropolimeri tipa PVC, CR, Stiren ili VT polimeri, metakrilati) [3, 12, 17].

U ovom radu će biti razmatrane glavne karakte-ristike premaza koji se koriste za zaštitu drveta u eks-terijeru, sa osvrtom na uretan-alkidne i vodorazredive premaze. Uretan-alkidni premazi izabrani su kao predstavnici klasičnih premaza, sušivih na vazduhu, baziranih na organskim rastvaračima, a vodorazredivi kao predstavnici relativno nove grupe ekoloških pre-maza.

2. USLOVI KOJE MORAJU DA ISPUNJAVAJU PREMAZI KOJI SE KORISTE ZA POVRŠIN-SKU ZAŠTITU DRVETA U EKSTERIJERU

Uzimajući u obzir svojstva drveta razmatrane u uvodu, premaz (celokupni sistem zaštite) koji će kva-litetno i efikasno da zaštiti drvo, čija se eksploatacija vrši u eksterijeru, mora da zadovolji sledeće uslove [2, 8, 18]: 1. film osušenog premaza treba da poseduje visoku

elastičnost i dugotrajnost u širokom tempera-turnom opsegu (od ispod 0°C do preko +60°C), bez ikakvih tendencija za lepljenjem, prljanjem i promenama pod dejstvom klimatskih uslova

2. premaz treba da sprečava prodiranje vode, kako u film premaza tako i u podlogu, u formi rose, kiše,



snega, ali i da dozvoli dovoljan prolaz vodene pare kroz film

3. posedovanje zaštitne funkcije, čime štiti drvo od delovanja buđi, gljiva i insekata

4. omogućavanje dubokog prodiranja (penetracije) osnovnog sloja premaza, odnosno impregnacije sa insekticidno-fungicidnom zaštitom u podlogu

5. zaštita drveta od dejstva UV zračenja. Tokom svog veka premazi su, takođe, izloženi

dejstvu atmosferskih uticaja, koji u zavisnosti od njihovog kvaliteta (pre svega od tipa veziva) mogu da uzrokuju različite pojave, kao što su gubitak sjaja, ispraškavanje, pojava kratera i pukotina, odlepljivanje i odslojavanje (ljušćenje premaza), žućenje premaza ili gubitak adhezije [6, 7, 13, 14].

3. KARAKTERISTIKE URETAN-ALKIDNIH I VODORAZREDIVIH PREMAZA

Jednokomponentni uretan-alkidni premazi formi-raju se reakcijom sušivih ulja ili alkida sa izocija-natima.

Uretan-alkidni premazi suše oksidacijom na vaz-duhu, kao i alkidni premazi. Uretanska veza koju sadrže pruža ovim premazima veću otpornost prema hidrolizi i dejstvu hemikalija, tako da u odnosu na alkidne premaze imaju povećanu otpornost prema delovanju vode i alkalija. Takođe, prisustvo uretanske veze ubrzava vreme sušenja u poređenju sa alkidnim premazima. Uretan-alkidni premazi odlikuju se i dobrom otpornošću na abraziju [1, 4, 15].

Vodorazredivi premazi su razvijeni kao posledica zahteva za smanjivanjem emisije organskih rastvarača na najmanju meru.

Vodorazredivi premazi ispitivani u ovom radu bazirani su na akrilnim vezivima modifikovanim aromatskim izocijanatima [11]. Ova veziva dobijaju se emulzionom polimerizacijom i nazivaju se lateksi. Odlikuju se dobrom adhezijom i odličnom fleksibil-nošću. Oni su sposobni da prate promene (″rad″) drveta kao podloge, kao što je dimenzionalno nesta-bilno drvo sa minimalnim promenama. Imaju dobru otpornost na ispraškavanje.

Važnije karakteristike uretan-alkidnih i vodoraz-redivih premaza prikazane su u Tabeli 1.

Tabela 1 - Uporedni prikaz karakteristika uretan-alkidnih i vodorazredivih premaza

URETAN-ALKIDNI PREMAZ VODORAZREDIVI PREMAZ

EMISIJA ORGANSKIH RASTVARAČA u okolnu sredinu EKOLOŠKI PREMAZ - minimalna emisija rastvarača, razređivanje vodom

NEZNATNO ŽUĆENJE PREMAZA NE ŽUTE (polimerni lanci sadrže zasićene veze)

DUŽE VREME SUŠENJA IZMEĐU SLOJEVA KRAĆE VREME SUŠENJA OD URETAN-ALKIDNIH PREMAZA

VREMENOM DOLAZI DO PUCANJA FILMA, JAVLJAJU SE KRATERI, FLEKSIBILNOST I ELASTIČNOST FILMA OPADAJU Makromolekuli u filmu vremenom postaju sve više i više razdvojeni, formirajući napetu mrežu, koja gubi elastičnost i postaje krta. To podiže Tg od ispod 0°C do +50°C i više

TERMOPLASTIČNI PREMAZI - VEĆA ELASTIČNOST BOLJE PRATE RAD DRVETA Oni se neće ili će se veoma slabo umrežavati, čak i posle 10 godina Tg će se povećati za samo 5°C - 10°C

POSTOJANI, OTPORNI NA ATMOSFERSKE UTICAJE POSTOJANIJI, OTPORNIJI NA ATMOSFERSKE UTICAJE OD URETAN-ALKIDNIH PREMAZA

ESTETSKI LEPŠI IZGLED - IZRAŽENIJA PUNOĆA FILMA PRUŽAJU ESTETSKI ″POSNIJE″ FILMOVE

LAKŠI ZA SKLADIŠTENJE I RUKOVANJE OSETLJIVIJI ZA SKLADIŠTENJE I RUKOVANJE STROŽIJI USLOVI PRIMENE

PRANJE ALATA I OPREME ORGANSKIM RASTVARAČIMA - skuplje PRANJE ALATA I OPREME VODOM - jeftinije

4. REZULTATI

Eksperimentalni deo istraživanja obuhvatio je određivanje sledećih svojstava površinski obrađenog drveta [1, 9]: 1) vreme sušenja premaza 2) otpornost na dejstvo UV zračenja

3) otpornost na temperaturne promene – “Cold-check” test

4) otpornost prema veštačkom starenju 5) otpornost prema upijanju vode 6) dubina prodiranja 7) propustljivost vodene pare



8) izlaganje premaza u slanoj komori i 9) adhezija premaza

Za ispitivanja pod brojevima 1-5 i 8-9 na uzorke od masivnog drveta smrče (Picea abies, L.) naneti su sledeći sistemi zaštitnih premaza:

I) Uretan-alkidni premaz:

Sistem 1. (ukupno 35 uzoraka) 1. nanos - impregnacija sa insekticidno-fungi-

cidnom zaštitom (sušenje 24 h), 2. nanos - osnovni beli premaz (150-200 g/m2,

sušenje 24 h, brušenje papirom N°220), 3. nanos - završni beli premaz (150-200 g/m2,

sušenje 24 h).

II) Vodorazredivi premaz:

Sistem 2 (ukupno 30 uzoraka)

1. nanos - vodorazrediva impregnacija sa insek-ticidno-fungicidnom zatitom (sušenje 4 h),

2. nanos - osnovni beli premaz (120-150 g/m2, sušenje 4 h, brušenje papirom N°220),

3. nanos - završni beli premaz (120-150 g/m2, sušenje 7 dana).

Sistem 3 je isti kao i sistem 2, ali sadrži 2 sloja osnovnog premaza (ukupno 30 uzoraka).

Za ispitivanje paropropustljivosti premaza, sis-temi 1, 2 i 3 naneti su na sečeni furnir bukve (Fagus silvatica L.) debljine 1,5 mm i na filter papir. Za ispitivanje dubine prodiranja premaza na podloge su nanete samo pigmentirane alkidna i vodorazrediva impregnacija (po 5 uzoraka).

Rezultati ispitivanja prikazani su u tabelama 2, 3, 4 i 5.

Tabela 2 - Rezultati ispitivanja sistema 1, 2 i 3 ISPITIVANJE/ STANDARD

KRATAK OPIS METODE SISTEM 1 SISTEM 2 SISTEM 3

VREME SUŠENJA na prašinu na dodir na brušenje potpuno (na

20°C, rel. vlažnost 65%)

impregnacija i osnovni sloj do brušenja 24 h završni sloj - potpuno nakon 48 h

impregnacija: na praši-nu 10 min., na dodir 30 min., brušenje posle 4 h osnovni sloj: na prašinu 30 min., brušenje posle 4 h završni sloj: na dodir 2h, potpuno 24 h, pa-kovanje posle 5-7 dana

impregnacija: na pra-šinu 10 min., na dodir 30 min., brušenje 4 h osnovni sloj: na pra-šinu 30 min., brušenje posle 4 h završni sloj: na dodir 2 h, potpuno 24 h, pakovanje 5-7 dana

Otpornost na delo-vanje UV zračenja (metoda DUGA 5140)

Grafitnom olovkom se obeleži deo koji se izlaže zračenju. Ostali deo se zaštiti crnom hartijom. Ploče se izlažu 3x8 h, ukupno 24 h i porede sa standardom

nakon 8 h - bez promene nakon 16 h - slabo vidljiva promena nakon 24 h - vidljiva promena

nakon 24 h - bez pro-mene nijanse (premaz je matirao)

nakon 24 h - bez promene nijanse (premaz je matirao)

Otpornost na delo-vanje temperatur-nih promena (COLD-CHEK TEST) 30 ciklusa (EN ISO 53231)

Ploče se izlažu temperaturi od 49±3°C 1h, kondicio-niraju 30 min. na sobnoj temperaturi, a zatim se 1 h izlažu temperaturi od ( - 21±1)°C

- nema promene - nema promene - nema promene

Otpornost prema veštačkom starenju

20 ciklusa (metoda IMS)

Ploče se izlažu: 0.5 h dejstvu kiše 4 h na temperaturi od 0°C do - 45°C 2 h na temperaturi od 0°C do + 50°C 1 h dejstvu UV zračenja 16,5 h dejstvu normalne klime Porede se sa pločama koje nisu izložene

- neznatna promena boje

- nema promena u otpornosti prema upijanju vode

- nema promena u stepenu prijanjanja filma

- nema promene boje - nema promena u

otpornosti prema upijanju vode


- nema promene boje - nema promena u

otpornosti prema upijanju vode




Otpornost prema upijanju vode (metoda IMS)

Ploče se izlažu dejstvu vo-denog stuba visine 15 cm (pritisak 150 Pa) u vreme-nu od 24 h. Porede se sa nezaštićenim uzorcima

- nije došlo do promene nivoa vodenog stuba



Izlaganje premaza u slanoj komori, 7 dana

(SRPS C.A1.554)

Vrši se vizuelno poređenje uzoraka izlaganih vlažnoj slanoj komori (slana mag-la) sa ne izlaganim uzorcima

- nema vidljivih promena



Adhezija premaza

(EN ISO 2409/2007)

Ručni višesecivi uređaj za zasecanje se drži normalno na podlogu i uz ujednačeni pritisak povlači po površini brzinom od 20-50mm/s

- odlupljen je mali broj rezova na mestima ukršta-nja rezova. Ošte-ćena površina nije veća od 5%

- odlupljen je mali broj rezova na mestima ukrštanja rezova. Otećena površina nije veća od 5%

- odlupljen je mali broj rezova na mes-tima ukrštanja rezo-va. Oštećena povr-šina nije veća od 5%

Tabela 3 - Rezultati ispitivanja dubine prodiranja

DUBINA PRODIRANJA

(metoda IMS)

Merenje prodiranja obo-jene impregnacije vrši se mikroskopom ″Amsler″ u smeru protezanja vlakana-ca, radijalnom smeru i tangencijalnom smeru

(vrsta drveta - smrča)

Za alkidnu impregnaciju:

radijalno: mm

tangencijalno: mm

podužno: mm

Za vodorazredivu impregnaciju:

radijalno: 0.15 -0.22 mm

tangencijalno: 0.10 - 0.20 mm

podužno: 1.00 - 4.50 mm

Tabela 4 - Rezultati ispitivanja propustljivosti vodene pare za uretan-alkidne premaze

PROPUSTLJIVOST VODENE PARE

UA – URETAN-ALKID, SISTEM - NA FILTER PAPIRU (FP) NA FURNIRU BUKVE (BF)

(SRPS G S2 815/1990) T 23°C, rel. vlažnost 0-85%, slobodna površina izložena vodenoj pari 50.25 cm

SISTEM WVT

[mg/s m2]

WVT/∆p

[mg/Pa s m2]

PERMEAB.

[mg/Pa s m3] x 0.0001

PVP

[g/m2 24] µ

d

[mm]

ETALON FP 7,529.13 31.50 6.46 650.75 293 0.210

ETALON BF 3,192.78 13.36 20.60 275.96 92 1.540

FP + IMPREGNACIJA 92.10 0.39 0.11 7.96 14461 0.298

FP+ IMPREG. + UA OSNOVNI PREMAZ + UA ZAVRŠNI PREMAZ

184.00 0.77 0.26 15.94 7159 0.343

BF+ IMPREGNACIJA 213.00 0.89 1.48 18.41 1296 1.652

BF + IMPREG. + UA OSNOVNI PREMAZ + UA ZAVRŠNI PREMAZ

667.92 2.8 4.42 59.71 449 1.578



Tabela 5 - Rezultati ispitivanja propustljivosti vodene pare za vodorazredive premaze

PROPUSTLJIVOST VODENE PARE

WB – VODORAZREDIVI PREMAZ, SISTEM – NA FILTER PAPIRU (FP), NA FURNIRU BUKVE (BF)

(SRPS G S2 815/1990) T 23°C, rel. vlažnost 0-85%, slobodna površina izložena vodenoj pari 50.25 cm

SISTEM WVT

[mg/s m2] WVT/∆p

[mg/Pa s m2]

PERMEAB.

[mg/Pa s m] x 0.0001

PVP

[g/m2 24 ] µ

d

[mm]

ETALON FP 7,529.13 31.50 6.46 650.75 293 0.210 ETALON BF 3,192.78 13.36 20.60 275.96 92 1.540 FP + IMPREG. + BELI WB OSNOVNI PRE-MAZ + BELI WB ZAVRŠNI PREMAZ

391.22 1.63 0.00 33.83 9209 8.274

FP + IMPREG. + BELI WB OSNOVNI PRE-MAZ 2 X + BELI WB ZAVRŠNI PREMAZ

345.37 1.44 0.43 29.85 4309 0.304

BF + IMPREG. + BELI WB OSNOVNI PRE-MAZ + BELI WB ZAVRŠNI PREMAZ

1,225.39 5.4 8.87 111.44 214 1.644

BF + IMPREG. + BELI WB OSNOVNI PRE-MAZ 2 X + BELI WB ZAVRŠNI PREMAZ

920.99 3.85 6.53 79.60 290 1.694

5. ZAKLJUČAK

Na osnovu dobijenih rezultata mogu se doneti sledeći zaključci: 1) Obe grupe ispitivanih premaza, u navedenim sis-

temima obrade, pružaju dosta kvalitetnu zaštitu drveta čija se eksploatacija vrši u eksterijeru.

2) Vodorazredivi premazi brže suše u međusloje-vima, ali imaju duže vreme potpunog sušenja. Pre montiranja obrađenih elemenata, na primer na objektu, moraju da odstoje najmanje 5-7 dana.

3) Vodorazredivi premazi su otporniji na dejstvo UV zračenja, u pogledu postojanosti nijanse, od uretan-alkidnih premaza.

4) Vodorazredivi premazi su pokazali nešto veću otpornost, nakon izlaganja veštačkom starenju, ali je za donošenje konačnih zaključaka potrebno ispitati premaze na većem broju ciklusa.

5) Obe grupe premaza su pružile kvalitetnu zaštitu drvetu prema dejstvu temperaturnih promena, upijanju vode i eksploataciji u uslovima slane klime.

6) Dubina prodiranja alkidne impregnacije je veća od vodorazredive. Dubina prodiranjasredstva za

impregnaciju najveća je u pravcu protezanja vlakanaca, a najmanja u tangencijalnom pravcu.

7) Vrednosti za stepen transmisije vodene pare (WVT), propustljivost vodene pare (WVP) i per-meabilnost vodene pare niže su kod uretan-al-kidnih premaza, nego kod vodorazredivih. Vred-nosti za navedene parametre niže su kod sistema u kojima se koriste dva sloja osnovnog vodoraz-redivog premaza, od sistema u kojima se koristi jedan sloj.

8) Obe grupe premaza imaju dobru adheziju, pre i posle veštačkog starenja.

9) Sistemi zaštite sa jednim i dva sloja osnovnog premaza kod vodorazredivih sistema nisu poka-zali različito ponašanje kod ispitivanja (osim na-pomene u tački 7). Primenom dva sloja osnovnog premaza, postiže se bolje zapunjavanje podloge i priprema za nanošenje završnog sloja, što je od posebnog značaja kod nanošenja završnog pre-maza visokog sjaja.

