When a heat exchanger operates in seawater, concentrated brine or an aggressive chemical stream, the material shortlist can move beyond stainless steels to titanium Grade 2 and Alloy 625 / UNS N06625. Both are premium corrosion-resistant materials, but their engineering strengths are different. Titanium is light and has a long industrial history in seawater heat-transfer equipment. Alloy 625 combines high mechanical strength with nickel-chromium-molybdenum-niobium chemistry that supports resistance across a broader range of severe corrosive environments.
Titanium vs Alloy 625: quick buyer comparison
| Procurement factor | Titanium Grade 2 | Alloy 625 / UNS N06625 |
|---|---|---|
| Typical buyer objective | Long-life seawater and chloride heat-transfer service | High strength plus broad corrosion resistance in severe chemical and marine service |
| Common tube standard | ASTM B338 | ASTM B704 welded tube; ASTM B444 seamless pipe/tube, subject to project specification |
| Density | Low; roughly 4.5 g/cm³ | Much higher; nickel-alloy density materially increases tube weight |
| Seawater performance | Established material for many continuous seawater heat exchangers | Strong marine corrosion resistance, including resistance to pitting and crevice attack in many conditions |
| Mechanical strength | Moderate compared with Alloy 625 | High |
| Chemical-service breadth | Excellent in many oxidizing chloride services, but chemistry must be checked carefully | Broad resistance to both oxidizing and nonoxidizing environments |
| Fabrication control | Requires clean tooling and high-quality inert-gas shielding | Requires qualified nickel-alloy welding procedures and control of heat input and filler system |
| Typical commercial position | Corrosion-life and low-weight option | High-strength, severe-chemistry nickel-alloy option |
This is a procurement overview, not a universal materials-selection chart. Final selection must follow the purchaser's approved corrosion review, process data, pressure design, fabrication requirements and applicable code.
What is Alloy 625 / UNS N06625?
Alloy 625 is a nickel-chromium-molybdenum-niobium alloy commonly identified as UNS N06625. INCONEL® alloy 625 is a registered trade name associated with Special Metals; in procurement documents, the UNS designation is useful because it identifies the alloy independently of a particular producer.
Special Metals describes Alloy 625 as resistant to a wide range of severe corrosive environments. Its nickel and chromium contribute resistance across oxidizing conditions, while molybdenum improves resistance to pitting and crevice corrosion. High nickel content also supports resistance to chloride-ion stress-corrosion cracking.
Typical project contexts include:
- marine and offshore equipment;
- seawater-cooled process systems;
- chemical processing;
- oil and gas equipment;
- high-chloride process streams;
- heat exchangers and condensers;
- components exposed to mixed corrosive media.
See our Inconel 625 / UNS N06625 Tube product page for related supply information.
Why titanium remains important in seawater heat exchangers
Commercially pure titanium, especially Grade 2, is a reference material for many seawater heat-transfer applications. Its stable oxide film gives strong resistance in aerated seawater and chloride-containing waters, while its low density reduces total bundle weight.
Typical applications include:
- seawater heat exchangers;
- surface condensers;
- desalination systems;
- marine coolers;
- coastal power plants;
- offshore cooling systems;
- industrial brine and chloride service.
For a seawater-focused guide, see Titanium Tube for Desalination & Seawater Service.
Seawater corrosion: both are strong, but the design question is different
Both titanium and Alloy 625 can perform well in marine environments. The purchasing decision should therefore move beyond a simple statement that each material is “seawater resistant.”
Evaluate:
- natural seawater vs concentrated brine;
- temperature;
- flowing vs stagnant conditions;
- biofouling and deposits;
- crevice geometry;
- cathodic-protection conditions;
- process contamination;
- cleaning chemistry;
- tube-to-tubesheet design;
- expected maintenance interval.
Special Metals' aqueous-corrosion handbook reports very low corrosion rates for Alloy 625 and other Ni-Cr-Mo alloys in flowing and stagnant seawater, while also noting that extremely tight crevice conditions can require even more highly alloyed nickel materials. That is an important procurement lesson: no alloy should be selected from a generic marketing statement alone.
Pitting and crevice corrosion: what buyers should ask
Localized corrosion often determines heat-exchanger reliability because the most severe location can be under a deposit or inside a tube-sheet crevice rather than in the bulk fluid.
For titanium, review whether the service remains within the approved corrosion envelope, including any reducing contaminants, fluoride exposure, crevice chemistry or hydrogen-related conditions.
For Alloy 625, review the actual crevice severity, temperature, oxidizing or reducing chemistry and whether the project uses cathodic protection. Alloy 625 has strong localized-corrosion resistance, but extremely severe crevice environments may still justify a higher-alloy nickel system.
