Metal Housing Oxidation Methods

Artificial methods are used to form an oxide film (Al₂O₃) on the surface of aluminum alloy housings and other alloy products, with various colors applied to improve the wear resistance of aluminum, extend its service life, and enhance appearance.
The basic process of oxidation coloring includes aluminum surface treatment, oxidation, coloring, followed by hydration sealing, organic coating, and other finishing processes.

Under natural conditions, aluminum alloy housings form a natural oxide layer of 10–100 Å. By contrast, artificial oxidation can control the oxide layer thickness between 0.5–250 μm to meet diverse application requirements.
Methods for forming artificial oxide films include chemical oxidation and electrochemical oxidation (anodizing).

After chemical oxidation and surface cleaning, aluminum or aluminum alloy reacts chemically in an oxidizing solution containing oxidants and activators to form an oxide film.
Activators partially dissolve the oxide film during formation to create pores, allowing the oxide film to continue growing and thickening.
Many types of oxidation solutions are available; chromate is commonly used as the oxidant and carbonate as the activator.

Anodizing is typically carried out in electrolytes such as sulfuric acid, oxalic acid, and chromic acid.
Using aluminum as the anode and lead or other metals as the cathode, anions (OH⁻) migrate toward the anode under a DC electric field, generating fresh oxygen at the anode and forming an aluminum oxide film.
In the electrolyte, the acid partially dissolves the oxide film to create pores, allowing the oxidation reaction to proceed inward.
This process produces a porous structure on the aluminum surface, suitable for various coloring treatments and as a primer for painting.

Coloring Methods for Oxide Films

Coloring methods include chemical coloring, electrolytic coloring, and natural coloring.

Chemical Coloring
The oxidized metal housing is immersed in an organic or inorganic dye solution.
Dyes penetrate into the pores of the oxide film and achieve coloring through chemical or physical adsorption.
Chemical coloring features simple and low-cost equipment and a wide color range, but has poor light resistance and corrosion resistance, making it suitable only for interior applications.

Electrolytic Coloring
After oxidation, the metal housing undergoes secondary electrolysis in a single or multi-metal salt solution.
Under the electric field, metal cations penetrate into the pores of the oxide film and deposit at the bottom of the pores, forming metal deposits that produce colors such as bronze, brown, gray, red, and blue.

Natural Coloring
This is a coloring method achieved during the anodizing process itself, including the alloy coloring method and electrolyte coloring method.

• The alloy coloring method obtains various colors by controlling the composition of the aluminum alloy.
• The electrolyte coloring method, also known as integrated electrolytic coloring, controls color by adjusting electrolyte composition and electrolysis parameters.

In industrial production, organic colorants are often used with organic acids as electrolytes, with a small amount of sulfuric acid added to adjust the pH value.o adjust the pH value.