Passivity 1 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 oxy- hydroxide passive film on the metal surface. However, passive metals are susceptible to local breakdown and accelerated localized attack. 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. barrier layer barrier layer deposit or porous layer
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Electrochemical Nature of Metallic Corrosion … · Passivity 1 Passivity is a loss of electrochemical reactivity (drastic decrease in corrosion rate) that many engineering alloys
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Passivity
1
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 oxy-
hydroxide passive film on the metal surface.
However, passive metals are susceptible to local breakdown and accelerated
localized attack.
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.
barrier layer barrier layer
deposit or porous layer
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.
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Passivity
Concentrated
HNO3
Add
Historical perspective on passivity - in 1836 Michael Faraday described the behavior
of Fe in nitric acid:
3
Passivity
Composition and thickness of the
passive film are functions of potential
and solution composition.
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).
Metal
O
M OH2 O
OH
O
M OH2 O
OH
O
M OH2 O
OH
Solution
Passive
Film
4
Stainless Steel
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:
Co
rrosio
n R
ate
, cm
/y
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, ….
Fe-Cr alloys
in
intermittent
water spray
H.H. Uhlig, Corrosion
and Corrosion Control.
5
Passivity Distinctive potential-current behavior of a passive metal:
ipass - passive current density
Epp - primary passivation potential
icrit - critical current density
Etrans - transpassive potential
Jones
Etrans -
• 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.
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Passivity
For certain systems, the critical potential values observed in
measured polarization curves may relate to boundaries in the
related Pourbaix diagram:
ipass
log i
Epp
Transpassive
Passive
Active
icrit
E
i0M/M+
i0H2/H+
icorr
Ecorr
Cathodic ErevM/M+
ErevH2H+
Etrans
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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.
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.
Jones
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Passivity The measured polarization curve can take different forms depending on the
relative positions of the anodic and cathodic half reactions: