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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 experience accelerated corrosion 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 untreated wood comes into contact with lead, the lead will be corroded. Prevention methods include using asphalt felt or paint as a barrier, or employing dry wood.