How Desalination Plants Work
A desalination plant removes dissolved salts from seawater to produce fresh water. Two technologies dominate: Thermal processes (MSF and MED) and Membrane processes (SWRO). Each process has distinct corrosion environments that demand different stainless steel grades.
Thermal Desalination: MSF and MED
Multi-Stage Flash (MSF) and Multi-Effect Distillation (MED) plants work on the same principle: seawater is heated, partially evaporates, and the vapour is condensed as fresh water. The difference is that MSF uses a series of pressure chambers (stages) at decreasing pressure, while MED uses multiple effects (vapour from one effect is used as the heating medium for the next).
MSF plants typically operate with a top brine temperature (TBT) of 90C-120C. MED plants operate at lower temperatures (60C-90C per effect), which is why MED is generally less corrosive than MSF.
Membrane Desalination: SWRO
Seawater Reverse Osmosis (SWRO) works by applying high pressure (55-80 bar) to seawater across a semi-permeable membrane. The membrane rejects salts; the concentrate (brine, ~70-80 g/L TDS) is discharged at high pressure. SWRO is more energy-efficient than thermal processes and now accounts for the majority of new desalination capacity worldwide.
Table 1: Desalination Technologies - Process Comparison and Corrosion Relevance
| Process | Principle | Typical Size | Energy Consumption | Top Brine Temp (C) | Number of Plants Globally |
MSF (Multi-Stage Flash)
| Evaporation + flash distillation in staged chambers
| 10-75 MLD
| 8-14 kWh/m3 (thermal + electric)
| 90-120
| ~350 (declining)
|
MED (Multi-Effect Distillation)
| Evaporation across multiple effects; vapour reuse
| 10-75 MLD
| 5-10 kWh/m3 (thermal + electric)
| 60-90
| ~200 (stable)
|
SWRO (Seawater RO)
| High-pressure membrane filtration
| 1-500 MLD
| 3-5 kWh/m3 (electric only)
| Ambient (~25C)
| ~17,000 (growing rapidly)
|
FO-MED (hybrid)
| Forward Osmosis + MED hybrid
| 5-30 MLD
| 2-4 kWh/m3
| < 40
| ~10 (early stage)
|
Sludge (Zero Liquid Discharge)
| Brine crystallisation + evaporation
| Project-specific
| 10-20 kWh/m3
| Variable
| ~50 (emerging)
|
The Five Corrosion Zones in a Desalination Plant
Every desalination plant can be divided into five distinct zones, each with different corrosion mechanisms and different stainless steel requirements.
Table 2: The Six Corrosion Zones in Desalination Plants - Key Parameters
| Zone | Location | Temperature (C) | Chloride (ppm) | Oxygen (ppm) | Flow Velocity | Microbiolog Activity | Pitting Risk |
Zone 1: Intake
| Raw seawater intake tunnel, screens, pumps
| 20-32
| 18,000-22,000
| 4-8
| 1-3 m/s (low)
| High (biofilm)
| Moderate (316L OK)
|
Zone 2: Pre-treatment
| Cartridge filters, flocculation tanks, dosing points
| 25-35
| 18,000-22,000
| 6-10
| 1-3 m/s
| Very high
| High (316L marginal)
|
Zone 3: High-Pressure Section
| SWRO: HP pump manifold, concentrate piping (55-80 bar)
| 25-40
| 25,000-35,000 (concentrate)
| < 2 (deoxygenated)
| High (3-5 m/s)
| Low (clean water)
| Very high (SS at risk)
|
Zone 4: Membrane Train
| SWRO: pressure vessels, membrane elements, product water
| 20-30
| < 500 (product water)
| < 0.1 (deoxygenated)
| Low (membrane)
| Low
| Very low (any SS OK)
|
Zone 5: Thermal Section
| MSF/MED: brine heater, evaporator shells, vapour space
| 60-120
| 40,000-70,000 (brine)
| < 0.1
| Low (gravity)
| Low (hot)
| Extreme (SS fails rapidly)
|
Zone 6: Brine Discharge
| Brine outfall tunnel, diffuser, mixing zone
| 30-40
| 50,000-70,000
| Variable
| Low (dilution zone)
| Moderate
| Moderate (dilution helps)
|
Stainless Steel Grades for Desalination Applications
Six stainless steel grades are commonly used in desalination plants, from 304L (lowest cost, lowest performance) to 2507 super duplex (highest cost, highest performance). Selecting the wrong grade wastes money or causes premature failure.
