When a plain austenitic stainless steel is the right material for a Coanda intake screen, life is simple. 304 is ADENCO's standard grade for fresh water: well-understood corrosion behaviour, abundant supply, well-priced relative to alternatives, weldable by every fabricator. Where the water carries chloride or seawater influence, 316L is selected instead. For fresh-water applications (fresh-water hydropower, mountain-stream diversions, agricultural intakes, low-chloride municipal water) one of these two grades is the answer.
This post focuses on water conditions that exceed the limits of both materials. When water chemistry crosses certain thresholds (sustained chloride exposure, seawater exposure, high-chloride industrial water, certain geothermal waters) 316L begins to corrode. The screen develops pitting corrosion, then crevice corrosion, then in extreme cases stress-corrosion cracking. A 25-year design life shrinks to 5-10 years, and the replacement costs become very high.
For these chemistries, the next step up is duplex stainless steel: Duplex 2205 for moderate chloride exposure, Super Duplex 2507 for severe chloride exposure. Both are mature, well-characterized alloys with mature supply chains. Both cost more than 316L. And both are routinely worth the cost premium when the water chemistry requires them.
This post is a practical guide for engineers writing the material specification. When does 316L stop being enough? What's the corrosion threshold for Duplex 2205? When do you need to step up to Super Duplex 2507? And what does the cost vs lifetime trade-off really look like?
When 316L stops being enough
The most useful single number for predicting the corrosion behaviour of stainless steel is the Pitting Resistance Equivalent Number (PREN). It's a weighted sum of the alloy's corrosion-fighting elements:
PREN = %Cr + 3.3 × %Mo + 16 × %N
For typical alloys:
- 304: PREN ≈ 19. Suitable for fresh water, low-chloride exposure only.
- 316L: PREN ≈ 25. Suitable for most fresh water and low-to-moderate chloride exposure.
- Duplex 2205: PREN ≈ 35. Suitable for brackish water and moderate chloride exposure.
- Super Duplex 2507: PREN ≈ 42. Suitable for seawater and high-chloride conditions.
PREN is one input. The other big input is Critical Pitting Temperature (CPT): the temperature at which the alloy starts to show pitting corrosion in a standardised chloride solution. Higher CPT = better hot-chloride resistance.
For practical Coanda intake design, the chloride thresholds are usually as follows:
| Chloride exposure | Recommended default steel grade |
|---|---|
| Fresh water (< 200 mg/L Cl⁻), ambient temperature | 304 (standard) |
| Sediment-laden fresh water (< 200 mg/L Cl⁻), ambient | 304 (chloride, not sediment, is what moves the choice to 316L) |
| Brackish or estuarine (200-2,500 mg/L Cl⁻), ambient | Duplex 2205 |
| Seawater (~19,000 mg/L Cl⁻) | Super Duplex 2507 |
| High-chloride industrial / geothermal (variable, often elevated temp) | Super Duplex 2507 |
The table above is a starting point, not a substitute for engineering review. Several factors push the threshold one way or the other:
- Operating temperature. Hotter water is more corrosive. A 25°C tropical river behaves differently from a 4°C alpine stream at the same chloride level.
- Sediment loading. Erosive abrasion strips passivating chromium oxide layers, accelerating pitting corrosion. Sediment-heavy water is more corrosive than the chloride number alone suggests.
- Stagnation cycles. Coanda screens that see intermittent flow (snowmaking, seasonal irrigation) experience wet-dry cycles that concentrate chlorides locally and accelerate localised corrosion.
- Galvanic exposure. Systems that combine different metals create galvanic cells.
If the project chemistry lies in a borderline zone, a review by an ADENCO engineer with a real water analysis quickly recovers its cost. The cost difference between 316L and Duplex 2205 across a single Coanda screen is meaningful, but it's a fraction of the cost of premature replacement.
