Atmospheric Contaminants
The primary sources of atmospheric contaminants come from chlorides in marine locations, and industrial and automobile pollutants. These contaminants deposit onto metal surfaces where they react primarily with oxygen, water, and free electrons producing metal compounds which have varying degrees of solubility; producing varying increased corrosion rates over non-corrosive environments.
The presence of chloride salts in the atmosphere significantly increased the corrosion rates of most metals. In the case of ferrous metals, chloride anions combine with ferrous cations to produce iron chloride. Iron chloride is more soluble than the ferrous hydroxide produced in a benign environment, leading to an increased corrosion rate. Other metals such as copper and zinc produce metal chlorides which are less soluble than ferrous chlorides. These metals therefore experience increased corrosion rates, but not to the extent of ferrous metals. It is notable that deicing salts used on roadways in winter months produce corrosivity in those environments similar to marine atmospheric environments.
Sulfur dioxide (SO2) and nitrous oxides (NOx) are found in industrial and urban environments from the burning of fossil fuels. Sulfur dioxide deposited on metal surfaces will react with oxygen and free electrons from the metal surface, producing sulfate ions, as expressed in Equation 1. The sulfate ions lead to the formation of metal sulfates, which in turn react with
water to complete the corrosion process, Equation 2.
There are a few additional atmospheric contaminants that are less abundant or may be found in special industrial environments. Hydrogen sulfide is extremely corrosive to most metals. This compound is readily found in oil-refining and petroleum industries. Hydrogen chloride and chlorine gas have been found to produce higher corrosion rates than chloride salt environments. Ammonia, sulfur trioxide, and smoke particles will also increase atmospheric corrosion of most metals. The typical concentrations of these major contaminants are found in Table 3.
