The most expensive component on a Coanda intake screen is the wedge wire panel: precision-formed V-wire with slot tolerances of ±0.1 mm. When that panel corrodes and fails, you do not repair it. You replace it. And you also pay for the intake structure downtime, the lost production, and the engineering hours spent finding out what went wrong.
In almost every case, the failure is caused by one decision: the wrong stainless steel grade for the water chemistry at the site.
This guide explains how to make that decision correctly: using water chemistry data, not assumptions.
Table of Contents
- Why Material Selection Matters for Intake Screens
- The Fundamentals: What Makes Stainless Steel "Stainless"
- 304 vs. 316: The Core Difference
- Understanding the Grade Variants
- Chloride Concentration: The Deciding Factor
- PREN: The Number That Predicts Pitting
- Beyond 316: When You Need Duplex or Higher
- Material Selection Decision Tree
- Cost vs. Risk: The Real Economics
- Frequently Asked Questions
- References
Why Material Selection Matters for Intake Screens
A Coanda intake screen is not a pipe or a plate. It is a hydraulic device built from fine wedge wire: individual V-shaped wires welded to support rods at precise intervals, with slot widths down to 0.5 mm. This geometry creates a very high ratio of exposed surface area to metal volume.
That matters for corrosion because pitting and crevice corrosion attack the surface. More surface area per kilogram of metal means faster degradation when the wrong grade is exposed to corrosive water. A 1.0 mm slot Coanda panel has roughly 3–4 times the exposed wedge wire surface per square metre compared to a flat plate of equivalent thickness.
We have reviewed screens returned from sites where 304 was chosen for water with chloride levels above 500 ppm. After 18–24 months in operation, the wedge wire cross-section was visibly reduced by pitting. Slot widths had widened from 1.0 mm to 1.4 mm, destroying both the hydraulic performance and the fish protection compliance of the screen.
The material selection must be right the first time. There is no coating, treatment, or retrofit that corrects a wrong grade choice once the screen is in operation.
The Fundamentals: What Makes Stainless Steel "Stainless"
All stainless steels resist corrosion through a passive chromium oxide layer: a self-healing film approximately 1–5 nanometres thick that forms spontaneously when chromium content exceeds approximately 10.5% [1]. This passive layer is what separates stainless steel from carbon steel, which rusts freely in water.
The passive layer is not permanent. It can be broken by:
- Chloride ions: which penetrate and destabilise the film, causing localised pitting
- Low pH: acidic conditions thin the passive layer
- Elevated temperature: accelerates all corrosion mechanisms
- Crevice geometry: oxygen depletion inside crevices prevents the film from re-healing
In water intake applications, chloride is the dominant threat. And the stainless steel grade determines how much chloride the passive layer can withstand.
304 vs. 316: The Core Difference
Both 304 and 316 are austenitic stainless steels: the most widely used family for water applications. Their compositions are similar, with one critical exception:
| Element | 304 | 316 |
|---|---|---|
| Chromium (Cr) | 18–20% | 16–18% |
| Nickel (Ni) | 8–10.5% | 10–14% |
| Molybdenum (Mo) | None | 2–3% |
| Carbon (C) | ≤0.08% | ≤0.08% |
The difference is molybdenum. The 2–3% molybdenum in 316 stabilises the passive layer against chloride attack, dramatically increasing resistance to pitting and crevice corrosion [2][3].
How dramatic? At a chloride concentration of 300 ppm, type 304 has a Critical Pitting Temperature (CPT) of just 40°C. At 500 ppm chloride (nearly double the concentration) type 316 still has a CPT of 70°C [2]. Molybdenum does not just add incremental resistance. It widens the whole range of conditions the steel can withstand.
This is why 316 is sometimes called "marine grade" stainless steel, though that term can be misleading: 316 is adequate for coastal and near-seawater applications, but not for continuous submersion in full seawater.
Understanding the Grade Variants
ADENCO manufactures Coanda screens in six stainless steel grades: 304, 304L, 316, 316L, duplex 2205, and super duplex 2507. The letter suffixes and variant names are not marketing labels: each solves a specific engineering problem, and it is worth understanding the variants you will see in tender specifications even where ADENCO's range answers the need differently.
The "L" Grades: 304L and 316L
The "L" stands for Low Carbon. Standard 304 and 316 contain up to 0.08% carbon. The L variants limit carbon to 0.03% maximum [4].
Why this matters: when stainless steel is heated to 480–820°C during welding, carbon migrates to grain boundaries and combines with chromium to form chromium carbides. This process (called sensitisation) depletes chromium from the zone adjacent to the grain boundary, creating a narrow zone with insufficient chromium to maintain the passive layer. The result is intergranular corrosion: attack along the grain boundaries that can cause cracking and structural failure months or years after fabrication [4][5].
