Two Nickel Alloy Families, Two Corrosion Strategies
The comparison that matters in practice is between the nickel-chromium-iron family and the nickel-chromium-molybdenum family. The first relies on a chromium-rich surface oxide to survive oxidising, sulphur-bearing and combustion environments; the second loads the alloy with molybdenum (and often tungsten) so that it survives reducing acids and chloride pitting. Both are austenitic, both are nickel-based, and both are frequently purchased for the same project on the same drawing set, yet they are not interchangeable.
Representative grades on the Ni-Cr-Fe side are UNS N06600, N06601, N06690 and the cobalt-bearing N06617. On the Ni-Cr-Mo side, UNS N10276, N06022, N06455 and N06625 are the common designations. Niobium-stabilised UNS N06625 sits between the two families: it keeps enough chromium for oxidising service while adding molybdenum and niobium for pitting resistance and creep strength.
Chemical Composition and Metallurgical Differences
| Grade | Ni, % | Cr, % | Mo, % | Fe, % | Design intent |
|---|---|---|---|---|---|
| UNS N06600 | 72.0 min | 14.0–17.0 | – | 6.0–10.0 | oxidation, carburisation, caustic service |
| UNS N06601 | 58.0–63.0 | 21.0–25.0 | – | remainder | cyclic oxidation above 1,000 °C; aluminium forms a protective scale |
| UNS N06690 | 58.0 min | 27.0–31.0 | – | 7.0–11.0 | high-chromium variant for nuclear steam-generator tubing |
| UNS N06617 | 44.5 min | 20.0–24.0 | 8.0–10.0 | 3.0 max | cobalt-bearing gas-turbine hot sections |
| UNS N06625 | 58.0 min | 20.0–23.0 | 8.0–10.0 | 5.0 max | pitting plus creep strength, niobium stabilised |
| UNS N10276 | balance | 14.5–16.5 | 15.0–17.0 | 4.0–7.0 | reducing acids, chlorides, tungsten added |
| UNS N06022 | balance | 20.0–22.5 | 12.5–14.5 | 2.0–6.0 | chromium raised for better oxidising performance |
The metallurgy explains the behaviour. Chromium above roughly 20 % allows a coherent chromium oxide film to form and reheal; molybdenum raises the pitting resistance equivalent number and stabilises the alloy against reducing acids but lowers the temperature at which detrimental sigma-type phases can precipitate.
Oxidising Versus Reducing Service
In oxidising environments, such as hot air, nitric acid, combustion gas and flue gas containing sulphur trioxide, the Ni-Cr-Fe family has the advantage because its chromium content builds and maintains a protective scale. Adding molybdenum to a nickel alloy actually degrades performance in strongly oxidising acids, which is why high-molybdenum grades are not specified for nitric acid plants.
In reducing environments, such as hydrochloric acid, wet hydrofluoric acid, phosphoric acid and many chloride-bearing process streams, molybdenum is the decisive element and the Ni-Cr-Mo family wins by a wide margin. Where one vessel must handle both oxidising and reducing streams, a balanced low-iron grade such as UNS N10276 or the higher-chromium UNS N06022 is normally the compromise choice.
High-Temperature Performance and Limits
Oxidation resistance: Ni-Cr-Fe grades are serviceable in air to about 1,095 °C; the highest-molybdenum grades should generally be held below about 650–700 °C in oxidising conditions.
Creep and rupture strength: UNS N06625 and UNS N06617 hold load-bearing capability to roughly 650–800 °C, whereas UNS N10276 is a corrosion alloy rather than a load-bearing high-temperature alloy.
Thermal stability: long exposure above about 700 °C can precipitate brittle phases in molybdenum-rich grades, so furnace hardware is usually made from the Ni-Cr-Fe family.
Thermal expansion: both families expand more than carbon steel, which must be allowed for in piping flexibility design to ASME B31.3.
Fabrication, Welding and Availability
Ni-Cr-Fe product forms are covered by ASTM B166, B167 and B564, with UNS N06625 bar and pipe also listed in B446 and B444. Low-iron Ni-Cr-Mo plate and pipe follow ASTM B575, B622 and B626. Filler metals are selected from AWS A5.14 / ASME SFA-5.14: ERNiCr-3 for Ni-Cr-Fe joints and ERNiCrMo-4 for the molybdenum-rich grades. Both families are readily machined with carbide tooling at low surface speeds and weld with matching nickel-base consumables using controlled heat input and interpass temperature. Ni-Cr-Fe grades are generally easier to source in heavy plate and large-diameter pipe; the molybdenum-rich grades cost more, need longer lead times and are often stocked only as sheet, bar and smaller tube.
Selection Checklist and Frequently Asked Questions
Q: Which is better for high-temperature oxidising service?
The Ni-Cr-Fe family, because chromium forms a stable, rehealing oxide film. High-molybdenum grades can suffer accelerated attack and phase instability above roughly 700 °C.
Q: Which is better for hydrochloric acid?
A high-molybdenum grade. UNS N10276 and UNS N06625 both resist hydrochloric acid, and the low-iron molybdenum-rich grades handle the widest range of concentrations and temperatures.
Q: Can the two families be welded to each other?
Yes. Dissimilar joints are normally made with a nickel-base filler such as ERNiCrMo-4 or ERNiCr-3, and the procedure should be qualified to ASME BPVC Section IX.
Q: Is UNS N06625 a good all-round compromise?
It is the most versatile single grade, combining 58 % minimum nickel, molybdenum and niobium for pitting resistance and creep strength, but it is not the best answer for strongly reducing acids at high temperature.
Q: How is corrosion resistance verified before purchase?
Suppliers typically qualify material with ASTM G48 Practice A for pitting, ASTM G28 for intergranular attack and ASTM G28-adjacent immersion tests agreed with the buyer, together with full chemical and mechanical certification.
Q: What should drive the final decision?
Start from the most aggressive combination of chemistry, temperature and chloride level the equipment will ever see, then compare the two families against that worst case rather than the nominal operating condition.
