The Influence of Copper Coatings on the Corrosion of Carbon Steel Substrates

Abstract

Within the multi-barrier system proposed for the permanent disposal of spent nuclear fuel, the primary engineered barrier is the sealed metallic container. The proposed Canadian container design involves a carbon steel vessel coated with 3 mm of Cu. Since they will be buried ~500 m below ground, the carbon steel provides the necessary structural support, while the Cu coating provides corrosion resistance.1 The coatings will be formed by either electrodeposition or a cold spray technique. Despite a considerable amount of research into the corrosion behaviour of the outer Cu coating,2 the impact of a defect in the coating on the corrosion of steel remains largely unstudied. Should a defect simultaneously expose both Cu and steel to groundwater, galvanically accelerated corrosion of the steel could increase the accumulation of corrosion damage, by either corrosion propagation through the steel or de-adhesion of the Cu coating from the steel. The goal of this research is to determine the influence of the coating process on these modes of damage accumulation when a through-coating defect is present. To simulate this scenario, Cu-coated carbon steel specimens with a small hole (0.5 mm in diameter) drilled through the coating to the Cu-steel interface are being used. Since strongly saline ground waters are possible, the specimens are exposed to 3 M NaCl solutions with various concentrations of dissolved O2. Using X-ray microtomography, a non-destructive 3D imaging technique, the progression of corrosion damage at the base of the simulated defect is being monitored both in-situ and ex-situ. The corrosion process is also being followed electrochemically by monitoring the corrosion potential and periodically recording the polarization resistance using the linear polarization technique. The potential reflects the relative rates of the chemical reactions occurring on the sample surface and the polarization resistance values reflect the changes in the overall corrosion rate. Results to date show that galvanically accelerated steel corrosion occurs, as expected, in oxygenated solutions, with the corrosion rate and distribution of damage being dependent on the dissolved O2 concentration. The corrosion behaviour also differs depending on the nature of the coating. Coatings produced by a cold spray process exhibited damage propagation along the Cu/steel interface, while electrodeposited coatings did not. Despite this difference in geometry, the overall extent of corrosion is very similar based on damage volume measurements obtained from the X-ray microtomography imaging. The long term goal of this project is to develop a computational model to determine the extent of corrosion damage which could occur on a waste container emplaced in a repository ~500 m below ground in which the redox conditions are evolving from oxic to anoxic. With this goal in mind, a finite element model (based on a COMSOL framework) is being developed to describe the galvanic corrosion process. (1) Scully, J. R.; Edwards, M. Review of the NWMO Copper Corrosion Allowance; NWMO TR-2013-04; NWMO: Toronto, ON, 2013. (2) King, F. Critical Review of the Literature on the Corrosion of Copper by Water; TR-10-69; SKB: Stockholm, Sweden, 2010.