Lecture 15 - ENGR 4250 Advanced Materials Engineering...

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ENGR 4250 – Advanced Materials Engineering Lecture 15 CORROSION AND DEGRADATION Ch 17 continued ; Callister 8/e Dr. A. Keshavarz - ENGR 4250U
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Polarizations • Activation polarization – Activation energy driven • Concentration polarization – Diffusion driven • Corrosion rates from polarization data Dr. A. Keshavarz - ENGR 4250U
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Activation polarization • Refers to the condition when the reaction rate is controlled by the one step in the series that occurs at the slowest rate – An activation energy barrier is associated with the slowest, rate-limiting step • Example: Dr. A. Keshavarz - ENGR 4250U
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H 2 on Zinc Electrode 1. Adsorption H + solution to zinc 2. Electron from zinc to form H atom H + + e - H 3. Atoms to molecule 2H H 2 4. Coalescence of molecules into bubbles Dr. A. Keshavarz - ENGR 4250U
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H 2 on Zinc Electrode Dr. A. Keshavarz - ENGR 4250U
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Over-voltage: Activation polarization β is constant for particular cell i o is exchange current density – In one half cell: – r red =r oxid = i o /nf o a i i log η ± = Dr. A. Keshavarz - ENGR 4250U
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Electrode Potentials Dr. A. Keshavarz - ENGR 4250U
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Over-voltage vs i o o a i i log β η ± = Dr. A. Keshavarz - ENGR 4250U
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Concentration Polarization Reaction rate limited by diffusion ) 1 log( 203 L c i i nf RT = η o a i i log β ± = Dr. A. Keshavarz - ENGR 4250U
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Reduction Reactions ) 1 log( 203 L c i i nf RT = η Dr. A. Keshavarz - ENGR 4250U
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Corrosion rates from polarization data Dr. A. Keshavarz - ENGR 4250U
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Two reactions are necessary: -- oxidation reaction: -- reduction reaction: Zn Zn 2 + + 2 e 2H + + 2 e H 2 (gas) Adapted from Fig. 17.1, Callister 7e . (Fig. 17.1 is from M.G. Fontana, Corrosion Engineering , 3rd ed., McGraw-Hill Book Company, 1986.) CORROSION OF ZINC IN ACID Zinc Oxidation reaction Zn Zn 2+ 2e - Acid solution reduction reaction H + H + H 2 (gas) H + H + H + H + H + flow of e - in the metal Dr. A. Keshavarz - ENGR 4250U
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Important Example Problem 17.2 Dr. A. Keshavarz - ENGR 4250U
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Passivity Normally active metals and alloys under particular environmental conditions become extremely inert Dr. A. Keshavarz - ENGR 4250U
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Environmental Effects • Fluid velocity, – Vel. corrosion, due to erosion • Temperature T corrosion rate chemical reaction • Composition concentration of corrosive species corrosion Dr. A. Keshavarz - ENGR 4250U
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Environmental Effects - 2 Cold working or plastically deforming ductile metals
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Lecture 15 - ENGR 4250 Advanced Materials Engineering...

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