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Corrosion

Learn about tarnish, corrosion, and various types of dental material degradation. Discover prevention techniques and surface coating methods to safeguard dental appliances.

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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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