Zr702 vs Zr705: Which Zirconium Grade Should You Specify?

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Reviewed and maintained by the Engineering Team, Shaanxi Qianyi Tuoda Technology Co., Ltd.

Zr702 vs Zr705: Which Zirconium Grade Should You Specify?

If you remember only one number from this page: Zr705 has a yield strength of 379 MPa, Zr702 only 207 MPa — almost double — yet both show a corrosion rate below 0.05 mm/year in boiling 20% HCl. So the selection logic is simple. For pure corrosion duty, complex shapes, or heavy welding, specify Zr702 (UNS R60702, unalloyed zirconium). For high mechanical load — pump shafts, agitator components, high-pressure valve stems, pressure-vessel internals — specify Zr705 (UNS R60705, zirconium–2.5%Nb). Both grades must stay away from hydrofluoric acid, oxidizing chlorides, low-water concentrated nitric acid, and hydrochloric acid above roughly 149 °C. Read the At a Glance table, then the failure-mode section before you write the grade into a purchase order.

At a Glance: Zr702 vs Zr705

Both grades carry the same passive ZrO₂ film. The only difference that matters in service is niobium and what it buys you — strength, at a small corrosion premium.

What Zr702 and Zr705 Actually Are

Zr702 is commercial-purity zirconium: Zr+Hf ≥ 99.2%, with no niobium. Hafnium is capped at 4.5% — not because it is harmful, but because zirconium and hafnium are mined together and their properties are so close that commercial (“chemical”) grade does not bother to strip hafnium below 4.5%. Zr705 is a zirconium–niobium alloy: Zr+Hf ≥ 95.5% with 2.0–3.0% Nb. The niobium forms fine precipitates that roughly double the yield strength.

Both are commercial grades. Do not confuse them with nuclear grades such as Zircaloy-2 / Zircaloy-4 (UNS R60802 / R60804), which carry tin, iron, chromium and nickel for fuel-cladding service. Zr702 and Zr705 are the workhorses of chemical-process equipment. If you are weighing zirconium against titanium, our comparison of titanium vs zirconium in acetic acid shows why zirconium wins in reducing acids where titanium fails.

Zirconium sits directly above titanium in the periodic table, which is why both form a tight, stable oxide film and both are valued for corrosion service. Commercial (chemical-grade) zirconium is made from zircon sand through carbothermal reduction and chlorination; hafnium, almost identical chemically, is either stripped out for nuclear use or left in (capped at 4.5%) for chemical use. That shared, difficult supply chain is a big part of why zirconium costs several times more than titanium.

Beyond chemical plants, zirconium’s very low neutron-absorption cross-section — about 0.18 barns, versus roughly 5 for titanium and far higher for most steels — makes it valuable wherever a material must sit near a reactor or a reprocessing line without soaking up neutrons or activating badly. Nuclear-grade variants (Zircaloy-2/4) are qualified for fuel cladding; the commercial grades Zr702 and Zr705 are not used for cladding but are common in reprocessing equipment such as dissolver vessels, centrifugal contactors and piping, where corrosion resistance and low activation matter together. That dual identity — corrosion metal in the plant, inert metal in the reactor neighborhood — is why zirconium keeps appearing in the most demanding process streams.

The “Pure Zirconium Resists Everything” Myth

A common assumption is that because Zr702 is “pure,” it resists every chemical. It does not. Zirconium is outstanding in reducing acids — hydrochloric, sulfuric, acetic, formic, lactic — but it is attacked rapidly by hydrofluoric acid, oxidizing chlorides, low-water concentrated nitric acid, aqua regia and several other oxidizers. Below roughly 149 °C, zirconium is excellent in hydrochloric acid at all concentrations up to the boiling point; above that temperature the risk of hydrogen pickup appears.

The accurate statement is not “pure zirconium resists everything.” It is “pure zirconium resists everything in the media where its passive film is stable — and fails fast where that film is destroyed.”

That is exactly why the failure-mode section further down is the most useful part of this page. Grade choice cannot fix a wrong-medium mistake.

