Duplex stainless steels comprise a mixture of ferrite and austenite, producing a alloy with many of the advantages offered by both structures. They generally have comparable or better corrosion resistance to austenitic steels, with more than twice the yield strength, and are especially resistant to pitting corrosion and stress corrosion cracking in chloride environments.
Duplex stainless steels have for years been providing excellent service in seawater on offshore platforms and in a wide variety of other chloride containing environments.
The pitting resistance equivalent number (PREN) is an empirical guide to the grading of alloys for resistance to seawater. Duplex stainless steel has been designed to exceed the commonly specified PREN of 40 and provide, for example, castings suitable for pumps and valves on offshore oil and gas platforms.