◌ِ ◌ِ 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.
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/2004
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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.
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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.
CONTENTS
Page
No.
Subject
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
1.1.2- Industrial preparation of the Electrodes
3
4
1.1.3- Structure of the Electrode Materials
6
1.1.3.1- PAM Structure
1.1.3.2- NAM Structure
6
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
a
Page
No.
Subject
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
b
Page
No.
Subject
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
4.6- Thermodynamics of Corrosion.
123
4.7- Kinetics of Corrosion.
149
134
CHAPTER FIVE:
CONCLUSIONS AND
SUGGESTIONS FOR FUTURE RESEARCH
5.1- Conclusions
166
5.2- Suggestions for future research
167
REFERENCES
168
c
Symbols and Abbreviations
Symbol
Definition
Units
Molecules. Cm-2.s-1
A
Pre-exponential factor
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
SCE
Gas constant
Saturated calomel electrode
J. mol-1.K-1
V
DS
Entropy change
J. mol-1.K-1
DS¹
Entropy of activation
J. mol-1.K-1
T
The temperature in Kelvin
a
Transfer Coefficient
aa
Anodic Transfer Coefficient
ac
Cathodic Transfer Coefficient
K
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) + SO42-(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)
discharge
charge
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
(1)
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.
(2)
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).
(3)
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).
(4)
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.
(5)
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:
(a)
(b)
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).
(6)
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).
(7)
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.
(8)
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
(9)
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).
(10)
Potential vs Hg/Hg2SO4
Microns
Microns
Fig.(1-6)
Representation of the multi-phase corrosion layer by Ruetschi
(11)
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
(12)
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
Anode(-)
H2SO4
solution
PbO2
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
Anode(-)
PbSO4
Cathode(+)
(13)
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:
discharge
PbSO4(s) + 2 H2O(l) (1-16)
PbO2(s) +4H+(aq)+SO4(aq)2- +2e…
charge
An important feature of the positive electrode discharge concerns the
nature of the PbSO4 deposit since the formation of dense, coherent layers
(14)
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)
discharge
charge
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
(15)
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
(16)
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 bPbO2. 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).
(17)
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.
(18)
Fig.(1-7): Discharge and charge processes of the battery
(19)
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.
(20)
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,
(21)
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.
(22)
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.
(23)
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,
(24)
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.
(25)
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 preexponential factors for the corrosion process have been determined.
(26)
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 EPL2B 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
(27)
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) .
(28)
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 Bartonpot 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).
(29)
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).
(30)
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
2Hg(l) + 2 Cl-
Hg2Cl2(s) + 2e
(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).
(31)
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).
(32)
Gas Outlet
Gas Inlet
Reference
Electrode
Working
Electrode
Auxiliary
Electrode
Fig. ( 2-2):- A schematic diagram of the polarization cell.
(33)
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).
(34)
NOBEL
Et
Applied Potential, E
Transpassive
G
EAP
passive
E
Active-Passive
Transition
EPP
D
Active
EC
C
Cathodic
B
ACTIVE
ip
A
icr
ic
Log Current Density
Fig.(2-3): A typical polarization curve showing Tofel, active-passive
transition, passive and transpassive regions and their
corresponding potentials and current densities.
(35)
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
(36)
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
(37)
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 .
(38)
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
(39)
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.
(40)
(41)
(42)
(43)
(44)
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.
(45)
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.
(46)
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
-Ec
ip/10-5
Ep
ba
aa
-bc
ac
Rp
2.75
0.513
7.20
0.820
0.04
1.38
0.16
0.37
53.46
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
298
2.75
0.520
7.40
0.810
0.04
1.41
0.45
0.13
60.33
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
298
2.60
0.530
7.00
0.770
0.05
1.16
0.16
0.37
64.52
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
298
0.65
0.516
7.30
0.610
0.03
1.76
0.43
0.14
207.49
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
298
0.55
0.520
12.00
0.560
0.05
1.09
0.38
0.16
374.74
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
298
0.50
0.510
11.00
0.620
0.05
1.28
0.45
0.13
362.50
1.20
0.500
11.50
0.600
0.10
0.61
0.59
0.10
308.23
1.60
0.480
12.50
0.580
0.11
0.57
0.62
0.10
256.71
T/K medium ic/10-4
298
0.56M 308
0.25M 308
0.1M
308
0.56M 308
0.25M 308
0.1M
308
318
stirred
stirred
stirred
unstirred
unstirred
unstirred
(47)
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
Ep
ba
7.5
0.518
6.80
1.010
0.07
0.84 0.16 0.36
284.50
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
298
7.2
0.510
6.20
0.670
0.05
1.29 0.18 0.33
220.76
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
298
7
0.500
6.00
0.630
0.07
0.79 0.24 0.25
353.36
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
298
5
0.519
5.70
0.980
0.04
1.47 0.44 0.13
320.66
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
298
2.55
0.520
6.00
1.000
0.04
1.34 0.21 0.29
617.43
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
298
1.65
0.518
6.20
0.780
0.09
0.69 0.47 0.13 1898.48
4.8
0.510
6.60
0.770
0.09
0.69 0.48 0.13
679.50
5.2
0.500
8.00
0.760
0.10
0.63 0.49 0.13
690.42
0.56M 308
0.25M 308
308
0.56M 308
0.25M 308
0.1M
αc
ip/10-5
298
0.1M
αa
-Ec
308
318
stirred
stirred
stirred
unstirred
unstirred
unstirred
(48)
-bc
Rp
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
-Ec
ip/10-5
Ep
ba
αa
-bc
αc
Rp
1.85
0.530
12.00
0.860
0.04
1.34
0.22
0.27
85.92
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
298
1.65
0.520
4.50
0.790
0.07
0.85
0.21
0.29 136.88
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
298
1.35
0.500
4.20
0.800
0.05
1.09
0.66
0.09 160.95
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
298
0.57
0.532
1.60
0.790
0.06
1.06
0.40
0.15 371.88
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
298
0.5
0.510
6.80
0.620
0.06
0.94
0.49
0.12 483.56
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
