Sour service refers to an environment containing Hydrogen Sulfide (H2S). The fact that a produced fluid may be defined as sour raises the possiblity of sulfide stress corrosion cracking also referred to as sulfide stress cracking (SSC). This article summarises the carbon and low alloy steel piping material considerations for use in sour service oil and gas environment as specified in NACE MR0175/ISO 15156 standards.
NACE MR0175/ISO 15156 specifies the requirements and recommendations for the selection and qualification of carbon and low-alloy steels, corrosion-resistant alloys, and other alloys for service in equipment used in oil and natural gas production and natural gas treatment plants in H2S-containing environments, whose failure could pose a risk to the health and safety of the public and personnel or to the equipment itself.
NACE MR0175/ISO 15156 consists of the following three parts:
- Part 1: General principles for selection of cracking-resistant materials
- Part 2: Cracking-resistant carbon and low alloy steels
- Part 3: Cracking-resistant CRAs (corrosion-resistant alloys) and other alloys
Mechanisms of Cracking due to H2S
Sulfide Stress Cracking (SSC) is a form of hydrogen induced cracking which occurs under the combined action of tensile stress and corrosion in the presence of water and hydrogen sulfide. SSC occurs when atomic hydrogen diffuses into the metal and collects at the internal laminations or other voids. Atomic hydrogen that collects at the laminations or voids combines to form hydrogen molecules. Eventually sufficient pressure forms inside the void to propogate into a crack. The tensile stresses could be in the form of directly applied stresses or in the form of residual stresses. Residual stresses can be induced due to cold deformation and forming, welding, heat treatment, machining and grinding. Failures due to SSC are classified as catastrophic as the presence of fine cracks due to SSC are difficult to detect and damage cannot be easily predicted.
NACE MR0175/ISO 15156 - Part 1 addresses mechanisms of cracking that can be caused by H2S including sulfide stress cracking (SSC), stress corrosion cracking (SCC), hydrogen induced cracking (HIC), stress oriented hydrogen induced cracking (SOHIC), soft zone cracking and galvanically induced hydrogen stress cracking. It does not cover damage mechanisms such as chloride stress corrosion cracking, amine stress corrosion cracking or carbonate stress corrosion cracking. The standard also cautions the user that metallic materials selected using this standard are resistant to cracking in defined H2S containing environments in oil and gas production, but are not necessarily immune under all service conditions.
Users of this standard are required to assess the service conditions to which the selected material will get exposed. Important factors to consider are the CO2 and Cl- content of the process streams and time of exposure with liquid water phase. As far as corrosion is concerned, the major components of produced fluids in oil and gas production are water, hydrogen sulfide, carbon dioxide and chloride ions. Each of these components contributes to corrosivity and to some extent their effects are synergistic. Increase in pressure or elevated temperatures will often exacerbate the problem.
Sour Gas Service
NACE MR0175/ISO 15156 - Part 1 defines the limits of H2S partial pressure above which precautions against sulfide stress cracking (SSC) are considered necessary. Materials are considered susceptible to sulfide stress corrosion cracking if the partial pressure of H2S gas exceeds 0.05 psia (0.3 kPaa). Crude oil storage and handling facilities as well as water handling facilities operating at a total pressure below 65 psia (448 kPaa or 4.3 bara) are excluded from requirements of NACE/ISO 15156-1. Thus, piping systems containing H2S gas, with H2S partial pressure exceeding 0.05 psia or operating at a total pressure of 65 psia (448 kPaa) or greater are considered sour and are susceptible to SSC.
Material Considerations - General
When a piping component is intended to be used for sour service, the piping material specification and material description should clearly state the intent for use in sour service and the applicability of NACE MR0175/ISO 15156 standards. Part 2 of the standard identifies pre-qualified carbon and alloy steels that may be selected to provide SSC resistance. Similarly Part 3 identifies CRAs and other alloys that may be selected to provide SSC resistance. Additional testing is generally not required for the pre-qualified materials.
Pipes fabricated from carbon steel plate material are required to be tested for resistance to Hydrogen Induced Cracking (HIC) in accordance with the method and procedure described in NACE TM 0284. For seamless piping HIC testing is not required.
As per section A.2.5.1 of ISO 15156 - Part 2, Metallic coatings (electroplated and electroless plated), conversion coatings, thermal spraying, plastic coatings and linings are not considered acceptable for preventing SSC. Some operators have considered the use of nonmetallic coatings such as epoxies or phenolics for non-critical sour service applications at operating temperatures < 93°C (200°F).
Material Considerations - Hardness Requirements
Resistance to SSC can be achieved by controlling the hardness of parent materials, their welds and heat affected zones. ASME B31.3 - Para F723.4.4 (b)(6) refers to NACE MR0175/ISO 15156 - Part 2 for specifying the piping material hardness limit for carbon steel, and low and intermediate alloy steels when it is exposed to cyanides, acids, acid salts, or wet hydrogen sulfide due to the possibility of stress corrosion cracking.
