High resolution characterisation of H on stress corrosion cracking in nuclear materials

Austenitic alloys are commonly used in the nuclear industry due to their high corrosion resistance and excellent mechanical properties. De- spite boasting an impressive service history, it is possible for critical structural components made from these alloys to undergo stress cor- rosion cracking (SCC) under pressurised water reactors (PWR) condi- tions. As the primary method of material degradation, SCC has been studied extensively for several decades, and many factors have been shown to affect SCC susceptibility, these include cold-work, tempera- ture and water chemistry. To develop a better mechanistic understand of SCC further research is required. This body of research evaluates the effect of hydrogen on mechanical deformation in SCC. Localised and high-resolution testing has been employed successfully to characterise key processes in SCC crack propagation. For this reason, nanoindenta- tion, transmission Kikuchi diffraction (TKD), NanoSIMS and Thermal desorption spectrscopy (TDS) have been used to characterise material deformation on a length-scale relevant for SCC.
The main objective of this thesis was to correlate the SCC crack growth rate’s (CGR) dependency on nickel content, first observed by Coriou, with both the localised mechanical deformation and hydrogen up-take at PWR temperatures. To satisfy this objective three alloys have been systematically tested in every chapter, these include SS316L (12%- Ni), A800 (32%-Ni) and A600 (72%-Ni) as they represent commercially available alloys with low, intermediate and high nickel contents. Room temperature micromechanical testing has been used to compare the mechanical properties of austenitic alloys on a length-scale that is relevant to SCC. The mechanical properties of all austenitic alloys are highly comparable at room temperature, and show no deviation with changing nickel content. High-magnification cross-sectional analysis shows that the deformation mechanisms in A800 differ from SS316L and A600 at room temperature. A800 does not undergo Σ3 defor- mation twinning, but instead deforms by dislocation glide. This is unexpected as the stacking fault energy of A800 is an intermediate of the other alloys. Deformation is a main SCC mechanism, and the ob- served alloy deformation correlates with Arioka’s reported SCC CGR in PWRs, i.e the slowest CGR undergoes no twinning. As the me- chanical properties of alloys with a 25-40% nickel content do not differ significantly from other austenitic alloys, it is thought that their corro- sion resistance is weighted towards diffusion-based SCC mechanisms. Such an explanation is consistent with findings at hotter temperatures, where diffusion-based mechanisms are more active. High temperature micromechanical testing supports these observations, and can be used to better understand the individual mechanism weights depending on alloy composition and temperature.
NanoSIMS and TDS were used to quantitatively and qualitatively ex- plore the hydrogen trapping in austenitic alloys exposed to pure heavy water at PWR temperatures. Hydrogen is found trapped by various microstructural features, such as carbides, grain boundaries and ox- ides by NanoSIMS. The total hydrogen content of SS316L and A800 is five-times greater than that of A600, which correlates with the en- thalpy of diffusion in the alloys. These results show an inverse trend with the SCC failure reported by Coriou, and a weaker inverse correla- tion to Arioka’s results. The weaker correlation with Arioka’s results is due to a change in the corrosive medium from pure water, used in this study and by Coriou, to simulated primary PWR water. It is suggested that the inverse relationship could be due to the elastic shielding pro- vided by the hydrogen enhancing dislocation motion, and preventing the stress intensity factor reaching a critical values that will initiate crack propagation.

Keywords: Electron Microscopy
Micromechanical Testing
NanoSIMS
Thermal Desorption Spectroscopy
Hydrogen
Austenitic alloys
TKD
Nanoindentation
A600
Stress corrosion cracking
Stainless Steel 316L
Advisor: Lozano-Perez, S
Tarleton, E