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TARNISH AND CORROSION

2CONTENTSINTRODUCTIONCAUSES OF TARNISH AND CORROSIONCLASSIFICATION OF CORROSIONELECTROCHEMICAL CORROSIONDISSIMILAR METALSHETROGENEOUS SURFACE COMPOSITION STRESS CORROSION CONCENTRATION CELL CORROSIONEROSION CORROSIONINTRAGRANULAR CORRROSIONCORROSION RATE DETERMINATIONPROTECTION AGAINST CORROSIONCLINICAL CONSIDERATIONS REVIEW OF LITERATURECONCLUSION3INTRODUCTION4For selecting a material for dental application, it is necessary to remember that the choice of material depends on a number of factors like corrosion behavior, mechanical properties including strength and fabricability, cost, availability, biocompatibility, and aesthetic values. The corrosion behavior of metallic materials usually is the most important property because of biocompatibility and cytotoxicity of the products of the corrosion process. However, from aesthetic considerations, appearance is the most important factor to be considered. The mechanical properties of the materials are equally important even though the materials may have enough corrosion resistance. Cost and availability are also important for the selection of the materials

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Jpd 2002 vol 875Jpd 2002 vol 87 Consequences of alloy corrosion for surrounding tissues. When corrosion is severe, ions from alloy can be released in sufficient quantities to discolor adjacent tissues (seen here lingual to molar and premolar).In dental metallurgy, 7 elements are recognized as noble: gold, platinum, palladium, iridium, rhodium, osmium, and ruthenium

5GPT 8 : TERMINOLOGIES

Tarnish - A process by which a metal surface is dulled in brightness or discolored through the formation of a chemical film, such as a sulfide or an oxide.

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

Corrosive : tending or having the power to corrode

66ORAL ENVIRONMENT AND EFFECTS ON METALS AND ALLOYS7Fusayama et al. [44] Gold and amalgam restorations Air, artificial saliva 1 day to 6 months In vitro and in vivo (a) Contact of gold inlays with oldamalgam fillings in air or saliva did not cause any significant corrosion of the restorations but with fresh amalgam fillings produced silver colored stains on the contact area (b) Contact with fresh amalgam fillings less than 1 h old in saliva produced silver-colored stains of negligible thickness (c) On amalgam filling surfaces no significant corrosion was found7Several aspects of the oral environment are highly conducive to corrosion

8Structures in the mouth are constantly exposed to an everchanging physical and chemical environment that includes temperature and components like saliva, salts, foods, liquids, and drugs. Plaques and food adhere to teeth and filling materials, which means several forms of bacteria and their products are always present inside mouth. Mechanical forces also vary with respect to the type of food consumed, temperament of person, and location in the mouth.

89Promote accelerated reaction between the corrosion products and the metal or alloy Several aspects of the oral environment are highly conducive to corrosion. The mouth is warm and moist, and is continually subjected to fluctuations in temperature. Ingested foods and liquids have a wide range of pH. Acids are liberated during the breakdown of foods, and the resulting debris often adheres tenaciously to the metallic restoration, providing a localized condition that promotes accelerated reaction between the corrosion products and the metal or alloy. Because it has the least tendency to become ionized, gold resists chemical attack very well. Thus it was natural that the most noble metal was employed early in modern dental history for the construction of dental appliances.

9Mechanical forces also vary with respect to the

Type of food/diet consumed

Temperament of person

Location in the mouth.

10. The mouth is warm and moist, and is continually subjected to fluctuations in temperature. Ingested foods and liquids have a wide range of pH. Acids are liberated during the breakdown of foods, and the resulting debris often adheres tenaciously to the metallic restoration, providing a localized condition that promotes accelerated reaction between the corrosion products and the metal or alloy. Because it has the least tendency to become ionized, gold resists chemical attack very well. Thus it was natural that the most noble metal was employed early in modern dental history for the construction of dental appliances.

