Valve corrosion is the leading cause of valve failure, and valve corrosion may lead to the valve’s short life and lousy performance.
For the corrosive reason, there are mainly six kinds of valve corrosive.
Galvanic corrosion
When two different metals are in contact and exposed to a corrosive liquid and electrolyte, a primary cell is formed. The current causes the anode to corrode, increasing the current. Corrosion is often localized near the point of contact. Reducing erosion can be achieved by plating dissimilar metals.
High-temperature corrosion
To predict the effect of high-temperature oxidation, we need to examine these data: 1) metal composition, 2) atmosphere composition, 3) temperature 4) exposure time. However, it is well known that most light metals (those lighter than their oxides) form a non-protective oxide layer that peels off as it gets thicker over time. There are also other forms of high-temperature corrosion, including sulfidation, carburization, etc.
Crevice corrosion
This happens all over the seam, which hinders oxygen diffusion, causing high and low oxygen areas, and creating a difference in solution concentration. In particular, the joint or welding joint defects may appear narrow gap. Its gap width (generally in 0.025 ~ 0.1mm) is enough to make the electrolyte solution into so that the metal inside the hole and the metal outside the gap constitute a short-circuit primary cell, and in the gap occurred in the strong corrosion of local corrosion.
Pitting corrosion
When the protective film is destroyed or corrosion products layer decomposition, it produces local corrosion or pitting corrosion. Film rupture forms the anode, and unbroken film or corrosion products as the cathode have established a closed circuit in the presence of chloride ions, some stainless steel prone to pitting corrosion. Corrosion occurs when the metal surface or rough areas, due to these are not for uniformity.
Intergranular corrosion
Intergranular corrosion occurs for a variety of reasons. The result is almost identical along with the metal’s grain boundaries and the destruction of mechanical properties. Without proper heat treatment or contact sensitization, 800-1500° Fahrenheit temperature of austenitic stainless steel intergranular corrosion is subject to many corrosives (427-816°C). This can be eliminated by pre-annealing and quenching 2000°F (1093°C) with low carbon stainless steel (c-0.03 max) or stabilized niobium or titanium.
Frictional Corrosion
The physical force from wear fracture dissolves the metal by protective corrosion. The effect relies mainly on pressure and speed. Excessive vibration or metal bending can also have similar results. A cavitation is a common form of corrosion pumps, stress corrosion cracking High tensile stress and corrosive atmosphere can cause metal corrosion. Tensile stress on the metal surface under static load exceeds the metal yield point, and the corrosion action concentrates the area of stress action. The result shows local corrosion. In the alternating metal corrosion and the establishment of high-stress concentration of parts, to avoid this corrosion can be through early stress relief annealing or the choice of appropriate alloy materials and design solutions. Corrosion fatigue, we usually associate static stress with deterioration.
There are two forms of valve corrosion for the corrosive area: uniform corrosion and local corrosion. Uniform corrosion rate can be used to evaluate the average annual corrosion rate. Metal materials, graphite, glass, ceramic, and concrete, according to the size of the corrosion rate into four grades:
1.Corrosion rate of less than 0.05mm, which is good;
2.Corrosion rate of 0.05 ~ 0.5mm , which is good;
3.Corrosion rate of 0.5 ~ 1.5mm, which can still be used; corrosion rate greater than 1.5mm / a for not applicable, the valve sealing surface, valve stem, diaphragm, small spring, and other valve Components generally with a level of material, valve body, valve cover and other applicable secondary or tertiary materials, for high-pressure, highly toxic, flammable, explosive, radioactive media valves, the choice of corrosive materials.
Uniform corrosion of metal valves
Uniform corrosion is carried out on the entire surface of the metal. Such as stainless steel, aluminum, titanium, etc. In the oxidizing environment produced by a protective film layer, the metal’s state under the film corrosion uniformity. There is also a phenomenon, the metal surface corrosion spalling. This corrosion is the most dangerous.
Localized corrosion occurs in the local location of the metal. It has various types of types, such as pitting corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, and selective corrosion, wear corrosion, blister corrosion, mezzanine corrosion, hydrogen corrosion, etc.
Pitting corrosion usually occurs in the passivation film or protective film of the metal. Due to defects in the metal surface, the passivation film of active ions can be destroyed. The passivation film local damage, and the damage reaches into the metal inside, and corrosion holes will be formed. Pitting corrosion is one of the most destructive and hidden corrosion forms of metal.
