<p>Plasma electrolytic oxidation (PEO) is commonly employed to enhance the corrosion resistance of magnesium and its alloys. However, the formation of pores during this process can compromise the corrosion resistance of the PEO coating. In this study, the PEO coating on an AZ91D magnesium alloy was subjected to post-treatment in Cr₂(SO₄)₃ solutions to further improve its corrosion resistance. The structural characteristics and corrosion resistance (evaluated in a 3.5% NaCl solution) after Cr₂(SO₄)₃ sealing treatment were investigated. The results revealed that, in the acidic sealing solution, oxides such as MgO and Mg₂SiO₄ in the PEO coating were dissolved, resulting in a local increase in pH. This pH increase promoted the precipitation of Cr₂O₃ and Cr(OH)₃, which filled the micropores of the PEO coating. As the sealing time progressed, an outer layer containing Cr₂O₃ and Cr(OH)₃ formed on the coating surface, enhancing the compactness of the PEO coating. The resistance of the PEO coating increased from 5.13 × 10⁵ Ω cm² to 1.56 × 10⁶ Ω cm² after sealing for 60&#xa0;min. The average hydrogen evolution volume decreased from 21.77&#xa0;ml/cm² to 0.2&#xa0;ml/cm² (after immersion in 3.5% NaCl solution for 7&#xa0;days); and no visible corrosion products were detected on the surface of the sealed samples after immersion for 100&#xa0;h. These findings suggest that trivalent chromium sealing is a promising method that can effectively enhance the corrosion resistance of PEO coatings on magnesium alloys.</p>