On the Shape of Some Polarization Curves in the Vicinity of the Corrosion Potential

Many calculated and real polarization curves can reasonably approximate a straight line in the neighborhood of the corrosion potential when the Stern-Geary simplifying assumptions are satisfied. This occurs although the curve exhibits a point of no curvature (point of inflection) which generally lies far from ϕ_k in the common case of unequal Tafel slopes. The expected valid range for approximately linear kinetics, moreover, is larger when Tafel slope values are high and close to each other, but not coincident. Overvoltages of about ±10 mV from ϕ_k, however, are not such that a satisfactory linearity in any case will be ensured close to ϕ_k. The fractional error involved in using a linearization method to calculate the corrosion current density, I_k, was estimated for pure iron and Armco iron electrodes corroding in sulfuric and chloridic media. For the studied Fe/HCl systems, linear kinetics (within 5%) are to be expected over several tens of millivolts from ϕ_k. Correspondingly, the error on I_k is very low. For the examined Fe/H₂SO₄ systems, instead, a reasonable linearity can only occur within a very narrow neighborhood of ϕ_k, due to unfavorable Tafel slope values. Even under such a circumstance, however, the error on I_k may be rather low if a suitable linearization method is carried out.