Requirements for Anodizing of Aluminum Alloy Materials

Requirements for Anodizing of Aluminum Alloy Materials

In general, black water marks appear on the anodized surface of aluminum alloy (as shown in the figure below). What is the reason for this? Today, precision casting manufacturer Tianyao Metal will discuss in detail the requirements of anodizing for aluminum alloy materials.

What is anodizing? Anodizing is the process of using electrochemical methods to oxidize the anode in an appropriate electrolyte, with alloy parts as the anode and stainless steel, chromium, or conductive electrolyte itself as the cathode. Under certain voltage and current conditions, the anode is oxidized, thereby obtaining an anodic oxide film on the surface of the product. According to the type of electrolyte and film properties, it can be divided into sulfuric acid, chromic acid, oxalic acid, mixed acid, hard and ceramic anodizing; According to the dyeing performance of various anodized films, only the oxide film obtained by sulfuric acid anodizing is most suitable for dyeing; Other materials such as oxalic acid and ceramic anodized films, although capable of coloring, have serious color interference; Chromic acid anodic oxide film or hard oxide film cannot be colored; Overall, to achieve the purpose of anodizing and coloring, only sulfuric acid anodizing can be used.

1、 Limitations of sulfuric acid anodizing on aluminum alloy materials

1.1. The presence of alloying elements can lead to a decrease in the quality of the oxide film. Under the same conditions, the oxide film obtained on pure aluminum is the thickest, has the highest hardness, corrosion resistance, and uniformity. For aluminum alloy materials to achieve good oxidation effects, it is necessary to ensure that the aluminum content is generally not less than 95%.

1.2. In alloys, copper can cause the oxide film to turn red, damage the quality of the electrolyte, and increase oxidation defects; Silicon can cause the oxide film to turn gray, especially when the content exceeds 4.5%, the effect is more significant; Iron, due to its inherent characteristics, will exist in the form of black spots after anodizing. The following figure shows the composition table of different cast aluminum alloys:

2、 For decorative anodizing, higher requirements are placed on aluminum and aluminum alloy materials. Generally, only pure aluminum, aluminum clad, aluminum magnesium, and aluminum manganese alloys can be dyed with bright colors. For alloys with high silicon and iron content, the resulting oxide film can only be dyed with a dark color. When decorative anodizing aluminum and aluminum alloy parts, the following points should be noted in material selection:

2.1. For parts that require a mirror like appearance, it is necessary to choose high-purity aluminum or 5A66 aluminum magnesium alloy with an aluminum content higher than 99.99%. After chemical or electrochemical polishing, parts can be anodized with sulfuric acid to obtain a colorless, transparent, and bright film layer that can be dyed with various bright colors.

2.2. For parts that require lower surface brightness than above, industrial high-purity aluminum with an aluminum content higher than 99.9% and LT65 LT67 aluminum magnesium alloy can be selected.

3、 When selecting aluminum materials, attention should also be paid to:

3.1. Some aluminum alloys should be heat treated using reasonable specifications. The size of the grains has a certain impact on the structure and properties of the oxide film. Coarse grains react unevenly during the oxidation process, often exhibiting an orange peel like appearance. Therefore, it is generally hoped that aluminum materials have a finer grain structure.

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