Effects of Zn addition on the microstructure and corrosion behavior of Mg-8%Al-(0-1)%Zn casting alloys were investigated. With increasing Zn content, the amount of β(Mg 17 Al 12 ) phase increased, while α-(Mg) dendritic cell size became reduced. The corrosion rate decreased continuously with the increase in the Zn content. The evaluation of the microstructural evolution indicates that the improved barrier effect of β particles formed more continuously along the dendritic cell boundaries and the incorporation of more ZnO into the surface corrosion product, by which the absorption of Cl − ions is impeded, are responsible for the better corrosion resistance in relation to the Zn addition.
The effects of the Zn content on the microstructure and corrosion behavior in 1M NaCl solution were investigated in Mg-(0~6)%Zn casting alloys. The MgZn phase was scarcely observed in the Mg-1%Zn alloy, while the Mg-(2~6)%Zn alloy consisted of α-(Mg) and MgZn phases. With an increase in the Zn content, the amount of the MgZn phase was gradually increased. Immersion and electrochemical corrosion tests indicated that the Mg-1%Zn alloy had the lowest corrosion rate among the alloys, and a further increase in the Zn content resulted in the deterioration of the corrosion resistance. Microstructural examinations of the corroded surfaces and EIS analyses of surface corrosion films revealed that the best corrosion resistance at 1%Zn was associated with the absence of MgZn phase particles in the microstructure and the contribution of Zn element to the formation of a protective film on the surface. A micro-galvanic effect by the MgZn particles led to the increased rate of corrosion at a higher Zn content.
The aim of this study is to investigate the effect of solution treatment on the corrosion behavior of Mg8%Al-(0-1)%Zn casting alloys in 1M NaCl aqueous solution. After the solution treatment, all alloys showed single α-(Mg) phase microstructure by dissolution of β(Mg 17 Al 12 ) phase into the α-(Mg) matrix. The H 2 evolution volume decreased with an increase in Zn content, which indicates that the addition of Zn plays a beneficial role in decreasing corrosion rate of the Mg-Al-Zn alloy in solution-treated state. The microstructural evaluations on the corrosion products and corroded surfaces after the immersion test in 1 M NaCl solution revealed that the incorporation of more Al 2 O 3 and ZnO into the corrosion product, by which the penetration of Cl − ions is impeded, are thought to be responsible for the better corrosion resistance in relation with the Zn addition.
Influence of trace amount of Ca addition on the corrosion resistance was comparatively investigated in solutionized Mg-4%Zn and Mg-4%Zn-0.1%Ca alloys. In as-cast state, the alloys were characterized by primary α-(Mg) dendrite with MgZn intermetallic compound particles. After solution-treatment, both alloys consisted of single α-(Mg) phase by dissolution of the compound particles into the matrix. It was found from the immersion and electrochemical corrosion tests that the Mg-4%Zn alloy had better corrosion resistance than the Mg-4%Zn-0.1%Ca alloy. Morphological and compositional analyses on the surface corrosion products indicate that the incorporation of Ca oxide with low PBR value into the surface corrosion products would be responsible for the decreased corrosion resistance of the Ca-containing alloy.
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