THE STABILITY OF THE TRANSPASSIVE FILM ON 304-STAINLESS STEEL WITH POSTTREATMENT

Investigation on the stability of transpassive film on 304 stainless s teel was carried out by ellipsometric measurement, AES and XPS analyse s and examination of the breakdown potential, E(b), in a Cl-containing solution and the active dissolution time t(a) in an acidic medium. It is found that the t(a), of properly transpassivated film is almost 2 orders of magnitude longer than that of normally passivated film, and that the E(b) of the film which is post-treated in a molybdate-contain ing solution is much higher than that of the passive film formed in th e passive region with the same post-treatment. The results of ellipsom etric measurement show that the thickness of the film on the stainless steel increases with the positive shift of the anodic polarization po tential, especially in the transpassive region, and the thickness of t he transpassive film increases much more sharply than in the passive r egion. Moreover, AES and XPS analyses indicate that the transpassivate d film consists of much more Fe3+ and much less Fe2+ than the passivat ed one, and this film with proper post-treatment contains much more Mo than the film formed in the passive region with the same post-treatme nt. It is believed that the increase of stability of the surface film obtained by transpassivation and post-treatment is ascribed to the inc rease of film thickness, the rise of the ratio Fe3+/Fe2+ and the adsor ption of Mo into the film during the post-treatment.