Influence of Moisture Exposure on the Composition of Oxides on Al-Zn-Mg Alloy: An Auger Electron Spectroscopy Study

Auger electron spectroscopy and depth profiling have been used to obtain information on the composition of oxide films formed on high purity Al-5.5Zn-2.5Mg alloys as a function of heat treatment, grain size, storage environment, and temperature. These studies show that in the as heat treated condition the oxide film is predominantly MgO. The Mg/O peak to peak height ratio of this oxide is sensitive to the heat treatment temperature and exhibits a maximum at 475 C. During subsequent storage, especially in a moist environment, this film undergoes substantial changes in composition. The Mg/Al ratio of the oxide decreases due to formation of an Al rich oxide on top of the as heat treated oxide. A model is proposed to account for the changes in the oxide film composition during growth of the Al rich oxide, and possible implications to pre-exposure embrittlement and stress corrosion cracking (SCO are discussed.