Technical Articles
Nine Anti-Corrosion Measures of Valves (Part Three)
Posted: 10/23/2017 15:08:45  Hits: 13
5. Spraying paint
Paint is most widely used for the corrosion resistant of valves, which is an indispensable anti-corrosion materials for valve products.

Paint belongs to the non-metallic material, which is usually made from synthetic resin, rubber slurry, vegetable oil, solvent, etc., covering the surface of the metal to isolate media, the air, etc. so as to protect the valve from corrosion. The spraying paint method is mainly used for the not too strong corrosive environments such as water, salt water, sea water, etc. The valve cavity commonly is sprayed with paint to prevent corrosion of water, air and other media. Different colors mixed with paint refers to the different materials the valves are made of. Valves should be sprayed with paint half a year or once a year.

6. Adding corrosion inhibitors
The mechanism for corrosion inhibitors to control corrosion is that corrosion inhibitors promote the polarization of cells. Corrosion inhibitors are mainly used for the medium and packing. The addition of corrosion inhibitor in the medium can slow down the corrosion on the equipment or the valve. For example, chromium nickel stainless steel will be seriously corroded if it is in the non-oxygenated sulfuric acid and with great solubility. But if a small amount of oxidants such as copper sulfate or nitric acid are added to the non-oxygenated sulfuric acid, the stainless steel will be changed to stainless steel and have a protective film on its surface to prevent the corrosion of the medium. Adding a small amount of oxidant to hydrochloric acid can reduce the corrosion of titanium.

Water is commonly used as the medium for the pressure test of the valve, which will easily cause the corrosion on the valve. Adding a small amount of sodium nitrite to water can prevent the corrosion of water on the valve.

Asbestos packing contains chloride, which can cause great damage to the valve stem.
Although the distilled water washing method can be used to reduce the chloride content in the asbestos packing, there will be great difficulties in the implementation of this method, which makes this method can not be widely adopted and is only used for special needs.

For the valve stem with the asbestos packing, it can be coated with the corrosion inhibitors and sacrificial metals in order to prevent the corrosion on the asbestos packing. Corrosion inhibitors such as sodium nitrite and sodium chromate can make the passive film form on the surface of the valve stem, which can improve the corrosion resistance of the valve stem.

Zinc can be added to asbestos as the sacrificial metal. Actually, zinc is also a kind of corrosion inhibitor, and it can combine with the chloride in asbestos, which will reduce the chance for the chloride to have any contact with the metal of the valve stem so as to achieve the purpose of anti-corrosion. If the paint mixed with the corrosion inhibitors such as the lead oxide is sprayed on the surface of the valve, it can prevent the valve from corrosion.