Learn about the functions of anti-corrosion coatings and the factors that influence them

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　　1. Functions 　　(1) Shielding effect. The paint film prevents corrosion agents from coming into contact with the material surface ； It blocks the path of the corrosion cell, increasing the resistance. 　　(2) Corrosion inhibition. Certain pigments, or the reaction products of these with film-forming substances or moisture, can have a corrosion-inhibiting effect on the substrate metal (including passivation). 　　(3) Cathodic protection effect. The electrode potential of the paint film is lower than that of the substrate metal; in a corrosion cell, it acts as the anode and \"sacrifices\" itself, thereby protecting the substrate metal (the cathode). 　　2. Influence of the film-forming substance in coatings The film-forming substance in anti-corrosion coatings must possess chemical stability in corrosive media, and the standards for this stability are related to the composition and chemical structure of the film-forming substance. It mainly depends on whether it reacts easily with corrosive agents under dry film conditions or decomposes into smaller molecules in such agents. 　　 Whether for preventing electrochemical corrosion or merely for providing isolation, the shielding effect of anti-corrosion coatings is very important, and the shielding capability of the paint film depends on the structural pores of its film-forming substances as well as the pores in the coating itself. 　　Water, oxygen, and ions have different penetration rates through the paint film. The permeation rate of water is much higher than that of ions. Oxygen permeation is complex and highly dependent on temperature. Water and oxygen can form a corrosion cell on the metal surface after passing through the paint film. Ions pass through the paint film in small amounts, so their direct effect on the base metal can be ignored; however, they increase the electrical conductivity of the paint film. 　　When the molecules in the film-forming substance have numerous functional groups, the paint film has few structural pores, and they can react with each other during the film formation process to create a highly cross-linked network structure, thereby enhancing the coating’s corrosion resistance. 　　The physical and mechanical properties of the paint film greatly influence the corrosion protection efficiency of anti-corrosion coatings. They are related to the molecular weight, chain segments, side groups, etc., of the film-forming substance. 3. Influence of pigments and fillers 　In coatings, coloring pigments are responsible for giving them color ； Body pigments are used to adjust the mechanical properties of the paint film or the flowability of the coating. For anti-corrosion coatings, in addition to the two types of pigments mentioned above, there are also pigments added for anti-corrosion purposes: one type is rust-inhibiting pigments that utilize their chemical properties to prevent metal corrosion ； Another category is flake pigments, which enhance the shielding properties of the coating through physical effects. 　　Anti-rust pigments can be further divided into alkaline pigments, soluble pigments, and metal powder pigments. 　　(1) Alkaline pigments. The reaction of basic pigments with oily film-forming substances can produce metal soaps, such as red lead, lead monoxide, lead cyanamide, basic lead chromate, calcium lead sulfate, and basic lead sulfate. When metal soaps come into contact with water, their decomposition products can act as corrosion inhibitors. Furthermore, the paint film that forms metal soaps enhances environmental shielding. Alkaline pigments keep the interface between the paint film and the metal slightly alkaline, which also provides corrosion protection. 　　(2) Soluble pigments. They are usually chromate pigments. When these pigments come into contact with water, chromate ions are dissolved. Chromate ions have a strong oxidizing effect, which can passivate the metal surface and thereby provide corrosion protection. 　　(3) Metal powder pigments. The widely used metal powder pigment is zinc powder. The coating obtained after drying a zinc-rich paint is primarily composed of zinc powder. Maintain direct contact between the zinc powder particles as well as between the substrate and the zinc powder. When moisture penetrates the coating, a battery composed of zinc powder and the base steel sheet is formed. Current flows from zinc to iron, thereby providing cathodic protection to the substrate. Furthermore, the corrosion products of zinc accumulate between the zinc powder and on the steel surface, enhancing the shielding effect of the coating. 　　Flake pigments in coatings can prevent corrosive agents such as water, oxygen, and ions from passing through, thereby blocking the capillaries within the coating. The mutually parallel and overlapping scales create a labyrinthine effect within the coating, prolonging the path taken by corrosive agents to penetrate it, thereby enhancing the coating’s corrosion resistance. The main flake pigments include mica powder, aluminum powder, iron oxide mica, glass flakes, stainless steel flakes, etc. 　　In addition to film-forming substances and pigments, other components of anti-corrosion coatings, such as fillers and additives, must also possess good chemical stability.