I'm no expert, but...
This is what I did last year.I sorted through all my old flares and pulled about a dozen that showed significant corrosion on the brass casings of the 12 ga. flares and showed any effects of moiture on the handhelds.On an evening after a rain shower, I shot the 12 ga. flares into the ground in a cleared area in my back yard away from any combustables and out about 10 feet from me.Over half fired properly and burned bright. One did not fire at all and the rest fired weakly and fizzled slowly.I lit the handhelds with a propane torch because the strikers were soft and would not work. A couple that showed obvious signs of moisture damage would not burn so I broke them open along with the 12 ga. that didn't fire spread the chemicals and burned them with the torch.I picked up the slag after it cooled and disposed of it.If you are wondering why I had so many old flares, I keep them because I believe that you can't have too many if you are in trouble and I also make it a practice to buy a new set each year so that I don't have to worry about checking barely legible dates to make sure I'm current. I no longer take them out of the packaging as that helps provide a measure of moisture protection. I do trim the cardboard down so they fit in the canister better.