Tarnish and Corrosion in Dentistry: Causes, Types, and Prevention

Tarnish And Corrosion

Tarnish and corrosion are chemically similar processes but the mechanical state or disposition of corrosion products vary. Clinical differentiation of tarnish and corrosion is very difficult

Tarnish

• Tarnish is defined as surface discoloration on a metal or even a slight loss or alteration of surface finish/Luster.
• The outermost layer of semiactive metals, i.e. copper, brass, silver, and aluminum undergo chemical reactions and form a thin layer called tarnish.
• It is the surface phenomenon that is self-limiting.
• Sulfur dioxide present in the air leads to tarnish.
• It should be manifested as a dull gray or black film over the metal surface.
• The occurrence of tarnish is a protective phenomenon that involves the reaction of only the top few layers of metal after this the layer of tarnish seals and protects the underlying layers.
• This layer of tarnish which seals and protects underlying layers is known as patina.
• An example of tarnish in dentistry is tarnish layers formed over well-polished amalgam and alloy restorations.

Causes of Tarnish:

The following are the causes of tarnish:

• Orally it occurs because of the formation of soft deposits, i.e. plaque and mucin, and hard deposits, i.e. calculus over the surface of the restoration.
• Discoloration occurs due to pigment-producing bacteria, drugs consisting of mercury or iron, and due to adsorbed food debris.
• Formation of thin films, i.e. oxide, sulfide or chloride can lead to surface discoloration. This is also an early indication of corrosion.

Corrosion

Corrosion is a chemical or electrochemical process where the metal is attacked by natural agents leading to partial or complete dissolution or deterioration of the metal.

Causes of Corrosion:

• Tarnish is the forerunner of corrosion
• Water, oxygen, and chlorine ions in saliva lead to corrosion
• Acidic solutions, i.e. phosphoric acid, lactic acid, and acetic acid present in the oral cavity at proper concentration and pH lead to corrosion.

Classifiation of Corrosion:

Corrosion is classified as:

1. Chemical/dry corrosion
2. Electrochemical/electrolytic/wet corrosion

• Galvanic corrosion
  • Dissimilar metals
  • Heterogeneous surface composition
• Stress corrosion
• Concentration cell/crevice corrosion
  • Electrolyte concentration cell corrosion
  • Oxygen concentration cell corrosion

1. Chemical or dry corrosion: In which the metal reacts to form oxides, and sulfides in the absence of electrolytes.

• Example: Formation of Ag2S in dental alloys containing silver.
• Oxidation of alloy particles in dental amalgam.

2. Electrolytic/Electrochemical/wet corrosions:  This requires the presence of water or other fluid electrolytes. There is a formation of free electrons and the electrolyte provides the pathway for the transport of electrons.

An electrolyte cell is:

M⁰ → m⁺+ e^–

The anode consists of positive ions. The metal surface corrodes since there is a loss of electrons. A reaction sometimes is known as an oxidation reaction.

M⁰ + e^– → M⁰

2H^– + 2 e^– → H²

2H₂O + O₂ + 4e^– →  4(OH)^–

At the cathode, the reaction will consume free electrons which are generated at the anode. The above three reactions occur at the cathode and are known as reduction reactions. So the anode loses electrons and the cathode consumes. The surface of the anode gets corroded due to the loss of electrons.

Types of Electrolytic Corrosion:

• Galvanic corrosion
• Stress corrosion
• Concentration cell corrosion
  • Electrolytic concentration cell
  • Oxygen concentration cell.

Galvanic Corrosion:

Galvanic corrosion occurs when two dissimilar metals lie in direct physical contact with each other:

• Saliva with its salt provides a weak electrolyte.
• If a gold restoration comes in direct contact with an amalgam restoration the amalgam forms the anode and starts corroding.
• The electric couple created when the two restorations touch causes sharp pain called “Galvanic shock”.
• It can be minimized by painting varnish on the surface of amalgam restoration. The best precaution is to avoid dissimilar metals in contact.

 

• Heterogeneous Surface Composition: It occurs inside the structure of restoration itself. Such compositions can cause galvanic corrosion.
  • The difficulty arises when a metal surface has both anode and cathodes on the same surface.
  • Metal ions go inside the solution or form corrosion products due to anodic reactions and other ions get reduced in cathodic reactions.
  • Both cathodic and anodic reactions occur at randomly distributed sites on metal, at the anode metal dissolves, and at the cathode, ions get discharged.

 

• Various examples of heterogenous composition are:
  • Solder joints present between dental alloys are prone to corrosion due to differences in the composition of the alloy and solder.
  • When a eutectic alloy is immersed in electrolyte metallic grains with lower electrode potential gets attacked and corrosion occurs.
  • In core structure, there is a difference in composition in alloy grains.
  • So a part of the grain acts as an anode and others act as a cathode.

Stress Corrosion:

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

Concentration Cell Corrosion:

Concentration cell corrosion occurs when a liquid corrosive or an electrolyte is entrapped in the narrow gaps between metals or either between metals or non-metals. There are two types of concentration cell corrosion, i.e.

•  Electrolyte concentration cell: In a metallic restoration that gets partly covered by food debris, the composition of electrolyte under debris will differ from that of saliva, and this leads to the corrosion of restoration.
• Oxygen concentration cell: This is due to the differences in oxygen tension between parts of the same restoration causing corrosion of restoration. Greater corrosion occurs in the part of restoration having a lower concentration of oxygen.