Hard Chrome Plating
What is
wrong with chrome plating?
Chrome
plating has been in use since the 1940’s and is a powerful,
simple and cheap process. The plating solution is very
simple (chromic acid, sulfuric acid, and sometimes
brighteners and other additives) and the process is quite
forgiving. Unfortunately chromic acid means lots of
hexavalent chrome (Cr6+), and the process is
quite inefficient, with most of the current going to
hydrolyze the water, producing copious amounts of hydrogen
and oxygen bubbles. When they rise to the surface these
bubbles burst, throwing a fine mist of hexavalent chrome
into the air. To protect workers and the environment, this
mist has to be sucked away in an efficient air-handling
system.
Chrome
plating generates several different waste streams:
-
Cr6+
mist air emissions – worker health and safety issue in
the plant, air pollution issue outside, must be trapped
in scrubbers
-
Cr6+-contaminated
waste water – must be treated before release to public
treatment plants or water courses
-
Solid
wastes – must be disposed of as hazardous waste:
-
Cr6+-contaminated
sludge and masking material
-
Cr6+-contaminated
air filters, solids and water from air-handling
systems
-
Groundwater contamination from leakage and spills (this
is rarely a problem with modern plants, which have
double-containment, but has been a serious source of
contamination in older brown-field plants).
Note that
the problem with chrome plating is the deposition process.
Chrome plating itself is benign.
What is
hard chrome plating used for?
In weapons
systems there are two primary uses for hard chrome
-
Wear resistance – Put down
by OEMs on hydraulic actuators, shaft journals, aircraft
landing gear, etc. for wear resistance (and to provide a
measure of corrosion protection)
-
Reclamation and rebuild –
This is the primary use for chrome plating in DoD.
Chrome plate is used by depots and overhaul shops to
bring worn or corroded parts back to dimensional
tolerances. This is done on many components that are
never plated by the OEM, as well as plated components
whose plating has become worn or scored.
(Numbers in
blue from EPA National Standards for Chromium Emissions,
1995.) |
