Microelectrochemical measurements of dissolution of MnS inclusions and morphological observation of metastable and stable pitting on stainless steel

The electrochemical dissolution of MnS inclusions and pit initiation processes on type 303 stainless steel were investigated by an electrochemical microcell technique. In 1 M Na2 S O4, the electrochemical dissolution of MnS inclusions started at 0.1-0.3 V vs AgAgCl (3.33 M KCl), which was followed by the initiation of a hemispherical and smooth wall pit at MnS/matrix boundary. It was suggested that the dissolution products of MnS inclusions induced the formation of a salt film, which caused localized electropolishing. In 0.1 M NaCl, the onset potential of electrochemical dissolution of MnS inclusions was 0.3-0.4 V and the stable pit growth occurred at a potential of around 0.5 V. Field-emission scanning electron microscopy observations revealed that many metastable pits with a diameter of approximately 1 μm were formed at MnS/matrix boundary on the specimen, of which the polarization measurement was stopped immediately after stable pit initiation. From a morphological point of view, the metastable pits were found to be very similar to flat-walled etch pits. The dissolution products of MnS inclusions and chloride ions would attack the metal surface newly exposed by dissolution of the inclusions. It is proposed that the chemistry of dissolved species from MnS inclusions have a substantial influence on pit initiation at MnS inclusions.