An X-Ray Photoelectron Spectroscopic Study of the Passive Film Formed on Pure Mo and MoSi₂ in 4MHCl

Electrochemical techniques and variable angle x-ray photoelectron spectroscopy (XPS) were used to study the passivation of Mo, and plasma-sprayed and hot-pressed MoSi₂ in a pitting environment (4M HC1). A thin (12 A), insoluble Mo oxychloride salt layer formed on the pure Mo electrode following 1 h of passivation at 0 mV vs. saturated calomel electrode. Both MoSi₂ samples in 4M HC1 displayed a significantly higher passive current density as well as a much higher breakdown potential, indicating a thicker but perhaps less dense passive layer. XPS of the film formed on the thermally sprayed sample revealed a 34 A thick SiO₂ film incorporating hexavalent Mo for an overall stoichiometry of Mo₀.₀₅SiO₂. The film formed on the hot-pressed disilicide had primarily the same stoichiometry with slight variation due to the presence of pores.