A system can be "empty" for quite some time without having corrosion problems. Just because it is empty, does not mean that it is OPEN. Typically a system that is considered empty is more correctly LOW on refrigerant. As long as there is even one pound of pressure in the system, then it is not getting any atmospheric air inside, thus it is not being contaminated.

Now a system that is OPEN is another matter. In the case of a system that has had a line disconnected or some such for a period of time, it probably should be flushed component by component, r/d replaced and then proper evacuation before charging.

Hope this helps,