A whole batch of comments here on why the thing is wet. Who cares if it's wet? Sure blow back, bad drains and all types of thing can cause that. It's a coil! It gets wet. Plain almost distilled water is not going to cause galvanized metal to rot out that fast! Look at the way that copper piping is discolored as well. This is just not the normal patina copper takes on in time.
There is something in the airstream (probably from the home itself) that is reacting with that water and turning it corrosive. I think of a few beauty shops I worked on years back when hair sprays had propellants that would rot the hell out of steel heat exchangers using inside air for combustion.
Even the worst pos galvanizing job on that coil would not account for the way that copper looks.