Effect of Relative Humidity on Corrosion Chemistry
Corrosion can normally occur at perceptible rates on surfaces where there is actually free moisture.
Moist air where no moisture is present on the surface is usually insufficient for corrosion to occur. Materials on the surface of metals such as corrosion products and salt deposits can be hydroscopic and can absorb sufficient amounts of water from moist air to allow corrosion to proceed.
The relative humidity of the environment affects the ability of such layers to provide sufficient moisture on the surface for corrosion to proceed and also affects the time required for the surfaces to dry after being wet by rain, dew, or spray. In general, corrosion is more severe in humid environments than in dry environments.
In order to arrest corrosion on equipment by storing in a dry atmosphere, the relative humidity must be reduced to below 50% with 35% being necessary to completely arrest corrosion on some materials.
When hydroscopic materials are on the metal surface, these must be removed before dehumidified storage will be effective unless very low humidities can be achieved.
Sea salts are particularly hydroscopic and must be removed in order to prevent rapid atmospheric attack.
Unless removed, the sea salts will effectively concentrate when dried and, unless very low humidities are achieved, will cause accelerated attack on many metals.
Source "Corrosion Control" NAVFAC MO307 september 1992