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Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Larry Zitko, ChromeTech, Inc.
Aeration in Chrome Tanks
Q. How much aeration is required for the chrome tanks. We seem to be over
aerating and producing more mist than our ventilation system can handle. We
are using balls to reduce the surface area of the tanks, however the
aeration keeps the balls pushed out of the way exposing as much as 1/3 of
the tank surface.
A. All hard chrome plating tanks benefit by some kind of solution agitation.
Otherwise, two undesirable events may occur.
- Temperature can stratify in the tank, with the top region warmer
than the bottom region. I have measured as much as 7-8 degrees F. difference
in some tanks. This can cause hardness/brightness problems with the chromium
deposits on ling parts that are vertically oriented in a deep vertical
- Plating chemicals can form a concentration gradient, rather than
having a uniform concentration throughout the bath.
I have successfully used air agitation in most of my plating system designs
through the years. But it's important to deliver the right kind of air to
the hard chromium plating tank. Here are some suggestions:
- Never use air from the big, oil-lubricated shop air compressor. Delivery
pressure is much too high (100+ psi), and the pressure regulators that are
commonly used to reduce the pressure to your plating tank can fail in a
manner that may blow the chrome solution out of the tank. This presents both
a health risk to building occupants and also a significant environmental
incident. Moreover, the entrained oil will contaminate the plating bath,
leading to higher-than-normal trivalent chromium concentrations, adhesion
problems and so forth.
- Instead, consider using mild, oil-free air delivered from a dedicated
rotary-vane, oil-less compressor. Size the unit so that the working pressure
is slightly higher than the weight of the column of fluid that must be
displaced by the air. Deeper tanks need higher pressure than shallower
tanks, assuming that the air sparger (see next bullet) is located at the
bottom of the tank. As an example of the ballpark pressure range, I have
installed many rotary-vane compressors that are rated for only 10 psi.
- Don't introduce the air agitation into the tank at the end of an open
pipe. Instead, fabricate a bottom air "sparger", or distribution manifold.
1-1/4" or 1-1/2" schedule 80 PVC or CPVC is a good choice. This horizontal
pipe is drilled with a series of very small holes (1/32" - 1/8") that
distribute the agitation air over the length of the tank, rather than only
in one location. You can put some 1/2" lead wire in the interior of the
sparger to keep it from floating.
- Rotary vane compressors can't be fitted with pressure regulators, because
they run very hot and fail quickly when the discharge is deadheaded or
restricted so much that discharge pressure exceeds the design working
pressure. Instead, you can install an adjustable pressure relief valve on
the discharge piping, and then adjust the degree of agitation to a slow,
rolling action which does not generate a lot of chrome mist at the liquid
surface. This wastes some air but works well.
- Mechanical agitation may be used instead of air agitation. The
over-the-side type vertical pumps are safest, because if a leak develops,
the leaking chromic acid will still be confined to the plating tank. As with
the air strategy previously discussed, the discharge port of the chemical
pump should be directed to a bottom air sparger. Be sure and consult the
pump manufacturer for chemical compatibility issues for the materials of
construction for the pump. Again, you want only mild agitation in a chrome
- I'm not fond of the floating balls. They seem to get in the way, get trapped
in the parts and fixtures, reduce the desirable evaporation of water vapor
from the top of the tank, and have a low efficiency at reducing chromium