Metal valve is easy to corrosion failure of the key parts in engineering equipment structure, usually, metal sealing face of valve, the valve stem, diaphragm, spring valve parts, materials, such as body, cover and so on for two or three materials, used in high pressure, highly toxic, flammable, explosive, radioactive medium valve, choose small materials subject to erosion.
Metal valves in the atmosphere or solution not only complex work conditions such as time going on uniform corrosion of the metal surface, in the local position of metal also prone to pitting corrosion, crevice corrosion, intergranular corrosion, delaminating corrosion, stress corrosion and corrosion fatigue, corrosion, selective corrosion, wear and cavitation corrosion, the corrosion of the vibration and local corrosion such as hydrogen corrosion.
Choose corrosion-resistant materials according to corrosive medium
In production practice, the corrosion of medium is very complex, even in the use of medium valve material sample, medium concentration, temperature, pressure is different, medium corrosion of material is not like. The corrosion rate increases 1~3 times when the temperature of the medium increases 10℃. Medium concentration has a great influence on the corrosion of valve materials, such as lead in sulfuric acid with a small concentration, the corrosion is very small, when the concentration exceeds 96%, the corrosion rises sharply. And carbon steel, on the contrary, in the sulfuric acid concentration of about 50% when the corrosion is serious, when the concentration increases to more than 6%, corrosion but sharply decreased. Like aluminum in the concentration of more than 80% of concentrated nitric acid corrosion is very strong, but in the middle, low concentration of nitric acid corrosion is serious. Although the corrosion resistance of stainless steel is very strong to dilute nitric acid, but in more than 95% of concentrated nitric acid corrosion is aggravated.
Non-metallic corrosion resistance, as long as the valve temperature and pressure meet the requirements of non-metallic materials, not only can solve the corrosion problem, but also can save precious metals. The valve body, valve cover, lining, sealing surface and other commonly used non-metallic materials are made of gaskets and fillers are mainly non-metallic materials. With polytetrafluoroethylene, chlorinated polyether and other plastics, as well as with natural rubber, neoprene, butadiene rubber and other rubber valve lining, and the valve body, valve cover body is made of cast iron, carbon steel. That is to ensure the strength of the valve, and ensure that the valve is not corroded. The clamp valve is also designed according to the corrosion resistance and variation of rubber. Now more and more of the use of nylon, polytetrafluoroethylene and other plastics, with natural rubber and synthetic rubber to do a variety of sealing surface, sealing ring, used in all kinds of valves, these non-metallic materials used as sealing surface, not only good corrosion resistance, and good sealing performance, especially suitable for the use of granular media. Of course, their low strength and heat resistance limit their range of applications. The appearance of flexible graphite makes the nonmetal enter the high temperature domain, solves the problem of leakage of packing and gasket which is difficult to solve for a long time, and is a good high temperature lubricant.
Valve connection bolt commonly used galvanizing, chrome plating, oxidation (blue) treatment to improve the ability of atmospheric and medium corrosion resistance. Other fasteners in addition to the above method of treatment, also according to the situation of phosphating surface treatment.
Sealing surface and closed parts of small diameter, often using nitriding, boronizing and other surface technology, improve its corrosion resistance and wear resistance. 38CrMoAlA made disc, nitriding layer ≥0. 4 mm.
Valve stem anticorrosion widely used nitriding, boriding, chromium plating, nickel plating and other surface treatment process, improve its corrosion resistance, corrosion resistance and abrasion resistance. Different surface treatment should be suitable for different stem material and working environment, in the atmosphere, water vapor medium and asbestos packing contact stem, can be used hard chromium plating, gas nitriding process (stainless steel should not use ion nitriding process); In the atmosphere of hydrogen sulfide, the valve adopts electroplated high phosphorus nickel coating, which has better protection performance. 38CrMoAlA can also resist corrosion by ion and gas nitriding, but hard chromium coating is not suitable. 2Cr13 can resist ammonia corrosion after quenching and tempering, and carbon steel with gas nitriding can also resist ammonia corrosion, while all phosphorous nickel coatings are not resistant to ammonia corrosion; After gas nitriding 38CrMoAlA material has good corrosion resistance and comprehensive performance, with it to make the valve stem for many.
Small diameter valve body and handwheel are often chrome plated to improve their corrosion resistance and decoration of the valve.
The thermal spraying
Thermal spraying is a kind of technology for coating preparation and has become a new technology for material surface protection. It is the use of high energy density of heat (gas combustion flame, arc, plasma arc, electric, gas combustion, etc.) after heating the molten metal or nonmetal material, in the form of atomized spray to the pretreatment of base surface, form the spraying layer, and at the same time the basic heating surface, the coating on the surface of the matrix, melt, forming the surface strengthening technology of the spray welding layers. Most metals and their alloys, metal-oxide ceramics, cermet complexes, and hard metal compounds can be coated on metallic or nonmetallic substrates by one or more thermal spraying methods.
Thermal spraying can improve the surface corrosion resistance, wear resistance, high temperature resistance and prolong the service life. Thermal spraying special functional coating, with heat insulation, edge (or heteroelectric), grinding seal, self-lubrication, thermal radiation, electromagnetic shielding and other special properties; Parts can be repaired by thermal spraying.