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9 Anti-Corrosion Measures for Industrial Valve

September 17, 2020

Valve corrosion is commonly viewed as damage to the metal material of the valve under the effect of a chemical or electrochemical environment. Since the “corrosion” occurs in the spontaneous reaction between the metal and the surrounding environment, the prevention of corrosion focuses on how to separate the metal from the surrounding environment or use more non-metallic synthetic materials. Here we are going to discuss ways to prevent the valve from corrosion

1. Electrochemical Protection: Both anodic and cathodic protection provide electrochemical protection. The so-called anode protection is to add an external direct current to protect the metal so that the potential for the anode increases in a positive direction. When raised to a certain point, a dense protective film is created on the metal anode surface, which is a passivation film. Metal cathode corrosion is reduced significantly. The anodic safety is for easily passivated materials.

Cathodic protection means the protected metal is being used as a cathode, and a direct current is applied in a negative direction to lower the potential. When a certain potential value is reached, the current corrosion speed is decreased, and now the metal is protected. Additionally, protected metal can be protected by cathodic protection with a metal having a higher electrode potential than protected metal. Zinc is corroded, and zinc is considered sacrificial metal if zinc is used to preserve iron. This cathodic protection method is used by large-scale valves and essential valves, which is an economical, simple, and effective process.

2. Use non-metallic material: Non-metallic corrosion resistance is good as long as the valve temperature and pressure match non-metallic material specifications, it will not only solve the corrosion issue but also save precious metals. The valve body, valve cover, lining, sealing surface and the rest are usually made from non-metallic materials. The filler for the gasket is made primarily of non-metallic materials. The valve is coated with plastics such as polytetrafluoroethylene or chlorinated polyether, and rubber, like natural rubber, neoprene, or nitrile, and the valve body and cover are made of standard cast iron and carbon steel. Nowadays, plastics like nylon and polytetrafluoroethylene are more commonly used and different sealing surfaces are made of natural and synthetic rubber. Sealing rings are used for different valve types. These non-metal materials used as sealing surfaces not only have good resistance to corrosion but also good sealing efficiency and are especially suitable for use in granular media. Of course, their strength and thermal resistance are poor and the range of applications is small. Flexible graphite has allowed non-metals to reach the high-temperature market, solve the long-term problem of filler and gasket leakage, and is a good high-temperature lubricant.

3. Metal surface treatment: Metal surface treatment processes include plating, surface penetration, passivation of surface oxidation, etc. It’s aim is to improve the resistance of metals to corrosion and improve the mechanical energy of metals. Surface treatments are commonly used in the field of the valve. The problem of anti-corrosion valve stems is a problem that people pay attention to, and people have obtained rich manufacturing experience. It also uses surface treatment methods such as nitriding, boronizing, chrome plating, and nickel plating to enhance corrosion resistance, corrosion resistance, and resistance to abrasion. The different surface treatment will be suitable for various valve stem materials and working environments. Valve stems in contact with air, water vapor medium, and asbestos packaging may be hard chrome-plated and gas nitride (stainless steel is not suitable for ion nitride); The small-diameter valve body and handwheel are often also chrome-plated to enhance their corrosion resistance and decorate the valve.

4. Choose corrosion-resistant materials according to the corrosive medium: The corrosion of the material is quite complicated in actual processing. Even if we’re using valve material for the same medium, the strength, temperature, and pressure of the liquid will be different and the corrosion of the medium to the substrate will be different. The rate of corrosion increases by around 1 to 3 times for every 10 ° C rise in the temperature of the material. The intensity of the medium has a major effect on the degradation of the valve material. As the concentration is increased to more than 6%, the corrosion drops significantly. Aluminum, for example, is extremely corrosive in concentrated nitric acid with a concentration of 80 percent or more but is heavily corroded at medium and low concentration of nitric acid. While stainless steel is resistant to dilute nitric acid, corrosion is more severe in more than 95% of concentrated nitric acid. It can be seen from the examples above that the right selection of valve materials will be based on particular conditions. Various corrosion factors should be evaluated and materials chosen according to the applicable anti-corrosion manual.

5. Spray coating: Coating is the most commonly used anti-corrosion process and is an important anti-corrosion tool and identification feature for valve products. Coatings are mostly non-metallic content. They are typically made of synthetic resin, rubber slurry, vegetable oil, solvent, etc., covering the metal surface, isolating the material, and the environment for anti-corrosion purposes. Coatings are primarily used in conditions where corrosion is not serious, such as water, salt water, seawater, or atmospheric. The interior of the valve is normally coated with anti-corrosive paint to prevent water, air, and other media from corroding the valve. The paint is combined with a variety of colors to reflect the materials used by Faine. The valve can generally be coated or painted in half a year or once a year.

