Probably something like heavy primer and then undercoating, or even tar foundation coating

Once the steel is 3" inside concrete, beyond oxygen, it is safe. The plate should probably have rebar studs welded all along it down in the concrete.
In a flitch plate beam a layer of steel is sandwiched between two bolted sideplates of dimensional lumber. The steel is carrying the load and the lumber is there to keep the steel plate from buckling.
That takes care of the "in plane" direction. The corners would need a 90 degree corner plate and then the endwall piers with their plates would brace the adjacent direction.
The bolts through the steel plates sandwiched in the beam will be trying to split the girder as the pier tries to rotate. The length of the plate in the girder is what reduces that tendency, longer is better.