When a steel structure is in contact with
an open aqueous environment or in underground soil , an electrochemical cell is
formed with following anodic and cathodic reactions.
Fe = Fe++ + 2e
Ea - Half cell
[O] + H2O + 2e = 2OH-
Ec - Half Cell potential
The cell potential E =
The free energy change associated with it
DG = -nFE ,
where n is the no of charge e
or electron involved and F is
Faraday. The half cell potentials Ea and Ec are related to
activities and standard potential by Nernst equation as follows.
Ea0 - RT/nF
ln (a Fe++ )
Ec0 - RT/nF ln (
( Assuming the solution behave ideally and activity equals to concentration )
If the resistance of the
aqueous electrolyte path in between the anode and cathode be R ( Fig.1.1 ), then
electrochemical corrosion current I0 can be written as
I0 = (Ea - Ec)/ R .1.7
Thus corrosion of steel
depends on electrode potential Ea and Ec , which in turn
are related to concentration Fe++ and OH- ions (pH) ,
dissolved oxygen concentration , temperature and resistivity of the
electrolyte. It has been
illustrated how on a same steel piece some regions act as anodic sites and
others as cathodic sites. This is a case of inseparable electrodes. The
equivalent electrical circuit shows a cell
and current flowing through
the resistance offered by the electrolyte. Unlike the normal electrical circuit
, here the current is the carrier of the charge through the electrolyte.
Electron flows from anodic sites to cathodic site through steel which also acts
as external conductor.If
a second metal M with electrode potential EM is present in the system
, it ionizes as follows
M = M++ + 2e
For example for iron
corroding in neutral aerated water , the cell potential is determined from under standard half cell electrode
E = (0.82)/2 (-0.440) = 0.85 volt
From equation 1.4
DG = -2ด 23061ด 0.85 = -39203.70
( F= 96500 coulombs converted
to 23061 cal/g equivalent/volt)
With Zn into the system DG = -2ด 23061ด ((0.82)/2
(-0.763)) = -54101.106 cals
Cathodic reaction 1.2 generates OH- ions and so
region in the vicinity of steel surface becomes alkaline with increase in pH. If
If the electrolyte here is stagnant, such as in under ground soil, soon a
different cathodic reaction takes place as follows with higher electrode
2H2O + 2e = H2 +
1.9 Ec =
The evolution of hydrogen in
the above reaction leads to embitterment of steel hydrogen. The free energy
change DG calculated for the cell
using this cathodic reaction will have higher negative value.
The corrosion product of
reactions 1.1 and 1.2 is hydrated
ferrous oxide which is not thermodynamically stable and in presence of aerated
aqueous media is converted to ferric form.
2Fe + 2H2O +
O2 = 2Fe(OH)2
4Fe(OH)2 + 2H2O + O2 = 4Fe(OH)3 1.11
So another electrochemical
reaction with ahalf cell potential
becomes active into the system.
Fe2+ = Fe3+ +e
. 1.12 Ea= -0.771
Instead of water or soil , if
corrosion takes place in acid the cathodic reaction will be of hydrogen
evolution reaction as follows.
2e = H2
Ea= -0.000 volt
It is seen from Nernst equation that electrode potential of steel in aqueous media are influenced by concentration of reacting ions associated with electrochemical reaction. Depending on the type of environment acidic, neutral, alkaline , oxidizing or reducing, various ions viz. Fe2+ , Fe3+, OH- , H+ as well oxides may be present. A graphical representation of electrode potential of a metal at different pH values with or without presence of these ions is called a E-PH diagram or Pourbaix diagram after M. Pourbaix* who first obtained for different metal-water systems. Detail E-PH diagram of iron with various reactions can be found in any corrosion text. Only utility of the diagram as far as corrosion control is concerned is discussed with a simplified form of it in fig. 1.2
Various environmental conditions such as acidic, alkaline , oxidizing and reducing are indicated by arrows. So for a highly acidic and oxidizing aqueous environment, one has to concentrate near top left portion of the diagram The diagram maps three major zones of corrosion, passivity and immunity with respective ions or compounds present. Corrosion of steel would take place into the corrosion zones in the acidic regions with formation of Fe3+ or Fe2+ ions as well under alkaline condition with formation of hypoferrite ion ( caustic embrittlement of steel). Steel corroding neutral aerated water is indicated by black circle as shown in fig. 1.2
Immunity is the zone where corrosion never occurs. Thermodynamically corrosion is not favorable here and DG comes out positive. In the passivity zone, on the other hand, corrosion does take place initially, but soon adherent, compact hydrated oxide layer forms over the surface. This oxide layer is passive to corrosive media and acts as a barrier between the metal surface and electrolyte. So the rate corrosion becomes negligibly small. So in this zone corrosion is thermodynamically favorable but not kinetically. To control corrosion of steel from, one can either move into passivity region shown by arrow from black circle (fig.1.2) by oxidizing the system that is dragging electron from the steel, known as Anodic protection or into the immunity region by reducing it that is pumping electron into the steel known as cathodic protection to be covered in later chapters.
