Frequent chloride corrosion? How should materials be selected for valves?

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In applications such as the chemical industry, seawater desalination, salt-fog coastal areas, and the chlor-alkali industry, chloride ions represent a major corrosive factor for valve materials. They have a strong ability to penetrate, easily destroying the metal’s passivation layer and causing pitting, crevice corrosion, and stress corrosion cracking, which in turn leads to valve leakage and failure. By combining industry selection guides with engineering practice, the material selection for valves should be based primarily on chloride ion concentration, temperature, pressure, and the pH value of the medium; the appropriate materials should be chosen by taking into account corrosion resistance, strength, and cost-effectiveness. For low concentrations of chloride ions (below 200 ppm), normal temperature and pressure, and neutral media conditions (such as municipal fresh water and lightly polluted circulating water), 304 stainless steel (CF8) can be used. This material contains 18% chromium and 8% nickel, offers good cost-performance, and is capable of forming a stable passivation film to resist normal corrosion. However, it is necessary to avoid dead zones where the medium can accumulate, as well as gaps at flanges and threaded connections, in order to prevent localized accumulation of chloride ions that could lead to corrosion ； When the medium temperature exceeds 60°C, its corrosion resistance decreases significantly. Under conditions of moderate chloride ion concentrations (200–5000 ppm) and moderate temperatures (not exceeding 100°C), such as in seawater desalination pretreatment, saline chemical solvents, and coastal process water, 316 or 316L stainless steel (CF8M) is the preferred material. It incorporates 2%–3% molybdenum into 304 stainless steel, which significantly enhances its resistance to pitting and crevice corrosion, allowing it to tolerate higher chloride ion concentrations. For more demanding operating conditions, 904L, 317 stainless steel, or 2205 duplex stainless steel can be used as an upgrade. Duplex stainless steel possesses both austenitic and ferritic structures, offering excellent resistance to chloride-induced stress corrosion cracking. Its strength is approximately twice that of ordinary stainless steel, making it suitable for use in high-pressure environments with chloride-containing media. For highly corrosive environments with high concentrations of chloride ions (over 5000 ppm) and high temperatures (over 100°C), such as seawater, concentrated brine, and hot chloride solutions, super austenitic stainless steels, duplex steels, or special alloys should be used. 254SMO and 2507 super duplex steels exhibit excellent corrosion resistance, with a pitting resistance equivalent (PREN) value of over 40, enabling them to withstand chloride erosion at seawater levels. Under extreme operating conditions, titanium and its alloys (TA1/TA2) are the best choice; their surface oxide layer is highly stable, and they suffer little corrosion in seawater environments at temperatures below 120°C. Their resistance to corrosion at high temperatures is significantly better than that of ordinary alloys ； If the medium contains hydrofluoric acid, nickel-based materials such as Hastelloy C-276 and Monel alloy must be used. Furthermore, for applications with severe corrosion and limited economic constraints, valves lined with fluorocarbon, rubber, or titanium can be used; the PTFE lining offers excellent chemical inertness and is suitable for various high-chloride media. When selecting materials, it is also necessary to ensure that the corrosion resistance of the valve body, valve stem, and sealing surface materials is consistent, in order to avoid galvanic corrosion. In summary, there is no universal material for selecting valves in an environment with chloride ions; it is necessary to follow the principles of determining the grade based on concentration, the upper limit based on temperature, and the specific details based on operating conditions. Additionally, taking into account the full life cycle cost is essential in order to achieve both safety, stability, and economic efficiency.