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Electrochemical Theory of Corrosion

Numerous workers^3-6 have employed electrochemical theory to evaluate the effectiveness of various preventive measures adopted to mitigate corrosion and others^7-10 has utilized it to explore the corrosion activity of various systems.

The mixed potential theory of corrosion along with Nearnst and Tafel equations form a basis of modern electrochemical theory of corrosion¹¹. This theory is now well recognized and in most of the cases of corrosion can be clarified on its bases.

According to this theory, it consolidated two hypotheses

1. All electrochemical reaction is separated in two or more fractional procedures.

2. During an electrochemical reaction of corrosion, there can be no net accumulation of electric charge.

Hence, the total rate of reduction must be equal to the total rate of oxidation in any corrosion process (reaction) and it can be experimentally revealed that electrochemical reaction are composed by two or more partial oxidation or reduction reactions. Oxidation or de - electronation of metal is defined as anodic reaction. Usually it can be written as

M   →  Mⁿ⁺  +  n e^-                        (1)

Reduction or electron consuming reaction is termed as cathodic reaction, which can be of numerous kinds depending on the nature of environment around the metal. Certain common ones are :-

Hydrogen evolution  :             2 H⁺ + 2 e^-  →  H₂                                    (2)

Oxygen reduction      :             O₂ +  4 H⁺ +  4 e^-  →  2 H₂O                  (3)

(In acidic solutions)

Oxygen reduction      :              O₂ + H₂O + 4 e^- → 4 OH^-                       (4)

(In basic or neutral solutions)

Metal ion reduction   :                 M³⁺  +  e^-   →   M²⁺                              (5)

Metal deposition       :                  M⁺   +  e^-   →   M                                 (6)

Hydrogen evolution (2) is a usual cathodic reaction since acid or acidic media are often encountered. Oxygen reduction reactions (3, 4) are also common as any aqueous solution open to air is deserved producing this reaction, whereas, metal ion reduction (5) and deposition (6) are not so common. In aerated acidic solution two cathodic reactions are feasible i.e. the evolution of hydrogen and the reduction of oxygen. Thereby aerated solution are ever more corrosive, Whether we consider the dissolution of Aluminium (Al) in aerated acidic solution.

The anodic reaction is

Al   →   Al³⁺  +  3 e^-                                                   (7)

And the reactions occurring at cathodic sites are

2 H⁺  +  2 e^-  →   H_{2 (g)}                                              (8)

O₂   +  2 H₂O  +  4 e^-  →  4 OH^-                                (9)

According to equation (7) a single Al³⁺ ion passing into solution and leaves three electrons on the metal surface. The separation of charge, the electrical double layer, gives rise to characteristic potential for the Al / Al³⁺.

Similar half- cell relationship, H₂ / H⁺ and O₂ / OH^- exists for equations (8) and (9).

This potential changes tends to impede the deposition of dissolved metal ions from the solution on the metal by a reverse process of equation (7). Continuation of the deposition and disintegration of metal ions would result in the metal attaining a stable potential such that the rate of deposition becomes equal to the rate of dissolution at equilibrium and a balancing cathodic current Al _ic for the reduction of Al³⁺ ions to Al (metal). At equilibrium stage Al_ia = Al_ic and this potential is called “ reversible potential E_r ˮ . Its value depends on the standard potential (E⁰) and concentration (activity) of the dissolved metal ions. The relation among the reversible potential (E_r) and standard potential (E⁰) can be expressed by the Nernst equation.

                                           E_r   =   E⁰   +                          (10)

Where a refers to the activity of the corresponding state, R is universal gas constant, n is the number of electrons involved in the reaction and T is absolute temperature.

Classification of Corrosion

Corrosion has been grouped into numerous various methods comprises low and high temperature, dry and wet, metallic and non-metallic, chemical and electrochemical, uniform and non-uniform etc.

