Why would it have to be disposed of? I still have good powder from my Father that he had before I was born. As long as it has been stored correctly, just keep using it.
That is almost true. Some of the manufacturing methods used in days gone by were good for the day but the components break down over time. This is especially true of the primers. I have powder dating back into the 1920s. It's still good to go. I also have some from the 30s and 40s that is fine. On the other hand, I had some late 30s made powder from the old Ammomart company that was extruded powder made by Nobel/Bofors for the Swede 6.5x55 and was very close to 3031 in speed. It was fine for years then about ten years after I bought it, the tried and tested loads became very erratic. The powder was sold as #44. I really liked it in the 257 Roberts and 6.5x55 as well as a couple of others. I had taken the 257rob out for a weekend of gopher shooting and put it back in the safe without cleaning it. MY BAD. When I remembered that I hadn't cleaned it, appx a month later, I went through the ritual cleaning process. I wasn't to worried, nothing to really worry about as none of the components were corrosive. WRONG. The first soaked patch was very difficult to push through the bore. When it made it through it was covered in dark brown rust. To say I was shocked would have been an understatement. I finished the cleaning but it was a waste of time. The bore was ruined. Not pitted as you would expect but it looked like someone had taken a ball peen hammer to it and dimpled it from the breach to the muzzle.
Went back to the primer batch and they looked fine. The next culprit to check would be the powder. I had about 25 unopened cardboard containers with metal bottoms and tops crimped onto the tubes. Ammomart had run a 2in wide strip of tape across the top of the can as a seal. I noticed right off that all but one of the containers were no longer straight walled but slightly swollen. In every can that showed swelling the powder had deteriorated. When I opened the first can a nasty acidic smell was released and the inside of the lid was very rusty. There was also a coat of rust that had fallen from the top of the can onto the top of the powder.
I took the rest of the cans outside to inspect. I cut one of the cans in half without breaking the seal. The powder looked normal but still had a strong acidic odor. Of course the powder on the bottom and top had rust from the residue from the metal.
That's what caused my bore to rust. It also explained the inconsistency while shooting.
I called my old friend Tom Higginson in Hawkesbury about the powder and he told me they had sold all of it quite a few years previously. I asked him for some history on the powder and he affirmed it was a lot that he got from a deal with International Firearms out of Montreal. Much of the powder came from cases with pulled 160grn round nose bullets as the primers were suspect. He didn't really expect the people that bought the components to set up a long term supply but at $4 per pound shipped with taxes in it was a steal even back then when most canister grade powders were running $15/lb.
The acidic state of the powder worried me so I burned it rather than put it into my wife's flower beds.
I had one other powder from Ammomart go bad on me as well. It was sold as N160 and was a bit slower than 4350 but was ball powder instead of extruded. It was really good consistent powder and after Tom sent me a 7lb bag to test I ordered another 21lb lot. It was perfect for the 8x57/338-06/35Whelen and 30-06 with heavy bullets. I had used up all but one 7lb box when the steel shelves I had the powder stored on started to show a coat of rust. NOT GOOD. Went through a similar inspection and found the powder in the bag was sweating/clumping. Of course, it was scrapped. By this time Tom was going through his battle with cancer.
Just to be clear. My powder is stored at a constant 22C and humidity controlled. Powder can and does destabilize. I am sure Ganderite can explain the reasons far better than I can.
I notice that Higginson's carries very limited amounts of surplus powders these days. To bad but I guess there is more money to be made in canister grades than surplus batches.