10) Vodorazredivi premazi su mnogo osetljiviji za skladištenje i primenu i zahtevaju strožije uslove u pogledu kompatibilnosti sa drvetom, vezujućim agensima, bajčevima....



11) Za donošenje konačnih zaključaka, potrebno je dugotrajnije izlaganje premaza veštačkom stare-nju i duže izlaganje direktnim atmosferskim uti-cajima, što će biti predmet daljih ispitivanja

Acknowledgement

Rad je finansiran sredstvima projekta Ministar-stva prosvete i nauke Republike Srbije broj 31041.

LITERATURA

[1] Bardage, S.L. and Bjurman, J.: European Coatings Journal, 3/98, page 39

[2] Banov, A.: Coatings Word, march/april 1997., Number 14

[3] Busato, F.: Modern Paint and Coatings, march 1997., Number 30

[4] Cassens L. D., Feist C. W.: Selection and Appli-cation of Exterior Finishes for Wood, North Central Region Extention publication #135, 1988, pp. 1-8

[5] Derbyshire H., Robson D.J.: Moisture conditions in coated exterior wood, Springer-Verlag, 1999., pp. 105-113

[6] Feist C. W., Mraz A. E.: Durability of Exterior Na-tural Wood Finishes in the Pacific Northwest, Forest Products Laboratory, Medison, Wisconcin, 1980.

[7] Feist C. W.: Weathering and Protection of Wood, Forest Products Laboratory, Medison, Wisconcin, 1993.

[8] Hackl, B., Dauth, J. and Dreyer, M.: European Coatings Journal, 11/97, page 1032

[9] Jaić, M., R. Živanović: Površinska obrada drveta : svojstva materijala, kvalitet obrade. SITZAMS, Beograd, 1993., str. 275

[10] Jaić, M., Živanović, R: Površinska obrada drveta, teorijske osnove, tehnološki procesi, autorsko izda-nje, Beograd, 2000., str. 400

[11] Jaić, M.: Površinska obrada drveta u eksterijeru vo-dorazredivim premazima. Drvo tehnika: ekologija, prerada, biznis, 4(2006)11, str. 58-60

[12] Jaić, M., Nikolić, M.: Površinska zaštita drveta u spoljašnjem prostoru, 05.–08. april 2011., planina Tara, Srbija, XIII YUCorr “Saradnja istraživača različitih struka na području korozije, zaštite mate-rijala i životne sredine”, str. 14-32

[13] Nussbaum, R. M., Sufcliffe, E.J., Hellgren, A.C.: European Coatings Journal, 3/98, page 49

[14] Schmid, E. V.: Exterior durability of organic co-atings, FMJ International publications limited, England, 1998.

[15] Surface coatings, Tafe educational books, Randwich Australia, 1983.

[16] Šoškić, B.: Svojstva drveta, Šumarski fakultet, Beograd, 1994.

[17] Williams S. R., Knaebe T. M., Feist C. W.: Finishes for Exterior Wood, Forest Products Laboratory, Medison, Wisconcin, 1996., pp. 1-24, 35-45, 85-103

[18] Williams S. R., Knaebe T. M., Feist C. W.: Why Exterior Finished Fail, Forest Products Laboratory, Medison, Wisconcin, 1997.

ABSTRACT

COMPARATIVE ANALYSIS OF URETHANE-ALKYD AND WATER BASED COATINGS FOR EXTERIOR WOOD

The protection of wood elements in the exterior use, such as window frames and doors, can be performed with various type of coatings. The most diffused in use are conventional coatings - solvent-based (alkyd, modified alkyds etc.). Last years, because of the restrictive normative regarding the solvent emission, the trend of coatings development leads to the reduction of volatile organic components. Organic solvents are reduced or substituted by environmentally safer solvents such as water or aliphatic solvents. Water based coatings are in constant development. Besides the reduced solvent emission, water based coatings have many other advantages, as well as some disadvantages in comparison with classic, solvent type of coatings. In this paper, the characteristics of water based (acrylic dispersions) and urethan modified alkyd coatings are analysed and compared. Key words: wood, exterior, surface protection, urethane-alkyd coatings, water based coatings, UV-radiation, adhesion, water penetration , water vapour permeability Paper received: 30.09.2011. Scientific paper

B. STOJANOVIĆ i ... KOROZIJA I ZAŠTITA RASHLADNOG SISTEMA


BOSILJKA STOJANOVIĆ1, BRANKO ĐUKIĆ2 Originalni naučni rad NEĐO STOJANOVIĆ1, SLAVKO SMILJANIĆ2 UDC:628.193.197:621.57

Korozija i zaštita rashladnog sistema Korozija u rashladnim sistemima je spontan,složen hemijski proces na koji utiču fizički i hemijski faktori. Javlja se u nekoliko oblika, a najpodmuklija je piting korozija. Mada različite materije se mogu koristiti kao inhibitori korozije, najviše efekta u zaštitu od korozije rashladnih sistemima imaju fosfonati, posebno sinergistički blendirani fosfonati i polimeri „all organic“. U Termoelektrani Ugljevik se fosfonati koriste nekoliko godina i u ovom radu će se komentarisati iskustva njihove primjene. Ključne riječi: korozija , inhibitori, rashladni system

1. UVOD

Specifičan pogonski problem predstavlja održava-nje opreme rashladnih sistema jer nečistoće površina rashladnih cijevi smanjuju ekonomičnost rada pogo-na, uslovljene mnogim formama korozije i grešaka. Raznolikost napada je uzrokovana razlikama u pro-jektovanju rashladnih sistema, temperaturi, protoku, osobinama vode, mješavini sastava i samim opera-cijama.

2. OSNOVI KOROZIJE

2.1. Mehanizmi korozije Korozija je elektrohemijski procesu pri kojem se

razlika elektropotencijala razvija između dva metala ili između dva različita dijela pojedinačnog metala. Razlika potencijala dopušta struji da prođe kroz metal izazivajući reakcije na anodnom i katodnom mjestu (korozionoj ćeliji). Šematski prikaz korozione ćelije je dat na slici 2.1.

Slika 2.1 - Koroziona ćelija

Intereakcijom produkata nastalih na anodi i katodi mogu se desiti reakcije pri čemu se formiraju čvrsti korozioni produkti na metalnoj površini.Tako se fero jon nastao rastvaranjem gvožđa transformiše u ferohi-droksid, a on dalje u feri oksid ,odnosno rđu (Fe2O3).

Adrese autora: 1Termoelektrana Ugljevik, Ugljevik, 2Tehnološki fakultet, Zvornik, Republika Srpska BiH

Rad primljen: 21. 09. 2011

Mehanizam nastajanja rđe se najbolje ojašnjava kiseoničnom korozijom prikazanoj na slici 2.2.

Slika 2.2 - Kiseonična korozija

Nagomilavanje rđe se dešava na anodnim mjesti-ma pri čemu se formira mulj poznat kao izrasli-ne.Ispod ovih muljeva lokalna korozija se nastavlja da ubrzava, slika 2.3.

Slika 2.3 - Podnanosna korozija

2.2. Tipovi korozije Korozioni procesi na metalnim površinama u ras-

hladnim sistemima mogu primiti mnoge forme pozna-te pod imenom: opšta korozija, eroziona, podnanosna, galvanska i piting (1-4).

Najpodmuklije djelovanje je piting korozija, a javlja se u malim odvojenim oblastima gdje je gubitak metala neznatan pri čemu se formira jamica (udub-



ljenje) koja predstavlja lokalno anodno mjesto na površini metala čiji se rast rapidno nastavlja, jer je u okruženju velika katodna oblast.

Piting koroziju veoma podstiču hloridni joni, jer se u udubljenju nakupi izvjesna koncentracija metal-nih hlorida koji hidrolizuju i snižavaju pH vrijednost u okruženju, a rastvor sa visokim sadržajem soli i malim sadržajem kiseonika ostaje nepokretan. Reak-cije u udubljenju postaju autokatalitičke sa vrlo malim tendecijom zaustavljanja, ultimativno uzro-kujući prodiranje kroz osnovu metala.

Piting korozija je združena sa podnanosnom i gal-vanskom korozijom.Tako meatalni nanos (npr. joni-ma bakra obložena površina čelika) može takođe formirati mjesta za piting djelovanje.

Faktori koji utiču na koroziju su fizički: tempe-ratura, brzina kretanja fluida, prenos toplote, mikro-struktura materijala oblast(formiran galvanski par), a hemijski faktori su: pH rashladne vode, izvjesne soli hloridi i sulfati, rastvorljivi gasovi CO2 i O2, sus-pendovane materije i mikrobiološki rast.

2.3. Korozioni inhibitori Najbolja zaštita od korozije su korozioni inhibi-

tori koji u zavisnosti koje korozione reakcije inhibi-raju su imenovani kao katodni ili anodni ( 5-8).

Inhibicija je obično rezultat jednog ili više od tri uopštena mehanizma.

Prvo molekuli inhibitora se adsorbuju na metalnu površinu pri procesu hemisorpcije, formirajući tanki zaštitni film ili samostalno ili u konjukciji sa metal-nim jonima.

Neki inhibitori ipak prosto uzrokuju da metal for-mira zaštitni film od metalnog oksida pa otuda pove-ćavaju svoju rezistentnost, što suštinski određuje se-kundarni mehanizam inhibicije.

U trećem mehanizmu inhibicije inhibitor reaguje sa potencijalno korozivnom supstancom u vodi.

Izbor podesnog inhibitora je određen projektnim parametrima rashladnog sistema i sastavom vode. Na izbor inhibitora će uticati vrsta metala u sistemu, uslovi napetosti,čistoća i projektna brzina vode, ali i drugi faktori kao: zahtjevani nivo tretmana, pH vri-jednost vode, sadržaj rastvorenog kiseonika, sadržaj i sastav soli i supendovanih materija u rashladnoj vodi.

Različiti hemijski materijali su pokazali efikas-nost pri inhibiciji korozionih procesa,a danas se naj-više koriste dvije klase fosfonatni materijali: amino-metilenfosfonat, nitrilotrismetilenfosfonska kiselina (AMP) i 1-hidroksi etil 1,1 –difosfonska kiselina (HEDP).

Da bi korozioni inhibitorski program bio prih-vatljiv moraju biti zaštićene sve izložene metalne

površine, koncentracija inhibitora mora biti efikasna i niska, ne smije stvarati depozit na metalnoj površini. Učinak inhibicije mora biti efektivan u širokim granicama pH-vrijednosti, temperature, kvaliteta vode i toplotnog fluksa, sa peventivnim djelovanjem na stvaranje kamenca i disperziju depozita i minimalnim uticajem na životnu sredinu.

U lepezi odabira najpovoljnijeg inhibitora za re-dukciju korozije prvi korak je razmatranje osobina vode odnosno njene korozivnosti ili sklonosti ka for-miranju kamenca. Za ovakva razmatranja se određuje Lanžeov indeks i Riznarov indeks stabiliteta (1,2,4).

Lanžeov indeks predviđa taloženje kalcijum kar-bonata, zavisi od pH vrijednosti i saturacione pH vri-jednost (pHS) rashladne vode , a izračunava se prema formuli:

LSI = pH – pHS

Lanžeova saturaciona pH vrijednost (pHS) je od-ređena relacijom između kalcijumove tvrdoće vode, ukupnog alkaliteta, rastvorenih čvrstih materija i tem-perature vode, a izračunava se prema formuli:

pHS = A+B -log[Ca+2] - log[ukupni alkalitet]

gdje su A i B konstante zavisne od temperature i rastvorenih čvrstih materija u vodi,a kalcijum i ukupni alkalitet su izraženi u mg/l kao CaCO3. Pri izračunavanju pHS vrijednost konstanti se uzima aproksimativno sa grafičkih prikaza datih na slikama 2.4-2.6 (1,2,4).

Da bi se poboljšala tačnost predviđanja tendecije vode ka stvaranju kamenca ili razvijanju korozije u kombinaciji sa Lanžeovim indeksom izračunava se Riznarov indeks stabiliteta(RSI) po jednačini:

RSI = 2pHS - pH

Zavisno od vrijednosti Riznarovog indeksa određuje se očekivana sklonost vode (tabela 2.1.) što i jeste indikator za doziranje inhibitora (2-4).

Tabela 2.1 - Kvalitet vode zavisno od Riznarovog indeksa

RSI Tendecija vode

4,0-5,0 preveliko stvaranje kamenca

5,0-6,0 slabo stvaranje kamenca

6,0-7,0 vrlo malo stvaranje kamenca i malo korozivna

7,0-7,5 zadovoljavajuće korozivna

7,5-9,0 isuviše korozivna

9,0 i više nedopustivo korozivna



Slika 2.4 - Grafički prikaz vrijednosti konstante A

Slika 2.5 - Grafički prikaz vrijednosti konstante B

Slika 2.6 - Grafički prikaz vrijednosti logaritma koncentracije kalcijuma i ukupnog alkaliteta

3. EKSPERIMENTALNI DEO

3.1. Iskustva u zaštiti od korozije rashladnog sistema u Termoelektrani Ugljevik Kao mjeru zaštite rashladnog sistema od korozije

projektant je predvidio samo brže kretanje fluida. Sa zapažanjem pojave korozije, u periodu do 1992 godi-ne, rashladnoj vodi se dodavao polifosfat bez anali-tičkog praćenja rasta korozije.

Nakon pauze u radu termoelektrane od nekoliko godina (1992-1995) nije rashladna voda tretirana inhi-bitorom korozije, ali je analitički praćena korozija.

Nekoliko godina kasnije je u rashladnu vodu do-ziran inhibitor na bazi fosfonata, organofos 30.00 (na-trijumova so 2-fosfono butan 1,2,4 trikarbonske kiseline).

Zatim je rashladni sistem tretiran sa Nalko hemi-kalijama, inhibitor za čelik na bazi fosfonata, N23210 i inhibitor za bakar, na bazi triazola, N73190.

Od 2009 godine tretman rashladne vode obavlja BK Đulini primjenjujući Gilufer 440B, inhibitor za

čelik na bazi fosfonata i Albafos 148, inhibitor za bakar na bazi triazola.

U periodu od 01.09.2009 godine do 16.04.2010 godine, rashladna voda je obrađivana Đulini inhi-bitorima, zbir no je dodato 6,5t biocida, 12%-nog rastvora natrijum hipohlorita i softeverski su praćene promjene karakteristika rashladne vode, količina inhibitora u njoj i njegova dostupnost.

4. DISKUSIJA REZULTATA

Mjera zaštite od korozije predviđena projektom nije dala rezultata (7). Učinjen je propust što anali-tički nije praćen rast korozije u periodu obrade rashladne vode sa polifosfatom.

Analitički zabilježen rast korozije bez inhibitora nakon pauze u radu termoelektrane je iznosila 1mm/god., a to je bio alarm da se pristupi ozbiljnijoj zaštiti rashladnog sistema od korozije.

Prilično dobri rezultati su postizani doziranjem organofosa 30.00. Zabilježen je rast korozije 0,1074-0,1260mm/god. Pri tom najpovoljniji rezultati su pos-



tizani kad se kao nadomirenje uvodila sirova voda i vrijednost m-alakalitet u rashladnoj vodi održavala oko 3, ali nedostatak ovog tretmana je bio povećanje količine susupendovanih materija koje su zapušavale prskalice u rashladnom tornju.

Pri tretmanu rashladne vode sa Nalko hemikali-jama su analitički zabilježeni rast korozije iznosili za

čelik <0,08mm/god. i za bakar < od 0,03mm/god., što je u potpunosti zadavoljavajuće (6-9).

Interesantno je porediti rast korozije pri tretmanu rashladne vode sa obje firme Nalko i Đulini kao i promjene rasta korozije uz doziranje biocida hipohlo-rita, tabela 4.1 i 4.2.

Tabela 4.1 - Uporedni rezultati rasta korozije bez biocidnog tretmana Nalco Đulini

TE Tip etalona*

Period posmatranja

Dani ekspozicije

Korozija mm/god

Period posmatranja

Dani ekspozicije

Korozija mm/god

U radu Fe-čelik 28.10.-08.11.2004 11 0,0631 20.05-

27.05.2008. 6,92 0,0515

U radu Cu-bakar 28.10.-08.11.2004 11 0,0216 20.05-

27.05.2008. 6,92 0,0002

Tabela 4.2 - Uporedni rezultati rasta korozije sa dodatkom biocida, hipohlorita (NaOCl) Nalco Đulini

TE Tip etalona*

Period posmatranja

Dani ekspozicije

Koro zija

mm/god

Dozi rano

NaOCL(t)

Period po-smatranja

Dani ekspo-zicije

Koro zija

mm/god

Dozi rano

NaOCL (t)

U radu Fe-čelik 05.07.-23.07. 2004 18,08 0,1712 4,7 30.05-

23.06.2008. 23,92 0,1156 3

U radu Cu-bakar 05.07.-23.07. 2004 18,08 0,0460 4,7 11.06-

23.06.2008 11,98 0,0258 3

Uporedna ispitivanja su vršena putem identičnih mjernih etalona, približnih radnih uslova i približnih dana ekspozicije, a rezultati iz tabela pokazuju da se rast korozije, za Fe 0,0515-0,0631mm/god. i Cu 0,0002-0,0216mm/god., u potpunosti prihvatljive kod oba proizvođača inhibitora. Razlika u rastu korozije između proizvođača je zanemarljiva.

