Abstract: Oxide films hinder diffusion and resist external forces, which determines the flame ignition mechanism of magnesium alloys. The effects of the continuity, compactness and mechanical properties of oxide films on the ignition mechanism were analyzed, by investigating the flame ignition behaviors of AZ80 (ZM5), EZ30K (ZM6) and WE43 Mg alloys. The results show that the rupture of the oxide films caused by liquid gravity was the key to causing ignition. According to thermodynamic calculations, compared with Mg, Al cannot be preferentially oxidized; while Nd can be preferentially oxidized through significant enrichment, resulting in a discontinuous Nd₂ O₃ inner layer in the ZM6 alloy; in contrast, Y has a strong preferential oxidation ability, which gives the WE43 alloy a continuous Y₂ O₃ inner layer and self-healing ability. In addition, the oxide film of the ZM5 alloy is loose and has poor mechanical properties, so it cannot effec-tively hinder diffusion and resist liquid gravity. Differently, the oxide films of the ZM6 and WE43 alloys are dense and have better mechanical properties, leading to higher ignition temperatures and longer igni-tion times. In addition, a criterion was proposed to predict the ignition time based on the law of energy conservation, and it was simplified to predict the ignition temperature. The errors between the predicted and measured values are within 11%.

Key words: Mg alloys, Flame ignition mechanis, Oxide film, Ignition criterion