METALS, NON-METALS, & METALLOIDS. Metals Metalloid Nonmetals.
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Chapter I
Introduction
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CHAPTER I
INTRODUCTION
I.1. Definition of corrosion
Most of the metals, except noble metals such as Au, Pt, etc, exist in nature in
combined form as their oxides, carbonates, hydroxyl carbonates, sulphides, chlorides
and silicates. These are reduced to their metallic states from their ores through
metallurgical processes, which require considerable amount of energy. Thus an
isolated pure metal attained higher energy state than their ores with lower energy
state. When metals are exposed to environment, the surface begins to decay and
becomes ores, through chemical or electrochemical reaction with the environment to
attain the stable lower energy state. Hence corrosion is a process exactly reverse of
extraction of metals from their ores. [1-3]
The term corrosion (Latin: Corrosio – fretting) is defined as the spontaneous
gradual destruction or deterioration of metals by chemical or electrochemical reaction
with its environment. Generally the term corrosion refers to metallic corrosion.
Corrosion is a costly and rigorous material science problem to the society. It degrades
the useful properties of metals like appearance, strength, rigidity and thermodynamic
instability, etc. It causes severe damages in an industrial sector such as cooling water
system, petroleum refineries and high pressure boilers, etc. In practice, it is extremely
dangerous process, which is often hard to identify deterioration well in advance.
The most familiar example of metallic corrosion is rusting of iron, when exposed to
the atmospheric conditions. As a result a layer of reddish scale and powder of oxide
(Fe3O4) is formed, and the iron becomes weak. Another common example is the
formation of green film of basic carbonate [CuCO3 + Cu(OH)2] on the surface of
copper, when exposed to moist-air containing carbon dioxide. Materials other than
metals, such as ceramics, wood and plastics, etc may also undergo corrosion. Though
corrosion is an adverse process, it has few useful applications such as electrochemical
milling, anodizing of aluminium, sacrificial anodic protection and batteries, etc. [4-6].
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I.2. Historical lessons on corrosion
In 1788, Austin noticed that water, initially neutral, which tends to become
alkaline when it reacts with iron. It was believed that the corrosion was an
electrochemical phenomenon by Thenard and Compartriot in the years 1819 and 1830
respectively. Dela Rive [7] certified the fact that acid attacks impure zinc more
quickly than the relatively pure varieties. The substantiation of the indispensable
association between chemical action and the generation of electric currents has been
studied in the period of 1834 and 1840 [8]. The protection of copper metal in sea
water using either iron or zinc metal has been proposed by sir. Humphrey Davy in
1847. In 1847, Richard Adie [9] established that difference in oxygen concentration in
a flowing stream could give rise to a flow of current between two metals, iron or zinc.
Trends in corrosion study changed rapidly over the years. Around 1950, an
electrochemical study provides extensive interest to study the field of corrosion
[10-12]. In 1970’s, corrosion research was intense in the mechanistic aspects of
various types of corrosion problems [13-19]. In recent years, the corrosion research
has been divided into a number of newer fields in order to make in-depth study.
The optical and surface analytical techniques play an important role in the
understanding of the nature and influence of surface oxides on corrosion of metals
and alloys. The thickness, structure and composition of the thin film formed on the
metal surface have been characterized by the optical and surface analytical
techniques. An electronic device such as computers and microprocessors finds
extensive applications the field of corrosion data analysis [20-22]. The ultimate goal
of all the above mentioned investigations is to reduce corrosion failures.
I.3. Cost of corrosion
According to the National Association of Corrosion Engineers (NACE) –
International Gateway India Section (NIGIS), the cost of corrosion in India alone,
estimated around Rs. 80,000 crore per annum which is may be 6.1% GDP (Gross
Domestic Product) of the nation [23]. It is very essential to reduce this unusual loss in
economy. Hence the industrial sectors must take corrosion prevention action, from the
beginning stage of the issue. The schematic Fig. I.1 shows the classification of
economic losses due to corrosion [24].
