AN X-RAY PHOTOELECTRON SPECTROSCOPIC STUDY OF THE PASSIVE FILM FORMEDON PURE MO AND MOSI2 IN 4M HCL

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