Corrosion is one of the important factors causing valve damage. Therefore, in the use of valves, anti-corrosion protection is the first consideration. Today, I would like to share with you the anti-corrosion measures of valves, including the selection of valve materials under common working conditions.
Principle of Valve Corrosion
The corrosion of metals is mainly caused by chemical corrosion and point chemical corrosion. The corrosion of non-metallic materials is usually caused by direct chemical and physical action.
1. Chemical Corrosion
In the absence of electric current, the surrounding medium directly acts with metals and causes them to be destroyed, such as the corrosion of metals by high temperature dry gas and non-electrolytic solution.
2. electro-chemical corrosion
The main form of corrosion is that metal contacts with electrolyte to produce electron flow, which destroys its electrochemical action.
Common acid-alkali solution corrosion, atmospheric corrosion, soil corrosion, seawater corrosion, microbial corrosion, stainless steel pitting corrosion and crevice corrosion are all electrochemical corrosion.
The electrochemical corrosion not only occurs between two chemically active substances, but also produces potential difference due to the concentration difference of solution, the concentration difference of surrounding oxygen, the tiny difference of material structure, etc. The power of corrosion is obtained and the metal in the position of anode plate with low potential is damaged.
General Measures for Valve Antisepsis
1. Choosing Corrosion Resistant Materials Based on Medium
Many mediums are corrosive, and their corrosion principle is very complex. Even if the same valve material is used in the same medium, the corrosion of the medium to the material will be different if the concentration, temperature and pressure of the medium are different.
The corrosion rate increases by 1-3 times with the increase of medium temperature by 10 C. Medium concentration also has a great influence on the corrosion of valve materials.
Selection of Valve Materials under Different Working Conditions
[Sulfuric acid medium]
Sulfuric acid of different concentration and temperature has great difference in corrosion of materials. Carbon steel and cast iron have better corrosion resistance for concentrated sulfuric acid with concentration above 80% and temperature below 80 C.
However, carbon steel and cast iron are not suitable for high-speed flow of sulfuric acid.
Ordinary stainless steel, such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti), are also limited in the use of sulphuric acid medium. Therefore, pumping valves for sulphuric acid transportation are usually made of high silicon cast iron (difficult to cast and process) and high alloy stainless steel (alloy 20).
Fluoro-plastics have good sulfuric acid resistance. It is a more economical choice to use fluorine-lined pump valve (F46). If the pressure is too high and the temperature rises, the point of use of the plastic valve will be impacted, so we can only choose ceramic ball valve which is more expensive than it.
[Hydrochloric acid medium]
Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), and ferrosilicon containing molybdenum can only be used in hydrochloric acid below 50 C and 30%.
Contrary to metal materials, most non-metallic materials have good corrosion resistance to hydrochloric acid, so lining rubber pump and plastic pump (such as polypropylene, fluoroplastics, etc.) is the best choice for transporting hydrochloric acid.
But if the temperature of such a medium exceeds 150 C, or the pressure is greater than 16 kg, any plastic (including polypropylene, fluoroplastics and even PTFE) will not be competent.
[Nitric acid medium]
Most metals are rapidly corroded and destroyed in nitric acid. 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 molybdenum-containing stainless steel (such as 316, 316L) to nitric acid is not as good as that of ordinary stainless steel (such as 304, 321).
For high temperature nitric acid, titanium and titanium alloys are usually used.
[Chlorine gas (liquid chlorine) medium]
Most metal valves have limited resistance to chlorine corrosion, especially when chlorine carries water, including various alloy valves.
For the chlorine tetrafluoro valve is a good choice, but the tetrafluoro valve with a slightly longer time, increased torque, tetrafluoro aging problems will be highlighted;
Replacing the original ordinary lined tetrafluoro valve with tetrafluoro ceramic ball core, using the self-lubrication of ceramics and the corrosion resistance of tetrafluoro will have a better effect.
[Ammonia (Ammonia Hydroxide) Medium]
Most metals and nonmetals corrode slightly in liquid ammonia and ammonia water (ammonia hydroxide). Only copper and copper alloys are not suitable for use.
