Galvanic corrosion of copper-coated carbon steel for used nuclear fuel containers
Abstract
Carbon steel vessels coated with ∼3 mm of Cu have been proposed for the permanent disposal of used nuclear fuel in a deep geological repository (DGR) in Canada. In the event of an undetected defect in the Cu coating that exposes the steel substrate, galvanically accelerated corrosion of steel is, in principle, possible. To investigate this scenario, the progression of steel corrosion at the base of novel simulated through-coating defects was monitored electrochemically and imaged non-destructively using X-ray micro-computed tomography (micro-CT) as a function of time, O2 availability (including anoxic conditions), and coating method (cold spray deposition (with and without heat treatment) and electrodeposition). The corrosion products and surface damage were analyzed using Raman spectroscopy and scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDX).
The results showed that the supply of O2 to the surface of the sample governed the corrosion rate, while the distribution of damage to steel at the base of a through-coating defect depended on the Cu-coated steel fabrication method and the resulting quality of the Cu/steel interface. Cold spray Cu-coated steel specimens exhibited a radial spread of corrosion along the Cu/steel interface, while electrodeposited Cu/steel specimens experienced preferential interfacial corrosion in the direction in which the steel substrate was machined prior to the electrodeposition of Cu. Less extensive corrosion along the Cu/steel interface was observed on samples that were shown to have a less stressed, more uniform, and more well-adhered interface by electron backscatter diffraction (EBSD) and adhesion tests. However, less extensive corrosion was typically at the expense of deeper penetration into the steel. In the absence of O2, both the volumes of corrosion damage to steel and the distribution of damage varied substantially with the Cu-coated steel fabrication method. The corrosion rates were significantly lower in anoxic conditions, compared to oxic conditions, and tended to decrease over time.
The influence of a wide range of cathode:anode area ratios and Cl− concentrations, and the availability of O2, was evaluated by monitoring the galvanic current passing between separate Cu and steel electrodes, connected through a zero-resistance ammeter (ZRA), and the galvanic potential of the couple. The galvanic corrosion of steel was most severe when it was exposed to air-sparged solution with a moderate [Cl−] as part of the couple with the largest Cu:steel area ratio.
The impact of porosity on the effective thickness of cold spray Cu coatings was evaluated by analyzing the size, distribution, and interconnectivity of the voids, and by using the distances between voids to determine the sequence of voids providing the shortest pathway through the coating. Small voids were predominant over large voids and the percentage of void space decreased with improvements to the cold spray deposition procedure. The effective Cu coating thickness was reduced by 25% through the shortest path. Further studies are required to determine how corrosion will proceed via voids.