Metal valves can be said to be the most vulnerable parts of engineering equipment that are likely to corrode failure.
Valve sealing surface, stem, diaphragm, small spring and other valve parts generally use a material, valve body, bonnet, etc. for two or three materials for high pressure, highly toxic, flammable, explosive, radioactive Medium valves, the choice of very corrosive materials.
Metal valves in the atmosphere or solution and other complex working conditions not only the moment the occurrence of uniform corrosion of metal surface, in the metal local location is also prone to pitting corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue Corrosion, selective corrosion, wear corrosion, cavitation corrosion, vibration corrosion, corrosion and other local corrosion.
Metal valve anti-corrosion measures
1 Corrosion resistant materials are selected according to corrosive media
In the actual production, the corrosion of the medium is very complicated, even in the use of a medium in the same valve material, the media concentration, temperature, pressure, the media corrosion of the material is not the same. Dielectric temperature increased by 10 ℃ , Corrosion rate increased by about 1 to 3 times the medium concentration on the valve material corrosion great impact, such as lead in the small concentration of sulfuric acid, the corrosion is very small, when the concentration exceeds 96%, the corrosion increased sharply, while the carbon steel, When the concentration of sulfuric acid is about 50% when the most serious corrosion, when the concentration increased to 6% or more, the corrosion but a sharp decline in the concentration of 80% or more concentrated nitric acid in the corrosive strong, but in the low concentration of nitric acid But the corrosion of dilute nitric acid is very strong, but more than 95% concentrated nitric acid corrosion is aggravated.
From the above cases can be seen that the correct choice of valve materials should be based on specific circumstances, 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 valve temperature and pressure in line with the requirements of non-metallic materials, not only can solve the corrosion problem, but also save precious metals. Valve body, bonnet, lining, sealing surface and other commonly used non-metallic materials , With the gasket, the main non-metallic filler material made with polytetrafluoroethylene, chlorinated polyether and other plastics, and with natural rubber, chloroprene rubber, NBR rubber and other rubber lining, and the valve body, The main body of the bonnet is made of cast iron and carbon steel, which ensures the strength of the valve and ensures that the valve is not subject to corrosion. The pinch valve is also designed according to the excellent corrosion resistance and excellent performance of the rubber. On the use of nylon, PTFE and other plastics, with natural rubber and synthetic rubber to do a variety of sealing surface, seals, for all types of valves, these used as a sealing surface of non-metallic materials, Good corrosion resistance, and good sealing performance, especially suitable for use in granular media.Of course, their strength and heat resistance are low, the scope of application is limited.Portical graphite appears, so that non-metallic into the high temperature field, Solved long-term difficult to solve the problem of packing and gasket leakage, and is a good high-temperature lubricant.
3 metal surface treatment
Valve connection screw machine commonly used galvanized, chrome, oxidation (blue) to improve the resistance to atmospheric resistance, resistance to medium corrosion. Other fasteners in addition to the above method of treatment, but also according to the situation using phosphating and other surface treatment.
Sealing surface and the diameter of the closing parts, often using nitriding, boronizing and other surface technology to improve its corrosion resistance and wear resistance .38CrMoAlA made of the valve, nitriding layer ≥ 0.4mm.
Stem anti-corrosion widely used nitriding, boronizing, chrome, nickel and other surface treatment process to improve its corrosion resistance, corrosion resistance and abrasion resistance performance. Different surface treatment should be suitable for different stem material and Working environment, in the atmosphere, the vapor medium and asbestos filler contact with the stem, can be used hard chrome plating, gas nitriding process (stainless steel should not use ion nitriding process); in the hydrogen sulfide atmospheric environment of the valve using electroplating high phosphorus nickel Coating has a better protective properties; 38CrMoAlA ion and gas nitriding can also be corrosion, but should not use hard chrome plating; 2Cr13 after quenching and tempering after the ammonia corrosion, the use of gas nitriding carbon steel can also resist the corrosion of ammonia, And all the phosphorous nickel coating is not resistant to ammonia corrosion; after gas nitriding 38CrMoAlA material has excellent corrosion resistance and comprehensive performance, with it made more stem.
Small diameter body and hand wheel are also chrome-plated, improve its corrosion resistance, decorative valves.
4 thermal spray
Thermal spraying is a kind of new technology for the preparation of coating. It has become one of the new technologies of material surface protection.It is the use of high energy density heat source (gas combustion flame, arc, plasma arc, electric heating, gas explosion, etc.) will be metal or The surface of the non-metallic material is heated and melted and sprayed onto the pre-treated base surface in the form of atomized form to form a spray coating, or the surface is heated at the same time to re-melt the coating on the surface of the substrate. Most metals and their alloys, metal oxide ceramics, cermet composites, and hard metal compounds can be coated on metal or nonmetallic substrates by one or more thermal spraying methods.
Thermal spraying can improve its surface corrosion resistance, wear resistance, high temperature and other properties, to extend the service life. Thermal spraying special functional coating, with insulation, insulation (or electricity), can be sealed, self-lubricating, Electromagnetic shielding and other special properties; the use of thermal spraying can repair parts.
5 spray paint
Paint is the most widely used anti-corrosion means, in the valve products is an indispensable anti-corrosion materials and identification marks. Paint is also a non-metallic materials, it is usually made of synthetic resin, rubber slurry, vegetable oil, , Covering the metal surface, isolation of the media and the atmosphere, to achieve the purpose of corrosion. Paint is mainly used for water, salt water, sea water, the atmosphere is not very strong corrosion environment. Valve cavity commonly used anti-corrosion paint brush to prevent water, air and other media The paint is mixed with different colors to indicate the material used by the law. The valve is sprayed with paint, usually in half a year to once a year.
