Study of the Cryolite Formation during the Zinc Phosphating of Aluminium

SUMMARYTo improve the corrosion resistance and the adhesion properties of aluminium, chemical conversion processes, such as zinc phosphating, are applied. During zinc phosphating of aluminium the co-precipitation of cryolite often occurs. Cryolite needs to be formed in solution to remove Al3 ions, which have a poisoning effect on the phosphating process, away from the surface. Cryolite formation on the surface must be avoided, since it leads to a decrease of the corrosion resistance and of the adhesion properties. In this paper, the influence of mass transport on this co-precipitation competition is reported. The resulting conversion layers are investigated by SEM/EDX. It can be concluded that the amount of mass transfer has a large influence on the formation of both the phosphate and cryolite crystals. Hence, this competition in precipitation is explained in terms of mass transport of the different species involved in the chemical reactions at the surface.