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.

2

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.

6

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

7

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

8

Passivity The measured polarization curve can take different forms depending on the

relative positions of the anodic and cathodic half reactions:

Jones

Active-passive Spontaneously passive Unstable passivity

1,2 left 4-6 left

4-6 right

3 left

1-3 right

9

Passivity

E

log i

HER

Dilute nitric

Conc. nitric

Nitric Acid is an oxidizer:

NO3- + 4H+ + 3e- = NO +2H2O E0 = 0.96 V SHE

Faraday experiment explained

10

Passivity

Jones

Alloys with different electrochemical behavior would be preferred for

environments with different oxidizing power:

11

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.

12

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:

ipass = C (dx/dt) = C’/t (4)

Integration yields the direct logarithmic growth law:

x = A + B log t (5)

Jones

13

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

14

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:

(7)

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.

(8)

Let B = anF/RT

(9)

i f ib nFkexp G*

RT

i if ib nFkexp G *aVnF/x

RT

nFkexp

G* aVnF/x RT

A nFk exp G *

RT

i AexpBV

x

Aexp

BV

x

2Asinh

BV

x

15

Theories of Passive Film Growth

There are two limiting approximations to Eqn 9:

1) High field, V/x is large: i = AexpBV

x

dx

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.

16

Theories of Passive Film Growth

Other theories predict direct logarithmic growth.

Fig 9 Sato and Cohen JES 111, 512

(1964)

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:

17

Theories of Passive Film Growth

Other theories predict direct logarithmic growth.

D. Macdonald JES 1992

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.