Crevice corrosion occurs in environments such as welding, riveting, gaskets, or underneath sediments. It is a particular form of hole corrosion. The prevention method is to eliminate the gap.
Intergranular corrosion is from the surface along the grain boundary deep inside the metal, so that the grain boundary is network corrosion. Austenitic stainless steel intergranular corrosion is expected and the most dangerous corrosion pattern. Prevent austenitic stainless steel valve intergranular corrosion methods are: “solution quenching” treatment, heated to about 1100 ℃ water quenching, the choice of containing titanium and niobium, while the carbon content of austenitic stainless steel below 0.03%, reducing the production of chromium carbide.
Delamination corrosion occurs in the laminar structure, breakdown first vertical inward development, erosion of the surface parallel to the material, and expanding the corrosive material. The body is laminar spalling.
Stress corrosion occurs in the breakdown and tensile stresses produced by the simultaneous action of the rupture. Prevent stress corrosion method; through heat treatment to eliminate or reduce the welding, cold processing in the pressure generated, improved valve structure can not be reasonable, avoid stress concentration, and use electrochemical protection, spraying anti-corrosion coating. Add corrosion inhibitor, apply compressive stress, and other measures.
Corrosion fatigue occurs at the joint action of alternating stress corrosion so that the metal rupture. Heat treatment can be carried out to eliminate or reduce stress. Surface shot blasting treatment and electroplating zinc, chromium, nickel, etc. However, pay attention to the plating layer must not have tensile stress and hydrogen diffusion phenomenon.
Selective corrosion occurs in materials with different compositions and impurities. Some of the elements are corroded leaching in a particular environment, leaving the uncorroded details in a spongy form. Commonly have brass dezincification, copper alloy de aluminum, cast iron graphitization, etc.
Wear corrosion is the fluid on the metal wear and corrosion of alternating effects of a corrosion form, is a common corrosion of the valve, this corrosion to occur in the sealing surface for more. Prevention methods: the use of corrosion-resistant, wear-resistant materials, improve the structural design, the use of cathodic protection.
Vacuolar corrosion, also known as cavitation and cavitation, is unique wear and tear corrosion. The bubble generated in the fluid, the shock wave generated when broken, the pressure can be as high as 400 atmospheres, so that the metal protective film damage, and even tear the metal particles. Then corrode again into a film, this process is constantly repeated, so that the metal corrosion. The method of preventing the bubble corrosion can be used to resist the bubble corrosion materials, the high degree of finish processing surface, an elastic protection layer, and cathodic protection.
Mo vibration corrosion is mutual contact between the two parts simultaneously under load, the contact surface due to vibration and sliding caused by the damage. Friction corrosion occurs in the bolt connection, the valve stem, the connection, ball bearings, and the parts’ shaft. It can apply lubricating grease, reduce friction, surface phosphating, carbide choice, and spray tile treatment or cold processing to improve the surface hardness method of protection.
Corrosion is the damage caused by the diffusion of hydrogen atoms produced in a chemical reaction to the inside of the metal in the form of hydrogen bubbles, hydrogen embrittlement, and hydrogen corrosion.
Strong steel and steel containing non-metals are prone to hydrogen bulging. Oil containing sulfides and hydrides are prone to hydrogen bulging. The use of non-cavitation of sedimentation steel instead of boiling steel with cavities, take rubber and plastic protection, add corrosion inhibitors, etc., can prevent blistering.
Strong steel in the lattice is highly denaturing, and hydrogen atoms into the four lattice should be changed to large, causing financial embrittlement. High temperature, high-pressure hydrogen into the metal, and a combination of elements will produce a chemical reaction and destruction, known as hydrogen corrosion. Austenitic stainless steel is entirely resistant to high-temperature hydrogen corrosion.
Non-metallic corrosion
the vast majority of non-metallic materials are non-electrical conductors, generally will not produce electrochemical corrosion, but purely chemical or physical effects of corruption, which is the main difference with metal corrosion. Non-metallic corrosion is not necessarily weighted loss and often weight gain. Weight loss is the main for metal corrosion. Many physical effects caused by the biological role of metal corrosion is infrequent. Non-metallic internal corrosion is a common phenomenon, while metal corrosion is dominated by surface corrosion.
