IMR powders from that era were stored in metal cans. The metal cans can and do corrode. I had a can of unsealed IMR 4227 that did the same thing. I had some Unique that was open and some that was unopened. I used up the one can and opened the other in the last two years and there was no problem with them The powder did not "rot", the can holding the powder did. Powder must be stored in a dry, cool area and be stored in a sunlight free container.