INFLUENCE OF SILICON ON GALVANIZING REACTIONS IN A ZINC-ALUMINUM BATH

Research and development of a new Zn-Al eutectoid alloy for batch galvanizing process was undertaken to develop coatings with improved corrosion resistance. The vigorous reactions between the substrate Fe and the molten Zn-Al bath created an interface controlled linear growth of the alloy layer, resulting in a coating which was rough, porous, and an order thicker than the commercially prevalent norm of approximately 80µm. The effect of ternary additions, such as Bi, rare-earth (RE), and Si, on the growth rate, miscrostructure and corrosion properties of the coatings were investigated using scanning electron microscopy, energy dispersive spectroscopy, x-ray diffraction, transmission electron microscopy, field exposure and electrochemical corrosion tests. Coatings obtained after ternary additions of Bi and RE continued to suffer from deficiencies such as porosities, outbursts, and thickness nearly an order higher than desired, which points towards fact that the Fe-Al interfacial layer did not have adequate inhibition effect. In contrast, the reaction and growth rate of the coatings were well controlled when Si was added into the eutectoid bath. The diffusion controlled growth rate yielded a much desired coating thickness of 10 to 40 ìm, which was smooth, bright, free from porosities and outbursts, and exhibit a corrosion resistance at least eight times greater than that of Zn-galvanized coatings. These coatings exhibit mainly two layers: the interfacial layer, resulting from the reaction between substrate Fe and the melt, was composed of ternary derivatives of Fe2Al5, FeAl3, and the T5c intermetallic phases containing varying amount of Si, and the top layer, resulting from the solidification of the drag out liquid layer, exhibit a pre-dominantly lamellar structure of Zn-rich and Al-rich phases, typical of eutectoid reaction. Presence of Si in all the phases in the interfacial layer emphasize the role played by this ternary addition in controlling the growth rate, eliminating the outbursts, and also in making the alloy layers adherent to the substrate. A solid state reaction mechanism was revealed in the interfacial region to effect formation and transformation of the phases in the following order: Solid Al-rich phase - T5 phase - è - ç. The product promises wide application.