Passivity
Passivity is a loss of electrochemical reactivity (drastic decrease in corrosion rate)
that many engineering alloys (e.g. stainless steel, Ni-based alloys, Al alloys)
exhibit under certain environmental conditions.
Passivation usually is the result of the presence of a thin protective oxide or oxyhydroxide passive film on the metal surface.
However, passive metals are susceptible to local breakdown and accelerated
localized attack.
barrier layer
deposit or porous layer
barrier layer
Passive films are a 3-dimensional oxide or oxyhydroxide, usually nm in
thickness, that acts as a barrier between the metal and the electrolyte.
Could be a bilayer structure with a porous or hydrated deposit layer on top of
barrier layer.
1
Passivity
Passive Oxide Layer on Al Thin Film
Sample was 150-nm thick Al film on quartz
substrate (Q).
Pit in thin film caused undermining of passive
film (P) at pit wall (W) - undermined passive film
is now lying on substrate surface.
Historical perspective on passivity - in 1836 Michael Faraday described the behavior
of Fe in nitric acid:
Add
Concentrated
HNO3
2
Passivity
Metal
Passive
Film
O
M
O
M
M
O
OH2
OH
O
O
OH2
OH
O
Solution
Composition and thickness of the
passive film are functions of potential
and solution composition.
OH2
OH
For alloys, usually one element is
enriched in the film (films on Fe-Cr
alloys are enriched in Cr).
Passive films can be either crystalline or
amorphous.
Films can be either insulators (e.g., Al,
Ti, and Ta) or semiconductors (e.g., Fe
and Ni).
3
Under most conditions, iron is
not very corrosion resistant.
Alloying with >12% Cr to
make stainless steel greatly
improves corrosion resistance
owing to the formation of
protective Cr-rich passive film:
Corrosion Rate, cm/y
Stainless Steel
Fe-Cr alloys
in
intermittent
water spray
H.H. Uhlig, Corrosion
and Corrosion Control.
Wt. % Cr
• Fe-based alloy with >12% Cr.
• Addition of >8% Ni increases corrosion resistance yet more and stabilizes the
fcc austenitic phase. 304 SS has about 18% Cr and 8% Ni.
• Further addition to 18-8 SS of about 2% Mo (316 SS) increases corrosion
resistance yet further.
• Other common alloying elements include N, W, Ti, Nb, ….
4
Passivity
Distinctive potential-current behavior of a passive metal:
Etrans -
ipass -
passive current density
Epp -
primary passivation potential
icrit -
critical current density
Etrans -
transpassive potential
Jones
• At the active-passive transition, the current density can decrease by many orders of
magnitude.
• The current density in the passive region, ipass, is often relatively independent of potential.
• Passive films may break down at the very high potentials, allowing high currents to pass
again. This is called the transpassive region.
• Transpassive current may be associated with oxygen evolution or dissolution - it is different
from currents associated with pitting.
5
Passivity
For certain systems, the critical potential values observed in
measured polarization curves may relate to boundaries in the
related Pourbaix diagram:
E
Transpassive
ipass
Etrans
Passive
icrit
i0H2/H+
Epp
ErevH2H+
Active
Ecorr
Cathodic
ErevM/M+
i0M/M+
icorr
log i
6
Passivity
Pourbaix diagrams are useful as guides in suggesting regions of passivity.
This shows that the range of passivity for stainless steels is increased over
that of Fe because of the influence of the added Cr.
Jones
However, Pourbaix diagrams are based on thermodynamics, and cannot be
used to predict behavior. For instance, Ni and Cr are passive in acid despite
thermodynamic predictions to the contrary because their oxides dissolve
very slowly.
7
Passivity
The measured polarization curve can take different forms depending on the
relative positions of the anodic and cathodic half reactions:
Jones
Active-passive
1,2 left
Spontaneously passive
Unstable passivity
4-6 left
3 left
4-6 right
1-3 right
8
Passivity
Faraday experiment explained
Nitric Acid is an oxidizer:
NO3- + 4H+ + 3e- = NO +2H2O
E0 = 0.96 V SHE
E
Conc. nitric
Dilute nitric
HER
log i
9
Passivity
Alloys with different electrochemical behavior would be preferred for
environments with different oxidizing power:
Jones
10
Kinetics of Passivity
Metal ions oxidized at a fresh metal surface can:
• Cross double layer as solvated ion  corrosion
• Form new solid phase:
• Deposit from solution of poorly-soluble ion to form nonprotective film
• Direct formation of film on surface without metal ions passing into solution
 passivity
First monolayer of passive film:
Formation of NiOHad intermediate:
Ni + H2O  NiOHad + H+ + e-
(1)
can be followed by either corrosion or oxide formation:
NiOHad + H+  Ni2+ + H2O + e-
(2)
NiOHad  NiO + H+ + e-
(3)
Whether Eqn. 2 or 3 proceeds will depend on E, pH, T, and solution
composition.
11
Kinetics of Passivity
After first monolayer, the rate of further oxidation slows, but does not stop. Thickening
occurs by migration of metal cations, oxide anions, or their vacancies under the influence of
the electric field (potential drop divided by film thickness, E/x). This leads to a reduction in
the field, and a further reduction in the rate of growth.
Steady state is when the rate of oxide growth = rate of dissolution = passive current density.
The current at a constant potential, or rate of passive film thickening, typically decreases
linearly in a log i/log t plot with a slope of about -1:
Jones
ipass = C (dx/dt) = C’/t
(4)
Integration yields the direct logarithmic growth law:
x = A + B log t
(5)
12
Kinetics of Passivity
At a constant potential, each decade of time and current density is seen to be
accompanied by an equal amount of film growth.
However, passive film growth data also fit inverse logarithmic kinetics:
1/x = A’ - B’ log t
(6)
Fig 8 Kruger and Calvert JES
114 43 (1967
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Theories of Passive Film Growth
Ions moving through the oxide must overcome an activation energy G* to jump distance
2a. At equilibrium, the rate in the forward direction equals the rate in backwards
direction:
 G *
i f  i b  nFk exp 
(7)
 RT 
where k is a rate constant.
Now apply an anodic overpotential V that is distributed linearly across the oxide of
thickness x resulting in an electric field of V/x.
The portion of that field along the jump path is 2aV/x. Therefore, the activation energies
in the forward and reverse directions each change by aVnF/x.
 G *  aVnF/x 
 G*  aVnF/x (8)
i  i f  i b  nFk exp 
 nFk exp 




