Titanium corrosion resistance by medium

Titanium's corrosion resistance is not a property of the metal itself. It is a property of a film a few nanometres thick — titanium dioxide — that forms instantly in air or water and repairs itself as long as there is enough oxygen or water at the surface to rebuild it. Everything else follows from that one sentence: where the film can repair, titanium is effectively inert; where it cannot, titanium corrodes like a much less exotic metal, and sometimes faster.

How the passive film behaves

The film is stable in oxidising conditions and in neutral aqueous solutions, including chlorides. It is destabilised by reducing conditions, by anhydrous environments where no water is available to repair it, by fluoride ions which dissolve titanium directly, and by sustained hydrogen charging. Temperature accelerates all of it, and so does a crevice, because a crevice consumes the available oxygen and then cannot rebuild the film it has lost.

This is why the same grade can be the correct choice in one plant and a failure in another plant handling the same nominal chemical. Concentration, temperature, aeration, flow and geometry are not secondary details — they are the specification.

Medium-by-medium summary

The ratings below reflect common industrial practice for the three grades most often quoted for corrosive duty. They are a starting point for engineering discussion, not a substitute for corrosion data or a coupon test in the actual medium.

Medium	Gr.2 (CP)	Gr.7 (Pd)	Gr.12 (Ni-Mo)	Notes
Seawater, ambient, flowing	Excellent	Excellent	Excellent	Gasketed joints, not the metal, are the weak point
Seawater / brine above ~70 °C	Limited	Good	Good	Crevice attack under deposits and seals
Wet chlorine gas	Excellent	Excellent	Excellent	Water must be present
Dry chlorine gas	Not recommended	Not recommended	Not recommended	Ignition / pyrophoric reaction risk
Sodium hypochlorite	Excellent	Excellent	Excellent	Watch temperature and chlorate content
Caustic soda (NaOH)	Excellent	Excellent	Excellent	Hydrogen pickup possible at high temperature
Nitric acid, up to ~65%	Excellent	Excellent	Excellent	Oxidising; the film is stable
Sulphuric acid, dilute and aerated	Good	Good	Good	Concentrated or hot: not recommended
Hydrochloric acid, dilute, ambient	Limited	Good	Good	Concentration and temperature limits are narrow
Hydrofluoric acid, any concentration	Not recommended	Not recommended	Not recommended	Dissolves the film and the metal
Wet H₂S / sour service	Limited	Limited	Limited	Hydrogen embrittlement, not general corrosion

Chlorides: the temperature ceiling nobody quotes

Commercially pure titanium is immune to uniform corrosion in seawater at any normal temperature. What limits it is crevice corrosion: under a gasket, a deposit, a lap joint or a tube-to-tubesheet seal, oxygen is consumed faster than it can be replenished, the film cannot repair, and attack starts. The practical threshold quoted in industry guidance is around 70 °C for chlorides; above that, crevice attack becomes the design case rather than an exception.

Two things shift the threshold. Palladium-bearing Gr.7 and the nickel-molybdenum Gr.12 both raise it substantially, which is why desalination and brine heat exchangers increasingly specify them instead of Gr.2. And geometry matters as much as grade: a design that eliminates crevices can survive where a poorly detailed one fails with a more expensive alloy.

Chlorine, hypochlorite and chlor-alkali

Wet chlorine, chlorine dioxide, hypochlorous acid and hypochlorite are all strongly oxidising, and titanium is the standard material for them. This is the basis of titanium's role in chlor-alkali plants: cell components, wet chlorine piping, cooler tube bundles, bleach equipment.

The exception is dry chlorine gas. With no moisture to maintain the film, titanium can react violently — a hazard well known in the industry and the reason dry chlorine service is explicitly excluded rather than rated "poor". If a line can be either wet or dry depending on operation, that is a design question, not a purchasing one.

Nitric, sulphuric and hydrochloric acid

Nitric acid is oxidising, so the film is stable and titanium is excellent up to roughly 65% concentration across normal temperatures. Red fuming nitric acid is a separate case and should not be treated as the same medium.

Sulphuric acid is oxidising when dilute and aerated, and titanium handles it. As concentration and temperature rise the medium becomes reducing, the film destabilises, and attack turns rapid. Dilute, cool, aerated streams are fine; hot concentrated acid is not, and no grade of titanium fixes that.

Hydrochloric acid is the medium where grade choice earns its price. Plain Gr.2 tolerates only dilute acid at ambient temperature, within a narrow window. Palladium (Gr.7) and nickel-molybdenum (Gr.12) shift the potential and extend that window, which is why both are common in pickling lines, chloride leach circuits and acid recovery.

Hydrogen embrittlement and cathodic protection

Titanium absorbs hydrogen, and above roughly 150 ppm in the metal the hydride phase makes it brittle. In service this happens in three ways: cathodic protection that over-protects the titanium, galvanic coupling to a less noble metal in an electrolyte, and environments that generate hydrogen at the surface — wet H₂S, acid pickling, and some high-temperature caustic duties.

The practical consequences for a buyer are simple. If the equipment will be cathodically protected, say so — the protection potential has to be controlled and the titanium should not be the cathode of an uncontrolled couple. If the medium is sour, expect the discussion to move from "which grade" to "is titanium appropriate at all". And if the part is pickled or acid-cleaned during fabrication, specify that hydrogen content is checked on the finished item, not just on the mill certificate.

Design details that decide the outcome

• Crevices. Full-penetration welds, no lap joints, no partial welds, gaskets that do not trap liquid. A crevice converts an excellent material into a limited one.
• Flow velocity. Titanium tolerates high velocities in seawater — higher than copper-nickel alloys — but abrupt changes of section and dead legs still cause erosion and deposit attack.
• Galvanic couples. Titanium is cathodic to steel, aluminium, copper alloys and stainless steel. Coupling accelerates attack on the other metal; insulate where it matters.
• Surface condition. Pickled or bright-finished surfaces perform better than as-rolled with embedded iron contamination, which is a common cause of "unexplained" pitting.
• Temperature margin. Design at the maximum excursion, not the normal operating point. A 20 °C excursion above the crevice threshold is where most failures originate.

How to validate a selection

For anything outside well-trodden service, three steps are cheaper than a failure. First, check published isocorrosion data for the actual concentration and temperature, not the nominal acid. Second, run an immersion test to ASTM G31 with both a plain coupon and a crevice assembly, so you learn which mechanism governs. Third, if the equipment is large or the duty is marginal, run a side-stream coupon in the plant for a few months before committing to the alloy.

Where a project is genuinely marginal, the honest answer is often that titanium is the wrong material and a lined steel or higher-alloy option is cheaper over life — that is a conclusion worth reaching before the purchase order, not after.

Corrosion-grade material is supplied as plate, tube and pipe for heat exchangers, bar and forged or machined components. If you send the medium, concentration, temperature and the standard you are working to, we will confirm what is available in Gr.2, Gr.7 or Gr.12.