In the petrochemical industry, hydrogen sulfide (H2S) corrosion is a widespread and highly hazardous phenomenon. By analyzing the corrosion mechanism of H2S, this blog introduces various forms of corrosion caused by wet H2S, including hydrogen embrittlement, hydrogen-induced cracking, sulfide stress corrosion cracking, and stress-induced hydrogen cracking. These types of corrosion are closely related to factors such as the mass concentration of H2S in the liquid phase, pH, temperature, and other conditions.
Pipeline Material Requirements and Selection
As essential energy sources for the development of the national economy, oil and natural gas have received significant attention from countries worldwide. Currently, the transportation of oil and natural gas resources in China primarily relies on pipelines, which are typically made of steel spiral-welded pipes. Due to the complex terrain and environmental conditions, these pipelines often suffer from various media corrosion over time, particularly from acidic media. To address this challenge, the text discusses the basic requirements for pipe materials from a design perspective and outlines the material selection standards applicable under various circumstances.
Importance of H2S Corrosion Research
With the large-scale development of high-sulfur gas fields, such as the Luojiazai gas field, there is a pressing need to study H2S corrosion-resistant pipe materials. Research on H2S corrosion-resistant pipe materials is crucial for extending the service life of pipelines, preventing accidents, and enhancing economic benefits. The text highlights the relevant standards and testing methods used in this field, underscoring the significance of this research for the petrochemical industry.
Principles for Selecting H2S Corrosion-Resistant Pipe Materials
When selecting materials for sulfur-containing oil and gas transportation pipelines, the possible occurrences of metal loss corrosion, hydrogen-induced cracking (HIC), and sulfide stress cracking (SSCC) must be considered. The general corrosion resistance and stress corrosion resistance of the pipe materials must be taken into consideration.
Russian experts believe that the greatest danger in pipelines exposed to H2S is not general corrosion (metal loss) but instead cracking related to hydrogen permeation in metals. The occurrence of SSCC is much faster than that of general corrosion. Therefore, in pipelines where H2S is present, the stress corrosion resistance of the pipe materials is significant.
To prevent H2S stress corrosion, the “NACE MR-0175” standard recommends that the hardness limit of pipelines in acidic media should not exceed 248 HV500 or 22 HRC. From the perspective of the chemical composition of the pipe materials, reducing the content of impurities such as oxygen and sulfur in steel can enhance the ability to resist sulfide stress corrosion.
The Grizzly Valley gathering system in Canada employs a sulfur-containing dry gas transportation process, with a pipeline length exceeding 1770 km, primarily composed of three diameters and wall thicknesses: ∅273 mm × 5.2 mm, ∅508 mm × 9.5 mm, and ∅610 mm × 11.4 mm. All pipelines are constructed according to the CSA-Z245.1 Grade 52 (359) standard, which specifies precise requirements for the chemical composition of the steel used for transporting sulfur-containing dry gas.
H2S Corrosion Resistance Table
Additionally, the maximum Rockwell hardness (HRC) of the steel pipe is required to be 20 HRC, and the maximum Vickers hardness (HV) value is required to be 238.
Texas, in the United States, also has clear regulations for natural gas pipelines with H2S concentrations exceeding 0.01%. To ensure the materials’ crack resistance, the pipes must undergo Charpy V-notch impact testing, and all welds should be subjected to radiographic testing to eliminate internal stresses. Currently, there are two internationally recognized standards for determining the stress corrosion resistance of pipe materials: the American NACE standard and the British BP standard.
Types of H2S Corrosion-Resistant Pipe Materials
Based on the development trends in natural gas pipeline construction, the following two types of pipelines show greater potential for development: one is a special alloy steel that is low-cost, corrosion-resistant, resistant to sulfur stress cracking, has good weldability, and high hardness; the other is artificially synthesized glass fiber-reinforced plastic pipes.
Special Alloy Steel
1 HDR Duplex Stainless Steel
HDR is an ultra-low carbon, high chromium (H), duplex (D), corrosion-resistant (R) stainless steel composed of approximately 50% austenite and 50% ferrite. It possesses the toughness of austenitic stainless steel and comparable stress corrosion cracking resistance to ferritic stainless steel. It exhibits high mechanical properties, along with good weldability, corrosion resistance, and wear resistance, making it suitable for use in acidic media.