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Schematic Fig. I.1. Cost of corrosion
I.4. Classification of corrosion
Corrosion of metals can be mainly classified into dry and wet corrosion
depending upon the nature of the corrosive environment [25-27].
I.4.1. Dry corrosion
It is also called chemical corrosion. It involves the direct chemical action of
environment / atmospheric gases such as oxygen, halogen, hydrogen sulphide, sulphur
dioxide, nitrogen or anhydrous inorganic liquid with metal surfaces in immediate
proximity. High temperature oxidation of metals and tarnishing of metals such as Cu,
Ag, etc, fall in this class. After the propagation of corrosion, it is also considered as an
electrochemical process with inward diffusion of oxygen and outward diffusion of
metal ions, through the metal oxide layer and electromotive force at the metal oxide
interface is considered as the driving force.
I.4.2. Wet corrosion
It is also called electrochemical corrosion. It occurs when a metal is in contact
with a conducting electrolytic solution or when two dissimilar metals / alloys are
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either immersed or dipped partially in a solution. The electrochemical reaction occurs
due to the existence of separate ‘anodic’ and ‘cathodic’ areas, between which current
flows through the conducting electrolytic solution. At anodic area, oxidation reaction
takes place, so anodic metal is destroyed by either dissolution in the combined form.
I.5. Types of corrosion
Corrosion of metals exists in various types. The type of corrosion is very
essential in order to identify the cause of corrosion and for the selection of most
efficient method to control the corrosion process. In most corrosion failure analysis, it
is necessary to know the type of corrosion, which has been responsible for the failure.
The different types of corrosion [24, 27] are shown in schematic Fig. I.2.
Schematic Fig. I.2. Types of corrosion
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I.6. Factors influencing corrosion
The rate and extent of corrosion depends on the nature of the metal and the
nature of the corroding environment [24-27].
Factors associated with the nature of the metal are as follow:
Purity of the metal.
Position in the galvanic series.
Relative areas of the anodic and cathodic parts.
Physical state of the metal.
Nature of the surface film.
Hydrogen over voltage.
Solubility of the corrosion products.
Volatility of the corrosion products.
Factors associated with the nature of the corroding environment are as follow:
Temperature.
pH.
Presence of impurities in atmosphere.
Presence of suspended particles in atmosphere.
Humidity of air.
Concentration of oxygen and formation of oxygen concentration cells.
Conductivity.
Presence or absence of an inhibitor.
Flow velocity of process stream.
Specific nature of the cation and anion present.
I.7. Theories of corrosion
In the beginning of 19th century, it was established that corrosion of metals is
very aggressive in an aqueous environment. Whitney [28] gave the most acceptable
electrochemical theory. The other theories such as acid theory, chemical attack theory,
colloidal theory and biological theory were proved to form a part of electrochemical
theory [29-31].
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I.7.1. Electrochemical theory
According to this theory the heterogeneity on the metal surface causes the
formation of galvanic cell, which is a basic requirement for corrosion of metals.
This suggests that ultra-pure metals are non-corrodible. There is no particular separate
anodic and cathodic area required for the corrosion to occur. The oxidation and
reduction reactions occur to a larger extent independent of each other on anodic and
cathodic sites. These sites occur indiscriminately over the metal surface and they have
a tendency to shift around on the entire surface thereby causing uniform corrosion.
I.7.2. Electrochemical process
The thermodynamic instability of metals, effects the conversion of metals into
their ores which thermodynamically stable with lower energy. In metallurgy, the
metallic iron is extracted from its chief ore haematite (Fe2O3) which is in low energy
state (Eqn.I.1)
Fe2O3 + 3 C → 4 Fe + 3 CO2 ↑ …………I.1
When steel is exposed to the environment containing moisture, it undergoes corrosion
by reverses to its combined form of thermodynamically stable lower energy state
(Eqn. I.2)
4 Fe + 3 O2 + 2 H2O → 2 Fe2O3 . H2O ………….I.2
Rust
Rusting is an electrochemical process. Microgalvanic cells [32, 33] with local anodes
and cathodes are formed on the metal surface due to heterogeneities in the
composition of metal during the metallurgical process. The heterogeneities of the
metal surface [34, 35] are due to surface defects, grain size of the particles, stress,
compatibility, orientation of the crystals and differential aeration, etc. The grain
boundaries generate centres with different potentials and develop cathodic and anodic
areas as shown in Fig. I.1.