With 0.03% maximum carbon, the L grades produce far fewer chromium carbides during welding. For any Coanda screen that will be welded during fabrication or installation (which is virtually all of them) the L grades are the choice where weld-zone corrosion resistance is a specified requirement; ADENCO supplies them on request alongside the standard 304.
316Ti: Titanium Stabilised
316Ti adds titanium to the 316 composition. Titanium has a stronger affinity for carbon than chromium does: it preferentially forms titanium carbides instead of chromium carbides during heating [6]. This provides a second line of defence against sensitisation, making 316Ti suitable for applications involving:
- Repeated thermal cycling (e.g., screens with electric anti-icing heating elements)
- Prolonged exposure to temperatures between 550°C and 800°C
- Post-weld heat treatment requirements
For most water intake applications at ambient temperatures, 316L provides sufficient sensitisation resistance, which is why ADENCO's range covers the low-carbon L grades rather than titanium-stabilised variants.
321: Titanium Stabilised (without Molybdenum)
321 is essentially 304 with titanium stabilisation. It provides excellent resistance to intergranular corrosion and high-temperature oxidation (up to 870°C) but lacks the molybdenum content that gives 316 its chloride resistance [6]. 321 is not part of ADENCO's range: for freshwater sites where thermal cycling matters, the low-carbon L grades cover the requirement.
304HC: High Carbon
304HC contains higher carbon (0.04–0.10%) for increased strength. Some fabricators use it in structural support components (not the wedge wire screening surface) where weldability is less critical and mechanical strength is the priority; ADENCO's range does not include it.
Chloride Concentration: The Deciding Factor
For water intake screen material selection, the single most important data point is the chloride ion concentration of the raw water. Everything else (temperature, pH, flow velocity, dissolved oxygen) modifies the chloride threshold, but chloride decides the grade.
The Nickel Institute and industry practice establish these approximate chloride limits for continuous exposure at neutral pH and ambient temperature [2][7][8]:
| Stainless Steel Grade | Maximum Chloride (ppm) | Typical Water Bodies |
|---|---|---|
| 304 / 304L | 200 | Clean rivers, reservoirs, mountain streams, groundwater |
| 316 / 316L | 1,000 | Estuaries, coastal rivers, brackish groundwater, streams affected by road salt |
| Duplex 2205 | 3,600 | High-salinity estuaries, industrial process water |
| Super-austenitic (e.g., 904L) | 8,500 | Intakes influenced by seawater, desalination pre-filtration |
Critical modifiers: these limits decrease when:
- Temperature exceeds 25°C: every 10°C increase roughly halves the safe chloride limit
- pH is below 6: acidic conditions thin the passive layer
- Crevices are present: stagnant zones deplete oxygen and concentrate chloride
- Flow velocity is very low: insufficient to flush corrosion products from the surface
A river that measures 150 ppm chloride in March may reach 300 ppm during low summer flow: safely within 304 range in spring, but above the pitting threshold by August. Material selection must be based on worst-case water chemistry, not averages.
PREN: The Number That Predicts Pitting
The Pitting Resistance Equivalent Number (PREN) is a single-value index calculated from alloy composition that predicts relative pitting resistance [9]:
PREN = %Cr + 3.3 × %Mo + 16 × %N
Higher PREN means greater pitting resistance. The 3.3× multiplier on molybdenum shows why even 2–3% Mo has such a large effect:
| Grade | Typical PREN | Relative Resistance |
|---|---|---|
| 304 / 304L | 18–20 | Baseline |
| 316 / 316L | 24–28 | ~40% higher than 304 |
| 316Ti | 23–28 | Similar to 316L |
| Duplex 2205 | 33–35 | ~75% higher than 304 |
| Super-duplex 2507 | 40–43 | ~120% higher than 304 |
PREN is a useful preliminary selection tool, but it has limitations. It does not account for microstructure, surface finish, crevice geometry, or real-world water chemistry variability. ADENCO uses PREN as a first check, then checks the selection with the full water chemistry data and site conditions.
Beyond 316: When You Need Duplex or Higher
For most freshwater and mildly brackish applications, the choice is between 304L and 316L. But some intake sites go beyond the limits of 316:
Duplex 2205 combines austenitic and ferritic microstructures, delivering roughly twice the yield strength of 316L with superior chloride resistance (PREN 33–35). ADENCO selects duplex for:
- Estuarine sites with tidal chloride variation exceeding 1,000 ppm
- Warm-water intakes (>30°C) with moderate chloride
- Applications requiring higher mechanical strength with thinner wedge wire cross-sections
Super-austenitic grades (904L, 254 SMO) push chloride tolerance to 8,500–15,000 ppm and are used for intakes influenced by seawater and for desalination pre-filtration [8]. These grades are significantly more expensive and are selected only when duplex does not meet the corrosion requirement.