A useful way to picture the failure is the oxide film. In reducing acids the ZrO2 film is stable and self-healing, so the metal is essentially inert. Introduce fluoride, a strong oxidizer, or an oxidizing metal ion, and the film is either dissolved or prevented from reforming, so the bare metal corrodes within hours. The grade you pick cannot change that chemistry; only the environment or a different alloy can.

Strength: How Zr705 Nearly Doubles Yield

The numbers translate to a +46% tensile and +83% yield for Zr705. For a designer this means thinner sections, fewer supports, or simply a part that will not deform under load. Zirconium is expensive, so the strength saving rarely pays for the material itself — Zr705 earns its premium when the part would otherwise be strength-limited, not when you are trying to save metal.

For a designer the yield gap is concrete: a Zr705 shaft carries roughly 80% more bending or torsional load than a Zr702 shaft of the same diameter, or reaches the same load at a smaller section. Against titanium, Zr705 yield of 379 MPa sits between commercial-purity Gr2 (about 275 MPa) and Ti-6Al-4V (about 880 MPa), but Zr705 keeps its corrosion resistance in HCl where even Ti-6Al-4V fails. Choose Zr705 when the component is strength-limited and the medium is reducing.

When zirconium itself is not enough, the next step up the corrosion ladder is tantalum, which resists hydrochloric and sulfuric acid at essentially all temperatures and concentrations, plus hot nitric acid containing fluoride. Tantalum is the metal engineers reach for only after zirconium’s temperature or concentration envelope is exceeded — but it can cost five to ten times as much as zirconium, and it fails in hot concentrated alkali and in free fluorine gas. Hastelloy C-276 covers many oxidizing and mixed environments that zirconium cannot, yet it is attacked by hydrochloric acid at high temperature where zirconium excels. The practical rule is to try zirconium first (cheapest of the exotic options), then tantalum, then a Hastelloy, in that order of cost.

Corrosion: Where the Two Grades Are the Same

Because both grades form the same ZrO₂ film, their corrosion behavior overlaps almost completely in the media where zirconium shines: hydrochloric acid (all concentrations to boiling), sulfuric acid (to about 70%, well above boiling), nitric acid (to about 70%, up to 250 °C), acetic / formic / lactic acids, urea and carbamate, hot alkalis, most organic acids, and reducing chloride environments such as seawater. In boiling 20% HCl both grades sit below 0.05 mm/year — the reason zirconium is the default for HCl distillation columns and reboilers.

By contrast, titanium collapses in hydrochloric acid: even at room temperature and low concentration, chloride ions destroy the TiO₂ film and the metal corrodes catastrophically. Our grade-by-grade HCl corrosion limits for titanium show why engineers reach for zirconium (or palladium-grade titanium) instead. For seawater duty the relevant comparison is Gr2 vs Gr7 titanium; for zirconium, chloride is only a problem when it is oxidizing chloride (see failure modes).

Typical corrosion rates tell the story: in boiling 20% HCl both grades stay under 0.05 mm/year; in 37% HCl zirconium remains serviceable well past 100 °C while 316L stainless and most nickel alloys are already attacking. Sulfuric acid is similar, good to roughly 70% and well above boiling. The two grades diverge only in the last fraction of edge cases, not across the broad band of normal chemical service.

The Strength–Corrosion Trade-off (the Part Vendors Skip)

Zr705 is not “more corrosion resistant” than Zr702. The niobium-rich precipitates can act as initiation sites for localized attack, so Zr705’s all-round corrosion resistance is slightly below Zr702’s. In reducing acids (HCl, H₂SO₄) Zr705 is not better than Zr702 — it is marginally worse — while costing more. Its only real selling point over Zr702 is strength.

Rule of thumb: never specify Zr705 to get “better corrosion.” Specify it only when you need the yield. For pure chemical resistance, stay with Zr702.