Due to the composition of HDR, its surface forms a dense oxide protective film rich in chromium, nickel, molybdenum, and nitrogen, effectively preventing corrosion of the internal matrix by ions. Over time, the protective film becomes more complete and dense, effectively preventing the expansion of corrosion, making it an excellent corrosion-resistant material. Additionally, tests have shown that HDR exhibits outstanding resistance to pitting, crevice corrosion, stress corrosion, and intergranular corrosion among stainless steel series.
2 Type Stainless Steel
The 18-8 type of stainless steel is the most basic form of stainless steel, represented by 304 (AISI grade). This type of material is prone to intergranular corrosion and pitting near the welds due to the precipitation of chromium compounds. Attention should be paid to the following points during the welding process: solid solution annealing, limiting carbon content, and using appropriate welding rods. A better approach is to add trace elements such as titanium and niobium, for example, 1Cr18Ni9Ti (widely used in Sichuan gas fields), 0Cr18Ni9Ti, and 00Cr18Ni9Ti.
The difference in the mechanisms of crevice corrosion and pitting is as follows: crevice corrosion occurs when an oxide film forms a protective layer; pitting occurs when the oxygen content decreases, making it difficult for the oxide film to form or causing it to peel off. Higher temperatures exacerbate pitting, and the quantity and intensity of pitting are mainly determined by the degree of attachment of biofouling. The occurrence of pitting in stainless steel in flowing media is significantly reduced.
Artificially Synthesized Glass Fiber Reinforced Plastic
1 PE Pipes
PE pipes are made primarily from polyethylene resin, with necessary additives, and are continuously extruded into shape on production lines. They have evolved from various materials, including CAB, PVC, ABS, PV, and PE. PE pipes exhibit strong corrosion resistance, capable of withstanding multiple chemical media except for a few strong oxidizers, and they do not suffer from electrochemical corrosion. They also offer a long service life, ultra-low friction, good impact resistance, reliable connections, and are lightweight, making them suitable for transporting sulfur-containing oil and gas.
2 Glass Fiber Reinforced Composite Pipes
Based on the high-pressure resistance characteristics of glass fiber-reinforced pipes, these pipes utilize heat-resistant, corrosion-resistant PVC as the inner lining, high-strength glass fiber as the intermediate layer, and high-density polyethylene as the outer layer, all of which are combined through specialized processes.
They are characterized by being lightweight (only one-third the weight of steel pipes), high-pressure resistance, corrosion resistance, high tensile strength, easy installation, and smooth inner walls. This design addresses the weaknesses of glass fiber pipes, including poor permeability and low mechanical strength.
The pipes can be connected using threaded, socket, or flange methods. The pressure resistance can be selected based on the thickness of the glass fiber intermediate layer, generally ranging from 2 to 16 MPa, suitable for crude oil gathering, water injection, polymer injection, and sewage pipelines in gas field wells. However, they are difficult to repair, and the entire pipe must be replaced if damaged.
3 Steel-Plastic Reinforced Composite Pipes
Depending on the reinforcement and manufacturing processes, these can be categorized into steel wire mesh-reinforced plastic composite pipes, steel plate mesh-reinforced plastic composite pipes, and steel wire woven mesh-reinforced plastic composite pipes.
Steel Wire Mesh Reinforced Plastic Composite Pipes:
These feature a welded steel wire mesh framework as the reinforcement, with high-density polyethylene plastic as the continuous base material, forming a new type of double-sided anti-corrosion pressure-resistant pipe. This effectively resolves delamination defects caused by discontinuities in physical properties (such as elastic modulus, thermal expansion coefficient, and thermal conductivity) and chemical properties at the interface of metal-based composite materials.
They have higher strength, rigidity, and impact resistance than plastic pipes, as well as similar low thermal expansion, creep resistance, and UV resistance. Additionally, they have lower thermal conductivity, smooth inner walls, and reduced head loss compared to mesh pipes by 30%. Connections typically use electric fusion or flange connections, and the pipes are lightweight, easy to install, with pressure ratings from 1.8 to 3.5 MPa and a service temperature range of -40 to 70°C, with a lifespan of up to 50 years.
However, they have poor resistance to external damage, limited temperature tolerance (generally below 75°C), and high repair requirements, needing specialized personnel and equipment.
