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, sputte r-deposited Cr2O3 films and passivated Cr were studied with in situ x- ray near-edge spectroscopy in 1 M H2SO4, berate 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 depende nce 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 spe ctra acquired during anodic potential steps reveal that, prior to tran spassive dissolution, Cr(VI) is trapped in the Cr2O3 film. There is no evidence of formation of intermediate Cr(IV); it appears that Cr2O3 i s directly oxidized to CrO42- (or Cr2O72-) (or Cr2O72- in acidic solut ions). X-ray near-edge spectroscopy measurements made during galvanost atic oxidation/dissolution show that the reaction Cr2O3 --> CrO42- (or Cr2O72-) takes place with a 100% current efficiency over the whole pH range (1 to 13). The results suggest that the transpassive dissolutio n of metalic chromium is a two-stage process CR --> Cr2O3 --> CrO42- ( Cr2O72-), with a faster kinetics of the first step; hence, the thermod ynamics and kinetics of the transpassive dissolution of Cr are complet ely determined by the surface oxide. The significance of present findi ngs for other experimental techniques and possible consequences for th e corrosion resistance of stainless steels are discussed.