I use electrolysis for rust removal and electroplating. I notice that when one of the electrodes is a closed tube or has other topological holes with a sufficiently large height:diameter ratio, the inside surface does not react.

My hypothesis is that current is not flowing through these surfaces because I didn't pay attention in the electromagnetism part of Physics 1 in high school, and karma has finally caught up with me 25 years later.

My questions are:

  • What is going on here? I vaguely remember something about Michael Faraday. Am I on the right track?

  • More importantly, how can I induce a reaction on the inside surface? Will running the opposite electrode through the hole work?

Here is a video of the effect: https://youtu.be/H6TrLn8TkS8

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Reaction visible on outer surface Calm, quiet inner surface

I don't think the setup details are that important, since the context here is physics not chemistry. However, for completeness, an example setup:

  • electrolyte is sodium bicarbonate and distilled water
  • both electrodes are mild carbon steel
  • cathode is a square tube
  • width of electrodes and distance between them is pretty much the same order of magnitude as the diameter of the tube, lengths of electrodes are one higher order
  • typically running 5-10A at 12-20V
  • electrolyte temperature held around 100-130F

Exactly. It's much easier for the current to flow to the outside of the piece than the inside. So almost no reactions take place there.

It can be tricky to plate the inside of something like that. You'll probably need to run auxiliary anodes inside the tube.

The other problem that can happen is the volume inside the pipe can be small and the solution inside can be stagnant. If so, you can deplete the solution of your plating material. You may need a pump or similar to force flow through the pipe and exchange with the rest of the tank.

  • $\begingroup$ Awesome and also thanks for the pump tip. Makes total sense. I'll experiment with a center electrode, I've got some sheet mild steel on hand that would be perfect. $\endgroup$
    – Jason C
    Oct 28 '21 at 1:14

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