Rate of corrosion
The rate of any chemical reaction r is amount reacted
in unit time per unit area . So the
amount reacted w on surface area s in t time can be represented as
r= Dw/st 1.14
From above equation unit of rate of corrosion is expressed in mdd ,which in mg per dm2 per day. But often change in weight is not a true representation of degree of representation. For example if very small holes develops into the hull of ship, the amount degraded is not much, but it may lean to entry of sea water into the hull , making it
accident prone. So it is felt thickness of penetration per unit time is more appropriate and unit of corrosion is also expressed in mils per year in short mpy. One mil is thousand of an inch.
Since corrosion is an electrochemical reaction with consumption or
production of electrons, the rate of corrosion is a measure of rate of electron
flow or current I. By Faradays law I can be related with the amount reacted w
on surface area s in t time.
w= Ita/nF 1.15
Where a is the atomic weight
r =w/s= (a/nF).(I/s)
Or r = (a/nF). i
where i is
current density that is current per unit area.
Now all the
terms within bracket in equation 1.15 is constant for a particular corrosion
reaction , the current density i is
proportional to the rate of corrosion. It is expressed in
mA/dm2 or mA/cm2.
Electrochemical determination of corrosion rate by i is more accurate and
precious than by weight change method mdd or thickness measurement mpy, since
very small current in terms of fraction of
can be measured. Corrosion rate i can be converted to mpy , knowing density and
atomic weight of the metal from the equation
mpy = 0.129 ai/nd 1.17 ( i is in mA/cm2, d gm/cc)
It is seen from equation 1.7 that the electrochemical current I0 flowing through the cell is proportional to (Ea - Ec). But as soon as current flows, the electrodes get polarized. Anode potential no more remains at Ea but increases in the positive direction as EP,a and cathode potential Ec shifts in the negative direction to EP,c as shown in fig.1.3. This phenomenon of shifting of electrode potentials from equilibrium values to polarized potentials is known as Polarization and magnitude of change is called overvoltage, designated by h.After polarization electrode potentials of anode and cathode come to very close to each other (within about 1-2 mV) and a much smaller current I indicated by following equation 1.14 flows.
I =( EP,c
- EP,a ) / R
Where resistive part R consists of electrolyte
resistance Rel and
external resistance Rext which is conductive resistance from
cathode to anode outside electrolyte so that equation can written as
Rel + I Rext =( EP,c - EP,a )
If the two electrodes are externally short
circuited which is the case for inseparable electrodes , Rext =0.
Rel =( EP,c -
If the electrolyte is of high conductivity
such as acid or sea water , Rel will be negligibly small and may be
approximated to very close to zero. Then right hand part of the above equation
is close to zero.
@( EP,c - EP,a )
EP,c @ EP,a = Ecorr
Under such conditions cathode potential becomes equal to anode potential and both of them are equal to corrosion potential Ecorr. At corrosion potential anodic reaction takes place with rate Ia known as anodic current which is equal and opposite in direction to Ic cathodic and both of them are equal to Icorr , corrosion current or rate of corrosion. This is illustrated in fig.1.4., with an example of iron corroding in acid. At Ecorr and Icorr, dissolution of iron takes place in acid at a rate which is equal to the rate of reduction of hydrogen ions so that there is no net accumulation of electron.Ecorr is the mixed potential of the two electrode potential. It will be seen later that both Ecorr and Icorr have huge implication in combating corrosion of materials
Why does polarization takes place ?
Before reactants start reacting, they
need to achieve the activation energy to surmount to activated state. This
energy requirement is proportional to the change in the electrode potential or overvoltage.
ΔGa# = nFha 1.22 and
ΔGc# = nFhc 1.23
Where ΔGa# and ΔGc# are the activation energies for anodic and Cathodic electrode reactions respectively with corresponding overvoltages ha and hc
For a solid iron surface to react with liquid
acid containing H+ ions following steps need to occur.