6. Add corrosion inhibitor: The addition of other special substances to corrosive media and corrosives will significantly slow down the rate of metal corrosion. This unique material is known as a corrosion inhibitor. Corrosion inhibitors are commonly used for media and packaging. The addition of a corrosion inhibitor to the medium can slow down the corrosion of equipment and valves. For example, in the oxygen-free sulfuric acid, chrome-nickel stainless steel is ignited in a wide range of solubility. Corrosion is very dangerous. However, if a small number of oxidants is applied, such as copper sulphate or nitric acid, the stainless steel may be converted into a passive state and a protective coating is formed on the surface to avoid the etching of the material. The valve pressure test generally uses water as a pressure test medium, which can easily cause valve corrosion. Adding a small amount of sodium nitrite to water will prevent the valve from being corroded by the water. The asbestos filler contains chloride, which is highly corrosive to the stem of the valve. The corrosion inhibitor is composed of sodium nitrite and sodium chromate to form a passivation film on the valve stem’s surface to improve the valve stem’s corrosion resistance. The solvent can slowly dissolve the inhibitor of corrosion and may provide lubrication. As sacrificial metal zinc powder is applied to asbestos. In reality, zinc is also an inhibitor of corrosion. First, it can be mixed with chloride in asbestos, thus significantly decreasing the risk of interaction between chloride and stem metal, thereby serving anti-corrosion purposes. If the coating is reinforced with a corrosion inhibitor such as red dan or calcium lead, the valve surface can prevent atmospheric corrosion.

7. Thermal spraying is a form of process block for coating preparation and has become one of the latest material surface protection technologies. It uses a heat source of high energy density (carbon combustion flame, flare, plasma arc, electric blast, gas explosion, etc.) to blast and melt the metal or non-metal substance and then spray it onto the pretreated basic surface in an atomized form to form a sprayed sheet, or at the same time heat the base surface to re-melt the coating on the surface of the substrate to form a spray-welded sheet.

By using one or more thermal spray processes, most metals and their alloys, metal oxide ceramics, cermet composites, and hard metal compounds may be sprayed onto a metal or non-metal substrate.

Thermal spraying will increase the resistance of the surface to corrosion, wear resistance, high-temperature resistance, and other properties and extend service life. Specially functioned thermal spray coating has unique properties such as heat insulation, isolation (or isoelectric), grinding seal, self-lubrication, heat emission, electromagnetic shielding. Thermal spraying is used to patch sections.

8. Control the corrosive environment: There are broad and narrow senses about the so-called environment. In a general sense, it refers to the atmosphere around the installation of the valve and its medium of internal circulation; in a narrow sense, the environment refers to the conditions around the installation of the valve. Most conditions are uncontrollable, and there are no improvements to the manufacturing processes. The method of regulating the atmosphere, such as boiler water deoxidation and the pH of the alkali in the refining process, may only be used when there is no damage to the product or process. The above-mentioned addition of corrosion inhibitors, electrochemical safety, etc. also belongs from this point of view to controlling corrosion environments. The environment is full of dust, smoke, and water vapors. It can cause varying degrees of corrosion on the valve, especially in the production setting, such as smoke and halogen, toxic gases, and fine powder emitted from equipment. The operator should periodically clean and purge the valve and refill regularly in the operating procedures as specified by the regulations. This is an important measure for managing corrosion inside the system. The valve stem is mounted with a protective cover, the ground valve is fitted with a well, the valve surface is coated with resin, which are methods to avoid corrosion by corrosive substances from the valve. Increased ambient temperatures and air pollution can accelerate corrosion, especially in closed environments. An open factory will be used as much as possible or ventilation and refrigeration steps should be taken to avoid corrosion in the environment.

9. Improve processing technology and valve structure: The valve’s anti-corrosion safety is a concern that has been considered from the start of the design. Without question, a valve product with a fair structure design and proper process method would have a positive impact on reducing valve corrosion.

Thus, the construction and manufacturing follow such unreasonable structure specifications, the process method is not right, it can be improved easily to avoid corrosion of the parts, to suit the requirements under different working conditions.

Summary:

Electrochemical corrosion corrodes different types of metals. It is acting in between the two metals. Also, due to the difference in the solubility of the solution, the difference in the solubility of oxygen, and the small difference in the internal structure of the metal, there is a possible difference that increases the corrosion rate. Some metals are not resistant to corrosion, but they are capable of producing a very good protective film, that is, a passivation film that can prevent the medium from corrosion. It can be seen that one is to remove electrochemical corrosion to achieve the function of the anti-corrosion of metal valves. when it doesn’t work; create a passivation film on the metal surface; the third is to use non-metallic materials instead of metal materials without electrochemical corrosion. The above-discussed methods are very helpful in the prevention of corrosion in valves.

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