Seawater service: the core challenge
Seawater contains approximately 35,000 ppm chloride, which destroys the passive oxide film on standard stainless steel (grades 304 and 316) that gives it corrosion resistance in fresh water and mildly corrosive environments. In seawater, standard 316 stainless will suffer pitting corrosion, crevice corrosion under gaskets and at flange interfaces, and in some conditions, stress corrosion cracking. It is not suitable for seawater service without protective coatings or specific alloy upgrades.
The three viable materials for direct seawater contact are copper alloys (bronze), high-alloy stainless steels (super duplex and 6Mo grades), and non-metallic options (plastic body valves for low-pressure service). Each has a distinct pressure capability, temperature limit and cost profile.
Bronze: the standard for small-bore marine valves
Bronze CC491K (gunmetal, formerly LG2) is the standard material for small-bore marine valves (DN15 to DN150): seacocks, sea inlet and overboard discharge valves, cooling water system isolation, and general shipboard service. Its corrosion resistance in seawater is excellent, and it is the material of choice in classification society rules (DNV, Lloyd's, Bureau Veritas) for sea water piping systems.
Dezincification-resistant brass (DZR brass) is sometimes used as a lower-cost alternative for valves not in direct seawater contact, but bronze is preferred wherever the medium is seawater or bilge water. Check that the specific grade of bronze meets the applicable classification society requirement for the vessel.
At larger bores, the cost and weight of bronze becomes prohibitive and ductile iron or carbon steel bodies with suitable coatings or linings are used instead.
Super duplex and 6Mo stainless: high-alloy options
Super duplex stainless steel (UNS S32750, also known as 2507) and 6-molybdenum austenitic grades (UNS S31254, also known as 254 SMO or similar) provide genuine seawater resistance through their high chromium, molybdenum and nitrogen content. Both grades have a PREN (Pitting Resistance Equivalent Number) above 40, which is the threshold generally considered necessary for seawater immersion resistance.
Super duplex is specified for offshore topside and subsea seawater lift systems, fire water mains, injection lines and other duties where large bore or high pressure makes bronze impractical. It is also standard for subsea production control systems and chemical injection headers where high pressure and chemical compatibility are both required.
6Mo grades offer similar corrosion resistance with better toughness at low temperatures, making them preferred for cryogenic and cold-climate offshore applications. Both super duplex and 6Mo are significantly more expensive than standard grades and require careful welding and heat treatment — a cost that is justified by the service life improvement in seawater duty.
Coated carbon and ductile iron for larger bores
Ductile iron GJS-500 with fusion-bonded epoxy (FBE) internal lining and polyurethane or epoxy external coating is used for large-bore marine and offshore water service: ballast piping, fire main distribution, cooling water headers and process water systems. The FBE lining protects the interior from corrosion while the external coating protects the body and flange faces from atmospheric marine corrosion.
Coating integrity is critical. Any holiday (pinhole or damage) in the internal lining creates a site for rapid localised corrosion, particularly at flange faces where the coating transitions to a gasket surface. Specify holiday detection testing (spark testing at the specified voltage for the coating thickness) on coated valves for seawater service.
Carbon steel with glass-reinforced epoxy (GRE) lining is used for larger-bore, higher-pressure seawater service where the GRE lining gives both corrosion resistance and smooth bore flow characteristics. Carbon steel with Inconel or titanium weld overlay at the seat and trim faces provides an upgrade where seat corrosion would otherwise limit valve life.