【2026 Valve Corrosion】Duplex stainless steel valves – The weakness of these \"corrosion-resistant champions\"
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Duplex stainless steel is becoming increasingly popular in the valve industry, especially in highly corrosive environments such as offshore engineering, desalination, and the chemical industry, where duplex stainless steel valves are almost standard. However, duplex stainless steel is not a universal material; it can still be damaged if used incorrectly.
Why is duplex stainless steel corrosion-resistant? The microstructure of duplex stainless steel consists of approximately 50% austenite and 50% ferrite. Austenite provides good corrosion resistance and toughness, while ferrite offers high strength and resistance to stress corrosion. When combined, 1+1>2.
The pitting resistance equivalent PREN value of duplex stainless steel can reach over 35, which is far higher than that of 304 stainless steel (PREN of about 19–24). In environments containing chloride ions, duplex stainless steel is much more corrosion-resistant than ordinary austenitic stainless steel.
The “weak point” of duplex stainless steel: Duplex stainless steel is not without weaknesses. Its maximum operating temperature is limited, generally not exceeding 300°C. Above this temperature, the harmful phase (σ-phase) precipitates, causing the material to become brittle and its corrosion resistance to decrease.
The processing and welding requirements for duplex stainless steel are also higher. If the welding heat input is not properly controlled, the ratio of ferrite to austenite becomes unbalanced, affecting the properties. Heat treatment is generally not required after welding, but the interlayer temperature must be controlled.
How to choose double-phase stainless steel valves? Common grades of duplex stainless steel include 2205 (the most widely used) and 2507 (super duplex, with higher corrosion resistance). The selection is determined based on the chloride ion concentration of the medium and the operating temperature. For applications with high chloride ion concentrations and high temperatures, 2507 is suitable; for ordinary seawater, 2205 is sufficient.
In marine environments containing chloride ions or in chemical processing processes, duplex stainless steel is the preferred choice due to its unique dual-phase structure. But note that duplex stainless steel is not suitable for use in strongly reducing acids (such as hot dilute sulfuric acid).
Common failures of duplex stainless steel valves: Although duplex stainless steel is corrosion-resistant, it is not immune to failure. Common failure modes include pitting and crevice corrosion—these often occur in areas where liquid can accumulate, such as on sealing surfaces and gasket contact points. Stress corrosion cracking (SCC) is relatively rare, but it does occur; it mainly happens when there are issues with the manufacturing quality. Another type is hydrogen-induced cracking, which occurs in marine environments with excessive cathodic protection.
Maintenance of duplex stainless steel valves: Duplex stainless steel valves require high surface cleanliness. If the surface is contaminated with rust (for example, due to contact with carbon steel), its corrosion resistance will be reduced. When installing, avoid direct contact of carbon steel tools with the valve surface. Avoid overheating during use, and regularly check the sealing surfaces and flange surfaces for signs of pitting.