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RT: 1 Introduction Material corrosion is a serious problem faced worldwide. Corrosion not only consumes the valuable wealth created by people, but also disrupts the normal operation of many activities such as production and daily life. The relationship between corrosion and environmental pollution is also attracting increasing attention. It is reported that the direct economic losses caused by corrosion worldwide each year amount to approximately 700 billion dollars, which is six times the total cost of natural disasters such as earthquakes, floods, and typhoons, and accounts for 2% to 4% of countries’ gross national product (GNP). Corrosion losses in our country account for 4% of the Gross National Product (GNP), and the amount of steel that becomes unusable due to corrosion makes up about 25% to 30% of the annual production volume. Corrosion of production equipment often leads to factory shutdowns, low labor productivity and rising costs, and even safety accidents such as fires and explosions. The American Challenger space shuttle exploded just a few minutes after liftoff due to corrosion in its combustion chamber, which was a very painful lesson. To prevent corrosion, various anti-corrosion methods have been developed, among which coating protection is the most widely used. Corrosion prevention is a costly process of struggle, with protection being the key focus. \"Applied current\" and \"sacrificial anode\" in electrochemical protection technologies aptly illustrate that corrosion prevention involves imposing additional conditions or sacrificing some alternative material. Anticorrosion coatings and coating technologies essentially involve finding coating materials that can take the lead in combating the effects of the surrounding environment, thereby replacing the metal surface and putting it in a secondary role, so as to preserve the integrity of the metal components or equipment and their proper functioning. In short, efforts are made in the research of anti-corrosion technologies, and the goal is by no means to eliminate corrosion; rather, it is to find ways to protect equipment, buildings, or engineering structures whose value is many times greater than that of the metal itself, at a lower cost. This article discusses the corrosion and protection of materials, starting from the principles of corrosion and anti-corrosion processes. 2 Mechanisms of corrosion All materials, especially metals, are subject to the problems of corrosion and protection. The principles of steel corrosion share many similarities with those of other metals; therefore, when discussing the principles of metal corrosion and its prevention, steel is often taken as the main subject of study. Based on corrosion factors, corrosion environment, and the condition of the corroded surface, as well as the mechanisms of metal corrosion, it can be classified into chemical corrosion, electrochemical corrosion, and multi-factor corrosion. 2.l Chemical corrosion: The chemical corrosion of metals is the result of chemical reactions that occur when metals are in contact with dry gases (such as oxygen, chlorine, hydrogen sulfide, etc.) and non-electrolytic solutions. Chemical reactions cause corrosion, and no electric current is generated during the corrosion process. Chemical corrosion of metals occurs only under specific conditions and is not universal. For example: the oxidation of metal, M+O→MO. 2.2 Electrochemical corrosion: Corrosion that occurs as a result of electrochemical reactions between metals and media. During this process, there are anodic and cathodic regions, and electric current can flow through the metal over a certain distance; this applies to the corrosion of metals in various electrolyte solutions such as seawater, acids, bases, salt solutions, and humid air. Under normal conditions, electrochemical corrosion mainly consists of microcell corrosion and concentration cell corrosion. 2.3 Corrosion caused by chemical or electrochemical factors combined with mechanical factors: This is corrosion resulting from multiple factors. Due to the interaction of various factors, very severe corrosion often occurs, which generally includes stress corrosion, corrosion fatigue, cavitation corrosion, etc. 3 Protection of materials: The main methods to prevent metal corrosion must be considered from both the metal and the medium perspectives. 3.1 Proper selection of materials and attention to processing techniques: The preservation of materials is a comprehensive task that constitutes an important part of production, requiring coordination across various aspects and seamless integration at every stage. In terms of the selection of equipment and engineering materials, while meeting the main technical, process, and economic requirements for production, materials with good stability under the given corrosion conditions should be used as much as possible. For example: In H2SO4 solution storage tanks, metal-lined lead and ceramic materials are used; when constructing outdoor structures, aluminum and its alloys are employed whenever strength permits, as aluminum does not corrode easily in normal air due to an oxide film layer that protects its surface ； In terms of design, the structure and combination of equipment and systems should comply with corrosion prevention principles. Try to avoid the overlapping and riveting of different metals. There should not be too many welds, and the stress on each part must be even. In terms of technical specifications and production processes, if there is a choice, conditions such as low reaction temperature, low pressure, minimal production of corrosive by-products, less stringent corrosion prevention requirements, and easy equipment maintenance should be given priority. 