298
0.4
0.510
1.20
0.740
0.05
1.10
0.38
0.16 508.97
1
0.500
8.50
0.680
0.05
1.12
0.39
0.16 207.74
1.35
0.500
8.80
0.600
0.05
1.27
0.43
0.15 143.45
T/K medium ic/10-4
298
0.56M 308
0.25M 308
0.1M
308
0.56M 308
0.25M 308
0.1M
308
318
stirred
stirred
stirred
unstirred
unstirred
unstirred
(49)
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
-Ec
ip/10-5
Ep
ba
αa
5.8
0.533
4.7
1.010
0.04
1.38
0.16 0.37
252.55
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
298
4.8
0.520
6.5
0.650
0.07
0.84
0.11 0.52
388.00
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
298
3.3
0.520
5.5
0.630
0.09
0.69
0.72 0.08 1011.65
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
298
3.3
0.536
5
1.000
0.11
0.55
0.21 0.29
922.61
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
298
5
0.510
4.5
0.950
0.05
1.21
0.38 0.16
375.49
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
298
1.3
0.530
5.1
0.870
0.03
1.71
0.35 0.17 1050.71
3.8
0.510
6
0.790
0.04
1.65
0.53 0.12
395.89
7.4
0.500
6.7
0.710
0.07
0.88
0.54 0.12
370.03
T/K medium ic/10-5
298
0.56M 308
0.25M 308
0.1M 308
0.56M 308
0.25M 308
0.1M 308
318
stirred
stirred
stirred
unstirred
unstirred
unstirred
(50)
-bc
αc
Rp
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
αc
Rp
0.78 0.21
0.28
241.56
0.09
0.72 0.23
0.26
246.28
0.920
0.09
0.68 0.24
0.26
207.02
8
0.710
0.07
0.85 0.13
0.47
430.43
0.500
9
0.680
0.09
0.66 0.16
0.39
387.60
8
0.500
1
0.640
0.10
0.63 0.16
0.39
334.68
5.1
0.500
7.5
0.970
0.06
0.91 0.62
0.10
499.76
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
298
3.7
0.528
7.5
1.070
0.03
1.97 0.15
0.41
291.92
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
298
2.5
0.520
7.3
1.050
0.04
1.33 0.22
0.26
554.15
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
298
2.1
0.510
1.15
1.010
0.06
1.05 0.10
0.61
737.49
4.5
0.500
8
0.840
0.10
0.59 0.16
0.39
600.80
8
0.500
8.5
0.830
0.10
0.65 0.17
0.37
335.67
T/K medium ic/10-4
-Ec
ip/10-5
Ep
ba
10
0.527
6.8
1.080
0.08
11
0.521
7.2
0.950
318
14
0.520
7.6
298
4.5
0.510
6.5
318
298
298
0.56M 308
0.25M 308
0.1M 308
0.56M 308
0.25M 308
0.1M 308
318
stirred
stirred
stirred
unstirred
unstirred
unstirred
(51)
αa
-bc
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
298
2.6
0.521
75
0.920
0.06
0.95 0.30
0.19
86.32
0.56M 308 stirred
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
298
2.4
0.500
5.7
0.950
0.07
0.91 0.17
0.35
85.32
0.25M 308 stirred
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
298
2.2
0.500
5.8
0.890
0.07
0.90 0.21
0.29
98.35
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
298
0.63
0.523
7.5
0.970
0.05
1.15 0.50
0.12
321.49
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
298
5
0.520
9.5
0.860
0.05
1.19 0.30
0.20
371.15
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
298
7.5
0.500
6.3
0.910
0.06
0.92 0.27
0.22
301.45
8.8
0.490
7.5
0.790
0.06
1.04 0.33
0.18
247.20
15
0.480
11.5
0.780
0.06
1.14 0.38
0.17
139.76
0.1M 308 stirred
0.56M 308
0.25M 308
0.1M 308
318
unstirred
unstirred
unstirred
(52)
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
-Ec
ip/10-3
Ep
ba
αa
-bc
1.2
0.474
5
0.810
0.05
1.31
0.34
0.18 1.65
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
298
1.05
0.510
2.2
0.760
0.08
0.72
0.15
0.40 2.18
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
298
0.375
0.520
2.75
0.770
0.05
1.22
0.17
0.34 4.38
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
298
1.1
0.478
4.8
0.780
0.05
1.25
0.17
0.35 1.61
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
298
1
0.520
2.75
0.770
0.07
0.83
0.22
0.27 2.35
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
298
0.45
0.500
2.9
0.770
0.05
1.09
0.27
0.22 4.38
0.5
0.500
3.2
0.780
0.05
1.13
0.27
0.22 3.93
0.61
0.500
3.5
0.750
0.09
0.74
0.30
0.21 4.71
T/K medium ic/10-2
298
0.56M 308
0.25M 308
0.1M 308
0.56M 308
0.25M 308
0.1M 308
318
stirred
stirred
stirred
unstirred
unstirred
unstirred
(53)
αc
Rp
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
Ep
ba
3
0.471
7.2
0.760
0.08
0.70 0.17 0.35
0.82
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
298
1.3
0.520
3
0.760
0.10
0.59 0.14 0.44
1.92
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
298
1.1
0.500
2.9
0.730
0.14
0.42 0.24 0.25
3.52
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
298
1.65
0.473
6.2
0.800
0.09
0.64 0.08 0.72
1.14
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
298
1.1
0.500
1.2
0.780
0.07
0.84 0.10 0.59
1.63
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
298
0.8
0.510
3.5
0.790
0.10
0.57 0.05 1.27
1.74
0.85
0.510
4.25
0.810
0.10
0.64 0.11 0.54
2.65
9
0.500
6.5
0.770
0.10
0.63 0.31 0.20
3.64
0.56M 308
0.25M 308
308
0.56M 308
0.25M 308
0.1M
αc
ip/10-3
298
0.1M
αa
-Ec
308
318
stirred
stirred
stirred
unstirred
unstirred
unstirred
(54)
-bc
Rp
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
Ep
ba
1.1
0.487
4.5
0.800
0.04
1.31 0.21 0.28
1.46
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
298
0.85
0.520
4
0.800
0.06
1.02 0.13 0.45
2.05
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
298
0.35
0.510
3.5
0.810
0.04
1.33 0.24 0.25
4.65
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
298
1
0.489
6.5
0.790
0.04
1.33 0.19 0.31
1.74
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
298
0.75
0.520
5.8
0.780
0.05
1.24 0.08 0.72
1.74
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
298
0.57
0.500
4.5
0.790
0.05
1.13 0.19 0.31
3.09
0.6
0.500
4.7
0.770
0.06
1.01 0.22 0.28
3.44
0.85
0.500
5.1
0.730
0.06
1.02 0.23 0.27
2.48
0.56M 308
0.25M 308
308
0.56M 308
0.25M 308
0.1M
αc
ip/10-3
298
0.1M
αa
-Ec
308
318
stirred
stirred
stirred
unstirred
unstirred
unstirred
(55)
-bc
Rp
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
Ep
ba
2.3
0.482
7
0.780
0.07
0.87 0.08 0.76
0.68
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
298
1.2
0.510
2.4
0.790
0.07
0.84 0.06 0.92
1.21
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
298
0.9
0.500
3.25
0.760
0.10
0.59 0.09 0.63
2.33
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
298
1.6
0.485
3.7
0.780
0.08
0.71 0.08 0.72
1.13
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
298
1.1
0.500
4.75
0.780
0.08
0.76 0.09 0.68
1.62
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
298
0.95
0.500
5.2
0.770
0.09
0.69 0.11 0.52
2.23
0.65
0.500
5.6
0.730
0.09
0.71 0.12 0.52
3.33
1.2
0.500
5.75
0.720
0.14
0.45 0.15 0.43
2.61
0.56M 308
0.25M 308
308
0.56M 308
0.25M 308
0.1M
αc
ip/10-3
298
0.1M
αa
-Ec
308
318
stirred
stirred
stirred
unstirred
unstirred
unstirred
(56)
-bc
Rp
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
Ep
ba
6.6
0.506
3.4
0.790
0.07
0.83 0.24 0.25 3.61
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
298
3
0.510
2.25
0.740
0.07
0.80 0.28 0.21 8.49
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
298
1.2
0.510
1.75
0.770
0.06
1.04 0.14 0.44 14.45
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
298
5.4
0.509
2.6
0.780
0.04
1.44 0.15 0.40 2.58
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
298
2.1
0.510
1.7
0.740
0.11
0.52 0.27 0.22 16.66
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
298
2
0.500
1.9
0.740
0.04
1.34 0.23 0.25 8.04
2.75
0.500
2
0.740
0.06
0.97 0.33 0.18 8.35
5.75
0.510
2.35
0.700
0.17
0.37 0.93 0.07 10.88
0.56M 308
0.25M 308
308
0.56M 308
0.25M 308
0.1M
αc
ip/10-3
298
0.1M
αa
-Ec
308
318
stirred
stirred
stirred
unstirred
unstirred
unstirred
(57)
-bc
Rp
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
αc
Rp
2.05 0.18
0.34
0.51
0.04
1.37 0.31
0.20
0.75
0.760
0.05
1.27 0.17
0.37
0.70
3
0.770
0.06
1.03 0.10
0.59
1.70
0.500
3.4
0.750
0.06
1.08 0.31
0.20
2.07
1.1
0.500
3.4
0.730
0.06
1.08 0.33
0.19
1.96
0.17
0.520
2
0.740
0.05
1.08 0.32
0.18
11.93
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
