Influence of surface preparation on performance of chromate conversion coatings on Alclad 2024 aluminium alloy Part II: EIS investigation

Chromate conversion layers, formed on Alclad 2024-T3 aluminium alloy after different surface preparation procedures, were investigated by means of EIS measurements performed in aggressive and middle aggressive electrolytes. T he morphology of the chromate layer varies depending on the type of solutio n and on the time of duration due to deposition of corrosion products insid e the defects and deterioration of the layer. Therefore, three slightly dif ferent equivalent circuits have been used to model the chromate film in the two electrolytes and for different times of exposure. The proposed equival ent circuits enable the study of the effect of variation in the aluminium s urface microstructure, caused by different surface pretreatment, on the per formance of the deposited chromate conversion coating. The removal of galva nic couples, mainly caused by precipitates, from the aluminium surface appe ars to be a necessary condition for the formation of a chromate layer with good corrosion resistance. Besides, the properties of the surface of the al uminium substrate largely influence the morphology and composition not only of the chromate film but also of the corrosion products formed at weak loc ations, which seem to play an important role in the protection provided by the chromate conversion coating. (C) 2001 Elsevier Science Ltd. All rights reserved.