The NACE Standard MR0175, "Sulfide Stress Corrosion Cracking Resistant Metallic Materials for Oil Field Equipment" is widely used throughout the world. The standard specifies the proper materials, heat treat conditions and strength levels required to provide good service life in sour gas and oil environments. NACE (National Association of Corrosion Engineers) is a worldwide technical organization which studies various aspects of corrosion and the damage that may result in refineries, chemical plants, water systems, and other industrial systems.

History

MR0175 was first issued in 1975, but the origin of the document dates to 1959 when a group of engineers in Western Canada pooled their experience in successful handling of sour gas. The group organized as NACE committee T-1B and in 1963 issued specification 1B163, "Recommendations of Materials for Sour Service." In 1965, NACE organized the nationwide committee T-1F-1 which issued 1F166 in 1966 and MR0175 in 1975. The specification is revised on an annual basis.

NACE committee T-1F-1 continues to have responsibility for MR0175. All revisions and additions must be unanimously approved by the 500-plus member committee T-1, Corrosion Control in Petroleum Production. MR0175 is intended to apply only to oil field equipment, flow line equipment, and oil field processing facilities where H₂S is present. Only sulfide stress cracking (SSC) is addressed. Users are advised that other forms of failure mechanisms must be considered in all cases. Failure modes, such as severe general corrosion, chloride stress corrosion cracking, hydrogen blistering or step-wise cracking are outside the scope of the document. Users must carefully consider the process conditions when selecting materials.

While the standard is intended to be used only for oil field equipment, industry has taken MR0175 and applied it to many other areas including refineries, LNG plants, pipelines, and natural gas systems. The judicious use of the document in these applications is constructive and can help prevent SSC failures wherever H₂S is present.

Requirements

Figure 1

Figure 2

Figures 1 and 2 taken from MR0175 define the sour systems where SSC may occur. Low concentrations of H₂S at low pressures are considered outside the scope of the document. The low stress levels at low pressures or the inhibitive effects of oil may give satisfactory performance with standard commercial equipment. Many users, however, have elected to take a conservative approach and specify NACE compliance any time a measurable amount of H₂S is present. The decision to follow MR0175 must be made by the user based on economic impact, the safety aspects should a failure occur, and past field experience. Legislation can impact the decision as well. MR0175 must now be followed by law for sour applications under several jurisdictions; Texas (Railroad Commission), off-shore (under U.S. Minerals Management Service), and Alberta, Canada (Energy Conservation Board).

The Basics of Sulfide Stress Cracking

Figure 3

SSC develops in aqueous solutions as corrosion forms on a material. Hydrogen ions are a product of many corrosion processes (Figure 3). These ions pick up electrons from the base material producing hydrogen atoms. At that point, two hydrogen atoms may combine to form a hydrogen molecule. Most molecules will eventually collect, form hydrogen bubbles, and float away harmlessly. Some percentage of the hydrogen atoms will diffuse into the base metal and embrittle the crystalline structure. When the concentration of hydrogen becomes critical and the tensile stress exceeds the threshold level, SSC occurs. H₂S does not actively participate in the SSC reaction; sulfides promote the entry of the hydrogen atoms into the base material.

In many instances, particularly among carbon and low alloy steels, the cracking will initiate and propagate along the grain boundaries. This is called intergranular stress cracking. In other alloy systems or under specific conditions, the cracking will propagate through the grains. This is called transgranular stress corrosion cracking. Sulfide stress cracking is most severe at ambient temperature, 20° to 120°F (-7° to 49°C). Below 20°F (-7°C) the diffusion rate of the hydrogen is so slow that the critical concentration is never reached. Above 120°F (49°C) the diffusion rate is so fast that the hydrogen passes through the material in such a rapid manner that the critical concentration is not reached. The occurrence of stress corrosion cracking above 120°F (49°C) is still likely and must be carefully considered when selecting material. In most cases, the stress corrosion cracking will not be SSC but some other form. Chloride stress corrosion cracking is likely in deep sour wells as most exceed 300°F (149°C) and contain significant chloride levels.