298
2
0.502
4.25
0.790
0.07
0.85 0.12
0.48
0.97
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
298
0.82
0.500
2.5
0.730
0.12
0.48 0.20
0.30
3.99
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
298
0.7
0.520
2
0.750
0.09
0.63 0.18
0.33
3.82
1.15
0.520
2.5
0.700
0.10
0.63 0.21
0.29
2.50
1.5
0.510
2.5
0.680
0.14
0.47 0.27
0.24
2.60
T/K medium ic/10-2
-Ec
ip/10-3
Ep
ba
2.1
0.500
3.4
0.780
0.03
2.25
0.500
4.5
0.770
318
2.4
0.500
5
298
0.93
0.510
1
318
298
298
0.56M 308
0.25M 308
0.1M
308
0.56M 308
0.25M 308
0.1M
308
318
stirred
stirred
stirred
unstirred
unstirred
unstirred
(58)
αa
-bc
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
Ep
ba
2
0.492
4.5
0.770
0.06
0.94 0.11 0.52
0.88
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
298
1.3
0.530
1.35
0.810
0.11
0.55 0.11 0.52
1.86
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
298
0.95
0.520
1.25
0.780
0.11
0.56 0.13 0.46
2.65
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
298
1.25
0.493
3.4
0.770
0.07
0.87 0.08 0.71
1.30
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
298
1.22
0.530
2
0.790
0.10
0.59 0.10 0.59
1.77
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
298
0.87
0.500
2.3
0.790
0.11
0.52 0.11 0.52
2.82
1
0.510
3.2
0.760
0.13
0.48 0.12 0.50
2.70
1.25
0.510
5.5
0.740
0.15
0.41 0.12 0.53
2.33
0.56M 308
0.25M 308
308
0.56M 308
0.25M 308
0.1M
αc
ip/10-3
298
0.1M
αa
-Ec
308
318
stirred
stirred
stirred
unstirred
unstirred
unstirred
(59)
-bc
Rp
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
-Ec
ip/10-3
Ep
ba
αa
-bc
αc
Rp
1.05
0.495
4.25
0.770
0.05
1.16
0.15
0.39
1.38
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
298
0.55
0.510
32
0.750
0.07
0.90
0.09
0.67
2.97
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
298
0.53
0.510
2.2
0.780
0.06
0.98
0.14
0.44
3.42
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
298
0.9
0.497
4.1
0.760
0.08
0.76
0.10
0.58
1.73
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
298
1.1
0.510
2.85
0.780
0.07
0.87
0.13
0.47
1.75
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
298
1
0.520
2.5
0.790
0.08
0.71
0.09
0.68
1.85
1.2
0.510
2.8
0.770
0.09
0.65
0.27
0.23
2.53
1.4
0.500
4
0.760
0.09
0.72
0.38
0.17
2.20
T/K medium ic/10-2
298
0.56M 308
0.25M 308
0.1M
308
0.56M 308
0.25M 308
0.1M
308
318
stirred
stirred
stirred
unstirred
unstirred
unstirred
(60)
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.
(61)
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.
(62)
(63)
(64)
(65)
(66)
(67)
(68)
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 unstirred 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.
(69)
(70)
(71)
(72)
(73)
(74)
(75)
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).
(76)
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).
(77)
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):
ac =
2.303RT
---bc F
(3.1)
aa =
2.303RT
---ba F
(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:
diffusion
H3O+(bulk solution)
H3O+ (metal / solution interface) ---- (3.3)
which is followed by the discharge step (D):
D
H3O+ + M + e
M – H + H2O
(78)
----(3.4)
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:
C-D
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 ratedetermining 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 ratedetermining 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:
E-D
2M + H + H O
.…(3.6)
M-H + H O++ M
3
(electrode)
2
2
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
(79)
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 = (
¶h
) T ,C at h ® 0
¶i
---- (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 =
b
Rp
----(3.8)
where b is a combination of the anodic and cathodic Tafel slopes (ba,
bc) as (89)(90):
b=
ba bc
2.303(ba + bc )
---- (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):
Rp =
ba bc
2.303(ba + bc ) i c
----(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.
(80)
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.
(81)
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
(82)
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.
(83)
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
-ΔG
ΔH
ΔS
99.00
-61.63
98.80
98.42
-59.80
98.23
318
96.49
-56.62
96.32
298
99.58
-53.57
99.40
98.42
-50.87
98.26
96.49
-47.39
96.34
T/K
medium
298
308
lead alloy
1
308
stirred
un-stirred
318
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
-ΔG
ΔH
ΔS
100.68
-53.41
100.63
100.16
-49.65
100.00
318
99.00
-46.85
98.86
298
101.72
-70.96
101.45
101.51
-68.74
101.29
100.55
-66.71
100.34
T/K
medium
298
308
lead grid
2
308
stirred
un-stirred
318
(84)
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
-ΔG
ΔH
ΔS
100.55
-68.93
100.32
99.39
-66.71
99.17
318
98.42
-64.68
98.22
298
100.93
-80.82
100.66
100.35
-79.56
100.10
99.58
-78.12
99.34
T/K
medium
298
308
pure lead
3
308
stirred
un-stirred
318
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
-ΔG
ΔH
ΔS
98.42
-69.67
98.19
97.46
-67.74
97.24
318
96.49
-65.81
96.29
298
96.88
-91.13
96.57
96.49
-90.55
96.20
96.49
-90.36
96.21
T/K
medium
298
308
Uncured
Positive
4
308
stirred
un-stirred
318
(85)
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
-ΔG
ΔH
ΔS
93.02
-35.51
92.90
92.63
-33.19
92.53
318
92.63
-31.26
92.54
298
93.60
-79.24
93.33
93.21
-78.37
92.96
92.63
-77.31
92.39
T/K
medium
298
308
Cured
Positive
5
308
stirred
un-stirred
318
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
-ΔG
ΔH
ΔS
94.95
-80.59
94.68
94.56
-79.72
94.31
318
93.99
-78.66
93.74
298
95.14
-86.53
94.85
95.14
-86.24
94.86
94.56
-85.37
94.30
T/K
medium
298
308
Uncured
Negative
6
308
Stirred
un-stirred
318
(86)
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
-ΔG
ΔH
ΔS
90.90
-88.04
90.60
90.90
-87.94
90.61
318
90.70
-87.65
90.43
298
91.28
-85.53
91.00
90.90
-84.95
90.62
90.90
-84.76
90.63
T/K
medium
298
308
cured
negative
7
308
stirred
un-stirred
318
(87)
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,
(88)
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.
(89)
(90)
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
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
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
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
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
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
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
grid lead
2
pure lead
3
uncured
positive
4
cured
positive
5
uncured
negative
6
cured
negative
7
(91)
(92)
(93)
(94)
(95)
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.
(96)
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
298
-bc
αc
Rp
0.8
0.840
0.12
0.50
0.27
0.22 886.77
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
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
298
4.6
0.518
1
0.690
0.11
0.56
0.13
0.46 549.60
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
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
298
6.3
0.516
3.5
0.710
0.07
0.81
0.25
0.23 390.78
12
0.514
4.3
0.700
0.08
0.79
0.27
0.22 219.22
15
0.513
6
0.700
0.09
0.73
0.29
0.21 193.51
19
0.513
5.2
0.690
0.07
0.82
0.22
0.27 123.92
31
0.511
7.8
0.690
0.10
0.62
0.23
0.26
318
33
0.511
8.3
0.670
0.31
0.20
0.27
0.23 191.57
298
5
0.517
2.5
0.640
0.05
1.30
0.15
0.39 304.38
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
13
0.513
4
0.630
0.09
0.66
0.18
0.34 203.11
20
0.511
5.3
0.630
0.11
0.57
0.26
0.24 168.45
308
308
308
NaCl
318
(4g dm3
)
298
308
308
FeSO4
(o.2g
dm-3)
αa
0.520
308
H3PO4
(11g
dm-3)
+
FeSO4
(o.2g
dm-3)
ba
4
308
H3PO4
(11g
dm-3)
ip/10-4 Ep
308
318
Unstirred
Stirred
Unstirred
Stirred
Unstirred
Stirred
unstirred
stirred
(97)
95.13
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).
