Charlie, Is it possible that you have an unbalanced system and the buss / terminals in question are getting hot? That would cause any condensate to evaporate, leaving behind mineral deposits.
white powder corrosion is an aluminium thing.check that the lugs used are compliant with the bus bar. id also clean everything up with a wire brush and make sure all the connections are tight. you say that the area isnt damp, but how far away from the sea are you. seaside areas suffer a lot from the salt in the air.
