Hangbo Alloy Group
Shanghai · Est. 2012
SWRO desalination pumps seawater at 55-82 bar through brine that pits ordinary steel. Which nickel alloys survive intakes, high-pressure pumps, ERDs and skids?
Seawater reverse osmosis (SWRO) is now the backbone of municipal and industrial water supply across the Gulf, North Africa, Australia, and parts of coastal China. A single train pumps raw seawater at 55-82 bar through semi-permeable membranes, and the reject (brine) stream that comes off the back end is even more aggressive than the feed. Pumps, energy-recovery devices, piping, and chemical-dosing skids in this service face chloride pitting, crevice corrosion, erosion, and biofouling - the exact conditions where nickel alloys earn their place over commodity stainless. Getting the grade right protects both availability and the capital tied up in the train.
Raw seawater runs 19,000-45,000 ppm total dissolved solids, dominated by chloride, with dissolved oxygen, a shifting pH, abrasive suspended sand, and a living biofilm. On the high-pressure (brine) side, chloride can climb past 70,000 ppm and temperature rises through later stages. That environment defeats ordinary stainless by pitting and crevice attack, then by under-deposit corrosion once biofilm or silt settles on a surface.
The design rule of thumb is to target a pitting resistance equivalent number (PREN) above 40 for warm seawater, and higher still for brine concentrate. This is the threshold where copper-nickel and the super-austenitic and nickel alloys take over from 316L and lean duplex.
Velocity matters as well. Swept intakes and high-pressure pump suction run at speeds that strip protective films and accelerate erosion-corrosion, so the alloy must hold up not only chemically but mechanically. Plants that ignore flow velocity in their specification pay for it in eroded fittings within the first few seasons of operation.
Type 316L begins to pit in warm seawater above roughly 20 C, and even 2205 duplex is only borderline in stagnant or creviced service. The super-austenitics lift PREN into the low 40s, while the nickel alloys sit well above 50 and add strength and fatigue resistance the stainless grades cannot match.
The alloy mix in a modern SWRO plant is deliberately tiered by duty. Copper-nickel handles the biofouling-prone low-pressure side, super-austenitics handle bulk piping, and the nickel alloys handle the high-stress, high-chloride components.
Cost engineering matters here. Super-austenitic piping costs a fraction of nickel-alloy pump internals, so the sensible design spends the premium only where stress, chloride, and fatigue actually meet - on shafts, pistons, and dosing lines - and uses copper-nickel and super-austenitics elsewhere. Over-specifying the whole train wastes budget, while under-specifying the high-stress parts costs the plant in outages.
A practical SWRO specification maps grade to location:
Material should be verified per heat with EN 10204 3.1 certification, because the gap between a borderline and a safe grade only shows up after a seasonal biofouling event or a dosing upset.
For intake and low-pressure seawater piping, copper-nickel 90-10 and 70-30 resist biofouling and chloride attack better than unalloyed stainless and avoid the pitting that plagues 316L. They remain the standard for screened intakes and manifold piping in coastal plants.
Target PREN at or above 40 for warm seawater. Alloy 926 and 254 SMO reach about 43; super-duplex 2507 about 42. Nickel alloy 625 and the C-family grades sit well above 50, covering brine-concentrate and mineralized streams. Compare the trade-offs in our 254 SMO vs 926 vs Inconel 625 guide.
Inconel 625 combines high strength with exceptional pitting and crevice resistance, so it is specified for high-pressure pump shafts, impellers, and energy-recovery pistons where fatigue and erosion meet chloride. See our Inconel 625 properties guide for composition, forms, and testing.
Brine concentrate can exceed 70,000 ppm chloride and runs hotter in later stages, pushing PREN demands higher than feed seawater. Here brine-service nickel alloys, Alloy 31, and Hastelloy 686 are preferred over borderline super-austenitics to avoid crevice and under-deposit attack.
As a dedicated nickel alloy manufacturer serving marine, power generation, chemical processing sectors, Hangbo Alloy Group supplies the grades discussed here across all standard product forms with full ASTM/ASME documentation and EN 10204 3.1 certification. Our material is specified for applications where corrosion resistance, high-temperature strength, and traceable quality are non-negotiable. See our production & testing facilities for more on how every heat is verified.