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Chemical Conversion Film Technology for Metal Surface Treatment

🕙 2023-09-01 👆

Chemical conversion film technology is the process of forming a stable compound film layer on the metal surface through chemical or electrochemical means.

Chemical conversion film technology is mainly used for anti-corrosion and surface decoration of workpieces, and can also be used to improve the wear resistance of workpieces. It is the use of a certain metal to come into contact with a specific corrosive liquid, and under certain conditions, the two undergo a chemical reaction. Due to concentration polarization and cathodic and anodic polarization, a film of corrosion products with good adhesion and insoluble properties is formed on the metal surface. These film layers can protect the base metal from the influence of water and other corrosive media, and also improve the adhesion and aging resistance of organic coatings. In production, the conversion film technology mainly includes phosphating treatment and oxidation treatment.

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1 Phosphating treatment

Phosphatization is the process of placing steel materials in a phosphate solution to obtain a phosphate film that is insoluble in water.

The process of phosphating treatment for steel materials is as follows: chemical degreasing → hot water washing → cold water washing → phosphating treatment → cold water washing → post phosphating treatment → cold water washing → deionized water washing → drying.

The phosphating film is composed of iron phosphate, manganese phosphate, zinc phosphate, etc., and appears as gray white or gray black crystals. The membrane is firmly bonded to the base metal and has a high electrical resistivity. Compared with oxide film, phosphating film has higher corrosion resistance, especially in atmospheric, oily, and benzene media, but has poor corrosion resistance in acid, alkali, ammonia, seawater, and water vapor.

The main methods of phosphating treatment are immersion method, spray method, and immersion spray combination method. According to different solution temperatures, phosphating can be further divided into room temperature phosphating, medium temperature phosphating, and high-temperature phosphating.

The impregnation method is suitable for high-temperature, medium and low-temperature phosphating processes, which can handle workpieces of any shape and obtain phosphating films of different thicknesses. The equipment is simple and the quality is stable. Thick phosphating film is mainly used for anti-corrosion treatment of workpieces and enhancing surface friction reduction. The spray method is suitable for medium to low temperature phosphating processes, and can handle large workpieces such as car shells, refrigerators, washing machines, and other large workpieces as paint substrates and cold deformation processing. This method has a short processing time and fast film forming speed, but can only obtain thinner and medium thickness phosphating films.

2 Oxidation treatment

Oxidation treatment of steel

The oxidation treatment of steel, also known as blueing, involves placing steel workpieces into certain oxidizing solutions to form a surface thickness of approximately 0.5~1.5 μ The process method for producing dense and sturdy Fe3O4 thin films. Bluing usually does not affect the precision of parts and is commonly used for the decoration and protection of tools and instruments. It can improve the corrosion resistance of the workpiece surface, help eliminate residual stress of the workpiece, reduce deformation, and also make the surface shiny and beautiful. The alkaline method is most commonly used for oxidation treatment.

The composition and process conditions of the solution used for the oxidation treatment of steel can be determined based on the material and performance requirements of the workpiece. The commonly used solution consists of 500g/L sodium hydroxide, 200g/L sodium nitrite, and residual water. It is treated for 6-9 minutes at a solution temperature of around 140 ℃.

Oxidation treatment of aluminum and aluminum alloys

Anodic oxidation method

Anodic oxidation method is a method of placing the workpiece in an electrolyte and then applying electricity to obtain a high hardness and strong adsorption oxide film. Commonly used electrolytes include sulfuric acid with a concentration of 15% to 20%, chromic acid with a concentration of 3% to 10%, and oxalic acid with a concentration of 2% to 10%. The anodic oxide film can be boiled in hot water to turn it into aqueous alumina, which is sealed due to volume expansion. It can also be sealed by treating with potassium dichromate solution to prevent corrosive solution from corroding the substrate through the crystalline gap of the oxide film.

Chemical oxidation method

Chemical oxidation method is the method of placing the workpiece in a weak alkaline or weak acid solution to obtain a strong oxide film that binds to the substrate aluminum. Mainly used to improve the corrosion resistance and wear resistance of workpieces, as well as for surface decoration of aluminum and aluminum alloys, such as rust resistant aluminum for construction and decorative films for signs.