Abstract
The anodic behaviour of 17–7 precipitation hardening stainless steel in 0.5 M H2SO4 and 0.5 M H2SO4 + Cl− solutions was investigated in detail. The coulometric areas of the two anodic peaks in the potentiodynamic curve in the chloride-free solution are dependent upon the duration of the free corrosion between the preliminary cathodic activation and the starting of the potentiodynamic scan. When the free corrosion time exceeds 6 minutes the peak at the more active potential disappears completely.
The influence of the free corrosion time was much less evident in the presence of chloride ions; in these conditions anodic currents were greater.
It was also established that the two anodic peaks correspond to the active-passive transitions relative to the two phases composing the steel: austenite and δ-ferrite. This was seen by potentiostatically etching the steel in the active dissolution ranges of both anodic peaks.
Finally it was shown, both micrographically and by comparing the anodic curves of pickled and polished specimens, that ordinary nitric–hydrofluoric pickling solutions selectively attack the austenite.
Get full access to this article
View all access options for this article.