Quantitative Evaluation of Chemical Stability of Dental Alloys

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The purpose of this study was to evaluate the chemical stability of dental alloys quantitatively.Corrosion resistivity was expressed by the parameter Q which correspond to the total amount of electrochemical reaction occurred on the specimen surface. Tarnish resistance was evaluated by color difference DELTAE between those of prior and after testing.Principal results were summarized as follows;(1) Corrosion behavior of alloys with single phase structure were principally determined by the nobility of the alloy.(2) Anodic potentiodynamic polarization curves of the alloys with multiphase structure were rather complicated. Characteristic potential (Ec) at which corrosion rate increased spontaneously appeared at two different potentials. But these complicated curves can be explained by superposing two simple curves of each single phase. Two different Ec correspond to those of each phase, and current density at a potential can be expressed by the arithmetical mean of those of each phase.(3) DELTAE of alloys. in which a part of gold was replaced with palladium up to about 10at%, decreased with palladium content. But further replacement increased AE. This increase in DELTAE was attributed to the decrease in chemical stability by the replacement of gold with palladium. Decrease in DELTAE by the addition of small amount of palladium may be explained by the mechanism of palladium enrichment just below the sulfide layer. This enrichment may suppress the diffusion of S^<--> ion. resulting in the decrease in the growth rate of sulfide layer.(4) Tarnish susceptibility of low-nobility alloy was very sensitive to its microstructure. Phase separation increased DELTAE to about two times of homogenized single phase state. But a small amount of palladium addition improved this tarnish sensitivity of dual phase alloy. This may be attributed to the enrichment of palladium in the copper-rich phase.