Steel Plate Mesh Reinforced Plastic Composite Pipes:
These use perforated steel strips as the reinforcement, formed into a framework on the production line, then welded and injected with high-density polyethylene to create a composite. Their characteristics are similar to those of steel wire mesh-reinforced plastic composite pipes, with typical working pressures ranging from 1 to 1.6 MPa. Connections are typically made using electric fusion or flange methods, which offer the advantage of lower material costs.
Steel Wire Woven Mesh Reinforced Plastic Composite Pipes:
Following the manufacturing process of high-pressure rubber hoses, they first extrude a plastic core pipe. Then, they wrap or weave single (or multiple) layers of high-quality steel wire around the core using a wire wrapping machine or weaving machine. A rubber layer is then extruded to ensure good bonding between the steel wire and the inner and outer layers, followed by extrusion of the outer plastic layer.
Their characteristics are similar to those of steel wire mesh and steel plate mesh-reinforced plastic composite pipes, with typical working pressures typically depending on the number of woven layers, generally around 1.6 MPa, providing significant cost advantages for large-diameter pipes.
Overall, alloy steel offers superior performance and is currently widely used; however, its cost and scale have not yet met industrial requirements. Conversely, artificially synthesized glass fiber reinforced plastic pipes have broader development prospects. They possess excellent properties, including high strength, good toughness, corrosion resistance, and resistance to mechanical damage, as well as low costs, convenient transportation, and ease of construction and maintenance.
Conclusion
For sulfur-containing oil and gas gathering pipelines, the electrochemical corrosion caused by hydrogen sulfide and the presence of hydrogen leads to a decrease in the fracture toughness and physical, chemical, and mechanical properties of the pipeline steel. This can easily result in stress corrosion cracking and hydrogen-induced cracking during transport, compromising the safety of gathering pipelines and affecting their service life.
Therefore, it is essential to carefully consider the stress corrosion resistance of pipe materials. We should learn from developed countries in oil and gas pipeline construction and align with international standards. At the same time, China should strengthen testing and research on H2S corrosion-resistant materials to develop new, applicable H2S corrosion-resistant pipe materials.
Sulfur-resistant steel pipes are suitable for acidic natural gas containing H2S (not exceeding 30 g/m³). The manufacturing materials for sulfur-resistant steel pipes primarily aim to prevent sulfide cracking and hydrogen-induced cracking.
According to “NACE MR0175,” the raw material hardness should not exceed 22 HRC, with nickel content not exceeding 1%. After undergoing hot rolling, annealing, normalizing, and tempering, they can be selected for use. For A105 material, after heat treatment, the hardness should not exceed 187 HBW, and for A234 material, it should not exceed 197 HBW. The domestic 20# steel corresponds to the American A234 grade steel, which can meet the sulfur resistance requirements after heat treatment.
H2S Corrosion-Resistant Oil Casing
H2S corrosion-resistant, special-threaded oil casing refers to oil and gas casing used in environments with H2S acid corrosion and specific gas sealing properties. The casing material is made from a specially developed chromium-molybdenum steel designed to resist H2S stress corrosion. This steel type exhibits high hardenability and temper stability, allowing tempering at specific temperatures to yield fine and uniform tempered martensite.
Through fine-tuning of composition and targeted heat treatment for different steel grades, it meets the required hardness and strength while exhibiting excellent resistance to sulfide stress corrosion cracking (SSSC). The oil casing connection joints utilize gas-tight threaded designs, independently developed by seamless steel pipe manufacturers, which are suitable for various complex and harsh working conditions.
Currently, seamless steel pipe manufacturers’ H2S stress corrosion-resistant, special-threaded oil casings have passed testing by authoritative domestic institutions and are widely used in major domestic oil fields. Among them, the H2S stress corrosion-resistant C110 steel grade CBS3 casing has met the stringent high evaluation requirements of the CAL IV test in the “ISO 13679 Petroleum and Natural Gas Industry. Procedures for Testing the Connections of Casing and Tubing.”
Common H2S Stress Corrosion-Resistant Steel Grades Used by Seamless Steel Pipe Manufacturers:
- CB80S, CB80SS, C90, CB90S, CB90SS, T95, CB95S, CB95SS, C110, CB110S, CB110SS.
