My post was mainly focused at electrical soldering. The difference between your copper pipe example and electrical is that obviously the electrical will be flowing current. That by its self will promote electrolysis but will be accelerated if flux residue is left behind. I have had Mil spec aerospace solder certification for 20 years now and have to be tested every 2 years to keep my cirtifications. The material is very specific when it comes to the use of flux and rosin core solders.