Duplex 2205: what it is, where it fits
Duplex 2205 is a "duplex" alloy: its microstructure is roughly half austenite (the structure of 316L) and half ferrite. The two-phase microstructure gives it both better corrosion resistance than 316L and roughly twice the yield strength.
Composition (typical):
- Chromium: 22%
- Nickel: 5-6%
- Molybdenum: 3%
- Nitrogen: 0.14-0.20%
- PREN: 34-36
What it's good for:
- Brackish water applications (estuarine intakes, partial seawater mixing zones)
- Sediment-laden water with moderate chloride content
- Industrial process water with chloride contamination
- Coastal raw-water intakes that don't see full seawater
What it's not for:
- True seawater conditions (chloride > 15,000 mg/L sustained). Duplex 2205 holds up better than 316L but still develops pitting corrosion eventually. Super Duplex is the right choice.
- Elevated temperature with high chloride. The CPT advantage over 316L narrows as temperature rises.
Cost premium over 316L: typically 1.5-2x the material cost. Total screen cost premium is usually 30-60% because labour and fabrication costs do not rise in proportion to the material price. Numbers vary with market conditions; don't quote these as fixed.
Welding and fabrication: Duplex requires more careful welding than 316L. The two-phase microstructure is sensitive to heat input: too much heat shifts the phase ratio and degrades the corrosion resistance. ADENCO's robotic welding maintains the controlled heat input needed for Duplex panels. On-site welding of Duplex requires qualified procedures and welders.
Real ADENCO use case: brackish-water municipal intake on the Black Sea coast. The original specification required 316L based on a default assumption; water analysis revealed elevated chloride from saltwater intrusion. We changed the specification to Duplex 2205. Material cost went up; expected service life increased by a much greater proportion.
Super Duplex 2507: what it is, where it fits
Super Duplex 2507 is the next tier up. Same two-phase austenite-ferrite microstructure as 2205, but more heavily alloyed for severe chloride conditions.
Composition (typical):
- Chromium: 25%
- Nickel: 7%
- Molybdenum: 3.5-4%
- Nitrogen: 0.24-0.32%
- PREN: 41-43
What it's good for:
- Seawater desalination intakes (the canonical use case)
- High-chloride industrial water (some geothermal applications, certain process waters)
- Severe marine conditions with sustained seawater contact
- High-temperature high-chloride combinations
What it's not for, usually:
- Fresh water and brackish water applications. The chemistry doesn't justify the cost premium. Use 304, 316L or Duplex 2205 as the chloride level dictates.
Cost premium over 316L: typically 2.5-4x the material cost. Total screen cost premium is in the range of 60-120%. As with 2205, exact numbers depend on market conditions for the alloying elements (especially nickel and molybdenum prices).
Welding and fabrication: more demanding than 2205. Tighter heat-input control. Specific welding consumables matched to the alloy. An inspection certificate to EN 10204 3.1 is mandatory; the mill test reports are included in the project documentation.
The main market: desalination in the Middle East and North Africa (MENA). Most coastal desalination projects in Saudi Arabia, the UAE, and along the North African coast require Super Duplex for any stainless steel components in contact with seawater. Coanda first-stage intakes for desalination plants fall squarely in this category.
Material decision matrix
The shortest path from "what water do I have" to "what alloy do I order" is a decision matrix. The rows are water chemistries; the columns are application contexts; the cells are the recommended steel grade.
| Water chemistry | Hydropower / SHP (small hydropower) | Drinking water | Industrial process | Desalination |
|---|---|---|---|---|
| Fresh water (< 200 mg/L Cl⁻) | 304 | 304 | 304 | n/a |
| Sediment-laden fresh water | 304 | 304 | 304 | n/a |
| Brackish (200-2,500 mg/L Cl⁻) | 316L if cool, Duplex 2205 if warm | Duplex 2205 | Duplex 2205 | n/a |
| Coastal estuarine (intermittent saltwater) | Duplex 2205 | Duplex 2205 | Duplex 2205 | Duplex 2205 (pre-treatment) |
| Seawater (~19,000 mg/L Cl⁻) | n/a | n/a | Super Duplex 2507 | Super Duplex 2507 |
| High-chloride industrial / geothermal | n/a | n/a | Super Duplex 2507 | n/a |
Cells marked 'n/a' indicate combinations that are unusual in practice (hydropower plants do not normally operate with seawater).