Metal materials and media contact, the solution or gas will gradually spread to the interior of the material, so that non-metallic a series of corrosion changes, according to the type of non-metallic materials and varieties of different, the form of corrosion are other. Corrosion forms are dissolution, swelling, bubbles, softening, decomposition, discoloration, deterioration, aging, hardening, fracture, and other phenomena. However, from a comprehensive perspective, non-metallic corrosion performance is superior to that of metal materials. In contrast, the strength of non-metallic materials, temperature resistance is lower than that of metal materials.
Anti-corrosion measures for metal valves
Electrochemical corrosion to various forms of corruption of metals acts between the two metals and the solution solubility difference, oxygen solubility difference, and small differences. It will also produce a potential difference in the corrosion intensified. Some of the metal itself is not corrosion-resistant, but it erodes to create a perfect protective film, namely passivation film, which can stop the medium’s corrosion:
1. To reduce metal valve corrosion, eliminating electrochemical corrosion.
2. If electrochemical corrosion can not be eliminated, make the metal surface produce passivation film.
3. The choice of non-metallic materials without electrochemical corrosion instead of metal materials.
The following is an introduction to several anti-corrosion methods.
1. According to the media selection of corrosion-resistant materials
In the “valve selection” section, we introduced the valve commonly used materials for the medium, but it is a general introduction. In production practice, the medium’s corrosion is very problematic; even in a medium with the same valve material, the concentration of the medium, temperature, pressure is different, the medium on the material corrosion is not the same. Media temperature increases every ten ℃; the corrosion rate increased by about 1 to 3 times. Media concentration on the valve material corrosion significantly impacts, such as lead in a small concentration of sulfuric acid. Corruption is minimal, and when the concentration exceeds 96%, corrosion rises sharply. Suppose it is carbon steel, when the sulfuric acid concentration of about 50% is the most severe corrosion, if the concentration increases to 6% or more, corrosion will decline. There are such as aluminum in the concentration of more than 80% of the concentrated nitric acid is very corrosive but in the medium and low concentrations of nitric acid corrosion instead of serious. Although the dilute nitric acid corrosion resistance is extreme, stainless steel is stable in more than 95% of the concentrated nitric acid corrosion but increased.
From the above examples, 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.The use of 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 to solve the corrosion problem but also to save precious metals. Valve cover, lining, sealing surface, etc., are commonly used non-metallic materials. The gasket and packing are mainly non-metallic materials made with polytetrafluoroethylene, chlorinated polyether, other plastics, natural rubber, neoprene, butyl rubber to do valve lining. The valve body, valve cover body, is generally cast iron, carbon steel made. That is to ensure the valve’s strength and ensure that the valve is not subject to corrosion. The clamp valve is also designed according to the excellent corrosion resistance of rubber and perfect change performance. Now more and more pairs of nylon, polytetrafluoroethylene and other plastics, natural rubber and synthetic rubber to do a variety of sealing surfaces, seals, used in various types of valves, these non-metallic materials used as sealing surfaces, not only good corrosion resistance, and good sealing performance, especially suitable for use with particles in the media. Of course, their strength and heat resistance are low, the scope of application is limited. The emergence of flexible graphite has brought non-metals into the high-temperature field, solving the long-standing problem of leaking fillers and gaskets, and is an excellent high-temperature lubricant.
3.Spraying paint
Paint is one of the most widely used means of corrosion protection and is an indispensable anti-corrosion material and identification mark on valve products. Color also belongs to non-metallic materials. It is usually prepared by synthetic resins, rubber slurry, vegetable oil, solvents, etc., covering the metal surface, isolated from the medium and atmosphere, to achieve anti-corrosion. The paint is mainly used in water, saltwater, seawater, atmosphere, and other corrosion environments. The valve’s inner cavity is commonly painted with anti-corrosion paint to prevent corrosion of the valve by water, air, and other media. The paint is mixed with different colors to indicate the materials used in Farn. Valve spray paint, generally in half a year to once a year.
4.Add corrosion inhibitor
Add a small amount of other particular substances in the corrosive medium, and corrosive materials can greatly slow down the metal corrosion rate. This specific material is called corrosion inhibitors.
Corrosion inhibitors control the mechanism of corrosion, is that it promotes the polarization of the battery. Corrosion inhibitors are mainly used in the media and fillers.
Adding corrosion inhibitor in the media, it can make the corrosion of valves slow down. Valve test pressure, commonly used as a test medium of water, easy to cause deterioration of the valve; adding a small amount of sodium nitrite in the water can prevent the corrosion of the valve.
