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

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.