Perhaps you could investigate storing the mill with some kind of sacrificial material. When I was crayfishing, all the cray pots would have a zinc block on them as a sacrificial material. This would cause corrosion to take place eating away the zinc block instead of the steel mesh the pot was made of, and was pretty effective. Bearing in mind that obviously you wouldn't be dipping your mill in salt water, and I'm not sure if the same principle would apply in air at all, but maybe if you were able to get hold of a small piece of zinc, or perhaps even a small amount of zinc / zinc coated steel and leave it in contact with one roller maybe? Does other mild steel rust up over time where you are? Maybe instead of using your mill in the experiment, cut a piece of scrap mild steel in half, and leave them both out, one with a piece of zinc on it and see what happens? It's all about the exchange of ions as Adam said, to balance electric charge during the corrosion process, and the zinc being more willing to give them up than the mild steel if I remember my chemistry....so it's important that the zinc is not insulated from the material you are attempting to protect.....or you could just not worry about the chemistry of it and just make sure you give it a good clean up after use so there's no flour for moisture to accumulate on. I don't think you should wet or oil it however, the minute you do that, you're going to get flour from the next milling gunking things up. Maybe get some silica crystals and store it in a closed container with them........
