Direct observation of thin membrane passive film over the growing pit on sputtered nanocrystalline austenitic stainless steel film

• Temperature controlled potentiostatic polarization was used to initiate single pit.
• Thin membrane passive film over the growing pit was directly observed on the SNAss.
• The passive film presents a distinctly duplex layer with an amorphous structure.
• The outer layer is a Fe-enriched oxide and the inner layer is a Cr-enriched oxide.
• The passive film of SNAss possesses higher elastic modulus and hardness.

Potentiostatic polarization with a temperature scan was applied to initiate formation of a single pit on a sputtered nanocrystalline austenitic stainless steel film and the thin membrane passive film over the growing pit was directly observed for the first time in this study. The results showed that the passive film consisted of two different layers, both of which exhibited an amorphous structure. The outer layer comprised of a Fe-enriched oxide, whereas the inner layer was a Cr-enriched oxide. Compared with conventional coarse-grained stainless steel, the passive film of sputtered nanocrystalline austenitic stainless steel possessed higher elastic modulus and hardness.