10Causes of Tarnish and Corrosion:

Tungsten rods with evaporated crystals, partially oxidised with colourful tarnish11Tungsten rods with evaporated crystals, partially oxidised with colourful tarnishRead more: http://www.answers.com/topic/tarnish#ixzz1DCfN7nAB

11Tarnish is observed as the surface discoloration on a metal, or as a slight loss or alteration of the surface finish or lustre.

SOFT DEPOSITS(PLAQUE)HARD DEPOSITS(CALCULUS)FILMS COMPOSEED MAINLY OF MICROORGANISMS AND MUCIN12Tarnish is observed as the surface discoloration on a metal, or as a slight loss or alteration of the surface finish or lustre. In the oral environment, tarnish often occurs from the formation of hard and soft deposits on the surface of the restoration. Calculus is the principal hard deposit, and its colour varies from light yellow to brown . The soft deposits are plaques and films composed mainly of micro organisms and mucin. Stain or discolouration arises from pigment producing bacteria, drugs containing such chemicals as iron or mercury, and adsorbed food debris. Although such deposits are main cause of tarnish in the oral environment, surface discolouration may also arise on a metal from the formation of thin films, such a s oxides, sulphides, or chlorides. This latter phenomenon maybe only a simple surface deposit, and such a film may even be protective. However, it can be an early indication of corrosion.

12STAINS OR DISCOLURATION CAN ALSO BE CAUSED BY

Pigment producing bacteria.Drugs containing Iron or Mercury.Formation of thin films of oxides, sulphides or chlorides.

13mutans streptococci and lactobacilli13Tarnish is a surface phenomenon, that is self-limiting unlike rust. Only the top few layers of the metal react.

Tarnish actually preserves the underlying metal in outdoor use and is called Patina. The formation of patina is necessary in applications such as copper roofing, bronze, brass statues and fittings

14Tarnish is a surface phenomenon, that is self-limiting unlike rust. Only the top few layers of the metal react, and the layer of tarnish seals and protects the underlying layers from reacting.Tarnish actually preserves the underlying metal in outdoor use and is called Patina. The formation of patina is necessary in applications such as copper roofing, and outdoor copper, bronze, and brass statues and fittings.

Read more: http://www.answers.com/topic/tarnish#ixzz1DCg1n7Bh

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Tarnish is often the forerunner of corrosionCorrosion is not merely a surface deposit. It is a process in which deterioration of a metal is caused by reaction with its environment.

15This disintegration of the metal is by the action of corrosion which occurs through the action of moisture, atmosphere, acid or alkaline solutions and certain chemicals.This disintegration of a metal by the action of corrosion may occur through the action of moisture, atmosphere, acid or alkaline solutions, and certain chemicalsThe film that produces tarnish may in time accumulate elements or compounds that chemical attack the metallic surface..Various sulphides such as hydrogen or ammonium sulphide, corrode silver, copper, mercury and similar metals present in dental alloys and amalgam. Also, water, oxygen, chlorine, ions are present in saliva and contribute to corrosion attack. Various acidic solutions such as phosphoric, acetic, and lactic acids are present at times and, at the proper concentration and pH, these can promote corrosion.the rate of corrosion attack may actually increase over time, especially with surfaces subjected to stress, with intragranular impurities in the metal, or with corrosion products that do not completely cover the metal surface.. In due course, corrosion causes severe and catastrophic disintegration of a metal body

15The rate of corrosion attack may actually increase over time, especially with surfaces subjected to stress, with intragranular impurities in the metal, or with corrosion products that do not completely cover the metal surface..

In due course, corrosion causes severe and catastrophic disintegration of a metal body. 16For example, eggs and certain other foods contain significant amounts of sulphur16Specific ions may play a major role in the corrosion of certain alloys

surface tarnish on casting alloys that contain silver

17Specific ions may play a major role in the corrosion of certain alloys. For example, oxygen, and chlorine are implicated in the corrosion of amalgam at the tooth interface and within the body of the alloy. is probably the most significant factor causing surface tarnish on casting alloys that contain silver, although chloride has also been identified as a contributor. Ammonium sulphide

17At times acidic solutions such as phosphoric, acetic, and lactic acids at the proper concentration and pH can promote corrosion.

