tarnish & corrosion in dentistry

Dr Mujtaba Ashraf 1

TARNISH AND CORROSION

Presented ByMujtaba AshrafMDS-1st yearDept. of Prosthodontics& Crown and Bridges

Dr Mujtaba Ashraf 2

ContentsIntroduction to Tarnish and Corrosion

Causes of Tarnish and Corrosion

Classification of Corrosion

Electrochemical Corrosion

Protection Against Corrosion

Dr Mujtaba Ashraf 3

Definitions

Corrosion: the action, process, or effect of corroding; a product of corroding; the loss of elemental constituents to the adjacent environment.

Tarnish: surface discoloration on a metal or as a slight loss or alteration of the surface finish or luster.

*GPT-8

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Introduction

Metals are used in dentistry in various forms, such as

metallic restorative materials in the mouth or as various

instruments to be used in the oral cavity or as numerous

tools and equipment associated with clinical and

laboratory work.

All metals used in dentistry undergo tarnish or

corrosion, the percentage of corrosion varying from

metal to metal.

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The mouth is moist, warm, salty, acidic, and is

continually subjected to fluctuations in temperature.

The food and liquid that we take have a wide range

of pH.

All these environmental factors contribute to the

degradation of the metals used in the oral cavity.

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However in the dental practice, a limited amount of

corrosion around the margins of dental amalgam

restorations may be beneficial, since the corrosion

products tend to seal the margin gap and inhibit the

ingress of oral fluids and bacteria.

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Tarnish

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The outermost surface layer of many semi-reactive metals such as copper, brass, silver, and aluminum undergo a chemical reaction forming a thin layer known as tarnish.This is a surface phenomenon that is self limiting unlike rust.It is mainly caused by chemicals in the air, such as sulfur dioxide.Tarnish is manifested as a dull gray or black film or coat over the metal surface.

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The formation of tarnish is a protective phenomenon that involves the reaction of only the top few layers of metal. The layer of tarnish then seals and protects the underlying layers. This layer of tarnished metal is called PATINA.

The formation of patina is necessary in applications such as copper roofing, and outdoor copper, bronze, and brass statues and fittings.

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Causes of Tarnish

In the oral environment, tarnish occurs due to the formation of soft deposits (plaque and mucin) and hard deposits (calculus) on the surface of the restoration.

Stain or discoloration arises from pigment producing bacteria, drugs containing chemicals such as iron or mercury, and adsorbed food debris.

Formation of thin films such as oxides, sulfides,or chlorides may cause surface discoloration


Corrosion


Corrosion is a chemical or electrochemical process wherein a metal is attacked by natural agents, resulting in its partial or complete dissolution or deterioration.Eg. Rust, the most familiar example of corrosion.

It differs from tarnish in that it is not merely a surface deposit but an actual deterioration of the metal due to a reaction with its environment.


Corrosion in the oral environment is either by chemical or electrochemical process through which a metal is attacked by natural agents, such as air and water, resulting in partial or complete dissolution, deterioration, or weakeningof any solid substance.


Causes of Corrosion

Tarnish is often the forerunner of corrosion.Water, oxygen, and chlorine ions present in saliva contribute to corrosion attack.Various acidic solutions such as phosphoric, acetic, and lactic acids often present in the oral cavity at proper concentrations and pH can promote corrosion.Examples:Eggs contain high amounts of sulfur. Various sulfides, such as hydrogen or ammonium sulfide, corrode silver, copper, and mercury.Ions such as oxygen and chlorine corrode amalgam.


Classification of Corrosion

Chemical/Dry corrosion Electrochemical/Wet corrosion

Galvanic

corrosion

Stress corrosi

on

Concentratio

n cell/cre

vice corrosi

on

-By Fontana and Jones


In chemical corrosion, there is a direct reaction between the metallic and nonmetallic elements to yield a chemical compound through processes such as oxidation, halogenation, or sulfurization in the absence of water or another fluid electrolyte.Electrolytes are absent This type of corrosion is less susceptible to occur in the mouth.

Non aqueous (dry) or Chemical corrosion:


Aqueous (wet) or Electrolytic corrosion

Electrochemical corrosion occurs in the presence of a fluid electrolyte such as water. Hence, it is also known as wet corrosion.Only this type of corrosion occurs in the oral cavity where the electrolyte is the saliva.


