1. When aluminum alloy is immersed in boiling water, the natural oxide film of aluminum will continuously thicken, eventually reaching 0.7–2 µm. The oxide film is colorless or milky white, and the water-formed oxide film is γ-alumina type hydrated aluminum oxide, which has a dense structure. The film is very stable at a pH between 3.5 and 9 and can be used as a substrate for paint. Superheated steam above 100°C is conducive to film formation. In practice, the process involves treating in pure water at 75–120°C for several minutes. To increase the film thickness, ammonia or triethanolamine can be added to the pure water to obtain a porous oxide film. The oxide film treated with ammonia is white in color and has a uniform tone. The optimal ammonia addition range is 0.3%–0.5%.
2. Zirconium Salt Oxidation Method
Using zirconium-containing solutions instead of chromates for the pretreatment of aluminum surfaces has been accepted, especially suitable for chemical conversion coating treatment before painting aluminum alloy parts. It can increase the adhesion between the coating and the substrate, enhance corrosion resistance, and at the same time, the oxide film itself also has a certain corrosion-protective ability.
3. Titanium Salt Oxidation Method
Titanium and chromium have very similar properties and do not corrode in almost all natural environments. Its excellent corrosion resistance comes from the continuous, stable, firmly bonded, and protective oxide film layer formed on its surface. Titanium's high reactivity and strong affinity for oxygen allow its metal surface to immediately form an oxide film when exposed to air or humid environments. In fact, just like chromate chemical oxide films, as long as trace amounts of oxygen or water (moisture) exist in the environment, due to titanium's strong affinity for oxygen, any damaged titanium oxide film can immediately self-repair.
4. Oxidation Method Using Rare Earth Metal Salts
The chemical oxide film of rare earth metal salts may potentially replace chromate chemical oxide films in the future. The material can be treated using an immersion method, and the treatment solution generally needs to be heated in order 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 rare earth treatment process for aluminum alloys generally uses a treatment method involving a mixed solution composed of rare earth metal salts, oxidants, 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., and oxidants include H2O2, KMnO4, (NH4)2S2O8, etc. In treatment processes where no oxidant is added, there is 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 into a rare earth salt solution to form a rare earth-containing bohmite layer. The feature of this process is that it does not require strong oxidants like H2O2 or KMnO4 to shorten the treatment time, but the treatment temperature is relatively high.
5. Potassium Permanganate Oxidation Method
Generally speaking, potassium permanganate is not only not a good corrosion inhibitor for aluminum and its alloys, but it can also accelerate corrosion. However, aluminum and its alloys can form a good protective film after proper treatment in a KMnO4 solution. The process includes continuously immersing in sodium bromate, distilled water, Al(NO3)3-LiNO3 solution, and KMnO4 solution, resulting in a film composed of Al2O3·MnO2. If the pores of the oxide film are further sealed using a K2SiO3 solution, the effect is even better. The protection offered by KMnO4 oxide film is approximately 70% of that of a chromate film. For pure aluminum and aluminum alloys with low copper, zinc, or iron content, a 1-minute treatment in an aqueous potassium permanganate solution can form a uniform yellow film similar to a chromate oxide film. For aluminum alloys with a higher tendency to corrode, to obtain a thicker protective film, they should first be treated in boiling water or steam to form an oxide layer, and then this film should undergo secondary or tertiary sealing treatments. One sealing step is done in aluminum salts and another in KMnO4 solution. The resulting oxide film's performance can be comparable to that of a chromate film. For aluminum alloys with high copper content that are not painted, to obtain the best protective film, an additional treatment in 95–100°C potassium silicate solution for 1.5 minutes can be applied. 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 after long-term storage. The protective performance of potassium permanganate oxide films and chromate films against underpaint filiform (fiber-like) corrosion is completely the same.
