Discuss the corrosion prevention role of paint coatings on metal surfaces

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Protecting metal structures in the atmosphere with organic coatings is the most widely used method of corrosion prevention. The coating is applied to the metal surface and forms a porous film upon drying. Although it cannot completely isolate the metal from the medium, it increases the diffusion resistance of the medium through the pores as well as the electrical resistance of the solution, thereby reducing the corrosion current. In mild environments such as the atmosphere and seawater, the corrosion of metal at the micropores occurs slowly; the corrosion products can block these micropores, resulting in a long service life. However, it is not suitable for highly corrosive solutions such as acids, as metal corrodes rapidly and hydrogen is generated, which can cause the paint coating to crack. The application procedure for coatings is as follows: The first step is surface treatment, which is the most important stage. Surface rust, oil stains, and other impurities must be thoroughly removed using methods such as sandblasting, shot blasting, and flame cleaning; otherwise, it will affect the adhesion between the coating and the metal, thereby reducing its lifespan ； Next is the selection of the primer; generally, an epoxy iron oxide primer is used, along with corrosion inhibitors such as calcium lead sulfate, zinc chromate, and zinc powder, which can provide corrosion protection once the medium penetrates into the micropores. Finally, there is the topcoat; in addition to corrosion resistance, aesthetics is also an important goal. Generally, several coats of topcoat are applied to minimize pores as much as possible. Common high-quality types of synthetic resins include: epoxy, vinyl chloride, polyurethane, chlorosulfonated polyethylene, chlorinated polyether, phenol-formaldehyde, furan (furfuryl alcohol), etc. Asphalt is an inexpensive yet high-performing coating, and it is often mixed with epoxy resins and similar materials for use in underground pipelines. Natural resin lacquer is a specialty of China; it possesses excellent acid and weather resistance, making it a high-quality coating material. Widely used in inorganic coatings are zinc-rich paints based primarily on zinc powder, with synthetic resins serving as binders; the zinc film formed on the surface after drying is conductive, and it functions in a similar manner to cathodic protection. Such coatings can be used for a long time in atmospheric conditions as well as in higher temperatures.