Basic knowledge of coating corrosion prevention

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Corrosion is a natural phenomenon, representing a return of materials from a high-energy state to a low-energy state; therefore, it is impossible to completely stop it. We can only control the corrosion rate by selecting appropriate materials and employing effective anti-corrosion measures, keeping it within acceptable and controllable limits. 1. Common methods for controlling corrosion include: 1.1 Proper material selection and structural design ； 1.2 Changing the environment (control and treatment of corrosive media, also known as process anti-corrosion measures) ； 1.3 Electrochemical protection. But we don’t have these three methods to choose from under all conditions. In many cases, it is not possible to change the corrosive environment, nor do we have the conditions for electrochemical protection; structural choices alone cannot resolve the issue either. In such situations, choosing appropriate materials is often the most effective and convenient solution. The selection of corrosion-resistant materials is sometimes influenced by the properties of the materials themselves; for example, when manufacturing equipment, metal materials with high strength, good temperature resistance, and excellent machinability are often required. However, metal materials often do not have corrosion resistance that meets the requirements, or materials with good overall performance are too expensive. At this point, people thought of a more effective method: covering the surface of the material that needed protection with a more corrosion-resistant material to create a protective layer. By taking advantage of this material’s corrosion resistance and barrier properties, the problem of corrosion could be addressed, and it was not necessary to invest more money in choosing materials with better overall performance in order to effectively control corrosion. 2. The protective effect of the anti-corrosion layer on the substrate is manifested in the following three aspects: 2.1 Isolation effect: It isolates the substrate from corrosive agents, thereby achieving anti-corrosion purposes ； 2.2 Corrosion inhibition: By reacting the internal components of the coating with the metal, a passivation or protective film is formed, thereby enhancing the protective effect of the anti-corrosion layer ； 2.3 Electrochemical protection: Active metal materials are added to the coating to act as a sacrificial anode, thereby slowing down corrosion. 3. The methods for corrosion protection using coating technologies include the following categories: 3.1 Coating-based corrosion protection ； 3.2 Glass fiber reinforced plastic lining and tape corrosion protection ； 3.3 Rubber or plastic lining ； 3.4 Corrosion-resistant metal lining ； 3.5 Metal spray coatings, plating layers ； 3.6 Enameled or glass-lined. 4. The anti-corrosion layer shall possess the following properties: 4.1 Stable resistance to chemical agents (including good water resistance) ； 4.2 It possesses sufficient mechanical properties such as impact resistance, bending resistance, and wear resistance ； 4.3 For corrosion protection under normal corrosion conditions, sufficient electrical insulation is required ； 4.4 Good adhesion and bonding strength to the substrate ； 4.5 It has good resistance to cathodic delamination, as well as decent resistance to aging and temperature ； 4.6 The coating shall not have an adverse effect on the base material and the media it comes into contact with, nor shall it pose any harm to humans or the environment during application ； 4.7 Good storage and construction stability ； 4.8 Easy to repair. Due to its advantages such as simple application, wide applicability (suitable for corrosion protection in environments like the atmosphere, water bodies, general liquids, and soil), ease of on-site application, suitability for large areas and equipment with complex shapes, relatively low overall cost for corrosion protection, short coating time, and the ability to achieve good corrosion protection results, this coating technology is the most widely used among various corrosion protection methods. However, coating corrosion protection also has some shortcomings: it has a thin thickness, weak resistance to mechanical impacts, a limited service life, restricted temperature tolerance, and is not suitable for highly corrosive environments. Therefore, today we will focus on introducing the relevant knowledge in this area. 5. Classification and characteristics of common anti-corrosion coatings 5.1 Definition and interpretation of coatings Definition: A series of liquid or solid materials applied to the surface of objects to form a solid coating layer that provides protection, decoration, or special functions such as insulation, corrosion resistance, or marking. Among them, liquid coatings account for the vast majority. Solid coatings are a new type of coating that has seen rapid development in recent years; they feature environmental friendliness, a short application time, and excellent performance, and their scope of use is continuously expanding. In liquid coatings, organic coatings hold an absolute dominance in the coating industry due to their excellent properties and good performance. Since the earliest paints were mostly made from plant oils and natural resins as their main ingredients, they were also referred to as varnishes. However, with the advancement of technology, more and more coatings are synthesized directly from chemical raw materials; they have nothing to do with \"oil\" at all. Moreover, many of these coatings are water-soluble or solid forms, so the term \"paint\" can no longer represent all types of coatings. 