Oxide-free phosphate surface films on metals studied by core and valence band X-ray photoelectron spectroscopy

Phosphate films with thicknesses on the order of the original native metal oxide have been formed on oxide-free aluminum and iron surfaces by electroc hemical treatment in 5M phosphoric acid. Electrochemistry was performed und er inert atmosphere in a previously described anaerobic cell. The metal pho sphate films formed under these conditions were studied by core level and v alence band X-ray photoelectron spectroscopy (XPS) with the valence band sp ectra interpreted by spectra generated from band structure calculations. Va lence band studies showed that the metal surfaces consisted of a metal phos phate film bonded directly to the metal substrate in the absence of any for m of metal oxide. An analysis of the inner valence band region around 25 eV indicates considerable difference between oxidized aluminum and iron compo unds and the metal phosphates. The phosphate film initially formed on alumi num is compatible with metaphosphate. This unusual surface is stable in atm osphere and may have significant potential based upon known advantages prov ided by phosphated surfaces. This study provides a detailed analysis of a r ecent patent report of oxide free phosphate surface films formed on metal s urfaces that are stable under ambient atmosphere.