Role of impurities on Mg surfaces under ambient exposure conditions

The long-term oxide growth kinetics under atmospheric ambient conditions has been studied for purr magnesium and several magnesium alloys. Oxide thicknesses in the range of 1-15 nm are measured using X-ray photoelectron spectroscopy, making use of the inelastic mean free paths for magnesium and MgO. On pure magnesium, the oxide growth rate is slow, following logarithmic kinetics during exposure periods as long as 10 months. On samples containing iron or manganese as inclusions, an increased oxidation rate war noted following exposure periods as short as several hours. Evidence of localised oxidation around such inclusions could be observed from secondary ion mass spectrometry (SIMS) images of the near-surface region. High purity aluminum-magnesium alloys exhibit lower oxide growth rates than pure magnesium; the initially thicker oxide film contains aluminum ions which are believed to increase the activation energy for ion movement. (C) 1998 Elsevier Science Ltd. All rights reserved.