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Inconel 625 is composed of molybdenum and niobium nickel alloy . It has excellent corrosion and oxidation resistance, good tensile and fatigue properties from cryogenic temperatures up to 980°C, and resistance to stress corrosion in salt spray. Therefore, it is widely used in the manufacture of aircraft engine components, aerospace structural components, and chemical equipment. 625 alloy has excellent machining and welding properties. JN can supply plates, bars, tubes, wire, strip, fittings, flanges, and forgings.
US brand: UNSN06625
Chinese brand: GH3625/GH625
French brand: NC22DNb
German brand: W.Nr.2.4856
The key feature of Inconel 625 is its high-temperature stability, maintaining excellent
mechanical properties above 800°C. The alloy exhibits excellent resistance to oxidation, corrosion, and fatigue in a variety of harsh environments, particularly in high-temperature, high-pressure, acidic, and alkaline media, often providing long-term reliability and durability.
| Composition | Value |
| C | ≤0.1 |
| Mn | ≤0.5 |
| Si | ≤0.5 |
| S | ≤0.015 |
| Fe | ≤5.0 |
| Ni | 58.0 |
| Cr | 20.0-23.0 |
| Co | ≤1.0 |
| Mo | 8.0-10.0 |
| Cr+Ta | 3.15-4.15 |
| P | ≤0.015 |
| Al | ≤0.4 |
| Properties | Value |
| Tensile Strength | 690MPa |
| Yield Strength | 276MPa |
| Elongation | 30% |
WERKSTOFF NR.: 2.4856
UNS: N06625
JIS: NCF 625
BS: NA21
GOST: ХН75МБТЮ
AFNOR: NC22DNB4M
EN: NiCr22Mo9Nb
Inconel 625 corrosion resistance is a three-layer defense system: (1) outer Cr2O3 chromium(III) oxide film blocks chloride and oxygen diffusion, (2) Mo-oxyhydroxide sublayer (Mo4+/Mo6+) repairs localized film breakdowns, (3) inner NiO layer maintains electrochemical stability. Together these give Inconel 625 a PREN of approximately 42-48, compared to ~24 for 316L stainless steel, explaining its superior performance in chloride-rich and acidic environments.
Layer 1 — Chromium(III) Oxide (Cr2O3) Passive Film
When chromium content exceeds 12% (Inconel 625 has 20-23%), the alloy spontaneously forms a 1-10 nm passive film of Cr2O3 on its surface when exposed to air or oxidizing environments. This film is tenaciously adherent, electrically insulating, and only 1/10,000th the thickness of a human hair, yet it reduces the bare-metal corrosion rate by factors of 10,000 to 1,000,000.
· Formation: Forms spontaneously in air at ambient temperature and rapidly at operating temperature.
· Reaction: Reaction: 2Cr + 3/2 O2 -> Cr2O3 (chromium diffuses to surface, reacts with oxygen)
· Properties: Highly adherent, continuous, and electrically insulating — blocks ion migration
· Temperature range: Stable in neutral and mildly acidic environments (pH 4-8) up to 980 deg C in air
· Seawater performance: In seawater, Cr2O3 resists Cl- penetration and prevents pitting initiation
[Source] Clayton, C.R. and Lu, Y.C., Corrosion Science, Vol. 29, No. 7, 1989.
Layer 2 — Molybdenum Oxyhydroxide Sublayer (Mo4+/Mo6+)
Molybdenum (8-10% in Inconel 625) integrates into the Cr2O3 lattice as Mo4+ and Mo6+ oxyhydroxide species [MoO2(OH)2, MoO4(2-)] that fill micro-pores and defects. This self-healing behavior is critical: when a pit nucleates and local pH drops below 2, molybdenum species migrate to the defect site and re-passivate the metal surface. This is why Inconel 625 resists pitting even in concentrated chloride solutions where 316L fails within days.
Molybdenum Role | Mechanism | Practical Benefit |
Self-healing at defects | MoO4(2-) fills micro-pores in Cr2O3 | Prevents pit growth after surface scratch |
Acid resistance | Mo species stable in 0-100% H2SO4 | Survives acidic process environments |
Chloride competition | Mo-oxyhydroxide competes with Cl- for surface sites | Reduces Cl- adsorption and penetration |
PREN contribution | PREN += 3.3 x Mo wt% (vs 1 for Cr, 30 for N) | PREN ~42 vs ~24 for 316L stainless steel |
[Source] Ikeda, A., et al., Stainless Steels, LKL Publishing, 1993; NACE Pub 34100.
