Why Titanium Performs Differently
Titanium owes its industrial value to two properties acting together. The first is density, 4.51 g/cm3, which is roughly 57 % of the density of steel and about half that of nickel, so a titanium component of equal strength weighs far less. The second is the passive film of titanium dioxide that forms spontaneously on a clean surface and repairs itself immediately when damaged, provided a small amount of oxygen or water is available. That film is stable across a wide range of pH and temperature, which is why titanium resists seawater, chlorides, hypochlorite and oxidizing acids in conditions that destroy stainless steel and copper alloys.
Grades and Composition
The commercially pure grades are distinguished mainly by oxygen and iron content, which control strength, while the alloyed grades add aluminium, vanadium, palladium or nickel for strength or corrosion performance. Composition ranges below follow ASTM B265.
| Grade | UNS | Typical composition | Minimum tensile strength |
|---|---|---|---|
| Grade 1 | R50250 | Ti balance, O 0.18 max, Fe 0.20 max | 240 MPa |
| Grade 2 | R50400 | Ti balance, O 0.25 max, Fe 0.30 max | 345 MPa |
| Grade 5 | R56400 | Ti balance, Al 5.5 - 6.75, V 3.5 - 4.5 | 895 MPa |
| Grade 7 | R52400 | Grade 2 base plus Pd 0.12 - 0.25 | 345 MPa |
| Grade 12 | R53400 | Ti balance, Ni 0.6 - 0.9, Mo 0.2 - 0.4 | 483 MPa |
Grade 2 is the default material for chemical and marine equipment. Grade 1 is used where maximum formability and ductility are needed. Grade 5, the Ti-6Al-4V alloy, is the structural workhorse for aerospace and medical implants. Grades 7 and 12 are additions of palladium or nickel-molybdenum that widen the corrosion range in reducing acids, and they carry the same mechanical properties as Grade 2 and a slightly stronger base respectively.
Corrosion Behaviour by Medium
In seawater and in chloride solutions titanium behaves better than any conventional stainless steel. It is immune to chloride pitting and crevice corrosion in natural waters up to at least 120 C and is one of the few materials suitable for high-velocity seawater in heat exchanger tubing. In oxidizing acids such as nitric acid, chromic acid and wet chlorine, the passive film is reinforced and the metal performs very well. In hypochlorite and chlorine dioxide service titanium is widely specified for bleach plant equipment.
The limitation concerns reducing acids. Unalloyed titanium is not suitable for hydrochloric or sulfuric acid beyond low concentration and near-ambient temperature, because the passive film cannot be maintained. Grade 7 and Grade 12, with palladium or nickel-molybdenum additions, extend the usable range but do not remove the limitation. Dry chlorine gas must also be avoided because it can cause ignition, and hydrogen uptake in cathodic or strongly reducing conditions can lead to hydride embrittlement in high-pressure components.
Mechanical Properties and Design Values
Grade 2 sheet to ASTM B265 has a minimum yield strength of 275 MPa and elongation of 20 % minimum, so it is comparable in strength to a mild steel but far lighter and far more corrosion resistant. Grade 5 reaches 828 MPa yield and 895 MPa tensile with 10 % minimum elongation, and is used where load-carrying capability matters. Titanium has a modulus of about 105 GPa, roughly half that of steel, so deflection rather than stress often governs the design of thin-walled components, and a higher section thickness may be needed than a steel equivalent. Fatigue strength is good, and the metal retains useful properties from cryogenic temperature up to about 300 C for the commercially pure grades and higher for the alloyed grades.
Principal Industrial Applications
Chemical processing uses titanium for reactors, columns, heat exchangers, piping, pumps and valves handling nitric acid, chlorine compounds, organic acids and oxidizing salt solutions. Desalination and power plant condensers use welded titanium tube because it resists flowing seawater and ammonia-bearing cooling water without the erosion problems of copper alloys. Marine engineering uses titanium for seawater piping, heat exchangers, propeller shafting components and subsea hardware. Aerospace and defence apply the alloyed grades to airframe structures and engine parts where weight reduction is critical. Medical devices use titanium for implants and surgical instruments under ASTM F67 for unalloyed grades and ASTM F136 for the extra-low-interstitial version of Ti-6Al-4V.
Fabrication and Applicable Standards
| Product form | Reference specification |
|---|---|
| Plate, sheet, strip | ASTM B265, ASME SB-265 |
| Seamless and welded tube | ASTM B338, ASME SB-338 |
| Bar, billet, forging stock | ASTM B348 |
| Welded pipe | ASTM B862 |
| Implant materials | ASTM F67, ASTM F136 |
Titanium is welded by gas tungsten arc and plasma processes under fully inert shielding, and the weld pool, heat-affected zone and even the hot back face must be protected from air until the metal cools below about 400 C, because oxygen and nitrogen contamination causes embrittlement. Forming is carried out cold, with allowance for the springback that results from the low modulus, and machining requires slow speeds, heavy feeds and copious cooling to avoid galling and ignition of chips.
Frequently Asked Questions
Q: Why is titanium used instead of stainless steel in seawater?
A: Titanium is immune to chloride pitting and crevice corrosion in natural waters up to at least 120 C, while austenitic stainless steels pit and can crack under stress, so titanium tube and piping deliver far longer service life in seawater systems.
Q: Which titanium grade should be used for corrosion service?
A: Grade 2 is the standard choice. For reducing acids, Grade 7 with palladium or Grade 12 with nickel and molybdenum extends the usable range considerably.
Q: Can titanium resist hydrochloric acid?
A: Only at low concentration and near-ambient temperature in the unalloyed grades. More severe hydrochloric or sulfuric acid service requires Grade 7, Grade 12 or a different alloy family such as a nickel-molybdenum grade.
Q: How strong is titanium compared with steel?
A: Commercially pure Grade 2 has a minimum tensile strength of 345 MPa, similar to mild steel, but at 4.51 g/cm3 it is far lighter. The Ti-6Al-4V alloy reaches 895 MPa tensile and 828 MPa yield, comparable to alloy steel at about half the weight.
Q: Why is titanium welding so demanding?
A: Because titanium absorbs oxygen and nitrogen above about 400 C and embrittles. The weld pool, heat-affected zone and back side must all be shielded with inert gas until the joint cools below that temperature.
Q: What is the maximum service temperature?
A: Commercially pure grades are used to about 300 C. Above that range, oxidation and creep become controlling and a nickel-base or other high-temperature alloy is normally selected.
