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What are the NACE MR0175 limits for alloy 718 (UNS N07718)?

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Question

Our question relates to ISO 15156-3, Table A.32: How should the table be interpreted in terms of the maximum allowable temperature for applications with less than 30 psi partial pressure of H2S? For example, in its current layout the table prohibits the use of UNS N07718 at temperatures higher than 450°F at any H2S pressure below 30 psi.

Answer:

ISO 15156-3, Table A.32 does not qualify UNS N07718 for use at higher temperatures than 450°F.
The limits on temperature, H2S, Cl-, pH, and sulfur defined in some of the tables of ISO 15156-3, Annex A apply collectively and reflect the knowledge available, usually from laboratory tests, at the time the standard was published. There were no data available related to the use of UNS N07718 at any temperature higher than 450°F.

ISO 15156 allows the qualification and use of materials, to an equipment user’s requirements, outside the limits stated in the tables. (See ISO 15156-3, Figure B.1, Column 2.)

A qualification to define an alternative temperature limit for UNS N07718 for a partial pressure of H2S less than 30 psi must be carried out in accordance with ISO 15156-3, Annex B.

This question is in relation to NACE MR0175/ISO 15156-3 A.9.2, Table A.32

Reference: ISO 15156 Maintenance Panel Inquiry #2005-20

Our corrosion consultants and oilfield metallurgists can help you evaluate the production environment and validate your alloy 718 selection.

UPDATE 2018-06-04: A recent Technical Circular brings relevant updates on NACE MR0175 including Inconel 718: https://oilandgascorrosion.com/nace-mr0175-iso-15156-technical-circulars-for-part-2-and-3-published-2018-06-01/

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