Written by : Neeraj Dubey
In oil, gas, and petrochemical industries, few terms carry more weight in fabrication and design than “sour service.” It’s not just technical jargon—it’s a critical safety and engineering consideration. If equipment isn’t properly designed or fabricated for sour service, it could suffer from serious damage like cracking, leaks, or even catastrophic failure.
This article breaks down what sour service means, why it’s a big deal, and what engineers, fabricators, and welders need to keep in mind when dealing with sour environments.
—
What is Sour Service?
Sour service refers to any operating environment where hydrogen sulfide (H₂S) is present in the process fluid. Even small amounts of H₂S—typically above 0.05 partial pressure—can make the environment “sour.”
H₂S is:
Highly corrosive
Toxic to humans (even at low concentrations)
A major cause of sulfide stress cracking (SSC) and hydrogen-induced cracking (HIC) in steels
Because of this, components used in sour service—like pressure vessels, pipelines, and valves—must be specially designed, manufactured, and tested to resist these conditions.
—
Why Is Sour Service Dangerous?
Sour service introduces two main challenges for metals:
1. Hydrogen-Induced Cracking (HIC)
Occurs when hydrogen atoms diffuse into steel and get trapped at inclusions or discontinuities, leading to internal cracks over time.
2. Sulfide Stress Cracking (SSC)
A form of brittle failure caused by the combined action of tensile stress and hydrogen (from H₂S corrosion) in susceptible materials.
Both of these mechanisms can lead to failure without warning, which is why prevention is absolutely critical.
—
How Is Sour Service Handled in Industry?
The global standard for dealing with sour service environments is NACE MR0175 / ISO 15156, which outlines:
Acceptable materials
Hardness limits
Welding requirements
Testing procedures
Let’s take a look at how it influences real-world fabrication.
—
Key Considerations in Sour Service Applications
1. Material Selection
Low-sulfur carbon steels (e.g., ASTM A516 Grade 70) are commonly used but must be HIC-tested.
Some applications require corrosion-resistant alloys like Inconel, Duplex stainless steel, or CRA claddings.
Materials must be selected not only for strength but also for resistance to cracking and embrittlement.
Comparison Table: Suitable vs. Unsuitable Materials for Sour Service Applications
| Criteria | Suitable Materials | Unsuitable Materials |
|---|---|---|
| Base Material Type | Low-sulfur carbon steels (e.g., ASTM A516 Grade 70 HIC-tested) | Untreated or high-sulfur carbon steels |
| Alloying Elements | Corrosion-resistant alloys (CRAs) like Inconel 625, Duplex SS, 316L SS | Steels with high hardness and no crack resistance certification |
| Hardness Compliance | Materials with controlled hardness ≤ 22 HRC (248 HV) | Materials exceeding hardness limits or lacking PWHT |
| Hydrogen Cracking Resistance | Steels tested per NACE TM0284 (HIC) and TM0177 (SSC) | Unqualified materials without hydrogen embrittlement resistance testing |
| Coating/Cladding Option | CRA Clad or Lined Steels (Corrosion-Resistant Alloy overlays) | Bare carbon steels exposed directly to sour environments |
| Heat Treatment History | Normalized, tempered, or PWHT-applied steels to control microstructure | As-rolled, quenched, or untempered high-strength steels |
| Service Provenance | Materials proven in previous sour service applications | Materials designed only for sweet service or general-purpose use |
| NACE Compliance | Fully compliant with NACE MR0175 / ISO 15156 standards | Non-compliant or uncertified materials |
—
2. Hardness Limits
Hardness is a key factor in cracking resistance. Industry standards limit the maximum allowable surface hardness to:
22 HRC (248 HV) for carbon and low-alloy steels used in sour environments
Proper heat treatment and controlled welding procedures help stay within these limits.
—
3. Welding Practices
Must use low-hydrogen consumables
Strict preheat, interpass temperature, and post-weld heat treatment (PWHT) controls
Welders must be qualified specifically for sour service procedures
Hardness testing of welds and heat-affected zones is mandatory
—
4. Testing and Certification
HIC Testing (per NACE TM0284) for base materials
SSC Testing (per NACE TM0177) for materials under stress
Hardness survey on welds, HAZ, and parent material
Often, third-party inspection and full documentation is required
—
Where Sour Service Is Commonly Found?
Oil and gas production wells (especially in sour fields)
Refineries handling sour crude or sour gas
Natural gas processing plants
Petrochemical facilities
As global exploration goes deeper and into more aggressive reservoirs, the need for sour service-compatible equipment continues to grow.
—
Conclusion
Sour service isn’t just about choosing a tougher metal—it’s about understanding a complete system of risks and defences. From material sourcing and welding to testing and inspection, every step must be carefully managed to ensure the safety and reliability of pressure equipment exposed to hydrogen sulfide.
For fabricators, engineers, and project managers, understanding and applying sour service standards is not optional—it’s a core part of delivering equipment that meets the demands of the modern energy industry.
—
Interested in more practical guidance on sour service welding, fabrication, or inspection? Follow our blog for updates and in-depth technical articles tailored for the fabrication and engineering community.
References
- NACE MR0175 / ISO 15156
- NACE TM0284 and TM0177
Related Article : What Is Hot Cracking (Solidification Cracking)?