Two Families, Two Design Intents
Most wrought nickel alloys used in process equipment fall into two broad families. The first is the Ni-Cr-Mo family, where molybdenum and often tungsten are added to nickel and chromium to resist reducing acids and chloride pitting. The second is the Ni-Cr-Fe family, where chromium and iron dominate and the alloy is designed for oxidising atmospheres, high temperature strength and thermal stability. The commercial names attached to individual grades carry no technical information, so selection should always be made from the UNS number and the product standard rather than from a family label.
Both families share the same base: a face-centred cubic nickel matrix that is inherently tough, ductile and resistant to chloride stress corrosion cracking. The differences that decide an application are the molybdenum and chromium balance, the strengthening mechanism, and the temperature at which useful strength is lost.
Composition and Strengthening Mechanisms
| Grade (UNS) | Typical Cr, % | Typical Mo, % | Strengthening |
|---|---|---|---|
| N10276 | 15.5 | 16 | Solid solution, Mo plus W |
| N06022 | 21 | 13.5 | Solid solution, Mo plus W |
| N06455 | 16 | 15.5 | Solid solution, Ti stabilised |
| N06625 | 21.5 | 9 | Solid solution plus Nb |
| N06600 | 15.5 | - | Solid solution, Ni-Cr-Fe base |
| N06690 | 29 | - | Solid solution, high Cr |
| N07718 | 19 | 3 | Precipitation hardened by Nb, Ti and Al |
Within the Ni-Cr-Mo family, molybdenum is the element that resists reducing acids; tungsten reinforces that effect and also raises pitting resistance. Within the Ni-Cr-Fe family, chromium forms the protective oxide that resists oxidising media, and additions of niobium, titanium and aluminium allow precipitation hardening in grades such as UNS N07718, where an ageing treatment develops very high yield strength. A widely used rule of thumb is that the element doing the work tells you the environment: high molybdenum for reducing acids and chlorides, high chromium for oxidising media and hot atmospheres.
Because the two families rely on different elements, their corrosion envelopes cross. A molybdenum-rich grade that resists hot hydrochloric acid will be outperformed by a high chromium grade in nitric acid or in a strongly oxidising salt solution. Selection on a single test result, or on the visual appearance of one sample, is a common and expensive mistake.
Corrosion Behaviour: Reducing versus Oxidising Service
Reducing acids: The Ni-Cr-Mo grades are the reference materials for hydrochloric and sulphuric acid service, including hot concentrated solutions and acid streams carrying chlorides. Chromium alone cannot protect the surface in these media, which is why the molybdenum content is decisive.
Oxidising media: Nitric acid, wet chlorine, chlorine dioxide, hypochlorite and ferric or cupric salt solutions are handled better by the higher chromium grades, including UNS N06022 within the Ni-Cr-Mo family and the high chromium Ni-Cr-Fe grades.
Chloride pitting and crevice corrosion: Ranked with the pitting resistance equivalent number, PREN = Cr + 3.3 x Mo + 16 x N, the Ni-Cr-Mo grades sit far above the Ni-Cr-Fe grades. Nominal compositions give approximately 68 for UNS N10276, 65 for UNS N06022, 51 for UNS N06625, 30 for UNS N06690 and 16 for UNS N06600. Only the molybdenum-bearing family is normally accepted for seawater and heavy brine duty.
Chloride stress corrosion cracking: All of these nickel-based grades resist the cracking that destroys austenitic stainless steel in hot chlorides, so in high chloride service the choice is driven by pitting resistance and cost rather than by cracking risk.
Localised attack in the heat affected zone: Thermal stability varies. Very low carbon grades and stabilised grades resist grain-boundary carbide precipitation during welding, while some high chromium Ni-Cr-Fe grades need a solution anneal after welding to restore corrosion resistance.
