Going from a cool (relatively) dry house to a warm moist environment (cool metallic surface allows for moisture to condense on the surface once outside) is what caused your ammunition to corrode. A waterproof case wil not solve that problem. Make sure your ammo is dry and at the temp of the house when you put it away and you will not have any (or will have fewer) problems with corrosion. What is the daily temperature change in the area the ammunition is stored? How quickly does the temperature change? Is it stored in a garage or a basement, in a "bonus room, or otherwise below ground level? A garage is suseptible to large daily temp swings that could cause condensation to form. Basements and other rooms below ground level tend to be much more humid (even when "finished") then the rest of the house. None of my ammunition is stored in waterproof cotainers and it all shines as much or more than factory brass. By the way, I live in Houston so battling humidity is 24/7/365.