TARNISH & CORROSION

Introduction

Metals undergo chemical reactions with non-metallic elements in the

environment to produce chemical compounds commonly known as corrosion

products. These compounds may accelarate ,retard or have no influence on the

subsequent deterioration of metal surface. The rusting of iron is a familiar

example of the effects that may be produced by such process.

One of the primary requests of any metal that is to be used in mouth is

that it must not produce corrosion products that will be harm full to the body.

If the corrosion process is not too marked, these products may not be

reorganised easily.

It is unfortunate that the oval environment is very conductive tot he

formations of corrosion products. The mouth is moist and certainly subjected

to fluctuation in temperature. The food and liquids ingested have wide ranges

of pH. Acids are liberated during the breakdown of foodstuffs. This food

debris often adheres tenaciously to the metallic restoration, thus producing a

localized condition that is extremely conductive to an accelerated reaction

between corrosion products and the metal or alloy. All of these environmental

factors contribute to the degrading process known as corrosion.

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Gold resist chemical attack of this nature very well, therefore it was

natural that this nobilest of the metals was employed early for the construction

of dental appliances.

Definition

Tarnish is a surface discoloration on a metal or even slight or activation

of the surface finish or luster.

Corrosion: In the specific sense is not merely a surface deposit but is an

actual determination of a metal by reaction with its environment.

Terminology

Tarnish: It is the process by which a metal surface is dulled in tightness or

discolored through the formation of a chemical film such as a Sulphide or an

Oxide.

Corrosion: A chemical or electrochemical process through which a metal is

attacked by a natural agents such as air and water, resulting in partial or

complete dissolution deterioration, or weakening of any solid substances.

Corrosion is an actual deterioration of the metal by reaction with its

environment it can be defined as a chemical reaction between a metal and its

environment to form metal compounds.

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Concentration Cell

An electrochemical corrosion cell in which the potential differences is

associated with the difference in the concentration of dissolved species, such

as oxygen, in the solution at different parts of the metal surface.

Crevice corrosion

Accelerated corrosion in a narrow spaces caused by localized

electrochemical processes and chemical changes, such as acidification and

depletion in its oxygen content crevice corrosion commonly occurs when a

leakage takes place between restoration and the tooth, under a pellicle or under

other surface deposits.

Galvanic Corrosion

An accelerated attack occurring on a noble metal when electrochemical

dissimilar metals are in electrical content is the presence of a liquid corrosive

environment.

Galvanic Shock

A pin sensation caused by electric current generated by a contact

between two dissimilar metals forming a battery in the oval environment.

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Pitting Corrosion

Is a localized corrosion occurring on baser metals such as iron, nickel,

and chromium, which are protected by a naturally forming a thin film of an

oxide. In the presence of chlorides in the environment the film locally breaks

down and rapid dissolution of the underlying metal occurs in the form of pits.

Stress corrosion

Degradation by the combined effects of mechanical stress and a

corrosive environment, usually in the form of cracking.

Causes of Tarnish and Corrosion:

Causes of tarnish

1) Formation of hard and soft deposits on a surface of the restoration.

2) Calculus is the principle hard deposit and its color varies from light yellow

to brown.

3) It’s color varies also with oval hygiene of the patient and is specially dark

in the mouth of a heavy smoker.

4) The soft deposits are plaques and films composed mainly of

microorganisms and mean.

Tarnish may be found anywhere in the mouth but are more optional to

be as surfaces that are protected from the abrasive actions of food and the

toothbrush.

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Causes of Tarnish and Corrosions

A differentiation should be shade between tarnish and corrosion. Even

though there is a definite technical difference, it is difficult clinically to

distinguish between the two phenomena, and the terms are often used

interchangeably in dental literature.

1. Corrosion is caused especially with surfaces under stress or with

intergranular rupurities is the metal or with corrosion products that do not

completely cover the substrate metal, the corrosion activate rate may

actually increase with time. In due course it causes severe and catastrophic

disintegration of to metal body.

In addition, corrosion attack that is extremely localized may cause rapid

mechanical factor at a structure even though the metal loss of material is quit

small.

2. Disintegration of a metal may occur through the action of moisture,

atmosphere, acid or alkaline solutions, and certain chemicals.

3. Tarnish is often the forerunner of corrosion the film that is deposited and

produces tarnish may in time forms or accumulate elements or compounds

that chemically attack the metallic surface for example, eggs and certain

other foods contain significant amount of sulfur various sulfhides, such as

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hydrogen or ammonium sulfide, corrode silver, copper, mercury, and

similar metals present in dental alloys and amalgam.

