Effect Of Mg On The Fe-Al Interface Structure And Coating Of Hot-dip Zn-Al-Mg Alloy

Hot dipping Zn and its alloy are widely used in steel anti-corrosion because of its simple process and good corrosion resistance.However,its performance needs to be further improved as the environment deterioration.Adding Al and Mg elements in Zn bath can effectively improve its corrosion resistance.However,the process of hot dipping Zn-Al-Mg alloy by solvent method is not mature,which leads to the poor quality of the coating.Study of the effect of Mg elements on the microstructure and properties of hot dipped Zn-Al-Mg coating is of great theoretical and practical significance for improving the quality of the coating.In this paper,Mg was added to the Zn-5 wt.%Al alloy,and the Zn-5 wt.%Al-x wt.%Mg?x=0,2,3,4,5,7?alloy for hot dip coating was smelted.The effects of process and Mg on the phase structure and microstructure of the reaction layer and the overlay of the Zn-5 wt.%Al-x wt.%Mg?x=0,2,3,4,5,7?alloy were analyzed.The effect of Mg on corrosion resistance and corrosion process of alloy coating was studied.The results showed that the fluidity of the bath decreased with the addition of Mg,resulting in the decrease of wettability between the matrix and the plating bath,and the thickness and grain size of the reaction layer decreased with the increase of Mg content.Grains were refined with the addition of Mg,however the phase composition was not changed.The Mg element had a significant effect on the composition of the solidified layer:When the content of Mg was 0%,the overlay consisted of the rich Zn phase,Zn/Al binary eutectic phase?more Zn content?and the Fe₄Al₁₃3 phase.When Mg content was 1%,the overlay was composed of Zn-rich phase,Fe₄Al₁₃3 phase and Zn/Al binary eutectic phase?more Zn content?.When the content of Mg was 3%,the overlay consisted of Zn-rich phase,Fe₄Al₁₃3 phase,MgZn₂ phase and Zn/Al binary eutectic phase?more Zn content?.When the content of Mg was 5%,the overlay consisted of Zn-rich phase,Fe₄Al₁₃3 phase,MgZn₂ phase,Zn/Al binary eutectic phase?more Zn content?,Al-rich dendrite phase,Zn/Al/MgZn₂ternary eutectic phase and Zn/Al binary eutectic phase?more Al content?.The corrosion resistance of the coating first increased and then decreased with the increase of Mg content,which was the best at 3%Mg.And the self corrosion potential of the coating is the highest at 3%Mg.With the addition of Mg,most of the corrosion products of the coating remained Zn₅?OH?₈Cl₂·2H₂O.But the products became finer and denser with the increase of Mg,resulting in the improvement of corrosion resistance.The eutectic phase of the coating was first corroded in the corrosion process,followed by Zn particles being eroded,and finally the MgZn₂ phase was corroded.