Late to the party but I'll add my 2¢.

I learned my lesson decades ago when I stored nearly a full case of 9mm ammo in what I would call a slightly humid, mildly corrosive, variable temperature environment. In less than 5 years, it was junk I considered too iffy to shoot myself and too iffy to give away.

Since then, all of my ammo is stored indoors in a temperature stable, dry, non-corrosive environment... in original boxes which are then locked in a large wooden ammo box or in ammo cans. Never another problem ever again.