Excerpt:
Titanium Grade 7 (Ti-0.15Pd, UNS R52400) technical guide from Hangbo Alloy (Shanghai Hangbo Alloy Group, nickel-alloy.com). Grade 2 CP titanium ennobled with 0.12-0.25% palladium - cathodic alloying that extends passive-film stability into reducing acids and hot chloride crevices. Covers ASTM B265/B348/B338 supply forms, electrochemistry, corrosion data, and the palladium verification checklist critical to procurement.
Titanium Grade 7 (Ti-0.15Pd / UNS R52400) — Plate, Bar and Tube Technical Guide | Hangbo Alloy
Technical Bulletin for Chemical Process, Heat-Exchanger, and Corrosion-Engineering Procurement
Introduction
Commercially pure titanium is justifiably famous for near-immunity to chloride pitting and stress-corrosion cracking, but every CP grade has one honest limitation: in hot, oxygen-starved crevices and in reducing acids, the protective TiO₂ passive film can fail, because the film's stability depends on an oxidizing environment. Titanium Grade 7 (UNS R52400, W.Nr. 3.7235) is the classic engineering answer to that limitation. It begins as Grade 2 CP titanium — the same base composition, the same 345 MPa minimum tensile strength, the same weldability — and adds a deliberate 0.12–0.25% palladium. That small addition of one of the most cathodic metals in the periodic table transforms the alloy's electrochemistry, extending titanium's passive behavior into reducing acids and hot chloride crevices where unalloyed titanium can corrode.
The mechanism is elegant and worth understanding before any procurement decision. In a reducing acid, or in an oxygen-starved crevice, the cathodic reaction on a titanium surface — hydrogen reduction — is slow, and the metal's potential drifts into the active region where the passive film dissolves. Palladium, present as a fine dispersion or in solid solution and enriched at the surface during service, provides highly efficient cathodic sites for hydrogen reduction. The result is that the mixed potential is pulled in the noble direction, back into the passive range, where the TiO₂ film remains stable and corrosion rates collapse to near zero. Engineers call this "cathodic alloying": rather than making the film stronger, palladium makes the environment effectively more oxidizing from the metal's point of view.
Shanghai Hangbo Alloy Group (nickel-alloy.com) supplies Titanium Grade 7 in plate, sheet, strip, bar, billet, and seamless/welded condenser tube, certified to ASTM B348, B265, and B338 with EN 10204 3.1 documentation, 100% PMI verification (palladium explicitly checked), and full heat traceability. Because palladium is a declared, price-critical element, verifying its presence — and its 0.12–0.25% range — on every certificate is one of the most important quality checks in titanium procurement. This guide covers the chemistry, electrochemistry, property envelope, specifications, corrosion performance, and economics of Grade 7.
Chemical Composition
The composition limits below are the acceptance baseline for Grade 7 mill products supplied by Hangbo Alloy to ASTM B265, B348, and B338. Titanium is the balance element.
| Element | ASTM Spec Limit (wt %) | Typical Heat Value | Metallurgical Role |
|---|---|---|---|
| Titanium (Ti) | Balance | ~99.0 | α-phase matrix; passive-film former |
| Palladium (Pd) | 0.12 – 0.25 | 0.14 – 0.18 | Cathodic modifier — the corrosion-control element |
| Oxygen (O) | 0.25 max | 0.10 – 0.18 | Interstitial strengthener (Grade 2 base level) |
| Iron (Fe) | 0.30 max | 0.05 – 0.15 | Trace |
| Carbon (C) | 0.08 max | 0.01 – 0.03 | Trace interstitial |
| Nitrogen (N) | 0.03 max | 0.008 – 0.015 | Trace interstitial |
| Hydrogen (H) | 0.015 max | 0.002 – 0.005 | Held low for hydride control |
| Other elements, each / total | 0.10 / 0.40 max | — | Per governing ASTM specification |
The palladium range is the contractual heart of the grade. Below 0.12% the cathodic modification weakens; above 0.25% nothing is gained except cost. A mill certificate that does not state palladium, or states it as "0.05% max residual," is not Grade 7 — it is Grade 2, and paying Grade 7 prices for it is exactly the kind of substitution that PMI verification exists to prevent. Hangbo Alloy performs XRF/arc-OES PMI on every Grade 7 item before release and declares the tested palladium value on the EN 10204 3.1 certificate.
Metallurgy and Electrochemistry: How Palladium Works
Understanding Grade 7 requires a corrosion-potential model rather than a simple "better alloy" mental model. Titanium's corrosion resistance is potential-dependent. In aerated, oxidizing, or neutral media, titanium sits in its passive range: the TiO₂ film is stable, and corrosion rates are typically below 0.02 mm/year. In reducing acids such as hydrochloric, sulfuric, or phosphoric acid without oxidizing impurities, and in crevices where the oxygen is consumed and the solution becomes acidic and reducing, the cathodic reaction is starved. The mixed potential falls into the active region, the film breaks down, and CP titanium corrodes at rates that can be severe.
