Download - MM454_Kinetics 80-116
-
8/18/2019 MM454_Kinetics 80-116
1/37
-
8/18/2019 MM454_Kinetics 80-116
2/37
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
1 10
-0.30
-0.25
-0.20
-0.15
-0.10
-0.05
0.00
E H
+ / H 2 ,
V
Log [H+], M
])([2
059.02/
H Log E H H
-
8/18/2019 MM454_Kinetics 80-116
3/37
If the cathodic reagent at the corrosion site (e.g., dissolved O2 in
the O2 reduction) is in short supply, mass transfer of the reagent
can become rate limiting.
e l e c t r o d e
Csurf = Cbulk
distant from electrode, x
under
activation control
no concentration
gradient
-
8/18/2019 MM454_Kinetics 80-116
4/37
conc. – distance profile at electrodes (mass transfer control)
e l e c t r o d e
Csurf
Cbulk
@anode
diffusion layer
Nernst layer
ordiffusion layer
diffusion layer thickness (δ)10 – 300 µm
can be changed by stirring
& changing bulk conc. e l e c t r o d e
Csurf =0
Cbulk
@cathodediffusion layer
Csurf ≠0
Csurf = conc. near
electrode surfacedistant from electrode, x
-
8/18/2019 MM454_Kinetics 80-116
5/37
Concentration profile builds up as redox process is
faster than the diffusion of oxidant and reducing species
This cause
@ anode
1. local saturation of metal ion at anode, which leads to
precipitation of corrosion products and salt films,which slow down the metal dissolution rate and hence
the anodic current density
@ cathode
2. depletion of oxidizing species (e.g. O2) at cathodewhich slow down the reduction rate at cathode and
hence the cathodic current density decreases
hence a limiting current density is seen at high η
-
8/18/2019 MM454_Kinetics 80-116
6/37
log i
E or η
log iL
Ecorr
ηa
ηaηc
ηaηc∞
1
23
i L
@ cathode iL = limiting current densityor limiting diffusion current
-
8/18/2019 MM454_Kinetics 80-116
7/37
iL = limiting current densityor limiting diffusion current
log i
E or η
log iL
Ecorr
ηa
ηaηc
ηa ηc∞
1
23
i L
@ anode
Tafel eqn line
-
8/18/2019 MM454_Kinetics 80-116
8/37
@point – 1
for a overpotential there is no limitation of reagent
(e.g. O2
) supply or accumulation of cations
Csurf = Cbulkcurrent remains in Tafel region & reaction is activation
controlled (only charge transfer controls the rate)
@point – 2
for a large overpotential there is short supply of reagent
(e.g. O2) or accumulation of cations
Csurf < Cbulkcurrent less than that expected on the basis of
activation control
Rate is partly activation & partly diffusion controlled
ηtotal = ηa + ηc
-
8/18/2019 MM454_Kinetics 80-116
9/37
-
8/18/2019 MM454_Kinetics 80-116
10/37
under limiting condition, Csurf = 0
bulk L C nFD
ii
0
max
C bulk = concentration of the solution you are using
concentration polarization means that the current densitycannot increase beyond i L no matter how large is the overvoltage
L
cii
nF RT 1ln
expression for
concentration
overpotential, ηc
at very large overpotential, i = i L, ηc ∞
-
8/18/2019 MM454_Kinetics 80-116
11/37
cathode reaction is under concentration polarization
-
8/18/2019 MM454_Kinetics 80-116
12/37
Effect of increasing mass transport rate
(e.g., by stirring the solution surrounding a corroding surface)
increasing mass transport decreases concentration overpotentialEcorr shifts noble direction (i.e. increases)
shows a cathodic reacn
.under mass transport
mass transport increases
from pt.1 to pt.3
ηc,1> ηc,2>ηc,3
If anodic reaction were mass-transfer controlled (difficulty of metal
ions diffusing away), improved stirring would decrease E corr . HOW?