4. Also water, oxygen and chloride ions are present in saliva and contribute to

corrosion attack. Various acids such as phosphoric, acetate and lactic acids

are present at times. At the proper concentrations and pH there can lead to

corrosions.

Classification of corrosion

There are two general classification of corrosion reactions.

1. Chemical corrosion

2. Electrolytic (or) electrochemical corrosion.

Under electrolytic corrosion we have 4 types of corrosions.

a) Corrosion found with a combination of dissimilar metals.

b) Corrosion due to heterogeneous compounds of the metal surface.

c) Corrosion due to presence of homogenous in homogenous surface texture.

d) Fourth type is concentrations cell corrosion.

Chemical corrosion

In chemical corrosion there is a direct combination of metallic and

nonmetallic elements.

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This type is exemplified by:

a) Oxidation

b) Stalogenation

c) Sulfurization reactions

1st Example

Discolouration of silver by sulfur. The formation of Ag S in this

reaction is chemical corrosion silver sulfide appears to be the principle

corrosions product of dental alloy that contains silver.

Such corrosion is also referred to as “dry” corrosion, since it occurs in

the absence of water or other fluid electrolytes.

Electrolytic or Electrochemical corrosion (or) Wet corrosion

This type is also referred as wet corrosion since it requires the presence

of water or other fluid electrolytes. It also requires a pathway for the transport

of electrons, an electrical current, if the process is to continue.

The starting point for the discussion of electrolytic corrosion is to

electrolytic cell composed of by components.

1. The auscle is the surface where positive ions a formed, that is the metal

surface which is corroding.

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2. At the cathode a reaction must occur that will consume the free electrons

produced at the anode.

3. The electrolytic sever to supply the ions that are needed at the cathode and

to early way the corrosion products at the anode.

4. The external circuit serves as a conduction path to early electrons (electric

current) from the anode to the cathode.

Current Consuming (Electrolytic cells)

Consider an electrolyte containing anions M+ and anions A- if two

electronic conductors or electrodes are placed in this electrolyte and an electric

potential difference established between then by battery, then movement of

ions will occur.

Contains more to the negatively charged electrode or cathode and

anions move to the anode or positively charged electrode. Such a process is

called electrolysis. The following reaction can be written for the above

discussion.

At the cathode : M+ + electrons = M

At the anode : A - = A+ electron

An anodic reaction, then is one that produces electrons whereas a

cathode process consumes electons.

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Current Producing (Galvanic Cells)

Consider now two electrodes of different metals. Eg. Copper and Zinc,

immersed in an electrolyte of sulphuric acid when electrical contact is made

between the electrodes, a current will flow.

As discussed previously, electrons are produced at the anode and

consumed at its cathode. This is an example of a galvanic or corrosion cell.

Zinc are being converted to zinc ions (i.e. essentially an oxidation reaction)

and passes into solution. Thus zinc in being corroded.

Thus according to this theory, if two metals are universal in an

electrolyte and are connected by an electrical conductor, an electrical cocycle

is formed. The metal with the lowest electrode potential goes into the solution.

The strength and direction of the current thus depends primarily upon

the electrode potential of the individual metals.

I. Galvanic Corrosion (Dissimilar metals)

Here the dental reference is to separate restorations in which the metal

surfaces are chemically dissimilar. The metallic combinations that produce

electrogalvanism (or) “galvanic currents” may or may not be intermittent

contact.

Examples of galvanic corrosion. Difference in composition of materials.

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1. If a gold inlay comes in contact with an amalgam restoration, because both

restorations are wet with saliva, an electrical couple exists, with difference

in potential between in dissimilar restorations.

Difference in alloy gram

2. In a cored structure, differ as in composition with in the alloy grain are

found. Thus part of a grain can be the anode and other part cathode.

When the teeth are not in contact, the difference in electric potential

(or) electromotive force between the two fillings still exists, thus a circuit also

exists. In this core, the saliva forms the electrolyte, and the hard and soft

tissues can consists the external circuit internal and external depending an a

contact.

3. A single metallic restorations showing two possible pathways between an

external surface exposed to saliva and interior surface exposed to dentinal

fluid. Because dentinal fluid contains higher Cl- concentrations than does

saliva it is assumed that the electrode potential of interior surface is more

active.

