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Corrosion form of valve

The corrosion of metals is mainly caused by chemical corrosion and electrochemical 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. electrochemical 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.

Corrosion rate of valve

The corrosion rate can be divided into six grades:

❶ Complete corrosion resistance

➨ Corrosion rate less than 0.001 mm/year

❷ Extreme corrosion resistance

➨ Corrosion rates of 0.001 to 0.01 mm/year

❸ Corrosion resistance

➨ Corrosion rates range from 0.01 to 0.1 mm/year

❹ Corrosion resistance

➨ Corrosion rates of 0.1 to 1.0 mm/year

❺ Poor corrosion resistance

➨ Corrosion rate 1.0 to 10 mm/year

❻ Non corrosion resistance

➨ Corrosion rate greater than 10 mm/year

Nine major anticorrosion measures

1. Selection of Corrosion Resistant Materials Based on Corrosive Medium

In production practice, the corrosion of medium is very complex. Even if the valve material used in a medium is the same, the concentration, temperature and pressure of medium are different, and the corrosion of medium to material is also different. The corrosion rate increases about 1-3 times with the increase of medium temperature by 10 degrees.

Medium concentration has a great impact on the corrosion of valve materials, such as lead in low concentration sulfuric acid, corrosion is very small, when the concentration exceeds 96%, corrosion rises sharply. On the contrary, carbon steel corrosion is the most serious when the concentration of sulfuric acid is about 50%, and when the concentration increases to more than 6%, corrosion decreases sharply.

For example, aluminium is highly corrosive in concentrated nitric acid with a concentration of more than 80%, but corrodes seriously in medium and low nitric acid. Although stainless steel has a strong corrosion resistance to dilute nitric acid, its corrosion is aggravated in concentrated nitric acid with a concentration of more than 95%.

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

2. Using non-metallic materials

Non-metallic corrosion resistance is excellent. As long as the temperature and pressure of the valve meet the requirements of non-metallic materials, it can not only solve the corrosion problem, but also save precious metals.

Valve body, valve cover, lining, sealing surface and other commonly used non-metallic materials, as for gaskets, fillers are mainly made of dead non-metallic materials.

Polytetrafluoroethylene, chlorinated polyether and other plastics, as well as natural rubber, chloroprene rubber, butadiene rubber and other rubber as valve lining, and valve body, valve cover body is made of general cast iron, carbon steel. That is to say, the strength of the valve is guaranteed, and the valve is not corroded.

Pinch valves are also designed according to the excellent corrosion resistance and excellent modification properties of rubber. Now more and more people use nylon, polytetrafluoroethylene and other plastics, natural rubber and synthetic rubber to make a variety of sealing surfaces, sealing rings, for all kinds of valves.

These non-metallic materials used as sealing surfaces not only have good corrosion resistance, but also good sealing performance. They are especially suitable for use in granular media. Of course, their strength and heat resistance are low, and their application scope is limited.

The emergence of flexible graphite has brought non-metals into the high temperature field, solved the long-term problem of leakage of fillers and gaskets, and is a good high temperature lubricant.

Application Conditions of Common Nonmetallic Materials.

3. Metal Surface Treatment

Valve connection

Valve connection screw commonly used galvanizing, chromium plating, oxidation (bluing) treatment to improve the resistance to atmospheric and medium corrosion. Other fasteners in addition to the above treatment, but also according to the situation using phosphating and other surface treatment.

Seal face and small caliber closing parts

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 nitriding layer of more than 0.4mm.

Stem anticorrosion

Nitriding, boronizing, chromium plating and nickel plating 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 contacted with asbestos filler in atmosphere and steam medium (ion nitriding process is not suitable for stainless steel).

The valves in hydrogen sulfide atmospheric environment have better protection performance by electroplating high phosphorus and nickel coating.

38CrMoAlA can also resist corrosion by ion and gas nitriding, but it is not suitable to use hard chromium coating.

After quenching and tempering, 2Cr13 can resist ammonia corrosion. Carbon steel nitrided by gas can also resist ammonia corrosion. All phosphorus-nickel coatings are not resistant to ammonia corrosion.

After gas nitriding, 38CrMoAlA material has excellent corrosion resistance and comprehensive performance, and it is widely used to make valve stem.

Small caliber body and handwheel

It is also chromium plated to improve its corrosion resistance and decorate valves.

4. thermal spraying

Thermal spraying is a kind of process block for coating preparation, and has become one of the new technologies for material surface protection.

It uses high energy density heat sources (gas combustion flame, arc, plasma arc, electrothermal, gas explosion, etc.) to heat and melt metal or non-metal materials and spray them into the pretreated basic surface in the form of atomization, forming spray coating, or heating the basic surface at the same time, so that the coating melts again on the base surface, forming a surface strengthening process of 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, prolong the service life. Thermal spraying special functional coatings, with heat insulation, insulation (or heteroelectricity), grindable sealing, self-lubrication, thermal radiation, electromagnetic shielding and other special properties; using thermal spraying can repair parts.

