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Applied Electrochemistry

Applied Electrochemistry. Dept. Chem. & Chem. Eng. Lecture 14 Metal Electrochemistry. Dept. Chem. & Chem. Eng. 1. Electrodeposition. 2. Corrosion. 3. Industrial electrolytic process. Outline. Definition.

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Applied Electrochemistry

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  1. Applied Electrochemistry Dept. Chem. & Chem. Eng.

  2. Lecture 14 Metal Electrochemistry Dept. Chem. & Chem. Eng.

  3. 1 Electrodeposition 2 Corrosion 3 Industrial electrolytic process Outline

  4. Definition Corrosion is the deterioration of materials by chemical interaction with their environment.  The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals.

  5. Corrosion: --the destructive electrochemical attack of a material. --Al Capone's ship, Sapona, off the coast of Bimini. Cost: --4 to 5% of the Gross National Product (GNP)* --this amounts to just over $400 billion/yr** Photos courtesy L.M. Maestas, Sandia National Labs. Used with permission. * H.H. Uhlig and W.R. Revie, Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, 3rd ed., John Wiley and Sons, Inc., 1985. **Economic Report of the President (1998).

  6. The Rusting Mechanism (Peel) • 4Fe + 6H2O + 3O2 4Fe(OH)3 • gives ferric hydroxide •  2Fe(OH)3 Fe2O3 3H2O • gives iron oxide (rust) and water Basic “rusting” or corrosion requirements 1. The metal is oxidized at the anode of an electrolytic cell 2. Some ions are reduced at the cathode 3. There is a potential or voltage difference between the anode and cathode 4. An electrolyte (fluid) must be present 5. The electrical path must be completed

  7. Corrosion of zinc in acid • Two reactions are necessary: -- oxidation reaction: Zn → Zn2+ + 2e- -- reduction reaction: 2H+ + 2e → H2(gas) Adapted from Fig. 17.1, Callister 6e. (Fig. 17.1 is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.) • Other reduction reactions: -- in an acid solution -- in a neutral or base solution

  8. - + Cd Ni 25°C 1.0 M 1.0 M 2 + 2+ Cd solution Ni solution Standard EMF • Metal with smaller V corrodes. • Ex: Cd-Ni cell • EMF series o V o metal metal metal Au Cu Pb Sn Ni Co Cd Fe Cr Zn Al Mg Na K +1.420 V +0.340 - 0.126 - 0.136 - 0.250 - 0.277 - 0.403 - 0.440 - 0.744 - 0.763 - 1.662 - 2.262 - 2.714 - 2.924 o DV = 0.153V Data based on Table 17.1, Callister 6e. 5

  9. Thermodynamic Driving Force • Like all chemical reactions – Thermodynamics • What is the driving force for the reaction? (otherwise stated as what is the electrochemical potential for the reaction) • Dissimilar metals • Different cold work states • Different grain sizes • Difference in local chemistry • Difference in the availability of species for a reaction (concentration cells) • Differential aeration cells

  10. For: 1 1

  11. Introduce: The total electropotential is • G = -nFE Where: F = Faraday’s constant (total charge on Avogadro’s number of electrons) n = the number of electrons transferred E = The electrode potential

  12. Nernst Equation: The Basic equation which describes ALL corrosion reactions For the Given Example: Note: pH = -log10[H+]

  13. Pourbaix Diagram Source: www.corrosionsource.com

  14. EFFECT OF SOLUTION CONCENTRATION • Ex: Cd-Ni cell with standard 1M solutions • Ex: Cd-Ni cell with non-standard solutions n = #e- per unit oxid/red reaction (=2 here) F = Faraday's constant =96,500 C/mol. • Reduce VNi - VCd by --increasing X --decreasing Y 7

  15. H 2H+ +2e- 0.1 for H2 io ANODIC 2 0 E on Fe A 0.0 A 2H+ +2e- H2 -0.1 CATHODIC -0.2 POTENTIAL VOLTS (SHE) -0.3 Fe Fe+2 +2e- ANODIC 0 io Fe E -0.4 B B -0.5 Fe+2 +2e- Fe CATHODIC -0.6 -0.7 -10 -9 -8 -7 -6 -5 -4 -3 -2 10 10 10 10 10 10 10 10 10 CURRENT DENSITY, A/cm² Kinetics Describes Rate of Reaction Evan’s Diagram

