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A chemical or electrochemical process through which a metal is attacked by natural agents such as air and water resulting in partial or complete dissolution, deterioration or weakening of any solid substance.
Types of Corrosion
Crevice corrosion - corrosion in narrow spaces caused by localized electrochemical process and chemistry changes such as acidification and depletion of oxygen content. Eg. Microleakage in dental restorations.
Galvanic corrosion - corrosion on a less noble metal when electrochemically dissimilar metals are in electrical contact in presence of corrosive liquid environment.
Pitting corrosion - Sharply localised corrosion in base metals like iron,nickel and chromium (which are naturally protected by a thin film of oxide) in the presence of chlorides in the environment ,the film locally breaks down causing dissolving of the metal underneath it in the form of pits.
Stress corrosion –Degradation by the combined effects of mechanical stress and a corrosive environment,usually in the form of cracking.
Tarnish - A process by which a metal surface is dulled in brightness or discolored through the formation of a chemical film,such as a sulfide and an oxide.
Concentration cell - An electrochemical corrosion cell,in which the potential difference is associated with the difference in the concentration of the solutions causing corrosion at different parts of the metal surface .
An important type of electrochemical corrosion occurs when dissimilar metals are in direct physical contact with each other. Hence the dental reference here is two dissimilar restorations in the oral cavity. this metallic combination may produce Electro galvanism or “Galvanic currents ”.
Eg.repeated removal and insertion of a partial denture will develop a severe stress pattern of certain alloys especially at the grain boundaries. Combined with the oral environment ,the appliance develops stress corrosion resulting in fatigue and failure.
Accumulation of food debris produces one type of electrolyte, normal saliva produces another electrolyte. This difference in electrolyte causes an electrochemical corrosion of the metal filling underneath the food debris.
As long as the film of chrome oxide is maintained, the stainless steel behaves like gold, silver or platinum, or in other words, it has a passive behavior. Stainless steel can also develop active sites of corrosion if the protective film is destroyed by scratches, nicks, stock deposits or contamination of steel by non-ferrous inclusions
Chromium passivated metal will be corroded by chloride, hence patients on removable dentures are instructed not to wash their appliances with household bleaches and cleansers
into a noble family, Count Volta was a physicist and pioneer
in the study of electricity. "Volt," named after Count Volta.
Inventor of the Battery. Around 1800, he invented a wet battery called a Voltaic Pile.
The Voltaic Pile consisted of discs of copper and zinc separated by discs of paper or cardboard (soaked in salt water). Attached to the top and bottom of this "Pile" was a copper wire. When Volta closed the circuit, electricity flowed through the pile.
Volta's battery was later refined by other scientists, and the French emperor, Napoleon, made Volta a "Count" for his discovery.