Characterization of cerium-based conversion coating on 7075-T6 Al alloy

Chromated coating systems are the current standard for corrosion protection of aluminum alloys, but chrome faces increased regulation because of its toxicity. One promising alternative is a nano-structured, cerium-based conversion coating deposited from aqueous solution that has been developed at UMR. Despite impressive progress in coating deposition, little is known about how cerium conversion coatings form or how coating structure affects performance. The objective of this work is to characterize and understand on the role of substrate pretreatment and the effect of deposition parameters on the structure and performance of cerium-based coatings on aluminum alloy 7075-T6. Pretreatment of 7075-T6 test panels prior to coating deposition had a significant effect on the surface morphology of the test panels, which, in turn, affected the performances of coatings. For spontaneous coatings, the best performance in salt spray testing was achieved for panels treated using an acid activation step developed at UMR specifically for Ce-based coatings. These coatings consisted of cerium-rich spherical precipitates in an amorphous-looking cerium-containing matrix. In most coatings, Ce was present only in its 4+ oxidation state, but Ce3+was detected in a few cases. CeO2 coatings were also prepared by sputter deposition. Coatings were characterized to determine the effects of substrate pretreatment and deposition parameters on structure and performance. Sputtered coatings were polycrystalline CeO2 with a grain size of less than 10 nm. Corrosion performance was poor due to the presence of pinholes in the coatings, regardless of deposition parameters. Characterization of spontaneous and sputtered coatings led to the conclusion that crystalline CeO2 acts only as a physical barrier during corrosive attack --Abstract, page iv