In NACE MR0175, are all bolts required to comply, even external flange bolts?

UPDATE 2017-12-22

ANSI / NACE MR0175 / ISO 15156 has been updated with a Technical Circular which is relevant to external bolts. We recommend to follow this link related to part 2 A.2.2.4 Bolting and Fasteners:


Below there is some further information for reference.

Bolt compliance to NACE MR0175 can sometimes cause confusion, for example flange bolts are not in direct contact with the transport fluids, however a leak may expose them to H2S. NACE MR0175 / ISO 15156 -2 section A.2.2.4 addresses this matter.

In 2011 an inquiry was sent to the maintenance panel as shown below, and the answer reinforces the fact that the equipment user is responsible for NACE MR0175 compliance.


I have a query in respect to the identification of the sour environment for deciding whether a bolting alloy shall conform to the general requirements of A.2.1 as reuired by A.2.2.4, where the bolting material is denied direct atmosphere exposure. An example best illustrates my question. In a pressure containing piping or vessel, it is clear that the partial pressure of H2S within the pressure containment determines whether the piping or vessel is exposed to sour service. Flange bolting is located ourside the vessel and the piping and flange protection systems are seldom pressure containing enclosures. If a leak occurs the partial pressure of H2S will be reduced according to the ratio of the external pressure (typically 1 atm or 100 kPa) to the total pressure internal to the vessel. My question then is whether it is the partial pressure of H2S in the external environment or the partial pressure of H2S in the internal environment that is used to determine whether the bolting material is in sour service.


The equipment user is responsible for defining the intended service environment and selecting materials in accordance with this standard.

This question is in relation to NACE MR0175/ISO 15156-2 Annex A.2.1 and A.2.2.4

Reference: ISO 15156 Maintenance Panel Inquiry #2011-12

At Oil & Gas Corrosion we help equipment users, operators and product manufacturers to comply with NACE MR0175 including bolt material selection.

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