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

Electrochemical behaviour and passive film characteristics of an ultra-thin 316 L foil with a thickness of20μm 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(~4μm)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 316 L foil than traditional wrought 316 L.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 316 L 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.