In modern oil and gas production, wells are reaching ever greater depths and encountering more aggressive environments. Among the most challenging are sour gas wells—reservoirs containing significant amounts of hydrogen sulfide (H₂S), carbon dioxide (CO₂), chlorides, and other corrosive agents.
Under these harsh conditions, choosing the right material for tubulars, valves, and completion equipment is vital for both safety and long-term reliability. One alloy that has proven exceptionally successful is Alloy N09935, a nickel-iron-chromium alloy engineered to resist the destructive combination of H₂S and chloride stress.
This article by SASAALLOY explains in detail why Alloy N09935 has become the preferred material for sour gas wells, its metallurgical advantages, corrosion performance, and practical benefits for field operations.
1. What Is a Sour Gas Environment?
A “sour” gas well refers to any well that produces hydrogen sulfide (H₂S). Even a small amount of H₂S can react with moisture and other chemicals to create highly corrosive conditions. In addition, CO₂ and chloride ions further accelerate corrosion and cracking.
Typical Conditions in Sour Gas Wells
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H₂S concentration: 10 ppm to several percent
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Temperature: up to 650°C (1200°F) in some wells
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Pressure: >20,000 psi in deep formations
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Chloride levels: up to 200,000 ppm in produced fluids
These factors make conventional stainless steels and low-alloy steels unsuitable due to rapid sulfide stress cracking (SSC), hydrogen-induced cracking (HIC), and pitting corrosion.
2. Alloy N09935 – Composition and Design Purpose
Alloy N09935 (UNS N09935) is a nickel-iron-chromium alloy developed to deliver superior resistance to H₂S, CO₂, and chloride attack, while maintaining high strength and good fabricability.
It is chemically balanced with elements that reinforce corrosion and mechanical stability:
| Element | Content (%) | Function |
|---|---|---|
| Nickel (Ni) | 35–40 | Base metal for corrosion resistance |
| Chromium (Cr) | 20–23 | Forms stable passive film against oxidation and pitting |
| Iron (Fe) | Balance | Strength and cost efficiency |
| Molybdenum (Mo) | 3.5–4.5 | Improves pitting and crevice corrosion resistance |
| Copper (Cu) | 1.5–2.5 | Enhances resistance to reducing acids |
| Niobium (Nb) + Titanium (Ti) | 0.8–1.5 | Enables precipitation hardening |
| Carbon (C) | ≤0.03 | Minimizes sensitization and intergranular attack |
This optimized composition provides a synergistic combination of mechanical strength, ductility, and corrosion protection—making Alloy N09935 particularly suited for sour-gas service.
3. Compliance with Sour-Service Standards
Alloy N09935 fully complies with NACE MR0175 / ISO 15156, the global standard governing materials used in H₂S-containing oil and gas environments.
This certification ensures that the alloy can safely withstand the combined effects of:
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Hydrogen embrittlement
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Sulfide stress cracking
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Stress corrosion cracking
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Localized attack (pitting and crevice corrosion)
The alloy’s proven compliance makes it an approved and trusted choice for oil companies and equipment manufacturers worldwide.
4. Mechanisms of Corrosion Resistance in Sour Gas Wells
4.1 Resistance to Sulfide Stress Cracking (SSC)
Hydrogen generated by H₂S reduces ductility in most steels, causing brittle fracture under stress. Alloy N09935’s high nickel content suppresses hydrogen absorption, while chromium and molybdenum stabilize the passive oxide film.
As a result, the alloy shows remarkably low susceptibility to SSC, even under high partial pressures of H₂S.
4.2 Resistance to Hydrogen-Induced Cracking (HIC)
HIC occurs when absorbed hydrogen collects in microvoids, leading to blistering and step-wise cracking. The fine-grained, homogeneous microstructure of Alloy N09935 prevents hydrogen trapping, ensuring long-term resistance to HIC in both annealed and aged conditions.
4.3 Chloride-Induced Pitting and Crevice Corrosion
Chlorides from produced water or drilling fluids can break down protective films on many metals. The Cr-Mo combination in Alloy N09935 enhances pitting resistance, while copper improves stability in acidic brines.
4.4 Stress Corrosion Cracking (SCC)
Nickel’s ability to retain ductility under strain means Alloy N09935 resists SCC better than traditional duplex stainless steels or martensitic alloys. It maintains structural integrity under combined mechanical and chemical stress.
5. Mechanical Strength in Sour-Service Conditions
| Property | Typical Value (Aged Condition) |
|---|---|
| Yield Strength | 900–950 MPa |
| Tensile Strength | 1000–1100 MPa |
| Elongation | 30–35% |
| Hardness | 28–32 HRC |
These values indicate a balance between high strength and ductility, ensuring safe operation under high internal pressures and mechanical loads typical of deep wells.
Even at elevated temperatures up to 650°C (1200°F), the alloy retains a high percentage of its room-temperature strength.
6. Comparison with Other Alloys in Sour Gas Wells
| Alloy | H₂S Resistance | Strength | Cost | Machinability | Remarks |
|---|---|---|---|---|---|
| Alloy N09935 | Excellent | High | Moderate | Good | Balanced performance |
| Alloy 925 | Good | Medium | Lower | Good | Economical but lower limits |
| Alloy 945 / 945X | Excellent | Very High | High | Moderate | Higher strength, higher cost |
| Alloy 718 | Moderate | Very High | High | Difficult | Limited sour-gas resistance |
| Alloy 625 | Excellent | Medium | High | Moderate | For high-temperature corrosion, not primarily sour service |
From this comparison, it’s clear that Alloy N09935 delivers an ideal balance of cost, machinability, and performance for the oil and gas industry, especially in H₂S-containing wells.