[Alcohols, ketones, esters, ethers]
Common alcohols, ketones, esters and ethers are basically non-corrosive. Common materials can be used. Reasonable selection should be made according to the properties of media and related requirements.
In addition, it is worth noting that ketone, ester and ether are soluble to many kinds of rubber, so mistakes should be avoided when choosing sealing materials.
2. Use of non-metallic materials
Non-metallic materials have excellent corrosion resistance. As long as the temperature and pressure of valves meet the requirements of non-metallic materials, the use of non-metallic materials can not only solve the problem of corrosion resistance, but also save precious metals and reduce the cost of valves.
Nowadays, more and more valves use nylon, polytetrafluoroethylene and other plastics as well as natural rubber and synthetic rubber to make various sealing surfaces and rings. These non-metallic materials have good corrosion resistance and sealing performance, and are especially suitable for use in medium with particles.
However, because of its low strength and heat resistance, its application scope is limited. Flexible graphite makes non-metallic materials enter the high temperature field, solves the problem of leakage of fillers and gaskets which is difficult to solve for a long time, and is a good high temperature lubricant.
3. Spray paint
Coatings are one of the most widely used anti-corrosion means, and they are indispensable anti-corrosion materials and identification marks in valve products.
Coatings are usually made of synthetic resin, rubber slurry, vegetable oil, solvents, etc. They cover the metal surface, isolate the medium and atmosphere, and achieve the purpose of anti-corrosion. The paint is mixed with different colors to represent the material of the valve.
Coatings are mainly used in water, brine, seawater or atmospheric environments where corrosion is not strong.
4. Adding Corrosives
The mechanism of corrosion control by corrosion inhibitor is that it promotes the polarization of battery. Corrosion inhibitor is mainly used in medium and filler. Adding corrosion inhibitor in medium can slow down the corrosion of equipment and valve.
Chromium-nickel stainless steel in oxygen-free sulfuric acid, in a large concentration range into active state, corrosion is serious, but adding a small amount of copper sulfate or nitric acid oxidant, can make stainless steel into passive state, the surface of a protective film, prevent the erosion of media.
In hydrochloric acid, if a small amount of oxidant is added, the corrosion of titanium can be reduced. Valves often use water as pressure test medium, which is easy to cause valve corrosion. Adding a small amount of sodium nitrite in water can prevent the corrosion of water to the valve.
The asbestos packing contains chloride, which corrodes the valve stem greatly. The chloride content can be reduced by washing with distilled water. However, this method is difficult to implement and should not be popularized widely. It is only suitable for special needs.
In order to protect the valve stem from corrosion of asbestos packing, corrosion inhibitors and sacrificial metals are coated in the asbestos packing and on the valve stem.
The corrosion inhibitor is composed of sodium nitrite, sodium chromate and solvent. Sodium nitrite and sodium chromate can form a passivation film on the surface of the valve stem and improve the corrosion resistance of the valve stem. Solvents can dissolve corrosion inhibitors slowly and lubricate them.
Zinc powder is added to asbestos as sacrificial metal. In essence, zinc is also a corrosion inhibitor. It can first combine with chloride in asbestos, so that chloride and valve stem metal contact opportunities are reduced, so as to achieve the purpose of corrosion protection.
Spraying on the valve surface can prevent atmospheric corrosion if some corrosion inhibitors such as red red red and calcium lead are added to the coating.
5. electro-chemical protection
There are two kinds of electrochemical protection: anodic protection and cathodic protection.
Anodic protection is to protect the metal as the anode to import the external direct current, so that the anode potential increases in the positive direction. When it increases to a certain value, a dense protective film is formed on the surface of the metal anode, that is, passivation film. At this time, the corrosion of the metal cathode decreases sharply. Anodic protection is suitable for metals that are easily passivated.
Cathodic protection is to use the protected metal as the cathode and direct current to reduce its potential in the negative direction. When it reaches a certain potential value, the speed of corrosion current decreases and the metal is protected. In addition, cathodic protection can protect the protected metal by metal whose electrode potential is more negative than that of the protected metal. If zinc is used to protect iron, zinc is corroded. Zinc is called sacrificial metal.
