Hangbo Alloy Group
Shanghai · Est. 2012
Nitric acid is a strong oxidizing acid — and it breaks the usual rules. This guide shows engineers why high-chromium, low-molybdenum nickel alloys (Alloy 600, Alloy 690, Alloy 800H) succeed where the famous molybdenum-bearing superalloys fail.
Nitric acid (HNO₃) is the second-largest commodity mineral acid after sulfuric acid, central to the production of fertilizers (ammonium nitrate), explosives, specialty chemicals and — ironically — the pickling of the very stainless and nickel alloys that must contain it. As a strong oxidizing acid, its material-selection logic is the opposite of reducing acids such as hydrochloric or pure sulfuric acid. Get it wrong and you will specify the most "corrosion-resistant" alloy in the catalogue and watch it fail.
This selection guide gives plant engineers and procurement teams a practical framework for specifying nickel alloys for nitric acid service. As a direct nickel alloy manufacturer and supplier, Hangbo Alloy Group produces and stocks Alloy 600, Alloy 690 and Alloy 800H in all product forms, with mill test certificates and third-party inspection.
Nitric acid is a strong oxidizing acid. In corrosion terms that is decisive:
Golden rule: For nitric acid, choose the highest-chromium, lowest-molybdenum alloy that meets your mechanical and cost requirements — not the alloy with the highest PREN or the highest molybdenum. Alloy 690 > Alloy 600 > 310L NAG > 304L, and a molybdenum-free path is almost always correct.
Corrosion behavior follows directly from chemistry. The nitric-acid winners are chromium-rich and essentially molybdenum-free; the referenced stainless grades are the economical baseline.
| Alloy (UNS) | Family | Ni % | Cr % | Fe % | Mo % | Other key |
|---|---|---|---|---|---|---|
| 304L (S30403) | Austenitic SS | 8–12 | 18–20 | Balance | ≤0.75 | Low C |
| Alloy 600 (N06600) | Ni-Cr-Fe | ≥72 | 14–17 | 6–10 | — | Low C |
| Alloy 690 (N06690) | Ni-Cr-Fe | 58–63 | 27–31 | 7–11 | — | Low C |
| Alloy 800H (N08810) | Fe-Ni-Cr | 30–35 | 19–23 | 39–46 | — | Ti/Al, low C |
The contrast is the whole story: Alloy 690 carries roughly 30% chromium — about double Alloy 600 and far above 304L — giving it the densest, most stable passive film in oxidizing nitric acid. It also carries no molybdenum, so there is no vulnerable reducing element to attack.
Typical room-temperature, annealed values help with pressure-part and structural sizing:
| Alloy | Density (g/cm³) | Tensile (MPa) | Yield (MPa) | Elongation % | Max Useful Temp* |
|---|---|---|---|---|---|
| 304L | 8.0 | 485–620 | 170–310 | ≥40 | ~870°C (struct.) |
| Alloy 600 | 8.47 | ≥550 | ≥240 | ≥30 | ~1093°C (oxidizing) |
| Alloy 690 | 8.19 | 550–760 | ≥240 | ≥35 | ~1000°C (oxidizing) |
| Alloy 800H | 8.0 | ≥450 | ≥180 | ≥30 | ~1093°C (struct.) |
*“Max useful temp” shown is the structural / high-temperature oxidation limit. In nitric acid the binding limit is corrosion, which is governed by concentration and temperature rather than by strength — always confirm against isocorrosion data for your exact conditions.
