15

2020

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04

These tips are enough to deal with valve corrosion. .

Author:


The corrosion of metals is mainly caused by chemical corrosion and electrochemical corrosion. The corrosion of non-metallic materials is generally damage caused by direct chemical and physical effects.

The corrosion of metals is mainly caused by chemical corrosion and electrochemical corrosion. The corrosion of non-metallic materials is generally damage caused by direct chemical and physical effects.

1.Chemical corrosion

The surrounding medium directly reacts chemically with the metal to destroy it without generating current, such as the corrosion of metal by high-temperature dry gas and non-electrolytic solution.

2.Electrochemical corrosion

When metal comes into contact with the electrolyte, electrons flow, causing electrochemical damage to the metal itself. This is the main form of corrosion.

Common acid, alkali and salt solution corrosion, 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 substances that can play a chemical role, but also occurs due to the difference in concentration of the solution, the concentration difference of surrounding oxygen, slight differences in material structure, etc., resulting in a difference in potential, which obtains the power of corrosion. , causing the metal with low potential and in the positive plate position to suffer losses.

 

Valve corrosion rate

 

Corrosion speed can be divided into six grades:

❶ Completely corrosion-resistant

 Corrosion rate is less than 0.001mm/year

❷ Extremely corrosion-resistant

 Corrosion rate 0.001 to 0.01 mm/year

❸ Corrosion resistant 

 Corrosion rate 0.01 to 0.1 mm/year

❹ Still corrosion-resistant

Corrosion rate 0.1 to 1.0 mm/year

❺ Poor corrosion resistance

 Corrosion rate 1.0 to 10 mm/year

❻ Not corrosion resistant 

 Corrosion rate is greater than 10 mm/year

 

Nine major anti-corrosion measures

1. Select corrosion-resistant materials according to corrosive media

 

In actual production , the corrosion of the medium is very complicated. Even if the valve material used in the same medium is the same, the concentration, temperature, and pressure of the medium are different, and the corrosion of the material by the medium is also different. Every 10°C increase in the medium temperature , the corrosion rate increases by approximately 1 to 3 times.

 

The concentration of the medium has a great influence on the corrosion of valve materials . For example, when lead is in sulfuric acid with a small concentration, the corrosion is very small. When the concentration exceeds 96%, the corrosion rises sharply. On the contrary, carbon steel corrodes most when the sulfuric acid concentration is about 50%. Seriously, when the concentration increases to above 6%, the corrosion drops sharply.

For example, aluminum is very corrosive in concentrated nitric acid with a concentration of more than 80%, but it corrodes seriously in nitric acid with medium and low concentrations. Although stainless steel is highly resistant to dilute nitric acid, it corrodes in concentrated nitric acid with a concentration of more than 95%. On the contrary, it gets worse.

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

 

2. Use non-metallic materials

 

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

The valve body, bonnet, lining, sealing surface, etc. of the valve are commonly made of non-metallic materials . As for the gaskets and packings, they are mainly made of non-metallic materials.

The valve lining is made of plastics such as polytetrafluoroethylene, chlorinated polyether, and natural rubber, neoprene, nitrile rubber, etc. , while the valve body and valve cover main body are made of general cast iron or carbon steel. This not only ensures the strength of the valve, but also ensures that the valve is not corroded.

Pinch valves are also designed based on the excellent corrosion resistance and deformability of rubber . Nowadays, it is more and more suitable to use nylon, polytetrafluoroethylene and other plastics, and use natural rubber and synthetic rubber to make various seals Face, sealing ring, used on various types of valves.

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

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

Commonly used non-metallic materials are suitable for working conditions

 

 

3. Metal surface treatment

 

Valve connection

Valve connecting screws are commonly treated with galvanizing, chromium plating, and oxidation (blue) to improve resistance to atmospheric and medium corrosion. In addition to the above methods, other fasteners are also treated with phosphating and other surface treatments according to the situation.