Layer 3 — Inner NiO/Ni(OH)2 Layer and SSC Immunity
The nickel oxide layer (NiO) forms at the metal-oxide interface and provides electrochemical stability. Nickel FCC crystal structure has very low hydrogen diffusivity (D_H ~ 10^-12 cm2/s at RT, vs 10^-6 for BCC carbon steel), meaning H2S-derived hydrogen atoms cannot penetrate the alloy lattice. With Ni >= 58%, Inconel 625 is SSC-immune per NACE MR0175 / ISO 15156-3. No carbon or low-alloy steel can match this property.
Alloy | UNS | Cr (%) | Mo (%) | PREN* | CPT (deg C)** |
Inconel 625 | N06625 | 20-23 | 8-10 | ~42 | >150 |
Hastelloy C-276 | N10276 | 14.5-16.5 | 15-17 | ~52 | >150 |
316L Stainless Steel | S31603 | 16-18 | 2.0-3.0 | ~24 | ~20 |
254 SMO | S31254 | 19.5-20.5 | 4.0-5.0 | ~42 | ~80 |
Super Duplex 32750 | S32750 | 24-26 | 3.0-5.0 | ~38 | ~65 |
[Source] PREN = Cr + 3.3xMo + 16xN (ASTM G48). CPT = Critical Pitting Temperature (ASTM G150).
Chloride stress corrosion cracking (Cl-SCC) requires: (1) tensile stress, (2) chloride ions (Cl-), (3) temperature above ~60 deg C. Inconel 625 breaks this chain: the high-nickel austenitic matrix (Ni >= 58%) prevents chromium-depleted zones. There are no grain boundary carbide precipitates (NbC replaces Cr3C2) and no active slip band attack. Inconel 625 is certified immune to Cl-SCC in all common seawater and brine environments. 316L stainless steel fails because Cr3C2 precipitation at grain boundaries creates Cr-depleted zones that dissolve under tensile stress in chloride solution above 60 deg C.
Above 600 deg C in air, Inconel 625 forms a duplex oxide scale: outer Cr2O3 layer plus inner Ni(Cr,Al)2O4 spinel oxide layer. Aluminum (0.4% max) contributes to the inner spinel, which is highly adherent and slows oxygen diffusion by ~100x vs pure NiO. This allows Inconel 625 to sustain 10,000+ hour exposure at up to 980 deg C in air with oxidation rate below 1 mg/cm2/h, qualifying it for aerospace combustor liners and gas turbine ducting.
[Source] Haynes International, INCONEL alloy 625 Technical Data Sheet HD EDM-A.
Excellent Overall Corrosion Resistance: Excellent resistance to pitting, crevice corrosion, stress corrosion cracking, and corrosion from both oxidizing and reducing media, making it suitable for a wide range of corrosive environments. It is one of the "all-rounders" among corrosion-resistant alloys.
Outstanding High-Temperature Strength: Higher strength, particularly creep strength, than many austenitic stainless steels and earlier nickel-based alloys in the 600°C to 900°C range.
Excellent Oxidation Resistance: Sustains long-term, stable operation in high-temperature air environments.
Good Weldability: Ease of welding, especially its excellent corrosion resistance, greatly simplifies manufacturing and maintenance.
Good Machinability: Compared to more complex precipitation-hardening superalloys, it is relatively easy to hot, cold, and machine.
Good Low-Temperature Toughness: Suitable for cryogenic environments.
Non-Magnetic: Suitable for applications requiring low magnetic permeability.
High Cost: High content of key elements such as nickel, chromium, molybdenum, and niobium results in significantly higher raw material and processing costs than stainless steel, a major limitation.
Relatively Low Room-Temperature Strength: As a solid-solution-strengthened alloy, its room-temperature yield strength of approximately 415 MPa is higher than that of stainless steel but lower than precipitation-hardening superalloys. This makes it less suitable for room-temperature structural parts requiring extremely high strength.
High Work-Hardening Rate: Rapid hardening during cold working requires intermediate annealing or increased working forces, increasing machining difficulty and cost. Its machinability is below average, making it more difficult to machine than austenitic stainless steel.
High-Temperature Endurance/Creep Strength Limit: While exhibiting good high-temperature strength, its endurance and creep strength for long-term use at extremely high temperatures are lower than those of specially designed precipitation-hardening superalloys or oxide-dispersion-strengthened alloys.
High Density: It is heavier than iron-based alloys and stainless steel.
Low Thermal Conductivity: It may not be the best choice for applications requiring good heat dissipation.
| Product | Standards | Show |
| Inconel 625 Bars | ASTM B446 AMS 5666 BS 3076 |  |
| Inconel 625 Sheets | ASTM B443 AMS 5599 Din 17750 |  |
| Inconel 625 Tube | ASTM B444 |  |
| Inconel 625 Pipe | ASTM B705 |  |
| Inconel 625 Fittings | ASTM B366 |  |
| Inconel 625 Forging | ASTM B564 Din 17754 |  |
Components of organic chemical processes containing chlorides, especially where acid chloride catalysts are used.
