Abstract: A decorative chemical conversion film was formed on A₃ mild steel by immersing it in a SiMo heteropoly-acid solution. The SEM photographs and the anodic polarization curves revealed the good anticorrosive effect of the film. The results of XPS and AES analyses showed that Fe existed as Fe(Ⅲ) and Fe (Ⅱ) in the film while Mo as Mo (Ⅵ) in the surface of the film but as Mo (Ⅵ) and Mo (Ⅳ) in the internal layer of the film. The relative atomic percent content(AC%) of the film obtained from the elemental composition constant region of the profile curves by AES was Fe 17.1%,Mo 13.5%,Si 8.8%, O 57.4% and C 3.2% respectively.

Key words: mild steel, Si-Mo heteropoly acid, chemical conversion film, anticorrosion property