Sealing surfaces and closing parts with small diameters

Surface processes such as nitriding and boronization are often used to improve its corrosion resistance and wear resistance. The valve disc made of 38CrMoAlA has a nitriding layer of ≥0.4mm.

Valve stem anti-corrosion

Surface treatment processes such as nitriding, boronizing, chromium plating, and nickel plating are widely used to improve its corrosion resistance, corrosion resistance, and wear and scratch resistance.

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Different surface treatments should be suitable for different valve stem materials and working environments . For valve stems in contact with the atmosphere, water vapor medium and asbestos filler, hard chromium plating and gas nitriding processes can be used (ion nitriding processes are not suitable for stainless steel);

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Valves exposed to hydrogen sulfide atmosphere are electroplated with high-phosphorus nickel coating to have better protective performance ;

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38CrMoAlA can also resist corrosion by ion and gas nitriding, but hard chromium plating is not suitable;

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2Cr13 can resist ammonia corrosion after quenching and tempering, and carbon steel using gas nitriding can also resist ammonia corrosion, while all phosphorus-nickel coatings are not resistant to ammonia corrosion;

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

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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 process for preparing coatings and has become one of the new technologies for surface protection of materials.

It uses high-energy-density heat sources (gas combustion flames, electric arcs, plasma arcs, electric heating, gas explosions, etc.) to heat and melt metal or non-metallic materials, and then spray them into atomized form onto the pretreated basic surface to form a spray coating. , or the surface strengthening process method of heating the basic surface at the same time to melt the coating again on the surface of the substrate to form a spray welding layer.

Most metals and their alloys, metal oxide ceramics, cermet composites and hard metal compounds can use one or several thermal spraying methods to form coatings on metal or non-metal substrates.

Thermal spraying can improve its surface corrosion resistance, wear resistance, high temperature resistance and other properties , and extend its service life. Thermal spraying special functional coatings have special properties such as heat insulation, insulation (or different electricity), grindable sealing, self-lubrication, thermal radiation, electromagnetic shielding, etc.; parts can be repaired using 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.

 

Coating is also a non-metallic material . It is usually made of synthetic resin, rubber slurry, vegetable oil, solvent, etc. It covers the metal surface to isolate the medium and atmosphere to achieve anti-corrosion purposes.

Coatings are mainly used in less corrosive environments such as water, salt water, sea water, and atmosphere. The inner cavity of the valve is often painted with anti-corrosion paint to prevent water, air and other media from corroding the valve. The paint is mixed with different colors to represent the materials used by Fahn. The valves are spray-painted, usually once every six months to a year.

 

6. Add corrosion inhibitor

 

The mechanism by which corrosion inhibitors control corrosion is that they promote polarization of the battery.

Corrosion inhibitors are mainly used in media and fillers. Adding corrosion inhibitors to the medium can slow down the corrosion of equipment and valves. For example, chromium-nickel stainless steel will become cremated in a wide solubility range in oxygen-free sulfuric acid, and the corrosion will be serious. However, adding a small amount of copper sulfate or nitric acid Oxidants such as stainless steel can transform the stainless steel into a passive state and form a protective film on the surface to prevent corrosion by the medium. In hydrochloric acid, if a small amount of oxidant is added, the corrosion of titanium can be reduced.

Water is often used as the medium for pressure testing of valves , which can easily cause corrosion of the valve. Adding a small amount of sodium nitrite to the water can prevent water from corroding the valve.

Asbestos filler contains chloride , which is very corrosive to the valve stem. If the distilled water washing method is used, the chloride content can be reduced. However, this method is very difficult to implement and cannot be generally promoted. Ester is suitable for special needs.

In order to protect the valve stem and prevent corrosion of the asbestos filler, the valve stem is coated with corrosion inhibitors and sacrificial metal in the asbestos filler. The corrosion inhibitor consists of sodium nitrite and sodium chromate, which can create a layer of passivation on the surface of the valve stem. film to improve the corrosion resistance of the valve stem; the solvent can slowly dissolve the corrosion inhibitor and act as a lubrication;

 

Zinc powder is added to asbestos as a sacrificial metal . In fact, zinc is also a corrosion inhibitor. It can first combine with the chloride in asbestos, greatly reducing the chance of contact between chloride and the valve stem metal, thereby achieving anti-corrosion purposes. If corrosion inhibitors such as red lead and calcium lead oxide are added to the paint, spraying on the valve surface can prevent atmospheric corrosion.

