The content of elements in aluminum has a decisive impact on the quality of clear anodized products. It directly affects the appearance, performance, and yield rate of the oxide layer.
These elements are added to enhance the mechanical properties of the aluminum material, but they significantly influence the clear anodizing process.
Magnesium (Mg)
Positive Impact: Magnesium is a primary element in the 5xxx series alloys (e.g., 5052, 5083). It can dissolve into the aluminum matrix and has a relatively minor effect on the color of the anodic film. Magnesium-containing alloys typically produce a uniform, colorless, transparent, or light gray-white oxide layer, making them a good choice for clear anodizing.
Note: Excessively high content (e.g., over 5%) may cause the oxide film to tend towards a milky white appearance.
Silicon (Si)
Significant Impact: Silicon is the main element in the 4xxx series and also an important element in the 6xxx series (e.g., 6061, 6063). It is almost insoluble during anodization and remains in the oxide film in a free state or as compounds.
Appearance Impact: Higher silicon content leads to a darker and duller oxide film. When the silicon content exceeds 0.5%, it results in a distinct gray-black or dark gray color. In 6xxx series alloys, silicon combines with magnesium to form Mg₂Si, which gives the oxide film a light to medium gray color – this is why anodized 6061 appears gray.
Performance Impact: Silicon particles can reduce the uniformity and corrosion resistance of the oxide layer.
Copper (Cu)
Highly Detrimental Impact: Copper is a primary element in the 2xxx series alloys and is extremely detrimental to the appearance of the anodic film.
Appearance Impact: Even at very low levels (0.1%), it can impart a yellowish or brownish tint to the oxide film. Higher content leads to darker colors, ranging from brownish to black, often with non-uniform coloration.
Performance Impact: Copper reduces the corrosion resistance and hardness of the oxide layer and makes the anodizing process difficult to control. Alloys with high copper content are generally not recommended for high-quality clear anodizing.
Manganese (Mn)
Detrimental Impact: Manganese is a primary element in the 3xxx series alloys (e.g., 3003).
Appearance Impact: Manganese causes the oxide film to appear brownish-gray or dark gray. The color deepens with increasing content. Its coexistence with copper intensifies the darkening effect.
Zinc (Zn)
Significant Impact: Zinc is a primary element in the 7xxx series alloys (e.g., 7075).
Appearance Impact: Zinc causes the oxide film to range from light gray to gray-black. At high concentrations, the color becomes very dark and can be non-uniform.
Chromium (Cr)
Minor Impact: Chromium is typically added as a trace element to improve the alloy’s resistance to stress corrosion cracking.
Appearance Impact: Small amounts of chromium (typically <0.2%) can impart a faint yellowish-green or golden-yellow hue to the oxide film. This color becomes more pronounced at higher concentrations.
The chemical composition of the aluminum material is the “genetic blueprint” determining the appearance of the clear anodized finish. Before proceeding with clear anodizing, it is essential to define the product’s appearance requirements (colorless, light gray, dark gray) and strictly select the appropriate aluminum alloy grade based on these requirements, while implementing tight controls on the content of key elements.
For die-cast products that require high-quality anodized colors, we control the entire process from start to finish. We source our own raw aluminum and refine it using a proprietary formulation to produce high-purity ingots. This allows us to fine-tune trace elements specifically to meet your exact color specifications for anodizing.