A colourimetric and microstructural study of the tarnishing of gold-based bulk metallic glasses

The tarnishing of the Au-Ag-(Pd)-Cu-Si bulk metallic glass system is studied based on the Yellowness Index detection and on microstructural investigations during various and prolonged low-temperature exposures. Polished surfaces tarnish faster than as-cast surfaces due to the removal of the native SiO2 film. The rate decreases with decreasing nominal Cu/Si and Si/Au ratios, with removing Pd, and with microalloying of Al. The tarnishing mechanism is controlled by the internal oxidation of the glassy matrix forming amorphous SiO2 dendrites. This triggers the partitioning of metallic elements that enhances the rate of out-diffusion and surface oxidation of Cu.