MODELING AL ENRICHMENT IN GALVANIZED COATINGS

Aluminum enrichment in galvanized coatings was shown to be due to the formation of an inhibition layer, consisting of Fe2Al5, at the substrate/coating interface. The formation of the inhibition layer is a two-stage process. The first stage, associated with a high rate of Al uptake, is nucleation controlled, and the successive stage is diffusional growth controlled. The critical nucleus size is approximately one molecule of the compound Fe2Al5, and the energy barrier for the heterogeneous nucleation is 0.94 eV. Aluminum uptake increases with increasing strip-entry temperature and thickness, because both work to increase the effective temperature for the nucleation and growth processes. A model was proposed in which Al enrichment was shown as a function of bath Al content, bath temperature, strip-entry temperature, strip thickness, immersion time, and coating weight. The model is in good agreement with experimental results available in the open literature.