I need to make a 90 degree turn down out the back of a urinal in a 2x4 wall. The urinal has a 2" female threaded brass coupling. Any PVC that I try to use ends up taking up more than the 3 1/2" of space I have in the wall. I need to end up with the standard 1 1/2" drain pipe. Either reducing down first or after a PVC elbow both take up too much space because the only coupling I've found for the brass is a male threaded 2" to a female 2", and that sticks out too far when you add the elbow. I've found a galvanized coupler to a galvanized threaded elbow that will get me to vertical within the gap OK, but -here's my question (finally): is the corrosive nature of urine something to worry about with respect to the galvanized coupler and elbow ? Thanks.