Corrosion Rate Measurement by Tafel Extrapolation

Unlike the cathodic portion of the polarization curve, the anodic portion of the curve in Fig. 3(b) does not exhibit clear Tafel-type behavior. The mechanism for Fe dissolution in acids is quite complex. A line can be drawn in the region just above the corrosion potential, giving a Tafel slope of 34 mV decade k Extrapolation of this line intersects the zero-current potential at 7 X 10 A cm , a considerably different value than the extrapolation of the cathodic portion of the curve. This is not uncommon in practice. When this happens, it is usually considered that the anodic portion of the curve is affected by changes on the electrode surface, that is, surface roughening or film formation. The corrosion rate is typically determined from the extrapolated cathodic Tafel region. 

As the applied potential increased further, a region of limiting current was observed. The reaction is limited in this region by mass transport of the dissolution product away from the surface, as described in Chapter 1.4. The surface was covered by a FeS04 salt film, and the solution in contact with the salt film was saturated with FeS04. Further increases 

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