Basic knowledge and preventive measures of valve corrosion
Corrosion is one of the important elements causing valve damage. Therefore, in the valve protection, valve anti-corrosion is an important issue to consider. What is the corrosion of valves? How to carry out valve anticorrosion? What is the basis for the selection of corrosion-resistant valves? Listen, how do you get here.
The corrosion of metals is mainly caused by chemical corrosion and electrochemical corrosion, and the corrosion of non-metallic materials is generally caused by direct chemical and physical effects.
Under the condition of no electric current, the surrounding medium directly acts with metal to destroy it, such as the corrosion of metal by high temperature dry gas and non electrolytic solution.
The main form of corrosion is the contact between metal and electrolyte, which produces electron flow and destroys itself in electrochemical action.
The common corrosion of acid-base and salt solution, atmospheric corrosion, soil corrosion, seawater corrosion, microbial corrosion, pitting corrosion and crevice corrosion of stainless steel, etc., are all electrochemical corrosion.
Electrochemical corrosion not only occurs between two kinds of substances that can play a chemical role, but also produces potential difference due to the concentration difference of solution, the concentration difference of surrounding oxygen, the tiny difference of material structure and so on, thus obtaining the power of corrosion and causing the loss of metal with low potential and in the position of anode plate.
Corrosion rate of valve
The corrosion rate can be divided into six grades:
❶ Complete corrosion resistance
➨ the corrosion rate is less than 0.001 mm / year
❷ Extreme corrosion resistance
➨ corrosion rate 0.001 to 0.01 mm / year
❸ Corrosion resistance
➨ corrosion rate 0.01 to 0.1 mm / year
❹ Corrosion resistant
➨ corrosion rate 0.1 to 1.0 mm / year
❺ Poor corrosion resistance
➨ corrosion rate 1.0 to 10 mm / year
❻ Corrosion resistant
➨ the corrosion rate is more than 10 mm / year
1. Select corrosion resistant materials according to corrosion medium
In production practice, the corrosion of medium is very complex. Even if the valve material used in a medium is the same, the corrosion of medium to material is different with different concentration, temperature and pressure of medium. The corrosion rate increases about 1 ~ 3 times with every 10 ℃ increase of medium temperature.
The medium concentration has a great influence on the corrosion of valve materials. For example, the corrosion of lead in sulfuric acid with low concentration is very small. When the concentration exceeds 96%, the corrosion rises sharply. On the contrary, the corrosion of carbon steel is the most serious when the concentration of sulfuric acid is about 50%. When the concentration increases to more than 6%, the corrosion drops sharply.
For example, aluminum is highly corrosive in concentrated nitric acid with concentration of more than 80%, but the corrosion is serious in medium and low concentration nitric acid. Although stainless steel has strong corrosion resistance to dilute nitric acid, the corrosion is aggravated in concentrated nitric acid with concentration of more than 95%.
It can be seen from the above examples that the correct selection of valve materials should be based on the specific situation, analyze various factors affecting corrosion, and select materials according to the relevant anti-corrosion manual.
2. Use non-metallic materials
Nonmetal corrosion resistance is excellent, as long as the valve temperature and pressure meet the requirements of nonmetal materials, not only can solve the corrosion problem, but also can save precious metals.
Valve body, valve cover, lining, sealing surface and other commonly used non-metallic materials, as for gasket, packing is mainly made of non-metallic materials.
PTFE, chlorinated polyether and other plastics, as well as natural rubber, chloroprene rubber, NBR and other rubbers are used as valve lining, while the main body of valve body and valve cover is made of general cast iron and carbon steel. The strength and corrosion resistance of the valve are ensured.
Pinch valve is also designed according to the excellent corrosion resistance and excellent denaturation of rubber. Now more and more nylon, polytetrafluoroethylene and other plastics are used, and natural rubber and synthetic rubber are used to make all kinds of sealing surfaces and rings for all kinds of valves.
These non-metallic materials used as sealing surfaces not only have good corrosion resistance, but also have good sealing performance. They are especially suitable for use in media with particles. Of course, their strength and heat resistance are low, and their application scope is limited.
