Anodized

Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of a metal workpiece.

Aluminum alloy is anodized to improve corrosion resistance, can be dyed (colored), and can also improve the lubricity and adhesion of the product.However, anodizing does not increase the strength of aluminum.The anode layer is non-conductive.

The aluminum layer of the anode is thickened after direct current is turned on in the electrolytic solution, and aluminum is used as the anode (positive electrode). The current releases hydrogen at the cathode (negative electrode) and oxidizes the surface of the aluminum anode to generate alumina.The voltage required for different solutions may range from 1 to 300 V (DC). Although most of the voltage is 15-21V, relatively high currents are usually required in sulfuric and organic acids to form thicker coatings.The anodizing current is different in the area where the aluminum is oxidized, usually 30-300 amps/square meter (2.8 to 28 amps/square foot).

Anodizing aluminum is usually done by slowly dissolving alumina in an acid solution.The acid reaction is balanced with the oxidation rate to form a coating with nanopores. The diameter of the holes is 10-150 nm. These holes allow the electrolytic solution and current to reach the aluminum substrate and continue to increase, and automatically passivate the thicker coating thickness, but these holes will then cause air and water to contact the substrate.If it is not closed, it will corrode the substrate.Colored dyes or corrosion inhibitors are usually filled before sealing.

Because the dye is only on the surface, even minor wear and scratches may cause the dyed layer to break, but the underlying oxide can continue to provide anti-corrosion protection.

The concentration, acidity, temperature and current of the electrolyte must be well controlled to ensure the continuous formation of the oxide layer. Harder, thicker films tend to be produced in the case of thinner solutions, lower temperatures, and higher voltages and currents.The thickness of the film can range from less than 0.5um (for bright decoration applications) to 150um (for architectural applications).

In recent years, manufacturers of 3C products such as mobile phones have used aluminum alloy extensively as the structural parts of their products, which has made the anodizing of aluminum alloy shine brightly.

3C product aluminum alloy anodizing process flow：

Feeding → degreasing → washing → alkali etching → washing → stripping the black film → washing → chemical polishing → washing → surface stripping treatment → washing → anodizing → washing → activation → dyeing → washing → sealing → washing → nitric acid ash removal → drying → cutting