China Aluminum Network The quality of the aluminum sulfate anodized oxide film is crucial for its corrosion resistance and overall reliability. This 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, the sealing and filling processes are also essential to ensure long-term performance. To minimize anodizing failures and enhance product quality, it's important to pay attention to details and implement effective measures at every stage.
(1) Different types of aluminum alloys require specific pretreatment methods depending on their form, such as casting, rolling, machining, or welding. For example, non-machined surfaces of cast aluminum alloys should typically be sandblasted or shot-blasted to remove existing oxide layers and impurities like sand. For high-silicon aluminum alloys, especially cast ones, etching with a nitric acid-based solution containing about 5% hydrofluoric acid is recommended to activate the surface effectively and maintain good oxide film quality. It's generally not advisable to anodize different aluminum materials, such as pure aluminum parts or varying sizes, in the same tank.
When dealing with lapped, spot-welded, or riveted aluminum assemblies, certain parts may trap air during anodizing, making sulfuric acid anodizing unsuitable from a quality standpoint.
(2) The material used for mounting fixtures must ensure good electrical conductivity. Hard alloy rods are usually preferred. The panel should have enough elasticity and strength, while the hooks should be made of copper or copper alloys. Whether using special or general-purpose fixtures, it's essential to remove any oxide layer from the surface to ensure proper contact. Fixtures must provide sufficient contact area and minimize any impressions on the part. If the contact area is too small, it can lead to erosion of the anodized surface.
(3) The temperature of the sulfuric acid anodizing solution must be strictly controlled, ideally between 15°C and 25°C. Aeration with compressed air is often required, and refrigeration equipment is recommended. In cases where refrigeration is unavailable, adding 1.5% to 2.0% of organic acids like citric, oxalic, or lactic acid to the sulfuric acid electrolyte can help keep the temperature within an acceptable range, preventing issues like flaking of the oxide layer. Studies and practical experience have shown that adding a small amount of carboxylic acid or glycerin can reduce heat effects without compromising the oxide film’s thickness or hardness. This allows for higher temperatures while maintaining quality and improving production efficiency. Additionally, under constant temperature control, careful management of current density is essential to ensure consistent oxide film quality.
(4) Water used in the sulfuric acid anodizing electrolyte and the presence of harmful impurities must be strictly controlled. Tap water should not 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, along with other halogens, can damage the oxide film or even prevent its formation. Instead, softened water, deionized water, or distilled water should be used, with Clâ» levels below 15 mg/L and total mineral content under 50 mg/L.
During the anodizing process, sulfuric acid solutions may produce oily foam and suspended impurities, which should be periodically removed. Common harmful impurities include Cu²âº, Fe³âº, and Al³âº. If these exceed allowable limits, the solution may need partial or full 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 conditions and careful execution, the quality of the anodized oxide film can be fully guaranteed.
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