Corrosion on valve body is considered that the metal materials are destructed under the environment of chemical or electrochemical environment. The corrosion is caused by spontaneous interaction between metal and surroundings. So, isolating metal from surroundings or using more nonmetal synthetic materials is the key of corrosion protection.
Valve body including bonnet account the most of weight of valve
and usually contact with medium. Therefore, choosing valve starts from the materials of valve body.
Corrosion on valve body is classified into two types: chemical corrosion and electrochemical corrosion. Corrosion rate depends on temperature, pressure and chemical properties of media and corrosion resistance of body materials. Corrosion rate is classified into 6 classes: completely corrosion resistance, extremely corrosion resistance, corrosion resistance, still corrosion resistance, poor corrosion resistance and no corrosion resistance.
Corrosion protection on valve body mainly accounts on correct materials. Although corrosion resistance materials are quite abundant, it is still difficult to choose proper materials. Corrosion is very complicated. For example, sulfuric acid having low concentration has strong corrosion on steel material. When it has high concentration, it can cause passive film on steel material which has corrosion resistance. Hydrogen only has strong corrosion on steel material under high temperature and pressure. Chlorine has poor corrosion under desiccation while has strong corrosion under definite humidity. For this, many materials cannot be used. The difficulty of body materials is not only considering corrosion, but also considering pressure resistance and temperature resistance.
Secondly, lining measures can be taken such as lean(lead), aluminum, engineering plastics, natural rubbers and synthetic rubbers. It is a kind of economical methods if conditions of media permit.
Lastly, under low pressure and temperature, using nonmetal as main materials can prevent corrosion effectively.
What’s more, outside surface of valve body is corroded by atmosphere. In general, steel material is painted for protection.
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