Nucleation and Growth of a Chromate Conversion Coating on Aluminum Alloy AA 2024-T3

The nucleation and growth of a chromate conversion coating on aluminum alloy AA 2024-T3 has been investigated by atomic force microscopy, scanning electron microscopy, and transmission electron microscopy techniques. Through the careful preparation of fresh, clean, and well-characterized alloy specimens through the novel use of an ultramicrotome, morphological changes associated with the initial stages of coating growth have been characterized and related to the microstructure of the alloy surface. Nonuniform growth of the conversion coating takes place locally at intermetallic particles and over the macroscopic alloy surface generally. For coating growth at intermetallic particles, coating development is influenced strongly by the size, shape, and composition of the particles and the composition of the neighboring matrix regions. Coating nucleation and growth is observed as discrete nodules of film material formed at particles, or halos of film material surrounding and S-phase particles. Film growth at S-phase particles is supported by anodic dissolution of the particles, while that at particles is accompanied by localized dissolution of a copper-depleted matrix zone immediately adjacent to the particles. Concerning the macroscopic alloy surface, coating growth occurs preferentially along microstructural defects possibly introduced by rolling during alloy processing and along cellular and grain boundaries, which are sites of alloying element segregation. Thus, the unique and careful approaches adopted for specimen preparation have enabled the anodic and cathodic sites associated with coating nucleation and growth to be identified and related to the substrate microstructure. Based on the results of this study, a mechanism of growth of a chromate conversion coating on aluminum alloy AA 2024-T3 is proposed.