CORROSION OF METALS

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CORROSION OF METALS
Corrosion is destructive and unintentional
attack of a metal by its environment.
Callister Anodes corrode. Alan J. Markworth
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CORROSION OF METALS IS AN ELECTROCHEMICAL PROCESS
Examples of anodic half-cell reactions Fe ?
Fe2 2e- Al ? Al3 4e- More
generally, M? Mn ne-
More cathodic half-cell reactions 2H 2e-
? H2 2H2O 2e- ? 2OH- H2 O2 4H
4e- ? H2 O2 2H2O 4e- ? 4OH-
Examples of cathodic half-cell reactions
Fe2 2e- ? Fe Al3 4e- ? Al More
generally, Mn ne- ? M
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EVERY HALF-CELL REACTION HAS A STANDARD POTENTIAL
V0
Standard conditions 25 C, 1 M ion
concentration, 1 atm gas pressure
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WE CAN STUDY CORROSION UNDER STANDARD CONDITIONS
Overall reaction Cu2 Fe ? Cu
Fe2 Half-cell reactions Cu2 2e- ?
Cu 0.340 V Fe ? Fe2 2e- 0.440
V overall reaction 0.780 V
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WE CAN STUDY CORROSION UNDER STANDARD CONDITIONS
Overall reaction Fe2 Zn ? Fe
Zn2 Half-cell reactions overall
reaction
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WHAT ABOUT NONSTANDARD CONDITIONS?
Nernst equation
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WHATS REALLY HAPPENING?
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CORROSION HAS BEEN CLASSIFIED INTO EIGHT FORMS (1
of 4)
Uniform attack (general corrosion) Where does it
happen? Why does it happen? What does it look
like? How can it be controlled? Galvanic
corrosion Where does it happen? Why does it
happen? What does it look like? How can it be
controlled?
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CORROSION HAS BEEN CLASSIFIED INTO EIGHT FORMS (2
of 4)
Crevice corrosion Where does it happen? Why
does it happen? What does it look like? How can
it be controlled? Pitting corrosion Where does
it happen? Why does it happen? What does it
look like? How can it be controlled?
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CORROSION HAS BEEN CLASSIFIED INTO EIGHT FORMS (3
of 4)
Intergranular corrosion Where does it
happen? Why does it happen? What does it look
like? How can it be controlled? Selective
leaching (dealloying, dezincification) Where
does it happen? Why does it happen? What does
it look like? How can it be controlled?
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CORROSION HAS BEEN CLASSIFIED INTO EIGHT FORMS (4
of 4)
Erosion-corrosion Where does it happen? Why
does it happen? What does it look like? How can
it be controlled? Stress corrosion (stress
corrosion cracking, SCC) Where does it
happen? Why does it happen? What does it look
like? How can it be controlled?
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AND THERE ARE STILL MORE FORMS OF CORROSION
Oxidation (high-temperature corrosion, dry
corrosion) Where does it happen? Why does it
happen? What does it look like? How can it be
controlled? Microbiologically influenced
corrosion (microbial corrosion, MIC) Where does
it happen? Why does it happen? What does it
look like? How can it be controlled?