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Chemical materials are the abbreviation of engineering materials required to build chemical equipment. The chemical machinery, chemical instruments, pipelines and structures that make up the chemical production equipment all operate under different temperatures, pressures and mechanical loads, and many of the materials they come into contact with are highly corrosive. Therefore, in addition to the properties of general engineering materials, chemical materials should also have excellent corrosion resistance. If the corrosion resistance is poor, it will not only directly affect the life of the device, but may also cause accidents such as fires and explosions, and may also affect product output and quality. In addition, according to different uses and conditions of use, chemical materials are sometimes required to have special properties such as high temperature resistance or low temperature resistance, thermal conductivity or heat insulation. Since the materials and process conditions of chemical production are different, there are many types of chemical materials with complex models and specifications, including general engineering materials and materials with special properties and special purposes dedicated to chemical equipment. Categories Chemical materials are divided into two categories: metallic materials and non-metallic materials. Metal materials used in chemical industry can be divided into ferrous metal materials and non-ferrous metal materials. Ferrous metal materials mainly refer to iron and steel. Most chemical machinery and equipment are made of cast iron and carbon steel. High alloy iron, high alloy steel (such as high ferrosilicon, high nickel iron and various stainless steels) as well as nickel, copper, aluminum, titanium, zirconium and their alloys are also widely used in chemical production. Non-metallic materials are increasingly used in the chemical industry, mainly plastics, rubber, glass, ceramics, enamel, impermeable graphite, etc. Plastics develop rapidly, have excellent corrosion resistance, and are the most widely used ； Impermeable graphite is heat-resistant, acid-resistant, has high thermal conductivity, and is used in large amounts. ； Ceramics, glass, etc. have good acid resistance, but are brittle and cannot be made into large-volume equipment, so their uses are limited. However, they can be used to enamel and line metal surfaces to make corrosion-resistant enamel equipment, enamel (or line) glass tanks, tanks, etc. Between metals and non-metals, there is also an amorphous (state) alloy, which has the structure of a supercooled liquid and has strong corrosion resistance. Some are close to tantalum and precious metals. It is a developing material. In order to take advantage of the advantages of different materials, a variety of composite materials have also been developed, including: ①Metal-to-metal composites, such as sandwich metal plates, use high-strength, low-corrosion-resistant materials (such as carbon steel, aluminum alloys) as the bottom layer, and materials with higher corrosion resistance (such as pure aluminum, nickel, titanium, stainless steel, etc.) as the surface layer in contact with corrosive media. ②Non-metal and non-metal composites, such as reinforced plastics. ③Non-metal and metal composites, such as alloy ceramics, have certain strength and can withstand high temperatures. Material Selection Principles Due to different usage conditions, the main material selection principles for chemical materials are:: Mechanical properties of materials Chemical machinery generally withstands certain pressure, tension or impact force, so chemical materials must first meet the requirements of mechanical properties, mainly tensile strength, yield strength, impact strength, creep strength, elastic modulus, elongation, hardness, fatigue limit, etc. Chemical machinery often operates in highly corrosive environments and media. If the strength standard value in the manual (or nominal strength, that is, the average value of the test in air) is used, it will sometimes be damaged when the stress is far below the maximum value. This is especially true in environments where stress corrosion cracking or corrosion fatigue may occur (see Corrosion and Anti-corrosion). Therefore, for environments where stress corrosion cracking may occur, consider using the stress corrosion cracking critical strength value. Corrosion resistance of materials Different materials have different corrosion resistance to various corrosive environments. The so-called excellent corrosion-resistant materials are only a relative concept. For example: Stainless steel is generally considered to be a more corrosion-resistant material than ordinary carbon steel, but this is only true in atmosphere, water and oxidizing environments ； In reducing acids, both are corroded equally ； In more than 70% concentrated sulfuric acid, carbon steel is better than stainless steel ； In water containing chloride ions, austenitic stainless steel may suffer from stress corrosion cracking, while carbon steel does not have this risk. The manual usually contains the corrosion data of various materials in various environments (acid, alkali, salt, gas, inorganic matter, organic solvent, atmosphere, soil, water, seawater, industrial products, food and vegetable oil, etc.). If the required data is not found in the manual, a corrosion test should be performed. The evaluation standard for metal corrosion resistance is divided into four levels according to the uniform corrosion rate (see the table Corrosion Resistance Levels of Metals and Alloys). For precision parts and high-pressure, dangerous equipment, first-grade materials with extremely low corrosion rates should be used. For general containers, equipment, etc., second-grade and third-grade materials can be used. Appropriately increasing the wall thickness can ensure a longer life. For metal materials, we should not only pay attention to uniform corrosion, but also pay attention to local corrosion. When the manual points out that this "material-environment" system may produce pitting corrosion, intergranular corrosion, and stress corrosion cracking, avoid using it or take necessary protective measures. There is no good assessment method for non-metals so far. For glass, ceramics, graphite, etc., metal standards can be used. For applicable standards for plastics and rubber, please refer to the following figures:: The decrease in flexural strength should be less than 25% ； Weight or size changes should be less than 5% ； The change in hardness (Rockwell M) should be less than 30%. When selecting materials, you must also consider whether the reaction between the material and the environment will have an adverse impact on the environment or production, such as whether it will pollute product quality, affect the process flow, produce side reactions, reduce yield, or even cause fires and explosions. For example, although the corrosion rate of some materials is very low, they can deteriorate the color, aroma, and taste of the product, and even produce toxicity (such as trace amounts of lead compounds). This must not be used in the food, medicine, fiber, and cosmetics industries. The processing properties of materials such as casting, cutting, welding, forging, heat treatment and other properties should meet the requirements. Among them, welding performance is particularly important for chemical equipment. General physical properties of materials such as density, melting point, thermal conductivity, coefficient of thermal expansion, etc. should also be considered. For example: Materials with strong thermal conductivity should be used to manufacture heat exchangers ； Choose materials with the lowest thermal conductivity as insulation materials. The purpose of economic efficiency material selection is to avoid premature damage to equipment, buildings, etc. and ensure normal production operation. This is essentially an issue of economic efficiency. It is hoped that the materials selected will have the best corrosion resistance and the lowest price, but often the materials with the strongest corrosion resistance are very expensive. It is usually necessary to comprehensively consider various aspects such as material price, equipment life, maintenance cycle, maintenance costs, production shutdown losses, accident possibility and scrap value to select the most suitable material. For general short-term intermediate test equipment, small, intermittent operating equipment and easily replaced parts, materials with lower price and corrosion resistance can be used. ； Large-scale continuous production equipment requires long service life and less frequent maintenance, and materials with higher prices and strong corrosion resistance are often used.