11-25-Lecture - Corrosion ENGR 213 1 CORROSION deteriorative properties response to environment unexpected atmosphere aqueous soln soil acid base etc

11-25-Lecture - Corrosion ENGR 213 1 CORROSION...

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1 Corrosion ENGR 213
CORROSION deteriorative properties response to environment; unexpected atmosphere, aqueous soln, soil, acid, base, etc metals May be highly susceptible ceramics relatively resistant polymers degradation
3 Electrochemical Corrosion Most common form of attack of metals Occurs when metals lose electrons and become ions Metal gradually consumed By-product(s) formed e.g., iron rust Occurs most frequently in water or moist air
4 Corrosion “opportunity”
CORROSION OF METALS Two important chemical steps Response to thermodynamic forces Minimises energy Oxidation Anode Gives electrons Reduction Cathode Receives electrons P M P M P ne P ne M M n n n n + + + + + + - + - +
Dissimilar Metals electrode potential (V) of galvanic couple driving force for oxidation-reduction rxns measured V = difference in potential
Half-cells standard hydrogen half-cell standard electromotive force (emf) series Can combine results to find direction of reaction
Example of forward reaction 0.323V 0.440 - 2 0.763V 2 2 2 2 2 Fe Zn Fe Zn Fe e Fe e Zn Zn + + + + + + - + - +

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