Valve Corrosion (2)

Valve parts anti-corrosion precautions

1. Stem corrosion and protection

Main cause of stem corrosion
  Corrosion damage of the valve body is mainly caused by corrosive media, and the problem of valve stem corrosion is mainly filler.

  Not only does the corrosive medium corrode the stem, but steam and water can also cause spots in the stem to contact the packing. Especially for valves stored in warehouses, stem corrosion can occur. This is the electrochemical corrosion of the filler to the valve stem.

  The most widely used filler is the asbestos-based packing. The asbestos material contains a certain amount of chloride ions, in addition to potassium, sodium and magnesium. These are all corrosive factors.


Valve stem anti-corrosion precautions

  Do not add filler during valve storage. Without packing, it loses the electrochemical corrosion of the valve stem and can be stored for a long time without being corroded.

  The stem is surface treated. Such as chrome plating, nickel plating, nitriding, boronizing, zinc and so on.

  Reduce asbestos impurities. Washing with distilled water can reduce the chlorine content in asbestos and reduce its corrosivity.

  Add a corrosion inhibitor to the asbestos packing. This corrosion inhibitor suppresses the corrosiveness of chloride ions. Such as sodium nitrite.

  Add sacrificial metal to asbestos. This is a metal that is lower than the stem potential as a victim. This corrosion of chloride ions first occurs on the sacrificial metal, thereby protecting the valve stem. It can be used as a sacrificial metal such as zinc powder.

  Protected with Teflon. Polytetrafluoroethylene has excellent chemical stability and dielectric properties, and current cannot pass. If the asbestos packing is impregnated with polytetrafluoroethylene, the corrosion will be reduced. The asbestos packing can also be wrapped with a Teflon tape and then filled with a stuffing box.

  Improve processing smoothness and also reduce electrochemical corrosion.


2. Closed parts corrosion and protection

The main reason for the corrosion of the closure
  The closure is often flushed with fluid, allowing corrosion to accelerate. Some discs, although using better materials, are still more corrosive than the valve body.

  The upper and lower closing members are usually threaded with the valve stem and the valve seat, and the joint is less oxygen-poor than the general part, which easily constitutes an oxygen concentration battery, causing corrosion damage.

  Some sealing parts are pressed into the sealing surface. Owing to the tightness of the fitting, there is a slight gap, and oxygen concentration battery corrosion may occur.

Close parts anti-corrosion precautions

  Use corrosion resistant materials as much as possible. The closing piece has a small weight, but plays a key role in the valve, as long as it is resistant to corrosion, even with a little expensive material.

  Improve the closure structure so that it is less subject to fluid erosion. Improve the connection structure to avoid the generation of oxygen concentration batteries.

  In valves below 200 ° C, the connection between the closing part and the sealing surface, using the Teflon raw material tape as a filler, can reduce the corrosion of these parts.

  While considering corrosion resistance, attention should also be paid to the erosion resistance of the closure material. Use a material that is resistant to erosion as a closure.