Is There a Maximum Partial Pressure Limit for Carbon Steels in NACE MR0175/ISO 15156-2 Section 7.1.2 and A.2?

The Question

This text includes a question regarding the partial pressure limitation for carbon steels in NACE MR0175/ISO 15156-2 section 7.1.2 and A.2. The question wants to know if there is a maximum partial pressure limit for carbon steels and where to find the reference.

The Answer

The partial pressure mentioned in Section 7.1.2 refers to the partial pressure of H₂S in the gas phase in equilibrium with the water in the production fluid. Annex C provides information on how to calculate H₂S partial pressure. Regions 1, 2, and 3 cover usual conditions above 0.3 kPa, and Note 1 mentions the unknown performance of steels above 1 MPa.

The maximum partial pressure limit for carbon steels depends on many variables, with a NACE TM0177 test with 100 kPa H₂S, ~2.7 pH, room temperature, 50 g/L NaCl currently considered to cover all normal production conditions for carbon steels.

Reference

This question pertains to NACE MR0175/ISO 15156-2 Clause 7.1.2. Reference: ISO 15156 Maintenance Panel Inquiry #2010-09.