Pitting corrosion around manganese sulfide (MnS) inclusions in stainless steel 303 (SS 303) were investigated using in situ Atomic Force Microscopy (AFM), Scanning Kelvin Probe Force Microscopy (SKPFM) and Scanning Electron Microscopy. In situ AFM experiments in 0.1 M sodium chloride solution at voltages near the pitting potential combined with post immersion energy dispersive X-ray spectroscopy (EDS) found copper (Cu) deposition on MnS inclusions but no pitting corrosion. SKPFM images before and after Cu deposition found that Cu ennobled the inclusion. Damage evolution at MnS inclusions was imaged in real-time with AFM at an applied anodic potential. During pit propagation the MnS inclusion remained "passivated" while a trench formed in the bulk SS 303 at the MnS/SS boundary. Focused Ion Beam (FIB) cross sectioning of trenches after this immersion period found no damage to the MnS and Cu deposition on the portion of the inclusion that was inside the trench. FIB images of unexposed samples found preexisting sub-surface trenches at MnS inclusions below the plastic deformation zone at the surface. From these results a proposed mechanism of corrosion propagation at the MnS / SS boundary is described. Over the course of the past century corrosion pits in austenitic stainless steels have been associated with manganese sulfide (MnS) inclusions. An excellent review of the early work in this area has been provided by Wranglen.1 It was proposed that MnS inclusions oxidized preferentially as they were ". . . less noble then the surrounding oxide film, and then [the oxidation] spreads to the active metal below the sulfide inclusions." The model also discussed the sulfur species that may be produced during MnS oxidation including sulfide (S 2− , HS − ), sulfite (SO 3 2− ) and sulfate (SO 4 2− ). As it relates to those species, Wranglen argued that the electrochemical oxidation of sulfur to sulfite produced local acidity. To the contrary, Eklund argued that local acidity was produced from the electrochemical oxidation of sulfide to sulfate.
2More recently, Stewart and Williams, in a metastable pitting study, proposed that MnS dissolution and the corresponding "supply of sulfur to the active pit surface" was a critical step for pit stability.3 They showed that laser re-melting of the surface greatly reduced metastable activity and pitting damage. Thus, larger MnS inclusions could supply sufficient S to maintain pit stability while smaller inclusions could not. Similar conclusions were drawn by Searson and Latanison for rapidly solidified SS 303. 4 Later work by Williams focused on the formation of a critical pitting solution at the site of inclusions as the trigger for pitting corrosion.5 It was proposed that the high dissolution rate of MnS inclusions resulted in the formation of a "sulfur crust" over the inclusion that concentrated chloride underneath it by electromigration. The result was the formation of an occluded environment in which stainless steel depassivates. Early work by Webb and Alkire proposed that the sulfide ...