【Common Issues in Haichuan Pipeline Design】How should valve materials be selected for applications involving strong acids and strong bases?

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During the design, installation, and inspection of pipelines, various minor issues that have been overlooked or not taken into consideration are often identified. These issues are compiled into a series; the list is being updated continuously. Everyone is welcome to actively participate in discussions. This is a compilation post on common problems in Hailuan pipeline design – it’s still being updated, and discussions are welcome: https://bbs.hcbbs.com/thread-5705828-1-1.html ------------------------------------------------------------------ How should valves be selected for use in environments with strong acids and strong bases? In industries such as chemicals, metallurgy, and pharmaceuticals, strong acid and strong alkali environments are the main factors causing valve corrosion and failure. Even slight changes in medium concentration, temperature, and pressure can significantly affect the corrosion resistance of materials. By combining the engineering selection manual with practical on-site experience, the material selection for valves must be determined based on the properties of the medium, as well as temperature and pressure parameters, and taking into account structural suitability. Safety and cost-effectiveness should both be considered, avoiding the mistake of using a single material for all operating conditions. For conditions of mild corrosion, including dilute acetic acid, low-concentration alkali solutions, and neutral media with a pH value between 5 and 9, 304 stainless steel (CF8) or carbon steel can be preferred. 304 stainless steel can form a stable passivation film in dilute nitric acid at room temperature as well as in weakly alkaline environments, offering good cost-performance ； Carbon steel is suitable for sodium hydroxide solutions with temperatures below 80°C and concentrations not exceeding 30%. Although there is slight corrosion, its low cost makes it appropriate for applications where strict requirements regarding iron ion content are not necessary. Under such operating conditions, the medium should be free of chloride ions or solid particles to prevent pitting and wear. For moderate corrosion conditions, such as 20% to 70% dilute sulfuric acid, medium-concentration alkalis, and temperatures not exceeding 100°C, 316 or 316L stainless steel (CF8M) can be used as the preferred material. Due to the addition of molybdenum, this material exhibits significantly better resistance to corrosion by non-oxidizing acids and alkaline solutions compared to 304 stainless steel, making it suitable for most medium- and low-concentration acid and alkali environments. For higher performance requirements, 2205 duplex stainless steel can be used; this material combines the advantages of austenitic and ferritic structures, offering high strength and excellent resistance to stress corrosion cracking, making it suitable for acidic or alkaline mixtures containing chloride ions. In highly corrosive environments involving media such as concentrated hydrochloric acid, concentrated sulfuric acid, concentrated nitric acid, and high-temperature strong bases, it is necessary to select materials carefully based on the specific type of medium. For reducing acids such as hydrochloric acid, Hastelloy C-276 or B3 should be the preferred choice, as these materials can withstand concentrated hydrochloric acid at boiling temperature and exhibit excellent resistance to pitting and crevice corrosion ； For oxidizing acids such as nitric acid, titanium alloy (TA2) is the best choice, as its surface oxide film is highly stable and suitable for high-temperature, concentrated nitric acid environments. For concentrated sulfuric acid, carbon steel can be used at normal temperatures and high concentrations thanks to passivation, but when the temperature exceeds 60°C, it is necessary to use Hastelloy C-276 or fluorinated-lined valves. For high-temperature, concentrated alkalis such as 50% caustic soda at temperatures above 100°C, nickel 200 or Monel alloy should be used to effectively prevent alkali embrittlement cracking. In medium and low pressure applications, at normal temperatures and in highly corrosive environments, PTFE-lined valves represent the most cost-effective solution. The lining made of polytetrafluoroethylene allows it to withstand most acid and alkali media. The valve body is made of carbon steel or cast iron to reduce costs; it can operate at temperatures up to 180°C and pressures up to 2.5 MPa. For corrosive slurries containing solid particles, ceramic valves are recommended, as they are hard, wear-resistant, and corrosion-resistant; however, their impact resistance is poor, so care must be taken when using them. During the material selection process, it is necessary to ensure that the corrosion resistance grades of the valve body, valve stem, and sealing surface materials are consistent; the seals should be made from corrosion-resistant materials such as fluororubber and EPDM. Adhering to the principle of selecting materials based on the medium, determining the grade based on temperature and pressure, and deciding the structure based on operating conditions is essential to ensure the long-term stable operation of valves in highly corrosive environments.