7. Typical Applications in Sour Gas Fields
7.1 Downhole Equipment
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Tubing and casing hangers
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Mandrels, packers, and safety valves
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Connector bodies and couplings
7.2 Wellhead Components
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Valve stems and flanges
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Flow control fittings
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Pressure housings and seals
7.3 Surface Facilities
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Chemical injection systems
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Pump shafts and impellers
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Piping exposed to sour water or H₂S vapor
In these components, Alloy N09935 ensures long operational life and minimal maintenance downtime.
8. Fabrication and Heat Treatment
Alloy N09935 offers good fabricability compared to other superalloys like 718 or 945. It can be:
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Machined using conventional carbide tooling
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Formed and welded with standard nickel-alloy procedures
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Aged to achieve desired mechanical properties
Typical heat treatment process:
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Solution anneal at 1050–1100°C
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Rapid cool to prevent carbide precipitation
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Age harden at 720–760°C
This treatment yields a fine precipitation-strengthened microstructure capable of resisting both stress and corrosion in service.
9. Field Performance – Proven Reliability
Case Example: Offshore Sour Gas Production
In a high-H₂S offshore well, Alloy N09935 replaced Alloy 925 in tubing and valve components. After five years of continuous exposure, inspections showed:
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No signs of pitting or cracking
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Minimal wall loss (<0.02 mm/year)
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Stable mechanical properties verified by tensile testing
This demonstrates the alloy’s superior long-term stability under severe sour conditions.
10. Economic and Operational Benefits
Choosing Alloy N09935 provides measurable advantages:
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Longer Equipment Life – Corrosion rates far below duplex or martensitic steels.
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Reduced Maintenance Costs – Fewer replacements and shutdowns.
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Improved Safety – Lower risk of hydrogen-related failure.
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High Strength-to-Weight Ratio – Allows thinner wall designs, reducing total mass.
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Compatibility – Can be welded to other nickel alloys with minimal galvanic issues.
In summary, Alloy N09935 minimizes the total cost of ownership in sour-service operations.
11. Comparison with Alloy 718 – Why N09935 Wins in Sour Service
While Alloy 718 is widely used for high-temperature aerospace and turbine parts, it is less suitable for sour-gas applications because of limited resistance to hydrogen embrittlement.
Alloy N09935, with its optimized Ni-Cr-Mo-Cu chemistry, provides:
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Better SCC and SSC resistance
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Easier welding and fabrication
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Lower cost per kilogram
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Smoother machinability and higher productivity
Thus, in oilfield applications where corrosion is more critical than extreme temperature, Alloy N09935 is the superior choice.
12. Recommended Working Limits
| Parameter | Value |
|---|---|
| Maximum Continuous Service Temperature | 650°C |
| Pressure Rating (Typical) | >20,000 psi |
| Chloride Concentration | Up to 200,000 ppm |
| pH Range | 3–8 |
| Environment | H₂S + CO₂ + Cl⁻ mixture |
Operating within these limits ensures long-term stability and mechanical reliability.
13. Quality Control and Certification
SASAALLOY supplies Alloy N09935 products under strict international standards:
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ASTM B649 / ASME SB-649
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NACE MR0175 / ISO 15156
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EN 10204 3.1 / 3.2 certification
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100% PMI verification
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Ultrasonic and eddy-current testing for defect control
Our production process ensures each bar, plate, and tube meets the highest industry requirements.
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14. Why Oil Companies Choose Alloy N09935
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Proven Reliability – Years of successful use in major offshore projects.
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Comprehensive Certification – Fully qualified for sour-gas compliance.
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Consistent Quality – Stable heat-to-heat mechanical and corrosion performance.
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Technical Support – SASAALLOY provides metallurgical assistance and machining guidance.
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Global Availability – Stock and production in multiple sizes and forms.
From exploration to completion, Alloy N09935 helps operators achieve higher uptime and reduced risk in challenging well environments.
15. SASAALLOY – Your Trusted Nickel Alloy Partner
SASAALLOY is a professional supplier of nickel-based and corrosion-resistant alloys, including Alloy N09935, 925, 945, and 718.
We provide:
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Bars, plates, tubes, and forged fittings
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Precision machining and custom dimensions
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PMI and mechanical testing with traceable MTCs
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Fast worldwide delivery
Our technical team supports customers from material selection to production to ensure every project achieves maximum performance and value.
SASAALLOY – Strength, Safety, and Stability in Every Alloy.
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16. Conclusion
The harsh conditions of sour gas wells demand materials that can resist hydrogen sulfide attack, chloride corrosion, and high pressure without sacrificing strength or ductility.
Alloy N09935 meets these demands through its:
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Nickel-rich composition for hydrogen resistance
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Chromium-molybdenum protection against pitting
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Copper stabilization in acidic brines
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Precipitation-hardened strength and toughness
Its proven track record in global oilfields, combined with cost efficiency and ease of fabrication, make it the preferred choice for sour gas applications.
When safety, reliability, and corrosion resistance matter most, Alloy N09935 from SASAALLOY delivers enduring performance and peace of mind.
Post time: Oct-29-2025