Atmospheric corrosion acidifying pollutants

In the present lecture a brief review will be given of the influence of acidifying air pollutants on the atmospheric corrosion of metals based mainly on more recent results from Europe and especially from Scandinavia. 

Ammonia (NH,) is emitted primarily from animal shelters, cleaning detergents, and fertilizer production. Ammonia in the aqueous phase establishes equilibrium with NH4, which results in increased pH. NH, affects the atmospheric corrosion chemistry by partly neutralizing acidifying pollutants, forming particulate ammonium sulfate [(NH4),S04] and acid ammonium sulfates such as NH4HSO4 and (NH4)4H(S04),. 

Principal sources of emission of NH3 are animal shelters, fertilizer production, and cleaning detergents. In the aqueous phase, NH3 establishes equilibrium with NH4+, which results in increased pH. An important role of NH3 in atmospheric corrosion chemistry is to partly neutralize acidifying pollutants by forming particulate (NH4)2S04 and acid ammonium sulfates, such as NH4HSO4 and (NH4)3H(S04)2. By increasing the pH of the aqueous phase, NH3 also increases the oxidation rate of S(rV) to S(V1), as discussed earlier. 

In the presence of atmospheric acidifying pollutants, such as SO2, the anode reaction is facilitated and, consequently, the total corrosion rate as well. Upon deposition of SO2, interaction with the aqueous phase proceeds with the following reactions ... 

With the increasing concern about acidifying pollutants and their influence on atmospheric corrosion rates, scientists interest has been focused on NO2 as an additional gaseous corrosion stimulant. Whereas the SO2 concentration has shown a significant decline over the past decades in many urban and industrial areas, estimated emission of NO2 has shown a continuous increase in the same type of environment [91]. Studies in laboratories using synthetic air have provided unambiguous evidence of increased corrosion rates when NO2 is added to air containing SO2 (Fig. 11). 

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