Interactions of the components of chromate conversion coating with the constituents of aluminum alloy AA2024-T3

Electrochemical techniques were employed to study the nature of individual interactions between the chemicals constituting a widely used commercial chromate conversion-coating treatment and the main constituents of AA2024-T3 alloy, namely, Al (99.999%), Al2Cu, Al-Al2Cu galvanic couple, and AA2024-T3 alloy. The samples were pretreated using chromate, ferricyanide, fluoride, tetrafluoroborate, and hexafluorozirconate, prior to electrochemical corrosion tests. The results were compared with untreated (control) samples. For the first time, the chemical interaction of aluminum with hexafluorozirconate has been studied. Maximum activation was observed in the case of hexafluorozirconate pretreatment, which also decreased the interfacial tension and increased surface wetting. The electrochemistry of the control and pretreated (except for hexafluorozirconate pretreatment) AA2024-T3 and Al2Cu were found to be similar. The cathodic electron transfer reaction rate for oxygen reduction was found to be enhanced by the presence of copper in the systems. The enhancement was proportional to the copper content. The cathodic reaction on all systems was inhibited by chromate. Aluminum was observed to undergo cathodic corrosion, leading to an enrichment of the surface copper content and subsequent enhancement of the cathodic electron transfer reaction. (C) 2004 The Electrochemical Society.