Electrochemical behavior of corroded and protected construction steel in cement extract

The corrosion behavior of corroded, cathodically protected and control (reference) construction steel, previously embedded in concrete, was studied in cement extract (pH 12.6, considered as concrete pore water), using cyclic voltammetry (CVA) and potentiodynamic polarization (PDP). The necessity for this investigation occurred from the previously observed and commented discrepancies in the recorded corrosion parameters for corroded and protected steel in embedded conditions [1]. Therefore this study aimed to evaluate how the naturally formed in concrete product layers (after 460 days) will influence the electrochemical behavior of the steel in cement extract. The PDP measurements reveal the lowest corrosion resistance to be for the previously corroded steel samples, for which the most active corrosion potential (-0.7 V SCE) and the highest anodic current in the potential region 0 to 0.6 V (SCE) were recorded. The CVA tests support the results from PDP and correlate the properties of the naturally formed layers with the recorded peak current densities and peak potentials with cycling. For all specimens, except the corroded ones, the peak potential initially shifts anodically, which denotes for a high corrosion resistance in the former and low corrosion resistance in the latter case. For the control and protected specimens, the passive current in the potential region of 0 to 0.6 V (SCE) remains almost unchanged with cycling, i.e. the protective properties of the initial layers remain unchanged. Thickening of the film with cycling does not influence the corrosion resistance of the previously formed layers. For the protected specimens, however, a tendency to reach a steady state and change of peak currents' height only were observed, without a pronounced shift to more anodic values. An increase in the peak current only, not accompanied by anodic shift of the peak potential, suggests that the layers in the cathodically protected specimens are more homogeneous and compact. Overall it can be stated that in cement extract, the product layer with lowest corrosion resistance is the one on the surface of the corroded steel reinforcement. The product layers in the protected specimens (although similar to control conditions) are with the highest corrosion resistance.