This is the same trade-off you see inside titanium families — compare Gr7 vs Gr12 titanium (palladium for corrosion vs molybdenum for strength) or Ni200 vs Ni201 nickel (carbon control vs high-temperature service). Alloying always trades one property for another.

The mechanism behind the small corrosion penalty is metallurgical. Niobium in Zr705 precipitates as fine beta-phase particles during cooling; these particles are slightly anodic to the alpha matrix and can act as pit-initiation sites in a few aggressive media. The effect is minor in most service and invisible in HCl, but real in strongly oxidizing conditions, which is another reason Zr702 is the default for pure corrosion duty.

How to Read the Mill Certificate

Both grades are covered by ASTM B551 / B551M (ASME SB551) for strip, sheet and plate. The chemistry and mechanical limits below are the values you should see on the EN 10204 3.1 MTC.

On acceptance, confirm Nb is inside 2.0–3.0% for Zr705, oxygen is within the cap, and hydrogen is at or below 0.005%. Hafnium at up to 4.5% is normal for both grades — do not reject a certificate just because hafnium is present. When you order Titanium Plate to ASTM B265 or titanium tube to ASTM B338, the same discipline of reading the MTC applies.

Beyond chemistry, insist on the right tests for the form: plate should carry ultrasonic testing and a flatness report, tube needs hydraulic or eddy-current testing, and bar should be checked for straightness and surface condition. The EN 10204 3.1 certificate ties every value back to the actual heat, which matters when the part goes into a pressure vessel or a nuclear-adjacent line.

Machining zirconium is closer to machining 316L stainless or titanium than to carbon steel: the metal work-hardens quickly, so use sharp, positive-rake tools, a rigid setup, moderate cutting speed and a generous feed to stay ahead of the hardened layer. Dwell, re-cutting of work-hardened surfaces and built-up edge all accelerate tool wear. Flood coolant is recommended, and grinding or wire-EDM dust must be collected and disposed of safely because fine zirconium is flammable. On acceptance, a flat, low-stress part with a clean, silver machined surface is the sign the shop controlled both the alloy and the process.

Welding and Forming: Why Zr702 Is Easier

Both grades are welded by GTAW (TIG) with full inert-gas shielding — trailing shield plus back purge with argon. Any air contact embrittles the weld; a bright silver bead means the shielding was good, a blue or grey bead means contamination. Zr702 has no second phase, so it welds and forms slightly more predictably, and its heat-affected zone stays more uniform. Zr705’s niobium demands tighter heat-input control to avoid Nb segregation; use matching ERZr705 filler for Zr705 and ERZr702 for Zr702 — never dilute across grades.

Two shop-floor cautions: zirconium fines and powder can ignite, so manage swarf carefully; and keep zirconium separate from carbon-steel tools and dust to avoid iron contamination. For thick vessels, zirconium is often explosion-clad onto steel to cut cost while keeping a corrosion-resistant face. Background on forming and cutting also applies to titanium bar and titanium wire supply.

Practical welding parameters: use argon of at least 99.999% purity, a trailing shield 40 to 60 mm wide, and a back-purge flow that keeps the root in inert gas until it cools below about 200 °C. Keep heat input moderate to limit the beta-phase band in Zr705. A qualified procedure (WPQ/PQR) and a certified welder are non-negotiable for pressure-retaining joints.

Commercial zirconium is supplied in the annealed condition, and for most fabrications no further heat treatment is needed beyond a stress-relief around 550–650 °C to relax forming and welding residual stresses. Any elevated-temperature step must be done in vacuum or inert gas — never in air above roughly 425 °C, or the surface oxidizes and embrittles. After welding a pressure-retaining joint, confirm the procedure was qualified (PQR/WPQ) and that the root and crown both show the bright silver color of uncontaminated zirconium before the part is released.

Limitations and Failure Modes

Grade choice cannot save a wrong-medium decision. Below are the conditions where zirconium — either grade — fails. Read this before specifying.

