Current fluctuations associated with localized corrosion: mechanistic studies of the early stages of pitting and stress corrosion cracking of stainless steel

In this paper, the applicability of a "feedback" model for the initiation of pitting corrosion of stainless steel are discussed, on the basis of measurements of current fluctuations associated with the initiation and temporary propagation of unstable micropits. The frequency of such fluctuations varies with the potential, potential scan rate and film thickness, but all such variations appear to reduce to a common curve if the passive current density is plotted as the independent variable. The effects of laminar and turbulent flow on the initiation of pitting corrosion are also discussed, and it is shown that turbulent flow exerts its effect by affecting the solution concentration field around the pit mouth and hence the resistance to current flow to the developing pit. Finally, it is show that current fluctuations occur during the early stages of stress corrosion cracking of a sensitised stainless steel, and that each pulse corresponds approximately to the movement of a crack across a single grain boundary facet. (from authors' abstract)