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 that 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 chemically prepared MnS. The dissolution forms a sulfur-rich crust extending over the inclusion and the surrounding metal. Photoelectrochemical and optical microscopy indicate that formation of a sulfur-rich stain 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 chloride under the crust, as a consequence of electromigration to support the current, and may also cause a significant decrease in the local pH as a consequence of the chemistry of the inclusion dissolution reaction, especially if the inclusion also contains some Cr. It is then further reasonable to propose that the conditions generated under the sulfur crust might be sufficiently extreme to cause the stainless steel to depassivate and a pit to trigger.