The corrosion of the valve is usually understood as the damage of the valve metal material under the action of the chemical or electrochemical environment. Since the phenomenon of “corrosion” occurs in the spontaneous interaction between metal and the surrounding environment, how to isolate the metal from the surrounding environment or use more non-metallic composite materials is the focus of anti-corrosion. The valve body (including the bonnet) of the valve occupies most of the weight of the valve and is in constant contact with the medium, so the selection of the valve often starts from the valve body material.
There are only two forms of corrosion of the valve body, namely chemical corrosion and electrochemical corrosion. Its corrosion rate depends on the temperature, pressure, chemical properties of the medium and the corrosion resistance of the valve body material. The corrosion rate can be divided into six grades:
1. Complete corrosion resistance: the corrosion rate is lower than 0.001 mm/year;
2. Extremely corrosion-resistant: the corrosion rate is 0.001 to 0.01 mm/year;
3. Corrosion resistance: the corrosion rate is 0.01 to 0.1 mm/year;
4. Still corrosion resistance: the corrosion rate is 0.1 to 1.0 mm/year;
5. Poor corrosion resistance: the corrosion rate is 1.0 to 10 mm/year;
6. No corrosion resistance: the corrosion rate is greater than 10 mm/year.
How to prevent valve corrosion?
The anti-corrosion of the valve body mainly depends on the correct selection of materials. Although the anti-corrosion information is very rich, it is not easy to choose the right one, because the problem of corrosion is very complicated. For example, sulfuric acid is very corrosive to steel when the concentration is low. Passivation film, can prevent corrosion; hydrogen only shows strong corrosiveness to steel under high temperature and high pressure, chlorine gas is not very corrosive when it is in a dry state, but it is very corrosive when there is a certain humidity, and many materials cannot be used. . The difficulty in choosing valve body materials is that not only the corrosion problem, but also the pressure and temperature resistance capability, whether it is economically reasonable, and whether it is easy to purchase must be considered. So you have to be careful. First, the second is to take lining measures, such as lining lead, lining aluminum, lining engineering plastics, lining natural rubber and various synthetic rubbers. If the media conditions permit, this is an economical method.
Second, once again, in the case of low pressure and temperature, the main material of fluorine-lined valve can often be very effective in preventing corrosion.
3. In addition, the outer surface of the valve is also corroded by the atmosphere. Generally, steel materials are protected by painting.