Duplex stainless steel

Duplex stainless steel is a two-phase alloy with roughly equal ferrite and austenite, combining the strength of ferritic grades with the corrosion resistance of austenitic ones. Standard grade 2205 runs about 22 percent chromium, 5 percent nickel, 3 percent molybdenum, with nitrogen; super-duplex grades such as 2507 add chromium and molybdenum for higher pitting resistance. Onboard it is used where chloride pitting and crevice corrosion attack ordinary 316: seawater pump impellers and casings, exhaust-gas scrubber wash-water piping, and ballast and cargo systems on chemical tankers. Yield strength near twice that of 316 lets thinner sections carry the same pressure. It needs controlled welding to keep the phase balance.