Tabela 4.2 pokazuje da je rast korozije uvećana i pri tretmanu Nalko i Đulini što je rezultat ometanja

reakcija inhibicije hloridnim jonima koji dodatno nastaju degradacijom hipohloritnog biocida.

Softeverski praćene promjene karakteristika ras-hladne vode, količina inhibitora u rashladnoj vodi i njegova dostupnost, pri trtetmanu rashladne vode Đulini inhibitorima uz dodatak hipohlorita su prika-zani na slikama 4.1-4.8.

Slika 4.1 - Promjene pH vrijednosti i provodljivosti u toku BK Đuluni tretmana rashladne vode

Slika 4.2 - Promjene ukupne tvrdoće u toku BK Đulini tretmana rashladne vode



Slika 4.3 - Promjene m-alkaliteta u toku BK Đulini tretmana rashladne vode

Slika 4.4 - Promjene CaT ekvilibrijuma u toku BK Đulini tretmana rashladne vode

Slika 4.5 - Promjene analitički nađenog i dostupnog „Gilufer440B“ u toku BK Đulini tretmana rashladne vode

Slika 4.6 - Promjene sadržaja suspendovanih materija u toku BK Đulini tretmana rashladne vode

Slika 4.7 - Promjene ugušćenja rashladne vode u toku BK Đulini tretmana rashladne vode

Slika 4.8 - Promjene doze „Gilufer 440B“ prema vodi za nadomirenje u toku BK Đulini tretmana rashladne vode



CaT ekvilibrijum je bezdimenzionalna vrijednost, a predstavlja odnos ugušćenja rashladne vode izraču-natog preko ukupne tvrdoće i ugušćenja rashladne vode izračunatog preko m-alkaliteta. Idealna vrijed-nost CaT ekvilibrijuma je 1, a cilj je da kreće od 0,9-1,1 jer niža vrijednost od 0,9 ukazuje da se taloži kal-cijum, a viša vrijednost od 1,1 ukazuje da se rastva-raju stare naslage (6,7,9).

Na osnovu grafičkih prikaza promjena u rashlad-nom sistemu se može zaključiti da su u posmatranom periodu vrijednosti m-alkaliteta i ugušćenja bile kore-ktne, sadržaj suspendovanih materija je pokazivao ve-like oscilacije preko limita, a da je dalje potrebno ra-diti na stabilizaciji sistema u smislu optimalne količine inhibitora (8,9).

Uzimajući u obzir kvalitet vode u posmatranom periodu : Ph = 8,71, T = 350C, Ca-tvrdoća = 103,21 mg/lCaCO3 i m-alkalitet = 128,5 mg/l CaCO3, izra-čunati indeksi su iznosili : Lanžeov indeks (LCI) -1,165 i Riznarov indeks (RSI) - 6,38 što ukazuje da je rashladna voda vrlo malo sklona stvaranju kamenca i malo korozivna.

5. ZAKLJUČAK

Korozija oduzima milione novčanih sredstva go-dišnje kroz gubitke ili kontaminaciju produkata, nak-nadu troškova, ponovno projektovanje opreme, sma-njenje efikasnosti, visoke troškove održavanja i rasi-panje dragocjenih resursa. Korozija, takođe, zadržava tehnološki progres i ugrožava ljudsku sigurnost.

Zaštita od korozije rashladnih sistema se postiže permanentnim inhibitorskim programom, najbolje multikomponentnim inhibitorima na bazi fosfonata.

Prihvatljiv inhibitorski program podrazumjeva zaštitu svih izloženih metalnih površina sa niskom koncentracijom inhibitora, čije je djelovanje efikasno

u širokim dijapazonima pH-vrijednosti, temperature, kvaliteta vode i temperaturnog fluksa bez stvaranja depozita na metalnoj površini, uz minimalno stva-ranje kamenca na stjenkama i minimalni uticaj na životnu okolinu u slučaju kad se iznosi iz rashladnih sistema.

LITERATURA

[1] Bennet P., Boffardi, Ph. d. Fundamentals of cooling water treatment, Corporation Calgon,1989.

[2] Principles of industrial water treatment, Published by Drew Chemical Plaza Boonton, New Jersey, third edition, 1979.

[3] Tehnološko-metalurški fakultet Beograd, Studija Obezbeđenje i osavremenjavanje postojećih režima voda–para i mogućnost uvođenja novih tehnologija u termoenergetskim objektima EPS-a, Beograd, 1994.

[4] F.N. Kremmer, The Nalco Water Handbook, second edition, New York, 1987.

[5] Kostić, S., Zaštita materijala, 43, 4(2002) 56-64. [6] Gajić, A., Tomić, M., Pavlović, Lj., Blagojević, B.,

Pavlović, M.: Značaj pripreme vode termoenergetskih postrojenja, XII YUCORR, Tara 2010, str. 145.

[7] M.Samardžić, B. Stojanović, M.Jotanović, S.Radić, Povećanje tehničke efikasnosti rashladnog sistema TE „Ugljevik“ –Ugljevik, odgovarajućim tretmanom rashladne vode, XXV Jugoslovenski majski skup, Održavanje industrijskih postrojenja, Beograd, 2001.

[8] Tomić, M., Pavlović, M., Tadić, G., Pavlović, Lj., Zaštita materijala, 50, 1(2009) 51-59.

[9] Đukić B., Smiljanić S.,Stojanović B.,Stojanović N., Zaštita materijala, 51, 4 (2010) 250-255.

ABSTRACT

CORROSION AND PROTECTION OF COOLING SYSTEM Corrosion in cooling systems is a spontaneous chemical complex process influenced by physical and chemical factors. It occurs in several forms, and most insidious is pitting corrosion. Although different materials can be used as corrosion inhibitors, the most effect in the corrosion protection of cooling systems are phosphonates, especially synergistic coupled phosphonates and polymers "all organic". The power plant Ugljevik phosphonates used for several years and this paper will comment on the experience of their application. Key words: corrosion, inhibitors, cooling system Paper received: 22.09.2011. Scientific paper

D. V. DIMITROVSKI et al ... ARSENIC REMOVAL THROUGH COAGULATION AND …


DEJAN V. DIMITROVSKI1, Scientific paper ZORAN LJ. BOZINOVSKI2 UDC:628.191/.196(497.11) KIRIL T. LISICHKOV1 STEFAN V. KUVENDZIEV1

Arsenic removal through coagulation and flocculation from contaminated water in Macedonia

Arsenic removal from contaminated water from Kozuf mountain region in R. Macedonia was examined through application of processes of coagulation and flocculation with ferric ions in this work. The experimental results have shown that arsenic-borne coagulates produced by coagulation with ferric ions were filterable, so filtration (filter paper as filter medium) was needed to remove the coagulates from water. The coagulation followed by conventional filtration achieved a very high arsenic removal rate (over 90%) from contaminated arsenic water (0.4 mg/l arsenic concentration), producing cleaned water with the residual arsenic concentration of 0.02 mg/l. In the process of arsenic removal, a zero residual arsenic concentration was achieved through appropriate adjustment of pH value. The results of the research have established the relations between the added flocculants, the contact time, and the pH change and it has been found that in certain cases efficiency for arsenic removal of 100% can be achieved.

Key words: coagulant, flocculent, arsenic, contact time, pH

1. INTRODUCTION

According to the WHO (World Health Orga-nization) recommendations, the presence of arsenic in the water supply system or commercially used mineral water is extremely harmful and dangerous. Based on the WHO recommendations, EU directives and domestic regulations that correspond to above-mentioned recommendations, the maximal allowed concentration of total arsenic is 10 µg/l. Lately, the arsenic concentration in commercially used water and water used through the public water supply system on the territory of Republic of Macedonia often sur-passes the MAC (Maximum Allowed Concentration) value [1-3]. Socio-economic conditions of R. Mace-donia demand low-cost as well as efficient treatment systems that could be implemented in the rural areas or cities. The main arsenic species present in natural waters are arsenate ions AsO4−3 (oxidation state V) and arsenite ions H3AsO3, H2AsO3 – and HAsO3 −2

Author's address: 1Faculty of Technology and Metallurgy, University "Ss. Cyril and Methodius", Skopje, 2"Water Supply and Sewage", Public Enterprise, Skopje, Center for Sanitary Control and Supervision


(oxidation state III). However, As(V) ions are most prevalent in oxygenatedwater while As(III) is found in anaerobic conditions, like in well water or in groundwater. The literature on arsenic oncludes that the most common valence states of arsenic in well water are As(V) or arsenate, and As(III) or arsenite. In the pH range of 4–10, the trivalent As(III) species are neutral in charge, while the As(V) are negatively charged. The removal efficiency for arsenic is often much lower for As(III) than for As(V) by using anyone of the conventional technologies for elimina-tion of arsenic from water [4, 5], so either elevation of pH [5] or oxidation of arsenite to arsenate [6] is con-sidered a prerequisite for any treatment method to be efficient. Therefore, treatment of arsenic contamina-ted well water through appropriate technology is one option to mitigate the arsenic problem. Various tec-hnologies have been used for removing arsenic from groundwater. The most commonly used technologies include co-precipitation with alum or iron, adsorption onto coagulated floc, ion exchange resin, reverse osmosis and membrane techniques. A review of these technologies along with their distinct advantages and disadvantages is shown in Table 1 [7–14].



Table 1 - Comparison of main arsenic removal technologies

Technologies Advantages Disadvantages Removal (%)

Oxidation/precipitation

Air oxidation

Relatively simple, low-cost but slow process In situ arsenic removal Also oxidizes other inorganic and organic constituents in water

Mainly removes arsenic (V) and accelerate the oxidation process 80

Chemical oxidation Oxidizes other impurities and kills microbes Relatively simple and rapid process Minimum residual mass

Efficient control of the pH and oxidation step is needed 90

Coagulation/co-precipitation

Alum coagulation

Durable powder chemicals are available Relatively low capital cost and simple in operation Effective over a wider range of pH.

Produces toxic sludges Low removal of arsenic Pre-oxidation may be required

90

Iron coagulation Common chemicals are available More efficient than alum coagulation on weigh basis

Medium removal of As(III) Sedimentation and filtration needed 94.5

Lime softening Most common chemicals are available commercially Readjustment of pH is required 91

Sorption techniques

Activated alumina Relatively well known and commercially available

Needs replacement after four to five regeneration 88

Iron coated sand Expected to be cheap No regeneration is required Remove both As(III) and As(V)

Yet to be standardized Produces toxic solid waste 93

Ion exchange resin Well-defined medium and capacity The process is less dependent on pH of water Exclusive ion specific resin to remove arsenic

High cost medium Requires high-tech operation and maintenance Regeneration creates a sludge disposal problem As(III) is difficult to remove Life of resins

87

Membrane techniques

Nanofiltration Well-defined and high-removal efficiency Very high-capital cost Pre-conditioning High water rejection

95

Reverse osmosis No toxic solid waste is produced High tech operation and maintenance 96

Electrodialysis Capable of removal of other contaminants Toxic wastewater produced 95

2. COAGULATION AND FLOCULATION Coagulation and adsorption processes are most

promising for arsenic removal from high-arsenic water because of the low cost and high efficiency, and are widely used in the developing world. But, they have not been shown to deeply eliminate arsenic from water and to produce cleaned water with a very low arsenic concentration. Coagulation process is traditio-nally realized by adding ferric or aluminum ions [14]. In this process, fine particles in water first aggregate into coagulates because added ferric or aluminum ions strongly reduce the absolute values of zeta poten-

tials of the particles. Then, arsenic ions (arsenate or arsenite) precipitate with the ferric or aluminum ions on the coagulates, and thus concentrate in the coa-gulates. After that, the coagulates are separated from water through filtration, eliminating arsenic from the water. The coagulates are termed arsenic-borne coa-gulates. Coagulation with ferric ions for arsenic re-moval can be traced back to the late 1960s in Taiwan to treat deep-well water with naturally elevated ar-senic concentrations [15]. Gulledge and O’Connor [16] also reported that arsenic could be readily remo-ved from water to a high degree by conventional wa-ter treatment using ferric or aluminum ions as coagu-



lants. Since then, there have been a lot of reports on coagulation process for arsenic removal. It has been found that the coagulation is much more effective for the removal of As (V) than As (III). In the case when only As (III) is present, oxidation to convert As (III) to As (V) is needed prior to coagulation. The effective pH for arsenic removal was reported to be 5–7 with aluminum ions, and 5–8 for ferric ions [17]. Besides iron and aluminum compounds, manganese, calcium and magnesium compounds are also of effective coa-gulants for eliminating arsenic from water in neutral medium [18,19]. Arsenic removal from water achie-ved by coagulation process depends on initial arsenic concentration in water [19,20]. The arsenic removal could reach 99% (Jiang, 2001). Recently, it was reported that modified coagulation/filtration could give a residual arsenic concentration of 2 mg/l or less for treated well water [21,22]. The arsenic removal is also dependent on the pore size of the membrane filter disks used for coagulation process [21-25], since coagulates smaller than the pore can pass through the filter and remain in water. As it is known, in filtra-tion, larger the filter pores, lower the capital and ope-ration costs, and higher the separation efficiency. Therefore, there is a great significance in enlarging arsenic-borne coagulates in order to improve coagula-tion/filtration process for arsenic removal. However, there is little information in this regard, although there are numerous reports on coagulation process for arsenic removal from water. Usually, coagulations are enhanced by adjusting pH and electrolyte concen-tration to reduce the absolute values of zeta potentials of particles, and by optimizing coagulation kinetics.

3. EXPERIMENTAL PART

The contaminated arsenic water sample used in this work was originally collected from the Kozuf mountain region, located in Gevgelija municipality, southern part of R. Macedonia. The water was filtrated with an Ahlstrom Grade 610 filter paper (2.5 mm aperture) to remove solid contaminants. The water past the filter paper was used for the tests, which gave the initial arsenic concentration of 0.3-0.4 mg/l. The chemical composition and some properties of the water sample are listed in Table 2.

Ferric chloride FeCl3 concentration of 40% , was used as coagulant; hydrochloric acid (HCl) (Fremont, analytic purity) and sodium hydroxide (NaOH) (J. T. Barker, analytic purity) was used to adjust pH.

Arsenic water (1000-4000 ml) was first mixed with a given amount of ferric coagulant in a flask, and then was adjusted for pH with HCl or NaOH by using a potential meter (Orion 720-A). After that, it was stirred on a magnetic agitator (Digital hot plate/stirrer

04644) at 400 rev/min for 20 min, while temperature was kept at 25 ± 0.5 0C. Arsenic-borne coagulates were formed during the agitation. After that, the suspension was filtrated through a Filtrak 391 filter paper (0.125 mm aperture). The filtrate was sent for arsenic analysis. Arsenic concentration was deter-mined by rather fast and less accurate colorimetric method provided by MERCK, while more precise results for the remaining concentration of arsenic are obtained by hydride method provided by AAS (atomic absorption spectroscopy) conducted in AAS Model Varian Spectra AA 55 apparatus with hydride generator. Each test was duplicated. The arithmetic average result of the two tests was reported in this paper. Table 2 - The chemical composition and some

properties of the water sample

Parameter Quantity units Sample

pH 6.6 blur NTU 0.45 Electrical conductivity µS 1316 dry residue mg/l 766.5 bicarbonate mg/l 951.6 alkalinity 156 hardness oDh 38.57 calcium mg/l 205.1 Magnesium mg/l 42.33 iron mg/l 0.0 Manganese mg/l 0.0 sodium mg/l 32.7 arsenic mg/l 0.3-0.4 Chromium mg/l <0.005 Chloride mg/l 9.0 sulfate mg/l 22.52 silicates mg/l 24.0 Nitrate mg/l 0.26 nitrite mg/l 0.0 ammonia mg/l 0.0 Consumption of permanganate mg/lO2 2.5

4. RESULTS AND DISCUSSION

4.1. Application of ferric chloride as a flocculent depending on the starting concentration Experimental results regarding the application of

ferric chloride in various concentrations as flocculants and the resulting effects are presented in Table.3.

Four concentrations of ferric chloride solution were used - 7.43 mg/l; 9.9 mg/l; 14.9 mg/l and 29.7 mg/l. Experimental reaction time was 5 minutes.



