Hydrogen Concentration Distribution in Surfaced Structures and Its Effect on Peel Fracture

Literature Overview

The paper by Xu Ying, Yuan Leping, Lin Dongliang, Lin Jianhong, Wang Zhengdong, and Wu Dongdi, published in Journal of Chinese Society for Corrosion and Protection (1995, Vol. 15, No. 2, pp. 112-118), addresses a critical issue in the integrity assessment of surfaced (clad) structures: the distribution of hydrogen concentration within the layered structure and its influence on peel fracture at the fusion line. This research, conducted at Shanghai Jiao Tong University and East China University of Science and Technology, employs finite element analysis (FEA) to model hydrogen diffusion behavior under both electrolytic charging and high-pressure autoclave conditions.

Core Technical Content

Surfaced structures, which consist of a base metal layer and a corrosion-resistant overlay layer bonded by fusion welding, are widely used in the chemical, petrochemical, and nuclear industries to combine the mechanical strength of a low-alloy base metal with the corrosion resistance of a high-alloy cladding material. However, these structures are susceptible to hydrogen-induced cracking (HIC) and blistering, particularly at the fusion line where the metallurgical and mechanical properties undergo abrupt changes.

The study employs finite element analysis to calculate the hydrogen concentration distribution within surfaced structures under two different hydrogen charging conditions:

The FEA model accounts for hydrogen diffusion governed by Fick's laws, with appropriate boundary conditions representing the hydrogen flux at the specimen surface. The model also considers the effect of microstructure on hydrogen diffusion, including the influence of grain boundaries, phase boundaries, and defects on hydrogen trapping and transport.

Key Findings and Technical Interpretation

The study's principal findings are of significant importance for the assessment of hydrogen damage susceptibility in surfaced structures:

1. Electrolytic charging equivalence: The FEA results demonstrate that electrolytic charging can produce hydrogen concentration peaks near the fusion line that are comparable in magnitude to those achieved by high-pressure autoclave charging. This finding is significant because it validates the use of electrolytic charging as a practical and cost-effective alternative to high-pressure autoclave testing for hydrogen damage assessment.
2. Time efficiency: The time required to achieve the peak hydrogen concentration near the fusion line is shorter under electrolytic charging conditions compared to high-pressure autoclave conditions. This is attributed to the higher hydrogen flux at the specimen surface under electrolytic charging, which drives faster hydrogen penetration.
3. Parameter sensitivity analysis: The study systematically examines the influence of various parameters on the hydrogen concentration peak and the resulting peel fracture susceptibility:

• Increasing the charging current density increases the hydrogen flux at the surface, leading to higher hydrogen concentration peaks and greater peel fracture tendency.
• Extending the charging time allows hydrogen to diffuse further into the specimen, increasing the hydrogen concentration at the fusion line.
• Reducing the base metal thickness decreases the diffusion distance to the fusion line, resulting in higher hydrogen concentrations at the interface.

These findings provide a quantitative framework for understanding and predicting hydrogen damage in surfaced structures, and they have direct implications for the design, testing, and qualification of clad equipment.

Engineering Practice Integration

The implications of this research for engineering practice are substantial:

1. Design considerations: The study's findings suggest that the base metal thickness is a critical parameter in determining hydrogen damage susceptibility. For applications involving hydrogen service, the base metal thickness should be optimized to minimize the hydrogen concentration at the fusion line while maintaining adequate mechanical strength.
2. Testing methodology: The validation of electrolytic charging as an equivalent alternative to high-pressure autoclave testing provides a practical and economical method for hydrogen damage assessment. This is particularly valuable for small-scale testing and for laboratories that do not have access to high-pressure autoclave facilities.
3. Qualification procedures: The parameter sensitivity analysis provides a basis for developing qualification procedures for surfaced structures in hydrogen service. The procedures should include hydrogen charging testing under conditions that simulate the maximum expected hydrogen exposure, with appropriate consideration of the effect of charging current density, time, and specimen geometry.
4. Mitigation strategies: The study's findings support the use of several mitigation strategies to reduce hydrogen damage susceptibility in surfaced structures, including the use of low-hydrogen welding consumables, post-weld heat treatment to relieve residual stresses, and the selection of cladding materials with high hydrogen permeation resistance.

Study Insights and Implications

This paper represents an important contribution to the understanding of hydrogen damage mechanisms in surfaced structures. The use of finite element analysis to model hydrogen diffusion is a powerful approach that provides insights that are difficult to obtain through experimental testing alone. The validation of the FEA model against experimental data from both electrolytic charging and high-pressure autoclave conditions provides confidence in the predictive capability of the model.

The study's emphasis on the fusion line as the critical location for hydrogen damage is consistent with the metallurgical understanding of surfaced structures. The fusion line is a region of microstructural and compositional heterogeneity, where the base metal and cladding material interact to form a complex microstructure that may include martensite, retained austenite, and various carbide phases. These microstructural features can act as hydrogen traps, increasing the local hydrogen concentration and promoting crack initiation.

The research also highlights the importance of considering the interaction between hydrogen concentration and mechanical stress in the assessment of hydrogen damage susceptibility. The peel fracture mode observed in the study is a result of the combined action of hydrogen embrittlement and tensile stress at the fusion line. The residual stresses from welding, the thermal stresses from the post-weld heat treatment, and the operational stresses all contribute to the stress state at the fusion line and must be considered in the assessment of hydrogen damage risk.

Reference Value and Outlook

This paper provides a rigorous analytical framework for understanding hydrogen damage in surfaced structures and offers practical guidance for the design, testing, and qualification of clad equipment. The finite element modeling approach presented here can be extended to more complex geometries and loading conditions, providing a valuable tool for the integrity assessment of surfaced components in hydrogen service. Future research could focus on the development of more sophisticated FEA models that incorporate the effect of microstructure on hydrogen trapping, the development of hydrogen permeation barrier coatings, and the investigation of novel cladding materials with improved hydrogen resistance. The integration of this research with modern digital twin technologies for equipment integrity management could further enhance the reliability and safety of surfaced structures in hydrogen service.