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E-coat throwing power, coverage inside tubes

Quickstart:
     Electrocoating is similar to dip painting in that the components are immersed in a solution. It is similar to electroplating in that the coating is created on the components by electrochemically converting that solution into a coating. But it differs from electroplating in that the paint coating it produces is insulative rather than conductive like metallic coatings.
     An advantage of electrocoating compared to either is that it can create very thin, very even, coatings because its insulative nature drives the current towards whatever areas of the surface are not yet covered.
     On the other hand there tends to be nearly no current flow deep inside a long tube, so the issue at point here is what can be expected.

Q. I've been trying to research coverage of e-coat inside tubes and have found very little information online. I hope that someone here can help shed some light on this. I am in the engineering and design world. Not in the finishing world. I had been under the impression that with e-coat, anything the liquid touches will get coated. We have learned differently after cutting open various tubes to find raw metal and rust.

One of the theories I am exploring is around current flow. I have read articles talking about powder coating and Faraday cage effect. I have also read about electroplating issues inside tubes where you would need an anode inside the tube in order to get any plating inside. Do both of these relate to e-coating as well? I have attached a photo showing a 1" tube that was e-coated and then cut open to show that the paint coverage goes about 8" inside both ends of the tube and then fades to nothing. I am going to do the same with a 2" tube to see the effect. I'm just looking to educate myself and others about what the expectations should be for different lengths and diameters.

Jamie Larson
Mechanical Designer - Bloomington, Indiana
December 4, 2024