A unifying framework has recently been developed to describe the critical conditions for pit growth stability and salt film formation ^1-3. It was shown that a salt film is not required for pit stabilization; it is just a consequence of a pit achieving diffusion-controlled growth. A salt film can form on the surface of both metastable and stable pits when the maximum pit dissolution current density, i_diss,max, exceeds the diffusion-limited current density, i_lim.

In this work, the unifying framework is further expanded to establish a mathematical model describing the electrochemical behavior of salt film after it precipitates, i.e. the situation when i_diss,max > i_lim. It can be mathematically shown that the main function of a salt film during diffusion-controlled pit growth is to accommodate the extra potential of (E_max - E_sat) by regulating its thickness to adjust the actual potential at the pit surface to E_sat and thus restricting the anodic dissolution current density at the diffusion-limited value ⁴. Therefore, the salt film will respond to any changes of the applied potential, temperature, pit depth, and perforation radius of the pit cover.

This new model can explain the current peak observed during the downward potential scan of a 1D pit at the transition point from diffusion-controlled region to charge-transfer-controlled region. According to this new model, the current peak results from the supersaturation generated by thinning of the salt film during the downward scan. The model predicts that the amplitude of the current peak decreases with decreasing scan rate, which is in agreement with the experimental results from 1D artificial electrode, providing strong support for the validity of the new model.

Acknowledgments: This work was supported as part of the Center for Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584.

Reference

1. G. S. Frankel, T. Li and J. R. Scully, Journal of the Electrochemical Society 164, C180-C181 (2017).
2. T. Li, J. R. Scully and G. S. Frankel, Journal of The Electrochemical Society 165, C484-C491 (2018).
3. T. Li, J. R. Scully and G. S. Frankel, Journal of The Electrochemical Society 165, C762-C770 (2018).
4. T. Li, J. R. Scully and G. S. Frankel, Journal of The Electrochemical Society 166, C115-C124 (2019).