Mixed chemical service: where Alloy 625 may gain an advantage
A heat exchanger may see more than seawater. The process side can contain acids, chlorides, oxidizers, reducing species, sulfur compounds or cleaning chemicals.
Alloy 625 is often considered where the process requires resistance across a wider mix of corrosive conditions. Its nickel-chromium-molybdenum chemistry gives it broader flexibility than materials selected primarily for seawater duty.
This does not mean Alloy 625 is always superior. If the service is predominantly clean seawater and the approved design already has a strong titanium service history, changing to a heavy nickel alloy can increase material and fabrication cost without solving a real project problem.
Mechanical strength: Alloy 625 has a clear advantage
Alloy 625 has substantially higher mechanical strength than commercially pure titanium Grade 2. This can matter in high-pressure process equipment, structural components, tube sheets, piping systems and designs where mechanical loads dominate wall-thickness selection.
However, a buyer must not automatically reduce tube wall because a stronger alloy is being considered. Wall thickness is controlled by the complete design basis, including:
- internal pressure;
- external pressure;
- tube vibration;
- support spacing;
- erosion allowance;
- manufacturing tolerance;
- tube-sheet joint;
- code minimum thickness;
- handling and installation damage.
Density: why titanium's lower weight matters commercially
Titanium is much less dense than Alloy 625. When an exchanger contains thousands of long tubes, that difference affects both engineering and commercial calculations.
A lighter bundle can affect:
- total metal mass;
- lifting requirements;
- support loads;
- transport cost;
- site installation;
- price comparison based on kilograms.
Heat transfer: use the exchanger calculation, not one material-property number
Tube thermal conductivity is only one element of the overall heat-transfer coefficient. A realistic titanium-vs-Alloy-625 comparison must include the actual wall thickness and operating conditions.
Overall duty is affected by:
- inside and outside film coefficients;
- tube wall resistance;
- fouling factor;
- fluid velocity;
- tube diameter;
- surface condition;
- temperature difference;
- flow arrangement;
- long-term deposit formation.
If one alloy allows a different approved wall thickness, the final exchanger performance and total material weight may change significantly.
Fabrication differences: titanium and Alloy 625 require different controls
Titanium fabrication
- clean, dedicated tools and work area;
- protection from iron contamination;
- high-quality inert-gas shielding;
- careful weld-zone cleanliness;
- qualified filler and procedure where welding is required.
Alloy 625 fabrication
- qualified nickel-alloy welding procedure;
- correct filler-metal selection;
- controlled heat input and interpass practice;
- attention to cleanliness and weld defects;
- appropriate post-fabrication inspection.
If the buyer is purchasing a complete fabricated exchanger rather than loose tubes, evaluate the fabricator's proven experience with the selected material family.
ASTM standards: titanium and Alloy 625 use different procurement routes
Titanium heat-exchanger tubing is commonly purchased under ASTM B338, which covers seamless and welded titanium and titanium-alloy tubes for condensers, evaporators and heat exchangers.
For Alloy 625, the correct tube specification depends on construction and project scope. ASTM B704-23 covers welded UNS N06625 nickel-alloy boiler, heat-exchanger and condenser tubes. ASTM B444 covers cold-worked seamless UNS N06625 pipe and tube.
| Material / route | Common ASTM basis | Buyer note |
|---|---|---|
| Titanium seamless or welded heat-exchanger tube | ASTM B338 | State grade, edition, route, OD, wall, length and inspection |
| Alloy 625 welded heat-exchanger / condenser tube | ASTM B704 | State UNS N06625, edition, dimensions, testing and documentation |
| Alloy 625 cold-worked seamless pipe / tube | ASTM B444 | Confirm that this is the project-approved product specification |
For titanium procurement details, see ASTM B338 Titanium Tube RFQ Guide.
When titanium is more likely to be preferred
Titanium may deserve stronger consideration when:
- continuous natural seawater is the main corrosive medium;
- long corrosion life is more important than very high strength;
- bundle weight matters;
- the owner has strong titanium service history;
- retubing is difficult or expensive;
- the project is a condenser, cooler or desalination exchanger;
- the approved process chemistry fits titanium's corrosion envelope.
When Alloy 625 may be more attractive
Alloy 625 may deserve stronger consideration when:
- the process contains more than seawater or chlorides;
- high mechanical strength is important;
- oxidizing and reducing species can both occur;
- a nickel-alloy piping or equipment system is already specified;
- high-temperature or high-pressure process conditions dominate;
- the purchaser requires broad chemical-service flexibility;
- the approved corrosion study specifies UNS N06625.
How does Alloy 625 compare with Super Duplex 2507?
Super Duplex 2507 offers high strength and excellent chloride resistance at a different alloy cost and fabrication basis. Alloy 625 generally extends the material envelope toward severe mixed chemistry and nickel-alloy service, while 2507 can be attractive when high strength and chloride-resistant stainless construction are the project priorities.
