19

2021

-

03

Do you know the top ten tips for anti-corrosion of metal valves?

Author:


Metal valves can be said to be key component structures that are prone to corrosion and failure in engineering equipment. Generally, the sealing surface, valve stem, diaphragm, small spring and other valve parts of metal valves are generally made of materials. The valve body, valve cover, etc. are suitable for two or three Materials. For valves used in high-pressure, highly toxic, flammable, explosive, and radioactive media, materials with little corrosiveness should be selected.

Metal valves can be said to be key component structures that are prone to corrosion and failure in engineering equipment. Generally, the sealing surface, valve stem, diaphragm, small spring and other valve parts of metal valves are generally made of materials. The valve body, valve cover, etc. are suitable for two or three Materials. For valves used in high-pressure, highly toxic, flammable, explosive, and radioactive media, materials with little corrosiveness should be selected.

        Under complex working conditions such as atmosphere or solution, metal valves not only undergo uniform corrosion on the metal surface at all times, but are also prone to pitting corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, and fatigue corrosion in local locations of the metal. , selective corrosion, abrasion corrosion, cavitation corrosion, friction corrosion, hydrogen corrosion and other local corrosion.

     

   Anti-corrosion measures for metal valves

 

 

01

Select corrosion-resistant materials according to corrosive media

       

        In actual production, the corrosion of the medium is very complicated. Even if the valve materials used in the medium are the same, the concentration, temperature, and pressure of the medium are different, and the corrosion of the material by the medium is also different. For every 10°C increase in medium temperature, the corrosion rate increases approximately 1 to 3 times. The concentration of the medium has a great influence on the corrosion of valve materials. For example, if lead is in sulfuric acid with a small concentration, the corrosion will be very small. When the concentration exceeds 96%, the corrosion will rise sharply. On the contrary, carbon steel suffers from serious corrosion when the sulfuric acid concentration is about 50%. When the concentration increases to more than 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 medium and low concentrations of nitric acid. Although stainless steel has strong corrosion resistance to dilute nitric acid, the corrosion becomes more severe in concentrated nitric acid of more than 95%.

        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.

        

02

Use non-metallic materials

 

        Non-metals have good corrosion resistance. As long as the operating temperature and pressure of the valve meet the requirements of non-metal materials, it can 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 good corrosion resistance and mutation properties of rubber. Nowadays, more and more plastics such as nylon and polytetrafluoroethylene are used, and natural rubber and synthetic rubber are used to make various sealing surfaces and sealing rings for various types of valves. These non-metallic materials are used as sealing surfaces. The material not only has good corrosion resistance, but also has good sealing performance, and is especially suitable for use in media with particles. Of course, their strength and heat resistance are low, and their range of applications is limited. The emergence of flexible graphite allows non-metals to enter the high-temperature domain, solving the long-term problem of filler and gasket leakage that has been difficult to solve, and it is a good high-temperature lubricant.

        

03

metal surface treatment

 

        Valve connection bolts 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 also use surface treatments such as phosphating according to the situation.

        Sealing surfaces and closing parts with small diameters often use surface processes such as nitriding and boronizing to improve their corrosion resistance and wear resistance. Valve disc made of 38CrMoAlA, nitrided layer ≥0.4mm.

        Valve stem anti-corrosion widely uses surface treatment processes such as nitriding, boronizing, chromium plating, and nickel plating to improve its corrosion resistance, corrosion resistance, and wear and scratch resistance. 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 should not be used for stainless steel); For valves in hydrogen sulfide atmosphere, electroplating high-phosphorus nickel coating has better protective performance; 38CrMoAlA can also resist corrosion by ion and gas nitriding, but it is not suitable to use hard chromium coating; 2Cr13 can resist ammonia corrosion after quenching and tempering. Gas nitrided carbon steel can also resist ammonia corrosion, while all phosphorus-nickel coatings are not resistant to ammonia corrosion; gas nitrided 38CrMoAlA material has good corrosion resistance and comprehensive performance, and is mostly used to make valve stems.

