Inconel 625 vs 718: Corrosion Champion vs Structural Workhorse

## Strengthening Mechanism The fundamental difference between 625 and 718 is how they derive their strength. **Inconel 625 (UNS N06625)** is strengthened by solid solution hardening from molybdenum and niobium dissolved in the nickel-chromium matrix. It cannot be age-hardened to significantly higher strength levels. This simplicity is an advantage: 625 retains good ductility and can be used in the as-welded condition without post-weld heat treatment. **Inconel 718 (UNS N07718)** is precipitation hardened by niobium, which forms gamma-double-prime (Ni3Nb) precipitates during aging at 620-720 degrees C. This aging response allows 718 to reach tensile strengths above 1240 MPa, roughly double that of 625. | Property | 625 (Annealed) | 718 (Aged) | |----------|----------------|------------| | Tensile Strength | 827 MPa | 1240 MPa | | Yield Strength | 414 MPa | 1036 MPa | | Elongation | 30% | 12% | | Hardness | 95 HRB | 44 HRC | | Density | 8.44 g/cm3 | 8.19 g/cm3 | | Max Service Temp | 980 degrees C | 650 degrees C | ## Composition | Element | 625 | 718 | |---------|-----|-----| | Nickel | 58% (min) | 50-55% | | Chromium | 20-23% | 17-21% | | Molybdenum | 8-10% | 2.8-3.3% | | Niobium | 3.15-4.15% | 4.75-5.5% | | Iron | 5% max | Balance (18%) | Note that 718 contains significantly more iron and less molybdenum than 625, which makes 718 less expensive but also less corrosion resistant. ## Corrosion Resistance 625 has superior corrosion resistance in virtually all environments. It resists pitting and crevice corrosion in seawater (PREN > 50), withstands reducing and oxidizing acids simultaneously, and is used as weld overlay cladding on carbon steel in oil and gas sour service per NACE MR0175. 718 has adequate corrosion resistance for many applications but falls short of 625 in aggressive chloride and acid environments. Its primary selection criterion is strength, not corrosion resistance. ## Temperature Capability 625 maintains useful strength up to 980 degrees C and retains oxidation resistance to even higher temperatures. It is used for exhaust systems, furnace components, and gas turbine transition ducts. 718 is limited to approximately 650 degrees C for sustained service because the gamma-double-prime precipitates coarsen and transform to delta phase above this temperature, causing strength loss. ## Cost 625 costs approximately 40-60 USD/kg. 718 costs approximately 30-50 USD/kg, somewhat less due to its higher iron content and lower molybdenum. ## When to Choose Each **Choose 625 when**: Corrosion resistance is paramount, the service temperature is above 650 degrees C, the application is a weld overlay or cladding, or you need an alloy that works in the as-welded condition. Key applications include subsea flowlines, chemical processing equipment, and marine exhaust systems. **Choose 718 when**: High structural strength is required at temperatures below 650 degrees C, the loading demands a material with yield strength above 1000 MPa. Key applications include jet engine turbine discs, high-strength fasteners, and oil-gas downhole tools.