Additiv
T/K medium ic/10-5
e
αa
-bc
0.08
0.72
0.25
0.24 894.56
0.860
0.15
0.41
0.31
0.20 878.74
8.5
0.840
0.16
0.40
0.52
0.12 592.40
0.527
7.2
0.900
0.18
0.33
0.22
0.27 1014.59
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
298
4.6
0.528
8.9
1.030
0.04
1.43
0.37
0.16 350.19
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
298
12
0.522
33
0.760
0.03
1.89
0.16
0.37 183.07
7.5
0.521
43
0.760
0.04
1.55
0.17
0.36 185.20
9
0.520
57
0.750
0.04
1.53
0.20
0.32 164.01
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
298
5
0.525
1.2
0.730
0.35
0.17
0.22
0.27 1179.48
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
16.5
0.522
33
0.610
0.12
0.52
0.31
0.20 229.78
-Ec
ip/10-4
Ep
3
0.529
7
0.980
5
0.527
8
318
8.8
0.526
298
4.2
298
H3PO4
(11g
dm-3)
308
unstirred
308 stirred
H3PO4
(11g
dm-3)
+
FeSO4
(o.2g
dm-3)
308
308
NaCl
(4g dm- 318
3
)
298
308
FeSO4
(o.2g
dm-3)
318
unstirred
unstirred
unstirred
(98)
ba
αc
Rp
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).
Additiv
T/K medium ic/10-5
e
αa
-bc
αc
Rp
0.05
1.13
0.12
0.50
3.15
0.820
0.05
1.23
0.14
0.45
2.89
2
0.810
0.06
1.05
0.22
0.29
3.25
0.537
1.2
0.810
0.05
1.21
0.15
0.39
1.79
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
298
5.6
0.527
1.4
0.840
0.09
0.67
0.11
0.56
3.72
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
298
10.6
0.485
3.4
0.730
0.04
1.42
0.15
0.39
1.34
18
0.484
3.4
0.700
0.05
1.13
0.16
0.37
0.98
20
0.483
3.9
0.710
0.06
1.03
0.18
0.34
1.00
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
298
6.3
0.518
3
0.740
0.05
1.26
0.13
0.45
2.37
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
-Ec
ip/10-4
Ep
5
0.539
1
0.830
5.5
0.539
1
318
6.3
0.538
298
9
308 stirred
298
308
H3PO4
(11g
dm-3)
H3PO4
(11g
dm-3)
+
FeSO4
(o.2g
dm-3)
308
308
unstirred
unstirred
unstirred
NaCl
318
(4g dm3
)
298
308
FeSO4
(o.2g
dm-3)
unstirred
(99)
ba
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
αa
αc
-Ec
ip/10-4
Ep
0.82
0.537
2.7
0.820
0.08
0.73
0.14 0.42
2.73
0.93
0.536
2.9
0.810
0.09
0.64
0.17 0.37
2.82
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
298
0.95
0.525
3.4
0.820
0.05
1.19
0.15 0.40
1.71
1.15
0.523
3.5
0.800
0.06
0.98
0.19 0.32
1.78
1.2
0.523
4.5
0.790
0.07
0.97
0.38 0.17
2.01
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
298
2
0.473
6.9
0.700
0.04
1.38
0.13 0.46
0.70
2.4
0.472
7
0.690
0.04
1.37
0.14 0.43
0.61
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
298
1
0.516
3.1
0.740
0.05
1.25
0.12 0.49
1.48
1
0.514
3.4
0.730
0.05
1.16
0.17 0.36
1.74
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
298
308
unstirred
318
H3PO4
(11g dm-3)
308
un-
H3PO4
stirred
(11g dm-3)
318
+
FeSO4
298
(o.2g dm-3)
308
unstirred
318
NaCl
-3
(4g dm )
308
unstirred
318
FeSO4
-3
(o.2g dm )
(100)
ba
-bc
Rp
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.
(101)
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.
(102)
(103)
(104)
(105)
(106)
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 unstirred 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.
(107)
(108)
(109)
(110)
(111)
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
(112)
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
(113)
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
(114)
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.
(115)
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):
ac =
aa =
2.303RT
----bc F
2.303RT
----ba F
(3.1)
(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 ratedetermining 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.
(116)
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).
(117)
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 =
ba bc
2.303(ba + bc )ic
---- (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.
(118)
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.
(119)
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.
(120)
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)
(121)
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.
(122)
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)
i
i1
P%= 100 [ 1- 2 ]
----- (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
(123)
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
(124)
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.
(125)
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
298
H3PO4
(H3PO4
(11g dm-3)+
(11g dm-3) FeSO4(0.2g
dm-3))
NaCl
FeSO4
(4g dm-3)
(0.2g dm-3)
38.46
29.23
3.08
23.08
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
68.97
62.07
-8.62
55.17
67.50
62.50
17.50
50.00
308
308
un-stirred
stirred
318
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
298
H3PO4
(H3PO4
(11g dm-3)+
(11g dm-3) FeSO4(0.2g
dm-3))
NaCl
FeSO4
(4g dm-3)
(0.2g dm-3)
52.38
26.98
1.59
20.63
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
80.00
65.71
57.14
61.43
80.80
68.00
56.00
67.00
308
308
318
un-stirred
stirred
(126)
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
298
H3PO4
(H3PO4
(11g dm-3)+
(11g dm-3) FeSO4(0.2g
dm-3))
NaCl
FeSO4
(4g dm-3)
(0.2g dm-3)
68.75
65.00
33.75
60.63
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
71.43
57.14
40.00
65.71
64.86
52.70
35.14
59.46
308
308
un-stirred
stirred
318
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
298
H3PO4
(H3PO4
(11g dm-3)+
(11g dm-3) FeSO4(0.2g
dm-3))
NaCl
FeSO4
(4g dm-3)
(0.2g dm-3)
50.30
42.42
9.09
39.39
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
71.97
65.15
27.27
48.48
72.00
65.71
5.71
35.71
308
308
318
un-stirred
stirred
(127)
(128)
(129)
(130)
(131)
(132)
(133)
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
(134)
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:
FeSO4, NaCl, (FeSO4+ H3PO4) mixture, and H3PO4 with lead
(i)
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
(135)
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.
