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In NACE MR0175, is 0.3kPa ppH2S the lower limit for sour service compliance? What happens at lower partial pressures of H2S?

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Question 1: I am preparing a “position” document regarding SSC on behalf of my company. I am trying to understand the NACE definition for sour/non-sour (with relation to NACE MR0175). The definition of sour service is provided by NACE. That states, “exposure to oilfield environment that contain H2S and can cause cracking by the mechanisms addressed by this part . . .” However, this needs further qualification as the environmental conditions (degree of H2S and pH) may determine whether cracking can result. This is found in 7.2.1.3 of part 2 within NACE MR0175/ISO 15156, the document that refers to carbon steel (ISO 15156-2). For environments with a partial pressure of H2S below 0.3 kPa (0.05 psi) the document states, “Normally, no precautions are required for the selection of steels for use under these conditions”

Answer 1: There is no “non-sour” limit as some steels can still be susceptible below the limit of 0.05 psi (0.3 kPa).

This question is in relation to NACE MR0175/ISO 15156-2 Clause 7.2.1.3

Reference: ISO 15156 Maintenance Panel Inquiry #2009-11, Part 1

Part 2: While not explicitly stated, the implication of that statement is that an environment with a partial pressure below 0.3 kPa is regarded as “non-sour.” This is reinforced later, where Annex C, C.2 defines H2S partial pressure isobars to determine “if a system is sour.” Line 1 of Figure C.1 (0.3 kPa) identifies the demarcation between “sour” and “non-sour” conditions (referred to as being in accordance with Option 1, where environments below 0.3 kPa require no precautions), while lines 2-5 identify the degree of sourness (Option 2—prequalification of material or specific testing needs to be performed).

Answer 2: In addition for CRAs, 15156-3 has no defined “non-sour” limit. Due to the wide range of environmental cracking resistance of CRAs, particularly those not listed in 15156-3, a non-sour limit would be so low (i.e., minimum detection level) that it would be useless.

Reference: ISO 15156 Maintenance Panel Inquiry #2009-11, Part 2

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