Titanium does not rust, and it does corrode. Those are two different statements and both are true. Rusting is a reaction of iron, and there is no iron phase in titanium for it to happen to, so the orange scale that eats carbon steel has no mechanism here. Corrosion is the wider word, and titanium has a short list of environments that get through its passive film. The useful part of the answer is that the list is short and the boundaries have numbers on them.
Why titanium resists corrosion at all
Titanium is a reactive metal. Left bare it oxidises almost instantly, and that is the point: the reaction produces a thin, tightly adherent titanium dioxide film that stops the reaction. Scratch it in the presence of oxygen or water and it reforms almost at once. Every corrosion answer below is really a question about whether the environment lets that film repair itself.
That also explains the one behaviour that surprises people. Titanium performs best in oxidising conditions and worst in hot reducing acids, where there is less oxygen available to rebuild the film. So titanium corrosion resistance is not one property with one value; it is the behaviour of that film in a named environment.
The three environments that actually attack titanium
| Environment | What happens | Where the boundary sits | The grade that answers it |
|---|---|---|---|
| Reducing acids, such as hydrochloric and sulphuric | The film cannot repair, and general corrosion proceeds | Concentration and temperature dependent; inhibiting metal ions move the boundary a long way, see below | Grade 7, titanium with palladium |
| Hot chloride in a tight crevice | Chemistry inside the crevice turns acidic and the film breaks down locally, while the open surface stays bright | Temperature banded in the table below | Grade 12 for hot brine, Grade 7 where the crevice is also acidic |
| Hydrofluoric acid, and acidic solutions carrying free fluoride | Attacks the passive film directly, even at very low concentration | Generally not a grade selection problem; fluoride speciation and any proven inhibitor effect have to be verified | Generally none of the titanium grades; the material or the fluoride has to change |
Those three cover most of the corrosion questions that reach an enquiry desk. Seawater, wet chlorine, chlorides in the open and most process waters are where titanium is chosen precisely because it does not corrode. A medium that is not named on this page has to be checked against corrosion data rather than assumed into one of these rows.
Crevice corrosion, band by band
This is the case worth getting right, because the failure is local and the rest of the part looks perfect. The figures below are for unalloyed titanium in sharp crevices in near-neutral saturated sodium chloride brine. The geometry qualifier is not decoration; it is what the figure applies to.
| Temperature | What the source states for unalloyed titanium in sharp crevices |
|---|---|
| Below 70 °C (158 °F) | Crevice attack is not likely |
| About 93 °C (200 °F) and above | Crevice attack becomes possible |
The source gives no figure between those two points, and we are not going to invent one. What we can say is where the grade decision starts to bite: our Grade 12 page puts the practical onset for tight crevices at roughly 70 to 80 °C, so above 70 °C in a tight crevice the chemistry has to be checked rather than Grade 2 assumed.
Two things can lower the onset temperature, and both are common in real service: lower pH, and high concentrations of calcium or magnesium ions. A brine that looks within range on temperature alone can sit outside it once the pH and the hardness are known, which is why an enquiry needs the chemistry and not only the duty temperature.
The grade answer splits at that point. Grade 12 extends useful crevice resistance well past where Grade 2 gives out, with reports in the corrosion literature of useful crevice resistance beyond 250 °C in high-temperature brine. That is an orientation figure from the literature, not a design guarantee. But Grade 12 is not a reducing-acid grade: below roughly pH 3 its advantage narrows and Grade 7 becomes the right call. Hot and acidic is a different problem from hot and neutral, and they take different grades.
The inhibitor effect, which is where the numbers get useful
Reducing acid service is not a fixed verdict, because small quantities of multi-valent metal ions inhibit titanium corrosion in hydrochloric acid, and process streams frequently contain them already. In the TIMET tests at 200 °F (93 °C) with 2 g/l ferric chloride added, dilute hydrochloric acid gives corrosion rates that would be acceptable in service on two of the three grades.
| Condition | Unalloyed, Grade 2 | Grade 12 | Palladium-bearing, Grade 7 |
|---|---|---|---|
| 3 per cent HCl, 200 °F (93 °C), 2 g/l FeCl3 | 0.2 mpy | 1.0 mpy | 0.1 mpy |
| 4 per cent HCl, 200 °F (93 °C), 2 g/l FeCl3 | 0.4 mpy | 2.0 mpy | 0.3 mpy |
Read the Grade 12 column before assuming the grades rank in a fixed order. In this acid Grade 12 is five times worse than plain Grade 2, not better. Grade 12 buys crevice resistance in hot neutral brine; it is not a reducing-acid grade, and a selection rule that treats the grades as a single ladder from Grade 2 up to Grade 7 will pick the wrong one here.
