The reactive element effect on high-temperature oxidation of magnesium

Abstract

It was discovered over 75 years ago that minor additions of elements with high affinity to oxygen exert a profound effect on oxidation resistance of many metals and alloys. In this report, progress in understanding the ‘reactive element effect’ (REE) and opportunities created for controlling the high-temperature oxidation of magnesium alloys were reviewed. Magnesium, the lightest structural metal, exhibits high affinity to oxygen and, as opposed to heat-resistant materials, does not form a dense, slow-growing and protective oxide layer. However, additions of small amounts of reactive elements (REs) to magnesium alloys produce a number of positive effects in improving their oxidation resistance. What is of crucial importance for magnesium is that RE benefits are also extended to its ignition, flammability, evaporation and liquid-state reactivity. The mechanism by which these elements influence oxidation and other related behaviour remains, so far, unclear. Although no specific theory explaining the REE on magnesium has been developed, growth kinetics, surface morphology and internal microstructure of magnesia films and scales, doped with REs show characteristics typical for high-temperature materials that form protective oxides.