18Specific ions may play a major role in the corrosion of certain alloys. For example, oxygen, and chlorine are implicated in the corrosion of amalgam at the tooth interface and within the body of the alloy. is probably the most significant factor causing surface tarnish on casting alloys that contain silver, although chloride has also been identified as a contributor.

18Other variables affecting corrosion process are.. Temperature. Movement or circulation of medium in contact with metal surface. Nature and solubility of corrosion product.19CLASSIFICATION OF CORROSION20

There are two general types of corrosion reactions :

Wet corrosion / Dry corrosion / electrochemical corrosion chemical corrosion

21There are two general types of corrosion reactions. In Chemical corrosion there is a direct combination of metallic and non-metallic elements to yield a chemical compound through processes such as oxidation, halogenations, or sulfurization reactions. A good example is the discoloration of silver by sulphur, where silver sulphide forms by chemical corrosion. It can also be a corrosion product of dental gold alloys that contain silver. This method of corrosion is also referred to as dry corrosion, since it occurs in the absence of water or another fluid electrolyte. Another example is the oxidation of silver copper alloy particles that are mixed with mercury to prepare certain dental amalgam products. These alloy particles contain a silver-copper eutectic phase, and oxidation limits their reactivity with mercury, thereby affecting the setting reaction of the dental amalgam product. This is why it is prudent to store the alloy in a cool, dry location to ensure an adequate shelf life. Chemical corrosion is seldom isolated and is almost invariably accompanied by electrochemical corrosion, which is also referred to as wet corrosion, since it requires the presence of water or some other fluid electrolyte. It also requires a pathway for the transport of electrons, if the process is to continue.

21In Chemical corrosion there is a direct combination of metallic and non-metallic elements to yield a chemical compound through processes such as oxidation, halogenations, or sulfurization reactions

22Specific ions may play a major role in the corrosion of certain alloys. For example, oxygen, and chlorine are implicated in the corrosion of amalgam at the tooth interface and within the body of the alloy. is probably the most significant factor causing surface tarnish on casting alloys that contain silver, although chloride has also been identified as a contributor.

22Commonly seen dry corrosion-- Oxidation

The principle step of an oxidation reaction :Dissociative oxygen adsorption: Metal and oxygen ion diffusion occurs through the growing oxide layer.

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24Examples of dry corrosion in dentistry :Corrosion of dental gold alloys that contain silver.Oxidation of silvercopper alloy particles that are mixed with mercury.

25ELECTROCHEMICAL CORROSIONAlso known as wet corrosion as it requires a fluid electrolyte or water. Requires a pathway for transport of electrons and electrical current.

English chemists John Daniell (left) and Michael Faraday (right), both are credited to be founders of electrochemistry as known today. wikipedia20102626wikipedia2010ELECTROCHEMICAL CORROSIONAnode -- Positive ions are formed with the production of free electrons.Cathode-- Reduction reaction must occur that will consume free electrons produced at the anode. The electrolyte supplies the ions needed at the cathode and carries away the corrosion products at the anode.

2727 The electrochemical cell is composed of three essential components: an anode, a cathode, and an electrolyte. An apparatus is employed to measure the volt In this example, the anode can be the dental amalgam restoration , a gold alloy restoration can represent the cathode and the saliva may serve as the electrolyte. The anode is the surface or sites on a surface where positive ions are formed with the production of free electrons. At the cathode or cathodic sites, a reduction reaction must occur that will consume free electrons produced at the anode. Numerous possibilities exist that are dependent on the environment. For example, metal ions maybe removed from the solution to form metal atoms, hydrogen ions maybe converted to hydrogen gas, or hydroxyl ions maybe formed.age and current between the two electrodesELECTROCHEMICAL CELL IN THE ORAL CAVITY WITH TWO DISIMILIAR FILLINGSAmalgam Anode+ ionGold alloy cathode - IONSaliva electrolyteAmmeter .......

