Marine Heat Exchanger Materials in Seawater: Carbon Steel, 316L, Titanium and Duplex 2205

Table of Contents

1. The $2.5 Trillion Problem Nobody Budgets For
2. Why Heat Exchangers Are the Front Line of Marine Corrosion
3. Material Selection 101: What Actually Touches Seawater
4. Carbon Steel — The Baseline That Sets the Floor
5. 316L Stainless Steel — The Default That Falls Short in Seawater
6. Titanium Grade 1 and Grade 2 — The Seawater Endgame
7. Duplex Stainless Steel 2205 — The Chemical Process Workhorse
8. Super Duplex 2507 — When 2205 Is Not Enough
9. Cu-Ni 90/10 — The Marine Traditionalist Under Pressure
10. Material Comparison Table: Properties, Cost, and Service Life
11. The Decision Tree: How to Choose the Right Material
12. Case Study: Titanium Cargo Heaters for VLGCs — 60+ Units Delivered
13. Case Study: Duplex 2205 for Organic Solvent Separation — 10-Unit Programme
14. Case Study: Cu-Ni Central Coolers — Multi-Class Certification
15. OEM Manufacturing: Titanium Seawater Pre-Heaters for a European Cruise Programme
16. Corrosion Mechanisms That Kill Heat Exchangers at Sea
17. Lifecycle Cost Analysis: Initial Price vs. Total Cost of Ownership
18. Classification Society Requirements and Material Certification
19. Welding Metallurgy: Why Material Selection Is Only Half the Battle
20. Alfa Laval, Tranter, and the Industry's Direction on Exotic Metallurgy
21. Frequently Asked Questions

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The $2.5 Trillion Problem Nobody Budgets For

NACE International (now AMPP — the Association for Materials Protection and Performance) published a landmark study estimating global corrosion costs at $2.5 trillion per year — roughly 3.4% of global GDP. That figure includes direct costs only: replacement parts, maintenance labour, downtime, and premature equipment disposal. Indirect costs — production losses, environmental remediation, safety incidents — push the real number considerably higher.

The maritime industry absorbs a disproportionate share of this burden. Every piece of equipment on a vessel operates in one of the most aggressive corrosive environments on earth: seawater at varying temperatures, biofouling, crevice conditions, galvanic couples between dissimilar metals, and continuous vibration. Heat exchangers sit at the intersection of all these factors.

A single tube failure in a central cooler does not just cost a tube. It costs emergency dry-docking, off-hire charges running into the hundreds of thousands of dollars, spare parts logistics, and — depending on the failure mode — potential cargo contamination or environmental release. For fleet operators running 20, 50, or 200 vessels, the aggregate corrosion cost of heat exchanger failures is a line item that rivals crew costs.

This article examines the material choices available for marine heat exchangers in 2026, analyses their corrosion performance in real-world service, and draws on our manufacturing experience across titanium, duplex stainless steel, copper-nickel, and conventional alloys to provide a practical framework for material selection decisions.

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Why Heat Exchangers Are the Front Line of Marine Corrosion

Heat exchangers are, by definition, designed to facilitate thermal energy transfer between two fluid streams. In marine applications, one of those streams is almost always seawater — the most consistently corrosive fluid an engineer will encounter in industrial service.

The Seawater Side

Seawater contains approximately 3.5% dissolved salts by weight, predominantly sodium chloride. Chloride ions are the primary driver of localised corrosion (pitting and crevice corrosion) in stainless steels and nickel alloys. But salinity alone does not tell the whole story:

• Temperature — Seawater temperature ranges from near-freezing in Arctic service to 35degC+ in tropical waters. Corrosion rates accelerate with temperature. A heat exchanger warming seawater from 25degC to 45degC creates a temperature gradient that concentrates attack at the outlet end of the tubes.
• Dissolved oxygen — Aerated seawater is more corrosive than deaerated. Surface seawater — the intake for most marine cooling systems — is fully aerated.
• Biofouling — Marine organisms colonise heat exchanger surfaces within days of commissioning. Biofilms create oxygen-depleted zones beneath them, establishing differential aeration cells that drive crevice corrosion. Macrofouling (barnacles, mussels) physically obstructs flow and creates under-deposit corrosion sites.
• Velocity — Flow velocity affects corrosion in two opposing ways. Too slow, and fouling accumulates. Too fast, and erosion-corrosion strips protective oxide films. Each material has an optimal velocity window.
• Galvanic coupling — Where dissimilar metals are connected in seawater, the less noble metal corrodes preferentially. Tube-to-tubesheet joints are classic galvanic coupling sites.

The Process Side

The non-seawater side introduces its own challenges:

• Cargo heating — LPG/LNG cargo at cryogenic temperatures creates thermal shock risks on startup.
• Lubricating oil cooling — Elevated temperatures, potential for sulphur compound formation.
• Freshwater cooling — Generally benign, but glycol-based coolants at elevated temperatures can become acidic.
• Chemical process streams — Organic solvents, acids, alkalis — each demanding specific metallurgy.

