On the stability of the passive film on carbon steel as indicated by electrochemical impedance spectroscopy

In this paper, we report an EIS study of the evolution of the passive state of carbon steel in contact with simulated concrete pore water. By mining the impedance data, we have determined that, with time, the defect structure and the defect-dependent properties of the barrier layer of the passive film on iron remain invariant, but that the change in the properties of the passive film, as a whole, can be accounted for in terms of changes in the thickness of the precipitated outer layer. The importance of this finding is that we may apply, with added confidence, the time-independent Point Defect Model for describing the barrier layer on carbon steel in contact with concrete pore water over periods of time that are well in excess of experimentally-accessible times. Thus, as envisaged in the supercontainer concept in Belgium's technology for the disposal of High Level Nuclear Waste (HLNW), the carbon steel over-pack will be exposed to concrete pore water for times exceeding 100,000 years, with the changes in the passive state of the metal being attributed to the increase in thickness of the precipitated, outer layer alone. The rate of formation of the outer layer film (80 °C) is calculated to be 4.6x10^-12cm/s (or 1.45x 10^-4 cm/year = 1.45 μm/year), which appears to be an eminently reasonable value.