Metal valves can be said to be the engineering equipment that is the most prone to corrosion failure of the key components of the structure. Usually, metal valve sealing surfaces, valve stems, diaphragms, small springs, and other valve parts are generally used in a class of materials, valve body, valve cover, such as the application of secondary or tertiary materials for high-pressure, highly toxic, flammable, explosive, radioactive media valves, the selection of corrosive materials is minimal.
Metal valves in the atmosphere or solution and other complex working conditions not only happen all the time the uniform corrosion of the metal surface, but in the local position of the metal is also very easy to occur pore corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, abrasion corrosion, cavitation corrosion, Mo vibration corrosion, hydrogen corrosion, and other local corrosion.
Anti-corrosion measures for metal valves:
1, According to the corrosion medium selection of corrosion-resistant materials
In production practice, the corrosion of the medium is very complex, even in a medium with the use of the same valve materials, media concentration, temperature, pressure, and media corrosion of materials are not the same. Medium temperature increases every 10 ℃, the corrosion rate of about 1 ~ 3 times, the medium concentration of the valve material corrosion has a great impact, such as lead in the concentration of small sulfuric acid, corrosion is minimal, when the concentration of more than 96%, the corrosion rises sharply.
Carbon steel, on the contrary, in the sulfuric acid concentration of about 50% of the most serious corrosion, when the concentration increased to more than 6%, the corrosion is sharply reduced. There are such as aluminum in a concentration of more than 80% of concentrated nitric acid corrosion is very strong, but in the medium and low concentrations of nitric acid, corrosion is serious. Stainless steel, although the dilute nitric acid corrosion resistance is very strong, but in more than 95% of concentrated nitric acid corrosion is aggravated. From the above examples can be seen, the correct choice of valve materials should be based on specific conditions, analyze the various factors affecting corrosion, according to the relevant anti-corrosion manual selection of materials.
2, The use of non-metallic materials
Non-metallic corrosion resistance is excellent, as long as the valve temperature and pressure in line with the requirements of non-metallic materials, not only to solve the corrosion problem, but also to save valuable metals. The valve body, valve cover, lining, sealing surface, and other commonly used non-metallic materials, as for the gasket, packing is mainly made of non-metallic materials. With polytetrafluoroethylene, chlorinated polyether and other plastics, as well as natural rubber, neoprene rubber, butyl rubber and other rubber to do the valve lining, and the valve body, the main body of the valve cover is generally made of cast iron, carbon steel. That is to ensure the strength of the valve, but also to ensure that the valve is not corroded.
Pinch valve is also based on the rubber’s excellent corrosion resistance and excellent performance and design out. Now more and more with nylon, PTFE and other plastics, natural and synthetic rubber to do a variety of sealing surfaces, seals, used in various types of valves, these non-metallic materials used as sealing surfaces, not only good corrosion resistance, but also good sealing performance, especially suitable for use with particles in the medium. Of course, their strength and heat resistance are lower, the scope of application is limited. The emergence of flexible graphite, so non-metallic into the field of high temperature, to solve the long-term difficult to solve the problem of packing and gasket leakage, but also a very good high temperature lubricant.
3, Metal surface treatment
Valve connection screw ambulance commonly used zinc plating, chromium plating, oxidation (blue) treatment to improve resistance to atmospheric and media corrosion resistance. Other fasteners in addition to the above methods of treatment, but also according to the situation using phosphate and other surface treatment.
Sealing surface and the caliber of the closure is not large, often using nitriding, boron infiltration and other surface processes to improve its corrosion resistance and wear resistance. 38CrMoAlA valve made of flap, nitriding layer ≥ 0.4mm.
Valve stem corrosion widely used nitriding, boriding, chrome plating, nickel plating and other surface treatment processes to improve its corrosion resistance, corrosion resistance and abrasion resistance. Different amount of surface treatment should be suitable for different stem materials and working environment, in the atmosphere, water vapor media and asbestos packing contact stem, can be used hard chrome plating, gas nitriding process (stainless steel is not suitable for ion nitriding process); in the hydrogen sulfide atmospheric environment of the valve using electroplating high phosphorus nickel plating has a better protection performance; 38CrMoAlA with a high phosphorus nickel plating, nickel plating and other surface treatment processes.