Table 3: Stainless Steel and Titanium Grades for Desalination - Chemistry, PREN, and Application Zone
| Grade (UNS) | Cr (%) | Ni (%) | Mo (%) | N (%) | PREN (approx) | PREN Formula | Max Temp in Seawater (C) | Desalination Use Zone | Relative Cost (vs 304L) |
304L (S30403)
| 18.0-20.0
| 8.0-12.0
| None
| None
| 18-22
| Cr only
| ~40 (limited)
| Zone 4 (product water only)
| 1.0x (base)
|
316L (S31603)
| 16.0-18.0
| 10.0-14.0
| 2.0-3.0
| None
| 24-30
| Cr + 3.3xMo
| ~60
| Zones 1-2 (low-temp, low-Cl)
| 1.3-1.6x
|
904L (N08904)
| 19.0-23.0
| 23.0-28.0
| 4.0-5.0
| None
| 35-40
| Cr + 3.3xMo
| ~80
| Zones 2-3 (chlorinated sections)
| 2.5-3.5x
|
2205 Duplex (S32205)
| 22.0-23.0
| 4.5-6.5
| 3.0-3.5
| 0.14-0.20
| 30-35
| Cr+3.3xMo+30xN
| ~80
| Zone 3 (HP piping, SWRO)
| 2.0-3.0x
|
2507 Super Duplex (S32750)
| 24.0-26.0
| 6.0-8.0
| 3.0-5.0
| 0.20-0.30
| 40-43
| Cr+3.3xMo+30xN
| ~100
| Zone 3 (HP pump, TBH inlets)
| 3.5-5.0x
|
Titanium Gr.2 (R50400)
| None
| None
| None
| None
| ~0 (not Cl-pitting)
| No pitting in seawater
| All temps
| Zone 5 (TBH, MSF/MED heat exchangers)
| 8-15x (but longest life)
|
PREN - The Key Number for Desalination Grade Selection
The Pitting Resistance Equivalent Number (PREN) is the most important single number for selecting stainless steel in chloride environments. It is calculated from the alloy chemistry and predicts the temperature at which pitting will occur in a given chloride concentration.
PREN = %Cr + 3.3 x %Mo + 16 x %N
PREN measures resistance to pitting initiation. A higher PREN means the alloy can withstand a higher chloride concentration or temperature before pitting begins. In seawater (~20,000 ppm Cl-), a PREN of at least 30 is generally recommended for continuous operation at ambient temperature.
Table 4: PREN Ranges and Seawater Application Limits for Stainless Steel Grades
| PREN Range | Grade(s) | Typical PREN | Seawater Application Limit | CIP Critical Temp (C) |
< 25
| 304L, 316L (standard)
| 18-30
| Pits at >40C in seawater
| < 25
|
25-35
| 316L (high-C), 904L, 2205 Duplex
| 30-40
| Safe up to 60-80C in seawater
| 25-35
|
35-43
| 904L, 2507 Super Duplex
| 35-43
| Safe up to 80-100C in seawater; SWRO HP section
| 35-45
|
43+
| 2507, Alloy 254 SMO
| 43-50
| Reserved for extreme conditions
| 45-55
|
Zero pitting risk
| Titanium Grade 2
| N/A (Ti immune to Cl-pitting)
| All seawater temperatures
| All temperatures
|
Stainless Steel Grade Selection by Plant Section
Raw Seawater Intake (Zone 1)
The raw seawater intake is the entry point for seawater. Temperature: 20-32C (varies by location and season). Chloride: 18,000-22,000 ppm. Dissolved oxygen: 4-8 ppm. Flow velocity: 1-3 m/s. The main corrosion risk is microbiologically influenced corrosion (MIC) from biofilm and low-velocity sediment accumulation.
Table 5: Stainless Steel Grade Selection - Raw Seawater Intake (Zone 1)
| Equipment | Recommended Grade | UNS Number | Justification | Alternative Grade |
Intake screens
| 316L
| S31603
| Standard for ambient seawater; adequate for low-temp intake
| 904L if biofouling severe
|
Intake pipe (concrete-lined)
| 316L (liner)
| S31603
| Concrete shell + SS liner; 316L adequate
| 2205 if pipe is fully buried SS
|
Intake pumps (casing)
| 316L or CD4MCu duplex
| S31603 / J93370
| 316L OK for pump casings at ambient temp
| CD4MCu for high-head pumps
|
Seawater storage tank (day tank)
| 316L
| S31603
| Ambient temp; 316L adequate for bulk storage
| 904L for tanks with high Cl-dose
|
Low-pressure wash water piping
| 316L
| S31603
| Low pressure; ambient temp; 316L adequate
| 2205 for larger plants
|
Pre-Treatment Section (Zone 2)
Pre-treatment adds coagulants (FeCl3, polyelectrolyte), acid (H2SO4 or HCl to adjust pH), and biocides ( NaOCl for chlorination, or chloramines). This section has the most complex chemistry in the plant. Chlorine is the key variable: 1-3 ppm continuous chlorination raises the pitting risk significantly for 316L.