Hydrofluoric Acid and All Fluoride-Bearing Media

Fluoride ions dissolve the ZrO₂ film. Hydrofluoric acid, fluoroboric acid and fluosilicic acid attack zirconium rapidly. Hard exclusion — no grade of zirconium is acceptable here.

Oxidizing Chlorides (Fe³⁺ / Cu²⁺, Wet Chlorine)

Reducing chlorides (e.g. sodium chloride, seawater) are fine. But oxidizing metal chlorides — ferric and cupric chloride solutions — and wet chlorine destroy the passive film. Note the nuance: zirconium resists bleaching liquors containing chlorine dioxide, yet free wet chlorine gas is aggressive. Confirm the exact species, not just “chloride.”

Fuming / Low-Water Nitric Acid (Fire Risk)

Zirconium is excellent in nitric acid up to about 70% concentration and 250 °C. The danger is concentrated nitric acid with very little water, which can ignite the metal. Keep nitric service within the ~70% / 250 °C envelope and avoid fuming grades.

Hydrochloric Acid Above ~149 °C

Below the boiling point at all concentrations, HCl is zirconium’s strongest case. Above roughly 149 °C the risk of hydrogen pickup appears, which leads to embrittlement. Stay within the boiling envelope for long-life HCl service.

Sulfuric Acid Above ~70% at Elevated Temperature

Zirconium resists sulfuric acid to about 70% concentration, well above boiling. Beyond that — especially fuming sulfuric acid — the corrosion rate climbs and Hastelloy or tantalum may be the better call.

Hydrogen Embrittlement (>300 °C or Cathodic Polarization)

Above about 300 °C zirconium begins to absorb hydrogen; the same happens under cathodic polarization or when galvanically coupled to a more noble metal. The result is hydrogen embrittlement. It is reversible by vacuum annealing near 1000 °C — but prevention at design stage is cheaper.

Air Oxidation Above 425 °C

In air, zirconium scales severely above 425 °C, forms white zirconia around 540 °C, and becomes oxygen-embrittled above 700 °C. Process temperatures must stay well below these thresholds; inert or vacuum atmospheres extend the limit.

Aqua Regia and Strong Mixed Oxidizers

Aqua regia and similarly strong mixed oxidizers break the passive film. Zirconium’s enemies are fluoride, free halogens, oxidizing metal ions and strong oxidizers — not chloride itself.

Hydrogen embrittlement is the failure mode engineers underestimate most. It appears slowly: a part looks fine, then cracks under load after months in hydrogen-rich, cathodically polarized, or very high-temperature service. Design margins, correct galvanic isolation, and a vacuum anneal can recover most ductility if caught early.

Five mistakes show up repeatedly in failed zirconium specs. First, specifying Zr705 to get “better corrosion” when it is only stronger. Second, forgetting that pickling or cleaning baths may contain hydrofluoric or fluoroboric acid — even trace fluoride destroys the film. Third, galvanically coupling zirconium to carbon steel in a conductive electrolyte: zirconium is noble, so the steel corrodes fast; use dielectric isolation. Fourth, stretching hydrochloric service past the ~149 °C hydrogen-pickup limit. Fifth, machining without accounting for work-hardening. Every one of these is a specification error, not a grade error.

Zr702 vs Zr705 by Application

For fabricated equipment — reactors, heat exchangers, agitators — see our non-standard titanium equipment buyer’s guide; the same corrosion logic applies when we build zirconium-lined or solid-zirconium units. Related hardware pages: Titanium Flanges, Titanium Fittings, Titanium Fasteners, and our nickel products line.

When you brief a fabricator, give the exact media, temperature, and whether the part is load-bearing. That single sentence decides the grade: corrosion-only and complex points to Zr702; load-bearing points to Zr705; and if the medium is on the exclusion list above, stop, because zirconium is the wrong metal and no grade change will fix it.