2828The electrolyle supplies the ions needed at the cathode and carries away the corrosion products at the anode. The external circuit serves as a conduction path to carry electrons (the electric current) from the anode to the cathode. If a voltmeter is placed into this circuit, an electrical potential difference i.e. A voltage (V), can be measured. This voltage has considerable theoretical importance . It should be understood that this simple electrochemical cell is , in principle, a battery since the flow of electrons in the external circuit is capable of lighting a light bulb in a flashlight or producing a physiological sensation such as pain.

1. M+ + e- Mo2. 2H+ + 2e- H2 3. 2H2O + O2 + 4e- 4(OH)-

Metal ion may be removed to form metal atoms

Hydrogen ions may be converted to hydrogen gas

Hydroxyl ions may be formed

POSSIBLE REACTIONS- REDUCTION REACTIONS2929Primary driving force for electrochemical corrosion:At the cathode or cathodic sites, a reduction reaction must occur that will consume free electrons produced at the anode.

30At the cathode or cathodic sites, a reduction reaction must occur that will consume free electrons produced at the anode. Numerous possibilities exist that are dependent on the environment. For example, metal ions maybe removed from the solution to form metal atoms, hydrogen ions maybe converted to hydrogen gas, or hydroxyl ions maybe formed.Often the cathodic reactions can be considered to be the primary driving force for the electrochemical corrosion. This is a very important consideration in determining the rate of corrosion process, and it can be used to advantage in order to reduce or eliminate corrosion.

30 The basis for any discussion of electrochemical corrosion of dental alloys is the Electromotive Series of the metals.31Electromotive force seriesThis classification for arrangement of the elements in the order of their dissolution tendencies in water.

Potential values are calculated with solutions containing one atomic weight ,in grams, of ions in 1000ml of water at 25 c.

Metals with a more positive potential have a lower tendency to dissolve in aqueous solutions.3232Metals

Ion

ElectrodePotential (V)

Gold

Au+

+ 1.50

Gold

Au3+

+ 1.36

Platinum

Pt2+

+ 0.86

Palladium

Pd2*

+ 0.82

Mercury

Hg2+

+ 0.80

Silver

Ag+

+ 0.80

Copper

Cu+

+ 0.47

Bismuth

Bi3+

+ 0.23

Antimony

Sb3+

+ 0.10Hydrogen

H+

0.00

Lead

Pb2+

-0.12

Tin

Sn2+

-0.14

Nickel

Ni2+

-0.23

Cadmium

Cd2+

-0.40

Iron

Fe2+

-0.44

Chromium

Cr2+

-0.56

Zinc

Zn2+

-0.76

Aluminum

Al3+

-1.70

Sodium

Na-+

-2.71

Calcium

Ca2+

-2.87

Potassium

K+

-2.92

33If two pure metals are immersed in an electrolyte an connected by an electrical conductor to form a galvanic cell.The metal with the lower electrode potential in the table becomes the anode and undergoes oxidation , that is, its ions go into solution.

ANODECATHODE3434

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Galvanic corrosionHeterogeneous compositionsStress corrosionConcentration cell corrosion Pitting corrosionCrevice corrosionIntergranular corrosionErosion corrosion.Electrochemical corrosion can be further classified into3636 GALVANIC CORROSION / DISSIMILAR METALS CORROSION

An important type of electrochemical corrosion occurs when dissimilar metals are in direct physical contact with each other.

This metallic combination may produce Electro galvanism or Galvanic currents.

AMALGAMGOLD3737GALVANIC SHOCKA pain sensation caused by electric current generated by a contact between two dissimilar metals forming a battery in the oral environment.

GOLD FILLING

ENAMELDENTINEPULPAMALGAM FILLINGS A L I V A3838Eg. Amalgam filling opposing a gold inlay as both the restorations are wet in saliva, an electrical couple exists, with a potential difference between the dissimilar restorations. When they come in contact, the potential is suddenly short circuited through the two alloys resulting in a sharp pain.

