Metal Corrosion

The terms corrosion resistance and chemical resistance to describe metal corrosion are synonymous. Meaning, when a surface has been "attacked", it is a chemical chemical reaction taking place between the metal surface and the environment, resulting in surface deterioration or a breakdown in properties.

Metal corrosion exists in a variety of forms. Here, we discuss the following:

Fretting refers to corrosion damage occurring at the asperities (irregularities) between the areas of contact. This damage is the result of high loads in the presence of relative motion at an interface typically not intended for movement. Vibration is an example. Pits, grooves, and oxidation debris are characteristic damage. Rubbing causes continued stripping away of protective films, exposing new metal to the corrosive condition.

Galvanic corrosion, also referred to as dissimilar metal corrosion, is the result of two dissimilar metals in the presence of a corrosive electrolyte. Damage here is typically local, accelerated by relative motion. Often, this type of corrosion is confused with coupled metals exhibiting increased corrosion based on their oxidation-reduction electromotive force potentials.

De-alloying is known as selective leaching of an element by a corrosion process. It is usually related to segregation of specific elements or formation of compounds in grain boundary regions.Uniform corrosion is an example of larger areas losing material over measurable, predictable periods of time. Here, catastrophic failure is unlikely.

Pitting corrosion is a localized form of corrosion. Often difficult to detect, this type of corrosion is considered more dangerous to design against. Pitting corrosion is initiated by local chemical or mechanical damage of protective oxide films or non metallic inclusions, theoretically forming a “battery” between an abnormal anode or cathode site.

Other forms of corrosion on metal, such as Intergranular, Exfoliation, Stress Corrosion and Hydrogen Embrittlement, can also result in localized cracking, severe loss in ductility, and increased residual tensile stresses. Failure is typically catastrophic.

Corrosion of metal is seen as electrochemical degradation of metals or alloys due to reaction with their environment, accelerated in the presence of acids or bases. In general, the ability of a metal or alloy to corrode depends on its position in the ‘Activity Series’ of the periodic table of the elements.

Corrosion products often take the form of metallic oxides. This is actually beneficial in the case of aluminum and stainless steel, because the oxide forms a strongly adherent coating which effectively prevents further degradation. Hence, these metals are used for structural purposes. The rusting of iron is a familiar form of corrosion which is catalyzed by moisture.

Copper, nickel, chromium, and zinc are among the more corrosion resistant metals and are widely used as protective coatings for corrosion control. But advancements in plating, anodize, thermal spray, vapor deposition, thermoplastics and other engineered polymers, offer an array of metal corrosion protection solutions, once the corrosive mechanism is clearly understood.

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