Influence of the chemical dissolution of MnS inclusions on the electrochemical behavior of stainless steels

Immersion of stainless steel containing a well-controlled density of MnS inclusions in 1 M NaCl, pH 3 leads to the chemical dissolution of these heterogeneities. This process was studied using in situ atomic force microscopy and the dissolution rate of MnS inclusions was estimated between 0.01 and 0.19 mu m(3)/min. The effects of MnS dissolution on the chemical composition and the local electrochemical behavior of the specimen surface were investigated using secondary ion mass spectroscopy, X-ray photoemission spectroscopy, and the electrochemical microcell technique. It was shown that stable CrS and unstable FeSO4 were formed. The size of the areas around MnS inclusions affected by the presence of sulfur-containing species depends on the immersion time and the composition of the native film. The local electrochemical measurements reveal that the chemical dissolution of MnS inclusions promotes pitting corrosion in the surrounding grains at moderate applied potentials for short immersion time and general corrosion of the whole specimen surface for long immersion time. (c) 2005 The Electrochemical Society. All rights reserved.