The
effects of additional elements on the measured
color difference (DE*) Ag-Cu-Zn-alloy is shown in
Figure 3. A lower DE value indicates better tarnish
resistance. However, the DE values in Figure 3
suggest that the samples with 0.1%Si provide
optimum tarnish resistance than that of 0.3%Si in
all testing time. The tarnishing is non-uniform and,
regarding severity, can be distinguished as three
types : 1) general tarnish which is light brown in
color, 2) local attack which is dark brown in color,
and 3) severe local attack which is very dark in
color. It appears that, although overall tarnish
resistance is improved, the alloy additions can
result in localized attack that is related with the
phase distribution in the microstructure. In general
it would concluded that an increase in alloying
elements (i.e. decrease in copper content or
decrease in eutectic structure) increases tarnish
resistance as also mentioned in a previous study.
However, it is also mentioned
(4)
that tarnishing is
possibly form by more than one chemical reaction,
several other tests and tarnish mechanisms exist
can lead to different results. It was reported that
silicon-oxide layer formation on the surface can be
formed on the silicon added silver sterling.
Microstructure-related mechanism from this layer
can be contributed to tarnish resistance.