Preparation of anti-corrosion films by microarc oxidation on an Al-Si alloy

Thick ceramic films over 140 μm were prepared on Al-7% Si alloy by ac microarc oxidation in a silicate electrolyte. The film growth kinetics was determined by an eddy current technique and film growth features in different stages were discussed. The microstructure and composition profiles for different thick films were analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy. Their phase components were determined by X-ray diffraction. The electrochemical corrosion behaviors of bare and coated alloys were evaluated using potentiodynamic polarization curves, and their corrosion morphologies were observed. In the initial stage of oxidation, the growth rate is slow with 0.48 μm/min due to the effect of Si element though the current density is rather high up to 33 A/dm². After the current density has decreased to a stable value of 11 A/dm², the film mainly grows towards the interior of alloy. The film with a three-layer structure consists of mullite, γ-Al₂O₃, α-Al₂O₃ and amorphous phases. By microarc discharge treatment, the corrosion current of the Al-Si alloy in NaCl solution was significantly reduced. However, a thicker film has to be fabricated in order to obtain high corrosion-resistant film of the Al-Si alloy. Microarc oxidation is an effective method to form an anti-corrosion protective film on Si-containing aluminum alloys.