What Is Inconel 625?
When to Specify Inconel 625
Inconel 625 (UNS N06625) is a nickel-chromium-molybdenum-niobium alloy with exceptional resistance to pitting, crevice corrosion, and stress corrosion cracking across various corrosive environments. It is the most commonly specified nickel alloy in offshore oil and gas production, particularly for sour service per NACE MR0175, subsea flowline components, and chemical process piping.
| Property | Value |
|---|---|
| UNS designation | N06625 |
| Common names | Inconel 625, Alloy 625 |
| Nickel | 58.0% min (balance) |
| Chromium | 20.0-23.0% |
| Molybdenum | 8.0-10.0% |
| Niobium + Tantalum | 3.15-4.15% |
| Iron | 5.0% max |
| Carbon | 0.10% max |
| Tensile strength (min) | 827 MPa (120 ksi); annealed |
| Yield strength (min) | 414 MPa (60 ksi); annealed |
| Elongation (min) | 30% |
| Hardness | Typically 180-220 HBW (annealed) |
| PREN | >50 |
| Density | 8.44 g/cm3 |
| Max service temp | 980°C (1800°F); oxidation limit |
| Heat treatment | Solution anneal 1095-1205°C |
Corrosion Resistance
The combination of high chromium (20-23%), molybdenum (8-10%), and niobium gives Inconel 625 a PREN above 50, far exceeding super duplex (PREN ~42) and austenitic stainless (PREN ~26). Key resistance areas:
| Environment | Performance |
|---|---|
| Seawater (ambient to 80°C) | Immune to pitting and crevice corrosion |
| Sour gas (H2S + CO2 + Cl-) | Excellent per NACE MR0175 |
| Hydrochloric acid | Resistant at dilute concentrations |
| Phosphoric acid | Excellent at all concentrations |
| Sulfuric acid | Good at low concentrations |
| High-temperature oxidation | Up to 980°C |
| Chloride SCC | Immune |
ASTM Specifications for Piping
| Product Form | ASTM Specification |
|---|---|
| Forgings (flanges, fittings) | B564 |
| Seamless pipe and tube | B444 |
| Welded pipe | B705 |
| Plate | B443 |
| Bar and rod | B446 |
| Castings | A494 CW-6MC |
Applications
- Subsea oil and gas: flowline connectors, wellhead hangers, compact flanges, clamp hubs
- Sour gas production: wellhead and Christmas tree components, production tubing
- Chemical processing: reactor vessels, heat exchangers, piping in chloride and acid environments
- Flue gas desulfurization: scrubber outlet ducts, dampers, and expansion joints
- Aerospace and power: gas turbine exhaust components, nuclear reactor core structures
- Weld overlay: Inconel 625 cladding on carbon steel vessels and pipe for internal corrosion protection
Testing
Inconel 625 products require solution annealing, chemical analysis, and tensile testing. Mill test certificates per EN 10204 3.1 or 3.2 document chemistry and mechanical properties. NDT includes ultrasonic examination and liquid penetrant inspection. Intergranular corrosion testing per ASTM G28 is commonly specified. Hydrostatic testing follows the applicable piping or flange standard.
Leave a Comment
Have a question or feedback? Send us a message.