Passivation characteristics of ultra-thin 316L foil in NaCl solutions

•A commercial ultra-thin 316L foil (20 μm thick) has fine grains with preferential orientation of (110), and no detectable MnS inclusion.•The 316L foil exhibits significantly lower passive current density and higher breakdown potential in NaCl solution than wrought 316L.•The passive film on the 316L foil is highly enriched in Cr and Mo oxides, and Ni is enriched underneath the passive film.•The enhanced diffusion of Cr and Mo leads to a Cr- and Mo-enriched passive film giving the superior corrosion resistance. Electrochemical behaviour and passive film characteristics of an ultra-thin 316L foil with a thickness of 20 μm in 3.5 wt.% NaCl solution were investigated using multiple techniques, focusing on the effect of microstructure, the applied potential, and the pH of the solution. The microstructure contains mainly fine grains (∼4 μm) with high-angle boundaries and preferential orientation of (220), and no MnS inclusion was detected. The electrochemical measurements show a significantly higher breakdown potential and lower passive current density for the 316L foil than traditional wrought 316L. The surface analyses using angle-resolved X-ray photoelectron spectroscopy (ARXPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) reveal that, compared to the wrought material, both the inner and out parts of the passive film on the 316L foil are more enriched in Cr- and Mo-oxides. The microstructure favourable for elemental diffusion and the absence of MnS inclusion facilitate the formation of a continuous compact Cr- and Mo-rich passive film, which effectively retards corrosion in NaCl solution and remains stable in acidic solution (pH 2) or at high polarised potential up to 600 mV vs Ag/AgCl.