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Technical Note: Sulfide Stress Cracking (SSC) – A Focus on Nickel Alloys and Stainless Steels
1. Definition and Mechanism
Sulfide Stress Cracking (SSC) is a form of “hydrogen embrittlement” occurring in susceptible materials under tensile stress in H₂S-containing aqueous environments. Hydrogen atoms generated by the sour corrosion reaction diffuse into the metal lattice, leading to crack initiation and propagation.
For nickel alloys and stainless steels, SSC resistance is generally superior to carbon/low alloy steels, but not all grades are immune. Proper material selection and heat treatment are critical for sour service applications.
Fig 1. Sulfide Stress Cracking (SCC) in metal
2. Why Nickel Alloys and Stainless Steels Excel in Sour Service
Most nickel alloys and austenitic stainless steels offer excellent resistance to SSC due to their face-centered cubic (FCC) structure, which exhibits lower hydrogen diffusivity and reduced susceptibility to hydrogen embrittlement compared to ferritic/martensitic steels .
Key Advantages:
Nickel-based alloys (e.g., Alloy 625, Alloy 825, C-276) are among the few metallic materials capable of withstanding severe sour environments (high H₂S, low pH, high chlorides)
Austenitic stainless steels (300 series) perform well in moderate sour conditions but may suffer SSC at high hardness levels or in cold-worked conditions
Duplex stainless steels(22%Cr, 25%Cr) provide an excellent balance of strength and SSC resistance when properly balanced between ferrite and austenite phases
Fig. 2: Nickel Alloy Strips for environment prone to Sulfide Stress Cracking
3. Nickel Effects on SSC Resistance – Critical Insights
Beneficial Effects of Nickel:
- Increasing Ni content in duplex stainless steels (from 3.5% to 7.5%) raises the critical chloride concentration for SSC initiation and increases threshold stress in HAZ regions
- High nickel coatings on low-alloy steels have been shown to increase SSCC resistance by reducing hydrogen permeation
Optimal Nickel Levels:
- There is an optimum Ni range (~6.5%) for SSC resistance in duplex steels; exceeding this may slightly decrease threshold stress in base metal
- For martensitic stainless steels (e.g., 13Cr-4Ni, super 13Cr with 5%Ni), proper heat treatment is essential to achieve NACE MR0175 compliance
Caution – Not All Nickel Alloys Are Equal:
- Nickel-copper alloys (Monel type) are generally SSC-resistant but may suffer hydrogen embrittlement under cathodic protection
- Precipitation-hardened nickel alloys (e.g., Inconel X-750) require specific heat treatments (overaging) for sour service; leaner Ni-base alloys may offer improved SSC resistance
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4. Hardness Limits per NACE MR0175/ISO 15156
For corrosion-resistant alloys (CRAs) including nickel alloys and stainless steels: | |
Material Type | Maximum Hardness Requirement |
Solid-solution nickel alloys | No specific hardness limit in NACE MR0175 |
Duplex stainless steels | <28 HRC (typically 250-280 HV10) |
Martensitic stainless steels | <23 HRC for sour service; |
Precipitation-hardened | Overaging treatment required; |
Carbon/low alloy steels | <22 HRC (~237 HB) |
Note: For nickel alloy welds, NACE MR0175 does not specify hardness limits for HAZ when tables/text do not address them . | |
5. Post-Weld Heat Treatment (PWHT) Requirements
Even for SSC-resistant alloys, welding can create local hard zones and residual stresses that increase SSC susceptibility.
For Nickel Alloys and Stainless Steels: | |
Alloy Type | PWHT Recommendation |
Solid-solution Ni alloys (625, C-276) | Generally not required; as-welded acceptable if hardness controlled |
Duplex stainless steels | Not required; proper filler metal and heat input control critical |
Martensitic stainless steels | Mandatory double tempering (first: 648-691°C; second: 593-621°C) to achieve <23 HRC |
9%Ni steel | Single tempering at 575°C reduces hardness from ~350 HV to ~257 HV and decreases SSC susceptibility |
Research Finding: | |
For 9%Ni low carbon steel (used in cryogenic CO₂ reinjection with H₂S traces), PWHT is strongly recommended – single tempering at 575°C proved as effective as double tempering for SSC mitigation. | |
Fig. 3: Effect of SSC on Ferrous Alloy vs Nickel Alloy
6.Environmental Severity Classification
For nickel alloy and stainless steel selection in sour service: | |
Condition | Recommended Material Classes |
Mild sour (H₂S < 0.05 psi, neutral pH) | 316/316L stainless steel |
Moderate sour (H₂S 0.05-1 psi, pH >3.5) | Duplex (2205), Alloy 825 |
Severe sour (H₂S >1 psi, low pH, chlorides) | Super duplex (2507), Alloy 625, C-276 |
High temperature + sour (>150°C) | Ni-Cr-Mo alloys (C-276, C-22) |
Critical threshold: SSC has been observed at H₂S concentrations as low as 1 ppmw in water – do not assume low H₂S means safe for all materials. | |
7.Summary Table – SSC Resistance by Alloy Family
Alloy Family | SSC Resistance | Key Limitation | Typical Applications |
Austenitic SS | Good (annealed) | Cold work >20% reduces resistance | Moderate sour, low Cl⁻ |
Duplex SS | Excellent | Ferrite/austenite balance critical | High Cl⁻, H₂S to 1 psi |
Martensitic SS | Moderate (with HT) | Requires double tempering (<23 HRC) | Mild sour, sweet service |
Ni-Cr-Mo alloys | Superior | Cost | Severe sour, high temp, all pH |
Ni-Cu alloys | Good | Avoid cathodic protection | Reducing acids, HF service |
Ni-Fe-Cr | Excellent | Lower strength than Ni-Cr-Mo | Sour high temperature |
Fig. 4: Nickel Based Alloy H2S Service Failure Schematic
References:
NACE MR0175/ISO 15156 (current edition)
ASM Handbook Vol. 13B – Corrosion of Nickel and Nickel-Base Alloys
TWI technical literature on duplex stainless steels
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