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| Corrosion & Environmental Degradation |
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An
example of the type of reaction show above is the corrosion of zinc
in an oxygen- free acidic environment containing H+ ions.
· The zinc atoms will be oxidized at the anode in the reaction: Zn -> Zn 2+ + 2e- · At the cathode, the hydrogen ions of the acidic electrolyte will be reduced to hydrogen gas: 2H+ + 2e- -> H2 (gas) · The overall corrosion reaction will be the sum of these two processes, provided no other reduction reactions occur. Zn + 2H+ -> Zn 2+ + H2 (gas) · In this reaction, the products of the corrosion process do not remain on the surface of the metal and corrosion will continue as long as the conditions are maintained. · For Iron corroding in oxygenated water (rusting), the corrosion products may remain on the surface of the metal and change the corrosion rate as the reaction continues. The overall rusting reaction has two steps. First, iron is oxidized to the 2+ state in the reaction: Fe + O + H2O -> Fe2+ + 2 OH _ -> Fe(OH)2 · In the second step, the iron changes from a 2+ ion to a 3+ ion, generating an insoluble compound that remains on the surface of the metal. 2Fe(OH)2 + O + H2O -> 2Fe(OH)3 The rust is non-metallic and interferes with further charge transfer from the metal. |
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