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1. Valve stem corrosion and protection
Main causes of valve stem corrosion:
The corrosion damage of the valve body is mainly caused by the corrosive medium, and the valve stem corrosion problem is mainly the packing.
Not only the corrosive medium causes corrosion damage to the valve stem, but also the steam and water can make the valve stem and the packing contact spot. In particular, valves kept in warehouses can also suffer from stem point corrosion. This is the electrochemical corrosion of the packing on the valve stem.
Now the most widely used filler is asbestos-based packing, asbestos material contains a certain amount of chloride ions, in addition to potassium, sodium, magnesium plasma, these are corrosion factors.
Valve stem anticorrosion precautions:
Do not fill the valve during storage. Without packing, it loses the factor of electrochemical corrosion of the valve stem, and can be preserved for a long time without being corroded.
Surface treatment of the stem. Such as chrome plating, nickel plating, nitriding, boriding, zinc and so on. Reduce asbestos impurities. The method of washing with distilled water can reduce the chlorine content of asbestos, thus reducing its corrosiveness. Add corrosion inhibitor to asbestos packing. This corrosion inhibitor inhibits the corrosion of chloride ions. Such as sodium nitrite. Adding sacrificial metals to asbestos. This is a metal with a lower potential than the valve stem to be sacrificed. In this way, corrosion of chloride ions first occurs on the sacrificial metal, thereby protecting the valve stem. Zinc powder can be used as a sacrifice metal. Protected by polytetrafluoroethylene. Polytetrafluoroethylene has excellent chemical stability and dielectric properties, the current can not pass through, such as impregnating asbestos disc polytetrafluoroethylene, corrosion will be reduced. Asbestos packing can also be wrapped with PTFE raw tape and then loaded into the stuffing box. Improve the finish of processing, but also reduce electrochemical corrosion.
2. Corrosion and protection of closing parts
Main reasons for corrosion of closing parts:
The closing parts are often washed by fluids, which accelerates the development of corrosion. Some valves, although the use of better materials, but corrosion damage is still faster than the valve body.
The upper and lower closing parts are commonly threaded with the valve stem and seat, and the connection point is hypoxic than the general part, which is easy to form an oxygen concentration battery and cause corrosion damage. Some closure sealing surface adopts the form of pressing, due to the lack of tight coordination, a little gap, will also occur oxygen concentration battery corrosion.
Closing parts anticorrosion precautions:
Try to use corrosion-resistant materials. The weight of the closure is small, but it plays a key role in the valve, as long as it can resist corrosion, even if a little valuable material is used. Improve the structure of the closing parts to make them less vulnerable to fluid erosion. Improved connection structure to avoid oxygen concentration battery. In the valve below 200℃, the connection of the closing part and the sealing surface are connected, and the use of polytetrafluoroethylene raw material belt as a filler can reduce the corrosion of these parts. While considering corrosion resistance, it is also necessary to pay attention to the erosion resistance of the closing materials, and use materials with strong erosion resistance as closing parts.