Glicerol Como Inhibidor en Cu

8/9/2019 Glicerol Como Inhibidor en Cu

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Research ArticleInhibition Effect of Glycerol on the Corrosion of Copper in NaClSolutions at Different pH Values

Santos Lorenzo Chi-Ucán,1 Andrea Castillo-Atoche,1

Pedro Castro Borges,1 José Antonio Manzanilla-Cano,2 Gerardo González-García,1

Rodrigo Patiño,1 and Luis Díaz-Ballote1

Departamento de F ́ısica Aplicada, Centro de Investigación y de Estudios Avanzados, Unidad M ́erida,

Antigua Carretera a Progreso Km. , M ́erida, YUC, Mexico Laboratorio de Electroquı́mica Anal ́ıtica, Facultad de Quı́mica, Universidad Aut ́onoma de Yucat ́an,Calle No. entre y , Colonia Industrial, M ́erida, YUC, Mexico

Correspondence should be addressed to Luis D́ıaz-Ballote; [email protected]

Received April ; Revised May ; Accepted June ; Published June

Academic Editor: Ying Zhou

Copyright © Santos Lorenzo Chi-Ucán et al. Tis is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

Te inhibitory effect o glycerol on copper corrosion in aerated NaCl (. M) solutions at three pH values (, , and ) was

evaluated. Inhibition efficiency was assessed with conventional electrochemicaltechniques: opencircuit potential, potentiodynamicpolarization, and electrochemical impedance analysis. Glycerol reduced the corrosion rate o copper in NaCl solutions. Te bestinhibition effect ( ≈ 83%) was produced in alkaline (pH ) chloride media. Tis effect can be ascribed to increased viscosity andthe presence o copper-glycerol complexes.

1. Introduction

As worldwide biodiesel production increases so does pro-duction o the byproduct glycerol [, ]. For every gallon(. L) o biodiesel produced approximately . Kg glycerolresults. By , the global biodiesel market is expected to be billion gallons [], consequently resulting in production

o . billion kilograms o glycerol. Tis looming glycerolglut means that new uses or glycerol need to be oundor conversion processes developed to convert it into more

valuable chemicals []. Finding new uses or glycerol wouldalso help to lower thecost o biodiesel, the main actor behinddecreasing production rates. Electrochemical studies haveshown glycerol to be useul as an additive in electrochemicalbaths or coating ormation [–]. It has also been used incombination with traditional corrosion inhibitors to improveefficiency, but very ew studies have ocused on glycerolalone as a corrosion inhibitor. Shaker and Abdel-Rahman []reported that the corrosion rate o copper decreases at higherglycerol proportions in water-glycerol solutions. Te authors

show that the reactions were controlled by diffusion. A key aspect o glycerol is its viscosity and its potential to orm ametal-glycerol complex. Viscosity in water-glycerol solutionsincreases as glycerol concentrationincreases []; thereore, anincrease in viscosity can also be expected to decrease masstranser o ions. A decrease in oxygen concentration canaffect

the cathodic reaction. In alkaline solutions, glycerol is knownto orm a Me-glycerol complex (Me = Zn, Fe, and Cu) [, ]which may act as a barrier to inhibit corrosion o metals.Te present study aim was to evaluate the inhibitory effecto glycerol (Figure ) alone on corrosion o copper exposedto NaCl solutions at three pH values.

2. Experimental Procedure

Deionized water and reagent grade NaCl, HCl, and NaOHwere used to prepare NaCl solutions with three different pH

values. Glycerol (.%) was purchased rom Sigma-Aldrich(CAS --). Samples were cut rom a pure (.%)

Hindawi Publishing CorporationJournal of Chemistry Volume 2014, Article ID 396405, 10 pageshttp://dx.doi.org/10.1155/2014/396405

http://dx.doi.org/10.1155/2014/396405http://dx.doi.org/10.1155/2014/396405


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Journal o Chemistry

Carbon

Oxygen

Hydrogen

F : Chemical structure o glycerol.

copper rod (Goodellow, . mm diameter) and embeddedin epoxy resin. A bar cross section was exposed to the NaClsolution. Beore each exposure, the sample was abraded witha series o different grade emery papers (up to ), rinsedwith water and ethanol, and dried with hot air.

