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The Fundamentals of Corrosion

Corrosion is a natural process that occurs when a material reacts to its environment. While some associate the term with the deterioration of metals, it also occurs with ceramics, polymers and composite-mechanical mixtures. Understanding the basics of this phenomenon allows you to take steps to ensure your corrosion control efforts are more effective.

Why Corrosion Happens

Materials corrode when they are chemically unstable environments. When a material interacts with this environment, its properties alter in an attempt to reach a chemically stable form. This thermodynamic reaction involves the gradual destruction of the material in question because of an electrochemical reaction or exposure to a destructive force, such as a chemical or radiation. Several materials can suffer the effects of corrosion, but the term is most often associated with metals made of ore or processed minerals, which are naturally unstable.   

Type of Corrosion and their Effects 

A 1999 NACE International study found that the direct cost of corrosion in the country was $276 billion annually, about 3.1 percent of the Gross Domestic Product. In 2016, the organization estimated that the global cost of corrosion was about $2.5 trillion, about 3.4 percent of the global GDP. From the weakening of polymers to hydrogen embrittlement on ships, corrosion control is vital in all industries. 

Scientists classify corrosion by the type of corrosive attack and the environment to which a surface is exposed. Common types of corrosion include:

  • Uniform, or general, corrosion: The most common type of corrosion, uniform corrosion occurs when there is a chemical attack on a surface. Rust is a type of general corrosion. 
  • Galvanic corrosion: An electrochemical process, galvanic corrosion occurs when two dissimilar metals are in contact or in the presence of an electron conductive path and an electrolyte.
  • Concentration cell corrosion: This type of localized corrosion occurs when a metal’s surface deteriorates at different rates because it contacts different concentrations of the same electrolytes.               
  • Pitting: Pitting is the result of microscopic defects caused by product accumulation, such as salt under a ship’s protective coating.               
  • Crevice, or contact, corrosion: Crevice corrosion happens where a metal contacts another metal or nonmetal, such as threaded joints. 
  • Filiform corrosion: This type of corrosion often occurs when moisture permeates a protective coating. 
  • Intergranular corrosion: This corrosion happens in the presence of tensile stress, resulting in the cracking along crystallite grain boundaries because of local differences in composition.
  • Stress corrosion cracking (SCC): SCC is the result of a metal’s corrosive environment and tensile strength. It’s common in condensate and steam lines exposed to excess moisture. 
  • Fretting: Fretting causes damage on two contacting surfaces that are not designed to move with each other. It’s the result of vibrations or repeated motions, which remove the protective film on metal surfaces.
  • Erosion: Erosion occurs when high-fluid surface velocities and an aggressive chemical environment remove a metal’s protective film or coating.
  • Dealloying: This rare type of corrosion occurs when an alloy, such as copper or gray cast iron, loses its reactive elements and keeps its corrosion-resistant elements in a porous state.
  • Hydrogen damage: This embrittlement-causing corrosion occurs in the presence of excess hydrogen because diatomic hydrogen molecules grow too large to escape, causing gas blisters that split metal.
  • Microbial-related corrosion: This type of corrosion takes many forms and is the result of microbes on or under a surface.

Proactive Corrosion Control

Because of the high costs associated with corrosion, prevention and control are more economical and effective. In addition to proper material design and selection, the most popular control techniques include surface preparation and coating paired with climate control. Coating a surface with a protective treatment creates a protective barrier between the surface and environment, which slows corrosion. A protective coating, however, is only as effective as the surface’s preparation and the environment surrounding it. The slightest defect could cause a coating to fail, resulting in extensive damage.

To aid with corrosion control, Polygon designs custom climate control systems that create the ideal environment for surface preparations and coatings. The controlled environments ensure the proper temperatures, ventilation and relative humidity levels for a project’s success. In turn, coating projects experience fewer delays, unnecessary expenses and premature failures. Get in touch with Polygon today to request a complementary consultation and learn how its affordable solutions will aid your corrosion control efforts.