Titanium is a reactive metal. What protects it is a titanium dioxide film only a few nanometres thick that forms the instant the metal meets air or water, and re-forms in milliseconds after damage as long as oxygen or moisture is present. The film fails where oxygen is excluded or where fluoride is present.
Titanium’s reputation for corrosion resistance is slightly misleading. The metal itself is reactive, very reactive. What protects it is a film about a few nanometres thick that forms the instant titanium meets air or water: a layer of titanium dioxide (TiO₂) bonded so tightly and reforming so fast that the metal underneath is effectively sealed off. Understand that film, how it works and the few places it doesn’t, and titanium grade selection stops being guesswork.
The passive film, and why it’s special
Bare titanium oxidises immediately. That sounds like a problem; it’s the whole defence. The oxide that forms is dense, well-adhered and chemically stable, and crucially it is self-healing: scratch through it, gouge it, machine it, and as long as a trace of oxygen or water is present it re-forms in milliseconds. There is no slow “repassivation” window for a crack to exploit, which is why titanium shrugs off conditions that pit and crevice-corrode stainless steel.
That film is why commercially pure Grade 2 titanium handles seawater, wet chlorine, oxidising acids and most natural waters almost indefinitely, including in environments where 316 stainless quietly fails. For marine and seawater specifics, see titanium for marine and seawater.
| Property of the passive film | What the guide states |
|---|---|
| Composition | Titanium dioxide (TiO2) |
| Thickness | A few nanometres |
| When it forms | The instant the metal meets air or water |
| Behaviour after damage | Re-forms in milliseconds as long as a trace of oxygen or water is present |
| Consequence for cracking | There is no slow repassivation window for a crack to exploit |
The three places the film struggles
The passive layer needs the right conditions to form and stay formed. Three situations take those conditions away:
| Condition | What happens | Practical rule |
|---|---|---|
| Strong reducing acids (HCl, H₂SO₄, phosphoric) without an oxidiser, especially hot | No oxygen to maintain TiO₂; the film thins and the metal corrodes | Move up to Grade 7 or Grade 12; check the temperature/concentration limit |
| Hydrofluoric acid (HF) or free fluoride in acidic aqueous service | Even dilute HF can attack titanium rapidly; free fluoride in acid can form HF | TIMET does not recommend titanium for HF or fluoride-containing solutions below pH 7. Fully complexed fluoride, or dilute-fluoride service with documented inhibiting ions, requires separate corrosion review |
| Tight crevices in hot chlorides | Oxygen is starved inside the gap. Per the TIMET corrosion manual, crevice corrosion in chloride brines is not likely below 70 °C (158 °F), and in sharp crevices in near-neutral brine it becomes possible for unalloyed titanium at about 93 °C (200 °F) and above; concentrated calcium or magnesium chloride brines can attack below the 70 °C figure | Grade 7/12 for hot brines; mind gasket and joint design |
How alloying fixes the weak spots
This is exactly why the corrosion-resistant grades exist. They don’t make titanium “stronger” against acid by brute force: they keep the passive film stable where pure titanium can’t.
- Grade 7 (Ti-0.15Pd): a tiny palladium addition shifts titanium’s corrosion potential into the passive range, so the film survives in reducing acids and hot chlorides that would strip Grade 2. That extra cost buys you a much wider safe envelope in aggressive chemical service.
- Grade 12 (Ti-0.3Mo-0.8Ni): molybdenum and nickel improve crevice-corrosion resistance at elevated temperature at lower cost than Grade 7, a common choice for hot brine and chloride process equipment.
The grade is a corrosion decision before it is a strength decision. Picking Grade 2 for a hot dilute-HCl line because “titanium resists acid” is how a titanium part fails. The right answer is Grade 7. We supply Grade 7 (Ti-0.15Pd) pipe and bar and Grade 12 stock for exactly these duties.
| Grade | Addition | What the guide states it does |
|---|---|---|
| Grade 7 | Ti-0.15Pd | A tiny palladium addition shifts titanium’s corrosion potential into the passive range, so the film survives in reducing acids and hot chlorides that would strip Grade 2 |
| Grade 12 | Ti-0.3Mo-0.8Ni | Molybdenum and nickel improve crevice-corrosion resistance at elevated temperature at lower cost than Grade 7, a common choice for hot brine and chloride process equipment |
What to give a supplier so the grade is right
- The chemistry: which acid/medium, concentration and whether any oxidiser is present.