(136)
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
-ΔG
ΔH
ΔS
100.35
-71.60
100.11
99.39
-69.67
99.16
318
98.42
-67.74
98.21
298
100.16
-88.66
99.86
99.39
-87.50
99.10
318
99.39
-87.11
99.11
298
99.97
-91.36
99.66
99.78
-90.88
99.49
318
99.40
-90.20
99.11
298
99.78
-85.42
99.49
99.20
-84.36
98.93
318
98.82
-83.49
98.55
298
99.59
-90.95
99.28
99.20
-90.27
98.91
318
99.01
-89.79
98.73
298
99.01
-90.40
98.71
98.62
-89.72
98.33
318
98.43
-89.24
98.15
298
99.78
-94.03
99.47
99.78
-93.84
99.48
318
99.40
-93.26
99.10
298
99.40
-85.00
99.11
98.43
-83.55
98.16
98.43
-83.07
98.17
T/K
medium
298
308
H3PO4
(11g dm-3)
308
308
FeSO4
(o.2g dm-3)
+
H3PO4
(11g dm-3)
308
308
NaCl
(4g dm-3)
308
308
FeSO4
(o.2g dm-3)
308
un-stirred
stirred
un-stirred
stirred
un-stirred
stirred
un-stirred
stirred
318
(137)
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
-ΔG
ΔH
ΔS
102.10
-93.46
101.78
101.71
-92.78
101.41
318
101.52
-92.30
101.23
298
101.71
-93.07
101.40
101.33
-92.39
101.03
318
101.13
-91.91
100.84
298
101.90
-93.29
101.59
101.71
-92.81
101.41
318
101.33
-92.13
101.04
298
101.52
-95.77
101.20
101.33
-95.38
101.02
318
101.13
-94.99
100.83
298
100.75
-94.99
100.43
100.55
-94.61
100.25
318
100.36
-94.22
100.06
298
100.55
-94.80
100.23
100.36
-94.42
100.05
318
100.17
-94.03
99.87
298
101.33
-92.71
101.01
101.13
-92.23
100.83
318
100.75
-91.56
100.46
298
100.94
-92.33
100.63
100.94
-92.04
100.64
100.75
-91.56
100.46
T/K
medium
298
308
H3PO4
(11g dm-3)
308
FeSO4
(o.2g dm-3)
+
H3PO4
(11g dm-3)
308
308
308
NaCl
(4g dm-3)
308
308
FeSO4
(o.2g dm-3)
308
un-stirred
stirred
un-stirred
stirred
un-stirred
stirred
un-stirred
stirred
318
(138)
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
-ΔG
ΔH
ΔS
104.03
-101.14
103.69
104.03
-101.04
103.70
318
103.83
-100.75
103.52
298
103.64
-100.75
103.30
103.64
-100.65
103.31
318
103.45
-100.36
103.13
298
101.71
-95.96
101.39
101.52
-95.57
101.21
318
101.33
-95.19
101.03
298
100.55
-97.66
100.23
100.36
-97.37
100.04
318
100.36
-97.28
100.05
298
93.61
-87.85
93.31
93.41
-87.47
93.13
318
93.22
-87.08
92.95
298
93.22
-87.47
92.93
93.22
-87.27
92.94
318
92.83
-86.70
92.56
298
99.97
-94.22
99.66
99.97
-94.03
99.67
318
99.59
-93.45
99.29
298
97.47
-88.82
97.17
97.08
-88.15
96.79
96.89
-87.66
96.61
T/K
medium
298
308
H3PO4
(11g dm-3)
308
FeSO4
(o.2g dm-3)
+
H3PO4
(11g dm-3)
308
308
308
NaCl
(4g dm-3)
308
308
FeSO4
(o.2g dm-3)
308
un-stirred
stirred
un-stirred
stirred
un-stirred
stirred
un-stirred
stirred
318
(139)
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
-ΔG
ΔH
ΔS
103.64
-97.89
103.31
103.45
-97.50
103.13
318
103.26
-97.12
102.95
298
102.87
-94.26
102.55
102.29
-93.39
101.99
318
102.29
-93.10
102.00
298
101.33
-95.57
101.00
100.94
-94.99
100.63
318
100.94
-94.80
100.64
298
100.55
-97.66
100.23
100.36
-97.37
100.04
318
100.36
-97.28
100.05
298
91.29
-85.54
91.00
91.10
-85.15
90.82
318
90.90
-84.77
90.64
298
91.10
-85.34
90.81
90.71
-84.77
90.43
318
90.71
-84.57
90.44
298
99.59
-90.95
99.28
T/K
medium
298
308
H3PO4
(11g dm-3)
308
FeSO4
(o.2g dm-3)
+
H3PO4
(11g dm-3)
308
308
308
NaCl
(4g dm-3)
308
FeSO4
(o.2g dm-3)
un-stirred
stirred
un-stirred
stirred
un-stirred
stirred
308
318
298
un-stirred
99.20
99.01
99.01
-90.27
-89.79
-93.26
98.91
98.73
98.70
308
stirred
99.01
-93.06
98.71
98.62
-92.49
98.33
318
(140)
(141)
(142)
(143)
(144)
(145)
(146)
(147)
(148)
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=
¹
KT
Ce DS / R
h
----- (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.
(149)
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
(150)
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) , preexponential 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≠
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
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
without
additive
H3PO4
(11g dm-3)
un-stirred
FeSO4
-3
(0.2g dm )
+
H3PO4
(11g dm-3) Stirred
14.15
1.40 X 10+14
24.65
25.78
12.06
1.12 X 10+12
20.06
-14.15
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
un-stirred
9.13
1.35 X 10+9
12.02
-70.22
NaCl
(4g dm-3)
FeSO4
-3
(0.2g dm )
(151)
stirred
11.29
1.95X 10+11
(152)
18.34
-28.89
Table (4.14):Values of activation Energies(Ea/k J mol-1) , preexponential 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
without
medium
log A
A
Ea
∆S≠
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
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
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
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
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
additive
H3PO4
(11g dm-3)
FeSO4
(0.2g dm-3)
+
H3PO4
-3
(11g dm )
NaCl
(4g dm-3)
FeSO4
(0.2g dm-3)
(153)
Table (4.15):Values of activation Energies(Ea/k J mol-1), preexponential 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≠
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
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
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
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
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
without
additive
H3PO4
(11g dm-3)
FeSO4
(0.2g dm-3)
+
H3PO4
-3
(11g dm )
NaCl
(4g dm-3)
FeSO4
(0.2g dm-3)
(154)
Table (4.16):Values of activation Energies(Ea/k J mol-1), preexponential 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≠
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
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
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
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
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
without
additive
H3PO4
(11g dm-3)
FeSO4
(0.2g dm-3)
+
H3PO4
-3
(11g dm )
NaCl
(4g dm-3)
FeSO4
(0.2g dm-3)
(155)
(156)
(157)
(158)
(159)
(160)
(161)
(162)
(163)
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.
(164)
(165)
(166)
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.
١٦٦(
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.
