X-ray photoelectron spectroscopic examinations of electrochemically formed passive layers on Ni-Cr alloys

The structure and composition of passive films electrochemically formed 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 within the passive range up to the transpassive range and the time of passivation from 10 ms to 1000 s. With angular-resolved x-ray photoelectron spectroscopy, the passive film was found to have a two-layer structure with an inner oxide part formed directly at the metal surface and an outer hydroxide part in contact with the electrolyte. The metal surface underneath is enriched in Ni compared to the metal bulk composition. Except for potentials approaching the transpassive range, the hydroxide 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 hydroxide layer. The maximum thickness of the passive layer is about 6 nm in NaOH 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 thermodynamic data.