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 as well.
It is
still permitted as a corrosion resistant
coating for aircraft and military components, applications
that are currently exempt from the rule. However,
since there are a growing number of alternatives, some of
which are proving to have as good or better performance, it
is unlikely that this exemption will last more than a few
more years.
Galvanic Series Chart
|
