Mg在多数服役环境(尤其含Cl⁻介质)中的耐腐蚀性能较差，含较多第二相的镁合金电偶腐蚀问题尤其突出，本工作采用3种不同轴肩直径(14、17和20 mm)的搅拌头对铸态Mg-5Zn合金进行搅拌摩擦加工(FSP)处理，研究具有不同第二相分布和晶粒特征的细晶组织的耐腐蚀性能。结果表明，FSP处理使合金中的第二相发生了不同程度的破碎和均匀分布，且随着轴肩直径增加，在固溶程度没有明显增加的前提下，第二相晶粒发生了长大，导致Mg-5Zn合金的微电偶效应增强，耐腐蚀性能逐渐降低。此外，FSP处理使Mg-5Zn合金形成了较强的(0001)基面织构，从而有利于耐腐蚀性能的提高。采用轴肩直径为14 mm的搅拌头，在转速为800 r/min、前进速率为40 mm/min及轴肩下压量为0.3 mm的FSP条件下，所制备Mg-5Zn合金的位错密度相对较低，其中的第二相被充分细化且呈均匀弥散分布状态，在3.5%NaCl溶液中的平均腐蚀电流密度相比铸态合金降低了近1个数量级。

Magnesium alloy is the lightest metal structure material and has one of the highest specific strength among metals. Thus, it has great potential in many applications, such as aerospace and automobile industry, to reduce the weight of components. However, magnesium alloys have very poor corrosion resistance that hinders their industrial application. Thus, several methods have been explored to improve the corrosion resistance of magnesium alloy to promote its application in industries such as automotive, aerospace, and electronics where lightweight materials are required. In this study, friction stir processing (FSP) has been applied to modify the microstructure of Mg-5Zn alloy to increase its corrosion resistance, which is a type of magnesium alloy used widely but has relatively poor corrosion resistance. Herein, three tools with different shoulder diameters of 14, 17, and 20 mm were selected to conduct FSP on the as-cast Mg-5Zn alloy. The microstructure has been observed and corrosion behavior has been investigated. The results reveal that the coarse grains of as-cast Mg-5Zn alloy are considerably refined by FSP treatment. The grain size reduces from hundreds of micrometers to a few micrometers. Furthermore, the coarse secondary phase in as-cast alloy is broken into small particles and distributed uniformly in the base material after FSP. Additionally, strong basal plane (0001) texture and low dislocation density have been observed in these FSP-treated samples, which are beneficial for increasing the corrosion resistance of magnesium alloy. Moreover, the size of the secondary phase increases with the increase of shoulder diameter, which leads to the increase in local cathode/anode area ratio, and the corrosion resistance of the three FSP-treated samples gradually reduces to 1520, 247, and 111 Ω·cm², respectively. Notably, under the FSP treatment at 800 r/min rotation speed, 40 mm/min traveling speed, and 0.3 mm plunge depth, when the tool with a shoulder diameter of 14 mm is employed, the precipitates in the Mg-5Zn alloy gets sufficiently fragmented and evenly dispersed, along with a relatively low dislocation density. The average corrosion current density of this friction stir processed sample in a 3.5%NaCl aqueous solution is reduced to 4.11 × 10^-6 A/cm² compared to that of the as-cast alloy (3.15 × 10^-5 A/cm²).