The effect of chlorides on the corrosion behaviour of weathered reinforcing bars

The present contribution is intended to provide an in depth approximation to the role plaid by the thick passive layers formed over years in atmospheric exposure in the electrochemical response of steel embedded in concrete. This information is expected to be of general interest for the interpretation of electrochemical data as the corrosion potential and the polarization resistance, commonly employed for assessing steel corrosion in this system. 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. Kinetic information for corrosion rate can be obtained in the low frequency region of the impedance spectrum. The polarization resistance, the DC resistance value, has to be corrected for the impedance at about 1 Hz. 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. Thus, the presence of chlorides decreases the corrosion protective property of the oxide layer.