University of Baghdad ِ◌◌ِ College of Science Chemistry Department

Corrosion of Lead-Acid Battery Electrodes

in Sulphuric Acid

A thesis

Submitted to the College of Science of Baghdad University as a partial fulfillment of the requirements

for the degree of Master of Science in Chemistry

By Bakhtiar Kakil Hamad

(B.Sc.) 2000

October 2004

Supervisors Certificate

I certify that this thesis was prepared under my supervision

at the College of Science, University of Baghdad in partial

fulfillment of the requirements for the degree of Master of Science in Chemistry.

Signature Prof. Dr. Jalal Mohammed Saleh Date: / /2004

In view of the above recommendation, I forward this thesis

for debate by Examining Committee.

Signature: Name : Address : Date : / /2004

Committee Certificate We certify that we have read this thesis and as examining

committee examined the student (Bakhtiar Kakil Hamad) in its

content and that in our opinion it meets the standard of a thesis

for the degree of Master of Science in Chemistry.

Chairman Member

Signature: Signature:

Name: Name:

Date: Date:

Member Member( Supervisor)

Signature: Signature:

Name: Name:

Date: Date:

In view of the above recommendation, I forward this thesis

for debate by Examining Committee.

Signature:

Name:

Address:

Date:

Acknowledgements

I would like to express my sincere thanks and gratitude to my

supervisor prof. Dr. Jalal Mhammed Saleh, Ph.D., C. Chem. D. Sc.,

FRSC. For his close supervision, encouragement and invariable

guidance throughout this research.

I wish to give my special thanks and appreciation to prof. Naema

Ahmed Hikmat, for her encouragement and care.

Special thanks are due to the staff of the State Company of

Battery Manufacturing for supplying the starting materials.

Finally, my sincere thanks are due to my family for their patience

and support during the duration of my studies. Above all my great

thanks to God for his mercy and blesses.

Bakhtiar

Summary

The present work involved the investigation of the polarization

behaviours of the following materials which consisted the electrodes and

components of the lead acid battery which were:

1, lead alloy working electrode,

2, Grid lead electrode,

3, Pure lead electrode,

4, uncured positive electrode,

5, cured positive electrode,

6, uncured negative electrode, and,

7, cured negative electrode.

In 0.1, 0.25 and 0.56 M sulphuric acid solution in the temperature

range (298-318)K in four different corrosion media which were:

1, un-stirred oxygenated sulphuric acid,

2, stirred oxygenated acid solution,

3, un-stirred deaerated acid solution, and,

4, stirred deaerated acid solution.

The major aspects of the work and the main results obtained may be

presented as follows:

1- The polarization behaviour studies were performed on the different

lead electrodes in the different media has been examined using a

potentiostat and a scan rate of (30)mm per minute. The potentioscan

covered a range from –2.0 to +2.0 Volt. The main results obtained

were expressed in terms of the corrosion potentials (Ec) which became

more negative in the un-stirred deaerated acid solution as compared

with the oxygenated acid solution, and also in terms of corrosion current

densities (ic) which became higher in the stirred oxygenated acid

solution. Thus, corrosion was more intense in the oxygenated acid

solution as compared with the deaerated acid solution.

2- The corrosion potentials and the corrosion current densities changed

considerably in the presence of the additives which involved :-

1, H3PO4 ( 11g dm-3),

2, A mixture of ( H3PO4(11g dm-3)+ FeSO4(0.2 g dm-3)),

3, NaCl (4 g dm-3) and ,

4, FeSO4 (0.2 g dm-3).

In the stirred and the un-stirred oxygenated 0.56M sulphuric acid solution

in the temperature range (298-318)K using the following working

electrodes:

1, lead alloy electrode,

2, grid lead electrode,

3, cured positive electrode, and ,

4, cured negative electrode.

Values of the corrosion potential (Ec) became more negative in the

presence of H3PO4 and less negative with NaCl additives, the values of the

corrosion current densities for all the electrodes were higher with NaCl

and lower with H3PO4 in the both media.

3- The protection efficiency (p%) was investigated for the additives in

the stirred and the un-stirred oxygenated 0.56M sulphuric acid

solution. Maximum values of p% were attained with H3PO4 and the

minimum with NaCl.

4- Values of the thermodynamic quantities (DG, DW and DH) were

estimated for the corrosion of the electrodes. DG values were more

negative in the deaerated acid solution in the absence of additives. In the

presence of the H3PO4, DG values were more negative while in the

presence of NaCl the values were less negative indicating a greater

corrosion feasibility in the former and smaller in the latter cases. DW

values extended over a wider range. Such variation of DW values

generally depended on the type and extent of the variation of DG vales

with temperature. As a result of such variations, values of DH were also

found to a quire appreciably negative values.

5- The kinetics of the corrosion followed Arrhenius type rate equation.

A linear relationship existed between the values of the activation energy

(Ea) and logarithm of the pre-exponential factor (log A) in the four

different media suggesting the operation of a compensation effect in the

kinetics of corrosion. This suggests that, the corrosion reaction proceed

on surface sites, which were associated with different energies of

activation (Ea). The corrosion reaction is assumed to start on sites with

lower Ea and log A values first, spreading thereafter to these sites on

which Ea and log A were higher.

a

CONTENTS

Subject Page No.

CHAPTER ONE: INTRODUCTION

1.1- Lead-Acid Storage Battery 1

1.1.1- The industrial production of Leady oxide 2

1.1.1.1- Barton-pot process 2

1.1.1.2- Ball-Mill Process 3

1.1.2- Industrial preparation of the Electrodes 4

1.1.3- Structure of the Electrode Materials 6

1.1.3.1- PAM Structure 6

1.1.3.2- NAM Structure 9

1.1.4-The Electrolyte 12

1.1.5- The cell structure and Reactions 13

1.1.6- The Positive Electrode 14

1.1.7- The Negative Electrode 15

1.1.8- Curing of the Battery Electrodes 16

1.1.9- Charging and Discharging Processes 17

1.2- Corrosion of Battery Electrodes 20

1.3- Corrosion of Lead and Lead Alloys 21

1.4- The Literature Survey 22

1.5- The Object and Scope of the Present Research 25

CHAPTER TWO: EXPERIMENTAL

2.1- The Experimental Set-Up 28

2.2- The Working Electrode 29

2.3- The Auxiliary Electrode 30

2.4- The Reference Electrode 31

2.5- The Corrosion Cell 32

b

Subject Page No.

2.6- Potentiostatic Measurement 34

2.7- The Experimental Techniques and Procedure 36

2.8- The Chemicals 38

CHAPTER THREE: RESULT AND DISCUSSION POLARIZATION IN SULPHURIC ACID IN THE

ABSENCE OF ADDITIVES

3.1-The Polarization Curves. 39 3.2- Results of the Polarization Curves. 45 3.2.1- Corrosion Potentials (Ec). 61 3.2.2- Corrosion Current Densities (ic). 69 3.2.3- Passive Potentials (Ep). 76 3.2.4- Passive Current Densities (ip). 77 3.3- Tafel slopes and Transfer Coefficients. 78 3.4- Polarization Resistance. 80 3.5-Thermodynamics of Corrosion. 82 3.6- Kinetics of Corrosion. 88

CHAPTER FOUR: RESULT AND DISCUSSION

POLARIZATION IN SULPHURIC ACID IN THE PRESENCE OF ADDITIVES

4.1- Results of the Polarization Curves. 96 4.1.1- Corrosion Potentials (Ec). 101 4.1.2- Corrosion Current Densities (ic). 107 4.1.3- Passive Potentials (Ep). 112

4.1.4- Passive Current Densities (ip). 114

4.2- Tafel slopes and Transfer Coefficients. 116 4.3- Polarization Resistance. 118

c

Subject Page No.

4.4- Effect of Additives. 120

4.4.1- Phosphoric Acid 120

4.4.2- Mixture of H3PO4 and FeSO4 121

4.4.3- Ferrous Sulphate (FeSO4) 122

4.4.4- Sodium Chloride 122

4.5- Protection Efficiency 123 4.6- Thermodynamics of Corrosion. 134 4.7- Kinetics of Corrosion. 149

CHAPTER FIVE: CONCLUSIONS AND SUGGESTIONS FOR FUTURE RESEARCH

5.1- Conclusions 166

5.2- Suggestions for future research 167

REFERENCES 168

Symbols and Abbreviations

Symbol Definition Units A Pre-exponential factor Molecules. Cm-2.s-1 ba Anodic Tafel slope V decade –1 bc Cathodic Tafel slope V decade –1 C Molar concentration Mole. dm-3 Ea Activation energy k J mol-1 Ec Corrosion potential V (S.C.E) Ecr Critical potential V (S.C.E) Ep Passive potential V (S.C.E) F Faraday constant C mol-1

DG Gibbs free energy change k J mol-1 DH Enthalpy change k J mol-1

i Current density A cm-2 ic Corrosion current density A cm-2 icr Critical current density A cm-2 ip Passive current A cm-2 n Number of electrons

NAM Negative active mass P Protection efficiency

PAM Positive active mass R Gas constant J. mol-1.K-1

SCE Saturated calomel electrode V DS Entropy change J. mol-1.K-1 DS¹ Entropy of activation J. mol-1.K-1 T The temperature in Kelvin K a Transfer Coefficient aa Anodic Transfer Coefficient ac Cathodic Transfer Coefficient

(1)

1.1-Lead –Acid Storage Battery

The lead acid battery was the first, commercially successful,

rechargeable battery. It was invented in 1859 by G. Plante and has

undergone steady improvement ever since(1).

A typical (12)V lead-acid car battery has six cells connected in series,

each of which delivers about 2V. Each cell contains two lead grids packed

with the electrode materials. The anode is spongy Pb, and the cathode is

powdered PbO2. The grids are immersed in an electrolyte solution of ~ 4.5

M H2SO4 fiberglass sheets between the grids prevent shorting by accidental

physical contact. When the cell discharges, it generates electrical energy as

a voltaic cell with reactions:

Anode (oxidation):

Pb(s) + SO42-

(aq) ® PbSO4(s) + 2e- (1-1)

Cathode(reduction):

PbO2(s) + 4H+(aq) + SO4

2-(aq)+ 2e ® PbSO4(s) + 2H2O(l) (1-2)

Note that both half-reactions produce Pb2+ ions, one through oxidation

of Pb, the other through reduction of PbO2. At both electrodes , the Pb2+

ions react with SO42- to form insoluble PbSO4(s)

(2).

The overall electrochemical process can be represented by the

equation(3):

Pb(s) + PbO2(s) + 2H2SO4(aq) 2PbSO4(s)+ 2H2O(l) (1-3)

The grids make an important part of the storage cell which act as

supports for the active materials of plates and conduct the electric current

developed. It also plays an important role in maintaining uniform current

distribution throughout the mass of the active material. Grids for both

discharge

charge

(2)

positive and negative plates are frequently of the same design, composition,

and weight.

The lead storage battery is the most widely applied storage battery in

the world today(4).

1.1.1-The industrial production of Leady oxide

The basic starting material for lead –acid battery plates is generally

referred to as “leady” or “ grey” oxide. This material is prepared by

reacting a lead feedstock with oxygen in either a Barton pot or a Ball Mill,

and usually comprises about one-part unreacted fine lead particles

(so-called ‘free lead’) and three parts lead monoxide (a-PbO and b-PbO).

A small amount of red lead (Pb3O4) can also be produced, but battery

manufactures generally prefer to add this oxide from a separate source. The

blending (or indeed complete substitution) of leady oxide with red lead is

particularly popular in the preparation of tubular positive plates(5). The

Barton-pot and Ball-Mill processes remained the principle methods for

producing leady oxide for lead–acid battery paste.

1.1.1.1-Barton-pot Process

In the Barton-pot approach to making battery oxide, lead is melted,

forced into a spray of droplets, and then oxidized by air at a regulated

temperature. Any accumulated bulk molten lead is broken up again into

droplets by a revolving paddle that directs the lead against a fixed baffle

arrangement inside the pot. By careful control of the :

¨ pot temperature,

¨ paddle rotation speed and,

¨ rate of air flow.

(3)

Battery oxide of the desired chemical composition and particle – size

distribution can be obtained(5). The oxide so produced is a mixture of

tetragonal ( a-PbO) and orthorhombic (b-Pbo) lead monoxide together with

some unreacted lead. The oxide usually consists of (65-80 w%) PbO(6,7).

The problem with the Borton-pot system is that of controlling the pot

temperature. If the temperature is excessive above 488oC a large amount of

b-PbO can be formed; which is considered undesirable in the final product

because of its effects on performance and life of the finished plate if the

amount of b-PbO exceeds 15%(8,9).

1.1.1.2-Ball-Mill Process

The alternative means for preparing battery oxide the Ball–Mill

process- involves tumbling lead balls, cylinders, billets or entire ingots in a

rotating steel drum through which a stream of air is passed. The heat

generated by friction between the lead pieces is sufficient to start oxide

formation. The reaction generates more heat and thus allows the lead

particles that are rubbed off by the abrasion to be converted to leady oxide

of the required composition.

The relative amounts of the oxide constituents can be controlled by

manipulation of the operational parameters governing the oxide-making

process, namely(5):

¨ mill temperature,

¨ mill speed,

¨ flow rate and temperature of the air steam, and ,

¨ amount of mill charge.

The oxide usually consists of (60-65wt %) of a-PbO, with remainder being

free lead(8).

(4)

1.1.2-Industrial preparation of the Electrodes

The pastes now commonly used in making the familiar pasted –plate

batteries are prepared by mixing some particular lead oxide or blend of

oxides with aqueous sulphuric acid (sp. gr. 1.4) and water. Free lead and

different basic lead sulphates have been found in the paste as the

monobasic lead sulphate, the dibasic lead sulphate, the tribasic lead

sulphate, and finally tetrabasic lead sulphate. In first, normal lead sulphate

is produced according to the following equation:(10)

PbO + H2SO4 ® PbSO4 + H2O (1-4)

and then the normal lead sulphate produced reacts with additional lead

oxide to form basic compounds. Both water and sulphuric acid serve

necessary functions in the pasting of battery oxide mixes.The water acts as

lubricant producing a lighter paste. As the plate dries the evaporation of

this water gives a desirable porosity. The sulphuric acid forms lead

sulphate which, in addition to expanding the paste and giving it great

porosity, supplies a necessary binding cement so that the dry plate can be

handled without loss of material.

The prepared paste is applied to the grid by machine pasting equipment. Freshly pasted plates are passed through a drying oven to harden their surface somewhat. They are left in the oven for 72hr to enable the so-called curing process to take place. During the curing operation the relative humidity in the curing ovens must be 100%, such humidity takes part in oxidation reaction. The temperature is responsible for the composition of cured plates, such plates cured at high temperature (more than 70oC) resulting in mainly tetrabasic lead sulphate 4PbO. PbSO4 (4BS) behave markedly different to those cured at low temperature having only tribasic lead sulphate 3PbO. PbSO4. H2O (3BS)(11,12). The surfaces are of active material and depend on curing temperature, as the suitable temperature in curing process is around (56-65oC)(13).

(5)

The final process for preparation of lead–acid battery plates is the formation. Formation of the plates is necessary to convert the inactive lead oxide-sulphate paste into the active electrode materials of the finished cell Essentially it is an oxidation reduction reaction wherein the positive plates are oxidized from lead oxide to lead dioxide, and the negative plates are reduced from lead oxide to sponge lead.

The negative plates are similarly made except that so-called “expanders” are added. Expanders are necessary in negative plates to activate the plates at low temperatures and high rates of discharge. Three materials constitute what are commonly called negative expanders. They are carbon black, barium sulphate, and organic materials such as lignin(14,15). The presence of the lignin, however, renders the lead sulphate film porous(16). A number of theories have been proposed to account for the reaction taking place in the lead-acid battery. The double- sulphate theory is now generally accepted. Gladstone and Tribe first proposed this theory in 1882(17). The double–sulphate theory is most conveniently stated by the equation(1-3)(18) ,which indicate that the overal reaction leads to the formation of the lead sulphate on the lead acid battery electrodes that is, both positive and negative plates will be converted to the lead sulphate at the end.

(6)

1.1.3-Structure of the Electrode Materials

1.1.3.1-PAM Structure

The structure of PAM (positive active mass) obtained during

formation of the plates consists of two structural levels and is presented as

in Fig.(1-1) and described in the following sections:

Fig.(1-1)

a- Microstructural. The smallest building element of PAM structure is the

PbO2 particle. A certain number of PbO2 particles interconnect into

agglomerates. At this microstructural level the electrochemical reaction

of discharge proceeds. This level determines the active surface area of

PAM.

b- Macrostructural level. A huge number of agglomerates, and in some

cases individual particles, interconnect to form aggregates (branches) or

porous mass. Micropores are formed between the agglomerates building

up the aggregates. Aggregates interconnect to form (i) Skeleton, which

is connected to the grid through an interface or (ii) porous mass.

Macropores are formed between the aggregates along which H2SO4 and

H2O flows move between the plate interior and the bulk of the

electrolyte(19,20).

(a) (b)

(7)

Fig.(1-2) presents structure of the three types of PbO2 particles:

spherical or egg-shaped, PbO2 crystal particles and needle-like particles.

Fig.(1-2)

A heterogenous mass distribution is observed in the bulk of PbO2 Particles

Fig.(1-3).

Fig.(1-3).

Dark zones have crystal structure (a or b PbO2) and sizes 20 to 40 nm.

More electron transparent zone are hydrated (gel zones). Hence PbO2

particles have crystal/gel structure. About 31-34% of PAM is

hydrated(21,22).

(8)

The mechanism of the formation of PbO2 particles involves

Pb4+ + 4H2O ® Pb(OH)4 + 4H+ (1-5)

Pb(OH)4 dehydrates partially as a result gel particles to form:

nPb(OH)4® [ PbO(OH)2]n + n H2O (1-6)

[PbO(OH)2]n stands for a gel particle. Further dehydration takes place and

PbO2 crystal zones are formed.

[PbO(OH)2]n¨ [kPbO2+(n-k)PbO(OH)2]n + k H2O (1-7)

Crystalzone gel zone

Hydrated zones exchange ions with the solution (PbO2 particle is an

open system). The ratio between crystal and gel zones influences the

capacity of the plate.

(9)

1.1.3.2-NAM STRUCTURE

NAM (negative active mass) structure consists of lead crystals

interlinked in a skeleton network Fig.(1-4) and secondary structure of

separated lead crystals which are precipitated on the lead skeleton surface

Fig.(1-5)(23,24).

Fig.(1-4) Fig.(1-5)

The skeleton structure is formed during the first stage of formation

when PbO and basic lead sulphates partially reduced to lead and partially

react with H2SO4 to give PbSO4.

These processes proceed at a neutral pH solution in the pores of cured

plates. The secondary structure is formed during the second stage of the

formation when PbSO4 crystals are reduced to lead crystals under acidic

conditions. Upon discharge, current is generated mainly at expense of the

oxidation of the secondary lead structure (energetic structure). The primary

(skeleton) structure serves both as a current collector and a mechanical

support of the energetic structure. The energetic structure participates

mainly in the charge discharge processes of the negative plates(25,26).

Pb2+ ions are formed at the lead/ anodic layer interface. Under the

action of the electric field they reach the second interface and return to the

solution. Since the solution is saturated with respect to PbSO4 the Pb2+ ions

(10)

diffuse to the growth front of some of the lead sulphate crystals and are

incorporated in it. Owing to these processes of transport of Pb2+ through the

anodic layer, microvoids are formed between the lead sulphate crystals and

the lead surface, Fig.(1-6)(27).

(11)

Fig.(1-6)

Representation of the multi-phase corrosion layer by Ruetschi

Microns

Microns

Pote

ntia

l vs H

g/H

g 2SO

4

(12)

1.1.4-The Electrolyte

Sulphuric acid of battery is a heavy transparent oily liquid having no

odour and easily soluble in water. When the acid is dissolved in water it

heats the solution very highly. This acid attacks leather, paper, cloth. It is

used for making the electrolyte for Lead-Acid batteries(28). The specific

gravity of sulphuric acid depends on the temperature and decreases with

increasing temperature.

During formation, the acid used to make the paste is released and

some water is lost due to gas evolution so that the concentration at the end

will be higher than at the beginning.

After charging, the batteries require leveling of the electrolyte. This is

primarily due to the fact that the batteries are normally not filled to the

correct level in the first time to allow for gassing and secondly to make up

for the water losses during charging. The leveling should be possible with

standard operation acid, i.e. specific gravity 1.28 g/ml. As shown in table

(1-1)(29).

Table(1-1): Specific gravity of sulphuric acid and charge conditions in lead-acid storage battery.

Electrolyte specific gravity The charge energy

1.28-1.25 Full charge

1.25-1.20 Suitable

1.20-1.16 Vacancy

1.16-1.08 Full vacancy

Less than 1.08 Un- rechargable

(13)

1.1.5- The cell structure and Reactions

The Pb-H2SO4 cell system for the fully–charged cell can be

represented it as in the following: (A)(7):

Pb H2SO4 PbO2

Anode(-) solution Cathod(+)

When the cell is connected to the external circuit (the process called cell

discharging )the left electrode (Anode) oxidized as :

L: Pb = Pb2+ + 2e (1-8)

Two electrons are generated and transferred within the external circuit

to the right electrode (cathode) and the reduction occur as:

R: PbO2 + 4H3O+ + 2e = Pb2+ + 6H2O (1-9)

The collection of (1-8) with (1-9) reactions may be made as:

Pb + PbO2 + 4H3O+ = 2Pb2+ + 6H2O (1-10)

adding (2SO42-) to the reaction (1-10) we obtains:

Pb + PbO2 + 2H2SO4 = 2PbSO4(s)+ 2H2O (1-11)

The symbol (s) for PbSO4(s) means an insoluble salt in electrolyte solution

which covers the plates surfaces. The reaction (1-11) is a discharge process

and converts the electrodes to lead sulphate. The measuring of the specific

gravity of the acid electrolyte solution during discharging process helps to

estimate the remaining life of the battery.

The chemical structure of the fully – discharged cell may be represented as

in B:

B

PbSO4(s) H2O PbSO4

Anode(-) Cathode(+)

(14)

Considering the remaining (un-reacted) lead and lead dioxide of the

electrodes are can re-write the cell (B) as cell(C):

C

Pb,PbSO4(s) ½ H2O ½PbSO4(s), PbO2

The left electrode is therefore made of lead and lead sulphate and the

right electrode is compared of lead dioxide and lead sulphate.

