The corrosion of metals is mainly caused by chemical corrosion and electrochemical corrosion. The corrosion of non-metallic materials is generally caused by direct chemical and physical damage.
1. Chemical corrosion
The surrounding medium directly reacts with the metal to cause damage under the condition that no current is generated, such as corrosion of the metal by the high-temperature drying gas and the non-electrolytic solution.
2. Electrochemical corrosion
The metal is in contact with the electrolyte, causing the flow of electrons, which destroys itself in electrochemical action, which is the main form of corrosion.
Common acid-base salt solution corrosion, atmospheric corrosion, soil corrosion, seawater corrosion, microbial corrosion, stainless steel pitting corrosion and crevice corrosion are all electrochemical corrosion.
Electrochemical corrosion occurs not only between two substances that can act as a chemical, but also because of the difference in concentration of the solution, the difference in the concentration of surrounding oxygen, the slight difference in the structure of the substance, etc., the difference in potential is generated, and the power of corrosion is obtained. The metal with low potential and in the positive position is lost.
Valve corrosion rate
Corrosion speed can be divided into six:
- Full corrosion resistance corrosion rate less than 0.001 mm / year
- Extreme corrosion resistance corrosion rate 0.001 to 0.01 mm / year
- Corrosion resistance corrosion rate 0.01 to 0.1 mm / year
- Corrosion resistance corrosion rate 0.1 to 1.0 mm / year
- Poor corrosion resistance Corrosion rate 1.0 to 10 mm / year
- Non-corrosion corrosion rate greater than 10 mm / year
Nine major anti-corrosion measures
1. Select corrosion resistant materials according to corrosive media
In the actual production, the corrosion of the medium is very complicated. Even in the case of the valve material used in a medium, 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, the lead is in the sulfuric acid with a small concentration, and the corrosion is small. When the concentration exceeds 96%, the corrosion rises sharply. 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 at a concentration of 80% or more, but it is severely corroded in medium and low concentrations of nitric acid. Although stainless steel has strong corrosion resistance 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 and sealing rings are used for natural rubber and synthetic rubber for various types of valves.
These non-metallic materials used as sealing surfaces not only have good corrosion resistance, but also good sealing performance, and are particularly suitable for use in granular media. 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. Metal surface treatment
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 also subjected to surface treatment such as phosphating depending on the situation.
Sealing surface and closing piece with small diameter
Surface processes such as nitriding and boronizing are often used to improve its corrosion resistance and wear resistance. The valve disc made of 38CrMoAlA has a nitrided layer ≥0.4mm.
Stem corrosion protection
Surface treatment processes such as nitriding, boronizing, chrome plating, and nickel plating are widely used to improve corrosion resistance, corrosion resistance and abrasion resistance.
Different surface treatments should be suitable for different valve stem materials and working environments. The stems that are in contact with the atmosphere, 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 be resistant to ammonia corrosion after quenching and tempering, carbon steel using gas nitriding 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 a valve stem.
Small diameter valve body and hand wheel
It is also often chrome-plated to improve its corrosion resistance and to decorate the valve.
4. 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 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 atomized form to form a sprayed layer. , or a method of surface strengthening which simultaneously heats the basic surface to re-melt the coating on the surface of the substrate to form a 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.
5. 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.
6. Add corrosion inhibitor
What is the mechanism of corrosion inhibitor control? It 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 is used, the chloride content can be reduced. However, this method is difficult to implement and cannot be generalized. The ester is suitable for special needs.
In order to protect the valve stem and prevent 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;
Zinc powder is added to asbestos 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.
7. Electrochemical protection
Electrochemical protection has both anodic protection and cathodic protection.
If zinc is used to protect iron, zinc is corroded and zinc is called sacrificial metal. In production practice, anode protection is used less and cathodic protection is used 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.
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 can be adopted, such as boiler water deoxidation, and the pH value of the alkali in the refining process. From this point of view, the above-mentioned addition of corrosion inhibitors, electrochemical protection, and the like are also controlled corrosion environments.
The atmosphere is filled with dust, water vapor, and smoke, especially in production environments, such as smoke and halogen, toxic gases and fine powder emitted from equipment, which can 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, and 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. A valve product with reasonable structural design and correct process method will undoubtedly have 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.
