I don't know a thing about electroplating, but the process seems simple enough. You have decent mixing and hence you're not depleted in zinc ions inside the ID, so what else could it be? It must be current flow distribution- the liquid column is at least 1.6 m in length to the midpoint of the ID from each end, which is an awfully long conductor. A counter-electrode on the ID may be required to get sufficient current flow to have proper coating all the way into the ID.

You need to run a wire through the tubing and keep it centralized to act as an electrode inside the tube.

I'm not convinced that 'a wire' will do as an electrode.

You may have to hang a round zinc bar in the center of each tube, and connect those bars as the counter-electrodes.

Mike Halloran
Pembroke Pines, FL, USA

The DC charge will 'push' the charge from the inside to the outside, the same way voltage is only on the outside surface of a wire. Also Van de Graff (sp?) generator has a 'dome' and the charge concentrates on the outside.

Dik