The heat-treated steel parts are soaked in a phosphate solution to form a phosphate film on the metal surface. This process is called phosphating. After phosphating treatment, the surface of the fastener is black and dark gray, and the thickness of the surface film is about 5-15μm, and the size of the workpiece is not changed. Its corrosion resistance is better than that of blackening treatment.
Phosphating film has a better protective effect on fasteners, and the corrosion resistance can be further improved by passivation or sealing treatment.
Phosphating at room temperature is 20~40℃, low chemical consumption, energy saving, low cost, stable solution, no pollution to the environment, good bonding force between phosphating film and paint film, it is a good bottom layer for paint coating. The following is a brief introduction to the tips of rapid phosphating.
1. Selection of phosphating solution group
Industrial phosphoric acid (H3PO4) 8%~10%, add 823-A phosphating agent 1%~1.5%, and the rest water. The orthogonal test method is used to optimize the combination to prepare various solutions with different contents to obtain the best components of high-quality membranes.
2. The impact of pretreatment
Experiments show that pretreatment has a great influence on the film formation process and quality. In addition to degreasing purification, rust removal and water washing, it is also activated, that is, immersed in 3~5g/L oxalic acid aqueous solution for 1 minute, so as to lay the foundation for the formation of a uniform, dense, and firmly bonded phosphate film.
3. The ratio of solution acidity
The ratio of the free acidity of the solution to the total acidity is 1: (20-30). The general rule is that the lower the phosphating temperature, the higher the acid ratio, which is equivalent to an increase in pH. The minimum principle of selection is: phosphating film can be formed at a lower temperature, increasing the total acidity can accelerate the phosphating reaction, making the film thin and detailed, the total acidity is too high, and the film layer is often too thin; the total acidity is too low , Phosphating speed is slow, and the film is thick and rough.
4. The influence of phosphating time and temperature
In the case of a certain phosphating temperature, the longer the phosphating time, the thicker the film, and the poorer the bond with the substrate and paint film; on the contrary, the film is too thin, although the bond with the substrate and paint film is good, But the corrosion resistance is poor. Therefore, it is advisable to control the time within 20-30min, and it will vary with different room temperature and the age of the phosphating solution.
Increasing the temperature can speed up the phosphating speed and improve the adhesion, hardness, corrosion resistance and heat resistance of the phosphating film. During operation, prevent the temperature from becoming too high. When the temperature is higher than 50℃, it is easy to cause the decomposition of nitrate. In order to prevent local overheating, steam heating should be selected. If a heater is used, it should be placed on the two walls or around the tank. Do not install it at the bottom of the tank. The heater cannot use aluminum or copper pipes, preferably stainless steel or iron pipes.
Medium-carbon steel and medium-carbon alloy steel parts are easier to phosphate, and the phosphating film is black and thick; the color of the phosphating film of low-carbon steel parts is lighter and the crystals are dense. In addition, the uneven phosphate layer, blooming and other phenomena are caused by the following factors: (1) The oil is not clean and the temperature is too low; (2) The surface of the part has a passivation state, strengthen pickling or sandblasting; (3) The parts are processed by different heat treatment methods.