The use of macroscopic modelling of intermetallic phases in aluminium alloys in the study of ferricyanide accelerated chromate conversion coatings

Chromate conversion coatings are used on aluminium alloys, primarily for th eir renowned corrosion resistant properties. Although these coatings are in common industrial use, neither the protection mechanisms, nor the coating interation with the intermetallic precipitation phases are fully understood . Macroscopic models have been developed in order to represent the galvanic cells present in aluminium alloys due to the presence of such intermetalli c particles. Particles modelled include CuAl2, FeAl3 and Cu2FeAl7, all know to be cathodic to the aluminium matrix. Variations in deposition, both in composition and thickness, are indicative of the mechanisms of deposition o ver each phase. Characterisation of the coating deposition was carried out using X-ray photoelectron spectroscopy, Rutherford backscattering spectrosc opy, Auger electron spectroscopy, scanning electron microscopy with X-ray a nalysis. Depositional characteristics have been determined for each phase. The coating on the intermetallic phases is primarily Al oxide, and is signi ficantly thinner than the coating on the matrix. This coating on the matrix consists mainly of a mixed Cr/Al oxide. The coating on the intermetallic p hases was only one tenth the thickness of the matrix coating, and contained higher levels of Fe, Al and O. Matrix coating chemistry predominated with Cr, O, Fe and N, indicative of a chromate conversion coating. The mechanism for reduced rates of deposition over intermetallic phases was found to be affected by fluorine ion attack leading to intermetallic de-alloying and de composition of Fe(CN)(6)(2-) accelerator into amide groups on the matrix. ( C) 2001 Elsevier Science Ltd. All rights reserved.