Shipyards Bet on Copper‑Nickel and Titanium to Keep Freshwater Flowing at Sea

Modern ships depend on compact distillers—shell‑and‑tube or plate‑type heat exchangers (metal tubes or plates that transfer heat between fluids)—to turn seawater into fresh water for washing, cooking, and drinking. Demand is not trivial: on board, each person may use on the order of 100 L/day (marineengineeringonline.com), and some units run as high as 30 tonnes/day (same source). The catch: seawater is unforgiving. Corrosion depends on dissolved oxygen, chlorides, velocity, and temperature (copper.org) (copper.org), so heat‑exchanger tubes must be specified with care.

Heat exchanger tube alloys and seawater performance

Copper–nickel (Cu–Ni) alloys—especially 70/30 and 90/10 compositions (copper 70–90%, nickel remainder, often with a few percent iron)—have long been the default for marine heat exchangers for their balance of conductivity, strength, biofouling resistance, and cost. Well‑formed Cu–Ni corrodes slowly in seawater: long‑term tests show 90/10 Cu–Ni (Alloy C70600) after stabilization (3+ years) has corrosion rates on the order of 0.001 mm/year (0.05 mils/yr) (copper.org). Typical 70/30 Cu–Ni (Alloy C71500) behaves similarly.

By contrast, 316L stainless steel can corrode on the order of 0.01–1 mm/yr under accelerated conditions; one seawater test logged ~0.25 mm/yr at high flow (≈8.2 m/s) for 70/30 Cu–Ni versus ~1.8 mm/yr for 316L under the same conditions (copper.org). Cu–Ni’s corrosion products form a protective cuprous‑oxide (Cu₂O) film that starts within hours and matures after roughly three months in flowing seawater—once mature, steady‑state corrosion is very low (copper.org) (copper.org).

Titanium (typically commercially pure Grade 1–2) sits at the top of the seawater‑resistance ladder. Thanks to a rapidly forming passive oxide film, it is essentially immune to general corrosion in seawater—even with chlorine or hydrogen sulfide present. In one desalinator where Cu–Ni tubes were failing in bromine‑rich seawater, a retrofit to titanium tubes yielded “no failure caused by corrosion” thereafter (titaniums.com.au). Titanium resists stress corrosion cracking, pitting, and crevice attack in chlorinated or warm seawater. Its trade‑off is cost—raw titanium can be 2–6× the price of Cu–Ni per kilogram—but its density is about half that of copper and tube walls can be much thinner (e.g., ~0.3 mm vs 1.0 mm), which moderates weight and even overall cost in practice (titaniums.com.au) (titaniums.com.au). Large desalination plants and naval units increasingly opt for titanium tube bundles for “maintenance‑free” longevity (titaniums.com.au) (titaniums.com.au).

Biofouling resistance and cleaning frequency

Cu–Ni’s biofouling resistance is a quiet advantage: cuprous ions discourage algal films, which reduces cleaning. In one study, 90/10 Cu–Ni tubes needed cleaning only once in ~110 days to maintain heat transfer, while titanium—lacking any biocidal effect—would require 12× more frequent cleaning over the same period (copper.org). In practice, copper–nickel exchangers often run without continuous chlorination, whereas stainless or titanium exchangers must be heavily chlorinated or cleaned frequently (same source). When chlorination is used to control macrofouling, operators may deploy electrochlorination systems; however, the timing of oxidants matters (see commissioning guidance below).

Alloy selection in marine evaporators

Designers typically choose 70/30 or 90/10 Cu–Ni for evaporator tubes, often pairing them with Cu–Ni or Monel tube sheets and waterboxes to avoid galvanic issues (marineengineeringonline.com) (copper.org). Where extreme durability is needed, titanium is specified. Regional practice echoes this: Indonesian industry sources note plate‑type freshwater generators often use SS 304 or 316, with titanium as the “best” (though costly) choice (migas-indonesia.com). Historically, tube‑type generators used aluminum‑brass tubing (marineengineeringonline.com), and some designs include knitted wire of Monel metal (same source).

Commissioning and protective film formation

Material selection is only half the battle. Proper commissioning and start‑up are critical for seawater service. Unprotected copper‑alloy surfaces must form their protective films under clean, oxygenated conditions (copper.org). For Cu–Ni tubes, that typically means an initial several‑week run with freshly filled, filtered seawater—avoiding polluted harbor water, oil, and sulfides—followed by careful flush and drain of acid test water and a light purge before operation (copper.org).

Many early failures occur when systems were never “salt‑water commissioned,” so no protective film formed (copper.org). During start‑up, operators often dose ferrous ion (Fe²⁺) via FeSO₄ (ferrous sulfate) to accelerate film formation; small continuous injections (~0.5–2 ppm, where ppm is parts per million) promote a brown Fe–Cu oxyhydroxide coating on Cu–Ni surfaces (copper.org) (same source). One guideline found that adding ~2 ppm Fe²⁺ for weeks reduced Cu–Ni tube failure rates from ~1.18% per 10,000 h to only 0.048%—about a 25× improvement (copper.org), and if iron anodes or sacrificial steel are present, they can naturally supply Fe²⁺ over time (same source). Many yards meter FeSO₄ with dedicated dosing pumps to maintain consistent ppm levels during this brief conditioning window.

