Types of Corrosion
Corrosion manifests in a number of different forms depending on a variety of factors. There are two types of corrosion that include the forms of corrosion that occur on metals: general and localized.
One of the types of corrosion results in a relatively uniform loss of material over an entire area and in a general thinning of that affected area and called general or uniform corrosion. General corrosion is relatively easy to detect and its effects predictable unless the affected area is hidden from sight. Cast irons and steels corrode uniformly when exposed to open atmospheres, soils and natural waters, leading to the rusty appearance. In some cases the rich color hues generated from general corrosion are integrated in architectural designs such building facades and outdoor sculptures.
Localized corrosion occurs at discrete sites on the metal surface. The areas immediately adjacent to the localized corrosion normally corroded to a much lesser extent, if at all. Localized corrosion often occurs in areas that are difficult to detect. This types of corrosion is less common in atmospheric exposure than in immersion or splash/spray exposures. Corrosion activity at localized corrosion sites may vary with changes such as long exposure to liquid water, defects in coatings, changes in contaminants or pollutants, changes in the electrolyte and galvanic cells The predominant forms of localized types of corrosion are pitting and crevice corrosion.
Corrosion does not proceed uniformly in pitting corrosion, but primarily occurs at distinct spots where deep pits are produced. The bottoms of pits are anodes in a small, localized corrosion cell, often aggravated by a large cathode-to anode area ratio. Pitting can be initiated on an open, freely-exposed surface or at imperfections in the coating. Deep, even fully penetrating pits can develop with only a relatively small amount of metal loss. Pitting can be isolated or a group of pits may coalesce to form a large area of damage. Pitting is especially prevalent in metals that form a protective oxide layer and in environments high in chloride contamination (where chlorides promote the breakdown of the oxide layer).
Crevice corrosion occurs in crevices where the environment differs from the surrounding bulk environment. The different environments result in corrosion because of differences in concentration (e.g., oxygen, pH, and ferric ions). If there is an oxygen concentration difference, corrosion will proceed at crevices where there is less oxygen than in the environment surrounding the crevice. Crevices are formed when two surfaces are in proximity to one another, such as when two metal surfaces are against one another, when a gasket is against a surface, or when angle plates are placed back to back. Crevice corrosion can also occur under deposits (e.g., barnacles, dirt, grease, and slime) on a metal surface. Such crevice corrosion is also known as poultice corrosion.
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