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Fig.I.1. Steel surface consists of anodic and cathodic areas.
The anodic reaction involves in dissolution of metal as corresponding metallic
ions with the liberation of free electrons (Eqn. I.3), where as the cathodic reaction
consumes electrons either by evolution of hydrogen or absorption of oxygen, based on
the nature of the corrosive environment.
Fe → Fe2+ + 2 e- …………I.3
Evolution of hydrogen-type corrosion usually occurs in acidic environments.
The electrons liberated from the anode move towards cathode, where H+ ion of the
acidic solution are eliminated as hydrogen gas (Eqn. I.4).
2 H+ + 2 e- → H2 …………I.4
This type of corrosion causes displacement of H+ ion from the acidic solution by
metal ions.
Absorption of oxygen-type corrosion occurs in neutral aqueous environment
like NaCl solution in the presence of atmospheric oxygen. The surface of the iron
metal is usually coated with a thin film of iron oxide. However, if this iron oxide film
develops some cracks, anodic areas are created on the metal surface, while the metal
parts act as cathodes. It follows that the anodic areas are small surface parts, while
nearly the rest of the surface of the metal forms large cathodes. The electrons
liberated from the anodic to cathodic areas, through metal, where electrons are
intercepted by the dissolved oxygen (Eqn. I.5).
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2 H2O + O2 + 4 e- → 4 OH- ………….I.5
Thus it is clear that the formation of anodic and cathodic areas, electrical
contact between the cathodic and anodic part to enable the conduction of electrons,
and an electrolyte through which the ions can diffuse are the essential requirements of
electrochemical corrosion.
I.8. Mechanisms of corrosion process
In most of the corrosion processes, the anodic reaction is always the metal
dissolution and the reduction reaction is either hydrogen evolution or oxygen
absorption. The metals can be categorized into three types based on the differences in
the mechanism of hydrogen evolution, exchange current densities and Tafel slope
values. The metals are with low, intermediate and high over voltages. It is not always
easy to predict corrosion rate of a metal on the basis of hydrogen evolution method
where as little is known about the oxygen absorption mechanism. This reaction occurs
in many steps and also on the oxide covered surface which a poor electron carrier. In
corrosion processes, the anodic reaction is equally complicated and involves the
migration of metal ions from the metal phase to the solution phase where salvation of
ions takes place through several steps. Bockris et al. studied the dissolution and
deposition of iron in various aqueous environments in detail [36].
The five possible mechanisms anticipated for the dissolution and deposition of
iron by Bockris et al. are given as follows:
Mechanism – 1
Fe + OH- + FeOH ↔ (FeOH)2 + e-
(FeOH)2 → 2 FeOH
FeOH ↔ FeOH+ + e-
FeOH+ ↔ Fe2+ + OH-
Mechanism – 2
Fe + H2O ↔ FeOH + H+ + e-
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FeOH ↔ FeOH+ + e-
FeOH+ + Fe → Fe2OH+
Fe2OH+ → Fe2+
+ FeOH + e-
FeOH → Fe2+ + H2O
+ e-
Mechanism – 3
Fe + OH- → Fe(OH)+ + 2 e-
Fe(OH)+ ↔ Fe2+ + OH-
Mechanism – 4
Fe + OH- ↔ FeOH + e-
FeOH + OH- → FeO + H2O
+ e-
FeO + OH- ↔ HFeO2-
HFeO2- + H2O ↔ Fe(OH)2 + OH-
Fe(OH)2 ↔ Fe2+ + 2 e-
Mechanism – 5
Fe + H2O ↔ FeOH + H+ + e-
FeOH → FeOH+ + e-
FeOH+ + H+ → Fe2+ + H2O
Kabanov [37] and Frumkin [38] studied the first indication of OH- ions are
taking part in metal dissolution although these are present in very small quantity
acidic environments.