Process of Electrochemical Corrosion

The electrochemical cell is made up of three main components: an anode, a cathode, and an electrolyte.The anode is the surface or sites on a surface where positive ions are formed.


ANODE: undergoes an oxidation reaction with the production of free electrons. Thus, the metal surface corrodes due to loss of electrons.

M0 → M+ + e-

CATHODE: The free electrons that are released by the anode are taken up by the cathode or the cathodic sites, where a reduction reaction occurs.

M+ + e- → M0


Electrolyte is the medium that carries the ions away from the anode, and these ions are then taken up by the cathode.It also carries away the corrosion products formed atthe anode.The pathway of transfer of electrons from the anode to the cathode forms the external circuits that serve as a conduction path.


Electromotive Series of Metals

All metals have a tendency to give away electrons; only their degree of tendency differs.The electromotive series of metals arranges them inorder of dissolution tendencies in water and classifies metals by their equilibrium values of electrode potential.


Metal with lower electrode potential has a greater tendency to give away electrons and undergo oxidation.

Thus the metal with the lower electrode potential becomes the anode and undergoes oxidation while the other metal with the higher electrode potential acts as the cathode and takes up the electrons.


Galvanic Corrosion/ Electrogalvanism

This type of electrochemical corrosion occurs when two or more dissimilar metals are in direct physical contact with each other, e.g. two adjacent or opposing restorations made of different alloys. Here, saliva acts asan electrolyte.


Galvanic corrosion occurs due to the galvanic couplingof dissimilar metals involved. Less corrosion-resistant metals become anode and usually corrode.

Schoonover and Souder reported that gold restorations were corroded by mercury released from amalgam fillings because of an electrochemical reaction.

Fusayama et al. observed that silver-colored stains formed on the surface of gold inlays that had got into contact with fresh amalgam mix or fillings.


A pain sensation caused by electric current generated by a contact between two dissimilar metal forming a galvanic cell in oral environment.Patient may also experience pain by touching the tine of a silver fork to a gold foil or inlayrestoration.

Galvanic Shock


Galvanic Corrosion cause: weakening of both the alloys discoloration of both the restorations, and presence of a metallic taste in the mouth.


Stress Corrosion

Stress corrosion is a complex form of corrosion that occurs when brittle cracks develop in irregularities such as notches and pits due to the combined effects of tensile stresses (bending or stretching stress) and corrosive medium.

Mechanical Stress

Corrosive Environment

Stress Corrosion


During mastication, restorations are subjected to heavy compressive shear, and bending forces.Also, burnishing of surfaces sometimes results in localized deformation. Thus, an electrolytic cell is formed between the stressed and unstressed metal portions, as stressed area become anode while unstressed become cathode.


Stress corrosion most likely to occur during fatigue or cyclic loading in the oral environment.Eg. Repeated removal and insertion of a partial denture will develop a severe stress pattern in the appliance causes stress corrosion resulting in fatigue and failure.


Concentration cell corrosion/ Crevice corrosion

This type of corrosion occurs when a liquid corrosive or an electrolyte is trapped in narrow gaps between metals or between metals and nonmetals.

A homogeneous metal or alloy can undergo electrolytic corrosion when there is a difference in electrolyte concentration across the specimen.


Example; 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.


Crevices are unavoidable with surgical implants where a screw or plate contacts the bone. The local flux of ions is drastically enhanced at crevice sites and tissue impregnation follows.

The contact region of a screw or implant material can form the crevice. In the small space the liquid and oxygen exchanges are severely limited, and surface in the crevice undergoes active corrosion and cause further deterioration.


EFFECTS OF CORROSION

Nobel Metal: Nobel metals resist corrosion because their electromotive force is positive when compared to any other metal used in the oral environment.

To protect an alloy from corrosion, alloys used in dentistry should have at least half the components as noble metals (gold, palladium, and platinum) – Lang et al

Gold, resist sulfide tarnishing. Palladium is found to be effective in resistance of silver alloys to sulfide tarnishing.


Base Metal Alloys: Generally, base metal alloys are very resistant to tarnishing.

Much of the corrosion resistance of the base metal alloys is due to the development of a thin, tough, adherent layer of oxide.This layer prevents the penetration of corrosive ions into the underlying metal and reduces electrochemical behavior to a passive state.


