Corrosion is one of the most important elements that cause valve damage. Therefore, in valve protection, valve anti-corrosion is an important issue to consider.
Valve corrosion form
The corrosion of metals is mainly caused by chemical corrosion and electrochemical corrosion, and the corrosion of non-metallic materials is generally caused by direct chemical and physical actions.
1. Chemical corrosion
Under the condition that no current is generated, the surrounding medium directly reacts with the metal and destroys it, such as the corrosion of metal by high-temperature dry gas and non-electrolytic solution.
2. Galvanic corrosion
The metal is in contact with the electrolyte, resulting in the flow of electrons, which causes itself to be damaged by electrochemical action, which is the main form of corrosion.
Common acid-base salt solution corrosion, atmospheric corrosion, soil corrosion, seawater corrosion, microbial corrosion, pitting corrosion and crevice corrosion of stainless steel, etc., are all electrochemical corrosion. Electrochemical corrosion not only occurs between two substances that can play a chemical role, but also produces potential differences due to the concentration difference of the solution, the concentration difference of surrounding oxygen, the slight difference in the structure of the substance, etc., and obtains the power of corrosion, so that the metal with low potential and the position of the dry sun plate is lost.
Valve corrosion rate
The rate of corrosion can be divided into six grades:
(1) Completely corrosion-resistant: the corrosion rate is less than 0.001 mm/year
(2) Extremely corrosion resistant: corrosion rate 0.001 to 0.01 mm/year
(3) Corrosion resistance: corrosion rate 0.01 to 0.1 mm/year
(4) Still corrosion resistant: corrosion rate 0.1 to 1.0 mm/year
(5) Poor corrosion resistance: corrosion rate 1.0 to 10 mm/year
(6) Not corrosion-resistant: the corrosion rate is greater than 10 mm/year
Nine anti-corrosion measures
1. Select corrosion-resistant materials according to the corrosive medium
In the actual production, the corrosion of the medium is very complicated, even if the valve material used in the same medium is the same, the concentration, temperature and pressure of the medium are different, and the corrosion of the medium to the material is not the same. For every 10°C increase in medium temperature, the corrosion rate increases by about 1~3 times.
The medium concentration has a great influence on the corrosion of the valve material, such as the lead is in the sulfuric acid with a small concentration, the corrosion is very small, and when the concentration exceeds 96%, the corrosion rises sharply. Carbon steel, on the contrary, has the most serious corrosion when the sulfuric acid concentration is about 50%, and when the concentration increases to more than 60%, the corrosion decreases sharply. For example, aluminum is very corrosive in concentrated nitric acid with a concentration of more than 80%, but it is seriously corrosive in medium and low concentrations of nitric acid, and stainless steel is very resistant to dilute nitric acid, but it is aggravated in more than 95% concentrated nitric acid.
From the above examples, it can be seen that the correct selection of valve materials should be based on the specific situation, analyze various factors affecting corrosion, and select materials according to the relevant anti-corrosion manuals.
2. Use non-metallic materials
Non-metallic corrosion resistance is excellent, as long as the temperature and pressure of the valve meet the requirements of non-metallic materials, it can not only solve the corrosion problem, but also save precious metals. The valve body, bonnet, lining, sealing surface and other commonly used non-metallic materials are made.
Plastics such as PTFE and chlorinated polyether, as well as natural rubber, neoprene, nitrile rubber and other rubbers are used for valve lining, and the main body of the valve body bonnet is made of cast iron and carbon steel. It not only ensures the strength of the valve, but also ensures that the valve is not corroded.
Nowadays, more and more plastics such as nylon and PTFE are used, and natural rubber and synthetic rubber are used to make various sealing surfaces and sealing rings, which are used on various valves. These non-metallic materials used as sealing surfaces not only have good corrosion resistance, but also have good sealing performance, which is especially suitable for use in media with particles. Of course, they are less strong and heat resistant, and the range of applications is limited.
3. Metal surface treatment
(1) Valve connection: The valve connection snail is commonly treated with galvanizing, chrome plating, and oxidation (blue) to improve the ability to resist atmospheric and medium corrosion. In addition to the above-mentioned methods, other fasteners are also treated with surface treatments such as phosphating according to the situation.
(2) Sealing surface and closed parts with small diameter: surface processes such as nitriding and boronizing are used to improve its corrosion resistance and wear resistance.
(3) Stem anti-corrosion: nitriding, boronization, chrome plating, nickel plating and other surface treatment processes are widely used to improve its corrosion resistance, corrosion resistance and abrasion resistance.
Different surface treatments should be suitable for different stem materials and working environments, in the atmosphere, water vapor medium and asbestos packing contact stem, can use hard chrome plating, gas nitriding process (stainless steel should not use ion nitriding process): in the hydrogen sulfide atmospheric environment using electroplating high phosphorus nickel coating has better protective performance; 38CrMOAIA can also be corrosion-resistant by ion and gas nitriding, but hard chrome coating is not suitable for use; 2Cr13 can resist ammonia corrosion after quenching and tempering, and the carbon steel using gas nitriding can also resist ammonia corrosion, while all phosphorus-nickel plating layers are not resistant to ammonia corrosion, and the gas nitriding 38CrMOAIA material has excellent corrosion resistance and comprehensive performance, and it is mostly used to make valve stems.
