Chemistry Reference
In-Depth Information
the surface of iron and steel exposed to the environment, for example, has
the same composition as the iron ores from which the metal is extracted.
“Lay not up yourselves treasures on earth where moth and rust doth
corrupt,” states an admonition in the New Testament, implying that the
inevitable course of corrosion was already familiar in ancient times. Not only
iron and steel but most metals corrode. Unprotected surfaces of copper
exposed to the atmosphere, for example, grow dull within a short time after
exposure; the dullness is generally due to the formation of a coating created
when the metal surface reacts with atmospheric components (oxygen, water
vapor, and carbon dioxide) and pollutants (such as hydrogen sulfide in the
atmosphere and salts carried by the wind). After more or less extended
periods of time a layer of copper carbonate and copper sulfide, gradually
growing in thickness, turns the initial dullness of the copper into an intensely
green, characteristic coloration on ancient copper surfaces. Even in relatively
unpolluted environments a copper surface will take on, within a few years of
exposure, a green bloom and will eventually turn green all over. Copper
roofs, for example, turn green in regions with relatively uncontaminated air
within 50-100 years; in polluted urban air, however, the change takes
place in less than about half that time. The corrosion of copper or bronze
buried in soil, or immersed in water, is generally faster. Most metals corrode
because in any natural environment all metals and alloys, with the exception
of only the noble metals, react chemically and are converted to more stable
compounds than the metals and alloys themselves. The process of corrosion
is, therefore, natural and unavoidable (Scully 1990; North and Macleod 1987).
The reaction that takes place during corrosion processes can be
expressed by the general equation:
oxidizing elements
corrosion products,
Metal or alloy
+
or compounds
→
stable to the environment
in the environment
All metals and alloys, except for the noble metals, have a natural tendency
to combine with oxidizing substances in their surroundings, particularly
oxygen, and to be corroded. During metal-smelting operations, this natural
tendency is reversed: through the application of large amounts of energy, the
metals are recovered from their ores. But this is only a temporary situation;
nature takes its unavoidable course, and in time, corrosion processes reestab-
lish the equilibrium that exists in nature. All that can be done to preserve
metals and alloys and prevent their return to the crust of the earth as metal
oxides is to slow down corrosion processes for shorter or longer periods of
time (Jones 1996; Scully 1990).
