Interpretation of the Second Anodic Current Maximum on Polarization Curves of Sensitized Chromium Steels in 1N H2 SO4

Abstract

Some data on the occurrence of the second anodic current maximum observed on polarization curves of chromium steels in 1N H2SO4 are presented and used in a critical interpretation of the results of other investigators. It is shown that the second anodic current maximum observed on specimens with special prepolarization treatments, i.e., holding the specimen for a long time either at the open circuit potential or at a cathodic potential, results from artificially imposed surface effects and does not represent the bulk material. Recent investigations on iron-chromium electrodes which did not have a prepolarization treatment correlate the second anodic current maximum to preferential attack along α-γ phase boundaries. In the latter experiments, the second anodic current maximum is sensitive to heat treatment (degree of sensitization) and the effect is explained entirely on the chromium impoverishment theory. Supporting evidence is provided by microprobe analyses and critical model experiments.