(4) Small-caliber valve body and handwheel: It is also often chrome-plated to improve its corrosion resistance and decorate the valve.
4. Thermal spraying
Thermal spraying is a kind of process method for preparing coatings, and has become one of the new technologies for material surface protection. It is a surface strengthening process method that uses high energy density heat sources (gas combustion flame, electric arc, plasma arc, electric heating, gas explosion, etc.) to heat and melt metal or non-metallic materials, and spray them to the pretreated basic surface in the form of atomization to form a spray coating, or heat the basic surface at the same time, so that the coating is melted again on the surface of the substrate to form a surface strengthening process of spray welding layer.
Most metals and their alloys, metal oxide ceramics, cermet composites and hard metal compounds can be coated on metal or non-metal substrates by one or several thermal spraying methods, which can improve the surface corrosion resistance, wear resistance, high temperature resistance and other properties, and prolong the service life. Thermal spraying special functional coating, with heat insulation, insulation (or abnormal electricity), grindable sealing, self-lubrication, thermal radiation, electromagnetic shielding and other special properties, the use of thermal spraying can repair parts.
5. Spray paint
Coating is a widely used anti-corrosion means, and it is an indispensable anti-corrosion material and identification mark on valve products. Coating is also a non-metallic material, which is usually made of synthetic resin, rubber slurry, vegetable oil, solvent, etc., covering the metal surface, isolating the medium and the atmosphere, and achieving the purpose of anti-corrosion.
Coatings are mainly used in water, salt water, seawater, atmosphere and other environments that are not too corrosive. The inner cavity of the valve is often painted with anticorrosive paint to prevent water, air and other media from corroding the valve
6. Add corrosion inhibitors
The mechanism by which corrosion inhibitors control corrosion is that it promotes the polarization of the battery. Corrosion inhibitors are mainly used in media and fillers. The addition of corrosion inhibitors to the medium can slow down the corrosion of equipment and valves, such as chromium-nickel stainless steel in oxygen-free sulfuric acid, a large solubility range into a cremation state, corrosion is more serious, but adding a small amount of copper sulfate or nitric acid and other oxidants, can make the stainless steel turn into a blunt state, the surface of a protective film to prevent the erosion of the medium, in hydrochloric acid, if a small amount of oxidant is added, the corrosion of titanium can be reduced.
Valve pressure test is often used as the medium for pressure test, which is easy to cause corrosion of the valve, and adding a small amount of sodium nitrite to the water can prevent the corrosion of the valve by water. Asbestos packing contains chloride, which corrodes the valve stem greatly, and the chloride content can be reduced if the steaming water washing method is adopted, but this method is very difficult to implement, and can not be popularized generally, and is only suitable for special needs.
In order to protect the valve stem and prevent the corrosion of the asbestos packing, in the asbestos packing, the corrosion inhibitor and sacrificial metal are coated on the valve stem, the corrosion inhibitor is composed of sodium nitrite and sodium chromate, which can generate a passivation film on the surface of the valve stem and improve the corrosion resistance of the valve stem, and the solvent can make the corrosion inhibitor slowly dissolve and play a lubricating role; In fact, zinc is also a corrosion inhibitor, which can first combine with the chloride in asbestos, so that the chloride and the stem metal contact opportunity is greatly reduced, so as to achieve the purpose of anti-corrosion.
7. Electrochemical protection
There are two types of electrochemical protection: anodic protection and cathodic protection. If zinc is used to protect iron, zinc is corroded, zinc is called sacrificial metal, in production practice, anode protection is used less, cathodic protection is used more. This cathodic protection method is used for large valves and important valves, which is an economical, simple and effective method, and zinc is added to the asbestos packing to protect the valve stem.
8. Control the corrosive environment
The so-called environment has two kinds of broad sense and narrow sense, the broad sense of environment refers to the environment around the valve installation place and its internal circulation medium, and the narrow sense of environment refers to the conditions around the valve installation place.
Most environments are uncontrollable, and production processes cannot be arbitrarily changed. Only in the case that there will be no damage to the product and process, the method of controlling the environment can be adopted, such as deoxygenation of boiler water, alkali addition in the oil refining process to adjust the PH value, etc. From this point of view, the addition of corrosion inhibitors and electrochemical protection mentioned above is also a way to control the corrosive environment.
The atmosphere is full of dust, water vapor and smoke, especially in the production environment, such as smoke brine, toxic gases and fine powder emitted by equipment, which will cause varying degrees of corrosion to the valve. The operator should regularly clean and purge the valve and refuel regularly according to the provisions of the operating procedures, which is an effective measure to control environmental corrosion. Installing a protective cover on the valve stem, setting a ground well on the ground valve, and spraying paint on the surface of the valve are all ways to prevent corrosive substances from eroding the valve.
The increase in ambient temperature and air pollution, especially for equipment and valves in a closed environment, will accelerate their corrosion, and open workshops or ventilation and cooling measures should be used as much as possible to slow down environmental corrosion.
9. Improve the processing technology and valve structure
The anti-corrosion protection of the valve is a problem that has been considered from the beginning of the design, and a valve product with reasonable structural design and correct process method will undoubtedly have a good effect on slowing down the corrosion of the valve. Therefore, the design and manufacturing department should improve the parts that are not reasonable in structural design, incorrect in process methods and easy to cause corrosion, so as to adapt them to the requirements of various working conditions.
Post time: Jan-22-2025
