Formation of stable passive film on stainless steel by electrochemical deposition of polypyrrole

Stable passive film has been formed on 304 stainless steel during the electrochemical deposition of polypyrrole (PPy) from sulfuric acid solution. The stability of passive film under PPy increases with aging in H2SO4 and this film has much higher resistance to pitting than that formed by anodic polarization under the same condition of aging. XPS studies indicate that the content of chromium components in the oxides under PPy layer is about twice that of the anodically formed passive film with larger value of the ratio Cr2O3/Cr(OH)3. Higher content of iron with a ratio of Fe2+/Fe3+ more than unity is also observed with a considerable lower hydration and sulfate content in the oxides under PPy layer. It is suggested that sulfate ion is consumed as a dopant in the formation of PPy film and the oxidation of stainless steel is achieved under the environment of lower concentration of water and sulfate molecules. This may result in the enhancement of formation of corrosion-resisting oxides rather than hydroxides and sulfates.