dental material, Basic sciences in dentistry, MDS

1. TARNISH AND CORROSION
Presented By
Mujtaba Ashraf
MDS-1st year
Dept. of Prosthodontics
& Crown and Bridges
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2. Contents
Introduction to Tarnish and Corrosion
Causes of Tarnish and Corrosion
Classification of Corrosion
Electrochemical Corrosion
Protection Against Corrosion
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3. Definitions
Corrosion: the action, process, or effect of corroding; a
product of corroding; the loss of elemental constituents to the
adjacent environment.
Tarnish: surface discoloration on a metal or as a slight loss
or alteration of the surface finish or luster.
*GPT-8
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4. Introduction
 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.
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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.
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6. 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.
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7. Tarnish
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8. 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.
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9. 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.
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10. 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
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11. Corrosion
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12. Corrosion is a chemical or electrochemical process
wherein a metal is attacked by natural agents, resulting in
its partial or complete dissolution or deterioration.
Eg. Rust, the most familiar example of corrosion.
 It differs from tarnish in that it is not merely a surface
deposit but an actual deterioration of the metal due to a
reaction with its environment.
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13. Dr Mujtaba Ashraf 13
Corrosion in the oral environment is either by chemical or
electrochemical process through which a metal is attacked by
natural agents, such as air and water, resulting in partial or
complete dissolution, deterioration, or weakening
of any solid substance.

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.
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15. Classification of Corrosion
Chemical/Dry corrosion
Electrochemical/Wet
corrosion
Galvanic corrosion Stress corrosion
Concentration
cell/crevice
corrosion
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-By Fontana and Jones

16. 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
This type of corrosion is less susceptible to occur in the
mouth.
Non aqueous (dry) or Chemical corrosion:
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17. Aqueous (wet) or Electrolytic corrosion
Electrochemical corrosion occurs in the presence of a
fluid electrolyte such as water. Hence, it is also known as
wet corrosion.
Only this type of corrosion occurs in the oral cavity
where the electrolyte is the saliva.
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18. Process of Electrochemical Corrosion
The electrochemical cell is made up of three main
components: an anode, a cathode, and an electrolyte.
The anode is the surface or sites on a surface where positive
ions are formed.
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19. ANODE: undergoes an oxidation reaction with the
production of free electrons. Thus, the metal surface
corrodes due to loss of electrons.
M0 → M+ + e-
CATHODE: The free electrons that are released by the
anode are taken up by the cathode or the cathodic sites,
where a reduction reaction occurs.
M+ + e- → M0
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20. Electrolyte is the medium that carries the ions away from the
anode, and these ions are then taken up by the cathode.
It also carries away the corrosion products formed at
the anode.
The pathway of transfer of electrons from the anode to the
cathode forms the external circuits that serve as a conduction
path.
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21. Electromotive Series of Metals
All metals have a tendency to give away electrons;
only their degree of tendency differs.
The electromotive series of metals arranges them in
order of dissolution tendencies in water and classifies
metals by their equilibrium values of electrode
potential.
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23. Metal with lower electrode potential has a greater tendency
to give away electrons and undergo oxidation.
Thus the metal with the lower electrode potential becomes
the anode and undergoes oxidation while the other metal with
the higher electrode potential acts as the cathode and takes up
the electrons.
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24. Galvanic Corrosion/ Electrogalvanism
This type of electrochemical corrosion occurs when two or
more dissimilar metals are in direct physical contact with
each other, e.g. two adjacent or opposing restorations made
of different alloys. Here, saliva acts as
an electrolyte.
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25. Dr Mujtaba Ashraf 25
Galvanic corrosion occurs due to the galvanic coupling
of dissimilar metals involved.
Less corrosion-resistant metals become anode and
usually corrode.
Schoonover and Souder reported that gold restorations
were corroded by mercury released from amalgam
fillings because of an electrochemical reaction.
Fusayama et al. observed that silver-colored stains
formed on the surface of gold inlays that had got into
contact with fresh amalgam mix or fillings.

26. A pain sensation caused by electric current
generated by a contact between two dissimilar metal
forming a galvanic cell in oral environment.
Patient may also experience pain by touching the tine
of a silver fork to a gold foil or inlay
restoration.
Galvanic Shock
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27. Galvanic Corrosion cause:
 weakening of both the alloys
 discoloration of both the restorations, and
 presence of a metallic taste in the mouth.
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28. Stress Corrosion
Stress corrosion is a complex form of corrosion that
occurs when brittle cracks develop in irregularities such
as notches and pits due to the combined effects of tensile
stresses (bending or stretching stress) and corrosive
medium.
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Mechanical
Stress
Corrosive
Environment
Stress
Corrosion

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During mastication, restorations are subjected to heavy
compressive shear, and bending forces.
Also, burnishing of surfaces sometimes results in localized
deformation.
Thus, an electrolytic cell is formed between the stressed and
unstressed metal portions, as stressed area become anode
while unstressed become cathode.

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Stress corrosion most likely to occur during fatigue
or cyclic loading in the oral environment.
Eg. Repeated removal and insertion of a partial
denture will develop a severe stress pattern in the
appliance causes stress corrosion resulting in fatigue
and failure.