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I.9. Expression for corrosion rate
The corrosion rate (CR) can be expressed in different ways are given as
follows:
ipy – inches per year
mpy – mils per year
μm/y – micrometre per year
mdd – milligram per square decimetre per day
mm/y – millimetre per year
Usually corrosion rates are expressed in mpy and mmpy. The corrosion rates in mpy
and mmpy scales can be calculated from the following expressions (Eqn. I.6 and Eqn.
I.7)
mpy = 534 W / DAT…………….I.6
mmpy = 13.56 W / DAT …………I.7
Where W – Weight loss in grams
A – Area of the specimen in square inches
D – Density of the specimen in gram / cm3 and
T – Immersion period in hours.
If the area is calculated in square centimetres then the expression for mpy and mmpy
are given (Eqn. I.8 and Eqn. I.9) as follows:
mpy = 82.75 W / DAT ……………I.8
mmpy = 87.6 / DAT……………I.9
Where W – Weight loss in milligrams
A – Area of the specimen in square centimetre
D – Density of the specimen; 7.87 gram / cm3 and
T – Immersion period in hours.
There is a monograph available for the calculation of corrosion rate [39].
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I.10. Methods of corrosion prevention and control
Corrosion is a major industrial problem. It is a natural phenomenon that
cannot be avoided completely, but it can be controlled and prevented. It destructs the
materials gradually by means of chemical or electrochemical reaction. It is a
spontaneous, silent and slow process. So that corrosion mitigation and control
methods shall be properly selected to meet out the specific environment and
operational condition [24, 40, 41]. The various methods employed for corrosion
prevention and control is shown in schematic Fig. I.3.
Schematic Fig. I.3. Corrosion prevention and control methods
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I.11. Corrosion control by inhibitors and their inhibition action
Inhibition is the most essential area in corrosion science, which has been
extensively studied. Several books [24, 25, 27, 40, 42-49] have been published on
corrosion inhibition of metals and alloys in various aqueous environments. The
University of Ferrara, Italy exclusively conducts a symposium on corrosion inhibition
of metals and alloys once in five years. In India, CECRI organizing various activities
such as International symposium on Advances in Electrochemical Science and
Technology, National Convention of Electrochemists (NCE) and National Congress
on corrosion control (NCC), etc every year on various topics for the development of
corrosion science. The Electrochemical Society of India (ECSI) conducts a national
symposium on Electrochemical Science and Technology every year in various topics
of corrosion science and engineering. Trabanelli and Carassiti [47] have reviewed the
phenomenon of inhibitors. The literature review revealed that various corrosion
inhibitors have been developed and applied to eliminate the corrosion [50].
I.11.1. Corrosion inhibitor
A corrosion inhibitor is a chemical substance that decreases the corrosion rate
of metals and alloys efficiently when added in small concentrations to an aqueous
corrosive environment. The inhibition can be caused by either adsorption or phase
layers on the metal surface [51]. Usually the inhibitors assembling near the phase
boundary system and hence the inhibition takes place. In a sense, an inhibitor can be
considered as a retarding catalyst. From a chemical kinetic point of view inhibition
[52, 53] is defined as the decrease in the rate of one or more partial steps of the entire
electrode reaction such as proton or electron transfer, charge or discharge of the
double layer, charge transfer, mass transports, partial chemical reactions and reaction
with transfer of metal ions and dissolution of metal crystals.
I.11.2. Types of corrosion inhibitors
There are numerous inhibitor types and compositions. Most inhibitors have
been developed by empirical experimentation. It is essential to classify the inhibitors
based on their mechanism, composition and applications.
Putilova et al. classified corrosion inhibitors into three types as follows:
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Type 1: Those, which form a protective film on the metal surface [54, 55].
Type 2: Those, which reduce the aggressiveness of the corrosive environment.
