Corrosion on metals is a process demanding both O2 and water. If you have none of these two components you have no corrosion. In case of humidity below 50% the water is not able to condense and corrosion is impossible. If the humidity is over 50% corrosion processes is started. Different pollutants increase the corrosion process as smoke, SO2, acids, salt, and so on. Other materials decrease the process and can be used as protective materials.