Base metals, such as stainless steels, nickel-chromium alloys, and cobalt-chromium alloys, are virtually immune to sulfide tarnishing, but these alloys are susceptible to chlorides.

Silver Points/Cones contains Cu & Ni cause corrosion when get in contact with periradicular tissue and cause cytotoxicity.


Titanium and its alloys are superior in their resistance to chloride attack, compared with the other dental base metal alloys, as titanium forms a relatively stable oxide layer, and this is the basis for the corrosion–resistance property and biocompatibility.

Lucas and Lemons have reported that the formation of oxide film on titanium provides corrosion-resistance under static conditions, but the oxide film is not sufficiently stable to prevent galling and seizing under loading conditions.


Stainless Steel: The corrosion resistance of stainless steel is attributed to the presence of chromium in thealloy. Approximately, 11% chromium is needed to produce corrosion resistance in pure iron.

Chromium resists corrosion well because of the formation of a strongly adherent coating of oxide on the surface, which prevents further reaction with the metal below the surface. The formation of such an oxide layer is called passivation.


Corrosion of AmalgamLow copper amalgam has much lower resistance to tarnish and corrosion as compared to high copper amalgam due to the formation of γ₂-phase, which has least resistance to corrosion.

It has been found that phosphate buffer solutions inhibit the corrosion process; hence, it can be said that saliva may provide some protection to dental amalgam from corrosion.


Amalgam restorations are subject to tarnish and corrosion in the oral environment.It has been suggested that the corrosion processalthough detrimental in some respects may also have beneficial effects.

Wagner has credited the corrosion process with sealingof leaky margins and mechanical anchoring of the restoration to cavity walls by deposition of corrosion products.


Schoonover and Souder have blamed the corrosion process for loss of mechanical strength in the surface regions of amalgam.

Jorgensen has claimed that corrosion is responsible for release of liquid mercury that diffuses into the amalgam and gives rise to mercuroscopic expansion and margin upheaval.

Use of amalgambond , 4-methacryloxyethyl trimetallic anhydrate (4-META) bonding agent with amalgam reduces microleakage.


Tissue discoloration: During placement of amalgam in cavity, some spillage tends to occur in the surrounding tissues. If not removed carefully, these remnants may corrode and lead to mucosal tissue discoloration known as amalgam tattoo or focal argyria.


Protection Against Corrosion

Passivation: This is the method by which certain metals develop a thin, adherent, highly protective film when they react with the environment. Such metals are known as passive metals.e.g: Thin coatings of electrolytic zirconium oxide (ZrO2) deposited on cobalt-chromium alloys reduce chromium-release levels in artificial saliva, as compared to uncoated cobalt-chromium alloys.


Electroplating: Electroplating is the application of electrolytic cells in which a thin layer of metal is deposited onto an electrically conductive surface.

The process used in electroplating is called electrodeposition.Boris Jacobi a Russian scientist developed electroplating.

Reasons for electroplating corrosion resistance. to improve the appearance and value of the items.to increase the thickness of an item.


Increasing noble metal content: Since noble metals have higher electromotive potential (positive) when compared to any of the other metals used in the oral environment, they can be added to alloys to reduce corrosion.

Polishing restorations

Avoiding dissimilar metal restorations

Maintenance of oral hygiene


Though corrosion is almost always undesirable, there are some instances where it is beneficial such as around the margins of dental amalgam restorations as it seals the marginal gap. It is difficult to prevent corrosion completely in oral cavity. Hence the selection of restorative material and good oral hygiene helps in minimizing corrosion process.

Conclusion


Phillips Science Of Dental Material 10th & 11th Edition

Restorative Dental Materials – Craig 13th Edition

Dental Materials And Their Selection- 3rd Edition By William J. O'brien

Schoonover, I.C. And SOUDER, W.: Corrosion Of Dental Alloys, JADA 28:1278-1291, 1941.

Jorgensen, K.D.: The Mechanism Of Marginal Fracture Of Amalgam Fillings, Acta Odont Scand 23:347-389, 1965.

Corrosion of alloys used in dentistry: A review. Materials Science and Engineering A 432 2006 Deepti Upadhyay

References