Table 3 – Removal of arsenic with flocculent FeCl3 (output concentration of arsenic)

Series 1 - Working volume 1000 ml with flocculent concentration 0.0272 g/l FeCl3 Series 2 - Working volume 2000 ml with flocculent concentration 0.0149 g/l FeCl3 Series 3 - Working volume 3000 ml with flocculent concentration 0.0099 g/l FeCl3 Series 4 - Working volume 4000 ml with flocculent concentration 0.00743 g/l FeCl3

time (min)

Series 1 As [mg/l]

Series 2 As-[mg/l]

Series 3 As-[mg/l]

Series 4 As-[mg/l]

0 0.38 0.35 0.35 0.33 3 0.009 0.01 0.025 0.025 5 0.004 0.006 0.025 0.025

4.2. Application of ferric chloride as a flocculent depending on pH value According to literature, it is evident that the pH

value of the treated water has certain influence on the removal efficiency of both forms of arsenic. A group of experiments was conducted in order to determine ferric chloride’s efficiency of arsenic removal depending on various pH values and flocculent concentration. The results of these experiments are given in Table 4.

Series of experiments were conducted with ferric chloride concentrations of 9.9 mg/l; 14.9 mg/l and 27.2 mg/l, and pH values of 5.5; 6.5; 7.5; and 8.5. Experimental reaction time was set for 20 minutes. Table 4 - Removal of arsenic with flocculent FeCl3

with change of the Ph value Series 1 - Working volume 1000 ml with flocculent

concentration 0.0272 g/l FeCl3 Time (min)

As [mg/l] pH=5,5

As [mg/l] pH=6,5

As [mg/l] pH=7,5

As [mg/l] pH=8,5

0 0.4 0.4 0.4 0.4 1 0.0 0.002 0.023 0.09 3 0.0 0.0 0.023 0.039 5 0.001 0.001 0.01 0.039

10 0.0 0.0 0.01 0.03 20 0.0 0.0 0.005 0.027

Series 2 - Working volume 2000 ml with flocculent concentration 0.0149 g/l FeCl3

time (min)

As [mg/l] pH=5,5

As [mg/l] pH=6,5

As [mg/l] pH=7,5

As [mg/l] pH=8,5

0 0.4 0.4 0.4 0.4 1 0.0 0.001 0.05 0.15 3 0.001 0.001 0.02 0.1 5 0.0 0.0 0.035 0.15

10 0.0 0.001 0.025 0.1 20 0.0 0.001 0.025 0.1

Series 3 - Working volume 3000 ml with flocculent concentration 0.0099 g/l FeCl3

time (min)

As [mg/l] pH=5,5

As [mg/l] pH=6,5

As [mg/l] pH=7,5

As [mg/l] pH=8,5

0 0.4 0.4 0.4 0.4

1 0.004 0.005 0.025 0.18

3 0.002 0.004 0.058 0.24

5 0.001 0.002 0.025 0.16

10 0.0 0.002 0.025 0.16

20 0.0 0.002 0.025 0.1

4.3. Analysis of the obtained results: a. Examinations were performed through discon-

tinuous setup of the experiments. b. Application of ferric chloride as a coagu-

lant/flocculent. - Four concentrations of ferric chloride were

utilized - 7.43 mg/l; 9.9 mg/l; 14.9 mg/l and 29.7 mg/l.

- The 90% or more arsenic removal in the experi-mental batch reactor was usually completed within 3min or less for most experiments.

- Following arsenic removal efficiency results were detected 92.4%, 92.9%, 98.3%, and 98.9%, accordingly.

- High removal efficiency was achieved in all the experiments, which resulted in decrease of the arsenic concentration in the treated water sam-ples. Experimental results regarding the 2 cases of water samples containing higher concentra-tions of arsenic suggest decrease below MAC values.

c. Usage of ferric chloride as a coagulant/flocculent at different pH values.

- Four different pH values were adjusted – 5.5, 6.5, 7.5 and 8.5.

- The 90% or more arsenic removal in the expe-rimental batch reactor was usually completed within 60s or less for most experiments.

- According to several literature sources, the pH value has a significant impact on the arsenic removal efficiency, especially regarding As3+ and As5+ [22-24].

- At higher pH values, only the highest con-centration of ferric chloride stimulates the arse-nic removal efficiency. This is evident from Table.4.

- On the other hand, decrease of arsenic concen-tration below MAC is achievable only at acidic or neutral pH values. This cannot be achieved at higher pH values.



5. CONCLUSIONS

Obtained results from conducted experiments indicate that the applied ferric ions can be efficiently used as a coagulant/flocculent. The coagulation/floc-culation followed by conventional filtration achieved a very high arsenic removal rate from contaminated arsenic water (0.4 mg/l arsenic concentration), produ-cing a cleaned water with no residual arsenic concen-tration (0.0 mg/l). The experimental results have shown that the process of coagulation/flocculation was finished in 60s or less. Furthermore, system’s pH value has significant influence on the arsenic separa-tion from its aqueous solutions, where higher efficie-ncy and better results are obtained for mildly acidic pH values as well as for neutral value.

Therefore, high arsenic removal efficiency can be accomplished by appropriate setup of the process parameters and in certain cases efficiency of 100% can be achieved.

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(2004) 17-30.

[9] J. Kim, M.M. Benjamin, Water Res. 38 (2004) 2053-2071.

[10] K.N. Ghimire, K. Inoue, H. Yamaguchi, K. Makino, M. Tohru, Water Res. 37 (2003) 4945-4958.

[11] Z.Lopicic,M.Stojanovic,C.Lacnjevac,J.Milokovic,M.mihajlovic,T.Sostaric, J. Protection of materials 52,3(2011) 189-195

[12] J.A. Jay, N.K. Blute, H.F. Hemond, J.L. Durant, Water Res. 38 (2004) 1155-1169.

[13] K.V. Hege, M. Verhaege, W. Verstraete, Water Res. 38 (2004) 1550-1567.

[1] N. Balasubramanian, K. Madhavan, Chem. Eng. Technol. 24 (5) (2001) 519-532.

[14] Hering, J.G., Chen, P.Y., Wilkie, J.A., Elimelech, M., Liang, S., Arsenic removal by ferric chloride. J. AWWA 88 (4), (1996) 155–167.

[2] Shen, Y.S., Study of arsenic removal from drinking water. J. AWWA 65 (8), (1973) 543-555.

[15] Gulledge, J.H., O’Connor, J.T., Removal of arsenic (V) from water by adsorption on aluminum and ferric hydroxides. J. AWWA 65 (8), (1973) 543-559.

[16] Sorg, J.T., Logsdon, G.S., Treatment technology to meet the interim primary drinking water regulations for inorganics. Part 2. J. AWWA 70 (7), (1978) 379–392.

[17] Raje, N., Swain, K.K., J. Radioanal. Nucl. Chem. 253, (2002) 77–80.

[18] Jiang, J.Q., Water Sci. Technol. 44 (6), (2001) 89–98. [19] Thirunavukkarasu, O.S., Viraraghavan, T., Subrama-

nian, K.S., Chaalal, O., Islam, M.R., Energy Source. 27, (2005) 209–219.

[20] Han, B., Zimbron, J., Runnells, T.R., Shen, Z., Wickramasinghe, S.R., New arsenic standard spurs search for costeffective removal techniques. J. AWWA 95 (10), (2003) 109–118.

[21] S.Kuvendziev, K.Lisichkov,D.Dimitrovski, Zastita materijala, 52, 4 (2011) 291-295

[22] J. G. Hering et al., J. Environ. Eng. 8 (1997) 800-807 [23] S. Karcher et al., J. Ciwern. 13 (1999) 164-168 [24] A. Ramaswami et al., Water Res. 35 (2001) 4474-4479

IZVOD

UKLANJANJE ARSENA PUTEM KOAGULACIJE I FLOKULACIJE IZ KONTAMINIRANIH VODA MAKEDONIJE Uklanjanje arsena iz zagadjene vode iz planinskog regiona Kozuf u R. Makedoniji je ispitan u ovom radu primenom procesa koagulacije i flokulacija sa feri jonima. Eksperimentalni rezultati su pokazali da se arsen zgrušava pri koagulacije sa feri jonima, tako da je potrebno izvršiti filtraciju vode i očistiti je od koagulanata. zgrušava. Konvencionalna filtracijom koagulanata ostvario se veoma visok stepen uklanjanja arsena (preko 90%) od kontaminiranog arsena u voda (0,4 mg / l, koncentracija arsena), i proizvedena je očišćena voda sa rezidualnom koncentracijom arsena od 0,02 mg / l. U daljem procesu uklanjanja arsena, nula rezidualna koncentracija arsena je ostvaren kroz odgovarajuće podešavanje pH vrednosti vode. Rezultati istraživanja su uspostavili odnos između dodatka flokulanta i vreme kontakta, kao i promene pH, pri čemu je uočeno da u nekim slučajevima može se postići efikasnost za uklanjanje arsena od 100%. Ključne reči: koagulanti, flokulanti, arsen, kontaktno vreme, pH Rad primljen: 25.07.2011. Originalni naučni rad

M. MIHAILOVIĆ et al ... THE POSSIBILITY OF BIOACTIVE COATINGS OBTAINING …


MARIJA MIHAILOVIĆ1, ALEKSANDRA PATARIĆ1 Scientific paper ZVONKO GULIŠIJA1, ZORAN JANJUŠEVIĆ1 UDC:617-007.843:669.14 MIROSLAV SOKIĆ1, ČASLAV LAČNJEVAC2

The possibility of bioactive coatings obtaining by electrophoretic deposition of HAp on the steel implants

Surgical metallic implants are usually made of stainless steel, titanium- and cobalt/chromium- based alloys which have superior structural and mechanical properties, and are corrosion resistant. Nevertheless, they are susceptible to local corrosion in the human body, releasing metal ions into the nearby body tissue and fluids. Keeping the advantage of their mechanical properties, their biocompatibility can be achieved through deposition the bioinert or even bioactive coating onto their surface. Hydroxyapatite is known by its bioactive behavior, originating from its chemical identity with human bones. The electrophoretic deposition is rather old technique used here with a novel nanostructured hydroxyapatite powder to obtain uniform and dense hydroxyapatite coatings, to overcome the restrictions of previously used high temperature coating techniques and to overcome the complex geometry metallic substrate problems. The coating of nanosized hydroxyapatite was electrophoretically deposited on blasted surface of stainless steel 316LVM samples at constant voltage, for different deposition times and subsequently sintered in argon atmosphere at 1000 oC. The HAp powder thermal stability was initially assessed using DTA-TG analyses over the temperature range of 23oC-1000oC. The microstructure characterization of the coating was accomplished using SEM, and phase composition was determined by XRD. Key words: hydroxyapatite, 316LVM stainless steel, EPD, coatings

INTRODUCTION

Hydroxyapatite is well known by its bioactive behavior, but structurally too weak to be used alone as an implant material [1, 2]. The adequate technique of hydroxyapatite coating depositing on the metallic substrate, i.e. on the implant’s metal material, has been the aim of investigation for almost two decades worldwide [1-10]. Usually used metal materials for hip implants, besides the 316LVM stainless steel, are titanium- and cobalt/chromium-based alloys. Altho-ugh bioinert, due to their corrosion resistance, they are not biocompatible. Besides, they are susceptible to local corrosion in the human body, releasing metal ions into the adjacent body tissue and fluids. Hydroxyapatite (HAp) is chemically identical with the mineral constituent of bones and teeth, and when deposited on the metallic implant it enables implant’s biocompatibility.

Author's address: 1Institut za tehnologiju nuklearnih i drugih mineralnih sirovina, Franše d'Eperea 86, Beograd, Srbija, 2Poljoprivredni fakultet Univerziteta u Beogradu, Nemanjina 6, Zemun


Numerous technological methods of HAp coa-tings deposition on metallic implants were developed. Some of the most applies are: ion beam deposition, plasma spraying, sputtering, sol-gel coating and elect-rophoretic deposition [2-14]. The electrophoretic de-position (EPD) of HAp on metal substrates have important advantages: efficiency in obtaining dense and uniform coatings on differently shaped and thick metal substrates, high purity of formed coating, the possibility of obtaining the desired coating thickness with relatively simple process control by different parameters variation [6-9].

This process, besides EPD of a coating, consists of subsequent annealing needed for HAp powder sintering. The major problem related to the high tem-perature process could be the HAp decomposition. Sintering temperatures needed to achieve highly den-se coatings could have caused HAp coatings phase changes, but due to the stoichiometric nanostructured hydroxyapatite powder used, the possibility to obtain a bioactive coating on 316LVM substrate, without the coatings phase changes is presented in this work.



316LVM stainless steel is commonly used as implant material due to its mechanical properties (strength, ductility), corrosion resistance, and low cost - which can be the decisive preference [6]. Since the substrate with deposited HAp coating should be subjected to relatively high sintering temperatures, the stainless steel advantage, compared to other metal alloys, is better matching of thermal expansion coefficient with hydroxyapatite. In theory, the thermal expansion coefficient of the coating should be somewhat lower than that of the substrate [4,5]. Such thermal coefficients correlation should result in com-pressive residual stresses in the coating during co-oling, in that way inhibiting the formation of cracks.

Sintering is unavoidable, but critical stage, beca-use the coating must be densified after the deposition, and it occurs at temperatures of at least 1200oC for the most of commercially available HAp powders [10-14]. During the sintering stage the coating den-sificates, but such a high sintering temperature can cause the thermal decomposition of the coating itself, as well as the degradation of the mechanical pro-perties of the metal implant [5]. There are studies in which authors suggested that concerning the metal implant mechanical properties, the densification tem-peratures should not exceed 1050oC, because up to 1050oC the tensile strength of 316LVM stainless steel was unaffected [4]. The same authors showed that HAp decomposes in the contact with metal at much lower temperatures. So, in contact with 316LVM stainless steel the typical HAp decomposition temperature of 1300o-1400oC is reduced to ~950oC [3,4]. Considering the metal substrate mechanical properties preservation, the lowering of the sintering temperature is desirable in HAp coating/metal system [8]. Minimization of the HAp densification tempe-rature requires the use of HAp powders with maximal specific surface area, so only with as-precipitated uncalcined powders the maximal density could be reached [8]. The present work investigated the nano-sturctured HAp powder obtained by a novel modified spray-dry method [12]. Having in mind the proven stability of the used HAp powder [12,14], it is de-cided to carry out the experiments at the temperatures above those reported for a HAp decomposition, but limited with the metal substrate properties. Sintering had to be performed in argon atmosphere to prevent the interface oxidation in the presence of oxygen, which results in weak bonding of ceramic coating to the metallic substrate [5,6,9].

The possibility to obtain a bioactive coating, made of non commercial home-synthesized nano-structured HAp powder on 316LVM substrate is presented here.

EXPERIMENTAL

The coating of nanosized hydroxyapatite was electrophoretically deposited on blasted surface of stainless steel 316LVM samples at constant voltage, for different deposition times and subsequently sintered in argon atmosphere, at 1000 oC.

The used HAp powder is synthesized by a novel modified precipitation method which is improved by spray-drying at 120±5 oC, as described elsewhere [12]. The Ca/P ratio of 1.67±0.01 was determined by ICP analyses. The powder phase composition, as well as XRD analysis of deposited and sintered HAp coatings was evaluated using X-ray diffractometer (Philips PW1710) with Cu Kα radiation and curved graphite monochromator, measuring angle 2θ in the range from 20o to 70o. The morphology of deposited and sintered HAp coatings was examined with a scanning electron microscope (Jeol JSM 5800). The estimation of the HAp powder thermal stability against the sintering temperature, the DTA-TG ana-lyses were carried out using a device NETZCH STA 409EP, at heating degree of 10oC/min in the tempe-rature range 23o-1000oC, i.e. up to the experimental sintering temperature.

The stainless steel 316LVM plates commonly used for hip implants, with dimensions of 40x15x2 mm, were used as both, cathode and anode, for ele-ctrophoretic deposition process. Metallic specimens were blasted for a better adhesion. It enabled also a clean surface; but the blasted specimens were rinsed with acetone and distilled water, dried at room tem-perature and stored in a desiccator before the EPD procedure.

Suspension of HAp particles was performed by agitation using magnetic stirrer of 0.5 g of the HAp powder in 100 ml of ethanol. For the suspension stability, the 10% HCl was added until pH=2.00 was reached. Both, cathode and anode were of the same dimensions at a distance of 15 mm. The deposition electrode was cathode.

The electrophoretic deposition of HAp particles on the 316LVM stainless steel substrate plates was performed at the constant voltage of 60 V. The depo-sition times were 30 s and 60 s. The coated specimens were drying at room temperature in desiccator before sintering. The subsequent sintering was carried out in argon atmosphere in an electric furnace, at 200 oC, previously degassed for 1 h, and heated up to 1000 oC, with heating rate of approximately 10 oC/min. The samples were held at the sintering temperature for 1 h, cooled in the furnace and taken out.



RESULTS AND DISCUSSION

The thermal stability assessment of the used HAp powder against the sintering temperature was carried out by the DTA-TG analyses, Figure 1.

