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In general, anti-corrosion methods can be divided into two main categories: one is the proper selection of anti-corrosion materials and other anti-corrosion measures； Second is to select appropriate process operations and equipment design. Strict adherence to the process specifications in chemical production can prevent corrosion that should not occur. Even when using good corrosion-resistant materials, failure to follow proper operating procedures can still lead to severe corrosion. Currently, the common anti-corrosion methods used in chemical production include the following: 01 Proper material selection and design. Understanding the corrosion resistance of different materials and selecting anti-corrosion materials in a proper and rational manner is the most effective approach. As is well known, there are many types of materials, and different materials corrode at different rates in various environments. Material selectors should choose materials that have a low corrosion rate, are relatively inexpensive, and possess physical and mechanical properties that meet the design requirements, so as to ensure that the equipment has an economical and reasonable service life. 02 Adjusting the environment: If various factors that cause corrosion in the environment can be eliminated, corrosion will cease or slow down. However, most environmental factors cannot be controlled; for example, moisture in the atmosphere and soil, as well as oxygen in seawater, cannot be removed, and chemical production processes cannot be changed at will. However, some local conditions can be adjusted; for example, oxygen can be removed from the water supplied to the boiler by adding deoxidizing agents such as sodium sulfite and hydrazine, which helps to protect the boiler from corrosion ； Similarly, removing moisture from air before it enters a sealed warehouse can also prevent stored metal parts from rusting. To prevent cooling water from causing scaling and perforation in heat exchangers and other equipment, alkalis or acids can be added to the water to adjust the pH value to an optimal range (near neutral) ； In refining processes, alkalis or ammonia are often added to keep the production fluid neutral or alkaline. When the temperature is too high, the wall of the container can be cooled down, or refractory bricks can be installed on the inner wall of the equipment for insulation, among other methods. These are methods that can be used to change the environment without affecting the products or processes. Where permitted, it is recommended to use milder media in place of highly corrosive media in the processes. 03 Addition of corrosion inhibitors. Generally, adding a small amount of corrosion inhibitor to a corrosive environment can **slow down the corrosion of metals**. These inhibitors are typically classified into three categories: inorganic, organic, and gaseous corrosion inhibitors; their mechanisms of action also vary. 3.1 Inorganic corrosion inhibitors Some corrosion inhibitors slow down the anodic process; they are known as anodic-type corrosion inhibitors. These include oxidants that promote anodic passivation (such as chromates, nitrites, iron ions, etc.) or agents that form a film on the anode surface (such as alkalis, phosphates, silicates, benzoates, etc.). They primarily react in the anodic area, thereby promoting anodic polarization. Generally, anodic corrosion inhibitors form a protective film on the anode surface, which results in good corrosion inhibition effects. However, there are also certain risks: if the dosage is insufficient, the protective film will be incomplete, and the exposed bare metal area at the defects in the film is small, leading to a high anode current density and an increased likelihood of perforation. Another type of corrosion inhibitor is involved in cathodic reactions; ions such as calcium, zinc, magnesium, copper, and manganese react with hydroxide ions at the cathode to form insoluble hydroxides, which cover the cathode surface in a thick layer. This prevents oxygen from diffusing to the cathode, thereby increasing concentration polarization. In addition, there are also mixed-type corrosion inhibitors that block both the anode and the cathode simultaneously, but the amount to be added generally needs to be determined through testing first. 3.2 Organic corrosion inhibitors Organic corrosion inhibitors are adsorptive in nature; they adhere to the metal surface to form an invisible film several molecules thick, which can inhibit both anodic and cathodic reactions, although their impact on each type of reaction is slightly different. Common inorganic corrosion inhibitors include organic compounds containing nitrogen, sulfur, oxygen, and phosphorus. The types of adsorption vary depending on the molecular structure of these organic compounds, and can be classified as electrostatic adsorption, chemical adsorption, and π-bond (delocalized electron) adsorption. Organic corrosion inhibitors have developed rapidly and are used in a wide range of applications. However, their use also comes with some disadvantages; for example, they can contaminate products, especially food items. While corrosion inhibitors may be beneficial in certain stages of the production process, they can become harmful in other stages. They may also inhibit reactions that are necessary, such as slowing down the rate of film removal during acid cleaning. 