RT
RT




Let
 G *
A  nFk exp 
 RT 
i  A exp
B = anF/RT
BV 
 BV 
BV 
 A exp 
 2A sinh
 x 
 x 
 x 
(9)
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Theories of Passive Film Growth
There are two limiting approximations to Eqn 9:
1) High field, V/x is large:
i = A exp
BV  dx

 x  dt
This is the Field Assisted Ion Migration (FAIM) theory in which film growth is limited
by ion migration driven by the high field in the oxide.
Integration of this equation leads to an expression that approximates inverse logarithmic
film growth, and thus fits the data rather well.
2) Low Field, V/x is small:
i = 2ABV/x
This is basically Ohms Law, and leads to parabolic film growth:
x = c’ + c” t1/2
Since passive films are usually thin, the high field approximation usually applies,
and FAIM is one of the primary theories for passive film growth.
15
Theories of Passive Film Growth
Other theories predict direct logarithmic growth.
The Place Exchange Mechanism describes passive film growth to occur by a
simultaneous exchange of position of metal and oxygen ions in the passive film,
resulting in a net movement of metal ions outwards and oxygen ions inwards:
Fig 9 Sato and Cohen JES 111, 512
16
Theories of Passive Film Growth
Other theories predict direct logarithmic growth.
The Point Defect Model for
passive film growth and
breakdown (developed at OSU in
early 80s) also predicts direct
logarithmic growth. Note that
passive films are often bilayers
with an inner barrier layer that is
responsible for the protection, and
an outer porous layer that is
unprotective.
D. Macdonald JES 1992
17