The Reasons Why 7-Series Aluminum Alloys Are Difficult To Anodize

Anodizing is a material protection technique that forms an oxide film on the surface of a metal by applying an electric current. It is also known as surface anodization. After aluminum alloy materials or products undergo surface anodization treatment, their corrosion resistance, hardness, wear resistance, insulation properties, and heat resistance are significantly improved. Aluminum is the metal material that undergoes anodization treatment most frequently.

01 7-Series Aluminum Alloys
7-series aluminum alloys belong to the Al-Zn-Mg-Cu series of super-hard aluminum. This alloy has been applied in the aircraft manufacturing industry since the late 1940s and is still widely used in the aviation industry as an ultra-high-strength wrought aluminum alloy. Its characteristics are as follows: After solution treatment, it has good plasticity. The effect of heat treatment strengthening is particularly good. It has high strength below 150 °C and excellent low-temperature strength. Its welding performance is poor. It has a tendency to stress corrosion cracking and needs to be clad with aluminum or undergo other protective treatments before use. Double-stage aging can improve the alloy's ability to resist stress corrosion cracking. In the annealed and freshly quenched states, its plasticity is slightly lower than that of 2A12 in the same state and slightly better than that of 7A04. The static fatigue, notch sensitivity, and stress corrosion performance of its plates are better than those of 7A04. Its density is 2.85 g/cm³.

Among the 7-series aluminum alloys, 7075 aluminum alloy is the most commonly used. 7075 is widely used in parts and components manufactured by CNC cutting and is suitable for aircraft frames and high-strength accessories. 7-series aluminum alloys contain zinc and magnesium elements. Zinc is the main alloying element in this series, so it has quite good corrosion resistance. With a small amount of magnesium alloy added, the material can achieve very high strength after heat treatment.

02 7-Series Aluminum Alloys & Anodization
The anodization of 7075 aluminum alloy is generally carried out in an acidic electrolyte with aluminum as the anode. During the electrolysis process, the anions of oxygen react with aluminum to form an oxide film. When this film is initially formed, it is not dense enough. Although it has a certain resistance, the negative oxygen ions in the electrolyte can still reach the aluminum surface to continue forming the oxide film. As the film thickness increases, the resistance also increases, and thus the electrolytic current becomes smaller. At this time, the outer oxide film in contact with the electrolyte undergoes chemical dissolution. When the rate of oxide formation on the aluminum surface gradually balances with the rate of chemical dissolution, this oxide film can reach the maximum thickness under these electrolytic parameters. The outer layer of the anodic oxide film on aluminum is porous and easily adsorbs dyes and colored substances, so it can be dyed to improve its decorative properties. After the oxide film is subjected to sealing treatment with hot water, high-temperature steam, or nickel salts, its corrosion resistance and wear resistance can be further improved.

03 Adverse Effects?
However, since 7075 aluminum alloy contains a large amount of zinc element, which is the chemical component that has the greatest impact on the oxidation effect, it is easy to cause adverse phenomena such as white edges, black spots, and uneven colors on the oxidized parts. Generally, the oxidation effect is better when it is oxidized to the original color of the aluminum alloy or black. When it is oxidized to bright colors such as red and orange-red, the performance is the most unstable.

7-series aluminum alloys (mainly aluminum alloys doped with zinc) have higher hardness and strength. As materials for the shells and middle frames of electronic products, 7-series aluminum alloys have the advantages of being lightweight and less likely to be bent or deformed, and are increasingly favored by electronic product manufacturers. However, when using the traditional anodization process for 6-series aluminum alloys (sulfuric acid anodization) to oxidize 7-series aluminum alloys, due to the relatively high content of 7-series aluminum alloys, especially Al-Zn-Mg alloys, the components of the aluminum alloys are inevitably distributed unevenly, resulting in segregation. After aluminum segregation undergoes sulfuric acid anodization, the pore sizes of the oxide film on the surface vary greatly, and the quality and adhesion of the oxide film at different positions will be different. The adhesion of the oxide film at some positions is weak and it is easy to fall off.

04 Oxidation Methods and Precautions
According to experiments, after 7-series aluminum alloys are anodized, the thickness of the oxide film can be 30 μm - 50 μm, and a hardness of 50 - 55 HRC can make the aluminum meet the requirements. (The electrolyte is oxalic acid, direct current, the electrolysis temperature is below 0 °C, and the oxidation color is a withered grass yellow-green - shit yellow with a metallic luster).

1 Preparations Before Oxidation
Oxidation should be carried out after all machining operations are completed and before the finished product is obtained.
2Oxidation Process
01 The first step is pickling. Before pickling, it is necessary to know whether any electrical discharge machining has been done before, such as electric spark discharge machining for splines, wire cutting machining, etc. If there is such machining, a hard oxide film will also be formed on the surface. Pickling must corrode this layer of film away. Otherwise, this part will not be conductive and will not be covered with an oxide film. However, this will affect the surface smoothness and luster of the workpiece and make the workpiece dull.
02 Calculate the surface area of the workpiece, as it has a certain proportional relationship with the current and also relates to the content of oxalic acid. Otherwise, problems such as slow film formation and thin film layers will occur.
03 During the electrolysis process, do not frequently take out the workpiece to check the thickness of the film, as this will cause the problem that it will not react when put back into the oxidation tank again.
04 Pay attention to controlling the current. If it is too large, it will cause ablation, and if it is too small, there will be no reaction.