Which oxide phases form on an aluminum surface when magnesium is present?

Which oxide phases form on an aluminum surface when magnesium is present?

• A. Aluminum oxide only
• B. Magnesium oxide only
• C. Aluminum oxide, magnesium oxide, or magnesium spinel MgAl2O4
• D. Silicon oxide

Answer: (C)

When aluminum is exposed to magnesium during oxidation, the oxide scale that forms can include the oxide of aluminum, the oxide of magnesium, and a mixed oxide known as magnesium aluminate spinel. Aluminum naturally forms a protective Al2O3 layer, while magnesium tends to form MgO. At higher temperatures, ions of Al3+ and Mg2+ diffuse and react to create the spinel phase MgAl2O4, which is a stable mixed oxide that can appear within or atop the oxide scale. Since there’s no silicon source in this system, silicon oxide is not expected. Therefore the surface can develop aluminum oxide, magnesium oxide, and the magnesium aluminate spinel MgAl2O4.

When aluminum is exposed to magnesium during oxidation, the oxide scale that forms can include the oxide of aluminum, the oxide of magnesium, and a mixed oxide known as magnesium aluminate spinel. Aluminum naturally forms a protective Al2O3 layer, while magnesium tends to form MgO. At higher temperatures, ions of Al3+ and Mg2+ diffuse and react to create the spinel phase MgAl2O4, which is a stable mixed oxide that can appear within or atop the oxide scale. Since there’s no silicon source in this system, silicon oxide is not expected. Therefore the surface can develop aluminum oxide, magnesium oxide, and the magnesium aluminate spinel MgAl2O4.