Metal surface conversion coating technology
Metal surface conversion coating technology involves chemically or electrochemically reacting the metal surface with a specific treatment solution, producing a thin compound film (conversion film) that bonds securely to the substrate. Conversion coatings are typically composed of metal oxides, hydroxides, phosphates, chromates, and silicates. They enhance the corrosion resistance, wear resistance, and decorative properties of the metal surface, as well as strengthen adhesion to subsequent coatings. Based on their chemical composition, conversion coatings can be categorized as oxide, phosphate, chromate, and silicate conversion films. These films are widely used in the automotive, machinery, electronics, aerospace, and other fields.
The formation mechanism of metal surface conversion films varies depending on the treatment method and type of metal, but generally involves the following steps: First, the treatment solution reacts chemically with the metal surface, dissolving the surface metal atoms; then, the dissolved metal ions combine with anions in the treatment solution (such as phosphate, chromate, and silicate) to form insoluble compounds; finally, these compounds gradually deposit on the metal surface, forming a continuous, uniform conversion film. For example, during the phosphating of steel, phosphoric acid reacts with iron to form iron phosphate, which gradually forms a phosphate film on the surface; while the anodization of aluminum uses electrolysis to form a dense aluminum oxide film on the aluminum surface.
There are numerous processes for metal surface conversion coatings, including oxidation, phosphating, chromating, and passivation. Oxidation treatments are categorized into chemical oxidation and electrochemical oxidation (anodizing). Anodizing is the most widely used method for aluminum and aluminum alloys, producing an aluminum oxide film with a thickness of 5-20μm. After dyeing and sealing, it exhibits excellent corrosion resistance and decorative properties, making it commonly used in architectural profiles and automotive parts. Phosphating is primarily used on steel surfaces. The resulting phosphate film is porous and can absorb lubricants or coatings, improving workpiece wear resistance and paint adhesion. It is a key process in automotive pre-painting.
Chromate conversion coating treatment was once widely used on metal surfaces such as zinc, cadmium, and aluminum. The resulting film layer has excellent corrosion resistance and self-healing ability. However, due to the toxicity and carcinogenicity of hexavalent chromium, this process is gradually being replaced by environmentally friendly processes. Passivation treatment is to form a thin and dense oxide film on the surface of the metal coating, such as passivation treatment after galvanizing, which can significantly improve the corrosion resistance of the galvanized layer. In addition, new environmentally friendly conversion film technologies such as silicate conversion film and rare earth conversion film are being gradually promoted and applied.
The development trend of metal surface conversion film technology is environmental protection, functionalization and composite. With increasingly stringent environmental regulations, conversion coating technologies that do not contain harmful substances such as chromium and phosphorus have become a research hotspot, such as silane treatment and titanium zirconium treatment. These processes are gradually replacing traditional phosphating and chromate treatments with their low toxicity and environmental protection characteristics. The research and development of functional conversion films (such as conversion films with antibacterial, conductive, thermal insulation and other functions) has expanded its application areas. Composite conversion coating technology (such as phosphating and silane composite treatment) combines the advantages of different conversion coatings and can significantly improve the overall performance of metal surfaces. In the future, metal surface conversion film technology will continue to improve film performance and process efficiency while ensuring environmental performance, providing more advanced technical support for the surface protection of metal materials.
