Tarnish and Corrosion
TARNISH AND CORROSION
TARNISH : Surface discoloration on a metal /even slight loss of surface finish.
CORROSION: Deterioration 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.
- 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
- When two dissimilar metal lie on direct contact with each other ,saliva acts as a electrolyte that provides pathway for transport of electrons .
- For example: If there is gold restoration in 26 and amalgam in 36 that is if there is dissimilar metal restoration in opposing tooth that comes in contact galvanism occurs.
- Amalgam forms an anode and start corroding .
- It produces sharp pain called galvanic shock.
- This galvanic that's produced can be minimised by painting varnish on amalgam restoration,therefore its advised to avoid dissimilar metal.
- Galvanic shock can also arise in a single metal restoration when a metallic surface is brought close to the tooth such as focke.
- concentration cell corrosion occurs when a liquid corrosive is entrapped in gaps between metals.
- i)Electrolyte concentration cell: in metallic restoration ,food debris partially cover,concentration of electrolyte differ from saliva because of corrosion.
- ii)Oxygen concentration cell: corrosion occurs on restoration having lower concentration of oxygen.
C) Stress corrosion:complex form which occurs when brittle crack develop in small surface irregularities,when stressed and unstressed metal contact in electrolyte it forms a galvanic cell
- example- failure of rubber dam clap,fracture of rpd framework.
FACTORS AFFECTING CORROSION OF RESTORATION
PROTECTION AGAINST CORROSION
oxide layer forms on surface of metal that protect from corrosion
2)INCREASING NOBLE METAL CONTENT:
The more positive electromagnetic force indicates its more corrosion and tarnish resistant.
polishing the metallic restoration to high lusture can act as a protection against corrosion.for example- amalgam,cast metal.
Dissimilar metal on opposing arch must be avoided in order to prevent galvanic corrosion.