Desalination plants are among the most demanding environments for metallic materials. Seawater contains 30,000–40,000 ppm chlorides, is at elevated temperatures in thermal processes, and contacts a wide range of chemical treatment agents. Selecting the wrong stainless steel grade results in catastrophic pitting, crevice corrosion, or stress corrosion cracking within months of commissioning. This guide covers which grades are proven in practice and why.
Why Chloride Resistance Is the Primary Selection Driver
Chloride ions are the primary corrosion risk in desalination plants. They destabilise the passive film on stainless steel surfaces, initiating pitting corrosion — small holes that penetrate deep into the material without spreading visibly across the surface. The Pitting Resistance Equivalent Number (PREN) is the key property:
PREN = %Cr + 3.3 × %Mo + 16 × %N
Grades with higher PREN are more resistant. For continuous seawater service, a PREN above 40 is generally required. For desalinated water (post-treatment, low chloride), PREN above 25 is typically adequate.
Desalination Technologies and Their Material Demands
SWRO (Seawater Reverse Osmosis)
SWRO is the dominant modern desalination technology, used in Saudi Arabia, UAE, Israel, and large-scale projects globally. Key material-intensive components include:
- High-pressure feed pumps and pressure vessels: Exposed to raw seawater at 60–80 bar. Super Duplex 2507 (PREN >40) or Duplex 2205 (PREN ~35) required for pump casings, impellers, and housings.
- Piping (brine and concentrate lines): The RO reject brine is more concentrated than seawater and more aggressive. Super Duplex 2507 or 6Mo alloys (UNS S31254) are standard.
- Permeate piping: After membranes, the permeate is low-chloride and low-TDS. Duplex 2205 or standard 316L is often acceptable for these lines.
- Pre-treatment filters and strainers: Duplex 2205 for bodies in direct seawater contact.
MED (Multi-Effect Distillation)
MED plants heat seawater through a series of effects at progressively lower pressures. Materials face high-temperature brine at 65–70°C in the first effect, with heavy scaling and corrosion potential. Key grades used:
- Evaporator tubes: Austenitic 316L acceptable in some effects; higher-alloy grades or titanium tubes for the most aggressive stages
- Flash chambers and heat exchanger plates: Duplex 2205 widely used
- Brine headers: Super Duplex 2507 or titanium Gr.2 for seawater at elevated temperature
MSF (Multi-Stage Flash)
MSF is the traditional thermal desalination technology still prevalent in the Gulf region (Saudi Arabia, UAE, Kuwait, Qatar). Operating at higher temperatures (90–110°C in the brine heater) than MED, MSF imposes severe material demands. Typical materials include:
- Heat exchangers and heat rejection sections: Duplex 2205 or Incoloy 825
- Brine heaters: Titanium or high-alloy austenitic steel
- Pumps, valves and piping: Super Duplex 2507 in seawater stages; 316L or duplex 2205 in post-treatment sections
Grade Selection Summary by Application
| Application / Location | Recommended Grade | PREN |
|---|---|---|
| Raw seawater intake piping | Super Duplex 2507 or 6Mo (S31254) | >40 |
| SWRO high-pressure pumps (seawater side) | Super Duplex 2507 | >40 |
| SWRO brine/concentrate piping | Super Duplex 2507 or 6Mo | >40 |
| SWRO permeate piping | SS 316L or Duplex 2205 | 24–35 |
| MED evaporators (first 2 effects) | Duplex 2205 or Titanium Gr.2 | >35 |
| MSF flash chambers | Duplex 2205 | ~35 |
| Chlorinated seawater (cooling towers, fire systems) | Super Duplex 2507 or 6Mo | >40 |
| Post-treatment product water distribution | SS 304L or 316L | 18–24 |
| Chemical dosing systems (scale inhibitor, chlorine) | Hastelloy C276 or 316L depending on chemical | varies |
The Chlorination Effect
Desalination intake seawater is frequently shock-chlorinated (typically 1–2 ppm Cl₂) to control biofouling. Even at low concentrations, active chlorine significantly increases the corrosivity of seawater toward stainless steel, particularly for grades with PREN below 40. Whenever active chlorine is present in seawater, use super duplex or higher-alloy grades — not Duplex 2205 alone.
Monel 400 in Desalination
Monel 400 (Ni-Cu alloy) has historically been used in flash evaporator tubes and brine heater tubes in MSF plants because of its excellent resistance to seawater and high-temperature brine. While its use has declined with the wider adoption of titanium and duplex, Monel 400 remains specified in retrofit and maintenance projects for existing MSF plants in Saudi Arabia and Kuwait.