Journal of Dental Research, Vol 64, 139-143, Copyright © 1985 by International & American Associations for Dental Research Online Journals
Tarnishing in vivo and in vitro of a low-gold alloy related to its structure
H. Hero and J. Valderhaug
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
Ag-rich 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 (approximately equal to 0.05 micron)
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 micron 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.
