Traditional anti-corrosion coating types and their protection mechanisms

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Anticorrosive coatings have a long history of development and a wide range of product types; for a long time, they have been a focus of attention within the coating industry. Anticorrosive coatings have a long history; it can be said that the development history of anticorrosive coating products is a microcosm of the development history of coating products in general. I. Traditional types of anti-corrosion coatings 1. Epoxy resin-based anti-corrosion coatings: Epoxy coatings exhibit excellent adhesion to metals, concrete, wood, glass, and so on. This is due to the presence of polar groups such as hydroxyl groups and ether bonds in their molecules, which allow for some level of adhesion even when applied wet-on-wet. Some resins (such as unsaturated polyesters) experience a volume contraction of up to 11% upon curing, resulting in high internal stresses that reduce adhesion. When curing, epoxy resin shrinks by only about 2%, and the ether bonds in the epoxy molecules make the molecular chains flexible and easy to rotate, which helps to eliminate internal stresses; as a result, its adhesion strength is high. Epoxy resins contain hydrophilic hydroxyl groups and ether bonds, and the amine groups in the curing agents also affect the water resistance of the coating film. However, the bisphenol A segments in these resins are hydrophobic; the rigidity of the two benzene rings shields the hydroxyl groups and ether bonds, thereby maintaining the water resistance of the overall coating film. The cured epoxy resin has a high glass transition temperature, which contributes to its water resistance. Epoxy resin molecules do not contain ester bonds, giving them excellent alkali resistance. The cathodic side of the corrosion microcell is alkaline, and ordinary oil-based paints or alkyd paints are easily destroyed by saponification. It can be said that epoxy resin coatings possess the alkali resistance and adhesion necessary for excellent anti-corrosion coatings. 2. Polyurethane-type anti-corrosion coatings: Polyurethane anti-corrosion coatings are generally of two-component formulation, one containing isocyanate groups—NCO, and the other containing hydroxyl groups. Before use in construction, the two components are mixed and reacted to cure, forming polyurethane. The properties of polyurethane anti-corrosion coatings are similar to those of epoxy coatings; they can cross-link indoors, and the resulting coating film is resistant to immersion in oils, salts, etc., offering excellent anti-corrosion performance ； It can also be made into solvent-free coatings or high-solid coatings, allowing for thick films to be achieved in a single application ； Can be made into powder coating ； It can be mixed with coal tar pitch to produce corrosion-resistant coatings that are resistant to saline water and other substances, at a reasonable cost (however, they are dark in color and lack decorative qualities). 3. Coal tar pitch coatings: Pitch is an important raw material for anti-corrosion coatings, and it can be divided into petroleum pitch, natural pitch, and coal tar pitch. The water absorption of coatings made from the first two types of asphalt is higher than that of coal tar asphalt coatings; therefore, coal tar asphalt coatings are the most important in the field of corrosion protection, as they possess excellent corrosion resistance. Coal tar pitch is aromatic in nature, with a carbon-to-hydrogen ratio of over 1.4:1, which gives it good water resistance; whereas petroleum pitch has a carbon-to-hydrogen ratio of around 0.9:1, resulting in poorer water resistance. Therefore, petroleum asphalt can be used in areas with less stringent corrosion requirements, such as the bottom of containers. 4. Chlorinated polyolefin anti-corrosion coatings: Chlorinated polyolefins (CPR) are resins that contain a high amount of chlorine in their composition, with a chlorine content typically ranging from 40% to 66%; they are commonly used to manufacture anti-corrosion coatings. The main types used in coatings in our country include chlorinated rubber (CR), chlorosulfonated polyethylene (CSPE), hypochloroethylene (HPVC), highly chlorinated polyethylene (HCPE), chlorinated polypropylene (CPP), vinyl chloride-vinyl acetate (CEVA), as well as chloroether resins (vinyl chloride-vinyl ether copolymers, BASF’s MP-series) and chlorinated EVA. 5. Phenolic resin-based anti-corrosion coatings: Thermosetting phenolic resins contain hydroxymethyl groups, which allow them to cross-link and cure upon baking. The resulting coating has a tight cross-linked structure, offering resistance to acids, heat, and solvents. However, the coating is relatively brittle; therefore, it must be applied in multiple thin layers, with each layer dried at moderate temperatures before the final layer is baked at high temperature to ensure thorough cross-linking and thus high corrosion resistance. In addition to being used alone, phenolic resin can also be combined with epoxy resin or polyvinyl butyral, as well as hydroxyl-containing resins, to create baking varnishes that are both corrosion-resistant and flexible. These varnishes are used for lining the inner walls of tanks, pipes, etc., with the phenolic resin serving as a crosslinking agent in this case. 6. Other common types of anti-corrosion coatings Besides the several common types of anti-corrosion coatings mentioned above, traditional anti-corrosion coatings available on the market include dried vegetable oils (such as linseed oil), alkyd resin paints, acrylic resin paints, and raw lacquer. II. Protection mechanism of anti-corrosion coatings (1) Isolation and shielding effect. A coating is formed on the metal surface using water-based coatings to prevent contact between the medium and the metal, thereby achieving anti-corrosion effects. (2) Passivation corrosion inhibition effect. The passivation and corrosion inhibition effect of a coating is achieved by using certain pigments in the coating to alter the properties of the metal surface, thereby passivating it and slowing down corrosion. (3) Electrochemical protection effect. By adding metals with a more positive electrode potential than the base metal as fillers to the coating, when an electrolyte penetrates into the coating and reaches the metal substrate, a corrosion cell is formed between the metal substrate and the electronegative metal fillers. The fillers act as the anode and dissolve first, thereby protecting the substrate; such coatings are known as sacrificial coatings. Traditional anti-corrosion coating systems are mainly solvent-based products. As society pays more attention to environmental pollution issues, the research on environmentally friendly anti-corrosion coatings has become a focus and an inevitable trend in this field of research.