Testing has shown that the new preservative chemicals used in pressure treated lumber are more corrosive to metals than CCA.  Because CCA was removed from the market in January 2004 galvanized aluminum joist hangers are no longer suitable to be used.  Today you must use only triple galvanized or stainless steel hangers for all outdoor applications.  Stainless steel hangers are the best choice but are cost prohibitive for most deck projects.  You will also now need to make certain that the nails you are using are compatible with the hangers.  Stainless steel nails must be used in combination with stainless steel hangers.