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This post was last edited by Wang Wei2 on 2024-4-18 at 10:01, introducing several types of low-surface-treatment coatings and their principles. Traditional high-VOC solvent-based low-surface-treatment coatings (commonly known as “rust-prone coatings”) were developed quite early on, and a range of such coatings have been created both domestically and internationally. Generally, they can be divided into 3 categories: penetrating, stable, and transforming low-surface-treatment coatings. 1. Transformative low-surface-treatment coatings are also known as reactive rust-on-coatings. In such coatings, those that do not contain film-forming substances are known as rust converters; since there is no film-forming substance, a primer must be applied separately ； If it contains film-forming substances, it is called a rust conversion primer. Rust conversion agents are not coatings; they merely render the rust layer inert, thereby allowing the primer applied on top to provide protection. Rust conversion primers contain film-forming substances, which enable them to convert rust into inert materials while also sealing off the rust and isolating it from the outside environment. They are coatings that transform the rust layer into something solid and inert, while ensuring good adhesion to the steel surface. The main mechanism of this type of coating is to rely on the rust conversion agents contained in the coating to transform active rust into harmless compounds or compounds that provide some level of protection, thereby enabling painting over rust. Transformative rust-inhibiting coatings generally use phosphoric acid and tannic acid to convert the rust layer into insoluble iron phosphate and iron tannate salts, thereby providing protection. However, it is difficult to control the amount of acid used; an excess of phosphoric acid will accelerate corrosion, leading to serious consequences. At the same time, conversion rust-resistant coatings contain less film-forming material, and it is best to use them in combination with other primers. Therefore, the use of conversion rust-resistant coatings must be handled with caution; they generally cannot be used alone and require a topcoat to ensure long-term corrosion protection. 2. Stable low-surface-treatment coatings: The basic principle of stable low-surface-treatment coatings is to rely on the reactive groups in the resins present in the coating, as well as the active pigments, to react with the reactive and harmful components in the rust layer, thereby forming stable inhibitory compounds. This transforms the entire rust layer into stable, protective filler components within the coating. Typical stable rust-resistant coatings use active mixed pigments such as chromates and phosphates; these pigments hydrolyze to produce corresponding chromate and phosphate ions, thereby forming heteropoly acids. This heteropolyacid forms heteropolyacid complexes with the reactive rust acid FeOOH present in the rust layer, thereby stabilizing the rust. The most commonly used active pigments include zinc chromate, barium chromate, calcium chromate, zinc oxide, zinc phosphate, aluminum tripolyphosphate, and organic nitrogen-alkali chromates; some formulations also contain sodium nitrite, and it is considered necessary to add strontium chromate as well. Chromate pigments hydrolyze to produce anionic species, while organic nitrogen-containing basic chromates hydrolyze to yield nitrogen-based ion groups; both have the ability to absorb reactive iron rust. Zinc oxide adjusts the pH, while barium and calcium ions combine with the anions formed by sulfur dioxide in the environment to form insoluble salts. These combined effects make rust stable. 3. Penetrating low-surface-treatment coatings: The basic principle of penetrating low-surface-treatment coatings is to use resins with strong penetration properties as the paint base; together with penetrants and polar solvents, this enables the coating to have good penetration capabilities, allowing it to penetrate the rust layer on the steel surface and seal it off. It can effectively isolate rust and external corrosive factors, enhancing the adhesion between the rust layer and the steel plate, between the rust layer and the coating, and between the coating and the steel plate