31. Concentration cell corrosion/ Crevice corrosion
This type of corrosion occurs when a liquid corrosive or an
electrolyte is trapped in narrow gaps between metals or
between metals and nonmetals.
A homogeneous metal or alloy can undergo electrolytic
corrosion when there is a difference in electrolyte
concentration across the specimen.
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32. Example; a metallic restoration which is partly covered by
food debris will differ from that of saliva, and this can
contribute to the corrosion of the restoration.
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Crevices are unavoidable with surgical implants
where a screw or plate contacts the bone. The local flux
of ions is drastically enhanced at crevice sites and tissue
impregnation follows.
The contact region of a screw or implant material can
form the crevice. In the small space the liquid and
oxygen exchanges are severely limited, and surface in
the crevice undergoes active corrosion and cause further
deterioration.

35. EFFECTS OF CORROSION
Nobel Metal: Nobel metals resist corrosion because
their electromotive force is positive when compared to
any other metal used in the oral environment.
To protect an alloy from corrosion, alloys used in
dentistry should have at least half the components as
noble metals (gold, palladium, and platinum) – Lang et al
Gold, resist sulfide tarnishing.
Palladium is found to be effective in resistance of silver
alloys to sulfide tarnishing.
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36. Base Metal Alloys: Generally, base metal alloys are
very resistant to tarnishing.
Much of the corrosion resistance of the base metal
alloys is due to the development of a thin, tough,
adherent layer of oxide.
This layer prevents the penetration of corrosive ions
into the underlying metal and reduces electrochemical
behavior to a passive state.
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Base metals, such as stainless steels, nickel-
chromium alloys, and cobalt-chromium alloys, are
virtually immune to sulfide tarnishing, but these
alloys are susceptible to chlorides.
Silver Points/Cones contains Cu & Ni cause
corrosion when get in contact with periradicular
tissue and cause cytotoxicity.

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Titanium and its alloys are superior in their resistance
to chloride attack, compared with the other dental base
metal alloys, as titanium forms a relatively stable oxide
layer, and this is the basis for the corrosion–resistance
property and biocompatibility.
Lucas and Lemons have reported that the formation
of oxide film on titanium provides corrosion-resistance
under static conditions, but the oxide film is not
sufficiently stable to prevent galling and seizing under
loading conditions.

39. Stainless Steel: The corrosion resistance of stainless
steel is attributed to the presence of chromium in the
alloy.
Approximately, 11% chromium is needed to
produce corrosion resistance in pure iron.
Chromium resists corrosion well because of the
formation of a strongly adherent coating of oxide on
the surface, which prevents further reaction with the
metal below the surface. The formation of such an
oxide layer is called passivation.
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40. Corrosion of Amalgam
Low copper amalgam has much lower resistance to
tarnish and corrosion as compared to high copper
amalgam due to the formation of γ₂-phase, which
has least resistance to corrosion.
It has been found that phosphate buffer solutions
inhibit the corrosion process; hence, it can be said
that saliva may provide some protection to dental
amalgam from corrosion.
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Amalgam restorations are subject to tarnish and
corrosion in the oral environment.
It has been suggested that the corrosion process
although detrimental in some respects may also have
beneficial effects.
Wagner has credited the corrosion process with sealing
of leaky margins and mechanical anchoring of the
restoration to cavity walls by deposition of corrosion
products.

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Schoonover and Souder have blamed the corrosion
process for loss of mechanical strength in the surface
regions of amalgam.
Jorgensen has claimed that corrosion is responsible for
release of liquid mercury that diffuses into the amalgam
and gives rise to mercuroscopic expansion and margin
upheaval.
Use of amalgambond , 4-methacryloxyethyl
trimetallic anhydrate (4-META) bonding agent with
amalgam reduces microleakage.

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Tissue discoloration: During placement of amalgam
in cavity, some spillage tends to occur in the
surrounding tissues. If not removed carefully, these
remnants may corrode and lead to mucosal tissue
discoloration known as amalgam tattoo or focal
argyria.

44. Protection Against Corrosion
Passivation: This is the method by which certain
metals develop a thin, adherent, highly protective film
when they react with the environment. Such metals are
known as passive metals.
e.g: Thin coatings of electrolytic zirconium oxide
(ZrO2) deposited on cobalt-chromium alloys reduce
chromium-release levels in artificial saliva, as
compared to uncoated cobalt-chromium alloys.
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45. Electroplating: Electroplating is the application of
electrolytic cells in which a thin layer of metal is deposited
onto an electrically conductive surface.
The process used in electroplating is
called electrodeposition.
Boris Jacobi a Russian scientist developed electroplating.
Reasons for electroplating
corrosion resistance.
to improve the appearance and value of the items.
to increase the thickness of an item.
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46. Increasing noble metal content: Since noble metals
have higher electromotive potential (positive) when
compared to any of the other metals used in the oral
environment, they can be added to alloys to reduce
corrosion.
Polishing restorations
Avoiding dissimilar metal restorations
Maintenance of oral hygiene
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47. Though corrosion is almost always undesirable, there are
some instances where it is beneficial such as around the
margins of dental amalgam restorations as it seals the
marginal gap.
It is difficult to prevent corrosion completely in oral
cavity. Hence the selection of restorative material and
good oral hygiene helps in minimizing corrosion process.
Conclusion
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48. Phillips Science Of Dental Material 10th & 11th Edition
Restorative Dental Materials – Craig 13th Edition
Dental Materials And Their Selection- 3rd Edition By
William J. O'brien
Schoonover, I.C. And SOUDER, W.: Corrosion Of Dental
Alloys, JADA 28:1278-1291, 1941.
Jorgensen, K.D.: The Mechanism Of Marginal Fracture Of
Amalgam Fillings, Acta Odont Scand 23:347-389, 1965.
Corrosion of alloys used in dentistry: A review. Materials
Science and Engineering A 432 2006 Deepti Upadhyay
References
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