Factors that affect the rate of corrosion
Four elements need to be present for corrosion to occur and collectively referred to as the corrosion cell: an anode (+), a cathode (-), a metallic conductor and an electrolyte. Changing the potency of the electrolyte affects the rate of corrosion. Corrosion rates are determined by a variety of factors; however, five factors do play an overwhelmingly important role in determining corrosion rates.
- Oxygen: Like water, oxygen increases the rate of corrosion. Corrosion can take place in an oxygen-deficient environment, but the rate of the corrosion reaction (and destruction of the metal) is generally much slower. In immersed conditions, if an electrolyte is in contact with one area of metal containing more oxygen than the electrolyte in contact with another area of the metal, the higher oxygen-concentration area is cathodic relative to the remaining surface. An oxygen concentration cell then forms, which results in rapid corrosion.
- Temperature: Corrosion reactions are electrochemical in nature and usually accelerate d with increasing temperature; therefore, corrosion proceeds faster in warmer environments than in cooler ones.
- Chemical Salts: Chemical salts increase the rate of corrosion by increasing the efficiency (conductivity) of the electrolyte. The most common chemical salt is sodium chloride, a major element of seawater. Sodium chloride deposited on atmospherically exposed surfaces also acts as a hygroscopic material (i.e., it extracts moisture from the air), which then increases the corrosion in non-immersed areas.
- Humidity: Humidity and time-of-wetness play a large role in promoting and accelerating corrosion rates. Time-of-wetness refers to the length of time an atmospherically exposed substrate has sufficient moisture to support the corrosion process. The wetter the environment, the more corrosion is likely to occur.
- Pollutants: Acid rain (a chemical by-product from manufacturing and processing plants), and chlorides (in coastal areas) promote corrosion. Acid gases, such as carbon dioxide, can also dissolve in a film of moisture in contact with the metal.
Protective coatings can control one of these elements-that of the electrolyte. Applying a tightly adhered continuous protective film over the surface of the metal, we control the rate of corrosion or eliminate it altogether. Working with a coatings professional you can identify the specific corrosion control needs of your infrastructure and select the appropriate coatings that will best serve your business.
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