Electrochemical adsorption and passivation on gold surface in alkaline thiourea solutions

An electrochemical method was used to examine Au surface absorption and passivation under the most commonly employed pH conditions in an alkaline Au leaching system. The polarization at different pH values was obtained through the current step curve. At pH 10, no absorption layer was formed. When the pH was increased to 11, an absorption layer was formed through the Au electrode reaction. At pH 12, the entire system could not be stabilized, even after long durations because of the thiourea oxidation and decomposition. The samples were analyzed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and atomic force microscopy (AFM). AFM observations of the Au plating surface and SEM–EDS analyses for the Au-coated graphite surface indicated that a passivation membrane was formed on the Au surface after its use in the alkaline thiourea Au leaching system. Two-step leaching was used to confirm the conclusions drawn from the experimental results. The two-stage experimental results further confirmed the existence of a passivation membrane.