Valve Corrosion Protection

Valve corrosion protection

  Corrosion is one of the important factors that cause valve damage. Therefore, in the use of valves, corrosion protection is the first consideration.

First, the shape of the valve corrosion
  Metal valve corrosion has two forms, namely uniform corrosion and local corrosion. The rate of uniform corrosion can be evaluated by the annual average corrosion rate. Metal materials, graphite, glass, ceramics and concrete, according to the corrosion rate, are divided into 4 grades: corrosion rate is less than 0.05mm/a; corrosion rate is good at 0.05~0.5mm/a; corrosion rate is 0.5~1.5 Mm/a can still be used; the corrosion rate is greater than 1.5mm/a is not applicable, the valve sealing surface, valve stem, diaphragm, small spring and other valve parts are generally made of first-grade materials, valve body, valve cover, etc. Grade or tertiary materials, for high pressure, highly toxic, flammable, explosive, radioactive media valves, use less corrosive materials.

1.Uniform corrosion
  Uniform corrosion is performed on the entire surface of the metal. Such as stainless steel, aluminum, titanium and other protective film produced in the oxidizing environment, the metal under the film is uniformly corroded. There is also a phenomenon in which the metal surface is corroded and peeled off, which is the most dangerous.

2. Local corrosion

Localized corrosion occurs at local locations of the metal. Its morphology is pitting, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, wear corrosion, cavitation corrosion, vibration corrosion, hydrogen Etc.

Pitting corrosion usually occurs on the metal of the passivation film or the protective film. Due to defects in the metal surface, the active ions of the passivation film can be destroyed in the solution, and the passivation film is partially destroyed and protrudes into the metal to become an etched hole. It is one of the most corrosive forms of metal destructiveness and hidden danger. Crevice corrosion occurs in environments such as welds, rivets, gaskets, or deposits, and is a special form of pitting corrosion. The prevention method is to eliminate the gap.

  Intergranular corrosion penetrates into the interior of the metal from the surface along the grain boundary, causing the grain boundary to reticulate. Intergranular corrosion is caused by the deposition of impurities in the grain boundary, mainly due to improper heat treatment and cold working. Austenitic stainless steel welds on both sides are prone to chromium-depleted areas and are subject to corrosion. Intergranular corrosion of austenitic stainless steels is a common and most dangerous form of corrosion. The method for preventing intergranular corrosion of austenitic stainless steel valve parts is as follows: "solution hardening" treatment, that is, heating to about 1100 ° C water quenching, using austenitic stainless steel containing titanium and niobium and having a carbon content of 0.03% or less. To reduce the production of chromium carbide.

  The delamination corrosion occurs in the layered structure. The corrosion first develops vertically inward, and the material which is parallel to the surface is corroded under the expansion force of the corrosive material to cause the surface to peel off layerwise.
  Stress corrosion occurs at the same time as corrosion and tensile stress. Method for preventing stress corrosion; eliminating or reducing welding, stress generated in cold working by heat treatment, improving irrational valve structure, avoiding stress concentration, using electrochemical protection, and spraying anti-corrosion coating. Add corrosion inhibitor, apply compressive stress and other measures.
  Corrosion fatigue occurs at the joint of alternating stress corrosion, causing the metal to rupture. Heat treatment can be used to eliminate or reduce stress, surface shot peening and electroplating of zinc, chromium, nickel, etc., but pay attention to the plating layer can not have tensile stress and hydrogen diffusion.
Selective corrosion occurs in materials with different compositions and impurities. In certain environments, some of the elements are etched out by corrosion, leaving the uncorroded elements spongy. Commonly, there are brass dezincification, copper alloy dealumination, and cast iron graphitization.
  Abrasion corrosion is a form of corrosion caused by the alternating action of fluids on metal wear and corrosion. It is a common type of corrosion of valves that occurs at the sealing surface. Prevention method: use corrosion-resistant and wear-resistant materials, improve structural design, and adopt cathodic protection.

  Cavitation corrosion, also known as cavitation and cavitation, is a special form of wear and corrosion. It is a bubble generated in a fluid. The shock wave generated when it is broken can be up to 400 atmospheres, which destroys the metal protective film and even tears the metal particles. Then it is etched into a film, and this process is repeated and the metal is corroded. For the method of preventing cavitation corrosion, a cavitation resistant material, a high-precision processing surface, an elastic protective layer and a cathodic protection can be selected. Frictional corrosion is the simultaneous contact of two parts in contact with each other, and the contact surface is damaged by vibration and sliding.   

