Pit Propagation at the Boundary between Manganese Sulfide Inclusions and Austenitic Stainless Steel 303 and the Role of Copper

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.