Lec 18 - ENGR ENGR 4250 Advanced Materials Engineering...

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ENGR 4250 ENGR 4250 – Advanced Materials Engineering Lecture 18 CORROSION AND DEGRADATION Continued Instructor: Ghaus Rizvi
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CHAPTER 17: SUES TO ADDRESS CORROSION AND DEGRADATION ISSUES TO ADDRESS. .. Why does corrosion occur ? What metals are most likely to corrode? How do temperature and environment affect corrosion rate? How do we suppress corrosion?
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Electro-Chemistry • If a chemical reaction is driven by an external applied voltage , as in electrolysis , or if a voltage is created by a chemical reaction as in a battery , it is an electrochemical reaction.
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CORROSION OF ZINC IN ACID Two reactions are necessary: -- oxidation reaction: duction action: Zn Zn 2 + + 2 e + as) -- reduction reaction: 2H + 2 e H 2 (gas) Oxidation reaction H + Zinc Zn Zn 2+ 2e - Acid solution H + H + H + flow of e - the metal Adapted from Fig. 17.1, Callister 7e . (Fig. 17.1 is from M.G. Fontana, Corrosion Engineering , 3rd ed., McGraw-Hill Book Company, 1986.) reduction reaction H + H 2 (gas) H + H + in the metal
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In Iron e +1/2O H e 2+ 2OH - e(OH) Fe +1/2O 2 + H 2 O Æ Fe + 2OH Æ Fe(OH) 2 – Then Fe(OH) 1/2O H Fe(OH) ust) • 2Fe(OH) 2 +1/2O 2 + H 2 O Æ 2Fe(OH) 3 (Rust) • Will it rust in pure water?
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STANDARD HYDROGEN (EMF) TEST Two outcomes: -- Metal sample mass -- Metal sample mass e - e - e - H + 2e - e - H 2 ( gas ) e - e - e - M n+ ions ne latinum etal, M H + M n+ ions ne 2e + H + 25°C 1M M n+ sol’n 1M H + sol’n P m 25°C 1M M n+ sol’n 1M H + sol’n H 0 o metal < V (relative to Pt) --Metal is the cathode (+) 0 o metal > V (relative to Pt) --Metal is the anode (-) Standard Electrode Potential Adapted from Fig. 17.2, Callister 7e .
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emon Battery Lemon Battery
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CORROSION IN A GRAPEFRUIT - + Anode Cathode Cu Zn Zn 2+ H + H + 2e - oxidation reduction + 2H + + 2 e H 2 (gas) Acid H + H H + O 2 + 4H + + 4 e 2 O H + H +
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Example problem 17.1 a) Standard cell-overall equation Cd Ni b) Compute the potential if Cd & Ni ion concentrations are 0.5 and 0.001 M respectively. Is spontaneous reaction direction still the same?
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Example problem 17.1 a) The Cd electrode is oxidized Ni reduced Cd Ni Cd Æ Cd 2+ +2e - Ni 2+ +2e - Æ Ni __________________ Ni 2+ +Cd Æ Ni + Cd 2+ Δ V 0 = +V Ni 0 -V Cd 0 = +0.153V = - 0.250V – (-0.403V)
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Influence of Concentration and emperature on Cell Potential Temperature on Cell Potential Cells of pure metal but T and molarity M 1,2 n+ ] [ + n M RT
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Lec 18 - ENGR ENGR 4250 Advanced Materials Engineering...

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