Atmospheric corrosion is highly visible compared to other corrosion processes. You can see its effects on flag poles, rusty bridges, monuments and buildings. Since it's pretty evident in almost all operations and industries, a huge portion of the paint industry is dedicated to the production of metal protectors that are doing their part in controlling atmospheric corrosion.
Here we'll take a look at the factors involved in atmospheric corrosion.
The Effects of Atmospheric Corrosion
Almost all kinds of general corrosion take place within the atmosphere. In most cases, the metals undergoing corrosion are not exposed to huge electrolyte quantities, so atmospheric corrosion in such conditions activates in corrosion cells that are highly localized. At times, atmospheric corrosion creates various patterns that are tough to explain, such as those seen on rusty galvanized roofs.
Factors that Affect Atmospheric Corrosion
There are different factors that are essential to atmospheric corrosion and these should be known and dealt with in a methodical manner in order to prevent its harmful effects. What are these factors? Take a look at the following:
1. Moisture, Dew and Condensation
Moisture, whether in the form of dew, rain or condensation, is a very significant factor when it comes to atmospheric corrosion. Although rain can wash away hazardous air pollutants in the atmosphere that have been deposited in exposed areas, such as in a marine environment, rain also collects in crevices and pockets. Rain can also hasten the corrosion process through constant wetness, especially in areas with galvanized bolts and steel parts or structures.
Moreover, condensation and dew are some of the unwanted types of moisture when they are not washed away by recurrent rain, which could eliminate or dilute the contamination. Dew films that have become saturated with acid sulfates, sea salt and other acids could produce an aggressive electrolyte environment that promotes the occurrence of corrosion.
In humid, tropical areas, condensation can be found on surfaces. On such surfaces, the moisture can become stagnant, which turns into an alkaline reaction with metal or absorbs carbon dioxide to create a dilute acid.
Temperature affects atmospheric corrosion. Essentially, every 50°F (10°C) increase in the temperature can double corrosion activity. Also, metallic objects suffer from something called temperature lag because of the heat capacity behind ambient temperature changes. When ambient temperature falls in the evening, the surfaces of metal objects or structures become warmer compared to the humid air that surrounds them. Thus, condensation will not begin until the dew peak has been attained. When the temperature increases within the air in the environment, the lag temperature in these metals will turn them into condensers that maintain a moisture film on their surfaces. This increases the wetness period compared to the period when the ambient air is below the dew point. This also depends on the metal thickness, structure, air currents and sun radiation.
Cyclic temperature can result in severe metal corrosion in tropical areas, especially in unheated warehouses, objects in plastic bags, metal tools and more. (The special case of corrosion protection in warehouses is discussed in Temporary Corrosion Protection during Storage, Transportation and Handling.) The dew point in the surrounding air is a sign of the evaporation and condensation equilibrium. With this, it is highly recommended to sustain the temperature about 50°F to 59°F (10°C to 15°C) higher than the dew point to guarantee that corrosion will not occur due to surface condensation that may be colder compared to the ambient air.
3. Relative Humidity
Relative humidity is defined as the water vapor quantity that can be found in the atmosphere relative to the quantity of saturation in a certain temperature. It is typically expressed as a percentage. Among the essential requirements in the process of atmospheric corrosion is the existence of an electrolyte in the form of a thin film that can develop on steel or metal surfaces that are exposed to critical humidity levels. Although the film is invisible, it can contain corrosive contaminants at high concentrations, particularly in situations where there is alternate drying and wetting.
The critical level of humidity is considered a variable that depends on the material undergoing corrosion. It also depends on the product’s tendency to corrode and the absorption of moisture by surface deposits as well as the existence of pollutants. For instance, the critical level of humidity is at 60% in cases where the environment is without pollutants. With the presence of electrolytes in thin films, atmospheric corrosion carries on through the combined cathodic and anodic reactions. Anodic oxidation involves corrosion attack on metals.
Typically, marine environments and aerosols that are salt-rich have high levels of relative humidity. There are various studies that revealed an adsorbed water layer on zinc can have increased thickness along with an increase in relative humidity. This results in increased corrosion rates. (Learn more in Five Key Factors in Understanding the Role of Soluble Salts in Coatings Failures.)
4. Aerosol Particle Deposition
Aerosol particle behavior in outdoor environments can be understood by the laws covering their movement, formation and capture. These can be found throughout the layers of the planetary boundary, and the concentrations rely on several factors such as time, location, local sources, atmospheric conditions, wind velocity and altitude. There are also studies that show that aerosol capture, deposition and wind speed are related. These studies involve saline winds that have an excellent correlation between deposition rates of chloride with a particular wind speed threshold.
Aerosols that are a main factor in atmospheric corrosion can be produced by either chemical processes in the atmosphere or via ejection. Some of the common aerosols are wind dust and sea spray. There are also secondary aerosols that are generated by condensing and reacting atmospheric gases, or through the conversion of gas into particles of cooling condensation. When aerosols remain suspended within the surrounding area, these can be removed, modified or destroyed. Aerosols will not stay in the surroundings indefinitely, as their average lifetime ranges from days to one week, but this still depends on the location and size of the particles.
Most of the aerosols are deposited near shorelines and these are typically huge particles that have a short lifespan and depend on gravitational forces. Aerosols are also influenced by gravity, wind resistance, solid surface impingement and dry-out droplets.
5. Presence of Pollutants
The existence of pollutants is a factor in atmospheric corrosion. For instance, sulfur dioxide, which is produced by fuel combustion that contains gasoline, diesel fuel, natural gas and sulfur, is considered one of the most harmful pollutants that can cause metal corrosion. (Learn more in the article The 6 Corrosive Components That Can Be Found in Crude Oil.)
Other pollutants include nitrogen oxides, which are also combustion products. These can be found in vehicle fumes and can form reactions with UV light and moisture to develop new chemicals that can be carried as aerosols. A typical example is the haze during summertime, especially in huge cities where most of the haze is the result of a combination of nitric and sulfuric acid.
When metals are exposed to the aforementioned factors, atmospheric corrosion may take place more rapidly and through various mechanisms. Thus, knowledge on these underlying factors could greatly help in managing corrosion and its harmful effects.