- Temperature: the single biggest variable; a grade fine at 25 °C may not be at 90 °C.
- Any fluorides: state the concentration, pH, temperature and whether the fluoride is free or fully complexed. TIMET does not recommend titanium for HF or fluoride-containing solutions below pH 7; complexed or specifically inhibited dilute-fluoride service needs a separate corrosion review.
- Geometry: crevices, gaskets and tight joints change the crevice-corrosion risk.
| What to give the supplier | Detail |
|---|---|
| The chemistry | Which acid or medium, the concentration, and whether any oxidiser is present |
| Temperature | The single biggest variable; a grade fine at 25 C may not be at 90 C |
| Any fluorides | State the concentration, pH, temperature and whether the fluoride is free or fully complexed. Do not default to titanium for HF or fluoride-containing solutions below pH 7; review complexed or specifically inhibited cases separately |
| Geometry | Crevices, gaskets and tight joints change the crevice-corrosion risk |
Send those with your Send an enquiry and the grade selection follows from the environment, not from a marketing line.
FAQ
Is titanium completely corrosion-proof?
No. It’s outstanding in oxidising, neutral and chloride environments because of its passive oxide film, but it corrodes in strong reducing acids without an oxidiser and is attacked by hydrofluoric acid. Grade and temperature decide the limit.
Why does titanium beat stainless steel in seawater?
Stainless relies on a chromium-oxide film that pits and crevice-corrodes under chlorides; titanium’s TiO₂ film is far more stable in chlorides and re-heals instantly, so it resists pitting and crevice attack that defeat 316.
Can I use titanium with hydrochloric or sulphuric acid?
Sometimes: it depends on concentration, temperature and grade. Grade 2 is limited; Grade 7 (Ti-0.15Pd) extends the safe range considerably. Always check against a corrosion chart for the exact conditions, and never use titanium with HF.
Does the oxide film wear off?
It can be mechanically removed (scratching, machining), but it re-forms immediately wherever oxygen or water is present. Continuous abrasion in an oxygen-free medium is the unusual case where passivity can’t keep up.
What the passive film explains, and what it does not
The oxide film explains why titanium behaves as it does in a given environment. It does not decide whether a delivered item is acceptable, because acceptance rests on composition and mechanical results that are measured rather than inferred.
| Explained by the film | Decided by measurement and the order |
|---|---|
| Why the metal is stable in oxidising conditions | Composition limits, reported per heat |
| Why reducing acids need a different grade | Which grade the order names, with its UNS number |
| Why surface damage matters | Surface condition, stated apart from the delivery condition |
How the reported figures behind grade selection are obtained
| What is reported | Method the specification references | What naming it settles |
|---|---|---|
| Oxygen and nitrogen | ASTM E1409 | Interstitials reported on a stated basis |
| Hydrogen | ASTM E1447 | Determined by a named method |
| Alloy composition | ASTM E539 and ASTM E2371 | The spectrometric routes named in the referenced documents |
Naming a method settles how a value was produced. It does not add an examination the order did not ask for, and a mechanism, however well understood, is not an acceptance basis.
Specification detail with sources
Chemistry windows for the corrosion-resistant titanium grades are set in the ASTM B-series tables, and they are narrow. ASTM B348 and B265 specify Grade 7 (UNS R52400) at 0.12 to 0.25 percent palladium and Grade 12 (UNS R53400) at 0.2 to 0.4 percent molybdenum with 0.6 to 0.9 percent nickel, and the lean-palladium Grade 16 (UNS R52402) carries 0.04 to 0.08 percent palladium for services where a smaller addition is specified. Those fraction-of-a-percent additions are the entire engineering difference between a passive film that survives hot reducing acid and one that thins.
Beiyu Titanium supplies these corrosion grades in the product forms the film question usually arrives in: the tube and pipe program spans 10 titanium grades quoted against ASTM B338 for tube and ASTM B861 and B862 for seamless and welded pipe, with the Grade 2, 7, 9 and 12 exchanger tubes covered in Beiyu’s B338 guide sitting inside that range, and Grades 16 and 17 available where a lean palladium chemistry is specified. Seamless tube runs 5 to 325 mm outside diameter and welded tube 6.35 to 1219 mm, so a small-bore crevice-prone exchanger joint and a large-bore header can be quoted from the same grade decision in one enquiry.
Send a technical RFQ
For corrosion-service titanium RFQs, include grade, ASTM standard, medium, temperature, chloride or crevice condition, dimensions and document requirement.