II. Corrosion in Heterogeneous Composition

A second type of electrolytic corrosion is that due to heterogeneous

compositions of the metal surface. Example of this type may be the intectic

and peritectic alloys.

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When an alloy containing a extentic is immersed is an electrolyte, the

metallic grains with the less electrode potential are attacked and corrosion

results.

Like wise, in a solid solution, any cored structure is less resistant to

corrosion than the homogenized structure because of difference in electrode

potential.

Even an homogenized solid solution in some what susceptible to

corrosion because of the difference in structure between the grains and their

boundaries.

The grains boundaries may act as anodes and the interior of the grains

as the cathode. This results in the corrosion of the material in the anodic region

at the grain boundaries.

III. Inhomogeneous Surface

A third conductors that produces electrolytic corrosion in the presence o

an inhomogeneous surface structure.

A common situation for this type of corrosion would be an amalgam

restorations with polished and unpolished area.

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This type is primarily with a stress condition in the alloy or metal. For

example, even in a pure metal not previously subjected for external forces, a

certain amount of stress is always present.

Stress corrosion

On most dental appliances, the deleterious effect of stress and corrosion

are most optional to occur because of fatigue of the metal when associated

with a corrosive environment. Reported removal and insertion of a partial

denture, for example, may build up a severe stress pattern in certain types of

alloys especially at the grain boundaries combined with an oral condition that

promotes corrosion, the stressed appliance develops stress corrosion.

Slight surface irregulation at that point, such as pit, can accelerate the

process, so that ordinary fatigue starts below the normal limit and failure

results.

Stress corrosion is characterized by the fast that external fracture occurs

intergranularly.

IV. Concentration cell composition or Crevice corrosion

This situation exists whenever there are variations in the electrolytic or

in the composition of the given electrolyte with the system. For example, there

are often occumulations of food debris in the interproximal areas of the mouth,

particularly if oral hygiene is poor.

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This debris produced one type of electrolyte in that area and the normal

saliva provides another electrolyte at the occlusal surface.

Therefore electrolyte corrosions occurs.

A similar type of attack may be produced from differences in oxygen

tension between parts of restorations. A cell is produced with greatest activity

occurring around the areas containing the least oxygen. Irregularities, such as

points, contribute to this phenomenon. The areas at the bottom of the surface

concavities do not have oxygen because they are covered with food debris and

mucin.

The material at the bottom of the pit then becomes the anode and the

material at the periphery the cathode.

In this manner, metal atoms at the base of the pit ionise and go into

solution, causing the pit to deeper. The rate of such corrosion may be very

rapid, and failure may occur much before what would be anticipated if only a

uniform surface attack were expected.

For this reason, all metallic dental restoration materials should be

polished for example, a polished amalgam restoration will corrode less than

one left unpolished.

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Multiple corrosion process / Complex corrosion

When different types of corrosion occurs together in a same restoration,

it is called as “complex corrosions”.

This situation can be illustrated by dissimilar metal corrosion between

an inlay and an amalgam restoration. Owing to the surface changes than can

occur during this type of electrolyte corrosion, difference in oxygen tension

arise. At same time, if the corrosion product layer is incomplete or porous, an

inhomogeneous surface results that produce new corrosion cells.

Corrosion Characteristics of Dental Amalgams

Dental amalgams are most susceptible to chloride corrosion then most

other dental materials.

The conventional 2 containing amalgams corrode along the networks

of the thermodynamically lean stable 2 (tin – mercury) phase.

The high copper 2 free amalgams are more corrosion resistant, but they

also suffer deterioration, the least stable being the (copper tin) phase. The 2

containing amalgams exhibit a clearly defined breakdown potential.

The protective film of tin oxide breaks down main corrosion products

are tin – oxide and tin chloride hydroxide. Corrosion resistance of are

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amalgams is decreased by porosity. The corrosion damage causes loss of

strength. The most intensive corrosion takes place in the areas of the margins.

Another type of attack observed on occlusal surface is the grain

boundary corrosion of the 1 (silver mercury) phase (Espevik and Mjor 1979).

Corrosion is thus thought to be one of the major causes of the

breakdown of restoration margins.

The high copper 2 free amalgams are most resistant to both corrosion

and breakdown of margins.

Corrosion Characteristics of Nobel Dental Alloys

Gold alloys conforming to ADA specification No. 5 (more than 75%

gold and platinum gp metals) are highly resistant in the oral environment to

electrochemical forms of deterioration.

