As long as you don't use petroleum based products on your weapon, the black powder, won't corrode the metal.
If you use any oil where it can come in contact with black powder, the two together, will become corrosive. That is why most people use a vegetable based product such as Bore Butter, and the like, on their weapons.
Here in Oklahoma, the humidity makes it almost impossible to keep loaded cylinders stored for very long. You can keep moisture out by putting grease on the front of the cylinder, but it is difficult to keep moisture from getting through the primer hole.
This week we have gone from 80 degree weather, with 30 % humidity, to 55 degrees with rain every day for three days, then back to 80 degrees, with 80% humidity. This makes it very hard to keep loaded cylinders anywhere but in the tightest of places.
If you have a very good gun cabinet to keep them in, it probably would be OK, but if you are like I am, and don't have one, it is probably just as easy to load them as you use them.