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[China Aluminum Industry Network] Analysis of the Three Main Causes of Uneven Color in Aluminum Oxide Films:
(1) One common cause is when the aluminum profile workpiece is too large, leading to excessive movement within the oxidation tank. This results in uneven contact, renewal, and exchange between the edges and the center of the workpiece with the solution, causing variations in the color of the oxide film. To prevent this, it's important to reduce the swinging amplitude during oxidation or consider static treatment. However, if the solution temperature is too low, a mottled appearance may occur, which looks unnatural and can be confusing for customers.
(2) Another issue arises when the aluminum cladding is damaged during processing, causing parts of the outer layer to break off. The outer cladding is typically high-quality aluminum, while the inner layer is often a mixed alloy. This difference in material composition leads to irregular spots on the oxide film, resembling a hurricane pattern. Many customers are unaware of this phenomenon, so manufacturers should clearly explain the reasons to avoid misunderstandings and ensure customer satisfaction.
(3) Operational issues during the oxidation process can also lead to uneven coloring:
- Incomplete alkali etching may leave residual oxide layers or dirt on the surface, preventing uniform film formation.
- If the workpiece is not rinsed immediately after alkali etching, it may remain alkaline, affecting the subsequent oxidation process.
- Contamination from foreign materials during transfer can also lead to discoloration.
When uneven colors appear, it’s essential to investigate multiple factors and implement targeted solutions to resolve the issue effectively.
A reader recently asked why it was difficult to form a conductive oxide film after alkali etching. After eliminating other possible causes, the problem might be due to an excess of aluminum ions in the alkali solution. The reader mentioned that the solution was very thick but not very corrosive. Based on experience, the author suggested replacing the solution, as prolonged use can cause aluminum ions to accumulate, making it hard for them to be removed from the surface. This can interfere with the contact between the aluminum part and the conductive oxide solution, hindering proper film formation. Another recommendation was to rinse the workpiece with hot water immediately after alkali etching, then treat it with concentrated nitric acid containing hydrofluoric acid before proceeding with oxidation. The reader later confirmed that this method significantly improved the results. From personal experience, it’s crucial to rinse the workpiece thoroughly in running water right after hot water washing to prevent unwanted oxidation before the actual oxidation process begins.
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