When the discharged cell is exposed to a charging process the

reactions which occur at the electrodes are:

L: PbSO4(s) + 2e = Pb + SO42- (1-12)

This is reduction process and the oxidation occurs at the right

electrode as:

R: PbSO4(s) +2H2O= PbO2+ SO42- + 4H++ 2e (1-13)

The overall reaction may be represented as summation of (1-12) and

(1-13) reactions as:

2PbSO4(s) +2H2O= Pb + PbO2+ SO42- + 4H+….. (1-14)

Or:

2PbSO4(s) +2H2O= Pb + PbO2+ 2H2SO42- …. (1-15)

This explain that the battery charging by an external current converts

the left electrode to lead and the right to lead dioxide and the water is

converted to sulphuric acid. Thus, the battery restores its original state by

such operation(30).

1.1.6- The Positive Electrode The electrochemical reactions at the positive electrode are usually

expressed as:

PbO2(s) +4H+(aq)+SO4(aq)

2- +2e… PbSO4(s) + 2 H2O(l) (1-16)

An important feature of the positive electrode discharge concerns the

nature of the PbSO4 deposit since the formation of dense, coherent layers

discharge

charge

(15)

can lead to rapid electrode passivation. Lead dioxide exists in two

crystalline forms, rhombic (a-) and tetragonal (b-), both of which are

present in freshly formed electrode structures.

Positive electrodes are manufactured in three forms, as plante plates,

pasted plates and tubular plates. In plante plates, the positive active

material is formed by electrochemical oxidation of the surface of a cast

sheet of pure lead to form a thin Layer of PbO2. The plate generally has a

grooved structure to increase its surface. Such plates have a very long life.

Since they have a large excess of lead which can subsequently be oxidized

to PbO2(31). Tubular plates consist of a row of tubes containing axial lead

rods surrounded by active material. The tubes are formed of fabrics such

as terylene or glass fibre or of perforated synthetic insulators which are

permeable to the electrolyte. Lead dioxide electrode system (Pb/ PbO2/

PbSO4) formed at potentials above +0.950 V(32,33).

1.1.7- The Negative Electrode:

The reactions of the negative electrode are generally given as:

PbO(s) +SO2-4(aq) PbSO4(s) + 2e (1-17)

Negative electrodes are almost exclusively formed of pasted plates , using

either fine mesh grids or coarse grids covered with perforated lead foil (box

plates) and the same paste used in positive plate manufacture. When the

paste is reduced under carefully controlled condition, highly porous sponge

lead is formed consisting of a mass of a cicular (needle-like) crystals which

give a high electrode area and good electrolyte circulation(31). Additives

such as very fine BaSO4, which is isomorphic with PbSO4, encourages the

formation of a porous non-passivating layer of lead sulphate. The precise

mechanism of the additive effects is complex and not completely

discharge

charge

(16)

understood. It is known that BaSO4 and the organic additives interact, since

together they are much more effective than the sum of their individual

contributions.

Lead/Lead sulphate electrode system (Pb/PbSO4) is formed within the

potential region from –0.950 to –0.400 V vs. a calomel reference

electrode(34).

1.1.8-Curing of the Battery Electrodes

The curing process consists of the conversion of wet pasted plates to

a dry, crack free, unformed plate of sufficient strength and adhesion to the

grid. During this process two steps proceed simultaneously and in

sequence:-

1. water loss by shrinkage.

2. Void formation.

Curing is an important part of manufacturing, for if it is not properly

carried out capacity and especially life expectancy are adversely

influenced. The curing can be done in different ways.

1. The plates are suspended individually on racks with small separation

according to a pre-established program, the plates are subjected to a

flow of damp or dry air and finally heated. The curing and drying lasts

about 16 to 24 hours.

2. The plates are hang on chains and moved through a tunnel kiln in

which temperature is increased and humidity is decreased. The kiln is

usually heated with CO2-containing combustion gas which passes

through the kiln.

3. The plates are flash-dried by gas heating or infrared heating so that they

may be packed densely 20 to 30 cm high without sticking. They are

covered to prevent the process from proceeding too rapidly, otherwise

(17)

small cracks will appear. For oxidation and drying in stacks 4 to 6 days

are required.

4. The plates are dipped in sulphuric acid or sprayed with sulphuric acid

to form a dense lead sulphate film on the surface, a process frequently

used for tubular plates but less often for grid plates.

After curing the paste in the plates must have sufficient dry strength

and adequate adhesion to the grid so that it does not detach during sub

sequent manufacturing steps and retains electrical contact with the grid

during formation.

Curing of positive plates take place when Pb oxidation of the

paste/grid contact and drying of the paste.

For operation duration of curing of negative plates has to be less than

8 hours too. Additive to the negative plate increases the rate of curing

process at 60Co and reduces the curing process to 8 hours. The expander

destroys at temperature higher than 65Co(35,3).

1.1.9- Charging and Discharging Processes

Formation of positive plates. It was found that formation of positive

active mass (PAM) takes place in two stages(36).

a. During the first stage, H2SO4 and H2O penetrate from the bulk of

the solution into the plate, As a result of chemical and

electrochemical reactions PbO and basic sulphates are converted to

a-PbO and b-PbO.

b. During the second period of formation PbSO4 is oxidized to b-

PbO2. H2SO4 originates and diffuses into the volume of electrolyte.

Taking into account specific conditions of chemical and

electrochemical reactions in porous electrodes a mechanism is suggested

for formation processes of the PAM(37,38).

(18)

Formation of the negative active mass: It was established that it takes

place also in two stages:

a. During the first stage electrochemical reduction of PbO and basic lead

sulphates occur and lead skeleton is formed. Beside, in these processes

chemical reactions of PbSO4 formation also proceed. PbSO4 crystal

remain included in lead skeleton. The (PbSO4 + Pb) zones are formed in

the both surfaces of the paste and advance into the interior of the plate.

b. During the second stage, reduction of PbSO4 to Pb occurs and the

obtained lead crystals are deposited on the lead skeleton surface in

strongly acidic solution. The mechanism of the elementary chemical and

electrochemical reactions as well as their mutual relationships are

determined. During formation, both the pore radii and the porosity of

the active mass increase(39,40). Fig.(1-7) Shows the Discharge and change

processes of the lead acid battery.

(19)

Fig.(1-7): Discharge and charge processes of the battery

(20)

1.2- Corrosion of Battery Electrodes

The grids of the electrodes which serve as carriers for the active

masses conductors for the electric current are manufactured from lead and

alloys by casting. Other methods such as punching or stretching are

common. The process of disintegration of a metal grid structure starting the

surface is called corrosion.

Each nonnoble metal suffers corrosion in aqueous solution in which

metal is dissolved anodically under hydrogen evolution or precipitated an

insoluble compound, depending on the constituents of the solution. This

reaction is small because of the high overvoltage of the hydrogen on lead

with negative electrodes the portion of the surface of the grid compared

with the total inner surface of the mass is small. Therefore a corrosion of

the grid is not noticeable. The lead sulphate forms a dense cover layer to

protect the grid. Failure of batteries due to corrosion of the negative grids is

rarely observed. The hydrogen corrosion occurs often in cavities in the

presence of organic substances and at higher operating temperatures.

On positive grids corrosion leads to solid oxidation products, to

reduction of the cross section of the grid rods, and thereby to a loss of

conductivity and grid breakage. Often a deformation or increased growth of

the grids is a related condition.

The local cell corrosion on positive plates plays a only minor role. The

corrosion under current, the anodic corrosion, however, is highly important.

With current flow the process becomes dependent on potential. A

schematic representation of the reaction products as a function of potential

is shown in Fig.( ). Included here is the dependence on the hydrogen and

sulphate ion concentration.

(21)

A sign of grid corrosion is a reduced number of ampere hours

obtained from the battery on discharge at the 10 hour rate. The positive

electrode always limits the capacity.

Cells containing plates destroyed by corrosion are no longer fit for

service. Usually, corrosion of the grids is a sign of long service of the given

cells.

1.3- Corrosion of Lead and Lead Alloys

Lead is used extensively in sulphuric acid in the lower concentration

ranges. Corrosion is practically nil in the lower concentrations but increases

as temperature and concentration increase. Rapid attack occurs in

concentrated acid because the lead sulphate surface film is soluble(46,47).

This is the lead used for corrosion applications. High purity lead is less

resistant particularly in the stronger and hotter acids and also exhibits

poorer mechanical properties.

Lead depends on the formation of a lead sulphate-lead protective

surface long life in sulphuric acid environments, and in many cases more

than 20 years service is obtained. Lead gains weight when exposed to

sulphuric acid because of the surface coating or corrosion product formed

except in strong acid wherein the lead sulphate is soluble and not

protective.

Lead forms protective films consisting of corrosion products such as

sulphates, oxides, and phosphates.

A more realistic model of the corrosion product layer formed on lead

has been proposed by Ruestchi as shown in Fig. (1-8)(48,49).

When corrosion resistance is required for process equipment,

chemical lead containing about 0.06% copper is specified, particularly for

sulphuric acid. This lead is resistant to sulphuric, chromic, hydrofluoric,

(22)

and phosphoric acid. It is rapidly attacked by acetic acid and generally not

used in nitric, hydrochloric, and organic acids(46).

Fig. (1-8)

Model of anodic layer

(a) In the lead sulphate reagion.

(b) In the lead monoxide region

(c) In the lead dioxide region

1.4- The Literature Survey

A study of the effect of corrosion of lead and lead alloys on the

performance of the batteries due to sulphuric acid concentration, is of

fundamental importance for increasing the useful life of these batteries(50).

Tedeschi (51) found that the rate of dissolution of lead prepared either

by the reduction of PbO2 or PbO increases with the concentration of

sulphuric acid solution.

(23)

Pourbaix(52) expected on the basis of potential –PH diagrams, that in

storage batteries of more than 6N. H2SO4 the solubility of the positive

electrode is greater than the negative electrode owing to the formation of

Pb4+ ions.

Lander(53) subjected lead to anodic corrosion at potentials near the

reversible PbO2/PbSO4. Results indicate that the first step in the corrosion

process was reaction of lead with water to form lead dioxide. Its potentials

just below the reversible PbO2/PbSO4 potential, the corrosion of lead

dioxide film to lead sulphate takes place.

Casey(54) described three modes of reaction of lead in sulphuric acid

depending on the acid strength, temperature, and the composition of the

lead. Firstly a slight attack with vigorous evolution of hydrogen and finally

complete decomposition with the evolution of sulphur dioxide.

Corrosion rate of refined lead in 50 to 80% sulphuric acid was

reported by Hohlstein and Pelzell who established the conditions of

passivation(55).

Local action increases rapidly when the concentration of the acid is

increased particularly for the negative plate. The temperatures to which the

battery is subjected in service have an important bearing on the specific

gravity of sulphuric acid. Battery exposed to low temperatures, such as

automobile batteries in cold climates, require a high density of acid to

permit their capacity to be utilized without depleting their electrolyte to so

low specific gravity that freezing occurs. On the other hand, batteries for

use in hot climates require a lower specific gravity because of the increased

chemical activity at the higher temperature(56).

Abdul Azim(57) in the course of studying the behaviour of Pb-Ca

alloys reported that the passive current for pure concentration increase as

in sulphuric acid concentration increases from 0.1 to 10 N.

(24)

Lead resists dilute sulphuric acid, even in presence of oxygen, owing

to the low solubility of lead sulphate (58).

Many materials, which exhibit passively effects, are only negligibly

affected by wide change in corrosive concentration. Other materials show

similar behaviour expect at very high corrosive concentration when

corrosion rate increases rapidly, lead shows this effect due to the fact that

lead sulphate, which forms a protection in low concentration of sulphuric

acid, is soluble in concentrated sulphuric acid(46).

Boctor(50) found that increasing temperature or sulphuric acid

concentration increases the rate of self-discharge.

Self discharge of positive plates is due to reaction between PbO2 in

the active material and Pb in the grid(59).

Self discharge of negative plates is due to the reaction between

sulphuric acid and the spong-lead, producing hydrogen gas and PbSO4(50).

Antimony was introduced into the electrode system either by alloying

it with the metal or by adding it to the H2SO4 solution. It was established

that Sb lowers the oxygen over voltage and increases the rate of anodic

corrosion of lead irrespective of the way in which it was introduced into the

system (60).

Study of electrodes of different active mass layer thickness shows that

with increase in thickness the corrosion rate decreases the corrosion rate

decreases(61).

Chloride in the electrolyte of lead-acid batteries has long been thought

to cause early failure due to accelerated corrosion of the positive-plate

group. This study investigates the effect of chloride species, added as either

hydrochloric acid or sodium chloride(62).

In the presence of H3PO4, the formation of soluble phosphate species

causes the decrease of corrosion layer thickness. Higher than 0.9%

concentration of H3PO4 negatively affect the behaviour of the electrodes,

(25)

higher potentials being required for the oxidation of PbSO4 to PbO2, when

the rate of oxygen evolution is also higher. Addition of FeSO4 with H3PO4

to the electrolyte as a Fe2+ ions prevents formation of Pb(IV) soluble ion

which is undesirable(63,64).

Takao(65) examined the effects of temperature, the concentration of

sulphuric acid, and the configuration of test specimens on negative

electrode corrosion. The reason for this corrosion seems corroded areas are

covered with electrolyte film that has a high resistance, so, they cannot be

polarized to the full cathodic protection potential.

Boctor(66) used an electrometric method for evaluation of the

corrosion of lead alloys, the lead electrode is subjected to electrolyte and

temperature condition , as well as to various states of polarization that

simulate the service of lead-acid batteries. The resulting corrosion layer is

first reduced to lead sulphate, then to sponge lead. A linear relation is

observed between the weight of the corroded lead and the surface area of

the sponge lead after cathodic reduction of the corrosion layer.

Dragan (67) studied the effect of Sn and Ca doping on the corrosion of

Pb anodes in lead-acid batteries and show that a small amount of Sn and Ca

which was deposited on Pb by electrodeposition minimizes the weight of

the anode corrosion.

1.5-The Object and Scope of the Present Research

The subject of this research included a number of important aspects

which may be summarized as:

1. Potentiostatic investigation of the corrosion behaviour of seven types

of specimens of lead-acid battery plates at three concentrations (0.1,

0.25 and 0.56 mol.dm-3) of stirred and un-stirred oxygenated sulphuric

acid solution, and also with stirred and un-stirred deaerated sulphuric

acid solution, at three temperatures 298, 308 and 318 K.

(26)

2. The additive effect of phosphoric acid(11g), mixture of (Phosphoric

acid(11g) + Ferrous Sulphate(0.2g)), Ferrous Sulphate (0.2g) and

sodium chloride(4g) in 1 litre of sulhuric acid has been tested for the

corrosion of four type of the following specimens.

1. Lead alloy electrode.

2. A cured positive plate of the battery.

3. Grid lead electrode.

4. A cured negative plate of the battery.

In stirred and un-stirred oxygenated sulphuric acid(0.56M) at 298K.

3. Investigation of the effect of oxygen and different media on the

corrosion and passivity of the battery plates.

4. Study of the thermodynamic quantities (DG, DH and DS) for the

corrosion of four type of the battery plates.

5. The kinetic study aspects of the corrosion of the four type of battery

plates have been investigated and the activation energies and pre-

exponential factors for the corrosion process have been determined.

(27)

2.1-The Experimental Set-Up

This instrument consists of a source of potential (an electronic

voltmeter) and a current source(68). The potentiostat measures the potential

V of the test electrode under study and compares this with the preselected

value V* from the potential source.

If there is a difference dV= V*- V between the measured and the

chosen potentials, potentiastate tells its current source to send a current i

between the auxiliary and the test electrode. The direction and magnitude

of this current is electronically chosen to keep the potential of test electrode

at the desired value, i.e, to make

dV= V*-V= 0(69).

The experiments on the electrodes in H2SO4 solution were performed

using a potentiostat of the type PRI 10-0.5L, which was obtained from sole

Tacussel (France) which had an output voltage of ± 10V and output

current of ± 500 mA and a response time of (2-3) ms.

The potentiostat was connected to a potentiostatic recorder, type EPL-

2B with an interchangeable plug- in pre-amplifier, type EPRL2, which

enabled the working electrode current to be recorded in either linear or

logarithmic coordinates. The potentiostat, which was termed commercially

as “ corroscript” contained a digital electronic millivoltmeter, type

MVN79. This instrument is intended for highly accurate potential

measurements from a few millivolts to some tens of volts, across sources of

very high resistance, all organized in a particularly way(70).

A simple electronic lay-out of the potentiostat is shown

diagrammatically in Fig. (2-1). The potential of the working electrode , Et,

is measured against another electrode Er, called reference electrode . A

third electrode, Ea, called the auxiliary electrode allows the electrical

(28)

current necessary to produce the desired potential difference to flow

through the circuit(71).

Fig. (2-1): The modern electronic instruments of potentiostate. Where : Ea = Auxiliary electrode, Er = Reference electrode, Et = Working electrode.

The working and the auxiliary electrodes are connected to the output

terminals of the potentiostat current through the circuit is automatically

controlled so that the potential difference between the working electrode

and the reference electrodes takes the desired value.

This process is carried out by means of a differential amplifier Ad,

one output of which e1, is connected to the reference electrode and the other

output, e2, to voltage source called pilot voltage (or control voltage). The

amplifier derives power, Ap, which controls the output current of the

potentiostat in such a manner that the potential difference between the

working electrode and the reference electrode remains equal to the applied

voltage, Ec.(72) .

(29)

2.2- The working Electrode

Seven types of specimens of different working electrodes have been

examined and these involved:

1. A spectroscopically standardized lead specimen which was obtained

from Johnson Matthery Co. Ltd (U.K).

2. A lead–antimony alloy, containing 2.7 wt % antimony. Such alloy is

used in Bable factories for industrial synthesis of lead oxide by Barton-

pot and Ball-Mill methods.

3. A lead-antimony alloy, containing more than 6% antimony. Such alloy

is used in Bable factories for industrial preparation of the grids of the

lead-acid battery plates.

4. Un-cured negative plate of the battery. This represented a grid, which

was coated with the paste of the negative plate prior to curing.

5. Un-cured positive plate of the battery. This represented a grid which

was coated with the paste of the positive plate and prior to curing stage.

6. A cured negative plate of step (4).

7. A cured positive plate of step (5).

Specimens of the steps (2-7) have been obtained directly from Bable

factory for manufacturing lead acid storage batteries in Baghdad.

The working electrode of the corrosion cell was made of plate

material of the battery (steps 1 to 7). The exposed surface area of the

material was circular with an apparent area of 1cm2. The working electrode

specimen of plate material was mounted in an appropriate plastic holder so

that a surface area of 1cm2 of the plate material remained exposed to the

test solution (H2SO4) when the Specimen was immersed in such

solution(70).

(30)

2.3- The Auxiliary Electrode

The auxiliary electrode was prepared from a high purity platinum rod

stock with an exposed surface area of 1.8cm2 (73).

Platinized auxiliary electrode was used in the experiments due to its

large surface area and high catalytic activity. Platinization of the electrode

was made after cleaning the surface of the platinum electrode in hot aqua

regia (3 parts concentrated HCl and 1 part concentrated HNO3), washing,

and then drying. The electrode was then platinized by immersion in

solution consisting 3 percent chloroplatinic acid and 0.02 percent lead

acetate and electrolyzing at a current density of 40 mA/cm2 for 5 min (74,75).

The polarity was reversed every minute. Occluded chloride was

removed by electrolyzing in a dilute (10 percent) sulphuric acid solution for

5 min, with a reversal in polarity every minute. The electrode was

thereafter rinsed thoroughly and stored in distilled water. The electrode

which was obtained by this procedure had a longer life and was less

susceptible to poisoning due to the presence of lead acetate in its surface

coating(72).

(31)

2.4- The Reference Electrode

A saturated calomel reference electrode (SCE) was used throughout

the whole work. The calomel electrode consisted of mercury, mercurous

chloride and chloride ion.

Pt, Hg(l), Hg2Cl2(s) / Cl- -----(2-1)

The reduction reaction which occurs in the calomel electrode, may be

represented as

Hg2Cl2(s) + 2e 2Hg(l) + 2 Cl-

(aq) -----(2-2)

The electrode is usually brought in contact with the electrolyte

through a glass tubing as “Luggin Cappillary” which is filled by the test

solution. The tip of the luggin capillary is placed in the electrochemical cell

very close to the working electrode through a Luggin Capillary bridge

which was filled with test solution(72).

The Calomel electrode could be prepared by grinding calomel

(Hg2Cl2), mercury and a small quantity saturated KCl solution together and

placing the resultant slurry in a layer about 1cm thick on the surface of

mercury contained in a clean test tube. External contact to the mercury was

usually made by a platinum wire which was sealed to glass(73).

(32)

2.5-The Corrosion Cell

The cell was made of Pyrex glass of 1 liter capacity with appropriate

necks to fit the electrodes (Fig.2-2 ) and to permit the introduction of gas

inlet and outlet tubes. A 750 ml of the test solution (H2SO4) was transferred

into the corrosion cell which was immersed in a thermostat at 25oC

(±0.01).

The Luggin Capillary was filled with the test solution. The tip of the

Luggin capillary was placed as close as possible to the surface of the

working electrode(70). About 1 mm apart to minimize the IR drop effect.

The electrode assembly of the cell was completed and placed in the

appropriate position. The test solution was purged for 30-60 min with

oxygen free nitrogen gas (purity 99.9%) at a rate of 150 cm3/min to remove

oxygen from the solution(73).

(33)

Gas Inlet

Working Electrode

Gas Outlet

Reference Electrode

Auxiliary Electrode

Fig. ( 2-2):- A schematic diagram of the polarization cell.

(34)

2.6- Potentiostatic Measurement

The potentiostatic scan started about 1 hour after the electrodes

immersion in the test solution, beginning at about –2.0V and proceeded

through to +2.0V versus the saturated calomel electrode.

The potential scan was fixed at a rate of 0.3 mV min-1. The potential

variation was monitered against log(current density) on an x-y recorder.

The recorder was of EPL series potentiostatic recorder with

interchangeable plug-in pre-amplifier, type EPL2, which enabled the

working electrode current density to be recorded in either linear or

logarithmic coordinates.

Both the cathodic and anodic curves were obtained with decreasing

and increasing polarization, and this was repeated several times. The

polarization curve obtained involved several regions covering the cathodic,

anodic, passive and transpassive regions. Extensive data could be derived

from the detailed analysis of each polarization region. Tangents to the

anodic and cathodic Tafel regions were extrapolated to the point of

intersection (Fig.2-3) from which both the corrosion current density (ic)and

corrosion potential (Ec) were determined using the four-point method.(76)

Cathodic (bc) anodic (bn) Tafel slopes, transfer coefficients (a), polarization

resistances (Rp) together with other data could be derived from the

polarization curves.

