NACE MR0175 and API 938-C have different limits for superduplex stainless steels. Why?

Please be informed that as per NACE MR0175/ ISO 15156, Super duplex stainless steel material can be used for temperature Up to 232 deg. C and partial pressure of H2s 3 Psi maximum with any combination of chloride concentration and in situ pH occurring in production environment is acceptable. However, based on API 938-C, the experimental data of critical pitting and crevice critical temperature of super duplex stainless steel material (Grade 2507) indicates that the maximum temperature it can be used as 80 deg. C and 50 deg. C respectively. Hence, would like to solicit the clarification on discrepancy in temperature limit for using of super duplex stainless material.
Revised Question: Regarding NACE MR0175/ISO 15156-3 Tables A.24 and A.25, super duplex alloys are listed for service up to 232C and 3 psi H2S with no limits on chloride and in-situ pH, the attached section of API 938-C shows lower maximum temperatures (80C and 50C respectively for pitting and crevice corrosion). Please explain the discrepancy.

Answer: NACE MR0175/ISO 15156-3 is a document that addresses environmental cracking in the presence of H2S; the standard title defines “Materials for use in H2S containing environment in oil and gas production”. The standard does not currently address pitting and crevice corrosion. The current values that are in the document are based on successful ballots and history. Note that the tables state “production environments” which are oxygen-free. The subject of API Technical Report 938-C is “Use of Duplex Stainless Steels in the Oil Refining Industry. You may need to employ a consultant to assist you with your particular application.

Reference MP Inquiry 2017-17

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