An Experimental Study on the Cathodic Protection Criteria of the 100 mV Depolarization for Chloride-Loaded Reinforced Concrete

The 100 mV depolarization value cathodic protection (CP) criterion is widely used to protect the steel bars in atmospherically exposed concrete structures. The CP on the steel in concrete creates a secondary effect by increasing OH- ions and decreasing Cl- ions near the surface of the steel, against steel corrosion. In this phenomenon, there is a possibility that the CP criteria of 100 mV could be decreased due to the environmental changes caused by these secondary effects. In this study, the effects of different depolarization values of 25, 50, and 100 mV for the protection of steel bars in concrete specimens with different chloride ion concentrations were experimentally evaluated, and their effects on the corrosion rate of steel were investigated. The concrete had a water-to-cement ratio of 0.55 and a sand-to-total-aggregate ratio of 49%, and chloride was admixed as NaCl to give Cl− contents of 2, 5 and 10 kg/m3, equivalent to 0.58%, 1.45%, and 2.90% by mass of cement. The steel bars were pre-corroded by an impressed current of 1.33 A/m2 and were then protected for 250 days at 20 ± 2 °C and 60% relative humidity, the protection current being adjusted to hold average depolarization values of 21–37 mV, 57–63 mV, and 117–121 mV. After 250 days, the corrosion rate under CP was 0.18–0.95 mA/m2, a reduction of 60–77% relative to the unprotected specimens, and raising the target depolarization from 25 mV to 100 mV improved that reduction by only 6–11 percentage points. The measured reduction factor of 2.5–4.3 is far below the factor of 50 predicted by the Tafel relationship for a 100 mV activation polarization, which is attributed to the partly passive state of the steel and to the non-activation components of the 24 h potential decay. Within the exposure conditions tested, a depolarization criterion below 100 mV therefore protected the steel as effectively as the conventional 100 mV criterion while requiring a markedly lower protection current density.