Grade 7 is commercially pure titanium with about 0.15% palladium. The palladium shifts the alloy into the passive range in reducing acids such as hydrochloric, sulfuric and phosphoric, where unalloyed Grade 2 corrodes. Chemistry, temperature and concentration decide whether Grade 7 or a different material family applies.
Grade 2 titanium handles most oxidizing and neutral media, but it loses ground in hot reducing acids such as hydrochloric and sulfuric acid, where the protective oxide film is harder to keep stable. Grade 7 titanium is the answer the chemical industry reaches for when the medium is reducing, hot, or chloride-rich and a plain commercially pure grade would corrode too fast. This guide explains what Grade 7 is, why the palladium addition matters, where it belongs, and how to specify it on an RFQ.
What Grade 7 titanium actually is
Grade 7 is commercially pure titanium with a small palladium addition, nominally 0.12 to 0.25% Pd, covered by UNS R52400. Mechanically it behaves almost exactly like Grade 2: the same modest strength, the same good formability and weldability, the same density. The palladium does not change the mechanics. What it changes is electrochemistry.
In reducing acids, titanium's passive oxide film becomes harder to maintain, and unalloyed grades can move into an active corrosion state. The palladium raises the alloy's corrosion potential into a more noble range, which keeps the film passive at acid concentrations and temperatures where Grade 2 would already be corroding. The result is the same titanium you already know how to fabricate, with a much wider window of chemical service.
Because the mechanical properties track Grade 2, Grade 7 is specified by corrosion need, not by strength. If the environment does not attack Grade 2, paying for palladium adds cost without benefit. The engineering question is always the medium, not the grade in isolation. The full chemistry and mechanical class sit on our Grade 7 titanium (Ti-0.15Pd, UNS R52400) spec page.
Why palladium helps in reducing acids
Titanium resists corrosion because it carries a thin, self-healing oxide film. In oxidizing conditions that film reforms instantly, which is why Grade 2 shines in seawater, wet chlorine, and nitric acid. In reducing acids there is less oxygen available to rebuild the film, so a scratch or a crevice can stay active and dissolve.
The palladium in Grade 7 acts as a catalyst for the cathodic reaction, shifting the metal's potential far enough to keep the oxide film stable even in these oxygen-starved conditions. That single change extends useful resistance to higher acid concentrations, higher temperatures, and the tight chloride-rich crevices where plain titanium is most vulnerable to pitting and crevice attack.
How far the resistance actually extends
Published titanium iso-corrosion data give a useful sense of the gap between grades. The figures below are approximate room-temperature limits for low corrosion rates and should be read as an orientation, not a design guarantee, because real service depends on temperature, aeration, chloride content, and impurities.
| Medium (room temperature, approximate) | Grade 2 | Grade 12 | Grade 7 |
|---|---|---|---|
| Hydrochloric acid, useful resistance to about | 7% | 9% | 27% |
| Sulfuric acid, useful resistance to about | low single digits | intermediate | about 45% |
| Crevice and pitting resistance in hot chlorides | limited | improved | best of the three |
Two points matter for specification. First, Grade 7 does not make titanium immune to acid; it widens the window, and boiling or concentrated conditions still need testing against the actual stream. Second, the ranking is consistent: for reducing-acid duty the order is Grade 7 first, then Grade 12, then unalloyed grades. Where a stream is borderline for Grade 2, Grade 7 usually converts it into safe titanium service.
The rates behind those concentration limits
Those percentages are room-temperature limits. The variable that actually decides a reducing-acid job is temperature, and the published rates show why. Same acid, same grade, room temperature against boiling:
| Wt percent HCl | Temperature | Unalloyed, ASTM Grade 2 | Ti-6Al-4V, ASTM Grade 5 | ASTM Grade 12 | Palladium-bearing, ASTM Grade 7 |
|---|---|---|---|---|---|
| 1 | Room | Nil | Not reported | 0.2 (0.005) | 0.1 (0.003) |
| 3 | Room | 0.5 (0.013) | Not reported | 0.5 (0.013) | 0.4 (0.010) |
| 5 | Room | 0.2 (0.005) | Not reported | 0.5 (0.013) | 0.6 (0.015) |
| 1 | Boiling | 85 (2.16) | Not reported | 1.4 (0.036) | 0.8 (0.020) |
| 2 | Boiling | 280 (7.11) | 260 (6.60) | 10.0 (0.254) | 1.8 (0.046) |
| 3 | Boiling | 550 (14.0) | 520 (13.2) | 400 (10.2) | 2.7 (0.069) |
| 5 | Boiling | 840 (21.3) | 1030 (26.2) | 1500 (38.1) | 10.0 (0.254) |
| 8 | Boiling | Over 2000 (over 50.8) | 1900 (48.3) | 3000 (76.2) | 24.0 (0.610) |
Three things fall out of this table that the concentration limits alone do not tell you.
