The distribution of stainless steel breakdown potentials: The effect of surface finishing degree and HNO 3 treatment

Abstract

A study has been carried out on the behaviour of AISI 304 or AISI 316 commercial or experimental stainless steels in 0.1 M HCl solution upon surface finishing with wet emery paper or HNO 3; treatment. Potentiodynamic tests performed at a very low potential scanning rate (1 μV s −1) in a cell capable of holding 96 specimens simultaneously, proved that: (a) the extrapolation of the survival probability vs potential curve to the unit survival probability value could lead, for most of the steels considered, to potential values of about 220 mV(NHE) regardless of the type of steel, of the surface finishing degree and of the HNO 3 treatment. (b) Both the minimum breakdown potential value, potentiodynamically measured (on a sample of limited number of specimens), and the survival probability <1, at equal potential, seem to slightly increase with the final mesh number of the grinding finishing. These effects can be clearly noted passing from a range of values corresponding to a coarser finish to that corresponding to a finer one. It is hypothesized that the difference between the two finishing degree ranges can be due to the different defect size that can be bigger or less than the critical size need for the onset of localized corrosion. (c) In corresponding high values (>0.8) of the survival probabilities, different samples of HNO 3-treated stainless steels tend to assume similar behaviours. Assuming a stochastic model for the localized corrosion mechanism, potentiostatic tests performed at various potentials for over 1000 h confirm that the localized corrosion mechanism itself could depend on the potential value. This suggests the idea of a potential range within which pit repassivation phenomena would be hampered. HNO 3 treatment can increase breakdown potentials and, at potentials corresponding to high survival probabilities of the samples, could hinder pit repassivation phenomena.