The closed loop systems are filled with a fluid that has corrosion inhibitors to drastically slow galvanic corrosion. It still takes place, just much much slower than if it had no corrosion inhibitors. Being completely sealed from outside contaminants also helps slow the process. I'd advise against integrating a closed loop system into a custom loop.
If you plan on doing a custom loop at any point in the future but want to start off with something small, you could get a starter kit, like the XSPC Raystorm kit. Or there is also the Swiftech H220, aka Coolermaster Glacer 240L
. It is an expandable system that allows you to add more radiators and blocks without having to completely disassemble other closed loops systems like you would with one of the Corsair Hydro Series systems