Figure 4

The susceptibility of a material to SSC is directly related to its strength or hardness level. This is true for carbon steels, stainless steels, and nickel based alloys. When carbon or alloy steel is heat treated to progressively higher hardness levels, the time to failure decreases rapidly for a given stress level (Figure 4). Years of field experience have shown that good SSC resistance is obtained below 22 HRC for the carbon and low alloy steels. SSC can still occur below 22 HRC, but the likelihood of failure is greatly reduced.

Carbon Steel

Carbon and low alloy steels have acceptable resistance to SSC provided their processing is carefully monitored. The hardness must be less than 22 HRC. If welding or significant cold working is done, stress relief is required. Even though the base metal hardness of a carbon or alloy steel is less than 22 HRC, areas of the heat effected zone will be harder. Post-weld heat treatment will eliminate these excessively hard areas.

ASME SA216 grades WCB and WCC are the most commonly used body casting materials. It is Fishers™ policy to stress relieve all WCB and WCC castings to MR0175 whether they have been welded or not. This eliminates the chance of a weld repair going undetected and not being stress-relieved.

ASME SA352 grades LCB and LCC are very similar to WCB and WCC. They are impact tested at -50°F (-46°C) to ensure good toughness in low temperature service. LCB and LCC are used in the northern U.S., Alaska, and Canada where temperatures commonly drop below the -20°F (-32°C) permitted for WCB. All LCB and LCC castings to MR0175 are also stress-relieved.

Cast Iron

Gray, austenitic, and white cast irons cannot be used for any pressure retaining parts, due to low ductility. Ferritic ductile iron to ASTM A395 is acceptable when permitted by ANSI, API, or industry standards.

Stainless Steel

UNS S41000 stainless steel (410 stainless steel) and other martensitic grades must be double tempered to a maximum allowable hardness level of 25 HRC. Post-weld heat treatment is also required. S41600 stainless steel is similar to S41000 with the exception of a sulfur addition to produce free machining characteristics. Use of free machining steels is not permitted by MR0175.

CA6NM is a modified version of the cast S41000 stainless steel. MR0175 allows its use, but specifies the exact heat treatment required. Generally, the carbon content must be restricted to 0.3 percent maximum to meet the 23 HRC maximum hardness. Post-weld heat treatment is required for CA6NM.

The austenitic stainless steels have exceptional resistance to SSC in the annealed condition. The standard specifies that these materials must be 22 HRC maximum and free of cold work to prevent SSC. The cast and wrought equivalents of 302, 304, 304L, 305, 308, 309, 310, 316, 316L, 317, 321, and 347 are all acceptable per MR0175.

Post-weld heat treatment of the 300 Series stainless steels is not required. The corrosion resistance may be effected by welding. However, this can be controlled by using the low carbon grades, or low heat input levels and low interpass temperatures.

Wrought S17400 (17-4PH) stainless steel is allowed, but must be carefully processed to prevent SSC. The standard now gives two different acceptable heat treatments for S17400. One treatment is the double H1150 heat treatment which requires exposing the material at 1150°F (621°C) for four hours followed by air cooling and then exposing for another four hours at 1150°F (621°C). A maximum hardness level of 33 HRC is specified. The second heat treatment is the H1150M treatment. First, the material is exposed for two hours at 1400°F (760°C), then air cooled and exposed for four hours at 1150°F (621°C). The maximum hardness level is the same for this condition.

CB7Cu-1 (Cast 17-4PH) is not approved per MR0175. However, many users have successfully applied it for trim parts in past years in the same double heat treated conditions as the wrought form.

Two high strength stainless steel grades are acceptable for MR0175. The first is S66286 (grade 660 or A286) which is a precipitation hardening alloy with excellent resistance to SSC and general corrosion. The maximum hardness level permitted is 35 HRC.

The second material is S20910 (XM-19) which is commonly called Nitronic 50R. This high strength stainless steel has excellent resistance to SSC and corrosion resistance superior to S31600 or S31700. The maximum allowable hardness is 35 HRC. The "high strength" condition, which approaches 35 HRC, can only be produced by hot working methods. Cold drawn S20910 is also acceptable for shafts, stems, and pins. It is our experience that the SSC resistance of S20910 is far superior to S17400 or other austenitic stainless steels at similar hardness levels. The only other materials with similar stress cracking resistance at these strength levels are the nickel-based alloys which are, of course, much more expensive. A few duplex stainless steels are now acceptable per MR0175. Wrought S31803 (2205) and S32550 (Ferralium 255) are acceptable to 28 HRC. Wrought S32404 (Uranus 50) is acceptable to 20 HRC. Only one cast duplex stainless steel is acceptable, alloy Z 6CNDU20.08M, NF A 320-55 French National Standard.

