Properties of Alloy Coatings on Surface of the Third Generation Automotive Steel

Three kinds of alloy coatings with different components were prepared on the surface of the third generation automobile steel by hot dip plating. The effects of Al content and Si addition on the phase composition, microstructure, micro-hardness, corrosion resistance of the coatings were studied. The results show that the main phases of Zn-0.6Al-1.6Mg, Zn-1.8Al-1.6Mg and Zn-1.8Al-1.6Mg-0.25Si alloy coatings were Zn, Al and MgZn₂, and black needle-like MgSi₂ phase also appeared in the Zn-1.8Al-1.6Mg-0.25Si coating. With the increase of Al content and Si addition, the grain size of the coatings was finer, and the structure uniformity of the coatings was improved. The microhardness of Zn-0.6Al-1.6Mg, Zn-1.8Al-1.6Mg and Zn-1.8Al-1.6Mg-0.25Si alloy coatings were 164.5, 186.1, 195.4 HV, respectively. The corrosion resistance of the alloy coatings with different composition from high to low was in the order of Zn-1.8Al-1.6Mg-0.25Si>Zn-1.8Al-1.6Mg>Zn-0.6Al-1.6Mg. The increase of Al content and Si addition in the coatings were helpful to increase the micro-hardness and corrosion resistance of the coatings.