I've pulled the projectiles (they can be reused) and, found a way to dispose of the powder

Now I'm left with the steel cases with primers in them. I can't/won't chamber the rounds and fire them because of the rust (probably not good for the rifle).
After googling I've read some dubious ways to dispose them and, don't want to simply bin them in case they end up going pop at some rubbish recycling depot!
I won't burn them either as I have a scar on my leg from when someone (NOT me) thought that was a good idea around a campfire.
Soaking them in something sounds good but that reportedly doesn't always work.
Any ideas...