The Composition of the Surface During Passivation of Stainless Steels^⋆

Molybdenum alloyed austenitic stainless steels were exposed in 0.1 M HCl + 0.4M NaCl and polarized in the active and passive regions of the alloys. The treated surfaces were analyzed by utilization of the electron spectroscopy for chemical analysis (ESCA) method. The reaction products formed on the surface after passivation consist mainly of chromium oxide and hydroxide. For the high Mo alloyed steel (6 w/o), even Mo is enriched in the passive film. Nickel is only present in the oxide to a very low extent. It is shown that polarization of the steels to their active potentials causes enrichment of the alloying elements—Cr, Ni, and Mo—in the outermost layers in the metal phase. Quantitative analysis indicates that the interphase, so formed, consists of only a few atomic layers. It is suggested that the interphase is formed by selective dissolution of iron during the initial period in the active state. The enrichment is considered possible by a mechanism similar to short range ordering. It is postulated that the beneficial effect of nickel and molybdenum is not related to their occurrence in the passive film. Instead the collection of the alloying elements on the surface lowers the dissolution rate in the active phase and thereby provokes formation of the passive film during passivation and during repassivation in conditions of local attacks.