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The corrosion behavior of AZ31 magnesium alloy with different Gd contents in 3.5%(mass fraction) Na₂SO₄ solution was investigated by mass loss method. The effect of rare earth Gd on microstructure,corrosion rate and corrosion morphology of AZ31 magnesium alloy was analyzed by optical microscope (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results show that adding rare earth Gd can improve the corrosion resistance of AZ31 magnesium alloy significantly. When the Gd content is less than 4.12%,the corrosion resistance of the alloy increases with the increase of Gd content, but when the Gd content is more than 4.12%,the corrosion resistance of the alloy declines at a certain degree with the increase of Gd content. This is mainly caused by the content of Gd, which exacerbates the micro-galvanic corrosion effect between the precipitation phase and matrix phase,and thereby results in the decline of the corrosion resistance of the alloy. So when the content of Gd is 4.12%,AZ31 magnesium alloy has the optimum corrosion resistance.