Fusayama et al. J Dent Rest 42 1963, compared gold and amalgam restorations in air and artificial saliva for a period of 1 day to 6 months in vitro and in vivo. Contact of gold inlays with old amalgam fillings in air or saliva did not cause any significant corrosion of the restorations but with fresh amalgam fillings produced silver coloured stains on the contact areaContact with fresh amalgam fillings less than 1 h old in saliva produced silver-coloured stains of negligible thickness.On amalgam filling surfaces no significant corrosion was found

39HETEROGENOUS SURFACE COMPOSITIONHeterogeneous compositions of metal surface. This occurs within the structure of the restoration itself.

Difference in compositions within the alloy grains

40Grains with lower electrode potential are attacked and corrosion resultsImpurities in alloy enhance corrosion

Microscopic structure of copper-zinc alloy41Microscopic structure of copper-zinc alloy41STRESS CORROSIONImposition of stress increases the internal energy of an alloy, either through elastic displacement of atom or the creation of micro-strained field associated with dislocation and the tendency to undergo corrosion will be increased.

42Stress corrosion most likely to occur during fatigue or cyclic loading in the oral environment.

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Eg. Repeated removal and insertion of a partial denture will develop a severe stress pattern of certain alloys especially at the grain boundaries. Combined with the oral environment ,the appliance develops stress corrosion resulting in fatigue and failure.

4444CONCENTRATED CELL CORROSIONIt occurs whenever there are variations in the electrolytes or in the composition of the given electrolyte within the system.

45CONCENTRATED CELL CORROSION46Pitting CorrosionPitting is a form of extremely localized attack that results in holes on the surface. The localized breakdown of the protecting passive layer causes pitting

47A similar type of attack may occur from differences in the oxygen concentration between parts of the same restoration, with the greatest attack at the areas containing the least oxygen

AnodeCathode48Anode Cathode48The rate of such corrosion may be very rapid , since the area of the anodic region is much smaller than that of the cathodic region and there must be a balance of charge transport in both regions.

49Materials Science and Engineering 2006 For this reason, all metallic dental restorative materials should be polished.

50Crevice CorrosionPreferential attack occurs at crevices in dental prostheses or at margins between tooth structure and dental restorations.

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52INTERGRANULAR CORROSION

Due to more reactive nature of grain boundaries, intergranular corrosion occurs adjacent to grain boundaries with relatively little corrosion of grains. This type of corrosion occurs due to the in homogeneity and mainly technological errors.

53It has been demonstrated that heating 18-8 stainless steel between 400 and 900 C may negate its resistance to corrosion R.G. Craig , Restorative Dental Materials, seventh ed. This corrosive tendency was attributed to the precipitation of chromium carbide at the grain boundaries at elevated temperatures.

Literature review

5454EROSION CORROSION

Erosion corrosion is the acceleration or increase in rate of deterioration or attack on a material because of relative movement between a corrosive fluid and the material surface.

55 Under erosion comes fretting corrosion. Fretting is described as the corrosion occurring at contact areas between materials under load subjected to vibration and slip.During insertion of an implant, some degree of abrasion between fixing screws and implants or between tools (screw drivers, grips, etc.) and implants is bound to occur

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Materials and science engineering 5th ed57The mechanical component of fretting, i.e., the rubbing movement, disrupts the protective surface films and removes metal particles. This action stimulates the chemical activities at the surface, leading to oxidation,or active corrosion, or repassivation as a result of the electrochemical aspect of the process.Materials and science engineering 5th ed

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Potentiodynamic polarization tests.

Three electrodesexperimental electrode-the dental alloycounter electrode -typically platinum.a reference electrode- saturated calomel electrode or saturated AgCI electrode

PROTECTION AGAINST CORROSIONGold coating is applied to enhance the appearance of many dental product.

But the noble metal is soft : when surface is scratched or pitted to such a depth that the base metal is exposed to the environment, the base metal will be corroded at a very rapid rate.

59because concentration cell have been created.Noble metal is applied on base metal.59Certain metal develop a thin adherent highly protective film by reaction with the environment and such a metal is said to be passive.

