Titanium Tube Applications by Industry and Service Duty

Titanium tubing has moved from a specialist aerospace material to a standard engineering choice in any system where chlorides, oxidising acids, seawater or high-purity process fluid would destroy steel and copper alloys within a few years. The reason is not simply that titanium does not corrode; it is that the metal lets the designer delete the corrosion allowance, raise the allowable flow velocity and stretch the replacement interval from years to decades. This review works through the applications industry by industry, linking each one to the service condition that drives it, the grade that is normally specified and the standard that governs the supply.

Property Basis for Application Engineering

Four properties decide whether titanium tube is the right answer for a given system. The first is the passive titanium dioxide film, which forms instantly in air and re-forms in any environment that contains water or oxygen, giving the metal virtual immunity to general corrosion in sea water and in chlorinated or hypochlorinated water. The second is the very high tolerance of flow velocity: titanium is not subject to the erosion-corrosion that limits copper-nickel tubes to a few metres per second, so seawater systems can be designed with smaller bores and fewer tubes for the same heat duty. The third is temperature capability in the region of 300 deg C for the unalloyed grades, which is well above the practical limit of aluminium and above what is required in most heat-rejection duty. The fourth is the density of about 4.51 g/cm3, roughly 56% of steel, which matters wherever weight is paid for, as it is on an aircraft, on a ship or on an offshore structure.

The same properties that make the metal attractive also define its limits, and those limits are the reason a selection decision is never automatic. Thermal conductivity is only about half that of copper alloys, so the designer has to earn back heat transfer through thinner wall and higher velocity. The elastic modulus of roughly 100-110 GPa is low compared with steel, which increases tube vibration sensitivity. And titanium behaves badly in a small number of specific chemistries, notably hydrofluoric acid and fluoride-bearing media, anhydrous methanol, dry chlorine and high-pressure oxygen, so any new service has to be screened against those exclusions before the grade is fixed.

Tube material	Critical seawater velocity, m/s	Chloride pitting and crevice risk	Thermal conductivity, W/m-K	Typical condenser wall, mm
Carbon steel, coated or cathodically protected	1.0-1.5	High without protection	about 50	2.0-3.0 plus allowance
90/10 copper-nickel	2.4-3.0	Moderate; sensitive to sulphide and ammonia	about 40	0.9-1.2
70/30 copper-nickel	3.0-4.5	Lower than 90/10 but still sulphide sensitive	about 29	0.9-1.2
Austenitic stainless steel type 316L	below about 1.5 in chlorinated sea water	High; pitting and crevice attack under deposits	about 16	0.7-1.0
Titanium Grade 2	above 30; erosion-corrosion is not the limiting factor	Effectively immune in clean oxidising sea water	17-22	0.5-0.9
Titanium Grade 12	above 30	Immune in clean sea water and resistant to crevice attack in hot brine	17-22	0.5-0.9

The comparison has a practical consequence that is often missed in first-cost discussions. A copper-nickel tube must be kept below roughly 2.5 m/s to avoid erosion of the protective film, while a titanium tube is not velocity limited, so a titanium bundle can be designed with a flow path that delivers a higher tube-side film coefficient. That partially offsets the lower conductivity of the metal, and the remaining gap is closed by the thinner wall, which shortens the conduction path. What titanium cannot do is match a copper alloy in a system that is already velocity limited by other equipment or by a very low approach temperature.

Aerospace and Airframe Fluid Systems

Aerospace was the first large user of titanium tubing, and the driver was weight with temperature capability rather than corrosion resistance. Aircraft hydraulic systems run at system pressures in the region of 21 MPa, with peak pulses well above that, and the lines must remain leaktight across a temperature range that reaches 150 deg C or more near engine and brake zones. Aluminium alloy tube is lighter still but loses strength at temperature and is difficult to join reliably; stainless steel tube has the strength and the fatigue behaviour but weighs roughly 40% more for the same burst capability. Titanium resolves the conflict, and the standard answer for hydraulic lines is the Ti-3Al-2.5V alloy, Grade 9 in the ASTM designation, supplied as seamless or welded tubing to the requirements of ASTM B338 and the associated aerospace material specifications for cold-worked and annealed hydraulic tubing.

That choice brings its own manufacturing discipline. Grade 9 tubing is bend radius limited and is supplied in the annealed condition to allow rotary draw bending without cracking, with flaring and flareless fitting performance verified by test. Wall thickness is selected on burst pressure, fatigue life under pressure cycling, and the ability of the tube to survive clamp and grommet loads in service. Cleanliness is an acceptance criterion in its own right: hydraulic tubing is flushed and capped, with limits on particulate and on residual manufacturing residue, because a particle that escapes flushing travels to a servo valve or an actuator.

