Tarnish And Corrosion
Tarnish: Tarnish is a surface discoloration on a metal surface or even a slight loss or alteration of the surface Finish/Luster.
Corrosion: It is not a surface discoloration but the actual deterioration of a metal by reaction with the environment.
- It can be defined as the deterioration of metals by chemical interaction with their environment.
- Water, oxygen, chloride ions, and sulfide contributes to corrosion attacks in the oral cavity.
Read And Learn More: Dental Materials Question And Answers
Protection Against Corrosion:
- Passivation: Certain metals readily form a strong adherent oxide film on their surface which protects them from corrosion. Such metals are said to be passive. Since this film is passive to oxidative chemical attack, their formation is called “passivation”.
- Chromium, titanium, and aluminum are examples of such metals.
- Example: In cobalt-chromium alloy chromium acts as a passive metal, protecting this alloy from corrosion by the formation of a layer of chromium oxide on the surface of these alloys.
- Increasing noble metal content: Noble metals tarnish and are corrosion-resistant because their electromotive potential is positive with regard to any of the common reduction reactions found in the oral environment. They can be added to alloys to reduce corrosion.
- Polishing: Polishing metallic restorations like amalgam and cast metal to a high luster to minimize corrosion.
Avoiding Dissimilar Metal Restorations:
When two dissimilar metals are in contact with each other galvanic corrosion is maximum. In such type of cases, a non–conductive film or paint can be applied over more noble metal.
This results in less surface area of more active metal reducing the process of corrosion. If any scratch occurs in such type of coat, this will not affect corrosion resistance as the active metal remains the same.
Corrosion Of Dental Materials
Stress corrosion is a complex form of corrosion that occurs when brittle cracks develop in small surface irregularities, i.e. notches or pits due to the combined effect of tensile stresses and corrosion.
- Cracks present in small surface irregularities have intergranular or transgranular morphology.
- This means that their paths lie at grain boundaries or at specific crystallographic boundaries inside the grains.
- When a metal has been stressed by cold working it becomes more reactive at the site of maximum stress.
- So when stressed and unstressed metals are in contact with an electrolyte they form a galvanic cell, the stressed metal will become the anode of a galvanic cell and will corrode, for example, Orthodontic wires.
- Various examples of stress corrosion are:
- Failure of rubber dam clamp
- Fracture of RPD framework.
Electrochemical corrosion occurs in the presence of a fluid electrolyte, i.e. water. That’s why it is also known as wet corrosion.
Biological Consideration:
Two dissimilar metals should not be restored in the oral cavity because they can lead to galvanism. Galvanism occurs due to the difference in potential between dissimilar filings in opposing or adjacent teeth. Such filings in conjunction with the saliva form an electric cell. When opposing filings contact with each other cells are short-circuited and the patient feels pain.
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