 

7.Electrochemical protection

 

There are two types of electrochemical protection: anode protection and cathodic protection.

If zinc is used to protect iron , the zinc will be corroded and the zinc will be called a sacrificial metal. In production practice, anode protection is used less often and cathodic protection is used more. This cathodic protection method is used for large valves and important valves, which is an economical and simple method. Another effective method. Adding zinc to asbestos filler to protect the valve stem is also a cathodic protection method.

 

8. Control corrosive environment

 

The so-called environment has two broad senses and a narrow sense. The broad sense environment refers to the environment around the valve installation place and its internal circulation medium; the narrow sense environment refers to the conditions around the valve installation place.

Most environments cannot be controlled , and production processes cannot be changed at will. Only when it does not cause damage to products, processes, etc., can environmental control methods be used, such as boiler water deoxygenation, alkali adjustment of pH value in the oil refining process, etc. From this point of view, the above-mentioned addition of corrosion inhibitors, electrochemical protection, etc. are also controlled corrosion environments.

The atmosphere is full of dust, water vapor, and smoke . Especially in the production environment, toxic gases and micro-powders emitted from fumes, equipment, etc. will corrode valves to varying degrees.

 

Operators should clean and purge valves regularly and refuel them regularly according to the regulations in the operating procedures. This is an effective measure to control environmental corrosion. Installing a protective cover on the valve stem, installing a well on the floor valve, spraying paint on the surface of the valve, etc. are all ways to prevent corrosive substances from corroding the valve.

Rising ambient temperature and air pollution , especially for equipment and valves in closed environments, will accelerate corrosion. Open workshops should be used as much as possible or ventilation and cooling measures should be adopted to slow down environmental corrosion.

 

9. Improve the processing technology and valve structure

 

The anti-corrosion protection of valves is an issue that is considered from the beginning of design . A valve product with reasonable structural design and correct process method will undoubtedly have a good effect on slowing down the corrosion of valves.

Therefore , the design and manufacturing departments should improve those parts that have unreasonable structural design, incorrect process methods, and are prone to corrosion to make them suitable for various working conditions.

 

Tips for different types of valve component corrosion

 

1. Methods to prevent intergranular corrosion of austenitic stainless steel valve parts include :

Carry out "solid solution quenching" treatment, that is, heat to about 1100°C for water quenching, and use austenitic stainless steel containing titanium and niobium with a carbon content of less than 0.03% to reduce the production of chromium carbide.

2. Stress corrosion occurs when corrosion and tensile stress cause cracks at the same time.

Methods to prevent stress corrosion: eliminate or reduce the stress generated during welding and cold working through heat treatment, improve the unreasonable valve structure to avoid stress concentration, use electrochemical protection, spray anti-corrosion paint, add corrosion inhibitors, apply compressive stress, etc. measure.

3. Wear corrosion is a form of corrosion caused by the alternating effects of fluid wear and corrosion on metal . It is a common corrosion in valves. This corrosion mostly occurs on the sealing surface.

Prevention methods : Use corrosion-resistant and wear-resistant materials, improve structural design, adopt cathodic protection, etc.

4. Frictional corrosion is when two parts in contact with each other bear load at the same time , and the contact surface is damaged due to vibration and sliding. Friction corrosion occurs at the bolt connection, the connection between the valve stem and the closing member, and between the ball bearing and the shaft.

Prevention methods: You can apply lubricating grease , reduce friction, surface phosphating, use cemented carbide, and use tile spraying or cold processing to improve surface hardness.

5. After welding, appropriate protective measures such as annealing treatment should be adopted as much as possible. Improve the surface roughness of the valve stem and other valve parts . The higher the surface roughness level, the stronger the corrosion resistance.

