(384j) Numerical Simulation of the Influence of Droplet Geometry on Corrosion of Copper

The Nuclear Waste Management Organization (NWMO) is responsible for the implementation of Adaptive Phased Management (APM), the federally-approved plan for the safe, long-term management of Canada’s used nuclear fuel.¹ Under APM, used nuclear fuel will ultimately be placed within a deep geological repository (DGR) in a suitable host rock formation. Part of evaluating the long-term performance and safety of the repository system is understanding the behavior of the copper coated container with respect to localized corrosion. Even though localized corrosion is not expected in the DGR environment, there is still a need to develop a mathematical model for localized corrosion of copper.

A two-dimensional axisymmetric model was developed using the concept of a classic Evans droplet.² Different droplet geometries in the model were used to explore the influence on localized corrosion kinetics and anode-cathode distributions. Geometries included a circular droplet, oval droplet and a droplet with secondary spreading zones. Studies have shown that initial deposition and secondary spreading of droplets could result in differences in the control mechanisms of local corrosion behavior under the droplet.³

The mathematical model was developed using the finite-element method (COMSOL Multiphysics). The model includes coupled, nonlinear, diffusion equations for ionic species, which include the contribution of migration, local electroneutrality, homogeneous reaction, and formation of precipitates. It accounts for six heterogeneous reactions on the metal surface and fifteen homogeneous reactions involving copper chloride complex, copper hydroxide complex, copper carbonate complex, and superoxide species. A total of twenty-eight dependent spatial-temporal variables including species’ concentrations, potentials and local corrosion rate were solved in this model. The influence of temperature was included on model parameters such as equilibrium rate constant, diffusion coefficient, Henry’s law constant, solubility product constant and kinetic parameters associated with electrochemical reactions. The associated equilibrium constants were obtained from PHREEQC thermodynamic software.⁴

Simulations were performed for a ten-year period. The droplet geometry was found to influence the corrosion rates and depths, the surface coverage for Cu₂O films, and concentrations of dissolved gaseous and ionic species. Nevertheless, the resulting corrosion was observed to be uniform for all droplet geometries. Uniform corrosion of copper is desirable for strategies that employ copper cladding to protect steel canisters that contain used nuclear fuel. In future work, we will continue exploring conditions that may lead to localized corrosion.

References

1. NWMO, “Choosing a Way Forward. The Future Management of Canada’s Used Nuclear Fuel. Final Study,” Nuclear Waste Management Organization, Toronto, Ontario, 2005. https://www.nwmo.ca/~/media/Site/Files/PDFs/2015/11/04/17/39/2680_nwmo_final_study_nov_2005.ashx.
2. U. R. Evans, The Corrosion of Metals, E. Arnold & Company, London, 1926.
3. Schindelholz, H. Cong, C. Jove-Colon, S. Li, J. Ohlhausen, and H. Moffat, “Electrochemical aspects of copper atmospheric corrosion in the presence of sodium chloride,” Electrochimica Acta, 276 (2018), 194-206. 10.1016/j.electacta.2018.04.184.
4. PHREEQC Version 3, United States Geological Survey, 202, https://www.usgs.gov/software/phreeqc-version-3.

Acknowledgement

This work was supported by the Nuclear Waste Management Organization, Canada, under project 2000904.