Table 6: Stainless Steel Grade Selection - Pre-Treatment Section (Zone 2)
| Equipment | Recommended Grade | UNS Number | Key Corrosion Concern | Alternative Grade |
Cartridge filter vessels
| 316L
| S31603
| Ambient temp; 316L adequate; check Cl dose
| 904L for high-Cl dosing
|
Flocculation tank (if SS-lined)
| 316L
| S31603
| FeCl3 coagulant is mildly corrosive; 316L OK
| 2205 for large tanks
|
Acid dosing line (H2SO4)
| 316L or 904L
| S31603 / N08904
| H2SO4 is reducing acid; 316L OK at low temp; 904L preferred for concentrated acid
| 316L adequate for dilute H2SO4
|
Sodium hypochlorite (NaOCl) line
| 904L or 2205
| N08904 / S32205
| NaOCl (1-3 ppm Cl2 equivalent) causes pitting in 316L above 30C; 904L minimum
| 2205 preferred for 3+ ppm Cl2
|
Anti-scalant dosing line
| 316L
| S31603
| Mild; 316L adequate
| 904L for scale inhibitor with Cl-
|
Degasifier (CO2 removal)
| 316L
| S31603
| Low temp; 316L OK; CO2 is not a corrosion driver
| 904L for high-temp degasifier
|
SWRO High-Pressure Section (Zone 3) - The Critical Zone
The SWRO high-pressure (HP) section is the heart of any SWRO plant and the highest-risk zone for stainless steel corrosion. Seawater enters at ~25C and exits the concentrate stream at 40-50C and 55-80 bar pressure. The chloride concentration in the concentrate (reject stream) reaches 25,000-35,000 ppm Cl-. The combination of high pressure, elevated temperature, and high chloride concentration is extremely aggressive.
Table 7: Stainless Steel Grade Selection - SWRO High-Pressure Section (Zone 3)
| Equipment | Recommended Grade | UNS Number | Critical Design Params | Alternative Grade |
HP pump manifold
| 2507 Super Duplex or 2205
| S32750 / S32205
| 55-80 bar pressure; 25-40C; concentrate stream; highest velocity (3-5 m/s)
| 2205 acceptable for < 70 bar; 2507 required for 70-80 bar
|
High-pressure piping (upstream of membranes)
| 2205 Duplex (S32205)
| S32205
| 55-80 bar; 25-40C; seawater + chlorination residual
| 2507 for high-Cl concentrate section
|
Concentrate piping (post-membrane, high-Cl)
| 2507 Super Duplex
| S32750
| 35,000-40,000 ppm Cl-; 35-50C; high pressure (55-65 bar)
| 2205 acceptable for < 30,000 ppm Cl- and < 35C
|
Pressure vessel (membrane housings)
| 316L or 2205 (internals)
| S31603 / S32205
| Interior wetted surface; product water (< 500 ppm Cl-); 316L OK here
| 2205 for high-salinity feed
|
Energy recovery device (ERI/PTEC)
| Duplex 2205 or 2507
| S32205 / S32750
| High velocity; abrasive particles; chlorination; 55-80 bar
| 2507 preferred for Erosive service
|
Interstage piping (booster pump)
| 2205 Duplex
| S32205
| Medium pressure (35-55 bar); 25-35C
| 316L OK for low-pressure stages
|
Sampling lines
| 316L
| S31603
| Low pressure; ambient temp; sampling only
| 904L for chlorinated samples
|
Thermal Desalination Section (Zone 5) - MSF and MED
The thermal section of MSF and MED plants is the most demanding corrosion environment in any desalination plant. Brine temperatures reach 90-120C (MSF top brine) and 70-90C (MED). Chloride concentrations reach 50,000-70,000 ppm in the concentrate. At these temperatures and chloride concentrations, no austenitic stainless steel (304L, 316L, 904L) is reliable for long-term service. Titanium Grade 2 is the standard for heat exchange tubes in MSF/MED plants.
