Chemomechanical modeling of hydrogen-induced degradation at high temperatures

Vijayvargia, Kshitij

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https://hdl.handle.net/2142/132751 Copy

Description

Title

Chemomechanical modeling of hydrogen-induced degradation at high temperatures

Author(s)

Vijayvargia, Kshitij

Issue Date

2025-11-10

Director of Research (if dissertation) or Advisor (if thesis)

Sofronis, Petros

Doctoral Committee Chair(s)

Sofronis, Petros

Committee Member(s)

• Sehitoglu, Huseyin
• Wharry, Janelle P.
• Krogstad, Jessica A.

Department of Study

Mechanical Sci & Engineering

Discipline

Theoretical & Applied Mechans

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Hydrogen
• chemomechanics
• creep, fracture
• lifetime prediction

Abstract

Degradation of structural materials by hydrogen gas at high temperatures poses serious challenges to the safety of hydrogen technologies operating at elevated temperatures such as solid oxide fuel and electrolyser cells, hydrogen gas turbines, etc. The objective of this dissertation is to develop mechanism-based methodologies for design of structural components exposed to hydrogen gas at high temperatures. Steel components exposed to hydrogen gas at high temperatures and pressures are particularly susceptible to a mode of degradation called High Temperature Hydrogen Attack (HTHA). During HTHA internal hydrogen reacts with carbides in the steel to form methane gas bubbles which results in decarburization and a loss in strength. These bubbles pressurized by methane gas can nucleate at grain boundaries and at particle/matrix interfaces. Adjacent bubbles grow and link up to form microcracks which can ultimately grow and can lead to fracture. The design of steel equipment against HTHA is primarily based on the use of the Nelson curves published by the American Petroleum Institute (API) which are empirical and do not account for the underlying failure mechanisms, the material microstructure, and the operating conditions, e.g., applied stress level and time. The API publication presents a series of Nelson curves specific to various steel compositions that are used extensively in the refining and petrochemical industries. These curves establish the safe operation regimes in a temperature vs. hydrogen pressure diagram. Despite the long-standing use of Nelson curves, recurring failures—notably the 2010 Tesoro accident—underscore the need for a deeper understanding of HTHA. With this pressing need for reliable design guidelines, this dissertation proposes methodology for mechanism-based assessment of material degradation. Recent experimental evidence suggests that the nucleation and growth of HTHA-induced methane bubbles near pre-existing cracks are the most detrimental to material integrity. To this end, a fracture mechanics based methodology is proposed for reliability assessment of steel components. Simulations are conducted to predict methane bubble growth and coalescence near crack tips in 2.25Cr–1Mo steel under specified crack sizes in the inner diameter surface of a vessel, hydrogen pressures up to 20 MPa, and operating temperatures in the range of 400 to 600 °C. The model accounts for the coupled effects of creep and grain boundary diffusion, incorporating the influence of stress triaxiality ahead of the crack tip on bubble evolution. By treating the time to microcrack formation as a conservative estimate of component failure, Nelson-type curves were generated, mapping time to failure on hydrogen pressure–temperature diagrams for various crack sizes. These flaw-sensitive design curves represent a promising advancement over the empirical API Nelson curves, offering improved predictive capability while preserving the diagram's simplicity. While most HTHA studies have focused on temperatures around 400–600 °C, carbon steels can experience HTHA damage at much lower temperatures as well—around 250 °C, at which the Tesoro accident occurred. We extend the fracture mechanics approach of design against HTHA to lower temperatures ~250 °C. To enable this design approach, two key developments are pursued. 1.) Existing models for methane gas generation during HTHA are based on very high-temperature (600–800 °C) experiments, which exceed typical HTHA conditions for carbon steels. We revisit the mechanism of methane formation in carbon steels, and a coupled chemical kinetics and micromechanics model is proposed which addresses methane and hydrogen gas formation along with simultaneous decarburization and bubble growth over a wide temperature range. The energetics of the chemical reactions taking place at the ferrite-matrix/bubble interface are established through atomistic calculations. Model calculations unveil the relationship between the rates of hydrogen migration to the bubble interface, carbon and hydrogen atom reactions for methane formation, and attendant volumetric bubble growth. 2.) Currently, little is known about the effect of hydrogen on the deformation of carbon steels at temperatures relevant to HTHA. To this end, a physically based constitutive model is developed which is based on the thermally activated motion of dislocations in the presence of hydrogen solutes. Through a series of stress relaxation and differential temperature experiments, the key role of hydrogen in aiding the motion of dislocations is identified. The constitutive model is implemented in a finite element program and is used to calculate the effect of HTHA on reduction of fracture toughness. The onset of crack growth is determined by the coalescence of a pre-existing crack tip with a nearby methane-filled grain boundary bubble, identifying a critical stress intensity factor. From an operational standpoint, shutdown and startup activities are known to elevate the risk of HTHA failure, yet their impact remains unaccounted for in previous studies. We investigate the impact of shutdown/startup cycles on HTHA progression, emphasizing the thermo-chemo-mechanical effects of operational temperature and pressure fluctuations. Notably, during shutdown, methane-filled bubbles experience a drop in internal pressure due to cooling, however upon restart, this pressure is rapidly restored, creating mechanical cycling that can drive fatigue-like damage. This mechanism—previously unaddressed in HTHA literature—is demonstrated in this dissertation through simulations showing plastic strain jumps near bubble surfaces under repeated thermal cycles. In addition to HTHA, hydrogen accelerates creep rupture in structural materials by increasing vacancy concentrations and enhancing dislocation climb. The last contribution of this dissertation is a physically based creep constitutive model rooted in the thermodynamics of hydrogen trapping at vacancies, providing a predictive framework for hydrogen-enhanced creep behavior at elevated temperatures. Based on experimental evidence, we posit that hydrogen’s influence on creep can be described by its effect on creep activation energy, specifically through its role in reducing the vacancy formation energy. The predictive capability of the proposed model is evaluated by comparison with creep experiments conducted on pure iron, which assess the effects of hydrogen pressure and temperature. The comparisons demonstrate excellent agreement of model predictions with experimental measurements, validating its applicability to predicting hydrogen-accelerated creep.

Graduation Semester

2025-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/132751

Copyright and License Information

Copyright 2025 Kshitij Vijayvargia

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