Cadmium Plating
Cadmium is
a sacrificial coating – i.e. in a corrosive environment the
cadmium dissolves, leaving the underlying steel intact.
Even when scratched, the surrounding coating still protects
the exposed steel. The only coating materials that have
this property are the electronegative elements – Cd, Zn, Al
(Mg and Be as well, but we do not use those as coatings).
You can replace Cd with a barrier coating such as Ni or a
polymer, but once it is scratched the protection is lost.
Cd
plating is forbidden under the European RoHS, ELV and WEEE
rules, and it is a worker and consumer exposure problem as
well as an environmental contamination problem. So very
little Cd plating is used in consumer products. It used to
be used for fasteners in cars and outdoor applications such
as decks, as well as for metal items used outside such as
padlocks. Now Cd has been replaced in most of these
applications, leaving it primarily as a corrosion resistant
coating for aircraft components. |
