A new framework has been developed in recent works to describe pit growth stability ^1-8, based on which many critical scientific issues in localized corrosion were addressed, such as the rate-controlling step in pit growth, the salt film growth and self-regulating mechanism, and the kinetics and the activation energy of metal dissolution in the local pit environment. Although many new insights were generated, there are still some phenomena that are not well understood, such as the pit repassivation temperature (T_rp). According to the new framework, T_rp is predicted to decrease with the increase of applied potential (E_app). However, T_rp sometimes exhibits an unexpected dependence on E_app that contradicts with the prediction, and the underlying mechanism is not clear.

In this study, T_rp of one-dimensional artificial pits made from 316L stainless steel was investigated in 3.5 wt.% NaCl by downward temperature scanning. For a given pit depth, T_rp exhibits an unexpected “S” dependence on applied potential, which has never been reported previously. Further analysis shows that this relationship is derived from repassivation underneath the salt film, which is different from repassivation that occurs when the pit surface concentration decreases below a critical value. This indicates that there is a reversal of the effect of the salt film on pit growth stability. The systematic study shows that such a reversal depends on temperature, E_app, and pit depth.

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.

References

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