Chloride Ion Barrier Properties of Small Electric Fields in the Protection of Steel in Concrete

Analysis of the Nernst-Plank relation suggested that a relatively small voltage drop would maintain a significant chloride concentration difference across an ideal electrolyte. This effect was used as the basis for a possible criterion for the protection of steel in concrete in which the chloride concentration at the steel is maintained at a level below that necessary to cause passive film breakdown. The applicability of ideal solution theory to the electrolyte in a porous cement matrix was investigated by driving chloride ions against a concentration gradient across a cement mortar disc using a small applied potential difference. Results indicated the potential difference was an effective barrier to chloride ions and that the theory presented can be used in the design of an active chloride barrier. This protective effect can be monitored by determining the voltage drop through the concrete between an installed anode and the protected steel. However, caution should be exercised when dealing with structures with existing severe chloride contamination. In that case, the chloride concentration at the depth of the steel first should be reduced to a value below a critical level. One method of doing so is to apply a minimum current density for an initial period of time. Experimental data suggested the efficiency of the applied current in moving free chloride against a concentration gradient was higher than expected.