http://www.ck12.org Chapter 23. Electrochemistry
fluorides and metals contaminants can also pollute water, soil, and air if not carefully managed.
A major expense in the refinement of aluminum is the cost of electrical power. Refining plants are usually located in
areas where electricity is abundant and less expensive. Producing aluminum from recycled materials uses only about
5% of the electricity needed to produce the aluminum from bauxite, which makes aluminum recycling profitable as
well as environmentally beneficial.
Corrosion
Corrosioninvolves the formation of an oxidized form of a metal by an electrochemical process. A common example
is the rusting of iron when exposed to water and oxygen. The tarnishing of silver and the red or green deposits formed
on copper are other examples. Corrosion in all its forms costs the U.S. millions of dollars each year in expenses for
metal replacement.
FIGURE 23.14
Rust formation involves a complex series of reactions that are still not well-understood. The first part of the overall
process appears to be the following reaction:
2 Fe + O 2 + 4 H+→2 Fe^2 ++ 2 H 2 OThe iron then undergoes further oxidation in the presence of O 2 to form ferric oxide (Fe 2 O 3 ). The complete process
is somewhat more complex.
The corrosion of copper involves a reaction with carbonic acid, which is formed when atmospheric carbon dioxide
dissolves in water. This acid is weak and unstable, but it can produce significant damage over time. The reaction
between copper and carbonic acid generates copper carbonate, which is responsible for the familiar blue-green layer
seen on structures such as the Statue of Liberty.
Silver will tarnish through a similar type of reaction with hydrogen sulfide, which is found in low concentrations in
the atmosphere and is often generated by foods with a high sulfur content, such as eggs. The tarnish in this case is
silver sulfide (Ag 2 S).