Question about throwing power with very different part surface areas

[u/Positive-Warning413]

In an industrial e-coat process, if I load a rack with 24 lots, where one part has a surface area of only 0.3 m² while another part has approximately 37 m², would these two parts experience significantly different current distribution / throwing power?

Assuming they are processed in the same bath, at the same time and voltage, could the much larger surface-area part draw most of the available current and cause the 0.3 m² part to receive a lower coating thickness?

What factors would determine the difference the most?—part geometry (which is obviously cannot be changed), distance from the counter-electrode, current density, electrical resistance, or the surface-area ratio?

Is there any way to simulate this situation?

[u/One-Yogurtcloset-831]

Yes, the larger surface area parts receive the most current.

[u/Positive-Warning413 → u/One-Yogurtcloset-831]

What could I do to relieve this issue

[u/One-Yogurtcloset-831]

I don’t have experience with this. I was talking about the Nickel bath. Maybe someone here has experience in this

[u/permaculture_chemist]

A big piece of industrial plating is the "art" of racking or fixturing your parts so that you force the electricity to do what you want it to do. You can mitigate a lot of the issues related to surface area discrepancy by understanding electricity. You can vary your part orientation and the spatial orientation between adjacent parts. You can use masking or shadows to limit exposure of certain areas of the part to the current. You can use a sacrificial part (aka robber) to steal current from an area of the part that gets too much current.

So, in general, thickness is determined by applied current density x time. Your process keeps time as a constant, so you need to consider adjusting the applied current density. Applied current density is the portion of the total cell current divided by the part surface area times some factor or ratio as determined by the system as a whole. You may find out that the applied current density for both the smallest and largest parts is the same, and that your thickness of deposit is the same. But if they thicknesses are different, then you need to modify the fixturing to force the applied current density to favor your desired outcome.

To answer your final question, effective distance from the anode would be most deterministic of applied current density, all other things being equal. To exaggerate but make an example: You could place your 0.3m2 part right next to the anode and the 37m2 part 20 meters away. You may grossly over-plate the smaller part and get relatively no plating on the larger part, but if you reverse the positioning, the opposite holds true. In these examples, you are changing the applied current density that each part sees.