Electrochemical corrosion corrodes metals in various forms. It acts between the two metals. Moreover, due to the different solubility of the solution, the different solubility of oxygen, and the slight difference in the internal structure of the metal, a potential difference is also generated, which will increase the corrosion. Some metals are not corrosion-resistant, but they can produce a very good protective film, that is, a passivation film, which can prevent the corrosion of the medium. It can be seen that to achieve the purpose of anticorrosion of metal valves, one is to eliminate electrochemical corrosion; when it doesn't work, to make a passivation film on the metal surface; the third is to use non-metallic materials without electrochemical corrosion instead of metal materials. Several anti-corrosion methods are described below.
1. Select corrosion resistant materials according to the medium
In the actual production, the corrosion of the medium is very complicated. Even if we use the same valve material for same medium, the concentration, temperature and pressure of the medium will be different, and the corrosion of the medium against the material will be different. For every 10 °C increase in the temperature of the medium, the corrosion rate increases by about 1 to 3 times. The concentration of the medium has a great influence on the corrosion of the valve material. For example, if the lead is in sulfuric acid with a small concentration, the corrosion is small, and when the concentration exceeds 96%, the corrosion sharply rises. On the contrary, the corrosion of carbon steel is most severe when the concentration of sulfuric acid is about 50%. When the concentration is increased to more than 6%, the corrosion drops sharply. For example, aluminum is highly corrosive in concentrated nitric acid with a concentration of 80% or more, but it is severely corroded in medium and low concentrations of nitric acid. Although stainless steel is highly resistant to dilute nitric acid, corrosion is more serious in more than 95% concentrated nitric acid.
It can be seen from the above examples that the correct selection of valve materials should be based on the specific conditions. We should analyze various corrosion factors, and select materials according to the relevant anti-corrosion manual.
2. Use non-metallic materials
The corrosion resistance of non-metallic is excellent, as long as the valve temperature and pressure meet the requirements of non-metallic materials, it can not only solve the corrosion problem, but also save precious metals. The valve body, valve bonnet, lining, sealing surface and the other are commonly made of non-metallic materials. As for the gasket, the filler is mainly made of non-metallic materials. The valve is lined with plastic such as polytetrafluoroethylene or chlorinated polyether, and rubber such as natural rubber, neoprene or nitrile rubber, and the body of the valve body and bonnet is made of general cast iron and carbon steel. That is to ensure the strength of the valve, and to ensure that the valve is not corroded. The pinch valve is also designed based on the excellent corrosion resistance and excellent properties of the rubber. Nowadays, plastics such as nylon and polytetrafluoroethylene are used more and more, and various sealing surfaces are made of natural rubber and synthetic rubber. Sealing rings are used for various types of valves. These non-metal materials used as sealing surfaces not only have good corrosion resistance, but also have good sealing performance, and are especially suitable for use in a granular medium. Of course, their strength and heat resistance are low and the range of applications is limited. The emergence of flexible graphite has enabled non-metals to enter the high-temperature field, solving the long-term problem of filler and gasket leakage, and it is a good high-temperature lubricant.
3. Spray coating
Coating is the most widely used anti-corrosion method, and it is an indispensable anti-corrosion material and identification mark on valve products. Coatings are also non-metallic materials. They are usually made of synthetic resin, rubber slurry, vegetable oil, solvent, etc., covering the metal surface, insulating the medium and the atmosphere to achieve anti-corrosion purposes. Coatings are mainly used in environments where the corrosion is not severe like water, salt water, sea water and atmosphere. The inner cavity of the valve is usually painted with anti-corrosive paint to prevent the water, air and other media from corroding the valve. The paint is mixed with different colors to represent the materials used by Faine. Valve should be coated or painted usually in half a year to once a year.
