Seawater and brine solutions account for approximately 3% of pump applications but represent some of the largest and most critical pumping installations on earth — desalination plants, power station cooling water intakes, offshore oil and gas platforms and aquaculture systems. Their high chloride content makes them uniquely aggressive toward conventional stainless steels, requiring careful material selection and ongoing corrosion monitoring.
Key Physical Properties
- Salinity: Typical seawater 32,000–38,000 mg/L total dissolved solids (TDS); hypersaline brine from desalination reject up to 70,000 mg/L
- Chloride content: ~19,000 mg/L Cl⁻ in standard seawater — primary driver of pitting and crevice corrosion in stainless steel
- Density: 1.025 g/cm³ for standard seawater; up to 1.20 g/cm³ for saturated brine — 20% heavier than fresh water, requiring upward power correction
- Viscosity: Approximately 1.07 cSt at 20 °C — marginally above fresh water; increases with salinity and decreases with temperature
- Biological fouling: Seawater is biologically active — barnacles, mussels, algae and microbial films colonise wetted surfaces rapidly in warm, sunlit zones
Corrosion Mechanisms
- Pitting corrosion: Chloride ions destroy the passive film on stainless steel at specific sites — creates deep pits that penetrate casing walls; PREN (pitting resistance equivalent number) must exceed 40 for seawater duty
- Crevice corrosion: Occurs under gaskets, in bolt holes and at impeller/wear-ring clearances — chloride concentration increases in stagnant crevice zones
- Galvanic corrosion: Dissimilar metals in electrical contact in seawater create a powerful galvanic cell — copper-nickel and bronze corrode when coupled to more noble metals
- Microbiologically influenced corrosion (MIC): Sulfate-reducing bacteria in biofilm produce H₂S that accelerates pitting under the biofilm layer
- Erosion-corrosion: High-velocity seawater strips protective corrosion product films — particularly severe at flow velocities above 3 m/s in carbon steel and above 5 m/s in copper-nickel
Biofouling and Anti-Fouling Measures
- Intermittent or continuous chlorination (0.1–0.5 mg/L residual) to inhibit biofilm formation
- Electrolytic chlorination systems generate hypochlorite in-situ from seawater
- Antifouling coatings on external surfaces of submerged structures
- Cathodic protection (sacrificial anodes or impressed current) for carbon steel and copper-alloy structures
- Rubber-lined impellers and casings resist biofouling better than metallic surfaces
Pump Type Selection
- Large-volume seawater intake (power, desalination): Vertical axial flow or mixed-flow centrifugal — high flow, low head, impeller submerged in intake screen
- High-pressure seawater (reverse osmosis feed): Multistage centrifugal with duplex or super-duplex wetted parts — pressures up to 80 bar
- Seawater lift pumps (offshore platform): Vertical turbine centrifugal — installed in a caisson, motor above waterline
- Brine recirculation (desalination): Centrifugal with titanium or super-duplex impeller — handles high chloride and elevated temperature
Materials of Construction
- Duplex stainless steel (SAF 2205, PREN ~35): Suitable for seawater at ambient temperature in flowing conditions; marginal for stagnant or warm service
- Super-duplex stainless (SAF 2507, PREN ~43): The standard choice for seawater duty — excellent pitting and crevice corrosion resistance
- Titanium (Grade 2): Outstanding resistance to all chloride concentrations and temperatures — the best all-round metallic material for seawater; expensive
- Copper-nickel (90/10 or 70/30): Good biofouling resistance, moderate seawater corrosion resistance — widely used in naval and marine applications
- Glass-fibre reinforced plastic (GRP / FRP): Non-metallic — immune to chloride corrosion; used for large-diameter pump casings in desalination intake
- Rubber lining: Natural rubber or EPDM lining over carbon steel — effective barrier; limits temperature and pressure rating
Applications
- Seawater reverse osmosis (SWRO) desalination — high-pressure feed, energy recovery and brine disposal
- Power station once-through cooling water — large-volume seawater intake and return
- Offshore oil and gas — firewater, cooling water, seawater injection for reservoir pressure maintenance
- Aquaculture — seawater circulation in fish farms and hatcheries
- Chlor-alkali industry — saturated brine electrolysis for chlorine and NaOH production
- Salt production — saturated brine handling in solar evaporation ponds
- Marine vessels — ballast water systems, cooling water and fire suppression