All electrochemical measurements were done using astandard three-electrode cell conguration. Electrochemicalexperiments were run in the ollowing order: open corrosionpotential (OCP), electrochemical impedance spectroscopy (EIS), and potentiodynamic scan. All experiments were car-ried out using a Gamry PCI Potentiostat/Galvanostat/ZRAinstrument with CMS and sofware. OCP wasmeasured or min prior to each impedance experiment,which was done at kHz to mHz with a mV peak-to-peakamplitude using ac signals at OCP. Te potentiodynamicscan wasbegun immediatelyafer EISby scanning rom −.

to . V versus OCP at a mVs−1 sweep rate. A standardcalomel electrode (SCE) was used as a reerence in all elec-trochemical experiments. Impedance data were examinedor causality, stability, and linearity with the Kramer-Kronigrelationship using the method described by Boukamp []and adapted in the Gamry Echem analyst sofware.

A scanning electron microscope (SEM, Philips XLESEM) was used to examine any inhibitory effect o glycerolon copper corrosion. Samples examined with SEM wereabraded with emery paper (up to ), polished with a. m diamond solution, rinsed with water and acetone,and dried with hot air. Copper samples were immersed or hours in . M NaCl (pH ) solution with and without M glycerol.

3. Results and Discussion

Te OCP (oc) recorded as a unction o time or copper inNaCl (. M) with and without different glycerol concentra-tions at three pH values shows that overall the presence o glycerol at pH shifed the OCP towards negative potentials(Figures (a)–(c)). Te drop in OCP during the rst seconds is requently ascribed to dissolution o a native oxide

previously ormed on the copper surace as a result o contactwith the atmosphere. A shif to the negative region is also anindication o an active surace. Te general behavior o oc atpH consisted o a shif to noble potentials and was similarwith or without glycerol (Figure (b)). At pH , increasingglycerol concentration had no effect on OCP. At pH ,glycerol concentration had a clear effect on oc, causing itto move gradually toward the positive potential region andindicating ormation o corrosion products or adsorption o species onto the copper surace.

Te polarization curves or copper in NaCl (. M) solu-tions with and without glycerol in different concentrationsat three pH values showed how the Cu corrosion potential(corr) changed slightly rom negative to positive as a unction

o pH (Figures (a)–(c)). Tis trend coincided with the shifobserved or OCPat allpH values. Te cathodic branch o thepolarization curves exhibited behavior typical o a reductionreaction with mass transer limitations. Reduction o dis-solved oxygen with ormation o hydroxide [], or reductiono water molecules, is commonly responsible or the cathodicreaction []. Another cathodic reaction in acid is thereduction o oxygen to orm water molecules. Reduction o hydrogen is an unlikely cathodic reaction because hydrogenis more active than copper in the electromotive series. Inthe present results, neither pH nor glycerol concentrationaffected the shape o the cathodic branch (Figures (a)–(c)),indicating that pH and glycerol did not inuence cathodic

reaction type. Glycerol concentration did reduce cathodiccurrent very slightly, a reaction more noticeable at pH . Tisreduction may be explained by reduction o mass transer dueto increased viscosity as glycerol concentration increased.

