http://www.ck12.org Chapter 22. Oxidation-Reduction Reactions
Corrosion Resistance
Some metals, such as gold and platinum, do not corrode easily because they are very resistant to oxidation by
common substances. Some other metals are easily oxidized, but the resulting metal oxide forms a relatively sturdy
coating on the surface of the metal that protects the interior of the structure from additional corrosion. For example,
aluminum reacts with oxygen to form aluminum oxide, which remains tightly packed on the surface. The aluminum
oxide prevents the interior of the aluminum from corroding. Not all corrosion is due solely to metal oxides. For
example, after copper is oxidized by oxygen gas, the resulting copper oxide reacts with carbon dioxide to form
copper(II) carbonate. This distinctive green compound is also called patina and prevents the copper underneath from
further corrosion (Figure22.3).
FIGURE 22.3
The green color of the Statue of Liberty is
from the coating of patina that forms when
its copper surface reacts with oxygen and
carbon dioxide in the atmosphere.Conversely, the iron oxides that form during the corrosion of iron form a surface that flakes off easily, continually
exposing the pure iron underneath to further oxidation. As a result, iron will eventually corrode completely through
until the object is destroyed.
One way to prevent corrosion is to protect the surface of the metal. Covering the surface of a metal object with paint
or oil will prevent corrosion by sealing the metal off from any surrounding oxygen. Unfortunately, these protective
coatings may eventually wear off or get scratched, exposing the metal surface and allowing corrosion to begin.
Corrosion-sensitive metals can also be coated with another metal that is resistant to corrosion. A “tin can” is actually
made of iron coated with a thin layer of tin, which protects the iron.
Corrosion can also be controlled by connecting the object to be protected to another object made of a metal that
corrodes even more readily. When an iron nail is wrapped with a strip of zinc and exposed to water, the zinc is
oxidized while the iron remains intact. Zinc is a more active metal than iron, so it is more easily oxidized. However,
if the nail remains exposed after all of the zinc is oxidized, the iron will then begin to corrode. The more active
metal in this type of setup is sometimes referred to as a sacrificial metal because it is sacrificed to corrosion in order
to protect the desired metal from unwanted oxidation. This technique, also called cathodic protection, is commonly
used to prevent the hulls of steel ships from rusting. Blocks of zinc or other active metals are attached to the underside
of the hull (Figure22.4). The zinc blocks preferentially corrode, keeping the hull intact. The zinc blocks must be
periodically replaced to maintain the protection of the iron.