Aggressive Anion

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Definition - What does Aggressive Anion mean?

An aggressive anion is a type of negatively charged particle that is closely linked to pitting corrosion. Pitting is a type of corrosion which leaves tiny holes in a metal's surface.

Pitting corrosion is further driven by depassivation in small surfaces that turn anodic, while a large surface becomes cathodic, resulting in galvanic corrosion. This type of corrosion breaks into the metal mass along with a limited number of diffusion ions.

Corrosionpedia explains Aggressive Anion

Gradient concentration of dissolved ions can have a downward orientation into the corrosion pit brought on by gravitational forces. The pit can be very acidic and is typically sustained by anodic and cathodic half reactions that result in movement of aggressive anions and potential gradients into the pit.

This kind of corrosion can cause severe damage to the deep structures, although only a tiny amount of damage is visible on the surface. This is because the surface pits are often hidden by the products of corrosion.

Alloys most susceptible to corrosion caused by aggressive anion migration include:

  • Aluminum
  • Nickel
  • Stainless steel

Metals that are prone to uniform types of corrosion generally do not undergo pitting. Therefore, carbon steel undergoes corrosion evenly when exposed to seawater, while metals like stainless steel form pits.

Other instances that involve aggressive anion movement include the presence of substances such as chlorides that can worsen the growth and formation of holes in the surface.

Various methods that are used to prevent the occurrence of corrosion brought on by aggressive anions and other factors include the use of methods such as:

  • Cathodic protection
  • Coating application
  • Addition of 2% molybdenum
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