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topic 10497

# Resistance of a chrome plating bath

What is an ordinary electrical resistance of a plating bath? I have to chrome plate a large object (silencer of a 30's motorcycle). I'm planning to use a welding transformer and making a rectifier, which gives me at maximum 5.2kW at 29 Volts. I need at least 170 Amps. However, the maximum resistance of the circuit can only be R=U/I=29V/170A= 0.17 Ohms. This is possible only if the plating bath is almost forming a short cut in the circuit.

Any chances of success?

Marko Mikkonen
- Oulu, Finland

Hi, Marko. Someone may be able to give you a better value for the resistivity of the chrome solution--the only figure I have right offhand is 3.18 ohm-cm for a 1 N solution at 77 °F. It could be half that at proper concentration and temperature. We would need to know the surface area of the silencer in order to determine the resistance (and to comment on whether your figure of 170 Amps sounds reasonable).

But the empirical answer is that the resistance of the circuit will be less than 0.17 ohms because 29 volts is way more than enough for chrome plating.

Ted Mooney, P.E.
finishing.com - Pine Beach, New Jersey
Striving to live Aloha

I'm not sure what you meant by saying 3.18 Ohms-cm... Do you mean 3.18 Ohms/(square centimeter of surface area) or perhaps 3.18 Ohms/(cm of distance between the cathode and the anode) or something else?

If, for example, the first assumption would be correct, I guess then the total resistance would be R(total)=3.18 Ohms/(surface area in cm^2), because you can think the total resistance as a resistance of multiple parallel equal resistances (for which the equation would be 1/R(total)=1/R'+1/R'+...=N/R'). The total surface area of the silencer is approximately 14dm^2=220square inches. With this number the total resistance according to the earlier equation would be only 0.0023Ohms. Then much less then 29V would be enough to chrome plate the thing. But tell me if I'm wrong.

I calculated the value 170 Amps on grounds of an article which I found from the web (web006.pavilion.net/users/nickfull/chrome.htm). In that article it was said that 12 Amps per dm^2 (0.75 Amps/square inch) should be the correct current per dm^2 in decorative chrome plating.

I also know about having to nickel plate first. Actually I have to nickel plate the silencer first (because copper plating bath would corrode the silencer - it's iron), then grow the thickness of the metal by copper plating (the metal is too thin to be buffed first because of corrosion), nickel plate again and then chrome plate. I also know the stuff about buffing and cleansing between each step. I have read a book called "Electroplating" by J. Boyner and a bunch of articles from the net. The biggest worry I had though whether I have enough power to chrome plate.

So do you still think I have little chances of success?

Marko Mikkonen [returning]
- Oulu, Finland

'Ohm-cm' is the unit of measure for resistivity.

Resistance is measured in ohms. R=rL/A. So, if the Area of the silencer is 220 square inches, and the equivalent length from the cathode to the anode is, say, 4 inches, the resistance is 0.058 ^ 0.0228 ohms, and 9.9 ^ 3.9 volts will be required for plating, although I suspect that the resistivity of a chrome plating solution at operating temperature is less than 3.18 ohm-cm. Good luck.

Ted Mooney, P.E.
finishing.com - Pine Beach, New Jersey
Striving to live Aloha

June 12, 2020

I have tried all variations of your math and I cannot come up with the same results as you have posted. From my research, the resistance is calculated R = ρ(L/A)

R = 3.18(10.6 cm/1419 sqcm) = 0.0238 Ohms

using .75 amps/sq in, it would take 3.8 volts to plate.

Can you verify that the equation that you have posted is correct? If it is correct, how did you arrive at those numbers? Where did you get the data for ρ(roh) of the chrome plating bath? Thanks!

- Portland, Oregon, USA

June 2020

A. Hi Dade. I did indeed make a mental slip :-(
Since I used Marko's English units for length and area, I should not have left his resistivity in the metric units of ohm-cm, but should have divided it by 2.54 to convert it to ohm-in. So your numbers for Resistance and Voltage are much closer to correct.

Regards,

Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
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