-
8/18/2019 MM454_Kinetics 80-116
13/37
Ohmic drop or resistance polarization
due to a voltage drop (iR ) in the solution results from the
passage of current (i ) through the resistive solution (R )
d iiRV
For a planar electrode with a current density I, the voltage
drop ΔV is given by
R = electrolyte resistance (ohm)
κ = the specific conductivity of the electrolyte (ohm-1 cm-1)d = the distance from the electrode surface (cm)
d Rwhere ,
Example, for i = 1 A /cm2 and κ = 22 Ω−1cm−1 for an electrolyte one
obtains for a planar electrode at a distance d = 0.5 cm, a value of
ΔV = 0.023 V = 23 mV (what will happen in i becomes 10A /cm2 ??)
-
8/18/2019 MM454_Kinetics 80-116
14/37
i 1
i 2i 3
R 1
R 2
R 3E c
E a
i R1 < R2 < R3
i1 > i2 > i3
ohmic polarization effect in galvanic couples
as Rsolution increases the driving force for
corrosion decreases
-
8/18/2019 MM454_Kinetics 80-116
15/37
Ohmic drop or resistance polarization depends on
1. Electrolyte conductivity2. Distance :
between two corrosion sites
between electrode and reference electrode
3. Current density
-
8/18/2019 MM454_Kinetics 80-116
16/37
Total polarization iRconact T
iRi
i
nF
RT
i
ib
Lcorr
cccathodeT
1lnlog,
-
8/18/2019 MM454_Kinetics 80-116
17/37
Passivity
Our modern industrial society is built upon the reactive metals
How is it possible ?
reactive metals (Fe, Ni, Cr, Al, Ti, Zr, etc) exhibit extraordinary
kinetic stability in oxidizing environments
How?
-
8/18/2019 MM454_Kinetics 80-116
18/37
Passivity
Passivity is a state of low corrosion rate due to the presence
of a thin surface oxide film, formed naturally or under a highanodic (+ve) potential (or high oxidizing condition)
passive film can form naturally and also by applying highanodic potential
passive metals are thermodynamically unstable, but their
low corrosion is due to oxide film formation
e.g. Fe, Cr, Ni, Mn, Al, Ti, W etc Au, Ag, Pt are passive but not due to formation of passive oxide
layer – they are thermodynamically stable
thickness of passive layer is in range of 1 – 10 nm
-
8/18/2019 MM454_Kinetics 80-116
19/37
Fe reaction in nitric acid
Passivity [ Faraday's paradox ]
conc. HNO3
no corrosion
dilute HNO3
freely corrodes
with H2 evolution
conc. HNO3
corrosion if scratched
-
8/18/2019 MM454_Kinetics 80-116
20/37
Fe react in nitric acid
Passivity
Faraday's paradox
D. D. Macdonald, Pure Appl. Chem., Vol. 71, No. 6, pp. 951-978, 1999
-
8/18/2019 MM454_Kinetics 80-116
21/37
C. Wagner: Corrosion Science 5, 751 (1965)
active vs passive metals
active metal
active – passive metal
-
8/18/2019 MM454_Kinetics 80-116
22/37
Polarization of passive metals
DA
B
C
F H J
IGE
Epass
active
active-
passive
transition transpassivepassive
i crit
i p
l o g i
E
oxide filmcontinue to grow
P l i ti f i t l
-
8/18/2019 MM454_Kinetics 80-116
23/37
Polarization of passive metals
AB : low anodic potential, active region; metal corrodes with current following
Tafel behavior (rise exponential)
@ B: corrosion current density reach maximum, icrit – critical passivation current
density; passivation starts. Potential at that point is called passivation potential (Epass)
BC: active – passive transition, passive film continues to form and metal dissolution is
gradually decreasing
@ C : passivation complete, current assumes a low value and potential at that point is
called Flade potential
CD : passive region – increasing potential doesn’t change the current andcurrent density at this region called passivation current (ip) density. One can use this
current density to calculate depth of passivation
@ D /F/H : transpassive region; current starts to increase; current rise can be due to
pitting of passive layer by oxidising agent, oxidative dissolution of
passive layer or O2 evolution.DE : oxide film brakedown due to pitting in presence of aggressive ions (Cl-); current
rises as anodic potential increases since pits grow and more pits form
FG : oxidative dissolution of oxide film to produce a soluble anion, and passivity fails
HI : the increasing rate of oxygen evolution (can only occur if the oxide film is an
electron conductor); Passivity here is not disturbed
-
8/18/2019 MM454_Kinetics 80-116
24/37
when passivity is attended:
1. Corrosion potential of metals changes in the +ve direction2. The corrosion rates of metals in the passive state
decreases drastically, usually a diminution of ∼ 1 million
times
3. Passivity is a metastable state, i.e. it is disturb by means
of scratch, grinding, or changes to the outer conditions:
temperature, concentration of aggressive variables, and
agitation.