5.2 Basic Components of Paints Regardless of the type or properties of a paint, it is composed of three or four basic components, as shown in the table below: Basic Component Typical Examples Main Function Film-forming Substance (also known as binder) Plant oils (such as tung oil), natural resins, various synthetic resins (such as epoxy resins, polyurethane resins) This substance enables the paint to form a continuous solid film that adheres firmly to the substrate; it constitutes the foundation of the paint and determines its basic characteristics. Dispersing Medium Volatile organic solvents (such as aromatics, esters), water This substance helps disperse the film-forming substance, resulting in a viscous liquid, and it contributes to improving the paint’s applicability and certain properties of the resulting film. The dispersion medium alone cannot form a film. Pigments and fillers (extenders) such as titanium white, chrome yellow, chrome green, iron red, talc, calcium sulfate, zinc powder, aluminum powder, etc., are used to color the material and improve the properties of the coating layer; they enhance its protective, decorative, and anti-corrosive functions, and can also help reduce costs. Additives: drying agents, levelers, anti-caking agents, curing agents, plasticizers – used to improve the properties of coatings in terms of production, application, storage, and use. 5.3 Requirements for anti-corrosion coatings: 5.3.1 Good adhesion is necessary; the adhesion of coatings, especially wet adhesion, enables the replacement of water and oxygen at the interface, thereby enhancing the protective capabilities of the coating layer. Therefore, coatings with good adhesion generally have better corrosion resistance. Anticorrosive coatings used as primers often require stronger adhesion. Therefore, epoxy and alkyd coatings are often used alone as primers. 5.3.2 Strong resistance to permeation: A high level of resistance to permeation can effectively prevent the penetration of corrosive agents. In addition to being related to the inherent properties of the coating resin itself, the resistance to penetration is often improved by adding fillers such as mica and iron oxide to enhance its performance. 5.3.3 Stability against corrosive media: It is required not only that the film-forming substances in the coating exhibit good stability, but also that added additives, fillers, etc., remain stable and do not undergo any chemical reactions or physical changes when exposed to the media. 5.4 Classification Methods of Coatings There are thousands of different coating products, and there are numerous classification methods. Common classifications include: 5.4.1 By the presence of pigments: clear coats and colored coats. 5.4.2 By form: water-based coatings, solvent-based coatings, solid powder coatings, high-solid-content coatings. 5.4.3 By application: architectural paint, wood paint, marine paint, water tank paint, etc. 5.4.4 Classified by construction sequence: primer, putty, intermediate coat, topcoat, and sealant. 5.4.5 Classified by performance: insulating paint, anti-rust paint, anti-fouling paint, anti-corrosion paint, fire-retardant paint. 5.4.6 Classified by the main film-forming substances of the coatings: natural resin paints, asphalt paints, phenolic paints, epoxy paints, polyurethane paints. 5.5 Performance characteristics, applications, and selection principles of common anti-corrosion coatings 5.5.1 Depending on the main film-forming substance, anti-corrosion coatings vary in terms of their anti-corrosion performance, applicable conditions, application properties, service life, and price level. Therefore, before selecting a paint, one should have a certain understanding of the characteristics of various paints. The table below shows the properties and main applications of commonly used synthetic resin coatings:

**Main film-forming substance**, **Coating properties**, **Main applications**
Alkyd resin: Glossy, weather-resistant, strong adhesion; however, soft in texture, with poor water and alkali resistance. Used in construction, vehicles, machinery, ships, and light industrial products.
Amino resin: Glossy, good light and color retention, hard; but requires heat curing. Used in vehicles, machinery, and mechanical products.
Perchloroethylene resin: Weather-resistant, corrosion-resistant, water- and oil-resistant, flame-retardant; however, has poor adhesion, is not temperature-resistant, has a low solid content, and results in a thin film layer. Used in chemical equipment.
Vinyl resin: Light in color, highly flexible, corrosion-resistant; but has a low solid content and results in a thin film layer. Used in cement and chemical equipment.
Acrylic resin: Glossy, light in color, good light and color retention, strong weather resistance. Used in building exteriors, cars, machinery, and light industrial products.
Polyester: Highly flexible, wear-resistant; difficult to apply. Used for wood products.
Epoxy resin: Durable, wear-resistant, excellent adhesion, corrosion-resistant; however, has a dark color and poor weather resistance. Used in chemical equipment.
Polyurethane: Durable, corrosion-resistant, good insulation properties. Used in airplanes, ships, chemical equipment, and wood products.
Silicone resin: Weather-resistant, high-temperature resistant, good insulation properties; however, has poor adhesion, is not resistant to gasoline, and is brittle. Used for coating electrical materials and heat-resistant equipment.
Inorganic polymers: High-temperature resistant, flame-retardant. Used in ships and bridges