Polarisation & Passivity

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Created by
Vithu Siva

Cards (12)

  • important factor that influences reactions rate/reaction kinetics of corrosion that good for corrosion and slows down corrosion
    1. Activation polarisation- high conc of reagents
    2. Concentraion polarisation- low conc of reagents
  • two types:
    activation polarisation
    concentration polarisation
  • Activation polarisation: electrochemical process controlled by reaction sequence (steps) at metal electrolyte interface
    slowest step of reaction limits whole reaction rate
  • Concentration Polarisation:
    Controlled by diffusion or mass transportation in the electrolyte.
    mass transfer=slow
    therefore limits reaction rate
    often when the concentration of reducing species is low
  • influencial concentration of polarisation:
    stir --> diffusion increases, polarisation decreases, corrosion rate increases
    temp increases, affect both conc of polarisation and activation polarisation
  • Measuring polarisation:
    corrosion behavious decides polarisation therefore measured by change of current- using potentiostat
  • E= applied potential/ driving force/oxidising power
    J= current density/corrosion rate
    E- logJ plot
  • 3 electrode cell
    1. working electrode- sample being investigated
    2. Counter electrode- created in circuit to carry the current
    3. Reference electrode- to provide stable potential against which the potential of the working electrode could be measured
  • Passivity:
    1.Normal State: E increase, corrosion rate increase
    2. Active-passive transition: E increase(oxidising power), corrosion rate decrease
    PROTECTIVE STATE
    3.Transpassive state:
    E increase to further level corrosion rate
    can no longer protect. passive state decrease
    PROTECTIVE BREAKDOWN
  • Passivation to develop resistant alloys/protection methods
  • how easy to reach passive :
    low Ep, ic
    Stability:
    wider passive state
    How low passive current:
    lower ip=lower corrosion rate
  • passivity is the phenomenon that certain metals/alloys lose their chemical reactivity under particular environmental conditions