Calcium carbonate scales with allotropic forms of aragonite, calcite and a mixture of both, were selectively induced onto carbon steel substrates in a synthetic cooling water. Electrochemical impedance spectroscopy (EIS) was successfully applied to characterize these scales. The resistance and capacitance of the high frequency loop of impedance diagrams for the aragonite and aragonite-calcite scales were related to the morphology of the deposits, and gave information which is in good agreement with scanning electron microscopy (SEM) observations. The impedance response of the scale with a calcite-aragonite mixture was dominated by the aragonite constituent. However, the calcite constituent has a strong influence on the scale morphology, so that the calcite crystals merged laterally with the aragonite agglomerates to form a more coherent and thicker scale, compared to that for a scale consisting of aragonite alone. For scales containing aragonite and a calcite-aragonite mixture, increasing the formation time lead to less porous and thicker deposits.