SEM and RBS characterization of a cobalt-based conversion coating process on AA2024-T3 and AA7075-T6

Samples of aluminium alloys AA2024-T3 and AA7075-T6 were treated with a chromate-based deoxidizer, then conversion coated with a commercial cobalt-based solution and finally sealed with a commercial vanadate-based product. The alloy specimens were examined using scanning electron microscopy, transmission electron microscopy and Rutherford backscattering spectroscopy. The thickness of the cobalt-based conversion coating increased rapidly up to 5 min of immersion but more slowly for longer times. Sealing resulted in penetration of vanadium through the oxide and a small increase in thickness due to the deposition of a thin sealing coating within the pores and on the external surface of the cobalt-containing coating.