4. Add corrosion inhibitor
The addition of small amounts of other special substances to corrosive media and corrosives can greatly slow the rate of metal corrosion. This special substance is called corrosion inhibitor. The mechanism by which corrosion inhibitors control corrosion is that it promotes the polarization of the battery. Corrosion inhibitors are mainly used in media and packing. Adding corrosion inhibitor to the medium can slow down the corrosion of equipment and valves. For example, chrome-nickel stainless steel is ignited in a large range of solubility in oxygen-free sulfuric acid. The corrosion is serious. But if a small amount of oxidants such as copper sulfate or nitric acid is added, the stainless steel can be transformed into a passive state, and a protective film is formed on the surface to prevent the etching of the medium. In the hydrochloric acid, if a small amount of the oxidizing agent is added, the corrosion of the titanium can be reduced. The valve pressure test commonly uses water as the medium for pressure test, which is easy to cause corrosion of the valve. Adding a small amount of sodium nitrite in water can prevent the water from corroding the valve. The asbestos filler contains chloride, which is very corrosive to the valve stem. The steamed water washing method can reduce the chloride content; this method is difficult to implement in practice, and the ester is suitable for special needs. In order to protect the valve stem from corrosion of the asbestos filler, in the asbestos filler, the valve stem is coated with a corrosion inhibitor and a sacrificial metal. The corrosion inhibitor consists of sodium nitrite and sodium chromate to form a passivation film on the surface of the valve stem to improve the corrosion resistance of the valve stem. The solvent can dissolve the corrosion inhibitor slowly and can provide lubrication. Zinc powder is added to asbestos as a sacrificial metal. In fact, zinc is also a corrosion inhibitor. It can be first combined with chloride in asbestos, so that the chance of contact between chloride and stem metal is greatly reduced, thus achieving anti-corrosion purposes. If a corrosion inhibitor such as red dan or calcium lead is added to the coating, the surface of the valve can prevent corrosion of the atmosphere.
5. Electrochemical protection
Electrochemical protection is available in both anodic and cathodic protection. The so-called anode protection is to introduce an external direct current for the purpose of protecting the metal, so that the anode potential increases in a positive direction. When it is increased to a certain value, a dense protective film is formed on the surface of the metal anode, which is a passivation film. The corrosion of the metal cathode is drastically reduced. The anodic protection is suitable for metal that is easily passivated. The so-called cathodic protection means that the protected metal is used as a cathode, and a direct current is applied to lower the potential in a negative direction. When it reaches a certain potential value, the corrosion current speed is reduced and the metal is protected. In addition, cathodic protection can protect the protected metal with a metal having a higher electrode potential than the protected metal. If zinc is used to protect iron, zinc is corroded and zinc is called sacrificial metal. In the production practice, the anode protection is used less, and the cathodic protection application is more. Large-scale valves and important valves use this cathodic protection method, which is an economical, simple and effective method. Zinc is added to the asbestos filler and the protective stem is also a cathodic protection method.
1. Select corrosion resistant materials according to the medium
In the actual production, the corrosion of the medium is very complicated. Even if we use the same valve material for same medium, the concentration, temperature and pressure of the medium will be different, and the corrosion of the medium against the material will be different. For every 10 °C increase in the temperature of the medium, the corrosion rate increases by about 1 to 3 times. The concentration of the medium has a great influence on the corrosion of the valve material. For example, if the lead is in sulfuric acid with a small concentration, the corrosion is small, and when the concentration exceeds 96%, the corrosion sharply rises. On the contrary, the corrosion of carbon steel is most severe when the concentration of sulfuric acid is about 50%. When the concentration is increased to more than 6%, the corrosion drops sharply. For example, aluminum is highly corrosive in concentrated nitric acid with a concentration of 80% or more, but it is severely corroded in medium and low concentrations of nitric acid. Although stainless steel is highly resistant to dilute nitric acid, corrosion is more serious in more than 95% concentrated nitric acid.
It can be seen from the above examples that the correct selection of valve materials should be based on the specific conditions. We should analyze various corrosion factors, and select materials according to the relevant anti-corrosion manual.
2. Use non-metallic materials
The corrosion resistance of non-metallic is excellent, as long as the valve temperature and pressure meet the requirements of non-metallic materials, it can not only solve the corrosion problem, but also save precious metals. The valve body, valve bonnet, lining, sealing surface and the other are commonly made of non-metallic materials. As for the gasket, the filler is mainly made of non-metallic materials. The valve is lined with plastic such as polytetrafluoroethylene or chlorinated polyether, and rubber such as natural rubber, neoprene or nitrile rubber, and the body of the valve body and bonnet is made of general cast iron and carbon steel. That is to ensure the strength of the valve, and to ensure that the valve is not corroded. The pinch valve is also designed based on the excellent corrosion resistance and excellent properties of the rubber. Nowadays, plastics such as nylon and polytetrafluoroethylene are used more and more, and various sealing surfaces are made of natural rubber and synthetic rubber. Sealing rings are used for various types of valves. These non-metal materials used as sealing surfaces not only have good corrosion resistance, but also have good sealing performance, and are especially suitable for use in a granular medium. Of course, their strength and heat resistance are low and the range of applications is limited. The emergence of flexible graphite has enabled non-metals to enter the high-temperature field, solving the long-term problem of filler and gasket leakage, and it is a good high-temperature lubricant.