Corrosion type for different valve components
1. Methods for preventing intergranular corrosion of austenitic stainless steel valve parts are as follows:
The solution treatment is carried out by solid solution quenching, that is, heating to about 1100 ° C for water quenching, and austenitic stainless steel containing titanium and niobium and having a carbon content of 0.03% or less is used to reduce the generation 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 generated by welding and cold working by heat treatment, improve the irrational valve structure, avoid stress concentration, and adopt electrochemical protection and spray anti-corrosion coating. Add corrosion inhibitor, apply compressive stress and other measures.
3. Wear corrosion is a form of corrosion caused by the alternating action of fluid on metal wear and corrosion. It is a common type of corrosion of valves, which occurs in the sealing surface.
Prevention method: use corrosion-resistant and wear-resistant materials, improve structural design, and adopt cathodic protection.
4. Vibration 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.
Prevention method: It can be applied by lubricating grease, reducing friction, surface phosphating, using hard alloy, and improving the surface hardness by spraying or cold working.
5. After welding, it is necessary to adopt corresponding protective measures such as annealing treatment. 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.
Prevention methods: 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 the stem and Corrosion of the flange sealing surface.
Valve parts anti-corrosion precautions
1. Stem corrosion and protection
Main causes of stem corrosion:
Corrosion damage of the valve body is mainly caused by corrosive media, and the problem of valve stem corrosion is mainly filler.
Not only does the corrosive medium corrode the stem, but steam and water can also cause spots on the stem to contact the packing. Especially for valves stored in warehouses, stem corrosion can occur. This is the electrochemical corrosion of the filler to the valve stem.
The most widely used filler is the asbestos-based packing. The asbestos material contains a certain amount of chloride ions, in addition to potassium, sodium and magnesium ions, which are all corrosive factors.
Valve stem anti-corrosion precautions:
Do not add filler during valve storage. Without filling, it loses the electrochemical corrosion of the valve stem and can be stored for a long time without being corroded.
The stem is surface treated. Such as chrome plating, nickel plating, nitriding, boronizing, zinc and so on.
Reduce asbestos impurities. Washing with distilled water can reduce the chlorine content in asbestos and reduce its corrosivity.
Add a corrosion inhibitor to the asbestos packing. This corrosion inhibitor suppresses the corrosiveness of chloride ions. Such as sodium nitrite.
Add sacrificial metal to asbestos. This is a metal that is lower than the stem potential as a victim. This corrosion of chloride ions first occurs on the sacrificial metal, thereby protecting the valve stem. It can be used as a sacrificial metal such as zinc powder.
Protected with Teflon. Polytetrafluoroethylene has excellent chemical stability and dielectric properties, and current cannot pass. If the asbestos packing is impregnated with polytetrafluoroethylene, the corrosion will be reduced. The asbestos packing can also be wrapped with a Teflon tape and then loaded into a stuffing box.
Improve processing smoothness and also reduce electrochemical corrosion.
2. Closed parts corrosion and protection
The main reason for the corrosion of the closure:
The closure is often flushed with fluid, allowing corrosion to accelerate. Some discs, although using better materials, are still more corrosive than the valve body.
The upper and lower closing members are usually threadedly connected with the valve stem and the valve seat, and the joint is deficient in oxygen than the general portion, which easily constitutes an oxygen concentration battery, causing corrosion damage. Some sealing parts are pressed into the sealing surface. Due to the lack of tightness and slight gap, oxygen concentration battery corrosion may occur.
Close parts anti-corrosion notes:
Use corrosion resistant materials as much as possible. The closing piece has a small weight, but plays a key role in the valve, as long as it is resistant to corrosion, even with a little expensive material.
Improve the closure structure so that it is less subject to fluid erosion.
Improve the connection structure to avoid the generation of oxygen concentration batteries.
In valves below 200 ° C, the joint between the closing part and the sealing surface, using a Teflon raw material tape as a filler, can reduce the corrosion of these parts.
While considering corrosion resistance, attention should also be paid to the erosion resistance of the closure material. Use a material that is resistant to erosion as a closure.
Corrosion resistant valve selection points
In the case of corrosive media conditions, anti-corrosion is the most critical place for chemical equipment. If the metal material of the chemical valve cannot be correctly selected, it is slightly inadvertent, and the equipment is damaged, which may cause accidents or even disaster.
How to choose the corrosion-resistant valve for some common chemical media?
