Application of semi-ellipse model in impedance studies of corrosion of 63–37 brass in neutral aqueous solutions containing sulphate,ammonium,and chloride ions

Abstract

The effects of NH⁺ ₄ and Cl^-ions on the corrosion of 63–37 brass have been investigated using the electrochemical impedance spectroscopy (EIS) technique. For this purpose, 10^-3, 10^-2, and 10^-1M sodium chloride, sodium sulphate, and ammonium sulphate solutions were used. Impedance data have been obtained at the corrosion potential with a two electrode system. The impedance diagrams (Nyquist plots) were evaluated according to the semi-ellipse model. A new equivalent circuit model has been suggested, and a differential capacitance definition has been created within this equivalent circuit model. The polarisation resistance has been divided into the diffuse layer resistance (R_d) and the charge transfer resistance (R_t). The R_t values, which have been calculated from impedance diagrams for all solutions, indicated that the ammonium ions are corrosive to 63–37 brass, the corrosive effect depending on the ammonium concentration. However, chloride ions did not show corrosive effects at the corrosion potential.