Thermal stability of the passive film formed on 316L stainless steel surface studied by ToF-SIMS

The modifications of the passive film formed on 316 L stainless steel surface during stepwise heating in ultra-high vacuum up to 300 degrees C have been studied in situ by ToF-SIMS. The pre-formed passive film (in 0.05 M H2SO4) has a bilayer structure, comprising Fe-rich, Mo-rich (outer) and Cr-rich (inner) layers. Below 100 degrees C, the passive film is stable. At 100 degrees C-250 degrees C, dehydroxylation and dehydration is observed. Above 250 degrees C, the main modification in the film is formation of chromium oxide at the expense of oxidized iron. At higher temperature, thicker Cr-rich inner layer with sharper inner/outer oxide interface is formed.