        Small-diameter valve bodies and handwheels are often chromium-plated to improve their corrosion resistance and decorate the valve.

        

04

Thermal Spray

 

        Thermal spraying is a process-like process for preparing coatings and has become one of the new technologies for material surface protection. It uses a high energy density heat source (gas combustion flame, electric arc, plasma arc, electric heating, gas explosion, etc.) to heat and melt metal or non-metallic materials, and then sprays them into atomized form onto the pretreated basic surface to form a spray coating. , or a surface strengthening process that heats 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 abnormal electricity), grindable sealing, self-lubrication, thermal radiation, electromagnetic shielding, etc.; parts can be repaired using thermal spraying.

   

05

spray paint

 

        Coating is a 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 and separates 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 Fine. Valves are spray-painted, generally every six months to once a year.

        

06

Add corrosion inhibitor

 

        The mechanism of corrosion inhibitors to control corrosion is that it promotes the degradation of batteries. 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 is composed of sodium nitrite and sodium chromate, which can form a passivation film on the surface of the valve stem to improve the corrosion resistance of the valve stem; the solvent can slowly dissolve the corrosion inhibitor and act as a lubrication; in asbestos Add zinc powder as a sacrificial metal. In fact, zinc is also a corrosion inhibitor. It can first combine with the chloride in the asbestos, greatly reducing the chance of contact between the chloride and the valve stem metal, thereby achieving the purpose of anti-corrosion. 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.

  

07

Electrochemical protection

 

        

         There are two types of electrochemical protection: positive protection and negative protection. If zinc is used to protect iron, and the zinc corrodes, the zinc is called a sacrificial metal. In production practice, positive protection is used less frequently and negative protection is used more frequently. This cathode protection method is used for large valves and important valves, which is an economical, simple and effective method. Adding zinc to asbestos filler to protect the valve stem is also a cathode protection method.

        

08

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 there is no damage to products, processes, etc., can environmental control methods be used, such as boiler water deoxygenation, alkali adjustment of PH value in the refining process, etc. From this point of view, the above-mentioned addition of corrosion inhibitors, electrochemical protection, etc. also belong to 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 temperatures 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.

       

09

Improve processing technology and valve structure

 

 

        The anti-corrosion protection of valves is an issue that is considered from the beginning of design. It is a valve product with reasonable structural design and correct process methods. There is no doubt that it has 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.

        

10

For different types of valve component corrosion, the editor has tips:

 

 

        The method to prevent intergranular corrosion of stainless steel valve parts is to perform "solid solution quenching" treatment, that is, heating to about 1100°C for water quenching, and selecting austenitic stainless steel containing titanium and niobium with a carbon content of less than 0.03% to reduce chromium carbide. of production.

        Stress corrosion occurs when corrosion and tensile stress cause rupture at the same time. The method to prevent stress corrosion is to eliminate or reduce the stress generated during welding and cold working through heat treatment, improve the unreasonable valve structure, avoid stress concentration, and use electrochemical protection, Spray anti-corrosion paint. Add corrosion inhibitors, apply compressive stress and other measures.

        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, use cathode protection, etc.

        Friction corrosion is when two parts in contact with each other are subjected to 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. It can be protected by applying lubricating grease to reduce friction, phosphating the surface, using cemented carbide, or using tile spraying or cold working to improve surface hardness.

        After welding, corresponding protective measures such as annealing treatment should be adopted as much as possible to improve the surface roughness of the valve stem and other valve parts. The higher the surface roughness, the stronger the corrosion resistance. Improving the processing technology and structure of fillers and gaskets, using flexible graphite and plastic fillers, as well as flexible graphite adhesive gaskets and PTFE-coated gaskets can improve sealing performance and reduce the need for valve stem and flange sealing Surface corrosion.