The extra cost of higher grades is real, but it is always less than the cost of replacing a corroded screen and the associated downtime.
Material Selection Decision Tree
ADENCO uses the following decision process for every project:
Step 1: Obtain Water Chemistry
Request a complete water analysis from the site, including chloride concentration (ppm), pH, temperature range (seasonal), and dissolved oxygen. If analysis is unavailable, ADENCO can advise on sampling protocol.
Step 2: Identify Worst-Case Chloride
Use the maximum recorded chloride concentration: typically during low-flow summer conditions or drought. If only a single measurement is available, apply a 1.5× safety factor.
Step 3: Match Grade to Chloride
- Chloride <200 ppm → 304 (freshwater standard; 304L on request)
- Chloride 200–1,000 ppm → 316L (brackish/estuarine)
- Chloride 1,000–3,600 ppm → Duplex 2205 (high-salinity estuarine)
- Chloride >3,600 ppm → Super-austenitic or super-duplex (consult ADENCO engineering)
Step 4: Apply Modifiers
- If water temperature exceeds 30°C → select the next higher grade
- If pH is below 6.0 → select the next higher grade
- If anti-icing heating is required → the low-carbon L grades resist sensitisation from thermal cycling
- If both temperature and pH modifiers apply → select a grade two steps higher
Step 5: Verify with PREN and CPT Data
Cross-check the selected grade's PREN value and Critical Pitting Temperature with the real water conditions. If the operating temperature approaches the CPT for the selected grade, select the next higher grade.
This process takes ADENCO's engineering team less than one hour when water chemistry data is available, and it would have prevented every material-related failure we have seen since ADENCO began manufacturing Coanda screens in 2013.
Cost vs. Risk: The Real Economics
The cost difference between 304L and 316L wedge wire is approximately 30–50% at current market prices (304L at $2.50–3.50/kg vs. 316L at $3.50–5.00/kg raw material). For a typical single-panel Coanda screen the 316L upgrade is a small share of the screen price [10].
What does the wrong grade cost?
| Failure Scenario | Typical Cost |
|---|---|
| Pitting causes slot widening → fish protection non-compliance | Screen replacement + regulatory delay |
| Crevice corrosion at support rod junctions → structural failure | Emergency replacement + downtime |
| Intergranular corrosion from sensitised welds → panel fracture | Full screen replacement |
| Accelerated corrosion in thermal cycling zones (anti-icing) | Panel replacement + anti-icing redesign |
The material upgrade from 304L to 316L costs a small fraction of the screen. A corrosion failure costs the screen itself, plus the downtime. On every project where there is any doubt about chloride, choose 316L: the cost of being wrong with 304 is many times higher than the cost of the upgrade.
Frequently Asked Questions
Which stainless steel is best for water intake screens?
The best grade depends entirely on your raw water chemistry. For clean freshwater with chloride below 200 ppm, 304L provides excellent corrosion resistance at the lowest cost. For estuarine, coastal, or brackish water with chloride between 200 and 1,000 ppm, 316L is required. For higher chloride concentrations, duplex 2205 or super duplex grades are needed. ADENCO's engineering team selects the grade based on a complete water chemistry analysis for every project.
What is the difference between 304 and 304L stainless steel for intake screens?
The difference is carbon content. Standard 304 allows up to 0.08% carbon, while 304L limits carbon to 0.03% maximum. This lower carbon content prevents sensitisation (the formation of chromium carbides at grain boundaries during welding) which can cause intergranular corrosion. Because all Coanda screen panels are welded during fabrication, ADENCO offers the L grades on request for sites where weld-zone corrosion resistance is a specified requirement; the standard grade is 304.
Can I use 304 stainless steel in brackish water?
No. Water with chloride concentration above 200 ppm will cause pitting corrosion on 304 stainless steel, particularly during warm months when water temperature rises. The pitting widens screen slots, reduces hydraulic performance, and compromises fish protection compliance. For brackish water (200–1,000 ppm chloride), 316L is the minimum acceptable grade. For higher salinity, duplex or super duplex grades are required.
Why does ADENCO offer six stainless steel grades?
Different water bodies present different corrosion challenges. A mountain stream feeding a micro-hydro system (clean freshwater, <50 ppm chloride) has completely different material requirements than a tidal estuary supplying a municipal intake (500–2,000 ppm chloride with temperature variation). By offering 304, 304L, 316, 316L, duplex 2205, and super duplex 2507, ADENCO can match the material precisely to each site: optimising both corrosion resistance and cost. No project pays for a higher grade than it needs, and no project receives a lower grade than it needs.
What is PREN and why does it matter for intake screens?
