In studies of the oxidation behavior of magnesium at ambient temperatures it was recognized that the initial oxidation proceeds in three stages:
1. Oxygen chemiesorption;
2. formation of the oxide layer (nucleation and lateral growth); and
3. oxide thickening.
As a result of exposure to air, a thin (20-50 nm), dense oxide film with an amorphous structure is formed. In a low humidity atmosphere, no further increase in film thickness is observed. An XPS study shows a mixture of elemental Mg, MgO and 50%-60% Mg(OH)2.
As with pure magnesium, the initial air-formed film on magnesium-aluminum alloys is amorphous and dense and in dry environments it offers good protection.
In summary, the interaction of oxygen with Mg leads to the formation of a thin, protective oxide layer. However, temperature and moisture accelerate the transition to a less protective film. At elevated temperatures the MgO film rapidly attains a critical thickness and cracks, allowing further oxidation. Water, if present in the environment, rapidly undermines the film as a result of formation of a less stable, hydrated oxide and electrochemical metal oxidation at the metal surface. |
