TARNISH AND CORROSIONTARNISH AND CORROSION

Dr LAKSHMI RAVI M.D.SAsst ProfessorDept of OrthodonticsSt.G.D.C

CORROSIONCORROSION

A chemical or electrochemical process through A chemical or electrochemical process through which a metal is attacked by natural agents such as which a metal is attacked by natural agents such as air and water resulting in partial or complete air and water resulting in partial or complete dissolution, deterioration or weakening of any solid dissolution, deterioration or weakening of any solid substance.substance.

Types of CorrosionTypes of Corrosion Crevice corrosionCrevice corrosion - corrosion in narrow spaces caused by - corrosion in narrow spaces caused by

localized electrochemical process and chemistry changes such localized electrochemical process and chemistry changes such as acidification and depletion of oxygen content. as acidification and depletion of oxygen content. Eg. Microleakage in dental restorations.Eg. Microleakage in dental restorations.

Galvanic corrosionGalvanic corrosion - corrosion on a less noble metal when - corrosion on a less noble metal when electrochemically dissimilar metals are in electrical contact in electrochemically dissimilar metals are in electrical contact in presence of corrosive liquid environment.presence of corrosive liquid environment.

Pitting corrosionPitting corrosion - Sharply localised corrosion in base - Sharply localised corrosion in base metals like iron,nickel and chromium (which are naturally metals like iron,nickel and chromium (which are naturally protected by a thin film of oxide) in the presence of protected by a thin film of oxide) in the presence of chlorides in the environment ,the film locally breaks down chlorides in the environment ,the film locally breaks down causing dissolving of the metal underneath it in the form of causing dissolving of the metal underneath it in the form of pits.pits.

Stress corrosionStress corrosion –Degradation by the combined effects of –Degradation by the combined effects of mechanical stress and a corrosive environment,usually in the mechanical stress and a corrosive environment,usually in the form of cracking.form of cracking.

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

Concentration cellConcentration cell - - An electrochemical corrosion cell,in An electrochemical corrosion cell,in which the potential difference is associated with the which the potential difference is associated with the difference in the concentration of the solutions causing difference in the concentration of the solutions causing corrosion at different parts of the metal surfacecorrosion at different parts of the metal surface ..

CONCENTRATION CELL

Causes of Tarnish and CorrosionCauses of Tarnish and Corrosion Formation of hard(calculus) and soft deposits(plaques and films Formation of hard(calculus) and soft deposits(plaques and films

composed of micro organisms and mucin) on the surface of the composed of micro organisms and mucin) on the surface of the restoration.restoration.

Formation of thin films of oxides, sulfides and chlorides on a Formation of thin films of oxides, sulfides and chlorides on a metal causing surface discoloration.metal causing surface discoloration.

Corrosion results in surfaces under stress or with irregular Corrosion results in surfaces under stress or with irregular impurities on the metal.impurities on the metal.

Disintegration of a metal may occur through the action of Disintegration of a metal may occur through the action of moisture, atmosphere, acid or alkaline solutions and certain moisture, atmosphere, acid or alkaline solutions and certain chemicals.chemicals.

ELECTRON MICROSCOPIC PICTURE OF CORROSION

SALT OF THE METAL

Tarnish that is formed in time can accumulate elements or Tarnish that is formed in time can accumulate elements or compounds that chemically attack the metallic surface. For compounds that chemically attack the metallic surface. For e.g. and certain foods contain amounts of sulfur.e.g. and certain foods contain amounts of sulfur.

Sulfides such an hydrogen and ammonium sulfides, corrode Sulfides such an hydrogen and ammonium sulfides, corrode silver, copper and similar metals present in dental alloys and silver, copper and similar metals present in dental alloys and amalgam.amalgam.

Water ,oxygen and chloride ions present in saliva contribute Water ,oxygen and chloride ions present in saliva contribute to corrosion attack.to corrosion attack.

Various acids such as phosphoric, lactic, acetic acids at Various acids such as phosphoric, lactic, acetic acids at proper concentration and pH can produce corrosion.proper concentration and pH can produce corrosion.

SCANNING ELECTRON MICROSCOPE PICTURE OF METAL WITH CORROSION

PIC. COURTESY PAX CAM

Variables affecting corrosionVariables affecting corrosion

Composition, physical state and surface condition of Composition, physical state and surface condition of the metalthe metal

Chemical components of the surrounding mediumChemical components of the surrounding medium TemperatureTemperature Temperature fluctuationsTemperature fluctuations Movement or circulation of the medium in contact Movement or circulation of the medium in contact

with the metal surfacewith the metal surface Nature and solubility of the corrosion productsNature and solubility of the corrosion products

Corrosion of steel under a droplet of water

TYPES OF CORROSION REACTIONTYPES OF CORROSION REACTION

1.CHEMICAL CORROSION1.CHEMICAL CORROSION Direct combination of metallic and non metallic elements. Direct combination of metallic and non metallic elements.

