The distribution of stainless steel breakdown potentials: experimental method and the effect of metallurgical conditions

Abstract

Potentiostatic tests up to 1000 h and potentiodynamic tests at a low potential scanning rate such as 1 μV s −1 have been carried out in 0.1 M HCl solution in a cell capable of holding 96 test specimens simultaneously. The steels tested were AISI 304 and 316 stainless steel, some specimens modified by cold working or by electron beam surface melting. Test methodology has shown: (a) the probability of localized corrosion onset can vary with time and potential in a discrete rather than continuum sequence of values and can also be reduced by increasing the potential, (b) potentiostatic tests, performed at a potential equal to the minimum breakdown potential detected potentiodynamically, showed that at this potential a little more than 25% of specimens remain uncorroded after 1000 h of exposure and (c) the mechanism itself of localized corrosion depends on the potential range. The study regarding different stainless steels suggests that: the structure ( γ, γ + a or γ + δ), the surface exposed (longitudinal or short transverse) and the Mo content influence the minimum breakdown potential value detected potentiodynamically and that grain boundary precipitates, grain size and cold working affect the shape of the survival probability vs potential curve.