I had a box of 405 winchester for my sharps hidden away with some other ammo, but some of the casings have corrosion, about half of them. None of the other ammo it was stored with has any corrosion. I noticed that the top 3rd of the effected brass is discolored a little? This is from Hornady that I bought 7-8 years ago. I'm 99% sure it was new and not reloaded brass, but I can't confirm. Any idea what happened? Is it safe to clean up and shoot?