Anodic oxidation process of aluminum and aluminum alloy

Anodized aluminum and aluminum alloy process can be divided into: surface pretreatment, anodizing, coloring (non-decorative products can not be colored) and closed processing.

(1) Sulfuric acid anodizing process

Ordinary sulfuric acid anodization can obtain a film layer with a thickness of 5 to 20 μm and good adsorption. This method has a low cell voltage, convenient maintenance, energy saving, low cost, and a wide range of allowable impurity content. It is mainly used for the protection and decoration of aluminum parts, but it is not suitable for large cast aluminum parts, spot welding and riveting assemblies. Commonly used DC electrolysis and AC electrolysis two processes, DC method using sulfuric acid 100 ~ 200gL, anode current density of 0.8 ~ 1.5A/dm2, temperature l5 ~ 25 °C, voltage l0 ~ 25V, time 20 ~ 40min; AC method The use of sulfuric acid l0% ~ 20%, the anode current density of 1 ~ 3A/dm2, the temperature is 20 °C, voltage 20 ~ 50V, time 20 ~ 40min.

(2) Hard anodizing

Hard anodic oxidation, also known as thick layer anodic oxidation, the thickness of the oxide film can reach 250μm. The film has high hardness, wear resistance, insulation, heat resistance, corrosion resistance and Other characteristics. Table 7-2 lists the features of the hard oxide film and the ordinary oxide film. A lot of electrolytes can be used to obtain hard anodic oxide films. Sulfuric acid, oxalic acid, malonic acid, malic acid, and sulfosalicylic acid are commonly used. Commonly used DC power, can also use AC, DC superposition and various pulse currents. In order to obtain a film with a high hardness and a thick film thickness, compressed air is used in the oxidation process and the lower temperature is generally kept within a range of 5 to +10°C. Table 7-3 lists the hard anodizing process specification.

For the sulfuric acid hard oxidation process, the concentration of sulfuric acid has a great influence on the oxidation process. When the concentration of sulfuric acid is high, the growth rate of the oxide film is slow, the hardness of the oxide film is reduced, and the porosity is large. However, when the concentration is low, the service life of the bath is short and the parts are easily burned. In order to increase the thickness of the oxide film, it is better to add a certain amount of oxalic acid, and chloride and tungsten salts and magnesium salts should not be present in the solution.

Temperature and current density are important factors affecting the quality of the oxide film. As the temperature rises, the thickness of the film decreases, and the temperature should also be determined according to the different alloys. If the current density is too low, the oxide film formation rate is slow, but when the temperature is too high, the temperature rises quickly, causing parts to "corrode" and "burn." The starting voltage and time of hard oxidation also have a great influence on the quality of the oxide film and should be determined based on the composition of the aluminum alloy. For aluminum alloys containing less than 2.5% copper, the starting voltage is 5 to 7V and should not be greater than 10V; for alloys containing more than 2.5% copper and manganese, the starting voltage is 20 to 24V, and the final voltage is determined according to the required current density. . (3) Anodizing of chromic acid

Anodized chromic acid film is thin, approximately 2 ~ 5μm, the film is relatively soft and elastic, corrosion resistance is not as good as the sulfate anodic oxidation film, opaque, the color from gray to dark gray, not easy to color. Pore ​​less, no closure treatment. This oxide film is suitable for precision parts and is rarely used for decoration. The film layer has good bonding force with organic materials and can be used as a good coating bottom layer, and is widely used in rubber adhesive pieces and honeycomb structure panels, and is mostly used in the aviation and aerospace industries. The electrolyte composition and process conditions are chromic anhydride 50-100gL, temperature 35°C±2°C, cell voltage 0-50V, time 30-60min, anode current density (average) 0.3-0.5A/m2, high aluminum content for copper Alloy, the temperature can be reduced to 25 ~ 30 °C, voltage 0 ~ 40V.

(4) Oxalic acid anodizing

The oxalic acid anodizing process results in a yellow oxide film with a higher hardness and thicker thickness up to 60 μm. You can get different colors without dyeing. When the aluminum alloy composition is different, the color of the film layer may be silver to brown. However, the coloring of the film is difficult, the corrosion resistance is not strong, the cost is high, the electrolyte is toxic, and it is not stable enough. At present, this process is mainly used for electrical insulation layer and daily surface decoration, commonly used electrolytes and processes for oxalic acid 3% ~ l0%, temperature 20 ~ 35 °C, voltage 40 ~ 60V, anode current density 1.0 ~ 2.0A/dm2 , time 40 ~ 60min.

(5) Phosphoric acid anodizing

Aluminum and aluminum alloys are anodized in a dilute acid solution to obtain a large-pore layer and can be used as a metal electrodeposition layer. Changes in the composition of the aluminum alloy have a great influence on the quality of the oxide film. Commonly used electrolytes and processes are 3% to 20% phosphoric acid, temperature 30 to 35°C, voltage 50 to 60V, and time 15 to 30 minutes. If the part is to be prepared for electroplating, it should be noted that the anodized film should not be dried before electroplating, and should be cleaned after phosphoric acid treatment, otherwise it will affect the bonding force, and the appropriate cooling and stirring methods should be adopted so that the electrolyte temperature cannot exceed 40 °C.

(6) Other Anodizing

The aluminum alloy anodized by the phosphoric acid solution has a good binding force with the electroplating layer; the aluminum alloy is anodized in a boric acid solution containing a small amount of borax or ammonia to obtain an oxide film excellent in electrical insulation; in chromic acid, oxalic acid, boric acid After the mixed solution or the mixed solution of oxalic acid and citric acid, and the mixed solution of boric acid and titanium oxalate are anodized, the aluminum alloy can obtain a so-called ceramic anodized film that imitates the glaze effect. The oxide film deposited on the pores of the oxide film by the hydrolysis of rare metals (such as titanium, tantalum, niobium, etc.) has good quality and high hardness, can maintain the high precision of parts and low surface roughness, but the price is expensive. The cycle is short. The mixed acid anodizing process is applicable to pure aluminum or aluminum alloys with lower copper and magnesium content. The film is silver-gray, translucent, and can be dyed, similar to the appearance of PVC plastic.