Parent Metal Hardness
As per section A2.1.2 of ISO 15156 - Part 2, Carbon and low alloy steels are acceptable at hardness less than 22 Rockwell hardness (scale C) subject to containing less than 1 % nickel, are not free-machining steels and are used in one of the following heat-treatment conditions:
- Hot-rolled (applicable to carbon steels only)
- Annealed
- Normalized
- Normalized and tempered
- Normalized, austenitized, quenched, and tempered
- Austenitized, quenched, and tempered
As per section A2.1.3
- ASTM A 105 forgings are acceptable if the hardness is less 187 Brinell hardness
- ASTM A 234 grades WPB and WPC fittings are acceptable if the hardness is less than 197 Brinell hardness
Hardness of Welds
NACE standard also calls for certain metallurgical requirements to be met which involves control of hardness in weldments. Carbon steel welds exposed to sour service can be made resistant to SSC by avoiding the presence of hard microstructures in the welds. Section 7.3 of NACE/ISO 15156-2 addresses the hardness requirements of materials. Carbon equivalent (CE) is a measure of hardenability of carbon and alloy steels as a result of welding. The Carbon Equivalent is dependent on specific elements in the parent metal and is defined by the International Institute of Welding (IIW) as follows:
Carbon Equivalent (CE) = \(\frac{Mn}{6}\) + \(\frac{(Cr + Mo + V)}{5}\) + \(\frac{(Ni + Cu)}{15}\)
The Carbon Equivalent is limited to 0.42-0.43 for sour service by engineering specifications. The hardness value of the welds shall not exceed 22HRC (237HB, 248HV). The qualification of welding procedures for sour service should include hardness testing to meet the limits of NACE MR0175/ISO 15156.
For piping in sour service, dissimilar welds are not recommended. The reason is that the dissimilar welds will most probably contain high hardness zones which are susceptible to sulfide stress cracking.
Material Considerations - Chemical Composition
What are the limitations in sulfur content for carbon steel material to be in compliance with NACE / ISO 15156 requirements?
The level of sulfur in the steel is of particular importance and is defined in ISO 15156 - Part 2.
Typical maximum acceptable sulfur levels are as follows:
- Flat-rolled products (plates) - 0.003 % mass fraction.
- Seamless products - 0.01 % mass fraction.
- Conventional forgings - 0.025 % mass fraction and
- Castings are not normally considered sensitive to HIC (hydrogen-induced cracking) or SOHIC (stress-oriented hydrogen-induced cracking).
Material Consideration for Bolts and Fasteners
As per ISO 15156-Part 2, bolting that is exposed to the sour environment or that is buried, insulated, equipped with flange protectors or otherwise denied direct atmospheric exposure shall comply with the requirement of this standard.
- Bolts shall be A193 Grade B7M for temperatures down to -29C or A320 Grade L7M for temperatures down to -45C.
- Nuts shall be A194 Grade 2HM or A194 Grade 7M respectively and used at lower temperature limits stated above.
ASTM A193 Grade B7M bolting has lower strength than A193 Grade B7 bolting and hence it may be necessary to lower the piping class pressure ratings in sour service applications. Other option is to choose a gasket with a lower seating stress.
Non-metallic Materials: Valve O-Rings and Seals
Elastomers in the oil and gas industry are susceptible to a phenomenon called Rapid Gas Decompression (RGD) or Explosive Decompression (ED). High pressure H2S or CO2 gas can permeate into the interior of an elastomer seal, where it remains in a compressed state in the voids until the system is depressurised or pressure is released. When the pressure is released, the absorbed gas expands rapidly causing failure of the elastomeric seal. Explosive Gas Decompression is a significant problem in the oil and gas industry. Hence elastomeric seals are required to be tested and certified for resistance to explosive decompression. O-rings that are formulated to resist this type of failure are commonly referred to as RGD, ED Resistant, or AED (anti-explosive decompression) resistant O-rings. Nitrile rubbers (NBR) in general are not considered suitable for use in sour service. However hydogenated nitrile rubbers (HNBR) and fluoroelastomers such as Viton, Kalrez or Chemraz can be made AED resistant. PTFE (Thermoplastic) is AED resistant. It has however lower sealing capabilities than elastomers. O-rings made from PTFE have a higher leakage rate than those made from elastomers. Elast-O-Lion (Hydrogenated Nitrile Elastomer) manufactured by James Walker is an example of RGD or AED resistant elastomer. Selection of appropriate elastomer material is dependent on the fluid composition including its H2S content. Viton may show embrittlement due to H2S induced vulcanization effects.
Threaded and Socket Weld Connections
Threaded and socket weld connections present an integrity threat due to possibility of crevice corrosion, are not recommended for use in sour service piping. Hence their use shall be minimized. Threaded connections may be used downstream of instrument isolation valves if required.