Man's physical universe

xanabras

METALLURGY 639

twenty years. Copper is sometimes used to coat iron, but copper is

so readily tarnished itself that it is not always a desirable coating for

iron, although it will prevent rust as long as it covers the whole surface.

Metallic coats may be applied by electrolysis; by electrochemical

displacement, as in the case of copper, zinc, tin, nickel, chromium, and

cadmium; by dipping the iron into the molten metal as in the case of

zinc, tin, and aluminum; by spraying with the recently invented

metallic-spray guns, which melt the metal and spray it on a surface

in one operation; by cladding, i.e.,

heating thin sheets of such metals

as copper, nickel, or aluminum into the surface; by using the finely

divided metal as a paint pigment; and by various other methods. As

mentioned above, tin and zinc may be applied to iron by dipping in

the molten metal. Tin is the favorite material for cans and pans. It

affords excellent protection as long as the surface is unbroken because

it reacts with the oxygen of the air to form on its surface a thin film

of tin oxide, which resists further action. Tin is below iron in the

electrochemical series and therefore accepts electrons from the iron,

once the surface is scratched, so that the iron then rusts even more

rapidly than it would in the absence of the tin. Zinc, on the other

hand, is above iron in the electrochemical series, and all of the zinc

will go into solution before the iron is attacked. Galvanized iron is

therefore preferred to tin for many purposes, but it

cannot be used in

contact with foods because it may dissolve to produce toxic zinc

compounds.

Cadmium, a metal similar to zinc, is now being used as a metallic

coating, especially of hardware articles.

Zinc may be applied to iron by the Sherardizing process, in which

the articles are heated with zinc dust in a tight drum to 800° F., thus

forming alloys at the surface.

Aluminum powder in fine flakelike form is now widely used as a

pigment in paints for the protection of bridges, oil tanks, and many

other metal structures. Aluminum, like tin, does not easily corrode,

because it forms a protecting layer of aluminum oxide on its surface;

if

it were not for this fact, aluminum would corrode more readily than

iron because it is above iron in the electrochemical series and, therefore,

has a greater tendency to give up electrons.

In 1907 Thomas Coslett, an English chemist, invented the process

which now is known as the " Parkerizing Process." It consists of producing

a coating of basic iron phosphate by dipping the iron into a

hot, dilute solution of iron phosphate. This process gives a pleasing

dull-black finish that serves as an excellent base for paint or enamel.

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