Mechanism of CO Corrosion Inhibitors

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Mechanism of CO
2

Corrosion
Inhibitors

Brian Kinsella

Institute for Corrosion and Multiphase Technology
(ICMT)

Russ College of Engineering and Technology

Ohio University


Srdjan Nesic, Director of ICMT

Yao Xiong (PhD Candidate) ICMT



Summary


Introduction CO
2

Corrosion Inhibitors


Thermodynamic of Inhibitor Adsorption using
Electrochemical Measurements


Inhibitor Molecular Properties from Quantum
Mechanical Calculations


Software


Relating Molecular Properties to
Thermodynamics of Adsorption


Atomic Force Microscopy (AFM) to Study
Inhibitor Films


Forces of Adhesion




3


Introduction


Surfactant
molecules are used to
prevent corrosion of carbon steel from
weak acids in oil and gas production.
Weak acids include: carbonic (CO
2
),
hydrosulfuric

(H
2
S) and organic acids, in
particular acetic acid

. Surfactant
molecules
feature a polar ‘head’ joined
to a long chain alkyl group (typically C
14

-

C
18
)


The polar head may be + or


charged.


Polar Group: Fatty acids, amines,
imidazolines, oxyalkylated amines,
oxygen, sulphur and phosphorous
(phosphate esters) containing species,
quaternary amines


Introduction

4

Hexadecylsuccinic

Anhydride

The surfactant molecule can adsorb
through coulombs type forces or by
chemisorption
. In the case of
hexadecylsuccinic

anhydride

the
molecule react with water to form a
di
-
carboxylic acid which in turn
forms an insoluble ferrous salt on
the iron surface.


Conventional Inhibitor Mechanism

Steel Surface

Surfactant Inhibitor
Molecules

Hydrophobic Film

Introduction
-

Inhibitor Mechanism

Oil
-

Hydrocarbons

Log

Inhibitor

Concentration

c

<

CMC

c

>

CMC

c

=

CMC

Oil/Water

Interfacial

Tension

(
mN
/m)


10

20

30

40

50

Critical

Micelle

[
Micro
-
emulsion
]

Concentration

(CMC)

Surfactant Corrosion Inhibitor Behaviour in Oil Water Systems


Nalco Energy Services


Spheres




Rods



Bilayer

AFM Images of Surfactant Molecules on
Mica

Similar Structures Adsorb on Steel

Images obtained using soft contact mode, Si/N cantilever spring constant 0.6
N/m,
NaCl

brine solution

300 nm

300 nm

DPC

CPC

Imidazoline

Shandelle

Bosenberg

Curtin
Univeristy

8

Thermodynamics of Inhibitor
Adsorption using Electrochemical
Measurements

Linear Polarization Corrosion
Measurements

9

Overview


The
corrosion

rate of steel
at different inhibitor
concentrations can be fitted to a
Temkin

adsorption
isotherm to determine
K
ads


and

G
ads
.


A Van’t Hoff plot, constructed from Temkin
isotherms,
i.e.
the
logarithm of inhibitor
concentrations to give 0.5


(surface coverage) at
different temperatures, is used to determine the
∆H

of adsorption.


Molecular
properties, obtained using molecular
modelling
software, is related to inhibitor
performance and the thermodynamics of
adsorption
.


10

Experimental
Equipment

11

Corrosion Rate Measurements


Linear polarisation
measurements
-

mild
steel electrode polarised
±

10 mV of the corrosion
potential using a voltage
scan of 0.1 mV/s.



Slope of the resulting
i/E curve gives the
polarisation resistance
Rp.

12

Calculation of Corrosion Rate


Using Rp, the corrosion current
density,
I
corr

is
calculated from the Stern Geary equation and inturn
the corrosion rate (
mm/y
) is calculated using
Faraday’s law.



b
a


and
b
c

= anodic and
cathodic

Tafel

slopes



= density of steel

13

Inhibitor Efficiency/Surface Coverage


The corrosion rate in the absence of inhibitor is
representative of the total number of corroding sites.
The corrosion rate in the presence of inhibitor is
representative of the number of available corroding
sites remaining after blockage of sites due to
inhibitor adsorption.

14

Adsorption Isotherms

Ln C vs


follows the
Temkin adsorption
isotherm which is used to
deduce fundamental
adsorption constants,

f
and K
ads
.


ln C = f


-

ln K
ads


Left: Temkin Adsorption
Isotherm at 30
o
C where C is
the concentration of the
inhibitor (mol/L),


is the
fractional surface coverage, f
is the molecular interaction
constant and K
ads

the
equilibrium constant of ads.

15

Van’t Hoff Plots


16


Enthalpy of Adsorption


The
isosteric

enthalpy of adsorption (

H
ads

at a
fixed surface coverage)
for inhibitor compounds is
determined using
Van’t
Hoff plots.





The
effect of temperature on inhibitor
performance is evident from Van’t Hoff plots.

Calculating Other Thermodynamic
Properties


LnK
ads


from the isotherms


Δ
H
ads

from Van’t Hoff plots


Δ
G
ads
and
Δ
S
ads

from the relationships

17


G
ads

=
-
30 to
-
40 kJ mol
-
1

18

The next step was to carry out quantum mechanical
calculations to determine various molecular
properties and relate these properties to the
∆H of
adsorption


PCSpartan and HyperChem


Enthalpy of adsorption was better related
to a combination of molecular properties,
average charge density (

av
), molecular
size (V
max
), dipole moment (

)
, and the
band energy gap (

)
of each molecule.