The second aerospace role is ducting and fluid transport in the airframe rather than in the hydraulic circuit. Bleed-air, anti-ice, inert-gas and fuel-system lines use unalloyed Grade 2 tube because it is formable, weldable and light, and because it tolerates the long service life demanded of a commercial airframe. The step that must never be taken is the substitution of titanium into high-pressure gaseous oxygen service. Titanium can ignite and burn in oxygen-enriched atmospheres under impact or particle impingement, and both aircraft and process-industry practice restricts titanium in oxygen systems to specific low-pressure cases that are individually assessed.

Chemical Processing and Petrochemical Service

Chemical plant is the most technically varied application area for titanium tube, because the metal is applied across a spectrum of chemistries in which some are entirely benign to the oxide film and others attack it. Oxidising media are the natural territory: nitric acid across a wide concentration range, chromic acid, hypochlorite, chlorine dioxide, chlorate and most organic acids in oxidising conditions. In these services Grade 2 tube is normally specified for shell-and-tube exchangers, bayonet heaters, spargers and transfer lines. Titanium is also the reference material for wet chlorine gas handling, which is a defining application in chlor-alkali plants, where coolers, dryers and cell-liquor exchangers are built with titanium tube bundles.

Where the chemistry is reducing rather than oxidising, the standard unalloyed grades are vulnerable, and the answer is a palladium-modified grade. Grade 7 contains between 0.12% and 0.25% palladium and behaves as self-repassivating in dilute hydrochloric and sulphuric acid, in acid chlorides and in mixed reducing media, which makes it the normal choice for acid coolers, hydrolysis and pickling services, and for exchangers in mineral-processing circuits. The palladium addition raises the price of the tube significantly because it is a precious metal, so Grade 7 is used where it is needed rather than as a general upgrade.

Hot chloride brine poses a different problem. General corrosion is negligible, but where the metal sits under a deposit, a gasket, a tube-sheet crevice or a spacer, the environment inside the crevice becomes acidic and oxygen-starved, and unalloyed titanium can begin to corrode there. Grade 12, alloyed with molybdenum and nickel, is designed for exactly that condition and is widely used in brine evaporators, crystallisers, salt and soda-ash plants and in exchanger service where fouling is expected. Its mechanical properties are also higher than Grade 2, which allows a slightly thinner wall for the same pressure rating.

The exclusion list is short but absolute. Titanium is attacked by hydrofluoric acid and by fluoride-containing media at essentially any concentration, so fluoride-bearing cleaning agents must never be used on a titanium surface. Anhydrous methanol causes stress corrosion cracking. Dry chlorine, meaning chlorine with a water content low enough that the protective film cannot form, can ignite titanium under some conditions; industry practice keeps a minimum moisture level in chlorine and in chlorine-bearing process streams, and designers specify titanium for wet chlorine but not for the dry gas. Where titanium is coupled to carbon steel in a seawater system and cathodic protection is applied, the titanium itself is not attacked at ambient temperature, but hydrogen uptake becomes a consideration at elevated temperature and at strongly cathodic potentials, and that combination is normally evaluated rather than assumed.

Seawater Desalination and Water Reuse

Desalination is the application in which titanium tube has displaced almost everything else. In multi-stage flash plants the brine heater and the heat recovery sections handle deaerated sea water at top brine temperatures typically in the range 90-120 deg C, dosed with anti-scale chemicals and periodically chlorinated. Copper alloys suffered from erosion and from the effect of sulphide and residual chlorine on their protective film; titanium tube allows design velocities of 2-2.5 m/s and above without erosion, which permits a higher heat transfer coefficient, and it tolerates the chlorination regime without loss of life. Grade 2 is standard, with Grade 7 selected for venting and deaerator sections where the oxygen level is low and the environment becomes reducing.

Multi-effect distillation plants use similar reasoning for their condenser and preheater bundles, and the trend to larger effects has favoured welded titanium tube because it can be produced in long lengths, which reduces the number of rolled joints in a bundle. Wall thickness in these plants is commonly in the 0.4-0.7 mm region for Grade 2, which is possible precisely because there is no corrosion allowance to add.

Reverse osmosis plants are built around spiral-wound membranes and high-pressure pumps that run at 55-80 bar, and their seawater piping operates in a brutally corrosive combination of concentrated brine, oxygen and variable flow. Titanium Grade 2 tube and pipe are used for high-pressure brine lines, permeate and interstage piping and for energy-recovery equipment connections, where the weight saving relative to duplex stainless steel is also a factor because high-pressure piping must be supported and restrained. Because the oxide film re-forms immediately, shutdowns and dry storage do not require the protective measures that stainless steel sometimes needs, which suits the intermittent duty of many membrane plants.