The thermodynamic feasibility of the corrosion has been judged from

the values of the corrosion potentials and of their dependencies on

temperature. The kinetic parameters were obtained from the corrosion

current densities and of their dependencies on temperature.

Data have also been obtained regarding the potentials and current

densities corresponding to the passive and transpassive regions(73).

(35)

Fig.(2-3): A typical polarization curve showing Tofel, active-passive

transition, passive and transpassive regions and their corresponding potentials and current densities.

NOBEL

Et G Transpassive

passive EAP

E

EPP

EC C

ACTIVE ip ic icr

A

B

Cathodic

Active

D

Active-Passive Transition

Log Current Density

App

lied

Pote

ntia

l, E

(36)

2.7- The Experimental Techniques and Procedure

The investigation was carried out using the standard CORROSCRIPT

potentiostat (TACUSSEL, France).

It consisted of the following parts:

a) A transistorized potentiostat, type PRT. 10.0.5L.

b) A digital electronic millivoltmeter, type MVN79.

c) A potentiometer recorder, type EPL-2B.

The recorder was fitted with a plug-in amplifier, type TILOG101,

enabling currents to be plotted on either linear or logarithmic

coordinates (68).

A pilot unit, type SYNCHOSCRIPT, was fitted on the right hand side

panel of the recorder. This unit was basically a 10-turn potentiometer,

coupled via an electromagnetic clutch to the chart drive shaft. It was used

to sweep the control potential supplied to the potentiostat, the sweep was

tied to chart speed with a maximum sensitivity of 100 mv/ cm. By the use

of an optional driver unit, type DIDT, the sensitivity could be set at 25, 50

or 100 mv/ cm.

The experimental procedure which was based on the standard

reference method for making potentiostatic polarization measurement,

which was under the jurisdiction of ASTM committee G-1 on corrosion of

Metals (77), and involved the following steps:

1- The specimen was mounted on the electrode holder and was further

cleaned just, prior to immersion, by degreasing for 5 min in hot

benzene, followed by acetone.

2- One liter of the sulphuric acid solution at a given concentration was prepared from Bable Battery Manufacturing Company and distilled

(37)

water. A 750 ml of the desired solution was transferred to the clean test cell.

3- The temperature of the solution was brought to the desired value by immersing the test cell in a controlled temperature water both with a precision of ± 0.1oC, a temperature regulator called Temp-unit, type HAAKE-KT-33, was used.

4- The platinzed auxiliary electrodes, the Luggin bridge and other components were placed in the test cell by the usual procedures(78) the tip of the luggin capillary was placed as close as physically possible to the surface of the working electrode in the corrosion cell. The Luggin bridge was filled with the test solution and temporarily close the center opening with a glass stopper.

5- The solution, prior to immersion of the test specimen, was purged for a minimum of 1h with oxygen –free nitrogen gas (purity, 99.9%) at a rate of 150 cm3/min to remove oxygen from the solution. In some experiments, the solution prior to immersion of the test specimen, was purged for a minimum of 1h with pure oxygen gas (purity, 99.9%) at a rate of 150 cm3 min.

In a series of experiments the solution was stirred at a constant rate in the range from 200 to 800 rpm.

6- The potential scan started 1h after the specimen immersion in the acid solution, beginning at about –2.0 V and proceeded through to + 2.0V versus the saturated calomel electrode (SCE). A potential against log (current density) was recorded by x-y recorder at a potential scan rate of 0.3V min-1.

Selected specimens at the end of the test were taken from the corrosion cell, rinsed carefully with distilled water and left to dry in a desicator for a bout 6 hour.

2.8 -The Chemicals

(38)

Sodium chloride was purum grade, obtained from Fluka, with a purity 99.5%.

Analar grade ferrous sulphate has been obtained from BDH, with purity exceeding 99%.

Ortho phosphoric acid 90% of 1.84 gm/ml density produced from Fluka.

Sulphuric acid solution with specific gravity of 1.4,which was obtained from Bable Battery Manufacturing Company .

(39)

3.1- The Polarization Curves

Figs. (3.1) to (3.7) show typical polarization curves for the corrosion

of seven types of lead specimens in 0.56M sulphuric acid at 298K.

In describing the various parts and regions of the polarization curves,

the following symbols have been adopted.

c, for the cathodic Tafel region,

a, for the anodic Tafel region,

p, for the passive region, and in cases where two passive regions were

present symbols p1 and p2 were used,

b, for the onset of the breaking of the passive layer.

It is also worthwhile to refer briefly to the processes, which take place

at various regions as follows:

c, also for the cathodic Tafel region in which reduction of hydrogen ions

occurs with subsequent evolution of hydrogen gas on the electrode surface.

a, also for the anodic Tafel region in which metal dissolution takes place.

Oxidation of OH- ions may also take place resulting in the evolution of

oxygen gas. Such a gas may be captured by the surface metal atoms

resulting in metal oxidation on passivation.

p, also for the passive region which involves the formation of an oxide

layer on the metal or the substrate surface. The passive layer may undergo

a chemical change giving rise to more than one passivity region (p1, p2,…).

b, also for breaking of the passivity corresponding to the onset of the

repture of the passive layer resulting in the liberation of the metal and

oxygen gas. The process is usually accompanied with the evolution of

oxygen gas.

Tables (3.1) to (3.14) present values of the corrosion current densities,

ic(A cm-2), corrosion potentials, Ec (volt), passivity current densities,

ip(A cm-2), passivity potentials, Ep (volt), cathodic, bc, and anodic, ba, Tafel

(40)

slopes (volt decade-1), cathodic, αc, and anodic, αa,transfer coefficients and

polarization resistances, Rp (W cm-2) for the polarization of the working

electrodes in different media in sulphuric acid solution at different

concentrations, c(mol dm-3) in the absence of additive at three temperatures

in the rang 298-318 K.

(41)

(42)

(43)

(44)

(45)

3.2- Results of the Polarization Curves

The corrosion potential (Ec) of a material in a certain medium at a

constant temperature is a thermodynamic parameter which is a criterion for

the extent of the corrosion feasibility under the equilibrium potential

(in opposite sign) of the cell consisting of the working electrode and the

auxiliary electrode when the rate of anodic dissolution of the working

electrode material becomes equal to the rate of the cathodic process that

takes place on the same electrode surface.

When Ec becomes more negative, the potential of the Galvanic cell

becomes more positive and hence the Gibbs free energy change (DG) for

the corrosion process becomes more negative. The corrosion reaction is

then expected to be more spontaneous on pure thermodynamic ground.

When the measured value of Ec becomes less negative, the potential of the

corresponding Galvanic cell becomes less positive, hence the (ΔG) value

for the corrosion process becomes less negative, and the process is thus less

spontaneous.

It is thus shown that Ec value is a measure for the extent of the

feasibility of the corrosion reaction on purely thermodynamic basis. Values

of Ec for the different electrode materials in four different media are

presented in tables (3.1–3.14) and are also plotted as in Figs.(3.8–3.18).

The corrosion current density (ic) on the other hand is a kinetic

parameter and represents the rate of corrosion under specified equilibrium

condition. Any factor that enhances the value of ic results in an enhanced

value of the corrosion rate (ic) on pure kinetic ground. Tables (3.1 – 3.14)

and Figs. (3.19- 3.29) give values of ic which have been derived from the

data of the polarization curves of the different working electrodes in the

four different corrosion media at different temperatures.

(46)

Other data have been obtained from the polarization curves which are

presented in the tables (3.1 – 3.14). These involved the cathodic (bc) and

anodic (ba) Tafel slopes and the corresponding cathodic (αc) and anodic (αa)

transfer coefficients. If the polarization curve involves a passivity region,

then values of the passive potential (Ep) and passive current density (ip)

may be obtained from the appropriate point in the passivity regions. Values

of Ep and ip have also been inserted in the data of tables (3.1– 3.14).

The results of Ec, ic, bc, ba, αc, αa, Ep, and ip which have been derived

from the polarization curves which have been given in tables (3.1 – 3.14)

will be further treated and discussed in the subsequent topics.

(47)

Table(3-1):Values of corrosion current densities, ic(A cm-2), corrosion potentials, Ec (volt), Passivity current density, ip(A cm-2), passivity potentials, Ep(volt), cathodic, bc, and anodic, ba, tafel slopes(volt decade-1) ,cathodic, αc, and anodic, αa, transfer coefficients and polarization resistance, Rp(W cm-2) for polarization of lead alloy working electrode in oxygenated sulphuric acid solution at different concentrations, c(mol dm-3) in the absence of additive .

C T/K medium ic/10-4 -Ec ip/10-5 Ep ba aa -bc ac Rp

0.56M

298

stirred

2.75 0.513 7.20 0.820 0.04 1.38 0.16 0.37 53.46

308 2.90 0.510 8.20 0.800 0.05 1.16 0.19 0.32 61.98

318 4.00 0.500 12.00 0.790 0.06 1.04 0.24 0.26 52.71

0.25M

298

stirred

2.75 0.520 7.40 0.810 0.04 1.41 0.45 0.13 60.33

308 2.80 0.517 7.70 0.800 0.04 1.48 0.48 0.13 59.05

318 4.00 0.500 9.00 0.770 0.05 1.27 0.54 0.12 49.52

0.1M

298

stirred

2.60 0.530 7.00 0.770 0.05 1.16 0.16 0.37 64.52

308 2.70 0.510 7.20 0.740 0.06 0.98 0.17 0.36 73.37

318 3.20 0.500 9.40 0.730 0.06 1.12 0.19 0.34 58.59

0.56M

298 un-

stirred

0.65 0.516 7.30 0.610 0.03 1.76 0.43 0.14 207.49

308 1.40 0.513 7.80 0.580 0.05 1.23 0.55 0.11 141.75

318 1.70 0.510 8.60 0.560 0.05 1.37 0.84 0.08 111.23

0.25M

298 un-

stirred

0.55 0.520 12.00 0.560 0.05 1.09 0.38 0.16 374.74

308 1.30 0.510 9.20 0.580 0.05 1.15 0.56 0.11 162.30

318 1.65 0.500 9.40 0.620 0.05 1.18 0.66 0.10 129.94

0.1M

298 un-

stirred

0.50 0.510 11.00 0.620 0.05 1.28 0.45 0.13 362.50

308 1.20 0.500 11.50 0.600 0.10 0.61 0.59 0.10 308.23

318 1.60 0.480 12.50 0.580 0.11 0.57 0.62 0.10 256.71

(48)

Table(3-2):Values of corrosion current densities,ic(A cm-2), corrosion potentials, Ec (volt),Passivity current density, ip(A cm-2),passivity potentials, Ep(volt), cathodic, bc, and anodic, ba, tafel slopes (volt decade-1), cathodic,αc,and anodic, αa,transfer coefficients and polarization resistance,Rp(W cm-2) for polarization of lead alloy working electrode in deaerated sulphuric acid solution at different concentrations, c(mol dm-3)in the absence of additive .

C T/K medium ic/10-5 -Ec ip/10-5 Ep ba αa -bc αc Rp

0.56M

298

stirred

7.5 0.518 6.80 1.010 0.07 0.84 0.16 0.36 284.50

308 8.1 0.514 7.80 0.910 0.08 0.76 0.16 0.37 289.55

318 9.5 0.510 8.50 0.750 0.08 0.82 0.20 0.31 254.69

0.25M

298

stirred

7.2 0.510 6.20 0.670 0.05 1.29 0.18 0.33 220.76

308 8.2 0.506 6.90 0.640 0.07 0.92 0.21 0.30 264.71

318 9.6 0.500 7.00 0.610 0.12 0.54 0.23 0.27 350.29

0.1M

298

stirred

7 0.500 6.00 0.630 0.07 0.79 0.24 0.25 353.36

308 7.9 0.490 7.90 0.620 0.12 0.51 0.26 0.23 452.11

318 8.3 0.480 8.50 0.610 0.15 0.43 0.31 0.20 518.03

0.56M

298 un-

stirred

5 0.519 5.70 0.980 0.04 1.47 0.44 0.13 320.66

308 6 0.510 9.00 0.930 0.05 1.30 0.22 0.28 279.89

318 10 0.500 9.60 0.900 0.05 1.21 0.45 0.14 202.00

0.25M

298 un-

stirred

2.55 0.520 6.00 1.000 0.04 1.34 0.21 0.29 617.43

308 4.8 0.510 6.90 0.960 0.06 0.95 0.23 0.26 455.70

318 6 0.500 7.50 0.880 0.07 0.93 0.40 0.16 418.86

0.1M

298 un-

stirred

1.65 0.518 6.20 0.780 0.09 0.69 0.47 0.13 1898.48

308 4.8 0.510 6.60 0.770 0.09 0.69 0.48 0.13 679.50

318 5.2 0.500 8.00 0.760 0.10 0.63 0.49 0.13 690.42

(49)

Table(3-3):Values of corrosion current densities,ic(A cm-2), corrosion potentials, Ec (volt),Passivity current density, ip(A cm-2),passivity potentials, Ep(volt), cathodic, bc, and anodic,ba, tafel slopes (volt decade-1), cathodic, αc, and anodic, αa, transfer coefficients and polarization resistance, Rp(W cm-2) for polarization of pure lead working electrode in oxygenated sulphuric acid solution at different concentrations, c(mol dm-3) in the absence of additive .

C T/K medium ic/10-4 -Ec ip/10-5 Ep ba αa -bc αc Rp

0.56M

298

stirred

1.85 0.530 12.00 0.860 0.04 1.34 0.22 0.27 85.92

308 3 0.519 12.50 0.840 0.04 1.42 0.32 0.19 54.69

318 3.5 0.513 15.00 0.820 0.06 1.06 0.38 0.17 63.62

0.25M

298

stirred

1.65 0.520 4.50 0.790 0.07 0.85 0.21 0.29 136.88

308 2.6 0.510 5.00 0.780 0.07 0.90 0.29 0.21 92.21

318 3.2 0.500 8.70 0.770 0.07 0.86 0.42 0.15 84.88

0.1M

298

stirred

1.35 0.500 4.20 0.800 0.05 1.09 0.66 0.09 160.95

308 2.4 0.490 4.20 0.760 0.05 1.19 0.84 0.07 87.64

318 3.2 0.490 5.40 0.750 0.05 1.30 0.69 0.09 61.38

0.56M

298 un-

stirred

0.57 0.532 1.60 0.790 0.06 1.06 0.40 0.15 371.88

308 1.1 0.526 7.00 0.750 0.06 1.11 0.48 0.13 195.35

318 1.65 0.521 11.00 0.540 0.06 1.13 0.69 0.09 136.11

0.25M

298 un-

stirred

0.5 0.510 6.80 0.620 0.06 0.94 0.49 0.12 483.56

308 1 0.500 6.90 0.600 0.06 0.97 0.33 0.18 229.62

318 1.4 0.500 7.00 0.590 0.07 0.85 0.25 0.25 177.79

0.1M

298 un-

stirred

0.4 0.510 1.20 0.740 0.05 1.10 0.38 0.16 508.97

308 1 0.500 8.50 0.680 0.05 1.12 0.39 0.16 207.74

318 1.35 0.500 8.80 0.600 0.05 1.27 0.43 0.15 143.45

(50)

Table(3-4):Values of corrosion current densities,ic(A cm-2), corrosion potentials, Ec (volt), Passivity current density, ip(A cm-2), passivity potentials, Ep(volt), cathodic, bc, and anodic, ba, tafel slopes (volt decade-1), cathodic, αc, and anodic, αa, transfer coefficients and polarization resistance, Rp(W cm-2) for polarization of pure lead working electrode in deaerated sulphuric acid solution at different concentrations, c(mol dm-3) in the absence of additive .

C T/K medium ic/10-5 -Ec ip/10-5 Ep ba αa -bc αc Rp

0.56M

298

stirred

5.8 0.533 4.7 1.010 0.04 1.38 0.16 0.37 252.55

308 7.5 0.530 5.7 0.940 0.04 1.53 0.27 0.22 201.38

318 20 0.526 7 0.925 0.04 1.51 0.28 0.23 78.98

0.25M

298

stirred

4.8 0.520 6.5 0.650 0.07 0.84 0.11 0.52 388.00

308 5 0.510 7 0.630 0.10 0.61 0.13 0.48 486.87

318 7.5 0.500 8 0.610 0.41 0.15 0.15 0.42 632.38

0.1M

298

stirred

3.3 0.520 5.5 0.630 0.09 0.69 0.72 0.08 1011.65

308 3.5 0.510 6 0.610 0.09 0.66 0.89 0.07 1033.29

318 7.2 0.500 6.4 0.610 0.21 0.30 0.95 0.07 1035.46

0.56M

298 un-

stirred

3.3 0.536 5 1.000 0.11 0.55 0.21 0.29 922.61

308 5.6 0.517 6 0.950 0.11 0.54 0.43 0.14 696.82

318 16.5 0.510 6.5 0.810 0.12 0.53 0.45 0.14 247.67

0.25M

298 un-

stirred

5 0.510 4.5 0.950 0.05 1.21 0.38 0.16 375.49

308 5.4 0.500 4.8 0.870 0.04 1.53 0.44 0.14 293.74

318 8.5 0.510 6 0.920 0.03 1.90 0.66 0.10 161.51

0.1M

298 un-

stirred

1.3 0.530 5.1 0.870 0.03 1.71 0.35 0.17 1050.71

308 3.8 0.510 6 0.790 0.04 1.65 0.53 0.12 395.89

318 7.4 0.500 6.7 0.710 0.07 0.88 0.54 0.12 370.03

(51)

Table(3-5):Values of corrosion current densities,ic(A cm-2), corrosion potentials, Ec (volt), Passivity current density, ip(A cm-2), passivity potentials, Ep(volt), cathodic, bc, and anodic, ba, tafel slopes (volt decade-1), cathodic, αc, and anodic, αa, transfer coefficients and polarization resistance, Rp(W cm-2) for polarization of grid lead working electrode in deaerated sulphuric acid solution at different concentrations, c(mol dm-3) in the absence of additive .

C T/K medium ic/10-4 -Ec ip/10-5 Ep ba αa -bc αc Rp

0.56M

298

stirred

10 0.527 6.8 1.080 0.08 0.78 0.21 0.28 241.56

308 11 0.521 7.2 0.950 0.09 0.72 0.23 0.26 246.28

318 14 0.520 7.6 0.920 0.09 0.68 0.24 0.26 207.02

0.25M

298

stirred

4.5 0.510 8 0.710 0.07 0.85 0.13 0.47 430.43

308 6.5 0.500 9 0.680 0.09 0.66 0.16 0.39 387.60

318 8 0.500 1 0.640 0.10 0.63 0.16 0.39 334.68

0.1M

298

stirred

5.1 0.500 7.5 0.970 0.06 0.91 0.62 0.10 499.76

308 6 0.490 7.8 0.840 0.07 0.92 0.75 0.08 441.65

318 7 0.490 10 0.840 0.10 0.61 0.95 0.07 576.14

0.56M

298 un-

stirred

3.7 0.528 7.5 1.070 0.03 1.97 0.15 0.41 291.92

308 6 0.515 7.8 0.930 0.03 1.76 0.15 0.39 204.76

318 15 0.500 9 0.900 0.04 1.50 0.15 0.42 94.76

0.25M

298 un-

stirred

2.5 0.520 7.3 1.050 0.04 1.33 0.22 0.26 554.15

308 7 0.510 8 0.910 0.04 1.58 0.27 0.22 209.73

318 14 0.500 9.2 0.900 0.04 1.47 0.31 0.20 116.91

0.1M

298 un-

stirred

2.1 0.510 1.15 1.010 0.06 1.05 0.10 0.61 737.49

308 4.5 0.500 8 0.840 0.10 0.59 0.16 0.39 600.80

318 8 0.500 8.5 0.830 0.10 0.65 0.17 0.37 335.67

(52)

Table(3-6):Values of corrosion current densities,ic(A cm-2), corrosion potentials, Ec (volt),Passivity current density, ip(A cm-2), passivity potentials, Ep(volt), cathodic, bc, and anodic, ba, tafel slopes (volt decade-1), cathodic, αc, and anodic, αa, transfer coefficients and polarization resistance, Rp(W cm-2) for polarization of grid lead working electrode in oxygenated sulphuric acid solution at different concentrations, c(mol dm-3) in the absence of additive .

C T/K medium ic/10-4 -Ec ip/10-5 Ep ba αa -bc αc Rp

0.56M

298

stirred

2.6 0.521 75 0.920 0.06 0.95 0.30 0.19 86.32

308 3.5 0.510 91 0.890 0.04 1.42 0.59 0.10 49.62

318 5 0.500 96 0.850 0.06 1.07 0.67 0.09 46.86

0.25M

298

stirred

2.4 0.500 5.7 0.950 0.07 0.91 0.17 0.35 85.32

308 3.3 0.490 6 0.810 0.05 1.29 0.25 0.24 52.58

318 4.2 0.500 6.5 0.780 0.11 0.58 0.54 0.12 93.50

0.1M

298

stirred

2.2 0.500 5.8 0.890 0.07 0.90 0.21 0.29 98.35

308 3 0.500 6.5 0.820 0.08 0.81 0.36 0.17 90.04

318 3.5 0.500 7 0.800 0.09 0.73 0.62 0.10 94.13

0.56M

298 un-

stirred

0.63 0.523 7.5 0.970 0.05 1.15 0.50 0.12 321.49

308 0.95 0.510 8 0.890 0.05 1.30 0.54 0.11 198.05

318 1.3 0.510 8 0.850 0.05 1.33 0.59 0.11 146.67

0.25M

298 un-

stirred

5 0.520 9.5 0.860 0.05 1.19 0.30 0.20 371.15

308 9 0.500 10 0.790 0.19 0.33 0.49 0.12 651.78

318 15 0.500 11 0.600 0.29 0.22 0.75 0.08 618.31

0.1M

298 un-

stirred

7.5 0.500 6.3 0.910 0.06 0.92 0.27 0.22 301.45

308 8.8 0.490 7.5 0.790 0.06 1.04 0.33 0.18 247.20

318 15 0.480 11.5 0.780 0.06 1.14 0.38 0.17 139.76

(53)

Table(3-7):Values of corrosion current densities,ic(A cm-2), corrosion potentials, Ec (volt), Passivity current density, ip(A cm-2), passivity potentials, Ep(volt), cathodic, bc, and anodic, ba, tafel slopes (volt decade-1), cathodic, αc, and anodic, αa, transfer coefficients and polarization resistance, Rp(W cm-2) for polarization of cured negative working electrode in deaerated sulphuric acid solution at different concentrations, c(mol dm-3) in the absence of additive .

