The Nature of Atmospheric Corrosion

Atmospheric corrosion is the attack of a metal (or an alloy) by the atmospheric environment to which it is exposed. This corrosion is caused by the simultaneous attack by rainwater or condensing water, oxygen contained in the air, and atmospheric pollutants. Atmospheric corrosion is a special type of corrosion because the electrolyte is represented by a thin film of moisture, whose thickness does not exceed a few hundred micrometres. It can be assumed that such a film is always saturated with oxygen, and that diffusion is not hindered. This type of corrosion may be intermittent, because it stops when the metal s surface is no longer humid. When immersed in water or in a salt solution, the metal is in permanent contact with the electrolyte, but the corrosion may be slowed down by the weak diffusion of oxygen to cathodic sites. 

The first theoretical explanation of atmospheric corrosion of metals was given by Vernon [5] and Hudson [6] starting in 1923 and was completed later by Rozenfeld in the 1960s [7] and by Graedel for aluminium in the 1980s [8]. Vernon introduced the concept of the critical degree of moisture, the threshold below which practically no corrosion will occur. The value of this threshold depends on several factors such as the nature and concentration of atmospheric pollutants and the metal s surface condition. 

---