Uniform erosion corrosion, also known as general erosion corrosion, is the most common type of corrosion caused by chemical or electrochemical reactions that cause deterioration of the entire exposed surface of the metal. Eventually, the metal will deteriorate to the point of failure. This corrosion accounts for the largest amount of metal damage, but is considered a safe form of corrosion because it is predictable, manageable, and usually preventable.
Different from uniform erosion corrosion, local corrosion is targeted at a specific area of the metal structure, which will eventually lead to pitting corrosion, crevice corrosion or stress corrosion cracking. Due to its latent incubation and rapid spread, this form is more dangerous and destructive. Pitting corrosion occurs most often in metallic materials of technical significance, such as carbon steel, low alloy and stainless steel, nickel-based alloys, aluminum, copper, and many other metals and alloys.
When small holes or cavities are formed in the metal, pitting corrosion occurs, which is usually the result of depassivation of small areas. This area becomes the anode, and a part of the remaining metal becomes the cathode, generating a local current reaction. Degradation in this small area can penetrate the metal and cause failure. Corrosion pits will continue to grow because the inside of the pit is naturally lack of oxygen, and the local pH value is reduced to a very low value, and the corrosion rate increases due to the autocatalytic process.
A special form of pitting corrosion is wormhole corrosion, which does not spread laterally to the exposed surface, but penetrates at 10 to 100 times the normal corrosion rate, usually at a 90¡ã angle to the surface.
The discs made of Super Duplex may suffer from wormhole corrosion, mainly due to improper Super Duplex casting methods and the presence of impurities on the metal surface (for example, iron particles released during flame cutting or welding nearby), but usually due to passivation insufficient. The valve discs and components made of Super Duplex must be pickled to remove impurities that may cause such corrosion during use.
It is very important to accurately understand the working conditions involved in order to design and recommend the most suitable valve technology.
In addition to the standard valve series made of metal or plastic materials, GEM Ü also offers a special product series for controlling and shutting off corrosive, corrosive or abrasive media.
GEM Ü’s lined diaphragm valves are versatile because they can withstand corrosive media and high temperatures. For example, they are used in water treatment, chemical industry, paper industry and mining industry and wherever harsh operating conditions may occur.
In many cases, valves with an all-metal valve body cannot be used because corrosive media will erode or even destroy the material. On the other hand, pure plastic bodies reach their limits, for example at higher temperatures. GEM Ü provides the right solution with its lined metal valve body, combined with the appropriate diaphragm material, it can meet even the most stringent requirements, thus ensuring a high degree of operational reliability. In these cases, the metal body (for example, made of ductile iron or investment cast stainless steel) is lined with materials such as PFA or PP.
Typical application examples include cutting off media such as sulfuric acid or hydrochloric acid, chlorine or sodium hydroxide.
When dealing with abrasive media (such as slurries from the mining industry or limestone suspensions used in flue gas desulfurization processes), metal valve bodies with hard rubber linings can be used. In this case, the lining protects the entire wetted part of the medium.
The GEM Ü 490 butterfly valve comes standard with a stainless steel disc with TFM lining and PFA encapsulation. TFM is a further developed version of classic PTFE, which further improves the outstanding performance of PTFE. Therefore, TFM has optimized surface quality, lower gas permeability and lower cold flow characteristics.
The design of the butterfly valve, especially the lining, shaft seal and backing, have been optimized to reduce driving torque while providing higher sealing. The technical characteristics of the sealing concept increase the service life and reduce the probability of failure. Because of this feature, the 490 butterfly valve is also suitable for temperatures up to 180¡ãC, which is common in roasting and smelting processes.
GEM Ü 490 butterfly valves are used in all applications that require the shut-off or control of corrosive liquids and gases. In addition to the chemical industry, such applications can also be used for the treatment of corrosive media in the mining industry. Due to its structure, it can also be used in explosive areas (ATEX).
According to the application details or the given operating conditions, GEM Ü’s expert team can assist in finding the most suitable technology to ensure the safety of the operators and the best price-performance ratio for the environment.
For more information, please contact Claudio Darpin, Gemme Valve Africa, +27 (0)11 462 7795, claudio.darpin@gemue.co.za, www.gemu-group.com
Post time: Jun-28-2021
