High Performance Alloys for Seawater Service

With its considerable industrial benefit as an effluent coolant, seawater remains a corrosive environment to many structural materials. Problems related to localized corrosion, erosion/wear, and biofouling represent limitations to the use of many metallic materials. Cumbersome cathodic protection techniques, unanticipated maintenance costs, along with reliability of equipment, safety to personnel, and environmental concerns are leading to a wider use of higher performance nickel-based alloys in seawater services.

Over the years, the nickel-chromium-molybdenum/tungsten alloys have proven to be among the most reliable and cost-effective materials for aggressive seawater applications. Of these, INCONEL® 625 is considered acceptable and the HASTELLOY® “C-type” alloys (C, C-4, C-276, C-22®) are considered optimum.

Nominal Composition Weight %

Alloy Nickel Chromium Molybdenum Tungsten Others
C Balance 16 16 4 -
625 Balance 21.5 9 - 3.6 Niobium + Tantalum
C-276 Balance 16 16 4 0.005 Carbon Typical
C-4 Balance 16 16 0 0.005 Carbon Typical
C-22® Balance 22 13 3 0.005 Carbon Typical


HASTELLOY® alloy shows unsurpassed resistance to corrosion in seawater atmosphere. Exposed at Kure Beach, N. C., since 1942, HASTELLOY® C sample continues to keep its original surface finish while the rest of the test allots have disintegrated long ago.

Localized Corrosion

The high nickel alloys offer excellent resistance to localized corrosive attack (pitting, crevice corrosion), when compared to stainless steels.

Critical Pitting Temperature (At and Above Which Pitting Occurred)
24 h Exposure 4% NaCl + 0.1% Fe2 (SO4)3 + 0.01 M HCl
Alloy °F °C
C-22® >302 >150
C-276 302 150
625 194 90
AL-6XN® 158 70
254SMO® 140 60
317LM 95 35
316L 68 20

Critical Crevice Corrosion Temperature
(At and Above Which Corrosion is Observed)
24 h Exposure in 6% FeCl3 Using Different Test Methods
Alloy ASTM G-48B MTL* Mod MTI*
C-22® 70°C 70°C 52.5°C
C-276 65°C 55°C 45°C
625 35°C 35°C 30°C
AL-6XN® 40°C - -
254SMO® 40°C - -
317LM 5°C 2.5°C 2.5°C
316L 0°C - -

*MTI: Material Technology Institute
Reference: E.L. Hibner (Inco) Materials Performance, page 3, March 1987

Seawater Corrosion

The excellent resistance of the high nickel super alloys to localized corrosion attack is maintained in seawater environments. When compared to stainless steels, these alloys offer better resistance to various seawater testing conditions.

Crevice Samples Tested for 180 Days (Atlantic Coast, 1989)
Alloy Natural Seawater Seawater Plus 1 ppm Cl2