Chapter 16 Corrosion and Degradation of Materials

Chapter 16 Corrosion and Degradation of Materials - Chapter...

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Chapter 16 Corrosion and Degradation of Materials Intro In metals actual material is loss either by dissolution/corrosion or by formation of nonmetallic scale or film (oxidation) Ceramic materials are relatively resistant to deterioration o Only occurs at elevated temperatures or in rather extreme environments For polymers mechanisms and consequences differ from those for metals and ceramics (degradation) o Polymers may dissolve when exposed to liquid solvent or they may absorb the solvent and swell o Electromagnetic radiation and heat may cause alterations in their molecular structures Corrosion = the destructive and unintentional attack of a metal; it is electrochemical and ordinarily begins at the surface Electrochemical Considerations Oxidation = metal atoms characteristically lose or give up electrons also known as anodic reaction o M M n+ + ne - o M becomes a positively charged ion and in the process loses its n valence electrons o Fe Fe 2+ + 2e - Anode = site at which oxidation takes place Reduction = electrons generated from each metal atom that is oxidized must be transferred to and become a part of another chemical species o For example some metals undergo corrosion in acid solutions which have a high concentration of hydrogen ions; H + ions reduced as follows: 2H + + 2e - H 2 o Redux reaction in an acid solution containing dissolved oxygen O 2 + 4H + + 4e - 2H 2 O o Redux reaction in a neutral or basic solution containing dissolved oxygen O 2 + 2H 2 O + 4e - 4(OH - ) o Redux of a multivalent metal ion to a lower valence state M n+ + e - M (n-1)+ o Redux of a metal ion to its electrically neutral atom M n+ + ne - M Cathode = location at which reduction occurs Half-reactions = an individual oxidation or reduction reaction A complete reaction = oxidation + reduction reaction o Zn Zn 2+ + 2e - oxidation o 2H + + 2e - H 2 reduction o Zn + 2H + Zn 2+ + H 2 complete reaction Electrode Potentials o Galvanic couple = two metals electrically connected in a liquid electrolyte
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o Standard half-cell = an electrochemical cell consisting of a pure metal immersed in a 1M aqueous solution of its ions, which is electrically coupled to the standard hydrogen electode Standard emf Series o Emf series = a ranking of metallic elements according to their standard electrochemical cell potentials o Electrochemical cell potential ΔV 0 =V 2 0 – V 1 0 Overall cell potential = reduction potential – oxidation potential Influence of Concentration and Temperature on Cell Potential o Nernst equation electrochemical cell potential for two half-cells that are electrically coupled and for which solution ion concentrations are other than 1 M ΔV=( V 2 0 – V 1 0 ) – RT/(nF)*ln([M 1 n+ ]/[M 2 n+ ] F = Faraday’s constant; 96500 n = number of electrons Simplified form at room temp: ΔV=( V 2 0 – V 1 0 ) – (.0592/n)*log([M 1 n+ ]/ [M 2 n+ ] Galvanic Series o Galvanic series = a ranking of metals and alloys as to their relative
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This note was uploaded on 01/02/2012 for the course ENME enme382 taught by Professor Bruck during the Spring '10 term at Maryland.

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Chapter 16 Corrosion and Degradation of Materials - Chapter...

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