Electrochemical and analytical (XPS and AES) study of passive layers formed on Fe-Ni alloys in borate solutions

Abstract

Passivation of Fe–Ni alloys (25, 50 and 75 Ni at.%) in borate buffer solution (pH 9.2) at +0.2 V/SCE has been studied by electrochemical methods (polarization curves, capacitance measurements and dissolution kinetics). The depth composition, chemical bonding and thickness of passive layers are determined by AES and XPS depth profiling experiments. Their structure consists of a thin enriched Ni(OH)2 outer layer above a thicker internal Fe-rich layer (Fe3+ and deeper Fe2+ oxidized species) of increasing thickness with the Fe alloy concentration. The presence of small amounts of internal NiO is also depicted in the Ni 75 at.% passivated alloy at short polarization times, in agreement with a potential analysis carried out during passive film dissolution kinetics in Na2SO4 solution (pH 3). In the underlying metallic alloy, a significant Fe depletion (and a simultaneous Ni enrichment) is observed after the first few minutes of polarization; afterwards, a slow decrease of the external Ni film content takes place, causing filmng film thinning. This can be interpreted by a two-step film formation mechanism consisting of fast initial selective oxidation of iron followed by slow preferential Ni dissolution. Copyright © 2000 John Wiley & Sons, Ltd.