Some years back I serviced a fleet of Caterpillar forklifts which constantly threw trouble codes and ate sensors, brass plugs and even the thermostats. To make a long story short;

First check for *AC* voltage in the coolant. If you have any (~5vac) have your alternator tested by a competent auto electric shop (not the NAPA spin-o-tron)
Secondly, as others have said be certain that the radiator, chassis, and engine are all thoroughly grounded together. I prefer to ground the chassis and radiator both to the same attachment point as the engine block negative. If your engine and radiator are completely grounded together, there won't be any juice to pass through the coolant.

Lastly, and importantly; what we discovered with the Caterpillar forklifts was that the alternator bodies were not grounded to the engines. The blocks and brackets were painted, and although the engine and radiator were both grounded to the frame, the ground loop found its way back to the alternator through the engine(starter ground)chassis(lights/gauges grounds)via the coolant. I still don't fully understand the path, but we grounded the alternators thoroughly and the problem went away.