a short and simplified presentation on the vast and important topic of tarnish and corrosion.

2. Tarnish and Corrosion.
Submitted by- Guided By-
NAYANJIT TALUKDAR. Dr.
RAJDEEP PAUL.
BDS 1st yr.
Roll No. 04
Regional Dental College.

3. CONTENTS.
 Introduction.
 Tarnish.
 Corrosion.
 Fundamental basis of corrosion.
 Classification of corrosion.
 Protection against corrosion.
 Conclusion.

4. INTRODUCTION.
Dentistry uses a wide variety of metals for restorations and
prosthetic devices, the successful performance and long term durability of
which require adequate corrosion resistance in the oral environment. The
oral environment is highly conductive to electrochemical action. It is
warm, moist and subjected to wide variety of fluctuations in temperature
and pH. So we need to understand the corrosion to minimise it and
corrosion induced problems in dentistry.

5. Tarnish.
 It is a surface discolouration on a metal or a slight loss of
alteration of the surface finish or lustre.
 It occurs due to formation of thin film of oxides, sulphides or
chlorides.
 This may be only a surface deposits.
 Tarnish may be even protective.
 It is often an early indication and precursor if corrosion.

7. Corrosion.
 It is a process whereby deterioration of the actual metal is
caused by reaction with its environment.
 Corrosion may cause the mechanical failure of a structure.
 Corrosion takes place due to the action of certain chemicals,
moisture, atmosphere etc.
 Chemicals such a ammonium of hydrogen sulphides initially
tarnish and eventually corrode silver, copper, tin, mercury etc.
present in their alloys.
 Acids such as Phosphoric, Acetic and Lactic acids are present
in a proper concentration and pH, can promote corrosion.

9. Tarnish of amalgam
restoration.
Corrosion of an amalgam
restoration.

10.  Corrosion occurs because most commonly used metals
and alloy are NOT in their energy state.
 Lowest energy state is thermodynamically the most
stable state.
 Thus pure metals spontaneously convert to a highly
reactive, oxidised state by reacting with for example
oxygen, sulphur or chlorine in order to revert to their
lowest energy state.
 Corrosion of metal is either a Chemical or
Electrochemical Process.

11. Classification of corrosion.
Corrosion.
Chemical/ dry
corrosion.
Electrochemical/
wet corrosion.
Galvanic/dissimil
ar metal
corrosion.
Heterogenou
s
composition.
Stress
corrosion.
Concentratio
n cell
corrosion.

12. CHEMICAL CORROSION.
 It is the direct combination of metallic and non-metallic
elements to yield chemical compound through oxidation
reactions.
 It occurs in the absence of water or fluid electrolyte.
 So, it is known as Dry corrosion.

13. ELECTROCHEMICAL CORROSION.
 It is known as Wet corrosion.
 It requires the presence of water or some other electrolyte
fluid.
 Electrochemical corrosion is seldom isolated and almost
invariably is accompanied by chemical corrosion.

15.  The electrolyte serves to supply the ions needed at the
cathode, and to carry away the corrosion products at
the anode.
 These process is referred to as reduction reactions.
 For corrosion to be an ongoing process at the anode
must be exactly balanced by the consumption of
electrons in the reduction at the cathode.

16. 1. Galvanic corrosion.
 It occurs when dissimilar metals are in direct contact with
each other.
 Here the dental reference is two separate restorations in
which the metal surface are chemically dissimilar.
 Due to this a electrochemical cell generates with saliva as the
electrolyte and corrodes the surface.
 The metallic combinations may produce electrogalvanism or
galvanic currents.

17. Galvanic corrosion takes place
between two different
restorative metals with saliva as
an electrolyte.

18.  This current is created due to dissimilar metallic
restorations.
 Because both the restoration are wet in saliva, an
electrical couple exists, with a difference in potential
between the dissimilar restorations.
 When the two restorations are brought in contact, the
potential is suddenly short-circuited through two alloys.
 This may results in sharp pain.

20. 2. Heterogenous
composition.
 This type of corrosion occurs due the heterogenous
composition of the metal surface.
 Examples of this type are eutectic and peritectic alloys.
 When an alloys containing eutectic is immersed in an
elecrtolyte, the metallic grains with lower electrode potential
are attacked and corrosion results.

22. 3. Stress corrosion.
 This type of corrosion is seen due to deleterious effects of stress
in combination with a corrosive environment.
 Slight surface irregularities such as notch or pit , can accelerate
the process so that ordinary fatigue starts below the normal limit
and failure results.
 If stressed an unstressed metals are in contact in an electrolyte,
the stressed metal will become anode of galvanic cell and will
corrode.
 Any cold working of an alloy by blending, burnishing, or
malleting localizes stress in some part which is easily dissolved
by the electrolyte.
 That is why unnecessary burnishing of the margins of metallic
restoration is contraindicated.

23. 4. Concentration cell
corrosion.
 This situation arise when there is a variation n the electrolytes
or in the composition of the given electrolyte in the solution.
 For example, there is often accumulation of food debris in the
interproximal areas of the tooth, this debris produce one tp of
electrolyte in that area, and the normal saliva produce another
type of electrolyte at the occlusal surface.
 Therefore, electrochemical corrosion occurs with preferential
attack of the metal occurring underneath the layer of food
debris.

25.  A similar type of attack may be produced due to difference in
oxygen tension between parts of the restoration.
 A cell is produced with the greatest activity occurring around the
areas containing the least oxygen.
 Irregularities s.a pits, fissures contributes to this phenomenon.
 The areas at the bottom o the surface concavities donot have
oxygen because they are covered with food debris and mouth.
 The material at the bottom of the pit then becomes the anode
and the material at the periphery the cathode.
 In this manner, metal atoms at the bottom of the pit ionize and
go into the solution, causing the pit to deepen.
 Generally two or more mechanisms act simultaneously and thus
exaggerating the corrosion.

26. Protection against corrosion.
Mainly there are four ways by which we can
protect the metal against corrosion.
 Passivation.
 Increasing noble metal content.
 Polishing.
 Other methods.

27.  PASSIVATION.
 The formation of a film which is passive to oxidative chemical
attacks, their formation is called passivation.
 Adding more than 12% Cr to iron or cobalt produces a
chromic oxides layer on the surface of stainless steel or cobalt
chromium alloys which is highly resistant.
 Examples of some passive alloy are chromium, titanium and
aluminium.

28.  INCREASING NOBLE METAL CONTENT.
 Alloy with noble metal content below 65% may tarnish, so I
has been suggested that at least 50% of the atoms in a dental
alloys should be gold, platinum or palladium to ensure against
corrosion.
 Noble metal resists corrosion because their EMF is positive
with regards to any of the common reduction reactions found
in the oral environment.

29.  POLISHING.
 Polishing metallic restorations like amalgam and cast metal to
a high lustre minimizes corrosion.
 It happens because of the reduction of surface irregularities.
 The patient also should maintain a good oral hygiene.

30.  OTHER METHODS.
 Dissimilar metal restorations should be avoided.
 High mercury containing amalgam should be avoided as it is
more susceptible to corrosion.
 Mercury tarnishes gold, thus care must be taken to protect the
gold ornaments worn by the operators, assistants or patient.

31. CONCLUSION.
Tarnish and corrosion are irreversible
phenomenon. They cant be stopped, but certainly they can be
controlled to some extent. With the help of some new
techniques and technology, we find that with the help of
tarnish also we can control corrosion. If we have a thorough
knowledge of tarnish and corrosion, we can surely increase
the durability of metallic restorations.