How to select materials for valves under strong acid and alkali conditions?

Thread Content

How to select materials for valves under strong acid and alkali conditions? In the chemical, metallurgical, pharmaceutical and other industries, strong acid and alkali media are the main factors leading to valve corrosion and failure. Subtle changes in medium concentration, temperature and pressure may significantly affect the corrosion resistance of the material. Combining the engineering selection manual and on-site practical experience, the valve material selection must accurately correspond to the medium characteristics, temperature and pressure parameters and structural matching, comprehensively consider safety and economy, and avoid the selection misunderstanding of using a single material to cover all working conditions. For weakly corrosive conditions, including dilute acetic acid, low-concentration alkali solution and neutral media with a pH value in the range of 5 to 9, 304 stainless steel (CF8) or carbon steel can be preferred. 304 stainless steel can form a stable passivation film in dilute nitric acid and weak alkaline environments at room temperature, and is cost-effective ； Carbon steel is suitable for sodium hydroxide solutions with temperatures below 80°C and concentrations not exceeding 30%. Although there is slight corrosion, the cost is low and it is suitable for occasions where there are no strict requirements for iron ion content. Under such working conditions, the medium should be avoided containing chloride ions or solid particles to prevent pitting corrosion and wear. For moderate corrosion conditions, such as 20% to 70% dilute sulfuric acid, medium concentration alkali solution and conditions where the temperature does not exceed 100°C, 316 or 316L stainless steel (CF8M) can be used as the basic selection material. Due to the addition of molybdenum, this material's ability to resist non-oxidizing acid and alkali corrosion is significantly better than that of 304 stainless steel, and it is suitable for most medium and low concentration acid and alkali environments. If the working conditions require higher requirements, 2205 duplex stainless steel can be selected. This material has the advantages of both austenite and ferrite structures, high strength, excellent resistance to stress corrosion cracking, and is suitable for acid-base mixed media containing chloride ions. Under strong corrosive conditions involving concentrated hydrochloric acid, concentrated sulfuric acid, concentrated nitric acid, high temperature and strong alkali and other media, materials must be accurately selected according to the specific media type. For reducing acids such as hydrochloric acid, Hastelloy C-276 or B3 should be preferred. This material can withstand concentrated hydrochloric acid in the boiling state and has excellent pitting and crevice corrosion resistance. ； For oxidizing acids such as nitric acid, titanium alloy (TA2) is the best choice. Its surface oxide film is extremely stable and suitable for high temperature and concentrated nitric acid environments. For concentrated sulfuric acid, carbon steel can be used with the help of passivation under normal temperature and high concentration conditions, but when the temperature exceeds 60°C, it needs to be upgraded to Hastelloy C-276 or fluorine-lined valves. For high-temperature concentrated alkali, such as 50% liquid alkali and the temperature exceeds 100°C, nickel 200 or Monel alloy should be used to effectively avoid alkali embrittlement cracking. In medium and low pressure, normal temperature and highly corrosive scenarios, fluorine-lined (PTFE) valves are the economical choice. The inner lining of polytetrafluoroethylene can withstand most acid and alkali media. The valve body is made of carbon steel or cast iron to reduce costs. The applicable temperature does not exceed 180°C and the pressure does not exceed 2.5MPa. For corrosive slurries containing solid particles, ceramic valves should be used. They have high hardness, wear resistance and corrosion resistance, but have poor impact resistance and need to be paid attention to during operation. During the material selection process, it should be ensured that the corrosion resistance levels of the valve body, valve stem and sealing surface materials are consistent. The seals must match corrosion-resistant materials such as fluorine rubber and ethylene-propylene rubber. Only by following the principles of determining the material by medium, determining the grade by temperature and pressure, and determining the structure by working conditions can we ensure long-term stable operation of the valve in a highly corrosive environment.