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TARNISH AND CORROSION

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CONTENTSINTRODUCTIONCAUSES OF TARNISH AND CORROSIONCLASSIFICATION OF CORROSION ELECTROCHEMICAL CORROSION DISSIMILAR METALS HETROGENEOUS SURFACE COMPOSITION  STRESS CORROSION  CONCENTRATION CELL CORROSION EROSION CORROSION INTRAGRANULAR CORRROSIONCORROSION RATE DETERMINATION PROTECTION AGAINST CORROSION CLINICAL CONSIDERATIONS REVIEW OF LITERATURECONCLUSION

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INTRODUCTION

CHOICE OF MATERIALCost

Mechanical properties

Biocompatibility

Availability

Aesthetic Values

Jpd 2002 vol 87 5

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GPT 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

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ORAL ENVIRONMENT AND EFFECTS ON METALS AND ALLOYS

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Several aspects of the oral environment are highly conducive to corrosion

Physical

Chemical

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pH

Promote accelerated reaction between the corrosion products and the metal or alloy

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Mechanical forces also vary with respect to the

Type of food/diet consumed

Temperament of person

Location in the mouth.

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Causes of Tarnish and Corrosion:

Tungsten rods with evaporated crystals, partially oxidised with colourful tarnish

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Tarnish 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 MUCIN

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STAINS OR DISCOLURATION CAN ALSO BE CAUSED BY

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

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Tarnish 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

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Tarnish is often the forerunner of corrosion

Corrosion is not merely a surface deposit. It is a process in which deterioration of

a metal is caused by reaction with its environment.

This 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.

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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.

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Specific ions may play a major role in the corrosion of certain alloys

surface tarnish on casting alloys that contain silver

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At times acidic solutions such as phosphoric, acetic, and lactic acids at

the proper concentration and pH can promote corrosion.

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Other variables affecting corrosion process are…..

Temperature.

Movement or circulation of medium in contact with metal surface.

Nature and solubility of corrosion product.

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CLASSIFICATION OF CORROSION

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There are two general types of corrosion reactions :

Wet corrosion / Dry corrosion /

electrochemical corrosion chemical corrosion

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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

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Commonly 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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Examples of dry corrosion in dentistry :

Corrosion of dental gold alloys that contain silver.

Oxidation of silver–copper alloy particles that are mixed with

mercury.

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ELECTROCHEMICAL 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.

wikipedia2010

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ELECTROCHEMICAL CORROSION

Anode -- 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.

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ELECTROCHEMICAL CELL IN THE ORAL CAVITY WITH TWO DISIMILIAR FILLINGS

Amalgam Anode+ ion

Gold alloy cathode - ION

Saliva electrolyte

Ammeter

………………………………..……………………………….……………………………….……………………………….………………………………..………………………………

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1. M+ + e- Mo

2. 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 REACTIONS

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Primary 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.

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The basis for any discussion of electrochemical corrosion of dental alloys

is the Electromotive Series of the metals.

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Electromotive force series

This 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.

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Metals 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+ + 0.10

Hydrogen 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

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If 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.

ANODECATHODE

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Galvanic corrosion

Heterogeneous compositions

Stress corrosion

Concentration cell corrosion

Pitting corrosion

Crevice corrosion

Intergranular corrosion

Erosion corrosion.

Electrochemical corrosion can be further classified into

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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”.

AMALGAMGOLD

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GALVANIC SHOCKA pain sensation caused by electric current generated by a contact

between two dissimilar metals forming a battery in the oral

environment.

GOLD FILLING

ENAMEL

DENTINE

PULP

AMALGAM FILLING

S A L I V A

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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 area

Contact 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

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HETEROGENOUS SURFACE COMPOSITION

Heterogeneous compositions of metal surface.

This occurs within the structure of the restoration itself.

Difference in compositions within the alloy grains

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Grains with lower electrode potential are attacked and corrosion results

Impurities in alloy enhance corrosion

Microscopic structure of copper-zinc alloy

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STRESS CORROSION

Imposition 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.

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Stress 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.

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CONCENTRATED CELL CORROSION

It occurs whenever there are variations in the electrolytes or in the composition of the given electrolyte within the system.

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PITTING

CORROSION

CREVICE CORROSION

CONCENTRATED CELL CORROSION

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Pitting 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

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A 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

Anode

Cathode

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The 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.

Materials Science and Engineering 2006

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For this reason, all metallic dental restorative materials should be polished.

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Crevice Corrosion

Preferential attack occurs at crevices in dental prostheses or at margins between tooth structure and dental restorations.

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INTERGRANULAR 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.

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It 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

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EROSION 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.

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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

57Materials and science engineering 5th ed

Potentiodynamic polarization tests.

Three electrodesexperimental electrode-the dental alloycounter electrode -typically platinum.a reference electrode-

saturated calomel electrode or

saturated AgCI electrode

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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.

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Certain metal develop a thin adherent highly protective film by reaction with the environment and such a metal is said to be passive.

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

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Chromium 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 .

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Taylor and Jordan 1967 tested in vitro chrome–cobalt alloy in domestic chlorine bleach for 9 months.

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

Brit dent Jr

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Yoshiki 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 Ti–6Al–4V and 17-4 stainless steel.

Bleaching agents were tested on commercially pure titanium, Ni–Co–Mo 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)

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Polishing metal restoration like amalgam and gold to a high luster minimizes corrosion.

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Paint application or coating with inorganic nonconductive coatings.

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Protective 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.

Dental Alloys and Corrosion Renata Poljak 2002

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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

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CLINICAL 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 PICTURE

MERCURY

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REVIEW 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 24°C.

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

Dent Mater J. 2003 Dec;22(4):482-93

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Elliott 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.

Dental Materials

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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 current–time 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.

Dental Materials

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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.

Dent Mater J

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A. 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.

J prosthet dent

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CONCLUSION

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REFERENCESPhillips science of dental materials 11th edition

Robert G Craig restorative dental materials 11th edition.

Corrosion of alloys used in dentistry: A review. Materials Science and

Engineering A 432 2006 Deepti Upadhyay

A 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, 823–831

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Corrosion of dental metallic materials by dental treatment agents. Yoshiki Oshida, Materials Science and Engineering C 25 (2005) 343 – 348

Dental alloys and corrosion Robert Poljak 2002.

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