Metals and Corrosion in Dental Biomaterials PDF

Metals and corrosion in dental biomaterials By Dr. Reham Mohammed Abdallah  II. Metals The pure metals that are commonly used in dentistry are gold, platinum, silver, copper and titanium.  Properties of metals 1) Metals are elements that ionize positi...

Metals and corrosion in dental biomaterials By Dr. Reham Mohammed Abdallah  II. Metals The pure metals that are commonly used in dentistry are gold, platinum, silver, copper and titanium.  Properties of metals 1) Metals are elements that ionize positively in solutions. 2) They are solids at room temperature (except Hg and gallium which are liquids and H2 which is a gaseous metal). 3) Luster: due to reflection of light waves by the free electrons. 4) All metals conduct heat and electricity because they have free electrons. 5) All metals have high strength, high hardness, and high melting temperature due to primary strong metallic interatomic bonding. 6) They are malleable (can be hammered into sheets) and ductile (can be drawn into wires). 7) Give a metallic ring sound when they are struck. 8) All metals have high density which is related to the atomic weight and to the type of lattice structure.  Shaping of metals 1) Casting “cast metal” This is performed by melting the metal and shaping it in a mould. In dentistry a molten metal is poured into a mould left from a molten wax pattern that was embedded in an investment material (lost wax technique). Surveying the lost wax technique 2) Cold working (wrought metals) A solidified block of a cast metal can be formed by mechanical working to produce a rod, wire, tube…etc. of other shapes Metal can be hammered into sheets or pulled through dies to form wires at room temp. Most dental appliances are cast structures, however orthodontic wires and clasps of partial dentures are wrought metals. 3) Electroforming The process of electrolysis is used to plate a metal on a conducting surface(corrosion in reverse) e.g. silver and copper plated dies. Metal Cold working Metal electroforming process Wrought metals 4) Sintering (powder metallurgy) It is a process of bonding of solid particles by heat in the absence of any liquid. It is an agglomeration process that involves not only bonding of powder particles but also the elimination of the initial porosity to give a denser product. This process requires also high pressure and accompanied by contraction. Amalgam tablets are made by sintering. Metal sintering process  Metal solidification  It occurs through a two-steps mechanism involving nucleus formation and crystallization. a) Nuclear formation  A molten alloy is cooled, and approaches its freezing temperature.  The atoms try to aggregate forming initial starting points of crystallization (nuclei of crystallization) at super cooling point. b) Crystallization The metals can solidify in single crystal (grain) which is very rare, or polycrystalline. As cooling continues, the nuclei of crystallization grow independently in three dimensions (up & down, anteroposteriorly and right to left) in the form of dendrites or branched structures (treelike branches to form crystals (grains).  Each crystal of a metal forms a part of grain  The growth is stopped when there is contact with adjacent growing crystal.  Each nucleus gives rise to one crystal or grain.  The tightly packed crystals are called grains and their boundaries are called grain boundaries. Grain boundary It is a region of transition between different oriented crystal lattice of the two adjacent crystals (grains)  Crystal imperfection  Real crystal structure usually contains a variety of defects.  Defect in crystals (point, line or plane) have a Considerable effect on the properties of the metal or alloy. a) Point defects 1- Impurities These can cause distortion of the crystal lattice. Impurities may be interstitial or substitution. 2- Vacancies These can allow atoms to move in the crystal. b) Line defects (dislocation) Dislocation is the movement of a row of atoms along each other in the lattice. This dislocation moves across the crystal, as shown in (A) deforming it in a series of single steps (B), and the dislocation finally moves out of the crystal (C). Point defects Line defect (dislocation)  All the techniques used for improving the strength of metal depend on the stop of the motion of dislocations. C) plane defect as grain boundaries defects.  Deformation of metals At stresses below the proportional limit, the atoms in the crystal lattice are displaced in amount yet, when the stress is relieved, they can return to their original positions (stretching of the bonds). However, once the proportional limit is exceeded, a permanent deformation takes place and the structure does not return to its original dimensions when the load is released, (dislocation) eventually occurred. This displacement becomes so great that the atoms are separated completely and a fracture results (loss of cohesion). Deformation of metal by dislocation movement Metals in dentistry ALLOYS Pure metals are not suitable for dental applications because they are too soft and ductile. The mechanical properties are improved by using mixtures of metals. These are called alloys. For example, if 5% copper is alloyed with gold, a relatively harder and stronger material will be obtained. Most alloys solidify over a range in temperature rather than a single temperature as does a pure metal. Tarnish and corrosion  What is tarnish? Definition Tarnish is a surface discoloration of a metal or even a slight loss of surface finish or luster. Causes of tarnish  Discoloration arises from pigment producing bacteria.  