Passivity of iron in alkaline solutions studied by in situ XANES and a laser reflection technique

Passivity of iron in alkaline solutions was investigated by in situ X-ray absorption near edge spectroscopy (XANES) and with a laser absorption technique. In 0.1 M NaOH, no significant material loss takes place during reduction of the iron passive film, but the ferric passive film is converted completely to a ferrous film. Subsequent anodic polarization leads to growth of a "new" passive film underneath the converted layer; By multiple potential stepping a thin iron film (100 Angstrom) (between reduction and oxidation potential range) can be completely converted from the metallic slate into an oxide. This thick oxide layer remains electrochemically active, i.e., during potential cycles the redox reactions Fe(II) --> Fe (III) and Fe(III) --> Fe (II) take place in the entire layer. The formation of such a porous oxide/hydroxide layer does not take place when ethylenediaminetetraacetic acid (EDTA) is added to electrolyte, as the ferrous layer formed during reduction is dissolved in presence of EDTA. The behavior of the Fe electrode during potential cycling in NaOH + EDTA becomes reversible with respect to oxide formation and reductive dissolution. (C) 1999 The Electrochemical Society. S0013-4651(98)08-118-X. All rights reserved.