They epoxy coat the beam ends even on what is called weathering (untreated) steel. Inevitable joint failures result in salt eating the beam ends. We even go to the effort of pressure washing the beam ends after the winter season when budget allows. Salt = bridge remover, plain and simple. Nasty stuff. As far as decks, they do use epoxy rebar which helps, but there "should" be enough cover over the concrete to prevent infiltration on newer decks. Older decks have less cover on average due to loss of wearing surface and poor workmanship back then, plus you can run into poor consolidation of the concrete itself. One thing is for sure, we see all kinds of stuff which keeps it interesting.
Salt degrades concrete as well as steel.
