THE SECONDARY PASSIVE FILM FOR TYPE 304 STAINLESS-STEEL IN 0.5 M H2SO4

An examination has been carried out of the secondary passive film on T ype 304 stainless steel in 0.5 M H2SO4. The characterization technique s used were electrochemical (potentiodynamic; potentiostatic, and film reduction experiments) and surface analytical. A bilayer model for th e secondary passive film is proposed. It appears that next to the meta l, there is a modified passive film which controls the electrochemical response; i.e., governs the current for any applied potential. On top of this modified passive film, the experimental data are consistent w ith a ''porous'' corrosion-product film which adds to the total film t hickness but has little influence on the electrochemical response. The composition of the secondary passive film corresponds most probably t o a mixed Fe/Cr oxide/hydroxide enriched in Cr3+, With a composition s imilar to a primary passive film.