Electrochemical study of calcium carbonate deposition on iron. Effect of the anion

Deposition of calcium carbonate on iron from supersaturated solutions containing 1 M sodium chloride, bromide, iodide, or nitrate as supporting electrolyte was studied at 60 °C under open-circuit conditions using impedance spectroscopy, chronopotentiometry, voltammetry, and scanning electron microscopy. The anions were found to fall into two groups with respect to their effect on scaling. On the one hand, chloride and, especially, nitrate favor faster scaling kinetics and lead to compact carbonate films composed of entangled aragonite crystals. On the other hand, in the presence of bromide and iodide the scaling rate is lower and the resulting films feature aragonite crystals more or less freely scattered on what appears to be a uniform sublayer of unknown structure. The experimental data are adequately described using quasi-uniform film model accounting for the cathodic and anodic electrode reactions. As deduced from the electrochemical measurements, the barrier properties of the carbonate films formed in different supporting electrolytes increase in the order of Cl− < NO3− ≈ Br− < I−.