Tarnish and Corrosion

TARNISH AND CORROSION TARNISH : S urface discoloration on a metal /even slight loss of surface finish. CORROSION: D eterioration of a metal by reacting with environment,water ,oxygen,chloride ion,sulfide contribute corrosion attack. CAUSES FOR TARNISH : Pigment producing bacteria formation of hard ,soft deposit on surface of restoration (eg-calculus) formation of thin film oxide and sulphide. CLASSIFICATION OF CORROSION CHEMICAL/DRY : In chemical type of corrosion metal reacts to form oxide,sulphide in the absence of electrolyte for an example : oxidation of alloy particle in amalgam. 2) Electrochemical /wet corrosion: In electrochemical type of corrosion there will be formation of free electron ,electrolyte that provide pathway for transport of electrons. types of electrochemical corrosion : A) galvanic corrosion B) concentration cell corrosion and C) stress corrosion A) Galvanic corrosion: When two dissimilar metal lie on direct contact with each other ,saliva acts