Treatment method and anticorrosion measures of valve corrosion 

Corrosion is the damage and deterioration of materials under the action of various environments. The corrosion of metal is mainly caused by chemical corrosion and spot chemical corrosion. The corrosion of non-metallic materials is usually caused by direct chemical and physical effects. 

1、 Corrosion pattern of valve 

There are two kinds of corrosion modes of metal valve, namely uniform corrosion and local corrosion. The average corrosion rate can be used to evaluate the rate of uniform corrosion. Metal materials, graphite, glass, ceramics and concrete, are divided into four grades according to the corrosion rate: the corrosion rate is less than 0.05mm/a, which is excellent; The corrosion rate is good from 0.05 to 0.5mm/a; The corrosion rate is 0.5-1.5mm/a; If the corrosion rate is greater than 1.5mm/a, the valve parts such as sealing surface, valve stem, diaphragm and small spring are generally made of grade I materials, and the valve body and bonnet are suitable for grade II or III materials. For valves with high pressure, highly toxic, inflammable, explosive and radioactive medium, the materials with small corrosion resistance are selected. 

1. Uniform corrosion 

Uniform corrosion is carried out on all surfaces of the metal. For example, stainless steel, aluminum, titanium and other products are produced in the oxidation environment, and the metal state under the film is corroded evenly. There is also a phenomenon that the metal surface is corroded and peeled off, which is the most dangerous. 

2. Local corrosion 

Local corrosion occurs in the local position of metal, and its forms include pore corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, wear corrosion, cavitation corrosion, friction corrosion, hydrogen corrosion, etc. 

Pitting usually occurs on the metal of passivation film or protective film. It is because of the defects on the metal surface. The active ions of passivation film can be destroyed in the solution, which causes the passivation film to be damaged locally and extends into the metal interior to become the corrosion hole. It is one of the most destructive and hidden dangers of metal. 

Crevice corrosion occurs in welding, riveting, gasket or sediment environment, which is a special form of hole erosion. The prevention method is to eliminate the gap. 

Intergranular corrosion is the corrosion of the grain boundary in a net form by penetrating the surface into the metal interior along the grain boundary. The intergranular corrosion is mainly caused by improper heat treatment and cold processing except for the deposition of impurities in grain boundaries. The welding joint of austenitic stainless steel is prone to corrosion due to the poor chromium zone. Intergranular corrosion of austenitic stainless steel is the most common and dangerous corrosion form. The methods to prevent intergranular corrosion of austenitic stainless steel valve parts include: to carry out "solution quenching" treatment, namely, to heat to 1100 ℃ water quenching, and to select austenitic stainless steel containing titanium and niobium, and carbon content below 0.03%, so as to reduce the production of chromium carbide. 

Delamination corrosion occurs in layered structure. Corrosion develops vertically and inward, then the material with parallel surface is corroded. Under the expansion force of corrosion, the surface is layered and flaking. 

Stress corrosion occurs when corrosion and tensile stress are simultaneously acting. Methods to prevent stress corrosion; The stress produced in welding and cold processing is eliminated or reduced by heat treatment, unreasonable valve structure is improved, stress concentration is avoided, and electrochemical protection and anti-corrosion coating are applied. The measures such as adding corrosion inhibitor and applying pressure stress are taken. 

Corrosion fatigue occurs in the joint action of alternating stress corrosion, which causes metal to break. Heat treatment can be used to eliminate or reduce stress, surface shot peening and galvanizing, chromium, nickel, etc., but it is necessary to note that the coating should not have tensile stress and hydrogen diffusion. 

Selective corrosion occurs in different components and impurities. In a certain environment, some elements are corroded and leached, and the remaining elements are sponge like. There are brass dezincification, copper alloy dealumination, graphite of cast iron, etc. 

Wear corrosion is a kind of corrosion form produced by fluid alternating action on metal wear and corrosion, and it is a common corrosion of valves. This corrosion occurs on the sealing surface. Prevention methods: choose corrosion resistant and wear-resistant materials, improve the structure design, adopt cathodic protection, etc. 

Cavitation corrosion, also known as cavitation and cavitation, is a special form of wear corrosion. It is a bubble in the fluid. The shock wave produced during the burst can reach 400 atmospheric pressure, which will destroy the metal protective film and even tear the metal particles. Then corrosion into film, this process is repeated, making metal corrosion. The methods to prevent cavitation corrosion can be made of cavitation resistant materials, high finish machining surface, elastic protective layer and cathodic protection. 

Friction corrosion is the damage caused by vibration and sliding of contact surface, which is caused by the two parts contacting each other. Friction corrosion occurs at the bolt connection, the connection between the stem and the closing part, the ball bearing and the shaft. It can be protected by applying lubricating grease, reducing friction, phosphating surface, selecting cemented carbide, and improving surface hardness by spraying tile or cold processing. 

Corrosion is the damage caused by the diffusion of hydrogen atoms in chemical reaction to the metal. The forms of corrosion are hydrogen bubble, hydrogen embrittlement and hydrogen corrosion. 