The heat exchanger must resist corrosion on both sides simultaneously, under thermal cycling, mechanical vibration, and — on vessels — continuous motion. This is why material selection for marine heat exchangers is a more consequential decision than for almost any other piece of equipment on board.

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Material Selection 101: What Actually Touches Seawater

Before choosing materials, it helps to understand which components of a shell-and-tube heat exchanger are exposed to which fluids:

Component	Seawater exposure	Process side exposure	Key concern
Tubes (inner surface)	Yes (tube-side seawater) or No	Depends on design	Pitting, erosion-corrosion
Tubes (outer surface)	No or Yes (shell-side seawater)	Depends on design	Crevice corrosion at baffles
Tubesheet	Both sides at the tube-to-tubesheet joint	Both sides	Galvanic corrosion, crevice attack
Shell	Typically process side only	Yes	General corrosion, stress corrosion
Baffles	Seawater side if shell-side SW	—	Erosion at baffle cuts
Channel/heads	Seawater side if tube-side SW	—	Erosion at inlet nozzle

In the most common marine configuration — seawater on the tube side — the tubes, tubesheets, and channel/heads see full seawater exposure. The shell, baffles, and tie rods see the process fluid.

This creates two distinct material selection zones:

1. Seawater-wetted components — Must resist chloride pitting, crevice corrosion, biofouling-induced corrosion, and erosion-corrosion.
2. Process-wetted components — Material selection driven by the specific process fluid chemistry, temperature, and pressure.

Many marine heat exchangers use mixed metallurgy: titanium or Cu-Ni tubes with carbon steel or stainless steel shells. The tube-to-tubesheet interface becomes the critical engineering challenge — a galvanic couple in a crevice geometry, wet with seawater, operating at elevated temperature. Get this wrong, and the heat exchanger fails at the joints, regardless of tube material quality.

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Carbon Steel — The Baseline That Sets the Floor

Carbon steel (SA516 Gr.70, SA106 Gr.B) remains the default shell material for most marine heat exchangers. Its role is primarily structural — containing pressure on the process side where the fluid is not aggressively corrosive.

Where Carbon Steel Works

• Shells containing lubricating oil, freshwater, or glycol at moderate temperatures
• Channel covers and floating heads with rubber-lined or epoxy-coated surfaces
• Support structures and saddles (not in direct fluid contact)

Where Carbon Steel Fails

• Any direct seawater contact without coating or lining
• Process fluids containing H2S (sour service — NACE MR0175 applies)
• High-temperature service above 400degC where creep becomes a concern (move to Cr-Mo)

Corrosion Rate Data

In aerated seawater, unprotected carbon steel corrodes at approximately 0.1-0.3 mm/year. With a typical 3mm corrosion allowance, the shell has a theoretical life of 10-30 years — but localised pitting can penetrate the wall in far less time. This is why carbon steel is never used unprotected on the seawater side.

Cost index: 1.0x (baseline reference)

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316L Stainless Steel — The Default That Falls Short in Seawater

Type 316L (UNS S31603) is the most widely specified austenitic stainless steel in process industries. Its 2-3% molybdenum content provides moderate chloride resistance, and the low carbon "L" grade reduces sensitisation risk during welding.

The PREN Problem

Pitting Resistance Equivalent Number (PREN) is the most widely used index for comparing chloride corrosion resistance:

PREN = %Cr + 3.3(%Mo) + 16(%N)

For 316L: PREN = approximately 23-25.

The general rule for seawater service: PREN > 40 required for reliable long-term performance. Type 316L falls well short of this threshold.

Where 316L Works in Marine Heat Exchangers

• Process side — Freshwater, clean oils, moderate chemical service
• Tubesheets clad or overlaid with a more corrosion-resistant alloy on the seawater face
• Internals (baffles, tie rods) in process-side service where seawater does not penetrate

Where 316L Fails

• Tube-side seawater service — crevice corrosion at tube supports within 2-5 years
• Stagnant seawater (during lay-up or low-flow conditions) — pitting initiates rapidly
• Warm seawater above 20degC — dramatically accelerates pitting kinetics

We regularly see 316L heat exchangers pulled from seawater service after 3-5 years with through-wall pitting. The material is not defective — it is simply being asked to do a job for which its metallurgy is inadequate.

Cost index: 2.0-2.5x carbon steel

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Titanium Grade 1 and Grade 2 — The Seawater Endgame

Titanium is, in terms of seawater corrosion resistance, essentially immune. The passive oxide film (TiO2) that forms on titanium surfaces is extraordinarily stable in chloride environments. There is no known case of titanium pitting or crevice corrosion in clean natural seawater at temperatures below 80degC.