Digesters and bleaching tanks for the pulp and paper industry.
Absorption tower, reheater, flue gas inlet baffle, fan (humid), agitator, deflector and flue in flue gas desulfurization system.
For the manufacture of equipment and components for use in sour gas environments. For example Acetic acid and acetic anhydride reaction generator.
Sulfuric acid condenser.
Client: Major North Sea operator (water depth: 380 m)
Challenge: A subsea production control system required hydraulic and chemical injection tubing that could withstand continuous exposure to seawater on the external surface and aggressive well fluids (containing H₂S and chlorides) on the internal surface, for a design life of 25 years without maintenance access.
Solution: Super-duplex stainless steel had been the incumbent material, but field data showed pitting corrosion after 8–12 years in warm seawater. The operator selected Inconel 625 (ASTM B704 / B705) seamless tubing in 12.7 mm OD × 1.65 mm wall thickness for the complete umbilical.
Key performance factors:
PREN ≥ 45 (super-duplex equivalent: ~42), providing a safety margin even under stagnant, oxygen-depleted conditions
Full compliance with NACE MR0175 for sour service (H₂S partial pressure up to 1.5 MPa)
Yield strength of 480 MPa guaranteed at design temperature of 90 °C
Result: After 12 years in service, an ROV (Remotely Operated Vehicle) inspection confirmed zero pitting, zero crevice attack at clamp interfaces, and no measurable wall thickness reduction. The operator has since specified Inconel 625 for all subsequent field developments.
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Client: Southeast Asian gas processing platform
Challenge: Shell-and-tube heat exchangers using seawater as the cooling medium suffered repeated tube failures. The original titanium tubes experienced hydrogen embrittlement in the cathodic protection zone, while 904L tubes failed by chloride pitting after only 3 years of service.
Solution: The engineering contractor retrofitted all four exchangers with Inconel 625 (ASTM B444 Grade 1) seamless U-tubes, 19.05 mm OD × 2.11 mm wall, in the annealed condition.
Design validation:
Critical Pitting Temperature (CPT) per ASTM G150: > 85 °C (service temperature: 45 °C max)
Critical Crevice Corrosion Temperature (CCT): > 70 °C
Thermal conductivity: 9.8 W/m·K at 100 °C (lower than titanium, but adequate with adjusted flow rates)
Erosion-corrosion resistance: Withstood suspended particle loading up to 50 ppm without measurable wall loss
Result: The retrofitted exchangers have operated continuously for 6 years without a single tube failure or planned shutdown for tube inspection, reducing annual maintenance costs by an estimated USD 340,000 compared to the previous material.
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Client: Gulf of Mexico deepwater development (water depth: 1,700 m)
Challenge: A steel catenary riser (SCR) required internal corrosion-resistant alloy (CRA) cladding to handle production fluid containing 3% CO₂, 200 ppm H₂S, and formation water with 55,000 ppm chloride. The design called for a 20-year service life at an operating temperature up to 120 °C.
Solution: The riser joints (API 5L X65 base pipe, 273 mm OD × 25.4 mm wall) were internally clad with 3 mm of Inconel 625 using the hot-isostatic pressing (HIP) bonding process, followed by a solution anneal at 1100 °C.
Metallurgical bond quality:
Shear strength at the clad interface: > 200 MPa (API 5LD minimum: 140 MPa)
No disbondment after thermal cycling test (150 cycles, 20 °C → 150 °C → 20 °C)
No sensitisation in the heat-affected zone, verified by ASTM A262 Practice A and E tests
Result: The SCR has been in continuous production since installation, with annual intelligent pigging showing zero internal corrosion features in the clad sections. The CRA-clad approach saved approximately 40% in material cost compared to a solid Inconel 625 pipe of equivalent dimensions.
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Client: Arctic-region offshore platform (design temperature: −46 °C ambient)
Challenge: Standard B7 bolting material failed Charpy V-notch requirements at the design minimum temperature, and the operator required a material resistant to seawater splash-zone corrosion without applying protective coatings (which were impractical at the remote installation schedule).
Solution: All wellhead connector and flange bolting was manufactured from Inconel 625 (ASTM B446, Grade 1 annealed) bar stock, machined to full-size ASME B18.2.2 hex bolts.
Low-temperature verification:
Charpy V-notch impact at −46 °C: 95 J average (project specification: ≥ 40 J)
No evidence of brittle fracture modes in fractographic examination
Yield strength retention at −46 °C: 105% of room-temperature value (i.e., marginally higher due to thermal hardening)
Result: After 8 years in service in the Arctic splash zone, the bolting shows no visible corrosion, no galling, and torque values remain within the original installation tolerance. No re-tightening or replacement has been required.
