Changes in the content of magnesium and zinc alloying elements during heat treatment in vacuum (1.3 × 10-3 Pa) and in air in the bulk and on the surface of specimens of AlMg, AlZn, AlMgSi and AlZnMg alloys have been investigated using an electron probe microanalyser. At the temperatures used (500 °C and above) in vacuum a zone depleted in magnesium and zinc is formed near the surface. After heat treatment in air in alloys containing magnesium (AlMg, AlMgSi and AlZnMg) a zone depleted in magnesium appears and the surface becomes enriched in magnesium. In alloys containing only zinc the concentration remains uniform after heat treatment in air; this indicates the formation of a layer which prevents oxidation. The results obtained with heat treatment in air emphasize the significance of the structure of the oxide layers for this process. The mechanism of depleted-zone formation is discussed.
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