Grade 1 vs. Grade 2

Property	Ti Grade 1 (UNS R50250)	Ti Grade 2 (UNS R50400)
Tensile strength	240 MPa min	345 MPa min
Yield strength	170 MPa min	275 MPa min
Elongation	24% min	20% min
Corrosion resistance	Identical	Identical
Weldability	Excellent	Excellent
Typical use	Thin-wall tubes	Tubes, tubesheets, plates

Grade 2 is the standard choice for marine heat exchanger tubes. Its higher strength allows thinner tube walls (0.5-0.7mm typical vs. 1.0-1.2mm for Cu-Ni), which improves heat transfer and reduces weight simultaneously.

Why Titanium Wins the Seawater Argument

1. Zero corrosion in seawater — Not "low" corrosion. Zero. The passive film self-heals instantly if damaged by erosion or mechanical impact.
2. Biofouling resistance — While organisms can attach to titanium, they cannot establish the underfilm corrosion cells that destroy other metals. The titanium remains intact beneath the fouling layer.
3. No velocity limit — Unlike Cu-Ni, which suffers erosion-corrosion above 2-3 m/s, titanium handles seawater velocities up to 20+ m/s without degradation.
4. Galvanic nobility — Titanium is noble in the galvanic series. It will not corrode when coupled with other metals. (This creates challenges for adjacent components — discussed in the welding section.)
5. Weight advantage — Density 4.51 g/cm3 vs. 8.9 g/cm3 for Cu-Ni 90/10. Roughly half the weight per unit volume.

Limitations

• Cost — Titanium raw material costs 5-8x carbon steel. Fabrication requires inert gas shielding (argon purge), adding to manufacturing costs.
• Galvanic attack on adjacent metals — In direct contact with carbon steel in seawater, the carbon steel corrodes rapidly. Careful design of insulation, cathodic protection, and material transitions is essential.
• Hydrogen embrittlement — In cathodically protected systems (common on vessels), excessive cathodic current can drive hydrogen into titanium, causing embrittlement. This requires coordination between the cathodic protection system design and the heat exchanger material specification.

Cost index: 5-8x carbon steel (tubes), 8-12x (plates/forgings)

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Duplex Stainless Steel 2205 — The Chemical Process Workhorse

Duplex stainless steel 2205 (UNS S32205/S31803) has a mixed microstructure — approximately 50% austenite and 50% ferrite. This dual-phase structure gives it roughly double the yield strength of 316L, significantly better chloride resistance (PREN 34-36), and good resistance to stress corrosion cracking (SCC).

Where Duplex 2205 Excels

• Chemical process heat exchangers — Organic solvents, fatty acids, phosphoric acid, caustic solutions
• Seawater-cooled process systems — Where seawater temperatures stay below 20degC and crevice geometries are minimised
• High-pressure applications — The higher yield strength allows thinner walls, reducing weight and cost
• SCC-prone environments — Where 316L would fail by stress corrosion cracking, duplex resists

Our Experience: 10-Unit 2205 Programme

A European specialty oils and fats company (a major global player in vegetable oil refining) contracted us for 10 duplex 2205 heat exchangers for their Zhangjiagang facility. These units process organic solvents in fatty acid fractionation — an environment that demands both chemical resistance and high mechanical strength.

Duplex 2205 was selected over 316L because the process fluid (hot fatty acid/solvent mixtures at 150-200degC) would cause SCC in austenitic grades. The duplex structure resists SCC up to approximately 120degC in chloride environments — and in organic solvent service (low chloride, moderate temperature), it performs excellently to much higher temperatures.

Limitations

• Seawater crevice corrosion — PREN 34-36 is better than 316L but still below the 40+ threshold for reliable long-term seawater crevice resistance. In warm seawater (>25degC) with tight crevices (tube-to-baffle contact), crevice corrosion can initiate.
• 475degC embrittlement — Prolonged exposure to 300-525degC causes spinodal decomposition of the ferrite phase, dramatically reducing toughness. Duplex heat exchangers must not operate in this temperature range.
• Welding complexity — Maintaining the 50/50 phase balance in weld metal requires careful control of heat input, interpass temperature, and filler metal nitrogen content. Overheating causes excess ferrite; insufficient heat input causes excess austenite. Both degrade corrosion resistance.

Cost index: 3-4x carbon steel

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Super Duplex 2507 — When 2205 Is Not Enough

Super duplex 2507 (UNS S32750) takes the duplex concept further: higher chromium, higher molybdenum, higher nitrogen. PREN 42-45 — above the seawater threshold.

Where 2507 Fits

• Seawater-wetted components requiring both high strength and corrosion resistance
• Tubesheets in titanium-tubed heat exchangers (where the tubesheet sees seawater on one face)
• High-pressure seawater systems — desalination, injection, ballast treatment
• Hot seawater (up to 40degC) in crevice geometries

Practical Considerations

Super duplex is significantly more expensive and more difficult to fabricate than standard duplex:

• Material cost — 2-3x that of 2205
• Welding — Even tighter control of heat input (0.5-1.5 kJ/mm), strict interpass temperature limits (max 150degC), and mandatory solution annealing after forming
• Machining — Work-hardens rapidly, requiring carbide tooling and rigid setups
• Availability — Longer lead times than 2205 or 316L, particularly for large forgings and plates

We specify 2507 selectively — typically for tubesheets in critical seawater service where titanium tubes meet a ferrous tubesheet. The 2507 tubesheet provides corrosion resistance compatible with the titanium tubes while allowing welded or expanded tube-to-tubesheet joints without the galvanic concerns of a carbon steel tubesheet.