Grade 7 defeats this by altering the cathodic kinetics. Palladium is one of the most efficient hydrogen-evolution catalysts known, and it is essentially immune to corrosion itself in these media. Palladium-rich sites on the surface — either from the alloy's own dispersion or from surface enrichment as titanium dissolves — support hydrogen reduction at a much higher rate than titanium does. That faster cathodic reaction pulls the corrosion potential upward (in the noble direction) by several hundred millivolts, back into the passive range, where the TiO₂ film is stable and the corrosion rate falls by orders of magnitude.
Three practical consequences follow:
- Reducing-acid resistance. Grade 7 tolerates dilute hydrochloric, sulfuric, and phosphoric acids at temperatures and concentrations that cause active corrosion of CP titanium — up to roughly the boiling point in the dilute range, depending on concentration and aeration.
- Crevice-corrosion immunity. In hot chloride service, where crevices generate the deoxygenated, acidified micro-environment that attacks CP titanium above roughly 70 – 80 °C, Grade 7 resists crevice initiation at temperatures far above the CP limit — the reason it is the standard titanium alloy for seawater heat exchangers with tight crevice geometries.
- Robustness to upset conditions. Because the passive state is maintained electrochemically, Grade 7 tolerates acid concentration excursions, localized oxygen depletion, and flow interruptions that would initiate attack on CP titanium.
Palladium does not change the base metal's mechanical behavior: the alloy is still single-phase α, still supplied annealed, and still carries Grade 2 mechanical minimums, so fabricators gain the corrosion upgrade with zero penalty in weldability, formability, or design allowables.
Governing Specifications and Product Forms
| Product Form | ASTM Specification | ASME Equivalent | Typical Range Supplied by Hangbo Alloy |
|---|---|---|---|
| Plate, sheet, and strip | ASTM B265 | ASME SB-265 | Plate 0.3 – 100 mm thick |
| Bar and billet | ASTM B348 | ASME SB-348 | Round bar Ø 6 – 300 mm |
| Seamless condenser / heat-exchanger tube | ASTM B338 | ASME SB-338 | Seamless tube Ø 6 – 89 mm OD |
| Seamless pipe | ASTM B861 | ASME SB-861 | 1/8" – 12" NB |
| Welded pipe | ASTM B862 | ASME SB-862 | 60 – 610 mm OD |
| Forgings | ASTM B381 | ASME SB-381 | Flanges, custom forgings |
| Wire | ASTM B863 | — | Welding wire |
Procurement callout discipline: "Titanium Grade 7, UNS R52400, seamless tube per ASTM B338 / ASME SB-338, annealed" — UNS plus product standard, with palladium range confirmed on the certificate.
Mechanical Properties
Grade 7 mechanical minimums are identical to Grade 2, because the base composition is Grade 2:
| Property | ASTM Minimum | Typical (Annealed) | Notes |
|---|---|---|---|
| Tensile strength (UTS) | 345 MPa (50 ksi) | 480 – 620 MPa | Per B265/B348/B338 tables |
| 0.2% yield strength (YS) | 275 MPa (40 ksi) | 380 – 520 MPa | Some product tables list a 450 MPa max for annealed stock |
| Elongation in 2 in. (50 mm) | 20% | 22 – 30% | Scales with section size |
| Hardness | — | 160 – 210 HV | Informational |
| Condition | Annealed | Recrystallized α | No heat-treatment response |
The mechanical equivalence to Grade 2 is a design convenience: a heat-exchanger designed on Grade 2 allowables can be re-specified in Grade 7 with no change in wall thickness or rating — only the corrosion margin improves. Where higher strength is also needed, the palladium addition is available on the Grade 3 and Grade 4 bases as well in related specifications, but Grade 7 as defined by the ASTM tables is the Grade 2-base product.
