Effect of air-formed film on corrosion behavior of magnesium-lithium alloys.

• Air-formed (AF) film is characterized under assistance of ultramicrotomy. • AF film is negligible on α but thick on β after prolonged air exposure. • AF film on β has a MgO/Mg(OH) 2 inner layer and a Li 2 CO 3 outer layer. • AF film on β elevates surface potential of the phase in air. • AF film on β retards filiform corrosion in LZ91 and LA141 alloys. It is recently suggested that air-formed film plays an important role in controlling corrosion resistance of Mg-Li alloys. However, the structure of the air-formed film and its effect on corrosion resistance of Mg-Li alloys has not been fully understood. Firstly, the air-formed films formed on α and β phases in a dual-phase LZ91 Mg-Li alloy after exposure to laboratory air for up to 48 h have been examined by SEM under the assistance of ultramicrotomy. Then, the effect of the air-formed film on surface potential and, consequently, corrosion/oxidation behavior of the alloy has been investigated. Finally, in order to exclude the influence from α phase, the structure of the air-formed film on β phase and its effect on corrosion/oxidation behavior of Mg-Li alloys have been studied based on a single-phase LA141 Mg-Li alloy. The results show that the air-formed film is thin and negligible on α phase but thick on β phase after prolonged exposure to laboratory air. The thick air-formed film on β phase has a multilayer structure with an inner layer consisting of MgO/Mg(OH) 2 and outer layer consisting of Li 2 CO 3 , which greatly elevates the surface potential of β phase in air. Both LZ91 and LA141 Mg-Li alloys firstly undergo uniform corrosion and then filiform corrosion when immersed in NaCl solution and the pre-existed air-formed film on β-Li phase can retard the occurrence of filiform corrosion in the alloys. [ABSTRACT FROM AUTHOR]