MEHRAN UNIVERISITY OF ENGINEERING AND TECHNOLOGY,JAMSHORO, SINDH, PAKISTAN

The half-cell potentials are thermodynamic parameters that relate to systems at equilibrium.

These half-cell potentials represent the magnitude of a driving force, or the tendency for the occurrence of the particular half-cell reaction.

How-ever, it should be noted that although these potentials may be used to determine spontaneous reaction directions, they provide no information as to corrosion rates.

That is, even though a ∆V potential computed for a specific corrosion situation using Nernst Equation is a relatively large positive number, the reaction may occur at only an insignificantly slow rate.

From an engineering perspective, we are interested in predicting the rates at which systems corrode; this requires the utilization of other parameters, as discussed below:

The corrosion rate, or the rate of material removal as a consequence of the chemical action, is an important corrosion parameter.

This may be expressed as the corrosion penetration rate (CPR), or the thickness loss of material per unit of time.

The formula for this calculation is

where W is the weight loss after exposure time t;

ρ and A represent the density and exposed specimen area, respectively.

K is a constant, its magnitude depending on the system of units used.

The CPR is conveniently expressed in terms of either mils per year (mpy) or millimeters per year (mm/yr).

In the first case, K = 534 to give CPR in mpy (where 1 mil 0.001 in.), and W, , A, and t are specified in units of milligrams, grams per cubic centimeter, square inches, and hours, respectively.

In the second case, K = 87.6 for mm/yr, and units for the other parameters are the same as for mils per year, except that A is given in square centimeters.

For most applications a corrosion penetration rate less than about 20 mpy (0.50 mm/y) is acceptable.

In as much as there is an electric current associated with electrochemical corrosion reactions, we can also express corrosion rate in terms of this current, or, more specifically, current density (that is, the current per unit surface area of material corroding) which is designated i.

The rate r, in units of mol/m2-s, is determined using the expressionwhere, again, n is the

number of electrons associated with the ionization of each metal atom, and F is 96,500 C/mol.

Consider the standard Zn/H2 electrochemical cell shown in Figure, which has been short-circuited such that oxidation of zinc and reduction of hydrogen will occur at their respective electrode surfaces.

The potentials of the two electrodes will not be at the values determined from emf Table because the system is now a nonequilibrium one.

The displacement of each electrode potential from its equilibrium value is termed polarization, and the magnitude of this displacement is the overvoltage, normally represented by the symbol η.

Overvoltage is expressed in terms of plus or minus volts (or millivolts) relative to the equilibrium potential.

For example, suppose that the zinc electrode in Figure has a potential of 0.621 V after it has been connected to the platinum electrode.

The equilibrium potential is 0.763 V (emf Table), and, therefore,

There are three types of polarization

Activation polarization

Concentration polarization

Resistance polarization(ohmic drop)

where:ηact is the activation overpotential, a complex function

describing the charge transfer kinetics of an electrochemical reaction.  η act is always present and mostly dominant at small polarization currents or voltages;

ηconc is the concentration overpotential, a function describing the mass transport limitations associated with electrochemical processes.  η conc is predominant at larger polarization currents or voltages;

iR is the ohmic drop. This function takes into account the electrolytic resistivity of an environment when the anodic and cathodic elements of a corrosion reaction are separated by this environment while still electrically coupled.

All electrochemical reactions consist of a sequence of steps that occur in series at the interface between the metal electrode and the electrolyte solution.

Activation polarization refers to the condition wherein the reaction rate is controlled by the one step in the series that occurs at the slowest rate.

The term ‘‘activation’’ is applied to this type of polarization because an activation energy barrier is associated with this slowest, rate-limiting step.

To illustrate, let us consider the reduction of hydrogen ions to form bubbles of hydrogen gas on the surface of a zinc electrode.

It is conceivable that this reaction could proceed by the following step sequence:

1. Adsorption of H+ ions from the solution onto the zinc surface.

2. Electron transfer from the zinc to form a hydrogen atom,

3. Combining of two hydrogen atoms to form a molecule of hydrogen

4. The coalescence of many hydrogen molecules to form a bubble.

The slowest of these steps determines the rate of the overall reaction.

For activation polarization, the relationship between overvoltage ηa and current density i is

where β and i0 are constants for the particular half-cell.

The parameter i0 is termed the exchange current density, which deserves a brief explanation.

Concentration polarization exists when the reaction rate is limited by diffusion in the solution.

For example, consider again the hydrogen evolution reduction reaction.

When the reaction rate is low and/or the concentration of H+ is high, there is always an adequate supply of hydrogen ions available in the solution at the region near the electrode interface

On the other hand, at high rates and/or low H+ concentrations, a depletion zone may be formed in the vicinity of the interface.

As much as the H+ ions are not replenished at a rate sufficient to keep up with the reaction

Thus, diffusion of H+ to the interface is rate controlling, and the system is said to be concentration polarized.

Concentration polarization generally occurs only for reduction reactions because for oxidation, there is virtually an unlimited supply of metal atoms at the corroding electrode interface.

The mathematical expression relating concentration polarization overvoltage ηc and current density i is

where R and T are the gas constant and absolute temperature, respectively, n and F have the same meanings as above, and iL is the limiting diffusion current density.

Concentration polarization data are also normally plotted as overvoltage versus the logarithm of current density.

It may be noted from this figure that overvoltage is independent of current density until i approaches iL;

at this point, ηc decreases abruptly in magnitude.

Both concentration and activation polarization are possible for reduction reactions.

Under these circumstances, the total overvoltage is just the sum of both overvoltage contributions.