Monitoring the corrosion of reinforcing steel in cement-based systems using impedance spectroscopy

Impedance spectroscopy was used to interrogate the electrical response of cement-based materials, the reinforcing steel contained therein, and the interdependence between the two. An equivalent circuit for the entire system is presented for the first time, which models the electrical properties of these systems and their relation to the microstructure. Emphasis was placed on determining the corrosion susceptibility of the system as a whole, through analysis of each of the microstructural components. The technique was found to be advantageous for these analyses, especially as a rapid, non-destructive, and in-situ test.; Several advantages of impedance spectroscopy were identified, the most important of which is the ability to separate the electrical responses of various transport phenomena within the system. This allows analysis of local variations in the microstructure. However, limitations of impedance spectroscopy were also identified, including restrictions on the ability to make multi-point measurements.; The underlying microstructure and chemistry of cementitious systems were found to greatly influence the corrosion of reinforcing steel, and thus, the durability of these systems. Distinct features of the system microstructure were identified and analyzed using impedance spectroscopy. Responses from the bulk paste and the paste/steel interface were observed for hydrating cement-based systems, with the paste/steel interface contributing responses from a charge transfer/double layer process and a passive iron oxide film. All three responses were observed when the frequency range of the experiment was 10 MHz to 10 mHz. From the bulk paste response, transport properties can be obtained which aid in the understanding of ingress mechanisms of corrosive species, and the effects of these species on corrosion of the steel. It was determined that the impedance response of the interface provides a measure of both the passive oxide film thickness, as well as providing insight into the near interfacial bulk microstructure. And, an intermediate frequency response was identified and explained for the first time in cement-based systems, which was related to corrosion reactions at the interface. Thus, the effects of changes in steel and cement chemistry on the corrosion of reinforcing steel in these systems were monitored.