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Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)

by Randy Taylor, Advanced Tooling Corporation.
July, 2006

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 an hour.

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 / 3600).

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.

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