Dynamic Change in Throwing Power for the Cu-to-Carbon Steel Galvanic Couple in the Presence of Bentonite

Abstract

In galvanic corrosion, the maximum distance from the periphery of the cathode to the anode is often defined as the throwing power. It dictates the surface area of the cathode involved in the galvanic corrosion and is also related to the extent of the accelerated anodic reaction. Therefore, its estimation is of great interest and importance in engineering applications, such as fastener and panel assembly [1], metal coatings [2], buried metallic structures [3], etc. Throwing power may be estimated experimentally by measuring the solution potential variation in close proximity to the electrode surface (Es) or through modelling, such as finite element analysis (FEA), provided the boundary conditions (polarization characteristics) are correctly determined. In this work, we investigated the throwing power for the Cu-to-carbon steel galvanic couple in the presence of bentonite clay, a scenario that could arise in the vicinity of a through-coating defect in the Canadian-designed nuclear waste container. Using a homemade micro-reference electrode array coupled to a Multichannel Microelectrode Analyzer (MMA), Es was measured simultaneously in 10 locations as a function of time in 0.1 M NaCl solution with and without bentonite clay, Fig. 1. Transient potential peaks were found when bentonite clay was added to the solution and are attributed to the increased galvanic activities driven by the reduction of air-formed Cu oxides.We further investigated this phenomenon using FEA. Boundary conditions were established experimentally under circumstances reflecting the actual surface states, i.e., with or without Cu oxides. Simulations were conducted considering secondary current distribution, using the appropriate boundary conditions. The obtained results match well with the experimental measurements, including the peak and the stable Es. Additionally, the dynamic change of the throwing power due to the evolution of the surface state was investigated through simulation and discussed. Reference [1] R.S. Marshall, A. Goff, C. Sprinkle, A. Britos, R.G. Kelly, Estimating the Throwing Power of SS316 when Coupled with AA7075 Through Finite Element Modeling, Corrosion, 76 (2020) 476-484.[2] J.R. Scully, F. Presuel-Moreno, M. Goldman, R.G. Kelly, N. Tailleart, User-Selectable Barrier, Sacrificial Anode, and Active Corrosion Inhibiting Properties of Al-Co-Ce Alloys for Coating Applications, Corrosion, 64 (2008) 210-229.[3] L. Liu, J. Li, M. Peng, W. Li, B. Lei, G. Meng, Macro-galvanic corrosion of tower grounding device consisting of graphite and Zn-coated steel in a simulated soil environment, Engineering Failure Analysis, 135 (2022) 106136. Figure 1