Corrosion of Dental materials

By By

Mohamed Mahmoud Abdul-MonemAssistant lecturer

Dental Biomaterials DepartmentAlexandria University

EgyptMohamed_mahmoud.biomaterials@yahoo.com

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.

Fundamental basis of corrosionChemical 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 .

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

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.

Forms of electrochemical corrosion

• Dissimilar metals.• Heterogeneous surface composition.• Stress corrosion• Concentration cell corrosion.

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.

Dissimilar metals

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

Heterogeneous surface

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)

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.

• 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.

Concentration cell corrosion

• Pitting corrosion :occurs in pits in restorations

• Crevice corrosion: at crevices and marginal gaps of crowns and bridges .

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)

Thank you