yTARNISH AND CORROSION
y y y y y y y y y y y y y y y
INTRODUCTION CAUSES OF TARNISH AND CORROSION CLASSIFICATION OF CORROSION ELECTROCHEMICAL CORROSION DISSIMILAR METALS HETROGENEOUS SURFACE COMPOSITION STRESS CORROSION CONCENTRATION CELL CORROSION EROSION CORROSION INTRAGRANULAR CORRROSION CORROSION RATE DETERMINATION PROTECTION AGAINST CORROSION CLINICAL CONSIDERATIONS REVIEW OF LITERATURE CONCLUSION
INTRODUCTIONBiocompatibility Mechanical properties Availability
CHOICE OF MATERIAL
Jpd 2002 vol 87
GPT 8 : TERMINOLOGIESy 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.
y Corrosion : the action, process, or effect of corroding, the loss of elemental constituents to the adjacent environment.
y Corrosive : tending or having the power to corrode
ORAL ENVIRONMENT AND EFFECTS ON METALS AND ALLOYS
Several aspects of the oral environment are highly conducive to corrosion
pHPromote accelerated reaction between the corrosion products and the metal or alloy
y Mechanical forces also vary with respect to the
Type of food/diet consumed
Temperament of person
Location in the mouth.
Causes of Tarnish and Corrosion:
Tungsten rods with evaporated crystals, partially oxidised with colourful tarnish11
y 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 MUCIN12
y 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.
y Tarnish is a surface phenomenon, that is self-limiting unlike rust. Only the
top few layers of the metal react.
y 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 fittings14
y Tarnish is often the forerunner of corrosion y 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.15
y The rate f c rr si
attac a act all i crease over ti e, es eciall ith s rfaces s jecte t stress, ith i tra ra lar i rities i the etal, r ith c rr si r cts that tc letel c er the etal s rface..
rse, c rr si .
ca ses se ere a
catastr hic isi te rati
Specific ions may play a major role in the corrosion of certain alloys
surface tarnish on casting alloys that contain silver
y At times acidic solutions such as phosphoric, acetic, and lactic acids at
the proper concentration and pH can promote corrosion.
Other variables affecting corrosion process are..y Temperature. y y
Movement or circulation of medium in contact with metal surface. Nature and solubility of corrosion product.
CLASSIFICATION OF CORROSION
y There are two general types of corrosion reactions :
Wet corrosion / electrochemical corrosion
Dry corrosion / chemical corrosion
y 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
y Commonly seen dry corrosion-- Oxidation
y The principle step of an oxidation reaction :
Dissociative oxygen adsorption: Metal and oxygen ion diffusion occurs through the growing oxide layer.
Examples of dry corrosion in dentistry :
y Corrosion of dental gold alloys that contain silver. y Oxidation of silvercopper alloy particles that are mixed with
ELECTROCHEMICAL CORROSIONy Also known as wet corrosion as it requires a fluid electrolyte or
water.y 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.wikipedia201026
y Anode -- Positive ions are formed with the production of free electrons. y Cathode-- Reduction reaction must occur that will consume free electrons
produced at the anode.y The electrolyte supplies the ions needed at the cathode and carries away the
corrosion products at the anode.27
ELECTROCHEMICAL CELL IN THE ORAL CAVITY WITH TWO DISIMILIAR FILLINGSAmmeter
Amalgam Anode + ion
.. . . . .. Saliva electrolyte
Gold alloy cathode - ION
POSSIBLE REACTIONS- REDUCTION REACTIONS 1. M+ + e2. 2H+ + 2e3. 2H2O + O2 + 4ey y y
Mo H2 4(OH)-
Metal ion may be removed to form metal atoms Hydrogen ions may be converted to hydrogen gas Hydroxyl ions may be formed
Primary driving force for electrochemical corrosion:y At the cathode or cathodic sites, a reduction reaction must occur that will
consume free electrons produced at the anode.
y The basis for any discussion of electrochemical corrosion of dental alloysis the Electromotive Series of the metals.
Electromotive force seriesy This classification for arrangement of the elements in the order of their
dissolution tendencies in water.
y Potential values are calculated with solutions containing one atomic
weight ,in grams, of ions in 1000ml of water at 25 c.
y Metals with a more positive potential have a lower tendency to dissolve in
Metals Gold Gold Platinum Palladium Mercury Silver Copper Bismuth Antimony Hydrogen Lead Tin Nickel Cadmium Iron Chromium Zinc Aluminum Sodium Calcium Potassium Au+ Au3+Pt2+
Electrode Potential (V) + 1.50 + 1.36 + 0.86 + 0.82 + 0.80 + 0.80
Pd2* Hg2+ Ag+ Cu+ Bi3+ Sb3+ H+ Pb2+ Sn2+ Ni2+ Cd2+ Fe2+ Cr2+ Zn2+ Al3+ Na-+ Ca2+ K+
+ 0.47+ 0.23
+ 0.100.00 -0.12 -0.14 -0.23 -0.40 -0.44 -0.56 -0.76 -1.70 -2.71 -2.87 -2.92 33
y If two pure metals are immersed in an electrolyte an connected by an
electrical conductor to form a galvanic cell.y The metal with the lower electrode potential in the table becomes the anode
and undergoes oxidation , that is, its ions go into solution.
Electrochemical corrosion can be further classified intoy Galvanic corrosion y Heterogeneous compositions y Stress corrosion y Concentration cell corrosion Pitting corrosion Crevice corrosion
y Intergranular corrosion y Erosion corrosion.36
GALVANIC CORROSION / DISSIMILAR METALS CORROSIONy An important type of electrochemical corrosion occurs when
dissimilar metals are in direct physical contact with each other.
y This metallic combination may produce Electro galvanism or
GALVANIC SHOCKy A pain sensation caused by electric current generated by a contact
between two dissimilar metals forming a battery in the oral environment.
S A L I V AAMALGAM FILLING ENAMEL DENTINE PULP
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. y 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 y Contact with fresh amalgam fillings less than 1 h old in saliva produced silver-coloured stains of negligible thickness. y On amalgam filling surfaces no significant corrosion was found
HETEROGENOUS SURFACE COMPOSITIONy Heterogeneous compositions of metal surface. y This occurs within the structure of the restoration itself.
y Difference in compositions within the alloy grains
y Grains with lower electrode potential are attacked and corrosion results y Impurities in alloy enhance corrosion
Microscopic structure of copper-zinc alloy41
y Imposition of stress increases the internal energy of an alloy, either
through elastic displacement of atom or the creation of microstrained field associated with dislocation and the tendency to undergo corrosion will be increased.
y Stress corrosion most likely to occur during fatigue or cyclic loading in the oral environment.
y 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.
CONCENTRATED CELL CORROSIONy It occurs whenever there are variations in the electrolytes or in the composition of the given electrolyte within the system.
CONC NTRAT D C LL CORROSION
CR IC C