30 Corrosion - 530.352 Materials Selection Lecture #29 -...

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530.352 Materials Selection Lecture #29 - Corrosion Tuesday November 22 nd , 2005
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Wet Corrosion Unless special precautions are taken, the life of most structures, from bicycles to bridges, buckets to battleships, is limited by wet corrosion The annual bill in the UK for replacing corroded components or preventing corrosion is ~ $6,000,000,000 .
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Example of wet corrosion Before After
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Wet Corrosion Iron Aerated Water Fe -> Fe ++ Fe -> Fe ++ O 2 + 2H 2 O +4 e - Fe(OH) 2 -> 4 OH -
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Wet Much Faster Than Dry: Hydrated oxides form away from or loosely deposit, giving little or no protection. M++ and OH- diffuse very fast in the liquid state. In conducting materials the electrons also move very fast.
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Measuring Driving Forces: Dry : Energy of formation Wet : Voltage scale related to the flow of electrons much easier to measure
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Cathodes and Anodes : + + + + + + - - - - - - O 2 + 2H 2 O 2 e - -> 4 OH - Fe ++ e - +.4 V -.4 V Cathode Anode
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Mg Al Ti Zn Cr Fe Cd Co Ni
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This note was uploaded on 04/08/2008 for the course MECHENG 352 taught by Professor Hemker during the Spring '08 term at Johns Hopkins.

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30 Corrosion - 530.352 Materials Selection Lecture #29 -...

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