60Iron, steel and certain other metals may be electroplated with nickel followed by chromium for corrosion protection .

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62Chromium passivated metal can be susceptible to stress corrosion and pitting corrosion, hence the patient should be warned against using household bleaches for cleaning partial denture frame work .

63[57] Chromecobalt alloy Domestic chlorinebleach9 months In vitro Steeping of chromecobalt denturesin domestic bleach had to be avoided,due to the inherently poor resistanceof this alloy to attack in solutionscontaining chlorine63Taylor and Jordan 1967 tested in vitro chromecobalt alloy in domestic chlorine bleach for 9 months.

They concluded that soaking of chromecobalt dentures in domestic bleach had to be avoided, due to the inherently poor resistance of this alloy attack in solutions containing chlorine

64Brit dent Jr 64Yoshiki Oshida et al, in 2005 did an in vitro study to evaluate and compare chemical and electrochemical corrosion behaviour of seven metallic dental materials in commercially available fluoride and bleaching agents. Fluoride treatment agents were tested on Ti6Al4V and 17-4 stainless steel. Bleaching agents were tested on commercially pure titanium, NiCoMo alloy, type IV gold alloy, and amalgam. Electrochemical corrosion tests were also conducted for these materials, using each treatment agent as an electrolyte. All tested materials suffered from discoloration, which mostly disappeared after brushing. The fluoride-treated 17-4 stainless steel indicated severe intergranular attack, causing localized Cr depletion. The bleaching agents made the passivation layer on the metallic materials unstable.65Materials Science and Engineering C 25 (2005)Yoshiki Oshida in 2005 did an in vitro study to evaluate and compare chemical and electrochemical corrosion behavior of seven metallic dental materials in commercially available fluoride and bleaching agents. Fluoride treatment agents were tested on Ti6Al4V and 17-4 stainless steel. Bleaching agents were tested on commercially pure titanium, NiCoMo alloy, type IV gold alloy, and amalgam. Electrochemical corrosion tests were also conducted for these materials, using each treatment agent as an electrolyte. All tested materials suffered from discoloration, which mostly disappeared after brushing. The fluoride-treated 17-4 stainless steel indicated severe intergranular attack, causing localized Cr depletion. The bleaching agents made the passivation layer on the metallic materials unstable. Materials Science and Engineering C 25 (2005)65Polishing metal restoration like amalgam and gold to a high luster minimizes corrosion.

66Paint application or coating with inorganic nonconductive coatings.

67Protective films :

Co-Cr alloys plated with a thin ZrO2 layer showed a considerable increase in corrosion resistance, as well as good quality of bonded hardness between the alloy and film, and for that reason they were recommended for widespread use.

Corrosive resistant amorphous Fe-Cr-P-C & Al-W alloys were recommended for widespread use as they showed a high level of corrosion resistance.68Dental Alloys and Corrosion Renata Poljak 2002 Co-Cr alloys plated with a thin ZrO2 layer showed a considerable increase in corrosive resistance, as well as good quality of bonded hardness between the alloy and film, and for that reasCorrosive resistant amorphous Fe-Cr-P-C & Al-W alloys were recommended for widespread use as they showed a high level of corrosive resistance. 68

Addition of noble metals like gold, platinum and palladium in dental alloyGold resist sulfide tarnish, Palladium resists sulfide tarnishing of silver.

Base metals alloys such a s Ni-Cr, Co-Cr and Ti are virtually resistant to sulfide tarnish although they are susceptible to chloride corrosion

6969CLINICAL CONSIDERATIONS

Application of varnish in the dentinal walls and on the surface of the filling to avoid galvanism in amalgam restorations.

Avoid giving amalgam restoration opposing gold filling because the mercury released from the silver amalgam will weaken and discolour both the fillings and also gives a metallic taste in the mouth.