Temperature dominates concentration. At 3 percent HCl the unalloyed grade runs 0.5 mpy at room temperature and 550 mpy boiling, a factor of about 1,100 on the same acid. A stream specified only by percentage is not specified.
Grade 12 is not a reducing-acid grade. In boiling acid the intermediate position does not hold. At 5 percent boiling HCl Grade 12 reads 1500 mpy against 840 for unalloyed titanium, so it is the worse of the two there. At 8 percent both are past 2000 mpy and the source reports the unalloyed value only as “over 2000”, which does not support a ranking between them at that point. Either way, Grade 12 earns its place in hot chlorides and crevice conditions rather than in boiling reducing acid, and specifying it for boiling HCl on the assumption that it is the intermediate grade is a real error.
The palladium grade holds where the others do not. At 8 percent boiling it reads 24 mpy against 3000 for Grade 12 and over 2000 for unalloyed. That is the gap the palladium addition buys, and it is why the concentration limit runs to about 27 percent rather than 7.
Ferric ion changes the answer, and it is quantified
Small additions of multi-valent metal ions inhibit titanium corrosion in hydrochloric acid. In the TIMET tests at 200 F (93 C) with 2 g/l ferric chloride added, 3 percent HCl gives 0.2 mpy on unalloyed titanium and 0.1 mpy on the palladium-bearing grade, and 4 percent gives 0.4 and 0.3 respectively. Compare that with the uninhibited boiling rows above.
| Condition | Unalloyed, ASTM Grade 2 | ASTM Grade 12 | Palladium-bearing, ASTM Grade 7 |
|---|---|---|---|
| 3 percent HCl, 200 F (93 C), 2 g/l FeCl3 | 0.2 (0.005) | 1.0 (0.025) | 0.1 (0.003) |
| 4 percent HCl, 200 F (93 C), 2 g/l FeCl3 | 0.4 (0.010) | 2.0 (0.050) | 0.3 (0.008) |
The same source maps the 5 mpy iso-corrosion line for unalloyed titanium against ferric ion content, and the same TIMET figures give the equivalences: 30 ppm of ferric ion is 0.0087 percent ferric chloride, 60 ppm is 0.0174 percent, 75 ppm is 0.0218 percent, 125 ppm is 0.0362 percent. Nitric acid, chlorine, sodium hypochlorite and chromate ions act as inhibitors in the same way, as do cupric, nickel, molybdenum and titanium ions. This is why titanium survives in many industrial hydrochloric streams that a pure-acid table would rule out: the process liquor is rarely pure acid.
The failure mode this data predicts, and it happens in cleaning not in service
Severe corrosion damage to titanium equipment has resulted from cleaning procedures using pure hydrochloric acid, or acid inhibited with amines. The reason is visible in the two tables above: the process stream that the equipment was designed for carries inhibiting metal ions, and the cleaning acid does not. The TIMET manual names amine-inhibited acid specifically as one of the cleaning procedures that has caused this damage, so an inhibitor package chosen for other metals cannot be assumed to protect titanium.
The published recommendation is direct: if hydrochloric or sulfuric acid is used to clean titanium surfaces, add sufficient ferric chloride to inhibit corrosion of the titanium. Put that on the cleaning procedure, not only on the material specification, because the equipment can survive years of service and be damaged in one wash.
