Differential Kelvin Probe Potential Mapping of Concrete for Reinforcing Steel Corrosion Assessment
Authors: Sagüés Alberto A. , Emmennegger Leonidas 
Published: February 2020

Chloride induced corrosion of steel reinforcement in concrete seriously limits civil infrastructure durability. Chloride ion intrusion through the concrete cover causes local passivity breakdown of the embedded reinforcement. Activation of the steel is manifested by a local open circuit potential drop reflecting increased polarization of the cathodic reaction , usually oxygen reduction, to match the increase in anodic current density. Local potential can be measured using a conventional Copper/Copper Sulfate reference electrode (CSE) with its wet junction tip physically contacting the external concrete surface at points on a grid pattern. The metallic terminal of the CSE is connected to the negative terminal of a high input impedance potential meter; the other terminal of the potential meter is connected to a point of the normally interconnected reinforcing steel (rebar) assembly, reached through a hole drilled in the reinforced concrete. The potentials measured at the grid are mapped revealing the locations of more negative readings, thus providing a diagnostic indication of where corrosion is occurring so local remedial action can be taken on an informed basis. While very useful, this type of survey can be time consuming. Delays occur because adequate contact between the moist CSE tip and the electrolyte in the concrete pore network may require some form of moisturizing surface preparation, and some time for stabilization of diffusional potentials that were altered by intrusion of newly introduced electrolyte in the pore network near the concrete surface. Such delays can become prohibitive in surveys that involve hundreds or thousands of m2 of highway bridge deck, especially if it becomes necessary to temporarily close traffic lanes. A radically new approach to conduct surface potential surveys in reinforced concrete uses a scanning array of mobile, macroscopic Kelvin probes. The Kelvin Probe (KP) uses a vibrating metallic capacitor disk hovering, but not touching, close to the concrete surface. In a metallic disk - metal surface system the KP yields the value of the difference of the Volta potential of both surfaces, obtained by finding the value of the potential difference that need to be applied in a connecting disk-metal surface circuit to null out the capacitive alternating current that would otherwise circulate there. On concrete, the probe provides the difference between the Volta potential of the metallic disk and an averaged, effective Volta potential of the heterogeneous concrete surface beneath the disk footprint. Because no surface preparation or electrolyte exchange is needed, the KP provides nearly instantaneous readings and obviates the need for stabilization delays thus opening the way for fast and practical surveying of large structures. Recent investigations [1, 2] have shown that the surface potential map obtained with an appropriately scaled macroscopic KP successfully reproduced the key features of the maps obtained with the conventional method, indicating that the spatial variations in the effective Volta potential of the concrete (and identification of corroding regions) can be used instead of those provided by the CSE survey, but at a much faster scanning speed. Until now KP operation was demonstrated only with wired contact to the rebar assembly. This investigation examines a differential mode, where anode identification is made without need for a trailing wire rebar contact, Potential measurements used a scanning array of two KPs 0.4 m from each other with galvanized steel sensing disks 0.1 m in diameter placed 0.01 m above the surface of an ~2.4x2.4 m reinforced concrete slab. The probes operated at a vibrating frequency of ~0.15 kHz with vibration amplitude > 1mm. The steel reinforcement in the slab was segmentally polarized via externally impressed currents to create temporarily anodic regions ~ 0.2x0.6 m on the external concrete surface. Differential operation was achieved over the entire concrete grid by means of an auxiliary temporary reference electrode that established periodic contact with the concrete surface, but serving only as a provisional link between contactless KP measurements thus keeping the overall scheme referenced to the metal disks. The results confirmed the suitability of the differential operation scheme and were contrasted against results of replicating conventional electrode measurements. Implications on deployment of this technique in highway applications and extension to larger KP arrays are discussed and presented.
[1] Sagüés. A. and Walsh, M. “Kelvin Probe electrode for contactless potential measurement on concrete – Properties and corrosion profiling application”, Corrosion Science, Vol 56, pp 26-35, 2012.
[2] Sagüés. A. and Emmenegger, L. “Contactless Electrode for Fast Survey of Concrete Reinforcement Corrosion” Final report, NCHRP IDEA Project 176, Transportation Research Board, National Research Council, Washington, DC, 2017 (In Press).