Valve corrosion and protection

1、 Corrosion and Protection of Closed Parts

Closed parts are often subjected to fluid erosion, which accelerates the development of corrosion. Some valve discs, although made of better materials, still corrode and damage faster than the valve body.

The upper and lower closing parts are usually connected with the valve stem and valve seat by threads. The connection is more oxygen deficient than ordinary parts, which is easy to form an oxygen Concentration cell, causing corrosion and damage. Some sealing surfaces of the closing elements are pressed in, and oxygen Concentration cell corrosion will also occur due to loose fit and slight gap.

Protective measures:

(1) Try to use corrosion-resistant materials. The weight of the closure component is very small, but it plays a crucial role in the valve. As long as it is corrosion-resistant, even using a little precious material is not harmful.

(2) Improve the structure of the closure component to minimize fluid erosion.

(3) The connection structure is improved to avoid the generation of oxygen Concentration cell.

(4) In valves below 200 ℃, polytetrafluoroethylene (PTFE) Thread seal tape is used as filler at the connection of closing parts and sealing faces to reduce the corrosion of these parts.

(5) While considering corrosion resistance, attention should also be paid to the erosion resistance of the closure material. Use materials with strong erosion resistance as closure components.

2、 Valve Stem Corrosion and Protection

The corrosion damage of the valve body is mainly caused by corrosive media; However, the corrosion situation of the valve stem is different, and its main problem is the packing. Not only can corrosive media cause damage to the valve stem, but general steam and water can also cause spots to appear at the contact between the valve stem and the packing. Especially problematic is that valves stored in warehouses can also experience valve stem pitting corrosion; Even stainless steel valve stems are difficult to avoid. Why is this? This is the electrochemical corrosion of the packing on the valve stem.

The current fillers used are asbestos based packing, such as graphite asbestos packing, oil immersed asbestos packing, and rubber asbestos packing. Asbestos materials contain certain chloride ions, in addition to potassium, sodium and magnesium plasma, and graphite contains impurities such as Iron(III) sulfide, which are all corrosion factors. Chloride ions can penetrate the passivation film on the metal surface, causing corrosion to continue. The presence of other impurities also contributes to the progress of corrosion. Water and steam are conductive substances that can further communicate with electrochemically corroded circuits. The valves in the warehouse also undergo electrochemical corrosion due to wet packing or humid air. In addition, there is a lack of oxygen between the packing and the valve stem. Compared with the surrounding areas, there is a difference in oxygen concentration, which constitutes an oxygen Concentration cell, another form of electrochemical corrosion.