Active and passive behaviors of gold in cyanide solutions

Active and passive behaviors of pure gold (Au) and roasted gold ore (RGO) electrodes were investigated at 25 °C in de-aerated agitated cyanide media. Cyclic voltammetry and potentiodynamic polarization with agitation at 100 r/min in 0.04 mol/L NaCN solution showed different peak positions and current densities. Potentiodynamic tests illustrate that the peak current densities increase greatly with increasing the cyanide concentration. Increasing the pH value from 10 to 11 resultes in a great decrease of current density, while it increases noticeably by decreasing the agitation from 100 to 60 r/min. In the presence of oxygen, Au and RGO electrodes show different characteristics of peak positions and corrosion rates. The potentiostatic studies show that increasing the potential from 1 to 1.4 V at pH value of 11 results in an 80% decrease of current density while decreasing the pH value from 11 to 10 at 1 V gives a 1.7 fold increase of current density, possibly due to more effective passive layer. Following polarization, electrochemical noise measurements (ENM) during decay periods show that Au results in more passive states at high potentials, showing pitting corrosion. The ENM results show that this technique can be a promising tool for a better understanding of gold leaching. The XPS studies prove the presence of passive oxides.