Effect of cooling rate and rare earth content on microstructure and corrosion resistance of Zn-Al-Mg alloys
In order to refine the microstructure and improve corrosion resistance,the influence of cooling rate on the Zn-Al-Mg alloys with different rare earth contents were theoretically investigated.The solidified microstructures and corrosion resistance of the La and Ce-added Zn-1.7％Al-1.1％Mg alloys were studied under different cooling rates(air cooling,water cooling,furnace cooling)and varied rare earth contents,and thermodynamic calculations,scanning electron microscopy(SEM),X-ray diffraction method(XRD)and electrochemical test were carried out in the present work.The results show that the thermodynamic calculations can predict the precipitated phases and the related precipitation sequence of the Ce-added Zn-Al-Mg alloys during the solidification process.The LaZn13/CeZn11 primary phases were initially precipitated from the La and Ce added Zn-Al-Mg molten alloys,and the Zn-rich phase,Mg2Zn11/MgZn2 phases and Al-rich phase were solidified from the molten alloys,respectively.With the increase of the cooling rate,more fined microstructures were obtained and the Mg2Zn11 phase would turn into MgZn2 phase.The reason lies on that the metastable phase transition Mg2Zn11 ↔MgZn2 takes place in the Zn-Al-Mg system when the cooling rate increases.The results of Tafel polarization test and electrochemical impedance spec-troscopy test(EIS)show that with the increase of rare earth contents,the corrosion resistance of the alloys elevates to a certain extent.When the cooling rate increases,the corrosion resistance of the Zn-Al-Mg alloys can be improved prominently.It can be concluded that refined microstructure benefits to the increase of corrosion resistance,thus cooling rate acts as the main controlling parameter of the alloy corrosion resistance of the alloy.
Zn-Al-Mg alloyssolidified microstructurecorrosion resistancecooling ratetrace earth amount of La and Ce element
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