Study On The Microstructure And Corrosion Resisitance Of Hot Dipping Zn-Al-Mg Alloy Coating

Hot-dipping is widely used in metal protection, which is a kind of economic and effective corrosion resistance technology. With the diversification and complexity of the application environment, pure Zn and Zn-5Al coatings are unable to meet the current requirements. In order to meet the increasing requirement of corrosion resistance, great efforts has been made to optimize the composition of the zinc coating by alloying. Since Zn-Al-Mg coatings have excelletnt corrosion properties and ductility, it became the research focus of international corrosion resistance of metal surface. The structure, surface quality of Zn-6Al-3Mg coating at different temperature, corrosion resistance and corrosion resistance mechanism of the coating were studied in this thesis.It is found that the coatings have high surface quality at 470~550℃, the thickness of the interfacial layer and the overlay increased with the increase temperature.MgZn2, Zn rich phase and Zn/Al/MgZn2 ternary eutectic were found in the coating surface at different dipping temperatures, which was no obvious effect on the surface composition. However, the cross section of Zn-6Al-3Mg coatings was affected by dipping temperature obviously. The coatings obtained at 450~480 ℃ consisted of Al rich phase and Zn/Al/MgZn2 ternary eutectic composition. While MgZn2 and Zn/Al/MgZn2 ternary eutectic were found in the coating dipped at 500~550℃.The results of NaCl neutral salt spray test showed that the corrosion resistance of Zn-6Al-3Mg and Zn-5Al coatings were better than zinc coating,and Zn-6Al-3Mg coating was the best. The reason is that Zn-6Al-3Mg coating surface is mainly consisted of Zn/Al/MgZn2 three fine eutectic structures, and the coating structure is more compact. The Zn5(OH)8Cl2?H2O was corrosion product of the three-phase eutectic composition, which covered more compact on the coating surface and forming a compact protective layer. This layer can further prevent the corrosion and delay the reaciton between the Zn-6Al-3Mg coatings and the NaCl electrolyte.