3.2 Coating protection: Methods that involve applying corrosion-resistant materials to the substrate to be protected in various ways, thereby preventing it from coming into direct contact with corrosive agents and avoiding corrosion, all fall under this category. The most common and simplest method is to coat the metal surface with non-metallic materials such as anti-corrosion coatings, plastics, rubber, enamel, ceramics, glass, and stone. Furthermore, a layer of metal or metal alloy with good corrosion resistance, such as Ni, Cr, Zn, Al, Sn, Cu, etc., can be deposited on the metal surface through chemical plating, electroplating, thermal spraying, or hot-dip coating. Metal coating protection can be divided into cathodic coating protection and anodic coating protection. The metal used as the coating layer on the anode should have a more negative electrode potential than the base metal; for example, Zn, Al, etc. can be used as coatings on iron ； The electrode potential of the metal in the cathodic coating is more positive than that of the base metal being protected. For example, when Ni, Cu, Sn, Pb, etc., are coated on iron, the base metal acts as the anode while the coating functions as the cathode; therefore, the coating must be intact in order to achieve protection of the substrate. 3.3 Electrochemical protection The necessary conditions for electrochemical corrosion are: anode, cathode, dielectric, and current path. Removing or changing any one of these conditions can prevent or slow down the progression of corrosion. The coating isolates the dielectric surrounding the metal, and it also serves an electrochemical anti-corrosion function. Electrochemical corrosion protection is mainly achieved through cathodic protection and anodic protection. 3.4 Treatment of Corrosive Media: Depending on the composition and properties of the corrosive media, appropriate treatments are applied to them. Generally, this involves removing substances that are corrosive or contribute to corrosion from the media, and in addition, corrosion inhibitors are added to them. 4 Coating protection for materials. Coating protection is one of the most direct, convenient, and effective methods for protecting metals. It has a long history, as it effectively combines protection with aesthetics, offering a variety of colors. Therefore, coating protection is irreplaceable by other methods in terms of corrosion prevention. 4.1 The role of coating protection: A large amount of metal powder acting as an anode is added to the coating, which is then applied to the surface of the metal using coating techniques to form a protective layer. During the corrosion process, it acts as an anode and is corroded, thereby protecting the metal of the structure. Zinc-rich primers are representative of this type. 4.2 Surface Treatment of Materials It is crucial to ensure proper surface treatment of the material to be coated before painting, so that its surface is smooth and free from defects such as weld slag, rust, acids, alkalis, moisture, and oil stains. For example, if the surface of the object to be coated is oily or wet, it may be difficult to form a continuous coating layer after painting. Even if a continuous coating layer is formed, it will severely affect the adhesion of that coating layer, causing it to peel off prematurely and losing its protective and decorative functions. Fouling on the surface of the object to be coated, such as pitting and acids or alkalis, if not removed, will also lead to the same consequences. Therefore, thorough surface treatment prior to coating plays an extremely important role in enhancing the adhesion of the coating film, maximizing its protective and decorative functions, and extending the service life of the product. The quality of corrosion resistance depends to 60%–70% on surface treatment and application methods; in other words, 40% comes from the coating itself and 60% from the way it is applied. This highlights the importance of surface treatment and proper application techniques. (1) Oil removal: There are several methods for oil removal, including removal using organic solvents, chemical methods, electrochemical methods, and emulsion cleaning. The most common methods are chemical degreasing and organic solvent degreasing. Chemical degreasing makes use of the saponification of fats in an alkaline medium to remove the saponified grease separately; therefore, it is also known as the hot-alkali degreasing method, and it offers the best degreasing results. Organic solvent degreasing is a method that uses the solvating power of solvents to remove oils, and it is only suitable for workpieces with mild oil contamination. Common solvents include gasoline, propane, benzene, and others. Solvents are highly toxic, flammable, and do not remove oil very effectively. (2) Rust removal (mainly for ferrous metals) There are several methods for rust removal: manual rust removal, mechanical rust removal, spray rust removal, chemical rust removal, etc. The first three methods yield generally poor rust removal results; if the object is large in size, it is difficult to clean it thoroughly. Chemical rust removal utilizes a chemical reaction between the oxides of rust and an acid solution to dissolve and remove the rust layer on the surface, thereby achieving rust removal; hence it is also known as \"acid washing\" for rust removal. The quality of surface rust removal has a significant impact on the durability of the coating. Swedish scientists conducted large-scale exposure tests, and the results showed