Influence of Microstructure of Coating on Corrosion Resistance of Zn-Al Alloy Coated Steel Wire Manufactured by Double Hot-Dip Process

In order to clarify the relation between the microstructure and the corrosion resistance of coating for the Zn-Al alloy coated steel wire manufactured by the double hot-dip process, the corrosion behavior of the alloy coated steel wires in the salt spray test has been investigated by varying the aluminum content of coating, the cooling conditions after coating, the reduction of area by drawing, and the bluing temperature after drawing.(1) The eutectoid phase, which contains approximately 22 mass per cent aluminum and consists of the fine complex mixture of α-Al phase and β-Zn phase, has the dominant effects on the corrosion behavior of the Zn-Al alloy coating.(2) The preferential corrosion occurs along the boundaries such as β-Zn primary crystals/eutectic phase, β-Zn matrix/eutectoid phase, and eutectic colony boundaries.(3) The corrosion of the β-Zn phase is preceded by that of the eutectoid phase.(4) The corrosion rate of coating increases with the increase in the cooling rate after coating.(5) Either the eutectic structure or the primary dendrite structure grows along the direction of solidification of coating as the cooling rate increases. These structures of the uniform direction from the surface to the base metal are considerd to accelarate the corrosion along the preferential corrosion paths resulting in the deterioration of the corrosion resistance of the Zn-Al alloy coating.