X-RAY PHOTOELECTRON SPECTROSCOPIC EXAMINATIONS OF ELECTROCHEMICALLY FORMED PASSIVE LAYERS ON NI-CR ALLOYS

The structure and composition of passive films electrochemically forme d on Ni-Cr alloys in 1 M NaOH and 0.5 M H2SO4 has been studied with x- ray photoelectron spectroscopy. The passivation potential was varied w ithin the passive range up to the transpassive range and the time oi p assivation from 10 ms to 1000 s. With angular-resolved x-ray photoelec tron spectroscopy, the passive film was found to have a two-layer stru cture with an inner oxide part formed directly at the metal surface an d an outer hydroxide part in contact with the electrolyte. The metal s urface underneath is enriched in Ni compared to the metal bulk composi tion. Except for potentials approaching the transpassive range, the hy droxide layer forms the dominant part of the passive film. Above 0.3 V in NaOK and 1.2 V in H2SO4, Cr(VI) ions are found within the hydroxid e layer. The maximum thickness of the passive layer is about 6 nm in N aOH and 3.5 nm in H2SO4. The growth and the pronounced changes of the composition of the passive layers reflect the electrochemical behavior of the pure metal components. The results are in agreement with the t hermodynamic data.