Duplex and Super Duplex in Seawater Service
PREN, ferrite balance and fabrication controls that determine duplex performance in chloride environments.
25 February 2026
Why PREN matters
The pitting resistance equivalent number gives a first-order comparison between grades. 22Cr duplex typically sits around 34, while 25Cr super duplex exceeds 40.
PREN is a guide, not a guarantee; actual resistance depends on temperature, chloride concentration and surface condition.
Ferrite balance and fabrication
Duplex grades rely on a roughly balanced austenite-ferrite microstructure. Incorrect heat input during welding shifts that balance and reduces both toughness and corrosion resistance.
Welding procedure qualification, interpass temperature control and ferrite measurement after fabrication are the practical safeguards.
Procurement checklist
Specify the grade by UNS number, not by trade name, and require solution annealing and quench records.
Where fabricated items are supplied, require ferrite content reporting on the finished component.
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