| Magnesium alloy is the lightest commercial structural metallic material. Compared with aluminum and steel, magnesium has obvious advantages in structure and performance due to its low density and light weight. However, Mg, which is the most active metal in engineering application, has a standard electrode potential of-2.37 V. Thus Mg alloy will suffer severe electrochemical corrosion in moist air or water solution. It is easy for magnesium to produce a thin layer(Mg O) in the air, which is porous, brittle, and far less dense than the oxide film on aluminum and aluminum alloys, resulting in poor corrosion resistance and restricting magnesium alloys in engineering applications. Mg-RE-Zn magnesium alloy is currently a hot topic in the field of magnesium alloy. The main reason is that the distinctive structural LPSO phase in the alloy can significantly improve the mechanical properties of magnesium alloys, which is expected to be applied to engineering field. However, prior to practical application, the poor corrosion resistance of Mg-RE-Zn magnesium alloy is the primary problem to be studied and solved.In this article, the two common LPSO phase magnesium alloys, Mg-Zn-Y and Mg-Zn-Gd alloys, were selected. Based on the two alloys, we tried to add small amounts of alloying element Sn. The content, morphology and distribution of LPSO phase were regulated by composition design, heat treatment and alloying, and its effect on corrosion was investigated to provide some references for composition design and microstructure regulation of high strength and high corrosion resistant magnesium alloy.Prior to study LPSO phase alloy, we investigated the influence of LPSO phase forming elements Zn, Y and Gd on corrosion of magnesium alloy. These three elements formed second phase with Mg respectively to promote micro-galvanic corrosion. The content of Mg Zn2 phase had little influence on corrosion of Mg-Zn alloy. Solid solution Y in the Mg matrix could be involved in the formation of oxide film, which improved the corrosion resistance of Mg-Y alloy. Solid solution Gd in the Mg matrix also improved the corrosion resistance of Mg-Gd alloy. However, the corrosion current density of Mg-Gd alloy was an order of magnitude higher than pure Mg.In the Mg-Zn-Y alloys, we designed the as-cast Mg(100-7x/3)ZnxY(4x/3)(x = 0.6,1,2,3 at.%) alloys, and Mg95.33Zn2Y2.67 alloy was extruded and heat treated. Under cast condition, there was no second phase but LPSO phase, its content increased with increasing Zn, Y content. LPSO phase had the similar role as β-Mg17Al12 in Mg-Al alloy. Namely, it could not only act as a cathode to accelerate corrosion, but could also be used as corrosion barriers to impede corrosion. Larger LPSO phase and its continuous distribution in Mg95.33Zn2Y2.67 alloy impeded corrosion. The content of LPSO phase in Mg93Zn3Y4 alloy was so high that lamellar and blocky LPSO phase connected into a large piece, resulting in the peeling off of LPSO phase during the corrosion process and weakening the blocking effect of LPSO phase. After heat treatment, extrusion, and extrusion heat treatment, the corrosion resistance of the as-cast Mg95.33Zn2Y2.67 alloy was improved in varying degrees, especially for the extrusion heat treatment.In the Mg-Zn-Gd alloys, we designed the as-cast Mg(100-7x/3)ZnxGd(4x/3)(x= 0.6,1,2 at.%) alloys, and performed various heat treatment processes. Under cast condition, there was no second phase but W phase, which had the similar role as β-Mg17Al12 phase in Mg-Al alloy. Namely, it could not only act as a cathode to accelerate corrosion, but could also be used as corrosion barriers to impede corrosion. Corrosion resistance of the alloy after heat treatment was improved in varying degrees, especially for the aging-treated alloys(T6 and T6F), whose corrosion resistance were increased by 98.5%(T6) and 97%(T6F), respectively, mainly due to the lamellar LPSO phase within the grains. After T6 treatment, a small amount of lamellar LPSO phase precipitated within the matrix of Mg98.6Zn0.6Gd0.8 and Mg97.67Zn1Gd1.33 alloys, but their contents were much lower than that in Mg95.34Zn2Gd2.66 alloy. The corrosion resistance of the alloys improved with increasing LPSO phase content.At last, we added a small amount of Sn into Mg95.33Zn2Y2.67 and Mg95.34Zn2Gd2.66 alloys. The addition of Sn was beneficial to promote the LPSO phase in the form of lamellar in Mg95.33Zn2Y2.67 alloy, so that the protective shell which was made of corrosion product layer and LPSO phase was more compact, efficiently hindered magnesium dissolution, and improved its corrosion resistance in a great degree. However, in Mg95.34Zn2Gd2.66 alloy, Sn not only promoted the formation of LPSO phase, but also combined with Gd element to form blocky Mg-Zn-Gd-Sn second phases, which acted as cathodes to facilitate the hydrogen evolution and reduced the corrosion resistance of Mg95.34Zn2Gd2.66 alloy. |
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