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While atmospheres can be classified into four basic types, most environments are mixed and present no clear demarcation. Furthermore, the type of atmosphere may vary with the wind pattern, particularly where corrosive pollutants are present, or with local conditions such as illustrated in the following Figure.
Bird damage to a 1931 bronze statue commemorating the 21st Battalion battles in WWI.
An industrial atmosphere is characterized by pollution composed mainly of sulfur compounds such as sulfur dioxide (SO2), a precursor to acid rain, and nitrogen oxides (NOx), the backbone of smog in modern cities. Sulfur dioxide from burning coal or other fossil fuels is picked up by moisture on dust particles as sulfurous acid. This is oxidized by some catalytic process on the dust particles to sulfuric acid which settles in microscopic droplets and fall as acid rain on exposed surfaces. The result is that contaminants in an industrial, atmosphere, plus dew or fog, produce a highly corrosive, wet, acid film on exposed surfaces.
In addition to the normal industrial atmosphere in or near chemical plants, other corrosive pollutants may be present. These are usually various forms of chloride which may be much more corrosive than the acid sulfates. The reactivity of acid chlorides with most metals is more pronounced than that of other pollutants such as phosphates and nitrates.
Are industrial sites near where you live more corrosive than adjacent locations? Provide some examples.
A marine atmosphere is laden with fine particles of sea mist carried by the wind to settle on exposed surfaces as salt crystals. The quantity of salt deposited can vary greatly with wind velocity and it may, in extreme weather conditions, even form a very corrosive salt crust, similar to what is experienced on a regular basis by sea patrolling aircraft or helicopters as shown in the following Figure.
a)
b)
Sea salt deposited on the external surface a) of a Cormorant sea and rescue helicopter radar antenna, and b) salt causing corrosion to the antenna internal components due to a broken seal. (courtesy Major S.J.R. Gigučre)
The quantity of salt contamination decreases with distance from the ocean, and is greatly affected by wind currents. The marine atmosphere also includes the space above the sea surfaces where splashing and heavy sea spray are encountered. The equipment exposed to these splash zones are indeed subjected to the worst conditions of intermittent immersion with wet and dry cycling of the corrosive agent as illustrated in the following Figure.
Aircraft carrier top deck. (courtesy Mike Dahlager, Pacific Corrosion Control Corporation)
Rural atmospheres are typically the most benign and do not contain strong chemical contaminants, that is unless one is close to a farm operation where byproducts made of various waste materials can be extremely corrosive to most construction materials.
Arid or tropical atmospheres are special variations of the rural atmosphere. In arid climates there is little or no rainfall, but there may be a high relative humidity and occasional condensation. This situation is encountered along the desert coast of northern Africa. In the Tropics, in addition to the high average temperature, the daily cycle includes a high relative humidity, intense sunlight, and long periods of condensation during the night. In sheltered areas, the wetness from condensation may persist long after sunrise. Such conditions may produce a highly corrosive environment.
Normal indoor atmospheres are generally considered to be quite mild when ambient humidity and other corrosive components are under control. However, some combinations of conditions may actually cause relatively severe corrosion problems. While there is no typical contaminant or set of conditions associated with an indoor atmosphere, any enclosed space which is not evacuated or filled with a liquid can be considered an indoor atmosphere. If not ventilated, such an environment may contain fumes, which in the presence of condensation or high humidity could prove to be highly corrosive.
Indoor corrosion has caused many unpleasant surprises. Find some examples close to your immediate surrounding.
Even in the absence of any other corrosive agent, the constant condensation on a cold metallic surface may cause an environment similar to constant immersion for which a component may not have been chosen or prepared for. Such systems are commonly encountered in confined areas close to ground level or, worse, below ground where high humidity may prevail. The following Figure shows the advanced corrosion of the frame and contacts in an electric junction box only four years after a building was completed. While the junction box in this example was only at the ground level, the wires coming to the box were buried without additional insulation and in constant contact with much cooler ground than ambient air in the room.
Electric junction box badly corroded only four years after a new residence was completed.
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