The appearance of flexible graphite makes non-metal enter the field of high temperature, solves the long-term difficult problem of packing and gasket leakage, and is a good high temperature lubricant.
3. Metal surface treatment
Valve connecting bolts are usually treated with zinc plating, chromium plating and oxidation (bluing) to improve the ability of atmospheric and medium corrosion resistance. In addition to the above methods, other fasteners are also treated with phosphating and other surface treatments according to the situation
Sealing surface and closing parts with small diameter
The surface technology of nitriding and boronizing is often used to improve its corrosion resistance and wear resistance.
Stem corrosion protection
Nitriding, boronizing, chromium plating, nickel plating and other surface treatment processes are widely used to improve its corrosion resistance, corrosion resistance and abrasion resistance.
- Different surface treatment should be suitable for different material and working environment of valve stem. Hard chromium plating and gas nitriding process can be used for valve stem contacting asbestos packing in atmosphere and steam medium (ion nitriding process is not suitable for stainless steel);
- In the atmosphere of hydrogen sulfide, the valve with high phosphorus nickel plating has better protection performance;
- 38CrMoAlA can also resist corrosion by ion and gas nitriding, but hard chromium coating is not suitable;
- 2Cr13 can resist ammonia corrosion after quenching and tempering, carbon steel with gas nitriding can also resist ammonia corrosion, but all phosphorus nickel coatings are not resistant to ammonia corrosion;
- After gas nitriding, 38CrMoAlA material has excellent corrosion resistance and comprehensive performance.
Small diameter valve body and hand wheel
It is also often chrome plated to improve its corrosion resistance and decorate the valve.
4. Thermal spraying
Thermal spraying is a kind of technology for coating preparation, which has become one of the new technologies for material surface protection.
It uses high energy density heat source (gas combustion flame, arc, plasma arc, electric heating, gas explosion, etc.) to heat and melt metal or non-metal materials, spray them on the pretreated basic surface in the form of atomization to form spray coating, or heat the basic surface at the same time to make the coating melt again on the substrate surface to form spray welding layer.
Most metals and their alloys, metal oxide ceramics, metal ceramic composites and hard metal compounds can be coated on metal or nonmetal substrates by one or more thermal spraying methods.
Thermal spraying can improve the surface corrosion resistance, wear resistance, high temperature resistance and prolong the service life. Thermal spraying special functional coating, with heat insulation, insulation (or heteroelectric), grindable seal, self lubrication, thermal radiation, electromagnetic shielding and other special properties; Parts can be repaired by thermal spraying.
Coating is the most widely used anti-corrosion means, and it is an indispensable anti-corrosion material and identification mark in valve products.
Coatings are also non-metallic materials, which are usually made of synthetic resin, rubber slurry, vegetable oil, solvent, etc. they cover the metal surface, isolate the medium and atmosphere, and achieve the purpose of anti-corrosion.
The coating is mainly used in water, salt water, sea water, atmosphere and other environment with less strong corrosion. The inner cavity of the valve is usually painted with anti-corrosion paint to prevent the corrosion of the valve by water, air and other media. The paint is mixed with different colors to indicate the materials used by Farne. The valve is usually sprayed with paint once every six months to a year.
6. Add corrosion inhibitor
The mechanism of corrosion inhibitor controlling corrosion is that it promotes the polarization of battery.
Corrosion inhibitors are mainly used in media and fillers. Adding corrosion inhibitor in the medium can slow down the corrosion of equipment and valves. For example, chromium nickel stainless steel in sulfuric acid without oxygen can be cremated in a large range of solubility, and the corrosion is serious. However, adding a small amount of oxidant such as copper sulfate or nitric acid can make the stainless steel change into a passive state, and form a protective film on the surface to prevent the corrosion of the medium, It can reduce the corrosion of titanium.
Water is often used as the medium of valve pressure test, which is easy to cause corrosion of the valve. Adding a small amount of sodium nitrite in water can prevent water corrosion of the valve.
Asbestos packing contains chloride, which causes great corrosion to the valve stem. The content of chloride can be reduced by using distilled water washing method. However, this method is difficult to implement and can not be popularized. Ester is suitable for special needs.