6 Add corrosion inhibitor
Corrosion inhibitor The mechanism of corrosion is that it promotes the polarization of the battery. The corrosion inhibitor is mainly used for media and filler. The addition of corrosion inhibitor in the medium can slow the corrosion of the equipment and the valve, such as chromium nickel Oxygen-containing sulfuric acid, a large range of melting within the melting range, more serious corrosion, but adding a small amount of copper sulfate or nitric acid and other oxidants, stainless steel can change the passive state, the surface layer of protective film to prevent the media dip Corrosion, in the hydrochloric acid, if adding a small amount of oxidant, can reduce the corrosion of titanium. Valve pressure test water commonly used for pressure test pressure, easily lead to corrosion of the valve, add a small amount of sodium nitrite in the water can prevent water corrosion of the valve. Asbestos packing contains chloride, the valve stem corrosion is great, if the use of distilled water washing method can reduce the chloride content, but this method is difficult in the implementation of many, not ordinary promotion, ester suitable for special needs.
In order to protect the stem, to prevent the corrosion of asbestos filler, in the asbestos filler, in the stem coated with corrosion inhibitor and sacrificial metal corrosion inhibitor sodium nitrite, sodium chromate can make a layer of passivation of the stem Membrane, to improve the corrosion resistance of the stem; solvent can slowly dissolve the corrosion inhibitor, but also from the lubrication; in the asbestos added zinc powder for sacrificial metal, in fact, zinc is also a corrosion inhibitor, it can First with the chloride in the asbestos, so that chloride and stem metal contact opportunities greatly reduced, so as to achieve the purpose of anti-corrosion. Paint if the red diamonds, calcium chloride and other corrosion inhibitors, spray on the surface of the valve to prevent Corrosion of the atmosphere.
7 Electrochemical protection
Electrochemical protection has both anodic protection and cathodic protection, such as zinc protection, zinc corrosion, zinc is called sacrificing metal.In the production practice, the anode protection is less, cathodic protection applications more. Large valves and important valves The use of this cathodic protection method is an economical and convenient method of adding zinc to the asbestos filler, and the protection of the stem also belongs to the cathodic protection method.
8 Control of corrosive environment
The environment in the narrow sense refers to the conditions around the valve installation, and most of the environment can not be controlled and the production process can not be arbitrary. The environment is in the broad sense and in the narrow sense. The general environment refers to the environment around the valve installation and its internal circulation medium. Change. Only in the case of no damage to products, processes, etc., can be used to control the environment, such as boiler water deoxidation, oil refining process in the home to adjust the PH value, etc. From this point of view, the above added corrosion Agents, electrochemical protection, etc. are also controlled corrosion environment.
The atmosphere is full of dust, water vapor, smoke, especially in the production environment, such as smoke and halogen, the equipment emits toxic gases and powder, will have different degrees of corrosion of the valve.The operator should be in accordance with the provisions of the operating procedures, Cleaning, purging the valve, regular refueling, which is effective measures to control the corrosion of the environment. Stem installment protective cover, the ground valve to set the ground, the valve surface spray paint, etc., this is to prevent corrosion of substances containing corrosion of the valve method. Elevated and air pollution, especially in closed environments, equipment and valves will accelerate its corrosion. Should try to use open plant or use ventilation, cooling measures to slow down environmental corrosion.
9 to improve the processing technology and valve structure
The corrosion protection of the valve is from the design began to consider the problem, a reasonable structure design, the correct method of valve products. Undoubtedly to reduce the corrosion of the valve is a good effect.
Therefore, the design and manufacturing department should deal with those parts that are unreasonable in the design and are not properly machined and easily corroded and should be modified to suit the requirements of different operating conditions. For different types of valve parts, Have a trick:
Austenitic stainless steel valve parts to prevent intergranular corrosion methods are: "solid solution quenching" treatment, that is heated to about 1100 ℃ water quenching, the choice of containing titanium and niobium, and carbon content of 0.03% or less of austenitic stainless steel , To reduce the production of chromium carbide.
Stress corrosion occurs under the action of corrosion and tensile stress generated under the same time to prevent stress corrosion method; by heat treatment to eliminate or reduce the welding, cold work stress, improve the irrational valve structure, to avoid stress concentration, the use of electrochemical protection, Spray anti-corrosion coating. Add corrosion inhibitor, the application of compressive stress and other measures.
Wear corrosion is a kind of corrosion caused by the effect of fluid on the wear and corrosion of metal. It is a kind of corrosion which is common to the valve. This kind of corrosion takes place on the sealing surface. Prevention method: Use corrosion-resistant, wear-resistant material , Improved structural design, the use of cathodic protection.
Vibration corrosion is the two parts that are in contact with each other at the same time bear the load, the contact surface due to vibration and sliding caused by the destruction of vibration corrosion occurred in the bolt connection, stem and closing parts connection, between the ball bearing and the shaft and other parts. Can be coated with grease, reduce friction, surface phosphating, the choice of cemented carbide, and with a shaving or cold processing to improve the surface hardness method of protection.
After welding, try to use the corresponding protective measures such as annealing.To improve the surface roughness of the valve stem and other valve surface roughness, the higher the surface roughness level, the stronger the resistance to improve the filler and gasket processing technology and structure , The use of flexible graphite and plastic fillers, as well as flexible graphite paste pads and polytetrafluoroethylene gasket, can improve the sealing performance and reduce the corrosion of the stem and flange sealing surface (according to the sound of the valve)