Corrosion fatigue of 304L stainless steel in simulated LWR water environment
Time: Mon 2025-09-29 10.00
Location: Kollegiesalen, Brinellvägen 8, Stockholm
Language: English
Subject area: Engineering Mechanics Solid Mechanics
Doctoral student: Mustafa Subasic , Material- och strukturmekanik, Solid Mechanics
Opponent: Professor Johan Moverare, Institutionen för ekonomisk och industriell utveckling (IEI) , Linköpings Universitet, Linköping, Sweden
Supervisor: Docent Pål Efsing, Material- och strukturmekanik; Docent Carl F. O. Dahlberg, Material- och strukturmekanik; Dr. Jean Smith, EPRI
QC 20256-09-09
Abstract
Fatigue and corrosion are two dominant mechanisms responsible for the failure of structural materials. Fatigue results from the progressive accumulation of damage under cyclic mechanical loading and is estimated to cause over 90 % of all mechanical service failures. Corrosion, caused by chemical or electrochemical reactions with the environment, leads to material loss and structural degradation. In Sweden, the annual cost of corrosion is estimated to be approximately 4 % of the national GDP. When these two phenomena interact, as in corrosion fatigue, the degradation process accelerates significantly. This is particularly critical for engineering components operating in harsh environments such as offshore structures, aerospace systems and industrial atmospheres.
In the context of the global transition toward a fossil-free society, the continued safe operation of existing nuclear reactors is essential to meet rising energy demands. Extending the service life of light water reactors (LWRs), without compromising human safety, is widely recognized as a critical step in this transition. This highlights the need for accurate predictive models that capture the complex interaction between mechanical fatigue and electrochemical corrosion of 304L stainless steel in LWR water environments.
A comprehensive mechanical characterization of 304L stainless steel was performed in air at both room temperature and 300 °C. This included tensile tests, fatigue tests, fatigue crack growth tests and cyclic stress-strain experiments in both the rolling and transverse directions. The results revealed isotropic behavior and reduced mechanical performance at elevated temperature. The cyclic plasticity response was modeled using a von Misesradial return-mapping algorithm incorporating Voce isotropic and Armstrong-Frederick kinematic hardening laws, demonstrating good agreement with experimental hysteresis loops. The mechanical characterization provided the foundation for the corrosion fatigue investigation.
To investigate mechanical-electrochemical degradation, strain-controlled corrosion fatigue tests were performed on hollow specimens in simulated boiling water reactor (BWR) water. The direct current potential drop (DCPD) technique was used in-situ to monitor crack initiation while corrosion potential was measured using a reference electrode. The results showed a pronounced reduction in fatigue life compared to air, especially at low strain amplitudes. Fractographic and microstructural analyses of fracture surfaces revealed that crack initiation preferentially occurred along shear planes within grains favorably oriented for slip, in conjunction with localized oxide film rupture and repassivation. These findings support an anodic slip dissolution mechanism and a clear relationship between strain amplitude and surface crack density was observed.
Based on the experimental insights, a crystal plasticity-based constitutive model was developed to simulate the coupled mechanical-electrochemical processes leading to corrosion fatigue crack initiation. Implemented in Abaqus through a UMAT, the model combines a phenomenological crystal plasticity framework with a corrosion damage law based on Gutman’s theory of mechanoelectrochemical interactions. The model parameters were calibrated using the experimental data and simulations were performed on a representative section of the hollow specimen. The model successfully captured key experimental features, including the localization of damage in favorably oriented grains. The predicted cycles to crack initiation agreed well with experimental results across multiple strain amplitudes.
The combined experimental program and modeling framework in this thesis enhance the predictive capabilities for corrosion fatigue crack initiation in nuclear environments. The work contributes to the long-term operation of nuclear reactor components and lays the groundwork for future research incorporating more complex environmental interactions.