Where Grade 7 belongs
Grade 7 is a chemical-processing grade first. It is specified for reactors, heat exchangers and acid coolers, columns, piping, and vessel linings in duties involving hydrochloric or sulfuric acid, chloride-bearing process streams, and fertilizer and pigment production. It also appears in metal-finishing and pickling lines, and anywhere hot brine or acidic chlorides would drive crevice corrosion in a leaner grade.
It is not the grade for structural or high-strength parts; that is where Ti-6Al-4V lives. And in strongly oxidizing service such as seawater, wet chlorine, or nitric acid, Grade 2 is already fully passive, so Grade 7 adds cost without adding life. The way to choose between commercially pure grades for a given medium is covered in our CP titanium grade selection guide, and the seawater and chloride side is covered in our titanium seawater corrosion resistance guide.
| Service | Grade to specify |
|---|---|
| Reactors, heat exchangers and acid coolers, columns, piping and vessel linings in hydrochloric or sulfuric acid duty | Grade 7 |
| Chloride bearing process streams, fertilizer and pigment production | Grade 7 |
| Metal finishing and pickling lines | Grade 7 |
| Hot brine or acidic chlorides that would drive crevice corrosion in a leaner grade | Grade 7 |
| Seawater, wet chlorine and nitric acid | Grade 2 |
| Structural and high strength parts | Ti-6Al-4V |
Standards, forms, and equivalents
Grade 7 is a mill-product grade available across the common wrought forms. Specify the form standard that matches the part.
| Property | Detail |
|---|---|
| UNS number | R52400 |
| Nominal palladium | 0.12 to 0.25% Pd |
| Bar and billet | ASTM B348 |
| Sheet, strip and plate | ASTM B265 |
| Seamless and welded tube and pipe | ASTM B338 (tube); ASTM B861 (seamless pipe) and B862 (welded pipe), which replaced the legacy ASTM B337 |
| Mechanical class | Comparable to Grade 2 (CP) |
| Certificate | EN 10204 3.1 with heat number and chemistry, including palladium |
When you order, make sure the mill test certificate reports the actual palladium content against the 0.12 to 0.25% range, not just a grade name, because palladium is the reason the grade costs more and it is the one element the certificate must confirm.
How to read the Grade 7 certificate when it arrives
Grade 7 costs more than Grade 2 for exactly one reason, and that reason has to be visible on the certificate:
- Palladium content. Confirm the certificate reports actual palladium in the 0.12 to 0.25% range by heat, not just the words Grade 7. Palladium is the element you are paying for and the only one that distinguishes Grade 7 from Grade 2, so a certificate that omits it does not confirm what you bought.
- Balance chemistry and mechanicals. The rest of the chemistry and the strength values track Grade 2, so read them against the same commercially pure limits and against the ordered form’s standard.
- Form and standard match. Check the certificate is written against the standard for the form you ordered, ASTM B348 for bar, B265 for plate, B338 for tube, since limits and test bases differ by form.
Close with the heat number matching the material marking. The trap in Grade 7 procurement is a certificate that names the grade but never prints the palladium result, which leaves you unable to prove you received a palladium-bearing heat rather than Grade 2 relabelled.
| What to check on the certificate | What good looks like | Why it matters |
|---|---|---|
| Palladium content | Actual palladium reported by heat, inside the 0.12 to 0.25 percent range | The only element that distinguishes Grade 7 from Grade 2, and the reason for the price |
| Balance chemistry and mechanicals | Read against the commercially pure limits and the ordered form’s standard | The rest of the chemistry and the strength values track Grade 2, so read them against the same commercially pure limits |
| Form and standard match | Written against the standard for the form ordered: ASTM B348 bar, B265 plate, B338 tube | Limits and test bases differ by form |
Grade 7 versus the leaner alternatives
Palladium is expensive, so the industry created leaner corrosion grades that aim for most of the benefit at lower alloy cost. Grade 16 uses a smaller palladium addition (nominally 0.04 to 0.08% Pd) and Grade 26 uses ruthenium instead of palladium; both target similar reducing-acid and crevice duties at a lower precious-metal loading. Grade 12, which adds molybdenum and nickel rather than a precious metal, is a further economical option for moderately aggressive hot brines and chlorides.
