Effects of alloyed molybdenum on the kinetics of repassivation on austenitic stainless steels

Abstract

The kinetics of repassivation of Type 316L austenitic stainless steel in aqueous solutions after in situ generation of the metal surface have been obtained. The results are compared with those obtained previously from similar experiments on Type 304L stainless steel. In alkaline and slightly acidic solutions both alloys repassivate at a rate controlled by ion migration through the growing oxide film under high electric field. The presence of alloyed molybdenum accelerates the rate in the early stages of film growth but in the later stages the kinetics for the two materials are identical. In strongly acidic solutions dissolution of the iron component of the oxide film accompanies film growth. This constructive dissolution process occurs at a rate controlled by the diffusion of iron ions through the oxide matrix. While the presence of molybdenum does not affect the diffusion coefficient significantly, it does allow the release of iron ions to the solution to commence at a far earlier stage of film growth, thereby hastening the onset of passivity. The associated kinetic parameters are presented and the implications of the proposed mechanisms discussed.