C T/K medium ic/10-2 -Ec ip/10-3 Ep ba αa -bc αc Rp

0.56M

298

stirred

1.2 0.474 5 0.810 0.05 1.31 0.34 0.18 1.65

308 1.1 0.470 5.2 0.800 0.05 1.35 0.34 0.18 1.58

318 1.35 0.480 5.5 0.770 0.05 1.33 0.38 0.17 1.35

0.25M

298

stirred

1.05 0.510 2.2 0.760 0.08 0.72 0.15 0.40 2.18

308 1 0.500 3.2 0.740 0.09 0.69 0.24 0.25 2.81

318 1.1 0.500 3.25 0.740 0.09 0.68 0.29 0.22 2.76

0.1M

298

stirred

0.375 0.520 2.75 0.770 0.05 1.22 0.17 0.34 4.38

308 0.42 0.510 2.8 0.750 0.07 0.91 0.31 0.20 5.70

318 0.525 0.510 3 0.730 0.07 0.96 0.38 0.17 4.64

0.56M

298 un-

stirred

1.1 0.478 4.8 0.780 0.05 1.25 0.17 0.35 1.61

308 1.55 0.470 5 0.760 0.08 0.73 0.20 0.31 1.66

318 1.6 0.470 5.6 0.740 0.08 0.80 0.21 0.31 1.54

0.25M

298 un-

stirred

1 0.520 2.75 0.770 0.07 0.83 0.22 0.27 2.35

308 1.4 0.520 3 0.740 0.08 0.80 0.31 0.20 1.90

318 1.5 0.510 3.5 0.720 0.09 0.72 0.43 0.15 2.10

0.1M

298 un-

stirred

0.45 0.500 2.9 0.770 0.05 1.09 0.27 0.22 4.38

308 0.5 0.500 3.2 0.780 0.05 1.13 0.27 0.22 3.93

318 0.61 0.500 3.5 0.750 0.09 0.74 0.30 0.21 4.71

(54)

Table(3-8):Values of corrosion current densities,ic(A cm-2), corrosion potentials, Ec (volt), Passivity current density, ip(A cm-2), passivity potentials, Ep(volt), cathodic, bc, and anodic, ba, tafel slopes (volt decade-1), cathodic, αc, and anodic, αa, transfer coefficients and polarization resistance, Rp(W cm-2) for polarization of cured negative working electrode in oxygenated sulphuric acid solution at different concentrations, c(mol dm-3) in the absence of additive .

C T/K medium ic/10-2 -Ec ip/10-3 Ep ba αa -bc αc Rp

0.56M

298

stirred

3 0.471 7.2 0.760 0.08 0.70 0.17 0.35 0.82

308 3.3 0.470 8.5 0.800 0.08 0.81 0.20 0.31 0.72

318 3.5 0.470 9.8 0.790 0.08 0.77 0.24 0.26 0.76

0.25M

298

stirred

1.3 0.520 3 0.760 0.10 0.59 0.14 0.44 1.92

308 1.4 0.520 2.5 0.750 0.10 0.61 0.20 0.30 2.06

318 1.5 0.510 2.8 0.740 0.11 0.57 0.21 0.31 2.07

0.1M

298

stirred

1.1 0.500 2.9 0.730 0.14 0.42 0.24 0.25 3.52

308 1 0.500 3 0.730 0.14 0.45 0.25 0.24 3.82

318 1.5 0.500 3.4 0.690 0.14 0.45 0.26 0.24 2.65

0.56M

298 un-

stirred

1.65 0.473 6.2 0.800 0.09 0.64 0.08 0.72 1.14

308 3 0.470 6.5 0.760 0.09 0.66 0.11 0.57 0.72

318 3.2 0.470 9 0.740 0.10 0.63 0.11 0.58 0.70

0.25M

298 un-

stirred

1.1 0.500 1.2 0.780 0.07 0.84 0.10 0.59 1.63

308 1.15 0.500 1.5 0.750 0.07 0.92 0.11 0.56 1.56

318 2.1 0.500 2.7 0.730 0.07 0.95 0.10 0.61 0.83

0.1M

298 un-

stirred

0.8 0.510 3.5 0.790 0.10 0.57 0.05 1.27 1.74

308 0.85 0.510 4.25 0.810 0.10 0.64 0.11 0.54 2.65

318 9 0.500 6.5 0.770 0.10 0.63 0.31 0.20 3.64

(55)

Table(3-9):Values of corrosion current densities,ic(A cm-2), corrosion potentials, Ec (volt),Passivity current density, ip(A cm-2), passivity potentials, Ep(volt),cathodic, bc, and anodic, ba, tafel slopes (volt decade-1), cathodic, αc, and anodic, αa, transfer coefficients and polarization resistance, Rp(W cm-2) for polarization of cured positive working electrode in deaerated sulphuric acid solution at different concentrations, c (mol dm-3) in the absence of additive .

C T/K medium ic/10-2 -Ec ip/10-3 Ep ba αa -bc αc Rp

0.56M

298

stirred

1.1 0.487 4.5 0.800 0.04 1.31 0.21 0.28 1.46

308 1.3 0.485 6.5 0.770 0.05 1.31 0.22 0.28 1.29

318 1.6 0.480 6.5 0.760 0.05 1.32 0.24 0.26 1.08

0.25M

298

stirred

0.85 0.520 4 0.800 0.06 1.02 0.13 0.45 2.05

308 0.9 0.510 4.9 0.780 0.06 1.11 0.19 0.32 2.06

318 0.93 0.500 5 0.770 0.06 1.07 0.19 0.33 2.10

0.1M

298

stirred

0.35 0.510 3.5 0.810 0.04 1.33 0.24 0.25 4.65

308 0.4 0.510 3.7 0.800 0.05 1.33 0.27 0.23 4.26

318 0.4 0.510 4.2 0.700 0.06 1.05 0.30 0.21 5.42

0.56M

298 un-

stirred

1 0.489 6.5 0.790 0.04 1.33 0.19 0.31 1.74

308 2.4 0.484 6.8 0.780 0.04 1.42 0.22 0.28 0.65

318 2.5 0.480 7 0.760 0.04 1.44 0.26 0.24 0.65

0.25M

298 un-

stirred

0.75 0.520 5.8 0.780 0.05 1.24 0.08 0.72 1.74

308 2 0.500 6.2 0.740 0.06 1.06 0.12 0.50 0.85

318 2 0.510 7 0.710 0.06 1.03 0.43 0.15 1.17

0.1M

298 un-

stirred

0.57 0.500 4.5 0.790 0.05 1.13 0.19 0.31 3.09

308 0.6 0.500 4.7 0.770 0.06 1.01 0.22 0.28 3.44

318 0.85 0.500 5.1 0.730 0.06 1.02 0.23 0.27 2.48

(56)

Table(3-10):Values of corrosion current densities,ic(A cm-2), corrosion potentials, Ec (volt), Passivity current density, ip(A cm-2), passivity potentials, Ep(volt), cathodic, bc, and anodic, ba, tafel slopes (volt decade-1), cathodic, αc, and anodic, αa, transfer coefficients and polarization resistance, Rp(W cm-2) for polarization of cured positive working electrode in oxygenated sulphuric acid solution at different concentrations, c(mol dm-3) in the absence of additive .

C T/K medium ic/10-2 -Ec ip/10-3 Ep ba αa -bc αc Rp

0.56M

298

stirred

2.3 0.482 7 0.780 0.07 0.87 0.08 0.76 0.68

308 3.5 0.480 8.5 0.770 0.11 0.57 0.10 0.59 0.65

318 3.7 0.480 8.8 0.750 0.08 0.76 0.05 1.22 0.37

0.25M

298

stirred

1.2 0.510 2.4 0.790 0.07 0.84 0.06 0.92 1.21

308 1.2 0.510 2.75 0.740 0.07 0.87 0.12 0.53 1.58

318 1.4 0.500 3 0.740 0.11 0.57 0.20 0.32 2.19

0.1M

298

stirred

0.9 0.500 3.25 0.760 0.10 0.59 0.09 0.63 2.33

308 1.2 0.500 4 0.740 0.10 0.61 0.15 0.42 2.15

318 1.35 0.500 4 0.720 0.12 0.54 0.18 0.34 2.30

0.56M

298 un-

stirred

1.6 0.485 3.7 0.780 0.08 0.71 0.08 0.72 1.13

308 1.7 0.480 4.1 0.770 0.10 0.59 0.09 0.67 1.24

318 1.9 0.480 5.5 0.750 0.10 0.64 0.09 0.69 1.08

0.25M

298 un-

stirred

1.1 0.500 4.75 0.780 0.08 0.76 0.09 0.68 1.62

308 1.5 0.510 5 0.750 0.08 0.81 0.13 0.48 1.37

318 1.5 0.500 5 0.740 0.09 0.73 0.14 0.44 1.55

0.1M

298 un-

stirred

0.95 0.500 5.2 0.770 0.09 0.69 0.11 0.52 2.23

308 0.65 0.500 5.6 0.730 0.09 0.71 0.12 0.52 3.33

318 1.2 0.500 5.75 0.720 0.14 0.45 0.15 0.43 2.61

(57)

Table(3-11):Values of corrosion current densities,ic(A cm-2), corrosion potentials, Ec (volt),Passivity current density, ip(A cm-2),passivity potentials, Ep(volt), cathodic, bc, and anodic, ba, tafel slopes (volt decade-1), cathodic, αc, and anodic,aa,transfer coefficients and polarization resistance, Rp(W cm-2) for polarization of uncured positive working electrode in deaerated sulphuric acid solution at different concentrations, c(mol dm-3) in the absence of additive .

C T/K medium ic/10-3 -Ec ip/10-3 Ep ba αa -bc αc Rp

0.56M

298

stirred

6.6 0.506 3.4 0.790 0.07 0.83 0.24 0.25 3.61

308 7.8 0.500 2.25 0.750 0.07 0.83 0.25 0.25 3.15

318 8.5 0.500 2.5 0.660 0.08 0.84 0.30 0.21 3.07

0.25M

298

stirred

3 0.510 2.25 0.740 0.07 0.80 0.28 0.21 8.49

308 4 0.500 2.3 0.720 0.08 0.80 0.31 0.20 6.67

318 7 0.510 2.4 0.710 0.08 0.84 0.33 0.19 3.81

0.1M

298

stirred

1.2 0.510 1.75 0.770 0.06 1.04 0.14 0.44 14.45

308 1.6 0.500 1.9 0.740 0.06 0.95 0.23 0.26 13.65

318 2.75 0.510 2 0.710 0.07 0.89 0.23 0.27 8.63

0.56M

298 un-

stirred

5.4 0.509 2.6 0.780 0.04 1.44 0.15 0.40 2.58

308 9.5 0.500 2.75 0.770 0.05 1.35 0.33 0.18 1.82

318 12 0.505 3.2 0.760 0.05 1.30 0.34 0.18 1.54

0.25M

298 un-

stirred

2.1 0.510 1.7 0.740 0.11 0.52 0.27 0.22 16.66

308 6.6 0.510 2.25 0.750 0.11 0.58 0.35 0.17 5.36

318 6.25 0.510 2.4 0.690 0.11 0.56 0.45 0.14 6.24

0.1M

298 un-

stirred

2 0.500 1.9 0.740 0.04 1.34 0.23 0.25 8.04

308 2.75 0.500 2 0.740 0.06 0.97 0.33 0.18 8.35

318 5.75 0.510 2.35 0.700 0.17 0.37 0.93 0.07 10.88

(58)

Table(3-12):Values of corrosion current densities, ic(A cm-2), corrosion potentials, Ec (volt),Passivity current density, ip(A cm-2), passivity potentials, Ep(volt), cathodic, bc, and anodic, ba, tafel slopes (volt decade-1), cathodic, αc, and anodic, αa, transfer coefficients and polarization resistance, Rp(W cm-2) for polarization of uncured positive working electrode in oxygenated sulphuric acid solution at different concentrations, c(mol dm-3) in the absence of additive .

C T/K medium ic/10-2 -Ec ip/10-3 Ep ba αa -bc αc Rp

0.56M

298

stirred

2.1 0.500 3.4 0.780 0.03 2.05 0.18 0.34 0.51

308 2.25 0.500 4.5 0.770 0.04 1.37 0.31 0.20 0.75

318 2.4 0.500 5 0.760 0.05 1.27 0.17 0.37 0.70

0.25M

298

stirred

0.93 0.510 3 0.770 0.06 1.03 0.10 0.59 1.70

308 1 0.500 3.4 0.750 0.06 1.08 0.31 0.20 2.07

318 1.1 0.500 3.4 0.730 0.06 1.08 0.33 0.19 1.96

0.1M

298

stirred

0.17 0.520 2 0.740 0.05 1.08 0.32 0.18 11.93

308 0.26 0.510 2.25 0.720 0.05 1.14 0.45 0.14 7.98

318 0.46 0.500 2.25 0.730 0.07 0.92 0.51 0.12 5.72

0.56M

298 un-

stirred

2 0.502 4.25 0.790 0.07 0.85 0.12 0.48 0.97

308 2.5 0.500 5 0.780 0.07 0.85 0.22 0.28 0.94

318 2.6 0.500 5.5 0.720 0.08 0.80 0.61 0.10 1.17

0.25M

298 un-

stirred

0.82 0.500 2.5 0.730 0.12 0.48 0.20 0.30 3.99

308 2.35 0.503 3.5 0.730 0.13 0.47 0.22 0.27 1.51

318 2.55 0.500 3.6 0.720 0.14 0.45 0.23 0.27 1.48

0.1M

298 un-

stirred

0.7 0.520 2 0.750 0.09 0.63 0.18 0.33 3.82

308 1.15 0.520 2.5 0.700 0.10 0.63 0.21 0.29 2.50

318 1.5 0.510 2.5 0.680 0.14 0.47 0.27 0.24 2.60

(59)

Table(3-13):Values of corrosion current densities, ic(A cm-2), corrosion potentials, Ec (volt), Passivity current density, ip(A cm-2), passivity potentials, Ep(volt),cathodic, bc, and anodic, ba, tafel slopes (volt decade-1), cathodic, αc, and anodic, αa, transfer coefficients and polarization resistance, Rp(W cm-2) for polarization of uncured negative working electrode in oxygenated sulphuric acid solution at different concentrations, c(mol dm-3) in the absence of additive .

C T/K medium ic/10-2 -Ec ip/10-3 Ep ba αa -bc αc Rp

0.56M

298

stirred

2 0.492 4.5 0.770 0.06 0.94 0.11 0.52 0.88

308 2.4 0.490 5.3 0.810 0.07 0.89 0.13 0.47 0.82

318 2.7 0.488 6.5 0.780 0.07 0.89 0.16 0.40 0.79

0.25M

298

stirred

1.3 0.530 1.35 0.810 0.11 0.55 0.11 0.52 1.86

308 2 0.530 1.75 0.750 0.11 0.54 0.12 0.49 1.28

318 2.6 0.520 1.9 0.740 0.13 0.48 0.16 0.38 1.22

0.1M

298

stirred

0.95 0.520 1.25 0.780 0.11 0.56 0.13 0.46 2.65

308 1.1 0.500 1.25 0.770 0.12 0.52 0.15 0.42 2.57

318 1.25 0.500 6.5 0.720 0.14 0.46 0.16 0.41 2.53

0.56M

298 un-

stirred

1.25 0.493 3.4 0.770 0.07 0.87 0.08 0.71 1.30

308 1.3 0.500 3.9 0.760 0.09 0.66 0.10 0.61 1.60

318 1.46 0.490 5 0.750 0.09 0.73 0.13 0.50 1.53

0.25M

298 un-

stirred

1.22 0.530 2 0.790 0.10 0.59 0.10 0.59 1.77

308 1.28 0.520 2.25 0.770 0.13 0.46 0.10 0.61 1.93

318 1.4 0.500 2.5 0.750 0.14 0.44 0.11 0.55 1.97

0.1M

298 un-

stirred

0.87 0.500 2.3 0.790 0.11 0.52 0.11 0.52 2.82

308 1 0.510 3.2 0.760 0.13 0.48 0.12 0.50 2.70

318 1.25 0.510 5.5 0.740 0.15 0.41 0.12 0.53 2.33

(60)

Table(3-14):Values of corrosion current densities,ic(A cm-2), corrosion potentials, Ec (volt), Passivity current density, ip(A cm-2), passivity potentials, Ep(volt), cathodic, bc, and anodic, ba, tafel slopes (volt decade-1), cathodic, αc, and anodic, αa, transfer coefficients and polarization resistance, Rp(W cm-2) for polarization of uncured negative working electrode in deaerated sulphuric acid solution at different concentrations, c(mol dm-3) in the absence of additive .

C T/K medium ic/10-2 -Ec ip/10-3 Ep ba αa -bc αc Rp

0.56M

298

stirred

1.05 0.495 4.25 0.770 0.05 1.16 0.15 0.39 1.38

308 1.9 0.495 4.9 0.760 0.05 1.14 0.19 0.33 0.95

318 2.2 0.490 5.5 0.730 0.07 0.96 0.25 0.26 1.02

0.25M

298

stirred

0.55 0.510 32 0.750 0.07 0.90 0.09 0.67 2.97

308 0.7 0.520 37.5 0.730 0.07 0.84 0.14 0.45 2.92

318 0.82 0.530 39 0.700 0.07 0.85 0.33 0.19 3.21

0.1M

298

stirred

0.53 0.510 2.2 0.780 0.06 0.98 0.14 0.44 3.42

308 0.6 0.510 2.85 0.770 0.06 0.97 0.17 0.36 3.32

318 0.7 0.510 3.25 0.690 0.08 0.82 0.26 0.24 3.70

0.56M

298 un-

stirred

0.9 0.497 4.1 0.760 0.08 0.76 0.10 0.58 1.73

308 2.75 0.491 3.8 0.800 0.08 0.74 0.11 0.57 0.74

318 2.7 0.490 4.5 0.770 0.08 0.77 0.15 0.42 0.86

0.25M

298 un-

stirred

1.1 0.510 2.85 0.780 0.07 0.87 0.13 0.47 1.75

308 1.5 0.510 3.1 0.780 0.07 0.86 0.16 0.38 1.42

318 1.6 0.500 4.1 0.740 0.08 0.81 0.31 0.20 1.68

0.1M

298 un-

stirred

1 0.520 2.5 0.790 0.08 0.71 0.09 0.68 1.85

308 1.2 0.510 2.8 0.770 0.09 0.65 0.27 0.23 2.53

318 1.4 0.500 4 0.760 0.09 0.72 0.38 0.17 2.20

(61)

3.2.1- Corrosion Potentials (Ec)

The results of Figs.(3.8-3.14) indicate that the sequence for the increasing negativity of the Ec values for the corrosion of the various working electrodes in the four different corrosion media was as:

4 > 3 > 2 >1 where the numbers refer to the different corrosion media:

1. for stirred oxygenated 0.56 M sulphuric acid, 2. for un-stirred oxygenated acid solution, 3. for stirred deaerated acid solution, and , 4. for un-stirred deaerated acid solution.

Thus, the corrosion attack on the different working electrodes was relatively more feasible on thermodynamic ground in un-stirred deaerated acid solution and less feasible in stirred oxygenated acid solution. The stirring and oxygenation of the acid solution may result in the formation of a protective passive oxide layer with ultimate depression of the thermodynamic feasibility for corrosion.

The results of Figs.(3.15-3.18) present different working electrode materials in the four different corrosion media. It is shown from the results of these figures that the sequence for the decreasing corrosion feasibilities in each medium was as:

2 > 3 > 1> 4> 6> 5> 7 where the numbers stand for:

1,lead alloy working electrode, 2,Grid lead, 3,Pure lead, 4,Uncured positive electrode, 5,Cured positive electrode, 6,Uncured negative electrode, and , 7,Cured negative electrode.

(62)

The grid lead showed greatest tendency for corrosion while the cured

negative electrode material had the least tendency. Curing of the positive or

the negative electrodes reduces its tendency for corrosion. The curing on

the other hand had greater influence on the reduction of the corrosion

tendency with negative electrode as compared with the positive electrode.

The lead alloy had less corrosion tendency in any corrosion medium than

grid lead. Pure lead on the other hand greater corrosion tendency than lead

alloy.

(63)

(64)

(65)

(66)

(67)

(68)

(69)

3.2.2- Corrosion Current Densities (ic)

Values of ic represent the corrosion rates of the working electrode material in the sulphuric acid solution at a constant temperature. The results of Figs.(3.19-3.25) indicate that the corrosion rates of all the electrode materials were highest in stirred oxygenated solution and lowest in un-stirred deaerated acid solution. The electrode materials differed in their corrosion rates in un-stirred oxygenated and stirred deaerated media. The largest corrosion rate in stirred oxygenated solution may be accounted for on the basis of the greater reactivity of the material surface towards oxygen. On the contrary, the smallest corrosion rate in un-stirred deaerated medium is expected when the corrosion medium is de-oxygenated (or deaerated).

The behaviours of the various electrode materials in each of the four corrosion media may also be compared with the aid of the Figs.(3.26-3.29). The corrosion rates of the materials in each medium may be presented in the following four sequences: sequence (1)- in stirred oxygenated acid solution (Fig. 3.26):-

7 > 5 > 4 > 6 > 1 > 3 >2 sequence (2)- in un-stirred oxygenated acid solution (Fig. 3.27):-

4 > 7 > 5 > 6 > 1 > 3 >2 sequence (3) – in stirred deaerated acid solution (Fig. 3.28):-

7 > 5 > 6 > 4 > 3 >1 > 2 sequence (4)- in un-stirred deaerated acid solution (Fig. 3.29):-

7 > 5 > 6 > 4 > 1 > 3 > 2 The largest corrosion rate was with cured negative electrode material in stirred oxygenated and in stirred and un-stirred deaerated acid solution, and with uncured positive electrode material in un-stirred oxygenated solution. The lowest corrosion rate was attained in all the four different corrosion media with grid lead material.

(70)

(71)

(72)

(73)

(74)

(75)

(76)

3.2.3- Passive Potentials (Ep)

Passivity is an unusual phenomenon observed during the corrosion of

certain metals and alloys, it can be defined as a loss of chemical reactivity

under certain environmental conditions (79).

Tables (3.1) to (3.14) show values of Ep in each case for the different

working electrodes in the absence of additives decreased with increasing

temperature in the four different corrosion media.