Cost index: 5-7x carbon steel

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Cu-Ni 90/10 — The Marine Traditionalist Under Pressure

Copper-nickel alloy 90/10 (UNS C70600) has been the standard material for marine heat exchanger tubes for decades. Its combination of reasonable seawater corrosion resistance, antifouling properties, and moderate cost made it the default specification for central coolers, oil coolers, and auxiliary heat exchangers across the global fleet.

The Cu-Ni Value Proposition

1. Self-policing corrosion film — Cu-Ni forms a protective oxide/hydroxide film in seawater that inhibits further corrosion. This film is self-healing under moderate flow conditions.
2. Antifouling — Copper ions released from the surface inhibit marine organism settlement. This is a genuine advantage over stainless steel and titanium, both of which foul readily.
3. Ease of fabrication — Cu-Ni is readily formed, welded, and expanded. No inert gas shielding required for welding. Standard workshop practices apply.
4. Established supply chain — Tube manufacturers worldwide produce Cu-Ni 90/10 to multiple national and international standards.

Why Cu-Ni Is Losing Ground

1. Erosion-corrosion — Cu-Ni is highly sensitive to flow velocity. Above 2-3 m/s in seawater, the protective film breaks down and erosion-corrosion accelerates dramatically. Inlet-end thinning is the most common failure mode.
2. Limited life in aggressive service — On large vessels operating in warm tropical waters, Cu-Ni tube life of 6-8 years is common. VLGCs with 4-6 cargo heaters face tube bundle replacement costs that, including off-hire, exceed the original equipment cost.
3. Sulphide attack — Exposure to polluted or sulphide-containing harbour waters can destroy the protective film, causing accelerated corrosion upon return to clean seawater.
4. Weight — Density 8.9 g/cm3 makes Cu-Ni the heaviest common tube material. On weight-sensitive vessels (VLGCs, container ships), this translates directly to reduced cargo capacity.
5. Environmental regulation — Increasing scrutiny of copper ion discharge is beginning to affect specification decisions, particularly in environmentally sensitive operating areas.

Where Cu-Ni Still Makes Sense

• Auxiliary coolers on vessels with moderate duty cycles and low seawater velocities
• Retrofit and replacement — Existing systems designed around Cu-Ni flow velocities and cathodic protection systems
• Cost-constrained projects where initial capital cost is the primary decision driver

Cost index: 3-4x carbon steel (tubes)

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Material Comparison Table: Properties, Cost, and Service Life

Property	Carbon Steel	316L	Ti Gr.2	Duplex 2205	Super Duplex 2507	Cu-Ni 90/10
PREN	N/A	23-25	N/A (immune)	34-36	42-45	N/A
Seawater pitting resistance	None	Poor	Immune	Moderate	Good	Moderate
Seawater crevice resistance	None	Very poor	Immune	Limited	Good	Moderate
Erosion-corrosion resistance	Poor	Moderate	Excellent	Good	Good	Poor >2.5 m/s
SCC resistance (chloride)	Good	Poor	Excellent	Excellent	Excellent	Excellent
Max seawater service temp	N/A (coated)	~15degC	80degC+	~20degC (crevice)	~40degC (crevice)	~30degC
Density (g/cm3)	7.85	8.0	4.51	7.8	7.8	8.9
Yield strength (MPa)	260	170	275	450	550	105
Antifouling	None	None	None	None	None	Good
Relative tube cost	1x	2-2.5x	5-8x	3-4x	5-7x	3-4x
Typical seawater tube life	<2 yr uncoated	3-5 yr	25+ yr	10-15 yr	15-20 yr	6-12 yr
Welding complexity	Low	Low	High (inert)	Medium-High	High	Low

Key takeaway: No single material dominates all categories. The selection decision depends on which combination of properties matters most for the specific application — and crucially, on total lifecycle cost rather than initial purchase price.