that the quality of rust removal can result in a difference of more than 4 times in the lifespan of the coating. (3) Phosphating treatment (ferrous metals) and anodizing treatment (aluminum and its alloys): After the surface of the workpiece has been degreased and derusted, painting it immediately is generally not possible. To prevent rust from forming again and to enhance the adhesion of the paint coating, certain chemical treatments are used to create a protective film on the surface of the substrate; common methods include oxidation and phosphating. For ferrous metals, this process is very important and generally cannot be omitted easily, as the oxide film and phosphating film also serve an anti-corrosion function. Phosphating is generally divided into high-temperature phosphating, medium-temperature phosphating, and low-temperature phosphating. The first two types yield better results; the zinc phosphate coating formed as a result of these processes consists of needle-shaped crystals, which enhance the adhesion between the coating and the metal. Additionally, this coating offers good corrosion and heat resistance. If strict corrosion protection is required after phosphating, passivation treatment is necessary, usually using a chromate solution – a process also known as chromium plating. This treatment aims to seal the pores in the phosphated layer and passivate any exposed areas, thereby preventing the corrosive effects of residual phosphating accelerants and further improving the protective properties. After phosphating is completed, the quality of the phosphated coating should be tested in accordance with the national standard GB6807-86. Generally, the failure of corrosion protection is mostly caused by defective phosphating coatings. For aluminum and its alloy materials, anodization should be carried out immediately after degreasing and derusting. 4.3 Selection of Coatings As mentioned earlier, there are many types of protective coatings, each with different properties. The objects to be protected vary, as do the operating conditions. In short, there is no \"universal paint\" that can be used for all purposes. Therefore, choosing the paint is very important. However, an excellent anti-corrosion coating must possess the following characteristics: (1) Good corrosion resistance. The corrosion resistance of a coating refers to the fact that its cured coating remains stable in both physical and chemical terms when in contact with corrosive agents such as water, acids, bases, salts, various chemicals, waste liquids, and industrial gases; it is not swollen or dissolved by these agents, nor is it damaged or decomposed by them, and it does not undergo any harmful chemical reactions with them. (2) Low air permeability and water permeability: Coatings generally have a certain degree of air and water permeability; therefore, it is necessary to use film-forming materials with low air permeability and coatings that provide strong shielding effects. (3) It must have good adhesion and sufficient mechanical strength. Whether the coating can adhere firmly to the metal substrate is one of the key factors determining its ability to provide corrosion protection ； In addition, the cured coating should also possess certain physical and mechanical strength to withstand the stresses under operating conditions. (4) Low coating costs and application expenses. Under normal circumstances, the cost of protection through coating is lower than that of other protection methods. The construction costs are higher than the cost of the paint itself. In short, in practice it often happens that a particular type of coating has excellent corrosion resistance, but its adhesion to the substrate and mechanical properties are poor, rendering it unusable. To resolve the conflict between corrosion resistance and mechanical properties, a combination of several coatings is often used. 4.4 Structure and Properties of Coatings It is well known that the basic characteristics of coatings are determined by the film-forming resin, which is primarily an organic polymer compound. The inherent properties of such organic polymer compounds are determined by the structural units of their molecular chains. Therefore, the main factor affecting the performance of anti-corrosion coatings can be attributed to the structure of the film-forming resin. For example, hydrocarbon resins containing methylene linkages such as phenolic resins and polyethylene resins possess good chemical resistance, water resistance, and corrosion resistance. Resins with ester-linked segments, such as resorcinol resin and polyester resin, are easily hydrolyzed by alkalis; therefore, they have poor resistance to chemical corrosion ； Epoxy resins and phenolic resins containing ether bonds exhibit excellent corrosion resistance ； Polyurethane resins containing carbamoylase linkages also exhibit excellent corrosion resistance. It is worth noting, however, that each resin contains various chemical structures and groups, and these structures and groups interact with one another. Additionally, the corrosive agents encountered and the corrosion environment vary as well. Therefore, when analyzing the impact of the structure of the film-forming resin on anti-corrosion coatings, a detailed analysis is necessary in order to achieve the desired results. In general, structurally, coatings formed from carbon-chain polymers exhibit better corrosion resistance than those made from hetero-chain polymers. When hydrogen atoms in the carbon chain are replaced by elements such as fluorine and chlorine, the smaller radius of these elements allows them to fill in the gaps between carbon-carbon bonds and