The matrix is a starting point. Borderline cases (chloride near a threshold, elevated temperature, sustained sediment) require engineer review with a real water analysis, not a chart lookup.
Cost vs lifetime trade-off
The simplest way to frame the materials decision is initial cost vs replacement avoidance.
A Coanda screen lasts up to 25 years in clean water when the material is right for the chemistry; in flood-prone, high-sediment rivers 15 to 20 years is the more realistic figure, with performance declining before function is lost. Choose too low a steel grade (say 316L for brackish water that requires Duplex 2205) and the same screen lasts 8-12 years. The replacement cost includes:
- A new screen (similar to the original, sometimes more expensive due to schedule pressure).
- Concrete and installation work to remove the corroded panel and install the replacement.
- Lost power generation or water supply during the replacement.
- Operator labour and project management overhead.
When you compare the upfront premium for the right steel grade against the discounted cost of premature replacement, choosing the right steel grade wins almost every time the chemistry justifies it. The trade-off only goes the other way for marginal cases: chemistry barely on the fence between two grades, where conservative engineering favours the higher steel grade if there's any uncertainty about the analysis.
The wrong question is "what's the cheapest material that will work." The right question is "which material does my water chemistry require, and is the cost premium acceptable for the design lifetime I need." Most project owners and EPC (engineering, procurement and construction) contractors answer the right question quickly when it's framed that way.
ADENCO material recommendations by application
We default to specific steel grades by application based on the typical chemistries we see. These are starting recommendations; project-specific water analysis is always the ultimate authority.
| Application | Default ADENCO recommendation |
|---|---|
| Run-of-river hydropower (mountain stream) | 304 |
| Run-of-river hydropower (silt-heavy lowland) | 304 (316L where chloride is elevated; occasionally Duplex 2205 for unusual chemistry) |
| Drinking water: municipal (fresh water) | 304 |
| Drinking water: coastal or brackish water | Duplex 2205 |
| Drinking water: seawater desalination first-stage | Super Duplex 2507 |
| Industrial process: fresh-water cooling | 304 |
| Industrial process: brackish or contaminated cooling | Duplex 2205 |
| Geothermal raw water (high chloride) | Super Duplex 2507 |
| Snowmaking: alpine fresh water | 304 |
| Agricultural irrigation: fresh diversion | 304 (316L where fertiliser salts or road salt have raised the chloride) |
Where you don't see your application in the table, contact engineering with a water analysis and we will recommend a steel grade.
What to send for an engineer-reviewed material recommendation
When the chemistry is borderline or unusual, an engineer-reviewed material recommendation is worth more than any chart. To get one, send us:
- Water analysis lab report with at minimum: pH, chloride (mg/L), total dissolved solids, sulfate, conductivity, and total suspended solids.
- Operating temperature range of the raw water across seasons.
- Application context: what's the water for, what equipment is downstream.
- Operating mode: continuous operation, intermittent (e.g. snowmaking), or seasonal.
- Design lifetime expectation: 20 years, 30 years, 40+ years.
A material recommendation comes back inside 1-2 business days. There's no charge for the recommendation itself.
The right material is the one your water chemistry really requires. 304 for fresh water. 316L where chloride or seawater influence enters the picture. Duplex 2205 for brackish and high-chloride water. Super Duplex 2507 for severe marine service. When the chemistry is borderline, ask. The cost of asking is zero; the cost of a wrong choice is close to the price of a new screen.