The Corrosion Case in Seawater
Titanium resists seawater by forming a stable titanium dioxide film that reforms within milliseconds of mechanical damage. In clean, flowing, oxygenated seawater the metal is effectively inert: uniform corrosion rates are immeasurably low and the metal does not pit. Two qualifications matter in practice. In hot, stagnant or chlorinated seawater, dissolved oxygen inside a tight crevice is consumed faster than it can diffuse in, the local potential drops, and the film breaks down, producing crevice corrosion. Titanium can also absorb hydrogen and embrittle if it is cathodically protected or coupled to a more active metal for long periods at elevated temperature.
Grade Selection for Marine Service
| Grade | UNS | Marine behaviour |
|---|---|---|
| Grade 2 (commercially pure) | R50400 | Standard choice for ambient and moderated seawater piping, tube bundles, coolers |
| Grade 7 (Ti-0.15Pd) | R52400 | Palladium raises breakdown potential; better in reducing acids and crevice-prone hot chloride service |
| Grade 12 (Ti-0.3Mo-0.8Ni) | R53400 | Improved resistance to hot chlorides and crevice corrosion, widely used in offshore heat exchangers |
| Grade 5 (Ti-6Al-4V) | R56400 | Structural components, subsea hardware, fasteners; not a corrosion upgrade |
As a working rule, unalloyed Grade 2 is comfortable in seawater below roughly 70 C where flow is continuous, while the palladium-bearing and molybdenum-nickel grades extend the safe envelope well above 100 C and cover the low-velocity pockets that cause crevice attack. Flow velocity should be kept high enough to prevent deposits while staying inside the erosion limit of thin-wall tube.
Heat Exchangers and Ocean Thermal Energy Conversion
Heat exchangers are the largest marine application. Titanium tube bundles are standard in offshore cooling circuits, platform utility systems, marine engine charge-air coolers and desalination plants, because a wall of 0.5 to 0.7 mm transfers heat efficiently while outlasting every copper-based alternative in chlorinated seawater.
Ocean thermal energy conversion (OTEC) is the most demanding version of the same problem. The cycle exploits the temperature difference between warm surface water and cold water drawn from roughly 700 to 1000 metres depth, using ammonia as the working fluid. That combination of ammonia, seawater and enormous heat transfer surface puts titanium in a class of its own: it resists both fluids, withstands the saline environment, tolerates high velocity, and its non-magnetic, acoustically transparent character suits sensors and acoustic equipment nearby. Cold-water pipes, condenser and evaporator tube bundles, and pumps for the seawater circuits are the main titanium consumers.
Offshore Systems: Piping, Firefighting and Utilities
Seawater lift and injection lines. Titanium piping replaces internally lined steel and avoids the liner failure that ends the life of conventional seawater lines.
Firefighting and deluge systems. Titanium pumps, monitors, nozzles and pipework tolerate long idle periods followed by full seawater flow, a duty cycle that corrodes steel and copper alloys.
Ballast water and desalination units. Titanium plate heat exchangers and electrolytic chlorination cells operate in the most aggressive streams on board.
Instrumentation. Non-magnetic behaviour and dimensional stability at temperature make titanium a natural material for acoustic and sensor housings.
Fabrication and Design Cautions
Welding. GTAW in a purge chamber or with a trailing shield; a grey or powdery white weld indicates oxygen contamination and must be cut out and redone.
Iron contamination. Carbon steel brushes, grinding wheels or lifting gear leave iron smears that corrode and can seed attack; use dedicated stainless or titanium tooling and pickle after fabrication.
Galvanic couples. Titanium is the cathode in most pairings and will accelerate corrosion of steel, aluminium and copper alloys. Insulate, use transition pieces or apply coatings.
Cathodic protection. Do not bond unprotected titanium directly to a sacrificial anode system at elevated temperature; check hydrogen uptake risk in the design.
Support and expansion. Allow for thermal expansion and design supports so that the tube or pipe is not clamped inside a crevice-forming sleeve.
FAQ
Q: Does titanium corrode in seawater?
No appreciable uniform corrosion occurs in clean, flowing seawater. The realistic risks are crevice corrosion in hot stagnant zones and hydrogen embrittlement under cathodic protection.
Q: Which grade should be used for a hot seawater heat exchanger?
Grade 12 (Ti-0.3Mo-0.8Ni) or Grade 7 (Ti-0.15Pd) where temperature exceeds roughly 70 C or where stagnant pockets and deposits are likely; Grade 2 is adequate for cooler, continuously flushed circuits.
Q: Why is titanium used instead of cupronickel in condensers?
Cupronickel releases copper ions and is vulnerable to erosion and ammonia attack, while titanium is immune to both and permits thinner walls and higher flow velocities.
Q: What role does titanium play in OTEC plants?
It is the preferred material for the large condenser and evaporator tube bundles and the seawater circuits, because it must survive ammonia on one side and warm or cold seawater on the other.
Q: Is titanium suitable for offshore firefighting equipment?
Yes. Pump bodies, pipework, monitors and nozzles made from titanium tolerate intermittent full-flow seawater service, which is the duty that most quickly destroys steel systems.
Q: What is the main practical risk when installing marine titanium?
Contamination and coupling. Iron smears from steel handling and galvanic contact with steel structures cause more field problems than the seawater itself.
