what would happy if galvanized pipe is connected directly to copper pipe?
Not a thing. However consider that with copper and plastic, water lines remain the same indefinitely, (PVC must be concealed from sun and weather) with galvanized, the flow of water through it creates electrolysis. This then causes the build up of scale inside the galv. pipe and reducing the inside diameter.
You will have a reaction called electrolosys which will cause the copper to turn green and will eat holes causing leaks either use a dialectric union or replace all the pipe with the same material
If you connect copper to galvanized directly, the galvanized will 'not be happy'. It will rot the steel over a period of time - a year or so. Use a 6 brass or stainless steel nipple in between the two to prevent problems. There are dielectric unions available, but they tend to clog up after a while.