PREN (Pitting Resistance Equivalent Number) is calculated from the alloy's chromium, molybdenum, and nitrogen content using the formula: PREN = %Cr + 3.3 × %Mo + 16 × %N. It predicts relative resistance to pitting corrosion: the primary failure mode for stainless steel in chloride-containing water. 304L has a PREN of 18–20, while 316L reaches 24–28 and duplex 2205 reaches 33–35. ADENCO uses PREN as a preliminary selection tool, then checks the result with the full water chemistry data.
How do I know the chloride level of my raw water?
A standard water chemistry analysis from any accredited laboratory will report chloride concentration in ppm (mg/L). The critical requirement is to sample during worst-case conditions (typically late summer during low flow) because chloride concentrations increase as river flow decreases. If only one sample is available, ADENCO recommends applying a 1.5× safety factor. If you are unsure about sampling, ADENCO's engineering team can advise on protocol and timing.
Does 316L resist seawater?
316L resists coastal and near-seawater conditions: splash zones, coastal humidity, and water with chloride up to approximately 1,000 ppm. It does not resist continuous immersion in full seawater (approximately 19,000 ppm chloride). For intakes influenced by seawater, duplex 2205 (up to 3,600 ppm) or super duplex grades are required. ADENCO's engineering team will select the appropriate grade based on your site's real chloride exposure.
Is the cost difference between 304L and 316L significant?
For the wedge wire panels, 316L adds approximately 30–50% to the raw material cost compared to 304L. This is a small part of the total project cost (which includes the concrete works, installation, and commissioning) and is negligible compared to the cost of replacing a corroded screen. When water chemistry data shows any chloride risk, the 316L upgrade is always justified economically.
References
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International Stainless Steel Forum (ISSF). "The Stainless Steel Family." Retrieved April 2026, from https://www.worldstainless.org/
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"Susceptibility of Type 304/304L and 316/316L Austenitic Stainless Steels to Chlorides in Cooling Water." Digital Refining. Retrieved April 2026, from https://www.digitalrefining.com/article/1002873/
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British Stainless Steel Association. "Selection of 316, 304, and 303 Types of Stainless Steels for Seawater Applications." Retrieved April 2026, from https://bssa.org.uk/bssa_articles/selection-of-316-304-and-303-types-of-stainless-steels-for-seawater-applications/
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"304 vs. 304L Stainless Steel: Understanding the Low-Carbon Variant." Mill Steel Company. Retrieved April 2026, from https://www.millsteel.com/news/304l-stainless-steel-vs-304-which-grade-is-right-for-you
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"304 vs. 304L: An Answer to Carbide-Precipitation Issues." Kay & Associates. Retrieved April 2026, from https://kaybrazing.com/brazing-articles/1000901-304-vs-304l-an-answer-to-carbide-precipitation-issues/
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"316Ti vs 321 Stainless Steel High-Temp Performance Analysis." Gangsteel. Retrieved April 2026, from https://gangsteel.net/News/316Ti_vs_321_Steel_High-Temp_Performance.html
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"Chloride and Chlorine Levels and Stainless Steel Alloy Selection." Penflex Engineering Bulletin #105. Retrieved April 2026, from https://www.penflex.com/news/chloride-chlorine-levels-and-stainless-steel-alloy-selection/
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"Stainless Steel Well Screen Materials: A Dynamic Study." Guangxing Water Well Screens. Retrieved April 2026, from https://www.gxscreen.com/water-well-screen/well-screen-materials-study/
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British Stainless Steel Association. "Calculation of Pitting Resistance Equivalent Numbers (PREN)." Retrieved April 2026, from https://bssa.org.uk/bssa_articles/calculation-of-pitting-resistance-equivalent-numbers-pren/
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Wahl, T.L. (2003). Design Guidance for Coanda-Effect Screens. U.S. Bureau of Reclamation, Research Report R-2003-03. Denver, CO.
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"The Resistance of Types 304 and 316 Stainless Steels to Crevice Corrosion in Natural Waters." Journal of Materials for Energy Systems, Springer. Retrieved April 2026, from https://link.springer.com/article/10.1007/BF02835718
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Nickel Institute. "Resistance of Stainless Steel to Corrosion in Naturally Occurring Waters." Retrieved April 2026, from https://nickelinstitute.org/media/8d91ba682b1c1d5/ni_inco_1262_resistanceofstainlesssteeltocorrosioninnaturallyoccurringwaters.pdf
Published by ADENCO: Advanced Engineering Coanda Intake Screens. ADENCO manufactures Coanda screens in six stainless steel grades to match every type of water chemistry, from clean mountain streams to brackish estuaries. Every screen is designed for your exact site conditions. Request a material consultation: tell us your water chemistry.