1. Immerse the aluminum alloy in boiling water, and the natural oxide film on the aluminum will gradually thicken, eventually reaching 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°C helps the film form, while the actual process involves treating it in pure water at 75–120°C for a few minutes. To increase the thickness of the film, ammonia water or triethanolamine can be added to the pure water to get a porous oxide film. The oxide film treated with ammonia water is white with an even tone. The optimal range for adding ammonia is 0.3%–0.5%.
2. Zirconium Salt Oxidation Method
Using zirconium-containing solutions 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 behaves very much like chromium and is highly resistant to corrosion in almost all natural environments. 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 instantly 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 moisture in the environment, the titanium oxide film can self-repair immediately due to titanium's strong affinity for oxygen.
4. Rare Earth Metal Salt Oxidation Method
The chemical oxide film formed by rare earth metal salts might eventually replace chromate chemical oxide films. The material can be treated using an immersion method, and the treatment solution usually 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, aluminum alloy rare earth treatment 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 like CeCl3, Ce(NO3)3, Ce(SO4)2, (NH4)2Ce(NO3)6, etc. Film-forming promoters include NaOH, HF, SrCl2, (NH4)2ZrF, etc., and oxidizers include H2O2, KMnO4, (NH4)2S2O8, etc.There is also a treatment process without adding oxidizers, called the rare earth bohmite layer process. This process involves first forming a bohmite layer on the aluminum alloy surface with hot water, and then immersing it in a rare earth salt solution to form a bohmite layer containing rare earths. The feature of this method is that strong oxidizers like H2O2 or KMnO4 are not needed to shorten the treatment time, but the treatment temperature is relatively high.
5. Potassium Permanganate Oxidation Method
Generally speaking, potassium permanganate is not only a poor corrosion inhibitor for aluminum and its alloys, but it can also accelerate corrosion. However, with proper treatment, aluminum and its alloys can form a good protective film in a KMnO4 solution. The process includes: continuously soaking in solutions of sodium bromate, distilled water, Al(NO3)3-LiNO3, and KMnO4. The resulting film consists of Al2O3·MnO2. If the pores of the oxide film are then sealed with a K2SiO3 solution, the effect is even better. The protection offered by a KMnO4 oxide 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 a potassium permanganate aqueous 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 perform comparably to chromate films. For high-copper, unpainted aluminum alloys, to achieve the best protection, an additional step of treating in 95–100°C potassium silicate solution for 1.5 minutes can be added. Compared with chromate films, the main advantage of this film is that its protective ability is not reduced by drying temperatures above 65°C or long-term storage. Potassium permanganate oxide films and chromate films offer identical protection against under-film filamentous corrosion.
