One of the main factors causing valve damage is corrosion. Consequently, valve anti-corrosion is a crucial factor to take into account when it comes to valve protection.
Valve corrosion form
Direct chemical and physical activities are typically the source of corrosion in non-metallic materials, while chemical and electrochemical corrosion are the primary causes of corrosion in metals.
1. Chemical corrosion
In the absence of current generation, the metal is directly reacted with and destroyed by the surrounding medium, as in the case of non-electrolytic solution and high-temperature dry gas corrosion.
2. Galvanic corrosion
Electrochemical activity, the primary mechanism of corrosion, damages the metal when it comes into contact with the electrolyte, causing electrons to flow.
Electrochemical corrosion includes pitting, crevice corrosion, microbiological corrosion, atmospheric corrosion, soil corrosion, seawater corrosion, common acid-base salt solution corrosion, and more. Both the metal with low potential and the position of the dry sun plate are lost as a result of electrochemical corrosion, which not only happens between two substances that can play a chemical role but also creates potential differences due to the concentration difference of the solution, the concentration difference of surrounding oxygen, the slight difference in the substance's structure, etc.
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. Choose materials that are resistant to corrosion based on the corrosive medium.
The corrosion of the medium is extremely complex in real production; even while the valve material is the same, the medium's concentration, temperature, and pressure vary, and the medium's corrosion to the material varies as well. About one to three times as much corrosion occurs for every 10°C increase in medium temperature.
The corrosion of the valve material is greatly affected by the medium concentration. For example, when lead is present in sulfuric acid at a low concentration, the corrosion is negligible, but when the concentration surpasses 96%, the corrosion increases significantly. On the other hand, carbon steel corrodes most severely when the sulfuric acid concentration is around 50%; corrosion drastically reduces when the concentration rises to over 60%. For instance, stainless steel is extremely resistant to diluted nitric acid, but it deteriorates in concentrated nitric acid that is higher than 95%. Aluminum, on the other hand, is extremely corrosive in concentrated nitric acid at concentrations of over 80%, but it is severely corrosive in medium and low concentrations of nitric acid.
The aforementioned examples demonstrate that the proper choice of valve materials should be determined by the particular circumstances, consider a variety of corrosion-affecting elements, and choose materials in accordance with the applicable anti-corrosion guides.
2. Use non-metallic materials
Excellent non-metallic corrosion resistance means that the valve may not only solve the corrosion issue but also save precious metals as long as its temperature and pressure match those of non-metallic materials. Commonly utilized non-metallic materials are used to make the valve body, bonnet, lining, and sealing surface.
The primary body of the valve body bonnet is composed of carbon steel and cast iron, while the valve lining is comprised of plastics like PTFE and chlorinated polyether, as well as natural rubber, neoprene, nitrile rubber, and other rubbers. It guarantees not just the valve's strength but also its resistance to corrosion.
Natural and synthetic rubber are utilized to create different sealing surfaces and sealing rings that are used on different valves, while plastics like nylon and PTFE are being employed more and more these days. In addition to having outstanding sealing performance and resistance to corrosion, these non-metallic materials make excellent sealing surfaces and are particularly well-suited for usage in particle-containing environments. Naturally, they have a smaller variety of uses and are less robust and heat resistant.
3. Metal surface treatment
(1) Valve connection: To increase its resistance to medium and atmospheric corrosion, the valve connection snail is frequently galvanized, chrome-plated, and oxidized (blue). Depending on the circumstances, other fasteners are subjected to surface treatments such phosphating in addition to the previously listed techniques.
(2) Sealing the surface and small-diameter closed parts: surface treatments like boronizing and nitriding are applied to increase the material's resistance to wear and corrosion.
(3)In order to increase the corrosion and abrasion resistance of stems, surface treatment techniques like as nitriding, boronization, chrome plating, nickel plating, and others are frequently employed.
Various surface treatments ought to be appropriate for various stem materials and operational settings. Hard chrome plating and gas nitriding are options for the atmosphere, water vapor medium, and asbestos packing contact stem (ion nitriding is not recommended for stainless steel). Electroplating a high phosphorus nickel layer provides superior protection in an atmosphere containing hydrogen sulfide; Ion and gas nitriding can also make 38CrMOAIA resistant to corrosion, however hard chrome plating is not appropriate for usage; While all phosphorus-nickel plating layers are not resistant to ammonia corrosion, 2Cr13 and carbon steel that has undergone gas nitriding can both withstand ammonia corrosion after quenching and tempering. The gas nitriding 38CrMOAIA material, which is primarily used to make valve stems, has exceptional corrosion resistance and all-around performance.
(4) Small-caliber handwheel and valve body: To increase its resistance to corrosion and to adorn the valve, it is frequently chrome-plated.
