X-RAY-ABSORPTION STUDY OF ELECTROCHEMICALLY GROWN OXIDE-FILMS ON ALCRSPUTTERED ALLOYS

The chemistry of chromium in the passive film on sputter-deposited chr omium and AlCr thin films has been investigated in situ in an electroc hemical cell under potential control by studying x-ray absorption near edge structure. At high potentials, Cr in the AlCr alloys was oxidize d to the 6-valent state. Depending on the rate of potential increase, 6-valent chromium either dissolved from the alloy or was trapped in th e passive film where it was electroactive, i.e., the valence state cou ld be reversibly switched between the 3- and 6-valent states by changi ng the applied potential. The kinetics of these processes were investi gated. Ex situ x-ray photoelectron spectroscopy measurements indicated that, during slow scanning at low potentials, the composition of both the surface oxide and underlying metallic layers changed. These chang es resulted in a structure that was susceptible to transpassive dissol ution of Cr at potentials above 0.2 V(MSE).