Choosing Alloys for Marine Environments

## The Marine Corrosion Challenge Seawater contains approximately 3.5% dissolved salts (primarily NaCl), dissolved oxygen, and biological organisms. Key degradation mechanisms include pitting corrosion (PREN must exceed approximately 32), crevice corrosion (requires 5-10 higher PREN than open seawater), galvanic corrosion between dissimilar metals, stress corrosion cracking in warm chlorides, and biofouling. ## Alloy Selection by Application ### Structural and Hull Carbon steel with coatings and cathodic protection (ASTM A131 grades) remains the most cost-effective choice. Corrosion allowances of 3-6 mm are supplemented by epoxy coatings and sacrificial zinc or aluminum anodes. ### Seawater Piping **90-10 Cupronickel (C70600)**: Naturally resistant to biofouling. Handles velocities up to 3 m/s. The workhorse for fire main, ballast, and cooling systems. **Super duplex 2507**: PREN 43. For critical process piping on offshore platforms where pressure ratings exceed copper alloy capability. ### Fasteners and Hardware **Alloy 625**: PREN > 50. The premium choice for critical subsea fasteners. **Monel K-500**: Precipitation hardened to 35 HRC. Excellent for pump and propeller shafts. **316 stainless**: Acceptable above waterline only. NOT suitable for immersed or crevice-prone applications without cathodic protection. ### Heat Exchangers **Titanium Grade 2**: Immune to chloride pitting and biofouling-related attack. Standard in naval and power plant condensers. ## The Galvanic Series When dissimilar metals must be coupled, keep them close on the galvanic series. Use insulating bushings at dissimilar metal joints. A large cathode-to-anode area ratio accelerates attack. ## Common Mistakes - Specifying 316 stainless for submerged fasteners without cathodic protection - Mixing brass fittings with aluminum piping (galvanic attack on aluminum) - Using carbon steel in splash zones without coatings (0.5-1.0 mm/year corrosion rate)