Inhibition layer breakdown by a fast diffusion procedure of zinc in galvanized coatings

Thick sheets of steel were galvanized in a bath containing Al additions. A Fe2Al5 layer is formed at the substrate steel sheet, which leads to a desirable transient inhibition of Fe-Zn reactions. Thus the more protective (eta) phase rich in zinc is favored. However an appreciable intergranular diffusion and a gradual formation of internal and surface oxide particles influence the growth and stability of the inhibition layer. The location of some oxide particles at the Fe2Al5 surface or inside of this layer, led to conclude that oxide particles might cause Fe-Zn outburst growths to form. This is because zinc diffuses along the oxide particle/Fe2Al5 interface. Moreover the mechanism of oxide formation causes a local depletion of the atoms concentration in the bath in the vicinity of the formed oxide. This in turn diminishes the probability of the formation of the Fe2Al5 layer. So the whole mechanism provides a fast diffusion bath for Zn, which reacts with Iron atoms forming Fe-Zn phases. The formation of the phases, were determined by XRD measurements (PHILIPS diffractometer CuK alpha radiation) while the dispersion of the elements was examined by SEM (20kV JEOL 840A equipped with an OXFORD ISIS 300 EDS analyzer.