Factors influencing the deposition of Ce-based conversion coatings, Part II: The role of localised reactions

In Part I it was demonstrated that the deposition of Ce-based conversion coatings onto the matrix of AA2024-T3 was well modeled by titrations of the Ce-coating solution with added Al³⁺ ions. In Part II the coating composition over the surface was imaged with Raman spectroscopy and X-ray elemental mapping. Multivariate analysis was used to interpret variance in the X-ray elemental maps. The coating composition was found to be determined by the underlying microstructure of the alloy. The microstructure is dominated by the distribution of Cu resulting from enrichment as a consequence of anodic etching of the matrix and dealloying of intermetallic particles, principally Al₂CuMg during coating. A model is presented that proposes two mechanisms for the deposition of the coating. For the matrix, coating deposition can be explained on the basis of a pH rise in the adjacent solution analogous to precipitation from solution during titration as described in Part I. For the rest of the coating, the mechanism of deposition is dictated by the heterogeneous surface of AA2024-T3.