<p>The influence of grain size or grain refinement on the corrosion of Zr alloy is clarified by employing a series of electrochemical analyses and characterization techniques. The corrosion resistance, as a function of exposure time, F⁻ concentration, and solution temperatures, of Zr alloys with different grain sizes is ascertained. The results confirm that refining the grain size can effectively enhance the short-time corrosion properties of Zr alloy in HNO₃ with F⁻. The fine-grained Zr alloy (∼10 µm in diameter) consistently exhibits a lower corrosion current density, ranging from 18% to 46% lower than that of the coarse-grained Zr alloy (∼44 µm). The enhanced corrosion resistance is attributed to the high-density grain boundaries, which promote oxide stability, and accelerate the creation of the protective layer. The high corrosion rate and pseudo-passivation behavior of Zr alloys in fluorinated nitric acid originate from the accelerated “dissolution-passivation” of the oxide film. However, the grain refinement does not provide enduring anti-corrosion for Zr alloys. To meet the operation of spent fuel reprocessing, additional systematic efforts are required to evaluate the long-term effect of grain refinement.</p>