Inconel 625 vs 825
Alloy 625 and alloy 825 are both nickel-based alloys used in highly corrosive piping services, but they serve different roles. Alloy 625 (UNS N06625) is a nickel-chromium-molybdenum alloy with exceptional strength and corrosion resistance at elevated temperatures. Alloy 825 (UNS N08825) is a nickel-iron-chromium-molybdenum alloy designed primarily for corrosion resistance in acid environments at moderate temperatures, at a significantly lower cost.
Definition and Classification
Alloy 625 (Inconel 625) contains approximately 58% nickel, 20-23% chromium, and 8-10% molybdenum. It combines high-temperature strength (solid-solution strengthened by Mo and Nb) with outstanding resistance to pitting, crevice corrosion, and chloride stress corrosion cracking.
Alloy 825 (Incoloy 825) contains approximately 38-46% nickel, 19.5-23.5% chromium, 2.5-3.5% molybdenum, and significant iron (balance). It is technically a nickel-iron alloy rather than a true nickel alloy, offering good resistance to sulfuric acid, phosphoric acid, and moderately chloride environments.
When Each Alloy Is Used
Alloy 625 is specified for the most aggressive environments: hot seawater, sour gas production with high H2S/CO2 and chlorides, flue gas desulfurization, and high-temperature chemical processing above 500°C. It is also widely used as weld overlay (cladding) on carbon steel to provide corrosion resistance at a fraction of the solid alloy cost.
Alloy 825 is used for sulfuric acid handling, phosphoric acid production, pickling systems, oil and gas production with moderate sour conditions, and nuclear fuel reprocessing. It is a cost-effective alternative where 625’s extreme strength and temperature capability are not required.
Specifications and Properties
| Property | Alloy 625 (N06625) | Alloy 825 (N08825) |
|---|---|---|
| Nickel | 58% min | 38.0-46.0% |
| Chromium | 20.0-23.0% | 19.5-23.5% |
| Molybdenum | 8.0-10.0% | 2.5-3.5% |
| Iron | 5.0% max | Balance (~30%) |
| Niobium | 3.15-4.15% | None |
| Titanium | 0.40% max | 0.6-1.2% |
| PREN | ~51 | ~30 |
| Tensile strength (min) | 827 MPa (120 ksi) | 586 MPa (85 ksi) |
| Yield strength (min) | 414 MPa (60 ksi) | 241 MPa (35 ksi) |
| Max service temp | 980°C (1800°F) | 540°C (1000°F) |
| Pipe spec | ASTM B444 | ASTM B423 |
| Fitting spec | ASTM B366 | ASTM B366 |
| Forging spec | ASTM B564 | ASTM B564 |
| Density | 8.44 g/cm3 | 8.14 g/cm3 |
| Relative cost | Very high (~5-6x CS) | High (~3-4x CS) |
Comparison: Corrosion Performance
| Environment | Alloy 625 | Alloy 825 |
|---|---|---|
| Seawater (ambient) | Excellent | Good |
| Hot seawater (60°C+) | Excellent | Marginal |
| Sulfuric acid (20-40%) | Excellent | Good |
| Phosphoric acid | Excellent | Good |
| Hydrochloric acid | Good (dilute) | Limited |
| Sour gas (high H2S/Cl) | Excellent | Moderate |
| Flue gas condensate | Excellent | Not suitable |
| High-temp oxidation (>500°C) | Excellent | Not suitable |
For complete specifications on nickel alloy piping products, see the full guide to nickel alloy pipes.
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