38CrMoAlA using ion and gas nitriding can also be corrosion resistant, but should not be used hard chromium plating; 20Cr13 after tempering can be resistant to ammonia corrosion, the use of gas nitriding of carbon steel can also be resistant to ammonia corrosion, and all the phosphorus nickel plating is not resistant to ammonia corrosion; through the gas nitriding of 38CrMoAlA material has excellent corrosion resistance and comprehensive performance, with which the production of the valve stem for more.
Small diameter valve body and handwheel is also often chrome-plated to improve its corrosion resistance, decorative valves.
4, Thermal spraying
Thermal spraying is the preparation of a coating of a class of technology square, has become one of the new technology of material surface protection. It is the use of high energy density heat source (gas combustion flame, electric arc, plasma arc, electrothermal, gas combustion explosion, etc.) will be heated metal or non-metallic materials melted, in the form of atomized spray to the basic surface of the pretreatment, the formation of the spray layer, or at the same time on the basic surface heating, so that the coating on the surface of the substrate again melted, the formation of the spray welding layer of the surface enhancement process method.
Most metals and their alloys, metal-oxide ceramics, metal-ceramic composites and hard metal compounds can be coated on metallic or non-metallic substrates by one or more thermal spraying methods.
Thermal spraying can improve the surface corrosion resistance, wear resistance, high temperature resistance and other properties, to extend the service life. Thermal spraying special function coating, with thermal insulation, insulation (or heteroelectricity), abrasion sealing, self-lubricating, heat radiation, electromagnetic shielding and other special properties; the use of thermal spraying can repair parts.
5, Spray paint
Paint is one of the most widely used means of anti-corrosion, in the valve products is a kind of anti-corrosion materials and identification mark. Paint also belongs to non-metallic materials, it is usually made of synthetic resins, rubber slurry, vegetable oils, solvents and other formulations, covered in the metal surface, isolated from the medium and the atmosphere, to achieve the purpose of corrosion. Paint is mainly used in water, salt water, seawater, atmosphere and other corrosive environment is not too strong.
Valve cavity commonly used anticorrosive paint to prevent water, air and other media on the valve corrosion. Paint mixed with different colors, to indicate the material used by the lawn. Valve painting, generally in half a year to once a year.
6, Add corrosion inhibitors
Corrosion inhibitors control the mechanism of corrosion, is that it promotes the polarization of the cell. Corrosion inhibitors are mainly used in the medium and filler. Add corrosion inhibitors in the medium, can make the corrosion of equipment and valves to slow down, such as chromium-nickel stainless steel in the sulfuric acid does not contain oxygen, a large range of solubility into the pyrophoric state, corrosion is more serious, but the addition of a small amount of copper sulfate or nitric acid and other oxidizing agents, stainless steel can be transformed into passivation, the surface of the generation of a layer of protective film, to prevent the medium of the corrosion of the impregnation in the hydrochloric acid, if you add a small amount of oxidizing agent, can reduce the corrosion of titanium.
Valve pressure test commonly used as a test medium of water, easy to cause corrosion of the valve, add a small amount of sodium nitrite in the water to prevent water corrosion of the valve. Asbestos packing contains chloride, the valve stem corrosion is very large, if the use of steam distillation water washing method can reduce the content of chloride, but this method in the implementation of many difficulties, can not be common promotion, ester is suitable for special needs.
In order to protect the valve stem, to prevent corrosion of asbestos packing, in the asbestos packing, in the valve stem coated with corrosion inhibitor and sacrificial metal. Corrosion inhibitor by sodium nitrite, sodium chromate can make the valve stem surface to generate a layer of passivation film, improve the corrosion resistance of the valve stem; solvent can make the corrosion inhibitor slowly dissolved, but also can play a lubricating effect; in asbestos add zinc powder for sacrificial metal, in fact, zinc is also a corrosion inhibitor, it can first and asbestos in the combination of chlorine, so that chlorine and metal contact with the valve stem opportunity to greatly reduce the corrosion prevention purposes. If the paint is added to the red Dan, calcium lead acid and other corrosion inhibitors, sprayed on the surface of the valve to prevent atmospheric corrosion.
