Role of sulphide inclusions on the pitting of stainless steels in chloride media

The resistance of stainless steels to the local breakdown of passive films was measured by using a potentiokinetic stochastic method, and was characterized by the pitting probability tu_E and the pit generation rate gE, depending on the potential E. This allowed a conventional pitting potential Eg to be obtained, corresponding to a given pit generation rate g°. Its evolution can be studied against the chloride contents in the solution. Measurements were taken from two ferritic stainless steels: AISI 430 and 430Ti, where the solubility of the sulphide inclusions differs greatly. In neutral chloride media, variations of the pitting parameters were observed with the changes to the solutions in terms of: - aeration or deaeration - presence or absence of molybdate ions - presence or absence of thiosulate ions. The results obtained were compared and discussed in terms of the mechanism of a possible oxidation of the sulphide inclusions by molybdate species, which can give an improvement in the passive films stability in neutral media through a change in the degree of solubility.