Nine measures to perfectly cope with corrosion of metal valves
Metal valves are arguably the most critical component structures in engineering equipment that are prone to corrosion failure. Generally, valve parts such as sealing surfaces, valve stems, diaphragms, and small springs of metal valves are generally made of primary materials. Valve bodies, valve covers, etc. are applicable to secondary or tertiary materials, and are used for high pressure, high toxicity, flammability, and easy Explosive, radioactive media valves use less corrosive materials.
Metal valves not only have uniform corrosion of metal surfaces at any time under complex working conditions such as atmosphere or solution, but also pitting, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion and fatigue at local locations of metals. Local corrosion such as corrosion, selective corrosion, wear corrosion, cavitation corrosion, vibration corrosion, and hydrogen corrosion.
Anti-corrosion measures for metal valves
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 are made of natural rubber and synthetic rubber. Sealing rings are used for various types of valves. These are used as non-metal sealing surfaces. The material not only has good corrosion resistance, but also has good sealing performance, and is especially suitable for use in a granular medium. 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.
The sealing surface and the closing member with small diameter often adopt surface treatment such as nitriding and boronizing to improve its corrosion resistance and wear resistance. The valve disc made of 38CrMoAlA has a nitrided layer ≥0.4mm.
The valve stem is widely used for surface treatment such as nitriding, boronizing, chrome plating and nickel plating to improve its corrosion resistance, corrosion resistance and abrasion resistance. Different surface treatments should be suitable for different valve stem materials and working environment. The valve stems in contact with atmospheric, water vapor medium and asbestos packing can be hard chrome plated and gas nitriding process (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 resist ammonia corrosion after quenching and tempering, Gas nitriding carbon steel 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 valve stems.
The small-diameter valve body and handwheel are also often chrome-plated to improve their 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
The mechanism by which corrosion inhibitors control corrosion 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; in asbestos Zinc powder is added as a sacrificial metal. In fact, zinc is also a corrosion inhibitor. It can be combined with the chloride in the asbestos first, so that the chance of contact between the chloride and the valve stem metal is greatly reduced, thereby achieving the purpose of anticorrosion. 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, and a valve product with reasonable structural design and correct process method. Undoubtedly, it has 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 of valves to prevent intergranular corrosion of austenitic stainless steel valve parts: “Solution hardening” treatment, that is, heating to about 1100 °C water quenching, using austenoids containing titanium and niobium, and containing less than 0.03% carbon Stainless steel to reduce the production of chromium carbide.
Stress corrosion occurs when cracks occur under the influence of both corrosion and tensile stress. A method for preventing stress corrosion; eliminating or reducing welding, stress generated in cold working by heat treatment, improving irrational valve structure, avoiding stress concentration, and adopting electrochemical protection and spraying anti-corrosion coating. Add corrosion inhibitor, apply compressive stress and other measures.
Abrasion corrosion is a form of corrosion caused by the alternating action of fluids on metal wear and corrosion. It is a common type of corrosion of valves that occurs at the sealing surface. Prevention method: use corrosion-resistant and wear-resistant materials, improve structural design, and adopt cathodic protection.
Frictional 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. Lubricating grease can be used to reduce friction, surface phosphating, use of hard alloys, and the use of spray tile treatment or cold working to improve surface hardness.
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. 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 ground stem and flange seals Corrosion of the surface.
Source: China Metal Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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