Dynamic change of the throwing power for the copper-to-carbon steel galvanic couple in the presence of bentonite: A combined experimental and numerical modelling study

Abstract

The throwing power of the copper-to-carbon steel galvanic couple in NaCl solutions with and without bentonite has been measured and simulated using finite element analysis (FEA). The presence of bentonite clay in the solution resulted in an initial high throwing power and a subsequent change to a stable low throwing power, captured by measuring the solution potential variation (E_s) close to the specimen surface using a micro-reference electrode array. The origin of the change was traced by studying the polarization characteristics of the copper and carbon steel electrodes using electrochemical polarization techniques. By varying the experimental parameters, polarization characteristics corresponding to the initial and stable stages were obtained. They were then used as the boundary conditions for finite element analysis to simulate the initial and stable throwing power. Simulation results showed good agreement with the experimental measurements of E_s. Finally, FEA models with extremely large copper-to-carbon steel area ratios were built, resembling a potential through-coating defect on a Canadian-designed container for used nuclear fuel. As predicted by the model, the presence of bentonite in the solution suppressed the throwing power in general, with thicker and compacted bentonite being more effective. However, the throwing power saw an uptick when the area of exposed carbon steel exceeded a critical dimension decided by the thickness of the bentonite. The enhanced throwing power was attributed to the increased kinetics of the oxygen reduction reaction on the reduced Cu cathode.