What is Anodizing?
Anodizing is an electrochemical surface treatment process that forms an oxide layer on the surface of metals, it can enhance physical and chemical properties such as corrosion resistance, hardness, and wear resistance. A typical anodic oxide layer can increase the surface hardness of a product to 200-300 HV; with special products, a hard anodic oxide layer can be produced, increasing the surface hardness to 400-1200 HV and providing excellent wear resistance.
Anodizing Process:
1）Cleaning: Before anodizing, metal parts must be thoroughly cleaned to remove any surface oils and impurities, usually using alkaline or acidic solutions.
2）Pre-treatment: Metal parts undergo pre-treatments like deburring, grinding, or polishing to enhance the smoothness of the surface after anodizing to some extent.
3）Electrolyte: The metal parts are immersed in an electrolytic solution containing oxidants and other additives. The composition depends on the metal being used and the desired surface treatment effect.
4）Anodizing: In the electrolyte, an oxide layer forms on the surface of the metal part — a process known as anodizing. The thickness and color of the oxide layer will depend on the electrolyte used and the current/time parameters.
5）Sealing: The micro-pores in the oxide layer are sealed by soaking in hot water or through other sealing agents, enhancing its corrosion resistance and hardness.
6）Cleaning and Drying: After treatment, parts need to be cleaned and dried to remove any excess electrolyte and sealing agent.
What Materials Suitable for Anodizing, and Those That Aren’t:
While anodizing is a common surface treatment technique, not all materials are suitable for this process. Anodizing is mainly used for aluminum and its alloys. Stainless steel and plastic materials cannot be anodized, as stainless steel is comprised mainly of chromium and nickel, which can be reduced during electrochemical reactions, preventing the formation of a stable oxide layer. Plastic is not conductive and thus can’t be directly anodized. Electroplating or vacuum plating will be recommended.
Advantages of Anodizing Aluminum and Aluminum Alloys:
Even though aluminum and its alloys can naturally form a thin and porous oxide layer in the atmosphere, this natural oxide layer is amorphous, non-uniform, and discontinuous, thus not providing reliable protection or decorative quality. The industrial development of aluminum products has increasingly utilized anodizing or chemical oxidation to generate an oxide layer on aluminum and its alloy components for protective and decorative purposes.
The oxide layer obtained from anodizing typically ranges from 5-20 micrometers, with hard anodic oxide layers reaching 60-2500 micrometers. This layer features the following characteristics:
1)High hardness: A pure aluminum oxide layer is harder than an alloy’s oxide layer. The hardness is related to the alloy’s composition and the electrolytic conditions during anodizing. Not only is the anodized layer hard, but it also possesses good wear resistance, especially the porous oxide film that can absorb lubricants, further improving its wear resistance.
2)High corrosion resistance: This is due to the high chemical stability of the anodic oxide layer. Pure aluminum’s oxide layer has been found to have better corrosion resistance than an alloy’s. This is due to alloying elements or metal compounds that cannot oxidize or dissolve, resulting in discontinuities or gaps which significantly reduce the oxide layer’s corrosion resistance. Generally, the oxide film obtained by anodizing must undergo a sealing process to improve its corrosion resistance.
3)Strong adsorption: The anodic oxide layer on aluminum and its alloys has a porous structure with strong adsorption abilities. Filling the pores with various pigments, lubricants, resins, etc., can further enhance the protective, insulating, wear-resistant, and decorative properties of aluminum products.
4)Excellent insulating properties: The anodic oxide layer on aluminum and aluminum alloys is no longer conductive and becomes a good insulating material.
5)Strong heat insulation and resistance: Since the thermal conductivity of anodic oxide layers is much lower than that of pure aluminum, they can withstand temperatures around 1500°C, while pure aluminum can only withstand 660°C.
Anodizing Colors:
Anodizing offers a vast array of colors, including various solid colors and gradient effects, with strong selectivity. The most common anodizing colors for prototypes in electronics, such as smartphones and computers, are black, while silver is widely used in kitchen equipment and automobile parts.
In summary, chemical treatments, especially anodizing, can form a protective and decorative oxide layer on the surface of aluminum and its alloys, hence widely applied in fields such as automotive parts, aerospace, kitchen utensils, and electronic product casings. Going forward, anodizing is expected to maintain its significance as a critical process within industrial applications.
