What causes the uneven color of the aluminum oxide film? There are several key factors that can lead to this issue. Here are three main reasons:
First, when the workpiece area of the aluminum plate is too large, and the swinging motion in the tank is excessive during the oxidation process, the contact, renewal, and exchange between the edge and center parts with the solution become inconsistent. This leads to an uneven oxide film in terms of color. To prevent this, it's important to minimize the movement of the workpiece during oxidation. Static treatment can also be used, but if the solution temperature is too low, map-like spots may appear, which look unnatural.
Second, during the processing of aluminum-clad sheets, some of the aluminum cladding layer may be damaged or cut off. The outer layer is usually high-quality aluminum, while the inner layer is a mix of different metals. This can result in color variations or "like spots" on the surface. Many customers are unaware of this phenomenon, so manufacturers should take the time to explain the cause clearly to avoid confusion and ensure customer satisfaction.
Third, issues in the oxidation process itself can also contribute to uneven coloring. For example:
1. Inadequate alkaline etching can leave behind original oxide layers and dirt, preventing proper surface preparation.
2. If the workpiece is not treated with light-emitting (anodizing) immediately after alkaline etching, residual alkali on the surface can interfere with the oxidation process.
3. Contamination from foreign objects during the transfer of the workpiece can also lead to uneven coloration.
When the oxide film shows uneven color, it's essential to investigate all possible causes and implement targeted solutions.
I have outlined the basic principles that lead to color inconsistencies in aluminum oxide films. I believe that technical staff at aluminum manufacturers already understand these concepts. With proper attention to these three key areas, most of the issues related to uneven coloration can be resolved effectively.
For example, one case involved a problem caused by excessive accumulation of aluminum ions in the alkaline etching solution. A technician from an aluminum plate manufacturer called in, asking why it was difficult to form a conductive oxide film after alkaline etching. They mentioned that the alkaline solution was very thick and not working well. At the time, I suggested replacing the solution, as prolonged use can lead to a buildup of aluminum ions, which are hard to remove from the surface. This can hinder the contact between the aluminum part and the oxidizing solution, affecting the formation of the oxide film.
Another suggestion was to rinse the workpiece in hot water immediately after rinsing in running water following alkaline etching, then treat it in concentrated nitric acid containing hydrofluoric acid, and finally perform anodic oxidation after thorough rinsing. The technician later reported that using hot water after etching made a significant difference. My experience has shown that after washing in hot water, the workpiece should be quickly transferred to running water to prevent oxidation during drying, which could otherwise affect the formation of the conductive oxide film.
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