Five Types of Aluminum Alloy Chromium-Free Oxidation Methods
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 of the boehmite type. Its structure is dense, and the film is very stable at pH 3.5–9, making it suitable as a primer layer 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 treatments before painting aluminum alloy parts. This can increase the adhesion between the coating and the substrate and improve corrosion resistance, while the oxide film itself also possesses a certain degree of corrosion protection.
3. Titanium Salt Oxidation Method
Titanium shares very similar properties with chromium and does not corrode in almost all natural environments. Its excellent corrosion resistance comes from the continuous, stable, firmly bonded, and protective oxide film formed on its surface. Titanium's high chemical reactivity and strong affinity for oxygen allow an oxide film to form immediately when its metal surface is exposed to air or humid environments. In fact, just like chromate chemical oxide films, as long as trace amounts of oxygen or water (moisture) are present in the environment, the titanium oxide film can self-repair immediately due to titanium's strong affinity for oxygen if it gets damaged.
4. Rare Earth Metal Salt Oxidation Method
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, 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., and oxidizers include H2O2, KMnO4, (NH4)2S2O8, etc. In treatment processes where no oxidizer 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 oxidizers 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 when properly treated in a KMnO4 solution. The process includes continuous soaking in sodium bromate, distilled water, Al(NO3)3-LiNO3 solution, and KMnO4 solution. The resulting film is composed of Al2O3·MnO2. If the pores of the oxide film are further sealed with a K2SiO3 solution, the effect is even better. The protection provided by the KMnO4 oxide film is about 70% of that of a chromate film (based on the same period in a 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 form a uniform yellow film similar to a chromate oxidation film. For aluminum alloys with higher corrosion tendencies, to obtain a thicker protective film, the alloy should first be treated in boiling water or steam to form an oxide film, and then this film undergoes secondary or tertiary sealing. One sealing is conducted in aluminum salt, and another sealing in KMnO4 solution. The resulting oxide film can perform comparably to a chromate film. For aluminum alloys with high copper content that are not painted, to achieve the best protective film, an additional step of treating with a 95–100°C potassium silicate solution for 1.5 minutes can be added. Compared with a chromate film, the greatest advantage of this film is that its protective performance does not decrease even when the drying temperature exceeds 65°C or during long-term storage. Potassium permanganate oxide films and chromate oxide films have identical protective performance against fiber-like corrosion under the paint.