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The Decision Tree: How to Choose the Right Material

The following decision framework reflects our manufacturing experience across 600+ welding procedure qualifications and equipment delivered to 50+ countries:

Step 1: Identify the Corrosion Environment

• Direct seawater contact? → Rule out carbon steel and 316L for wetted surfaces
• Process fluid chemistry? → Identify organic solvents, acids, alkalis, H2S, ammonia
• Temperature range? → Max seawater temperature directly affects material options
• Velocity range? → Eliminates Cu-Ni above 2.5 m/s

Step 2: Define the Service Life Requirement

• 5-10 years (short-term/replaceable) → Cu-Ni 90/10 may be acceptable
• 15-20 years (vessel design life) → Super duplex or titanium
• 25+ years (zero replacement) → Titanium, which is the material that reliably reaches this life in seawater

Step 3: Evaluate Constraints

• Weight-critical? → Titanium (half the weight of Cu-Ni or steel)
• Cost-critical? → Cu-Ni or duplex 2205 (but evaluate lifecycle cost, not just purchase price)
• Certification requirements? → Classification society approvals, ASME stamps, PED certification — verify material and welding procedure availability
• Cathodic protection system? → Titanium requires coordination; Cu-Ni benefits from it

Step 4: Select the Material Combination

Most marine heat exchangers use mixed metallurgy. Common combinations:

Application	Tubes	Tubesheet	Shell	Channel
VLGC cargo heater	Ti Gr.2	Ti Gr.2 or clad	SA516 / 304L	Ti Gr.2 or clad
Central cooler (large vessel)	Ti Gr.2 or Cu-Ni	2507 or NAB	Carbon steel	2507 or Cu-Ni
Oil cooler	Cu-Ni 90/10	NAB or 2205	Carbon steel	Cu-Ni or CS lined
Chemical process HEX	Duplex 2205	2205	2205 or CS + clad	2205
High-performance seawater HEX	Ti Gr.2	2507	Carbon steel	Ti Gr.2 lined CS

Step 5: Verify Weldability and Fabrication Feasibility

This step is where many projects encounter problems — particularly when exotic materials are specified without confirming that the fabricator has qualified welding procedures. A qualified procedure record (PQR) for Ti-to-Ti welding, or for duplex 2205 tube-to-tubesheet expansion, requires specific equipment, gas shielding systems, and operator qualifications that not every workshop possesses.

At Lmart, we maintain 600+ PQR records covering the full range of materials discussed in this article. This is not a marketing number — it reflects decades of systematic welding procedure development across carbon steel, stainless steels, duplex grades, titanium, Cu-Ni, and exotic alloys for ASME, PED, and classification society certification.

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Case Study: Titanium Cargo Heaters for VLGCs — 60+ Units Delivered

Application: Seawater-heated cargo heaters for 82,000-93,000 cbm Very Large Gas Carriers

Client: A leading international marine systems integrator (a global leader in marine cargo handling systems)

Material specification:
- Tubes: Ti Grade 2, OD 19.05mm, wall 0.7mm
- Tubesheet: Ti Grade 2 forging
- Shell: SA240 Type 304L
- Channel: Ti Grade 2

Key parameters:
- Unit weight: 3,660-4,000 kg
- Shell diameter: DN890-900mm
- Tube length: 5,900-7,000mm
- Classification: ABS, KR, NK, DNV, BV, CCS

Why titanium was selected:

The VLGC fleet previously used Cu-Ni 90/10 cargo heaters. Fleet operators reported tube bundle life of 6-8 years in tropical seawater service. With 4-6 heaters per vessel and dry-docking costs for tube bundle replacement exceeding $200,000 per unit (including off-hire), the lifecycle economics clearly favoured titanium despite its higher initial cost.

Titanium tubes in this application last the full 25-year vessel design life with zero maintenance. Across the VLGC fleet, this has eliminated mid-life replacement programmes entirely.

Manufacturing considerations:

All Ti-to-Ti welding was performed in controlled atmosphere chambers with trailing argon shielding. Tube-to-tubesheet joints used a combination of hydraulic expansion and seal welding. Each weld was inspected by radiographic or ultrasonic testing per classification society requirements.

Over a production run of 60+ units for this single programme, we developed welding procedures and QA protocols that reduced the reject rate to below 0.1% on tube-to-tubesheet welds — a critical quality metric given that a single leaking joint compromises the entire heat exchanger.

Vessels and shipyards served: Units installed on VLGCs built at Hyundai Heavy Industries, Kawasaki Heavy Industries, COSCO Shipping, and Jiangnan Shipyard — operating for international fleet owners.

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Case Study: Duplex 2205 for Organic Solvent Separation — 10-Unit Programme

Application: Organic solvent heat exchangers for fatty acid fractionation

Client: A major European specialty oils and fats group (one of the world's largest vegetable oil processors), Zhangjiagang facility

Material specification:
- Tubes: Duplex 2205
- Tubesheet: Duplex 2205 forging
- Shell: Duplex 2205
- All internals: Duplex 2205

Why duplex 2205 was selected:

The process fluid — a mixture of organic solvents and fatty acids at 150-200degC — would cause stress corrosion cracking (SCC) in conventional austenitic grades (304L, 316L) within months. The chloride content of the process stream, combined with elevated temperature, made SCC the primary failure mechanism to guard against.

Duplex 2205 provides SCC resistance up to approximately 120degC in chloride-bearing environments, and significantly higher in organic solvent systems where water activity is low. The client's previous experience with 316L exchangers confirmed failure by SCC within 2-3 years.