3.3 Vapor-phase corrosion inhibitors: These inhibitors are highly volatile substances that contain corrosion-inhibiting groups; they are generally used to protect metal components during storage and transportation, and are mostly applied in solid form. Its vapor decomposes the water in the atmosphere to produce effective corrosion-inhibiting groups, which adhere to the metal surface thereby reducing corrosion. Additionally, it is also an adsorptive corrosion inhibitor, so the metal surface to be protected does not require rust removal. 04 Cathodic protection: Cathodic protection is a method that relies on an external direct current or sacrificial anodes to make the metal to be protected act as a cathode, thereby reducing or eliminating corrosion of the metal. Because before applying cathodic protection, most metal structures subject to corrosion have cathodic and anodic areas; if all of the anodic areas can be converted into cathodic areas, thereby turning the entire metal component into a cathode, corrosion can be eliminated. For a specific project, there are many factors to consider before selecting a cathodic protection system: 4.1 Total protection current required – To implement cathodic protection, it is necessary to know the total current required. The required current can be determined using a temporary testing setup. If the required protection current is not high (less than 1.5–2 A), sacrificial anode protection is the preferred option; if a higher protection current is needed, impressed current protection is more economical. 4.2 Changes in the environment: In soils with poor air permeability, metals are relatively easy to polarize; whereas in soils where oxygen can easily reach the surface of the structure, a larger current is required for the structure to become polarized. Furthermore, the areas with the lowest soil resistivity are the most suitable for installing four-electrode anodes or anode systems with external current supply. The movement of water plays a significant role; if the water is stationary, the protective current can be kept at a lower value. On the contrary, turbulent water can erode the surface of the structure, thus requiring a very strong mechanical depolarization effect. 4.3 Electrical shielding: In components with small spacing, complex structures, and those that are under cathodic protection, electrical shielding can occur quite easily. Current coming from a remote cathodic protection power source is easily absorbed by the outer components; only a small amount of current reaches the inner components. Thus, the outer components act as an electrical shield. At this time, the number and arrangement of cathodes should be such that their distances from various parts of the structure to be protected are roughly equal, in order to achieve a more even distribution of current. 4.4 Economic factors When using cathodic protection, it is necessary to consider whether it is economically viable. If cathodic protection is an economical way to address corrosion problems, then the cathodic protection system chosen should be the one with the lowest cost, taking into account design and installation costs, power supply costs, as well as system maintenance costs. 4.5 Protection life: During design, it is necessary to know the expected service life of the structure to be protected. In practical applications of cathodic protection, the design life of the cathodic protection system should be matched to the lifespan of the structure being protected. A too short lifespan results in inadequate protection, while a too long lifespan increases costs and leads to waste. 4.6 Impact of stray currents Before designing a cathodic protection system, it is necessary to determine whether there are any stray currents in the area. It mainly comes from DC power sources such as electrified railways, mining machinery, and electric welding. Stray currents cause rapid corrosion of the structure being protected, usually more severe than corrosion caused by other environmental factors. Therefore, when designing cathodic protection, the location of the anode system should be carefully chosen to avoid stray currents as much as possible. 4.7 Temperature The temperature affects the resistance of a medium; generally, the resistance of soil and water decreases as the temperature rises. This is the principle behind why the resistivity of tropical seawater is much lower than that of seawater in colder regions. 4.8 Sacrificial Anode Materials Materials suitable for use as sacrificial anodes include aluminum, magnesium, and zinc. Anode materials can be cast into various shapes of sacrificial anodes to meet the requirements of cathodic protection design. 4.9 External current anodes: These are used in external current cathodic protection systems; it is desirable that they have a practical minimum corrosion rate when delivering current. Waste steel pipes, bars, and similar waste steel materials can all be used as anodes for impressed current protection systems; although they are consumed in large quantities, they are readily available. In general, cathodic protection is more suitable for media with moderate corrosion levels, such as seawater, soil, and neutral salt solutions. In highly corrosive media, it is generally not used due to the high consumption of electrical energy and protective materials. 