Five Types of Chromium-Free Anodizing for Aluminum Alloys

1. Immerse aluminum alloy 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. The water oxide film is a γ-alumina type of hydrated aluminum oxide, its structure is dense, and the film is very stable at a pH between 3.5 and 9, making it suitable as a primer layer for paint. Overheated steam above 100°C is beneficial for film formation. The actual process involves treating it in pure water at 75–120°C for several minutes. To increase the film thickness, ammonia water or triethanolamine can be added to the pure water to obtain a porous oxide film. The oxide film treated with ammonia water is white and has a uniform tone. The optimal ammonia addition range is 0.3%–0.5%.

2. Zirconium Salt Oxidation Method

The use of zirconium-containing solutions instead of chromates for pretreating aluminum surfaces has been accepted, especially suitable for chemical conversion coating treatment before painting aluminum alloy parts. This can increase the adhesion between the coating and the substrate, improve corrosion resistance, and the oxide film itself also has certain anti-corrosion capabilities.

3. Titanium Salt Oxidation Method

Titanium is very similar to chromium in properties and does not corrode in almost all natural environments. Its excellent corrosion resistance is due to the formation of a continuous, stable, tightly bound, and protective oxide film layer on its surface. Titanium’s high reactivity and strong affinity for oxygen allow its metal surface to form an oxide film immediately when exposed to air or a humid environment. In fact, just like chromate chemical oxide films, as long as trace amounts of oxygen or water (moisture) are present in the environment, the destroyed titanium oxide film can immediately self-repair due to titanium’s strong affinity for oxygen.

4. Rare Earth Metal Salt Oxidation Method

The chemical oxidation film of rare earth metals may potentially replace chromate chemical oxidation films in the future. The material 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 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 　　　　

In general, potassium permanganate is not only not a good corrosion inhibitor for aluminum and its alloys, but can also accelerate corrosion. However, aluminum and its alloys can form a good protective film after appropriate treatment in KMnO4 solution. The process includes: continuously immersing in solutions of sodium bromate, distilled water, Al(NO3)3-LiNO3, and KMnO4, resulting in a film composed of Al2O3·MnO2. If the pores of the oxide film are sealed with K2SiO3 solution, the effect is even better. The protective effect of KMnO4 oxide film is about 70% of that of a chromate film (based on salt spray test conducted simultaneously). For pure aluminum and aluminum alloys with low copper, zinc, or iron content, a 1-minute treatment in aqueous potassium permanganate can produce a uniform yellow film similar to a chromate oxide film. For aluminum alloys with higher corrosion tendency, 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 should undergo a secondary or tertiary sealing process. One sealing is performed in aluminum salt, and another is in KMnO4 solution. The resulting oxide film can have performance comparable to a chromate film. For aluminum alloys with high copper content and 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 with chromate films, the greatest advantage of this film is that its protective effect is not reduced even with drying temperatures above 65°C and long-term storage. The protective performance of potassium permanganate oxide films and chromate films against filamentous corrosion under paint is exactly the same.