Tarnish and Corrosion

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 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
  1. 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 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.
B)Concentration cell corrosion:
  • 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

1) Diet
2)Drug
3)Smoking
4)Bacterial activity
5)Oral hygiene,habits.

PROTECTION AGAINST CORROSION 

1)PASSIVATION:
oxide layer forms on surface of metal that protect from corrosion
example-titanium,chromium
2)INCREASING NOBLE METAL CONTENT:
The more positive electromagnetic force indicates its more corrosion and tarnish resistant.
3)POLISHING:
polishing the metallic restoration to high lusture can act as a protection against corrosion.for example- amalgam,cast metal.
4)OTHER:
Dissimilar metal on opposing arch must be avoided in order to prevent galvanic corrosion.
                           THANK YOU 

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