Table 8: Stainless Steel / Titanium Grade Selection - Thermal Desalination Section (Zone 5)
| Equipment | Recommended Grade | Alternative Grade | Why Not Stainless Steel |
MSF evaporator brine heater (shell)
| Carbon steel (epoxy-lined or rubber-lined)
| -
| High temp (120C) brine; CS adequate with lining
|
MSF heat exchange tubes (primary)
| Titanium Grade 2 (UNS R50400)
| Copper-nickel 90/10 (for lower temp stages)
| No SS survives 90-120C seawater long-term
|
MSF heat exchange tubes (later stages)
| Titanium Grade 2 or CuNi 90/10
| 316L (only for stages below 60C, last 2-3 stages)
| 316L fails in < 3 years above 70C in seawater
|
MED evaporator tubes (effects 1-3)
| Titanium Grade 2
| -
| Effects 1-3 run at 70-90C; no SS adequate
|
MED evaporator tubes (effects 4+)
| CuNi 90/10 or 316L
| Titanium Grade 2 for last effect
| Lower temperature (50-70C); 316L marginally OK
|
Vapour head space (MSF/MED)
| 316L or 2205 (for non-condensable gas zones)
| Titanium Grade 2
| High temp vapour + non-condensables; 316L for condenser shell
|
Brine recirculation pump casings
| CD4MCu (duplex cast) or 316L (coated)
| Titanium Grade 2 (for hot brine)
| Hot brine (80-120C); 316L fails; CuNi inadequate
|
MSF distillate trays
| 316L
| 2205 Duplex (for high velocity)
| 60-90C; 316L adequate; 2205 for high-velocity sections
|
Cost Comparison and Lifecycle
Stainless steel costs more upfront than carbon steel or lined carbon steel, but the total lifecycle cost (LCC) strongly favours stainless steel in desalination applications. This section quantifies the economics.
Table 9: Lifecycle Cost Comparison - Stainless Steels and Alternatives in Seawater Service
| Material / Grade | Density (g/cm3) | Relative Cost Index (Base=CS) | Design Life in Seawater (years) | Annualised Cost Index | Maintenance Cost (per year) |
Carbon Steel (uncoated)
| 7.85
| 1.0x
| 1-3
| HIGH (replacement every 2 yr)
| High (corrosion rate 0.3-1.0 mm/yr)
|
Carbon Steel (rubber-lined)
| 7.85
| 1.5-2.0x
| 10-15
| Moderate
| Low (inspect lining every 5 yr)
|
316L Stainless Steel
| 7.98
| 3.5-4.5x
| 20-30
| Low
| Very low
|
2205 Duplex Stainless Steel
| 7.80
| 5.0-7.0x
| 25-35
| Low
| Very low
|
2507 Super Duplex Stainless Steel
| 7.80
| 8.0-12.0x
| 30-40
| Low
| Negligible
|
Titanium Grade 2
| 4.51
| 15.0-25.0x
| 30-50+
| Low (despite high upfront)
| Negligible (zero active corrosion)
|
CuNi 90/10
| 8.94
| 4.0-6.0x
| 15-25
| Moderate
| Low (minor biofouling)
|
Production and Material Standards for Desalination Stainless Steel
Table 11: Production and Material Standards for Desalination Stainless Steel
| Standard | Full Name | Applies To | Key Requirements | When Required |
ASTM A240/A240M
| Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip
| Sheet and plate for pressure vessels
| Chemistry limits, mechanical properties, hardness, mill test report
| Mandatory for all stainless steel sheet and plate
|
ASTM A789/A789M
| Standard Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Tubing
| Seamless duplex and super duplex tubing
| Chemistry, mechanical properties, HIC test (for sour environments)
| Mandatory for all duplex tube
|
ASTM A790/A790M
| Standard Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Pipe
| Seamless duplex pipe
| Same as A789; plus dimensional tolerances, hydrostatic test
| Mandatory for all duplex pipe
|
ASTM A268/A268M
| Standard Specification for Seamless and Welded Ferritic Stainless Steel Tubing
| Ferritic SS tube (not used in desalination)
| -
| Not applicable
|
ASME BPVC Section II Part A
| ASME Boiler and Pressure Vessel Code, Section II, Part A (Ferrous Materials)
| Materials for ASME-stamped vessels
| Material qualification, heat treatment, testing
| For pressure vessels with ASME stamp
|
EN 10028-7
| Flat Products for Pressure Purposes - Part 7: Stainless Steels
| Plate and sheet for pressure equipment (EU)
| Same as ASTM A240
| EU projects; EN-standard countries
|
ISO 21457
| Petroleum and Natural Gas Industries - Materials for Seawater Injection Systems
| Seawater piping systems (general)
| PREN minimum; corrosion testing requirements
| International oil & gas; desalination reference
|
DNVGL-RP-B101
| Recommended Practice for Corrosion Resistant Alloys in Seawater
| Seawater piping and heat exchangers
| PREN, Mo%, N%; temperature limits; test requirements
| Nordic projects; DNV-classified vessels
|
NACE MR0175 / ISO 15156
| Materials for H2S-Containing Environments
| Sour service desalination (rare)
| Material requirements for H2S service
| Only if H2S present (oil-field water co-production)
|
Frequently Asked Questions
Q: What is the most common stainless steel grade used in desalination plants?
