Corrosion and electrochemical properties of dental materials

1. Corrosion of Dental materials
By
By
Mohamed Mahmoud Abdul-Monem
Assistant lecturer
Dental Biomaterials Department
Alexandria University
Egypt
Mohamed_mahmoud.biomaterials@yahoo.com

2. Corrosion
• Is an electrochemical process
and is dependant on the
ability of metals to conduct
electrical current.
• It causes metal deterioration
and subsequent mechanical
failure.
• It causes pulpal pain(galvanic
shock).
• Toxic and allergic responses
to metal ions.
• Metallic taste
• It may be desirable as in
dental amalgam ?? As
corrosion products seal the
marginal gap.(limitied
corrosion)
Tarnish
• Surface discoloration on a
metal or slight loss or
alteration of the surface
finish or luster.
• Occurs due to formation of
deposits ,oxides, sulfides
and chlorides on the
restoration surface.
• It is an early indication and
precursor of corrosion.

4. Fundamental basis of corrosion
Chemical corrosion
• (Dry corrosion) It occurs in
absence of water or any fluid
electrolyte
• Direct combination of a metallic
and nonmetallic element to yield
a chemical compound through
oxidation reactions.
• E.g (1) Oxidation of Ag-Cu
eutectic alloy particles of dental
amalgam. Oxidation limits their
reactivity with mercury.
• (2) Sulfurization of silver in
amalgam (Ag2S)
Electrochemical corrosion
• Galvanic corrosion/wet corrosion
: requires the presence of water
or any fluid electrolyte and an
electric current .

5. Electrochemical corrosion
• When a metal is in contact with a fluid
electrolyte ,ions dissolve to form a saturated
solution and produce free electrons.
• The loss of electrons by a metal is Oxidation.
• The ease with which a metal gives up
electrons and oxidizes is the Oxidation
potential

6. Electrochemical cell
• Anode : surface where positive ions are formed
(surface undergoing oxidation and corroding).
• Cathode : surface where ions are deposited
(Reduction)
• Electrolyte : supplies ions needed at the cathode
and carries away corrosion products at the
anode.
• External circuit (conduction path) of electric
current.

8. Forms of electrochemical corrosion
• Dissimilar metals.
• Heterogeneous surface composition.
• Stress corrosion
• Concentration cell corrosion.

9. Dissimilar metals
• When combinations of dissimilar metals are in
direct physical contact or between
restorations of similar alloys with different
surface composition.
• Example : Dental amalgam restoration
opposing a gold inlay.
• This results in sharp pain (galvanic shock).
• Saliva acts as an electrolyte and hard and soft
tissues as the external circuit.

10. Dissimilar metals

11. Heterogeneous surface composition
• Example:
1. Eutectic alloys (two phase solid solution) corrosion
between lamellae of the alloy.
2. Cored single phase solid solutions.
3. Homogenized solid solution at grain boundaries
(anodic(high energy) to the cathodic grain interiors) .
4. Solder joints
5. Impurites in the alloy segregated at grain boundaries.
6. Polished (cathode)and unpolished (anode) surfaces

12. Heterogeneous surface

13. Stress corrosion
• Stress increases internal energy of an alloy either
through elastic displacements or dislocations.
• It occurs due to fatigue or cyclic loading.
• Small surface irregularities (notches and pits) act
as sites of stress concentration.
• Deformed part of alloy (anode) and undeformed
part (cathode).
• Example: (1) excessive burnishing of margins in
metallic crowns .
• (2) Excessive carving (deep and sharp pits)

14. Concentration cell corrosion
• It occurs whenever there are variations in the
composition of electrolytes.
• Example(1): Accumulation of food debris in
interproximal areas .The debris produce an
electrolyte in this area that is different from
the electrolyte produced by saliva at the
occlusal surface.

15. • Example (2) : Differences in oxygen
concentration between parts of the same
restoration. The greatest attack happens at
the areas containing less oxygen (Anode).
• Pits, scratches and cracks in a restoration are
important example of this phenomenon.

16. Concentration cell corrosion

17. • Pitting corrosion :occurs
in pits in restorations
• Crevice corrosion: at
crevices and marginal
gaps of crowns and
bridges .

18. Protection against corrosion
• Passive metals as chromium in RPD base metal
alloys. (Thin, adherent ,highly protective film).
• Stainless steel (Fe,C,Cr)
• Nickel electroplating
• Titanium forms passivating titanium oxide film.
• Noble metals (especially palladium )
• Highly polished restorations
• Varnish coating on metallic restoration (isolate
from saliva)

19. Thank you