The summary below reflects well-established industry and manufacturer guidance for pure nitric acid. Rates illustrate typical selection bands; confirm against isocorrosion curves and, for nitric acid, the Huey (ASTM A262) test for your exact conditions.
| HNO₃ Condition | Recommended Alloy | Notes |
|---|---|---|
| ≤ ~60%, to atmospheric boiling | 304L / 310L NAG | Economical baseline; L-grade or stabilized for welds |
| Medium conc. (≤65–70%), moderate T | Alloy 600 (N06600) | Good Ni-Cr-Fe resistance; lower cost than 690 |
| ≥70%, ≤80°C, concentrated | Alloy 690 (N06690) | Excellent; highest-Cr nickel alloy |
| HNO₃ + HF (pickling) | Alloy 690 (N06690) | Best combined HNO₃/HF resistance |
| Fuming (>90%) | Aluminium / high-Si SS | Not a nickel-alloy option |
| Mixed HNO₃+HCl (oxidizing+halogen) | Alloy 690 / C-22 (coupon test) | Chromium + nickel defense |
| AVOID in pure/boiling HNO₃ | 625, C-276, C-22 (Mo-bearing) | Molybdenum accelerates attack |
Nitric acid is oxidizing, which inverts the normal alloy logic:
The highest-chromium (27–31%) nickel alloy used in pressure equipment, and the top choice for hot, concentrated nitric acid and for nitric/hydrofluoric pickling service. Its dense, chromium-rich oxide film is essentially immune in boiling and concentrated HNO₃ where lower-chromium alloys are attacked. Also the nuclear-grade material of choice where HNO₃/F⁻ resistance and immunity to chloride stress corrosion cracking are both required.
Nickel-chromium-iron alloy whose chromium addition extends usefulness into oxidizing nitric acid. Gives good, reliable service at medium concentrations up to about 65–70°C and moderate temperature, and is widely used for nitric-acid condensers, heat-exchanger tubing and vessel components. The cost-effective nickel choice when Alloy 690's ultra-high chromium is not required.
Iron-nickel-chromium alloy with ~21% chromium, used where nitric acid service coincides with high temperature (furnaces, reformers, nitrate production equipment). Its high chromium and good creep strength make it suitable for structural nitric-acid duty up to elevated temperatures.
The economical baseline. 304L handles ≤60% HNO₃ to the boiling point; the low-carbon 310 variant (310L NAG) is engineered for boiling 50–65% nitric acid and is used above ~70% concentration. Always use the L-grade or a stabilized grade (321/347) where welding is involved.
| Your Service Condition | First Choice | Upgrade When… |
|---|---|---|
| ≤60% HNO₃, clean, to boiling | 304L / 310L NAG | Welded & sensitized → 600/690 |
| Medium HNO₃ (≤65–70°C) | Alloy 600 (N06600) | Higher T / conc → Alloy 690 |
| ≥70% concentrated, ≤80°C | Alloy 690 (N06690) | Fuming → Al / high-Si SS |
| HNO₃ + HF pickling | Alloy 690 | Start here (Mo alloys fail) |
| High-temp + nitric acid | Alloy 800H | Conc. rises → 690 |
| Reducing acid instead (HCl/H₂SO₄) | Inconel 625 / C-276 | Only if NOT nitric |
| Pure nickel / Monel | Not for HNO₃ | Use for caustic / reducing |
In stainless-steel and nickel-alloy pickling, nitric acid is mixed with hydrofluoric acid — one of the most aggressive combinations in the plant. The correct long-life material is Alloy 690, whose high nickel resists the HF component while its high chromium resists the HNO₃ component; tanks, heaters and acid pumps lined or built from 690 routinely outlast 304L by many times. For nitrate production, fertilizer and explosives plants where nitric acid meets elevated temperature, Alloy 800H and Alloy 690 give the needed combination of oxidation resistance and structural strength.
