Aging degradation characteristics and long-term performance of structural materials for energy conversion systems

Degradation of material used in the safety boundary components of energy conversion systems can be regarded as critical phenomena, which affects the structural integrity and safety and reduces the utilization factor of the power plant resulting in higher economic costs. A hollow cylindrical specimen was designed to evaluate the effect of the surface condition on the initiation and short crack growth behavior of environmentally assisted cracking (EAC), including stress corrosion cracking and environmental fatigue in connection with the structural integrity assessment. A typical environment of pressurized high-temperature water and flow was considered in the cylindrical specimens, where the inner surface was exposed to the environment. Drilling and honing were used to achieve the surface finish of the specimens. Drilled specimens exhibited a higher EAC initiation and a short crack growth behavior compared with the honed specimens. This reveals that the work-hardened layer is critical in EAC. The oxidation behavior of the hardened layer and the oxide film with respect to hydrogen-accelerated oxidation (HAO) was investigated to examine the effect of hydrogen in the metal on the oxidation in high-temperature water. A chemical fatigue test was proposed for the evaluation of oxidation under cyclic exposure to high dissolved hydrogen (DH) and high dissolved oxygen (DO). The result showed that the oxide film was affected by the cyclic exposure to high DH and high DO, suggesting HAO. A mechanical study suggests that a vacancy introduced by deformation, e.g., by cold work or machining at the surface, can trap a significant amount of hydrogen atoms, which can be important for the HAO and deteriorate the strength of the material. Another typical material degradation mode is microstructural degradation. A nondestructive detection method of the Laves phase, a detrimental precipitate, was demonstrated, in which a novel electrochemical method was developed to detect and quantify the Laves phase, and the method can be applicable to plant components. A novel ultra-high purity iron-based alloy was developed for advanced ultra-supercritical applications by adding minor elements.