The practical decision comes down to how aggressive the stream is and how the palladium price sits when you buy. For the most demanding reducing-acid duty, Grade 7 remains the reference. For borderline streams, a leaner grade can carry the load at lower cost. Beiyu supplies Grade 7, Grade 12, and the leaner corrosion grades Grade 16 and Grade 26, so the grade can be matched to the actual stream rather than defaulting to the most expensive option. When even Grade 7 is not enough because the medium attacks titanium itself, the conversation moves to nickel alloys such as Alloy 625, which is why we keep both material families in scope for chemical equipment.
| Grade | Alloying addition | What it targets |
|---|---|---|
| Grade 7 | Nominally 0.12 to 0.25 percent palladium | The reference for the most demanding reducing acid duty |
| Grade 16 | Nominally 0.04 to 0.08 percent palladium | Similar reducing acid and crevice duty at a lower precious metal loading |
| Grade 26 | Ruthenium in place of palladium | Similar reducing acid and crevice duty at a lower precious metal loading |
| Grade 12 | Molybdenum and nickel, no precious metal | Moderately aggressive hot brines and chlorides, as a further economical option |
How to specify Grade 7 on an RFQ
Because Grade 7 is chosen by corrosion duty, the most useful RFQ describes the service, not just the part. State the medium and concentration, the operating temperature, the form and dimensions, the governing standard, the quantity, and the certificate requirement. Naming the actual acid stream lets the supplier confirm that Grade 7 is the right call rather than a leaner or heavier grade. The full field list is in our titanium bar RFQ checklist. Beiyu supplies Grade 7 bar, sheet and plate, and tube on an RFQ basis with EN 10204 3.1 documentation that reports palladium against the specified range.
| Field to state | What to give |
|---|---|
| Medium and concentration | The medium and concentration, with the actual acid stream named |
| Operating temperature | The operating temperature |
| Form and dimensions | The form and dimensions |
| Governing standard | ASTM B348 for bar and billet, B265 for sheet and plate, B338 for tube, B861 or B862 for pipe |
| Quantity | The order quantity |
| Certificate requirement | EN 10204 3.1 documentation reporting palladium against the specified range, with positive material identification where the RFQ calls for it |
Fabrication and welding: palladium changes the chemistry, not the workshop
Grade 7 forms, machines and welds like Grade 2, because mechanically it is Grade 2; the palladium addition works in the corrosion cell, not in the press brake. Welding follows standard titanium practice with full inert gas shielding, and the filler question matters here more than anywhere: a Grade 7 vessel welded with plain Grade 2 filler has Grade 2 weld seams facing the acid, which defeats the point of paying for palladium. Specify matching Ti-Pd filler on fabrication-bound orders and keep weld colour acceptance in the fabrication file.
| Fabrication point | Grade 7 practice | Why |
|---|---|---|
| Forming and machining | As Grade 2 | Mechanically it is Grade 2; the palladium works in the corrosion cell, not in the press brake |
| Welding | Standard titanium practice with full inert gas shielding | The palladium addition works in the corrosion cell, not in the workshop |
| Filler and weld colour | Matching Ti-Pd filler on fabrication bound orders, with weld colour acceptance kept in the fabrication file | Grade 2 filler leaves Grade 2 weld seams facing the acid, which defeats the point of paying for palladium |
Verifying palladium: the certificate and the PMI check
Palladium is invisible: Grade 7 bar looks exactly like Grade 2, and the price difference makes misidentification expensive in both directions. Two controls close the gap. The certificate must report palladium in the chemistry table, in the 0.12 to 0.25 percent band the standards define. Positive material identification by X-ray fluorescence can confirm palladium in the delivered metal at receiving, which is why Ti-Pd grades are a textbook case for adding a PMI line to the RFQ on safety-critical acid service. A buyer who orders Grade 7 and never verifies palladium is trusting the label on the most label-dependent grade in the family.