provide shielding effects. Additionally, the bond energies of fluorine-carbon and chlorine-carbon bonds are higher than that of carbon-carbon bonds, which further enhances their corrosion resistance. Furthermore, the cured coating should contain as few low-molecular-weight compounds as possible, and the presence of reactive groups such as hydroxyl, carboxyl, aldehyde, and amino groups should be avoided. The number of double bonds in the cured coating film should also be reduced to a minimum, in order to avoid the creation of too many sites that can act as catalysts for corrosion, thereby accelerating its progression. Once the film-forming resin is determined, an increase in its polymerization degree leads to improved strength, wear resistance, softening point, chemical stability, and weather resistance of the paint film. 4.5 Coating Design In the design of protective coatings, different applications are often assigned based on the specific properties of the film-forming substances. For example: using chemically active film-forming substances as primers, such as epoxy resins. The topcoat uses chemically inert polymers, such as acrylates, aliphatic polyurethanes, and vinyl resins containing chlorine and fluorine. By incorporating reactive resins or using catalysts, chemically active film-forming substances undergo chemical reactions between their functional groups during the drying process after coating, resulting in a networked three-dimensional structure; this approach also helps to achieve both good adhesion and weather resistance. In general, in most coating design scenarios, the coating system consists of a primer, an intermediate coat, and a topcoat. The primer provides strong adhesion and bonding to both the substrate and the topcoat, as well as offering corrosion and rust prevention properties ； The intermediate coat is a transitional layer that provides water resistance ； The topcoat serves to resist corrosive agents and external stresses, and the coating formed by these three components delivers the overall effect. 4.6 Painting Homework Once the previous processes have been completed and passed inspection, the painting Homework can be carried out. With the advancement of coating technology and the increasing requirements for new types of coatings, there are currently over a dozen coating methods available. For example: brush coating, dip coating, curtain coating, roll coating, air spraying, high-pressure airless spraying, electrostatic spraying, electrophoretic coating, self-propelled coating, powder sulfurization bed coating, and so on. Each method has its own characteristics and specific scope of application. It is necessary to select the appropriate coating method based on factors such as the object to be coated, technical requirements, conditions of the coating equipment, and the processing environment, in order to achieve good coating quality, high efficiency, and low costs. Coatings are applied to the surface of objects, and the process of converting the wet film or dry powder deposit into a continuous solid film requires drying or curing. There are generally three methods of drying: natural drying, heat drying, and high-energy radiation drying. Natural drying has strict requirements for the environment, especially regarding temperature and humidity. Under normal circumstances, the painting temperature should be kept between 15 and 30°C, with humidity below 65%; it is also advisable to avoid carrying out the work on rainy or overcast days. When working indoors, attention should be paid to ventilation, waste disposal, and fire prevention. For heat drying, it is an efficient drying method both in terms of coating performance quality and economics. For the same type of coating, the corrosion resistance of the dried paint film is generally better than that of self-drying paint. Drying at temperatures below 100°C is considered low-temperature drying ； 100–150°C is considered medium-temperature drying ； Drying at temperatures above 150°C is considered high-temperature drying. High-energy radiation drying technology, which currently refers primarily to the use of ultraviolet light and electron beam radiation to cure organic release agents, is a relatively new coating drying technique. They are characterized by an extremely fast curing speed; the drying process can be completed within a few seconds to a few minutes. They offer high energy efficiency and drying effectiveness, low coating costs, and cause little to no environmental pollution. Although there are many new technologies in coating. But to date, anti-corrosion coating still relies mainly on natural drying and heat drying. 5 Conclusion The corrosion and protection of materials have always been a pair of contradictions. Corrosion is a natural phenomenon, especially in harsh environments; for instance, in coastal areas with severe salt fog and warm air, in regions with acidic fog (such as those containing H2S, SO2, CO2, CO, NO2, etc.), around large chemical and petrochemical plants, and near large thermal power plants, where the degree of corrosion is even more severe. It is not possible to completely prevent corrosion from occurring, but it is possible to take measures to protect the material against corrosion and thus slow down its rate of corrosion. In this process, various methods of corrosion prevention were developed, among which coating-based corrosion protection is the most widely used. To form a proper anti-corrosion coating, the following should be done: ① Select an appropriate coating system ； ②Develop the optimal coating process ； ③Perform proper surface treatment ； ④Strictly enforce the management of painting procedures.