0 200 400 600 800 1000

-28

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-16

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-8

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(400-1000)oC-1,81%

(200-400)oC-1,91%

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DTA/TG HAP

Temperature,oC

TG, %

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(25-200)oC-4,97%112oC E

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Figure 1 - DTA-TG curves of the HAp powder

The TG curve from Fig.1 demonstrates a rapid weight loss up to 200oC and a continuous slight weight loss above 200oC. Accordingly, the DTA curve shows an endothermic change at the 112oC cau-sed by humidity release, while over the temperature range of 200oC-1000oC there was no significant DTA change accompanied to the TG change. The DTA curve has slight exothermic tendency, which does not indicate any phase change, and TG curve in the temperature range of 800o-1000oC, has the relative change of -0.62%. This is the temperature range in which, in contact with 316LVM stainless steel, the phase transformation of HAp into the detrimental β-TCP (tricalcium phosphate) could have been oc-curred, due to HAp/metal interfacial decomposition reaction [3-5]. The typical HAp decomposition tem-perature of 1300o-1400oC is reduced to ~950oC [3,4]. Here obtained results may be considered in favour of the HAp powder thermal stability over the tem-perature range of 23oC-1000oC, and at the experi-mental sintering temperature. The absence of β-TCP peaks at XRD pattern of the HAp coatings can also be interpreted in favour of the HAp powder stability in contact with 316LVM.

The Figure 2, presenting XRD pattern of the HAp nano-powder, exhibited peaks corresponding to the hydroxyapatite phase, indicating a low crystallinity. Figure 3 presents the XRD pattern of electropho-retically deposited HAp powder for 30 s, and sub-sequently heat treated for sintering in Ar atmosphere, also exhibits a very low crystallinity. The only peaks registered at Figures 2 and 3 were those correspon-ding to the hydroxyapatite phase. All the peaks perfectly matched the JCPDS pattern 9–432 for HAp, suggesting that the pure HAp powder was obtained

[12] and that the phase composition of the coating after EPD was unaffected after sintering processes. The compatibility of the recognized peaks at XRD patterns of the powder and of the coating leads to the conclusion that there was no phase transformation during the sintering process.

The exhibition of wide XRD peaks from the Figure 2 could be due to the low crystallinity or the result of the crystal size effect [13]. The evaluated degree of crystallinity for samples used in [13] was higher than expected from XRD pattern appearance. Accordingly, the wide peaks at XRD pattern of the HAp powder used in this investigation could be caused by very small, i.e. nanosized rod-shaped HAp powder particles, 50-100 nm in size, which were observed in previous TEM image analysis of the here used HAp powder [12]. Besides influencing the low crystallinity, this nanostructured starting powder may favour the densification process and their high thermal stability, i.e. the absence of thermal decomposition products [4,8-10,12].

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nsity

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Figure 2 - XRD pattern of HAp powder

Figure 3 - XRD pattern of HAp coating

The HAp peaks were not such a wide in Figure 3, meaning that sintered powder has a better crystalli-nity. For samples electrophoretically deposited for 60



s and subsequently heat treated for sintering Ar atmo-sphere, the thicker coating was obtained, but XRD patterns were analogue to these for shorter deposition time.

The morphology has a significant impact on the electrophoretic coating quality, namely the best coatings were obtained by the rounded particles, the worst was with platy particles, while acicular particles (185nm), similar to here used powder with nanosized rod-shaped particles (sizing from 50 to 100 nm) resulted in some cracking of the coating during drying [8]. The shrinkage due to drying could be minimized by the use of regularly shape particles that can pack more efficiently. The nano-rods in the used HAp powder are almost one order of magnitude smaller than those in reported investigation, although with similar aspect ratio.

When electric field is constant, the preferential deposition of finer particles can be expected due to their higher mobility comparing to the larger particles. Knowing that the closest to the substrate the finer particles can be observed and that with shorter deposition time, only the very fine nano particles were reached the substrate surface, it can be assumed that the coating formed for shorter deposition time of 30s is made of very fine particles with low susceptibility to cracking.

Figure 4a - Surface morphology of HAp coating

obtained by EPD for 60s

The absence of γ-Fe peak or the peaks corres-ponding to detrimental structure phases, i.e. trical-cium phosphate (TCP) is evident in the presented XRD pattern of a coating sintered in Ar atmosphere at 1000oC, Figure 3. The absence of γ-Fe peak indicates that coating covers the substrate continuously, without pores or cracks, through which the peak originated from the substrate, could be recorded. This also means that the starting Ca/P ratio of 1.67 ±0.01 remains unchanged, favorizing the bioactivity of the coating.

Figure 4b - Surface morphology of the 30s EPD coated and in Ar sintered sample

Figure 4a presents the morphology of a sample EPD coated for 60s, while Figure 4b presents the morphology of a sample EPD coated for 30s, and afterwards sintered in Ar atmosphere. Here, the morphology of sintered coating is visible all over the visible area. The obtained coating is comparatively uniform and free of cracks, which may be concluded by both XRD and SEM. At the XRD patterns of these Ar-sintered samples, just HAp reflections are detected. It can be concluded that the metal catalyzed decomposition of the HAp coating was not observed here, having in mind the assessed thermal stability of the powder by DTA-TG and since the XRD detected reflections belong to the original HAp.

CONCLUSIONS The stoichiometric Ca/P ratio of the HAp powder,

along with required chemical and phase composition were important quality requirements. Here is presen-ted a successful attempt to maintain the stoichio-metric Ca/P ratio even after deposited HAp powder sintering at temperatures as high as metal substrate could resist in terms of structural and mechanical properties. The thermal stability of the used HAp powder was assessed by DTA-TG analyses and did not show any characteristic change over the tempe-rature range of 23oC-1000oC, i.e. at the or nearby experimental sintering temperature.

The XRD patterns of the nanosized HAp powder showed the presence of characteristic HAp peaks, while the detrimental high temperature phase which could be the result of HAp decomposition was not registered. XRD patterns of the coatings sintered in Ar-atmosphere have shown just characteristic HAp peaks, meaning that comparatively continuous and crack-free HAp coatings were produced after sintering in Ar atmosphere at 1000 oC for 1 h.

The use of the stable, stoichiometric nano-structured HAp powder, synthesized by a method which produced the nanosized rod-shaped particles,

20 µm

20 µm



enabled obtaining the continuous HAp coatings onto the 316LVM stainless steel, where the only inherent ceramic phase is HAp. Acknowledgement

The authors wish to acknowledge the financial support from the Ministry of Science and Technolo-gical Development of the Republic of Serbia through the project 34002.

REFERENCES [1] W. R. Lacefield, in Bioceramics: Materials Characte-

ristics Versus in Vivo Behavior, Ed. P. Ducheyne and J.E. Lemons, New York Academy of Science, New York (1988), p.73-80

[2] C. Y. Tang, P. S. Uskokovic, C. P. Tsui, Dj. Veljovic, R. Petrovic, Dj. Janackovic, Influence of microstructu-re and phase composition on the nanoindentation cha-racterization of bioceramic materials based on hydroxyapatite, Ceram. Int. 35 (2009), 2171–2178

[3] M. Wei, A. J. Ruys, B. K. Milthrope, C. C. Sorrell, Solution ripening of hydroxyapatite nanoparticles: Effects on electrophoretic deposition, J. Biomed. Mater. Res.Part A, 45, (1999), 11-19.

[4] M. Wei, A. J. Ruys, M. V. Swain, S. H. Kim, B. K. Milthrope, Interfacial bond strength of electropho-retically deposited hydroxyapatite coatings on metals, J. Mater. Sci.: Mater. Med. 10 (1999), 401-409.

[5] M. Wei, A. J. Ruys, B. K. Milthorpe, C. C. Sorell, J. H. Evans, Electrophoretic Deposition of Hydroxy-apatite Coatings on Metal Substrates: A Nanoparti-culate Dual-Coating Approach J. Sol-Gel Sci. Technol. 21(2001), 39–48

[6] T. M. Sirdhar, U. Kamachi Mudali, M. Subbaiyan, Preparation and characterisation of electrophoretically

deposited hydroxyapatite coatings on type 316L stainless steel Corros. Sci. 45 (2003), 237–252

[7] N. Eliaz. T.M. Sridhar, U. K. Mudali, B. Raj, Electro-chemical and electrophoretic deposition of hydroxyl-apatite for prthopaedic applications, Surf. Eng. 21, 3 (2005), 1-5

[8] M. Wei, A. J. Ruys, B. K. Milthrope, C. C. Sorrell, Precipitation of hydroxyapatite nanoparticles: Effects of precipitation method on electrophoretic deposition, J. Mater. Sci.: Mater. Med. 16 (2005), 319-324.

[9] M. Javidi, S. Javadpour, M. E. Bahrololoom, Elecrtophoretic deposition of natural hydroxyapatite on medical grade 316L stainless steel, J. Ma, Mater. Sci. Eng., C28 (2008), 1509-1515

[10] Dj. Veljović, I. Zalite, E. Palcevskis, I. Smiciklas, R. Pertović, Dj. Janaćković, Microwave sintering of fine grained HAP and HAP/TCP bioceramics, Ceram. Int. 36 (2010), 595–603

[11] F.Korać, S.Ćatić, M.Cacan, S.Gutić, S.Ismailović, The dotted corrosion of orthopedic implants in dissolution, Zastita materijala, 51, 2 (2010) 99-105

[12] Dj. Veljović, B. Jokić, R. Petrović, E. Palcevskis, A. Dindune, I. N. Mihailescu, Dj. Janaćković, processing of dense nanostructured HAP ceramics by sintering and hot pressing, Ceram. Int. 35 (2009), 1407–1413

[13] E. Landi, A. Tampieri, G. Celotti, S. Sprio, Densif-cation behaviour and mechanisms of synthetic hydro-xiapatites, J. Eur. Ceram. Soc. 20 (2000), 2377-2387

[14] M. Mihailović, A. Patarić, Z. Gulišija, Dj. Veljović, Dj. Janaćković, Electrophoretically deposited nano-sized hydroxyapatite coatings on 316LVM stainless steel for orthopaedic implants, Chemical Industry and Chemical Engineering Quarterly,

DOI:10.2298/CICEQ100326052M

IZVOD

MOGUĆNOST DOBIJANJA BIOAKTIVNIH PREVLAKA ELEKTROFORETSKIM DEPONOVANJEM HIDROKSIAPATITA NA ČELIČNE IMPLANTATE Metalni implantati za ortopedsku hirurgiju izrađuju se najčešće od nerđajućeg čelika, legura na bazi titana, kobalta ili hroma, koje imaju odlične konstrukcione i mehaničke osobine i otporne su na koroziju. Ipak, ovakvi implantati poldožni su lokalnoj koroziji u ljudskom telu jer je zapaženo prisustvo njihovih metalnih jona u okolnim tkivima i telesnim tečnostima. Da bi se iskoristila prednost njihovih mehaničkih karakteristika, a postigla biokompatibilnost, moguće je na njihovu površinu naneti bioinertnu ili čak bioaktivnu prevlaku. Hidroksiapatit je poznat po svom bioaktivnom ponašanju, koje duguje hemijskoj istovetnosti sa ljudskim kostima. Elektroforetska deopzicija je relativno stara tehnika, koja je ovde primenjena uz upotrebu novog materijala – nanostrukturnog hidroksiapatita, kako bi se dobile ravnomerne i kompaktne hidroksi-apatitne prevlake i tako prevazišla ograničenja prethodno korišćenih visokotemperaturnih tehnika nanošenja prevlaka i problemi sa složenim oblikom metalnog supstrata. Prevlake nanostrukturnog hidroksiapatita eletroforetski su deponovane na peskiranu površinu uzoraka od nerđajućeg čelika 316LVM, pri konstantnom naponu, za različito vreme deponovanja, a zatim su sinte-rovane u atmosferi argona na 1000oC. Toplotna stabilnost HAp praha prvo je ocenjena DTA/TG analizom u temperaturnom intervalu 23oC-1000oC. Za mikrostrukturnu karakterizaciju prevlake korišćen je SEM, a za određivanje faznog sastava XRD analiza. Ključne reči: hidroksiapatit, nerđajući čelik, EPD, prevlake Rad primljen: 15.09.2011. Originalni naučni rad

V. NOVAKOVIĆ i ... UTICAJ UPOTREBE KOMPRIMOVANOG VAZDUHA PRI IZVOĐENJU ...


VASO NOVAKOVIĆ1 Stručni rad MILADIN GLIGORIĆ2 UDC:628.112.161 RANKO GRUJIĆ1

Uticaj upotrebe komprimovanog vazduha pri izvođenju bunara na rezultate fizičko-hemijskih analiza vode

Za potrebe bušenja ili kvalitetne razrade i ispiranja pijezometara i bunara, često se koristi kompri-movani vazduh. Ovim radom se ukazuje na to, da se rastvoreni kiseonik, ubačen u podzemne vode, može zadržati na velikom prostoru i dugo vremena nakon toga. To objektivno može uticati na rezultate ispitivanja fizičko-hemijskog sastava uzoraka podzemne vode, uzetih tokom prvog testiranja bunara. U radu su prezentovani rezultati ispitivanja ovog uticaja, u akviferu neogene starosti u Stanarima kod Doboja i u aluvijalnoj izdani u Kozluku kod Zvornika, Republika Srpska, korelacijom rezultata ispitivanja odmah nakon izvedbe bunara i kasnije u toku njegovog korišćenja. Kako bi se u budućnosti eliminisao ovaj problem, u radu su date preporuke za promjenu uobičajene procedure hidrogeoloških istraživanja, da se sagleda efekat aeracije vode u akviferu, tokom razrade bunara. Ključne riječi: podzemne vode, gvožđe, mangan, amonijak, razrada bunara

UVOD

Bušenje za potrebe izvođenja pijezometara u fazi hidrogeoloških istraživanja, kao i izvođenja bunara, često podrazumijeva korišćenje vazduha, kao fluida za izbacivanje nabušenog materijala, što je dobro objasnio K. Rafferty 2001 [1]. Razlog tome su uglavnom povećana brzina bušenja, očuvanje i poboljšanje filtracionih karakteristika pribušotinske zone u toku bušenja i mogućnost detekcije intervala dubine sa prisutnom podzemnom vodom, već u toku bušenja. Funkcije ispirnog fluida su: održavanje stabilnosti zidova bušotine i sprečavanje obrušavanja, čišćenje dleta i bušotine od krhotina stijene i hlađenje dleta.

Cilj korišćenja komprimovanog vazduha u toku ispiranja i razrade pijezometara i bunara je svakako efikasnije ispiranje i razrada pribunarske zone. Metode i svrha su objašnjeni u stručnoj literaturi i publikovanim radovima od strane T. Hartera 2003. [2]. Od uspješnosti izvedene razrade direktno zavisi kapacitet bunara u datim uslovima.

Korišćenje komprimovanog vazduha pomoću kompresora, za ispiranje i razradu bunara se izvodi utiskivanjem vazduha u bunar i direktno u akvifer, kroz cijevnu konstrukciju (aer-lift sistem), sekcioni ispirač i otvore bunarskog filtera. Otvaranjem ventila na odvodu, smješa mutne vode sa česticama mulja i pijeska iz pribunarske zone i bunarske cijevne konstrukcije se izbacuje van bunara.

Adrese autora:1DOO „IPIN“ Institut za primijenjenu geologiju i vodoinženjering, 2 Univerzitet u Istočnom Sarajevu, Tehnološki fakultet Zvornik, R. Srpska


Imajući u vidu da se pri razradi bunara uz koriš-ćenje aerlifta, naizmjenično vazduh utiskuje direktno u sloj uz minimalne gubitke, a kada se otvori ventil na odvodu smješa vazduha i vode djelimično izbacuje iz bunara, neophodno je sagledati orijentacione vrijed-nosti prečnika zone utiskivanja vazduha oko bunara, i intenzitet procesa aeracije u samom vodonosnom slo-ju, koji se jednokratno dešava isključivo pod uticajem antropogenog faktora, odnosno primjenom metoda bušenja, razrade i ispiranja bunara.

Vazduh je smješa gasova od kojih su neki u vrlo promjenjivom sastavu. Suvi vazduh je smješa: azota (78,1 %), kiseonika (20,9 %), argona (0,934 %) i u vrlo malim količinama ugljendioksida, neona, heliju-ma, kriptona i drugih komponenata što je objasnio Randall D. 2010 [3].

Dakle, sa injektiranjem jednog m3 vazduha, u kojem je prosječno sadržano oko 0,2095 m3 ili 299,3755 g kiseonika, teoretski je moguće stvoriti zonu zasićenosti kiseonikom od 50 %, podzemne vo-de zapremine oko 60 m3.

Kapacitet kompresora koji se pri tom koriste je najčešće oko 8 m3/min sa prosječnim pritiskom od 8 bara.

Ukupno vrijeme, neophodno za razradu bunara se kreće od nekoliko sati za pijezometre na rudnicima, do nekoliko nedelja za velike bunare posebne namje-ne sa dugim filterima.

