SSM perspective
Background
Stress corrosion cracking (SCC) in load bearing structures has been a longstanding issue both in pressurized and boiling water reactors. The specific environment in nuclear power applications consists of high temperatures and pressures that together with the effects of the radiation field on the coolant composition provide tough conditions for the materials used in some of the applications. Thus, it is important to improve the understanding of the corrosion process and to gain insight regarding the effect of coolant composition on the chromia forming nickel-base alloys that are used in Light Water Reactors.
This project concerns first principles atomistic modelling of essential sensitizing processes that would render load bearing nickel-base alloys prone to stress corrosion cracking in nuclear power applications.
Results
A sensitization process has been proposed and validated by means of density functional theory, for the chromia forming nickel base alloys that are usually used for strength as well as corrosion resilience.
The protective chromia scale is subject to chromate dissolution as well as intermittent microcracking, where healing is provided by outward diffusion of Cr in the alloy grain boundaries to Cr depleted regions such as crack tips. Corrosion ensues if Cr is not supplied by the alloy. This may be owing to Cr depletion in front of the crack tip into the oxide or by Cr mobility mitigation in the alloy.
In this work, the researchers articulate possible reaction pathways for water, as oxidizing agent, accessing the metal/oxide interface as well as the fate of hydrogen, either as H2(g), as hydrides ions in the oxide scale, or being incorporated in the alloy. In the case of the latter, hydrogen is preferentially accommodated adjacent to alloy atom vacancies thus mitigating their mobility. This reaction channel becomes important because the Cr depleted oxide scale readily conveys water, by nickel oxide converting into nickel hydroxide, to the reaction zone beyond the crack tip, where chromium becomes oxidized. There, the H2 evolution reaction becomes suppressed owing to the confining environment, and therefore hydrogen pick-up in the alloy becomes enhanced. Sensitization toward SCC ensues as mitigation of outward diffusion of chromium favors internal oxidation. These processes are elucidated by means of the electrochemical Wagner theory for order of magnitude assessment.
Finally, the impact of lithium to corrosion is addressed and support is found for the chemistry of lithium with the nickel oxy-hydroxide scale mirroring that of hydrogen, yet being more detrimental owing to a more ionic nature of the [Li-O]- bond as compared to the [H-O]- bond.
Relevance
Knowledge of the environmental sensitizing processes that affect load bearing nickel-base alloys prone to stress corrosion cracking is essential in order to increase radiation safety at nuclear power plants in the long term. SSM has contributed to the development of models that increase the understanding of environmental sensitizing of nickel-base alloys in both BWR and PWR. Through funding a group of researchers at Chalmers University of Technology, SSM has also contributed to the maintenance of national competence within nuclear radiation safety.
Need for further research
This report provides a theoretical foundation for further development of models that could help develop better nickel-base alloys for nuclear applications in a long term.