Transpassive dissolution of Cr and sputter-Deposited Cr oxides studied by in situ X-ray near-edge spectroscopy

The kinetics and mechanism of transpassive dissolution of thin, sputter-deposited Cr₂O₃ films and passivated Cr were studied with in situ x-ray near-edge spectroscopy in 1 M H₂SO₄, borate buffer (pH 8.4), and 1 M NaOH. The onset potentials of the transpassive dissolution and the plateau potentials during galvanostatic oxidation and their pH dependence are very similar for passive Cr and sputter-deposited Cr₂O₃ films and indicate that the mechanism of transpassive dissolution of Cr can be experimentally modeled with Cr₂O₃. X-ray near-edge spectroscopy spectra acquired during anodic potential steps reveal that, prior to transpassive dissolution, Cr(VI) is trapped in the Cr₂O₃ film. There is no evidence of formation of intermediate Cr(IV); it appears that Cr₂O₃ is directly oxidized to CrO₄ ^2- (or Cr₂O₇ ^2- in acidic solutions). X-ray near-edge spectroscopy measurements made during galvanostatic oxidation/dissolution show that the reaction Cr₂O₃ → CrO₄ ^2- (or Cr₂O₇ ^2-) takes place with a 100% current efficiency over the whole pH range (1 to 13). The results suggest that the transpassive dissolution of metallic chromium is a two-stage process Cr → Cr₂O₃ → CrO₄ ^2- (Cr₂O₇ ^2-), with a faster kinetics of the first step; hence, the thermodynamics and kinetics of the transpassive dissolution of Cr are completely determined by the surface oxide. The significance of present findings for other experimental techniques and possible consequences for the corrosion resistance of stainless steels are discussed.