Ukoliko razrada bunara traje samo 12 sati (mini-malno trajanje razrade), pod pretpostavkom da se 90 % komprimovanog vazduha utisnutnog u bunar izbaci tokom ispiranja van bunara, pri kapacitetu kompreso-ra od 8 m3/min u akvifer se utisne oko 576 m3 vaz-duha. Ova količina je dovoljna za stvaranje zone zasi-



ćenosti od 50 %, podzemne vode kiseonikom zapre-mine oko 34.500 m3.

Upravo tako se može objasniti pojava koja je utvrđena na primjeru akvifera u Kozluku kod Zvornika i Stanara kod Doboja, da se neposredno nakon završetka procesa bušenja, razrade i ispiranja bunara dobijaju nerealno niske vrijednosti sadržaja gvožđa, mangana i amonijaka, iako je prirodno visok sadržaj ovih parametara hemijskog sastava [4-8].

Na osnovu izloženog proračuna, se jasno može zaključiti, da se kiseonik u akviferu može zadržati na velikom prostoru i dugo vremena nakon izvođenja i ispiranja bunara, što objektivno može uticati na rezultate ispitivanja fizičko-hemijskog sastava podzemne vode u toku prvog testiranja bunara.

Za vrijeme razrade i ispiranja bunara, u pijezo-metre i bunare se prosječno utisne od 1000 m3 (za pijezometre na rudnicima) do 100.000 m3 vazduha (za bunare velikog prečnika, posebne namjene sa dugim filterima). Ako se tome doda i vazduh koji se utisne u akvifer pri bušenju nekih pijezometara i bunara (metoda "down the hole"), onda to povećava količinu utisnutog vazduha. Naravno manji dio vazduha utis-nutog u bunar, završi u akviferu, a značajno veći dio se u vidu smješe vode i vazduha izbaci tokom ispira-nja bunara.

Korišćenjem vazduha koji se utiskuje u akvifer, kroz bunar, pri bušenju nekim metodama bušenja i u toku ispiranja i razrade bunara, odvija se intenzivno uklanjanje gvožđa iz podzemne vode u široj zoni, zahvaljujući tome što prilikom oksidacije Fe2+ u Fe3+ i zatim hidrolize nastalih feri jedinjenja sa vodom, do-lazi do nastajanja ferihidroksida, koji se izdvajaju u vidu taloga u samom akviferu:

2 32 22 0,5 2 2Fe O H Fe H O+ + ++ + → +

(oksidacija)

( )32 3

3 3Fe H O Fe OH H+ ++ → ↓ +

(hidroliza i taloženje)

Stehiometrijski, prema gornjim jednačinama je lako pokazati da je za oksidaciju l mg/1 Fe2+ potrebno 0,143 mg/1 kiseonika, što je lako ostvariti, obzirom da je rastvorljivost kiseonika u vodi oko 10 mg/1 (na 15 °C). Drugim riječima, ako se voda zasiti kiseo-nikom iz injektiranog vazduha, do 50 %, tj. do 5 mgO2/l, obezbjediće se uslovi za oksidaciju i izdvaja-nje čak 5/0,143 = 35 mg/1 gvožđa.

U toku oksidacije, iz vode se izdvaja i mangan kao manganioksihidrat:

( )22 2 2

2 4 4Mn O H O MnO OH H+ ++ + → +

Prema ovoj reakciji, za oksidaciju l mg/1 Mn2+ u Mn4+ treba utrošiti 0,291 mg/1 rastvorenog kiseonika, što znači da se sa oko 5 mg/1 rastvorenog kiseonika (zasićenost oko 50 % na 15 °C) moglo oksidisati oko 17 mg Mn2+/l.

U stručnoj praksi na više terena je uočeno da re-zultati prvih laboratorijskih ispitivanja fizičko-hemij-skog sastava uzoraka podzemne vode, koji se uzimaju u toku testiranja probnim crpljenjem novih bunara, ne prikazuju realan sadržaj gvožđa, mangana, i amonija-ka u podzemnoj vodi [4-8]. Naime, često su dobijeni rezultati sadržaj manji od očekivanih i poznatih za taj akvifer. To može dovesti neiskusnog istraživača i in-vestitora u situaciju da donose ishitrene odluke o nas-tavku ulaganja, u objekte za zahvatanje i tretman vo-da ili da sumnjaju u ispravnost izvedenih laborato-rijskih analiza fizičko-hemijskog sastava vode.

Imajući u vidu vlastitio iskustvo u pogledu po-navljanja ove pojave na više različitih terena, lite-raturnih podataka o sličnim pojavama u prošlosti, kao i vremensku podudarnost takvih rezultata laboratorij-skog ispitivanja, sa terminom završetka izvođenja i razrade bunara, u ovom radu je izvršena analiza ovog uticaja na dva primjera. Za ispitivanje su izabrana dva akvifera za koja je već utvrđen povišen sadržaj gvožđa i povremeno mangana, nitrita i amonijaka. To su akvifer neogene starosti u Stanarima kod Doboja i akvifer od šljunkovito-pjeskovitih sedimenata kvarta-rne starosti u Kozluku kod Zvornika. Izvršena je fizi-čko-hemijska analiza uzoraka vode i korelacija rezul-tata ispitivanja uzoraka vode uzetih odmah nakon iz-vedbe bunara i uzoraka uzetih kasnije u toku njego-vog korišćenja. U radu su dati objašnjenje uzroka ovog uticaja i preporuke kako bi se u budućnosti eli-minisao ovaj problem kod uobičajenog postupka hid-rogeoloških istraživanja.

METODE

U radu se razmatra trajanje ubacivanja kompri-movanog vazduha, pritisak i količina vazduha koja se ubacuje u akvifer tokom izvođenja bunara i pijezo-metara, efekti i prognozno trajanje aeracije vode, kao i udaljenost od bunara ovih procesa u pribunarskoj zoni akvifera.

Uzorkovanje je vršeno uz obaveznu konzervaciju uzoraka. U Kozluku je uzorkovanje vršeno na pijezo-metarskim bušotinama BV-2 i BV-3 (01.07.2002.), te na bunarima EB-1, EB-2 i EB-3 koje eksploatiše fabrika mineralne vode Vitinka iz Kozluka (u periodu od 2006. do 2012). Sve bušotine i bunari su plići od 14 m tj. zahvataju vodu iz pjeskovito-šljunkovitog akvifera kvartarne starosti u kome je nivo vode na dubini oko 4 m od površine terena. Zbog visokog sadržaja gvožđa i povremeno mangana, nitrita i amo-



nijaka u podzemnoj vodi na ovoj lokaciji, fabrika vrši deferizaciju radi korišćenja ove vode u tehničke svrhe.

Na prostoru Stanara kod Doboja je vršeno uzor-kovanje na bunarima kod upravne zgrade rudnika i na lokaciji platoa buduće termoelektrane. Svi bunari na ovoj lokaciji zahvataju vodu iz šljunkova pjeskovitih pliocenske starosti. Podzemna vode iz ovog akvifera ima povišen sadržaj gvožđa i ponekad mangana i amonijaka [3,5,6]. Uzorkovanje na prostoru platoa TE Stanari iz bunara IB-1 je izvršeno 06.09.2006.g, bunara B-2 i B-3 2007.g. te bunara BS-1/10 2010.g. kada su tek izvedeni odn. kada je izvršena razrada, ispiranje i prvo testiranje bunara. Navedeni podaci su

upoređeni sa sadržajem ovih parametara u podzemnoj vodi tokom eksploatacije ovih bunara.

REZULTATI I DISKUSIJA

Izdan akumulirana do dubine 10 m, u aluvijalnim sedimentima rijeke Drine u Kozluku kod Zvornika, ima povišen sadržaj gvožđa (0,47 do 1,58 mg/l) i mangana (0,13 do 0,15 mg/l), te povremeno i amoni-jaka (0,43 mg/l), a nakon tretmana, se koristi za teh-ničke potrebe fabrike Vitinka. U narednoj tabeli se daje prikaz sadržaja gvožđa, mangana, nitrita i amoni-jaka u vodi nakon izvođenja istražnih bušotina i kas-nije u fazi eksploatacije bunara.

Tabela 1 - Sadržaj gvožđa, mangana, amonijaka i nitrita, neposredno nakon izvođenja bunara u Kozluku i

njihovog sadržaja tokom eksploatacije istih bunara

Oznaka bušotine/bunara Amonijak (mg/l) Nitriti (mg/l) Gvožđe (mg/l) Mangan (mg/l)

BV2 i BV-3 odmah nakon izvođenja 0,00 0,000 0,02 0,00

VEB-1 0,17 0,000 0,54 0,03 VEB-2 0,41 0,006 1,58 0,13 tokom eksplotacije VEB-3 0,43 0,001 0,47 0,10

Prosječne vrijednosti u eksploataciji 0,34 0,002 0,86 0,26

Kao što se iz tabele 1 vidi, nakon izvođenja, raz-rade i testiranja pijezometara, BV-2 i BV-3, prvi re-zultati laboratorijskih analiza (01.07.2002.g.) su uka-zivali da je voda ispravna za piće sa sadržajem gvož-đa 0,02 mg/l, mangana 0,00 mg/l i amonijaka 0,00

mg/l. Očigledno je da rezultati u fazi prvih hidro-geoloških istraživanja nisu dali realne rezultate u pog-ledu sadržaja pojedinih parametara hemijskog sastava vode. Jedino sadržaj nitrita nema značajnijih odstu-panja.

Slika 1 - Uporedni dijagram sadržaja pojedinih parametara hemijskog sastava podzemne vode u aluvijalnom

akviferu u Kozluku kod Zvornika: 1) Odmah nakon izvođenja pijezometarskih bušotina; 2) Tokom višegodišnje eksploatacije bunara



Akvifer neogenog basena u Stanarima kod Dobo-ja, prosječne debljine 16,8 m ima povišen sadržaj gvožđa (do 3,27 mg/l), mangana (0,30 mg/l) i amo-nijaka 0,195 mg/l, a koristi se za tehničke potrebe rudnika Stanari. Međutim, u vrijeme nakon izvođenja i razrade, tj pri završetku prvog testiranja bunara

probnim crpljenjem, prvi rezultati laboratorijskih ana-liza su ukazivali da je prosječan sadržaj gvožđa 0,196 mg/l, mangana 0,037 mg/l, i amonijaka 0,058 mg/l. Rezultati hemijskih analiza sadržaja gvožđa, manga-na, amonijaka i nitrita su dati u tabeli 2.

Tabela 2 - Sadržaj gvožđa, mangana, amonijaka i nitrita, neposredno nakon izvođenja bunara u Stanarima i

sadržaja istih parametara tokom eksploatacije tih bunara

Oznaka bunara Datum uzorkovanja Amonijak (mg/l) Nitriti

(mg/l) Gvožđe (mg/l)

Mangan (mg/l)

IB-1 06.09.2006. 0,04 0,000 0,25 0,04 B-2 25.07.2007. 0,1 0,000 0,18 0,05 B-3 25.07.2007. 0,035 0,000 0,098 0,03 BS-1/10 13.08.2010. <0,05 <0,005 0,257 0,028 Prosječne vrijednosti 0,058 0,000 0,196 0,037 Izmjerene vrijednosti u toku eksploatacije bunara 0,195 <0,007 3,27 0,30

Iz abele 2 može se uočiti da je, tokom eksplo-atacije bunara, došlo do povećanja sadržaja gvožđa, mangana i amonijaka u vodi.

Evidentno je da je sadržaj gvožđa i mangana u uzorcima vode, uzetim odmah po izvođenju i ispira-nju pijezometara i bunara, mnogo manji nego u priro-dnim uslovima tokom višemjesečne i višegodišnje eksploatacije bunara.

Slika 2 - Uporedni dijagram sadržaja pojedinih parametara hemijskog sastava podzemne vode u neogenom

akviferu u Stanarima kod Doboja: 1) Odmah nakon izvođenja i razrade bunara; 2) Tokom višegodišnje eksploatacije bunara

Za donjepliocenski šljunkovito-pjeskoviti akvifer u neogenom basenu Stanara kod Doboja, može se usvojiti prosječna debljina od 16,8 m, a koeficijent efektivne poroznosti 0,93 %.

To znači da je razradom bunara kompresiranjem tokom 12 sati, u području Stanara kod Doboja, mo-guće stvoriti zonu zasićenja 50 % kiseonikom u polu-prečniku od 265 m. Naravno ovaj proračun ostaje sa-

mo u sferi idealno ravnomjernog rasprostranjenja u akviferu. Naime, stvaranje zone zasićenja podzemne vode kiseonikom je uslovljeno i drugim faktorima (nivo-om vode u akviferu, prisustvom tzv. "vadozne zone" u akviferu, pritiskom i protokom vazduha iz kompresora, smjerom i brzinom podzemnog toka, promjenom vrijednosti poroznosti vodonosnog sloja i njegove hidrauličke provodljivosti u različitim smje-rovima i na različitim udaljenostima od bunara).



Imajući u vidu da će se podzemna voda u bliskoj pribunarskoj zoni, brzo zasititi gasovima, to će se nas-tavkom utiskivanja vazduha, nove količine vazduha nagomilavati u vadoznoj zoni, privremeno povećava-jući geostatički pritisak ili će, kod arteskih akvifera, formirati vazdušni jastuk u njegovom gornjem dijelu. Kiseonik iz ove zone će se tokom vremena, usporeno rastvarati u vodi i tokom crpljenja bunara, odn. Ra-dijalno usmjerenog kretanja vode ka bunaru vršiti nje-nu aeraciju. Aeracija kao proces direktno utiče na obaranje sadržaja gvožđa, mangana, nitrita i amonija-ka u vodi.

ZAKLJUČCI Ovim radom se ukazuje na to, da se kiseonik u

podzemlju može zadržati na velikom prostoru i dugo vremena nakon izvođenja i ispiranja pijezometara i bunara, što objektivno može uticati na rezultate ispi-tivanja fizičko-hemijskog sastava podzemne vode u toku prvog testiranja bunara.

U radu su prezentovani podaci o ovoj pojavi u akviferu neogene starosti u Stanarima kod Doboja i u aluvijalnoj izdani u Kozluku kod Zvornika.

Na osnovu provedenih ispitivanja sadržaja gvož-đa, mangana, amonijaka i nitrita, može se zaključiti da rezultati laboratorijskih ispitivanja fizičko-hemij-skog sastava uzoraka podzemne vode, ne prikazuju realan sadržaj gvožđa, mangana, nitrita i amonijaka u podzemnoj vodi u toku prvog testiranja probnim crp-ljenjem novog bunara, ukoliko se ispitivanja izvode, odmah nakon izvođenja, ispiranja i razrade bunara metodama uz korišćenje komprimovanog vazduha.

Naime, često su dobijeni rezultati manji od očeki-vanih i poznatih za taj akvifer, što može dovesti neis-kusnog istraživača i investitora u situaciju, da donose ishitrene odluke o nastavku ulaganja, u objekte za zahvatanje i tretman voda ili da sumnjaju u ispravnost izvedenih laboratorijskih analiza fizičko-hemijskog sastava vode.

Kako bi se u budućnosti eliminisao ovaj problem u proceduri, hidrogeoloških istraživanja, neophodno je izvršiti fizičko-hemijske analize podzemne vode, prije i poslije razrade bunara aerliftovanjem (nakon ispiranja čistom vodom), kako bi se mogao sagledati efekat aeracije vode u akviferu, tokom razrade bunara.

REFERENCE [1] Rafferty K., "Specification of Water Wells"

ASHRAE Transactions, Vol. 107, Pt. 2. 2001 Ame-rican Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. pp 487-493 [Available at http://geoheat.oit.edu/pdf/tp112.pdf]

[2] Harter T., "Water well design and construction" Groundwater cooperative extension program, Uni-versity of California, 2003. Publication 8086 [Available at http://groundwater.ucdavis.edu/ Publi-cations/ Harter_FWQFS_8086.pdf]

[3] Randall D., "The Composition of Air" Quick Stu-dies in Atmospheric Science, Center for Multiscale Modeling of Atmospheric Processes CMMAP, 2010. [Available at: http://kiwi.atmos. Colo-state.edu/group/dave/pdf/Composition_of_Air.pdf]

[4] Gligorić M., "Priprema vode za piće", Tehnološki fakultet Zvornik, Zvornik 2010.

[5] Novaković V., Gligorić M., Grujić R., "Uticaj reži-ma izdani na sadržaj gvožđa, mangana, nitrita i amonijaka u podzemnim vodama" Zbornik radova, 29. stručno-naučni skup "Vodovod 2009" Savez inženjera i tehničara Srbije, Zlatibor, 2008.

