Composition and structure of coloured oxide films on stainless steel formed by triangular current scan and cathodic hardening treatment

The influence of the electrochemical hardening treatment on the composition and structure of coloured porous films formed by triangular current scan on AISI 304 stainless steel has been examined by impedance spectroscopy and also by SEM, AES, XPS and AFM. It was observed that the films, originally consisting of a porous chromium–iron oxide, became more compact and enriched in chromium after the hardening treatment. The development of a high frequency semi-circle in the impedance spectra is explained by the formation of an outer oxide layer.