For that comparison path, read Titanium vs Super Duplex 2507 Heat Exchanger Tubes.
What about Hastelloy C276?
If the service involves very aggressive reducing acids, extreme mixed chemistry or severe crevice conditions, Alloy C276 may enter the material shortlist. The correct choice depends on the exact process, not the prestige of the alloy name.
Related product page: Hastelloy C276 Tube.
Titanium vs Alloy 625 RFQ checklist
| RFQ field | What to provide |
|---|---|
| Application | Heat exchanger, condenser, brine heater, cooler or replacement bundle |
| Process medium | Seawater, concentrated brine, acid, chemical stream and impurities |
| Chloride concentration | Normal and maximum value |
| Temperature | Operating, maximum and design temperature |
| pH / chemistry | Normal range plus oxidizing or reducing conditions where relevant |
| Pressure | Operating and design pressure |
| Material option | Titanium Grade 2, UNS N06625 or approved alternate quotation |
| Tube standard | ASTM B338, B704, B444 or approved project specification |
| Construction | Seamless / welded / approved alternatives |
| Dimensions | OD × wall thickness × finished length |
| Quantity | Pieces by size plus approved spare quantity |
| Inspection | NDT, pressure test, PMI, dimensional inspection and witness points |
| Documents | MTC, heat traceability, inspection reports and TPI requirements |
| Packing | Export bundle/crate, end protection, identification and lifting restrictions |
| Delivery | Country, port, incoterm and required arrival date |
How to request alternate quotations
ASTM B338 Grade 2 · OD ___ mm × WT ___ mm × L ___ mm · ___ pcs · seamless/welded ___ · inspection ___
Option B — Alloy 625:
UNS N06625 · ASTM B704 welded tube or project-approved seamless specification · OD ___ mm × WT ___ mm × L ___ mm · ___ pcs · inspection ___
If engineering requires different wall thicknesses for the two materials, keep those differences visible. A technically normalized comparison is more useful than forcing both alloys into the same nominal dimensions.
Supplier evaluation questions
Before placing an order, confirm whether the supplier can support:
- the exact UNS / ASTM material designation;
- seamless or welded construction;
- fixed-length production;
- thin-wall dimensional control;
- NDT and pressure-test requirements;
- PMI where specified;
- heat and lot traceability;
- MTC and inspection reports;
- third-party inspection;
- technical deviation control;
- export packing for long tubes.
For a broader checklist, see Titanium Tube Supplier Selection Guide.
Frequently asked questions
Is titanium better than Alloy 625 for seawater?
Not universally. Titanium is a proven option for many continuous seawater heat exchangers, while Alloy 625 combines marine corrosion resistance with higher strength and broader chemical-service flexibility.
What is UNS N06625?
UNS N06625 is the standardized designation for Alloy 625, a nickel-chromium-molybdenum-niobium alloy used in severe corrosive and high-strength applications.
Which material is lighter?
Titanium is substantially lighter. For large tube bundles, this affects total metal mass, lifting, freight and the meaning of a price-per-kilogram comparison.
Which ASTM standards apply?
ASTM B338 is common for titanium condenser and heat-exchanger tubes. ASTM B704 covers welded UNS N06625 boiler, heat-exchanger and condenser tubes, while ASTM B444 covers cold-worked seamless Alloy 625 pipe and tube.
Can suppliers quote both materials?
Yes, if engineering permits alternates. Each option should use the correct product standard, wall thickness, construction, inspection and documentation basis.
Related titanium and nickel-alloy procurement guides
- Titanium Heat Exchanger Tube
- Titanium Heat Exchanger Tubes Buyer Guide
- Inconel 625 / UNS N06625 Tube
- Titanium vs Super Duplex 2507
- Titanium vs 316L Stainless Steel
- Grade 2 vs Grade 12 for Chloride Service
- Titanium Tube for Desalination & Seawater Service
Reference sources
Titanium tube standard: ASTM B338-17(2026) — Seamless and Welded Titanium and Titanium Alloy Tubes for Condensers and Heat Exchangers.
Alloy 625 corrosion and composition reference: Special Metals — INCONEL® alloy 625 Technical Bulletin.
Marine corrosion context: Special Metals — Aqueous Corrosion Handbook.
Alloy 625 welded heat-exchanger tube standard: ASTM B704-23 — Welded Nickel Alloy Tubes.
Send your heat exchanger tube RFQ
Send the process medium, chloride level, temperature, pressure, OD × wall × length, quantity, required standard and inspection scope. If your project is comparing titanium and Alloy 625, attach the exchanger datasheet or approved tube schedule.
Scan to chat on WhatsApp