|
I was doing some research on copper in the web & came
across this interesting article that might be of some help to all
readers.
Date: Tue Nov 23 15:34:50 2004
Posted by: John Christie, Faculty, Dept. of
Chemistry
It all depends! Here are several bits of information that might help:
“Rust” is a term that is normally used for iron-based metals only. It
is the name for the familiar dusty or flaky red-brown iron oxide that
slowly builds up on these metals. On copper, bronze, or silver, we
talk about a “tarnish” if we are thinking about the degradation of the
metal surface, or a “patina” if we are thinking about the pretty and
sometimes artistic effects that a tarnish can produce on the surface
of these metals.
“Steel” is the name of a large number of different metal alloys,
containing mostly iron, a little carbon, and varying amounts of
different other metals. Different steels are designed and made for
different purposes. “Tungsten steel” is designed to be very hard and
strong for use in tools; your tools will rust quite badly if you leave
them out in the weather for a few days. “Stainless steel” is designed
to resist rusting and other sorts of attack by chemicals in a wide
range of situations, but it is quite soft. So it is almost impossible
to compare the rusting rate of “steel” with anything — it depends what
sort.
The process of rusting or tarnishing occurs when a metal reacts with
something in the air. It is not always the same component of the air.
Air varies greatly in its water content, its content of salt spray
from the ocean, and the presence and amounts of various pollutants.
Iron reacts very slowly with ordinary oxygen, but the reaction is
catalyzed by water, and especially by acid or salt. So iron in a damp
climate rusts very quickly if it is exposed to acid rain or to ocean
spray. It rusts much more slowly in damp mountain climates with clean
air. It hardly rusts at all in the desert.
Copper hardly reacts with oxygen at all. It does react quite slowly
with carbon dioxide and water and oxygen to form a green tarnish
(basic copper carbonate), more quickly with salt spray and acid to
form a different green tarnish (basic copper chloride) or very quickly
with sulfur-containing gases to form a black tarnish (copper sulfide).
Silver does not tarnish at all in ordinary air. But if it is used in
the kitchen, where traces of sulfur containing gases are produced
during cooking, it rapidly forms a black tarnish of silver sulfide.
Bronze is an alloy of copper and tin. It will react very slowly with
ordinary oxygen to produce a whitish tin oxide tarnish, or more
rapidly in the various types of polluted air to form the same products
as copper.
BRASS
Brass has a muted yellow color, somewhat
similar to gold. It is relatively resistant to tarnishing, and is
often used as decoration and for coins. In antiquity, polished brass
was often used as a mirror. Brass has great corrosion resistance
capability too.
Brass and
Hygiene:
Copper and brass are playing a leading role in the fight against
hospital-acquired infections such as MRSA and Clostridium difficile.
It has been shown that these pathogens, which can be spread by touch,
will die in a few hours on copper/brass surfaces. This does not happen
on stainless steel or plastic - quoted from http://www.copperinfo.co.uk/alloys/brass/
|