Also called Also called dry corrosiondry corrosion As it occurs in the absence of As it occurs in the absence of water or any other fluid electrolyteswater or any other fluid electrolytes

Eg.OxidationEg.Oxidation- Discoloration of silver by sulfur by formation of silver Discoloration of silver by sulfur by formation of silver

sulfides.sulfides.- It also corrodes the gold alloy that contain silver.It also corrodes the gold alloy that contain silver.

- oxidation of alloy particles (siver-copper)in the dental oxidation of alloy particles (siver-copper)in the dental amalgam.This prevents proper amalgamation with mercury amalgam.This prevents proper amalgamation with mercury

Modern low-copper amalgams have a powder component composed of Modern low-copper amalgams have a powder component composed of 69.4% 69.4% silversilver, 3.6% , 3.6% coppercopper, 26.2% , 26.2% tintin, and 0.8% , and 0.8% ZincZinc..They have a liquid component of 42% to 45% They have a liquid component of 42% to 45% mercurymercury by weight. by weight.

The principle steps of an oxidation reaction:The principle steps of an oxidation reaction:Dissociative oxygen adsorption, metal and Dissociative oxygen adsorption, metal and oxygen ion diffusion through the growing oxygen ion diffusion through the growing oxide layer oxide layer

2.ELECTROCHEMICAL CORROSION2.ELECTROCHEMICAL CORROSION Also known as wet corrosion as it requires a fluid electrolyte or Also known as wet corrosion as it requires a fluid electrolyte or

water.water.

Requires a pathway for transport of electrons an electrical Requires a pathway for transport of electrons an electrical

current.current.

English chemists John Daniell (left) and Michael Faraday (right), both credited to be founders of electrochemistry as known today.

ELECTRO CHEMICAL CELL

APPLIED IN THE ORAL CAVITY WITH TWO DISIMILIAR APPLIED IN THE ORAL CAVITY WITH TWO DISIMILIAR FILLINGSFILLINGS

Amalgam Anode+ ion

Gold alloy cathode - ION

Saliva electrolyte

Ammeter

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

POSSIBLE REACTIONS- REDUCTION REACTIONSPOSSIBLE REACTIONS- REDUCTION REACTIONS

1. M1. M++ + e + e-- M Moo

2. 2H2. 2H+ + + 2e+ 2e-- H2 H2 3. 2H3. 2H22O + OO + O2 2 + 4e+ 4e- - 4(OH)4(OH)--

Metal ion may be removed to form metal atomsMetal ion may be removed to form metal atoms

Hydrogen ions may be converted to hydrogen gasHydrogen ions may be converted to hydrogen gas

Hydroxyl ions may be formedHydroxyl ions may be formed

The electrolyte provides the ions needed at the cathode to The electrolyte provides the ions needed at the cathode to carry away the corrosion products to anode.carry away the corrosion products to anode.

Electromotive force seriesElectromotive force series

This classification for arrangement of the elements This classification for arrangement of the elements in the order of their dissolution tendencies in water.in the order of their dissolution tendencies in water.

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

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

Galvanic corrosion / Dissimilar metals corrosion Galvanic corrosion / Dissimilar metals corrosion

An important type of electrochemical corrosion occurs when An important type of electrochemical corrosion occurs when dissimilar metals are in direct physical contact with each dissimilar metals are in direct physical contact with each other. Hence the dental reference here is two dissimilar other. Hence the dental reference here is two dissimilar restorations in the oral cavity. this metallic combination may restorations in the oral cavity. this metallic combination may produce Electro galvanism or produce Electro galvanism or “Galvanic currents“Galvanic currents”.”.

AMALGAMGOLD

Galvanic ShockGalvanic Shock A pain sensation caused by electric current generated by a A pain sensation caused by electric current generated by a

contact between two dissimilar metals forming a battery in contact between two dissimilar metals forming a battery in the oral environmentthe oral environment

GOLD FILLING

ENAMEL

DENTINE

PULP

AMALGAM FILLING

S A L I V A

Eg. Amalgam filling opposing a gold inlay as Eg. Amalgam filling opposing a gold inlay as

both the restorations are wet in saliva, an both the restorations are wet in saliva, an

electrical couple exists, with a potential electrical couple exists, with a potential

difference between the dissimilar restorations.difference between the dissimilar restorations.

When they come in contact, the potential is When they come in contact, the potential is

suddenly short circuited through the two alloys suddenly short circuited through the two alloys

resulting in a sharp pain.resulting in a sharp pain.

Stress corrosionStress corrosion Eg.repeated removal and insertion of a partial Eg.repeated removal and insertion of a partial

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

Concentration cell corrosionConcentration cell corrosion Accumulation of food debris produces one type of Accumulation of food debris produces one type of

electrolyte, normal saliva produces another electrolyte, normal saliva produces another electrolyte. This difference in electrolyte causes an electrolyte. This difference in electrolyte causes an electrochemical corrosion of the metal filling electrochemical corrosion of the metal filling underneath the food debris.underneath the food debris.

