Metal cans or plastic ??? I have seen some IMR/Duont powders turn to brown dust, that was from the late 70's cans and a home that did not use much a/c or a dehumidifier.
That is somewhat depressing.Stored away from heat and large temperature fluctuations it will outlive you. Really.
On another note, anybody else notice that single base powder seems impervious to degradation? While the only powder I ever had to turn into fountain sparklers was double base.