Effect of manganese content on the corrosion resistance of AZ31 Mg alloys fabricated from high-purity magnesium

The widespread application of AZ31 magnesium (Mg) alloys is severely limited by their inconsistent and often poor corrosion. Manganese (Mn) is routinely added to AZ31 alloys to mitigate the adverse effects of impurity elements, particularly iron (Fe). Its efficacy depends critically on the Fe/Mn ratio, which varies with both composition and processing. Although international standards specify the allowable Mn content, the optimal Fe/Mn ratio for AZ31 alloy remains unclear, especially when fabricated from high-purity Mg feedstock. Such AZ31 alloys inherently possess low impurity levels and already exhibit improved corrosion resistance, warranting a re-evaluation of both the necessity and optimal content of Mn addition. Here, we systematically investigate the impact of Mn (0–0.4 wt.%) content —within the bounds of current standards—on the corrosion resistance of AZ31 alloys derived from high-purity Mg, and identify a critical Fe/Mn ratio of 0.0075 yields the most pronounced improvement. The resultant AZ31–0.05Mn alloy achieves a low corrosion rate (P_W=0.25 mm·y^-1, in 3.5 wt.% NaCl solution), comparable to that of high-purity Mg. This performance enhancement is attributed to the promotion of uniform corrosion and formation of a compact double-layer protective film (the outer: Mg(OH)₂+spinel and inner: MgO), effectively suppressing further corrosion progression. These findings not only offer crucial guidance for designing high-quality Mg alloys, particularly high corrosion-resistant Mg-Al alloys, but also underscore the need to revise alloy standards when high-purity Mg is employed.