1. Soak the aluminum alloy in boiling water, and the natural oxide film on the aluminum will keep thickening until it reaches 0.7–2 µm. The oxide film can be colorless or milky white. The water-formed oxide film is γ-alumina type, with a dense structure, and is very stable at a pH between 3.5 and 9, making it suitable as a base layer for paint. Superheated steam above 100℃ helps the film form, while the actual process involves treating it in pure water at 75–120℃ for a few minutes. To increase the thickness of the film, you can add ammonia or triethanolamine to the pure water to get a porous oxide film. The oxide film formed with added ammonia is white and evenly toned. The optimal amount of ammonia to add is 0.3%–0.5%.
2. Zirconium Salt Oxidation Method
Using a zirconium-containing solution instead of chromates for pretreating aluminum surfaces has been widely accepted, especially suitable for chemical conversion coating before painting aluminum alloy parts. This method can improve the adhesion between the coating and the substrate, enhance corrosion resistance, and the oxide film itself also has some anti-corrosion properties.
3. Titanium Salt Oxidation Method
Titanium is very similar to chromium in nature and does not corrode in almost any natural environment. Its excellent corrosion resistance comes from the continuous, stable, tightly bound, and protective oxide film formed on its surface. Titanium's high chemical reactivity and strong affinity for oxygen allow its metal surface to form an oxide film immediately when exposed to air or humid conditions. In fact, just like chromate chemical conversion films, as long as there is a small amount of oxygen or water (moisture) in the environment, the titanium oxide film that gets damaged can repair itself immediately due to titanium's strong affinity for oxygen.
4. Rare Earth Metal Salt Oxidation Method
The chemical oxide film made from rare earth metal salts may eventually replace chromate chemical oxide films. Materials can be treated using the immersion method, and the treatment solution generally needs to be heated to form a protective layer on the base metal surface. Its corrosion resistance comes from the rare earth oxide film formed on the metal surface. Currently, the aluminum alloy rare earth treatment process generally uses a mixed solution consisting of rare earth metal salts, oxidizers, film-forming promoters, and auxiliary film-forming agents. Rare earth salts mainly refer to cerium salts such as CeCl3, Ce(NO3)3, Ce(SO4)2, (NH4)2Ce(NO3)6, etc.; film-forming promoters include NaOH, HF, SrCl2, (NH4)2ZrF, etc.; oxidizers include H2O2, KMnO4, (NH4)2S2O8, etc. There is a treatment process without adding oxidizers, known as the rare earth bohmite layer process. This process involves first forming a bohmite layer on the surface of aluminum alloy using hot water, and then immersing it in a rare earth salt solution to create a bohmite layer containing rare earth elements. The feature of this process is that it doesn't require strong oxidizers like H2O2 or KMnO4 to shorten the treatment time, but it does require higher treatment temperatures.
5. Permanganate Oxidation Method
Generally speaking, permanganate is not a good corrosion inhibitor for aluminum and its alloys; in fact, it can accelerate corrosion. However, aluminum and its alloys can form a good protective film when properly treated in a KMnO4 solution. The process involves continuously soaking in solutions of sodium bromate, distilled water, Al(NO3)3-LiNO3 solution, and KMnO4 solution. The resulting film is composed of Al2O3·MnO2. If the pores in the oxide film are then sealed with a K2SiO3 solution, the effect is even better. The protective effect of a KMnO4-oxidized film is roughly 70% that of a chromate film (based on a simultaneous salt spray test). For pure aluminum and aluminum alloys with low copper, zinc, or iron content, treatment in an aqueous potassium permanganate solution for 1 minute can produce a uniform yellow film similar to a chromate film. For aluminum alloys more prone to corrosion, a thicker protective film can be obtained by first treating in boiling water or steam to form an oxide film, then performing secondary or tertiary sealing treatments. One sealing is done in an aluminum salt solution, and another in KMnO4 solution. The resulting oxide films have properties comparable to chromate films. For high-copper, unpainted aluminum alloys, to achieve the best protective film, an additional step of treating in 95–100°C potassium silicate solution for 1.5 minutes can be added. Compared to chromate films, the greatest advantage of this film is that its protective properties are not reduced even when the drying temperature exceeds 65°C or during long-term storage. Potassium permanganate oxide films and chromate films provide identical protection against fibrous corrosion under paint.
