The main components of atmosphere are almost invariant. However, the content of water vapor is an exception that it varies with the changing of location, season and time. Oxygen and water vapor plays a significant role on the processes of atmospheric corrosion. Therefore, atmospheric corrosion can be classified in the following three categories: Dry corrosion Damp corrosion Wet corrosion Component Percent/% Component Percent/% Component Percent/% Atmosphere 100 H2O.
Components of atmosphere (Impurity excluded, 10? ) 2. 1 Dry corrosion In the absence of significant water vapor, many common metals develop films of oxide. In the presence of traces of gaseous pollutants, copper, silver and other non-ferrous metals undergo film formation which is known as tarnishing. The tarnishing of silver in air is well-known. Tarnishing by hydrogen sulfide may be retarded by moisture if present in very small amounts. 2. 2 Damp Corrosion Damp corrosion would occur only when the relative humidity reaches 70% which is considered generally as the critical value for the onset of corrosion.
The precise level of critical humidity varies with the type of contaminants, such as dust and salt particles, and the composition of metals. For instance, in the presence of marine salts corrosion is stimulated at lower values of relative humidity. The difference between the damp and wet environment is very narrow and it is more representative of a climatic condition rather than the magnitude of corrosion. Damp environments promote the corrosion of most metals. Water saturated with dissolved gases, such as CO2, H2S and SO2, cause severe corrosion of iron and steels, copper, nickel, silver and other non-metallic materials and alloys.
For example, silver loses its luster and develops a tarnished film of sulfide on coming in contact with H2S, and copper develops tints and becomes black. In agricultural areas abundance of ammonia, particularly during the rainy seasons, subjects copper fittings to seasonal cracking and causes serious damage to water distribution systems. 2. 3 Wet Corrosion This is the most frequently observed form of atmospheric corrosion, where the water layers or pockets are formed on the metal surface, and the metal surface remains constantly in contact with water.
The rate of corrosion would depend on the solubility of the corrosion product. Higher solubility means a higher rate of corrosion, because the dissolved ions increase the electrolytic conductivity. In case of alternate dry and wet conditions, the dry corrosion product film may absorb moisture from the air which increases the rate of corrosion of the metal by bringing the moisture in contact with the metal surface. Patina formation on copper, such as brochantite, and corrosion of iron and steel structures are common examples of corrosion caused by wet atmosphere.