lecture 27 - ME 330 Engineering Materials Lecture 27...

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Lecture 27 Corrosion & Oxidation Stephen D. Downing Mechanical Science and Engineering © 2001 - 2010 University of Illinois Board of Trustees, All Rights Reserved ME 330 Engineering Materials
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ME 330 - Lecture 27 © 2010 Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 1 of 50 Environmental Degradation ± Metals ± Corrosion ± Oxidation ± Electrochemical reactions ± Polymers ± Chemical reactions ± Ultraviolet light ± Physiochemical reactions ± Ceramics ± Oxidation
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ME 330 - Lecture 27 © 2010 Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 2 of 50 Electrochemical Cell Corrosion requires an electrical circuit, involves flow of current via free electrons Any two metals will cause a current to flow due to relative tendencies to ionize Fe 2+ Fe Cu 2+ Cu e - e - 1M solution of Cu 2+ 1M solution of Fe 2+ Anode: Oxidation reaction Corrosion Cathode: Reduction reaction Electrodeposition Voltage or Current + - Semi-porous barrier keeps solutions separate
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ME 330 - Lecture 27 © 2010 Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 3 of 50 Reactions ± Electrochemical : chemical reaction in which there is a transfer of electrons ± Anode (oxidation): M M n+ + ne - ± Cathode (reduction): ± Metal ion reduction: M n+ + e - M (n-1)+ ± Metal deposition: M n+ + ne - M ± Hydrogen Evolution: 2H + + 2e - H 2 gas ± Oxygen Evolution: ± Acidic solutions: O 2 + 4H + + 4e - 2H 2 O ± Basic solutions: O 2 + 2H 2 O+ 4e - 4(OH) -
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ME 330 - Lecture 27 © 2010 Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 4 of 50 Rust Iron rust Fe 2+ H 2 O e - ± Different parts of steel surface serve as anode and cathode ± Anode: Fe Fe 2+ + 2e - ± Cathode: O 2 + 2H 2 O+ 4e - 4(OH) - ± 2Fe + 2H 2 O + O 2 2Fe 2+ + 4(OH) - 2Fe(OH) 2 ± Ferrous hydroxide is unstable ± 4Fe(OH) 2 + 2H 2 O+ O 2 4Fe(OH) 3 (rust) water
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ME 330 - Lecture 27 © 2010 Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 5 of 50 Standard EMF Series ± Want to rank materials in terms of corrosion potentials ± Compare a metal to a “standard reference cell” ± Platinum electrode (relatively inert) ± 1 M solution of hydrogen ± Acts as cathode
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ME 330 - Lecture 27 © 2010 Stephen Downing, University of Illinois at Urbana-Champaign, All Rights Reserved 6 of 50 Electromotive Force (EMF) Series Au 3+ + 3e - Au +1.420V Pt 2+ + 2e - Pt +1.200V Ag + + e - Ag +0.800V Cu 2+ + 2e - Cu +0.340V 2H + + 2e - H 2 +0.000V Pb 2+ + 2e - Pb -0.126V Ni 2+ + 2e - Ni -0.250V Fe 2+ + 2e - Fe -0.440V Cr 3+ + 3e - Cr -0.744V Al 3+ + 3e - Al -1.662 V Mg 2+ + 2e - Mg -2.363V K + + e - K -2.924V Electromotive force series: More inert (cathodic) More active (anodic) ± When two materials form a corrosion cell ± More positive will plate ± More negative will corrode ± Voltage driving corrosion: ± Generated under highly idealized conditions ± But potential also depends on electrolytic solution ± Salt increases ions in solution and conductivity M 1 M 1 n+ +ne - M 2 n+ +ne - M 2 M 2 n+ + M 1 M 2 +M 1 n+ Δ V= V 2 -V 1 = V red -V ox Zn 2+ + 2e - Zn -0.763V
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ME 330 - Lecture 27
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lecture 27 - ME 330 Engineering Materials Lecture 27...

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