Effect of cathodic protection on macrocell currents: A viable method to monitor and control the efficiency of CP?

Abstract

Corrosion protection of steel reinforcement in concrete structures by cathodic protection (CP) is a cost effective, reliable, and widely accepted method to stop and prevent the corrosion of the steel reinforcement. The efficiency of CP is usually monitored by the ''24 h, 100 mV depolarization criterion,'' a purely empirical criterion whose implementation is cost and labor intensive and that does not allow online control of CP. Within an extended research project on CP applied to concrete members of a highway bridge exposed to penetrating moisture, three sets of macrocells (MC), each composed of five MC sensors, were installed in conjunction with concrete resistance sensors and silver/silver chloride reference cells. Chloride profiles were determined from the cored or drilled powder originating from the installation of sensors and from drilling cores. Corrosion currents, steel potentials, and concrete resistance were monitored over a period of 1 year before, after installing the CP systems (which remained switched off for half a year for evaluating the effect of the conductive coating), and after startup of the CP systems. The CP systems applied consist of a moisture resistant conductive coating. As expected CP has a pronounced effect on local MC currents: Anodic MC currents were reduced or changed into cathodic currents, whereas cathodic currents were only weakly influenced. By adjusting the applied protection current all anodic MC currents may be changed into cathodic MC currents. Results indicate that the 100 mV depolarization criterion is a conservative criterion in atmospherically exposed concrete; it is not reliable in strongly wetted concrete. There was no consistent correlation between the CP induced changes in the local MC currents and 24 h depolarization values indicating that large potential shifts induced by CP do not necessarily imply overprotection. Results show that monitoring MC currents before, during, and after CP operation allows to demonstrate in a transparent way the effect of CP on the corrosion of the steel reinforcement. Online monitoring of MC currents is proposed as a viable and comprehensible method to monitor and control the efficiency of CP.

Maintenance and repair of steel reinforced concrete structures by galvanic corrosion protection – field experiences over 10 years

“…Adhesion strength after 24 hours is in the range of 0.6 -1.0 MPa, after 7 days > 2 MPa and after 28 days about 2.5 -3.0 MPa at ambient temperatures of 20 -25°C and depending on substrate strength. The efficiency of corrosion protection of the steel reinforcement by the EZA may be evaluated according to the procedure described in EN 12696 -the 24 h depolarization criterion and/or by macro cell current measurements [8]. To monitor the efficiency of the installed EZA systems, reference cells, macro-cell-, concrete resistivity, temperature and humidity sensors were installed and monitored with an automated monitoring system.…”

“…Three ERE 20 reference cells and one set of macro-cell sensors [8] were installed to monitor the corrosion protection efficiency. The galvanic current, the steel potentials and the macro-cell currents are monitored and every 4 weeks 24 hour depolarization measurements are performed by the CAS modular data acquisition and control (MODAC) system.…”