Electrochemical Behaviour of Passive Layers in Weathered Reinforced Mortars

Electrochemical methods are widely employed for assessing the corrosion performance of steel rebars embedded in concrete. Corrosion potential and polarization resistance are the main parameters considered for this purpose; under the assumption that the cathodic process corresponds to oxygen reduction and the anodic process to steel corrosion. Although these assumptions are mostly true, they are not able to explain some transitions from passive to active states, as well as some corrosion potential evolutions. The present contribution is intended to provide an in depth approximation to those questions based on the study of thick passive layers formed over years in atmospheric exposure. Results for blank samples and samples containing chlorides are discussed based on findings from electrochemical impedance spectroscopy. Temperature was considered in the study as a parameter for acting on both the chemistry of the pore electrolyte and the specific volume of the oxides layer. By this approach, we were able to identify the contribution of the oxides layer to the impedance spectra in the 10 kHz-1 Hz frequency range. The oxides layer supports the oxygen cathodic reaction, but also the anodic and cathodic processes corresponding to the re-oxidation and reduction of the layer itself. Compared to layers formed in the absence of chlorides, the presence of chlorides in the concrete mix leads to the formation of a more oxidised oxide layer, with decreased redox activity and more prone to cracking under thermal cycling