My copper electrodes are discoloring, and I'm not sure why

[Hexnovae]

For a high school school project I'm trying to build a device that will separate ions dissolved in water. I'm pumping water trough a box that has copper plates in it, and I'm putting that in a magnetic field. Because of Lorentz force the ions should bend off to different sides and heap up, creating a charge. Theoretically.

In practice it isn't that easy (it never is, is it?). I built the box, and it appears to work, however there are other things happening that neither I nor my teacher can explain. Take a look at the first image of the imgur album. It's a graph that has the voltage on the vertical axis and time on the horizontal axis. From t=0 until t=40 nothing was on, it was just the box. At t=40 we started pumping water with NaCl dissolved in it and a bit later we turned on the magnet. At the bump at t=120 we inverted the flow of the water and the current of the electromagnet. At the peaks we tried to discharge the plates by plugging it into a battery. It had a negative voltage after that, which went positive and then back to the voltage it had during the first part (25 mV). The copper plates got a dark color after plugging in the battery, which faded as the current went back to 24 mV. They became a matte orange, as you can see in the picture.

When we stop the water flow the charge stays. Even if we short the circuit the charge stays. What is happening here? Obviously some kind of reaction with the copper plates, because they discolor as you can see in the pictures. But what's going on exactly?

 

[Borek]

Lorentz force (as long as the voltages involved are measured in volts, not in megavolts) is way too weak to overcome thermal movements of ions. What you observe is most likely some electrochemical reaction (dark coloration may suggest copper was oxidized to CuO) followed by voltage of a galvanic cell.

You may try to estimate charges that can build assuming your copper plates are just a capacitor - estimate its capacity, see what voltage you apply and that's all you need to calculate theoretical charge. You won't get higher charges than that, as these are what is needed to neutralize the electric field created by the potential difference.