Corrosion And Protection Of Valve Stem

The corrosion damage of the industrial valve shell is mainly caused by the corrosive medium. However, the corrosion situation of the valve stem is different, and its main problem is the packing. Not only corrosive media can cause corrosion and damage to the valve stem, but also general steam

Water can also cause spots on the contact between the valve stem and the packing. Particularly problematic is that pitting corrosion of the valve stem will occur in the valves stored in the warehouse, even the stainless steel stem is unavoidable. This is the filler

Electrochemical corrosion of the valve stem.

Now the most widely used packing is asbestos-based packing, such as graphite asbestos packing, oil-impregnated asbestos packing, and rubber asbestos packing.Asbestos materials contain a certain amount of ammonia ions, in addition to potassium, sodium, and magnesium ions, and graphite contains impurities such as iron sulfide, which are all corrosive factors. Ammonia ions can penetrate the passivation of metal surfaces

Membrane, so that the corrosion continues. 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 because of the filler tide, humidity or humid air will cause electrochemical corrosion. In addition, there is a lack of oxygen between the material and the valve stem. Compared with the surrounding environment, there is a difference in oxygen concentration. This constitutes an oxygen concentration difference battery and becomes another kind of battery.

A form of electrochemical corrosion.

As we all know, the valve stem is often made of high-quality materials, its corrosion damage is a pity, and affect production, so we must take anti-corrosion measures.

The measures that can be taken are as follows.

① Do not add packing during the preservation of the valve. Without packing, the electrochemical corrosion factor of the valve stem is lost, and it can be stored for a long time without being corroded.

②Surface treatment of the valve stem. Such as chromium plating, nickel plating, nitriding, boronizing, zincing and so on.

③Reduce asbestos impurities. The method of washing with distilled water can reduce the ammonia content in asbestos, thereby reducing its corrosiveness.

④ Add corrosion inhibitor to asbestos packing. This corrosion inhibitor can inhibit the corrosiveness of ammonia ions, such as sodium nitrite.

⑤Add sacrificial metal to asbestos. It is to use a metal with a lower potential than the valve stem as the victim, so that the corrosion of ammonia ions will first occur to the sacrificial metal, thereby protecting the valve stem. Can be sacrificed metallic zinc powder and so on.

⑥Use Teflon protection. Polytetrane ethylene has excellent chemical stability and dielectric properties, and current cannot pass. If asbestos packing is impregnated with polytetrane ethylene in combination, the corrosion will be reduced.

The asbestos packing can also be wrapped with Teflon raw material tape, and then loaded into the material box.