Damp Atmospheric Corrosion Adsorption Layers

Aqueous phase layers consist of water adsorbed on the metal surface. The amount formed is dependent on the relative humidity of the atmosphere and the chemical and physical properties of the corrosion products. 

Water may be absorbed from the atmosphere and wet the metal smface if hygroscopic salts are deposited or formed by corrosion. This absorption will take place when the relative humidity exceeds the critical relative hmnidity. The value of the critical relative humidity is dependent on the specific metal and the specific metal contaminants. When the relative humidity exceeds the value at which the salt starts to absorb water and dissolve, the corrosion rate increases sharply. This critical relative humidity corresponds to the vapor pressure above a saturated solution of the salt present. Adsorption layers of electrolyte on the surface of the metal may also be the result of capillary condensation. 

The corrosion rate is directly influenced by the amount of water present on the corroding surface. Laboratory studies have shown that the corrosion rate above the critical value sharply increases with increasing relative humidity. The amount of water present on a metal surface has been roughly estimated as follows  

Atmospheric corrosion resulting from the reaction of water vapor with a metal surface is a serious problem. Most clean metal surfaces will permit the bonding of water in molecular form. The oxygen atom bonds to the metal surface and acts as a Lewis base (donating an electron pair) because the bonding is coimected with a net charge transfer from the water molecule to the surface. The water adsorbs on electron-deficient adsorption sites. 

It is also possible for water to bond in dissociated form. In this case, the driving force is the formation of metal-oxygen or metal-hydroxyl bonds. The end-products formed as a result of the water adsorption are adsorbed hydroxyl, atomic oxygen, and atomic hydrogen. When metal oxides are present, water may adsorb in either dissociative or molecular form. Lattice defect sites seem to facilitate dissociation, as observed, for instance, on monocrystalline 1102, NiO, and a-Fc203. The dissociation of water forms a mono-molecular thick film of surface hydroxyl groups that is relatively protective and reduces the subsequent reaction rate of water. The first monolayer of water adsorbed to the hydroxylated oxide surface is highly immobile, whereas the second and third layers are more randomly oriented and less immobile. 

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Atmospheric corrosion can be defined as the corrosion of materials exposed to air and its pollutants, rather than immersed in a hquid. Atmospheric corrosion can further be classified into dry, damp, and wet categories. This chapter deals only with the damp and wet cases, which are respectively associated with corrosion in the presence of microscopic electrolyte (or moisture ) films and visible electrolyte layers on the surface. The damp moisture films are created at a certain critical humidity level (largely by the adsorption of water molecules), while the wet films are associated with dew, ocean spray, rainwater, and other forms of water splashing. 

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