Structure and properties of aluminum alloy composite conversion coating

The intrinsic corrosion resistance of aluminum alloy is relatively poor and needs to be enhanced through anodic oxidation treatment. Traditional anodic oxide films predominantly consist of aluminum oxide and are characterized by low toughness and poor impact resistance, making them unsuitable for environments with high temperatures, large temperature variations, and stringent corrosion resistance requirements. This study employs oxalic acid and trace amounts of ammonium fluoride as the electrolyte for anodic oxidation treatment of 6061 aluminum alloy. Through electrochemical oxidation and chemical co-deposition, a composite oxide film is prepared. The composition of the oxide film includes Al2C6O12, AlF3, AlOOH, and amorphous Al2O3, exhibiting a micro-porous structure with an average pore diameter of 10–20 nm. The surface of the micropores is covered by a relatively dense chemical conversion film, which seals the micropores and enhances the corrosion resistance of the oxide film. When the concentration of oxalic acid is 10 g/L and the concentration of ammonium fluoride is 0.5 g/L, the composite oxide film does not crack when bent at 90° at room temperature. Furthermore, no cracks are observed after heating to 100°C and holding for 30 min or even after maintaining at 300°C for 6 h. The electrochemical impedance reaches 5.891 × 106 Ω cm2, while the sulfuric acid anodic oxide film develops cracks when bent at 30° at room temperature or heated to 58°C, with a low-frequency impedance value of 3.649 × 105 Ω cm2.