Marine Alloys: Fighting Saltwater Corrosion

## Why Seawater Is So Corrosive Seawater contains approximately 3.5% dissolved salts (predominantly NaCl), dissolved oxygen, biological organisms, and a pH around 8.1. Chloride ions attack passive films, dissolved oxygen drives cathodic reactions that sustain corrosion, and biofouling creates crevices and differential aeration cells. The combination makes seawater far more aggressive than fresh water or atmospheric exposure. The velocity of seawater matters: stagnant seawater promotes pitting and crevice corrosion under deposits, while high-velocity flow (above 1-2 m/s for copper alloys, above 5-7 m/s for stainless steels) causes erosion-corrosion and impingement attack. ## Carbon and Low-Alloy Steels Unprotected carbon steel corrodes at 0.1-0.3 mm/year in seawater. For ship hulls, offshore platforms, and marine piling, carbon steel is used with protection systems: **Coatings**: Marine paint systems (epoxy primer + antifouling topcoat) protect hull steel. Modern antifouling coatings use controlled-release biocides (cuprous oxide) or silicone-based foul-release surfaces. **Cathodic protection (CP)**: Sacrificial anodes (zinc or aluminum alloy blocks welded to the hull) corrode preferentially, protecting the steel. Alternatively, impressed current cathodic protection (ICCP) uses a rectifier to force protective current onto the steel from inert anodes. CP is mandatory for all immersed steel structures per standards like DNV-RP-B401. Offshore platform jackets use structural steel grades like S355 (EN 10025) or ASTM A572 Grade 50 with CP and coatings in the splash zone (the most corrosive zone, alternately wet and dry). Corrosion allowance of 3-6 mm per side is added to structural members in the splash zone. ## Stainless Steels for Marine Service Not all stainless steels resist seawater. The critical metric is the Pitting Resistance Equivalent Number (PREN): **PREN = %Cr + 3.3(%Mo) + 16(%N)** A PREN above 40 is generally required for reliable performance in warm seawater: | Grade | PREN | Seawater Resistance | |-------|------|---------------------| | 304 | 18 | Poor — pits rapidly | | 316 | 24 | Marginal — pits in crevices and warm water | | 2205 Duplex | 34 | Good — most seawater service | | 2507 Super Duplex | 42 | Excellent — subsea, desalination | | 254 SMO (6%Mo) | 43 | Excellent — heat exchangers, piping | | Alloy 625 | 51 | Outstanding — critical subsea | **316 stainless steel is not a marine alloy** in the common understanding. It performs well in atmospheric coastal exposure but pits severely when continuously immersed in seawater, particularly in crevices (under gaskets, flanges, barnacles). Many marine failures stem from specifying 316 where a super duplex or 6%Mo grade was needed. **Super duplex 2507** (UNS S32750, 25%Cr, 7%Ni, 4%Mo, 0.27%N) combines high strength (YS 550 MPa, double that of 316L), excellent pitting resistance (PREN 42), and resistance to chloride stress corrosion cracking. It is the standard alloy for seawater-cooled heat exchangers, fire water piping, and subsea manifolds on offshore platforms. ## Copper Alloys Copper alloys have been used in marine service for centuries due to a unique combination of corrosion resistance, biofouling resistance, and thermal conductivity: **90-10 Cupronickel (C70600)**: 90%Cu, 10%Ni. The standard alloy for seawater piping, heat exchanger tubes, and condenser tubes in naval and commercial vessels. Corrosion rate in clean seawater is 0.02-0.05 mm/year, and the copper surface is naturally antifouling (copper ions are toxic to marine organisms). Maximum seawater velocity: 3.5 m/s. **70-30 Cupronickel (C71500)**: Higher nickel content improves strength and erosion-corrosion resistance in high-velocity and high-temperature seawater. Used for condenser tubes in power plants and naval vessels where seawater velocity reaches 4.5 m/s. **Aluminum Bronze (C95800, Nickel Aluminum Bronze - NAB)**: 81%Cu, 9%Al, 5%Ni, 4%Fe. The primary alloy for marine propellers, pump impellers, and valve bodies. UTS 620 MPa, excellent cavitation resistance, and good seawater corrosion resistance. The US Navy specifies NAB (MIL-DTL-15159) for submarine and surface ship propellers. ### Copper and Cathodic Protection Incompatibility Copper alloys and cathodic protection do not mix well. CP drives the potential of copper below its natural corrosion potential, which can cause dealloying, hydrogen damage, and loss of antifouling effectiveness. Marine systems must segregate copper alloy components from cathodically protected steel zones using insulating flanges and gaskets. ## Nickel Alloys for Severe Service **Alloy 625 (UNS N06625)**: 22%Cr, 9%Mo, 3.5%Nb. PREN 51. Used for the most critical subsea components: wellhead equipment, flowline connectors, and downhole tools where the combination of seawater, H₂S, CO₂, and high pressure creates extremely aggressive conditions. **Alloy C-276 (UNS N10276)**: 16%Cr, 16%Mo, 4%W. The ultimate corrosion-resistant alloy for chemical processing and marine service. Resists pitting, crevice corrosion, and stress corrosion cracking in virtually all seawater conditions. ## Titanium in Marine Titanium is virtually immune to seawater corrosion at all temperatures and velocities encountered in marine service. Grade 2 CP titanium is used for: - Heat exchanger plates in desalination plants (operating life 30+ years vs. 10-15 for super duplex) - Submarine hull valves and fittings (US Navy) - Fire pumps and seawater cooling systems on offshore platforms The drawback is cost: titanium seawater piping costs 5-8x more than super duplex per meter installed. It is specified only where the combination of reliability, maintenance reduction, and service life justifies the premium. ## Galvanic Corrosion in Multi-Metal Systems Marine structures inevitably combine different metals (steel hulls, bronze propellers, stainless shafts, aluminum superstructures). The seawater electrolyte connects them galvanically. Proper design requires: 1. Selecting metals close together in the galvanic series (or actively protecting the anodic member) 2. Insulating dissimilar metal joints with nylon bushings, rubber gaskets, or epoxy coatings 3. Maintaining favorable area ratios: small cathode area relative to anode area minimizes galvanic acceleration 4. Cathodic protection of the steel structure, with isolation of copper components