Phenomenological Model for Gold-Copper Electrodeposition: Application to Thick Coatings

Pure gold-copper alloys are known to be difficult to electrodeposit because of a strong variation in composition after a few microns have been deposited. Commissariat à l’Energie Atomique (CEA) studied the phenomenon and showed that the decrease in gold’s content is accompanied by an evolution of the microstructure that could be attributed to the free cyanide released near the cathode. During electrolysis, free cyanides provoke a decrease of the copper overpotential (until copper reduction is stopped) and promote the formation of Cu(CN)₄³⁻ that conduct to an instantaneous three-dimensional nucleation of copper. This phenomenological model well explains why the growth mechanism changes and why only gold is deposited for thick deposits. On the basis of this model, CEA has developed a specific process using ultrasonic waves in order to remove the free cyanides from the cathode. This process allows CEA to perform thick gold-copper deposits with a constant concentration in copper through all the thickness. By controlling the applied potential, different thick alloys with a concentration of copper between 0 wt% up to 40 wt% can be deposited.