Effect of scale inhibitors on the structure and morphology of CaCO₃ crystal electrochemically deposited on TA1 alloy

The deposition of CaCO₃ scale in circulating cooling water on metal surface is a major concern in industry. This paper focuses on the feasibility of electrochemical methods to study the scale inhibition performance of 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA), Polyacrylic Acid (PAA), including linear sweep voltammetry, chronoamperometry, and electrochemical impedance spectroscopy (EIS). In addition, the coverage, morphology and structure of deposited CaCO₃ crystal on titanium alloy surface in the absence and presence of inhibitors were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). A new method for calculating the efficiency of scale inhibitors was proposed so that it can be calculated by using the change in residual current density (i_r). In order to prove the feasibility and accuracy of such method, the efficiencies of inhibitors were evaluated using i_r and charge transfer resistance (R_ct), respectively. In addition, molecular dynamics (MD) simulations were performed to evaluate the interaction between the scale inhibitor molecule and the CaCO₃ crystal. The experimental results show that both the residual current density obtained by chronoamperometry and the charge transfer resistance obtained by electrochemical impedance spectroscopy can be used to evaluate the efficiency of scale inhibitors, and there is high consistency from the calculation results. It is also confirmed by X-ray diffraction and scanning electron microscopy studies that the presence of inhibitor reduces the surface coverage of CaCO₃ at metal electrode and that the crystal structure of CaCO₃ is transformed from the original aragonite into the most unstable vaterite. The best inhibition efficiency of PBTCA for CaCO₃ deposit is confirmed by the results of MD simulations.