Type 3: Those, which will form a protective film on the metal surface as well as
reducing the aggressiveness of the corrosive environment.
Based on the corrosive environment, inhibitors can be classified into three
types. They are acid inhibitors, neutral and alkaline inhibitors and vapour phase
inhibitors [56, 57]. Based on the practical applications, inhibitors classified into sever
types those used in refrigeration system, oil refineries, cooling water system and in
radiators [58].
Based on the inhibition mechanism inhibitors classified as follows:
Barrier layer formers:
It includes oxidizers, adsorbed layer formers and conversion layer formers.
These inhibitors are very effective in reducing the rate of both anodic and cathodic
reaction except the oxidizing inhibitors, because which shift the corrosion potentials
of the metal to more positive value where a stable oxide or hydroxide layer is formed
which will be more stable [59, 60].
Neutralizing inhibitors:
These inhibitors remove the H+ ions from the corrosive environment and
hence the aggressiveness of the corrosive environment is reduced [61]. They find
applications in petroleum refineries, hydraulic liquid, boiler treatment and condenser,
etc.
Scavengers:
These inhibitors used to remove corrosive reagents from solution. Examples of
this type of inhibitor are sodium sulphite and hydrazine, which remove dissolved
oxygen from aqueous solution.
Miscellaneous:
These inhibitors include materials such as scale prevention inhibitors and
microbiological growth inhibitors, which reduce rate of corrosion by interference with
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other chemical reactions. The potentiodynamic polarization data is very much helpful
to classify the inhibitors into anodic [41, 62, 63], cathodic [64, 65] and mixed type
[66, 67]. Fig.I.2. illustrates the significant terms for a freely corroding metal. The line
EaD and EcD represents the anodic and cathodic reactions respectively. The point ‘D’
at which the anodic and cathodic lines intersects each other which results open circuit
corrosion potential (Ecorr) of the metal. This indicates the magnitude of corrosion
current subscript. Fig. I.3. illustrates [24] that the relation of metallic corrosion (D),
protection (F and G) and inhibition (P). Generally the immersed metal may corroding
by reaction under anodic control (EaF) using anodic type inhibitors [62], cathodic
control (EcG) using cathodic type inhibitors [64] and mixed type [66] control (Ea –
EcP) where the inhibitor controls both anodic and cathodic reactions. It very clear
those inhibitors, which control both the reactions, are more effective. It is found that
both anodic and cathodic type inhibitors reduce the corrosion current (anodic: ∆i1,
cathodic: ∆i2) and the mixed type inhibitor reduces the corrosion current more
effectively (∆i3).
Anodic and cathodic inhibitions result greater shifts in anodic and cathodic
Tafel slopes respectively. But in the case of mixed type inhibitors both anodic and
cathodic slopes are shifted to an equal extent or there is not much change in the Tafel
slopes. The anodic [62] and cathodic [64] inhibitors shift the corrosion potential
towards anodic and cathodic sides respectively where as in mixed type inhibitor both
shifts will be observed [67].
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Fig. I.2. Polarization curve for significant terms for freely corroding metal.
Fig. I.3. Polarization curve relating metallic corrosion, protection and inhibition.
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I.12. Theories of corrosion inhibition
There were so many national and international conferences and symposiums
have been conducted exclusively on corrosion inhibitors for the searching of
innovation, development and novel ideas of new cost-effective, eco-friendly and high
efficient corrosion inhibitors [40, 68-70]. The mechanism and action of inhibitors in
various corrosive environments such as acidic, alkaline and neutral, etc has been
explained on the basis of adsorption and protective film formation. Especially organic
inhibitors such as carboxylates and amides show very good corrosion inhibition of
metals in contact with aqueous environment. In addition to this, they are
environmentally safe, as they have low toxicities and are readily biodegradable.
The mechanism and action of corrosion inhibitors have been explained by different
theories.
I.12.1. Adsorption theory
Machu [71] puts forward the adsorption theory which predicts the formation
of a porous thin layer of inhibitive molecules with high electrical resistance.
