Although atmospheres can be classified into four basic types, most of them are mixed and present no clear lines of demarcation. Furthermore, the type of atmosphere may vary with the wind pattern, particularly where corrosive pollutants are concerned.
An industrial atmosphere is characterized by pollution composed mainly of sulfur compounds and nitrogen oxides. Some important atmospheric components are anhydrous acids. For example, sulfur dioxide (SO2), released from burning coal or other fossil fuels, is anhydrous sulfurous acid (H2SO3). This gas may combine with moisture on dust particles to form sulfurous acid. A portion of this weak acid may be catalyzed with oxygen to form the stronger sulfuric acid (H2SO4), which settles in microscopic droplets 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.
Urban conditions represent a special case of industrial atmosphere, where the exhaust from internal combustion engines provides the primary source of airborne pollutants.
A marine atmosphere is laden with fine particles of sea-salt mist carried by the wind to settle on exposed surfaces. Chlorides from some of this salt may remain in solution (ionized) within water droplets or may ionize as droplets collect on metallic surfaces. 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. Some splash zones could be classified as intermittent immersion.
A rural atmosphere does not contain strong chemical contaminants but does contain organic and inorganic dusts. Its principal corrosive constituent is moisture and, of course, gaseous elements such as oxygen and carbon dioxide (CO2).
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 moisture from condensation may persist long after sunrise. Such conditions may contribute to a highly corrosive environment.
The indoor atmosphere, although originally considered quite mild, actually can sometimes be quite severe. However, there is no typical contaminant or set of conditions associated with an indoor atmosphere. Any enclosed space that is not evacuated or filled with a liquid can be considered an indoor atmosphere. If not ventilated, it may contain fumes, which in the presence of condensation or high humidity could prove to be highly corrosive.
When considering atmospheres of any type, it must be understood that not only acidic contaminants are aggressive, but that alkaline materials (e.g., dusts from limestone or seashells) can also be corrosive when in contact with amphoteric metals (e.g., aluminum, zinc, and lead).
This article is adapted from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 86-87.