11332 Determination of Corrosion Rate

It can be seen that by plotting the corrosion current as a function of potential (Figs. 11.3 and 11.4), four important parameters can be determined, that is, corrosion potential Ecorr, corrosion current /corr, and the anodic (Pa ) and cathodic (Pc) Tafel constants. Figure 11.3 also shows that by moving the potential away from Ecorr by a set value +AE(= E — Ecorr) a straight line is obtained:

Similarly moving the potential by a value —AE, a corresponding equation is obtained:

,c = pclog(M, corr where a and b represent anodic and cathodic terms and n is designated as the overpotential or polarization. Hence the kinetics of the anodic or cathodic reaction may be obtained by plotting n versus log (iapp) for either reaction. It should be recognized that the above description of the Stern and Geary electrode kinetic expression is somewhat simplified and a full treatise can be found in reference [12].

This method of polarizing the sample and measuring the corresponding change in corrosion current (or current density for a known area) is commonly used to determine the corrosion rates of metals in given electrolytes. The plots obtained by this method are known as polarization curves.

Although polarization curves are commonly used to provide kinetic data for corrosion reactions, several complications arise due to both solution resistance and concentration polarization effects. The first of these is associated with the ohmic drop between the sample and reference electrode. This arises primarily due to (a) the resistivity of the solution. (b) the magnitude of the applied current, and (c) the location of the reference electrode. The result of ohmic errors is that the measured potential requires correction [13].

Concentration polarization occurs when the rate of removal of reacting species from the electrode surface, or rate of transfer of species to the electrode, becomes diffusion limited. such effects occur at large over potentials [14] and cause the anodic and cathodic Tafel lines to deviate from linearity. Figure 11.4 shows a typical polarization curve for the aluminum alloy 2024/NaCl system.

Figure 11.4 shows that at low values of overpotential, n the E versus log i curve deviates from a straight line. If, however, E versus I values are plotted on linear axes the following relationship holds [15]:

corr

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