Q1: How does Inconel 625 resist corrosion in seawater compared to 316L stainless steel?
Inconel 625 resists seawater corrosion far more effectively than 316L in three ways. First, its PREN of ~42 (vs ~24 for 316L) means it will not pit in seawater at any offshore operating temperature. Second, its Mo-oxyhydroxide self-healing sublayer repairs micro-defects in the passive film, while 316L has no such mechanism. Third, Ni >= 58% prevents Cl-SCC at all temperatures, while 316L is susceptible above 60 deg C. In practice, 316L seawater tubes require replacement every 3-5 years at velocities above 1.5 m/s, while Inconel 625 tubes last 20+ years with zero maintenance.
Q2: What is the passive film on Inconel 625 made of, and how does it protect the metal?
The passive film is a three-layer system: (1) outer Cr2O3 chromium(III) oxide (1-5 nm), electrically insulating, blocks Cl- ion penetration; (2) middle Mo-oxyhydroxide sublayer (Mo4+/Mo6+), fills micro-pores and self-repairs localized breakdowns; (3) inner NiO/Ni(OH)2 layer, provides electrochemical stability at the metal-oxide interface. The Cr2O3 film forms spontaneously in air at room temperature and self-repairs within minutes in oxygenated environments.
Q3: Can Inconel 625 be welded, and what filler wire should be used?
Yes. Inconel 625 welds by GTAW (TIG), GMAW (MIG), SMAW (stick), and SAW. Standard filler wire is AWS A5.14 ERNiCrMo-3 (UNS N06625), same composition as base metal with equal corrosion resistance. PWHT is generally NOT required. For aggressive environments (concentrated HCl, elemental sulfur), solution annealing at 1093-1171 deg C + water quench may be specified. JN Alloy supplies ERNiCrMo-3 filler wire in all standard sizes.
Q4: Is Inconel 625 suitable for high-temperature sour gas (H2S) service?
Yes. Inconel 625 (UNS N06625) is the primary nickel alloy for API 6A Class HH wellhead equipment in sour gas. Its Ni >= 58% makes it immune to sulfide stress cracking (SSC) per NACE MR0175 / ISO 15156-3:2020. It operates up to 980 deg C in air and up to 180 deg C in H2S service (API 6A Class HH upper limit). Common applications: Christmas tree valve bodies, tubing hangers, wellhead connectors, and subsea safety valve housings.
Q5: What is the difference between Inconel 625 Grade 1 and Grade 2?
ASTM B443 Grade 1: C <= 0.10% (low-carbon, maximum corrosion resistance and weldability, preferred for marine and sour gas). Grade 2: C <= 0.15% (slightly higher yield strength, for structural applications where corrosion resistance is still required but maximum toughness is less critical). For most marine and sour gas applications, Grade 1 is the standard specification.
Q6: Does Inconel 625 require special surface preparation for marine service?
Inconel 625 should be supplied in the solution-annealed + pickled condition (standard per ASTM B443/B564), which naturally forms the protective Cr2O3 passive film. No additional passivation acid treatment is required. For seawater heat exchangers, electropolishing to Ra <= 0.2 um on tube ID is recommended (reduces friction, minimizes biofouling). Avoid galvanic contact with carbon steel or 316L SS without insulation spacers.
Q7: What is the PREN of Inconel 625, and why does it matter for marine applications?
PREN of Inconel 625 is approximately 42 (Cr + 3.3xMo + 16xN = ~51 with W correction). Critical Pitting Temperature (CPT) exceeds 150 deg C in 6% FeCl3 per ASTM G150, meaning it will not initiate pitting in natural seawater (35,000 ppm Cl-) at any offshore production temperature. This is why Inconel 625 is specified for seawater heat exchangers, riser hydraulic lines, and subsea hardware in deepwater fields.
Q8: What certifications does JN Alloy provide with Inconel 625 shipments?
Every JN Alloy Inconel 625 shipment includes: (1) EN 10204 3.1 Mill Test Certificate with heat chemistry (Ni, Cr, Mo, Nb, C, Fe, Si, Mn, P, S), mechanical test results, and heat treatment record; (2) NACE MR0175 / ISO 15156-3 qualification letter; (3) optional ASTM B443/B564/B574/B622 certificates; (4) PMI test report on receipt; (5) API 6A PSL 3 third-party inspection available on request. Contact: jnalloy123@gmail.com.
Q9: What is the price of Inconel 625 vs alternatives?
Inconel 625 is approximately 8x the cost of 316L stainless steel per kg (indicative range: USD 30-45/kg for plate/bar). Despite the higher material cost, Inconel 625 typically has the lowest total lifecycle cost over 20 years offshore due to 20+ year seawater service life and elimination of sour gas SSC risk. Contact jnalloy123@gmail.com for current stock and RFQ pricing.