The sequence of the decreasing Ep values for the different working

electrodes in stirred oxygenated 0.56 M H2SO4 solution was as following:

2 > 3 > 1 > 4 > 5 > 6 >7

and in un-stirred oxygenated 0.56 M H2SO4 solution was as:

2 > 1 > 7 > 4 > 3 > 5 > 6

in stirred deaerated acid solution the sequence was as:

2 > 3 > 1 > 7 > 5 > 4 > 6

and in un-stirred acid solution the sequence was as:

2 > 3 > 1 > 5 > 4 > 7 > 6

The largest passive potential was acquired in all the four different

corrosion media with grid lead material. The lowest passive potentials were

attained in stirred and un-stirred deaerated and un-stirred oxygenated acid

solution with uncured negative electrode, and with cured negative in

stirred oxygenated acid solution.

The passive potentials of a passive film on a metal or alloy depend

upon the nature of the metal or the alloy, it becomes more or less positive

depending on the stability of the existing oxide film. The presence of

certain anions destroys the passivity and results in localized corrosion(80)(81).

(77)

3.2.4 – Passive Current Densities (ip)

Values of ip represent the passive current densities of the working

electrode material in the sulphuric acid solution at a constant temperature.

The behaviours of the various electrode materials in each of the four

corrosion media may also be compared with the aid of the tables (3.1) to

(3.14). The passive current densities of the working electrodes in each

medium may be presented in the following four sequences:

sequence (1)- in stirred oxygenated acid solution:

7 > 5> 6> 4> 2> 1> 3

sequence (2)- in un-stirred oxygenated acid solution:

7 > 4 > 5 > 6 > 3 > 1 > 2

sequence (3)- in stirred deaerated acid solution:

7 > 5 > 6 > 4 > 3 > 1> 2

sequence (4)- in un-stirred deaerated acid solution:

4 > 6 > 7 > 5 > 3 > 1 >2

The largest passive current density was obtained with cured negative

electrode material in stirred and un-stirred oxygenated and in stirred

deaerated acid solution, and also with uncured positive electrode material

in un-stirred deaerated acid solution. The lowest passive current density

was obtained with grid lead electrode in stirred and un-stirred deaerated

and in un-stirred oxygenated acid solution, and also with pure lead

electrode material in stirred oxygenated acid solution.

The decreasing passive current density(ip) for the electrodes in each

sequence may be connected with the increasing stability of the oxide

films, while the increasing in ip for implies a decrease in the stability of the

oxide film which tends to dissociate at and close to the transpassive

potential (82).

(78)

3.3-Tafel Slopes and Transfer Coefficients

Values of the transfer coefficients for the cathodic (ac) and anoxic

(aa) processes have been calculated from the corresponding cathodic (bc)

and anodic (ba) Tafel slopes using the relationships (83)(84):

FbRT

cc

303.2=a ---- (3.1)

FbRT

aa

303.2=a ---- (3.2)

Tables (3.1) to (3.14) show the cathodic (bc) and anodic (ba) Tafel

slopes which are obtained from the polarization curves for the corrosion of

the electrode materials in deaerated and oxygenated solution of the

different sulphuric acid concentrations and temperatures.

Values of Tafel slopes (ba or bc) for the both anodic and cathodic

reactions were generally close to 0.120 V decade-1 and the corresponding

values of the transfer coefficients (aa and ac) were close to 0.5. The main

exception to this result was the relatively some higher or lower values of

the Tafel slopes (ba and bc) or of the transfer coefficients (aa and ac) for

certain specimens in sulphuric acid solutions. Increasing the temperature

from 298 to 318 K caused only a slight change in the values of ba and bc.

A value of the cathodic transfer coefficient (ac) of @ 0.5, or of the

cathodic Tafel slope of –0.120V decade-1, may be diagnostic of a proton

discharge-chemical desorption mechanism in which the proton discharge is

the rate- determining step (r.d.s).

The two basic reactions paths for the hydrogen evolution reaction are:

H3O+(bulk solution) H3O+ (metal / solution interface) ---- (3.3)

which is followed by the discharge step (D):

H3O+ + M + e M – H + H2O ----(3.4) D

diffusion

(79)

where M is the metal electrode and M-H represents a hydrogen atom which

is adsorbed on the metal surface. The discharge (D) step is usually

followed by a chemical desorption (C-D) step as:

M-H + M-H 2M + H2 .…(3.5)

in which two adjacent adsorbed hydrogen atoms unite together to form one

molecule of gaseous hydrogen. If the chemical desorption is the rate-

determining step, the rate would be independent of the overpotential since

no charge transfer occurs in such a step and the rate becomes directly

proportional to the concentration or the coverage (q) of adsorbed hydrogen

atoms. On the other hand, if the discharge process is followed by a rate-

determining step involving chemical desorption, the expected value of a

should be 2.0.

In some cases, the previous two steps (D) and (C.D) may unite

together to form one electrochemical desorption (E.D) step as:

M-H + H3O++ M(electrode) 2M + H2 + H2O .…(3.6)

When electrochemical desorption becomes the rate-determining step

for hydrogen evolution reaction on the casthode, the expected value of a

will be 1.5.

The results of the tables (3.1-3.14) indicate that the variation of the

Tafel slopes and of the corresponding transfer coefficients could be

interpreted in terms of the variation in the nature of the rate-determining

step from charge transfer process to either chemical–desorption or to

electrochemical desorption.

The variation of the anodic Tafel slopes (ba), or of the anodic transfer

coefficient (aa), as shown in tables (3.1-3.14) may be attributed to the

variation of the rate-determining step throughout the metal dissolution

reaction(85)(86).

Two mechanisms have been proposed for the formation of precursor

passive film on the materials. The first is the precipitation-oxidation

C-D

E-D

(80)

mechanism and the second is the solid state mechanism, the latter

mechanism would not be mass transfer affected, but would account for the

formation of the precursor film (87)(88).

3.4- Polarization Resistance

The polarization resistance, Rp, of according electrode is defined as

the slope of a potential (E)-current density (i) plot of the corrosion potential

(Ec) as :

Rp = 0 at )( , ®¶¶ hh

CTi ---- (3.7)

where h =E-Ec, is the extent of polarization of the corrosion potential

and i is the current density (c.d.) corresponding to a particular value of h.

From the polarization resistance, Rp the corrosion current density (c.d) ic

can be calculated as:

ic = pR

b ----(3.8)

where b is a combination of the anodic and cathodic Tafel slopes (ba,

bc) as (89)(90):

)(303.2 ca

ca

bbbb

+=b ---- (3.9)

For the general case, by inserting equation (3.8) into equation (3.9)

one obtains the so-called the stern-Geary equation(91):

ci )(303.2 ca

cap bb

bbR

+= ----(3.10)

The results of tables (3.1) to (3.14) show that the polarization

resistance for the corrosion of the working electrodes in an un-stirred

sulphuric acid solution is greater than its values in the stirred sulphuric

acid solution indicating an increase in the resistance o the interface in the

absence of stirring.

(81)

In general, the polarization resistance (Rp) decreased with increasing

sulphuric acid concentration, and Rp values of the first three types of the

working electrodes were greater than for the other working electrodes as

given in tables (3.1) to (3.14).

The polarization resistance (Rp) of the materials in each medium may

be presented in the following four sequences:

sequence (1)- in stirred oxygenated acid solution

2 > 3 > 1> 6 > 7 > 5 > 4

sequence (2)- in un-stirred oxygenated acid solution

3 > 2 > 1 > 6 > 7 > 5 > 4

sequence (3)- in stirred deaerated acid solution

1 > 3 > 2 > 4 > 7 > 5 > 6

sequence (4)- in un-stirred deaerated acid solution

3 > 1 > 2 > 4 > 5 > 6 > 7

The largest polarization resistance was obtained with pure lead

electrode in un-stirred oxygenated and in deareated acid solution, and with

lead alloy electrode in stirred deaerated acid solution and also with grid

lead in stirred oxygenated acid solution. The lowest polarization

resistance was obtained with uncured positive electrode in stirred and in

un-stirred oxygenated acid solution and also with uncured negative

electrode in stirred deaerated acid solution and also with cured negative in

un-stirred deaerated acid solution.

(82)

3.5- Thermodynamics of Corrosion

When a metal undergoes corrosion there is a change in Gibbs free

energy, DG, of the system, which is equal to the work associated with the

corrosion reaction. The performance of such a work is accompanied usually

by a decrease in the Gibbs free energy of the system, (-DG)(20).

If the metal tends to corrode, the work done ( the free energy change

of the corrosion process) may be expressed in terms of the corrosion

potential, Ec using the equation:

DG = - n FEc --- (3.11)

where F is the Faraday constant and n is the number of electrons involved

the corrosion reaction.

The equation indicates that the free energy change is directly measurable

from electrochemical corrosion potential determination. From the value of

DG at several temperatures, the change in the entropy (DS) of the corrosion

could be derived according to the thermodynamic relation:

-d(DG) / dT = DS ---- (3.12)

Values of DG are usually plotted against temperature (T); thus at any

temperature (T) the value of -d(DG) / dT is equal to DS which corresponds

the slope of the -DG versus T plot at a constant temperature.

The change in the free energy, DG, is related to DH, the change in the

enthalpy, and DS, the change in entropy, of the corrosion reaction at a

constant temperature, T, by the equation (72):

DG = DH - TDS ---- (3.13)

Tables (3.15) to (3.21) give values of the thermodynamics quantities

DG, DH and DS for the corrosion of all the working electrodes and indicate

that the values of DG and DH in the un-stirred oxygenated 0.56 M H2SO4

(83)

solution are more negative than the corresponding values in stirred

oxygenated 0.56 M H2SO4 solution while DS was more positive.

The more negative DG values implies generally a greater corrosion

feasibility on thermodynamic ground, while DH values indicate exothermic

or endothermic nature of the corrosion reaction.

The Gibbs free energies obtained for working electrodes, in stirred

and un-stirred oxygenated 0.56M H2SO4 solution as shown in tables

(3.15 –3.21) may be arranged in more negativity as in the sequence:

2 > 3 > 1 > 4 > 6 > 5 > 7

The sequence shows that the more corrosion feasibility was obtained

with grid lead electrode of more negative DG value, while the less

corrosion feasibility was obtained with the cured negative electrode, which

had less negative value of DG.

(84)

Table(3-15):Values of the thermodynamics quantities -ΔG, ΔH (k J mol-1) and ΔS(J mol-1K-1)for the corrosion of the lead alloy working electrode in (0.56M) oxygenated H2SO4 solution in the absence of additives.

Electrode T/K medium -ΔG ΔH ΔS

lead alloy 1

298

stirred

99.00 -61.63 98.80

308 98.42 -59.80 98.23

318 96.49 -56.62 96.32

298

un-stirred

99.58 -53.57 99.40

308 98.42 -50.87 98.26

318 96.49 -47.39 96.34

Table(3-16):Values of the thermodynamics quantities -ΔG,ΔH

(k J mol-1) and ΔS(J mol-1K-1)for the corrosion of the grid lead working electrode in (0.56M) oxygenated H2SO4 solution in the absence of additives.

Electrode T/K medium -ΔG ΔH ΔS

lead grid 2

298

stirred

100.68 -53.41 100.63

308 100.16 -49.65 100.00

318 99.00 -46.85 98.86

298

un-stirred

101.72 -70.96 101.45

308 101.51 -68.74 101.29

318 100.55 -66.71 100.34

(85)

Table(3-17):Values of the thermodynamics quantities -ΔG,ΔH (k J mol-1) and ΔS(J mol-1K-1)for the corrosion of the pure lead working electrode in (0.56M) oxygenated H2SO4 solution in the absence of additives.

Electrode T/K medium -ΔG ΔH ΔS

pure lead 3

298

stirred

100.55 -68.93 100.32

308 99.39 -66.71 99.17

318 98.42 -64.68 98.22

298

un-stirred

100.93 -80.82 100.66

308 100.35 -79.56 100.10

318 99.58 -78.12 99.34

Table(3-18):Values of the thermodynamics quantities -ΔG,ΔH

(k J mol-1) and ΔS(J mol-1K-1)for the corrosion of the uncured positive working electrode in (0.56M) oxygenated H2SO4 solution in the absence of additives.

Electrode T/K medium -ΔG ΔH ΔS

Uncured Positive

4

298

stirred

98.42 -69.67 98.19

308 97.46 -67.74 97.24

318 96.49 -65.81 96.29

298

un-stirred

96.88 -91.13 96.57

308 96.49 -90.55 96.20

318 96.49 -90.36 96.21

(86)

Table(3-19):Values of the thermodynamics quantities -ΔG,ΔH (k J mol-1) and ΔS(J mol-1K-1)for the corrosion of the cured positive working electrode in (0.56M) oxygenated H2SO4 solution in the absence of additives.

Electrode T/K medium -ΔG ΔH ΔS

Cured Positive

5

298

stirred

93.02 -35.51 92.90

308 92.63 -33.19 92.53

318 92.63 -31.26 92.54

298

un-stirred

93.60 -79.24 93.33

308 93.21 -78.37 92.96

318 92.63 -77.31 92.39

Table(3-20):Values of the thermodynamics quantities -ΔG,ΔH

(k J mol-1) and ΔS(J mol-1K-1)for the corrosion of the uncured negative working electrode in (0.56M) oxygenated H2SO4 solution in the absence of additives.

Electrode T/K medium -ΔG ΔH ΔS

Uncured Negative

6

298

Stirred

94.95 -80.59 94.68

308 94.56 -79.72 94.31

318 93.99 -78.66 93.74

298

un-stirred

95.14 -86.53 94.85

308 95.14 -86.24 94.86

318 94.56 -85.37 94.30

(87)

Table(3-21):Values of the thermodynamics quantities -ΔG,ΔH

(k J mol-1) and ΔS(J mol-1K-1)for the corrosion of the cured negative working electrode in (0.56M) oxygenated H2SO4 solution in the absence of additives.

Electrode T/K medium -ΔG ΔH ΔS

cured negative

7

298

stirred

90.90 -88.04 90.60

308 90.90 -87.94 90.61

318 90.70 -87.65 90.43

298

un-stirred

91.28 -85.53 91.00

308 90.90 -84.95 90.62

318 90.90 -84.76 90.63

(88)

3.6- Kinetics of Corrosion

The rate(r) of the corrosion of the working electrode material

increased with increasing temperature from 298 to 318 K and the behaviour

may be described by Arrhenius equation as (93):

r= A exp(-E/RT) ----(3.14)

where A and Ea are the Pre-exponential factor and the energy of activation

respectively. The value of (r) at any temperature (T) was taken to be

proportional to the corrosion current density (ic). The values of Ea were

derived from the slopes of the log(ic)(85) versus 1/T plots of Fig.(3.30),

while those of A were obtained from intercepts of the such plots at 1/T =

zero; values of A, were expressed in term of A cm-2, and have then been

converted into molecules per cm-2 per second(94).

Table (3.22) show the resulting values of Ea and A for the corrosion of

the working electrode material.

The activation energy values obtained from working electrodes, in

stirred oxygenated 0.56 M H2SO4 Solution as shown in table (3.22) may be

arranged in a sequence as:

2 > 3 > 5 > 1 > 6 > 7 > 4

The pre-exponential values may also be arranged as in the sequence:

2 > 3 > 1 > 5 > 6 > 7 > 4

The sequence of the activation energies and pre-exponential values in

the un-stirred oxygenated 0.56 M H2SO4 solution was as:

3 > 1 > 2 > 7 > 4 > 5 > 6

In stirred oxygenated 0.56 M H2SO4 solution, the highest value of the

activation energy and of the pre-exponential was found with the grid lead

electrode, while the lowest value of Ea and A was with obtained the

uncured positive electrode. In un-stirred oxygenated 0.56 M H2SO4

solution the highest value of Ea and A was with the pure lead electrode,

(89)

while the lowest value of Ea and of A was with uncured negative electrode.

Thus, the corrosion reaction proceeded on special surface sites, which are

associated with different energies of activation (Ea). When the corrosion

occurs on sites with low values of Ea, then log A is expected to be also low.

On the other hand, when the activation energy of the surface site was high,

the corresponding value of A was also high.

Two mechanism have been proposed for the corrosion of lead grids,

the first is based on the release of divalent lead ions (pb2+) through a porous

lead dioxide layer(50) and the second assumes the growth of a relatively

impervious lead dioxide layer through ionic diffusion(95)(96). Figs. (3.31–

3.37) show the influence of temperature on corrosion rates which are

expressed as corrosion current densities.

(90)

(91)

Table (3-22):Values of activation Energies(Ea/k J mol-1) , pre-exponential factors(A/molecules cm-2 s-1) and Entropy of activation(DS≠/J mol-1K-1) for the corrosion working electrodes in (0.56M)oxygenated H2SO4 solution in the absence of additives.

Electrode medium log A A Ea ΔS≠

lead alloy 1

stirred 9.47 2.96 X 10+9 14.64 -64.71

un-stirred 19.57 3.71 X 10+19 38.09 129.55

grid lead 2

stirred 13.97 9.34 X 10+13 25.72 22.39 un-stirred 15.73 5.41 X 10+15 28.56 56.14

pure lead 3

stirred 13.72 5.20 X 10+13 25.24 17.52 un-stirred 20.9 7.92 X 10+20 41.95 154.99

uncured positive

4

stirred 3.77 5.91 X 10+3 5.26 -172.79 un-stirred 5.8 6.38 X 10+5 10.41 -133.88

cured positive

5

stirred 9.29 1.82 X 10+9 18.87 -67.77 un-stirred 4.52 3.30 X 10+4 6.74 -158.48

uncured negative

6

stirred 6.4 2.52 X 10+6 11.84 -122.45 un-stirred 4.34 2.20 X 10+4 6.08 -161.88

cured negative

7

stirred 3.98 9.54 X 10+3 6.08 -168.80 un-stirred 12.4 2.54 X 10+12 26.30 -7.58

(92)

(93)

(94)

(95)

(96)

4.1- Results of the Polarization Curves:

Four types of working electrodes have been used in the research and

these involved:

1, cured positive electrode,

2, cured negative electrode,

3, lead alloy electrode, and ,

4, grid lead electrode.

Four different additives have been added separately into the 0.56M

sulphuric acid solution and these were:

1, H3PO4 (11g dm-3),

2, H3PO4 (11g dm-3) + FeSO4 (0.2g dm-3) mixture,

3, NaCl (4 g dm-3), and ,

4, FeSO4 (0.2 g dm-3).

Addition of the additive to the corrosion medium caused numerous

alterations in the polarization behaviours of the working electrodes.

These involved changes which occurred in the values of ic, Ec, ip, Ep,

ba, bc, aa, ac and Rp as compared with the corresponding values which have

been obtained in the absence of additives.

Tables (4.1) to (4.4) show results of the polarization curves for the

corrosion of the four types of the working electrodes, both in the stirred and

in the unstirred oxygenated 0.56M H2SO4 solution in the presence of

additive.

(97)

Table(4-1):Values of ic,Ec,ip,Ep,ba,bc,αa,αc and Rp for the polarization of lead alloy working electrode in 0.56M oxygenated sulphuric acid solution in the presence of additives .Symbols were defined in Tables(3-1 to 3-14).

additive T/K Medium ic/10-5 -Ec ip/10-4 Ep ba αa -bc αc Rp

H3PO4 (11g dm-3)

298 4 0.520 0.8 0.840 0.12 0.50 0.27 0.22 886.77

308 Un-stirred 6.2 0.515 0.98 0.810 0.12 0.49 0.23 0.27 560.23

318 7.4 0.513 1.5 0.800 0.17 0.36 0.14 0.47 447.03

298 8.5 0.519 1.3 0.790 0.07 0.81 0.24 0.25 286.39

308 Stirred 9 0.515 1.4 0.780 0.08 0.77 0.33 0.19 309.38

318 1.3 0.515 1.85 0.750 0.10 0.63 0.43 0.15 269.65

H3PO4 (11g dm-3)

+ FeSO4 (o.2g dm-3)

298 4.6 0.518 1 0.690 0.11 0.56 0.13 0.46 549.60

308 Un-stirred 6.4 0.517 2.1 0.690 0.13 0.46 0.14 0.45 452.86

318 8.6 0.515 2.3 0.670 0.15 0.43 0.17 0.37 398.39

298 9 0.517 2.7 0.700 0.04 1.39 0.17 0.35 164.45

308 Stirred 11 0.514 3.9 0.670 0.04 1.42 0.19 0.32 138.13

318 15 0.510 4.8 0.600 0.04 1.43 0.23 0.27 107.06

NaCl (4g dm-

3)

298 6.3 0.516 3.5 0.710 0.07 0.81 0.25 0.23 390.78

308 Un-stirred 12 0.514 4.3 0.700 0.08 0.79 0.27 0.22 219.22

318 15 0.513 6 0.700 0.09 0.73 0.29 0.21 193.51

298 19 0.513 5.2 0.690 0.07 0.82 0.22 0.27 123.92

308 Stirred 31 0.511 7.8 0.690 0.10 0.62 0.23 0.26 95.13

318 33 0.511 8.3 0.670 0.31 0.20 0.27 0.23 191.57

FeSO4 (o.2g dm-3)

298 5 0.517 2.5 0.640 0.05 1.30 0.15 0.39 304.38

308 un-stirred 5.3 0.517 3 0.630 0.05 1.29 0.16 0.38 298.44

318 6.8 0.515 5 0.620 0.05 1.22 0.17 0.37 253.27

298 12.5 0.515 3.6 0.630 0.07 0.88 0.16 0.37 164.69

308 stirred 13 0.513 4 0.630 0.09 0.66 0.18 0.34 203.11

318 20 0.511 5.3 0.630 0.11 0.57 0.26 0.24 168.45

(98)

Table(4-2):Values of ic,Ec,ip,Ep,ba,bc,αa,αc and Rp for the polarization of lead grid working electrode in 0.56M oxygenated sulphuric acid solution in the presence of additives.Symbols were defined in Tables(3-1 to 3-14).