Nonferrous Alloys

The final category in MR0175 is the nonferrous materials section. In general, the nickel-based alloys are acceptable to a maximum hardness level of 35 HRC. All have excellent resistance to SSC. Commonly used acceptable materials include nickel-copper alloys N04400 (alloy 400) and N04405 (alloy 405) and the precipitation hardening alloy N05500 (K500). The nickel-iron-chromium alloys include alloys N06600 (alloy 600) and N07750 (alloy X750). The acceptable nickel-chromium-molybdenum alloys include alloys N06625 (alloy 625), and N10276 (alloy C276). The precipitation hardening grade N07718 (alloy 718) is also acceptable to 40 HRC. Where high strength levels are required along with good machinability, Emerson Process Management Regulator Technologies uses N05500, N07718, N07750, or N09925 (alloy 925). They can be drilled or turned, then age hardened. Several cobalt based materials are acceptable, including R30035 (alloy MP35N), R30003 (Elgiloy), and R30605 (Haynes 25 or L605).

Aluminum based and copper alloys may be used for sour service, but the user is cautioned that severe corrosion attack may occur on these materials. They are seldom used in direct contact with H₂S.

Several wrought titanium grades are now included in MR0175. The only common industrial alloy is R50400 (grade 2).

Springs

Springs in compliance with NACE represent a difficult problem. To function properly, springs must have very high strength (hardness) levels. Normal steel and stainless steel springs would be very susceptible to SSC and fail to meet MR0175.

In general, very soft, low strength materials must be used. Of course, these materials produce poor springs. The two exceptions allowed are the cobalt based alloys, such as R30003, which may be cold worked and hardened to a maximum hardness of 60 HRC and alloy N07750 which is permitted to 50 HRC.

Coatings

Coatings, platings, and overlays may be used provided the base metal is in a condition which is acceptable per MR0175. The coatings may not be used to protect a base material which is susceptible to SSC. Coatings commonly used in sour service are chromium plating, electroless nickel (ENC) and ion nitriding. Overlays and castings commonly used include CoCr-A (StelliteR or alloy 6), R30006 (alloy 6B), and NiCr-C (ColmonoyR 6) nickel-chromium-boron alloys. Tungsten carbide alloys are acceptable in the cast, cemented, or thermally sprayed conditions. Ceramic coatings such as plasma sprayed chromium oxide are also acceptable.

ENC is often used by Emerson Process Management Regulator Technologies as a wear-resistant coating. As required by MR0175, it is applied only to acceptable base metals. ENC has excellent corrosion resistance in sour, salt containing environments.

Stress Relieving

Many people have the misunderstanding that stress relieving following machining is required by MR0175. Provided good machining practices are followed using sharp tools and proper lubrication, the amount of cold work produced is negligible. SSC resistance will not be affected. MR0175 actually permits the cold rolling of threads, provided the component will meet the heat treat conditions and hardness requirements specified for the given parent material. Cold deformation processes such as burnishing are also acceptable.

Bolting

Bolting materials must meet the requirements of MR0175 when bolting is directly exposed to a sour environment. Standard ASTM A193 grade B7 bolts or A194 grade 2H nuts can be used per MR0175 provided they are outside of the sour environment. If the bolting will be deprived atmospheric contact by burial, insulation, or flange protectors, then grades of bolting such as B7 and 2H are unacceptable. The most commonly used fasteners for "exposed" applications are ASTM A193 grade B7M bolts and A194 grade 2M nuts. They are tempered and hardness tested versions of the B7 and 2H grades. HRC 22 is the maximum allowable hardness.

Many customers use only B7M bolting for bonnet, packing box, and flange joints. This reduces the likelihood of SSC if a leak develops and goes undetected or unrepaired for an extended time. It must be remembered, however, that use of lower strength bolting materials such as B7M often requires pressure vessel derating.