Timing with oxidants is key: do not chlorinate during the iron‑dosing period because chlorine erodes nascent oxide layers (copper.org). Once the film is mature, regular (lower‑dose) chlorination may be resumed to control macrofouling, if needed (same source).

Galvanic compatibility and flow velocity limits

Where dissimilar metals meet, galvanic cells can accelerate attack on the more active metal. Designers keep tubing, sheets, and fittings galvanically compatible—e.g., 90/10 Cu–Ni tubes with Cu–Ni waterboxes/tubesheets; if stainless is used, steel or zinc anodes are added for cathodic protection (copper.org). Special care is needed when titanium components are adjacent to Cu–Ni; titanium is much nobler and can induce rapid Cu–Ni corrosion unless isolated or provided with an anode (same source). In practice, evaporator tube bundles and condensers are made of uniform or similar alloys to avoid such cells (marineengineeringonline.com) (copper.org).

Flow matters. Cu–Ni protective films are shear‑sensitive: impingement or erosion‑corrosion begins if design velocities exceed ~3–4 m/s for 90/10 Cu–Ni; systems are sized to keep velocities below this limit (copper.org). Stainless steels are less sensitive to impingement yet may pit at low flow if stagnant.

Cleaning routines, monitoring, and standards

Even corrosion‑resistant tubes foul. Operators schedule periodic cleaning of evaporator bundles. For Cu–Ni, mild mechanical cleaning (sponge balls, brushing) removes sediments while preserving the protective film (copper.org). For stainless or titanium, more aggressive cleans (dilute acids or chelants) may be needed more frequently. Inline salinity monitors and alarms ensure product water stays below safe limits; if salinity creeps up, tube leaks or film deterioration are likely.

Robust inspection programs align with outage or dry‑dock cycles. Tasks include visual inspection of tubesheets, ultrasonic thickness checks at tube ends (impingement zones), and verification of welds or brazed joints. Anodes (if fitted) are replaced as they deplete. Product water is sampled for conductivity and microbiology; international rules (IMO SOLAS, WHO ship‑water guidelines) set potable standards—for example, salinity < ~10 ppm NaCl equivalent—so any deviation triggers remedial action. Shipyards often rely on spare parts and consumables to keep inspection kits and anodes in rotation.

Corrosion cost and maintenance economics

Because seawater corrosion can progress slowly, inspections are easy to defer—with outsized costs. NACE’s corrosion impact study puts the global cost of corrosion at ~$2.5 trillion (3.4% of GDP), and good corrosion control can cut maintenance costs by up to 30% (ampp.org). In shipping specifically, one analysis estimates U.S. fleet corrosion costs at ~$2.7 billion/year, with ~$800 million in downtime costs (rustbullet.com). Avoiding a single tube leak can avert emergency port calls.

Case experiences echo the data: failing to form a protective film in a new distiller led to repeated clogging and failures; a few weeks of controlled start‑up and sponge‑ball runs thereafter delivered years of steady operation.

Material performance summary (corrosion rates)

• 70/30 Cu–Ni (C71500): forms protective Cu₂O/Fe‑oxide film; corrosion ≪0.02 mm/yr in calm water (copper.org); ~0.25 mm/yr at high flow (~8.2 m/s) (copper.org). Excellent biofouling resistance. Limited by erosion‑corrosion with a “breakaway” velocity ~3–4 m/s (copper.org).
• 90/10 Cu–Ni (C70600): similar behavior; corrosion ≈0.012 mm/yr after 1 yr and ≈0.001 mm/yr after 3–7 yr (copper.org). Excellent heat transfer (thermal conductivity ~29–47 W/mK, where W/mK is watts per meter‑kelvin).
• Titanium (Grade 2, commercially pure): near‑zero corrosion (stable passive film); unaffected by flow. Requires thin wall due to lower conductivity (~17 W/mK) (titaniums.com.au). Cost ~2–6× Cu–Ni by mass but ~½ density; minimal maintenance (titaniums.com.au) (titaniums.com.au).
• 316L stainless steel: ~0.02 mm/yr in still water to ~1.8 mm/yr at 8 m/s in seawater tests; pitting/crevice risk if chlorides are high; heavy chlorination or frequent cleaning needed; thermal conductivity ~two‑thirds of Cu–Ni (copper.org).
• Monel 400 (≈67Ni–33Cu): excellent all‑round seawater resistance, including against sulfides and ammonia; no pitting; typically reserved for harshest duty due to cost (copper.org).

Source: empirical seawater tests and industry associations, including copper.org, copper.org, copper.org, and titaniums.com.au.

Operating takeaway and lifecycle emphasis

In practice, operators follow an “alloy + maintenance” strategy: select Cu–Ni or Ti for tubes; install sacrificial anodes or coatings where needed; commission with iron‑assisted film formation; apply controlled chlorination; schedule weekly/quarterly cleanings and yearly inspections. A fully passive Cu–Ni tube system can operate with virtually zero metal loss (<0.001 mm/yr) after commissioning (copper.org), whereas a poorly maintained steel system might lose 1–2 mm/yr and become unsafe within a few years. The cost case is equally clear: corrosion control cuts maintenance and downtime, preserves potable‑water integrity, and avoids sudden failures (ampp.org) (rustbullet.com).