١٦٧(
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(173)
‫ﺍﻟﺨﻼﺻﺔ‬
‫ﻳﺘﻨﺎﻭﻝ ﻣﻮﺿﻮﻉ ﺍﻟﺮﺳﺎﻟﺔ ﺩﺭﺍﺳﺔ ﺍﻟﺴﻠﻮﻙ ﺍﻻﺳﺘﻘﻄﺎﺑﻲ ﻟﺘﺄﻛﻞ ﺳﺒﻌﺔ ﻧﻤﺎﺫﺝ ﻣﻦ‬
‫ﺍﻻﻗﻄﺎﺏ ﺍﻟﻤﺴﺘﺨﺪﻣﺔ ﻓﻲ ﺻﻨﺎﻋﺔ ﺍﻟﻮﺍﺡ ﻭﻣﻜﻮﻧﺎﺕ ﻧﻀﻴﺪﺓ ﺍﻟﺮﺻﺎﺹ‬
‫ﺍﻟﺤﺎﻣﻀﻴﺔ ﻭﻫﻲ ‪:‬‬
‫‪ -۱‬ﻗﻄﺐ ﺳﺒﻴﻜﺔ ﺍﻟﺮﺻﺎﺹ ‪.‬‬
‫‪ -۲‬ﻗﻄﺐ ﻣﺸﺒﻚ ﺍﻟﺮﺻﺎﺹ ‪.‬‬
‫‪ -۳‬ﻗﻄﺐ ﺍﻟﺮﺻﺎﺹ ﺍﻟﻨﻘﻲ ‪.‬‬
‫‪ -٤‬ﺍﻟﻘﻄﺐ ﺍﻟﻤﻮﺟﺐ ﻏﻴﺮ ﺍﻟﻤﻌﻤﺮ ‪.‬‬
‫‪ -٥‬ﺍﻟﻘﻄﺐ ﺍﻟﻤﻮﺟﺐ ﺍﻟﻤﻌﻤﺮ‪.‬‬
‫‪ -٦‬ﺍﻟﻘﻄﺐ ﺍﻟﺴﻠﺐ ﻏﻴﺮ ﺍﻟﻤﻌﻤﺮ‪.‬‬
‫‪ -۷‬ﺍﻟﻘﻄﺐ ﺍﻟﺴﺎﻟﺐ ﺍﻟﻤﻌﻤﺮ‪.‬‬
‫ﻋﻨﺪ ﻏﻤﺮﻫﺎ ﻓﻲ ﻣﺤﻠﻮﻝ ﺣﺎﻣﺾ ﺍﻟﻜﺒﺮﻳﺘﻴﻚ ﺑﺘﺮﺍﻛﻴﺰ)‪ (0.56,0.25,0.1‬ﻣﻮﻝ‬
‫ﻟﻠﺪ ﺳﻤﺘﺮ ﺍﻟﻤﻜﻌﺐ ﻋﻠﻰ ﻣﺪﻯ ﻣﻦ ﺩﺭﺟﺎﺕ ﺍﻟﺤﺮﺍﺭﺓ ﺗﺮﺍﻭﺣﺖ ﻣﻦ ‪ ۲۹۸‬ﺍﻟﻰ‬
‫‪ ۳۱۸‬ﻛﻠﻔﻦ ﻭﻳﻤﻜﻦ ﺗﻘﺴﻴﻢ ﺍﻟﻨﻤﻂ ﺍﻟﻌﺎﻡ ﻭﺍﻟﺼﻴﻐﺔ ﺍﻟﻌﻤﻠﻴﺔ ﻟﻠﺪﺭﺍﺳﺔ ﺍﻟﻰ ﺍﺭﺑﻌﺔ‬
‫ﺍﻗﺴﺎﻡ ﻭﻛﻤﺎ ﻳﺄﺗﻲ ‪:‬‬
‫ﺃ‪ -‬ﺩﺭﺍﺳﺔ ﺍﻟﺴﻠﻮﻙ ﺍﻻﺳﺘﻘﻄﺎﺑﻲ ﻟﻠﻨﻤﺎﺫﺝ ﺑﻮﺟﻮﺩ ﻏﺎﺯﺍﻷﻛﺴﺠﻴﻦ ﻓﻲ ﻭﺳﻂ‬
‫ﺍﻟﺘﺄﻛﻞ‪.‬‬
‫ﺏ – ﺩﺭﺍﺳﺔ ﺍﻟﺴﻠﻮﻙ ﺍﻻﺳﺘﻘﻄﺎﺑﻲ ﻟﻠﻨﻤﺎﺫﺝ ﺑﻮﺟﻮﺩ ﻏﺎﺯ ﺍﻷﻭﻛﺴﺠﻴﻦ ﻭﻓﻲ ﺟﻮ‬
‫ﺗﺤﺮﻳﻜﻲ ﻟﻮﺳﻂ ﺍﻟﺘﺎﻛﻞ ‪.‬‬
‫ﺝ‪ -‬ﺩﺭﺍﺳﺔ ﺍﻟﺴﻠﻮﻙ ﺍﻻﺳﺘﻘﻄﺎﺑﻲ ﻟﻠﻨﻤﺎﺫﺝ ﺑﻮﺟﻮﺩ ﻏﺎﺯﺍﻟﻨﺘﺮﻭﺟﻴﻦ ﻓﻲ ﻭﺳﻂ‬
‫ﺍﻟﺘﺄﻛﻞ‪.‬‬
‫ﺩ‪ –-‬ﺩﺭﺍﺳﺔ ﺍﻟﺴﻠﻮﻙ ﺍﻻﺳﺘﻘﻄﺎﺑﻲ ﻟﻠﻨﻤﺎﺫﺝ ﺑﻮﺟﻮﺩ ﻏﺎﺯ ﺍﻟﻨﺘﺮﻭﺟﻴﻦ ﻭﻓﻲ ﺟﻮ‬
‫ﺗﺤﺮﻳﻜﻲ ﻟﻮﺳﻂ ﺍﻟﺘﺎﻛﻞ ‪.‬‬
‫‪ -۱‬ﺗﻤﺖ ﺩﺭﺍﺳﺔ ﺍﻟﺴﻠﻮﻙ ﺍﻻﺳﺘﻘﻄﺎﺑﻲ ﻟﻠﻨﻤﺎﺫﺝ ﺑﺎﺳﺘﺨﺪﺍﻡ ﺟﻬﺎﺯ ﺍﻟﻤﺠﻬﺎﺩ‬
‫ﺍﻟﺴﻜﻮﻧﻲ )‪(Potentiostat‬ﺍﻟﻤﺴﻤﻰ)‪(CORROSCRIPT‬ﺍﻟﻤﺴﺘﺤﺼﻞ ﻣﻦ‬
‫ﺷﺮﻛﺔ ﺗﺎﻛﻮﺳﻴﻞ )‪(Taccussel‬ﺍﻟﻔﺮﻧﺴﻴﺔ ﻭﺃﻣﻜﻦ ﺑﻮﺍﺳﻄﺘﻪ ﺍﻟﺤﺼﻮﻝ ﻋﻠﻰ‬
‫ﻣﻨﺤﻨﻴﺎﺕ ﺍﻻﺳﺘﻘﻄﺎﺏ )‪(Polarization Curves‬ﻋﻠﻰ ﻣﺪﻯ ﻣﻦ ﺍﻟﺠﻬﻮﺩ‬
‫ﺗﺮﺍﻭﺣﺖ ﻣﻦ ‪-2.0‬ﺍﻟﻰ ‪+2.0‬ﻓﻮﻟﺖ ﺑﺄﺳﺘﻌﻤﺎﻝ ﺳﺮﻋﺔ ﻣﺴﺢ ) ‪Scan‬‬
‫‪(Rate‬ﻟﺠﻬﺎﺯ ﺍﻟﺘﺴﺠﻴﻞ )‪ (x-y Recorder‬ﺑﻠﻐﺖ ‪ 30‬ﻣﻠﻤﺘﺮ ﻓﻲ ﺍﻟﺪﻗﻴﻘﺔ‬
