F. Basile et al., Electrochemical and analytical (XPS and AES) study of passive layers formed on Fe-Ni alloys in borate solutions, SURF INT AN, 30(1), 2000, pp. 154-157
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.