| Control | Where it happens | What it establishes |
|---|---|---|
| Chemistry on the mill certificate | At the mill | Palladium reported inside the 0.12 to 0.25 percent band the standards define |
| Positive material identification by X-ray fluorescence | At receiving | That palladium is present in the delivered metal |
| Physical segregation and legible heat marking | In stores and through fabrication | That two visually identical grades do not get mixed |
| Re-marking of offcuts | Before offcuts return to stores | An unmarked Ti-Pd offcut is economically stranded; an unmarked commercially pure offcut in the Grade 7 rack is a latent acid service failure |
Handling and segregation: two invisible grades, one rack
The plant-side risk with Grade 7 is mix-up, not corrosion of the stock itself. Because Grade 2 and Grade 7 are visually identical, segregate them physically, keep heat-number marking legible through cutting and fabrication, and re-mark offcuts before they return to stores. An unmarked Ti-Pd offcut is economically stranded, and an unmarked CP offcut in the Grade 7 rack is a latent acid-service failure. The general titanium rules also apply: keep the material clear of carbon steel contact so embedded iron does not stain the surface or seed local attack.
Procurement notes from the export desk
Palladium makes Grade 7 the most price-volatile grade in the CP family, so quotes carry shorter validity than Grade 2 and comparing offers weeks apart misleads. Three habits keep orders clean: state the acid, concentration and temperature on the RFQ so the mill’s review confirms the grade fits the duty; ask whether Grade 12 or Grade 16 covers the condition at lower cost, because the honest answer is sometimes yes and the existing section on leaner alternatives sets out when; and put the PMI requirement on the order rather than negotiating it after delivery. Beiyu reviews each Ti-Pd enquiry line against mill capability before quoting, with EN 10204 3.1 documentation and heat-number traceability by order scope.
FAQ
What is the difference between Grade 2 and Grade 7 titanium?
Grade 7 is Grade 2 titanium with 0.12 to 0.25% palladium added. The mechanical properties are essentially the same; the palladium greatly improves resistance to reducing acids such as hydrochloric and sulfuric acid and to crevice corrosion in hot chlorides. Use Grade 2 when the medium is oxidizing or neutral and Grade 7 when it is reducing, hot, or chloride-rich.
What is the UNS number for Grade 7 titanium?
Grade 7 titanium is UNS R52400. It is supplied as bar and billet to ASTM B348, as sheet and plate to ASTM B265, as tube to ASTM B338, and as pipe to ASTM B861 or B862.
Is Grade 7 titanium resistant to hydrochloric acid?
Yes, within limits. Grade 7 offers useful resistance to substantially higher hydrochloric acid concentrations than Grade 2 at room temperature, but boiling or concentrated acid still needs testing against the actual stream, including its temperature and chloride content.
When should I choose a leaner grade instead of Grade 7?
If the stream is only moderately aggressive, a lower-palladium grade such as Grade 16, a ruthenium grade such as Grade 26, or the molybdenum-nickel Grade 12 can carry the duty at lower precious-metal cost. Grade 7 remains the reference for the most demanding reducing-acid service. Share the medium and temperature and the supplier can confirm the leanest grade that still meets the duty.
For a view of who else publishes this material and what each one states, see Grade 7 Titanium Forging Suppliers: Five Company Pages Compared.
For the question behind this page, where titanium corrodes at all and how reducing acids compare with hot chloride crevices and fluoride, see does titanium corrode.
Specification detail with sources
Grade 7 (UNS R52400) is Grade 2 with a deliberate palladium addition: ASTM B265 specifies 0.12 to 0.25 wt% Pd, about 0.15 wt% in typical mill practice. The palladium promotes passivation, keeping the protective oxide film stable in reducing acid conditions that actively attack unalloyed grades.
Mechanically, nothing changes at the design level: ASTM B265 assigns Grade 7 the same minimum tensile requirements as Grade 2 (ultimate tensile strength 345 MPa min, yield strength 275 to 450 MPa), so substituting Grade 7 for corrosion performance does not require requalifying strength allowables.
Nitric acid is the opposite case to the reducing acids above, and the grade logic inverts with it. The published rates, the concentration peak and the red fuming exclusion are set out in titanium in nitric acid.
Reference sources
- ASTM B265 and ASTM B348, titanium and titanium alloy product specifications covering UNS R52400
- TIMET, Corrosion Resistance of Titanium (technical manual)
- ASM Handbook, Volume 13B, Corrosion: Materials
- ASTM G31, Standard Guide for Laboratory Immersion Corrosion Testing of Metals
- NACE / AMPP literature on palladium-stabilised titanium in reducing acids