Tarnish and corrosion Dr. Waseem Bahjat Mushtaha Specialized in prosthodontics

Introduction : Metals undergo chemical reactions with nonmetallic elements in the environment to produce chemical compounds known as corrosion products e.g. rusting of iron. One of the primary requisites of any metal or alloy to be used in the mouth is that it should not produce corrosion products that will be harmful to the metallic structure (weaken the restoration and spoil its esthetic). However corrosion products do not only give metallic taste sensation but also are toxic to the patient.

The oral environment: The mouth is an ideal environment for aqueous corrosion of metals and alloys to occur. The presence of moisture, temperature fluctuations, and the changing PH caused by diet and the decomposition of foodstuffs, can all contribute to this phenomenon.

Tarnish : Tarnish : is a surface discoloration of the metal or even a slight loss of the luster. This surface discoloration is due to either, the formation of hard and soft deposits on the surface, or the formation of thin films of oxides, sulfides or chlorides. Tarnish is often the first step of corrosion.

Tarnish :

Corrosion : Corrosion : is not only a surface discoloration but is a disintegration of a metal by reaction with its environment. For example, eggs contain significant amount of sulphur that corrode silver, copper, tin, mercury and similar metals which are present in dental gold alloys and amalgam.

Corrosion :

Classification of corrosion: 1) Non aqueous (dry) or chemical corrosion: In which there is a direct combination of metallic and non-metallic elements e.g. oxidation, halogenations, or sulfarization reaction. This type of corrosion is less susceptible to occur in the mouth. Example of chemical corrosion is the oxidation of metal surface during soldering and heat treatment procedures.

2) Aqueous (wet) or electrolytic corrosion: The basis for any discussion of electrolytic corrosion is the electromotive force series. It is an arrangement of elements in the order of their dissolution tendency. If two metals are immersed in an electrolyte and are connected by an electrical conductor, an electric cell is formed. The metal with the lowest electrode potential goes into solution

Aqueous (wet) or electrolytic corrosion

Types of electrolytic corrosion: 1) Galvanic cell corrosion. 2) Stress cell corrosion. 3) Concentration cell corrosion. 4) Corrosion due to heterogeneous composition of the metal surface. 5) Stress corrosion

1) Galvanic cell corrosion: It occurs when two dissimilar metals are combined resulting in corrosion of one metal. For example, assume that an amalgam restoration is placed on the occlusal surface of a lower tooth directly opposing a gold inlay in an upper tooth. Because both restorations are wet with saliva, an electrical couple (cell) exists, with a difference in potential between the dissimilar restorations.

When the two fillings are brought into contact, an electrical current exists and the result is sharp pain. When the teeth are not in contact, the difference in electromotive force between the two fillings still exists. N.B. Galvanism can also occurs between restorations of similar metals which are never exactly comparable in surface composition e.g. low and high copper amalgams.

Galvanic cell corrosion:

2) Stress cell corrosion: a) The grain boundaries of pure metal or any alloy are attacked in preference to the grain it self because of the lattice distortion in the grain boundary which produces a stressed condition. b) Any cold working of an alloy localizes stress in some parts of the structure. A cell composed of the stressed metal. Saliva, and the unstressed metal is thus formed. The stressed area is more readily dissolved by the electrolyte.

3) Concentration cell corrosion (difference in composition of electrolyte) A homogeneous metal or alloy can undergo electrolytic corrosion where there is a difference in electrolyte concentration across the specimen. 1) Consider the case of a metallic restoration which is partly covered by food debris will differ from that of saliva, and this can contribute to the corrosion of the restoration

2) Where there are difference in the concentration of oxygen in an electrolyte, an oxidation- type concentration cell is formed. Corrosion is greater at portions of the metal or alloy with lower concentrations of oxygen. This can happen in an unpolished metallic restoration. The surface concavities become filled readily with food debris, this lower the concentration of oxygen at those parts, which corrode, and lead to pitting of the filling.

4) Corrosion due to heterogeneous composition of the metal surface a) When an alloy containing an eutectic is immersed in an electrolyte, the metallic grains with the lower electrode potential are attacked and corrosion results. b) In a solid solution any cored structure is less resistant to corrosion than the homogenized structure because of the difference in the electrode potential caused by variation in composition between the individual dendrites. 3) The solder joint (denture or alloys)

5) Stress corrosion A combination of corrosion and stress condition. Can cause failures of a metal by stress corrosion. At an anodic portion of a surface, electrolytic action can form a minute crack. This can increase in size as a result of stress concentration. More corrosion can occur in the enlarged crack, and so on, eventual failure occurs.

Stress corrosion

Passivity : Certain metals develop a protective oxide coating which protects them form further corrosion, such a metal is said to be passive. It is a form of tarnish in which the coating protects the metal underneath from further tarnish and corrosion. Chromium is the best example of passivity.

Resistance to corrosion in dental restorations: 1) Choice of alloy: Alloys for long-term use in the mouth must be either noble or passive. a) Noble metals are those such gold, platinum and palladium. Dental gold alloys contain some copper, which has poorer corrosion resistance, however, such alloys should ideally contain at least 70-75% noble metals

b) Passive alloys, such as those containing chromium are widely used dentally. Example : stainless steel, cobalt-chromium and nickel-chromium. 2) Avoid using dissimilar metals. 3) Base metal alloys containing chromium are resistant to corrosion because of the passivity of chromium.

cobalt-chromium

5) Titanium is another metal that passivates and very recently used in dentistry. 6) Also, the surface of any dental restoration should be smooth and lustrous. A polished, smooth surface provides easier cleaning and prevents accumulation of debris.

Titanium