Their resistance is due to thermodynamic stability of the noble

elements, which dominates the behaviour of the essentially single phase

materials.

The resistance to chloride corrosion decreases as the gold and platinum

gp metal content of the alloy decreases.

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Silver is attacked by the oral environment, but its resistance to chloride

corrosion can be substantially improved by alloying with palladium, which

includes passivation.

Noble dental alloys are relatively resistant to occluded cell corrosion

because the electrochemical reactions that take place do not result in an

increase in acidity with in the cell.

Corrosion Characteristics of Base Metal Alloys

Cobalt, Nickel, Iron and Titanium as well as the major elements used in

their alloys. Such as chromium molybdenum, vanadium and aluminium are

base metals that are not thermodynamically stable in oral environment.

The corrosion resistance of there alloys is because of the formation of a

protective passive film. Due to the presence of chlorides in the oral

environment, passivating alloys are potentially susceptible to pitting corrosion.

The corrosion resistance of nickel based dental alloys is mainly a

function of the chromium content.

Titanium and its alloys are highly resistant to chloride corrosion

because of the superior properties of the passive film. Base alloys are not

susceptible to sulphade tranishing.

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Protection Against Corrosion

Choice of alloy

Alloy for long term use in mouth must be either.

1) Noble or 2) Passive

1) Noble

Noble metals are those such as gold, platinum and palladium. Dental

alloys contain some copper which has poorer corrosion resistance, however,

such alloys must contain at least 70-75% noble metals. For this reason it is

estimated that at least half the metals in dental alloy should be gold, with

platinum and palledium to insure against corrosion.

Silver Sulphide appears to be the common corrosions product of gold

alloys that contain silver, palladium tends to retard the formation of silver

sulphide and thus has become a common addition to silver containing dental

gold alloys.

2) Passive alloys

Certain metal develop a protective coating by oxidation, or some other

chemical reaction, which protects them from further corrosion, such a metal is

said to be passive chromium is the best example of passivity. This important

metal does not readily corrode because it has already corroded, so rapidly and

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so uniformly that the film of corrosion product formed does not mar its

reflectivity.

It is probably, that this film consists of either a continuous layer of

adsorbed oxygen or a closely packed chromic oxide with each molecule being

oriented so that the oxygen is on the outside.

A coating of a noble metal applied to the surface of a second metal may

be used to prevent corrosion. The coating material must be less active than the

base metal.

If a noble metal eating is applied to the base metal surface and becomes

scratched or pitted to such a depth that the base metal is exposed to the

environment, the base metal will be corroded at a rapid rate.

Points or other types of inorganic or organic coatings behave as noble

coatings.

Clinical Significance of Galvanic Currents

It has been proved that small galvanic currents associated with electro

galvanium are continually present in the oral cavity. As long as metallic dental

restorative materials are employed, these seems to be little possibility that

these galvanic currents can be eliminated.

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The cement base itself, although it is a good thermal insulator has little

effect in minimizing the current that is carried into the tooth and through the

pulp.

Until materials or techniques are developed that will provide perfect

adaptation to the cavity walls, the possibility of blocking such currents is

highly unlikely.

In a situation in which metallic restoration is extremely close to the

pulp, the current concentration and pulpal stimulation may be reduced some

what by replacing the deepest portion of the metal restoration with a lower

conduction base material.

Although the post operative pain due to galvanic shock is not a

common occurrence in the dental office, it can be real source of discomfort to

patients. However, such post operative pain usually occurs immediately after

insertion of a new restorative and generally, it gradually subsides and

disappears in a few days.

It has often been suggested that the reason the pain does not last

indefinitely is became of the formation of a layer of tarnish on the restoration

or that the cemetal base such as Zn-oxide eugenol cement, becomes a better

insulator as setting progresses.

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Practically the best method for reducing or eliminating galvanic shock

seems to be a paint or external varnish on the surface of the restoration. As

long as the varnish remains, the restorations is insulated from saliva and no

cell is established. By the time the varnish has worn away. The pulp has

usually healed sufficiently so that no pain is experienced by the patient.

Summary and Conclusion

Corrosion is obviously undesirable, as it can spoil the aesthetics of an

alloy, and in extreme cases can severely weaken the material.

Hence the very important requirement of any metal or the alloy that is

to be used in the mouth is that it should not produce corrosions products that

will be harmful to the structure.

In addition to choosing the best materials, alloy must be used correctly,

and is situations which are likely to lead to corrosion must be avoided as far as

possible.

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