In order to protect the valve stem and prevent the corrosion of asbestos packing, the valve stem is coated with corrosion inhibitor and sacrificial metal. The corrosion inhibitor consists of sodium nitrite and sodium chromate, which can form a passive film on the surface of the valve stem and improve the corrosion resistance of the valve stem; The solvent can make the corrosion inhibitor dissolve slowly and play a lubricating role;
Zinc powder is added to asbestos as sacrificial metal. In fact, zinc is also a kind of corrosion inhibitor. It can first combine with chloride in asbestos, so that the chance of chloride contacting with valve rod metal is greatly reduced, so as to achieve the purpose of corrosion prevention. If red lead, calcium lead and other corrosion inhibitors are added to the coating, spraying on the valve surface can prevent atmospheric corrosion.
7. Electrochemical protection
Electrochemical protection includes anode protection and cathode protection.
If zinc is used to protect iron and zinc is corroded, zinc is called sacrificial metal. In production practice, anodic protection is less used, while cathodic protection is more used. This cathodic protection method is an economical, simple and effective method for large valves and important valves. Adding zinc in asbestos packing to protect valve stem also belongs to cathodic protection method
8. Control of corrosive environment
The so-called environment, there are two broad and narrow sense, broad environment refers to the valve installation place around the environment and its internal circulation medium; Narrow sense of the environment refers to the conditions around the valve installation.
Most of the environment can not be controlled, and the production process can not be arbitrarily changed. Only when there is no damage to the product and process, can the environmental control methods be used, such as boiler water deoxidation, pH value adjustment in refining process, etc. From this point of view, the addition of corrosion inhibitor and electrochemical protection are also controlled corrosion environment.
The atmosphere is full of dust, water vapor, smoke, especially in the production environment, such as bittern, poisonous gas and micro powder emitted by equipment, which will cause different degrees of corrosion to the valve.
The operators should clean and purge the valve regularly and add oil regularly according to the regulations in the operation procedures, which is an effective measure to control environmental corrosion. Installation of protective cover for valve stem, installation of ground well for ground valve and painting on valve surface are all methods to prevent corrosive substances from eroding the valve.
The increase of environmental temperature and air pollution, especially for the equipment and valves in the closed environment, will accelerate their corrosion. Open plant or ventilation and cooling measures should be adopted as far as possible to slow down the environmental corrosion.
9. Improve the processing technology and valve structure
The anti-corrosion protection of valve is considered from the beginning of design. A valve product with reasonable structure design and correct process method undoubtedly has a good effect on reducing the corrosion of valve.
Therefore, the design and manufacturing departments should improve those parts which are unreasonable in structure design, incorrect in process method and easy to cause corrosion, so as to make them suitable for various working conditions.
Unique measures for different corrosion types of valve components
1. Methods to prevent intergranular corrosion of austenitic stainless steel valves are as follows:
The austenitic stainless steel with titanium and niobium but less than 0.03% carbon content is selected to reduce the production of chromium carbide.
2. Stress corrosion occurs under the action of both corrosion and tensile stress.
Methods to prevent stress corrosion: eliminate or reduce the stress in welding and cold working by heat treatment, improve unreasonable valve structure, avoid stress concentration, adopt electrochemical protection, spray anti-corrosion coating, add corrosion inhibitor, apply pressure stress and other measures.
3. Wear corrosion is a kind of corrosion form produced by the alternate action of fluid on metal wear and corrosion. It is a common corrosion of valves. This kind of corrosion mainly occurs on the sealing surface.
Prevention methods: select corrosion-resistant and wear-resistant materials, improve structural design, adopt cathodic protection, etc
4. Friction and vibration corrosion is the damage of contact surface caused by vibration and sliding when two parts in contact bear load at the same time. The friction corrosion occurs at the bolt connection, the connection between the valve stem and the closing piece, and between the ball bearing and the shaft.
Prevention method: it can be protected by applying lubricating grease, reducing friction, phosphating the surface, selecting hard alloy, and improving the surface hardness by spraying tile or cold working.
5. After welding, annealing treatment and other protective measures should be adopted as far as possible. To improve the surface roughness of valve stem and other valve parts, the higher the surface roughness level is, the stronger the anti-corrosion ability is.
