Corrosion Rate Measurement by Fitting Polarization Curve to Wagner-Traud Equation

2 Corrosion Rate Measurement by Fitting Polarization Curve to Wagner-Traud Equation 

For a situation in which the anodic and cathodic reactions are both under pure activation control, the net current measured as a function of potential is the difference between two exponential expressions, as given by the Wagner-Traud equation [13]  

This expression was derived in Chapter 1.3. An experimental polarization curve, such as that shown in Fig. 3(b), can be fitted by nonlinear least squares fitting to this expression. Such a fit will yield values for the corrosion rate, corrosion potential, and anodic and cathodic Tafel slopes. Most modern software packages for analysis of corrosion data have this capability. 

In this approach, the behavior is assumed to be perfect activation polarization for both reactions, so that the extrapolated Tafel regions intersect the corrosion potential at the same value of current density. As was seen for Fe in H2SO4, this is often not the case. The fit usually provides 

The corrosion rate determined by the fit for the polarization curve shown in Fig. 3(b) is 1.4 X 10 A cm , and the fitted anodic and cathodic Tafel slopes are 47 and 98 mV decade respectively. The corrosion rate is between the values determined by extrapolation of the anodic and cathodic Tafel regions, and the Tafel slopes are different than those determined manually. 

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