Tarnishing in vivo and in vitro of a Low-gold Alloy Related to Its Structure

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Abstract
Studies on tarnishing of a low-gold alloy, both as-cast and solid-solution-annealed, have been carried out in vivo and compared with that of a type III gold alloy. Small polished samples were partly embedded in complete dentures for nine months. Generally, the low-gold alloy in the as-cast state was found to be more prone to tarnishing than were the other two materials, followed by the same alloy, solid-solution-annealed. These results are in agreement with both previous and present in vitro studies using 2% solutions of Na2S. The tarnish attacks followed the pattern of Cu segregations in vitro, and frequently also in vivo. Previous findings in vitro, on the other hand, show that tarnishing occurs predominantly on Agrich phases. The link is to be sought in the structural details. In the Cu-rich regions along grain boundaries and in interdendritic positions, eutectic particles with thin lamellae (~0.05 μm) of alternating Ag and Cu enrichments occurred. In these areas, we found a more unstable matrix with increased tendency to splitting into similar, but even thinner, lamellae. Most likely, these lamellae of alternating compositions act as galvanic cells. The cathodic reduction of oxygen probably takes place mainly on the Cu-rich lamellae, which are Au-enriched on the surface due to initial selective dissolution of Cu. With increasing time, attacks up to several μm in depth comprising both kinds of lamellae tend to occur at the grain boundary regions in an in vivo test, probably by a fretting corrosion mechanism.

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