The 10-unit programme was delivered on schedule, with all units passing hydrostatic testing and helium leak testing at the factory. Classification: ASME U stamp + PED CE marking (2014/68/EU Module H).

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Case Study: Cu-Ni Central Coolers — Multi-Class Certification

Application: Marine central cooling systems — seawater-cooled central coolers for vessel machinery cooling circuits

Material specification:
- Tubes: Cu-Ni 90/10 (UNS C70600)
- Tubesheet: Aluminium bronze (NAB) or Cu-Ni
- Shell: Carbon steel, internally coated
- Channel: Cu-Ni or rubber-lined carbon steel

Certification: Multiple classification society approvals — CCS, BV, ABS, DNV, LR

Design considerations:

Central coolers represent the interface between the vessel's seawater intake and the freshwater cooling circuit that serves main engines, generators, compressors, and other machinery. They operate continuously, processing seawater at flow velocities that must be carefully controlled to stay within Cu-Ni's acceptable range (1.5-2.5 m/s).

Our Cu-Ni central coolers are designed with attention to:

1. Inlet turbulence management — Impingement plates and flow distributors at the seawater inlet reduce localised high-velocity zones that cause erosion-corrosion.
2. Tube support spacing — Closer baffle spacing reduces unsupported tube spans, limiting flow-induced vibration that can cause tube fatigue and baffle-hole fretting.
3. Cathodic protection compatibility — Zinc or iron anodes in the waterbox provide cathodic protection to the Cu-Ni tubes without over-protecting (which can cause dezincification of brass components if present).
4. Tube expansion quality — Consistent tube expansion into tubesheets is critical for leak-tight performance. We use servo-controlled mechanical expansion with real-time torque monitoring.

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OEM Manufacturing: Titanium Seawater Pre-Heaters for a European Cruise Programme

Application: Titanium seawater pre-heaters for a major European cruise shipbuilder's newbuilding programme

OEM partner: A global leader in heat transfer, separation, and fluid handling (the heat exchanger brand with the largest installed base in the marine industry)

Configuration: Titanium tubes and waterbox components in a compact plate-and-shell or shell-and-tube design, pre-heating seawater for downstream HVAC and freshwater generation systems.

Significance:

This programme illustrates the trend toward titanium in applications beyond traditional cargo heating. Cruise ships operate in diverse water conditions — from Caribbean tropics to Mediterranean harbours to Northern European coasts. The seawater pre-heaters must perform reliably across this full spectrum of temperatures, salinities, and biofouling conditions.

The OEM selected us as the manufacturing partner based on our titanium welding capability, classification society certifications, and track record of series production consistency. Each unit underwent factory acceptance testing per the OEM's specifications, which exceed classification society minimums in several areas (helium leak test sensitivity, radiographic acceptance criteria).

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Corrosion Mechanisms That Kill Heat Exchangers at Sea

Understanding failure mechanisms is essential for material selection. Here are the six most common corrosion modes in marine heat exchangers:

1. Pitting Corrosion

What it is: Highly localised attack creating small, deep pits that can penetrate tube walls.

Vulnerable materials: 316L, 304L in seawater. Cu-Ni under stagnant conditions.

Immune materials: Titanium. Super duplex 2507 (highly resistant).

Prevention: Select materials with PREN > 40 for seawater service. Avoid stagnant conditions. Ensure continuous flow during operation and dry-lay during shutdown.

2. Crevice Corrosion

What it is: Corrosion within confined spaces — tube-to-baffle gaps, tube-to-tubesheet interfaces, gasket faces — where stagnant solution becomes depleted of oxygen and enriched in chloride.

Vulnerable materials: All stainless steels including 2205 (at elevated temperature). 316L is particularly susceptible.

Resistant materials: Titanium (immune). Super duplex 2507 (resistant to ~40degC). Cu-Ni (moderate resistance).

Prevention: Minimise crevice geometries. Use tube-to-tubesheet welding rather than expansion alone. Select PREN > 40 for crevice-prone joints.

3. Erosion-Corrosion

What it is: Removal of protective films by high-velocity fluid, exposing fresh metal to corrosive attack.

Vulnerable materials: Cu-Ni (critical above 2.5 m/s). Carbon steel. Brass.

Resistant materials: Titanium (resistant to 20+ m/s). Duplex and super duplex (resistant to 10+ m/s).

Prevention: Control flow velocity. Avoid turbulence at inlet ends. Use impingement protection. Select velocity-resistant materials for high-flow applications.

4. Stress Corrosion Cracking (SCC)

What it is: Cracking caused by the combined effect of tensile stress and a corrosive environment. Can cause sudden catastrophic failure without warning.

Vulnerable materials: Austenitic stainless steels (304L, 316L, 321, 347) in chloride environments above 50-60degC.

Resistant materials: Duplex 2205, super duplex 2507, titanium, Cu-Ni.

Prevention: Avoid austenitic grades in hot chloride service. Use duplex or titanium. Stress-relieve fabricated components.