Dry Corrosion

It take places above the dew point of the environment such as gases and vapours are commonly the corrodent or in the absence of the liquid phase (aqueous environment). It occurs caused by direct chemical attack of environment gases and vapours on the metal surface most often associated at high temperature. An illustrations are the corrosion of Silver material by H₂S gas and Iron metal undergoes chemical corrosion by HCl gas.

Oxidation Corrosion (Corrosion by Oxygen)

Oxidation corrosion is defined as oxidation of metal caused by direct attack of air containing oxygen in absence of moisture at low or high temperature. Alkali and Alkaline earth metals are quickly oxidized at low temperature. Approximate all the metals (except Pt, Au and Ag) are oxidized at high temperature.

Mechanism

                                     2 M   →  2 Mⁿ⁺  +  2 n e^-

                                      O₂  +  2 n e^-  →  n O^2-

Overall Reaction          2 M  +    O₂  →  2 Mⁿ⁺  +  n O^2-

                                Metal  +  Oxygen   →   Metal Oxide  (Corrosion Product)

A protective invisible oxide film is created over the metal surface during the oxidation as well as nature of oxide layer may be unstable, volatile, stable or porous.

Liquid Metal Corrosion

The flowing corrosive liquid metal chemical action on highly stressed (solid) metal surface or alloy at high temperature is called liquid metal corrosion.

For instance, Devices employed in order to nuclear power affected by liquid metal corrosion.

Corrosion by other Gases

Hydrogen Embrittlement corrosion occurs when metal is immersed in hydrogen environment. H₂S attacks on the iron at high temperature resulting in creation of FeS and evolves atomic hydrogen which is porous.

At higher temperature, when steel or Al is immersed to hydrogen environment then atomic hydrogen may be produced. When chemical reaction among atomic hydrogen and carbon in steel to generates methane (CH₄) gas. Decarburization is a technique that diminishes the extent of carbon constituents in steel and also strength and hardness of the steel decreases.

Electrochemical Corrosion (Wet Corrosion)

When metal is electrically contact with conducting liquid or electrolytic solution then wet corrosion occurs. Electrochemical corrosion comprises –

i. Existence of conducting medium (electrolytic solution) as electrolyte (Moisture, Soil and Water etc.).

ii. Oxidation reaction (metal atoms lose electron) where corrosion occurs in the presence of anode.

iii. Reduction reaction takes place, no corrosion in the existence of cathode.

iv. Existence of an electrically contact among anode and cathode (via metal wall and wire etc.).

These four factors are inevitable in order to wet corrosion by removing any one of the above factors wet corrosion can be prevented.

Mechanism– Corrosion of metals is an electrochemical process in aqueous solution. In such type of corrosion, flow of electron current occurs among anodic and cathodic region in the existence of electrolytic solution. Whitney¹² introduced the extremely acceptable electrochemical theory in 1903.

Reaction at anode - Oxidation is termed as anodic reaction with liberation of free electrons from metal. These free electrons move toward cathode from anode.

                                 M (Metal)  →  Mⁿ⁺  +  n e^- (Oxidation Reaction)

                                  Mⁿ⁺ (Metal ion)  →  Dissolves in solution

                 or

                                Mⁿ⁺  (Metal ion)   →   creates oxide compound

In electrolyte cell, the anodic metal abraded by either changing in combined state as oxide or dissolving in electrolytic solution etc. Consequently corrosion always occur at anodic region.

Reaction at Cathode

Reduction is recognized as cathodic reaction. Reduction reaction (electronation or electron consuming process) does not impress the metal since almost all metal cannot be further diminished. Consequently, the dissolved ingredient gains the electrons in the conducting electrolyte and converts into certain ion such as OH^- and O^2- etc. Electrons are used in two methods during cathodic reaction depending on the nature of corrosion environment.

