Metallic corrosion under aqueous conditions can take place by many mechanisms with quite varied results. A brief general description of each type of corrosion is presented here as an introduction. General Corrosion – General corrosion is a uniform attack and is the most commonly encountered type of corrosion. It is characterized by a chemical or electrochemical reaction which proceeds uniformly over the entire exposed surface area. The metal becomes thinner and eventually fails.
Localized Corrosion – Conditions may exist that cause corrosive attack in a very specific location or under a specific set of conditions. Such attack is normally referred to as localized. This type of corrosion takes several forms.Pitting, crevice attack, and microbially influenced corrosion (MIC) are all types of localized corrosion.
Pitting – Pitting is a form of very localized attack that results in the formation of holes in the metal. While the holes formed are generally small they can propagate very quickly leading to material perforation and failure in a very short period of time. The micro-environment within the pit itself can be autocatalytic making this a particularly dangerous form of attack.
Crevice Corrosion – Crevice attack usually takes place in a very tight gap between two surfaces (e.g. as with gaskets or fasteners or under mineral deposits). As with pitting, the micro-environment within the crevice can greatly differ from the general medium. Concentration cells can cause this type of corrosion to progress at a very rapid rate.
Microbially Influenced Corrosion (MIC) – MIC is a recently identified form of crevice attack. Certain types of bacteria form dome-shaped colonies on the metallic surface. The inside of the structure is sealed from the outside. The life cycle of the bacteria produces a corrosive environment within the colony which causes a crevice attack of the metal. While MIC usually takes place on components exposed to natural water (either fresh or brackish), it has also been encountered in process water in industrial systems.
