Pre-treatment of aluminum and aluminum alloy parts before coating can enhance the bonding force between the coating and the workpiece. Pretreatment methods include chemical oxidation, anodization, and phosphating. The phosphate treatment of aluminum is similar to the phosphating treatment of steel parts, but an appropriate amount of fluoride should be added to the phosphating solution, and the main component of the obtained film is zinc phosphate. However, the phosphate film on the surface of aluminum and aluminum alloys has poor corrosion resistance and cannot be used for protection, but is used for the bottom layer of electrophoretic paint. Aluminum and aluminum alloys are relatively easy metals for phosphating, and the chromium phosphating method in which zinc-based phosphating and chromate-phosphate work together can be used. The two methods differ in solution characteristics and film properties, but the film formation process is very similar.
Phosphating treatment of titanium and titanium alloys
Titanium and titanium alloys are light in weight and high in mechanical strength, and have applications in industry, national defense, aviation, aerospace, etc., and are mainly used to manufacture shells and parts of various devices. Titanium and titanium alloys are easily oxidized to form a protective oxide film in the natural state, and the film structure is dense, so the bonding force when the organic coating is applied on the surface is poor. In addition, the natural oxide film on the surface of titanium and titanium alloys is thin, which is easily damaged during processing and transportation, resulting in more serious corrosion. In order to improve the wear resistance and corrosion resistance of titanium and titanium alloys, conversion coating treatment can be carried out on the surface, such as chemical oxidation, anodization, phosphating, etc. The phosphate film of titanium and titanium alloys can enhance the bonding force between the workpiece surface and the organic paint coating. In addition, the phosphate film on the surface of titanium and titanium alloy has a good lubricating effect, which can be used for stamping and drawing of magnesium alloy workpieces to achieve good lubrication and wear resistance. If it is simply used as a protective layer, it should be After the phosphating treatment, the phosphating layer is sealed, and the phosphatized parts can be immersed in oil or soap for a period of time to achieve the purpose of sealing.
Magnesium alloy phosphating treatment
Magnesium has relatively active chemical properties, poor corrosion resistance to the environment, and the surface is easily oxidized. It is often mixed with other metals to make magnesium alloys. The density of magnesium alloy is also small, the strength is high, and the mechanical properties are good. At the same time, it also has a good shielding effect on electromagnetic waves, which can effectively reduce noise and vibration. At present, magnesium alloys have been widely used in automobile manufacturing, electronic communication, decoration, household appliances, aviation, aerospace and other fields.
In order to improve the decoration of magnesium alloy products and enhance the corrosion resistance of magnesium alloys, it is necessary to carry out finishing and coating treatments on the surface, such as oxidation, passivation, phosphating, anodizing, etc., to form a conversion film, or to coat organic coating.
In general, the oxide film obtained by treating the magnesium alloy with chromate has good corrosion resistance; the phosphating film obtained after phosphating the magnesium alloy with phosphide salt has worse performance than that of the chromate film. , seldom used before. In recent years, as people pay more and more attention to environmental protection, the use of chromate has been restricted in various ways. Therefore, the use of phosphating treatment to replace chromate oxidation treatment is the key direction of the development of magnesium alloy surface treatment.
A. Phosphating formula and process conditions of magnesium alloys
Phosphate conversion coatings on the surface of magnesium alloys include phosphate/potassium permanganate, zinc phosphate, zinc-calcium phosphate, etc.
In addition to the chemical phosphating method to treat the magnesium alloy, the electrolytic phosphating method can also be used to obtain a better quality phosphating film. The formula and process conditions of its electrolytic phosphating solution are shown in Table 2-32.
B. Properties of Magnesium Alloy Films
Magnesium alloys are treated with phosphate conversion or phosphate/permanganate conversion. The biggest disadvantage is that the solution is consumed quickly, and the concentration and acidity of the phosphating solution need to be adjusted continuously. Therefore, the application of permanganate/phosphoric acid system greatly restricted.
Depending on the composition of phosphate, two different types of films are formed on the surface of magnesium and magnesium alloys when they are phosphated. When the alkali metal salt or ammonium salt of phosphoric acid is used as the treatment liquid, the phosphate obtained on the surface of the workpiece is an oxide film, that is, a phosphating conversion film. When the phosphating solution contains accelerators such as free phosphoric acid and dihydrogen phosphate (eg ), a film composed of divalent metal ion monohydrogen salt or orthophosphate can be obtained on the surface, which is called a phosphating pseudo-conversion film.