TARNISH AND CORROSIONPresented ByMujtaba AshrafMDS-1st yearDept. of Prosthodontics& Crown and BridgesDr Mujtaba Ashraf1
ContentsIntroduction to Tarnish and CorrosionCauses of Tarnish and CorrosionClassification of CorrosionElectrochemical CorrosionProtection Against CorrosionDr Mujtaba Ashraf2
DefinitionsCorrosion: the action, process, or effect of corroding; a product of corroding; the loss of elemental constituents to the adjacent environment.Tarnish: surface discoloration on a metal or as a slight loss or alteration of the surface finish or luster.*GPT-8Dr Mujtaba Ashraf3
Introduction Metals are used in dentistry in various forms, such as metallic restorative materials in the mouth or as various instruments to be used in the oral cavity or as numerous tools and equipment associated with clinical and laboratory work.All metals used in dentistry undergo tarnish or corrosion, the percentage of corrosion varying from metal to metal.Dr Mujtaba Ashraf4
The mouth is moist, warm, salty, acidic, and is continually subjected to fluctuations in temperature. The food and liquid that we take have a wide range of pH. All these environmental factors contribute to the degradation of the metals used in the oral cavity.Dr Mujtaba Ashraf5
However in the dental practice, a limited amount of corrosion around the margins of dental amalgam restorations may be beneficial, since the corrosion products tend to seal the margin gap and inhibit the ingress of oral fluids and bacteria.Dr Mujtaba Ashraf6
Dr Mujtaba Ashraf7
The outermost surface layer of many semi-reactive metals such as copper, brass, silver, and aluminum undergo a chemical reaction forming a thin layer known as tarnish.This is a surface phenomenon that is self limiting unlike rust.It is mainly caused by chemicals in the air, such as sulfur dioxide.Tarnish is manifested as a dull gray or black film or coat over the metal surface.Dr Mujtaba Ashraf8
The formation of tarnish is a protective phenomenon that involves the reaction of only the top few layers of metal. The layer of tarnish then seals and protects the underlying layers. This layer of tarnished metal is called PATINA.
The formation of patina is necessary in applications such as copper roofing, and outdoor copper, bronze, and brass statues and fittings.
Dr Mujtaba Ashraf9
Causes of TarnishIn the oral environment, tarnish occurs due to the formation of soft deposits (plaque and mucin) and hard deposits (calculus) on the surface of the restoration.
Stain or discoloration arises from pigment producing bacteria, drugs containing chemicals such as iron or mercury, and adsorbed food debris.
Formation of thin films such as oxides, sulfides,or chlorides may cause surface discoloration
Dr Mujtaba Ashraf10
Dr Mujtaba Ashraf11
Corrosion is a chemical or electrochemical process wherein a metal is attacked by natural agents, resulting in its partial or complete dissolution or deterioration.Eg. Rust, the most familiar example of corrosion.
It differs from tarnish in that it is not merely a surface deposit but an actual deterioration of the metal due to a reaction with its environment.Dr Mujtaba Ashraf12
Dr Mujtaba Ashraf13Corrosion in the oral environment is either by chemical or electrochemical process through which a metal is attacked by natural agents, such as air and water, resulting in partial or complete dissolution, deterioration, or weakeningof any solid substance.
Causes of CorrosionTarnish is often the forerunner of corrosion.Water, oxygen, and chlorine ions present in saliva contribute to corrosion attack.Various acidic solutions such as phosphoric, acetic, and lactic acids often present in the oral cavity at proper concentrations and pH can promote corrosion.Examples:Eggs contain high amounts of sulfur. Various sulfides, such as hydrogen or ammonium sulfide, corrode silver, copper, and mercury.Ions such as oxygen and chlorine corrode amalgam.Dr Mujtaba Ashraf14
Dr Mujtaba Ashraf15-By Fontana and Jones
In chemical corrosion, there is a direct reaction between the metallic and nonmetallic elements to yield a chemical compound through processes such as oxidation, halogenation, or sulfurization in the absence of water or another fluid electrolyte.Electrolytes are absent This type of corrosion is less susceptible to occur in the mouth.Non aqueous (dry) or Chemical corrosion:Dr Mujtaba Ashraf16
Aqueous (wet) or Electrolytic corrosionElectrochemical corrosion occurs in the presence of a fluid electrolyte such as water. Hence, it is also known as wet corrosion.Only this type of corrosion occurs in the oral cavity where the electrolyte is the saliva.Dr Mujtaba Ashraf17
Process of Electrochemical CorrosionThe electrochemical cell is made up of three main components: an anode, a cathode, and an electrolyte.The anode is the surface or sites on a surface where positive ions are formed.
