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When the aluminum alloy is immersed in boiling water, the natural oxide film of aluminum will continuously thicken, reaching 0.7-2 μm later. The oxide film is colorless or milky white. The water oxide film is gamma diaspore type alumina, and its structure is dense. The pH is between 3.5 and 9. The film is very stable and can be used as the bottom layer of the paint. The superheated steam exceeding 100°C is favorable for the formation of the film, and the actual process is a few minutes at 75120°C in pure water. In order to increase the film thickness, a porous oxide film can be obtained by adding ammonia water or triethanolamine to pure water. The oxide film treated with ammonia is white in color and has a uniform color tone. The addition range of ammonia is 0.3% to 0.5%.
2 zirconium salt oxidation
The use of zirconium-containing solutions instead of chromates for the pretreatment of aluminum-based surfaces has been accepted by people, and is particularly suitable for the chemical conversion film-forming treatment before the application of aluminum alloy parts, which can increase the adhesion between the coating and the substrate and increase the corrosion resistance. Performance, while the oxide film itself also has a certain degree of corrosion resistance.
3 titanium salt oxidation
Titanium and chromium are very similar in nature and do not corrode in almost all natural environments. Its excellent corrosion resistance stems from the continuous, stable, strong and protective oxide film formed on its surface. Titanium's high reactivity and strong affinity with oxygen make it possible to form an oxide film immediately after exposure of its metal surface to air or a tidal temperature environment. In fact, as with the chromate chemical oxide film, as long as trace amounts of oxygen or water (moisture) are present in the environment, the titanium oxide film destroyed by the strong affinity of titanium and oxygen can be immediately self-repaired.
4 Rare earth metal salt oxidation
The rare earth metal salt chemical oxide film may replace the chromate chemical oxide film in the future, and the material may be treated by a dipping method. The treatment solution generally needs heating to produce a protective layer on the basic metal surface. Its corrosion resistance is provided by the formation of rare earth oxides on the metal surface. At present, the rare-earth aluminum alloy rare-earth treatment process generally uses a mixed solution consisting of a rare-earth metal salt, an oxidant, a film-forming accelerant, and a film-forming agent. Rare-earth salts mainly refer to cerium salts such as CeCl3, Ce(NO3)3, Ce(SO4)2, (NH4)2Ce(NO3)6, etc., and film-forming accelerators include NaOH, HF, SrCl2, (NH4)2ZrF, etc. H2O2, KMnO4, (NH4)2S2O8, etc. The treatment process without an oxidant in the treatment liquid is a rare earth bohmite layer process. This process involves the formation of a bohmite layer on the surface of an aluminum alloy with hot water and then immersing it in a rare earth salt solution to form a bohmite layer containing a rare earth. The feature of this process is that it does not need to use H2O2, KMnO4 and other strong oxidants to shorten the processing time, but the processing temperature is higher.
5 Permanganate oxidation
In general, permanganate is not only a good corrosion inhibitor for aluminum and alloys, but also accelerates corrosion. However, aluminum and its alloys can form a good protective film by proper treatment in KMnO4 solution. The process includes: continuously immersing in sodium bromate, distilled water, Al(NO3)3-LiNO3 solution and KMnO4 solution, and the composition of the obtained film is Al2O3·MnO2. If you use K2SiO3 solution to close the pores of the oxide film, the effect is better. The protection of the KMnO4 oxide film is approximately 70% of that of the chromate film (in the same period as the salt spray test). For pure aluminum and aluminum alloys containing no copper, zinc, or iron, a uniform yellow film similar to the chromate oxide film can be formed in a potassium permanganate solution for 1 min. For aluminum alloys with higher corrosion propensities, in order to obtain a thicker protective film, it should be treated in boiling water or steam first to form an oxide film, and then this film is subjected to secondary or tertiary sealing treatment. Once in the aluminum salt is closed, once in the KMnO4 solution is closed, the oxide film thus formed can be compared with the chromate film, for the aluminum alloy with high copper content, and not coated, to obtain better Protective film, can add a 95 ~ 100 °C potassium silicate solution to process 1.5min, compared with chromate film, the film has the advantage of drying temperature greater than 65 °C and long-term storage will not reduce it Protective. The potassium permanganate oxide film and the chromate oxide film have the same protective performance against silk (fiber) corrosion under the paint film.
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