Most evaporator failures I get called in to look at are not process failures. They are material failures. The wrong alloy gets specified at the proposal stage, and eighteen months later someone is cutting out a heat exchanger bundle because the tubes are pitted through. The three alloys you will see quoted most often for industrial evaporator wetted parts are 316L stainless steel, duplex stainless steel (typically 2205), and titanium (Grade 2). Choosing between them comes down to four things: chloride concentration, pH, operating temperature, and how often the unit will be cleaned. This article walks through how I actually make that call on real projects, including the cases where the cheaper alloy is the correct answer and the cases where it is a costly mistake.
Start With the Chemistry, Not the Price Tag
Material selection for an evaporator is a corrosion problem before it is a cost problem. The evaporator concentrates everything that was dissolved in the feed. Chlorides, sulfates, fluorides, and organics all increase in concentration as water leaves the system, and the boiling temperature adds a thermal acceleration factor on top of that.
Before I look at any alloy table, I want the following from the client:
- Complete ion analysis of the feed, not just TDS and conductivity
- Chloride concentration at the concentrated end, not the feed end
- pH range across the full operating cycle, including cleaning
- Maximum metal wall temperature, not just the boiling point
- Presence of fluorides, bromides, ammonia, or oxidizing agents
- Cleaning chemicals and frequency
That last point catches people out constantly. A unit running neutral pH wastewater can still destroy 316L if the CIP program uses an acidic descaling agent at 60 °C. The alloy has to survive the cleaning cycle, not just the process.
The Three Alloys in Practical Terms
316L Stainless Steel
316L is the default starting point for a reason. It is weldable, widely available, reasonably priced, and easy to fabricate. The molybdenum content gives it better chloride resistance than 304, and the low carbon grade reduces sensitization at weld seams.
Its limits are well documented. Above roughly 60 °C, 316L becomes progressively vulnerable to chloride pitting and stress corrosion cracking. The practical ceiling most engineers work to is around 200 mg/L chloride at moderate temperature, dropping sharply as temperature rises. I have seen 316L heat exchangers fail in under a year at 500 mg/L chloride and 90 °C. I have also seen it run for fifteen years at 150 mg/L chloride and 55 °C with no issues. Temperature matters as much as concentration.
Duplex 2205
Duplex 2205 sits between 316L and the high-nickel alloys in both performance and price. Its two-phase microstructure gives it roughly twice the yield strength of austenitic stainless, which means thinner walls for the same pressure rating and often a lower total material weight.
On corrosion, duplex handles chloride stress corrosion cracking far better than 316L. It resists pitting in chloride environments at higher temperatures and concentrations, and it stands up well to the mildly acidic conditions common in industrial wastewater. The trade-off is fabrication. Duplex needs controlled heat input during welding, correct filler selection, and proper interpass temperature control. If your fabricator has never welded duplex, that is a real risk, not a theoretical one.
For a broader look at how these decisions play out across different evaporator configurations, the wastewater evaporator overview is a reasonable starting reference.
Titanium Grade 2
Titanium is the answer when chlorides are high and the environment is oxidizing. It forms a tenacious passive oxide layer that survives chloride concentrations where stainless steels simply cannot operate. In seawater and high-chloride brine service, titanium heat exchanger tubing is standard practice.
The catch is that titanium has specific weaknesses that catch people off guard. It performs poorly in reducing acids and in anhydrous or oxygen-starved conditions. It is vulnerable to fluoride attack even at low concentrations. It has poor thermal conductivity compared to stainless, which means more surface area for the same duty. And it costs significantly more per kilogram, with longer lead times and a smaller pool of qualified fabricators.
Side-by-Side Comparison
| Property | 316L | Duplex 2205 | Titanium Grade 2 |
|---|---|---|---|
| Chloride resistance (typical working limit) | Low, roughly <200 mg/L at moderate temp | Moderate, roughly 200–1000 mg/L depending on temp | High, suitable for seawater-level chloride |
| Temperature ceiling for chloride service | About 60 °C | About 100–120 °C | Limited mainly by process, not chloride |
| Pitting and crevice corrosion | Susceptible above threshold | Good resistance | Excellent resistance |
| Stress corrosion cracking | High risk in hot chloride | Low risk | Not a typical failure mode |
| Fluoride tolerance | Moderate | Moderate | Poor |
| Thermal conductivity | Reference | Slightly lower than 316L | Roughly half of 316L |
| Relative material cost | Low | Medium | High |
| Fabrication difficulty | Easy | Moderate, needs qualified welders | Moderate to high |
| Typical best-fit applications | Low-chloride, neutral pH, moderate temperature | Mid-chloride, wide pH, higher temperature | High chloride, oxidizing conditions, seawater |
The numbers in that table are indicative ranges, not hard limits. Every project needs its own corrosion assessment. The thresholds shift with pH, dissolved oxygen, flow velocity, and the presence of trace species.