05 Anodic protection: Using the equipment as an anode, electricity is supplied from the outside; this generally accelerates corrosion, and the corrosion current increases as the anode becomes more polarized. However, for metals that can be passivated, a different situation occurs: as the potential rises with the current, once it reaches the passivation potential, the corrosion current drops sharply, by as much as tens of thousands of times; thereafter, as the potential continues to rise, the current remains constant until the passivation region is reached. Using this principle, with the device to be protected as the anode and current being applied to maintain the potential in the middle of the passivation region, the corrosion rate can be kept very low; the amount of current applied indicates the corrosion rate of the device. 06 Alloying: Alloying elements that promote passivation are added to the base metal; when the addition amount reaches a certain proportion, a material with excellent corrosion resistance is obtained. If more than 12% chromium is added to iron, it is called stainless steel ； Adding nickel to chromium steel can expand the passivation range as well as improve mechanical properties ； For example, adding 14% silicon to iron yields high-silicon iron with excellent acid resistance, and so on. Furthermore, adding trace amounts of cathodic noble metals with low overvoltage to certain active metals can promote passivation. Materials such as stainless steel and titanium are active in sulfuric acid at certain concentrations and temperatures; by adding 0.1–0.15% palladium or platinum to the base metal, numerous microcathodes are formed on the surface of the alloy, which facilitates the operation of local corrosion cells. As a result, the cathodic current increases rapidly, allowing the material to reach a passivation state and thereby enhancing its corrosion resistance. 07 Surface Treatment: Before coming into contact with the operating environment, metals are treated with passivators or film-forming agents, resulting in the formation of a stable and dense passivation film that enhances their corrosion resistance. It differs from the corrosion inhibitor method in that no further addition of corrosion inhibitors is required in the subsequent operating environment. Aluminum, when subjected to anodic treatment, develops a film on its surface that is denser than one formed in atmospheric conditions; such films exhibit excellent corrosion resistance in mild corrosive environments. The blueing of steel components also follows this same principle. 08 Metal coatings and claddings: A thin layer of a more corrosion-resistant metal can be used to protect the steel substrate. The common method is electroplating, typically 2 to 3 layers are applied, each with a thickness of only a few dozen micrometers; as a result, pores are inevitably present, and the solution can penetrate these pores, creating a corrosion cell between the plating layer and the underlying layer. If the coating is made of a precious metal, its potential is higher than that of iron; it then acts as a cathode, accelerating the corrosion of the underlying iron. Therefore, such coatings are not suitable for highly corrosive environments, but can be used in environments such as the atmosphere and water; the slowly formed corrosion products can block the micropores, increasing resistance and thus providing a certain service life. If a cheap metal is used, the polarity of the corrosion cell becomes opposite to that described above, providing cathodic protection for the steel and thus extending its service life. In addition to electroplating, hot-dip plating (melting immersion plating), flame spraying, vapor plating, and full-metal sheet cladding are also commonly used. The latter has no micropores, exhibits strong corrosion resistance, and has a longer lifespan; however, it is slightly more expensive. 09 Coating: Protecting metal structures in the atmosphere with organic coatings is the most widely used method for corrosion prevention. The coating is applied to the metal surface and forms a porous film upon drying; although it cannot completely isolate the metal from the medium, it increases its value by reducing the corrosion current through the diffusion resistance of the micropores and the electrical resistance of the solution. In mild environments such as the atmosphere and seawater, the corrosion of metal at the micro-pores occurs slowly; the corrosion products can clog these micro-pores, resulting in a long service life. However, this material is not suitable for highly corrosive solutions, as the metal corrodes more rapidly and hydrogen is generated, which can cause the paint coating to crack. 10 Rubber sheet lining: Anti-corrosion lining refers to unvulcanized, pre-vulcanized, or vulcanized rubber sheets or plates used to prevent equipment corrosion. The technique of forming a continuous insulating coating on the working surface of metals or other materials using lined rubber sheets is known as rubber lining technology. Lining rubber compounds can be divided into soft rubber, hard rubber, and semi-hard rubber; they are generally manufactured using materials such as NR, CR, and SBR depending on the application conditions. The manufacturing of linings involves processes such as surface treatment of the metal substrate, processing of the lining adhesive sheet, cutting, bonding, and vulcanization. Lined rubber sheets are widely used as materials for chemical corrosion protection and mechanical wear resistance; they are employed as linings for chemical equipment, as well as for equipment in the mining and metallurgy industries such as slurry pumps, flotation machines, grinders, and cement grinders in the building materials industry.