Let’s talk about contact corrosion
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When materials come into contact with each other, gap corrosion or galvanic corrosion can occur; in addition, there are other adverse factors as well. Metal-to-nonmetal contact also often promotes the corrosion of the former. The formation of gaps is one of the causes of corrosion. On the other hand, metals protected by an oxide film, such as aluminum and stainless steel, experience extreme oxygen deficiency on their surfaces covered by non-metals, making it difficult for the film to form and potentially leading to severe corrosion. The composition and properties of non-metals also play an important role in corrosion; for example, gaskets containing ammonium salts or amine compounds can cause stress corrosion cracking in brass flanges and bolts, materials containing graphite can lead to electrochemical corrosion of the metals in contact, and sulfur-containing materials can as well accelerate corrosion. Insulating materials containing chlorides, when wet and in contact with stainless steel, can cause stress corrosion cracking of the stainless steel. Some metals corrode more rapidly when in contact with plastics and rubbers; for example, certain sulfur-free rubbers can accelerate metal corrosion, possibly because these materials adsorb the corrosion products of the metals, preventing them from being protected by a protective layer. It is particularly harmful when metals come into contact with porous materials. If the material is pore-free and in tight contact with metal, it can prevent corrosive agents from penetrating between them. However, porous materials can often retain a certain amount of solution, keeping their electrical conductivity high and accelerating the corrosion of the metals in contact with them. Water-absorbing fiber gaskets are generally not used between stainless steel flanges, as they can cause localized corrosion; instead, gaskets made of polyethylene, polytetrafluoroethylene, neoprene, etc., should be used. Materials containing acids or capable of producing acids, when in contact with metals, cause the metals to corrode. When unair-dried wood comes into contact with lead, the lead is corroded. The preventive measure is to separate them with asphalt felt or paint, or use dry wood.
Contact corrosion refers to the corrosion phenomenon that occurs when different materials are in contact, mainly including crevice corrosion and galvanic corrosion. For example, when a metal comes into contact with a non-metal, it may be difficult to form a protective oxide layer due to lack of oxygen, leading to corrosion. The use of non-metallic gaskets containing harmful components, such as those with ammonium salts or sulfur-based materials, can exacerbate stress corrosion and galvanic corrosion in metals. Furthermore, the contact of certain plastics or rubbers with metals can also promote corrosion, as they may adsorb and concentrate corrosive agents. To prevent contact corrosion, appropriate isolation materials and dry operating conditions should be selected, and porous or hygroscopic materials should be avoided as gaskets or spacers. .
I would like to ask the teacher: In actual production, when gaskets made of polytetrafluoroethylene are used on the sealing surfaces of flanges made of TA2 material, corrosion of those sealing surfaces was detected. Does this constitute contact corrosion? How can such corrosion be prevented?
In my opinion, this is considered gap corrosion; the preventive measures are as follows: 1. Material selection and treatment: 1. Optimize the TA2 material: Ensure the purity and quality of TA2 material. 2. Treat the surface of TA2. Surface treatment methods such as anodization and electroless plating can be employed. 3. Use high-quality PTFE gaskets to ensure that their material is uniform and free of impurities. 4. Consider using modified PTFE gaskets. For example, PTFE gaskets filled with reinforcing materials such as glass fiber or carbon fiber may experience further improvements in mechanical strength and corrosion resistance. II. When designing the flange sealing structure, the size of the gap should be minimized as much as possible