Corrosion
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| Corrosion |
What do you understand by Corrosion?
Corrosion, partial or complete wearing away, dissolving, or softening of
any substance by chemical or electrochemical reaction with its environment. The
term corrosion specifically applies to the gradual action of natural
agents, such as air or salt water, on metals.
The most
familiar example of corrosion is the rusting of iron, a complex chemical
reaction in which the iron combines with both oxygen and water to form hydrated
iron oxide. The oxide is a solid that retains the same general form as the
metal from which it is formed but, porous and somewhat bulkier, is relatively
weak and brittle.
Three
methods may be used to prevent the rusting of iron: (1) alloying the iron so
that it will be chemically resistant to corrosion; (2) coating it with a
material that will react with the corroding substances more readily than the
iron does and thus, while being consumed, protect the iron; and (3) covering it
with an impermeable surface coating so that air and water cannot reach it. The
alloying method is the most satisfactory but the most expensive. A good example
is stainless steel, in which chromium or chromium and nickel are alloyed with
the iron; this alloy is not only absolutely rustproof but will even resist the
action of such corrosive chemicals as hot, concentrated nitric acid. The second
method, protection with an active metal, is also satisfactory, but expensive.
The most common example of this method is galvanizing, in which iron is covered
with zinc. In the presence of corrosive solutions, an electric potential is set
up between the iron and the zinc, causing the zinc to dissolve but protecting
the iron as long as any zinc remains. The third method, protection by coating
the surface with an impermeable layer, is the least expensive and therefore the
most common. It is satisfactory as long as no crack appears in the coating.
Once the coating cracks, however, rusting proceeds at least as fast as it would
have with no protection. If the protective layer is an inactive metal, such as
tin or chromium, an electric potential is set up, protecting the layer but
acting on the iron and causing the rusting to proceed at an accelerated rate.
The most satisfactory coatings are baked enamels; the least expensive are such
paints as red lead.
Some metals,
such as aluminum, although very active chemically, appear not to corrode under
normal atmospheric conditions. Actually, aluminum corrodes rapidly, and a thin,
continuous, transparent layer of oxide forms on the surface of the metal,
protecting it from further rapid corrosion. Lead and zinc, although less active
than aluminum, are protected by similar oxide films. Copper, a comparatively
inactive metal, is slowly corroded by air and water in the presence of such
weak acids as carbonic acid, producing a green, porous, basic carbonate of
copper. Green corrosion products, called verdigris or patina, appear on such
copper alloys as brass and bronze, as well as on pure copper.
Some metals,
called noble metals, are so inactive chemically that they do not suffer
corrosion from the atmosphere; among them are silver, gold, and platinum. A
combination of air, water, and hydrogen sulfide will act on silver, but the
amount of hydrogen sulfide normally present in the atmosphere is so small that
the degree of corrosion is negligible except for the black discoloration,
called tarnishing, produced by the formation of silver sulfide.
The
corrosion of metals is more of a problem than that of other materials. Glass is
corroded by strongly alkaline solutions and concrete by sulfate-bearing waters.
The corrosion resistance of glass and concrete can be greatly increased by
changes in their composition.
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