Journal of Dental Research, Vol 65, 1303-1307, Copyright © 1986 by International & American Associations for Dental Research Online Journals
Tarnishing in vivo of Ag-Pd-Cu-Zn alloys
H. Hero and L. Niemi
NIOM, Scandinavian Institute of Dental Materials, Oslo, Norway.
Pre-polished specimens of four Ag-Pd-Cu-Zn alloys have been tested for
tarnishing while embedded in full acrylic dentures for from six to eight
months in vivo. The results have been compared with those from previous
immersion tests using 2% Na2S solutions. In both types of tests, a
multiphase alloy consisting of a Pd-Cu-Zn-rich compound in a Ag-rich matrix
displayed less corrosion and tarnish resistance than did alloys with
compositions similar to the Pd-Cu-Zn-rich compound or the Ag-rich matrix.
It is suggested that the presence of local microgalvanic cells due to
chemical inhomogeneities caused the observed susceptibility to tarnishing.
Sulfur was detected by a microprobe in the tarnish layer developed in the
oral cavity. The discoloration on this multiphase alloy covered the
interdendritic areas with short distances between the chemical
inhomogeneities. This microstructural location was somewhat different from
the many dark spots created predominantly on Ag-rich areas during in vitro
testing. This difference is most likely due to dissimilar conditions with
regard to sulfide ion concentrations, the presence of protein films, and
the length of exposure time.