- Sulfur Resistance Level: “S” indicates ordinary sulfur resistance; “SS” indicates high sulfur resistance.
The oil casing connection joints use gas-tight threaded designs independently developed by seamless steel pipe manufacturers:
- CBS1: Seamless steel pipe manufacturer, gas-tight type one special thread (for tubing and casing).
- CBS2: Seamless steel pipe manufacturer, gas-tight type two special thread (for tubing and casing).
- CBS3: Seamless steel pipe manufacturer, gas-tight type three special thread (for tubing and casing).
- CBSJ: Economic low-pressure sealing special thread (for tubing and casing).
- CB-NUFJ: Seamless steel pipe manufacturer’s direct connection with a gas-tight special thread (for tubing and casing).
- EU-HT/BC-HT: High-torque resistance top special-threaded oil casing.
Standards (partial):
- API Spec 5CT Specification for Casing and Tubing.
- ANSI/NACE MR0175/ISO 15156 Materials for Use in H2S-Containing Environments in Oil and Gas Production.
- ANSI/NACE TM0177 Laboratory Evaluation of Metals for Stress Corrosion Cracking and Sulfide Stress Cracking in H2S Environments.
- API Spec 5B Specification for the Processing, Measurement, and Inspection of Casing, Tubing, and Line Pipe Threads.
- ISO 13679/GB/T 21267 Procedures for Testing the Connections of Casing and Tubing in the Petroleum and Natural Gas Industry.
| Steel Grade | Name | Yield Strength (MPa) | Tensile Strength (MPa) | Elongation | Impact Toughness | Hardness Max (HRC) | Applicable Range (PH2S) |
|---|---|---|---|---|---|---|---|
| 80 | CB80S | 552 | 655 | 655 | According to the API 5CT formula | 23 | 0.34 KPa – 100 KPa |
| CB80SS | PH2S > 100 KPa | ||||||
| 90 | C90 | 621 | 724 | 689 | According to API 5CT C90 requirements | 25.4 | ≥ 0.34 KPa |
| CB90S | 0.34 KPa – 100 KPa | ||||||
| CB90SS | PH2S > 100 KPa | ||||||
| 95 | T95 | 655 | 758 | 724 | According to API 5CT T95 requirements | 25.4 | ≥ 0.34 KPa |
| CB95S | 0.34 KPa – 100 KPa | ||||||
| CB95SS | PH2S > 100 KPa | ||||||
| 110 | CB110S | 758 | 862 | 793 | According to API 5CT C110 requirements | 30 | ≥ 0.34 KPa |
| 0.34 KPa – 100 KPa | |||||||
| CB110SS/C110 | PH2S > 100 KPa |
| Steel Grade | Corrosion Resistance Requirements |
|---|---|
| H2S Stress Corrosion Test (SSC) | According to NACE TM 0177 Method A solution, no cracking after 720 hours |
| ф6.35 Sample | |
| ф3.81 Sample | |
| C90 T95 CB110S | 80% |
| CB80S CB90S CB95S C110 | 85% |
| CB80SS CB90SS CB95SS | 90% |
| C90 T95 (PSL-2) | 90% |
| Joint Type | Internal Pressure and Crush Resistance | Tensile Efficiency | Compression Efficiency | Applicable Well Conditions |
|---|---|---|---|---|
| CBS1 | Equivalent to the pipe body | 100% pipe body | 60% pipe body | Less severe well conditions, shallow and medium-depth oil and gas wells |
| CBS2 | 100% pipe body | 60% pipe body | Severe, medium, and deep oil and gas wells | |
| CBS3 | 100% pipe body | 60% pipe body | More severe well conditions, deep wells, ultra-deep wells, horizontal wells | |
| CBS3C | 100% pipe body | 100% pipe body | ||
| CBSJ | 100% pipe body | 60% pipe body | Low-pressure gas-sealed wells, ordinary horizontal wells, and large displacement wells | |
| CB-NUFJ | 45%~66% pipe body | 45%~66% pipe body | Repair wells, tail pipes, etc. | |
| EU-HT/BC-HT | Equivalent to API EU/BC performance | Ordinary oil wells |
Product Specification Range:
- Outer Diameter: Φ60.32mm to Φ219.08mm;
- Wall Thickness: 4.83mm to 22.2mm.
