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.
The hard chrome plating process.
(Numbers in blue from EPA National Standards for Chromium Emissions,
1995.)
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