Pasivity

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Pasivity
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Faradays Experiment (1840s)
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Faradays Experiment (Contd..)

• We can have some observations from this set of
  experiments
• Corrosion of a metal, showing active-passive
  behavior, in the passive state is very low
• In the active state, the corrosion of the same
  metal would be 104 to 106 times more
• The passive state may not be always stable. The
  unstable state of passivity in the above
  experiments is demonstrated by the effect of
  scratching of passive iron in the same
  environment
• We should be able to use this passivation
  phenomenon in different metal/alloy-environment
  situations but ought to be very careful about the
  unstable nature of the phenomenon
• Because of the prospect of important engineering
  applications, passivity has been studied and
  researched extensively since its first
  demonstration

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Definition of Passivity

• Two types of passivity have been defined by Uhlig
  and Revie
• Type 1 "A metal is passive if it substantially
  resists corrosion in a given environment
  resulting from marked anodic polarization" (low
  corrosion rate, noble potential).
• Type 2"A metal is passive if it substantially
  resists corrosion in a given environment despite
  a marked thermodynamic tendency to react" (low
  corrosion rate, active potential).

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Galvanostatic Polarization
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Galvanostatic Polarization Curve
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Potentiostatic Polarization
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Potentiostatic Anodic Polarization Curve
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Flade Potential
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Flade Potential

• If -fF is the potential for the reaction then
• Where fFo is the Flade potential at pH 0. This
  equation is valid for Fe, Ni, Cr and alloys of Fe

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Flade Potential and Stability of Passive Film

• Stability of passivity is related to the Flade
  potential
• The lower the fFo, easier it is for passivation
  to occur and bigger is the stability of the
  passive film formed
• For Cr-Fe alloys, value ranges from 0.63V for
  pure iron (Cr-0) to 0.10 V for 25 Cr. Thus
  increasing Cr content increases the stability of
  passivation

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Passivators

• It is interesting to note that the same Flade
  potential is reached whether Fe is passivated by
  anodic polarization in H2SO4 or passivated by
  immersion in Conc. HNO3
• Fe can be passivated in solutions of chromates
  (CrO4 ), nitrites (NO2-), molybdates (MoO4),
  tungstates (WO4 ), etc. These inorganic
  oxidizing agents hence are called passivators.
• Passivators act as anodic inhibitors. They cause
  corrosion of the metal to shift in the noble
  direction. They themselves get reduced at the
  anodic sites on the metal surface producing
  current density necessary for passivation.

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Theories of Passivity Oxide-Film Theory

• This theory holds that the initial corrosion
  product e.g. a metal oxide provides the diffusion
  barrier thus reducing corrosion
• The oxide layer virtually separates the metal
  from the surrounding environment
• Effectiveness of this barrier in reducing
  corrosion depends on the nature and the
  properties of the protective film
• A visible PbSO4 film on Pb exposed to H2SO4 and
  FeF2 film on steel immersed in aqueous HF are two
  examples of such protective film
• Films formed on Cr or stainless steels by anodic
  polarization are too thin and invisible

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Theories of Passivity Adsorption Theory

• According to this theory, passivity is achieved
  by a chemisorbed film of O2 or other passivating
  agents
• This layer displaces the adsorbed H2O molecules
  from the metal surface and prevents anodic
  dissolution by hydration of metal ions. The
  adsorbed O2 decreases io and increases anodic
  polarization (overvoltage) for the anodic
  reaction
• Mo ? M 2e-
• Some authors do point out that the oxide-film
  theory and the adsorption theory are not
  contradictions, rather they supplement each
  other. The adsorbed film while getting thicker
  gradually changes to an oxide film. Thus these
  authors mention a combined oxide-film adsorption
  theory of passivity.

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Passivity and Chloride Ions

• Chloride ions and to a lesser degree other
  halogen ions break down passivity or prevent
  passivation in Fe, Cr, Ni, Co and stainless
  steels
• According to the oxide-film theory, Cl- ions
  penetrate the oxide film through pores or
  discontinuities. Chloride ions may also
  colloidally disperse the oxide film thus
  increasing its permeability.
• According to the adsorption theory, chloride ions
  adsorb on the metal surface faster than dissolved
  O2 or OH-. While in contact with the metal
  surface, Cl- ions favour hydration of metal ions
  and help the metal ions go into solution. Whereas
  adsorbed O2 decreases the rate of metal
  dissolution.
• Thus adsorbed Cl- ions increase io, decrease
  overvoltage for anodic dissolution of the metal.
  This is so effective, that iron and the stainless
  steels are not passivated in aqueous environments
  containing appreciable amount of Cl- ions.
• Breakdown of metal passivity by chloride ions is
  local and hence leads to pitting type of attack.