The Effect of Gaseous Impurities on the High‐Temperature Oxidation of Magnesium

Abstract

The kinetics of the oxidation of magnesium in oxygen at temperatures between 475° and 570°C have been investigated. It is found that the course of the oxidation is grossly different according to whether the oxidation is carried out on a vacuum microbalance in static oxygen at 10 cm pressure or on a thermal balance in flowing oxygen at 76 cm pressure. This discrepancy cannot be attributed to different surface pretreatments. It is shown that the ready film breakdown (and subsequent nonprotective oxidation) in flowing oxygen is brought about by the reaction of the impurities in the oxygen with the magnesium. If these impurities are removed from the oxygen by gettering, either with magnesium itself or with a mixture of copper and iron oxides, before the magnesium is oxidized on the microbalance, then the amount of oxidation is very small, e.g., at 525°C only 3 µg/cm2 after 50 hr. The impurities are thought to be hydrocarbons which are catalytically cracked on the oxide surface to deposit carbon which becomes incorporated in the growing oxide layer. The rate of film thickening is dependent on the partial pressure of impurities in the oxygen, and as a consequence the rate of oxidation shows an apparent dependence on the pressure of oxygen.