1999 Ford F-150 Heater Core Failure

I am about to replace the heater core for the 4th
time in my '99 F-150 (V8, 281 cid FI). The last time
it was replaced. My mechanic indicated (after reading
Ford messages) that there may be an electrolysis
problem and he changed radiator fluid, checked PH
and voltages. He also tried to find a copper core
but couldn't find one, so went with aluminum. Which
he didn't want to do. The present core lasted less
than one year.

Any suggestions would be appreciated. This is a very
expensive problem for me!

Thank you, Craig Wilson