No signicant change was observed in the polarizationcurve anodic current at pH , indicating that the anodicreaction was unaffected by addition o glycerol. Glycerol’seffect became apparent at pH and even more obvious atpH . In all cases, a linear portion o the polarization curvewas not easily determined. Extrapolation o both sides o the polarization curves to calculate corrosion parameters ispreerable, although occasionally just one side is used, mainly when a ael linear region is toosmall or is difficult to observe


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Journal o Chemistry

pH 4−0.15

−0.20

−0.25

−0.300 500 1000 1500 2000

Time (s)

P o t e n t i a l ( V v e r s u s S C E )

(a)

−0.15

−0.20

−0.25

−0.300 500 1000 1500 2000

Time (s)

pH 7


(b)

pH 10

−0.15

−0.20

−0.25

−0.30 0 500 1000 1500 2000

Time (s)


0.5 M) − blank

0.5 M) + G (0.1 M)

0.5 M) + G (0.5 M)

NaCl (

NaCl (

NaCl (

0.5 M) + G (1 M)

0.5 M) + G (2 M)

NaCl (

NaCl (

(c)

F : Open circuit potential (OCP) or Cu in NaCl (. M) solutions with and without glycerol at three pH values: (a) pH , (b) pH ,and (c) pH .

[]. Tereore, in the present study the cathodic branch

was chosen to calculate corrosion parameters using the aelextrapolation method [, ]. O note is that at pH theanodic branch exhibited a major reduction in anodic currentas glycerol concentration increased. Te corrosion param-eters calculated rom extrapolation o the cathodic branchshowed that, at all pH values, corrosion current decreasedas glycerol concentration increased (able ). Te percentageinhibition efficiency was calculated using corrosion currentand the ollowing equation [, ]:

(%) =0corr − corr

corr× 100, ()

where 0corr is the corrosion current measured in a NaCl

solution in absence o glycerol and corr is the corrosioncurrent measured in a NaCl solution containing glycerol asinhibitor.

Te best inhibition efficiency was attained at pH ,possibly due to ormation o a layer o copper-glycerolcomplexes near the copper surace, effectively reducing thereaction area and producing an inhibitory effect []. Te

two main copper-glycerol complexes are CuGl(OH)3−2 and

CuGl2(OH)2−2, where Gl− is a glycerol anion [].

In the example shown in Figures (a)–(c), the calculatedKramer-Kronig data t well in both the Bode and Nyquistdiagrams. Residual error was within .%, and goodness-o-

t was 24.86 × 10−6. However, residual error was generally


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Journal o Chemistry

−2

−4

−6

−8

−10

−0.6 −0.4 −0.2 0.0

pH 4

Potential (V versus SCE)

l o g i ( A )

(a)

−2

−4

−6

−8

−10

−0.6 −0.4 −0.2 0.0

pH 7


l o g

i ( A )

(b)

−2

−4

−6

−8

−10−0.6 −0.4 −0.2 0.0

pH 10


0.5 M) − blank

0.5 M) + G (0.1 M)

0.5 M) + G (0.5 M)

0.5 M) + G (1 M)

0.5 M) + G (2 M)

l o g i ( A )

NaCl (

NaCl (

NaCl (

NaCl (

NaCl (

(c)

F : Polarization curves or Cu in NaCl (. M) solutions with and without glycerol at three pH values: (a) pH , (b) pH , and (c) pH.


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Journal o Chemistry

: Electrochemical parameters determined by ael extrapolation o the cathodic branch o the potentiodynamic polarization curvesor copper in aerated NaCl (. M) with and without glycerol at three pH values.

pH Glycerol

(M)corr

mV versus SCEcorr

A⋅cm−

mV⋅d−1

mV⋅d−1%

pH

. − . − . Blank

. − . − . .

. − . − . .

− . − . .

− . − . .

pH

. − . − . Blank

. − . − . .

. − . − . .

− . − . .

− . − . .

pH

. − . − . Blank

. − . − .

. − . − . .

− . − . .

− . − . .

: Electrochemicalimpedance parameters obtained by tting theNyquistplots or Cu in aerated NaCl (. M) with and without glycerolat three pH values.

. M NaCl(pH)

GC(M)

Rs

(Ω cm)

1(Ω cm)

CPE 2 (Ω⋅cm)

CPE W

0 (Ω−1 cm− S.)

