Pasivity

Faraday’s Experiment (1840s)

Faraday’s 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

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).

Galvanostatic Polarization

Galvanostatic Polarization Curve

Potentiostatic Polarization

Potentiostatic Anodic Polarization Curve

Flade Potential

Flade Potential

If -φF is the potential for the reaction then

Where φFo

is the Flade potential at pH = 0. This equation is valid for Fe, Ni, Cr and alloys of Fe

pHoFF 059.0

Flade Potential and Stability of Passive Film Stability of passivity is related to the Flade potential

The lower the φFo, 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

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.

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

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.

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.