Power Generation: Surface Condensers and Nuclear Service

The seawater-cooled surface condenser is the largest single application for titanium tube. In a large coastal plant the condenser may contain hundreds of kilometres of tubing, and the material decision determines the maintenance regime for the whole station. Titanium replaced brass and copper-nickel largely because of two failure mechanisms: erosion-corrosion at inlet ends and in high-velocity zones, and ammonia attack, which cracks copper alloys when ammonia carried in the steam condenses in the air-cooled sections. Titanium resists both, and it also tolerates the chlorination used to control biofouling, which is stopped or reduced in copper-alloy plants to protect the tube material.

Nuclear service imposes additional requirements rather than a different metallurgy. Grade 2 tube is used for the main condensers of seawater-cooled stations and for various cooling and heat-exchanger duties, with cleanliness, dimension and documentation requirements set by the plant specification on top of the ASTM tube standard. Because titanium retains its strength and its oxide film in high-temperature water, it also appears in auxiliary exchanger and safety-related cooling circuits, where its freedom from chloride stress corrosion cracking is a design argument in itself. In fossil plants, flue-gas desulphurisation systems use titanium for the reheat exchangers and for critical lining and duct components downstream of the absorber, because the flue gas there is a saturated, acidic, chloride-bearing atmosphere that attacks most metals.

What causes titanium condenser tube failures is not corrosion. The published failure experience across the industry points consistently to mechanical mechanisms: tube vibration and fretting at support plates, damage during installation, tube-to-tubesheet joint defects, and water-side erosion from debris carried in the cooling water. That is why the engineering package for a titanium bundle concentrates on support plate spacing, inlet-end protection, debris filters, flow distribution and inspection during assembly, and why the low elastic modulus of titanium is treated as a design constraint rather than a footnote.

Marine, Offshore and Shipboard Systems

Shipboard seawater systems are a natural fit, because the piping must survive continuous exposure to flowing, aerated, chlorinated sea water for the life of the vessel and because space and weight are at a premium. Titanium is used for seawater cooling circuits, ballast and bilge systems on high-value vessels, fire-fighting and wash-down lines, wet exhaust and scrubber discharge piping, and the tube bundles of onboard desalination and refrigeration condensers. The velocity tolerance is the decisive advantage afloat: a titanium line can be sized smaller than a copper-nickel line for the same flow, which saves both weight and space in a crowded machinery space.

Offshore installations extend the same logic to the subsea and topside levels. Grade 2 and Grade 9 tubing are used for hydraulic control and chemical injection lines, for instrumentation tubing, for seawater lift and injection piping, and for heat exchangers on floating production facilities. Here the weight saving compounds with the elimination of a corrosion allowance and with the reduction in maintenance offshore, where the cost of a repair is dominated by the intervention rather than by the material. Two cautions apply: anhydrous methanol injection lines need specific evaluation because of the cracking risk, and piping that carries sand or other solid particles at high velocity is designed with the erosion resistance of titanium in mind but still with attention to local geometry at bends and reducers. Classification society rules for seawater piping systems, rather than any single manufacturer recommendation, are normally the governing reference for wall thickness and joint design in these projects.

Medical, Pharmaceutical and High-Purity Gas Service

Medical applications exploit a different set of properties: biocompatibility, corrosion resistance in body fluid, high strength-to-weight ratio, non-magnetic behaviour and the absence of nickel. Unalloyed titanium for surgical implants is specified by ASTM F67, and the wrought Ti-6Al-4V ELI alloy for load-bearing implants by ASTM F136, with the corresponding ISO 5832-2 and ISO 5832-3 metallurgical standards referenced in international device submissions. Corrosion performance of implant materials is characterised in the laboratory by tests such as the cyclic potentiodynamic polarisation method of ASTM F2129, which is used to screen surface treatments and finishes.

Tube and tubular components appear in medical practice as cannulae, catheter and instrument shafts, endoscopic components, biopsy and aspiration needles, and the internal channels of devices that must be cleaned and sterilised repeatedly rather than used once. Titanium's resistance to autoclave cycles, to disinfectants including chlorine-releasing agents, and to the saline and acidic media encountered clinically, is the reason it is selected for these parts. The same reasoning carries into pharmaceutical and bioprocessing plants, where titanium tube is used for corrosive process streams, for brine and acid duties in API manufacturing, and for ultrapure water and chemical distribution lines where a metallic contamination risk has to be eliminated. Its use in ultrapure water and in oxidising chemicals such as ozone and hydrogen-peroxide-bearing streams follows from the stability of the oxide film; the corresponding warning is that fluoride-containing cleaning agents, which appear in some bioprocess cleaning regimes, are incompatible with titanium and must be replaced with an alternative chemistry.

