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Corrosion

Corrosion. Prepared by: Dental Materials Department Yenepoya Dental College, Yenepoya University, Mangalore. Objectives. Understand the tarnish and corrosion process and can apply the knowledge in dentistry Know how to prevent the corrosion process in dentistry.

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Corrosion

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  1. Corrosion Prepared by: Dental Materials Department Yenepoya Dental College, Yenepoya University, Mangalore.

  2. Objectives • Understand the tarnish and corrosion process and can apply the knowledge in dentistry • Know how to prevent the corrosion process in dentistry

  3. Tarnish = a surface discoloration on a metal or a slight loss or alteration of the surface finish or luster. • Corrosion = an deterioration of a metal by reaction with its environment.

  4. Causes • The formation of hard and soft deposits on the surface of the restoration • Chemical attack

  5. Classification of Corrosion • Chemical corrosion (Dry corrosion) • Direct combination of metallic and nonmetallic elements • Oxidation, halogenation, sulfurization • Electrochemical corrosion (Wet corrosion) • Requires the presence of water or other fluid electrolytes, electrical current

  6. Electrochemical Corrosion • Electrochemical cell • Anode (oxidation) • Mo M+ + e- • Cathode (reduction) • M+ + e-  Mo • 2H+ + 2e-  H2 • 2H2O + O2 + 4e-  4(OH)- • EMF series

  7. EMF series Cathode More noble Anode Less noble

  8. I. Galvanic Corrosion (1) Dissimilar metals are in direct physical contact with each other. • e.g. amalgam restoration vs. gold inlay, spoon vs. amalgam

  9. Galvanic Corrosion (cont.) (2) One metal, dissimilar electrolytes • Saliva, tissue fluid

  10. Galvanic Corrosion (cont.) (3) Heterogeneous Composition of the metal surface • Multiphases alloys: Eutectic and peritectic alloys (e.g. amalgam alloy) • Grain boundaries of homogenized solid solution (boundaries = anode) • Impurities

  11. II. Stress Corrosion • Stressed material + corrosive environment  stress corrosion • Surface irregularities pit, burnishing, bending (cold working)  stress in appliance • Causes a material to fail at stress < its fatigue strength.

  12. III. Concentration Cell Corrosion low O2, anode • “Crevice corrosion” • Variations in the electrolytes or in the composition of the given electrolyte within the system • Differences in O2 tension between parts of the same restoration  All metallic restorative materials should be polished.

  13. Protection • Surface polishing  irregularities • Avoid creating stress in appliances • Use noble metal e.g. gold, platinum,… • Surface coating with paint/ other types of coating • Coating/Electroplating with a material having lower EMF or higher EMF • Coating/Electroplating with an element that resists corrosion e.g. chromium • Alloying with chromium, aluminum

  14. Surface coating with paint/ other types of coating Problem: Any pit or scratch in the paint or coatings may lead to rapid corrosion of the base metal.

  15. Coating/Electroplating with a material having higher EMF • Coating of a more noble metal on a base metal • Can cause rapid corrosion if the surface becomes scratched --WHY? • (1) set up a concentration cell • (2) produce a galvanic cell • (3) unfavorable anode:cathode surface area ratio

  16. Coating/Electroplating with a material having lower EMF

  17. A B Question? • In case of dissimilar metal corrosion, a paint or other nonconductive film can be used to advantage if it is applied to the more or the less noble of the two metals.—WHY? • The more noble metal! • The surface area available for the reduction reaction has been decreased. • A scratch in this coating does not lead to a rapid attack on the active (less noble) metal.

  18. “Passivating” metals • e.g. Cr, Al, Ti • Chromium: forms a film of a corrosion product which consists of oxide. • The oxide film is very stable, therefore, it protects the metal from further corrosion. • Need to have a sufficient amount of Cr to passivate the alloy • Tensile stresses and chlorides can disrupt the protective film and rapid corrosion may occur. • *Do not use household bleaches for cleaning RPD framework or orthodontic appliances that are alloyed with chromium (Ni-Cr, Co-Cr).

  19. Summary • Types of corrosion: Galvanic, stress, and concentration cell corrosions. • How to protect the corrosion process?

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