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TARNISH AND CORROSION.pptx
1.
2. Contents
Introduction to Tarnish and Corrosion Causes of
Tarnish and Corrosion Classification of Corrosion
Electrochemical Corrosion
Protection Against Corrosion
3. Tarnish—Process by which a metal surface is dulled or
discolored when a reaction with a sulfide, oxide,
chloride, or other chemical causes surface discoloration
through formation of a thin oxidized film.
Corrosion—Chemical or electrochemical process in
which a solid, usually a metal, is attacked by an
environmental agent, resulting in partial or complete
dissolution.
4. Metals are used in dentistry in various forms, such as
metallic restorative materials in the mouth or as various
instruments to be used in the oral cavity or as numerous
tools and equipment associated with clinical and
laboratory work.
All metals used in dentistry undergo tarnish or
corrosion, the percentage of corrosion varying from
metal to metal.
5. The mouth is moist, warm, salty, acidic, and is
continually subjected to fluctuations in temperature.
The food and liquid that we take have a wide range of
pH.
All these environmental factors contribute to the
degradation of the metals used in the oral cavity.
However in the dental practice, a limited amount of
corrosion around the margins of dental amalgam
restorations may be beneficial, since the corrosion
products tend to seal the margin gap and inhibit the
ingress of oral fluids and bacteria.
6. The outermost surface layer of many semi-reactive
metals such as copper, brass, silver, and aluminum
undergo a chemical reaction forming a thin layer
known as tarnish.
This is a surface phenomenon that is self limiting
unlike rust.
It is mainly caused by chemicals in the air, such as
sulfur dioxide.
Tarnish is manifested as a dull gray or black film or
coat over the metal surface.
7.
8. Tarnish is a surface discoloration on a metal or a slight
loss or alteration of the surface finish or luster.
In the oral environment, tarnish often occurs from the
formation of deposits on the surface of a restoration.
Tarnish also arises from the formation of thin films, such as
oxides, sulfides, or chlorides.
The latter phenomenon may be only a simple surface
deposit, and such a film may even be protective, as
discussed subsequently.
However, it is often an early indication and precurser of
corrosion.
It is manifested as dull gray or black film or coat over the
metal surface.
9.
10. The formation of tarnish is a protective phenomenon
that involves the reaction of only the top few layers of
metal. The layer of tarnish then seals and protects the
underlying layers. This layer of tarnished metal is
called PATINA.
The formation of patina is necessary in applications
such as copper roofing, and outdoor copper, bronze,
and brass statues and fittings.
11. Causes of Tarnish:
In the oral environment, tarnish occurs due to the
formation of soft deposits (plaque and mucin) and hard
deposits (calculus) on the surface of the restoration.
Stain or discoloration arises from pigment producing
bacteria, drugs containing chemicals such as iron or
mercury, and adsorbed food debris.
Formation of thin films such as oxides, sulfides, or
chlorides may cause surface discoloration
12.
13. Corrosion is a process whereby deterioration of a metal
is caused b reaction with its environment.
In due course, corrosion can cause severe and
catastrophic disintegration of metals.
Even if highly localized, corrosion may cause
mechanical failure of a structure even though the actual
volume of material lost is quite small.
Corrosive disintegration can take place through the
action of moisture, atmosphere, acid or alkaline
solutions, and certain chemicals
14. Causes of Corrosion
Tarnish is often the forerunner of corrosion.
Water, oxygen, and chlorine ions present in saliva
contribute to corrosion attack.
Various acidic solutions such as phosphoric, acetic, and
lactic acids often present in the oral cavity at proper
concentrations and pH can promote corrosion.
Examples: Eggs contain high amounts of sulfur.
Various sulfides, such as hydrogen or ammonium
sulfide, corrode silver, copper, and mercury. Ions such
as oxygen and chlorine corrode amalgam.
15. Gold is unique: it does not corrode and is available in
nature in pure forms.
Ceramics, on the other hand, do not corrode because
they are already fully oxidized and in their lowest
energy states.
However, glass-phase ceramics and feldspathic
porcelains, and glazes can dissolve in acidic and basic
media.
17. In chemical corrosion, there is a direct reaction
between the metallic and nonmetallic elements to
yield a chemical compound through processes such as
oxidation, halogenation, or sulfurization in the absence
of water or another fluid electrolyte.
Electrolytes are absent – that’s why referred as dry
corrosion.
This type of corrosion is less susceptible to occur in the
mouth.
18. A good example is the discoloration of silver by sulfur,
where silver sulfide forms by chemical corrosion.
It can also be a corrosion product of dental gold alloys
that contain silver.