[6] Novaković V., Gligorić M., "Uticaj proizvodnje glinice u fabrici "Birač" Zvornik na kvalitet pod-zemnih voda", Međunarodna konferencija industrije aluminijuma Jugoslavije 1997. Banja Koviljača, Zbornik radova

[7] Fondovska dokumentacija DOO "IPIN" Institut za primijenjenu geologiju i vodoinženjering, Bijeljina

[8] Grujić R., Novaković V., Gligorić M., Zaštita materijala, 49, 4 (2008) 60-65

ABSTRACT INFLUENCE OF USING COMPRESSED AIR FOR PERFORMING WELLS, TO THE RESULTS OF PHYSICAL-CHEMICAL ANALYSIS OF WATER The compressed air is often used for purpose of drilling or quality developing and flushing the piesometers and water wells. This paper suggests, that the dissolved oxygen, which is injected in groundwater, can retain in a large area for a long time after the performing and cleaning of the piezometers and water wells, which objectively, might have an affect on the the testing results of physical and chemical composition of groundwater samples when we test water wells in the first time. The paper presents results of testing that influence, in the Neogene aquifer in the Stanari settlement, near of city of Doboj, and in the alluvial aquifer in the Kozluk village near city of Zvornik, by correlation of test results immediately after the performing of water wells and later, during their usage. In order to eliminate this problem in the future, this paper offers proposals, cchange of the usual procedure of hydrogeological research. It will bi possible to examine the effect of aeration of groundwater in the aquifer, during the performing of the wells. Key words: groundwater, iron, manganese, ammonia, development of water wells Paper received: 20.09.2011. Proffesional paper

S. STANKOV REGULACIJA RADA I NADGLEDANJE STANICA ZA KATODNU ZAŠTITU


STANKO STANKOV Strucni rad UDC :620.197.5:621.355.002.58

Regulacija rada i nadgledanje stanica za katodnu zaštitu

Efikasan način zaštite od korozije metalnih konstrukcija, opreme i uređaja koje se instaliraju u zemlji ili vodi je primena katodne zaštite. Katodna zaštita se izvodi dovođenjem odgovarajućeg negativnog potencijala na štićenu konstrukciju, u odnosu na tlo. Katodnom polarizacijom metalne konstrukcije u elektrolitičkoj sredini smanjuje se ili potpuno zaustavlja proces korozije. Izvori jednosmerne struje su instalirani u ormarima – stanicama katodne zaštite. Predmet katodne zaštite su najčešće ukopani rezervoari, vodovodi, gasovodi, naftovodi, kablovi, uzemljivači i sl., koji se prostiru na velikim površinama, tako da su i stanice katodne zaštite dosta razuđene. Problem nadzora i upravljama stanicama katodne zaštite rešava se najčešće primenom digitalnih radio uređaja ili GPRS/GSM komunikacijom. Na ovaj način stanice su povezane sa dispečerskim centrom u kome se vrši kontinualno praćenje parametara katodne zaštite i optimizacija režima rada samih stanica. Ključne reči: katodna zaštita, izvor jednosmerne struje, prenos podataka, radio, GPRS/GSM

1. UVOD

Korozija objekata instaliranih u zemlji je osnovni uzrok njihovog oštećenja i razaranja (npr. Uzemlji-vači) i havarija (cevovodi za transport vode, nafte, gasa i sl. gde dolazi do nekontrolisanog isticanja). Borba protiv korozije traje već više od sto godina (godine 1895. u SAD se pojavljuje patent u vezi s katodnom zaštitom ukopanih čeličnih cevi koje se na-laze u zoni električnih pruga s jednosmernom stru-jom). Tridesetih godina XX veka katodna zaštita je prihvaćena kao vid zaštite od korozije podzemnih čeličnih cevovoda i konstrukcija [1-5, 8-11, 13].

Uzrok korozije (rđanja) čeličnog materijala je oksidacija gvožđa pod dejstvom vlage i raznih neči-stoća, koje izazivaju i ubrzavaju proces. Od mnogo-brojnih agenasa koji prouzrokuju proces rđanja naj-značajniji su zagađena atmosfera, industrijske oblasti zagađene sumporom, morska obala, slane sredine i sl. Reakcija rđanja se odvija prema relaciji:

αε rđ

vlaga

vazduhgvožđOFeOFe 322 234 →→

Za ovaj proces značajno je: - u slučaju da nestane jedan od elemenata u

reakciji, reakcija rđanja se usporava ili prestaje - za nastajanje reakcije potrebni su vlaga i vazduh

(kiseonik). Čelik ne rđa na suvom vazduhu (iskustvo pokazuje da su čelične konstrukcije u uslovima pustinjske klime dugotrajnije)

- u prisustvu vlage i vazduha proces rđanja traje sve do potpunog razaranja čelika.

Adresa autora: Univerzitet u Nišu, Elektronski fakultet, Niš, Beogradska 14


Brzina korozije je znatno povećava ako se pod-zemni objekti nalaze u oblasti delovanja lutajućih stru-ja. Poznato je da pri jačini struje od 1A može godišnje da se odnese do 6 kg metala. Imajući u vidu da na po-jedine objekte, instalirane pod zemljom, deluje lutaju-ća struja jačine do nekoliko desetina ampera, biće i brzina korozije prilično veća.

Ove struje su povratne struje kroz zemlju i metal-ne konstrukcije. Potiču od raznih električnih uređaja koji su uzemljeni na više od jednog mesta. Uređaji i oprema koji nisu uzemljeni mogu generisati lutajuće struje u slučaju pojave zemljospoja na najmanje dva različita mesta. Najčešći izvori lutajućih struja su: jednosmerna električna vuča, uređaji za zavarivanje, dalekovodi visokog napona jednosmerne struje, pos-trojenja za elektrolizu, livnice s elektrolučnim peći-ma, pogoni za cinkovanje, razni izvori jednosmerne struje u industriji, distributivne električne mreže s vi-šestruko uzemljenim neutralnim provodnikom, a i sa-me električne instalacije su generatori lutajućih struja.

Postoje dve metode zaštite od korozije: pasivna i aktivna. Pasivni način zaštite se ogleda u stvaranju barijere između metala i okolne sredine upotrebom raznih zaštitnih sredstava.

CEVOVOD

ŽRTVENAANODA

TLO

KA

BL

TOK JONA (+)

TOK ELEKTRONA ( )

Slika 1 - Princip zaštite sa žrtvenim anodama



Ovim načinom nije moguća potpuna zaštita, imajući u vidu činjenicu da nema savršenih izola-cionih premaza. S druge strane, aktivna metoda za-štite sastoji se u katodnoj polarizaciji, koja smanju-je razaranje metala u korozivnoj sredini. Kadodna zaštita se može realizovati na dva načina: galvan-skim putem pomoću “žrtvenih” anoda (slika 1), pri čemu se koristi osobina da različiti metali u elek-trolitu imaju različit elektrohemijski potencijal. Kao žrtvena anoda najčešće se koristi anoda od ma-gnezijuma zbog relativno visokog radnog poten-cijala ovog metala. Ukoliko se štićeni objekat nala-zi u slabo provodnom tlu, žrtvene anode se polažu u punilu od gipsa/bentonita/natrijum sulfata. Kada se merenjem utvrdi manja vrednost katodnog potencijala, koja je posledica smanjenja kapaciteta magnezijumske anode, potrebno je zameniti anodu.

Drugi način je primena spoljnjeg izvora jedno-smerne struje (slika 2), čiji se minus vezuje za štićeni metal, a plus pol za anodu, pri čemu nastaje sma-njenje potencijala u defektnim zonama ispod granice od -0,85V, što omogućava smanjenje brzine korozije do 10 mikrona po godini.

TLO

CEVOVOD

KA

TO

DN

I KA

BL

ME

RN

I KA

BL

ME

RN

I ST

UB

IÆ

ANODNO

ANODNO LEŽIŠTE

MERNI STUBIÆANODNOG LEŽIŠTA

REFERENTNA

MERNA SONDA

JEDNOSMERNIIZVOR

KOLO

ELEKTRODA

Slika 2 - Princip zaštite sa spoljnjim izvorom

Zaštitna struja koja protiče kao posledica razlike potencijala na relaciji štićeni cevovod – zemlja, nije ravnomerno raspoređena duž cevovoda. Maksimalna apsolutna vrednost zaštitnog potencijala meri se u tzv. drenažnoj tački. Potencijal se menja sa udaljavanjem od ove tačke. Povećanje potencijala iznad dozvoljene granice dovodi do izdvajanja vodonika s površine če-ličnog materijala, što izaziva stvaranje pukotina u ce-vi, narušavanja athezije izolacije i pojave “džepova”. Smanjenje zaštitnog potencijala ispod potrebnih vred-nosti onemogućava pouzdanu zaštitu od korozije. Sigurna zaštita podzemnih metalnih objekata zahteva da se zaštitni potencijal održava striktno u određenim granicama.

Ova zaštita se može primeniti kako na novim konstrukcijama tako i na starim. Na postojećim konstrukcijama može se sagledati stepen ugroženosti pojedinih objekata preko merenja agresivnosti tla, prisustva lutajućih struja, uticaja morske vode i drugih parametara koji prouzrokuju proces korozije.

2. IZVOR JEDNOSMERNE STRUJE

Karakteristične veličine sistema za katodnu zaš-titu sa spoljnjim izvorom (blok šema data na slici 3) su jednosmerni napon i struja. Snaga ovih izvora kreće se obično u opsegu od 100W do 5000W, a izbor se vrši prema konkretnom objektu koji se štiti. Sastavni delovi jednosmernog izvora, koji služi za katodnu zaštitu metalnih konstrukcija su transformator, osigurači, odvodnici prenapona i upravljačka jedinica na bazi mikrokontrolera ATmega16 s pratećom elek-tronikom[7, 12] .

0 L1

ELEK

TRIC

NO

BR

OJI

LO

25A/0,03A

Zaštit. ureðajdiferencijalne

struje

ODVODNIKPRENAPONA

+

L1 0

- A

V

F1 F2

Q1

10A 6A

F4

F5

ODVODNIKPRENAPONA

ANODAŠTICENAKONSTRUKCIJA

F316A =

+ -

UPR

AV

LJA

CK

A J

EDIN

ICA

REG

ULA

CIJ

AN

APO

NA

I ST

RU

JE

GPRS/GSM

MODUL

-+

--

DigitalniRADIO

MODUL

-

Slika 3 - Blok šema sistema za katodnu zaštitu Osim napona i struje značajni parametri su: sta-

tusi osigurača i zaštitnog uređaja diferencijalne stru-je, status odvodnika prenapona, zatvorenost vrata ormara.

Promena otpornosti tla i prisustvo lutajućih struja, koje dodatno otežava katodnu polarizaciju štićenog objekta, nameću potrebu za regulacijom struje.

Upravljačka jedinica (slika 4) je u suštini izvor konstantne struje, čija vrednost ne zavisi od fluktu-acije ulaznog napona i promene otpornosti tla i elek-troda, u određenim granicama, u čemu se ogleda nje-gova robusnost, tako da u potpunosti odgovara zahte-vima katodne zaštite. Vrednost struje (u intervalu od



0 do 20A DC) zadaje se preko potenciometra P2. Sa ovog potenciometra se „skida“ naponski signal, koji se vodi na pin PC5 mikrokontrolera. Na otporniku Rs meri se napon (srazmeran struji), koji se vodi u opera-cioni pojačavač (IC5). Ovaj naponski nivo se dalje vodi na pin PC3 mikrokontrolera ATmega16, gde se obrađuje.

Ukoliko je struja manja od zadate, podatak se pre-nosi preko D/A konvertora (IC 3), čiji je izlaz pove-zan sa operacionim pojačavačem (IC4). Izlaz ovog OP pobuđuje gejt FET-a Q4 pri čemu se povećava napon na gejtu FET–a, koji radi u aktivnom režimu, a time se povećava i vrednost struje za katodnu zaštitu.

14

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PD2PD3PD4PD5PD6

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383940

242322

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R210Ω

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FC4100 nF

GND

TL431IC2 (2.5V)

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PB0

RES

PB1PB2PB3PB4PB5

PA2PA1PA0

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AREF

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9

re1re2re3

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100nFC5 GND

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B7 (DAC 08)B8 (DAC 08)

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PB6PB7 B6 (DAC 08)

272625

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B4 (DAC 08)B5 (DAC 08)

B3 (DAC 08)

2928PC7

PC6B1 (DAC 08)B2 (DAC 08)

3637PA4

PA3

5V

PD7PD7

Slika 4 - Šema upravljačke jedinice

Ako struja dostigne veću vrednost od željene napon na gejtu FET–a se smanjuje. Sistem uzima po potrebi veći napon od transformatora pomoću relea d1, d2 i d3, čiji su radni i mirni kontakti vezani u sekundarnim namotajima transformatora. Namotaji pomenutih relea su vezani u kolektorima NPN tra-nzistora, koji se uključuju od strane mikrokontrolera, preko pinova PB2, PB3, PB4 respektivno. Poveziva-nje digitalnog radio uređaja ili GPRS modula vrši se preko pinova PD0 i PD1, korišćenjem RS 232 pro-tokola komunikacije.

Digitalni ulazi i izlazi su realizovani na pinovima PB3 ÷ PB7 i PD2 ÷ PD7. Na digitalne ulaze su dove-deni: status mikroprekidača na vratima ormara, statusi osigurača, prekidača, zaštitnog uređaja diferencijalne struje i katodnih odvodnika prenapona. Jedan digital-ni signal (izlazni) služi za alarm u slučaju dostizanja kritičnih vrednosti napona i struje. Napon i struja jednosmernog izvora prikazuju se na LCD displeju.

Za upravljanje displejom upotrebljen je drugi mikrokontroler ATmega8 (pinovi PD0 ÷ PD7), što je prikazano na slici 5.

Slika 5 - Šema upravljanja LCD displejom

Slika 6 - Šema upravljanja LCD displejom

Na slici 6 prikazane su tekuće vrednosti karakte-rističnih parametara uređaja katodne zaštite: napon Uiz, struja Iiz i napon Urr-ref.el između štićenog ob-jekta (u ovom slučaju rezervoara) i referentne elek-trode.



3. REALIZACIJA SISTEMA ZAŠTITE

U praktičnom određivanju sistema katodne zaštite primenjuju se teorijski rezultati i praktična iskustva, ali nema univerzalnih rešenja, jer svaki objekat koji je predmet zaštite ima svoje specifičnosti. Te osobenosti su najčešće neponovljive kod drugog objekta, tako da je ovo potrebno imati u vidu prilikom projektovanja katodne zaštite, uz primenu postojećih standarda i propisa koji regulišu ovu oblast. Poznato je da je čelična konstrukcija zaštićena ako je nivo zaštitnog potencijala negativniji od – 850 mV (bez greške merenja) na celoj površini (deonici) štićene konstruk-cije u odnosu na referentnu elektrodu Cu/CuSO4, pod uslovom da sistem radi neprekidno. Sledeći primer zaštite ukopanih rezervoara pokazuje postupak di-menzionisanja sistema katodne zaštite. Na slici 7 pri-kazan je princip zaštite jednog ukopanog rezervoara. Ovde su instalirane tri ferosilicijumske anode čiji je izgled dat na slici 10. Anode su + polovi u strujnom kolu uređaj katodne zaštite – anoda – zemlja – ukopani metalni objekat – uređaj katodne zaštite. Ovakav smer struje obezbeđuje smanjenje korozije, pri čemu se anoda delimično troši. Indikacija za zamenu anode je porast otpornosti anodnog ležišta iznad 10Ω. Ugrađuje se i jedna referentna elektroda, koja služi za merenje potencijala katodne zaštite. Ispravnost ove elektrode se utvrđuje merenjem potencijala u odnosu na prenosnu referentnu elektrodu[6, 7, 21].

Predmet katodne zaštite u primeru je određeni broj ukopanih čeličnih rezervoara, ukupne površine P =1760m2. Prilikom transporta, skladištenja, montaže i eksploatacije, dolazi do oštećenja i do starenja zaštitne izolacije. Efikasnost zaštitne izolacije definiše se kao odnos gustine struje potrebne za zaštitu izolovane površine sa oštečenjima i gustine struje potrebne za zaštitu iste površine bez izolacije (gole površine). Za vek trajanja konstrukcije od 30 godina, iskustveno se određuje oštećenje izolacije (tipična vrednost 25%).

Za pomenute rezervoare potrebna je gustina zaštitne struje do 20 mA/m2. Jačina zaštitne struje, koja direktno zavisi od površine štićenog objekta, procenjenog nivoa oštećenja izolacije i veka trajanja konstrukcije, iznosi:

AI 8.825,0*10*20*1760 3 == −

Na sličan način se određuje zaštitna struja i za druge objekte poput cevovoda, uzemljivača i drugih elemenata ukopanih u tlu.

Strujni kapacitet jednosmernog izvora planira se na osnovu objekata koji su predmet katodne zaštite.

