NACE MR0175 / ISO 15156 Compliance for Sample Probes in Sour Service
How to specify NACE MR0175 / ISO 15156-compliant sample probe assemblies for H2S service. Material restrictions, hardness limits, heat treatment requirements, and the difference between MR0175 and MR0103.
TL;DR
NACE MR0175 / ISO 15156 is the governing standard for materials in upstream H2S-bearing service. It restricts not just material grade but also hardness, heat treatment, and cold work. A "316L" probe is not automatically NACE-compliant — it must also satisfy a maximum hardness of HRC 22 in the wetted condition. NACE MR0103 is the parallel standard for refinery sour service. The two are not interchangeable.
What the Standard Covers
NACE MR0175 / ISO 15156 is divided into three parts:
| Part | Scope |
| Part 1 | General principles |
| Part 2 | Cracking-resistant carbon and low-alloy steels |
| Part 3 | Cracking-resistant CRAs (corrosion-resistant alloys) and other alloys |
A sample probe in sour service almost always falls under Part 3 because the wetted alloy is typically 316L, a duplex grade, or a nickel-base alloy.
Failure Modes Targeted
NACE MR0175 was written to prevent three related failure modes:
1. Sulfide Stress Cracking (SSC) — fast brittle failure of high-strength steels in wet H2S
2. Hydrogen-Induced Cracking (HIC) — internal blistering of plate steels
3. Stress-Oriented HIC (SOHIC) — stepwise cracking under residual or applied stress
The dominant concern for cantilevered sample probes is SSC. Probes fail at the gland-air interface where condensed sour fluid combines with the static tensile stress of the probe's own cantilever bending.
Hardness Limits
The single most important rule:
All wetted components must be ≤ HRC 22 (or equivalent) in the as-installed condition.
This includes the probe tube, any welds, the gland body, and any fasteners that contact the sour stream. Cold-worked, age-hardened, or precipitation-hardened conditions are generally not allowed unless specifically listed in Tables A.x of the standard.
Common Sample Probe Materials and NACE Status
| Material | NACE MR0175 Status |
| 316L SS, solution-annealed | Acceptable, with hardness limit |
| 316L SS, cold-worked | Not acceptable at high cold work |
| Duplex 2205 | Acceptable within partial-pressure limits |
| Hastelloy C276 | Acceptable, solution-annealed |
| Inconel 625 | Acceptable, solution-annealed |
| Inconel 718, age-hardened | Restricted; check H2S partial pressure |
| Monel 400 | Acceptable, but watch for free oxygen |
| Carbon steel | Acceptable below hardness; HIC concerns |
MR0175 vs MR0103
| Aspect | MR0175 | MR0103 |
| Industry | Upstream oil & gas production | Refinery & petrochemical |
| Wet H2S definition | ≥ 0.05 psia partial pressure | Process-specific |
| Test methods | More restrictive | Less restrictive |
| Heat treatment | Strict for CRAs | Less strict |
A refinery sample probe specified to MR0175 will satisfy MR0103, but not vice versa. If the probe will see both upstream and refinery service, default to MR0175.
Documentation Requirements
A NACE-compliant probe ships with:
- Material test report (MTR) traceable to heat number
- Hardness test results (Rockwell or Vickers)
- Statement of compliance to MR0175 / MR0103 with edition year
- Weld procedure specification (WPS) and procedure qualification record (PQR) for any welds in the wetted boundary
Configurator Behavior
When the SPA Configurator detects sour service in the process inputs (presence of H2S above the threshold partial pressure), it:
1. Restricts the material list to NACE-acceptable options
2. Flags any cold-worked variants as non-compliant
3. Adds the MR0175 statement to the generated bill of materials
4. Cross-references the sour crude SSC blog in the wizard guidance pane