In production practice, less anode protection is used and more cathodic protection is used. This cathodic protection method is an economical, simple and effective method for large valves and important valves.
6. Surface Coating Treatment
Metal surface treatment technology includes surface coating, surface penetration, surface oxidation passivation and so on. Its purpose is to improve metal corrosion resistance and mechanical properties. Surface treatment is widely used in valves.
Valve connection bolts are commonly treated with galvanizing, chromium plating and oxidation (bluing) to improve their resistance to atmospheric or dielectric corrosion.
In addition to the above methods, other fasteners can also be treated by phosphating passivation and other surface treatment processes according to the situation.
Sealing surfaces and small caliber closures are often treated by nitriding or boronizing to improve their corrosion resistance and wear resistance. If the valve disc is made of 38CrMoAlA, the nitriding layer thickness is more than 014mm.
Valve stem is usually treated by nitriding, boronizing, chromium plating and nickel plating to improve its corrosion resistance, wear resistance and abrasion resistance.
Different surface treatment is suitable for different material and working environment of valve stem. Hard chromium plating and gas nitriding process can be used for valve stem contacting with asbestos filler in air or steam medium (ion nitriding process is not suitable for stainless steel).
In hydrogen sulfide gas environment, the use of electroplated high phosphorus nickel coating has better protection performance.
The corrosion resistance of 38CrMoAlA can be improved by ion and gas nitriding, but hard chromium coating is not suitable. 2Cr13 can resist ammonia corrosion after quenching and tempering, and carbon steel nitrided by gas can also resist ammonia corrosion, but all phosphorus-nickel coatings are not resistant to ammonia corrosion.
38CrMoAlA material nitrided by gas has excellent corrosion resistance and comprehensive performance, and is often used to make valve stem. Small caliber valves and handwheels are often chrome plated to improve their corrosion resistance and decorate valves.
7. Thermal Spraying
Thermal spraying is a process for preparing coatings and has become one of the new technologies for surface protection and strengthening of materials.
Thermal spraying is a surface strengthening process that uses high energy density heat sources (gas combustion flame, arc, plasma arc, electrothermal, gas explosion, etc.) to melt metal or non-metallic materials, spray them onto the pretreated substrate surface in the form of atomization, form spray coating, or at the same time heat the substrate surface, make the coating melt again on the substrate surface, and 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 non-metal substrates by one or more thermal spraying methods.
Thermal spraying can improve the surface corrosion resistance, wear resistance, high temperature resistance and other properties, extend the service life.
Thermal spraying special functional coatings, with heat insulation, insulation (or conductivity), sealing, self-lubrication, thermal radiation and electromagnetic shielding and other special properties. Components can also be repaired by thermal spraying.
8. Environmental Control
The atmosphere is full of dust, steam and smoke. Especially in the production environment, toxic gases and dust emitted from chimneys and equipment will cause varying degrees of corrosion to valves.
The valves should be cleaned and purged regularly and refueled regularly according to the rules of operation, which is an effective measure to control environmental corrosion.
Valve stem mounting protective cover, ground valve mounting wells and valve surface painting are also effective ways to prevent corrosive substances from corroding valves.
Rising ambient temperature and air pollution will accelerate the corrosion of equipment and valves in closed environment. Open workshop or ventilation measures should be adopted to reduce environmental corrosion as far as possible.
9. Improvement of process and structure
The anti-corrosion protection of the valve should be considered from the design. If the structure of the valve is reasonable and the processing method is correct, the corrosion of the valve can be greatly reduced.
Therefore, the parts which are easy to cause corrosion in valves should be improved to meet the requirements of different working conditions.
Oxygen concentration battery corrosion can occur in the gap of the valve connection. Therefore, the threaded connection should not be used in the connection between the valve stem and the closing parts as far as possible.
Spot welding and lap welding are prone to corrosion. Double-sided welding and continuous welding should be adopted for valve welding.