H+ ions must be available in the vicinity of solid
Formation of hydrogen atom by H+ + e = H
Adsorption of H to the metal surface as Had
Molecular hydrogen by Had + Had= H2
Formation of gas bubble and its detachment .
Availability of iron surface in contact with acid
Fe+2 + 2e
Of all steps of Cathodic reduction reaction (
i- iv), one which will need maximum energy will control the electrode reaction
rate and contribute for activation polarization. In general reaction (ii ) and
(vii) account for activation polarization for cathodic and anodic reactions
The activation polarization or overvoltage
is related to current density ia or ic by Tafel s equation.
ha = +ba log(ia/i0) 1.24
hc = -bc
where i0 is the exchange current
For any single electrode at equilibrium say Fe/Fe2+ , rate of forward reaction Rf is equal to the rate of backward reaction Rb.
Fe2+ + 2e ฎ Fe
Fe= Fe2+ + 2e
Rf = Rb = i0
The exchange current is related to rate of forward reaction and backward reaction by the equation 1.28. Though the rate of forward 1.26 and backward reaction 1.27 are equal, there is no net accumulation Fe2+ or Fe , still there is a rate which is represented by i0
Consider iron two electrodes iron and platinum dipped in acid as shown in fig.1.6.
At anode iron ionizes releasing electrons that pass through the external conductor to other electrode platinum and these electrons discharge hydrogen ions, when they are available in the vicinity of the platinum electrode surface. Once hydrogen ions adjacent to the platinum electrode have been consumed by electron , ions from the bulk of the solution need to diffuse through the solution towards the electrode surface for further reaction. Now the speed of electron movement through the external conductor from anode to cathode is much faster than the diffusional ionic speed of ions through the bulk solution towards the cathode surface. There is concentration deficiency of H+ ions near cathode surface and build up of negative charge electron, when the electrode potential of the cathode decreases or moves in the negative direction. This phenomenon is known as concentration polarization or Diffusion polarization. At times situation may arises, no H+ ions are available near cathode surface, no cathodic reaction takes place and overvoltage
approaches to negative infinity( see Fig.1.7).
The rate at which it occurs is called limiting current density iL.
The concentration overvoltage can be given by the equation.
hcon = 2.3RT/nF log ( 1- i/iL)
iL = DnFC/d 1.30
is the diffusivity of the reaction ion of concentration C, d
is the thickness of the concentration gradient. Thus iL increases (fig.1.7) with higher
concentration, higher temperature and solution agitation due to increase in
The total polarization h is the combined effect of activation and concentration polarization. Concentration polarization is normally absent at anode , since there is unlimited supply of the metal for ionization at anode , Thus total polarization for cathode and anode can be written as ;
ha = +ba
hc = -bc
2.3RT/nF log ( 1- ic/iL) 1.32
From the above equations , it is seen that for a particular system of a metal corroding in an environment, the parameters ba, bc , i0 and iL define the system and rate of corrosion is decided by them. In practice there may be more than one anodic reactions and cathodic reactions and each of the equation must follow either of the above two equation
This polarization arises in cases where electrolyte resistance is very high. For example if an insulating coating which separates anodic and cathodic areas, is applied to steel structure. In this case the anodic line and cathodic line instead of intersecting , is separated by resistance polarization IR (fig.1.9).
Mixed potential theory
According to Mixed potential theory, the total rate of oxidation or sum of anodic currents must be equal to total rate of cathodic current or the sum of cathodic currents and this would occur at the potential, Ecorr and current, icorr . Activation polarization normally occurs in the initial stage of any reaction that is when current or rate is not very high, whereas concentration polarization generally occurs when at a later stage when the current is high. For most cases the point Ecorr and icorr in the region of activation polarization. Hence concentration polarization is normally not considered for determination of corrosion rate. However for corrosion of steel in aerated aqueous media or soil represented by following equations , concentration
Cathodic O2 +
2H2O + 4e =
Anodic Fe = Fe 2+ + 2e 1.34
Polarization occurs at very early stage. Since the cathodic reduction reaction is controlled by concentration of dissolved oxygen and rate controlling step is the diffusion of dissolved oxygen. So the point of Ecorr and icorr is on the concentration polarization line of cathodic reaction (fig. 1.8). Corrosion current icorr approaches limiting current density iL
Prepared by S Paul