CHROMATE CONVERSION COATINGS ON 2024-AL-ALLOY

X-ray photoelectron spectroscopy (XPS), scanning electron microscopy ( SEM) and potentiodynamic measurements have been made on chromate conve rsion-coated Al 2024-T3 alloy. X-ray photoelectron spectroscopy measur ements indicated that the conversion coating had a surface of CrOOH an d Cr-(VI), enriched in ferricyanide. The bulk of the coating was an eq ual mixture of CrOOH and Cr2O3 with significant levels of F- and Fe, t he latter implying the presence of ferricyanide throughout the coating . Copper(II) ion was present at the interface between the conversion c oating and the alloy, as well as Al3+. During ageing experiments, pote ntiodynamic measurements indicated that the corrosion current (i(corr) ) decreased from similar to 0.4 to similar to 0.04 mu A cm(-2) during the first 40 h after preparation but thereafter slowly increased. No s ignificant changes were observed in the chemistry of the coating by XP S for ageing times longer than 40 h, although morphological changes we re observed with SEM. As the coating aged, a network of microcracks de veloped across the surface. It is believed that Cr6+ is consumed in th e process in which plugs of hydrated chromium oxide form at the base o f these cracks. (C) 1997 by John Wiley & Sons, Ltd.