Composition of the near surface regions of conversion coated aluminium

Abstract

An X-ray photoelectron spectroscopy study has been made of aluminium, initially supporting electropolished and barrier-type anodic films, after treatment in the so-called chromate and chromate-phosphate conversion coating baths. Within the detection limits of the technique, general analysis of the near surface regions after conversion coating indicates only the presence of aluminium on anodized specimens whereas chromium is associated with electropolished specimens. Similarly phosphate is detected only for the electropolished specimen treated in the chromate-phosphate bath. A relatively small concentration of fluorine is associated with all the specimens. Where detected, chromium species are mainly in the Cr(III) state, other than about 10% Cr(VI) associated with treatment in the chromate bath. Further analysis of the spectra suggests that the outer surface regions are composed of CrPO 4. nH 2O and CrO x (OH) y . nH 2O for treatment of electropolished aluminium in the chromate-phosphate and chromate baths, respectively; the water content associated with the former film region is greater than the latter. For treatment of aluminium supporting alumina in the respective baths, no significant conversion coating develops over the immersion period employed and the outer region of the chemically thinned alumina may be considered as aluminium oxyhydroxide.