The two floors of my deck rest on 100x100mm timber. They are nailed, and
I had galvanised plates to hold the whole thing together in case of
hurricanes etc. The building inspector seemed happy with what I have
done, but I'm not that happy.

http://i58.tinypic.com/1zcll5x.jpg

The galvanised plates have now rusted, and I was thinking of putting
stainless steel brackets under the deck, bolted through to a 50x90mm
stainless plate on the outside of the studs.
I have been told that stainless steel will corrode in the absence of air.
Is that true and does it matter? The galvanised plates lasted about 15
years. They are against treated timber.