X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and potentiodynamic measurements have been made on chromate conversion-coated Al 2024-T3 alloy. X-ray photoelectron spectroscopy measurements indicated that the conversion coating had a surface of CrOOH and Cr(VI), enriched in ferricyanide. The bulk of the coating was an equal mixture of CrOOH and with signiÐcant levels of F-and Fe, the latter implying the Cr 2 O 3 presence of ferricyanide throughout the coating. Copper(II) ion was present at the interface between the conversion coating and the alloy, as well as Al3'. During ageing experiments, potentiodynamic measurements indicated that the corrosion current decreased from ¿0.4 to ¿0.04 lA cm-2 during the Ðrst 40 h after preparation but (i corr ) thereafter slowly increased. No signiÐcant changes were observed in the chemistry of the coating by XPS for ageing times longer than 40 h, although morphological changes were observed with SEM. As the coating aged, a network of microcracks developed across the surface. It is believed that Cr6' is consumed in the process in which plugs of hydrated chromium oxide form at the base of these cracks.
1997 by John