Hydrogen bubbling is easy to occur in strong steel and non-metallic steel. Hydrogen bubble is easy to be produced when sulfide and hydrides are contained in oil. The cooling steel without holes is used instead of boiling steel with holes. Rubber and plastic are used to protect the steel from bubbling. 

The lattice of strong steel is highly variable, and after hydrogen atom enters, the 4 lattice should be changed greatly, which will cause financial embrittlement. Alloy steel containing nickel and lead should be selected to avoid high strength steel with high hydrogen brittleness, and hydrogen embrittlement should be avoided or reduced in welding, electroplating and pickling. Hydrogen enters into metal at high temperature and high pressure, and it will react with a combination of elements and destroy it. It is called hydrogen corrosion. Austenitic stainless steel is completely resistant to high temperature hydrogen corrosion. 

3. Nonmetallic corrosion 

The non-metallic corrosion is the same as the metal corrosion. Most non-metallic materials are non-electrical conductors, which will not produce electrochemical corrosion, but pure chemical or physical corrosion, which is the main difference from metal corrosion. Non-metallic corrosion is not necessarily weightless but often increases weight. For metal corrosion, weight loss is the main, non-metallic corrosion, many of which are caused by physical action, but metal corrosion physical action is rare; The internal corrosion of non-metallic is common, while the metal corrosion is mainly surface corrosion. 

After metal materials contact with medium, solution or gas will gradually spread into the interior of the material, which will cause a series of corrosion changes to non-metal. According to the different types and varieties of nonmetallic materials, the corrosion forms are different. The corrosion forms include dissolution, swelling, bubble, softening, decomposition, discoloration, deterioration, aging, hardening, fracture and other phenomena. However, from a comprehensive point of view, the corrosion resistance of nonmetal is much better than that of metal materials, but the strength and temperature resistance of non-metallic materials are lower than that of metal materials. 

2、 Corrosion protection measures of metal valves 

Electrochemical corrosion corrodes metals in various forms, which not only acts between the two metals, but also results in potential difference due to the poor solubility of solution, poor solubility of oxygen and small difference of internal structure of metal, which will also lead to the aggravation of corrosion. Some metals are not corrosion resistant, but they can produce very good protective film after corrosion, that is, passivation film, which can prevent the corrosion of the medium. Therefore, to achieve the purpose of metal valve corrosion prevention, the first is to eliminate electrochemical corrosion; Second, when electrochemical corrosion can not be eliminated; Passivation film should be produced on the metal surface; Third, non-metallic materials without electrochemical corrosion are used instead of metal materials. Several anticorrosive methods are introduced below. 

1. Choose corrosion resistant materials according to the medium 

In the section "selection of valves", we introduce the medium that the common materials of valves are applicable, but only general introduction. In the production practice, the corrosion of medium is very complex. Even if the valve materials used in one medium are the same, the medium has different concentrations, temperature and pressure, and the medium corrodes different materials. The corrosion rate of the medium increases by about 1-3 times every 10 ℃ rise. The medium concentration has a great influence on the corrosion of valve materials, such as lead in sulfuric acid with small concentration, and the corrosion is very small. When the concentration is over 96%, the corrosion rises sharply. On the contrary, the corrosion of carbon steel is the most serious when the sulfuric acid concentration is about 50%, while when the concentration increases to more than 6%, the corrosion decreases sharply. For example, aluminum has strong corrosiveness in concentrated nitric acid with concentration of more than 80%, but it is serious in medium and low concentration nitric acid. Although stainless steel has strong corrosion resistance to dilute nitric acid, it is worse in more than 95% of concentrated nitric acid. 

From the above examples, it can be seen that the correct selection of valve materials should be based on the specific conditions, analysis of various factors affecting corrosion, and selection of materials according to relevant anti-corrosion manual. 

2. 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, bonnet, lining, sealing surface and other non-metallic materials are commonly used for manufacturing. As for gasket, packing is mainly made of non-metallic materials. The valve liner is made of polytetrafluoroethylene, chlorinated polyether and other plastics, as well as natural rubber, neoprene, and rubber, and the body and cover body are made of general cast iron and carbon steel. It ensures the strength of the valve and the valve is not corroded. The pipe clamp valve is also designed according to the excellent corrosion resistance and excellent denaturation energy of rubber. Nowadays, more and more plastics such as nylon, polytetrafluoroethylene, natural rubber and synthetic rubber are used as various sealing surfaces and sealing rings. They are used on various valves. These nonmetallic materials used as sealing surfaces are not only corrosion resistant, but also have good sealing performance, especially suitable for use in granular media. Of course, they are both of low strength and heat resistance and their application is limited. The appearance of flexible graphite has made nonmetallic enter the high temperature field, solved the problem of packing and gasket leakage which is difficult to solve for a long time, and is a good high temperature lubricant.

3. Spray paint

Coating is the most widely used anti-corrosion means, and it is an indispensable anti-corrosion material and identification mark in valve products. Coating is also a non-metallic material, which is usually made of synthetic resin, rubber slurry, vegetable oil, solvent, etc., covering the metal surface, isolating medium and atmosphere, and achieving the purpose of anti-corrosion. The coating is mainly used in the environment with low corrosion such as water, salt water, sea water and atmosphere. The valve cavity is usually coated with anticorrosive paint to prevent the corrosion of the valve by water, air and other media. The paint is mixed with different colors to indicate the material used by Farne. The valve is painted once a year from half a year.

