Atmospheric corrosion and its severity is essentially determined by four variables:

• Air pollution (both man made and natural such as volcanic gases)
• Airborne salt spray or droplets
• Temperature and
• Moisture

Atmospheric corrosion is caused primarily by moisture and oxygen; but it is amplified by contaminants such as sulphur compounds and sodium chloride, or salt spray.

The presence of industrial pollutants in the polar ice caps demonstrates that even the most remote corners of the Earth are not immune to its effects. Nitrogen and sulfur compounds can form acids when discharged or acidic material may be released to the atmosphere directly by some industries accelerating the corrosion process on metal these substances encounter. Of course urban areas and those areas in close proximity to or downwind from "smokestack type" industries and power plants are subject to the most corrosive effects of airborne pollution.

Salt spray and airborne saltwater droplets introduce chloride ions to metal surfaces with a corresponding deleterious effect on the metal. But one does not have to live on the beach to experience these corrosive effects as it is estimated that airborne sea spray saltwater micro droplets can carry as much as seven miles inland from the coast. The presence of moisture is an absolute necessity for most corrosion processes and when combined with elevated temperatures and salt or pollutants further enhances the atmospheric corrosion process.

Corrosion of steel on a seacoast is 400 to 500 times greater than in a desert. One researcher has shown that steel samples located eighty feet from a coastline corroded 12 times faster than those eight hundred feet from the coastline.

Atmospheric corrosion is also an electrochemical process, requiring the presence of an electrolyte. Invisible electrolytes in the form of a thin film tend to form on metallic surfaces when a certain critical humidity level is reached. For iron, this humidity level is approximately 60% in unpolluted areas.

Atmospheric corrosion is responsible for more metal damage (both on a cost accounting basis and an actual quantitative basis) then any other form of environmental corrosion. After all, everything on the Earth's surface is exposed to the atmosphere be it vehicles, buildings, bridges, etc.

Atmospheric Corrosion Monitoring:
Atmospheric corrosion accounts for the highest overall cost and metal loss of all the fundamental corrosive environments. A defining feature of atmospheric corrosion is the thin aqueous layer between the surface of the corroding material and the atmosphere. Between the following three phases and the interfaces are therefore important and can be used in corrosion monitoring principles:

• Solid [corroding substrate],
• Liquid [thin aqueous layer] and
• Gaseous [atmosphere]

Corrosion monitoring in outdoor and indoor atmospheres poses specific challenges related to characterizing corrosion damage (generally taking place at a low rate) in a short (practical) time frame. Three basic approaches to corrosion monitoring are available:

• Direct measurement/monitoring of corrosion damage. Examples include exposing actual components or coated coupons to corrosive atmospheres and evaluating these for corrosion damage periodically.
• Indirect measurement/monitoring of corrosion damage with corrosion sensors. Examples include thin film electrochemical sensors embedded under paint coatings and smart coatings.
• Classification of atmospheric corrosivity by categories and correlating such classifications to actual in-service performance and corrosion rates.

The main drawbacks of the direct measurement approach are the lengthy exposure time period usually required and that only a "snapshot" of cumulative damage is obtained when detailed analysis of corrosion damage is performed periodically.

The approach of atmospheric corrosivity classification is generally one of a simple, low-cost measurement in a short time frame. Ultimately, such simplistic measurements require correlation to actual long term service performance and therefore need to be linked to the other measurement methodologies.

This monitoring is generally employed to quantify this type of damage and corrosion risk, rather than merely resorting to broadly descriptive atmospheric classifications such as industrial, rural, etc.