ELUCIDATION OF A TRIGGER MECHANISM FOR PITTING CORROSION OF STAINLESS-STEELS USING SUBMICRON RESOLUTION SCANNING ELECTROCHEMICAL AND PHOTOELECTROCHEMICAL MICROSCOPY

Scanning electrochemical microscopy with submicron resolution shows th at the local current density for dissolution of certain MnS inclusions in stainless steel can be extremely high (>1 A cm(-2)) and appears to be chloride-catalyzed, a result not anticipated by previous work on c hemically prepared MnS. The dissolution forms a sulfur-rich crust exte nding over the inclusion and the surrounding metal. Photoelectrochemic al and optical microscopy indicate that formation of a sulfur-rich sta in around an inclusion is a necessary preliminary to the initiation of a pit and show attack on the metal underneath the stain. Therefore it is reasonable to propose that the very high local current density of inclusion dissolution leads to a significant local concentration of ch loride under the crust, as a consequence of electromigration to suppor t the current, and may also cause a significant decrease in the local pH as a consequence of the chemistry of the inclusion dissolution reac tion, especially if the inclusion also contains some Cr. It is then fu rther reasonable to propose that the conditions generated under the su lfur crust might be sufficiently extreme to cause the stainless steel to depassivate and a pit to trigger.