Inconel 718 vs 625: Which Nickel Alloy for Your Application?

Bottom line up front: Inconel 718 gives you precipitation-hardenable strength to 650°C — it’s the turbine disc alloy. Inconel 625 gives you universal corrosion resistance to 980°C — it’s the seawater and chemical processing alloy. If your application needs both, 718 wins on strength; 625 wins on corrosion. But don’t guess. I’ve seen guys swap them and regret it.

TL;DR — 718: Ni-Cr-Nb, age-hardenable, 650°C max, 1275 MPa UTS, AMS 5662. 625: Ni-Cr-Mo, solid-solution, 980°C max, outstanding pitting resistance, NACE MR0175.

Chemical Composition: What’s Actually Different?

Inconel 718 derives its strength from gamma-double-prime (Ni3Nb) precipitation — that 5% niobium is the key. Without proper solution treatment at 980-1010°C, the strengthening phase never fully dissolves and creep life drops by 60%. True story: we had a furnace thermocouple off by 12°C last year. 200 kg of turbine disc blanks looked fine, but LCF tests failed at 30% of spec life. The Ni3Nb never fully went into solution. Just a tiny temperature miss. So watch your furnace profiling like a hawk.

Inconel 625 relies on molybdenum (8-10%) and niobium (3-4%) in solid solution for both strength and corrosion resistance. No heat treatment required for corrosion performance. That’s a huge plus on the shop floor. You weld it, you’re done (mostly). For 718, you’ve got that double-aging dance.

Element	718	625	What It Does
Ni	50-55%	58% min	Base — corrosion resistance
Cr	17-21%	20-23%	Oxidation resistance
Mo	2.8-3.3%	8-10%	Pitting resistance (625 wins)
Nb	4.75-5.5%	3.15-4.15%	718: precipitation hardening; 625: solid-solution
Ti	0.65-1.15%	0.4% max	718: gamma-prime former
Al	0.2-0.8%	0.4% max	718: gamma-prime former

Temperature Limits: Where Each Alloy Excels

Inconel 718: Rated for -253°C to 650°C. Above 650°C, gamma-double-prime coarsens and strength drops rapidly. For 700°C+ service, consider Waspaloy or Inconel X-750. We’ve had customers try to push 718 to 700°C in a hot spot — not pretty. Rupture life nose-dived.

Inconel 625: Rated from cryogenic to 980°C. Oxidation resistance holds to 980°C, but mechanical strength drops significantly above 650°C. The alloy was designed for corrosion resistance, not creep strength. It’ll hold its shape, but not under high stress. Think furnace muffles, not turbine blades.

Corrosion Resistance: The Deciding Factor

1. Seawater: 625 wins decisively. PREN (Pitting Resistance Equivalent Number) >50 vs 718’s ~30. For offshore and subsea, 625 is the standard. Only Monel K-500 competes in this environment. I’ve seen 718 spool pieces in Gulf of Mexico platforms get crevice corrosion after 8 years, while the 625 piping is pristine at 30. That molybdenum difference? That’s real life.

2. Sour Gas (H2S): Both are NACE MR0175 compliant. 625 has better resistance to chloride SCC. 718 is commonly used for high-strength wellhead components where 625 lacks the required mechanical properties. So if you need a hanger that holds 100 tons, you’ll spec 718. The corrosion is handled by the cladding.

3. Oxidizing Acids: 625 outperforms 718 in nitric and phosphoric acid. 718 is acceptable in most organic acids but should be avoided in reducing acids like HCl. Here’s a shop tip: if you pickle 625, don’t use straight HCl. Use nitric-hydrofluoric mix at 50-60°C, or you’ll get intergranular attack. Ask me how I know.

Cost Comparison (Relative)

At time of writing, Inconel 718 typically costs 10-15% less than 625 per kilogram due to lower molybdenum content. However, 718 requires heat treatment (solution + two-step aging) which adds processing cost. For components requiring post-weld heat treatment, the total fabrication cost difference narrows considerably. So don’t just stare at the raw material price. Factor in the oven time. (And the reworks if you mess up the aging, like I’ve done.)

Selection Checklist

• Operating temperature above 650°C? → 625 (or consider 617/X-750 above 700°C)
• Seawater or high-chloride environment? → 625
• Need maximum tensile strength (>1200 MPa)? → 718 (heat treated)
• Sour gas with H2S? → Both NACE compliant; 625 for corrosion, 718 for strength
• Welding without post-weld heat treat? → 625 (easier to weld. But if you must weld 718 in the field, use solution-annealed material and a 625 filler for a quick fix — just know you’ll sacrifice some strength)
• Fatigue-critical rotating component? → 718 (superior fatigue strength)

Bottom line: If you’re building a turbine disc — 718. If you’re building a seawater piping system — 625. If you need both and can’t decide — send us your operating conditions. Our metallurgy team will recommend the optimal grade.

Standards: AMS 5662/5663 (718), ASTM B443/B444 (625), NACE MR0175.

Related alloy pages on our site:

• Inconel 718 full datasheet
• Inconel 625 corrosion guide
• Waspaloy vs. Inconel 718 comparison
• Inconel X-750 for high-temperature springs
• NACE MR0175 compliant alloys overview