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 electrochemical treatment in 5M phosphoric acid. Electrochemistry was performed under inert atmosphere in a previously described anaerobic cell. The metal phosphate films formed under these conditions were studied by core level and valence band X-ray photoelectron spectroscopy (XPS) with the valence band spectra interpreted by spectra generated from band structure calculations. Valence band studies showed that the metal surfaces consisted of a metal phosphate film bonded directly to the metal substrate in the absence of any form of metal oxide. An analysis of the inner valence band region around 25 eV indicates considerable difference between oxidized aluminum and iron compounds and the metal phosphates. The phosphate film initially formed on aluminum is compatible with metaphosphate. This unusual surface is stable in atmosphere and may have significant potential based upon known advantages provided by phosphated surfaces. This study provides a detailed analysis of a recent patent report of oxide free phosphate surface films formed on metal surfaces that are stable under ambient atmosphere.