Material Selection for a Wet Sour Gas Separator
A new two-phase separator will handle wet sour gas at 65 °C and 45 bar, with H2S partial pressure of 0.35 bar, CO2 partial pressure of 8 bar, and produced water chloride of 60,000 mg/L. The current design uses carbon steel with 3 mm corrosion allowance. The process engineer proposes upgrading to 316L stainless steel to solve both corrosion and sour-service concerns. The mechanical engineer notes that 316L is readily available and cheaper than duplex. The owner wants a defensible material selection with a clear rationale for the regulator.
1. Evaluate whether 316L stainless steel is suitable for this service, and explain the governing degradation mechanisms.
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316L is not suitable. The service is wet sour gas with high CO2 and high chloride:
- Sulfide stress cracking (SSC) / sour service: At 0.35 bar H2S partial pressure, this is squarely in sour service. Austenitic stainless steels like 316L are susceptible to chloride stress corrosion cracking and, in sour service, to SSC depending on hardness and stress. 316L is generally not accepted for sour service at this H2S level without qualification.
- Chloride pitting and crevice corrosion: 60,000 mg/L chloride at 65 °C greatly exceeds the practical chloride limit for 316L (pitting resistance equivalent number ~25 is marginal). Pitting and crevice attack are likely.
- CO2 corrosion: 8 bar CO2 is aggressive; 316L offers some resistance but combined with chlorides and H2S the environment is beyond its envelope.
The process engineer's assumption that 316L 'solves sour service' is incorrect - it may actually be worse than carbon steel with corrosion allowance in terms of cracking risk.
2. Propose a defensible material selection and justify it against the governing mechanisms.
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A defensible selection is duplex stainless steel (e.g., 22Cr duplex, UNS S31803/S32205) or a corrosion-resistant alloy such as Alloy 825/625 depending on qualification, with the following rationale:
- Duplex stainless offers higher chloride pitting resistance (PREN ~34) and better SSC resistance than 316L, and is commonly used in wet sour service at moderate H2S. It must be qualified per NACE MR0175/ISO 15156 for the specific H2S partial pressure and temperature.
- Carbon steel with corrosion allowance may be acceptable for the CO2 corrosion if inhibited and if the corrosion rate is managed, but the H2S sour service and high chlorides make inhibition challenging; a corrosion allowance of 3 mm is likely inadequate.
- Alloy 825 or 625 would be selected if duplex cannot be qualified for the H2S level or if chlorides/temperature exceed duplex limits.
The regulator-facing rationale should cite NACE MR0175/ISO 15156 qualification for the chosen alloy, the calculated CO2 corrosion rate for carbon steel, and the chloride pitting resistance (PREN) comparison.
3. What additional data or testing would you require before finalizing the material selection?
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Before finalizing, require:
- NACE MR0175/ISO 15156 compliance review for the selected alloy at the specific H2S partial pressure (0.35 bar), temperature (65 °C), and chloride level.
- CO2 corrosion rate calculation (e.g., de Waard-Milliams or similar) for carbon steel to confirm whether inhibition plus corrosion allowance is viable.
- Chloride SCC and pitting testing (e.g., ASTM G48) for any stainless option.
- Water chemistry and flow regime - confirm chloride, pH, and whether wetting is continuous; stratified flow can concentrate chlorides.
- Inhibition feasibility study if carbon steel is retained.
- Welding and hardness qualification for duplex (ferrite/austenite balance, hardness limits per NACE).
Only after these are complete should the material be locked in for the regulator submission.