1. Sulfuric acid medium
As one of the strong corrosive media, sulfuric acid is an important industrial raw material with a wide range of uses. Different concentrations and temperatures of sulfuric acid have a great difference in corrosion of materials. For concentrated sulfuric acid with a concentration above 80% and temperature less than 80 °C, carbon steel and cast iron have good corrosion resistance, but it is not suitable for high-speed flowing sulfuric acid. Not suitable for use as a valve material.
Ordinary stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) are also limited in their use for sulfuric acid media.
Therefore, valves for transporting sulfuric acid are usually made of high-silicon cast iron (difficult to cast and process) and high-alloy stainless steel (20 alloy). Fluoroplastics have better resistance to sulfuric acid, and a fluorine-lined pump valve (F46) is a more economical option. If the pressure is too high and the temperature rises, the point of use of the plastic valve is impacted, and only a ceramic ball valve that is more expensive than it is selected.
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 at 50 ° C and 30%.
Contrary to metal materials, most non-metallic materials have good corrosion resistance to hydrochloric acid, so rubber pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) are the best choice for transporting hydrochloric acid.
However, if the temperature of the medium exceeds 150 ° C, or the pressure is greater than 16 kg, any plastic (including polypropylene, fluoroplastic or even Teflon) will not be competent, and there is no ideal valve on the market. .
However, you can try the emerging ceramic ball valve. The advantages of this valve are self-lubricating, low torque, no aging, and much longer life than normal valves. Its disadvantage is that the price is much higher than plastic valves.
3. Nitric acid medium
Generally, most metals are quickly 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 normal temperature. It is worth mentioning that it contains molybdenum-containing stainless steel (such as 316 and 316L). The corrosion resistance to nitric acid is not only not superior to ordinary stainless steel (such as 304, 321), and sometimes even worse.
For high temperature nitric acid, titanium and titanium alloy materials are usually used.
4. Acetic acid medium
It is one of the most corrosive substances in organic acids. Ordinary steel is severely corroded in acetic acid of all concentrations and temperatures. Stainless steel is an excellent acetic acid resistant material. Molybdenum-containing 316 stainless steel can also be used for high temperature and dilute acetic acid vapor. .
High-alloy stainless steel or fluoroplastic valves are available for demanding requirements such as high temperature, high concentration acetic acid or other corrosive media.
5. Alkali (sodium hydroxide)
Steel is widely used in sodium hydroxide solutions below 80 ° C and 30% concentration. Many petrochemical plants still use ordinary steel at 100 ° C and below 75%. Although the corrosion is increased, the economy is good.
The corrosion resistance of ordinary stainless steel to lye has no obvious advantages compared with cast iron. Stainless steel is not recommended as long as a small amount of iron is allowed in the medium. For high-temperature alkaline liquids, titanium and titanium alloys or high-alloy stainless steels are often used.
6. Ammonia (ammonium hydroxide)
Most metals and non-metals are slightly corrosive in liquid ammonia and ammonia (ammonium hydroxide), and only copper and copper alloys are not suitable.
7. Chlorine (liquid chlorine)
Most metal valves are very resistant to chlorine corrosion, especially in the case of chlorine with water, including various alloy valves. In this case, PTFE valves are a good choice, but chemical plants that produce chlor-alkali will It was found that the time required for the PTFE valve was slightly longer, the torque was increased, and the problem of PTFE aging was highlighted. The leakage occurred in this case was fatal.
It can be considered to replace the original ordinary lining PTFE valve with a PTFE-filled ceramic core, which has a perfect effect by utilizing the self-lubricating property of the ceramic and the corrosion resistance of the PTFE.
8. Salt water (sea water)
Ordinary steel is not too corrosive in sodium chloride solution and seawater or salt water, and generally needs to be protected by paint.
All types of stainless steel also have a very low uniform corrosion rate, but may cause local corrosion due to chloride ions, usually 316 stainless steel is better.
9. Alcohols, ketones, esters, ethers
Common alcohol media are methanol, ethanol, ethylene glycol, propanol, etc., ketone media are acetone, methyl ethyl ketone, etc., ester media have various methyl esters, ethyl esters, etc., ether media are methyl ether, ether, Butyl ether, etc., they are basically non-corrosive, and commonly used materials can be applied. When selecting specifically, they should also make reasonable choices according to the properties of the medium and related requirements.
It is also worth noting that ketones, esters, and ethers are soluble in a variety of rubbers and avoid errors when selecting sealing materials.
Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)