Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Randy Taylor, Advanced Tooling Corporation.
Hydrogen generation in hard chrome plating
Q. By knowing the amperage and voltage
used during the chrome plating process, how can the amount of hydrogen
generated be calculated? We want to do a mass balance for hydrogen and know
where the generated H2 ended up.
A. (Answer from Paul Chalmer, NCMS) If you know how much current is flowing through the cell, you know how
many electrons are going into the cathode. For every amp, you're
putting a coulomb of charge into the cell every second, or 3600 coulombs
According to Faraday's law, it takes 96,490 coulombs to plate an
"equivalent" of metal. That works out to about 27 amp-hours (96,490 /
An "equivalent" depends on the weight and charge of what you're
plating. An equivalent of chrome, plated from hex chrome, is about 8.7
grams of metal.
If all the current in a chrome bath went into plating chrome, then you
would get the full 8.7 grams of chrome for every 27 amp-hours that
passed through the bath. But chrome baths are pretty inefficient. If
you figure that maybe only 20% of the current goes into plating chrome,
and most of the rest goes into hydrogen, you would expect 0.2
equivalents (1.7 grams) of plated metal, and 0.8 equivalents (0.8 gram)
of hydrogen for every 27 amp-hours. That's the same as one gram of
hydrogen for every 34 amp-hours.
At room temperature and pressure, a gram of hydrogen will occupy about
11.2 liters, or 0.4 cubic feet. To create one cubic foot of hydrogen,
you would need (34 / 0.4), or 85 amp-hours.
So the bottom line is, you can divide the number of amps passing through
the bath by 85, and figure that the process is producing roughly that
many cubic feet of hydrogen per hour.
Note that if you have a completely covered tank that is not being
ventilated, and if you're passing a few hundred amps through the tank,
you will produce enough hydrogen to displace all of the air in several
cubic feet of head space in less than half an hour. With the other
electrode in the tank producing oxygen, and with the potential for
arcing, it's not immediately clear to me why the potential hazard
doesn't seem to be perceived as more of a concern, as tanks are being
retrofitted with full covers.