Page 28 - Curriculum Visions Dynamic Book

(Above) Copper and alloys
that contain a high proportion
of copper develop a greenish patina of copper carbonate when exposed to the atmosphere (see also the Statue of Liberty on page 14). The patina protects the metal below it. But if it is regarded
as unsightly, it can be removed temporarily using cleaning agents.
Making metal resist corrosion
corrosion is a surface chemical reaction that takes place between a metal and its environment – usually a combination of air and water. Some reactive liquids, such as acids and strong alkalis like potassium hydroxide, can also cause severe corrosion.
When a metal corrodes in water, for example, by being left out in the rain, the metal reacts chemically with water in the presence of air, and the metal atoms lose some electrons to the water. In time holes form in the corroding part, and it become useless or dangerous.
Of the common metals only gold does not react with air or water at all. Others, like chromium, react very poorly and so are almost corrosion free. Copper is typical of metals that react in such a way as to protect themselves. Copper develops a protective green coat. Thus a newly roofed building using copper may start out as an orangy colour, but it soon changes to green. This is called a patina. Once formed, the coating is stable.
(Left) Paint has traditionally been used to protect steel. Once it wears away, however, the steel quickly rusts.
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