Urine (whether human or any other animal) is highly corrosive.
One of the most labor-intensive areas during heavy maintenance checks on airliners are the structural elements under the lavs. I work with plenty of sheet-metal guys with airline MX experience; they've all got "horror stories" of finding ribs/stringers/skins under lavs that were basically powder under the paint, from piss-poor aim (pun intended), blue-juice leaks, etc. In one case, a guy pulled the lav floor in a 727, only to find the rib that was supposed to be under it had literally corroded to dust and was gone.