Additive T/K medium ic/10-5 -Ec ip/10-4 Ep ba αa -bc αc Rp

H3PO4 (11g dm-3)

298 3 0.529 7 0.980 0.08 0.72 0.25 0.24 894.56

308 un-stirred 5 0.527 8 0.860 0.15 0.41 0.31 0.20 878.74

318 8.8 0.526 8.5 0.840 0.16 0.40 0.52 0.12 592.40

298 4.2 0.527 7.2 0.900 0.18 0.33 0.22 0.27 1014.59

308 stirred 7 0.525 7.9 0.850 0.23 0.26 0.28 0.22 793.62

318 9.6 0.524 9 0.840 0.35 0.18 0.29 0.22 710.72

H3PO4 (11g dm-3)

+ FeSO4 (o.2g dm-3)

298 4.6 0.528 8.9 1.030 0.04 1.43 0.37 0.16 350.19

308 un-stirred 6.2 0.527 9.3 0.880 0.05 1.28 0.22 0.27 275.61

318 9.1 0.525 9.8 0.860 0.10 0.63 0.21 0.31 322.63

298 10 0.526 8 0.930 0.04 1.53 0.57 0.10 156.78

308 stirred 12 0.525 8.1 0.870 0.04 1.42 0.59 0.10 115.86

318 16 0.524 10 0.840 0.04 1.51 0.93 0.07 108.72

NaCl (4g dm-

3)

298 12 0.522 33 0.760 0.03 1.89 0.16 0.37 183.07

308 un-stirred 7.5 0.521 43 0.760 0.04 1.55 0.17 0.36 185.20

318 9 0.520 57 0.750 0.04 1.53 0.20 0.32 164.01

298 12 0.521 55 0.740 0.12 0.51 0.20 0.30 263.65

308 stirred 15 0.520 8 0.730 0.15 0.40 0.25 0.25 275.31

318 22 0.519 9.6 0.720 0.18 0.35 0.45 0.14 251.59

FeSO4 (o.2g dm-3)

298 5 0.525 1.2 0.730 0.35 0.17 0.22 0.27 1179.48

308 un-stirred 6.5 0.524 2 0.700 0.41 0.15 0.24 0.25 1012.93

318 8.2 0.520 2.5 0.710 0.51 0.12 0.37 0.17 1128.33

298 11 0.523 25 0.720 0.09 0.63 0.25 0.24 266.46

308 stirred 15 0.523 27.5 0.710 0.10 0.61 0.28 0.22 212.73

318 16.5 0.522 33 0.610 0.12 0.52 0.31 0.20 229.78

(99)

Table(4-3):Values of ic,Ec,ip,Ep,ba,bc,αa,αc and Rp for the polarization of cured positive lead alloy working electrode in 0.56M oxygenated sulphuric acid solution in the presence of additives .Symbols were defined in Tables (3-1 to 3-14).

Additive T/K medium ic/10-5 -Ec ip/10-4 Ep ba αa -bc αc Rp

H3PO4 (11g dm-3)

298 5 0.539 1 0.830 0.05 1.13 0.12 0.50 3.15

308 un-stirred 5.5 0.539 1 0.820 0.05 1.23 0.14 0.45 2.89

318 6.3 0.538 2 0.810 0.06 1.05 0.22 0.29 3.25

298 9 0.537 1.2 0.810 0.05 1.21 0.15 0.39 1.79

308 stirred 10 0.537 1.3 0.790 0.06 1.11 0.19 0.32 1.86

318 13 0.536 1.7 0.760 0.08 0.80 0.22 0.28 1.95

H3PO4 (11g dm-3)

+ FeSO4 (o.2g dm-3)

298 5.6 0.527 1.4 0.840 0.09 0.67 0.11 0.56 3.72

308 un-stirred 6 0.526 1.52 0.840 0.10 0.61 0.11 0.57 3.75

318 6.7 0.525 1.9 0.830 0.17 0.37 0.11 0.56 4.41

298 9 0.521 1.12 0.790 0.05 1.22 0.11 0.54 1.62

308 stirred 15 0.520 1.25 0.770 0.07 0.93 0.13 0.47 1.27

318 17.5 0.520 1.65 0.730 0.09 0.69 0.14 0.47 1.35

NaCl (4g dm-

3)

298 10.6 0.485 3.4 0.730 0.04 1.42 0.15 0.39 1.34

308 un-stirred 18 0.484 3.4 0.700 0.05 1.13 0.16 0.37 0.98

318 20 0.483 3.9 0.710 0.06 1.03 0.18 0.34 1.00

298 13 0.483 3.6 0.740 0.04 1.40 0.15 0.38 1.11

308 stirred 21 0.483 6.3 0.730 0.05 1.29 0.18 0.35 0.77

318 24 0.481 9 0.720 0.05 1.21 0.19 0.33 0.74

FeSO4 (o.2g dm-3)

298 6.3 0.518 3 0.740 0.05 1.26 0.13 0.45 2.37

308 un-stirred 6.9 0.518 3.3 0.740 0.05 1.23 0.18 0.33 2.46

318 8 0.516 5 0.720 0.05 1.17 0.28 0.22 2.46

298 10 0.505 3.2 0.720 0.04 1.55 0.12 0.51 1.25

308 stirred 12 0.503 3.4 0.710 0.04 1.46 0.12 0.50 1.13

318 15 0.500 3.55 0.710 0.05 1.25 0.17 0.37 1.12

(100)

Table(4-4):Values of ic,Ec,ip,Ep,ba,bc,αa,αc and Rp for the polarization of cured negative lead alloy working electrode in 0.56M oxygenated sulphuric acid solution in the presence of additives .Symbols were defined in Tables(3-1 to 3-14).

Additive T/K medium ic/10-5 -Ec ip/10-4 Ep ba αa -bc αc Rp

H3PO4 (11g dm-3)

298 0.82 0.537 2.7 0.820 0.08 0.73 0.14 0.42 2.73

308 un-stirred 0.93 0.536 2.9 0.810 0.09 0.64 0.17 0.37 2.82

318 0.95 0.535 3.2 0.810 0.10 0.62 0.18 0.35 2.97

298 0.825 0.533 3.1 0.790 0.23 0.26 0.15 0.39 4.83

308 stirred 0.925 0.530 3.1 0.780 0.27 0.22 0.17 0.37 4.84

318 0.98 0.530 4.25 0.780 0.33 0.19 0.18 0.35 5.23

H3PO4 (11g dm-3)

+ FeSO4

(o.2g dm-3)

298 0.95 0.525 3.4 0.820 0.05 1.19 0.15 0.40 1.71

308 un-stirred 1.15 0.523 3.5 0.800 0.06 0.98 0.19 0.32 1.78

318 1.2 0.523 4.5 0.790 0.07 0.97 0.38 0.17 2.01

298 1.1 0.521 3.3 0.800 0.08 0.78 0.09 0.63 1.65

308 stirred 1.15 0.520 3.8 0.790 0.08 0.76 0.16 0.39 2.01

318 1.2 0.520 3.9 0.790 0.10 0.61 0.19 0.33 2.42

NaCl (4g dm-3)

298 2 0.473 6.9 0.700 0.04 1.38 0.13 0.46 0.70

308 un-stirred 2.4 0.472 7 0.690 0.04 1.37 0.14 0.43 0.61

318 3.3 0.471 8.9 0.690 0.05 1.38 0.16 0.41 0.46

298 1.5 0.472 6.5 0.690 0.05 1.27 0.11 0.54 0.95

308 stirred 1.9 0.470 6.9 0.690 0.06 1.04 0.12 0.53 0.89

318 2 0.470 9.5 0.670 0.06 1.00 0.13 0.49 0.92

FeSO4 (o.2g dm-3)

298 1 0.516 3.1 0.740 0.05 1.25 0.12 0.49 1.48

308 un-stirred 1 0.514 3.4 0.730 0.05 1.16 0.17 0.36 1.74

318 1.2 0.513 3.5 0.730 0.06 1.10 0.19 0.33 1.60

298 1.2 0.513 5 0.740 0.04 1.47 0.14 0.44 1.12

308 stirred 1.7 0.513 5.25 0.720 0.04 1.58 0.21 0.29 0.83

318 2.25 0.511 5.7 0.710 0.05 1.30 0.52 0.12 0.85

(101)

4.1.1- Corrosion Potentials (Ec):

Tables (4.1) to (4.4) show values of the Ec which have been obtained

from the polarization curves in the presence of the additives which may be

summarized as in the following:

1. Values of Ec for the different working electrodes shifted to more

negative values in the presence of additive as compared with the

corresponding values in the absence of additive indicating an increasing

tendency of the electrode for corrosion.

2. Values of Ec in stirred oxygenated 0.56 M sulphuric acid solution

was less negative than the values in the un-stirred oxygenated 0.56M

H2SO4 solution.

3. Values of Ec for the corrosion of the four working electrodes

increased at constant H2SO4 concentration with increasing temperature.

The results of Figs.(4.1) to (4.8) show the effect of additives on the

values of the corrosion potentials of the electrode materials in stirred

oxygenated 0.56M sulphuric acid solution which may be arranged from

more negative to less negative in a sequence as:

2 > 3 > 5 > 4 > 1

The sequence in un-stirred oxygenated acid solution on similar basis

was as:

2 > 3 > 5 > 1 > 4

Thus, in both stirred and un-stirred oxygenated 0.56 M sulphuric acid

solution, the addition of H3PO4 had a greater influence on shifing the

corrosion potential to a more negative value. The addition of sodium

chloride shifted the Ec to least negative value in un-stirred acid solution. In

stirred oxygenated acid solution, the values of Ec were less negative in the

absence of additives as compared with the presence of the additives.

(102)

The grid lead showed the greatest tendency for corrosion, while the

cured negative electrode material had the least tendency for corrosion.

Curing of the positive electrode or of the negative electrode reduced the

tendency for corrosion. The additives reduced the corrosion tendency as

compared with the tendency in the absence of additives.

(103)

(104)

(105)

(106)

(107)

4.1.2- Corrosion Current Densities (ic)

The corrosion current density (ic) represents the rate of corrosion

under equilibrium condition.

Tables (4.1) to (4.4) show values of ic, and hence of corrosion rates,

of the electrode materials in the presence of additives in the both unstirred

and stirred oxygenated acid solutions. In general, values of ic were higher

in stirred solution than in unstirred solution.

The behaviours of the four electrode materials in the presence of the

various additives may be compared with each other with the aid of the

Figs.(4.9) to (4.16). The corrosion rates of the materials in stirred and un-

stirred oxygenated acid solution may be presented in the following

sequence:

1 > 4 > 5 > 3 > 2

Thus, the highest corrosion inhibition was caused by the addition of

phosphoric acid (11g dm-3) in both stirred and un-stired 0.56 M H2SO4

solution with respect to all the electrode materials. The less corrosion

inhibition was caused by ferrous sulphate (0.2g dm-3) and sodium chloride

(4g dm-3) in stirred and un-stirred oxygenated 0.56 M H2SO4 solution as

compared with acid solution without additives.

The largest coorosion rate in stirred oxygenated solution may be

accounted for on the bases of the greater reactivity of the material surface

towards oxygen.

(108)

(109)

(110)

(111)

(112)

4.1.3- Passive Potentials (Ep)

The passive potential (Ep) is the potential at which a stable passive

layer is formed on the electrode surface. The greater value of Ep, the more

noble is the passive potential and hence a greater work should be required

to attain such state. When the value of Ep is low, a relatively smaller

electrical work is needed to lay down a compact passive layer on the

electrode surface. When two values of Ep are compared for two different

electrodes or under two different experimental conditions, the lowest value

of Ep then corresponds to a smaller work that is required to achieve

passivity as compared with the larger value of Ep. The passive lay is

expected in all cases to be an oxide or sulphate layer on the electrode

surface.

Values of passive potentials (Ep) for the corrosion of the four electrode

materials decreased with increasing temperature and the values of (Ep) in

un-stirred oxygenated acid solution were generally greater than the values

in stirred oxygenated acid solution in the presence of the additives as

shown in tables(4.1) to (4.4).

The sequence of Ep values for the corrosion of lead alloy electrode in

un-stirred oxygenated acid solution may be arranged as in the following :

2 > 1 > 3 > 5 > 4

the sequence in stirred oxygenated acid solution was:

1 > 2 > 3 > 5 > 4

For grid lead the sequence of Ep values in un-stirred oxygenated acid

solution was:

2 > 3 > 1 > 5 > 4

and in stirred oxygenated acid solution the sequence was:

2 > 1 > 3 > 5 > 4

(113)

For cured positive and cured negative electrodes the sequence of Ep values

in un-stirred acid solution was:

2 > 3 > 1 > 5 > 4

In the stirred oxygenated acid solution the sequence of Ep values for

cured positive and cured negative electrodes was:

3 > 2 > 1 > 5 > 4

(114)

4.1.4- Passive Current Densities (ip)

The passive current density (ip) represents the corrosion rate of the

electrode surface in the passive state. The lower the value of ip the more

stable is the passive layer on the electrode surface and hence the lower is

the corrosion rate when the electrode surface attains such state. On the

other hand, the greater the value of ip, the less stable is the passive layer,

and hence the higher is the corrosion rate of the electrode when it attains

such state. When two values of ip are compared, the greater ip value

corresponds to less stable passive state and hence to larger corrosion rate as

compared with the lower value of ip. The ip value is the corrosion current

density of the electrode surface subsequent to its coating with the surface

passive (oxide or sulphate) layer.

Tables (4.1) to (4.4) show values of the passive current densities (ip)

of the four working electrode materials in the stirred and un-stirred

oxygenated 0.56 M H2SO4 solution in the presence of additives.

Values of ip for all electrode materials increased with increasing

temperature and the values of ip in stirred oxygenated acid solution were

greater than the corresponding values in un-stirred oxygenated acid

solution.

Tables (4.1) to (4.4) indicate the effect of the various additives on the

values of ip for the electrodes as compared with ip values in the absence of

additives. The effect of additive for lead alloy in stirred and un-stirred

oxygenated acid solution may be arranged as:

4 > 5 > 3 > 2 > 1

For grid lead, the sequence in stirred oxygenated acid solution was:

1 > 4 > 5 > 3 > 2

In un-stirred oxygenated acid solution the sequence was:

4 > 5 > 3 > 1 > 2

(115)

The sequence for the cured positive electrode and cured negative electrode

in the both stirred and un-stirred oxygenated acid solution was as:

1 > 4> 5> 3 > 2

The greatest effect was obtained in the presence of NaCl, while the

smallest effect was observed in the presence of H3PO4.

The passive current density (ip) decreased generally on moving from

the left to right in the sequences. The decrease of (ip) should be connected

with the increasing stability of the oxide film, while the subsequent

increase in ip implies a decrease in the stability of the oxide or sulphate film

which is formed on the electrode surface.

(116)

4.2- Tafel Slopes and Transfer Coefficients

Tables (4.1) to (4.4) show the influence of temperature (T) and

concentration (C) of the additives on the cathodic (bc) and anodic (ba) Tafel

slopes which have been obtained from the polarization curves of the

working electrodes in stirred and un-stirred oxygenated 0.56M H2SO4

solution over the temperature range 298-318K.

Values of the transfer coefficients for the cathodic (ac) and anodic

(aa) processes have been calculated from the corresponding cathodic (bc)

and anodic (ba)Tafel slopes using the relationships (97):

FbRT

cc

303.2=a ----- (3.1)

FbRT

aa

303.2=a ----- (3.2)

where R is the gas constant and F the Faraday constant.

A cathodic Tafel slope of -0.120V (or of ac =0.5) may be diagnostic

of a discharge–chemical desorption mechanism for hydrogen evolution

reaction of the cathode in which the proton discharge is the rate-

determining step. If chemical desorption is the rate- determining step, the

rate will then be independent of the overpotential since no charge transfer

occurs in such step and the rate becomes directly proportional to the

concentration or the coverage (q) of the adsorbed hydrogen atoms, and

may occur at coverages ranging from very small values to almost full

surface layer formation (88). The expected Tafel slope in such step would

then be –0.03V decade-1 and ac=2.0.

When electrochemical desorption becomes the rate–determining step

for the hydrogen evolution reaction on the cathode, the expected value of

bc is –0.05 decade-1 and ac=1.5.

(117)

Values of the anodic Tafel slopes (ba) are shown in the tables of

chapter III (Tables (3.1-3.14)) and IV(Tables(4.1-4.4)) to be close to 0.120

decade-1 in some cases, and those of the corresponding anodic transfer

coefficients (aa) were also close to 0.5, indicating that the metal dissolution

reaction to be the rate- determining step for the dissolution reactions taking

place at the anode.

The results of the tables indicate that the variation of the Tafel slopes

and of the corresponding transfer coefficients could be interpreted in terms

of the variation of the rate-determining step from charge transfer process to

either chemical-desorption or to electrochemical desorption.

The variation of the anodic transfer coefficients (aa) may be attributed

to the variation of the rate-determining step in the metal dissolution

reaction. A change in mechanism as well as in the rate- determining step,

cannot be ignored throughout the anodic processes(98).

(118)

4.3- Polarization Resistance

Another approach to the problem of electrochemical corrosion rate

measurement is to apply only a small potential difference to the specimen

and then measure the current density. The potential –current density plot is

approximately linear in the region within ±10mV of the corrosion potential.

The slope of this plot in terms of potential divided by current density has

the units of resistance area and is often called the polarization resistance

(Rp). The polarization resistance (Rp) is related to the corrosion current

density by the relationship:(99)

Rp = cca

ca

ibbbb

)(303.2 + ---- (3.10)

Where ic is the corrosion current density, and ba and bc are the magnitudes

of the Tafel slopes of the anodic and cathodic Tafel lines respectively.

The measurement of polarization resistance has very similar

requirements to the measurement of full polarization curves and it is

particularly useful as a method to rapidly identify corrosion upsets and

initiates remedial action.(100)

The results of Tables (4.1-4.4) indicate the following:

1. Values of the polarization resistance were higher in un-stirred

oxygenated acid solution than in stirred oxygenated solution in all cases

and this may be accounted for on the basis of the smaller reactivity of

the material surface towards oxygen.

2. For the four working electrodes, the values of the polarization

resistance in the presence of additives were generally greater than in the

absence of additives in the both media, except in certain cases, where

the reverse was true, and such cases were:

a. for lead alloy in the presence of NaCl in stirred acid solution,

b. for cured negative electrode in the presence of NaCl.

(119)

3. The greatest values of the polarization resistance which were observed

for the electrodes in some cases were:

a. for lead alloy and for cured negative electrode in the presence of

H3PO4,

b. for grid lead and cured positive electrode in un-stirred acid solution

in the presence of H3PO4,

c. for grid lead in stirred acid solution in the presence FeSO4,

d. for cured positive electrode in stirred acid solution in the presence of

(H3PO4 + FeSO4 ) mixture.

4. The smallest values of the polarization resistance were observed in the

following cases:

a. for cured positive and cured negative electrodes in the presence of

NaCl,

b. for grid lead in stirred acid solution in the presence of NaCl,

c. for lead alloy in the presence of FeSO4,

d. for grid lead in un-stirred acid solution in the presence (H3PO4 +

FeSO4) mixture.

(120)

4.4 – Effect of Additives

4.4.1- Phosphoric Acid

Several attempts have been made to improve the corrosion resistance of lead and lead-antimony alloy electrodes. Lead forms an insoluble phosphate that provides protection in phosphoric acid (101). Boctor (102) stated that the addition of few grams per liter of phosphoric acid in relatively higher concentration of sulphuric acid in the storage batteries, is useful to inhibit corrosion specially under high temperature conditions. Bullock and McClelland (103) have shown that phosphoric acid decrease the rate of the self-discharge reaction of the positive electrode:

PbO2 + H2SO4 ® PbSO4 + H2O + ½ O2 ------ (4.1) in sealed lead-acid cells with pure lead grids. Visscher(104) confirmed that adding small quantities of phosphoric acid to approximately 5M H2SO4 modifies the kinetics of the PbO2/ PbSO4 couple reactions. Bullock (105) studied the effect of H3PO4 on the constant potential corrosion of pure lead positive grid in the lead acid batter and found that the PbO2 film formed in the presence of phosphoric acid requires longer time to self-discharge to PbSO4 than the PbO2 film formed in pure electrolyte. Phosphoric acid reduces corrosion rate, it is apparent that the greatest effect is in going from zero to 0.2% H3PO4. Further increase in H3PO4 concentration decreases the rate only slightly. Thus, it may be concluded that (105): 1- Phosphorate modifies the morphology of PbO2 formed by grid

corrosion. 2- Phosphate is incorporated in the PbO2 structure during corrosion

process. 3- These effects occur on pure lead, antimonial and non-antimonial lead

alloys as well.

(121)

4.4.2- Mixture of H3PO4 and FeSO4

The addition of (11g) of phosphoric acid (H3PO4) to one litre of

sulphuric acid electrolyte results in a solution in which the H3PO4 is

subjected to a change during discharging and charging processes of the

lead-acid battery. It is established that H3PO4 undergoes some absorption

by the positive plates (PbO2) through the charging process of the battery

and ah part of the absorbed H3PO4 will return and transfer to the

electrolyte.

It was proved(106) that the addition of phosphoric acid to the

electrolyte causes formation of Pb(IV) ions on charging of the positive

plates and he particles of Pb(IV) may precipitate as a jelly like mass of

white colour in the bottom of the electrolyte. The particles may oxidize

some organic materials, which are present in the battery structure resulting

in the formation of Pb at he negative plates of the battery.

As a result it will cause premature failure of the negative plates. The

formation of Pb(IV) is therefore undesirable whether as soluble ions or

jellylike particles and in order to prevent this an amount of Fe+2 ions

(0.2 g) is usually added to the electrolyte to reduce the corrosion of

Pb(IV) particles to Pb(II).

As a conclusion, the H3PO4 decreases the rate of corrosion of the

positive plates and hence the rate of dropping of the active mass of the

plates. The presence of Fe2+ with H3PO4 in the acid solution reduces the

conversion of Pb (IV) to Pb(II).(107)

(122)

4.4.3- Ferrous Sulphate (FeSO4)

Adding an amount of Fe+2 ions (0.2g dm-3) as FeSO4 to the electrolyte

is necessary to control the extent of the formation of Pb(IV) and it is

confirmed that :-(108)

1- The Fe+2 ions should be added to the electrolyte as a soluble ferrous

sulphate (FeSO4).

2- Adding 0.05 g of FeSO4 to the electrolyte has no effect on the

capacity of the battery and on the dropping of active mass of the plates.

3- The formation of Pb(IV) particles may be presented only when the

concentration of FeSO4 in the acid solution becomes 0.2 g for each litre

of the sulphuric acid electrolyte.

At this concentration of FeSO4, the capacity of the battery may

decrease by 5% and the rate of dropping of the active mass of the positive

plates decreases by about 80%.

4.4.4- Sodium Chloride (NaCl):

Adding (4g) of NaCl to one litre of the sulphuric acid electrolyte may

cause corrosion of lead by the formation of PbCl2 film(48).

