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The electrochemical behaviour of microcrystalline pure aluminium coating (mc-Al), fabricated by a magnetron sputtering technique, has been investigated in NaF as well as NaF + NaCl aqueous solutions. Results indicate that the anodic polarization characteristic changes with NaF concentration. F(-) anions promote the formation of a passive film, whose semiconducting properties were investigated. When the F(-) concentration is high ([F(-)] >= 0.03 mol/L) the passive film is mainly composed of aluminium fluoride, which is an n-type semiconductor on mc-Al and a p-type semiconductor on polycrystalline Al (pc-Al). In NaF + NaCl aqueous solutions, Cl(-) and F(-) ions compete in affecting the type of semiconducting passive films formed on mc-Al. (C) 2008 Elsevier Ltd. All rights reserved.