A complete purchase order line should name the grade (UNS R60702 or R60705), the product standard (ASTM B551 for plate, B523 for seamless tube, B658 for welded tube, B493 for bar/wire, B350/B351 for forgings), the temper (annealed), exact dimensions and quantity, the MTC type (EN 10204 3.1), and any required NDT (ultrasonic, eddy-current or hydrostatic). For explosion-clad plate, state both the base thickness and the zirconium cladding thickness. Send the fabricator the media, temperature and whether the part is load-bearing — those three facts settle the grade faster than any corrosion chart.

Summary: Choose Zr702 if / Choose Zr705 if

Choose Zr702 if:

• ☐ The service is reducing-acid dominated (HCl, H₂SO₄, acetic, formic, urea).
• ☐ The shape is complex and welding / forming volume is high.
• ☐ Cost is sensitive and you do not need very high strength.
• ☐ Purity matters — pharma, food, nuclear reprocessing.

Choose Zr705 if:

• ☐ The component carries high mechanical load (shaft, agitator, stem, vessel internals).
• ☐ You need a 379 MPa yield grade — stronger than most titanium, far better in reducing acids.
• ☐ You accept a slightly lower all-round corrosion rating and a higher price for the strength.

One-line rule: Zr702 is the default. Zr705 is the premium you pay only when the load demands it.

Frequently Asked Questions

Is Zr705 more corrosion resistant than Zr702?

No. Zr705's all-round corrosion resistance is slightly below Zr702's because the niobium-rich precipitates can initiate localized attack. In reducing acids like HCl and H2SO4, Zr705 is not better than Zr702. Zr705's only real advantage over Zr702 is strength (yield 379 MPa vs 207 MPa).

Can I substitute Zr705 for Zr702 on an existing drawing?

Usually yes, because Zr705's properties envelope Zr702's. But confirm the design did not rely on Zr702's slightly easier welding and forming, and that budget and lead time allow the niobium grade. The reverse — dropping Zr705 to Zr702 on a high-load part — is not safe without re-rating.

What welding wire do I use for Zr702 and Zr705?

Use matching filler: ERZr702 for Zr702, and ERZr705 for Zr705. Never dilute across grades, because mixing changes the corrosion and strength balance. Both require GTAW with full inert-gas shielding (trailing shield plus back purge); a bright silver bead indicates good shielding.

Why is zirconium so much more expensive than titanium?

Zirconium ore is scarcer, hafnium must be separated or tolerated, and fabrication needs strict inert-gas protection. As an approximate market range, commercial zirconium runs about 3 to 5 times the price of titanium per kilogram, which is why it is specified only where corrosion or purity forces it.

Does zirconium resist nitric acid?

Yes, within limits. Zirconium is excellent in nitric acid up to about 70% concentration and 250 °C. The danger is concentrated nitric acid with very little water, which can ignite the metal. Stay inside the ~70% / 250 °C envelope and avoid fuming grades.

Is zirconium safe for food and pharmaceutical contact?

Yes. Zirconium is non-toxic and biocompatible, and it does not shed metal ions into the process stream, which is why it is used in pharmaceutical bioreactors and food-contact equipment under cGMP and FDA-compliant plant designs. Zr702 is the usual grade for these duties.

What ASTM standards cover Zr702 and Zr705?

Plate, sheet and strip: ASTM B551 / B551M (ASME SB551). Seamless tube: ASTM B523. Welded tube: ASTM B658. Bar and wire: ASTM B493 (also referenced as B550). Forgings: ASTM B350 / B351. Always confirm the current edition and the mill certificate values before acceptance.

Need Zr702 or Zr705 in stock, cut to size, or built to drawing? As a titanium / nickel / zirconium stockholder we supply plate, tube, bar and fittings to ASTM B551 / B523 / B658 / B493, offer zero-cut small batches, machine to customer drawings, and ship internationally with fast turnaround. Request a quote →

Images to add in CMS (alt text in English):

• Alt: “Zr702 vs Zr705 zirconium grade comparison table — yield strength 379 MPa vs 207 MPa”
• Alt: “Zirconium 705 pump shaft and agitator component in chemical service”
• Alt: “Mill test certificate checkpoints for ASTM B551 Zr702 and Zr705 plate”