The characterisation of Ce-Mo-based conversion coatings on Al-alloys .1.

Scanning Electron Microscopy (SEM), X-Ray Photo-electron Spectroscopy (XPS) and Scanning Auger Electron Spectroscopy (AES) have been used to study the surface chemistry of a multistep process for the generation of cerium- and molybdenum-containing conversion coatings on aluminium alloys. After standard pretreatment, the conversion coating was produced on 2024-T351 and 6061-T6 Al alloys by (i) treatment for 24 h in air saturated with water vapour at 100 degrees C, (ii) 2 h in 10 mM Ce(NO3)3 at 90-100 degrees C, (iii) 2 h in 5 mM CeCl3 at 90-100 degrees C and (iv) anodic polarisation for 2 h in Na2MoO4 at 500 mV (SCE). XPS indicated that the coating generated on both alloys was predominantly a hydrated aluminium oxide. It appeared from SEM and Scanning AES that the cerium was concentrated locally on the oxide. XPS indicated that the cerium was present in a mixture of Ce(III) and Ce(IV) oxidation states. Many of these cerium rich areas were associated with the constituent particles (large intermetallics). Mo(VI) was detected with XPS and there was evidence from SEM and AES to suggest that there was local enrichment of Mo around the intermetallic particles. Copyright (C) 1996 Elsevier Science Ltd