Drugs producing stains such as  Food debris, dental plaque and calculus deposited on the surface of the restorations  Formation of thin films such as oxides, sulfides or chlorides. So, tarnish is the forerunner of the more serious condition of corrosion.  What is corrosion? Definition  Corrosion is the destruction of a metal or an alloy by chemical or electrochemical reaction with its surrounding environment or medium. Causes of corrosion  Oral environment which contains water, salt, dissolved oxygen, bacteria, proteins and various ions such as chloride and hydroxide  Types of corrosion 1. Chemical corrosion (dry corrosion): Occurs in the absence of fluids or water. 2. Electrochemical corrosion: It is a combination of 1. Chemical corrosion (dry corrosion): Occurs as a result of direct combination of metallic and non metallic elements forming a chemical compound through processes of oxidation, halogenation Examples Sulfur causes chemical corrosion of silver forming silver sulfide. Oxidation of silver-cupper alloy particles of dental amalgam. So, the alloy must be stored in a cool, dry place to provide good shelf life. 2. Electrochemical corrosion Also referred as “wet corrosion” (requires the presence of water or some other fluid electrolyte). In order to continue the process, it requires the pathway for the transport of electrons (electric current).  Electrolytic corrosion can be predicted by using the electromotive force series or standard potential series for metal reduction listed in the table. This is an arrangement of the elements in the order of their electrode potential So, If two metals are present in an Components of electrochemical cell 1. ANODE …. It is the surface of lower electrode potential where + ions are formed (oxidation reaction free electrons are liberated). 2. CATHODE … It is the surface of higher electrode potential where +ions are gained (reduction reaction  free electrons are consumed). i.e. Anode dissolves (corrodes) while the cathode remains intact. 3. ELECTROLYTE … supplies ions needed at cathode. Components of electrochemical cell Cathodic reactions is considered to be the primary deriving force for electrochemical corrosion. i.e If the consumption of electrons at cathode is higher than production of electrons at anode Types of electrochemical corrosion 1.Galvanic corrosion  Occurs when dissimilar metals of different compositions are in physical contact.  One metal will become anodic with reference to the other and the potential difference will create an electric current An example of this, anode can be dental amalgam, cathode may be gold alloy restoration and saliva as electrolyte. It produces galvanic shock.  Varnish coating eliminates galvanic shock. 2. Stress cell corrosion  Presence of stresses in a metallic restoration increases its tendency for corrosion because it increases its internal energy. Ex: Cold working of an alloy by bending or burnishing causes localized permanent deformation and stresses in The stressed areas act as anode and the unstressed areas act as cathode with the presence of saliva, electrochemical cell is formed and corrosion of the anodic part of a restoration occurs. 3. Concentration cell corrosion (crevice corrosion) Is a type of localized corrosion which occurs as a result of difference in electrolyte composition or difference in oxygen tension. Example1. Difference in oxygen concentration between parts of the same restoration. Electrochemical cell is formed because the bottom of the pit (area of least oxygen tension) act as anode, the alloy surface at the margins of the pit act as cathode and saliva act as electrolyte. So, corrosion at the base of the pit occurs leading to deepening of the pit. Example 2. An electrochemical cell is formed in a single isolated metallic Protection from corrosion: 1. Passivation: Passivation is the formation of stable surface oxide layer that prevents further oxidation of the metal. Passivation corresponds to the transformation of an active surface which is corroding to an inactive surface, by formation of a passivation layer. Chromium is the best example for 3.Dental alloys that are used in oral cavity must be highly polished to attain smooth lustrous surface to overcome crevice corrosion. 4.In case of two dissimilar metals: Increase anode/cathode ratio to decrease corrosion process as consumption of electrons at the cathode will be less than production of electrons Effect of corrosion: Corrosion products (ions) released in the body may lead to: 1.Weakening the restoration and spoiling the esthetics 2.Metallic taste sensation and may lead to toxicity of the patient. 3.Roughening of the surface of the restoration with bacterial and plaque 4. local pain and swelling in the region of the restoration, with no evidence of infection. 5. Metal ions may migrate to other organs of the body causing inflammation. 6. Cracking or flaking of the restoration may lead