The film of poorly soluble PbCl2 is formed via the process:

Pb + 2Cl- ® PbCl2 + 2e ----- (4.2)

This reaction may be compared with the formation of the sulphate

system as represented by the reaction:

Pb + SO42- ® PbSO4 + 2e ----- (4.3)

The analogy ends when it is realized firstly that the chloride of lead is

some 300 times more soluble in water or in an aqueous media than lead

sulphate, and secondly, in practice, the importance of lead in sulphate

media focuses on solutions high in sulphate concentration (strong sulphuric

acid) which are relatively quiescent.

(123)

4.5- Protection Efficiency

The corrosion current densities in the presence and absence of

additives in the corrosion medium have been used to determine the

protection efficiency (P%) using the relation (109,110,94)

P%= 100 [ 1-1

2

ii ] ----- (4.4)

where i1 and i2 are the corrosion current densities in the absence and

presence additive in the corrosion medium respectively.

A positive value of P% indicates inhibition of corrosion by the added

additive while a negative value of P% implies corrosion stimulation or

corrosion acceleration.

The results of Tables (4.5-4.8) and of Figs.(4.17-4.28) reveal the

following:

1. Values of the protection efficiency (P%) were higher in stirred

oxygenated acid solution than in unstirred oxygenated solution in the

all cases except for cured positive electrode where the reverse case was

holding. Stirring in the former cases may cause a closer contact

between the additive and the electrode surface resulting in a higher

percentage of protection efficiency. Stirring may also result in the

formation of a more compact passive layer by the dissolved oxygen.

For cured positive electrode, the values of P% were higher in unstirred

oxygenated solution than in the stirred solution in the presence of the

additives 1, 2 and 4. Such behaviour may be attributed to the different

nature of this electrode which was mainly in the form of PbO2 and not as

metallic lead.

2. The order of the variation of P% values in most cases lied in the

sequence :

1 > 2 > 4 > 3

(124)

irrespective of the type of the additive which was present in the oxygenated

acid solution, and also irrespective of the presence or the absence of

stirring.

3. The lowest P% values were obtained in such cases as:

a- for grid lead, lead alloy and cured negative electrode in unstirred

oxygenated acid solution containing dissolved NaCl,

b- For grid lead in unstirred oxygenated acid solution containing

dissolved FeSO4,

c- NaCl caused the lowest protection efficiency than the other three

additives in the both stirred and unstirred oxygenated solution with

respect to all the four types of electrode materials.

4. Stirring of the acid solution had largest effect in enhancing the value

of protection effect in enhancing the value of protection efficiency P%

in the case of grid lead and lead alloy electrodes in the presence of NaCl

as additive.

5. H3PO4 was most effective in raising P% values:

(a) with grid lead, lead alloy and cured negative electrode in the stirred

oxygenated solution.

(b) with cured positive electrode in unstirred oxygenated acid solution.

6. The (H3PO4 + FeSO4) mixture was less effective than H3PO4 alone in

raising the value of P% and such effect was more pronounced in the

case of lead alloy in the stirred oxygenated solution.

7. The temperature dependencies of P% values were as follows:-

a. for lead alloy, were lowest in the presence of NaCl. The temperature

dependence remained almost constant in stirred oxygenated solution; it

may be specified as almost independent of temperature in such cases.

With the other three cases, the temperature dependence in unstirred

oxygenated acid solution was markedly temperature dependent and the

(125)

dependence attained maximum values at 308K. NaCl presence in certain

cases caused corrosion acceleration.

b. P% values with grid lead in the presence of H3PO4 in stirred

oxygenated solution were almost independent of temperature. With the

other additives including NaCl, values of P% increased by some 5%

with temperature in the stirred oxygenated solution. In the unstirred

solution, P% became highest at 298K in the presence of H3PO4 but

decreased sharply with the rise of temperature. With (H3PO4 + FeSO4

)mixture, P% values increased with temperature from 298 to 308K and

then decreased with further increase in temperature. With the other

additives, there was an increase in P% with the rise of temperature.

c. with cured positive electrode, the variation of P% with temperature

in the unstirred solution was almost similar to that obtained with H3PO4

for lead alloy electrode in the stirred oxygenated solution. In the

unstirred oxygenated solution, the variation of P% values was almost

similar to the behaviour of lead alloy in the unstirred oxygenated

solution.

d. With cured negative electrodes, the variation of P% values with

temperature had certain similarities with those for lead alloy with the

exception of NaCl behaviour. While NaCl caused an increase in P%

values with temperature in the unstirred oxygenated solution, it resulted,

on the other hand, in a decrease of P% values with increasing

temperature.

(126)

Table(4-5):Protection efficiencies(P%) for the corrosion of the lead alloy in (0.56M) oxygenated H2SO4 solution in the presence of additives.

p%

T/K medium H3PO4 (H3PO4 (11g dm-3)+

NaCl FeSO4

(11g dm-3) FeSO4(0.2g dm-3))

(4g dm-3) (0.2g dm-3)

298 38.46 29.23 3.08 23.08

308 un-stirred 55.71 54.29 14.29 62.14

318 56.47 49.41 11.76 60.00

298 69.09 67.27 30.91 54.55

308 stirred 68.97 62.07 -8.62 55.17

318 67.50 62.50 17.50 50.00

Table(4-6):Protection efficiencies(P%) for the corrosion of the grid

lead in (0.56M) oxygenated H2SO4 solution in the presence of additives.

p%

T/K medium H3PO4 (H3PO4 (11g dm-3)+

NaCl FeSO4

(11g dm-3) FeSO4(0.2g dm-3))

(4g dm-3) (0.2g dm-3)

298 52.38 26.98 1.59 20.63

308 un-stirred 47.37 34.74 21.05 31.58

318 32.31 30.00 30.77 36.92

298 83.85 61.54 53.85 57.69

308 stirred 80.00 65.71 57.14 61.43

318 80.80 68.00 56.00 67.00

(127)

Table(4-7):Protection efficiencies(P%) for the corrosion of the cured positive in (0.56M) oxygenated H2SO4 solution in the presence of additives.

p%

T/K medium H3PO4 (H3PO4 (11g dm-3)+

NaCl FeSO4

(11g dm-3) FeSO4(0.2g dm-3))

(4g dm-3) (0.2g dm-3)

298 68.75 65.00 33.75 60.63

308 un-stirred 67.65 64.71 -5.88 59.41

318 66.84 64.74 -5.26 57.89

298 60.87 60.87 43.48 56.52

308 stirred 71.43 57.14 40.00 65.71

318 64.86 52.70 35.14 59.46

Table(4-8):Protection efficiencies(P%) for the corrosion of the cured

negative in (0.56M) oxygenated H2SO4 solution in the presence of additives.

p%

T/K medium H3PO4 (H3PO4 (11g dm-3)+

NaCl FeSO4

(11g dm-3) FeSO4(0.2g dm-3))

(4g dm-3) (0.2g dm-3)

298 50.30 42.42 9.09 39.39

308 un-stirred 69.00 61.67 36.67 66.67

318 70.31 62.50 37.50 62.50

298 72.50 63.33 33.33 60.00

308 stirred 71.97 65.15 27.27 48.48

318 72.00 65.71 5.71 35.71

(128)

(129)

(130)

(131)

(132)

(133)

(134)

4.6- Thermodynamics of Corrosion

Thermodynamic laws tell us that there is a strong tendency for high

energy states of metals to transform into low energy states. It is this

tendency of metals to recombine with components of the environment that

leads to the phenomenon which is known as corrosion(111).

Tables (4.9) to (4.12) give values of the thermodynamic quantities

(DG, DH and DS) for the corrosion of the working electrodes in the stirred

and unstirred oxygenated sulphuric acid solution. Figs.(4.29-4.32) represent

the temperature dependencies of DG in the both media.

The results of tables (4.9-4.12) and of Figs.(4.33-4.44) indicate the

following:-

1- Values of DG were generally negative suggesting the existence of

thermodynamic feasibility for the corrosion of the electrodes materials

in oxygenated sulphuric acid solution in the absence or the presence of

the additives in the acid solution.

Values of DG for the different working electrodes were slightly more

negative in the unstirred oxygenated acid solution than in the stirred

oxygenated acid solution. The presence of additives in the oxygenated acid

solution caused a shift to more negative DG values as compared with the

case where no additive was present in the acid solution.

It is also shown that the effect of shifting DG to more negative values

may be arranged in a sequence as:

2 > 3 > 5 > 4

Thus, H3PO4 and its mixture with FeSO4 were the most effective in

shifting the DG to more negative values.

2- DH values were generally negative indicating a stronger bonding of

the metal ions, resulting from electrode corrosion, with the species that

are present in the corrosion medium as compared with the bonding of

(135)

the metal atoms in the crystal lattice of the solid electrode. Values of DH

were more negative in the presence of additives in the corrosion

medium than when such additives were absent.

Stirring of the oxygenated acid solution resulted in a less negative DH

values, except in certain cases, where the reverse behaviour was true, and

such cases were:-

a. for lead alloy in the presence of H3PO4,

b. for grid lead, for cured positive and cured negative electrodes in the

presence of H3PO4 + FeSO4 mixture in the corrosion medium, and,

c. for cured negative electrode in the presence of FeSO4.

The most effective additives in altering the DH values in the more

negative direction were:

(i) FeSO4, NaCl, (FeSO4+ H3PO4) mixture, and H3PO4 with lead

alloy,

(ii) (FeSO4 + H3PO4) mixture, NaCl, H3PO4 and FeSO4 with grid

lead,

(iii) H3PO4, (H3PO4 + FeSO4) mixture, FeSO4 and NaCl with cured

positive electrode, and ,

(iv) H3PO4, (H3PO4 + FeSO4)mixture, FeSO4 and NaCl with cured

negative electrode.

These results suggest stronger bonding in the presence of these

additives of the resulting metal ions with the existing charged species

which are present in the oxygenated acid medium as compared with the

state of the metal atoms while they are present in the surface lattices of the

corroding electrodes.

3- Values of DS were generally positive due to greater negativity of DG

values than the corresponding DH values. This suggests a smaller order

(136)

in the solvated states of the metal ions as compared with the state of

metal atoms in the crystal lattice of the corroding electrodes.

Values of DS were more positive in the presence of additives than in

their absence. The only exception to this statement was for grid lead and

cured negative electrode in the presence of NaCl in the corrosion medium.

Stirring of the acid solution caused a slight decrease in the values of DS

with respect to all the working electrodes.

The H3PO4 and its mixture with FeSO4 were the most effective

additives in increasing the values of DS.

(137)

Table(4-9):Values of the thermodynamics quantities -ΔG,ΔH(k J mol-1) and ΔS (J mol-1K-1)for the corrosion of the lead alloy working electrode in (0.56M) oxygenated H2SO4 solution in the presence of additives.

Additive T/K medium -ΔG ΔH ΔS

H3PO4 (11g dm-3)

298

un-stirred

100.35 -71.60 100.11

308 99.39 -69.67 99.16

318 98.42 -67.74 98.21

298

stirred

100.16 -88.66 99.86

308 99.39 -87.50 99.10

318 99.39 -87.11 99.11

FeSO4 (o.2g dm-3)

+ H3PO4

(11g dm-3)

298

un-stirred

99.97 -91.36 99.66 308 99.78 -90.88 99.49

318 99.40 -90.20 99.11

298

stirred

99.78 -85.42 99.49

308 99.20 -84.36 98.93

318 98.82 -83.49 98.55

NaCl (4g dm-3)

298

un-stirred

99.59 -90.95 99.28

308 99.20 -90.27 98.91

318 99.01 -89.79 98.73

298

stirred

99.01 -90.40 98.71

308 98.62 -89.72 98.33

318 98.43 -89.24 98.15

FeSO4 (o.2g dm-3)

298

un-stirred

99.78 -94.03 99.47

308 99.78 -93.84 99.48

318 99.40 -93.26 99.10

298

stirred

99.40 -85.00 99.11

308 98.43 -83.55 98.16

318 98.43 -83.07 98.17

(138)

Table(4-10):Values of the thermodynamics quantities -ΔG,ΔH(k J mol-1) and ΔS (J mol-1K-1)for the corrosion of the grid lead working electrode in (0.56M) oxygenated H2SO4 solution in the presence of additives.

additive T/K medium -ΔG ΔH ΔS

H3PO4 (11g dm-3)

298

un-stirred

102.10 -93.46 101.78

308 101.71 -92.78 101.41

318 101.52 -92.30 101.23

298

stirred

101.71 -93.07 101.40

308 101.33 -92.39 101.03

318 101.13 -91.91 100.84

FeSO4 (o.2g dm-3)

+ H3PO4

(11g dm-3)

298

un-stirred

101.90 -93.29 101.59

308 101.71 -92.81 101.41

318 101.33 -92.13 101.04

298

stirred

101.52 -95.77 101.20

308 101.33 -95.38 101.02

318 101.13 -94.99 100.83

NaCl (4g dm-3)

298

un-stirred

100.75 -94.99 100.43

308 100.55 -94.61 100.25

318 100.36 -94.22 100.06

298

stirred

100.55 -94.80 100.23

308 100.36 -94.42 100.05

318 100.17 -94.03 99.87

FeSO4 (o.2g dm-3)

298

un-stirred

101.33 -92.71 101.01

308 101.13 -92.23 100.83

318 100.75 -91.56 100.46

298

stirred

100.94 -92.33 100.63

308 100.94 -92.04 100.64

318 100.75 -91.56 100.46

(139)

Table(4-11):Values of the thermodynamics quantities -ΔG,ΔH(k J mol-1) and ΔS (J mol-1K-1)for the corrosion of the cured positive working electrode in (0.56M) oxygenated H2SO4 solution in the presence of additives.

additive T/K medium -ΔG ΔH ΔS

H3PO4 (11g dm-3)

298

un-stirred

104.03 -101.14 103.69

308 104.03 -101.04 103.70

318 103.83 -100.75 103.52

298

stirred

103.64 -100.75 103.30 308 103.64 -100.65 103.31

318 103.45 -100.36 103.13

FeSO4 (o.2g dm-3)

+ H3PO4

(11g dm-3)

298 un-stirred

101.71 -95.96 101.39

308 101.52 -95.57 101.21 318 101.33 -95.19 101.03

298

stirred

100.55 -97.66 100.23

308 100.36 -97.37 100.04

318 100.36 -97.28 100.05

NaCl (4g dm-3)

298

un-stirred

93.61 -87.85 93.31

308 93.41 -87.47 93.13

318 93.22 -87.08 92.95

298

stirred

93.22 -87.47 92.93

308 93.22 -87.27 92.94

318 92.83 -86.70 92.56

FeSO4 (o.2g dm-3)

298

un-stirred

99.97 -94.22 99.66

308 99.97 -94.03 99.67

318 99.59 -93.45 99.29

298

stirred

97.47 -88.82 97.17

308 97.08 -88.15 96.79

318 96.89 -87.66 96.61

(140)

Table(4-12):Values of the thermodynamics quantities -ΔG,ΔH(k J mol-1) and ΔS (J mol-1K-1)for the corrosion of the cured negative working electrode in (0.56M) oxygenated H2SO4 solution in the presence of additives.

additive T/K medium -ΔG ΔH ΔS

H3PO4 (11g dm-3)

298

un-stirred

103.64 -97.89 103.31

308 103.45 -97.50 103.13

318 103.26 -97.12 102.95

298

stirred

102.87 -94.26 102.55

308 102.29 -93.39 101.99

318 102.29 -93.10 102.00

FeSO4 (o.2g dm-3)

+ H3PO4

(11g dm-3)

298 un-stirred

101.33 -95.57 101.00

308 100.94 -94.99 100.63 318 100.94 -94.80 100.64

298

stirred

100.55 -97.66 100.23

308 100.36 -97.37 100.04

318 100.36 -97.28 100.05

NaCl (4g dm-3)

298

un-stirred

91.29 -85.54 91.00

308 91.10 -85.15 90.82

318 90.90 -84.77 90.64

298

stirred

91.10 -85.34 90.81

308 90.71 -84.77 90.43

318 90.71 -84.57 90.44

FeSO4 (o.2g dm-3)

298 un-stirred

99.59 -90.95 99.28

308 99.20 -90.27 98.91 318 99.01 -89.79 98.73 298

stirred

99.01 -93.26 98.70

308 99.01 -93.06 98.71

318 98.62 -92.49 98.33

(141)

(142)

(143)

(144)

(145)

(146)

(147)

(148)

(149)

4.7- Kinetic of Corrosion

The rate(r) of the corrosion of lead-acid battery plates and components

increased with increasing temperature form 298 to 318K. The behaviour

obeyed Arrhenius type equation as:

ic = A exp(-Ea/RT) ----- (4.5)

where ic is the rate of corrosion in terms of corrosion current density, A and

Ea are the pre- exponential factor and energy of activation of the corrosion

process respectively.

Values of Ea were derived from the slopes of the log ic versus 1/T

linear plots as in Figs. (4.45-4.52), while those of A were obtained from the

intercepts of the plots at 1/T = zero; values of A, expressed in term of Am-2,

have then been converted into molecules per m2 per second . A was defined

as:

A = RSCeh

KT /¹D ----- (4.6)

where

K =Boltzman constant,

h= Planck constant,

T= temperature on Kelvin Scale, and,

C= concentration of corrosion sites per m2 of the surface.

Tables (4.13-4.16) and Figs. (4.53-4.60) show the following results:

1- Values of the activation energy (Ea) and the pre-exponential factor (A)

had the same kinetic effects of the additives in both media and these were:

a. for lead alloy and cured negative electrode in the un-stirred acid

solution, the additive caused a decrease in Ea and A values as compared

with the values in the absence of the additive,

b. H3PO4 caused an increase in the values of Ea and A to maximum for

grid lead in the both media.

(150)

c. The lead alloy in the stirred acid solution and the cured positive

electrode in the un-stirred solution in the presence of NaCl resulted in

the maximum values of Ea and A.

d. The (H3PO4+ FeSO4) mixture attained maximum values for Ea and

A in the stirred oxygenated acid solution for the cured positive

electrode.

e. For cured negative electrode in the presence FeSO4 showed greatest

values of Ea and A in stirred the acid solution.

2- The minimum values of Ea and A were attained in such cases as:

a. for lead alloy in the un-stirred acid solution in the presence of

FeSO4,

b. in the presence of FeSO4 in the stirred solution for the grid lead

electrode,

c. in the absence of additive for lead alloy in the stirred solution and

for the cured positive electrode in the un-stirred acid solution,

d. H3PO4 caused a decrease in Ea and values to minimum for the cured

positive and negative electrodes in the both stirred and un-stirred media

respectively,

e. in the presence of (H3PO4 + FeSO4) mixture for the cured negative

electrode in the stirred solution,

f. for the grid lead electrode in the presence of NaCl in the un-stirred

solution.

Thus, the smaller activation energy of a reaction which was attained

by additives (the lower the height of the energy barrier) the more rapid is

the reaction at a given temperature, corrosion reaction proceeded on

special surface sites starting on sites with low values of Ea and proceeded

to others with higher Ea.(111,84)

4- DS¹ values in the presence of different amounts of additives shifted

to more positive or to less negative values than the corresponding

(151)

values in the absence of additives indicating a decrease in the rate

corrosion of electrodes.

Table (4.13):Values of activation Energies(Ea/k J mol-1) , pre-exponential factors(A/molecules cm-2 s-1)and Entropy of activation(ΔS≠/J mol-1 K-1) for the corrosion of lead alloy working electrode in(0.56M) oxygenated H2SO4 solution in the presence and absence of additives.

additive medium log A A Ea ∆S≠

without additive

un-stirred 19.57 3.71 X 10+19 38.09 129.55

stirred 9.47 2.96 X 10+9 14.64 -64.71

H3PO4 (11g dm-3)

un-stirred 14.03 1.08 X 10+14 24.33 23.59

stirred 10.75 5.58 X 10+10 16.59 -39.29

FeSO4

(0.2g dm-3) +

H3PO4 (11g dm-3)

un-stirred 14.15 1.40 X 10+14 24.65 25.78

Stirred 12.06 1.12 X 10+12 20.06 -14.15

NaCl (4g dm-3)

un-stirred 17.78 6.09 X 10+17 34.33 95.39

stirred 12.36 2.28 X 10+12 21.91 -8.46

FeSO4 (0.2g dm-3)

un-stirred 9.13 1.35 X 10+9 12.02 -70.22

(152)

stirred 11.29 1.95X 10+11 18.34 -28.89

(153)

Table (4.14):Values of activation Energies(Ea/k J mol-1) , pre-exponential factors(A/molecules cm-2 s-1) and Entropy of activation(ΔS≠/J mol-1K-1) for the corrosion of grid lead working electrode in (0.56M) oxygenated H2SO4 solution in the presence and absence of additives.

additive medium log A A Ea ∆S≠

without

additive

un-stirred 15.73 5.41 X 10+15 28.56 56.14

stirred 13.97 9.34 X 10+13 25.72 22.39

H3PO4

(11g dm-3)

un-stirred 21.40 2.51 X 10+21 42.34 164.58

stirred 17.33 2.14 X 10+17 32.63 86.71

FeSO4 (0.2g dm-3)

+ H3PO4

(11g dm-3)

un-stirred 15.04 1.09 X 10+15 26.82 42.84

stirred 11.37 2.37 X 10+11 18.46 -27.28

NaCl

(4g dm-3)

un-stirred 10.58 3.78 X 10+10 15.98 -42.53

stirred 13.43 2.71 X 10+13 23.79 12.10

FeSO4

(0.2g dm-3)

un-stirred 12.06 1.14 X 10+12 19.49 -14.23

stirred 10.31 2.03 X 10+10 16.05 -47.72

(154)

Table (4.15):Values of activation Energies(Ea/k J mol-1), pre-exponential factors(A/molecules cm-2 s-1) and Entropy of activation(ΔS≠/J mol-1 K-1) for the corrosion of cured positive working electrode in (0.56M) oxygenated H2SO4 solution in the presence and absence of additives

additive medium log A A Ea ∆S≠

without additive

un-stirred 4.52 3.30 X 10+4 6.74 -158.48

stirred 9.29 1.82 X 10+9 18.87 -67.77

H3PO4 (11g dm-3)

un-stirred 5.93 8.45 X 10+5 9.08 -131.54

stirred 7.82 6.59 X 10+7 14.41 -95.33

FeSO4 (0.2g dm-3)

+ H3PO4

(11g dm-3)

un-stirred 5.07 1.16 X 10+5 7.04 -148.02

stirred 12.46 2.86 X 10+12 26.33 -6.58

NaCl (4g dm-3)

un-stirred 11.91 8.14 X 10+11 25.17 -17.03

stirred 11.48 3.05 X 10+11 24.28 -25.18

FeSO4 (0.2g dm-3)

un-stirred 5.95 8.89 X 10+5 9.38 -131.11

stirred 8.36 2.29 X 10+8 15.94 -84.98

(155)

Table (4.16):Values of activation Energies(Ea/k J mol-1), pre-exponential factors(A/molecules cm-2 s-1) and Entropy of activation(ΔS≠/J mol-1 K-1) for the corrosion of cured negative working electrode in (0.56M) oxygenated H2SO4 solution in the presence and absence of additives.

additive medium log A A Ea ∆S≠

without additive

un-stirred 12.40 2.54 X 10+12 26.3 -7.58

stirred 3.98 9.54 X 10+3 6.08 -168.80

H3PO4

(11g dm-3)

un-stirred 4.39 2.45 X 10+4 5.84 -160.97

stirred 4.78 6.03 X 10+4 6.80 -153.47

FeSO4

(0.2g dm-3) +

H3PO4 (11g dm-3)

un-stirred 5.68 4.77 X 10+5 9.26 -136.29

stirred 3.32 2.10 X 10+3 3.43 -181.37

NaCl (4g dm-3)

un-stirred 9.56 3.61 X 10+9 19.66 -62.06

stirred 6.33 2.12 X 10+6 11.40 -123.89

FeSO4

(0.2g dm-3)

un-stirred 4.86 7.22 X 10+4 7.18 -151.98

stirred 11.77 5.95 X 10+11 24.78 -19.63

(156)

(157)

(158)

(159)

(160)

(161)

(162)

(163)

(164)

The results of Figs.(4.61-4.62) indicate the existence of a linear

relationship between the values of log A and the corresponding values of

log A and the corresponding values of Ea, which may be expressed as(112):

Log A = mE + I ----- (4.7)

where m and I are respectively the slope and intercept of the plots in

Figs.(4.61-4.62) such a behaviour is referred to as “compensation effect”

which describes the kinetics of a great number of catalytic and tarnishing

reactions on metals (113,114). Equation (4.7) indicates that simultaneous

increase or decrease in Ea and log A for a system tend to compensate from

the standpoint of the reaction rate.

