Propellants exposed to metal(oid)s like aluminum can decompose at much faster rates than when stored in containers made of polymer or other materials. The metal acts as a "template" on which the propellant's organic molecules (nitroglycerine, nitrocellulose) can "sit" and be acted upon by elements in the aluminum alloy, or even surface contaminants accreted to the metal despite repeated rinsing.
I doubt that any propellant so stored would develop a fast chain reaction and get hot enough to do anything (don't leave it in the garage over the summer), but I'd darn sure expect the burn characteristics to be altered.
Best bet would be to find an empty propellant bottle still capable of sealing and store it in there. Make sure you relabel it in an obvious way, also.
ADDENDUM: I'D be willing to bet that the old-style metal cans in which smokeless propellants were once sold, were also pre-coated on the inside with some sort of "rock-stable" polymer/sealant to separate new powder from bare metal.