China Aluminum Network The quality of the anodized oxide film formed on aluminum sulfate is crucial for its corrosion resistance and overall performance. This reliability depends on several factors, including the composition of the aluminum alloy, the thickness of the film, and the conditions during the anodizing process such as temperature, current density, and water quality. After anodization, a filling and sealing process is typically carried out to enhance durability. To minimize defects and improve product quality, it's essential to focus on small details and implement effective measures.
(1) Different aluminum alloys require specific pretreatment methods depending on their form—whether cast, rolled, machined, or welded. For example, non-machined surfaces of cast aluminum should generally be sandblasted or shot-blasted to remove the existing oxide layer and any residual sand. For high-silicon aluminum alloys, especially cast types, etching with a nitric acid-based solution containing about 5% hydrofluoric acid is recommended to maintain a clean, active surface and ensure a high-quality oxide layer. It’s also important to note that different aluminum materials, bare aluminum parts, or varying sizes of aluminum components are usually not suitable for anodizing in the same tank.
Assemblies made from lapped, spot-welded, or riveted aluminum parts may face issues during anodizing due to trapped air bubbles, which can lead to poor results. In such cases, sulfuric acid anodizing is often avoided to maintain quality standards.
(2) The materials used for mounting fixtures must provide good electrical conductivity. Hard alloy rods are commonly used, while the panel needs to have sufficient elasticity and strength. Hooks should be made of copper or copper alloys. Reused fixtures must be thoroughly cleaned to remove any oxide layers on their surfaces to ensure proper contact. Fixtures should not only offer enough conductive area but also minimize visible marks on the parts. If the contact area is too small, it could cause erosion on the anodized surface.
(3) Temperature control is critical in sulfuric acid anodizing, with an ideal range between 15°C and 25°C. Aeration using compressed air is necessary, and refrigeration equipment is usually required. If no refrigeration is available, adding 1.5% to 2.0% of tricarboxylic acids like oxalic or lactic acid to the electrolyte can help manage higher temperatures up to 35°C, preventing the oxide film from becoming brittle or powdery. Studies and practical applications have shown that adding carboxylic acids or glycerin to the sulfuric acid solution can reduce heat-related damage without compromising the oxide film’s thickness or hardness. This allows for higher operating temperatures while maintaining quality and improving production efficiency. Additionally, consistent temperature control must be paired with careful management of current density to ensure optimal oxide film quality.
(4) Water quality and impurities in the electrolyte play a significant role in the anodizing process. Tap water should never be used, especially if it contains high levels of Ca²âº, Mg²âº, SiO₃²â», or Clâ». Even a Clâ» concentration of 25 mg/L can negatively affect the anodizing process. Chloride ions can damage the oxide film, leading to defects. Instead, softened, deionized, or distilled water should be used, with Clâ» levels kept below 15 mg/L and total mineral content under 50 mg/L.
During the anodizing process, the sulfuric acid solution may develop oily foam and suspended impurities, which should be removed regularly. Common harmful impurities include Cu²âº, Fe³âº, and Al³âº. If their concentrations exceed acceptable limits, the solution may need partial or complete replacement to maintain the quality of the anodized aluminum sulfate.
Sulfuric acid anodizing is a widely used and well-established method for corrosion protection and surface decoration. With strict adherence to process parameters and careful execution, the quality of the anodic oxide film can be reliably ensured.
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