Theoretical investigation of the evolution of the passive state on Alloy 22 in acidified, saturated brine under open circuit conditions

It is observed in many systems that as a metal or alloy is exposed to an aqueous solution over a long period of time, the corrosion potential drifts in the positive direction and the corrosion rate decreases. This implies the possibility that the potential may become sufficiently positive that it will exceed the transpassive potential and result in depassivation of the surface and hence to massive corrosion. On the other hand, if depassivation does not occur, the corrosion rate is predicted to continuously decrease with time, such that experiments carried out over short periods of time may seriously over-estimate the true corrosion rate over extended periods. This is an important issue in the selection of materials for the containment of high-level nuclear waste (HLNW), because of the excessively long performance horizon (up to 106 years), and in interpreting corrosion data from archeological artifacts. The theory developed in this work provides a physico-electrochemical explanation of the ennoblement phenomenon that is in good agreement with experiment and, in principle, allows one to optimize the model on short-term data and then predict the corrosion behavior over the very long term.