EIS characterization of the evolution of calcium carbonate scaling in cooling systems in presence of inhibitors

Electrochemical impedance spectroscopy (EIS) was used to assess the relative effects of scaling and corrosion for steel electrodes in cooling water media and to obtain information on corrosion inhibition and scale inhibition properties of 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) and hydroxyphosphonoacetic acid (HPA). Steel electrodes were preliminary scaled with CaCO ₃ in simulated cooling water and then immersed in the characterization solution. Analysis of the impedance spectra with a simple model allowed following of the time evolution of physical parameters corresponding to the calcium carbonate islands and to the corrosion products accumulated in areas not covered by the scale. In uninhibited solutions, the main effect was the progressive deposit of corrosion products with no additional scaling and little restructuring of the initial carbonate islands. When the solution contained HEDP alone, part of the initial scale was detached from the surface, but the presence of HPA or the mixture HPA+HEDP only induced structural modifications of the initial scale. Moreover, the impedance analysis also showed that HPA exhibited better corrosion inhibition properties than HEDP.