  16. Aa = Area Inside Crevice (Anodic) Ac = Area Outside Crevice (Cathodic) Aa << Ac M M+ +e- Aa A io i c + o H+ H Aa io E O2 Ac ia E O2 O2 /OH + ½ O2 + H2O + 2e- 2OH- E M/M+ icorrin very aggresive environment Crevice Effect No Crevice PLUS Crevice Effect Effect Log i Area Effects

  17. Effect of Oxidizer Concentration (e.g., Oxygen) on the Electrochemical Behavior of an Active - Passive Metal Increasing Oxidant Concentration M M+ Log i [Fontanna and Greene, Corrosion Engineering, McGraw-Hill, 1967]

  18. Passivity Passivity is defined as corrosion resistance due to formation of thin surface films under oxidizing conditions with high anodic polariza tion. For example Fe is passivated in concentrated nitric acid.

  19. Effect of Temperature and Dissolved O2 35 LEGEND 30 FRESH WATER @ 50°F FRESH WATER @ 90°F FRESH WATER @ 120°F 25 VELOCITY = 2.5 FPS pH=7, R=100M-0HM 20 pH=7, R=2500M-0HM CORROSION RATE (MPY)) 15 10 5 0 2 3 4 5 6 7 0 1 8 9 10 11 DISSOLVED O2 (PPM) 0 0.7 1.4 2.1 2.8 3.5 4.2 4.9 5.6 6.3 7.0 7.7 DISSOLVED O2 (Mg/H2O)

  20. Forms of corrosion • Stress corrosion Stress & corrosion work together at crack tips. • Uniform Attack Oxidation & reduction occur uniformly over surface. • Erosion-corrosion Break down of passivating layer by erosion (pipe elbows). • Selective Leaching Preferred corrosion of one element/constituent (e.g., Zn from brass (Cu-Zn)). • Pitting Downward propagation of small pits & holes. Fig. 17.8, Callister 6e. (Fig. 17.8 from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.) • Intergranular Corrosion along grain boundaries, often where special phases exist. • Galvanic Dissimilar metals are physically joined. The more anodic one corrodes.(see Table 17.2) Zn & Mg very anodic. • Crevice Between two pieces of the same metal. Fig. 17.6, Callister 6e. (Fig. 17.6 is courtesy LaQue Center for Corrosion Technology, Inc.) Fig. 17.9, Callister 6e.

  21. Types of Aqueous Corrosion Cells • a. General Corrosion • b. Localized Corrosion • Pitting • Crevice Corrosion • Under-deposit Corrosion • MIC • c. Tuberculation • d. Galvanic Corrosion

  22. Factors Affecting Corrosion • Material properties • Metallurgical factors • Passivity • Environment • Metallurgical factors • Chemical segregation • Presence of multiple phases • Inclusions • Cold Work • Non-uniform stresses • Passivity • Example with steel in nitric acid…dilute solutions will cause rapid attack, strong solutions have little visible effect. • Surface film can be formed • Some types of steel may do this with rust • Aluminum does this • Need to watch passive film, but can be used for simple protection

  23. Controlling corrosion • Self-protecting metals! --Metal ions combine with O2 to form a thin, adhering oxide layer that slows corrosion. • Reduce T (slows kinetics of oxidation and reduction) • Add inhibitors --Slow oxidation/reduction reactions by removing reactants (e.g., remove O2 gas by reacting it w/an inhibitor). --Slow oxidation reaction by attaching species to the surface (e.g., paint it!). • Cathodic (or sacrificial) protection --Attach a more anodic material to the one to be protected. Adapted from Figs. 17.13(a), 17.14 Callister 6e. (Fig. 17.13(a) is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Co., 1986.)

  24. Introduction Corrosion Processes Corrosion of Metals Passivity of Metals Atmospheric Corrosion of Steels

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