Uhlig [72] investigated that the inhibitor molecules get adsorbed at the surface of the
metal and followed by blocking the active sites and influence the potential of the
metal by virtue of their net charge.
Riggs [73] confirmed that the organic inhibitors forms a very effective
protective layer due to the presence of hetero atoms such as nitrogen, oxygen, sulphur
and phosphorus, etc and the adsorption depends on the nature of metal, environment
and the electrochemical potential of the metal-solution interface. Further the
adsorption is classified into three types as, Π bond orbital adsorption, electrostatic
adsorption and chemical adsorption.
The various studies on corrosion inhibition reflect the relationship between the
characteristics of the electronic interaction at the metal-solution interface and the
structure of the inhibitor molecules in detail [74]. It has been reported that several
carboxylates such as ethylenediaminetetraacetate [75], sodium salicylate [76], sodium
cinnamate [77], anthranilate [78], adipate [79], citrate [80], succinate [81], tartrate
[82] and oxalate [83] have been used as inhibitors. It was also found that amides as an
inhibitor forming a protective layer on metal surface by strong adsorption.
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I.12.2. Hydrogen over potential theory
This theory explains the action of acid inhibitors. It was believed that
inhibitors increase the hydrogen over potential, which cause increase in cathodic
polarization [84]. Zhu Yan et al. have pointed out that amides are adsorbed on the
anodic and cathodic regions on the metal surface [85]. The organic inhibitors shift the
corrosion potential of the metal in cathodic [64] and anodic [62] sides, which
indicates that their action on cathodic and anodic sites respectively. This theory fails
to explain the inhibitive action of all types of systems.
I.12.3. Protective film formation theory
Evans [86] who considered the “Father of corrosion science” explained the
film formation by the inhibitor molecules over metal surface immersed in neutral and
alkaline environment and he pointed out the formation of insoluble film which
inhibits corrosion extensively. Putilova et al. [40] have identified that the corrosion
inhibition of metals in acidic environment due to formation of a thin layer of insoluble
or slightly soluble corrosion product on the metal surface.
I.12.4. Electrochemical polarization theory
Stern [87] explained the action of passivating inhibitors in various corrosive
environments and suggested that the inhibitors such as chromate get reduced at the
cathodic sites and increase the electrode potential to the noble direction which results
passivation. It has been reported that a very small quantity of the total corrosion
current be associated with cathodic reduction of passivating inhibitors [40].
I.13. Methods of evaluation of corrosion
The methods of evaluation of corrosion include gas volumetric study [88],
weight loss study [63, 90, 91], electrochemical technique such as polarization and AC
impedance spectra [63, 66, 89], determination of surface coverage [64], Fourier
Transform Infrared spectroscopy [90-92], surface characterization studies such as
scanning electron microscopy (SEM) [93-95] and atomic force microscopy (AFM)
[96-105] and biocidal study [106-109].
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I.13.1. Gas volumetric study
In gas volumetric study the corrosion of metals and alloys in a non-oxidizing
acid environment in the presence of inhibitors can be evaluated by finding the volume
of hydrogen gas released in the presence and absence of additives. The corrosion rate
(CR) obtained from this study is not very accurate due to the appreciable
decomposition reaction of inhibitors. Usually the ability of the inhibitor expressed in
percentage inhibition efficiency (IE) which can be obtained from the formula (Eqn.
I.10).
I E % = (Uninhibited CR – Inhibited CR) x 100 / Uninhibited CR ……..I.10
I.13.2. Weight loss study
The weight loss can be estimated by the immersion of identical specimens for
a constant immersion period and temperature the corrosive environment chosen for
the investigation. The IE is calculated by using the formula (Eqn. I.11).
I E % = [(Wo – W1) / Wo] x 100 ………I.11
Where Wo and W1 are weight loss in the absence and presence of inhibitor
respectively.