SEPAGE OF MECURY FROM AN AMALGAM FILLINGELECTRON MICROSCOPE PICTUREMERCURY7070REVIEW OF LITERATURE :

Demetrios M. Sarantopoulos et al 2011. Evaluated the corrosion properties of 2 new CoPdCr and one new NiPdCr alloy and compare them to traditional CoCr and NiCr alloys.The cast and oxidized specimens were assessed electrochemically in phosphate-buffered saline at 24C.

The palladium containing alloys had corrosion effects with pitting, the inclusion of palladium in CoCr and NiCr alloys had deleterious effects on electrochemical corrosion properties

71Dent Mater J. 2003 Dec;22(4):482-93Elliott J. Sutow et al, In April (2006) studied the maximum variability of the corrosion potential of aged dental amalgam restorations, for 28 days.

The corrosion potentials of 148 aged dental amalgam restorations in 12 human subjects were measured at t=0 and at t=4 h, for 4, 7, 14, 21 and 28 days. Measurements were made with a high impedance voltmeter and a Ag/AgCl micro-reference electrode. It was shown that the corrosion potential of aged dental amalgam restorations varies substantially over time, and that a single measurement is not representative of short- or long-term electrochemical behaviour.

72Dental Materials Elliott J. Sutow et al, (2004) studied In vivo galvanic currents of intermittently contacting dental amalgam and other metallic restorations.A convenience sample of 106 human subjects was studied. A Ag/AgCl micro-reference electrode and a high impedance voltmeter were used to measure the pre-contact corrosion potentials. Galvanic couples (n = 194) were next formed by simultaneously contacting occluding restorations with gold-plated probe tips for 15 s.The resulting currenttime transients were measured with a zero-resistance ammeter. The vast majority of couples measured contained at least one dental amalgam restoration. A wide range of galvanic currents resulted from electrical contact of restorations in vivo. These currents were influenced by restoration age and total surface area of the galvanic couple.

73Dental Materials Dong H, Nagamatsu Y 2003 analysed the corrosion behaviour of dental alloys was examined in electrolyzed strong acid water, weak acid water and neutral water using a 7-day immersion test. The precious metal alloys, gold alloy. Au-Ag-Pd alloy and silver alloy showed the greatest surface color change and dissolution of constituents in the strong acid water and the smallest in the neutral water. The release of Au from gold alloy was especially marked in the strong acid water. Co-Cr alloy showed greater corrosion and tarnish resistance in the strong acid water rather than in the weak acid water and the neutral water.

74Dent Mater JDong H, Nagamatsu Y74A. B. Linke et al in 1991 studied the Tarnish of dental alloys by oral microorganisms.

Five dental alloys, on exposure to blood and chocolate media with and without inoculated microorganisms, showed varying degrees of tarnish. The results indicated a composition-dependent tarnish behaviour of alloys in microorganism inoculated media, indicating a potential role for the oral microorganisms in inducing clinically observed tarnish of dental alloys. Actinomyces viscosus and periodontal pocket specimens show a similarity in their activity to induce tarnish in base metal-containing dental alloys.

75J prosthet dent CONCLUSION

76REFERENCES

Phillips science of dental materials 11th editionRobert G Craig restorative dental materials 11th edition.Corrosion of alloys used in dentistry: A review. Materials Science and Engineering A 432 2006 Deepti UpadhyayA proposal for the classification of dental alloys according to their resistance to corrosion. Claire Manaranche. Dental materials 23(2007)Corrosion-fatigue life of commercially pure titanium and Ti-6Al-4V alloys in different storage environments Ricardo A. Zavanelli, (J Prosthet Dent 2000;84:274-9.)Corrosion of CoCr and NiCr dental alloys alloyed with palladium. Demetrios M. Sarantopoulos, (J Prosthet Dent 2010;105: 35-43)In vivo galvanic currents of intermittently contacting dental amalgam and other metallic restorations. Elliott J. Sutow Dental Materials (2004) 20, 823831

77Corrosion of dental metallic materials by dental treatment agents. Yoshiki Oshida, Materials Science and Engineering C 25 (2005) 343 348Dental alloys and corrosion Robert Poljak 2002.

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