22/07/2023
Carbon Steel for HIC Resistance and Sour Service Applications
Carbon steel is a versatile and widely used material in various industries, including oil, gas, and petrochemical sectors. However, in environments containing hydrogen sulfide (H2S), carbon steel is susceptible to hydrogen-induced cracking (HIC), which can lead to catastrophic failures. To ensure the integrity and safety of equipment and pipelines, it is crucial to use carbon steel that is resistant to HIC and suitable for sour service applications.
Understanding HIC and Sour Service Environments
HIC is a form of hydrogen-induced cracking that occurs when carbon steel is exposed to H2S. The presence of hydrogen atoms from the corrosion reaction between the environment and the steel can lead to the formation of cracks. These cracks can be difficult to detect and can result in sudden failures, making HIC a significant concern in industries dealing with sour service environments.
Sour service refers to environments containing H2S gas, with a partial pressure exceeding 0.05 psia or operating at a total pressure of 65 psia (448 kPaa) or greater. In these environments, carbon steel piping and equipment are susceptible to sulfide stress corrosion cracking (SSC), a type of HIC that occurs under the combined action of tensile stress and corrosion.
NACE MR0175/ISO 15156 Standards for Sour Service
To address the challenges posed by HIC and SSC, the National Association of Corrosion Engineers (NACE) developed the MR0175/ISO 15156 standards. These standards provide guidelines for the selection and qualification of materials, including carbon and low-alloy steels, corrosion-resistant alloys (CRAs), and other alloys, for use in oil and gas production and natural gas treatment plants in H2S-containing environments.
NACE MR0175/ISO 15156 consists of three parts:
Part 1: General principles for the selection of cracking-resistant materials
Part 2: Cracking-resistant carbon and low-alloy steels
Part 3: Cracking-resistant CRAs and other alloys
These standards outline the requirements and recommendations for material selection based on the specific H2S conditions and potential risks to health, safety, and equipment integrity.
Carbon Steel Grades for HIC Resistance
Part 2 of NACE MR0175/ISO 15156 focuses on cracking-resistant carbon and low-alloy steels suitable for sour service applications. These steels are pre-qualified based on their composition, heat treatment, and mechanical properties to ensure resistance to HIC and SSC.
Low Carbon Steel
Low carbon steel, also known as mild steel, is the most common type of carbon steel used in various applications. It has a carbon content ranging from 0.05% to 0.25% and exhibits low hardness and cost. Low carbon steel is highly ductile, tough, and weldable, making it suitable for a wide range of structural and industrial uses.
Medium Carbon Steel
Medium carbon steel contains a carbon content ranging from 0.30% to 0.50%. It has higher strength and hardness compared to low carbon steel but sacrifices some ductility and toughness. Medium carbon steel is commonly used in machinery parts, gears, axles, and crankshafts that require a balance between strength and machinability.
High Carbon Steel
High carbon steel has a carbon content ranging from 0.60% to 1.00%. It offers the highest hardness and strength among carbon steels but has reduced ductility. High carbon steel is commonly used in applications such as cutting tools, springs, and components that require wear resistance and hardness.
HIC Resistant Carbon Steel Grades
In sour service applications, it is crucial to use carbon steels that have been tested and proven to be resistant to HIC. NACE MR0175/ISO 15156 provides guidelines for the selection of HIC resistant carbon steel grades. These grades undergo specific testing to ensure their suitability for sour service environments.
Testing for HIC Resistance
To verify the HIC resistance of carbon steel, various testing standards are followed, including NACE TM0284. This testing standard provides guidelines for evaluating the resistance of carbon and low-alloy steels to HIC.
The HIC testing process involves subjecting steel samples to a test solution that simulates the conditions of the sour service environment. The samples are then evaluated for the presence of hydrogen-induced cracks using ultrasonic procedures and metallographic evaluation techniques.
The latest revision of NACE TM0284 in 2016 introduced fitness-for-purpose testing, which takes into account the specific application and test environment when determining the acceptance criteria for HIC testing. This allows for more accurate evaluations based on the intended use of the material.
Material Considerations for Sour Service
When selecting carbon steel for sour service applications, several factors must be considered to ensure the material's suitability and HIC resistance. These considerations include:
Steel Quality
The quality of the steel plate used plays a significant role in its resistance to HIC. Well-controlled manufacturing processes and the absence of impurities contribute to better HIC resistance. Steel suppliers and manufacturers should adhere to strict quality control measures to ensure the integrity of the material.
Hardness Requirements
Controlling the hardness of carbon steel is crucial for HIC resistance. Hardness can be controlled through proper heat treatment and alloying elements. NACE MR0175/ISO 15156 provides specific hardness limits for parent materials, welds, and heat-affected zones to prevent the formation of hard microstructures that are susceptible to HIC.
Welding Considerations
Welding can introduce residual stresses and microstructural changes in carbon steel, affecting its HIC resistance. Proper welding techniques, including preheating, post-weld heat treatment, and filler material selection, are essential to minimize HIC risks in welded joints.
Coatings and Linings
While metallic coatings are not considered effective in preventing SSC, non-metallic coatings such as epoxies or phenolics may be suitable for non-critical sour service applications at lower operating temperatures. The compatibility of coatings and linings with the sour service environment should be thoroughly evaluated.
Benefits and Limitations of Carbon Steel for Sour Service
Carbon steel offers several advantages for sour service applications, including its widespread availability, cost-effectiveness, and ease of fabrication. Carbon steel is also highly versatile, with different grades and compositions available to meet specific requirements.
However, carbon steel does have limitations in sour service environments. It is susceptible to HIC and SSC under certain conditions, necessitating careful material selection, testing, and maintenance. Regular inspections, monitoring, and preventive measures are crucial to ensure the integrity and safety of carbon steel equipment and pipelines in sour service applications.
Carbon Steel vs. Stainless Steel for Sour Service
While carbon steel is commonly used in sour service applications, stainless steel is another option worth considering. Stainless steel offers excellent corrosion resistance, including resistance to HIC, making it suitable for more demanding sour service environments.
The choice between carbon steel and stainless steel depends on various factors, including the severity of the sour service environment, temperature, pressure, and the specific requirements of the application. Consulting with corrosion and materials experts can help determine the most suitable material for a particular sour service application.
Conclusion
Carbon steel is a widely used material in various industries, including sour service applications in the oil, gas, and petrochemical sectors. To ensure the integrity and safety of equipment and pipelines in sour service environments, it is essential to select carbon steel grades that are resistant to HIC. Compliance with NACE MR0175/ISO 15156 standards and proper testing can help mitigate the risks associated with HIC and ensure the long-term performance of carbon steel in sour service applications.
By considering the quality of steel, hardness requirements, welding considerations, and appropriate coatings and linings, the risks of HIC in carbon steel can be minimized. Regular inspections, monitoring, and preventive maintenance are crucial for the ongoing integrity of carbon steel equipment in sour service environments.
While carbon steel is a commonly used material for sour service applications, stainless steel offers enhanced corrosion resistance and may be suitable for more severe sour service conditions. The choice between carbon steel and stainless steel should be based on careful evaluation of the specific requirements and risks associated with the sour service environment.
With proper material selection, testing, and maintenance, carbon steel can provide reliable performance in sour service applications, contributing to the overall safety and efficiency of the oil, gas, and petrochemical industries.
