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GRADES 1-8
OXIDATION AND CORROSION
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GRADES 1-8
OXIDATION AND CORROSION
BACKGROUND INFORMATION:
"OH NO, DON'T CLEAN IT!", exclaims the
antique dealer. "It has a beautiful patina," he
says. The bronze statue looks dirty. You look puzzled.
You know the patina is actually a form of corrosion.
Tarnish, rust, patina and other forms of corrosion are
formed when certain metals are exposed to the atmosphere. Oxygen
in the air or water chemically reacts with these metals and
they corrode. This chemical reaction is called oxidation. Although
many chemical reactions occur very quickly, the chemical reactions which
corrode metals often occur very, very slowly over a period of months or years.
Although the patina on an object of art helps identify its
age and may increase its value, the corrosion process is
destructive and causes millions of dollars of damage, safety concerns
and the loss of valuable artifacts and personal treasures. Corrosion
damages buildings, bridges, railroad tracks and electrical components --
all of these objects have metal components. Corrosion is of
great concern to museums and other organizations entrusted to preserve our
cultural heritage. Artistic and historical artifacts as well as statues,
monuments and historical buildings are also affected by corrosion.
Preserving historical treasures can be even more difficult for a museum.
A beautiful leather or cloth belt could be decorated with a
metal belt buckle. Although a low humidity (drier air)
environment is ideal for the metal buckle, a moister humidity
will prevent the leather from drying out and cracking.
Here are some definitions of common terms used in this field:
Scientists and engineers have studied and developed methods to detect,
prevent, restore and preserve objects affected by oxidation and corrosion.
Different procedures are used for different metals. You know from
your own experience certain cleaners work better than others on different
objects. You might use a glass cleaner on a mirror,
but a steel wool soap pad on a dirty pot.
Similarly, some restoration, preservation and cleaning techniques work better
than others do on certain metals. Some techniques do not
work at all on certain metals -- so it is important
to know exactly what metal you have.
Recognizing Metals:
Some metals occur in nature like silver, gold and nickel.
Other metals are actually combinations of metals called alloys. For
example, brass is composed of copper and zinc, bronze
is primarily composed of copper and tin. Some metals like
aluminum and tin are used to coat or make other metals
more corrosion resistant.
Here is a list of metals, the metals periodic chart
symbol (meaning it is a metal which occurs in nature)
or the combination of metals, its natural color and oxidation
or corrosion color. Don't be fooled! Although silver
in color dimes, quarters are made of nickel.
Corrosion: Gradual wearing away through chemical changes.
Oxidation: To combine with oxygen. Both air and water
(H2O) contain oxygen.
Rust: The reddish brittle coating that forms on iron which
has reacted with moist air; steel has a brown rust and zinc a white rust.
Patina: A corrosion layer on copper or copper alloys
(like bronze). It can be brown or green.
Tarnish: Dull, stained or discolored metal surface, due
to a thin layer of corrosion.
| Metal | Element Symbol Or Composition |
Natural Color | Oxidation Color |
| Aluminum | Al | Silvery White | Black |
| Brass | Copper and Zinc | Gold | Dark Brown to Black |
| Bronze | Copper and Tin | Yellow to Olive Brown | Dark Brown to Black |
| Copper | Cu | Light Red Brown | Green |
| Iron | Fe | Lustrous Silvery White | Reddish Brown |
| Silver | Ag | White-Grey | Black |
Prevention and Preservation: Most metals have been buried underground inside the earth. When exposed to the earth's atmosphere metals can begin to corrode. This can happen outside or indoors! Metals unsheltered from moisture or pollutants will corrode. Lunar rocks collected during NASA missions are enclosed in special cases. Although the moon has no atmosphere, the earth does and moon rocks would begin to corrode if exposed to the earth's atmosphere!
One form of prevention is to remove, shelter, or cover the object from a harsh environment. Of course, that is not always possible --- for example a large outdoor statue. Another alternate is to seal the metal with a protective coating that prevents corrosion.
Restoration: If the metal has already oxidized the sections can be cleaned. When the corrosion is more extensive, the damaged areas need to be removed or replaced. Once the damage is removed the piece can go through one of the prevention processes to stop further damage.
Restoration processes are very important to a museum, where irreplaceable treasures can't have replacement parts. If we had a dirty plate, we could just wash the dirt away. The food and liquids are separated from the plate. The plate doesn't change. Corrosion actually wears away part of the object. As you remove corrosion you are actually removing part of the object. There is no easy answer for a museum. Unlike a bridge or railway track where a corroded part and be replaced, replacing a portion of a cultural treasure changes a piece of history.
REFERENCES:
Leygraf, C. and Graedel, T. E., Atmospheric Corrosion, John Wiley & Sons, Inc., New York, 2000.