Hydrogen Evolution Type 

The liberation of electrons by metal atom in acidic corrosion environment at anode are occupied by hydrogen ion at the cathode. The hydrogen ions (H⁺) are enlisted from acidic substances in water. Corrosion of aluminium in acid medium proceeds with following steps¹³.

At Anodic sites          

Al_(S)  +  H₂O   →   AlOH_(ads)  +  H⁺  +  e^-                                                   (1)

AlOH_(ads)  +  5 H₂O  +  H⁺   →   Al³⁺  +  6 H₂O  +  2 e^-                              (2)

Al³⁺  +  H₂O   →   [AlOH]²⁺  +  H⁺                                                              (3)

 [AlOH]²⁺  +   X^-   →   [AlOHX]⁺                                                                  (4)

The controlling step in the metal dissolution is the complexation reaction between the hydrated cation and the anion present in Eq.(4). In the presence of chloride ions (Cl^-) the reaction will correspond to

                                               [AlOH]²⁺  +  Cl^-  →  [AlOHCl]⁺

At Cathodic sites hydrogen evolution is according to the following steps

                                                     H⁺  +  e^- →  H_(ads)

                                        H_(ads)  +  H_(ads)   →   H₂ ↑       (acidic medium)

                                       O₂ + 2H₂O + 4 e^-  →  4OH^-     (neutral / alkaline medium)

Consequently in electrochemical series, all metal above hydrogen have propensity to find decomposed in acidic solution ( H⁺ ) with simultaneous liberation of hydrogen. Hence owing to dislocation of H⁺ from the acidic solution by metal ions then such type of corrosion occurs.

Oxygen Absorption Type

The aluminium metal surface is commonly coated with aluminium oxide in the presence of air containing oxygen and in view of the few cracks on the oxide layer anodic area is generated on the Al surface and Al metal area act as cathode.

At anode                 Al  →  Al³⁺  +  3 e^-                          (Oxidation Reaction)

At cathode                O₂  +  2 e-  +  H₂O  →  2 OH^-       (Reduction Reaction)

Al³⁺ ions and OH^- ions expand and when they react forming of corrosion product (aluminium hydroxide) insoluble in water which precipitates as a white gel.

Al³⁺ + 3 OH^- → Al (OH)₃ ( insoluble in water precipitates as a white gel ).

Environmentally Induced Cracking (EIC)

EIC may be further classified into following three kinds.

(i) Stress Corrosion Cracking (SCC): When there is a conjoint action of stress and environment stress corrosion cracking occurs. The alloy is susceptible to stress corrosion cracking only when conspicuous ions are present as in to pitting corrosion.

(ii) Corrosion Fatigue Cracking (CFC): Moving interfaces under load causes damage as in to wear known as fretting damage. Cyclic stress under corrosive environment result in corrosion fatigue cracking. Existence of corrosive environment commonly increases the susceptibility and rate of fatigue cracking without corrosion but pure metal and alloy are equally.

(iii) Hydrogen Induced Cracking (HIC): Hydrogen induced cracking is the reaction among hydrogen and carbides frequently present as impurity to create methane resulting in decarbonisation, voids and surface blisters.

Flow –Assisted Corrosion

In view of combined influence of agents, corrosion media and existence of a flowing liquid to form flow-assisted corrosion. Flow – assisted corrosion may be classified as three types of corrosion.

(i) Erosion Corrosion: When there is a relative movement of the corrosive environment with respect to the alloy it can proceed to erosion corrosion. Pipelines and heat exchangers are subjected to such a kind of failure.

(ii) Cavitation Corrosion: This kind of corrosion is owing to bubble formation and collapse when there is hydrodynamic variation in pressure difference along the line.

At low pressure liquid (water) vaporizes when same is subjected to higher pressure bubbles forms and subsequently implodes. This proceed to plastic deformation and formation of cavities.

(iii) Fretting Corrosion: Moving interfaces under load causes damage akin to wear called fretting corrosion.

