nickel alloy UNS N06600 and 625 at a Glance
nickel alloy UNS N06600 (UNS N06600) and nickel alloy UNS N06625 (UNS N06625) are both nickel-chromium alloys with austenitic structures, non-magnetic behavior, and good fabricability, but their chemistry steers them to different services. nickel alloy UNS N06600 is a nickel-chromium-iron alloy with a minimum of 72% nickel; it is supplied in annealed to cold-worked conditions and is prized for high-temperature oxidation resistance, resistance to chloride and caustic environments in specific combinations, and stable performance in heat-treating and furnace equipment. nickel alloy UNS N06625 is a nickel-chromium-molybdenum-niobium alloy with at least 58% nickel; its higher chromium plus molybdenum and niobium provide exceptional corrosion resistance and a significant strength advantage. Selecting between them means deciding whether the priority is maximum high-temperature economy or maximum corrosion resistance with strength.
Chemical Composition Compared
Key registered limits for the two alloys are shown below (percent by weight).
| Element | nickel alloy UNS N06600 (N06600) | nickel alloy UNS N06625 (N06625) |
|---|---|---|
| Nickel | 72.0 min | 58.0 min |
| Chromium | 14.0 - 17.0 | 20.0 - 23.0 |
| Iron | 6.0 - 10.0 | 5.0 max |
| Molybdenum | - | 8.0 - 10.0 |
| Niobium + Tantalum | - | 3.15 - 4.15 |
| Carbon | 0.15 max | 0.10 max |
| Aluminum / Titanium | - | 0.40 / 0.40 max |
The absence of molybdenum and niobium in 600 means moderate strength and no precipitation response; their presence in 625 drives solid-solution strengthening, carbide stability, and chloride corrosion resistance.
Temperature and Mechanical Capability
Both alloys form a protective chromium-oxide scale in air. nickel alloy UNS N06600 is commonly rated for oxidation resistance to about 1093°C (2000°F), while 625 is rated to about 980°C (1800°F). For load-bearing service, 625 is the stronger alloy: its solid-solution strengthening provides roughly twice the yield strength of annealed 600 at room temperature and, more importantly, far superior creep resistance at elevated temperature, which is why 625 appears in hot structural parts while 600 appears in lightly stressed furnace components. Both alloys retain excellent toughness down to cryogenic temperatures, and their austenitic structures do not embrittle in cold service.
Corrosion Resistance Profiles
nickel alloy UNS N06600 is highly resistant to chloride stress corrosion cracking in many environments and has outstanding resistance to caustic stress corrosion cracking, for example in hot sodium hydroxide; it is, however, susceptible to chloride stress corrosion cracking in hot chloride solutions and should be selected with the exact chemistry in mind. nickel alloy UNS N06625 resists pitting, crevice corrosion, and chloride stress corrosion cracking across a much broader range, including seawater, oxidizing and reducing acids, and high-chloride process streams, making it the preferred alloy where media are aggressive and mixed. In mild media such as clean fresh water and food processing environments, both perform adequately, and cost, not corrosion, decides the choice.
Applications and Selection
nickel alloy UNS N06600 is standard in furnace components, heat-treating equipment, chemical and petrochemical heat exchangers and evaporators, food processing equipment, and nuclear steam-generator tubing, where high-purity water and caustic resistance matter. nickel alloy UNS N06625 is specified for jet-engine components and heat shields, chemical and pollution-control equipment such as acid scrubbers, marine seawater piping and subsea hardware, flue gas desulfurization systems, and power-plant heat exchangers in aggressive service. The governing standards reflect the split: ASTM B168 covers 600 plate, sheet, and strip, while ASTM B443 covers 625 plate, sheet, and strip. Specify the UNS number, product form, condition, and standard, and let the severity of the medium and the operating temperature drive the alloy choice.
Frequently Asked Questions
Which nickel-based alloy has more nickel, 600 or 625? nickel alloy UNS N06600, with a minimum of 72% nickel. nickel alloy UNS N06625 has a minimum of 58% nickel but adds chromium, molybdenum, and niobium for strength and corrosion resistance.
Is nickel alloy UNS N06625 stronger than nickel alloy UNS N06600? Yes. Solid-solution strengthening from molybdenum and niobium gives 625 roughly double the yield strength of annealed 600, plus superior creep resistance at elevated temperature.
Which alloy resists seawater better? nickel alloy UNS N06625. Its molybdenum content provides excellent resistance to pitting and crevice corrosion in chlorides and seawater, while 600 offers no such protection.
What are the maximum service temperatures? For oxidation in air, 600 is commonly rated to about 1093°C and 625 to about 980°C; for load-bearing design, 625 retains strength at higher temperatures than 600.
Which standards cover their plate and sheet? ASTM B168 (ASME SB-168) covers nickel alloy UNS N06600 plate, sheet, and strip, and ASTM B443 (ASME SB-443) covers nickel alloy UNS N06625 plate, sheet, and strip.