Physical Properties
| Property | Value | Notes |
|---|---|---|
| Density | 4.51 g/cm³ (0.163 lb/in³) | Palladium addition does not measurably change density |
| Melting range | ~1660 – 1670 °C | Titanium-base melting behavior |
| Beta transus | ~890 – 920 °C | Oxygen-dependent, as for CP titanium |
| Modulus of elasticity | ~103 – 110 GPa | RT |
| Coefficient of thermal expansion | ~8.6 – 9.2 × 10⁻⁶ /°C | Steel-like |
| Thermal conductivity | ~16 – 19 W/m·K | |
| Electrical resistivity | ~0.55 µΩ·m | |
| Magnetic behavior | Non-magnetic |
Corrosion Performance: Where Grade 7 Earns Its Premium
| Environment | CP Titanium (Grade 2) | Grade 7 (Ti-Pd) |
|---|---|---|
| Seawater and chloride brines | Excellent | Excellent |
| Hot chloride crevices (>70 – 80 °C) | Crevice attack possible | Resistant well above the CP limit |
| Dilute HCl (non-oxidizing) | Active corrosion possible | Passive — usable to moderate concentrations/temperatures |
| Dilute H₂SO₄ (non-oxidizing) | Active corrosion possible | Passive — markedly extended envelope |
| Phosphoric acid | Moderate, temperature-limited | Extended — wider concentration/temperature window |
| Oxidizing acids (nitric, chromic) | Excellent | Excellent |
| Organic acids | Excellent | Excellent |
| Wet chlorine, hypochlorite | Excellent | Excellent |
| Hydrofluoric acid | Not recommended | Not recommended (Pd does not defeat HF) |
| Oxidizing impurities (Fe³⁺, Cu²⁺, O₂) in reducing acids | Improve CP performance | Further improve Grade 7 — synergy with aeration |
The practical message is that Grade 7 should be considered whenever a design is CP-titanium-limited by crevice temperature or by reducing-acid excursions rather than by general strength. Typical installations are seawater-cooled exchangers with tube-to-tubesheet crevices running above ambient, brine heaters, acid-recovery systems, and any chloride service where process upsets can create acidic, deoxygenated conditions. Because its weld metal can be made with matching Pd-bearing filler, full crevice and weld-zone protection carries through fabricated joints.
Fabrication, Welding, and Machining
- Welding: Grade 7 welds exactly like Grade 2. GTAW with argon shielding and backing purge is standard; matching Grade 7 filler preserves the corrosion upgrade in the weld, while Grade 2 filler is sometimes used where weld-metal crevice duty is secondary. No preheat and no post-weld heat treatment are required for the α structure.
- Hot and cold forming: Identical practice to CP titanium — excellent cold formability; hot forming in the 650 – 925 °C band with a final anneal at 650 – 760 °C where needed.
- Machining: Free-cutting relative to α+β alloys; sharp tooling, positive rake, flood coolant, and rigid setups avoid galling and work hardening.
- Surface and cleanliness: Prevent iron contamination; pickle or passivate per specification after fabrication. The passive film re-forms instantly on cleaned surfaces.
- Quality verification: The single most important check is palladium content. Hangbo Alloy verifies every item by PMI and confirms the declared Pd value against the 0.12–0.25% range on the mill certificate, so the corrosion upgrade is real, not assumed.
Grade 7 vs. the Low-Cost Alternatives
Palladium is expensive, and Grade 7 commands a significant premium over CP titanium. That premium has driven the development of alternative "crevice-resistant" titanium alloys — Grade 12 (Ti-0.3Mo-0.8Ni), the ruthenium-bearing Grades 13/14/15 (Ti-Ni-Ru), and the Grade 16/17/18 Pd-modified variants with lower palladium — which Hangbo Alloy also supplies:
| Alloy | Key Addition | Strength Level | Relative Cost | Typical Selection Logic |
|---|---|---|---|---|
| Grade 2 (R50400) | None | Low | Baseline | General chloride service, no crevice issue |
| Grade 7 (R52400) | 0.12 – 0.25% Pd | Grade 2 base | High | Maximum reducing-acid + crevice margin |
| Grade 12 (R53400) | 0.3% Mo + 0.8% Ni | Elevated (483 MPa UTS min) | Moderate | High-temperature reducing acid + higher strength |
| Grade 15 (R53415) | 0.5% Ni + ~0.05% Ru | Grade 3 base (483 MPa UTS min) | Moderate | Crevice resistance at ~Grade 3 strength, lower cost |
| Grade 17 (R52252) | ~0.05% Pd | Grade 1 base | Moderate | Soft, formable Pd grade for tube and strip |
Selection should follow coupon testing in the actual process stream wherever the duty is borderline: the palladium grades set the maximum-performance benchmark, while the Mo-Ni and Ni-Ru grades capture most of the benefit at lower alloy cost. Hangbo Alloy routinely supplies side-by-side coupons for in-plant verification.
Applications Summary
- Seawater and brackish-water heat exchangers, condensers, and coolers with tight crevice geometries running above ambient temperature.
- Chemical-process reactors, columns, and piping handling dilute hydrochloric, sulfuric, or phosphoric acid with intermittent oxidizer exposure.
- Brine heaters, evaporator heat-transfer surfaces, and chlor-alkali cell components.
- Pulp-bleach and chlorine-dioxide equipment where chloride crevice duty is severe.
- Pharmaceutical and fine-chemical reactors requiring total metallic-ion freedom and crevice immunity.
- Geothermal brine handling equipment and produced-water systems in oil and gas.
- Marine propulsion heat exchangers and desalination plant heat-rejection circuits.
