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

Passivation of Fe-Ni alloys (25, 50 and 75 Ni at.%) in berate buffer soluti on (pH 9.2) at +0.2 V/SCE has been studied by electrochemical methods (pola rization curves, capacitance measurements and dissolution kinetics). The de pth composition, chemical bonding and thickness of passive layers are deter mined 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 l ayer (Fe3+ and deeper Fe2+ oxidized species) of increasing thickness with t he Fe alloy concentration. The presence of small amounts of internal NiO is also depicted in the Ni 75 at.% passivated alloy at short polarization tim es, 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 obs erved after the first few minutes of polarization; afterwards, a slow decre ase of the external Ni film content takes place, causing film thinning. Thi s can be interpreted by a two-step film formation mechanism consisting of f ast initial selective oxidation of iron followed by slow preferential Ni di ssolution, Copyright (C) 2000 John Wiley & Sons, Ltd.