Prevention methods: Improve the processing technology and structure of fillers and gaskets , use flexible graphite and plastic fillers, as well as flexible graphite adhesive gaskets and PTFE-coated gaskets, which can improve the sealing performance and reduce the number of valve stems and valve stems. Corrosion of flange sealing surface.

 

Precautions for anti-corrosion of valve parts

 

1. Valve stem corrosion and protection

The main causes of valve stem corrosion:

The corrosion damage of the valve body is mainly caused by corrosive media, while the corrosion problem of the valve stem is mainly caused by the packing.

Not only corrosive media can corrode and damage the valve stem, but steam and water can also cause spots on the contact between the valve stem and the packing. Especially valves stored in warehouses will also suffer from valve stem pitting corrosion. This is the electrochemical corrosion of the valve stem by the packing.

The most widely used filler now is packing based on asbestos. The asbestos material contains a certain amount of chloride ions, as well as potassium, sodium, magnesium and other ions, which are all factors of corrosion.

Valve stem anti-corrosion precautions:

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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 being corroded.

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Surface treatment of valve stem. Such as chromium plating, nickel plating, nitriding, boronizing, zinc plating, etc. 

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Reduce asbestos impurities. Washing with distilled water can reduce the chlorine content in asbestos, thereby reducing its corrosiveness.

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Add corrosion inhibitor to asbestos packing. This corrosion inhibitor inhibits the corrosive properties of chloride ions. Such as sodium nitrite. 

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Add sacrificial metal to asbestos. This is a sacrifice of a metal with a lower potential than the valve stem. In this way, chloride ion corrosion occurs first on the sacrificial metal, thereby protecting the valve stem. Examples of sacrificial metals include zinc powder.

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Protected with PTFE. PTFE has excellent chemical stability and dielectric properties, and current cannot pass through it. If asbestos packing is impregnated with PTFE, corrosion will be reduced. The asbestos packing can also be wrapped with polytetrafluoroethylene raw material tape and then installed into the stuffing box.

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Improving the processing finish can also reduce electrochemical corrosion. 

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2. Corrosion and protection of closing parts

The main reasons for corrosion of closing parts:

Closing parts are often washed away by fluids, which accelerates the development of corrosion. Some valve discs, although made of better materials, are still corroded and damaged faster than the valve body.

The upper and lower closing parts are often threaded to the valve stem and valve seat. The connection is less oxygen than the general part, and it is easy to form an oxygen concentration cell, causing corrosion and damage. The sealing surface of some closing parts is press-fitted. Due to the loose fit and a slight gap, oxygen concentration difference battery corrosion may occur.

Precautions for anti-corrosion of closing parts:

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Use corrosion-resistant materials whenever possible. The closing piece weighs very little, but plays a key role in the valve. As long as it can resist corrosion, even a little expensive material does not matter.

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Improve the closure structure to make it less susceptible to fluid erosion.

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Improve the connection structure to avoid oxygen concentration difference in the battery.

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In valves with a temperature below 200°C, using polytetrafluoroethylene raw material tape as filler at the connection between the closing parts and the sealing surface can reduce corrosion in these parts.

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While considering corrosion resistance, attention should also be paid to the erosion resistance of the closure material. Use materials with strong erosion resistance for closing parts.

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Key points for selecting corrosion-resistant valves

 

For valves operating in corrosive media, anti-corrosion is the most critical aspect of chemical equipment. If the metal materials of chemical valves cannot be selected correctly, a slight carelessness may damage the equipment at least, or cause accidents or even disasters at worst.

 

How to choose corrosion-resistant valves for some commonly used chemical media ?

 

 

1. Sulfuric acid medium

As one of the highly corrosive media, sulfuric acid is an important industrial raw material with a wide range of uses. The corrosion of materials by sulfuric acid at different concentrations and temperatures is quite different. For concentrated sulfuric acid with a concentration above 80% and a temperature less than 80°C, carbon steel and cast iron have good corrosion resistance, but they are not suitable for high-speed flowing sulfuric acid. Not suitable for use as valve material.

