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Sour service is defined by hydrogen sulfide in a wet environment, and the failure it causes is not a leak. It is a brittle, invisible crack that can develop in a sound-looking component with no corrosion to show for it. That is why sour service selection is not a corrosion-resistant-alloy question in the way chloride service is. It is a cracking-resistance question, governed by ISO 15156, which sits alongside the ASME material specifications rather than replacing them. This article is organised by service environment: what breaks in gathering systems, why the grades that handle seawater and chloride service are not automatically right, and how the hardness controls in the forging and bolting specifications interact with the sour service limits. It is a selection framework, not a project record.
What the sour environment actually is
ISO 15156 — published jointly as ANSI/NACE MR0175 and ISO 15156, currently the 2020 ISO parts with the 2021 NACE edition, and under revision — applies to equipment in oil and gas production and natural gas sweetening plants, in hydrogen sulfide-containing environments where failure could harm people or the environment. It is not the document for refining and downstream processes; those are covered by NACE MR0103 and ISO 17945, which carry different limits.
The threshold most often used — and the one used here — is a hydrogen sulfide partial pressure of 0.05 psia [0.0003 MPa] in the gas phase with free water present. Above that the environment is treated as sour and the standard’s selection rules apply. Free water is the operative qualifier: a system dry in normal operation but able to form condensate at shutdown must be assessed as wet.
Four cracking mechanisms are covered. Sulfide stress cracking is a brittle failure under tensile stress in the presence of hydrogen sulfide, and it governs the hardness rules. Hydrogen-induced cracking is the formation of hydrogen bubbles within the steel, leading to laminar cracks, and stress-oriented hydrogen-induced cracking is multi-layer cracking along the principal stress directions. Chloride stress corrosion cracking operates in the presence of chlorides, which is why a system handling brine can fail for two reasons at once. The standard addresses cracking only, so a sour service selection is never complete on its own.
Service environment one: sour gas gathering
Sour gas gathering systems are the canonical sour service application: raw gas from wells containing hydrogen sulfide, at pressure, flowing through gathering lines, test separators and inlet manifolds before treatment. The gas is wet, the pressure is high, and the temperature sits where aqueous sulfide chemistry is active.
For carbon and low alloy steel, the controlling requirement is hardness. The maximum permitted hardness in the standard for carbon and low alloy steel in sour service is 22 HRC, and the limit applies to the base metal, weld metal and heat-affected zone alike — not just to the material as delivered. This is why the heat treatment condition and the hardness test on the certificate matter as much as the grade itself: a grade meeting its chemistry and tensile requirements can still be non-compliant if it arrives in the wrong condition, or if the weld has hardened beyond the limit. Specifying “carbon steel” is therefore incomplete specification — you need the grade, the heat treatment condition, the maximum hardness and the test result.
Where the service also carries chlorides — and a sour gathering system handling brines will — the cracking and corrosion assessments diverge. Chloride stress corrosion cracking of austenitic stainless steel is a live mechanism at moderate chloride levels and elevated temperature, and it is why a 316L fitting specified purely for “sour service” can be wrong even when it satisfies the hardness rule.
Service environment two: sour water and acid gas
Acidic water streams — the separated water phase from a sour separator, or the acid gas stream itself — are a harsher environment than the gas phase, and the two are frequently confused when a system is specified as a single package. The distinction is chlorides: acid gas handling involves water that has contacted the reservoir and dissolved the chloride salts present. Chloride concentration and pH jointly determine whether an alloy is acceptable, and the limits are set per alloy family and condition. For duplex stainless grades the conditions add phase balance and a hardness limit, because phase balance is not a formality: corrosion resistance depends on the austenite/ferrite ratio being right.
| Material family | What governs the choice | What the certificate must show |
|---|---|---|
| Carbon steel, low alloy steel | Maximum 22 HRC across base metal, weld and HAZ | Heat treatment condition, hardness, weld procedure, consumables |
| Austenitic stainless steel | Limits on chloride concentration, pH, temperature and H2S partial pressure | Solution treatment condition, hardness results |
| Duplex stainless steel | Environmental limits plus phase balance and a hardness limit | Solution annealing and quench, phase balance, hardness |
| Nickel alloys | Alloy-specific limits; highest cracking and chloride SCC resistance | Heat treatment condition, hardness results |
| Bolting and fasteners | Same environment limits, applied to the fastener | Hardness, and class or condition where the standard distinguishes them |
How the hardness controls in the ASME specifications interact
The ASME specifications carry their own hardness requirements, and these are what a fabricator works to. For low alloy steel forgings the tensile and hardness table sets a maximum Brinell hardness of 170 for the Class 1 condition and a 156 to 207 range for Class 3. Both sit below the roughly 237 Brinell equivalent of the 22 HRC sour service ceiling, so the ASME specification is not the binding constraint here — the heat treatment condition and the documented hardness result close the gap between a compliant forging and a compliant installation.
For duplex forgings the requirements table gives a maximum Brinell hardness of 310 for the super duplex grade, with the duplex grades specified by tensile and yield strength plus elongation and reduction of area minima. The relationship inverts here: the ASME figure is a ceiling on what the material may be, and the sour service limit is set separately and per alloy, so a duplex component arriving at the ASME maximum is not thereby acceptable. This is where the two documents diverge, and where a certificate reporting only chemistry and tensile strength is not enough evidence.
For bolting the contrast between ferritic and austenitic grades is directly useful. The low alloy chromium-molybdenum bolt grades carry a maximum hardness of 321 HBW or 35 HRC, and a variant carries 235 HBW or 99 HRB — the second is the one to look at in sour service, because a 99 HRB ceiling is a softer material and sits inside the sour service envelope. The austenitic bolt grades are specified carbide solution treated, with a maximum hardness of 223 HBW or 96 HRB for Class 1 and 192 HBW or 90 HRB for Class 1A, where Class 1A is solution treated in the finished condition for corrosion resistance.
The alloys that solve the problem, and the ones that only postpone it
The temptation in sour service is to jump straight to the most resistant alloy available. For a fitting or flange in an aggressive, hot, wet, chloride-bearing sour service, that is often right — our Hastelloy C-276 butt weld fittings and Inconel 625 butt weld fittings ranges are specified to those grades for this class of service. But the standard’s limits, not a supplier’s enthusiasm, decide: nickel alloys carry the highest cracking resistance in common use, while 316L may be adequate in mildly sour gas service and grossly inadequate in acid water with high chlorides at elevated temperature.
What to specify
Four items, in the order they cause problems when omitted.
State the service conditions. Hydrogen sulfide partial pressure, chloride concentration, pH, temperature and whether the service is upstream production or downstream processing. Without these no one can select a material against the standard, because its limits are functions of them. State the standard and the edition too: ISO 15156 or NACE MR0175, with the technical circulars that apply. The 2020 ISO parts with the 2021 NACE edition are current, with a revision in progress, so naming the edition removes an argument at inspection.
Require the hardness limit, the heat treatment condition, the hardness test and the certification wording. Not just the grade. The 22 HRC ceiling for carbon and low alloy steel, the condition of the material as delivered, and the test result on the certificate — chemistry and tensile strength alone do not evidence sour service conformance. For austenitic and duplex grades that means the solution treatment and quench as specified; for bolting, name the class. Cover the welds too: weld metal and heat-affected zone carry the same hardness limit as the parent metal.
If you want a quotation for sour service fittings and flanges, send the service conditions, material family, sizes and certification requirements to our contact page and we will quote the material and documentation against them, and tell you plainly where a condition needs confirming against the standard you work to. Our flange product range lists the forms we hold.