Application	Service environment	Typical grade	Governing specification	Selection driver
Aircraft hydraulic line	Mineral hydraulic fluid, 21 MPa class, cycling to 150 deg C	Grade 9 (Ti-3Al-2.5V)	ASTM B338 plus aerospace tubing specifications	Strength-to-weight ratio, fatigue and flare performance
Mineral acid cooler	Dilute hydrochloric or sulphuric acid, reducing	Grade 7 (Ti-0.12-0.25Pd)	ASTM B338	Self-repassivation in reducing acid
Chlor-alkali wet chlorine exchanger	Moist chlorine gas, chlorine-containing brine	Grade 2	ASTM B338	Stability of the oxide film in wet oxidising chlorine
Multi-stage flash brine heater	Deaerated sea water, 90-120 deg C, chlorinated	Grade 2, Grade 7 in vent sections	ASTM B338, GB/T 3625	Chloride immunity with high allowable velocity
Brine evaporator and crystalliser	Hot concentrated chloride brine with deposits	Grade 12 (Ti-0.3Mo-0.8Ni)	ASTM B338	Crevice corrosion resistance under fouling
Seawater-cooled surface condenser	Chlorinated sea water, steam side with ammonia	Grade 2	ASTM B338, ASME SB338	Immunity to erosion-corrosion and to ammonia cracking
Reverse osmosis high-pressure line	Concentrated brine at 55-80 bar	Grade 2	ASTM B861 or ASTM B862	Pressure service with no corrosion allowance
Shipboard seawater cooling	Flowing aerated sea water, biofouling control	Grade 2	ASTM B338, classification society rules	Weight saving and unrestricted velocity
Implant and surgical device tubing	Body fluid, sterilisation and disinfection cycles	Grade 2, Grade 23 (Ti-6Al-4V ELI)	ASTM F67, ASTM F136, ISO 5832-2 and 5832-3	Biocompatibility, nickel-free, non-magnetic

FAQ

Q: Can titanium tube be used in dry chlorine gas service?
No. Titanium relies on a water-bearing oxide film for passivity, and dry chlorine can react with the bare metal to the point of ignition. Chlor-alkali and chemical practice keeps a controlled minimum moisture content in chlorine and specifies titanium for wet chlorine, while dry gas duty is given to other materials. The moisture limit is set by the process design rather than by the tube supplier and should be written into the operating envelope.

Q: Why can titanium tubing crack in methanol service?
Anhydrous methanol promotes stress corrosion cracking in titanium, and cracking can occur in the presence of residual stress from bending, welding or forming. Water content in the methanol is the controlling variable, so methanol injection lines on offshore facilities are either operated with a defined water content, designed with low residual stress and stress relief, or allocated to a different material.

Q: How does titanium tube compare with copper-nickel tube in a seawater condenser?
Titanium is immune to the erosion-corrosion and ammonia cracking that limit copper alloys, allows much higher water velocity and needs no corrosion allowance, so a thinner wall does the same mechanical job. Its thermal conductivity is roughly half that of 90/10 copper-nickel, which is partly offset by the thinner wall and by the higher velocity the material permits. The remaining differences are that titanium requires careful vibration design because of its lower elastic modulus, and that its first cost is higher while its service life is normally much longer.

Q: Is titanium tube suitable for high-pressure oxygen service?
Titanium can ignite and burn in oxygen-enriched atmospheres when it is impacted or when particles impinge on it at speed, so it is not a general-purpose material for high-pressure gaseous oxygen. Applications involving oxygen require individual assessment of pressure, velocity, particle content and ignition energy, and the outcome is frequently a different alloy.

Q: How should titanium tubes be cleaned without damaging them?
Mechanical cleaning with soft brushes and water jetting, and chemical cleaning with nitric-acid-based or citric-acid-based formulations, are the conventional routes. Cleaning agents containing hydrofluoric acid or any fluoride salt must be avoided because they attack titanium directly, and carbon steel brushes or tooling should not be used because embedded iron creates sites for localised attack. Steam or alkaline cleaning is acceptable where the chemistry of the process allows it.

Q: Does titanium tube need cathodic protection in sea water?
Titanium does not require cathodic protection and is not consumed by it, which is why titanium condensers are often paired with protected carbon steel water boxes and tubesheets. The point to evaluate is hydrogen uptake: at ambient temperature titanium tolerates cathodic potentials without difficulty, but at elevated temperature and strongly cathodic conditions hydride formation and embrittlement become possible, so the combination of hot seawater, cathodic protection and a titanium pressure boundary is assessed case by case. Galvanic coupling to carbon steel or to a sacrificial anode is otherwise acceptable and does not require electrical isolation of the titanium.