a high current density may be required to cause passivation
(> icrit), a small current density (ip) is required to maintain it
-
8/18/2019 MM454_Kinetics 80-116
25/37
criteria of passivation
ia > icrit | low Epass | Epass < Ecorr < Etrans
321
log icorr,1
log icorr,2
log icorr,3
Ecorr,1Ecorr,2 Ecorr,3Epass
log ia > log icrit
1,2 & 3 are three different cathodic reactions with increasing power of oxidation or
same oxidizing agent with increasing conc. (hence increasing strength)
1 < 2 < 3
icorr,3 = ipass
-
8/18/2019 MM454_Kinetics 80-116
26/37
which alloy would you select?
alloy1 alloy2
cathode recn.1log I
E
cathode recn.2
-
8/18/2019 MM454_Kinetics 80-116
27/37
effect of aeration
log I
E
increasing
aeration
-
8/18/2019 MM454_Kinetics 80-116
28/37
Fe corrosion in aerated water
- pH dependence
below pH 4
H2 evolution
pH 4 – 10
limited by O2 diffusion
oxide film soluble,not protective
above pH 10
protectiveoxide film formation
NACE
-
8/18/2019 MM454_Kinetics 80-116
29/37
diffusion control effect
log I
E
increasing
agitation
-
8/18/2019 MM454_Kinetics 80-116
30/37
Elements forming passive oxide film
-
8/18/2019 MM454_Kinetics 80-116
31/37
Passivity of Fe & Ti in acid
Fe: icrit for Fe of the order of 1 A/cm2
Ti: very small critical current densities (icrit lower than 1 µA/cm2)
even in very acidic condition (extremely stable passivation)
Fe Ti
-
8/18/2019 MM454_Kinetics 80-116
32/37
Iron passivation in an acetic acid/ sodium acetate
solution – Effect of water
No formation of oxide films
an absence of water
Oxygen reduction is not
sufficient to form a passive
film
-
8/18/2019 MM454_Kinetics 80-116
33/37
Why stainless steel with Cr ≥ 12 wt.% passivates easily
Revie & Uhlig, pg-101
i c r i t
polarization diagram of Fe – Cr alloys
in O2 saturated 3% Na2SO4 solution
Icrit of Fe – Cr alloys in deaerated
3% Na2SO4 solution
P i ti f C Ni St l
-
8/18/2019 MM454_Kinetics 80-116
34/37
Passivation of Cr – Ni Steels
Type 304 SS 18% Cr & 8% Ni
-
8/18/2019 MM454_Kinetics 80-116
35/37
effect of different alloying element on passive
behavior of steel
E
log I
ThyssenKrupp Fortinox S.A.
ic
ff t f t t & Cl i b h i f t l
-
8/18/2019 MM454_Kinetics 80-116
36/37
effect of temperature & Cl- conc. on passive behavior of steel
-
8/18/2019 MM454_Kinetics 80-116
37/37
How to passivate metals and alloys:
1. Anodization: e.g. Al
2. Immersion of metals in specific media:
e.g Fe in aqueous solutions of HNO3 (>70%), H2SO4(>96%), Na2SO4 (1 M)
3. presence of oxidizers : O2 in soln, P2O84-, NO2-, WO42-
, CrO4-
4. addition of noble metals: Pd, Pt, Rh, Ir to Cr or Ti