The following are several methods of corrosion resistance of valves.
1.We can choose corrosion-resistant materials according to the medium
The corrosion of the medium is very complicated in production practice. Even in the same medium used in the same valve material, if the concentration of the medium, temperature, and pressure are different, the medium on the material corrosion is not the same. Media temperature every 10 ℃, the corrosion rate increased by about 1 to 3 times. Media concentration on the valve material corrosion has a significant impact. If the lead is in the concentration of small sulfuric acid, it is minimal corrosion. And when it is more than 96% concentration, corrosion will rise sharply. The opposite is true for carbon steel, which rusts most severely when the sulphuric acid concentration is about 50%. When the concentration increases to 6%, it is corrosion instead of a sharp decline. Another example is aluminum, in a concentration of more than 80% of the concentrated nitric acid in the corrosion, which is very strong, but in the medium and low concentrations of nitric acid in the corrosion but serious. In stainless steel, although the corrosion resistance of dilute nitric acid is extreme, in more than 95% of the concentrated nitric acid, corrosion increased.
From the above examples, the correct choice of valve materials should be based on specific circumstances, analysis of various factors affecting corrosion, and according to the relevant anti-corrosion manual selection of materials.
2. Use of non-metallic materials
Non-metallic corrosion resistance is excellent. As long as the valve uses temperature and pressure in line with the requirements of non-metallic materials, it can solve the corrosion problem and save precious metals. Valve body, valve cover, lining, sealing surface, etc., commonly used non-metallic materials. With respect to the gasket, it is mainly a packing made of non-metallic materials.We use plastics such as PTFE, chlorinated polyether, and rubber such as natural rubber, neoprene, and nitrile rubber for valve lining. The valve and cover bodies are generally cast iron and carbon steel. It ensures the strength of the valve and that the valve is not subject to corrosion. The clamp valve is also designed according to the rubber’s excellent corrosion resistance and change performance. Now more and more use of nylon, polytetrafluoroethylene, and other plastics, there are natural rubber and synthetic rubber to do various sealing surfaces and sealing rings. They are also used on all types of valves. These non-metallic materials used as sealing surfaces have not only good corrosion resistance but also good sealing performance. They are particularly suitable for use in media with particles. Of course, their strength and heat resistance are low, and their range of application is limited. The advent of flexible graphite, on the other hand, has brought non-metals into the high-temperature sector. It solves the long-standing problem of leaking fillers and gaskets and is a very good high-temperature lubricant.
3. Spray paint
Paint is an essential anti-corrosion material and recognition symbol on valve products. Paint is also a non-metallic material. It is usually prepared with synthetic resin, rubber slurry, vegetable oil, solvent, etc. It can cover the metal surface, and isolate the medium and atmosphere, thus achieving the purpose of corrosion protection. The paint is mainly used in environments where corrosion is not too strong, such as water, salt water, seawater, and the atmosphere. The inner cavity of the valve is commonly painted with anti-corrosion paint, which prevents the corrosion of the valve by water, air, and other media. The paint is mixed with different colors to indicate the material used for the valve.
4. Add corrosion inhibitor
In the corrosive medium and corrosive substances, adding a small amount of other particular substances can greatly slow down the rate of metal corrosion. This unique material is called a corrosion inhibitor.
Corrosion inhibitors control the mechanism of corrosion, is that it promotes the polarization of the battery. Corrosion inhibitors are mainly used as media and fillers. Corrosion inhibitors are added to the medium to slow down the corrosion of equipment and valves. Such as chromium-nickel stainless steel in non-oxygenated sulphuric acid, in a large range of solubility, will become a pyrophoric state. Therefore, it is more severe corrosion. But if you add a small amount of copper sulfate, nitric acid, and other oxidants, we can make stainless steel to change the blunt state. Its surface will generate a protective film to prevent the leaching of the medium. In hydrochloric acid, if a small amount of oxidizing agent is added, it can reduce the corrosion of titanium. Valve test pressure is commonly used for the test pressure of the medium, easy to causes corrosion of the valve. Adding a small amount of sodium nitrite in the water can prevent water corrosion of the valve. Asbestos packing contains chloride, and the valve stem corrosion is excellent. Suppose the use of a steam-distilled water washing method can reduce the content of chloride. But this method, in the implementation of many difficulties, can not be commonly promoted, only suitable for special needs.