3. Spray coating
Coating is the most widely used anti-corrosion method, and it is an indispensable anti-corrosion material and identification mark on valve products. Coatings are also non-metallic materials. They are usually made of synthetic resin, rubber slurry, vegetable oil, solvent, etc., covering the metal surface, insulating the medium and the atmosphere to achieve anti-corrosion purposes. Coatings are mainly used in environments where the corrosion is not severe like water, salt water, sea water and atmosphere. The inner cavity of the valve is usually painted with anti-corrosive paint to prevent the water, air and other media from corroding the valve. The paint is mixed with different colors to represent the materials used by Faine. Valve should be coated or painted usually in half a year to once a year.
4. Add corrosion inhibitor
The addition of small amounts of other special substances to corrosive media and corrosives can greatly slow the rate of metal corrosion. This special substance is called corrosion inhibitor. The mechanism by which corrosion inhibitors control corrosion is that it promotes the polarization of the battery. Corrosion inhibitors are mainly used in media and packing. Adding corrosion inhibitor to the medium can slow down the corrosion of equipment and valves. For example, chrome-nickel stainless steel is ignited in a large range of solubility in oxygen-free sulfuric acid. The corrosion is serious. But if a small amount of oxidants such as copper sulfate or nitric acid is added, the stainless steel can be transformed into a passive state, and a protective film is formed on the surface to prevent the etching of the medium. In the hydrochloric acid, if a small amount of the oxidizing agent is added, the corrosion of the titanium can be reduced. The valve pressure test commonly uses water as the medium for pressure test, which is easy to cause corrosion of the valve. Adding a small amount of sodium nitrite in water can prevent the water from corroding the valve. The asbestos filler contains chloride, which is very corrosive to the valve stem. The steamed water washing method can reduce the chloride content; this method is difficult to implement in practice, and the ester is suitable for special needs. In order to protect the valve stem from corrosion of the asbestos filler, in the asbestos filler, the valve stem is coated with a corrosion inhibitor and a sacrificial metal. The corrosion inhibitor consists of sodium nitrite and sodium chromate to form a passivation film on the surface of the valve stem to improve the corrosion resistance of the valve stem. The solvent can dissolve the corrosion inhibitor slowly and can provide lubrication. Zinc powder is added to asbestos as a sacrificial metal. In fact, zinc is also a corrosion inhibitor. It can be first combined with chloride in asbestos, so that the chance of contact between chloride and stem metal is greatly reduced, thus achieving anti-corrosion purposes. If a corrosion inhibitor such as red dan or calcium lead is added to the coating, the surface of the valve can prevent corrosion of the atmosphere.
5. Electrochemical protection
Electrochemical protection is available in both anodic and cathodic protection. The so-called anode protection is to introduce an external direct current for the purpose of protecting the metal, so that the anode potential increases in a positive direction. When it is increased to a certain value, a dense protective film is formed on the surface of the metal anode, which is a passivation film. The corrosion of the metal cathode is drastically reduced. The anodic protection is suitable for metal that is easily passivated. The so-called cathodic protection means that the protected metal is used as a cathode, and a direct current is applied to lower the potential in a negative direction. When it reaches a certain potential value, the corrosion current speed is reduced and the metal is protected. In addition, cathodic protection can protect the protected metal with a metal having a higher electrode potential than the protected metal. If zinc is used to protect iron, zinc is corroded and zinc is called sacrificial metal. In the production practice, the anode protection is used less, and the cathodic protection application is more. Large-scale valves and important valves use this cathodic protection method, which is an economical, simple and effective method. Zinc is added to the asbestos filler and the protective stem is also a cathodic protection method.
Next: The corrosion prevention of metal valves - part two
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