4. Add corrosion inhibitor

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

The mechanism of corrosion control by corrosion inhibitor is that it promotes the polarization of the battery. The corrosion inhibitor is mainly used in the medium and the filling. The corrosion of equipment and valves can be slowed down by adding corrosion inhibitor in medium. For example, chromium nickel stainless steel is in a large range of solubility in sulfuric acid without oxygen, and the corrosion is serious. However, adding a small amount of oxidants such as copper sulfate or nitric acid can make stainless steel dull, and a protective film will be formed on the surface to prevent the corrosion of medium. If a small amount of oxidant is added in hydrochloric acid, It can reduce the corrosion of titanium. The pressure test of valves is usually used as the medium for pressure test, which can easily cause the corrosion of the valve. Adding a small amount of sodium nitrite in the water can prevent the corrosion of the valve by water. Asbestos packing contains chloride, which has a great corrosion on the stem. If the washing method of distillation water is used, the chloride content can be reduced. However, this method is difficult to implement, and it can not be popularized generally. The ester is suitable for special needs.

 In order to protect the stem and prevent the corrosion of asbestos packing, the valve stem is coated with corrosion inhibitor and sacrificial metal. The corrosion inhibitor can make a passivation film on the surface of the valve stem, and improve the corrosion resistance of the valve stem; The solvent can dissolve the corrosion inhibitor slowly and can play the role of lubrication; Zinc powder is added to asbestos as sacrificial metal. In fact, zinc is also a corrosion inhibitor. It can first combine with chlorides in asbestos, which can reduce the contact chance between chloride and valve rod metal, so as to achieve the purpose of corrosion protection. If red lead, calcium lead acid and other corrosion inhibitors are added in the coating, spray brush on the valve surface can prevent the corrosion of the atmosphere.

5. Electrochemical protection 

There are two kinds of electrochemical protection, anode protection and cathodic protection. The so-called anode protection is to introduce the protective metal as the anode and add the direct current to increase the anode potential to the positive direction. When the value is increased to a certain value, a dense protective film is formed on the surface of the metal anode, namely, the passivation film. At this time, the corrosion of the metal cathode decreases sharply. Anode protection is suitable for metals that are easy to passivate. The so-called cathodic protection is to use the metal as the cathode and add DC to reduce its potential to the negative direction. When the potential reaches a certain potential value, the corrosion current speed decreases and the metal is protected. In addition, cathodic protection can protect the metal with a negative electrode potential than the metal being protected. If zinc is used to protect iron, zinc is corroded, zinc is called sacrificial metal. In the production practice, the anode protection is less and the cathodic protection is more applied. This cathodic protection method is an economical and effective method for large and important valves. Zinc is added to asbestos packing, and the protection valve rod is also a cathodic protection method. 

6. Metal surface treatment 

The surface treatment technology of metal is better than sleeping coating, surface penetration, surface oxidation passivation, etc. The purpose of this paper is to improve the corrosion resistance of metal and improve the mechanical energy of metal. Surface treatment valves are widely used. 

Valve connecting screw is usually galvanized, chromium plated and oxidized (blue) to improve the ability of atmospheric and medium corrosion resistance. In addition to the above methods, other fasteners shall be treated with phosphating and other surface treatment according to the situation. 

Sealing surface and close parts with small diameter often adopt surface technology such as nitriding and boronizing to improve its corrosion resistance and wear resistance. The nitriding layer of 38CrMoAlA valve disc is ≥ 0.4mm. 

The problem of stem corrosion protection is a problem that people pay attention to, and has accumulated rich production experience. It often adopts surface treatment technology such as nitriding, boronizing, chromium plating and nickel plating to improve its corrosion resistance, corrosion resistance and abrasion resistance. Different surface treatment should be suitable for different valve rod materials and working environment. For stem contacting asbestos packing in air and steam medium, hard chromium plating and gas nitriding process can be adopted (ion nitriding process is not suitable for stainless steel); The valve in hydrogen sulfide atmosphere has better protective performance by electroplating high phosphate nickel coating; 38CrMoAlA can also be corrosion-resistant by ion and gas nitriding, but hard chromium coating is not suitable; 2Cr13 can resist ammonia corrosion after tempering, carbon steel with gas nitriding can also resist ammonia corrosion, while all phosphorus nickel coatings are not ammonia corrosion resistant; The 38CrMoAlA material has excellent corrosion resistance and comprehensive performance. It is used to make many stem. 

Small diameter valve body and handwheel are often chrome plated to improve its corrosion resistance and decorate the valve. 

7. Thermal spraying 

Thermal spraying is a kind of process block for coating preparation, and has become one of the new technologies of surface protection of materials. It is a key national promotion project. It uses high energy density heat sources (gas combustion flame, arc, plasma arc, electric heating, gas ignition, etc.) to heat and melt metal or non-metallic materials