Calcium carbonate deposits grown under cathodic polarization in the conditions of cathodic protection were characterized by ne and EHD impedances. The transition between a regime dominated by the germination of calcium carbonate crystals and the progressive development of a 3-D (three dimensional) porous layer occurs when the actual current has decreased to half its value measured at time zero where the interface is bare. In particular from the EHD impedance data, it was possible to estimate the average value of the crystals size in the first phase to about 30-50 mu m and the covered fraction from the high frequency capacitance, in perfect agreement with direct SEM examination.