Mate replaced a rad on a 2002 4.4 6 months ago and the owner has bought it back to his garage complaining of rad leak. So rad off and sure enough small pin hole leak from a couple of places. Took it back under warranty
to the independent supplier who he bought it off and was told it had been caused by electrolysis. First he'd ever heard of that and a new one on me. Given the following fact sheet.

Testing for electrolysis in cooling systems
A voltmeter capable of reading both AC and DC currents is required to test cooling systems. The meter needs to read zero to the maximum voltage of the system being tested in tenths of a volt. The meter leads must be long enough to reach between the coolant and the groundside of the battery. An ohm function of a voltmeter is very helpful to pinpoint areas of resistance in as electrical system that will cause an electrical current to ground through the coolant rather than the engineered electrical circuit.
Procedure

Attach the proper meter lead to the groundside of the battery, negative-to-negative or positive-to-positive.
Install the second lead in the coolant touching the coolant only.
Read the DC and AC voltage with all systems off. If a block heater is present, also take a reading with the heater turned on. If an automatic battery charger is present, as a standby system, also take a reading with this system running.
Read the DC and AC voltage with the electrical starter engaged.
Read the DC and the AC voltage with the engine running and all systems turned on: lights, coolers, fans, heaters, air conditioning, cell phone, two-way radio, including the phone and radio on both standby and transmit.
The above procedure will test a complete system except for an electrical current, which can be generated by the rear end transmission. This is particularly true with air bag suspensions, rubber pad suspensions and rubber-mounted transmissions. Any current generated will travel up to the drive shaft to ground through the engine coolant. Grounding rear ends and transmissions is strongly recommended.
Voltage of zero to .3 is normal in a coolant of cast iron engine. Such an engine will be destroyed with time by .5 volts, and engine manufactures are reporting .15 volts will destroy an aluminum engine.
The current will be AC if the problem is due to static electricity.
If the coolant shows an electrical problem with all the equipment turned on; turn off one system at a time until you finally turn off the system that stops the electrical current. When the current stops, this will indicate the electrical system causing the problem.
Be partially careful of starters. They can cause as much damage to a cooling system as a direct connection to an arc welder. This is due to the amperage present.
Always change the coolant if a current is detected. The electrical current will destroy the protecting chemicals in a properly inhibited coolant.

As I had a rad replaced in a 2003 4.4 just weeks after he had done the one mentioned above, we got the voltmeter out and checked mine yesterday... sure enough .7 volts. We then went and checked his Jeep ( sorry about the language ! ) . Sure enough .5 volts. Next a customers P38... around .6 volts. Put a ware to the various rads and then earthed to body..... no change in the voltages shown. So to conclude, none the wiser ???

Any one else come across this ?

I've not come across it before but just measured mine out of interest with the car "asleep" and get 0.17V. ie coolant to battery -ve terminal.
Also checked engine block to battery -ve terminal and got 0.00V.
I can only assume therefore that the aluminium radiator core and coolant is acting like a battery and producing a nominal voltage ie 0.17V
I have no idea what to do about it other than possibly changing the coolant.
Does the coolant become slightly acidic over time?

As the coolant 'breaks down with age and cycles' dirt is often picked up from within the system. this dirt as it rubs across surfaces is like a small balloon being rubbed over a wool jumper.

It builds up a little bit of charge over time.

Changing the coolant and flushing the system may help a bit, and ive seen what this does in poorly designed heater matrix's where the metal is very thin on some bends and literally gets worn thru over time, leading to a leak! FFRR MY06 facelift With TDV8 Alloys Zeros/ATR's
Mantec Sump Guard, Rigid Load liner, MY10 BT upgrade.

Old fluid is unlikely to be the problem this time though as it must've been changed when the rad was installed so would only be 6 months old.

But if it's not flushed fully how much of the old fluid or dirt is still in there? FFRR MY06 facelift With TDV8 Alloys Zeros/ATR's
Mantec Sump Guard, Rigid Load liner, MY10 BT upgrade.

DaveK - Will's questioning your flushing.

I'm a 2 flush man myself as you know David  FFRR MY06 facelift With TDV8 Alloys Zeros/ATR's
Mantec Sump Guard, Rigid Load liner, MY10 BT upgrade.

....er, how would I know?

Its ok David, we cant hide it forever  FFRR MY06 facelift With TDV8 Alloys Zeros/ATR's
Mantec Sump Guard, Rigid Load liner, MY10 BT upgrade.

Had an auto spark look at the car in question and he found a reading in the cooling system that would be enough to get electrolysis going... mind you, as he went on to demonstrate, he could get a reading of the plastic trim in the engine bay !!. Several vehicles later and we had yet to find one that didm't give a reading in the coolant. Then he posed the question... how come the heater matrix hadn't been destroyed as well seeing as the aluminium in that is really thin to start with ?
Looks like the rad manufactures have discovered a bit of dodgy science to ensure they never have to pay out under warranty.