Nickel takes a bright polish and keeps it well, so nickel plating has
become the favorite method of protection for small objects where the
expense is not prohibitive. Copper plating is used for fine wires. A
sheet of iron dipped in melted tin comes out coated with a thin adhesive
layer of the latter metal. Such tinned plate commonly known as "tin" has
become the favorite material for pans and cans. But if the tin is
scratched the iron beneath rusts more rapidly than if the tin were not
there, for an electrolytic action is set up and the iron, being the
negative element of the couple, suffers at the expense of the tin.
With zinc it is quite the opposite. Zinc is negative toward iron, so
when the two are in contact and exposed to the weather the zinc is
oxidized first. A zinc plating affords the protection of a Swiss Guard,
it holds out as long as possible and when broken it perishes to the last
atom before it lets the oxygen get at the iron. The zinc may be applied
in four different ways. (1) It may be deposited by electrolysis as in
nickel plating, but the zinc coating is more apt to be porous.
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