5. Galvanic Corrosion

What it is: Accelerated corrosion of the less noble metal in a bimetallic couple immersed in an electrolyte (seawater).

Common problem pairs: Titanium + carbon steel (carbon steel corrodes rapidly). Cu-Ni + steel (steel corrodes). Stainless steel + carbon steel (carbon steel corrodes).

Prevention: Use electrical insulation between dissimilar metals. Select compatible metals for connected components. Design cathodic protection systems to account for galvanic interactions.

6. Microbiologically Influenced Corrosion (MIC)

What it is: Corrosion initiated or accelerated by marine microorganisms (sulphate-reducing bacteria, iron-oxidising bacteria, manganese-oxidising bacteria).

Vulnerable materials: Carbon steel, 316L (biofilm creates under-film oxygen depletion cells).

Resistant materials: Titanium (biofilm cannot establish corrosion cells on TiO2 surface). Cu-Ni (copper ion toxicity inhibits bacterial colonisation).

Prevention: Antifouling coatings. Biocide dosing (marine growth prevention systems). Material selection (Ti or Cu-Ni).

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Lifecycle Cost Analysis: Initial Price vs. Total Cost of Ownership

The most expensive heat exchanger is the one you have to replace. Consider a simplified lifecycle comparison for a typical VLGC cargo heater:

Scenario: 25-Year Vessel Life, 4 Cargo Heaters

Cost element	Cu-Ni 90/10	Ti Grade 2
Initial equipment cost (4 units)	$280,000	$420,000
Mid-life tube bundle replacement (Year 8)	$160,000	$0
Second replacement (Year 16)	$160,000	$0
Off-hire costs (2 dry-dock events x 5 days)	$200,000	$0
Logistics and yard mobilisation	$60,000	$0
25-year total	$860,000	$420,000

The titanium option costs 51% less over the vessel's life — despite costing 50% more at initial purchase. This calculation does not include the risk of unplanned failure between scheduled replacements, which would add emergency repair costs and potentially insurance implications.

This lifecycle perspective is why the VLGC fleet has moved almost entirely to titanium cargo heaters over the past decade. It is not a technology preference — it is an economic calculation.

When Cu-Ni Lifecycle Costs Are Competitive

• Short vessel design lives (10-15 years)
• Low seawater temperature service (Arctic/sub-Arctic — corrosion rates drop significantly)
• Auxiliary exchangers with low duty hours
• Systems where tube bundle replacement is straightforward (easy access, standard tooling, no classification survey required)

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Classification Society Requirements and Material Certification

Every material used in a marine heat exchanger on a classified vessel must be tested and certified according to the relevant classification society rules:

Material Certification Levels

• 3.1 Inspection Certificate (EN 10204) — Mill test certificate from the material manufacturer, confirming chemical composition and mechanical properties from the actual heat of material used.
• 3.2 Inspection Certificate — Third-party inspection body witnesses the testing and co-signs the certificate.
• Classification society material approval — Some societies require specific material grade approvals for marine service (e.g., DNV-approved titanium grades, BV-approved Cu-Ni specifications).

Welding Procedure Qualification

Each material and material combination requires a qualified welding procedure per the classification society's welding rules. Key requirements include:

• Base metal grouping — Classification societies group materials differently than ASME. A PQR qualified for one group may not cover another.
• Filler metal approval — Welding consumables must be approved by the classification society for the specific material and application.
• Welder qualification — Individual welders must hold valid classification society welder approval certificates.
• Workshop approval — The manufacturing facility must hold current workshop approval from each classification society whose certificates it issues.

At Lmart, we maintain current workshop approvals from CCS, BV, ABS, DNV, LR, NK, and KR — covering the seven major classification societies. This allows us to manufacture heat exchangers for vessels registered with any of these societies without requiring project-specific workshop audits.

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Welding Metallurgy: Why Material Selection Is Only Half the Battle

Selecting the right material is necessary but not sufficient. The welding process must preserve the material's corrosion resistance — and in several important cases, welding can degrade it.

Titanium Welding

Titanium reacts aggressively with oxygen, nitrogen, and hydrogen above approximately 500degC. Welding must be performed under complete inert gas (argon) shielding — not just at the weld pool, but also trailing behind it until the metal cools below the contamination threshold.

Our setup:
- Primary argon shielding at the torch
- Trailing argon shield (minimum 150mm behind the weld pool)
- Backing argon purge (inside the tube or on the reverse side of the joint)
- Oxygen monitoring — maximum 50 ppm O2 in the purge atmosphere before welding begins

Contaminated titanium welds show characteristic discolouration: straw yellow (minor contamination), blue (moderate), white/flaky (severe). Any discolouration beyond light straw requires grinding and re-welding.