(%)0 (Ω

−1 cm−2 S1 ) 1 0 (Ω−1 cm−2 S1 ) 2

. . × −5 . . . × −4 . . × −5 bk

. . . × −5 . . . × −4 . . × −5

. . . × −5 . . . × −4 . . × −5

. . × −5 . . . × −4 . . × −4

. . × −5 . . . × −4 . . × −4

. . × −6 . . × −3 . × −4 . . × −2 bk

. . . × −6 . . × −3 . × −5 . . × −2

. . . × −6 . . × −3 . × −5 . . × −2

. . × −5 . . × −3 . × −5 . . × −1

. . × −6 . . × −3 . × −5 . . × −1

. . × −6 . . × −3 . × −6 . . × −3 bk

. . . × −6 . . × −3 . × −6 . . × −3

. . . × −6 . . × −3 . × −6 . . × −3

. . × −6 . . × −3 . × −6 . . × −3

. . × −6 . . × −3 . × −6 . . × −3

GC = glycerol concentration.

improve model t. Te characteristic parameters o theconstant phase elements are and . Estimated romthe semicircle diameter corresponded to the impedance o an anodic reaction occurring in two stages with magnitudesrepresented as 1 + 2 []. Although an exact differencebetween both resistances has been neither ully understoodnor ully described [], a number o studies use theseparameters or modeling corrosion behavior o copper in thepresence o an inhibitor. Inhibition efficiency was calculated

using polarization resistance (1 + 2) and the ollowingequation [, ]:

(%) = −

0

× 100, ()

where 0 is the polarization resistance measured in a NaCl

solution in the absence o glycerol and is the polarization


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Journal o Chemistry

0 10 20 30 40 50 60 70 80

80

70

60

50

40

30

20

10

0

Nyquist

Experimental

KK evaluated

− Z i m

( k Ω c m )

ZRe (k Ωcm)

(a)

100

10

1

0.1

0.1 1 10 100 1000 100000

20

10

0

−10

−20

−30−40

−50

−60

−70

−80

Bode

Frequency (Hz)

| Z | ( Ω )

10000

Experimental, KK calculated,

Experimental, |Z|KK calculated, |Z|

( ∘ )

(b)

0.1 1 10 100 1000 100000

Frequency (Hz)

10000

25

20

15

10

5

0

−5

−10

−15

20

15

10

5

0

−5

−10

−15

Goodness of t: 24.9 ×10−6

R e s i d u a l Z i m

× 1 0 3

R e s i d u a l Z R e × 1 0 3

Residual ZReResidual Zim

(c)

F : ypical Kramer-Kronig analysis results or Cu in NaCl (. M, pH ) solution containing glycerol ( M). (a) Nyquist diagram, (b)Bode diagram, and (c) residual error (Δ/) or the real and imaginary components o the impedance data.

resistance measured in a NaCl solution containing glycerol as

inhibitor.Te t was generated using the equivalent circuit in

Figure . ypical results o tting analysis are shown inBode (Figure (a)) and Nyquist (Figures (b)-(c)) plots(symbols represent measured data and solid lines representtted curves). In general, there was a good t between thecalculated and experimental impedance data.

Te most signicant parameters are total polarizationresistance ( = 1 + 2), the constant phase element,which represents the double layer capacitance (CPE1), andinhibition efficiency. Other parameters also exhibit glycerol’sinhibitory effect but are less notable. Polarization resistanceincreased and CPE1,2 values decreased in response to pH

levels and glycerol concentration (able ). Te and

CPE behaviors agree with the potentiodynamic polarizationmeasurement results, and the values o both parameterssuggest the presence o a lm thickening process that couldact as a barrier to corrosion and thus be responsible or theinhibitoryeffect. Presenceo a lm was more evident at pH ;indeed, the best agreement between the inhibition efficiency

values determined by the ael extrapolation o the cathodiccurrent and the impedance data was observed at pH . Tesendings support the assumption o ormation o a copper-glycerol complex on the copper surace.