Otpornost uzemljenja jedne anode AR je

)14(ln2

−=dl

lrRA

gde se za konkretne podatke r=5000 [Ωcm] - specifična otpornost tla l =100 [cm] - dužina anode d =20 [cm] – prečnik anode s punilom dobija: Ω= 9.49AR Otpornost rasprostiranja anodnog ležišta je

Fn

RR A

AL =

gde je

)*66,0ln(*1 naR

rFa

+= faktor zasenjenja

a =500 [cm] - srednje rastojanje između anoda n=16 je broj anoda

BB

PRES

EK B

-B

A1, A2, A3-ferosilicijumske anode s punjenjemMO - merno oknoRM - referentna elektroda i merna sonda

RR - rezervoar

MO

RR

RM

A3

A2

MOA2

RR

RM

A3A1

Slika 7 -Katodna zaštita jednog metalnog rezervoara



Zamenom se dobija: 472,1=F

Ω= 6,4ALR

Izlazni parametri uređaja za katodnu zaštitu su:

8.8I A= ,

VVRRIU VAL 426.0)(* =++= Napon U iz bezbednosnih razloga mora da zado-

volji uslov U≤50V. Merenje potencijala rezervoara ili drugog štiće-

nog objekta u odnosu na referentnu elektrodu (slika 11 prikazuje način postavljanja ove elektrode), sadrži grešku, koja je veća ukoliko je izolacija rezervoara bolja.

a) b)

Slika 8 - Izgled a)merne sonde b)referentne elektrode Merna sonda (slike 8a i 9) je čelična pločica Φ 50

mm, koja je kablom povezana s rezervoarom i pred-stavlja ogoleli segment rezervoara. Ona se postavlja najčešće u paru s referentnom elektrodom (slika 7b).

50 mm x 10 mmCELICNA PLOCICA EPOKSIDNA SMOLA

BITUMEN

PVC CEV 50 mm

KABL PP00 2x2,5 mm2

O

Slika 9 - Postavljanje merne sonde

Merenje potencijala merne sonde prema referent-noj elektrodi daje podatak o polarizaciji rezervoara na mestima oštećenja, koja su veličine merne sonde.

Slika 10 - Izgled ferosilicijumske anode

Merna sonda se izrađuje od ugljeničnog čelika debljine 10mm i služi za merenje “OFF” potencijala posle dugotrajne polarizacije. S priključenim mernim instrumentom kratkotrajno se prekine metalna veza između merne sonde i rezervoara i očitava vrednost potencijala u vremenu manjem od jedne sekunde.

Merni instrument mora imati ulaznu otpornost naj-manje 10 MΩ kako bi mogla da se zanemari otpornost rasprostiranja merne sonde i referentne elektrode i da se zatvaranjem strujnog kola kroz tlo ne dođe do depolarizacije, dok traje vreme očitavanja potencijala.

KABL PP00 2*2,5mm 2

ZEMLJA IZ ISKOPA

REFERENTNA

NABIJENA PROSEJANAZEMLJA

oko 40cm

oko

30cm

oko

100c

m

ELEKTRODA

Slika 11- Postavljanje referentne elektrode

4. PRENOS PODATAKA POMOĆU DIGITALNIH RADIO UREĐAJA

Podaci o vrednostima struje, napona i o statusi-ma prekidačkih elemenata sistema katodne zaštite prenose se do komandnog centra najčešće putem radio UKT (ultra kratki talasi) uređaja ili uz pomoć GPRS (General packet radio service) veze. Razvoj izuzetno kvalitetnih digitalnih radio uređaja (DR), sa vrlo malom potrošnjom, otpornošću na meteorološke uslove, velikim stepenom sigurnosti i tačnosti u pre-nosu podataka, nametnuo je ove uređaje u novije vre-me, kao veoma pouzdane i prihvatljive za eksploata-ciju. DR je primopredajnik, koji radi u VHF (Very high frequency) i UHF (Ultra High Frequency) frek-ventnim opsezima i omogućava prenos telemetrijskih podataka neophodnih za rad sistema za daljinski nad-zor i upravljanje [7, 14, 18, 19]. Namenjen za rad u okruženjima koja zahtevaju point - to multipoint pre-nos podataka što je slučaj sa stanicama za katodnu zaštitu. Brzina prenosa podataka može biti 4800, 9600 ili 19200 bps. Standardna funkcija (FEC) Forward Error Correction daje mogućnost korekcije veoma velikog broja grešaka u prenosu signala, koje



se javljaju usled pojave šuma, čime se povećava pouzdanost DR. Blok šema digitalnog radio uređaja prikazana je na slici 12.

Slika 12 - Blok šema radio uređaja PDR 121 Ovi uređaji podržavaju rad sa širokim spektrom

postojećih komunikacionih protokola, koji se koriste u sistemima za daljinski nadzor i upravljanje, uključujući DNP3, IEC870, RP570, COMLI, Modbus, Cooper/PG&E 2179, L&G Telegyr 8979, Twacs by DCSI, Turtle by Hunt, Exoline, Cactus, PRIP, Aquacom by ITT Flygt.

Dispečerskicentar

Slika 13 - Peer-To-Peer komunikacije pomoću DR

Od savremenih sistema za prenos podataka zah-teva se besprekorna bezbednost i pouzdanost. Radio uređaji obezbeđuju sigurno šifriranje radio poruka i to 128 bit key XTEA cipher, pogodni su za primenu u sistemima koji rade na principu prozivke, kao i u sistemima s prozivkom koja je pokrenuta događajem. Na ovaj način se značajno ubrzava komunikacija između uređaja u sistemu. Uređaj takođe podržava i Peer-To-Peer način komunikacije (slika 13) – proto-kol ravnopravnih korisnika.

U proceduri slanja podataka, dodaju se FEC bito-vi podacima koji se prenose, pre nego što se konver-tuju u signal osnovnog opsega sa 4 nivoa. Pri prijemu podataka, na osnovu informacije iz FEC bitova, pri-menjuje se FEC algoritam, čime se ispravlaju greške nastale u prenosu signala. Kod slanja signala, na kraj poruke tj. niza podataka, dodaje se CRC (Cyclic Redundancy Checksum) bajt ili bajtovi, koji se izra-čunavaju na osnovu sadržaja poruke. Prijemnik pro-verava CRC bajt prema primljenim podacima i tako

detektuje greške nastale u prenosu signala, pri čemu se eliminišu sve oštećene poruke. DR može da radi kao master ili slave repeater u sistemu za prenos po-dataka. U svakom radio uređaju u sistemu, konfigu-rišu se informacije o putanji za prenos signala, adrese udaljenih krajnjih stanica (RTU – remote terminal unit), identifikacije radio uređaja i repeater – a. Kada upravljački sistem pošalje poruku master uređaju, ovaj prepoznaje adresu poruke, odnosno njenu desti-naciju i proverava da li u tabeli radio uređaja u siste-mu postoji slave uređaj sa identifikacijom koja odgo-vara adresi iz poruke.

5. PRENOS PODATAKA PREKO GPRS

Visina troškova koji su potrebni za osiguravanje infrastrukture neophodne za UKT radio prenos doprineli su sve većoj primeni GPRS veze. Ovaj način prenosa ne zahteva posebnu infrastrukturu jer koristi već postojeće komunikacije, i karakteriše se pouzdanim i brzim prenosom podataka. GPRS čija je blok šema prikazana na slici 14 je u suštini jedna vrsta usluge na GSM (Global System for Mobile Communications) mreži koja svojim proto-kolom za prenos podataka u vidu paketa, omo-gućuje kraće vreme uspostavljanja veze s brzim i pouzdanim prenosom podataka [7, 15-17, 20].

RF - BB INTERFACEPOWER MANAGMENT/CHARGE

ADCUART

CPU & MEMORI INTERFACEMIDI CONTROLLER

VOCODERKEYPAD - CONTROLLER

SIM CONTROLLERRTCSPI

GPIO CONTROLLER

60 P

IN A

RA

Y

CHARGER

FLA

SH/ S

RA

M

RF - TRANSCIEVERANTENA

BATTERY

PD0, PD1

AUXEARKEY PADRTCUART PORTLCDGPIO

Slika 14 - Blok šema GPRS/GSM modula

GPRS podržava brzine prenosa od 20 do 30 kbps (teorijski je moguće 171,2 kbps), obezbeđuje stalnu vezu, pri čemu se ne naplaćuje vreme trajanja veze, već količina prenesenih podataka.

Ovaj sistem omogućava daljinsku kontrolu i cen-tralizovano upravljanje različitim uređajima, korišće-njem mreže mobilne telefonije i Interneta.

Kontrola se ostvaruje preko računara, PDA (per-sonal digital assistant) uređaja ili mobilnog telefona koji su povezani sa Internetom.



GPRS modul podržava automatski prenos podata-ka između centralnog računara i uređaja u polju, na primer upis podataka u uređaj ili očitavanje i slanje podataka ka serveru korisnika. Ovaj modul ima sop-stvenu IP (internet protocol) adresu tako da svaki ure-đaj povezan s modulom preko RS232 porta postaje deo TCP/IP (Transmission Control Protocol/Internet Protocol) mreže korisnika. Time se rešava problem svakodnevne komunikacije s velikim brojem uređaja koji su instalirani na udaljenim ili teško pristupačnim lokacijama. Na slici 15 prikazana je šema dvosmer-nog prenosa podataka između dispečerskog centra i stanica za katodnu zaštitu, korišćenjem GPRS sistema i konvencionalnog interneta.

Slika 15 - Prenos podataka preko GPRS/GSM sistema

Za uspostavljanje veze i prenos podataka u real-nom vremenu koriste se GPRS, CSD (circuit-swit-ched data) i SMS (Short Message Service) servisi. Takođe je moguće uspostaviti Internet dial up vezu s modulom koji u tom slučaju radi kao GPRS modem. Osnovne odlike GPRS – a su:

- Efikasna komunikacija centralnog servera ili druge kontrolne jedinice sa svim uređajima u IP mreži korisnika,

- Dvosmerni prenos podataka preko GSM mreže i Interneta,

- Automatski upis podataka u uređaje i automat-sko očitavanje i slanje izveštaja ka serveru korisnika,

- Centralizovano upravljanje i kontrola, - Univerzalna primena – GPRS moduli mogu up-

ravljati različitim uređajima preko RS232 interfejsa, - Stalna veza računara sa svim kontrolisanim

uređajima - Komunikacija sa uređajima na celoj teritoriji

koju pokrivaju mobilni operatori, - Mogućnost pristupa pomoću računara ili PDA

uređaja povezanog na Internet ili pomoću mobilnog telefona koji ima GPRS i WAP (Wireless Application Protocol) browser,

- Bezbedan prenos podataka – centralni server, druge kontrolne jedinice i uređaji povezani preko GPRS modula pripadaju zaštićenoj TCP/IP mreži,

- Samostalan rad – GPRS modulima se može za-dati vremenski raspored izvršavanja određenih aktiv-nosti, npr. slanje izveštaja,

- Minimalni troškovi korišćenja – troškovi preno-sa podataka srazmerni su količini prenetih podataka, a ne proteklom vremenu za prenos i uspostavljanje veze,

- Jednostavna i brza instalacija i lako korišćenje. Od tehničkih karakteristika najznačajnije su:

– Eksterna antena (tipično 2 dB) – Napajanje 220 V +10%/ -15%, 50 Hz – Serijski RS232 interfejs – Priključak RJ12 – Časovnik realnog vremena (RTC – real time

clock) sa internom baterijom – Hardverski Watch - dog reset – Daljinska promena firmware – SMS: Point-to-point, SM-MT (short message -

mobile terminated) i SM-MO (short message – mo-bile originated)

– General Purpose Input/Output (GPIO) sa eksternim tasterom

– Cell broadcast, tekst i PDU (protocol data unit) mod

– Skup AT komandi (inicijalizacione komande za modem) i AT komande za TCP/IP pristup

– Fiksna brzina prenosa: od 300 bps do 115000 bps

– Autobauding: od 1200 bps do 115000 bps – RTS/CTS (Request to Send and Clear to Send) i

ON/XOFF



6. SIGURNOST PRI PRENOSU PODATAKA

GPRS tehnologiju odobrava Evropski institut za standard u telekomunikacijama-ETSI (European Te-lecommunications Standards Institute). To je skup standarda koji propisuje određene norme obavezujuće za proizvođače opreme, mrežnih elemenata, mobilnih uređaja i same operatore mobilnih telekomunikacija. Prema propozicijama iz standarda mora da se obezbedi nekoliko nivoa sigurnosti kako bi se zaštitila “privatnost” podataka i njihov integritet, uz proveru autentičnosti pošiljioca [14-20]. Putem SMS poruka, sistem obaveštava operatera u Dispečerskom centru o statusu konkretne stanice katodne zaštite. Te poruke mogu biti sledeće sadržine:

- Alarm (adresa SKZ) – nestanak mrežnog napajanja

- Alarm (adresa SKZ) – vrata ormara otvorena - Alarm (adresa SKZ) – uspostavljeno mrežno

napajanje - Alarm (adresa SKZ) – mala vrednost DC

napona - Alarm (adresa SKZ) – mala vrednost DC struje - Alarm (adresa SKZ) – Ispao neki od prekidačkih

(sigurnosnih elemenata) - Podatak o radnim satima uređaja

7. ZAKLJUČAK

Proces korozije (rđanja) dovodi do razaranja ma-terijala od koga je napravljena konstrukcija usled de-lovanja fizičkih, fizičko – hemijskih i bioloških age-nasa. Korozija nastaje usled hemijskih reakcija pod čijim se dejstvom vrši razaranje materijala. Gotovo da nema materijala koji pod određenim okolnostima nisu podložni koroziji. Troškovi izazvani korozijom mogu se podeliti na direktne i indirektne. Direktni troškovi se ogledaju u zameni korodirane opreme, održavanju i sprovođenju mera zaštite, dok indirektni troškovi iza-zivaju zaustavljanje pogona (proizvodnje), gubitke u proizvodnji (curenja iz rezervoara i cevovoda), sma-njenje stepena korisnog dejstva, kontaminaciju proiz-voda i okoline, predimenzionisanje konstrukcija.

Višedecenijsko iskustvo pokazuje da je katodna zaštita najefikasniji način zaštite od korozije čeličnih konstrukcija i objekata koji su položeni u tlu. Uređaj za katodnu zaštitu je pod nadzorom upravljačke jedi-nice koja je instalirana u stanici katodne zaštite, radi u lokalnom režimu i reguliše jednosmerni napon i stru-ju. U novije vreme za nadzor i upravljanje udaljenim objektima uglavnom se koriste dva načina prenosa informacija i upravljačkih signala: digitalni radio ure-đaji i GPRS sistem. Savremeni digitalni radio uređaji, namenjeni za industrijske komunikacije, zadovolja-

vaju zahteve pouzdanosti i tačnosti u prenosu poda-taka, ali je potrebno dodatno ulaganje (izgradnja od-govarajućih repetitora). U svakoj stanici za katodnu zaštitu instalira se odgovarajući radio modem, koji se povezuje sa upravljačkom jedinicom uređaja za kato-dnu zaštitu preko RS 485 (RS 232) protokola.

Primena GPRS – a u nadgledanju pomenutih sta-nica i dvosmernoj razmeni podataka s dispečerskim centrom odvija se putem interneta i mobilne telefo-nije, čime je obezbeđena mobilnost veze između ko-mandnog centra i udaljenih stanica s bilo kog mesta, bez obzira na udaljenost i bez potrebe za ulaganjem u infrastrukturu. Oba načina prenosa podataka omogu-ćavaju obaveštavanje operatera aktiviranjem zvučnog i svetlosnog alarma i ispisivanjem odgovarajuće poru-ke, u slučaju poremećaja u radu same stanice, što do-prinosi smanjenju troškova vezanih za obilazak, kon-trolu i održavanje.

LITERATURA

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[3] Cathodic protection of buried or immersed metalic structures, General principles and

[4] Applicions of Pipelines EN 13509 [5] Cathodic protection measurement Techniques EN

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instalacija i antikorozivne zaštite – kataloški podaci

ABSTRACT

CONTROL AND MONITORING A STATIONS FOR CATHODIC PROTECTION

An effective way to of protection against corrosion of metal structures, equipment and devices that are installed in the ground or water is the application of cathodic protection. Cathodic protection is performed by bringing an appropriate negative potential in the construction of manned, to the ground. Cathodic polarization of metal constructions in the electrolytic environment reduces or completely stops the corrosion process. DC sources are installed in racks - cathodic protection stations. The subject of cathodic protection is usually buried reservoirs, water mains, gas lines, oil pipelines, cables, grounding and the like., which are spread over large areas, so that both the cathodic protection station quite jagged. The problem of monitoring and control the cathodic protection stations is usually solved using digital radio or GPRS/GSM communication. In this way cells are connected to the dispatch center where it is continuously monitoring the parameters of cathodic protection and optimization mode stations themselves. Key words: cathodic protection, DC source, data transmission, radio, GPRS/GSM Paper received: 30.09.2011. Proffesional paper

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ZAŠTITA MATERIJALA - sitzam.org.rssitzam.org.rs/zm/2012/ZASTITAMATERIJALA 1-2012.pdf · ZAŠTITA MATERIJALA G o d i n a LIII B e o g r a d, 2012. ... Uporedna svojstva uretan-alkidnih