The effects of (i) potential, (ii) the electrolyte's concentrations of LiPF6 and H2O, (iii) the aluminum's air-formed oxide, (iv) an anodized layer of Al2O3, and (v) the purity of aluminum, on the formation of a protective layer of AlF3 during anodic polarization of aluminum were investigated. Most tests were conducted in lithium-ion battery electrolytes with 1.0 M LiPF6. At potentials above a critical value, a film of AlF3 forms on top of the air-formed oxide, creating a duplex, or two-layered film. The thickness of the AlF3 increases with applied potential. The critical value of potential for formation of AlF3 is independent of the composition of the aluminum alloys investigated, and increases with the thickness of pre-existing surface films, such as the native air-formed oxide. There is a threshold value of concentration of LiPF6 (=0.05 M LiPF6) for the formation of a protective layer of AlF3. Protective films of AlF3 are formed in 1 M LiPF6 electrolytes with concentrations of H2O in the range of 2 to 4000 ppm. LiTFSI causes severe pitting corrosion of aluminum that is covered by only its air-formed oxide. A thin film (< 2 nm) of AlF3 significantly improves the resistance of aluminum to pitting corrosion in 1:1 ethylene carbonate+dimethyl carbonate with 1 M LiTFSI. (c) 2006 The Electrochemical Society.