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The influence of an ultrasound intensity (28 kHz, 1.11-7.48 W/cm²), water hardness (as CaCO3, 125-275 mg/l) and temperature (15-30°C) on the nucleation-growth process of calcium carbonate on the surface of a cylinder mild steel electrode rotated at 500 rpm was studied. The deposition kinetics was analyzed by chronoamperometry; the calcareous layer was characterized by gravimmetry and scanning electron microscopy. The water hardness was found to determine the crystallization kinetics of calcium carbonate influencing both the deposition rate and crystal morphology, so that a porous layer of vaterite was formed in soft water and highly protective calcite scale was built up in hard water. While temperature accelerates crystallization, faster crystallization leads to the formation of a more porous layer with vaterite inclusions due to limitations in calcium supply. Ultrasound accelerates formation of a calcite at intensities lower than 2.23 W/cm². At higher ultrasound intensities, a significant change in carbonate crystallization occurs. Insufficient calcium concentration in the pre-electrode layer promotes formation of vaterite crystals in the growing calcite layer increasing porosity and scaling time. The results obtained could be applied in industry to solve both scale and corrosion problems of heat-exchange equipment by regulation carbonate layer thickness and porosity. [ABSTRACT FROM AUTHOR]