Prevention methods: improving the processing technology and structure of packing and gasket, using flexible graphite and plastic packing, as well as flexible graphite bonded gasket and PTFE wrapped gasket can improve the sealing performance and reduce the corrosion of valve stem and flange sealing surface.
Precautions for corrosion protection of valve components
1. Stem corrosion and protection
The main causes of stem corrosion are as follows:
The corrosion damage of valve body is mainly caused by corrosive medium, while the corrosion of valve stem is mainly caused by packing.
Not only the corrosive medium causes the corrosion damage of the valve stem, but also the steam and water can cause the spots at the contact between the valve stem and the packing. In particular, the valve stored in the warehouse will also have stem pitting corrosion. This is the electrochemical corrosion of the packing to the valve stem.
Now the most widely used filler is packing based on asbestos. Asbestos materials contain certain chloride ions, in addition to potassium, sodium and magnesium plasma, which are the factors of corrosion.
Precautions for corrosion protection of valve stem:
Do not add packing during valve storage. Without packing, the electrochemical corrosion factor of the valve stem is lost, and it can be stored for a long time without corrosion.
Surface treatment of valve stem. Such as chromium plating, nickel plating, nitriding, boronizing, zinc and so on.
Reduce asbestos impurities. The chlorine content in asbestos can be reduced by washing with distilled water, so as to reduce its corrosiveness.
Add corrosion inhibitor into asbestos packing. The corrosion inhibitor can inhibit the corrosion of chloride ion. Such as sodium nitrite.
Adding sacrificial metals to asbestos. This is a metal with a lower potential than the valve stem as a victim. In this way, chloride corrosion occurs first on the sacrificial metal to protect the valve stem. Zinc powder can be used as sacrificial metal.
PTFE protection is adopted. Polytetrafluoroethylene (PTFE) has excellent chemical stability and dielectric properties, so the current can not pass through. If the asbestos packing is impregnated with PTFE, the corrosion will be reduced. Asbestos packing can also be wrapped with Teflon raw material tape and then packed into stuffing box.
The improvement of machining finish can also reduce electrochemical corrosion.
2. Corrosion and protection of closing parts
The main causes of corrosion of closing parts are as follows:
The closure is often scoured by fluid, which accelerates the development of corrosion. Some discs, though made of better materials, are corroded faster than the valve body.
The upper and lower closures are usually connected with the valve stem and valve seat by screw thread, and the connection is oxygen deficient compared with the general parts, so it is easy to form the oxygen concentration cell and cause corrosion damage. The sealing surface of some closing parts is pressed in. Due to the loose fit and slight gap, oxygen concentration cell corrosion may also occur.
Precautions for anticorrosion of closing parts:
Try to use corrosion-resistant materials. The closing piece is small in weight but plays a key role in the valve. As long as it is corrosion-resistant, even a little valuable material can be used.
The structure of closing part is improved to make it less eroded by fluid.
The connection structure is improved to avoid oxygen concentration difference battery.
In the valve below 200 ℃, the use of PTFE raw material belt as packing can reduce the corrosion of these parts.
In consideration of corrosion resistance, attention should also be paid to the erosion resistance of closure materials. Materials with strong erosion resistance shall be used as closing parts.
Selection of corrosion resistant valve
In the corrosive medium condition of the valve, anti-corrosion is the most critical part of chemical equipment, if you can not correctly select the metal material of chemical valve, slight carelessness, light damage to the equipment, serious accident or even disaster.
In view of some commonly used chemical media corrosion-resistant valve in the end how to choose?
1. Sulfuric acid medium
As one of the strong corrosive media, sulfuric acid is an important industrial raw material widely used. Sulfuric acid with different concentration and temperature has great difference in corrosion of materials. For sulfuric acid with concentration above 80% and temperature below 80 ℃, carbon steel and cast iron have better corrosion resistance, but they are not suitable for sulfuric acid with high speed flow and are not suitable for valve materials.
Common stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) are also limited in use for sulfuric acid medium.
