Electrochemical behaviour of microcrystalline aluminium in neutral fluoride containing solutions

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.