Duplex Welding

The challenge is maintaining the 50/50 austenite-ferrite phase balance in the weld metal and heat-affected zone (HAZ):

• Heat input — Too low: excess ferrite (reduced toughness and corrosion resistance). Too high: excess austenite (reduced strength) and potential sigma phase precipitation.
• Interpass temperature — Maximum 150degC for 2205, 100degC for 2507.
• Filler metal — Over-alloyed in nickel (to promote austenite formation in the weld) and nitrogen (to stabilise the austenite).
• Post-weld testing — Ferrite content measurement (ferritescope or point count method) on production welds, confirming 35-65% ferrite.

Cu-Ni Welding

Relatively straightforward, but requires:
- Clean joint preparation (Cu-Ni is sensitive to contamination)
- Matching filler metal (ERCuNi or ECuNi)
- Moderate heat input (Cu-Ni has high thermal conductivity — requires higher amperage than steel)
- No post-weld heat treatment

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Alfa Laval, Tranter, and the Industry's Direction on Exotic Metallurgy

The heat exchanger industry is moving decisively toward wider use of corrosion-resistant alloys:

Alfa Laval's Olmi acquisition introduced a dedicated "exotic metallurgy" production line — manufacturing heat exchangers in titanium, zirconium, tantalum, and high-nickel alloys. This signals market demand: end users are increasingly willing to pay the initial premium for materials that eliminate replacement and maintenance costs over the equipment's life.

Tranter has been promoting titanium plate heat exchangers specifically for marine seawater cooling applications, positioning titanium as a replacement for traditional Cu-Ni shell-and-tube central coolers. Their argument centres on the same lifecycle economics we have described — titanium's zero-maintenance seawater performance eliminates the recurring costs of Cu-Ni tube bundle replacement.

What this means for equipment buyers:

The exotic metallurgy trend is not a fashion — it reflects a rational economic calculation by fleet operators and industrial plant owners. As raw material prices for titanium have moderated (Ti sponge prices have declined from their 2022 peaks), and as fabrication expertise has become more widely available, the cost gap between titanium and traditional materials is narrowing.

For equipment buyers, the key question is no longer "Can we afford titanium?" but "Can we afford not to specify it?" In seawater service with a 20-25 year design life, the answer is increasingly clear.

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Frequently Asked Questions

Q1: What is the best material for marine heat exchanger tubes in seawater service?

Titanium Grade 2 is the highest-performing material for seawater tube service, offering complete immunity to pitting, crevice, and erosion corrosion in natural seawater at temperatures below 80degC. For applications where titanium's initial cost is prohibitive, Cu-Ni 90/10 remains a viable option for moderate-duty service with shorter design lives (8-12 years), and super duplex 2507 provides a middle ground for demanding seawater applications requiring high strength and corrosion resistance.

Q2: How much does corrosion cost the marine industry per year?

According to NACE International (now AMPP), global corrosion costs are estimated at $2.5 trillion per year — approximately 3.4% of global GDP. The maritime sector bears a disproportionate share due to the aggressive seawater environment. For individual vessel operators, heat exchanger corrosion-related costs (replacement parts, dry-docking, off-hire) can exceed $50,000-$200,000 per vessel per replacement cycle, with multiple cycles expected over a 25-year vessel life when using Cu-Ni tubes.

Q3: Can duplex stainless steel replace titanium in seawater heat exchangers?

Standard duplex 2205 (PREN 34-36) does not provide reliable long-term resistance to crevice corrosion in warm seawater (above 20-25degC). Super duplex 2507 (PREN 42-45) approaches titanium's performance in many seawater applications but is not immune — crevice corrosion can still occur in tight geometries at elevated temperatures. For critical seawater applications with 25-year design lives, titanium remains the only material that provides a genuine "fit and forget" solution. Duplex grades are better suited to process-side applications where the primary corrosion challenge is chemical rather than seawater-based.

Q4: Why do some heat exchangers use different materials for tubes and shell?

Mixed metallurgy — such as titanium tubes in a carbon steel shell — optimises cost and performance. The tubes see the corrosive fluid (seawater) and require maximum corrosion resistance. The shell sees the process fluid (oil, freshwater, gas) and can use less expensive materials. The tube-to-tubesheet joint is the critical engineering interface, requiring careful design to manage galvanic effects and crevice geometries between the dissimilar materials.

Q5: What certifications should a marine heat exchanger manufacturer hold?

For classified vessels, the manufacturer should hold workshop approval from the relevant classification society (DNV, BV, LR, ABS, CCS, NK, KR). For pressure-containing equipment, ASME U stamp (for ASME Code vessels) and PED CE marking (for EU-flagged or EU-destined vessels) are standard requirements. ISO 9001 quality management certification provides the framework for consistent production quality. Beyond certifications, the critical factor is the number of qualified welding procedures (PQRs) the manufacturer holds — particularly for exotic materials like titanium and duplex stainless steel.

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Related Reading

• Tube Material Selection: When Titanium or Duplex?
• Fixed Tubesheet vs. Floating Head vs. U-Tube Heat Exchangers
• Heat Exchangers — Complete Product Range
• Projects & References

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