Inter Granular Corrosion (IGC)

This type of corrosion occurs as a result of selective attack of the grain boundaries when either grain boundary becomes highly active or phase prone to selective attack are produced. Welding, a common practice in fabrication causes such an inter granular corrosion attack.

Stainless steel alloys carbon is precipitated out at grain boundaries at higher temperature 565⁰ C to 870⁰C during welding. Stainless steel alloys carbon converts in to chromium carbide (Cr₃C₂) applying metallurgical combining with chromium of stainless steel.

Mitigation of Corrosion

In order to monitoring of corrosion, there are so numerous techniques mentioned.

The techniques adopted to combating corrosion can be classified under following heads:

Material Selection

Metallic selection plays prominent role to inhibit or mitigate the corrosion process.

(I) Use of noble metals – Ti, Pt and Au etc used as precious metal is first preference in accordance to material selection. Such type of metals are noble, expensive and cannot be utilized in order to general purpose but these metals are most resistant to corrosion.

(II) Use of pure metals – To avoid corrosion we should exploit pure metal since impurities in metal are the prime cause in order to heterogeneity which reduces corrosion resistance of using metal. Consequently the corrosion resistance of given metal (Al, Mg etc.) can be improved by increasing the purity.

(III) Use of metal alloys – To mitigate the corrosion process applying metal alloy (such as ferrite stainless steels) is the better preference. We can very enhance the corrosion resistance of maximum metals through alloying them with appropriate constituent. In order to maximum corrosion resistance alloys should be perfectly homogeneous. Few illustrations of corrosion resistant alloys are as follows -

(a) Chromium in iron is known as stainless steel. Alloying metal like chromium in stainless steel or iron generates impervious and coherent oxide film which protects the iron or steel from further attack.

(b) In gas turbine components, aircraft parts and internal combustion engines, Nimonic Alloys (Ni, Cr, Ti, Al) are employed.

(c) In order to heat exchanger or condenser tubes, Cupronickel alloys (70% Cu + 30% Ni) are applied.

Reforming The Environment

Numerous environment parameters namely, temperature, velocity, pressure, pH etc. which are to be affected corrosion rate of any material. Consequently peripheral environmental corrosion nature can be diminished by following parameters

1. By applying expulsion of harmful ingredients from environment

2. By addition of conspicuous materials which nullifies the influence of corrosion ingredients of peripheral environment.

(I) Oxidizer (De–aeration):

Oxidizer is a greatly chronic corrosion – monitoring technique. In recent practice, this is accomplished by vacuum treatment, inert gas spraying or through the exploit of oxygen scavengers and by use of deoxidizing reagents such as hydrazine hydrates (NH₂NH₂.H₂O) and sodium sulphite (Na₂SO₃). This technique also isolate CO₂ gas. Although de-aeration finds widespread application, it is not recommended in order to active-passive metals or alloys. These metals require oxidizers to produce and maintain their protective oxide films. Procedures to diminish oxygen content in corrosive or surrounding environment can monitoring the rate of corrosion of metal.

(II) Changing Concentration:

Diminishing corrosive concentration is extremely dominant. The existence of corrosive is incidental in numerous processes. For illustration, eliminating chloride ions reduces corrosion by coolant water in nuclear reactor. Numerous acids like phosphoric acid and sulphuric acid are practically inert at higher concentration as well as at moderate temperature. In these circumstances, increasing acid concentration can inhibit corrosion.

Acids which more concentrated endow large magnitude of active species (H⁺ ions) and consequently enhance the corrosion rate. In acid medium, we can mitigate corrosion through neutralizing the acidic behaviour of corrosion medium. Similarly we can prevent corrosion by injecting alkaline neutralizer such as NaOH and NH₃ in liquid or gaseous forms.

(III) Lowering Temperature:

The influence of temperature on corrosion rate has been extensively studied^14-16. Temperature also plays considerable role in order to corrosion rate. Commonly, corrosion rate decreases with the fall of temperature.

