Combined surface analytical and electrochemical study of the formation of passive layers on Fe/Cr alloys in 1 <scp>M</scp> NaOH

Abstract

Abstract Combined XPS and electrochemical studies of the passivation of Fe/Cr alloys have been performed in 1 M NaOH. Argon ion sputtering and a specimen transfer within a closed system provide a reliable situation with well‐controlled starting conditions for passivation transients and surface analytical studies with sufficient time resolution, especially at negative electrode potentials. The results for Fe/Cr are discussed on the basis of previous studies with the pure elements. The extremely slow dissolution of Cr(III) oxide and its relatively negative formation potential are responsible for the excellent passive behaviour of these alloys. Chromium(III) stabilizes Fe(II) and Fe(III) within the passive layer. Any Cr(III) enrichment is a consequence of preferential Fe dissolution. In 1 M NaOH the reduced solubility of Fe(III) oxide leads to an overlayer consisting mainly of Fe(III)oxide/hydroxide. The formation and oxidation of Fe(II) follows similar characteristics as found for oxide films on pure iron. Iron oxide components may, however, not be reduced to metal, as in the case of passive layers on pure Fe. These studies show that not only the final stationary state but also the process of passivation and the chemical changes of passive layers with potential and time can be investigated with XPS. Thus, one can obtain more reliable insight into the complicated processes leading to passivity.