The non-destructive monitoring of the corrosion of steel in concrete using indirect electrical techniques is gaining interest as it does not require any connection to the rebar. Here, we provide insights into the ability of the indirect galvanostatic pulse (GP) method, using four-electrode configuration placed on concrete surface, to locate corroding areas in macrocell corrosion in the Wenner configuration, in which C1 and C2 inject a direct current and P1 and P2 measure the resulting potential difference (Figure 1a). This technique differs from the conventional one in three-electrode configuration with a connection to the rebar, as the current that polarizes the rebar is not constant but decreases over time. Consequently, the temporal evolution of the potential difference measured between P1 and P2 can be explained by the increase of the potential difference related to concrete resistance and the decrease of the potential difference related to the rebar (Figure 1b, c).
Using numerical simulations considering macrocell corrosion, it was shown that the presence of an anodic area disturbs the current distribution in the material, which is explained by the different capability of anodic and cathodic areas to consume the impressed current. Consequently, the evolution of the transient potential is slowed down as compared to uniform corrosion (Figure 2). This effect is notably dependent on the position of the anode with respect to the current probes, with a higher sensitivity when the corroding areas are below or in their vicinity. Hence, the interpretation of spatial measurements of transient potentials made over the length of the rebar can provide insights in the localization of the anodic areas (Figure 3). For the estimation of their length, which is one of the main current problematic issue when performing any measurement on reinforced concrete (RC) structures, adjusting the probe spacing during the spatial measurement appears interesting to modulate the sensitivity of the technique to macrocell corrosion [1].