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Offshore oil & gas fasteners must resist seawater corrosion, H2S-induced cracking, and extreme pressures up to 15,000+ psi. NACE MR0175/ISO 15156 governs material selection for sour service environments. Inconel 718 (1379 MPa tensile) is the primary choice for high-strength subsea structural bolting, while Super Duplex 2507 (PREN ≥42.5) provides cost-effective chloride resistance for subsea connectors and topside applications.
Offshore oil and gas operations impose some of the most demanding conditions on fastener materials: continuous seawater immersion, exposure to hydrogen sulfide (H2S) and carbon dioxide (CO2), extreme pressures up to 15,000+ psi downhole, and temperatures ranging from -46°C to over 200°C. Selecting the wrong alloy can lead to sulfide stress cracking (SSC), stress corrosion cracking (SCC), or hydrogen-induced cracking (HIC) with catastrophic consequences.
NACE MR0175/ISO 15156 is the governing standard for metallic materials in H2S-containing (sour) environments in oil and gas production. It addresses sulfide stress cracking (SSC), stress corrosion cracking (SCC), hydrogen-induced cracking (HIC), stepwise cracking, stress-oriented hydrogen-induced cracking (SOHIC), and galvanically induced hydrogen stress cracking (GHSC). Compliance is mandatory for all fasteners in sour service.
The following table provides alloy recommendations for common offshore oil & gas fastener applications, based on industry practice and NACE MR0175 compliance requirements.
| Component | Recommended Alloys | Why |
|---|---|---|
| Subsea wellhead / tree bolting | Inconel 718, Inconel 625 | High strength + seawater resistance; 718 for structural loads, 625 for weld overlays |
| Subsea connector bolts | Super Duplex 2507, Inconel 718 | Chloride resistance + high strength |
| Topside flange bolting | Duplex 2205, Super Duplex 2507 | Cost-effective corrosion resistance |
| Downhole (sour service) | Inconel 718, Inconel 625 | H₂S + high temp + high pressure |
| Riser bolting | Super Duplex 2507, Inconel 625 | Long-term seawater immersion |
| Heat exchanger fasteners | Inconel 625, Hastelloy C-276 | Process fluid compatibility |
NACE MR0175 specifies maximum hardness values to prevent sulfide stress cracking: Inconel 625 (annealed) max 22 HRC (237 HBW), Inconel 718 (age-hardened) max 40 HRC, Duplex 2205 max 28 HRC, and Super Duplex 2507 max 32 HRC. Exceeding these limits in sour service environments risks SSC failure.
Subsea and topside fasteners operate at temperatures from -46°C to 250°C, with downhole exceeding 200°C. Pressures reach 5,000–10,000 psi subsea and 15,000+ psi downhole. Media include seawater, H2S, CO2, brine, and production fluids. Material selection must account for all these variables simultaneously.
Inconel is very strong in a wide temperature range from sub-zero to approx. 980°C. The material also has very good resistance to mineral and organic acids.
View specificationsHastelloy has high resistance to corrosion in both oxidizing and reducing environments. The material has excellent resistance to localized corrosion such as pitting as well as crevice and stress corrosion cracking.
View specificationsCompared to standard stainless steel grades, Duplex is twice as strong and more resistant to localized corrosion. Corrosion resistance is excellent in environments with phosphoric acid, organic acids, and chloride.
View specificationsOur team has decades of experience supplying exotic alloy fasteners for critical applications. Contact us for alloy selection guidance and competitive quotes.