5. 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 also non-metallic materials. They are usually made of synthetic resin, rubber slurry, vegetable oil, solvents, etc. They cover the metal surface, insulate the medium and atmosphere, and achieve the purpose of anti-corrosion.

Coatings are mainly used in water, brine, seawater, atmospheric and other environments where corrosion is not strong. Anti-corrosive paint is commonly used in the inner chamber of the valve to prevent the corrosion of water, air and other media to the valve. The paint is mixed with different colors to represent the materials used by Farn. Valve spray paint is usually used every six months to one year.

6. Adding Corrosion Inhibitor

The mechanism of corrosion control by corrosion inhibitor is that it promotes the polarization of batteries.

Corrosion inhibitors are mainly used in media and fillers. The corrosion of equipment and valves can be slowed down by adding corrosion inhibitors in medium, such as chromium-nickel stainless steel in oxygen-free sulphuric acid, which is cremated in a wide range of solubility and corroded seriously. However, adding a small amount of oxidants such as copper sulphate or nitric acid, the stainless steel can be passivated, and a protective film can be formed on the surface to prevent the corrosion of medium. In hydrochloric acid, if adding a small amount of oxidant, the corrosion of titanium can be reduced. Eclipse.

Valve pressure test usually uses water as the medium of pressure test, 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.

Asbestos packing contains chloride, which corrodes the valve stem greatly. If the chloride content can be reduced by washing with distilled water, this method is difficult to implement and can not be popularized generally. Esters are suitable for special needs.

In order to protect the valve stem and prevent the corrosion of asbestos packing, corrosion inhibitors and sacrificial metals are coated on the valve stem in asbestos packing. Inhibitors can form a passivation film on the surface of the valve stem by sodium nitrite and sodium chromate, which can improve the corrosion resistance of the valve stem. Solvents can dissolve the corrosion inhibitor slowly and lubricate the valve stem.

Zinc powder is added to asbestos as sacrificial metal. In fact, zinc is also a corrosion inhibitor. It can combine with chloride in asbestos at first, which greatly reduces the contact chance between chloride and metal of valve stem, thus achieving the purpose of anti-corrosion. If red red, calcium lead acid and other corrosion inhibitors are added to the coating, spraying on the valve surface can prevent atmospheric corrosion.

7. electrochemical protection

There are two kinds of electrochemical protection: anodic protection and cathodic protection.

If zinc is used to protect iron and zinc is corroded, zinc is called sacrificial metal. In production practice, less anodic protection is used and more cathodic protection is used. It is an economical, simple and effective method for large valves and important valves to adopt this cathodic protection method. Adding zinc to asbestos packing also belongs to the cathodic protection method.

8. Controlling Corrosion Environment

The so-called environment, there are two broad and narrow sense, the broad sense of the environment refers to the environment around the valve installation and its internal circulation medium; the narrow sense of the environment refers to the conditions around the valve installation. Most environments are uncontrollable and production processes cannot be changed at will. Only when there is no harm to the products and processes, can we adopt the methods of controlling the environment, such as deoxidation of boiler water, alkali adjustment of PH value in refining process, etc. From this point of view, the above-mentioned additions of corrosion inhibitors and electrochemical protection also belong to the control of corrosion environment. The atmosphere is full of dust, water vapor and smoke. Especially in the production environment, such as bittern, poisonous gas and powder emitted by equipment, will cause varying degrees of corrosion to valves. Operators should regularly clean, purge and refuel the valves according to the rules of operation, which is an effective measure to control environmental corrosion. Stem mounting protective cover, ground valve mounting wells, valve surface painting, etc. are all methods to prevent corrosion of the valve. Rising ambient temperature and air pollution, especially for equipment and valves in enclosed environment, will accelerate their corrosion. Open workshop or ventilation and cooling measures should be adopted as far as possible to alleviate environmental corrosion.

9. Improving Processing Technology and Valve Structure

Anti-corrosion protection of valves is considered from the beginning of design. A valves product with reasonable structure design and correct process is undoubtedly effective in slowing down the corrosion of valves. Therefore, the design and manufacturing departments should improve the parts which are unreasonable in structure design, incorrect in process method and easy to cause corrosion, so that they can meet the requirements of different working conditions.

For different corrosion types of valve components:

The methods to prevent intergranular corrosion of austenitic stainless steel valves are as follows:

1. The austenitic stainless steel with titanium and niobium and less than 0.03% carbon content is selected to reduce the production of chromium carbide by "solution quenching" treatment, i.e. water quenching at about 1100 C.

2. Stress corrosion occurs when corrosion and tension stress are combined to produce cracks.

The methods of preventing stress corrosion are: eliminating or reducing the stress in welding and cold working by heat treatment, improving the unreasonable valve structure, avoiding stress concentration, adopting electrochemical protection, spraying anti-corrosion coating, adding corrosion inhibitor, applying pressure stress and other measures.

3. Wear corrosion is a kind of corrosive form produced by the alternating action of fluid on metal wear and corrosion. It is a common corrosion of valves. This kind of corrosion occurs mostly on the sealing surface.