7, Electrochemical protection
Electrochemical protection has anodic protection and cathodic protection two kinds. Such as zinc protection of iron, zinc is corroded, and zinc is called the sacrificial metal. In production practice, the use of anodic protection is less, cathodic protection is applied more. Large valves and important valves using this cathodic protection method, is an economical and simple, and effective method. Asbestos filler with zinc to protect the valve stem also belongs to the cathodic protection method.
8, Control corrosion environment
The so-called environment, there are two kinds broad and narrow, broad environment refers to the valve installation around the environment and its internal circulation medium; narrow environment refers to the valve installation around the conditions. Most of the environment can not be controlled, the production process can not be changed arbitrarily. Only in the case of not causing damage to the product, process, etc., can be used to control the environment of the method, such as boiler water deoxygenation, or refining process of alkali pH value adjustment. From this point of view, the above-mentioned addition of corrosion inhibitors, electrochemical protection, etc. also belongs to the control of corrosive environments.
The atmosphere is full of dust, water vapor, and smoke, especially in the production environment, such as smoke brine, equipment emitting toxic gases, and micro-powder, which will produce different degrees of corrosion on the valve. Operators should be in accordance with the provisions of the operating procedures, regular cleaning, blowing the valve, and regular refueling, which is an effective measure to control environmental corrosion.
Valve stem installation of protective cover, ground valve set up wells, valve surface painting, etc., which are to prevent the corrosive substances contained in the valve erosion methods. Rising ambient temperatures and air pollution, especially in the closed environment of the equipment and valves, will accelerate its corrosion. Should try to use open-type plants or ventilation, and cooling measures to slow down environmental corrosion.
9, Improve the processing technology and valve structure form
Valve corrosion protection from the beginning of the design to consider the issue of a reasonable structural design, and the correct method of valve products. Undoubtedly slowing down the corrosion of the valve is a good effect.
Therefore, the design and manufacturing departments should be those structural design is not reasonable, the process is not correct, easy to causes corrosion of the components, and should be improved, so that it is suitable for various working conditions under the requirements.
For different types of valve parts corrosion:
Prevent intergranular corrosion of austenitic stainless steel valve parts are: “solution quenching” treatment, that is, heated to about 1100 ℃ water quenching, the choice of titanium and niobium, and the carbon content of 0.03% below the austenitic stainless steel, reduce the production of chromium carbide.
Stress corrosion occurs in the breakdown and tensile stress at the same time under the action of the rupture. Methods to prevent stress corrosion; through heat treatment to eliminate or reduce welding, cold working in the stress, improving the structure of the valve that can not be rationalized, avoiding stress concentration, the use of electrochemical protection, spraying anti-corrosion paint. Add corrosion inhibitors, applying compressive stress and other measures.
Wear corrosion is the fluid on the metal wear and corrosion of alternating effects produced by a corrosion pattern, is a common valve corrosion, this corrosion occurs in the sealing surface for more. Prevention methods: selection of corrosion-resistant, wear-resistant materials, improve structural design, cathodic protection.
Mo vibration corrosion is mutual contact between the two parts at the same time to bear the load, the contact surface due to vibration, and sliding caused by the damage. Friction corrosion occurs in the bolted joints, the connection between the valve stem and the shut-off parts, between the ball bearing and the shaft, and other parts. It can be protected by applying lubricating grease, reducing friction, surface phosphatization, selecting hard alloy, and improving surface hardness by spraying tile treatment or cold machining.
After welding, annealing treatment should be used as much as possible and other corresponding protective measures. Improve the surface roughness of the valve stem processing and other valve surface roughness, the higher the surface roughness level, the stronger the corrosion resistance. Improve the packing and gasket processing technology and structure, the use of flexible graphite and plastic packing, as well as flexible graphite adhesive gaskets and polytetrafluoroethylene gaskets, can improve the sealing performance, reduce the corrosion of the stem and flange sealing surface.