Hangbo Alloy Group manufactures and supplies nitric-acid-service alloys in a full range of forms, with mill test certificates and third-party inspection (SGS, TÜV) available:
| Alloy (UNS / W.Nr) | Plate | Rod/Bar | Pipe/Tube | Forgings |
|---|---|---|---|---|
| Alloy 600 (N06600 / 2.4816) | ASTM B168 | ASTM B166 | ASTM B167/B163 | ASTM B564 |
| Alloy 690 (N06690 / 2.4642) | ASTM B168 | ASTM B166 | ASTM B167 | ASTM B564 |
| Alloy 800H (N08810 / 1.4958) | ASTM B409 | ASTM B408 | ASTM B407 | ASTM B564 |
| 304L (S30403) | ASTM A240 | ASTM A276 | ASTM A312 | ASTM A182 |
All grades are also referenced by EN/DIN (W.Nr.), ISO, AMS and GB standards, and are covered by ASME pressure-vessel codes. Alloy 690 additionally carries RCC-M and VdTÜV approvals for nuclear-grade nitric/fluoride service. For nitric-acid duty, materials are typically qualified with the Huey test (ASTM A262 Practice C) for intergranular-corrosion susceptibility.
Not sure whether your nitric acid stream calls for Alloy 690, Alloy 600 or Alloy 800H — or whether 304L/310L stainless is sufficient? Send us your concentration, temperature and impurity profile and our metallurgists will recommend the most cost-effective certified alloy. As a direct manufacturer and supplier, we offer competitive price quotes, full material certification (including Huey-test support) and worldwide delivery.
Email: [email protected] | Phone / WhatsApp: +86-136-1165-6360
For nitric acid (a strong oxidizing acid) the best nickel alloys are high-chromium, molybdenum-free grades. Alloy 690 (UNS N06690, ~30% Cr) is the top choice for hot, concentrated nitric acid and for HNO3+HF pickling service. Alloy 600 (UNS N06600) is an excellent, lower-cost option for medium concentrations up to about 65–70°C, and Alloy 800H (UNS N08810) is used where high temperature and oxidizing acid coincide. Critically, molybdenum-bearing nickel alloys such as Inconel 625 and Hastelloy C-276 are NOT recommended for pure/strong nitric acid because molybdenum accelerates attack.
Nitric acid is a strong oxidizing acid. Chromium is the element that builds the protective oxide (passive) film, while molybdenum is a reducing-acid element. In strongly oxidizing nitric acid, molybdenum can be oxidized and preferentially attacked, and sigma phase / molybdenum-rich transitional phases in these alloys are selectively corroded (intergranular attack). So the very grades that resist hydrochloric and sulfuric acid — 625, C-276, C-22 — perform poorly in pure nitric acid. Choose chromium-rich, low-molybdenum alloys (600, 690, 800H) instead.
Yes, for many services. Type 304/304L austenitic stainless steel is the workhorse for handling and storing nitric acid up to about 60% concentration and at temperatures up to the atmospheric boiling point. The low-carbon 'L' grade (or a stabilized grade such as 321/347) must be used where welding is involved, because nitric acid readily attacks chromium-depleted regions caused by carbide precipitation at the weld heat-affected zone. For concentrations above about 70%, the low-carbon 310 variant (310L NAG / S31002) is preferred.
Alloy 690 (UNS N06690, W.Nr 2.4642) is the highest-chromium (27–31% Cr) nickel alloy used in pressure equipment. That ultra-high chromium content forms an exceptionally stable, dense oxide film in strong oxidizing media — including boiling and concentrated nitric acid — where lower-chromium alloys would be attacked. It is rated excellent for ≥70% HNO3 at up to about 80°C and is the leading alloy for nitric-acid / hydrofluoric-acid (HNO3+HF) stainless-steel pickling lines because its high nickel content also resists HF while its chromium resists HNO3.
Yes. Inconel 600 (UNS N06600, W.Nr 2.4816) is a nickel-chromium-iron alloy whose chromium addition expands its usefulness into oxidizing environments. It gives good, reliable service in nitric acid at medium concentrations (roughly up to 65–70%) and moderate temperature, and is widely used for nitric-acid condensers, heat-exchanger tubing and vessel components. For hot, concentrated nitric acid, step up to Alloy 690.
