Medium Aggressivity and Inhibitive Species in Pitting Corrosion of Stainless Steels

Abstract

The presence of halide ions in an electrolyte can provoke local breakdown of the passive film of stainless steels and initiate the localized corrosion by different exposed mechanisms, resulting in stress corrosion cracking, crevice corrosion or pitting. The chloride ions play a considerable role in these phenomena, but several oxyanions can be used to balance the influence of the halide ions presence and prevent the risk of pitting corrosion. Their efficiency is shown by the use of polarisation curves or impedance measurements, and calculated by statistical studies of the pitting potential values. Several possible mechanisms can explain their action on the passive film. The results obtained show that there is a competition between aggressive and inhibitive species present in the medium. The corresponding mechanisms -adsorption on active sites, diffusion and transport - are discussed, and it appears that the thickness of the film and the limiting step of the corrosion process depend on the medium composition. If the austenitic or ferritic structure does not play a great role in these mechanisms, the steel cleanliness is an important parameter which can modify the properties of inhibitive species, and the sulphide-containing species - in the metal or in the medium - can provoke changes in the kinetics of the pitting corrosion or inhibition process, by modification of the localized stability of the passive film.