<p>The corrosion susceptibility of magnesium alloys is the most formidable challenge in large-scale applications. In this work, a highly corrosion-resistant coating was fabricated on a magnesium-alloy surface to protect the underlying substrate. Unlike conventional hydrothermal methods, this approach does not improve corrosion resistance by altering the coating’s precursor materials; instead, it focuses on investigating grain behavior at different temperatures to regulate the coating’s corrosion resistance. By extending the heating duration, the coating exhibited increased thickness, grain refinement, and a denser, more ordered arrangement, which effectively blocked corrosive ions and consequently led to a significant improvement in corrosion resistance. To verify corrosion-resistance performance, the as-prepared coatings were immersed in a saturated sodium chloride solution for 15&#xa0;days; the coatings exhibited almost no corrosion, whereas the magnesium substrate had nearly completely dissolved by day 10. Additionally, when coated samples were immersed in an HCl solution at pH 2 for 15&#xa0;days, only slight surface corrosion was observed on the coatings. Electrochemical tests indicated that when the heating duration was extended to 5&#xa0;h, corrosion resistance reached its maximum: the coating’s impedance was six orders of magnitude higher than that of the substrate, and its corrosion current density decreased by five orders of magnitude, consistent with the immersion tests. Here, grain refinement and dense packing played critical roles by reducing porosity and crack density, thereby blocking the ingress of chloride and other corrosive ions. These results further confirm that grain refinement achieved by prolonging the heating duration provides a novel strategy for enhancing the performance of corrosion-resistant coatings.</p> Graphical Abstract <p></p>