Effects Of Mg Content On Interface Microstructure Of Hot Dipped Zn-Al Alloy

As a high efficient anti-corrosion technology,hot-dipping is widely used for the iron and steel protection.The properties of the pure Zn and Zn-Al coatings hot dipped on iron and steel materials need to be improved due to the complex natural environment.Hot-dipped Zn-Al-Mg coatings have become a hotspot because of their excellent corrosion resistance recently.Hot-dipped alloy coatings are composed of the reaction layer and the solidified layer.An appropriate reaction layer is the key to ensure the quality and corrosion resistance of the coating.In this paper,the Zn-5Al alloy was used as a starting material with different Mg added.The microstructure,microhardness and bonding force of the hot-dipped Zn-Al-Mg coatings were investigated.Results showed that the reaction layers consist of Fe₄Al₁₃3 and Fe₂Al₅Zn_0.4,and no obvious new phase is found which indicates that Mg dose not react with the Fe-Al compounds.The thickness of the reaction layer increased with the increasing of the dipping temperature.However,the increase of the Mg content resulted in the increase of the Fe₄Al₁₃phase,the decrease of grain size and the straightness of interface between the coating and steel matrix.When the content of Mg was greater than 3%,cracks were formed in the interface layer due to the increase of loose Fe₄Al₁₃ phase.Mg promoted the nucleation and growth of Fe₄Al₁₃.The growth of the reaction layer was mainly controlled by the diffusion reaction.With the increasing of Mg content,the growth rate constant of the reaction layer decreased and the diffusion activation energy of Al in Fe-Al intermetallics increased.The lattice parameters of the Fe-Al intermetallics were changed by Mg resulted in the distortion of lattice which inhibited the growth of Fe₂Al₅Zn_0.4.With the increasing of Mg content,the microhardness of the reaction layer increased.When the content of Mg was greater than 2.5%,the homogeneity of the reaction layer became worse.When the content of Mg was less than 1%,the powder content of the coating was small,which indicated that the resistance to pulverization and bonding properties between the coating and the steel substrate are improved.