A: 316L stainless steel (UNS S31603) is the most widely used grade in desalination plants, covering approximately 60-70% of all stainless steel usage (intake, pre-treatment tanks, product water piping, and low-pressure sections). For high-pressure SWRO sections and thermal sections, 2205 duplex and Titanium Grade 2 respectively are the standards.
Q: Can 316L stainless steel be used in seawater?
A: Yes, but only in specific conditions: ambient temperature (below 30-40C), low flow velocity (below 2 m/s), no chlorination or low chlorination (below 1 ppm), and no under-deposit conditions. In practice, 316L is adequate for raw seawater intake (Zone 1) and product water sections (Zone 4). It is NOT adequate for chlorinated pre-treatment (Zone 2), SWRO HP sections (Zone 3), or thermal sections (Zone 5).
Q: What is PREN and why does it matter for desalination?
A: PREN (Pitting Resistance Equivalent Number) = %Cr + 3.3 x %Mo + 16 x %N. It predicts the chloride concentration and temperature at which pitting will initiate. In seawater (~20,000 ppm Cl-), a PREN of at least 30 is recommended for continuous operation. 316L (PREN ~24-30) is marginal; 2205 (PREN ~30-35) is adequate; 2507 (PREN ~40-43) is preferred for the most aggressive zones.
Q: Why is titanium used in MSF/MED desalination plants?
A: Titanium Grade 2 (UNS R50400) is immune to pitting and crevice corrosion in seawater at all temperatures up to 260C. At the top brine temperature of MSF plants (90-120C) and MED plants (70-90C), no stainless steel grade can survive long-term. Titanium has zero active corrosion in seawater: the titanium oxide film (TiO2) is completely stable and self-healing if damaged. Its only limitation is cost (8-15x over 316L) and sensitivity to crevice corrosion in stagnant conditions at elevated temperature.
Q: What is the difference between 2205 duplex and 2507 super duplex stainless steel?
A: The key differences are: (1) 2507 has higher chromium (24-26% vs 22-23%), higher molybdenum (3-5% vs 3.0-3.5%), higher nitrogen (0.20-0.30% vs 0.14-0.20%); (2) 2507 has a higher PREN (40-43 vs 30-35); (3) 2507 has higher tensile strength (800 MPa vs 620 MPa for 2205); (4) 2507 is used for the most demanding zones (SWRO HP pump manifolds, concentrate piping at 40C+); 2205 is used for general SWRO HP piping.
Q: How does chlorination affect stainless steel selection?
A: Chlorination (NaOCl dosing) is the most significant operational variable in desalination stainless steel selection. Chlorine raises the oxidation potential of seawater, which accelerates pitting. 316L pitting occurs at 1 ppm Cl2 at 30C and at 0.3 ppm Cl2 at 50C. For continuously chlorinated seawater (2-3 ppm), use 904L minimum (PREN 35-40). For shock chlorination (5-10 ppm), use 2205 duplex or higher.
Q: What causes under-deposit corrosion in desalination plants and how is it prevented?
A: Under-deposit corrosion (UDC) occurs when sediment, scale, or biofilm accumulates on the pipe wall, creating a differential aeration cell (oxygen-depleted zone under the deposit corrodes while the surrounding surface is protected). In desalination, UDC is the primary failure mode for 316L in intake and pre-treatment sections. Prevention: (1) maintain flow velocity above 1.5 m/s to prevent sediment settling; (2) specify smooth-bore piping (sanitary pipe rather than welded pipe with internal bead); (3) use 904L or 2205 in areas prone to deposit accumulation (bends, dead legs, low-velocity sections).
Q: What is the design life of stainless steel in desalination applications?
A: 316L: 20-30 years in Zones 1-2; 2205 duplex: 25-35 years in Zone 3; 2507 super duplex: 30-40 years in Zone 3; Titanium Grade 2: 30-50+ years in Zone 5. Actual service life depends on: temperature, chloride concentration, flow velocity, chlorination regime, and fabrication quality.
