Got corrosion?
Got High Humidity? 
Thanks. 
I guess a better question is do you see anything in the pictures that would be a contributing (and correctable) factor.
No, the condensate line didn’t appear to be leaking.
Nope… looks like everyday excessive moisture to me.
Guess my question would be why?
I assume the humidifier (above the BIP in photo #2) is NOT leaking … bad float/small crack in the jacket, did you check the feed for the humidifier? Be it a flare, commpression fitting possible small leak you may have overlooked? is this space conditioned? dehumidifier installed? Tough to tell without being there. (overstating the obvious) :mrgreen:
Could have been the piping was sprayed with a gas leak detection solution and was not cleaned off.
Don’t I see a drip on the bottom left of the union? both pic# 1 & 3.
Induced draft leak? Or other leak from above.
Chemical storage (chlorine) nearby?
Old piping used for drip leg and short leg?
Wire brush the rust off and paint it.
That looks like standard galvanized pipe. Anywhere that I have worked that gas has been common, only black iron pipe or copper were allowed. Is that standard galvanized?
I’m going to guess higher humidity than recommended (why?), and/or possible stored corrosive agent, and/or possibly leaking corrosive condensate (past or present) leaking onto the piping. I don’t see combustion design provisions in the pics. This could be a contributing/possible culprit.
Let us know what you finally determine, after further inspection by a Prof. HVAC Contr.
Looks like black pipe and someone didn’t clean off the leak detector solution.
Oops, sorry, I see Gary already said that.
It IS black pipe.
not a problem with the gas pipe, but is the condensate line traped?