Composition Materials

MR0175 does not address elastomer and polymer materials. However, the importance of these materials in critical sealing functions cannot be overlooked. User experience has been successful with elastomers such as nitrile, neoprene, fluoroelastomer (FKM), and perfluoroelastomer (FFKM). In general, fluoropolymers such as teflon (TFE) can be applied without reservation within their normal temperature range.

Codes and Standards

Applicable ASTM, ANSI, ASME, and API standards are used along with MR0175 as they would normally be used for other applications. The MR0175 requires that all weld procedures be qualified to these same standards. Welders must be familiar with the procedures and capable of making welds which comply.

The Commercial Application of NACE

Special documentation of materials to MR0175 is not required by the standard and NACE itself does not issue any type of a certification. It is the producer's responsibility to properly monitor the materials and processes as required by MR0175.

It is not uncommon for manufacturers to "upgrade" standard manufactured components to MR0175 by hardness testing. This produces a product which complies with MR0175, but which may not provide the best solution for the long-term. If the construction was not thoroughly recorded at the outset, it may be difficult to get replacement parts in the proper materials. The testing necessary to establish that each part complies is quite expensive. And, due to the "local" nature of a hardness test, there is also some risk that "upgraded" parts do not fully comply.

With proper in-house systems, it is quite simple to confidently produce a construction which can be certified to MR0175 without the necessity of after manufacture testing. This eliminates many costly extras and additionally provides a complete record of the construction for future parts procurement. An order entry, procurement, and manufacturing system which is integrated and highly structured is required in order to confidently and automatically provide equipment which complies.

Due to its hierarchical nature and its use by all company functions, Emerson Process Management Regulator Technologies system is ideal for items such as MR0175 which requires a moderate degree of control without undue cost. In order to illustrate the system used by Emerson Process Management Regulator Technologies, an example will be used.

Most products produced by Emerson Process Management Regulator Technologies (including products to MR0175) will be specified by a Fisher Standard (FS) number. These numbers (e.g. FSED-542) completely specify a standardized construction including size, materials, and other characteristics. The FS number is a short notation which represents a series of part groups (modules) describing the construction. One module may represent a 3-inch WCB valve body with ANSI Class 300 flanges, another may specify a certain valve plug and seat ring. The part numbers which make up these modules are composed of a drawing number and a material/finish identifier. The drawing describes the dimensions and methods used to make the part, while the material/finish reference considers material chemistry, form, heat treatment, and a variety of other variables. The part number definition also includes a very specific "material reference number" which is used to identify a material specification for purchase of materials. The material specification includes the ASME designation as well as additional qualifiers, as necessary, to ensure compliance with specifications such as NACE MR0175.

For NACE compliant products, an FS number and a NACE option are generally specified. The FS number establishes the standard construction variation. The option modifies the construction and materials to comply totally with MR0175 requirements. The option eliminates certain standard modules and replaces them with NACE suitable modules. Each part in a NACE suitable module has been checked to assure that it complies to the specification in form and manufacturing method and that it is produced from an appropriate material.

It is due to this top-to-bottom system integrity that Emerson Process Management Regulator Technologies can be confident of MR0175 compliance without the need for extensive test work. At each level of the system documentation, there are specific references to and requirements for compliance to MR0175. Further, since the construction is permanently documented at all levels of detail, it is possible to confidently and simply procure replacement parts at any future date.

Test documentation is available in a variety of forms, including certificates of compliance, hardness test data, chemical and physical test reports, and heat treat reports. Since these items will have some cost associated with them, it is important to examine the need for documentation in light of the vendor's credibility and manufacturing control systems. Emerson Process Management Regulator Technologies's normal manufacturing processes and procedures assure that all NACE specified products will comply without the need for additional test expense.

Emerson Process Management Regulator Technologies has been producing equipment for a variety of sour conditions and specifications since the mid-1950's and has thousands of devices in service. MR0175 has been shown to be an excellent technical reference for solving the complex application problems found in the handling of sour fluids. As more sour hydrocarbons are produced, it grows in importance and applicability.