  Frictional corrosion occurs at the bolted joint, between the stem and the closure, between the ball bearing and the shaft. Lubricating grease can be used to reduce friction, surface phosphating, use of hard alloys, and protection by surface treatment with cold spray or cold work.

  Corrosion is the destruction caused by the diffusion of hydrogen atoms generated in the chemical reaction into the interior of the metal. The form is hydrogen bubbling, hydrogen embrittlement and hydrogen etching.

  Strong steel and non-metallic steels are prone to hydrogen bubbling. When oil contains sulfides or hydrides, hydrogen bubbling is likely to occur. The use of void-free killed steel instead of boiling steel with holes, protection by rubber and plastic, plus corrosion inhibitors can prevent bubbling.

  In the strong steel, the lattice is highly denatured. After the hydrogen atoms enter, the 4 lattice strain is larger, causing financial embrittlement. Alloy steel containing nickel and lead should be used to avoid high-strength steel with high hydrogen embrittlement, avoiding or reducing hydrogen embrittlement in welding, electroplating and pickling. At high temperature and high pressure, hydrogen enters the metal and is destroyed by a chemical reaction with a combination of elements, called hydrogen etching. Austenitic stainless steel is completely resistant to high temperature hydrogen etching.

3. Non-metallic corrosion

  Non-metallic corrosion is the same as metal corrosion. Most non-metallic materials are non-electrical conductors. Generally, they do not produce electrochemical corrosion, but pure chemical or physical corrosion, which is the main difference from metal corrosion. Non-metallic corrosion does not necessarily lose weight and is often weight gain. For metal corrosion, weight loss is the main, non-metallic corrosion, many are caused by physical effects, and metal corrosion physical effects are rare; non-metallic internal corrosion is a common phenomenon. Metal corrosion is dominated by surface corrosion.

  After the metal material is in contact with the medium, the solution or gas will gradually diffuse into the interior of the material, causing a series of corrosion changes in the non-metal. The corrosion forms vary depending on the type and variety of the non-metallic material. Corrosive forms include dissolution, swelling, air bubbles, softening, decomposition, discoloration, deterioration, aging, hardening, and fracture. However, from a comprehensive point of view, the non-metallic corrosion performance is much better than that of the metal material, and the strength of the metal material is lower than that of the metal material.

Second, the corrosion of metal valves
  Electrochemical corrosion corrodes metals in various forms. It acts not only between the two metals, but also because of poor solubility of the solution, poor solubility of oxygen, and small differences in the internal structure of the metal. . 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; the other is to eliminate electrochemical corrosion; 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.According to the medium selected corrosion resistant materials
  In the section "Selection of Valves", we introduce the medium to which the materials commonly used for valves are applied. It is just a general introduction. In the actual production, the corrosion of the medium is very complicated, even when used in a medium. Like the valve material, the concentration, temperature and pressure of the medium are different, and the medium is not corroded to the material. 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, carbon steel is the 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, analyze various corrosion factors, and select materials according to the relevant anti-corrosion manual.

2.Using non-metallic materials
  Non-metallic corrosion resistance 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 cover, lining, sealing surface and other common non-metallic materials are used. 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 are used as non-metal sealing surfaces. The material not only has good corrosion resistance, but also has good sealing performance, and is 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 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., covering the metal surface, insulating the medium and the atmosphere to achieve anti-corrosion purposes. Coatings are mainly used in environments where water, salt water, sea water, and the atmosphere are not corrosive. 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 spray paint, 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. Corrosion is 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 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. If 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; in asbestos Zinc powder is added 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
  Telephone protection is available in both anodic and cathodic protection. The so-called anode protection is to introduce a direct current into the anode with the protective 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 anode protects the metal suitable for easy passivation. 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.