Mechanical Strength at Temperature
| Grade (UNS) | Tensile strength, MPa | Yield strength, MPa | Condition |
|---|---|---|---|
| N10276 | 690 min | 283 min | Annealed bar |
| N06022 | 690 min | 283 min | Annealed plate |
| N06625 | 827 min | 414 min | Annealed plate |
| N06600 | 550 min | 240 min | Annealed plate |
| N06690 | 586 min | 240 min | Annealed plate |
| N07718 | 1240 min | 1030 min | Solution treated and aged |
The strength ranking at room temperature is not the ranking that matters at 700 °C. Precipitation hardened grades such as UNS N07718 reach minimum tensile strengths above 1240 MPa when aged and retain useful load capacity to roughly 700 °C, which is why they are chosen for turbine and high pressure hot-section hardware. The solid-solution Ni-Cr-Fe grades such as UNS N06600 and UNS N06690 hold their oxidation resistance to higher temperatures but their strength falls steadily; UNS N06600 annealed plate is specified at 550 MPa tensile and 240 MPa yield at room temperature and these figures drop sharply above about 500 °C. Reported room-temperature strengths in the range of 710 to 1100 MPa describe the precipitation hardened family, not the annealed solid-solution grades, and mixing the two sets of figures is a frequent error in selection documents.
Selection Guidance by Service Condition
Hot hydrochloric or sulphuric acid, acid chlorides, or seawater: select from the Ni-Cr-Mo family, choosing between the molybdenum plus tungsten grades according to the required pitting margin and thermal stability.
Nitric acid, wet chlorine, hypochlorite, or mixed oxidising and reducing streams: select the higher chromium grade, which may be either a chromium-rich Ni-Cr-Mo alloy or a high chromium Ni-Cr-Fe alloy.
Continuous service above 650 °C in air or combustion gas where strength is secondary: the high chromium Ni-Cr-Fe grades, including UNS N06690, are the standard selection because oxide spalling resistance improves as chromium rises.
High temperature plus high mechanical load, such as fasteners, turbine hardware and hot high pressure tubing: use a precipitation hardened grade such as UNS N07718 and apply the specified ageing treatment.
Welded fabrication where no post-weld anneal is possible: prefer very low carbon or stabilised grades so that grain-boundary precipitation does not degrade the joint.
Every selection should be qualified against the actual medium, concentration, temperature and stress state, because published corrosion tables cover single chemistries and rarely reproduce real plant conditions.
Frequently Asked Questions
Q: Is a molybdenum-bearing nickel alloy always more corrosion resistant?
No. Molybdenum improves resistance to reducing acids and chlorides but is not beneficial in strongly oxidising media, where high chromium content governs performance. The correct family depends on the chemistry of the medium.
Q: What does a PREN number actually tell you?
It ranks relative resistance to pitting in chloride environments based on chromium, molybdenum and nitrogen content. Values of roughly 40 and above indicate good seawater pitting resistance, which places most Ni-Cr-Mo grades comfortably in that range.
Q: Why do some nickel alloys need ageing and others do not?
Only precipitation hardened grades rely on a deliberate ageing treatment to form strengthening particles. Solid-solution grades such as UNS N10276 and UNS N06600 obtain their properties from annealing and rapid cooling, and ageing them serves no purpose.
Q: Which family performs better at very high temperature?
Strength and oxidation resistance must be separated. Oxidation resistance improves with chromium content, while usable strength at temperature depends on whether the grade is precipitation hardened. UNS N07718 delivers the highest strength of the group to about 700 °C.
Q: Can the two families be welded to each other?
Yes, dissimilar joints between nickel-based grades are made with matching or over-alloyed nickel filler metals. The joint then behaves as the less resistant of the two alloys plus the filler, so corrosion testing of the actual joint is advisable for critical duty.
Q: How should the correct grade be documented for procurement?
Specify the UNS number, the product form and the governing product standard, together with the required condition of supply and any corrosion testing. Family names or trade designations alone are not sufficient to define a material.