4. Thermal spraying
One of the newest technologies for protecting the surface of materials is thermal spraying, a type of coating preparation procedure. Using high energy density heat sources (such as gas combustion flames, electric arcs, plasma arcs, electric heating, gas explosions, etc.), metal or non-metallic materials are heated and melted before being sprayed onto the pretreated basic surface in the form of atomization to create a spray coating. Alternatively, the basic surface is heated simultaneously so that the coating melts again on the substrate's surface to create a spray welding layer.
Using one or more thermal spraying techniques, the majority of metals and their alloys, metal oxide ceramics, cermet composites, and hard metal compounds can be coated on metal or non-metal substrates to increase their resistance to wear, corrosion, high temperatures, and other conditions while also extending their service life. Thermal spraying is a unique functional coating that can be used to repair parts because it has unique properties such as heat insulation, insulation (or anomalous electricity), grindable sealing, self-lubrication, thermal radiation, electromagnetic shielding, and other distinctive qualities.
5. Spray paint
An essential anti-corrosion material and identification mark for valve products, coating is a commonly used anti-corrosion technique. Another non-metallic substance is coating, which is often composed of synthetic resin, rubber slurry, vegetable oil, solvent, etc. and is used to cover the metal surface, isolate the medium and atmosphere, and accomplish anti-corrosion goals.
Water, salt water, seas, the atmosphere, and other non-corrosive conditions are the primary applications for coatings. To stop water, air, and other media from corroding the valve, anticorrosive paint is frequently applied to the interior cavity.
6. Add corrosion inhibitors
Corrosion inhibitors work by encouraging the battery's polarization, which prevents corrosion. Media and fillers are the primary applications for corrosion inhibitors. Equipment and valve corrosion can be slowed down by adding corrosion inhibitors to the medium. For example, chromium-nickel stainless steel in oxygen-free sulfuric acid has a large solubility range and can deteriorate into a cremation state. A small amount of copper sulfate, nitric acid, and other oxidants can turn the stainless steel into a blunt state, preventing the medium's surface from eroding. In hydrochloric acid, titanium corrosion can be decreased if a small amount of oxidant is added.
A small amount of sodium nitrite added to the water can stop water from corroding the valve, which is why valve pressure tests are frequently employed as the medium for pressure tests. Because asbestos packing includes chloride, which severely corrodes the valve stem, the chloride content can be decreased by using the steamed water cleaning method. However, this approach is very difficult to use, cannot be widely adopted, and is only appropriate for specific situations.
In order to protect the valve stem and stop the asbestos packing from corroding, the valve stem is coated with a corrosion inhibitor and sacrificial metal. The corrosion inhibitor is made of sodium nitrite and sodium chromate, which can create a passivation film on the valve stem's surface and increase the valve stem's resistance to corrosion. The solvent can cause the corrosion inhibitor to dissolve gradually and act as a lubricant. In fact, zinc is also a corrosion inhibitor, which can initially combine with the chloride in asbestos to significantly reduce the possibility of contact between the chloride and the stem metal.
7. Electrochemical protection
Anodic and cathodic protection are the two forms of electrochemical 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
There are two types of "environment": broad and narrow. The former relates to the surroundings of the valve installation location and its internal circulation medium, while the latter describes the conditions around the valve installation location.
Production processes cannot be adjusted at will, and most settings are unmanageable. Environmental control techniques, like deoxygenating boiler water or adding alkali to oil refining to change the PH value, can only be used if there won't be any harm to the final product or the process. According to this perspective, controlling the corrosive environment can also be achieved by adding corrosion inhibitors and the electrochemical protection described above.
Dust, water vapor, and smoke are all present in the ambient, particularly in the production setting. Toxic fumes, smoke brine, and fine powder released by machinery can cause the valve to corrode to differing degrees. In order to effectively prevent environmental corrosion, the operator should clean and purge the valve and refill on a regular basis in accordance with the operating regulations. To stop corrosive materials from eroding the valve, apply paint to the valve's surface, install a protective cover on the valve stem, and place a ground well on the ground valve.
Open workshops or ventilation and cooling methods should be employed as much as possible to slow down environmental corrosion because rising ambient temperatures and air pollution will accelerate the corrosion of equipment and valves, especially when they are in a closed environment.
9. Improve the processing technology and valve structure
Since the beginning of the design process, the issue of the valve's anti-corrosion protection has been taken into account. A valve product with a sensible structural design and the right process method will surely have a beneficial effect on slowing down the corrosion of the valve. Thus, in order to adapt them to the demands of different working conditions, the design and production department should alter the parts that are irrational in structural design, inaccurate in process methods, and prone to corrosion.