I.13.3. Polarization method
This method is widely used for the testing of corrosion inhibitors. It involves
with the estimation of corrosion current during the electrochemical process in the
presence and absence of the inhibitor. The value of corrosion current (Icorr) can be
obtained by intersection of the extrapolated anodic and cathodic Tafel lines using
potentiodynamic method and the IE is calculated from Eqn. I.12.
I E % = [(Icorr – I*corr) / Icorr] x 100 ………I.12
Where Icorr and I*corr are corrosion current in the absence and presence of the
inhibitor.
The IE may also be calculated from the polarization resistance (Eqn. I.13) and
impedance techniques (Eqn. I.14)
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I E % = [{(1 / Rp*) – (1 / Rp) } / (1 / Rp*)] x 100 ………I.13
Where Rp* and Rp are the polarization resistance in the absence and presence
of the inhibitor respectively.
I E % = [{(1 / Rt*) – (1 / Rt) } / (1 / Rt*)] x 100 ………I.14
Where Rt* and Rt are the charge transfer resistance in the absence and
presence of the inhibitor respectively.
The linear polarization results should always be compared with weight loss
study or other corrosion rate measurements to ensure the correctness of the technique
and its fitness for a particular environment.
I.13.4. Determination of surface coverage
The degree of surface coverage can be calculated using polarization study
using the following relationship (Eqn. I.15).
θ = [1 – (i / io)] ………….…I.15
Where i and io are the corrosion current in the presence and absence of the
inhibitor respectively.
The constancy of slopes of the polarization curve with increasing
concentration of inhibitor is a necessary condition for the calculation of θ, i and io.
The surface coverage values can be calculated from the charge transfer resistance in
the absence and presence of inhibitor (Eqn. I.16).
θ = 1 – [(1 / Rt) / (1 / Rt*)]….………………. I.16
Where Rt* and Rt are the charge transfer resistance in the absence and
presence of inhibitor.
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I.13.5. Spectroscopic technique
The ultra-violet (UV) [110] and infrared (IR) spectroscopic techniques are
very much useful for the understanding of arrangement of inhibitor molecules on the
metal surface. The IR study interprets the extent of adsorption and the orientation of
inhibitor molecules on the metal surface. The UV spectroscopy is highly useful in the
determination of amount of inhibitors adsorbed over a metal surface. But the same can
be determined exactly by a new technique, reflection spectra. Raman spectroscopy
[111], Auger electron spectroscopy (AES) [112], X-ray diffraction spectroscopy
(XRD) [113], X-ray photoelectron spectroscopy (XPS) [114], Mossbauer
spectroscopy [115], nuclear magnetic resonance (NMR) [116] and fluorescence
spectroscopy [109] are the other techniques to study the influence of corrosion
inhibitors.
I.14. Surface characterization study
The scanning electron microscopy (SEM) provides 2D pictorial representation
of the metal surface. It is very much useful to understand the nature of the protective
film formed on the metal surface in the absence and presence of the inhibitor and to
know the extent of corrosion inhibition.
The atomic force microscopy (AFM) provides 3D images of the metal surface.
It gives a statistical roughness parameters such as average roughness, root-mean-
square roughness and peak-to-valley heights in nano scales, which are very much
helpful to understand the extent of corrosion inhibition.
I.15. Biocidal study
Biocides are used in controlling microbial corrosion of metals and alloys in an
aqueous medium through micelle formation. Biocide such as CTAB and SDS finds
extensive applications in corrosion control of metals in high chloride medium.
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References
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REFERENCES
1. D. Pletcher and F.C. Walsh, “Industrial Electrochemistry”, Chapman and
Hall (1993).
2. L.L. Shreeir, R.A. Jarman and G.T. Burstein, “Corrosion and Corrosion
control”, Vol. I, II, Butterworth-Heinemann, London (1976).
3. R. Winston Revie, “Uhlig’s Corrosion Hand Book”, 2nd Edition, John
Wiley & Sons, INC (2000).
4. J.O’ M. Bockris and A.K.N. Reddy, “Modern Electrochemistry”, Plenum
press, New York, 2 (1977) 1267.
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