‫)‪ (mm/min‬ﻭﻗﺪ ﺗﺒﻴﻦ ﻣﻦ ﺍﻟﻨﺘﺎﺋﺞ ﺍﻟﻤﺴﺘﺤﺼﻠﺔ ﺃﻥ ﺟﻬﺪ ﺍﻟﺘﺄﻛﻞ )‪(Ec‬ﻳﺼﺒﺢ‬
‫ﺃﻛﺜﺮﺳﺎﻟﺒﻴﺔ ﻓﻲ ﻣﺤﻠﻮﻝ ﺣﺎﻣﺾ ﺍﻟﻜﺒﺮﻳﺘﻴﻚ ﺑﻮﺟﻮﺩ ﻏﺎﺯﺍﻟﻨﺘﺮﻭﺟﻴﻦ ﻓﻲ ﻭﺳﻂ‬
‫ﻏﻴﺮ ﻣﺘﺤﺮﻙ ﻭﺃﻗﻞ ﺳﺎﻟﺒﻴﺔ ﺑﻮﺟﻮﺩ ﻏﺎﺯ ﺍﻻﻭﻛﺴﺠﻴﻦ ﻓﻲ ﻭﺳﻂ ﻣﺘﺤﺮﻙ ﻭﺃﻥ‬
‫ﺗﻐﻴﺮﺍﺕ ﻛﺜﺎﻓﺔ ﺗﻴﺎﺭ ﺍﻟﺘﺄﻛﻞ )‪ (ic‬ﻋﻤﻮﻣﺎ ﻛﺎﻧﺖ ﻋﺎﻟﻴﺔ ﻓﻲ ﺣﺎﻣﺾ ﺍﻟﻜﺒﺮﻳﺘﻴﻚ‬
‫ﺑﻮﺟﻮﺩ ﻏﺎﺯ ﺍﻻﻭﻛﺴﺠﻴﻦ ﻭﻓﻲ ﺟﻮ ﻣﺘﺤﺮﻙ ﻣﻘﺎﺭﻧﺔ ﺑﺠﻮ ﺍﻟﻨﺘﺮﻭﺟﻴﻦ ﻏﻴﺮ‬
‫ﺍﻟﻤﺘﺤﺮﻙ ﻭﺍﻟﺘﻲ ﻛﺎﻥ ﻓﻴﻬﺎ ﺗﻐﻴﺮ )‪ (ic‬ﻣﻨﺨﻔﻀﺎ‪.‬‬
‫‪ -۲‬ﺗﻤﺖ ﺩﺭﺍﺳﺔ ﺗﺄﺛﻴﺮ ﻋﺪﺩ ﻣﻦ ﺍﻟﻤﻀﺎﻓﺎﺕ ﺃﺷﺘﻤﻠﺖ ﻋﻠﻰ‪:‬‬
‫‪ -۱‬ﺣﺎﻣﺾ ﺍﻟﻔﻮﺳﻔﻮﺭﻳﻚ)‪۱۱‬ﻏﻢ ﻟﻠﺪﺳﻤﺘﺮ ﺍﻟﻤﻜﻌﺐ(‪.‬‬
‫‪ -۲‬ﻣﺰﻳﺞ ﻣﻦ ﺣﺎﻣﺾ ﺍﻟﻔﻮﺳﻔﻮﺭﻳﻚ )‪۱۱‬ﻏﻢ( ﻣﻊ ﻛﺒﺮﻳﺘﺎﺕ‬
‫ﺍﻟﺤﺪﻳﺪﻭﺯ)‪0.2‬ﻏﻢ( ﻟﻠﺴﻤﺘﺮ ﺍﻟﻤﻜﻌﺐ ‪.‬‬
‫‪ -۳‬ﻛﻠﻮﺭﻳﺪ ﺍﻟﺼﻮﺩﻳﻮﻡ)‪4‬ﻏﻢ ﻟﻠﺪﺳﻤﺘﺮﺍﻟﻤﻜﻌﺐ(‪.‬‬
‫‪ -٤‬ﻛﺒﺮﻳﺘﺎﺕ ﺍﻟﺤﺪﻳﺪﻭﺯ)‪0.2‬ﻏﻢ ﻟﻠﺪﺳﻤﺘﺮ ﺍﻟﻤﻜﻌﺐ(‪.‬‬
‫ﻓﻲ ﺍﻟﻤﺤﻠﻮﻝ ﺍﻷﻟﻜﺘﺮﻭﻟﻴﺘﻲ ﻟﺤﺎﻣﺾ ﺍﻟﻜﺒﺮﻳﺘﻴﻚ )‪(0.56‬ﻣﻮﻻﺭﻱ ﻓﻲ ﻭﺟﻮﺩ‬
‫ﺍﻻﻭﻛﺴﺠﻴﻦ ﻭﺟﻮﻣﺘﺤﺮﻙ ﻭﺳﺎﻛﻦ ﻋﻠﻰ ﺃﺭﺑﻌﺔ ﻧﻤﺎﺫﺝ ﻣﺨﺘﻠﻔﺔ ﻣﻦ ﺍﻻﻗﻄﺎﺏ‬
‫ﻭﻛﻤﺎ ﻳﺎﺗﻲ ‪:‬‬
‫‪ -۱‬ﻗﻄﺐ ﺳﺒﻴﻜﺔ ﺍﻟﺮﺻﺎﺹ‪.‬‬
‫‪ -۲‬ﻗﻄﺐ ﻣﺸﺒﻚ ﺍﻟﺮﺻﺎﺹ‪.‬‬
‫‪ -۳‬ﺍﻟﻘﻄﺐ ﺍﻟﻤﻮﺟﺐ ﺍﻟﻤﻌﻤﺮ‪.‬‬
‫‪ -٤‬ﺍﻟﻘﻄﺐ ﺍﻟﺴﺎﻟﺐ ﺍﻟﻤﻌﻤﺮ‪.‬‬
‫ﻓﻲ ﻣﺪﻯ ﺍﻟﺪﺭﺟﺎﺕ ﺍﻟﺤﺮﺍﺭﻳﺔ ﻣﻦ)‪(318-298‬ﻛﻠﻔﻦ ﻭﻗﺪ ﺩﻟﺖ ﺣﺴﺎﺑﺎﺕ‬
‫ﻛﻔﺎﻳﺔ ﺍﻟﺤﻤﺎﻳﺔ)‪ (Protection Efficiency‬ﻋﻠﻰ ﺑﻠﻮﻍ ﺍﻗﺼﻰ ﺣﻤﺎﻳﺔ‬
‫ﻣﻤﻜﻨﺔ ﺑﺎﺳﺘﻌﻤﺎﻝ ﺍﻟﻤﺜﺒﻂ ﺣﺎﻣﺾ ﺍﻟﻔﻮﺳﻔﻮﺭﻳﻚ ﻭﺍﺩﻧﻰ ﺣﻤﺎﻳﺔ ﻣﻤﻜﻨﺔ‬
‫ﺑﺄﺳﺘﻌﻤﺎﻝ ﻛﻠﻮﺭﻳﺪ ﺑﺎﻟﻨﺴﺒﺔ ﺍﻟﻰ ﺟﻤﻴﻊ ﺍﻻﻗﻄﺎﺏ‪.‬‬
‫‪ -۳‬ﺣﺴﺒﺖ ﺍﻟﻜﻤﻴﺎﺕ ﺍﻟﺜﺮﻣﻮﺩﺍﻳﻨﻤﻴﻜﻴﺔ ﻟﺘﻔﺎﻋﻼﺕ ﺍﻟﺘﺄﻛﻞ)‪(∆S،∆H،∆G‬‬
‫ﻓﻲ ﺣﺎﻟﺔ ﻏﻴﺎﺏ ﻭﻭﺟﻮﺩ ﺍﻟﻤﻀﺎﻓﺎﺕ ﻭﻗﺪ ﺃﻅﻬﺮﺕ ﺍﻟﺪﺭﺍﺳﺔ ﺃﻥ ﻗﻴﻢ ﻁﺎﻗﺔ‬
‫ﻛﻴﺒﺰ ﺍﻟﺤﺮﺓ ‪ ∆G‬ﻛﺎﻧﺖ ﻋﻠﻰ ﺍﻟﻌﻤﻮﻡ ﺃﻛﺜﺮ ﺳﺎﻟﺒﻴﺔ ﻓﻲ ﺍﻟﻮﺳﻂ ﺍﻟﺤﺎﻣﻀﻲ ﻓﻲ‬
‫ﻭﺟﻮﺩ ﻏﺎﺯ ﺍﻟﻨﺘﺮﻭﺟﻴﻦ ‪،‬ﺃﻣﺎ ﻓﻲ ﺣﺎﻟﺔ ﻭﺟﻮﺩ ﺍﻟﻤﻀﺎﻓﺎﺕ ﻓﺄﻅﻬﺮﺕ ﺍﻟﺪﺭﺍﺳﺔ‬
