Method for preparing trivalent chromium chemical conversion film of aluminum or aluminum alloy

The natural oxide film on the surface of aluminum and aluminum alloys is about 5mm. Due to its thin thickness, it is prone to wear and scratches, and its corrosion resistance is poor. If exposed to severe corrosive environments such as outdoor and seaside, the surface will quickly generate a Layer of aluminum rust. In order to improve the corrosion resistance of the oxide film on the surface of aluminum and aluminum alloys, it is necessary to manually treat the aluminum and aluminum alloy surfaces to increase the thickness of the oxide film. At present, the most widely used is the chemical conversion film and anodizing. Compared with anodizing, the chemical conversion film has the advantages of simple operation, low cost, low energy consumption, short film forming time, low requirements on the material of the substrate, less influence on fatigue performance of aluminum and aluminum alloy, and application to complex parts such as slender tubes. , spot welding and riveting parts and other surface treatment of a variety of advantages.

Traditionally, hexavalent chromium is used as the main component to perform chromium chemical conversion coating on the surfaces of aluminum and aluminum alloys. However, hexavalent chromium is highly toxic and therefore has been banned or restricted by environmental regulations in various countries, such as EU RoHS regulations. . In order to replace the hexavalent chromium conversion film, there have been some related studies, such as the chemical conversion film of ruthenium (electrochemical research on the formation of ruthenium oxide film on aluminum alloys. Material Protection, 1995, 28(3): 1~3), chromium-free. Chemical Conversion Coatings (Study of Chromium-free Chemical Conversion Coatings on Aluminum Alloy Surfaces, Material Protection, 2005, 34(6):38-39), etc., but the chemical conversion coating of niobium not only has poor corrosion resistance, but also has a colorless and transparent film. Inconvenient to identify, the corrosion resistance of chromium-free chemical conversion coating is still poor, and it can only withstand 35% NaC1 salt water soaking for 3 days, which still cannot meet people's requirement for corrosion resistance of aluminum alloy surface.

From Zhongnan University Yu Huicheng;Chen Baizhen;Xu Hui;Yang Xiyun;Shi Xichang Invented and designed an aluminum or aluminum alloy trivalent chromium chemical conversion film preparation method: The aluminum or aluminum alloy workpiece in 50 ~ 100g/L sodium hydroxide Activated in aqueous solution for 1 to 2 minutes. Activation temperature is 5 to 30°C. Wash with deionized water. Chromate 6 to 10 minutes after cleaning the workpiece into soluble trivalent chromium salt and phosphoric acid mixed aqueous solution. The ion concentration is 1-20 g/L, the phosphoric acid concentration is 1-25 g/L, the pH of the reaction system is adjusted to 1.6-4.0 with dilute sodium hydroxide or dilute sulfuric acid, and the chromizing temperature is 10-60°C. The invention solves the disadvantages of the prior art that the hexavalent chromium has great toxicity, and the technical process and products meet the environmental protection requirements. The trivalent chromium chemical conversion film and the hexavalent chromium chemical conversion film obtained have the same quality.

Compared with the current technology, the present invention has the following technical advantages and effects:

1 The present invention adopts a low-toxicity trivalent chromium compound as a raw material to perform chemical conversion film treatment on the surfaces of aluminum and aluminum alloys, thereby solving the disadvantages of a large toxicity of the process of the surface conversion film of aluminum and aluminum alloys originally containing hexavalent chromium as a main component. Processes and products meet environmental requirements, such as compliance with RoHS requirements of EU environmental regulations;

2 The anti-corrosion performance of the trivalent chromium chemical conversion film of the present invention is basically the same as that of the hexavalent chromium chemical conversion film, and the film is light green or green, and aluminum rust is not immersed in 3.5% NaCl salt solution for more than 10 days;

3 The process of the invention is simple and easy, the film forming time is short, the cost is low, the influence on the fatigue performance of the aluminum and the aluminum alloy is small, the requirement for the material of the substrate is low, and the material quality of the substrate is not affected.