Dr Mujtaba Ashraf18
ANODE: undergoes an oxidation reaction with the production of free electrons. Thus, the metal surface corrodes due to loss of electrons.M0 M+ + e-
CATHODE: The free electrons that are released by the anode are taken up by the cathode or the cathodic sites, where a reduction reaction occurs.M+ + e- M0 Dr Mujtaba Ashraf19
19Tarnish & Corrosion
Electrolyte is the medium that carries the ions away from the anode, and these ions are then taken up by the cathode.It also carries away the corrosion products formed atthe anode.The pathway of transfer of electrons from the anode to the cathode forms the external circuits that serve as a conduction path.Dr Mujtaba Ashraf20
Electromotive Series of MetalsAll metals have a tendency to give away electrons; only their degree of tendency differs.The electromotive series of metals arranges them inorder of dissolution tendencies in water and classifies metals by their equilibrium values of electrode potential.Dr Mujtaba Ashraf21
Dr Mujtaba Ashraf22
Metal with lower electrode potential has a greater tendency to give away electrons and undergo oxidation.
Thus the metal with the lower electrode potential becomes the anode and undergoes oxidation while the other metal with the higher electrode potential acts as the cathode and takes up the electrons.Dr Mujtaba Ashraf23
Galvanic Corrosion/ Electrogalvanism
This type of electrochemical corrosion occurs when two or more dissimilar metals are in direct physical contact with each other, e.g. two adjacent or opposing restorations made of different alloys. Here, saliva acts asan electrolyte.Dr Mujtaba Ashraf24
Dr Mujtaba Ashraf25Galvanic corrosion occurs due to the galvanic couplingof dissimilar metals involved. Less corrosion-resistant metals become anode and usually corrode.
Schoonover and Souder reported that gold restorations were corroded by mercury released from amalgam fillings because of an electrochemical reaction.
Fusayama et al. observed that silver-colored stains formed on the surface of gold inlays that had got into contact with fresh amalgam mix or fillings.
A pain sensation caused by electric current generated by a contact between two dissimilar metal forming a galvanic cell in oral environment.Patient may also experience pain by touching the tine of a silver fork to a gold foil or inlayrestoration.
Galvanic ShockDr Mujtaba Ashraf26
Galvanic Corrosion cause:weakening of both the alloysdiscoloration of both the restorations, andpresence of a metallic taste in the mouth.Dr Mujtaba Ashraf27
Stress CorrosionStress corrosion is a complex form of corrosion that occurs when brittle cracks develop in irregularities such as notches and pits due to the combined effects of tensile stresses (bending or stretching stress) and corrosive medium.
Dr Mujtaba Ashraf28Mechanical Stress
Dr Mujtaba Ashraf29During mastication, restorations are subjected to heavy compressive shear, and bending forces.Also, burnishing of surfaces sometimes results in localized deformation. Thus, an electrolytic cell is formed between the stressed and unstressed metal portions, as stressed area become anode while unstressed become cathode.
Dr Mujtaba Ashraf30Stress corrosion most likely to occur during fatigue or cyclic loading in the oral environment.Eg. Repeated removal and insertion of a partial denture will develop a severe stress pattern in the appliance causes stress corrosion resulting in fatigue and failure.
Concentration cell corrosion/ Crevice corrosionThis type of corrosion occurs when a liquid corrosive or an electrolyte is trapped in narrow gaps between metals or between metals and nonmetals.
A homogeneous metal or alloy can undergo electrolytic corrosion when there is a difference in electrolyte concentration across the specimen.
Dr Mujtaba Ashraf31
Example; a metallic restoration which is partly covered by food debris will differ from that of saliva, and this can contribute to the corrosion of the restoration.
Dr Mujtaba Ashraf32
Dr Mujtaba Ashraf33
Dr Mujtaba Ashraf34Crevices are unavoidable with surgical implants where a screw or plate contacts the bone. The local flux of ions is drastically enhanced at crevice sites and tissue impregnation follows.
The contact region of a screw or implant material can form the crevice. In the small space the liquid and oxygen exchanges are severely limited, and surface in the crevice undergoes active corrosion and cause further deterioration.
EFFECTS OF CORROSIONNobel Metal: Nobel metals resist corrosion because their electromotive force is positive when compared to any other metal used in t