How to Prevent The Valve From Corrosion

Electrochemical corrosion corrodes metals in various forms. It not only acts between two metals, but also produces a potential difference due to the poor solubility of the solution, the poor solubility of oxygen, and the slight difference in the internal structure of the metal, which aggravates the corrosion. . Some metals themselves are not corrosion-resistant, but they can produce a very good protective film after corrosion, that is, a passivation film, which can prevent the corrosion of the medium. It can be seen that in order to achieve the purpose of anti-corrosion of metal valves, one is to eliminate electrochemical corrosion; the other is to eliminate electrochemical corrosion; the passive film must be formed 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. Choose corrosion-resistant materials according to the medium

In the section of "Valve Selection", we introduced the suitable medium for the common materials of the valve, but it is only a general introduction. In actual production, the corrosion of the medium is very complicated, even if it is used in a medium The valve material is the same, the concentration, temperature and pressure of the medium are different, and the corrosion of the medium to the material is also different. When the temperature of the medium increases by 10°C, the corrosion rate increases by about 1 to 3 times. The medium concentration has a great influence on the corrosion of valve materials. For example, when lead is in sulfuric acid with a small concentration, the corrosion is very small. When the concentration exceeds 96%, the corrosion rises sharply. On the contrary, carbon steel has the most serious corrosion when the concentration of sulfuric acid is about 50%, and when the concentration increases to more than 6%, the corrosion decreases sharply. For example, aluminum is very corrosive in concentrated nitric acid with a concentration of more than 80%, but it is seriously corroded in medium and low concentrations of nitric acid. Although stainless steel has strong corrosion resistance to dilute nitric acid, the corrosion is aggravated 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 specific conditions, analyze various factors affecting corrosion, and select materials according to relevant anti-corrosion manuals.

2.Using non-metallic materials

Non-metallic corrosion resistance is excellent. As long as the valve operating 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, bonnet, lining, sealing surface, etc. are commonly made of non-metallic materials. As for gaskets, packings are mainly made of non-metallic materials. The valve lining is made of plastics such as polytetrafluoroethylene and chlorinated polyether, as well as rubber such as natural rubber, neoprene and nitrile rubber, while the valve body and valve cover are generally made of cast iron and carbon steel. It not only ensures the strength of the valve, but also ensures that the valve is not corroded. The pinch valve is also designed based on the excellent corrosion resistance and excellent variable performance of rubber. Nowadays, it is more and more correct to use nylon, PTFE and other plastics, and natural rubber and synthetic rubber to make various sealing surfaces and sealing rings, which are used on various types of valves. These non-metallic materials used as sealing surfaces Material, not only good corrosion resistance, but also good sealing performance, especially suitable for use in media with particles. Of course, their strength and heat resistance are low, limiting the range of applications. The emergence of flexible graphite has brought non-metals into the high-temperature field, solved the long-term difficult to solve the problem of packing and gasket leakage, and is a good high-temperature lubricant.

3. Spray paint

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., and cover the metal surface to isolate the medium and the atmosphere to achieve anti-corrosion purposes. Coatings are mainly used in environments that are not very corrosive, such as water, salt water, seawater, and atmosphere. The inner cavity of the valve is usually painted with anti-corrosion paint to prevent water, air and other media from corroding the valve. The paint is mixed with different colours to represent the materials Fahn uses. The valve is sprayed with paint, generally once every six months to one year.

4. Add corrosion inhibitor

Adding a small amount of other special substances to the corrosive medium and corrosive substances can greatly slow down the speed of metal corrosion. This special substance is called corrosion inhibitor.

The mechanism by which the corrosion inhibitor controls corrosion is that it promotes the polarization of the battery. Corrosion inhibitors are mainly used in media and fillers. Adding a corrosion inhibitor to the medium can slow down the corrosion of equipment and valves. For example, chromium-nickel stainless steel in oxygen-free sulfuric acid has a wide range of solubility in a cremated state, and the corrosion is more serious, but a small amount of copper sulfate or nitric acid is added. When the oxidant is used, the stainless steel can be transformed into a passive state, and a protective film is formed on the surface to prevent the corrosion of the medium. In hydrochloric acid, if a small amount of oxidant is added, the corrosion of titanium can be reduced. Water is often used as the pressure test medium for valve pressure testing, which is easy to cause valve corrosion. Adding a small amount of sodium nitrite in water can prevent water from corroding the valve. The asbestos packing contains chlorides, which corrode the valve stem greatly. If the method of washing with distilled water is used, the content of chlorides can be reduced. However, this method is difficult to implement and cannot be generally promoted. The ester is suitable for special needs.

In order to protect the valve stem and prevent the corrosion of the asbestos packing, the valve stem is filled with corrosion inhibitor and sacrificial metal in the asbestos packing. The corrosion inhibitor is composed of sodium nitrite and sodium chromate, which can form a passivation film on the surface of the valve stem to improve the corrosion resistance of the valve stem; the solvent can slowly dissolve the corrosion inhibitor and play a lubricating role; in asbestos Zinc powder is added as a sacrificial metal. In fact, zinc is also a corrosion inhibitor. It can first combine with the chloride in the asbestos, so that the contact between the chloride and the valve stem metal is greatly reduced, so as to achieve the purpose of anti-corrosion. If a corrosion inhibitor such as red red and calcium lead acid is added to the paint, spraying on the surface of the valve can prevent atmospheric corrosion.