Therefore, valves conveying sulfuric acid are usually made of high silicon cast iron (difficult to cast and process) and high alloy stainless steel (No. 20 alloy). Fluoroplastics have good resistance to sulfuric acid. It is a more economical choice to use fluorine lined pump valve (F46). If the pressure is too high and the temperature rises, the use point of the plastic valve will be impacted, so we can only choose the more expensive ceramic ball valve.
2. Hydrochloric acid medium
Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), and molybdenum containing high silicon iron can only be used for hydrochloric acid below 50 ℃ and 30%.
Contrary to metal materials, most non-metal materials have good corrosion resistance to hydrochloric acid, so rubber lined pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) are the best choice for transporting hydrochloric acid.
However, if the temperature of such medium exceeds 150 ℃ or the pressure is greater than 16 kg, any plastic (including polypropylene, fluoroplastics and even polytetrafluoroethylene) will not be competent. At present, there is no ideal valve on the market.
However, you can try the new ceramic ball valve. The advantages of this valve are self lubrication, small torque, no aging, and its service life is much longer than that of ordinary valves. Its disadvantage is that its price is much higher than that of plastic valves.
3. Nitric acid medium
Stainless steel is the most widely used nitric acid resistant material. It has good corrosion resistance to all concentrations of nitric acid at room temperature. It is worth mentioning that the corrosion resistance of stainless steel containing molybdenum (such as 316, 316L) to nitric acid is not better than that of ordinary stainless steel (such as 304, 321), sometimes even worse.
For high temperature nitric acid, titanium and titanium alloy are usually used.
4. Acetic acid medium
It is one of the most corrosive substances in organic acids. Ordinary steel will be seriously corroded in acetic acid of all concentrations and temperatures. Stainless steel is an excellent acetic acid resistant material. 316 stainless steel containing molybdenum can also be used in high temperature and dilute acetic acid steam.
For high temperature and high concentration acetic acid or containing other corrosive media and other harsh requirements, high alloy stainless steel or fluoroplastic valve can be selected.
5. Alkali (sodium hydroxide)
Iron and steel are widely used in sodium hydroxide solution with concentration below 80 ℃ and within 30%. Many petrochemical plants still use ordinary iron and steel at 100 ℃ and below 75%. Although the corrosion is increased, the economy is good.
Compared with cast iron, ordinary stainless steel has no obvious advantage in corrosion resistance to alkaline solution. As long as a small amount of iron is allowed to be added into the medium, stainless steel is not recommended. For high temperature alkaline solution, titanium and titanium alloy or high alloy stainless steel are mostly used.
6. Ammonia (ammonia hydroxide)
The corrosion of most metals and nonmetals in liquid ammonia and ammonia water (ammonia hydroxide) is very slight, only copper and copper alloys are not suitable for use.
7. Chlorine (liquid chlorine)
Most metal valves have limited resistance to chlorine corrosion, especially in the case of chlorine with water, including all kinds of alloy valves. In this case, Teflon valve is a good choice, but the chemical plants producing chlor alkali will find that the problem of Teflon aging will be highlighted when the time of Teflon valve is slightly longer and the torque force increases, In this case, the leak is fatal.
It can be considered to replace the original ordinary PTFE lined valve with PTFE lined ceramic ball core. The perfect effect can be achieved by using the self-lubricating property of ceramics and the corrosion resistance of PTFE.
8. Salt water (sea water)
The corrosion rate of ordinary steel in NaCl solution, seawater and salt water is not very high, so it is generally necessary to use coating protection.
All kinds of stainless steel also have very low uniform corrosion rate, but it may cause local corrosion due to chloride ion. 316 stainless steel is usually better.
9. Alcohols, ketones, esters and ethers
Common alcohol media include methanol, ethanol, glycol, propanol, etc., ketone media include acetone, butanone, etc., ester media include all kinds of methyl ester, ethyl ester, etc., ether media include methyl ether, ether, butyl ether, etc., they are basically non corrosive, common materials can be applied, specific selection should be made according to the properties of the media and relevant requirements.
In addition, it should be noted that ketones, esters and ethers are soluble to a variety of rubbers, so mistakes should be avoided in the selection of sealing materials.
Source: Network Arrangement – China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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