Hw. Hoppe et al., COMBINED SURFACE ANALYTICAL AND ELECTROCHEMICAL STUDY OF THE FORMATION OF PASSIVE LAYERS ON FE CR ALLOYS IN 1-M NAOH/, Surface and interface analysis, 21(8), 1994, pp. 514-525
Combined XPS and electrochemical studies of the passivation of Fe/Cr a
lloys have been performed in 1 M NaOH. Argon ion sputtering and a spec
imen transfer within a closed system provide a reliable situation with
well-controlled starting conditions for passivation transients and su
rface analytical studies with sufficient time resolution, especially a
t negative electrode potentials. The results for Fe/Cr are discussed o
n the basis of previous studies with the pure elements. The extremely
slow dissolution of Cr(III) oxide and its relatively negative formatio
n potential are responsible for the excellent passive behaviour of the
se alloys. Chromium(III) stabilizes Fe(II) and Fe(III) within the pass
ive 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 form
ation 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. The
se 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 passi
vity.