[physicsdude]

I hope this is the right place for this - if not, please forgive me.

Real life situation - we have a vented, galvanized metal deck with concrete poured directly on top of it - creating a pH at the interface of approximately 13. 

On the bottom of the deck, when the humidity in the room is very high, it condenses and creates a brownish, soapy liquid that is attacking the primer on the metal joists.

No moisture is coming from above, only below due to humidity in the air. 

Any ideas on what this reaction is creating (what the brownish liquid could be) ?

Just searching for ideas here...thanks!

[MOTOBALL]

If galvanized means zinc-coated iron, then it sounds like you have a solution of zinc hydroxide [Zn(OH)2], formed by NaOH eluting from concrete at a typically high pH (13).

I know that concentrated NaOH solution has a soapy feel.

A little colour goes a very long way--so don't worry about the brown colour (other than the primer being eaten away).  Your problem is the VERY corrosive (presumed) NaOH solution.

WEAR safety glasses and Watch your eyes !!!!!!! when you are looking up at the under surface of deck.