On the origin of passive film breakdown and metastable pitting for stainless steel 316L

Passivity breakdown and metastable pitting of stainless steel 316 L was investigated in chloride-containing solution using in-situ microscopy and atomic spectroelectrochemistry (ASEC). X-ray photoelectron spectroscopy revealed the presence of Cl- ions in the outer layer of the passive film. The film was found to undergo dielectric breakdown during anodic polarisation, resulting in the formation of metastable or stable pits. ASEC revealed that Cr and Mo can aid in the early repassivation of ‘open’ pits, whereas ‘undercutting’ pits were observed to exhibit delayed repassivation later accompanied by rupture of the perforated cover due to the formation of hydrogen gas bubbles inside the pit.