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1. Tarnish and Corrosion
INDIAN DENTAL ACADEMY
Leader in continuing dental education
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2. Metals do not exist in their pure form
naturally.
They are always bound with non-metals and
exist as oxides and sulfides.
So, to produce pure metals a great deal of
energy is required.
So pure metals exist in high energy form and try
to return to their normal combined state.
Process by which this occurs is corrosion.
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3. What is tarnish ?
It is a surface discoloration on a metal
or even a slight loss or alteration of the
surface finish or luster.
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4. What is corrosion ?
It is not the surface discoloration but actual
deterioration of a metal by reaction with
environment.
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5. Tarnish is often a forerunner of corrosion.
H2S /NH4S2 Ag, Cu, Hg
O2& Cl Amalgam
S
Casting alloys containing
silver
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6. Corrosion
Dry (chemical)
Wet
(electrolytic/electrochemical)
galvanic stress Crevice/concentration cell
electrolyte oxygen
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7. Chemical/dry corrosion
Metal reacts to form oxides and sulfides
in absence of electrolytes.
Eg- silversulfide in silver alloys
oxidation of amalgam
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8. Electrolytic corrosion
Requires presence
of water and
other fluids
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9. Anode – metal which looses electrons
- forms +ve ions
- undergoes oxidation reaction
M0
M+
+ e-
Cathode-metalwhichacceptselectrons
M+
+e-
M0
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10. Electromotive force series
it shows the tendency
of a metal to give out
electrons when
inserted in an
electrolyte
Metal with lowest
EMF values corrodes
metal ion Electro
de
potenti
al
Gold Au +1.50
Platinu
m
Pt +0.86
Silver Ag +0.80
Copper Cu +0.47
Hydroge
n
H +
0.00
Cobalt Co -0.28
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11. Galvanic corrosion
Occurs when dissimilar metals come in contact
with each other
Eg- amalgam in contact with gold restoration
Galvanic shock
Varnish application
Due to heterogenous composition
-impurities in any alloy
-solder joints
-in alloys due to difference in composition
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12. Stress corrosion
Cold working makes the metal more
reactive
Metal acts as anode and corrodes.
Eg- ortho wires
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13. Crevice corrosion/concentration cell
corrosion
Electrolyte concentration cell –
- In metallic resto which is partially covered,the
composn of electrolyte under the food debris will
differ from that of saliva which can contribute
to corrosion.
Oxygen concentration cell –
- Differences in oxygen tension in between parts
of the same restoration causes corrosion of the
restoration
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14. Protecion against corrosion
Passivation
Increase noble metal content
Polishing restoration
Avoid dissimilar metal restoration
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15. www.indiandentalacademy.com
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