‫ﺃﻥ ﻗﻴﻢ‪ ∆G‬ﻛﺎﻧﺖ ﺃﻛﺜﺮ ﺳﺎﻟﺒﻴﺔ ﻓﻲ ﻭﺟﻮﺩ ﺣﺎﻣﺾ ﺍﻟﻔﻮﺳﻔﻮﺭﻳﻚ ﻭﺃﻗﻞ ﺳﺎﻟﺒﻴﺔ‬
‫ﻓﻲ ﻭﺟﻮﺩ ﻛﻠﻮﺭﻳﺪ ﺍﻟﺼﻮﺩﻳﻮﻡ ‪.‬ﻛﻤﺎ ﻭﺟﺪﺕ ﺗﻐﻴﺮﺍﺕ ﻓﻲ ﻗﻴﻢ‬
‫ﺍﻻﻧﺘﺮﻭﺑﻲ)‪ (∆S‬ﺑﻤﻘﺪﺍﺭ ﻣﻠﺤﻮﻅ ﻭﻳﺪﻝ ﻫﺬﺍ ﺍﻟﺘﻐﻴﺮ ﻋﻠﻰ ﺣﺪﻭﺙ ﺗﺒﺎﻳﻦ ﻓﻲ‬
‫ﻧﻮﻉ ﻭﻣﺪﻯ ﺍﻋﺘﻤﺎﺩﻳﺔ ﺗﻐﻴﺮﺍﺕ ﺍﻟﻄﺎﻗﺔ ﺍﻟﺤﺮﺓ ﺍﻟﻌﻤﻠﻴﺔ ﺍﻟﺘﺄﻛﻞ ﻋﻠﻰ ﺩﺭﺟﺔ‬
‫ﺍﻟﺤﺮﺍﺭﺓ ‪.‬ﻭﺃﺩﺕ ﺗﻐﻴﺮﺍﺕ ﻗﻴﻢ ﺍﻻﻧﺘﺮﻭﺑﻲ )‪ (∆S‬ﺍﻟﻰ ﺣﺪﻭﺙ ﺗﻐﻴﺮﺍﺕ‬
‫ﻣﻨﺎﻅﺮﺓ ﻓﻲ ﺍﻻﻧﺜﺎﻟﺒﻲ)‪ (∆H‬ﺃﻳﻀﺎ‪.‬‬
‫‪ -٤‬ﺧﻀﻌﺖ ﺣﺮﻛﻴﺎﺕ)‪(Kinetics‬ﺗﻔﺎﻋﻼﺕ ﺍﻟﺘﺄﻛﻞ ﻓﻲ ﺣﺎﻟﺔ ﻭﺟﻮﺩ ﻭﻋﺪﻡ‬
‫ﻭﺟﻮﺩ ﺍﻟﻤﻀﺎﻓﺎﺕ ﻟﻤﻌﺎﺩﻟﺔ ﺃﺭﻳﻨﻮﺱ ﺍﻟﺘﻲ ﺗﻘﻀﻲ ﺑﻮﺟﻮﺩ ﻋﻼﻗﺔ ﺧﻄﻴﺔ ﺑﻴﻦ‬
‫ﻗﻴﻢ ﻟﻮﻏﺎﺭﻳﺘﻢ ﺳﺮﻋﺔ ﺍﻟﺘﺄﻛﻞ )‪(Log ic‬ﻭﻣﻘﻠﻮﺏ ﺩﺭﺟﺔ ﺍﻟﺤﺮﺍﺭﺓ ﺍﻟﻤﻄﻠﻘﺔ‬
‫)‪ (1/T‬ﻭﺍﻟﺘﻲ ﺃﻣﻜﻦ ﻣﻨﻬﺎ ﺣﺴﺎﺏ ﻗﻴﻢ ﻁﺎﻗﺔ ﺍﻟﺘﻨﺸﻴﻂ )‪Energy ,Ea‬‬
‫‪ ( Activation‬ﻭﻗﻴﻢ ﻣﺴﺒﻮﻕ ﺍﻟﻤﻘﺪﺍﺭ ﺍﻻﺳﻲ)‪(pre-exponential,A‬‬
‫ﻭﺍﻧﺘﺮﻭﺑﻲ ﺍﻟﺘﻨﺸﻲ)≠‪( Entropy of Activation, ∆S‬ﻛﻤﺎ ﺗﺒﻴﻦ ﻣﻦ‬
‫ﺍﻟﺪﺭﺍﺳﺔ ﻭﺟﻮﺩ ﻋﻼﻓﺔ ﺧﻄﻴﺔ ﺑﻴﻦ ﻟﻮﻏﺎﺭﻳﺘﻢ ﻣﺴﺒﻮﻕ ﺍﻟﻤﻘﺪﺍﺭ ﺍﻻﺳﻲ ) ‪Log‬‬
‫‪ (A‬ﻭﻗﻴﻢ ﻁﺎﻗﺔ ﺍﻟﺘﻨﺸﻴﻂ )‪(Ea‬ﺍﻟﻤﻨﺎﻅﺮﺓ ﻟﻬﺎ‪.‬ﻭﺗﺪﻝ ﻫﺬﻩ ﺍﻟﻌﻼﻗﺔ ﺍﻟﺨﻄﻴﺔ ﺑﺄﻥ‬
‫ﺗﻔﺎﻋﻞ ﺍﻟﺘﺄﻛﻞ ﻳﺤﺪﺙ ﻋﻠﻰ ﻣﻮﺍﻗﻊ ﻣﺘﺒﺎﻳﻨﺔ ﻋﻠﻰ ﺳﻄﺢ ﻧﻤﺎﺫﺝ ﺃﻟﻮﺍﺡ‬
‫ﻭﻣﻜﻮﺍﻧﺎﺕ ﻧﻀﻴﺪﺓ ﺍﻟﺮﺻﺎﺹ ﺍﻟﺤﺎﻣﻀﻴﺔ ﻣﻦ ﺣﻴﺚ ﻗﻴﻢ ﻁﺎﻗﺔ ﺍﻟﺘﻨﺸﻴﻂ ﻭﺃﻥ‬
‫ﺗﻔﺎﻋﻞ ﺍﻟﺘﺄﻛﻞ ﻳﺒﺪﺍ ﺃﻭﻻ ﺑﺎﻟﻤﻮﺍﻗﻊ ﺍﻟﺘﻲ ﺗﺘﻤﺘﻊ ﺑﻄﺎﻗﺔ ﺗﻨﺸﻴﻂ ﻭﺍﻁﺌﺔ ﺛﻢ ﻳﻨﺘﺸﺮ‬
‫ﻣﻨﻬﺎ ﺍﻟﻰ ﺍﻟﻤﻮﺍﻗﻊ ﺍﻟﺘﻲ ﺗﺘﻤﺘﻊ ﺑﻄﺎﻗﺎﺕ ﺗﻨﺸﻴﻂ ﺃﻋﻠﻰ‪.‬‬
‫ﺟﺎﻣﻌﺔ ﺑﻐﺪﺍﺩ‬
‫ﻛﻠﻴﺔ ﺍﻟﻌﻠﻮﻡ‬
‫ﻗﺴﻢ ﺍﻟﻜﻴﻤﻴﺎء‬
‫ﺗﺂﻛﻞ ﺍﻟﻮﺍﺡ ﻧﻀﻴﺪﺓ ﺍﻟﺮﺻﺎﺹ ﺍﻟﺤﺎﻣﻀﻴﺔ‬
‫ﻓﻲ‬
‫ﺣﺎﻣﺾ ﺍﻟﻜﺒﺮﻳﺘﻴﻚ‬
‫ﺭﺳﺎﻟﺔ ﻣﻘﺪﻣﺔ ﺍﻟﻰ‬
‫ﻛﻠﻴﺔ ﺍﻟﻌﻠﻮﻡ ﺑﺠﺎﻣﻌﺔ ﺑﻐﺪﺍﺩ ﻛﺠﺰء ﻣﻦ ﻣﺘﻄﻠﺒﺎﺕ ﻧﻴﻞ ﺩﺭﺟﺔ‬
‫ﺍﻟﻤﺎﺟﺴﺘﻴﺮ ﻋﻠﻮﻡ ﻓﻲ ﺍﻟﻜﻴﻤﻴﺎء‬
‫ﻣﻦ ﻗﺒﻞ‬
‫ﺑﺨﺘﻴﺎﺭ ﻛﺎﻛﻞ ﺣﻤـــــﺪ‬
‫ﺑﻜﺎﻟﻮﺭﻳﻮﺱ‬
‫‪۲۰۰۰‬‬
‫ﺷﻌﺒﺎﻥ‬
‫‪۱٤۲٥‬ﻫــ‬