(IV) Decreasing Velocity:

Usually, corrosion attack increases with velocity, although there are certain significant exceptions. Very high velocity should always be avoided in order to minimize corrosion damage. Decrease in velocity is frequently used as practical technique to control corrosion.

[C] PROTECTING COATING:

It can be further separated into under mentioned two groups.

(I) Inorganic Coating:

Coating of metallic and inorganic materials can endow a satisfactory barrier amongst the metal and its environment. Inorganic coating can be further classified in to two classes.

(a) Metallic Coating – Metallic corrosion is inhibited through applying coating the inhibit metal with another metal. There are few techniques are as follows:–

(i) Hot dipping – It is the process of coating of low melting point metals mainly Zn, Al, Pb and Sn etc. with a metal of high melting point such as Fe and Cu etc. at temperature of around 450⁰ C. Galvanised sheet is produced by hot-dipping. Thin coats are difficult to produce.

(ii) Galvanizing – Galvanizing is the process of applying a protective Zn coating to a more noble metal (Fe or steel) to prevent corrosion (rusting). Dried metal is dipped in molten Zn and thin layer of Fe is coated.

(iii) Metal Cladding – It is a very cheap technique in order to protecting against corrosion. The cladding material acts as a barrier among the substrate and the corrosive environments. Some metals such as Ni, Ti, Cu, Al and stainless steels are used as cladding for steel.

(iv) Tinning – Tinning is the method of thinly coating plates of wrought iron or steel with tin and the findings product is called tinplate. Term ^“tinplate^ˮ is extensively applied in order to the different technique of coating a metal with solder before soldering.

(v) Platting – It is a surface covering process endow aesthetic, decorative finishes in which a weak film of metal( Sn, Cr, Ni ) that has been added to the outside of a base metal by passing a direct current via an electrolytic solution having soluble salts of coating metal. 

1. Philip Schweitzer, “Corrosion and Corrosion Protection Hand bookˮ, Morcel Dekker, 1983.

2. H.H Uhlig, “Corrosion and Corrosion Control – An Introduction to Corrosion Science and Engineeringˮ John Wiley and Sons, New York, 2, 1971.

3. E. Deltombe and M. Pouribiax, Corrosion, 14, (1958),496

4. E. Heitz and Wscrewenk, Br. Corros.J., 11,(1976), 74.

5. A.S. Fouda, Indian J. Technol., 20, (1982), 412.

6. S. Sathyaanarayana and R. Ramesham, Indian J. Technol., 24, (1986), 529.

7. M.S. Abdetal, A.A.A.Wahab and A.El-Saied, Corrosion, 37, (1981), 557.

8. P. Singh, L.Bhadur and R.N.Singh, Corrosion, 42, (1986), 64.

9. A.A. Mazher, E. El-Talib Hackal and A.G.G. Allah, Corrosion, 44, (1988), 705.

10. R.N. Singh, N.Verma and W.R.Singh, Corrosion, 45, (1989), 222.

11. L.S.Lisae, M.Metikos-Hukovie, D. Lencie, J. Vorkapic and K. Berkovic, Corrosion, 49, (1992), 924.

12. W. R. Whitney, Journal of the American chemical Society, 25(4), (1903), 394-406.

13. E.E.Oguzie, B.N. Okolue, E.E. Ebenso, G.N. Onuoha and A.I. Onuchukwu, Materials Chemistry and Physics, 87, (2004),401.

14. O. L. Riggs JR, C.E. Locke and N. E. Hammer, Anodic Protection of Industrial Equipment in Anodic Protection (pp.17-48 ). Springer US (1981).

15. L.I. Shreir, R.A. Jarman and G.T. Burstein, “Corrosionˮ 3^rd Editors, Buttuerworth, London, (1994).

16. F. Mansfeld, Corrosion, 44, (1988), 558.