The distinctive colors of copper and copper alloys make
them prized for architectural and consumer items and
objects of art. Their natural metallic tones range from red to
yellow to silvery gray (Figure 6). A number of other colors
can be obtained by chemical or electrochemical processing
of their surfaces. Copper and its alloys are extremely resistant
to atmospheric corrosion, but over time a superficial
discoloring or tarnish layer eventually forms. All metals
tarnish or form an oxide layer when exposed to the atmosphere.
The thickness and chemical content of the layer
varies as a function of exposure time, atmospheric conditions
and base alloy chemistry.
Most metals develop a darkened surface which, as with
stainless steel, may make it difficult to see the underlying
base metal color. The appearance of the underlying base
metal color can be preserved by applying thin, clear protective
coatings. These coatings are organic chemicals which
harden at room temperature or with baking and are usually
applied in a solvent vehicle. Coatings do, however, interfere
with and negate the fundamental antimicrobial nature of the
copper alloy surface.
The tarnish film that forms on copper alloys is generally
a copper oxide which, when thin, creates a darkened base
color but, with time, can develop into a gray film. Although
such oxides change the physical appearance of the surface,
they can be removed with standard cleaning solutions.
Testing indicates that oxidation and tarnishing do not
interfere with copper alloy antimicrobial performance but,
in fact, enhance surface efficacy. These oxides play the
crucial role of interacting with membranes of bacteria* to
establish the antimicrobial efficacy of copper alloy surfaces.
Base alloy chemistry and atmospheric conditions dictate the
kinetics and nature of surface oxidation. Tarnishing of the
alloy surface is considerably less in an indoor environment
as compared with outdoor exposures. Some copper alloys,
copper nickels, silicon-containing alloys and nickel silvers,
in particular, show a resistance to tarnishing and retention of
the base color.
Architectural applications such as roofing and hardware
(lock sets, doorknobs, kick plates, hand rails, and etc.)
capitalize on the atmospheric corrosion resistance of copper
and copper alloys.