How to use materials with good stability under given corrosion conditions?

I. Protection of materials 1. Protective coating protection: Methods that involve covering the substrate to be protected with corrosion-resistant materials using various techniques, so as to prevent it from coming into direct contact with corrosive agents and thus avoiding corrosion, 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. 2. Electrochemical protection: The necessary conditions for electrochemical corrosion are: anode, cathode, dielectric, and electrical circuit. Removing or altering 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. Treatment of corrosive media: Depending on the composition and properties of the corrosive media, appropriate treatment is carried out; generally, substances that are corrosive or contribute to corrosion are removed from these media, and corrosion inhibitors are added to them. II. Coating Protection for Materials. Coating protection is one of the most direct, convenient, and effective methods for protecting metals. It has a long history, and it effectively combines protection with aesthetics, offering a variety of colors. Therefore, coating protection is irreplaceable by other methods in terms of corrosion prevention. 1. Surface treatment of the material: 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 contaminants such as slag, rust, acids, alkalis, moisture, and oil. 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 the coating, 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, four parts relate to the coating itself and six parts to the application process, which highlights the importance of surface treatment and proper application. (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 takes advantage of the saponification of fats in an alkaline medium to remove the saponified grease; 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 oil stains, and it is only suitable for workpieces with mild oil contamination. Common solvents include gasoline, propane, benzene, etc. The solvent is highly toxic, flammable, and does 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, jet rust removal, chemical rust removal, etc. The first three methods have generally poor rust-removal effects; 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-like crystals, which enhance the adhesion between the coating and the metal. Additionally, this coating offers good corrosion and heat resistance. If high levels of corrosion protection are required after phosphating, passivation treatment is necessary, usually using a chromate solution – a process also known as chromium plating. This treatment serves 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 anti-corrosion measures is mostly caused by substandard phosphating coatings. For aluminum and its alloy materials, anodization should be carried out immediately after degreasing and derusting. III. Selection of Coatings (1) Good Corrosion Resistance The corrosion resistance of a coating refers to the ability of its cured coating to remain stable, both in terms of physical and chemical properties, in contact with corrosive agents such as water, acids, alkalis, salts, various chemicals, waste liquids, and industrial gases. In other words, it does not get swollen or dissolved by these corrosive agents, nor is it damaged or decomposed by them, and no harmful chemical reactions occur between it and these agents. (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 a certain level of 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.

The main methods for preventing metal corrosion must be considered from both the metal and the medium perspectives. 1. Select materials appropriately and pay attention to the processing techniques. The protection of materials is a comprehensive task that constitutes an important part of production; it requires coordination from various aspects and seamless integration across different stages. When selecting equipment and engineering materials, it is necessary to use materials with good stability under the given corrosion conditions, as much as possible, while meeting the main technical, process, and economic requirements for production. 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 lapping 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. 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# lthlycyj Methods for controlling corrosion: The main purpose of our research on the mechanisms and patterns of metal corrosion is to prevent and control corrosion. According to the principles of metal corrosion, the main ways to control corrosion are as follows: Proper material selection – Different materials exhibit significantly varying degrees of spontaneous corrosion and corrosion rates in different environments; therefore, in specific environments, it is necessary to choose materials that meet the required performance standards and have low levels of spontaneous corrosion and slow corrosion rates. Passivation – As mentioned earlier, once a passivation layer forms on the metal surface, the diffusion resistance increases greatly, and corrosion essentially stops. For metals that can be passivated, the following methods can be employed: ① Increasing the oxidizing capacity of the solution by adding oxidants; ② Applying anodic current to raise the potential of the solution, thereby achieving anodic protection; ③ Alloying by adding elements such as Cr and Ni; ④ Surface passivation, such as phosphating steel to form a protective layer on its surface, or forming a layer on aluminum through anodic oxidation. In addition, elements that are prone to passivation (such as Cr and Al) are incorporated into the surface, making it easier for the surface to become passivated. Corrosion inhibitors: Their function is to **reduce the corrosion rate** by being added to the solution. Cathodic protection: ① Utilization of an external electric current; ② Sacrificial anode method. Coatings: This is one of the most widely used methods for corrosion prevention; they are typically made from synthetic resins, vegetable oils, rubber, solvent-based substances, etc., and are applied to the metal surface to form a thin, porous film once dried. Although it cannot completely isolate the metal from the corrosive medium, it increases the diffusion resistance of the medium through the micropores as well as the electrical resistance of the solution, thereby reducing the corrosion current. Metal plating: ①. Precious metal plating: Plating one or more layers of corrosion-resistant metals (such as Cr, Ni, etc.) can protect the underlying Fe, but the plating layer must be dense; otherwise, a corrosion cell with a large cathode and a small anode will form, which will instead accelerate the corrosion of Fe. ②. Base metal protective layer: A metal with a lower potential is plated on the surface of the metal (such as through electroplating or hot-dip galvanizing with Zn), and its protection mechanism relies on sacrificial anode action; therefore, the presence of minor pores in the coating is not a problem. Non-metallic lining: Linings such as rubber, plastic, and tiles that are widely used in chemical processing equipment. Control the corrosive environment: By eliminating the factors in the environment that cause corrosion, directly or indirectly, corrosion will cease. However, this is possible only on the condition that changing the environment does not have any harmful effects on the products or manufacturing processes.