Non-metallic corrosion resistance is excellent. As long as the valve uses temperature and pressure in line with non-metallic materials’ requirements, it cannot only solve the corrosion problem and save precious metals. Valve body, valve cover, lining, sealing surface, etc. commonly used non-metallic materials, as for the gasket, packing mainly dead non-metallic materials made. With polytetrafluoroethylene, chlorinated polyether and other plastics, natural rubber, neoprene, butyl rubber, and other rubber to do valve lining, the valve body, valve cover body is generally cast iron, carbon steel made. That is to ensure the valve’s strength and ensure that the valve is not subject to corrosion. The clamp valve is also designed according to the excellent corrosion resistance of rubber and perfect change performance. Now more and more pairs of nylon, polytetrafluoroethylene and other plastics, natural rubber and synthetic rubber to do a variety of sealing surfaces, seals, used in various types of valves, these non-metallic materials used as sealing surfaces, not only good corrosion resistance, and good sealing performance, especially suitable for use with particles in the media. Of course, their strength and heat resistance are low, the scope of application is limited. The emergence of flexible graphite has brought non-metals into the high-temperature field, solving the long-standing problem of leaking fillers and gaskets, and is an excellent high-temperature lubricant.
Small-diameter valve bodies and handwheels are often chrome-plated to improve their corrosion resistance, decorative valves.
5.Thermal spraying
Thermal spraying is a process square class for preparing coatings, which has become one of the new technologies for material surface protection. Most metals and their alloys, metal oxide ceramics, metal-ceramic complexes, and hard metal compounds can be coated on metal or non-metal substrates by one or more thermal spraying methods.
Thermal spraying improves its surface resistance to corrosion, abrasion, and high temperature and extends its service life. Thermal spraying special function coating, with heat insulation, insulation (or isoelectric), wearable seal, self-lubrication, thermal radiation, electromagnetic shielding, and other unique properties; the use of thermal spraying can repair parts.
6.Control the corrosive environment
In the so-called climate, there are two kinds of broad and narrow sense. The environment’s general-purpose refers to the ground around the valve installation and its internal circulation medium; the environment’s little definition refers to the valve installation conditions. Most environments cannot be controlled, and the production process cannot be changed arbitrarily. Only in the case of no damage to the product, process, etc., can be used to control the environment, such as boiler water de-oxygenation, refining process home alkali to adjust the PH value. From this point of view, the addition mentioned above of corrosion inhibitors, electrochemical protection, etc., also belongs to the control of corrosion environment.
The atmosphere is full of dust, water vapor, smoke, especially in the production environment, such as smoke and brine, equipment emitting toxic gases and micronized powder, will produce different degrees of corrosion of the valve. Operators should be according to the operating procedures’ provisions, regular cleaning, blowing valves, and regular refueling, which is an effective measure to control environmental corrosion. The method to prevent the erosion of the valve containing corrosive substances is as below.
A.The Stem installation protection cover,
B.Ground valve set ground well,
3. Valve surface spray paint, etc.
Ambient temperature rise and air pollution, especially for the equipment and valves’ closed environment, will accelerate corrosion. Should try to use open plant or ventilation, cooling measures to slow down environmental deterioration.
7.Improve the processing technology and valve structure form
Valve anti-corrosion protection is a problem from the design, an excellent structural design, and valve products’ correct process method. Undoubtedly to slow down the corrosion of the valve is a sound effect.
Therefore, the design and manufacturing department should be those structural design is not reasonable. The process method is not correct, easy to cause corrosion of the components, and should be improved to make it suitable for various working conditions under the states’ requirements.
The gap at the valve connection is the right environment for oxygen concentration corrosion.
Different metal contact will constitute a point couple, promote anodic metal corrosion, choose materials, and not be free of the potential metal gap. It can not produce passivation film metal contact. In the production and processing, welding, and heat treatment in the stress corrosion should improve the processing method after welding to use annealing treatment and other corresponding protective measures. Improve the surface roughness of the valve stem processing and different valve surface roughness. The higher the surface roughness level, the stronger the corrosion resistance. Improve the processing technology and structure of packing and gasket, use flexible graphite and plastic packing, and flexible graphite sticky gaskets and gaskets wrapped with PTFE, improving the sealing performance reduce the corrosion of the ground stem and flange sealing surface.
