An introduction to contact corrosion, which is often overlooked
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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. Contact between metals and non-metals 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 rubber; 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 reaching 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 that contain acids or produce acids can cause corrosion in metals when in contact with them. 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 that occurs when different materials are in contact with each other. This type of corrosion is not limited to interactions between metals alone; contact between metals and non-metals can also promote corrosion. For example, metals such as aluminum and stainless steel, which rely on an oxide film for protection, find it difficult to form such a protective film when in contact with non-metallic materials under oxygen-deficient conditions, which exacerbates corrosion. Furthermore, non-metallic materials containing components such as ammonium salts, amine compounds, graphite, or sulfur can also cause galvanic corrosion or stress corrosion cracking in the contacting metals. Porous materials are particularly dangerous because they tend to retain moisture, maintaining high electrical conductivity and thus accelerating metal corrosion. Measures to prevent contact corrosion include using non-absorbent materials such as polyethylene or polytetrafluoroethylene as gaskets, as well as avoiding direct contact between metal and materials prone to corrosion such as untreated wood. .