Stainless and nickel alloy pickling uses mixed nitric + hydrofluoric acid, which is extremely aggressive. Alloy 690 (UNS N06690) is the preferred long-life material for pickling tanks, heaters and acid pumps because its high nickel resists the HF component and its high chromium resists the HNO3 component. Lower-cost 304L/310L is used for mild, well-controlled pickling, while plain carbon or coated steel is common for bulk dilute lines. Molybdenum-bearing alloys are avoided here.
Corrosion resistance is driven by whether a stable chromium oxide passive film can be maintained. 304L is reliable to about 60% concentration at the boiling point; above ~70% concentration or at higher temperatures the film can break down and a higher-chromium alloy (310L NAG, Alloy 600 or especially Alloy 690) is needed. Always size the alloy to the worst-case process upset (highest temperature, highest oxidizing condition), not the design average, and confirm against isocorrosion data for your exact concentration.
The Huey test (ASTM A262 Practice C) is a five-period, 48-hour boiling nitric acid immersion test used to measure a material's susceptibility to intergranular corrosion. Because nitric acid is so sensitive to sensitization (chromium carbide precipitation at grain boundaries), alloys intended for nitric acid service are routinely qualified with the Huey test. A low, stable weight-loss rate after five boiling periods is the acceptance criterion — and a key reason 'L' grades and proper solution annealing are mandated for nitric-acid equipment.
During welding, chromium near the grain boundaries can combine with carbon to form chromium carbides, depleting the surrounding metal of chromium. In oxidizing nitric acid this chromium-depleted zone is preferentially attacked (intergranular corrosion). Using low-carbon 'L' grades (304L, 316L, low-C Alloy 600/690) or stabilized grades (321, 347, with Nb/Ti) prevents sensitization. Where required, a post-weld solution anneal restores full corrosion resistance.
Alloy 600 = UNS N06600 (W.Nr 2.4816); Alloy 690 = UNS N06690 (W.Nr 2.4642); Alloy 800H = UNS N08810 (W.Nr 1.4958); 304L = UNS S30403; 310L NAG = UNS S31002. These grades are cross-referenced in ASTM/ASME, EN/DIN (W.Nr.), ISO, GB (China) and AMS standards. For nuclear-grade nitric/fluoride service, Alloy 690 also carries RCC-M and VdTÜV approvals.
Yes. Alloy 600 is welded with ERNiCr-3 (AWS A5.14) / ENiCrFe-3 (AWS A5.11) filler. Alloy 690 uses matching ERNiCrFe-7 (Inconel 52) or the modified ERNiCrFe-7A (Inconel 52M) consumable. Alloy 800H is welded with ERNiCr-3 or 800-type filler. Use clean, low-carbon procedures and proper argon purge; for nitric acid service, qualify the weld with the Huey test and apply a solution anneal where sensitization is a risk.
Yes. For large nitric-acid vessels and towers, explosion-clad or weld-overlay construction (carbon steel + Alloy 600/690/800H liner) provides the strength of carbon steel with a corrosion-resistant alloy on the wetted surface at a fraction of the solid-alloy cost. Hangbo Alloy Group supplies both solid and clad forms with full material certification and third-party inspection.
Send your nitric acid concentration, temperature, any impurities (HF, HCl, metal ions), pressure and required form to Hangbo Alloy Group at [email protected] or call/WhatsApp +86-136-1165-6360. As a direct manufacturer and supplier we provide competitive price quotes, mill test certificates, Huey-test support and worldwide delivery of bars, plates, tubes, forgings and welding consumables.
For help choosing between Alloy 690, Alloy 600, Alloy 800H or stainless alternatives for your nitric acid service — or for a quotation on bars, plates, tubes, forgings or welding consumables — contact Hangbo Alloy Group. We typically respond within 10 minutes and ship worldwide from our Shanghai facility.
Email: [email protected]
Phone: +86-136-1165-6360
WhatsApp: +86 13611656360
Send us your concentration, temperature and impurity profile and we will recommend the most cost-effective certified nickel alloy — with a competitive quote and full Huey-test-capable certification.