Passivity
Passivity is defined as a condition of corrosion resistance due to formation of thin surface film
under oxidizing conditions , some metals and alloys having simple barrier films with reduced
corrosion of active potential .For example in presence of concentrated “fuming” nitric acid ,
iron is virtually inert despite the highly oxidizing conditions of the solution when the acid is
diluted with water the iron remains inert, while initially corrodes vigorously evolving brown ,
nitrous oxide gas when the surface is lightly scratched .Passivity, is displayed by chromium,
Aluminum, iron ( in some environment) nickel, titanium, and many of their alloys. It is felt that
this passive behavior results from the formation of a highly adherent and very thin oxide film on
the metal surface, which serves as a protective barrier to further corrosion. Stainless steels
(iron alloy) are highly resistant to corrosion in a rather wide variety of atmospheres as a result
of passivation .Chromium is noted for formation of very stabile, thin resistant surface in less
oxidizing conditions when alloys with other metals especially iron as shown in stainless steel
which is having minimum 12% Cr and it is passive in most aerated solutions.
Note :usual corrosion conditions are not sufficiently oxidizing to form passive state on iron.
Active passive behavior
Transition metal such as Fe, Cr , Ni , Al ,and Ti demonstrate an active –passive behavior in
aqueous solutions such metals called active-passive metals and they exhibit S-shaped
polarization curves see figure 1
At low potentials, corrosion rates measured by anodic current density are high and increase
further with potential in active state above the primary passive potential E pp. The passive film
becomes stable and corrosion rate falls to very low value , In passive state the reduced
corrosion rate in the passive state may be as much as 10 6 times lower than the maximum in
active state at
icritical
.
Definition of Important Parameters for Active-passive Metals
following are the definition of important terms related to potential and current as shown in
active-passive polarization diagrams
1- Equilibrium potential (Eeq or EM/M+):The potential of an electrode in an electrolyte when the
forward rate of reaction is balanced by the rate of reverse reaction (M+z +2e- = M) .It can be
defined only with respect to a specific electrochemical reaction .This is also written as E ° and
must not be confused with Ecorr
2- Passive Potential ( E passive) the potential of an electrode where a change from an active to a
passive state occurs
3- Flade Potential (EF) the potential at which a metal changes from a passive state to an active
state
4-Transpassive Potential (ETranspassive) the potential corresponding to the end of passive region
which corresponds to the initial point of anodic evolution of Oxygen >This may correspond
either to breakdown ( electrolysis) voltage of water or to the pitting potential
5- Critical current Density (i critical ) The maximum current density observed in active region for
metal or alloy that exhibits an active –Passive behavior
6- Passive current density (i p) The minimum current density required to maintain the thickness
of film in the passive range
7- Pitting Potential (Ep) It is the potential at which there is a sudden increase in the current
density due to breakdown of passive film on the metal surface in the anodic region.
Figure 2 illustrates how a metal can experience both active and passive behavior depending on
the corrosion environment. Included in this figure is the S-shaped oxidation polarization curve
for an active–passive metal M and, in addition, reduction polarization curves for two different
solutions, which are labeled 1 and 2. Curve 1 intersects the oxidation polarization curve in the
active region at point A, yielding a corrosion current density (A). The intersection of curve 2 at
point B is in the passive region and at current density (B). The corrosion rate of metal M in
solution 1 is greater than in solution 2 since (A) is greater than (B) and rate is proportional to
current density according to below equation
r
i
zF
This difference in corrosion rate between the two solutions may be significant—several
orders of magnitude—when one considers that the current density scale in Figure 2
is scaled logarithmically.
The passive oxide film is a solid interfacial oxide compound that protects the metal against further
oxidation and ranges from to
in thickness. A metal that exhibits passivity is thermodynamically unstable within a potential range
independent or nearly independent of current or current density. This means that the metal is unstable
in the passive state since a slight disturbance may increase the passive potential to or above the pitting
potential causing film breakdown.
KINETICS OF PASSIVATION
Assume a defect-free single crystal and a mechanism of oxide film growth by vacancy migration. Thus,
the rate of film formation for a single crystal, related to Faraday’s law, can be approximated as
𝑑𝑥
𝑖𝑀
=
𝑑𝑡 𝑧𝐹𝜌
where
x= Film thickness (cm)
𝑑𝑥
= Rate of film formation (cm/s)
𝑑𝑡
ip = Passive current density (A/cm2 )
z= Valence
F = 96,500 C/mol (= A.s/mol)
𝜌 = Density of metal (g/cm3)
Note that eq. (6.1) mathematically