A number of interpretations (115) have been offered for the

phenomenon of the compensation effect in surface reaction, among which

the effect could be ascribed to the presence of energetically heterogeneous

reaction sites on the electrode surface, which suffered corrosion in the

electrolytic solution. A decrease in Ea at constant log A implies a higher

rate, while an increase in Ea at constant log A implies a lower rate;

simultaneous increase in Ea and log A therefore tend to compensate from

the standpoint of the corrosion rate. When such a compensate operates, it is

possible for striking variations in Ea and log A through a series of surface

sites on a metal or an alloy to yield only a small variation in reactivity.

(165)

(166)

(١٦٦

5.1- Conclusions:

The conclusions that could be drawn from the experimental results and the related discussions may be put as:

1. The rate of corrosion was generally higher in the stirred oxygenated acid solution than in the corresponding unstirred deaerated media.

2. The grid lead showed greatest tendency for corrosion while the cured negative electrode material had the least tendency for corrosion.

3. The grid lead showed the lowest and the cured negative the greatest rate of corrosion in the stirred oxygenated acid solution.

4. The higher protection efficiencies (p%) were attained for the corrosion of the working electrode materials in the stirred oxygenated acid solution than in the unstirred oxygenated acid solution in the presence of H3PO4 in the corrosion medium.

5. The additives had an inhibiting effect on the corrosion of battery plates and components in the stirred and un-stirred oxygenated sulphuric acid solution and the corrosion potential shifted in the noble direction. The stimulating effect of the additives was noticed in the some cases in the presence of sodium chloride in the acid solution.

6. The Gibbs free energy changes (DG) for the corrosion of the battery plates and components was always negative indicating the thermodynamic feasibility of the corrosion of the battery plates and components. The dependencies of such changes on temperature (-dDG/dt) were either negative or positive resulting either in negative or positive value of DH and DS for the corrosion process.

7. The corrosion processes for battery plates and components in the absence or the presence of the various additives followed kinetically Arrhenius type rate equation. Positive values have been derived for the energy of activation (Ea). A linear relationship existed between the values at Ea and the logarithm of the pre- exponential factor (log A) suggesting the operation of a compensation effect in the kinetics of corrosion.

(١٦٧

5.2- Suggestions for Future Research

1. The corrosion medium may be extended to other concentrations of the sulphuric acid in the presence and the absence of different other salts and organic inhibitors.

2. The corrosion investigations may be carried out for the same electrodes under stirred and un-stirred conditions of the corrosion medium which should also be subjected to the both aeration and deaeration conditions.

3. The corrosion medium may also be subjected to thorough chemical analysis after corrosion tests in order to identify the types and extends of the various metallic ions that may be formed throughout anodic dissolution of the working electrode.

4. The working electrodes may also be examined carefully by scanning electron microscope, ESCA and other sophisticated techniques subsequent to all the corrosion experiments.

5. Other additives may be used in examining the corrosion behaviours of the electrodes and these may involve picric acid, boric acid and chromates.

6. The additives may also be added to the paste or to the grid of battery plates and to the other battery components prior to corrosion experiments in the various corrosion media.

(168)

References

1. R.J. Brodd, Batteries for cordless Appliances, (John Wiley and Sons,

Inc. New York, 1987).P.(154-155).

2. M. Silberberg, Chemistry, The Molecular Nature of Matter and

change, (John Wiley and Sons, New York, 1996).

3. H. Bode, Lead-Acid Batteries, (John Wiley and Sons, New York,

London,1977).

4. Gregory M.Williams,Chemistry,The Molecular Science,(John Wiley

and Sons,Inc.New York 1999).

5. D.A.J.R and , J. Power Sources, 1989,28, 107.

6. M.Barak, Electrochemical Power Sources, (Stevenage, U.K., 1980).

7. D. Linden, Handbook of Batteries and Fuel Cells, (McGraw-Hill

Book Co, 1984).

8. J.E.Dix, J. Power Sources , 1987, 19, 157.

9. D.A.J. Rand , The Battery Man, 1987, September, 14.

10. Kirk- Othmer, Encyclopedia of chemical Technology, 2nd edition,

(John Wiley and Sons, Inc. New York , 1964), vol.3.

11. P. Scharf and R. Wagner, J. Power Sources, 2000, 35, 117.

12. V.Iliv and D. Pavlor, J. Appl. Electrochem., 1979, 9, 555.

13. T.Inoue and N. Koura, Electrochemistry , 2000, 68, 789.

14. N.K. Grigaluk, J.Apply. Chem. USSR, 1979, 52, 742.

15. B.K.Mahato, J. Electrochem. Soc., 1980, 127, 1679.

16. G.J. Szava, J. Power Sources, 1988, 23, 119.

17. G.W.Vinal, Storage Batteries, 4th edition, (John Wiley and Sons,

Inc. New York , 1955).

18. R.J. Hill, J. Power Sources, 1983, 9, 55.

19. D.Pavlov , E. Bashtavelova, J. Electrochem. Soc., 1984, 131, 1468.

20. E. Bashtavelova, J. Electrochem. Soc., 1986, 133, 241

(169)

21. D. Pavlov, J. Electrochem. Soc., 1992, 139, 3075.

22. M.Dimitrov, J. Power Sources, 2001, 93, 234.

23. D. Pavlov, and V. Iliev, J. Power Sources, 1981, 7, 153.

24. V.Iliev and D.Pavlov, J.Appl. Electrochem., 1985, 15, 39.

25. D. Pavlov, J. Electrochem. Soc., 1974, 121, 854.

26. D. Pavlov, Power Source for Electric vehicles, B.D. McNicol, D.A.

J. Rand . eds, Chapter 5. Lead Acid Batteries. P. 313-327. ( Elsevier,

Amsterdam, 1984).

27. N.Iordonov, J. Electrochem. Soc., 1970, 117, 9.

28. L. Semynov, Storage Batteries, 1967, P. 78.

29. E.S.Napoleon,J.Power Sources,1987,19,169.

30. D.J.G.Ives, Reference Electrodes, (Acadeic Press, New York,

London, 1961).

31. Colin A. Vincent and B. Scrosati, Modern Batteries An Introduction

to electrochemical Power Source, 1997, P. 145.

32. D.Pavlov, Berichfe der Bunsengeselchaft, 1967, 71, 398.

33. C.N. Poulieff and N. Iodanov, J. Electrochem. Soc., 1969,116, 316.

34. D. Pavlov, J. Electrochem. Soc., 1970, 117, 1103.

35. S. Ruevski, J. Power Sources, 1990, 31, 217.

36. D. Pavlov, G. Papazov, J. Electrochem. Soc., 1972, 119, 8.

37. G. Papazov, J. Electrochem. Soc. , 1980, 127, 2104.

38. E. Bashtavelova, J. Power Sources, 1990, 31, 243.

39. D.Pavlov, J. Electroanal. Chem., 1976, 72, 319.

40. V.Iliv, J. Electrochem. Soc., 1974, 121, 854.

41. V.S. Bagotzky, Chemical Power Sources, (Academic Press, New

York, London, 1980).

42. R.J.Ball,J.Power Sources,20002,111,23.

43. B. Monahov, J.Appl. Electrochem., 1993, 23, 1244.

44. T. Laitinen, K. Salmi, Electrochem. Acta, 1991, 36, 605.

(170)

45. D. Pavlov, B. Monahov, J. Electroanal. Chem., 1991, 305, 57.

46. D.Pavlov,J.Appl.Electrochem.,1997,27,6.

47. U.R. Evans, Metallic corrosion, Passivity and protection, (Arnold,

London, 1947).

48. A.T.Kuhn, The Electrochemistry of Lead, (John Wiley and Sons,

Inc. New York, 1979).

49. P.Ruestschi and R.T. Angstadt, J.Electrochem. Soc., 1964, 12, 111.

50. W.H.Boctor, Proceding Corrosion Seminar, 13-16 Dec., (1976)

Baghdad.

51. G.Tedeschi, Mat. Nat., 1937, 25, 641.

52. P. Delalay, M.Pourbaix, J. Electrochem. Soc., 1951, 98, 97.

53. J.Lander, J. Electrochem Soc., 1951, 98, 6.

54. J. Casey and K.M. Camprey, J. Electrochem Soc., 1955, 102, 219.

55. G. Hohlstein and E. Pelzell, Metall., 1960, 14, 765.

56. G.W. Vinal, Storage Batteries, (John Wiely Inc. 1962).

57. A .A. Abdul Azim and K.M. El-Sobki, Corr. Sci., 1971, 11, 821.

58. U.R. Evans, The corrosion and Oxidation of Metals., (Edward

Arnold, 1971).

59. P.Ruetschi, Electrochem. Acta., 1963, 8, 333.

60. T. Rogachev, J.Power Sources, 1988, 23, 4.

61. G. Papazov, J. Power Sources, 1983, 10,3.

62. R.H.Newnham, D.A.J.Rand, J. Power sources, 1995, 53, 93.

63. Guo-Lin Wei, Jia-Rong Wang, J. Power sources, 1994, 52, 25.

64. W.A.Badaway, S.S.El-Egamy, J.Power Sources, 1995, 55, 11.

65. Takao. Omae, J.Power Sources, 1997, 65,2.

66. W.H. Bactor, S.Zhong, J. Power Sources, 1999, 77, 56.

67. D. Slavkov, J. Power Sources, 2002, 112, 199.

68. J.A. Van Frawnhofer and G.H. Banks , potentiostat and Its

Applications, (Butter Worths, London, 1972).

(171)

69. J.O.M. Bockris and A. K. Reddy, Modern Electrochemistry , Press,

New York, 1970), Vol. 2, P. 1315.

70. A. I. Karim, M.Sc. Thesis, College of Science, University of

Baghdad, Jan., (2003).

71. Kh. S. Abed, Ph. D. Thesis, College of Science, University of

Baghdad, October, 1997.

72. Q.A. Yousif, M.Sc. Thesis, College of Science, University of

Baghdad, July,(2001).

73. N.A. Hikmat, Ph. D. Thesis, College of Science, University of

Baghdad, Jan., 2002.

74. N.D. Green, Experimental Electrode Kinetics, (Troy, New York,

1965), P. 64.

75. J.W.D. France, Mat. Res., 1969, 9,21.

76. S.Glasstone,Principle of Elrctrochemistry,Van Nastrand, New York,

1942,P.448.

77. American Society for Testing and material, Annual Book of ASTM

standard, 1980, Part 10.

78. American Society for Testing and Material Annual Book of ASTM

Standard, 1978-G5

79. H.H.Uhlig, Corrosion and Corrosion Control, (Wiley, New York),

(2000), P. 411.

80. L. L. Shrier (ed.), Corrosion; Metal/Environment Reactions, (Butter

Worths, Boston, Mass., 1978), Vol.1.

81. F. Mansfeld, Corrosion science, 1973, 29, 397.

82. Stern. M., and Roth, J. Electrochem.Soc., 1957, 104, 390.

83. E. Heitz and W. Schwenk, Br. Corros. J., 1976, 11, 2.

84.

85. I.G. Murgulessue and O. Radorici, 2nd-Inter. Congr. Metal corrosion,

(London, 1961), P. 202-205.

(172)

86. A.M. Farhan, Ph.D., Thesis, College of Science, University of

Baghdad, June, 2000.

87. W.J. Lorenz and F. Mansfeld , Proc. 8th . Intr. Congr. Of Metal

Corrosion (Dechema,Germany, 6-11 September, 1981).

88.

89. B.N. Kabanov, Electrochemica, Acta, 1964, 9, 1197.

90. J. Lander, J. Electrochem. Soc., 1951, 98, 213.

91. Pierre R. Roberge, Handbook of Corrosion Engineering, (Mc Graw-

Hill, New York, 1999).

92. G.B. Roger, Determination of pH (Wiley, New York, 1972), P. 307.

93. L.M. Al-Shama’a , Ph. D., Thesis, College of Science, University of

Baghdad, April, 1999.

94. J.O.M. Bockris and A. K. Reddy, Modern Electrochemistry , Press,

New York, 1970), P.176.

95. Mars G. Fontana and Norbert D. Greene, Corrosion Engineering,

(Mc Graw-Hill, New York, 1978), P. 319.

96. H.Y. Chen, Journal of Power Sources, 2000, 88, 78.

97. E. Rocca, J. Electronal. Chem. 2003, 543, 153.

98. K.R. Trethewey and J. Chamberlain, Corrosion for science and

Engineering , 2nd . ed. (Addision Wesley Longman Ltd. 1996).

99. M. G. Fontana, N.D. Green, Corrosion Engineering, 2nd Edn.,

(McGraw-Hill, New York, 1963).

100. W.H. Boctor, J. Electrochem. Soc., 1976, 123, 12.

101. K.R. Bullock and D.H. Mcclelland, J. Electrochem. Soc., 1976, 123,

327.

102. W. Visscher, J. Power Sources, 1977, 1, 257.

103. K.R. Bullock, J. Electrochem. Soc., 1979, 126, 3.

104.

105. E. Kunse and K. Schwabe, Corrosion Sci., 1964, 15, 419.

(173)

106. A.M. Farhan, M.Sc., Thesis, College of Science, University of

Baghdad, 1989.

107.

108. G. Chond, Catalysis by Metals, (Academic Press, New York, 1962),

p.140.

109. Y.K. Al-Haydari, J.M. Saleh and M.H. Matloob, J. Phys. Chem. ,

1985, 89, 3286.

110. S.A. Isa and J.M. Saleh, J. Phys. Chem., 1972, 76, 2530.

111. E.Cremer, Advances in Catalysis, (Academic Press, New York, 1955)

vol 7, p. 75.

112. H.H.Uhlig, Corrosion and Corrosion Control, (Wiley, New York,

2000), P. 74-76.

113. M. Stern and A.L. Grary , J. Electrochem Soc., 1957, 56, 104.

114. K.R. Trethewey and J. Chamberlain, Corrosion for Science and

Engineering, 2nd . ed. (Addision Wesley Longman Ltd. 1996).

115. T.A. Sakman, Thesis, College of Science, Saddam University,

January, 1996.

116. Driver, R., and Meakins, R.J., Br. Corros. J., 1974, 9, 227.

117. Jsseling, F.P., Corros. Sci., 1974, 14, 97.

118. D. Pavlov and N. Kapkov, J. Electrochem. Soc., 1990, 137, 28.

119. L.A. Beckctaeva and K.V. Rybalka, J. Power Sources, 1990, 32, 143.

120. R.J. Hill, J. Power Sources, 1988, 22, 175.

الخالصة

يتناول موضوع الرسالة دراسة السلوك االستقطابي لتأكل سبعة نماذج من االقطاب المستخدمة في صناعة الواح ومكونات نضيدة الرصاص

الحامضية وهي : قطب سبيكة الرصاص . -۱ قطب مشبك الرصاص . -۲ طب الرصاص النقي .ق -۳ القطب الموجب غير المعمر . -٤ عمر.القطب الموجب الم -٥ القطب السلب غير المعمر. -٦ القطب السالب المعمر. -۷

مول )0.56,0.25,0.1(محلول حامض الكبريتيك بتراكيز عند غمرها فيالى ۲۹۸للد سمتر المكعب على مدى من درجات الحرارة تراوحت من

كلفن ويمكن تقسيم النمط العام والصيغة العملية للدراسة الى اربعة ۳۱۸ تي :اقسام وكما يأ

دراسة السلوك االستقطابي للنماذج بوجود غازاألكسجين في وسط -أ التأكل.

ستقطابي للنماذج بوجود غاز األوكسجين وفي جو دراسة السلوك اال –ب تحريكي لوسط التاكل .

دراسة السلوك االستقطابي للنماذج بوجود غازالنتروجين في وسط -ج التأكل.

نماذج بوجود غاز النتروجين وفي جو دراسة السلوك االستقطابي لل –-د تحريكي لوسط التاكل .

تمت دراسة السلوك االستقطابي للنماذج باستخدام جهاز المجهاد -۱)المستحصل من CORROSCRIPT)المسمى(Potentiostatالسكوني (

بواسطته الحصول على )الفرنسية وأمكنTaccusselشركة تاكوسيل ()على مدى من الجهود Polarization Curvesمنحنيات االستقطاب (

Scanفولت بأستعمال سرعة مسح (2.0+الى 2.0-تراوحت من Rate) لجهاز التسجيل(x-y Recorder ملمتر في الدقيقة 30) بلغت

)mm/min) وقد تبين من النتائج المستحصلة أن جهد التأكل (Ec يصبح(في وسط أكثرسالبية في محلول حامض الكبريتيك بوجود غازالنتروجين

غير متحرك وأقل سالبية بوجود غاز االوكسجين في وسط متحرك وأن ) عموما كانت عالية في حامض الكبريتيك ic(تغيرات كثافة تيار التأكل

بوجود غاز االوكسجين وفي جو متحرك مقارنة بجو النتروجين غير ) منخفضا.icالمتحرك والتي كان فيها تغير (

المضافات أشتملت على:تمت دراسة تأثير عدد من -۲ غم للدسمتر المكعب).۱۱حامض الفوسفوريك( -۱كبريتات مع ) غم۱۱مزيج من حامض الفوسفوريك ( -۲ للسمتر المكعب . غم)0.2حديدوز(ال غم للدسمترالمكعب).4(كلوريد الصوديوم -۳ غم للدسمتر المكعب).0.2كبريتات الحديدوز( -٤

)موالري في وجود 0.56األلكتروليتي لحامض الكبريتيك ( في المحلولاالوكسجين وجومتحرك وساكن على أربعة نماذج مختلفة من االقطاب

وكما ياتي : قطب سبيكة الرصاص. -۱ قطب مشبك الرصاص. -۲ القطب الموجب المعمر. -۳ القطب السالب المعمر. -٤

)كلفن وقد دلت حسابات 298-318في مدى الدرجات الحرارية من() على بلوغ اقصى حماية Protection Efficiencyاية الحماية(كف

ممكنة باستعمال المثبط حامض الفوسفوريك وادنى حماية ممكنة بأستعمال كلوريد بالنسبة الى جميع االقطاب.

)G،∆H،∆S∆حسبت الكميات الثرموداينميكية لتفاعالت التأكل( -۳أن قيم طاقة في حالة غياب ووجود المضافات وقد أظهرت الدراسة

كانت على العموم أكثر سالبية في الوسط الحامضي في G∆كيبز الحرة وجود غاز النتروجين ،أما في حالة وجود المضافات فأظهرت الدراسة

كانت أكثر سالبية في وجود حامض الفوسفوريك وأقل سالبية G∆أن قيم.كما وجدت تغيرات في قيم في وجود كلوريد الصوديوم

حدوث تباين في ) بمقدار ملحوظ ويدل هذا التغير على S∆(االنتروبينوع ومدى اعتمادية تغيرات الطاقة الحرة العملية التأكل على درجة

) الى حدوث تغيرات S∆الحرارة .وأدت تغيرات قيم االنتروبي ( ) أيضا.H∆(مناظرة في االنثالبي

)تفاعالت التأكل في حالة وجود وعدمKineticsخضعت حركيات( -٤وجود المضافات لمعادلة أرينوس التي تقضي بوجود عالقة خطية بين

)ومقلوب درجة الحرارة المطلقة Log icقيم لوغاريتم سرعة التأكل ()1/T التنشيط () والتي أمكن منها حساب قيم طاقةEnergy ,Ea

Activation )وقيم مسبوق المقدار االسي (pre-exponential,A ()كما تبين من ≠Entropy of Activation, ∆S (وانتروبي التنشي

Logالدراسة وجود عالفة خطية بين لوغاريتم مسبوق المقدار االسي (A) وقيم طاقة التنشيط (Ea المناظرة لها.وتدل هذه العالقة الخطية بأن(

تفاعل التأكل يحدث على مواقع متباينة على سطح نماذج ألواح حيث قيم طاقة التنشيط وأن ومكوانات نضيدة الرصاص الحامضية من

يبدا أوال بالمواقع التي تتمتع بطاقة تنشيط واطئة ثم ينتشر تفاعل التأكل منها الى المواقع التي تتمتع بطاقات تنشيط أعلى.

جامعة بغداد كلية العلوم قسم الكيمياء

تآكل الواح نضيدة الرصاص الحامضية في

حامض الكبريتيك

رسالة مقدمة الىكلية العلوم بجامعة بغداد كجزء من متطلبات نيل درجة

الماجستير علوم في الكيمياء

من قبل ـــــدبختيار كاكل حم بكالوريوس

۲۰۰۰

هــ۱٤۲٥ شعبان