Where Temperature Changes Everything
The single biggest mistake I see is selecting material based on feed chemistry alone. The evaporator does not operate at feed chemistry. It operates at the concentrated end, and that is where corrosion happens.
A feed with 300 mg/L chloride that concentrates tenfold puts 3,000 mg/L chloride against the heat exchanger wall. At 80 °C, that is well outside 316L territory. Duplex might hold. Titanium will hold comfortably.
This is why I always ask for the concentration factor and the maximum wall temperature before recommending anything. On a mechanical vapor recompression system, the compressor adds heat and the boiling point elevation can push wall temperatures meaningfully above the nominal boiling point. That extra 10 or 15 degrees is often the difference between a 316L bundle lasting ten years and lasting eighteen months.
If you are working through a concentration project and want a second opinion on the wetted material call, that is the kind of thing worth talking through early rather than after the purchase order. Our engineering team reviews material selection against your actual water analysis before anything gets fabricated.
Cleaning Chemistry Is Part of the Design Basis
I have watched a plant destroy a perfectly good duplex heat exchanger with an aggressive acid clean. The process side was fine. The cleaning side was not.
Here is what I tell clients to check:
- Acidic descaling agents at elevated temperature attack the passive layer on stainless and duplex. Titanium tolerates many of these better, but not all.
- Caustic cleaning is generally safe for all three, but watch for caustic stress corrosion cracking in stainless at high temperature and concentration.
- Chlorinated cleaning agents are the fastest way to pit 316L. Even brief exposure at the wrong concentration can initiate pits that grow in service.
- Fluoride-containing cleaners will attack titanium. This is a common and expensive oversight.
The cleaning program and the material selection have to be developed together. If your operations team is going to use a particular descaling product, the alloy has to be chosen around it.
How I Actually Decide
After enough projects, the decision tree becomes fairly mechanical.
- Get the concentrated-end chemistry and the maximum wall temperature. Everything else follows from these two numbers.
- If chloride is low and temperature is moderate, use 316L. Do not over-specify. The savings are real and 316L is easier to fabricate and repair.
- If chloride is moderate or temperature is high, move to duplex. This is the sweet spot for a lot of industrial wastewater, including many plating and chemical streams.
- If chloride is high, or the stream is seawater-derived, or the temperature is high enough that duplex is marginal, go to titanium. Pay the premium once rather than replacing a bundle twice.
- Check the cleaning chemicals against the chosen alloy. If there is a conflict, either change the alloy or change the cleaning program.
- Confirm the fabricator can actually work with the material. A duplex design that gets welded badly is worse than a 316L design that gets welded well.
There is a related consideration for systems that combine membrane concentration with thermal finishing. On a zero liquid discharge train, the evaporator sees the reject from the membrane stage, which is already at high concentration. That changes the material call. The ZLD system design page covers how the stages interact, and the heat exchanger selection discussion goes deeper on the thermal side.
Common Failure Modes and What They Tell You
Pitting Under Deposits
This is the classic 316L failure. Scale or organic deposits create stagnant zones where chloride concentrates and oxygen is depleted. The pit starts under the deposit where you cannot see it, then penetrates the wall. If you are seeing pitting in a 316L unit, the material is probably underspecified for the concentrated-end chloride level, and the deposit is just accelerating what was going to happen anyway.
Stress Corrosion Cracking at Welds
Cracks that run along the heat-affected zone in hot chloride service are almost always stress corrosion cracking. In 316L, this points to a temperature and chloride combination beyond the alloy’s range. In duplex, it usually points to a fabrication problem: wrong filler, excessive heat input, or inadequate shielding gas.
