Zhuojin Pipe Fitting Co., LtdFailure Analysis of Synthesis Tower Gas Outlet Elbow Due to Hydrogen Attack
Literature Overview
The paper by Li Chao (2007), published in Petroleum and Chemical Machinery, presents a failure analysis of a gas outlet elbow from the No. 2 synthesis tower at Hebei Qian'an Fertilizer Co., Ltd. The elbow experienced a burst failure after prolonged service at temperatures above 300°C. The analysis revealed that the failure was caused by hydrogen attack, a form of hydrogen-induced cracking that occurs when hydrogen reacts with carbon in the steel to form methane. The paper provides a detailed investigation of the failure mechanism, including the chemical reactions involved, the metallurgical changes in the material, and the resulting loss of mechanical properties.
Hydrogen attack is a well-known degradation mechanism in high-pressure hydrogen service, particularly in the petroleum refining and chemical industries. It is classified under the broader category of environmentally assisted cracking (EAC) and is distinct from hydrogen embrittlement and hydrogen blistering in its mechanism and characteristics.
Core Technical Content
The failure mechanism involves a series of chemical and metallurgical processes. At elevated temperatures (above 200°C), atomic hydrogen generated by the dissociation of molecular hydrogen at the pipe wall surface diffuses into the steel. Inside the steel, the atomic hydrogen reacts with carbon atoms or with iron carbide (Fe₃C) to form methane (CH₄). This reaction is described by the following equations:
| Reaction | Equation | Condition |
|---|---|---|
| Hydrogen dissociation | H₂ → 2H (atomic) | At steel surface, T > 200°C |
| Reaction with carbon | 4H + C → CH₄ | Inside steel, T > 200°C |
| Reaction with Fe₃C | 4H + Fe₃C → 3Fe + CH₄ | Inside steel, T > 200°C |
The formation of methane creates internal pressure at the reaction sites, leading to the formation of microvoids and microcracks. As the reaction progresses, these microvoids coalesce into larger voids and cracks, leading to a loss of mechanical integrity. The affected region exhibits a characteristic microstructure with decarburization and void formation, which can be observed through metallographic examination.
The key factors influencing hydrogen attack susceptibility are:
- Temperature: The reaction rate increases with temperature, with significant attack occurring above 200°C
- Hydrogen partial pressure: Higher hydrogen partial pressures increase the driving force for hydrogen absorption
- Steel composition: Carbon content, manganese content, and the presence of alloying elements affect susceptibility
- Microstructure: Pearlite, bainite, and martensite structures have different susceptibilities
- Residual stress: Stress concentrations can accelerate crack propagation
Failure Analysis Methodology
The failure analysis followed a systematic approach consistent with established practices in the field:
- Visual examination: Initial assessment of the failure location, fracture surface morphology, and external condition of the elbow
- Dimensional measurement: Verification of the elbow geometry, wall thickness, and any deformation or thinning
- Metallographic examination: Examination of the microstructure at the failure location and in unaffected regions
- Chemical analysis: Determination of the chemical composition of the material
- Mechanical property testing: Measurement of hardness, tensile strength, and impact energy
- Fractography: Examination of the fracture surface using optical microscopy and scanning electron microscopy (SEM)
- Service condition review: Analysis of the operating conditions, including temperature, pressure, and hydrogen partial pressure
The metallographic examination revealed decarburization and void formation in the region adjacent to the inner surface of the elbow. The voids were predominantly located at prior austenite grain boundaries and at the interface between ferrite and cementite phases. The fracture surface exhibited a mixed mode of failure, with both ductile and brittle characteristics, indicating that the material had undergone significant degradation prior to failure.
NACE MR0175 and Hydrogen Attack Resistance
The susceptibility of steel to hydrogen attack is evaluated according to NACE MR0175/ISO 15156, which provides guidelines for materials selection and testing for equipment in hydrogen and sour environments. The Nelson curve, a widely used tool for predicting hydrogen attack resistance, plots the maximum allowable operating temperature as a function of hydrogen partial pressure for different steel grades.
| Steel Grade | Maximum Temperature at 100 psia H₂ | Maximum Temperature at 1000 psia H₂ |
|---|---|---|
| Carbon steel (0.2% C) | ~200°C | ~150°C |
| Low alloy steel (Cr-Mo) | ~370°C | ~300°C |
| 9Cr-1Mo | ~425°C | ~370°C |
| 12Cr-1Mo | ~480°C | ~425°C |
The failure in this case occurred at temperatures above 300°C, which exceeds the recommended operating limits for carbon steel according to the Nelson curve. The material selection was therefore inadequate for the service conditions, leading to progressive degradation and eventual failure.
Countermeasures and Prevention
Several measures can be taken to prevent or mitigate hydrogen attack:
- Material selection: Use of hydrogen-resistant materials such as 9Cr-1Mo or 12Cr-1Mo steels for high-temperature hydrogen service
- Temperature control: Maintaining operating temperatures below the threshold for significant hydrogen attack
- Hydrogen partial pressure management: Reducing the hydrogen partial pressure through process modifications
- Regular inspection: Implementing a comprehensive inspection program including ultrasonic testing for internal voids and cracks
- Corrosion allowance: Providing adequate corrosion allowance to compensate for progressive wall thinning
- Hydrogen permeation barriers: Application of diffusion barrier coatings or overlays to reduce hydrogen absorption
Study Insights and Implications
This failure analysis provides a clear demonstration of the consequences of inadequate material selection for high-temperature hydrogen service. The systematic approach to failure analysis — combining visual examination, metallographic analysis, chemical analysis, and mechanical testing — is essential for understanding the root cause and implementing effective corrective measures. The case underscores the importance of adhering to material selection guidelines such as NACE MR0175 and the Nelson curve in design. For engineers involved in hydrogen service applications, the key lessons are the critical importance of temperature and pressure limits, the need for regular inspection of hydrogen-exposed equipment, and the value of failure analysis in improving design and material selection practices. The paper serves as a valuable case study for training and reference in the field of environmentally assisted cracking and materials selection for hydrogen service.