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 coatings for 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 the coating, the cooling conditions after coating, the reduction of area by drawing, and the bluing temperature after drawing. The eutectoid phase, which contains approximately 22 mass percent aluminum and consists of the fine complex mixture of α-Al phase and β-Zn phase, has the dominant effect on the corrosion behavior of the coating. Preferential corrosion occurs along the β-Zn primary crystals/eutectic phase, β-Zn matrix/eutectoid phase, and eutectic colony boundaries. The corrosion of the β-Zn phase is preceded by that of the eutectoid phase. The corrosion rate increases with the increase in the cooling rate after coating. Either the eutectic structure or the primary dendrite structure grows along the direction of solidification of the coating as the cooling rate increases. These structures accelerate the corrosion.