NACE MR 0175/ISO 15156 for Corrosion Resistant Alloys for Sulphide Service
Introduction
NACE MR 0175/ISO 15156 is a Materials Standard issued by the National Association of Corrosion Engineers.
It is originally a US standard intended to assess the suitability of materials for oilfield equipment where sulphide (sulfide) stress corrosion cracking may be a risk in hydrogen sulphide (sour) environments. However, the world standards body ISO has issued it under its own “brand”. The latest edition includes technical corrigenda from 2005. Discussions about the standard can be found on the NACE website.
The standard specifies the types of corrosion resistant materials including stainless steels that can be used in specific oilfield environments and places limits on the hardness of the material. This applies both to parent and weld material. The maximum hardness is usually defined in terms of the Rockwell ‘C’ scale.
No conversion to other hardness scales is given in MR 0175 which presents one problem as softened stainless steels hardnesses are measured using either the Rockwell ‘B’, Vickers or Brinell scales.
Approximate conversions are available.
Summary of MR 0175 Requirements
A wide range of materials is covered by the standard including most types (families) of stainless steels. The table below shows some of these grades. However, this summary is intended to only give a general idea of this complex standard and is not a substitute for the original document.
| Steel Type | Grades Included | Comments |
| Ferritic | 405,430, 409, 434, 436, 442, 444, 445, 446, 447, 448 | Hardness up to 22 HRC |
| Martensitic | 410, 420 | Hardness up to 22 HRC |
| Martensitic | F6NM | Hardness up to 23 HRC |
| Martensitic | S41425 | Hardness up to 28 HRC |
| Austenitic | 201, 202, 302, 304, 304L, 305, 309, 310, 316, 316L, 317, 321, 347, S31254(254SMO), N08904(904L), N08926(1925hMo) | Solution annealed, no cold work to enhance properties, hardness up to 22 HRC |
| Austenitic | S20910 | Hardness up to 35 HRC |
| Duplex | S31803 (1.4462), S32520 (UR 52N+), S32750 (2507), S32760 (Zeron 100), S32550(Ferralium 255) | PREN >30 solution annealed condition, ferrite content 35% to 65%, or 30 to 70% in welds. Note that the general restriction of 28 HRC in previous editions is not found in this latest edition of the standard. There is a specific restriction on HIP’d S31803 to 25HRC. For some applications cold worked material is allowed up to 36HRC |
| Precipitation Hardening | 17-4 PH | 33 HRC Age hardening at 620 deg C |
| Precipitation Hardening | S45000 | 31 HRC Age hardening at 620 deg C |
| Precipitation Hardening | S66286 | 35 HRC |
Free machining grades such as the 303 and 416 types are excluded from of NACE MR 0175/ISO 15156
Help on Materials Selection for Sour Gas Service
The selection of the correct corrosion resistant alloy for a specific set of conditions is quite a complex subject. There are a number of consultancies which specialise in this work. Typical of these is Intetech who have developed Electronic Corrosion Engineer software which guides the user to the correct alloy.