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