The issue of stem corrosion resistance is an issue that is taken seriously. After accumulating rich production experience, we often use surface treatment processes such as nitriding, boriding, chrome plating, and nickel plating. It can improve its corrosion resistance and wear and abrasion resistance. Different surface treatments should be suitable for other stem materials and working environments. In the atmosphere, water vapor media and asbestos packing contact stems can use hard chrome plating and gas nitriding process (stainless steel should not use an ion nitriding process). In the hydrogen sulfide atmosphere valve, electroplated high phosphorus nickel plating has a better protective performance. 38CrMoAlA material, if used ion and gas nitriding, can also be corrosion resistant but should not use hard chrome plating. After tempering, 2Cr13 material can be resistant to ammonia corrosion. It is also resistant to corrosion by ammonia using gas-nitrided carbon steel. And all phosphor-nickel plating is not resistant to ammonia corrosion.
Small-diameter valve bodies and handwheels are also often chrome-plated. It can improve its corrosion resistance performance and decorate the valve.
5. Thermal spraying
Thermal spraying is a process square for preparing coatings, which has become one of the new technologies for material surface protection. It is a national key promotion project. It is the use of a high energy density heat source (gas combustion flame, electric arc, plasma arc, electric heat, gas combustion explosion, etc.) that it will heat and melted metal or non-metallic materials in the form of an atomized spray to the basic surface by pretreatment. This results in the formation of a sprayed layer, or the simultaneous heating of the basic surface so that the coating is again molten on the surface of the substrate, and a surface strengthening process method in which a sprayed weld layer is formed. Most metals and their alloys, metal oxide ceramics, metal-ceramic complexes, and hard metal compounds can be coated with one or more thermal spraying methods to form a coating on a metallic or non-metallic substrate.
Thermal spraying improves their surfaces’ corrosion, wear, and heat resistance and extends their service life. The unique functional coating of thermal spraying has special properties such as heat insulation, insulation (or isoelectric), abradable sealing, self-lubrication, thermal radiation, and electromagnetic shielding. Therefore, we can use thermal spraying can repair parts.
6. Control the corrosive environment
There is a broad and narrow sense of two in the so-called environment. A general sense of the environment refers to the valve installation around the environment and its internal circulation medium. We cannot control most environments, and we cannot change production processes at will. Only if there is no damage to the product, process, etc., can we use the method of controlling the environment. For example, the boiler water is de-oxygenated, and the PH value is adjusted by home alkali in the oil refining process. From this point of view, the addition mentioned above of corrosion inhibitors, electrochemical protection, etc., also belongs to the control of the corrosion environment.
Dirt, water vapour and fumes fill the atmosphere, especially in the production environment.Such as smoke and brine, equipment emitting toxic gases, and micronized powder, which will produce different degrees of corrosion on the valve. The operator should regularly clean and purge the valve as specified in the operating procedures and refuel regularly, which is an effective measure to control environmental corrosion. Stem installation protection cover, ground valve set ground well, valve surface spray paint, etc., are methods to prevent the erosion of the valve containing corrosive substances. If the ambient temperature rises and air pollution, especially in the closed environment of the equipment and valves, will accelerate its corrosion. We should use open plants or ventilation and cooling measures to slow down environmental corrosion.
7. Improvement of machining processes and valve construction forms
Valve corrosion protection from the design of the problem, a good structural design, and the correct method of technology valve products.
Therefore, the design and manufacturing department should be those structural design is not reasonable, the process method is not correct, and easy to cause it should improve corrosion of the components. And make it suitable for various working conditions under the requirements.
The gap at the valve connection is a suitable environment for corrosion of the oxygen concentration difference cell.
Therefore, the valve stem and the closure of the connection, as far as possible not to use li, threaded connection form. Valve welding applications, double-sided butt welding, and continuous welding are suitable. Spot and lap welds tend to cause corrosion. Valve thread connection, the use of polytetrafluoroethylene raw tape and pad. It not only can have a good seal but also can prevent corrosion. Media that do not flow easily at dead ends tend to corrode the valve. In addition to the use of the valve is not installed backward and paying attention to the discharge of deposited media, we should try to avoid a free-of-depression structure when manufacturing valve parts. And we should also try to set up drainage holes for the valve.
Contact between different metals can constitute a point couple and promote anodic metal corrosion. When we choose materials, we should avoid contact with metals that have a large difference in potential and cannot produce a passivation film. In the process of fabrication and processing, especially in welding and heat treatment to make stress corrosion, we should pay attention to improving processing methods. After welding, we should try to use corresponding protective measures such as annealing treatment. We need to improve the surface roughness of the valve stem machining and the surface roughness of other valve parts. The higher its surface roughness level, the better the corrosion resistance. We need to improve the processing and construction of packing and gaskets. We can use flexible graphite and plastic packing, as well as flexible graphite stick-on gaskets and gaskets wrapped in PTFE. They all improve sealing performance and reduce corrosion of the ground stem and flange sealing surfaces.
