As early as the 16th century, it has been found that asphalt materials have good adhesion, water resistance and impermeability, and are very suitable for use as anti-corrosion materials for the outer wall of steel pipes. There are two types of asphalt: petroleum asphalt and coal asphalt. Petroleum asphalt is composed of alkanes, and its chemical properties are relatively unstable; coal asphalt is composed of aromatic hydrocarbons, and its chemical properties are extremely stable. Coal asphalt has been modified to overcome the sensitivity to temperature; the softening point and toughness have been improved to become a material that is as bright and corrosion-resistant as enamel, so it is called coal tar enamel. Coal tar enamel has better adhesion than petroleum asphalt, its water absorption rate is low, it can resist microbial erosion and plant rootstock penetration, and it has low price, small daily maintenance and long life.
The research of modern polymer chemistry has led to a high-performance coating compounded with epoxy resin and coal tar, especially suitable for the protection of the outer wall of buried steel pipelines. This kind of coating is divided into primer and topcoat. The primer contains more powder, which requires strong adhesion to the surface of the steel pipe; the topcoat has the function of filling and penetrating the pinholes of the primer, and it is smooth and smooth. The design of long-life heavy-duty anti-corrosion coatings is often not only a one-side one-side approach, but also three-oil one-cloth and four-oil two-cloth methods. Epoxy coal asphalt coatings have a longer construction and maintenance period, and are difficult to cure when the construction environment temperature is below 5e. Areas of that condition must use polyurethane anticorrosive coatings.
The high-purity metals Mg, Al, and Zn are corrosion-resistant and easily available materials, and their potentials are more negative than those of steel pipes. The pure Al wire, pure Zn wire or zinc aluminum alloy wire, aluminum magnesium alloy wire is melted into aerosol with acetylene flame and sprayed onto the surface of the steel pipe to form a uniform and continuous metal film with a minimum local thickness of 100Lm-200Lm to shield the surface of the steel pipe Get up, and then carry out multi-layer anticorrosive paint protection on the sprayed metal film, this is the metal spray protection method. The metal spray protection method not only has the protective properties of paint, but also has the role of cathodic protection because the potential of the sprayed metal is negative to the steel pipe. Metal spraying anticorrosion requires strict pretreatment of the surface of the steel pipe, otherwise it is difficult to meet the quality requirements. If zinc spraying is used, it is toxic and unfavorable to environmental protection.
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