5. Electrochemical protection

There are two types of electrochemical protection: anodic protection and cathodic protection. The so-called anodic protection is to use the protective metal as the anode to introduce an external direct current to increase the anode potential in the positive direction. When it increases to a certain value, a dense protective film is formed on the surface of the metal anode, which is a passivation film. Corrosion of metal cathodes is drastically reduced. Anodic protection is suitable for metals that are easily passivated. The so-called cathodic protection means that the protected metal is used as the cathode, and direct current is applied to reduce its potential in the 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 whose electrode potential is more negative than that of the protected metal. If zinc is used to protect iron, zinc is corroded, and zinc is called a sacrificial metal. In production practice, anodic protection is used less, and cathodic protection is used more. Large valves and important valves use this cathodic protection method, which is an economical, simple and effective method. Zinc is added to the asbestos filler to protect the valve stem, which also belongs to the cathodic protection method.

6. Metal surface treatment

Metal surface treatment processes are better than dormant coating, surface penetration, surface oxidation passivation, etc. Its purpose is to improve the corrosion resistance of metals and improve the mechanical energy of metals. Surface-treated valves are widely used.

The valve connecting screw is usually galvanized, chrome-plated, and oxidized (blued) to improve the resistance to atmospheric and medium corrosion. In addition to the above-mentioned methods for other fasteners, surface treatments such as phosphating are also used according to the situation.

The sealing surface and the closing parts with small caliber often use surface processes such as nitriding and boronizing to improve its corrosion resistance and wear resistance. The valve disc made of 38CrMoAlA, the nitrided layer is greater than or equal to 0.4mm.

The problem of valve stem anti-corrosion is a problem that people pay attention to. We have accumulated rich production experience. Surface treatment processes such as nitriding, boronizing, chrome plating and nickel plating are often used to improve its corrosion resistance, corrosion resistance and abrasion resistance. injury performance. Different surface treatments should be suitable for different valve stem materials and working environments. The valve stem in contact with the atmosphere, water vapor medium and asbestos packing can be plated with hard chrome and gas nitriding process (stainless steel is not suitable for ion nitriding process); In the atmosphere of hydrogen sulfide, the valve is electroplated with high phosphorus nickel coating, which has better protection performance; 38CrMoAlA can also resist corrosion by ion and gas nitriding, but it is not suitable to use hard chromium coating; 2Cr13 can resist ammonia corrosion after quenching and tempering. Carbon steel using gas nitriding is also resistant to ammonia corrosion, while all phosphorus-nickel coatings are not resistant to ammonia corrosion; after gas nitriding, 38CrMoAlA material has excellent corrosion resistance and comprehensive performance, and it is used for many valve stems.

Small-diameter valve bodies and hand-wheels are also often chrome-plated to improve their corrosion resistance and decorate the valve.

7. Thermal spraying

Thermal spraying is a type of process block for preparing coatings and has become one of the new technologies for material surface protection. It is a national key promotion project. It uses a high energy density heat source (gas combustion flame, electric arc, plasma arc, electric heat, gas explosion, etc.) to heat and melt metal or non-metallic materials, and then spray it onto the pretreated basic surface in the form of atomization to form a spray coating. , or heating the basic surface at the same time, so that the coating is melted again on the surface of the substrate, and the surface strengthening process of the spray welding layer is formed. Most metals and their alloys, metal oxide ceramics, cermet composites, and hard metal compounds can be coated on metallic or non-metallic substrates by one or more thermal spray methods.

Thermal spraying can improve its surface corrosion resistance, wear resistance, high temperature resistance and other properties, and prolong its service life. Thermal spray coating with special functions has special properties such as heat insulation, insulation (or different electricity), grindable sealing, self-lubricating, heat radiation, electromagnetic shielding, etc.; parts can be repaired by thermal spraying.

8. Control the corrosive environment

The so-called environment has two broad senses and narrow senses. The broad environment refers to the environment around the valve installation site and its internal circulation medium; the narrow sense environment refers to the conditions around the valve installation site. Most environments cannot be controlled and production processes cannot be changed arbitrarily. Only in the case that it will not cause damage to the product, process, etc., the method of controlling the environment can be used, such as deoxidizing boiler water, adjusting the pH value of domestic alkali in the refining process, etc. From this point of view, the above-mentioned addition of corrosion inhibitors, electrochemical protection, etc. are also controlled corrosion environments.

The atmosphere is full of dust, water vapor and smoke, especially in the production environment, such as smoke halogen, toxic gases and fine powder emitted by equipment, which will corrode the valve to varying degrees. Operators should regularly clean and purge valves and refuel regularly according to the regulations in the operating procedures, which are effective measures to control environmental corrosion. The valve stem is installed with a protective cover, the ground valve is installed in the ground well, and the valve surface is sprayed with paint, etc., which are all methods to prevent the corrosion of the valve from containing corrosive substances. Elevated ambient temperature and air pollution, especially for equipment and valves in closed environments, will accelerate their corrosion. Open workshops or ventilation and cooling measures should be adopted as far as possible to slow down environmental corrosion.

9. Improve processing technology and valve structure

The anti-corrosion protection of the valve is a problem that is considered from the design, a valve product with reasonable structural design and correct process method. There is no doubt that it has a good effect on slowing down the corrosion of the valve.