Comparison o the inhibition efficiency obtained by potentiodynamic polarization and the impedance data showsthat both results ollow the same trend. Te very slight


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Journal o Chemistry

80

60

40

20

0

0 20 40 60 80

4

2

0

0 2 4

pH 4

ZRe (k Ω)

Z R e

( k Ω

)

ZRe (k Ω)

− Z i m

( k Ω )

(a)

80

60

40

20

0

0 20 60 80

pH 7

40

ZRe (k Ω)

− Z i m

( k Ω )

(b)

80

60

40

20

0

0 20 60 80

0.5 M) − blank

0.5 M) + G (0.1 M)

0.5 M) + G (0.5 M)

pH 10

0.5 M) + G (1 M)

0.5 M) + G (2 M)

NaCl (

NaCl (

NaCl (

NaCl (

NaCl (

40

− Z

i m

( k Ω )

ZRe (k Ω)

(c)

F : Nyquist plot or Cu in NaCl (. M) at our glycerol concentrations as a unction o three pH levels: (a) pH , (b) pH , and (c) pH.

difference in values was probably caused by differences inmeasurement time []. Te order o the electrochemi-cal measurements was open circuit potential, impedance,potentiodynamic polarization, meaning the layer o adsorbedmolecules during each measurement would be slightly differ-ent.However, impedance was most effective at demonstratingglycerol’s inhibitory effect on copper corrosion.

Micrographs (SEM) o copper samples exposed to . MNaCl at pH with and without added glycerol showedno visible indications o ion chloride attack on the coppersurace in the presence o glycerol (Figure (a)). In contrast,the copper exposed to NaCl in the absence o glycerolexhibited evidence o localized corrosion, such as micro- andmacropitting across the entire copper surace (Figure (b)).


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Journal o Chemistry

Rs

CPE1

CPE2

W

Rp1

Rp2

F : Equivalent circuit model used to t the impedance data rom the copper/NaCl (. M) interace with and without glycerol.

60

50

40

30

20

10

0

0 10 20 30 40 50 60

Fitting

− Z i m

( k Ω )

ZRe (k Ω)

ZimZim

(a)

0

−10

−20

−30

−40

−50

−60

−70

−80

10−1 101 102 103 104 105

Frequency (Hz)

Phase

Fitting Phase

P h a s e

( d e g

)

100

(b)

( Ω )

10−1 101 102 103 104 105

Frequency (Hz)

102

103

104

105

Fitting

100

|Z|

|Z|

| Z |

(c)

F : Equivalent circuit t or Cu in .M NaCl (pH ) + . M glycerol; (a) Nyquist, (b) phase, and (c) modulus.


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Journal o Chemistry

(a)

(b)

F : SEM micrographs o copper surace afer hours o immersion at room temperature (a) in aerated . M NaCl (pH ) + Mglycerol and (b) in aerated . M NaCl (pH ).

Tese observations agreewith the electrochemical results andconrm that glycerol effectively inhibits copper corrosion.

4. Conclusions

Glycerol inhibits corrosion o copper in aerated NaCl (. M)solutions. Inhibitionefficiency increases with increasing glyc-erol concentration andis especially notable at high pH values.Glycerol does not affect the anodic or cathodic reactions,suggesting that increased viscosity reducing mass transportapparently explains the reduction in corrosion rate rom acidto neutral solutions. Te reduced anodic corrosion rate atpH is probably due to two causes: an increase in solution

viscosity and presence o a lm on the copper surace, mostlikely composed o copper-glycerol complexes. SEM images

conrm the effectiveness o glycerol as an inhibitor o coppercorrosion in NaCl solutions.

Conflict of Interests

Te authors declare that there is no conict o interestsregarding the publication o this paper.

Acknowledgments

Te authors thank the Consejo Nacional de Ciencia y ecnoloǵıa or nancial support via Grants no. ,FOMIX-Yucatán -, and CONACY LAB--no. . Te authors also thank Biol. Ana Ruth CristobalRamos or her technical assistance.

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Journal o Chemistry

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Glicerol Como Inhibidor en Cu