Effect of palladium addition on the tarnishing of dental gold alloys

Tarnishing tests were carried out on Au-Cu-Ag and Au-Cu-Pd alloys. This study focused on the individual and combined effects of nobility, palladium content and microstructure. Tarnish resistance was almost perfect for the alloys with nobility higher than 50 at%, but it seemed to relate to the palladium : gold atomic ratio for the alloys with low nobility. Palladium inclusion reduced the tarnish susceptibility up to about 10 at%. This decrease in the degree of tarnishing was attributed to the decrease in the diffusion rate of S² ion which resulted in a decrease in the growth rate of sulphide layer. Tarnishing of the alloy with low nobility was very sensitive to its microstructure. The tarnishing susceptibility of dual-phase Au-Cu-Ag alloy was twice as high as that of the single-phase alloy. However, palladium-bearing alloy showed no increase in the degree of tarnishing by phase separation. This may be attributed to the enrichment of palladium in the copper-rich phase.