Uniform Thinning in Acidic Service
If the metal is thinning evenly rather than pitting, the problem is general corrosion from low pH. This is where titanium can actually be the wrong choice, because titanium performs poorly in reducing acid environments. Duplex or a higher-alloy stainless is often the better answer here.
Hydrogen Embrittlement in Titanium
Titanium can absorb hydrogen in certain conditions, particularly in cathodic or oxygen-starved environments, and become brittle. This is uncommon in typical evaporator service but shows up in specific chemical streams. It is another reason titanium is not automatically the safe choice.
For streams with high organic content or oil carryover, the corrosion picture gets more complicated. The oily wastewater evaporation discussion covers how organics affect both fouling and material performance.
Cost Reality: It Is Not Just the Metal Price
Everyone focuses on the per-kilogram cost difference. That is only part of the story.
- Fabrication cost scales with difficulty. Duplex and titanium both require more careful welding, more qualified labor, and more inspection.
- Lead time matters. Titanium plate and tube can add weeks to a project schedule.
- Repair cost is higher for exotic alloys because fewer shops can do the work and the filler metals cost more.
- Downtime cost usually dwarfs all of the above. A bundle replacement on a running plant means lost production, not just a parts bill.
When I run a lifecycle comparison, the material that looks expensive at the quotation stage often wins over a ten-year horizon once you account for one avoided bundle replacement and the associated downtime. But not always. On a low-chloride stream at moderate temperature, 316L is genuinely the right answer and specifying duplex is just burning money.
Practical Recommendations
A few things I would tell any engineer specifying an evaporator:
- Do not accept a material recommendation without a water analysis and a maximum wall temperature. If the vendor did not ask for both, they are guessing.
- Ask specifically about the concentrated-end chloride level, not the feed level.
- Get the cleaning chemical compatibility confirmed in writing.
- If you go with duplex, verify the fabricator’s welding procedure and welder qualifications before the order is placed.
- If you go with titanium, confirm there is no fluoride in the stream and no fluoride in the cleaning chemicals.
- Consider whether a hybrid design makes sense. It is common to use different materials for different sections of the same unit, with the more exotic alloy only where the aggressive conditions actually exist.
The goal is not to buy the most corrosion-resistant alloy available. It is to buy the least expensive alloy that will still be in service when you expect it to be. Getting that call right requires real chemistry data and an honest look at how the unit will actually be operated, not just how it is designed to be operated.
Frequently Asked Questions
Can I use 316L for an evaporator if my feed chloride is under 200 mg/L?
Probably, but only if the concentrated-end chloride stays low and the wall temperature stays under roughly 60 °C. If the unit concentrates tenfold, your 200 mg/L feed becomes 2,000 mg/L at the heat exchanger, which is outside 316L range. Always calculate the concentrated-end value, not the feed value.
Is duplex 2205 always better than 316L?
No. Duplex is better in chloride and higher-temperature service, but it costs more and is harder to fabricate. On a low-chloride, moderate-temperature stream, 316L will outlast the equipment’s design life and cost less. Over-specifying is a real waste of capital.
When is titanium the only correct choice?
When chloride concentrations are high enough that duplex becomes marginal, particularly at elevated temperature, or when the stream is seawater-derived. Titanium is also the right call when the process is oxidizing and chloride levels rule out stainless steels entirely.
What cleaning chemicals should I avoid with each alloy?
With 316L, avoid chlorinated cleaners and strong acids at high temperature. With duplex, the same cautions apply, plus watch for caustic stress corrosion cracking at high temperature. With titanium, avoid fluoride-containing cleaners entirely, and be careful with reducing acids.
How do I know if my fabricator can handle duplex or titanium?
Ask for welding procedure specifications and welder qualification records for the specific alloy before the order is placed. If they cannot produce them, find a shop that can. Bad welding on a good alloy is worse than good welding on a cheaper alloy.
Does the evaporator type affect the material choice?
Yes. Mechanical vapor recompression units typically run higher wall temperatures than multi-effect units because of the compression heat, which pushes the material requirement up. Falling film and forced circulation designs also expose the metal to different flow regimes and different scaling behavior, which changes the corrosion risk.
What is the most common material selection mistake you see?
Specifying based on feed chemistry instead of concentrated-end chemistry, and forgetting to check the cleaning chemicals against the chosen alloy. Both mistakes are avoidable with a proper water analysis and a short conversation with the operations team.