Preventive methods: Choose corrosion-resistant and wear-resistant materials, improve structural design, use cathodic protection, etc.

4. Friction-induced corrosion is a kind of damage caused by vibration and sliding of two parts which are in contact with each other and bear loads at the same time. Friction corrosion occurs at bolt connection, valve stem and closure connection, and between ball bearing and shaft.

Prevention methods: lubricating grease, reducing friction, surface phosphating, cemented carbide, spray tile treatment or cold working can be used to improve surface hardness.

5. Relevant protective measures such as annealing should be adopted as far as possible after welding. Increasing the surface roughness of the stem and other valve parts, the higher the surface roughness level, the stronger the corrosion resistance.

Preventive methods: Improving the processing technology and structure of fillers and gaskets, using flexible graphite and plastic fillers, flexible graphite sticking gaskets and polytetrafluoroethylene thin-packed gaskets can improve sealing performance and reduce corrosion of ground valve stem and flange sealing surface.

Points for Attention to Anticorrosion of Valve Parts

1. 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.

2. 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 the valve stem and seat. The connection is more oxygen-deficient than the common parts, which is easy to form oxygen concentration battery and cause corrosion damage. 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.

Key Points for Selection of Corrosion Resistant Valves:

In the case of corrosive medium, corrosion protection is the most critical part of chemical equipment. If the metal materials of chemical valves can not be selected correctly, slightly careless and slightly damaged equipment will cause accidents or even disaster. How to choose corrosion-resistant valves 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 wide application. The corrosion of sulfuric acid at different concentration and temperature is quite different. For concentrated sulfuric acid with concentration above 80% and temperature below 80 C, carbon steel and cast iron have better corrosion resistance, but they are not suitable for high-speed flow sulfuric acid, and they are not suitable for valve materials. Ordinary stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) are also of limited use in sulphuric acid media. Therefore, valves conveying sulphuric acid are usually made of high silicon cast iron (difficult to cast and process) and high alloy stainless steel (alloy 20). Fluoroplastics 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.

2. 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 for 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. However, if the temperature of such medium exceeds 150 C or the pressure is greater than 16 kg, any plastic (including polypropylene, fluoroplastics and even PTFE) will not be able to compete, and there is no ideal valve on the market at present. However, you can try the new ceramic ball valves, which have the advantages of self-lubrication, small torque, non-aging, long life than ordinary valves, and its disadvantage is that the price is much higher than plastic valves.

3. Nitric acid medium

Most metals are rapidly corroded in nitric acid. Stainless steel is the most widely used nitric acid resistant material. It has good corrosion resistance to nitric acid of all concentrations at room temperature. It is worth mentioning that the corrosion resistance of molybdenum-containing stainless steel (such as 316 and 316L) to nitric acid is not better than that of ordinary stainless steel (such as 304 and 321), sometimes even worse. For high temperature nitric acid, titanium and titanium alloys 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 at all concentrations and temperatures. Stainless steel is an excellent acetic acid resistant material. 316 stainless steel containing molybdenum can also be applied to high temperature and dilute acetic acid vapor. High alloy stainless steel or fluoroplastic valves can be selected for high temperature and high concentration acetic acid or other corrosive media.

5. Alkali (sodium hydroxide)

Iron and steel are widely used in NaOH solutions below 80 C and 30% concentration. Many petrochemical plants still use ordinary steel under 100 C and 75%. Although corrosion increases, it has good economy. The corrosion resistance of common stainless steel to alkali solution 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. Titanium and titanium alloys or high alloy stainless steel are often used for high temperature alkali liquor.

6. Ammonia (ammonia hydroxide)

Most metals and nonmetals corrode slightly in liquid ammonia and ammonia water (ammonia hydroxide). Only copper and copper alloys are not suitable for use.

7. Chlorine (liquid chlorine)

Most metal valves have limited resistance to chlorine corrosion, especially when chlorine carries water, including all kinds of alloy valves. In this case, tetrafluoro valves are a good choice. However, chemical plants producing chlor-alkali will find that the problem of aging of tetrafluoro valves will be highlighted when the time of using tetrafluoro valves is slightly longer and the torque increases. It's deadly. It can be considered that the original ordinary lined tetrafluoro valve can be replaced by a tetrafluoro ceramic ball core. The self-lubrication of ceramics and the corrosion resistance of tetrafluoro will have a perfect effect.

8. Salt water (sea water)

The corrosion rate of common steel in sodium chloride solution, seawater and saline water is not very high, so it should be protected by coating.

All kinds of stainless steel also have very low uniform corrosion rate, but may cause local corrosion due to chloride ions, usually 316 stainless steel is better.

9. Alcohols, ketones, esters and ethers

Common alcoholic media are methanol, ethanol, ethylene glycol, propanol, acetone, butanone, ester media have a variety of methyl esters, ether media have methyl ether, ether, butyl ether and so on, they are basically non-corrosive, commonly used materials are applicable, the specific selection should be based on the properties of the media and related requirements to make reasonable choices. 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.