6.Metal surface treatment
  Metal surface treatment processes are better than sleeping coatings, surface penetration, surface oxidation passivation, and the like. 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 connection screw is usually galvanized, chrome-plated, and oxidized (blue) to improve the resistance to atmospheric and medium corrosion. In addition to the above methods, other fasteners are treated with a surface treatment such as phosphating.
  The sealing surface and the closing member with small diameter often adopt surface treatment such as nitriding and boronizing to improve its corrosion resistance and wear resistance. The valve disc made of 38CrMoAlA has a nitrided layer ≥0.4mm.
  The problem of valve stem anti-corrosion is a problem that people pay attention to, and it has accumulated rich production experience. It often uses surface treatment processes such as nitriding, boronizing, chrome plating and nickel plating 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 environment. The valve stems in contact with atmospheric, water vapor medium and asbestos packing can be hard chrome-plated and gas nitriding (stainless steel is not suitable for ion nitriding); The valve in the hydrogen sulfide atmosphere has better protection performance by electroplating high-phosphorus nickel plating; 38CrMoAlA can also resist corrosion by ion and gas nitriding, but it is not suitable to use hard chrome plating; 2Cr13 can resist ammonia corrosion after quenching and tempering, Gas nitriding carbon steel can also resist ammonia corrosion, and all phosphorous nickel coatings are not resistant to ammonia corrosion; the gas nitriding 38CrMoAlA material has excellent corrosion resistance and comprehensive performance, and it is used to make valve stems.
  The small-diameter valve body and handwheel are also often chrome-plated to improve their corrosion resistance and to 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 surface protection of materials. It is a national key promotion project. It uses a high energy density heat source (gas combustion flame, arc, plasma arc, electric heat, gas explosion, etc.) to heat and melt the metal or non-metal material, and then sprays it onto the pretreated basic surface in an atomized form to form a sprayed layer. , or at the same time heating the basic surface to re-melt the coating on the surface of the substrate to form a surface strengthening process of the spray-welded layer. Most metals and their alloys, metal oxide ceramics, cermet composites, and hard metal compounds can be coated on a metal or non-metal substrate using one or more  thermal spray methods.
  Thermal spraying can improve the surface corrosion resistance, wear resistance, high temperature resistance and other properties, and prolong the service life. Thermal spray special function coating with special properties such as heat insulation, insulation (or isoelectric), grindable seal, self-lubricating, heat radiation, electromagnetic shielding, etc.; parts can be repaired by thermal spraying.

8.Control the corrosive environment
  The so-called environment, there are two broad and narrow senses, the generalized environment refers to the environment around the valve installation and its internal circulation medium; the narrow environment refers to the conditions around the valve installation. Most environments are uncontrollable and production processes are not subject to change. Only when there is no damage to the product or process, the method of controlling the environment, such as boiler water deoxidation and the pH of the alkali in the refining process, can be used. 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 and halogen, toxic gases and fine powder emitted from equipment, it will cause different degrees of corrosion on the valve. The operator should periodically clean and purge the valve and regularly refuel according to the regulations in the operating procedures. This is an effective measure to control environmental corrosion. The valve stem is installed with a protective cover, the ground valve is provided with a well, the surface of the valve is painted with paint, etc., which are methods for preventing corrosion of the valve by corrosive substances. Increased ambient temperatures and air pollution, especially in closed environments, can accelerate corrosion. Floors should be used as much as possible or ventilation and cooling measures should be adopted to reduce environmental corrosion.

9.Improve processing technology and valve structure
  The anti-corrosion protection of the valve is a problem that has been considered from the design, and a valve product with reasonable structural design and correct process method. Undoubtedly, it has a good effect on slowing the corrosion of the valve. Therefore, the design and manufacturing department should improve the components that are unreasonable in structural design, incorrect in process methods, and prone to corrosion, and should be improved to suit the requirements of various working conditions. The gap at the valve connection is a good environment for corrosion of the oxygen concentration cell. Therefore, the connection between the valve stem and the closing member should be as far as possible without the use of the inner and the threaded connection; the valve welding should be double-sided butt welding and continuous welding, spot welding and lap welding are prone to corrosion, and the valve thread connection is made of polytetrafluoroethylene. Thin tape and pad. Not only can it have a good seal, but it can also corrode. The medium whose dead angle is not easy to flow is easy to corrode the valve. Except when the valve is used, it does not need to be flipped and the discharge medium is discharged. When manufacturing the valve parts, the recessed structure should be avoided as much as possible, and the valve should be provided with drain holes as much as possible. Different metal contacts will constitute a point couple, which promotes corrosion of the anode metal. When selecting materials, it is inevitable that the metal potential difference is large and the metal contact of the passivation film cannot be produced. In the production and processing process, special stress corrosion is generated during welding and heat treatment. Attention should be paid to improving the processing method. After welding, appropriate protective measures such as annealing treatment should be used. Improve the surface roughness of the valve stem and the surface roughness of other valve parts. The higher the surface roughness level, the stronger the corrosion resistance. Improve the processing and structure of fillers and gaskets, use flexible graphite and plastic fillers, as well as flexible graphite adhesive gaskets and PTFE gaskets to improve sealing performance and reduce ground stem and flange seals Corrosion of the surface.