Ordinary stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) also have limited use in sulfuric acid media.

Therefore, valves for transporting sulfuric acid are usually made of high-silicon cast iron (which is difficult to cast and process) and high-alloy stainless steel ( No. 20 alloy). Fluorine plastic has good sulfuric acid resistance, and using fluorine-lined pump valve (F46) is a more economical choice. If the pressure is too high and the temperature rises, the use point of the plastic valve will be affected, and you will have to choose a ceramic ball valve that is much more expensive.

2. Hydrochloric acid medium

Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), and molybdenum-containing high-silicon ferrosilicon 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 rubber-lined pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) are better choices for transporting hydrochloric acid.

However, if the temperature of such a medium exceeds 150°C, or the pressure is greater than 16 kg, any plastic (including polypropylene, fluoroplastics or even polytetrafluoroethylene) will not be able to handle it, and there is currently no ideal valve on the market. .

However, you can try the emerging ceramic ball valve. The advantages of this kind of valve are self-lubricating, small torque, no aging, and a much longer service life than ordinary valves. Its disadvantage is that the price is much higher than that of plastic valves.

3. Nitric acid medium

Most general 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 molybdenum-containing stainless steel (such as 316, 316L ) The corrosion resistance to nitric acid is not only no better than ordinary stainless steel (such as 304, 321), but 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 among organic acids. Ordinary steel will be severely corroded in acetic acid at all concentrations and temperatures. Stainless steel is an excellent acetic acid-resistant material. 316 stainless steel containing molybdenum is also suitable for high temperatures and dilute acetic acid steam. .

For demanding requirements such as high temperature, high concentration acetic acid or other corrosive media, high alloy stainless steel or fluorine plastic valves can be selected.

5.Alkali (sodium hydroxide)

Steel is widely used in sodium hydroxide solutions below 80°C and with a concentration of 30%. Many petrochemical plants still use ordinary steel at temperatures below 100°C and 75%. Although corrosion increases, the economy is good.

The corrosion resistance of ordinary stainless steel to alkali liquid has no obvious advantage compared with cast iron. As long as a small amount of iron is allowed to be incorporated into the medium, stainless steel is not recommended. For high-temperature alkaline solutions, titanium and titanium alloys or high-alloy stainless steel are often used.

6. Ammonia (ammonia hydroxide)

Most metals and non-metals corrode very slightly in liquid ammonia and ammonia (ammonium hydroxide). Only copper and copper alloys are not suitable for use.

7. Chlorine gas (liquid chlorine)

The corrosion resistance of most metal valves to chlorine is very limited, especially when chlorine is mixed with water, including various alloy valves. In this case, PTFE valves are a very good choice, but chemical plants that produce chlor-alkali will It was found that if the PTFE valve is used for a little longer, the torque will increase, and the problem of PTFE aging will become prominent. In this case, leakage will be fatal.

You can consider replacing the original ordinary PTFE-lined valve with a PTFE-lined ceramic ball core. Taking advantage of the self-lubricating properties of ceramics and the corrosion resistance of PTFE will have perfect results.

8. Salt water (sea water)

The corrosion rate of ordinary steel in sodium chloride solution, sea water, and salt water is not very high, and it generally needs to be protected by paint.

Various types of stainless steel also have very low uniform corrosion rates, but may cause localized corrosion due to chloride ions. Generally, 316 stainless steel is better.

9. Alcohols, ketones, esters, ethers

Common alcohol media include methanol, ethanol, ethylene glycol, propanol, etc.; ketone media include acetone, butanone, etc.; ester media include various methyl esters, ethyl esters, etc.; ether media include methyl ether, diethyl ether, etc. Butyl ether, etc., they are basically non-corrosive and can be used with commonly used materials. The specific selection should also be based on the properties of the medium and related requirements to make a reasonable choice.

It is also worth noting that ketones, esters, and ethers are soluble in a variety of rubbers, so avoid mistakes when selecting sealing materials.