Those are corrosion rates in the range normally treated as negligible, in an acid that would otherwise be a clear exclusion. The practical consequence is that a stream analysis showing ferric or cupric ions can change the grade decision, and sometimes changes whether titanium is a candidate at all.
The source maps that inhibiting effect against ferric ion content, so the equivalences matter when reading a stream analysis:
| Ferric ion content | Equivalent ferric chloride |
|---|---|
| 30 ppm | 0.0087 per cent |
| 60 ppm | 0.0174 per cent |
| 75 ppm | 0.0218 per cent |
| 125 ppm | 0.0362 per cent |
Common follow-up questions
So does titanium rust or not? Not in the literal sense the word carries. ASTM B348 caps iron in Grade 2 at 0.30 per cent as a residual, and at that level it is in solution in the titanium rather than present as the separate iron phase rusting requires. Orange staining on a titanium part is therefore contamination rather than the titanium rusting, but it is not cosmetic: iron smeared into a titanium surface during handling is a documented starting point for localized attack in service, which is why material is kept clear of carbon steel contact and why the fix is pickling the iron out rather than either ignoring it or rejecting the material.
Does titanium corrode in seawater? Not in open seawater service, which is one of the applications it is specified for. The qualifier is crevice geometry rather than the seawater itself: flanged joints, gasket faces, tube-to-tubesheet gaps and deposits under scale are where the temperature bands above start to matter.
Does titanium corrode faster than stainless steel? In the chloride and oxidising service where titanium is specified against stainless, no, and that is usually why titanium is on the drawing. In hot reducing acid neither is a default answer and the grade has to be chosen against the stream. The comparison is set out on the titanium against stainless steel page.
Will galvanic contact corrode titanium? Not by corrosion of the titanium. Titanium is the more noble metal in most practical couples, so the other metal carries the increased attack. The area ratio decides how bad that is for the other metal, and a small carbon steel or stainless part bolted into a large titanium surface is the arrangement to avoid.
Can a certificate confirm corrosion performance? No. ASTM B265, B338 and B348 set chemistry, mechanical properties and dimensions. Corrosion behaviour follows from the grade and the environment, and where a corrosion test is required it is agreed on the order rather than assumed from the specification.
Specification detail with sources
The crevice corrosion bands quoted above are the figures Beiyu Titanium publishes on its titanium wetted parts page, drawn from the TIMET corrosion manual: crevice attack on unalloyed titanium in sharp crevices in near-neutral saturated sodium chloride brine becomes possible at about 93 °C (200 °F) and above and is not likely below 70 °C (158 °F), with lower pH or high concentrations of calcium or magnesium ions able to lower the onset temperature. The hydrochloric acid rates at 200 °F with 2 g/l ferric chloride, 0.2, 1.0 and 0.1 mpy at 3 per cent acid and 0.4, 2.0 and 0.3 mpy at 4 per cent acid for Grade 2, Grade 12 and Grade 7 respectively, and the ferric ion equivalences of 30 ppm to 0.0087 per cent, 60 ppm to 0.0174 per cent, 75 ppm to 0.0218 per cent and 125 ppm to 0.0362 per cent, are the TIMET figures carried on the Grade 7 reducing acid page, where the equivalences come from that source’s 5 mpy iso-corrosion line for unalloyed titanium. The Grade 12 figure of reported useful crevice resistance beyond 250 °C in high-temperature brine, which that page itself labels an orientation value from the corrosion literature rather than a design guarantee, and the pH 3 boundary below which the palladium grade is preferred, are carried on the Grade 12 hot brine page. The hydrofluoric acid row, including the wording that titanium is generally not the material and that fluoride speciation and any proven inhibitor effect have to be verified, is the row carried on that same wetted parts page.
What the enquiry has to carry
A corrosion question cannot be answered from the grade alone, and the gaps are predictable. State the medium and its concentration, the metal temperature rather than the bulk temperature, the pH, the chloride level, whether calcium or magnesium are present in quantity, whether any inhibiting metal ions are in the stream, and whether the design contains tight crevices such as gasket faces or tube-to-tubesheet joints. With those, the grade follows and the quotation can be built against it. Beiyu Titanium supplies titanium in Grade 1, 2, 5, 7, 9, 12 and 23 as bar, plate, sheet, tube and forgings to ASTM and AMS specifications, with an EN 10204 3.1 mill test certificate by order scope.