Mechanism of action and failure mechanism of anti-corrosion paint

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The mechanism of action and failure of anti-corrosion paint: Paints for corrosion protection offer an attractive appearance and a simple application process, and have been widely used for many years. With the development of coating technologies, heavy-duty anti-corrosion coatings have become the mainstream for protecting steel bridges. In heavy-duty anti-corrosion coatings, whether they are well-known foreign brands or domestic brands, the anti-corrosion coating processes and types of coatings are quite similar. That is, the coating process involves a multi-layer system consisting of a primer, an intermediate coat, and a topcoat. The types of coatings include epoxy (inorganic) zinc-rich primers, epoxy mica-iron oxide intermediate coats, as well as epoxy polyurethane or epoxy topcoats in various colors, or chlorinated rubber topcoats. Mechanism of action of anti-corrosion paint: ① Shielding effect: The paint coating mechanically separates steel from the corrosive environment. ②Passivation and corrosion inhibition: In paint coating systems, the first shop primer provides passivation and corrosion inhibition for steel, enhancing the adhesion of the paint layer; its anti-corrosion effect is quite weak. ③Cathodic protection effect: The addition of zinc powder to anti-corrosion primers (such as zinc-rich primers) provides cathodic protection for steel. The failure mechanism of anti-corrosion paint: The corrosion protection provided by paint for steel structures relies primarily on mechanical shielding. The aging and flaking of the paint coating weaken or eliminate this protective effect. The zinc particles that provide cathodic protection are able to bond with steel only thanks to the organic binders in the paint; as these organic substances break down and age, the zinc particles can no longer bond with steel, and thus the cathodic protection effect is lost. Secondly, the paint coating itself contains countless tiny pores. When exposed to salt spray and humid environments over time, ions such as chloride and water molecules can penetrate through these pores and corrode the base metal. At the interface between the paint layer and the base metal, the volume of corrosion products accumulates and expands, causing the paint layer to flake off. The corrosion then spreads rapidly around the areas where the paint has peeled off, resulting in the failure of the entire anti-corrosion system.