Abstract
Potentiostatic polarization experiments with type 410 stainless steel in ferric sulfate solutions show that the reduction curve for inhibitor and the oxidation curve of the metal can be superimposed without distortion from the corrosion potential to the mixed potential established by the inhibitor. The inhibitor concentration required to produce passivity is determined by the critical current density of the metal; it exceeds the concentration required to maintain passivity by about two orders of magnitude. Ferric ion does not protect pure iron in acid solution because of the very large critical current density of this metal. However, an iron electrode first made passive with impressed anodic current remains passive in ferric sulfate solutions after the current is discontinued. In this sense ferric ion is an inhibitor for pure iron as well as for stainless steel.
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