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What are the surface treatment processes for aluminum alloys and aluminum profiles?

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2019-09-25 15:28

What are the surface treatment processes for aluminum alloys and aluminum profiles?

Aluminum alloys and profiles usually require surface treatments to meet various needs. Common surface treatments for aluminum alloys include electroplating, spraying, brushing, anodizing, sandblasting, passivation, polishing, and oxide film treatment.

  1. Sandblasting: The primary function is surface cleaning. Sandblasting before coating (painting or plastic spraying) can increase surface roughness and enhance adhesion to some extent. However, its contribution is limited and less effective than chemical pre-treatment.
  2. Coloring: There are two main processes for coloring aluminum: anodic coloring and electrolytic coloring. This process creates various colors on the oxide film to meet specific requirements. For example, optical instrument parts are often colored black, while commemorative medals are colored gold.
  3. Conductive Oxidation (Chromate Conversion Coating): Used in situations where both protection and conductivity are required.
  4. Chemical Oxidation: The oxide film formed is relatively thin, about 0.5 to 4 microns, porous, and soft, with good adsorption properties. It can serve as a base layer for organic coatings. However, its wear resistance and corrosion resistance are inferior to those of anodic oxide films. Chemical oxidation processes for aluminum and aluminum alloys can be classified into alkaline and acidic oxidation methods based on the nature of the solution. According to the film properties, they can be divided into oxide films, phosphate films, chromate films, and chromic-phosphate films.
  5. Electrochemical Oxidation: This process involves simpler equipment, convenient operation, and high production efficiency. It does not consume electrical energy and has a wide application range, unaffected by the size and shape of parts. The oxide film thickness ranges from 5 to 20 microns (with hard anodic oxide films reaching up to 60 to 200 microns). It has high hardness, good heat resistance, insulation properties, and higher corrosion resistance than chemical oxide films. The porous structure offers excellent adsorption capacity.
  6. Spraying: Used for external protection and decoration of equipment, typically applied over an oxide layer. Proper pre-treatment is required to ensure strong adhesion between the coating and the workpiece. There are three common methods:
  7. Phosphating (phosphate method)
  8. Chromating (chromate-free chromating)
  9. Chemical oxidation.
  10. Anodizing: This process uses electrolysis to coat a thin layer of another metal or alloy on the surface of a specific metal. Brush plating is used for localized plating or repairs, while barrel plating is suitable for small items like fasteners, washers, and pins. Electroplating provides decorative, protective, and functional surface layers on mechanical products. It can also repair worn or incorrectly processed parts. Electroplating solutions include acidic, alkaline, and those containing complexing agents in acidic and neutral solutions. Regardless of the plating method, plating tanks and fixtures that contact the workpiece and plating solution should have a certain degree of versatility.
  11. Chemical Polishing: This method uses selective self-dissolution of aluminum and aluminum alloys in an acidic or alkaline electrolyte solution to level and polish the surface, reducing its roughness. This process features simple equipment, no power requirement, unrestricted by the shape and size of the workpiece, high polishing speed, and low processing costs. The purity of aluminum and aluminum alloys significantly affects the quality of chemical polishing. The higher the purity, the better the polishing quality, and vice versa.
  12. Passivation: This process converts the metal surface into a state that is not easily oxidized, thereby slowing down the corrosion rate. An active metal or alloy, whose chemical activity is significantly reduced to resemble a noble metal state, is said to be passivated. When a dense film (often invisible) forms on the metal surface due to corrosion products, it alters the surface state, causing a significant positive shift in the metal's electrode potential, making it corrosion-resistant. This state is called the passivation state. For example, when Fe→Fe++ with a standard electrode potential of -0.44V is passivated, it shifts to +0.5 to 1V, exhibiting the properties of a corrosion-resistant noble metal. This film is known as a passivation film.

If the corrosion products generated by the metal due to the action of the medium have a dense structure and form a thin film (often invisible), which closely covers the surface of the metal, the surface state of the metal is changed, and the electrode potential of the metal is greatly changed to the positive direction. Jump, and become a passive state of corrosion resistance. For example, when Fe→Fe, the standard potential is -0.44V, and it jumps to 0.5~1V after passivation, showing corrosion-resistant precious metal properties. This thin film is called passivation film.

Aluminum anodizing process, metal surface treatment, aluminum alloy, electroplating, spraying, wire drawing, anode, sandblasting, passivation, polishing