The PTFE raw material belt should be used at the threaded connection of the valve, which not only has good sealing, but also can prevent corrosion.
Where the medium in the valve is not easy to flow, it is easy to be corroded. In addition to discharging sedimentary medium when installing and using the valve, when manufacturing the valve parts, the sag structure should be avoided as far as possible, and the discharge hole should be set as far as possible.
Different metal contacts will form galvanic couples and promote corrosion of anodic metals. When selecting materials, attention should be paid to avoid metal contacts with large potential difference and no passivation film.
Stress corrosion occurs in the process of welding and heat treatment. Attention should be paid to improving the processing methods. After welding, the corresponding protective measures such as annealing treatment should be adopted as far as possible.
Improve the surface finish grade of the stem and other valve parts, with good surface finish and corrosion resistance.
Improve the processing technology and structure of fillers and gaskets, and use flexible graphite, plastic fillers, flexible graphite sticking gaskets and PTFE gaskets to improve sealing performance and reduce the corrosion of valve stem and flange sealing surface.
Points for Attention to Anticorrosion of Valve Parts
Stem Corrosion and Protection
The main causes of valve stem corrosion are as follows:
Corrosion damage of valve body is mainly caused by corrosive medium, while corrosion of valve stem is mainly caused by packing.
Not only the corrosive medium causes the stem to corrode, but also the steam and water can cause spot on the contact between the stem and the filler. In particular, the valve stored in the warehouse will also occur spotting corrosion of the valve stem. This is the electrochemical corrosion of the filler on the stem.
At present, the most widely used filler is asbestos-based packing, asbestos materials contain chloride ions for a certain time, in addition to potassium, sodium, magnesium plasma, these are corrosion factors.
Points for Attention to Stem Antisepsis:
Do not add packing during valve storage. Without packing, the electrochemical corrosion factor of the valve stem is lost and can be preserved for a long time without being corroded.
Surface treatment of valve stem
Such as chromium plating, nickel plating, nitriding, boronizing, ginseng zinc, etc.
Reduce asbestos impurities. The chlorine content in asbestos can be reduced by washing with distilled water, thus the corrosivity of asbestos can be reduced.
Corrosion inhibitor was added to asbestos pan root. The corrosion inhibitor can inhibit the corrosiveness of chloride ions. Such as sodium nitrite, adding sacrificial metals to asbestos. This is a metal that is lower than the stem potential as a victim. In this way, the corrosion of chloride ions occurs first at the sacrificial metal, thus protecting the valve stem. Zinc powder can be used as sacrificial metal.
It is protected by polytetrafluoroethylene. Polytetrafluoroethylene (PTFE) has excellent chemical stability and dielectric properties, and the current can not pass through. If the asbestos root is impregnated with PTFE, the corrosion will be reduced. Polytetrafluoroethylene raw material belt can also be used to wrap asbestos cockroaches and then pack them into packing box.
Improving the processing finish can also reduce the electrochemical corrosion.
Corrosion and Protection of Closed Parts
The main causes of the corrosion of the shut-off parts are as follows:
Closure parts are often eroded by fluids, which accelerates the development of corrosion. Some discs, although made of better material, are corroded faster than the body.
The upper and lower shut-off parts are usually connected with threads of valve stem and seat. The connection is more oxygen-deficient than the common parts, which is easy to form oxygen concentration battery and make it corroded and damaged. Some of the sealing surfaces of the closures are pressed in. Oxygen concentration battery corrosion will also occur due to the loose fit and slight cracks.
Anti-corrosion precautions for closures:
Use corrosion resistant materials as far as possible. Closure parts are small in weight, but play a key role in the valve, as long as they can resist corrosion, even if a little valuable material is used.
Improve the closure structure to make it less susceptible to fluid erosion.
Improve the connection structure to avoid oxygen concentration battery.
In valves below 200 C, the use of raw PTFE tape as filler at the joint of closure and sealing surface can reduce the corrosion of these parts.
In consideration of corrosion resistance, attention should also be paid to the erosion resistance of the closure material. The material with strong erosion resistance should be used as the closing part.