PROTECTION AGAINST CORROSIONPROTECTION AGAINST CORROSION Highly polished restorationsHighly polished restorations

A coat of noble metal over the base metal in dental casting(noble A coat of noble metal over the base metal in dental casting(noble metals resist corrosion because their EMF is positive)metals resist corrosion because their EMF is positive)

Paint application or coating with inorganic nonconductive Paint application or coating with inorganic nonconductive coatings.coatings.

Passivating metals-some metals form a thin protective layer Passivating metals-some metals form a thin protective layer when exposed to the environment .Eg,chromiumwhen exposed to the environment .Eg,chromium

Electroplating with nickel followed by chromium for protective Electroplating with nickel followed by chromium for protective film.eg.stainless steelfilm.eg.stainless steel

•As long as the film of chrome oxide is maintained, the stainless As long as the film of chrome oxide is maintained, the stainless steel behaves like gold, silver or platinum, or in other words, it has steel behaves like gold, silver or platinum, or in other words, it has a passive behavior. Stainless steel can also develop active sites of a passive behavior. Stainless steel can also develop active sites of corrosion if the protective film is destroyed by scratches, nicks, corrosion if the protective film is destroyed by scratches, nicks, stock deposits or contamination of steel by non-ferrous inclusions stock deposits or contamination of steel by non-ferrous inclusions

•Chromium passivated metal will be corroded by chloride, hence Chromium passivated metal will be corroded by chloride, hence patients on removable dentures are instructed not to wash their patients on removable dentures are instructed not to wash their appliances with household bleaches and cleansers appliances with household bleaches and cleansers

Corrosion of dental restorationsCorrosion of dental restorations

Affecting FactorsAffecting Factors DietDiet DrugDrug Smoking Smoking Bacterial activityBacterial activity Oral hygiene and habitsOral hygiene and habits

Prevention of corrosion in dental Prevention of corrosion in dental restorationsrestorations

Addition of noble metals like gold, platinum Addition of noble metals like gold, platinum and palladium in dental alloyand palladium in dental alloy

Gold resist sulfide tarnish, palladium resists Gold resist sulfide tarnish, palladium resists sulfide tarnishing with silversulfide tarnishing with silver

Base metals alloys such a s ni-cr,co-cr and ti Base metals alloys such a s ni-cr,co-cr and ti are virtually resistant to sulfide are virtually resistant to sulfide tarnish(Orthodontic wires) although they are tarnish(Orthodontic wires) although they are susceptible to chloride corrosionsusceptible to chloride corrosion

Clinical considerationsClinical considerations Application of varnish in the dentinal walls and on the Application of varnish in the dentinal walls and on the

surface of the filling to avoid galvanism in amalgam surface of the filling to avoid galvanism in amalgam restorations.restorations.

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

SEPAGE OF MECURY FROM AN AMALGAM FILLINGELECTRON MICROSCOPE PICTURE

MERCURY

Luigi Galvani -1737 - 1798Luigi Galvani -1737 - 1798 Galvani was born, educated and taught anatomy Galvani was born, educated and taught anatomy in Bologna. The Italian physiologist made in Bologna. The Italian physiologist made one of the early discoveries that advanced the one of the early discoveries that advanced the study of electricity. His work with frogs led tostudy of electricity. His work with frogs led to his discovery in 1781 of galvanic or voltaic electricity. his discovery in 1781 of galvanic or voltaic electricity.

Galvani found he could make the muscles of a dead Galvani found he could make the muscles of a dead frog twitch when he touched them with different frog twitch when he touched them with different metals or the current from a nearby static electric generator. metals or the current from a nearby static electric generator. But he incorrectly thought fluid in the frog's body was the But he incorrectly thought fluid in the frog's body was the source of the electricity. This discovery soon led to anothersource of the electricity. This discovery soon led to another by Allesandro Volta, who invented the electric cell or early battery.by Allesandro Volta, who invented the electric cell or early battery.

Alessandro Volta - 1745 - 1827Alessandro Volta - 1745 - 1827 Count Alessandro Volta was born in Como, Italy,Count Alessandro Volta was born in Como, Italy, into a noble family, Count Volta was a physicist and pioneer into a noble family, Count Volta was a physicist and pioneer in the study of electricity. "Volt," named after Count Volta. in the study of electricity. "Volt," named after Count Volta.

Inventor of the Battery. Around 1800, he invented a wet battery called a Inventor of the Battery. Around 1800, he invented a wet battery called a Voltaic Pile.Voltaic Pile.

The Voltaic Pile consisted of discs of copper and zinc separated by discs of The Voltaic Pile consisted of discs of copper and zinc separated by discs of paper or cardboard (soaked in salt water). Attached to the top and bottom paper or cardboard (soaked in salt water). Attached to the top and bottom of this "Pile" was a copper wire. When Volta closed the circuit, electricity of this "Pile" was a copper wire. When Volta closed the circuit, electricity flowed through the pile.flowed through the pile.

Volta's battery was later refined by other scientists, and the French Volta's battery was later refined by other scientists, and the French emperor, Napoleon, made Volta a "Count" for his discovery.emperor, Napoleon, made Volta a "Count" for his discovery.