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Explain with the Help of Diagram Wet Corrosion in Neutral Medium. - Applied Chemistry 2

Numerical

Explain with the help of diagram wet corrosion in neutral medium.

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Solution

Electrochemical corrosion occurs:
i)When conducting liquid is in contact with metal.
ii)Two dissimilar metals or alloys are dipped in electrolyte.

  • The corrosion can take place by H2 evolution mechanism or O2 absorption, mechanism.
  • The wet corrosion in neutral medium takes place by O2 absorption mechanism.

Take an example of Fe in contact with water.

  • Fe covered with oxide film acts as cathode and a crack in coating acts as an anode.
  • At room temperature the water consists of 8ppm of O2.
  • Being larger area of cathode,protected layer of Fe2+ ions.
  • At crack,anode Fe sheds e- & goes into the water as Fe2+ ions.
    Fe → Fe2++2e-

O2 in water accepts e- and OH- is formed.
`1/2`O2H2O+2e-→2OH-
Fe2++2OH- combine to form Fe(OH)2 & further precipitates
Fe(OH)2 Ferric hydroxide.
Fe2++2OH-→Fe(OH)2
Fe(OH)2+`1/2` O2H2O →2Fe(OH)2
Ferric Hydroxide

Concept: Numerical on Atom Economy
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