- Anode and cathode hardware in electro-winning and metal-finishing lines exposed to acid mist and chloride.
Hangbo Alloy Supply Program
| Product Form | Specification | Typical Size Range | Testing / Documentation |
|---|---|---|---|
| Plate / Sheet / Strip | ASTM B265 / ASME SB-265 | 0.3 – 100 mm thick | EN 10204 3.1, PMI (Pd verified) |
| Bar / Billet | ASTM B348 / ASME SB-348 | Ø 6 – 300 mm | EN 10204 3.1, PMI, UT on request |
| Seamless Tube | ASTM B338 / ASME SB-338 | Ø 6 – 89 mm OD | Hydrostatic, eddy current, PMI |
| Pipe (seamless / welded) | ASTM B861 / B862 | 1/8" NB – 610 mm OD | Hydrostatic, flattening, PMI |
| Forgings / Fittings | ASTM B381 / B363 | Custom | Dimensional, penetrant, PMI |
Every Hangbo Alloy Grade 7 heat is released with the tested palladium value documented, 100% PMI of delivered items, heat-lot traceability, and, on request, third-party witness testing (SGS, BV, TÜV) at our Shanghai mill.
Technical FAQ — Titanium Grade 7 (UNS R52400)
1. What is Titanium Grade 7?
Grade 7 is Grade 2 CP titanium with 0.12–0.25% palladium deliberately added. The palladium acts as a cathodic modifier that extends titanium's passive-film stability into reducing acids and hot chloride crevices where unalloyed titanium can corrode. All other properties — strength, weldability, formability — match Grade 2.
2. How does palladium improve corrosion resistance?
Palladium provides highly efficient sites for the hydrogen-reduction cathodic reaction, pulling titanium's corrosion potential back into the passive range even in deoxygenated, reducing environments. The TiO₂ film therefore stays stable where it would otherwise break down — the principle of cathodic alloying.
3. Which acids does Grade 7 handle better than CP titanium?
Dilute hydrochloric, sulfuric, and phosphoric acids — especially non-oxidizing, deaerated solutions where CP titanium corrodes actively. The improvement is largest in the dilute-to-moderate concentration range up to moderate temperatures; oxidizing impurities in the acid further improve performance.
4. Does Grade 7 resist crevice corrosion?
Yes — this is its signature advantage. CP titanium can suffer crevice attack in hot chloride service above roughly 70 – 80 °C; Grade 7 resists crevice initiation at substantially higher temperatures, making it the standard choice for seawater exchangers with tube-to-tubesheet crevices.
5. What are the mechanical properties of Grade 7?
Identical to Grade 2: minimum 345 MPa tensile, 275 MPa yield, 20% elongation in the annealed condition. A design rated for Grade 2 can be upgraded to Grade 7 with no change in thickness or allowables.
6. Which ASTM specifications cover Grade 7?
ASTM B265 (plate, sheet, strip), ASTM B348 (bar and billet), and ASTM B338 (seamless condenser and heat-exchanger tube), with ASME SB equivalents for code construction. Seamless pipe is B861, welded pipe B862, forgings B381, wire B863.
7. Is Grade 7 weldable, and does the weld retain corrosion resistance?
Grade 7 welds like CP titanium — GTAW with argon shielding and backing purge, no preheat, no post-weld heat treatment. Using matching Pd-bearing filler preserves the corrosion upgrade through the weld and heat-affected zone.
8. Why is Grade 7 expensive, and what are the alternatives?
Palladium is a precious metal, so Grade 7 carries a large premium over CP titanium. Grade 12 (Mo-Ni), Grade 15 (Ni-Ru), and the low-palladium Grade 16/17 family capture much of the crevice-resistance benefit at lower cost. Hangbo Alloy supplies the full family and advises on the economic crossover for each duty.
9. How do I verify I actually received Grade 7?
Check the mill certificate for a declared palladium value within 0.12–0.25% and confirm by independent PMI on arrival. A certificate without a stated palladium figure is not proof of Grade 7. Hangbo Alloy declares the tested Pd value on every EN 10204 3.1 certificate and PMI-checks every delivered item.
10. Does Hangbo Alloy supply Grade 7 tube for heat exchangers?
Yes. Hangbo Alloy supplies Grade 7 seamless condenser and heat-exchanger tube to ASTM B338, plus plate, bar, pipe, and forgings, with full traceability and third-party inspection options at our Shanghai mill.
This page is part of the Titanium Alloy Technical Reference series by Hangbo Alloy (Shanghai Hangbo Alloy Group, nickel-alloy.com). Data presented are typical engineering values compiled from recognized industry sources and are provided for material selection guidance; the governing documents for any purchase are the applicable ASTM/ASME specifications (B265, B348, B338 and SB equivalents) and the certified mill test report. Contact Hangbo Alloy for current stock, mill certificates, and application engineering support.
