Direct evidence of passive film growth on 316 stainless steel in alkaline solution

Evolution of the oxide films formed on the surface of 316 stainless steel needles during the electrochemical polarization in 0.1 M NaOH solution is investigated by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). During the polarization process, the films can grow from 4 nm to dozens of nanometers and are composed of crystalline structures incorporated with amorphous phases. Additionally, Cr (III) species that are formed at low potentials will transform into Cr (VI) species in secondary passive region. Fe element is present as Fe3O4-like spinel oxides while no Fe2O3 is found in the films. The Fe/Cr ratio increases with the increase of applied potential.