That particular system was fitted late sixtys and was full of weird stuff like triacs and thyristors all connected with great big bus bars ... complete mystery to me. I'm sure modern systems are very different. My visits to QE2 was while she was still steam driven!
The theory, as I understand it (but don't take it as gospel, it was a long time ago), is that steel is a mix of metals and you get potential differences between one area and another (on a very small scale). Therefore, by galvanic action, rusting occurs. By impressing current through the hull, you make the whole hull negative and prevent the little mini currents between areas of the hull.
Luckily the cabinets didn't go wrong too frequently. It was, as you say, the annodes and especially the connections within the cofferdams that used to give the most trouble. I never found one where the platinised titanium had corroded/eroded but plenty where the hull penetration had leaked filling the cofferdam with water. Before they started using solid filled cable, it wasn't uncommon to find sea water leaking into the control cabinet!
To try and keep the system going until the next dry dock, the trick was to turn the flange on the cofferdam through 180° so the drain plug was at the top and then fill it with hot paraffin wax. It used to work surprisingly well, but was a bugger to get out when it came time to change the anode. I spent hours sitting in a stinking bilges with a hammer and screwdriver chiseling the stuff out!
There was a way we used to check the hull potential at various places which should have highlighted if an anode was malfunctioning. We had things they called copper/copper sulfate half cells. It was a tube with a copper electrode in the center and a porous plug in the end. Copper sulfate was packed in the tube around the copper rod. A long wire was attached to the copper rod and the cell dangled in the water. A multi-meter was then attached between the wire and the hull if I remember correctly, 200 mv indicated all was well. I often used to get into trouble for chipping paint off the hand rails to make a circuit! As with the cabinets, I've no idea how or why this worked.
I use to carry the cell and all my tools in hand luggage ... can't see that being allow these days!