H
ads

= k
1

av

+ k
2
V
max

+ k
3


+ k
4


+k
5




19

Negatively charged
molecules

A plot of the experimentally
determined
∆H vs the
∆H
determined from molecular
properties

20

Positively charged
molecules

21

Comparison of experimental and predicted concentrations for 50% surface
coverage determined from the four
-
variable fits

22

Conclusions


Corrosion rates from linear polarisation
measurements can
be fitted to adsorption isotherms
allowing determination of enthalpies of adsorption
and thus possible modes of adsorption. The free
energy and entropy of adsorption, along with the
molecular interaction constant can also be
determined.


A reliable semi
-
empirical model based on coulombic
and steric forces has been developed to describe the
efficiency of inhibitor molecules. This model can be
used in the development of “green inhibitors”

ATOMIC FORCE MICROSCOPY

Determining Properties of Inhibitor Films

1990s Performance of inhibitors with fluid flow or shear stress
indicated a critical velocity in which the inhibitor film would be
removed


G
ads

=
-
30 to
-
40 kJ mol
-
1

The free energy of adsorption suggests that fluid flow alone
cannot remove an inhibitor film

Using a jet impingement cell/loop we performed measurements
up to 1200 pa on 21 commercial inhibitor formulations and
never reached a critical flow velocity

Critical Micelle
Concentration

Surfactant

cmc pure
water

cmc brine

Dodecylpyridinium
chloride

(DPC)

1.58 x 10
-
2
M

1.96 x 10
-
4
M

Benzyldimethyl
-
tetradecyl ammonium
chloride

(BDMAC)


2 x 10
-
3
M

Cetylpyridinium
chloride

(CPC)

9.02 x 10
-
4
M

3.21 x 10
-
6
M

Imidazoline

3.54 x 10
-
3
M

1.05 x 10
-
4
M

Phosphate Ester

144 ppm

29.5
ppm

Shandelle

Bosenberg
, Curtin University

Ratio to CMC

The performance of surfactant corrosion inhibitors can be related to their CMC
which inturn is related to their chain length and the dissolved salt concentration.

Shandelle

Bosenberg
, Curtin University

Atomic Force
Microscopy

Shapes of Surfactants on
Mica (2 CMC)



DPC



CPC


C14BDMAC

film break through

Shandelle

Bosenberg
, Curtin University


Atomic Force Microscopy

Shapes
of Surfactants on Steel



DPC



CPC


C14BDMAC

Shandelle

Bosenberg
, Curtin University

Corrosion inhibitors

TOFA/DETA
Imidazoline

2 x 10
-
3

mol/liter

1.0 x 10
-
4

mol/liter

Alkylbenzyl

dimethyl

ammonium chloride
(
Quat
)

1.3 x 10
-
3

mol/liter

3 x 10
-
4
mol/liter

Sodium
thiosulphate

Non
-
surfactant

TOFA/DETA
Imidazoline
-
Sodium
thiosulphate

2.0 x 10
-
3
mol/liter

6.0 x 10
-
4

mol/liter

Alkylbenzyl

dimethyl

ammonium
chloride
Sodium

thiosulphate

1.3 x 10
-
3

mol/liter

8.0 x 10
-
4

mol/liter

Components

CMC
(DI water)

CMC
(1wt%NaCl)

Corrosion inhibitor

TOFA/DETA

Imidazoline

Coco quaternary amine

Hydrophilic head


Corrosion inhibitors are surface active compounds
(surfactants)
that protect
metal surfaces by forming adsorption films.

Hydrophilic head

Molecular length calculated by Chem Office, Cambridge soft.

Quaternary amine

Alkyl
Benzyl
Dimethyl

Ammonium Chloride


Compositions: C12
-
C16 alkyl benzyl
dimethyl

ammonium
chloride (ABDAC)



Reported CMC:


C12 benzyl
dimethyl

ammonium chloride: 9 X 10
-
3
M


C14 benzyl
dimethyl

ammonium chloride: 2 X 10
-
3
M


C16 benzyl
dimethyl

ammonium chloride: 5 X 10
-
4
M



Inhibitor contains 28% C12 compound, 64% C14 compound,
8% C16 compound. A CMC. of 2 X 10
-
3
M was used as C14 is
the dominant component.



Penetrating inhibitor film vertically

No interaction

In contact with the film


Break through”

Stress = Force/cross
-
section area


=
1
nN
/(
3.14*10 nm*10 nm*0.5
)


= 1.6
Mpa

2~6nm

Diameter = ~10 nm

Lateral scratching of inhibitor film

Imidazoline
, 0.5 CMC; on Mica




Substrate mica




Averaged film thickness: 2.4 nm




Film structure: Monolayer

Film thickness from lateral scratching

Imidazoline

2
CMC
on Mica



On mica substrate



Averaged film thickness: 4.4 nm



Film structure: double layer

Film thickness of inhibitors on mica

K1
Imidazoline

K2
Quat

K4
Imidazoline

+
Thiosulphate

K5
Quat

+
Thiosulphate

Film
thickness
(nm)

0.5CMC

2CMC

0.5CMC

2CMC

2CMC

2 CMC

2.4

4.4

1.9

2.9

4.1

3.0

Thiosulphate

has little influence
on film thickness

Summary of Penetration Measurements (
nN
) at 2 CMC

Imidazoline

Quat

Imidazoline

+
Thiosulphate

Quat

+
Thiosulphate

Mica (
nN
)

2.1

----

2.2

----

Au (
nN
)

1.2

1.0

1.1

0.8

Pt (nN)

0.5

0.3

0.7

0.6

Similar inhibitor types have a
similar penetration
force while
the sodium
thiosulphate

doesn’t
significantly increase the force, probably because they have
similar
structures
.

Increased surface roughness appears to reduce the force to
penetrate the inhibitor film.

Same penetration force

Na
2
S
2
O
3


Roughness

Lateral Force Measurements



Scratching inhibitor molecules


Lateral force contributes to the
scratching of inhibitor molecules.


Lateral force is proportional to the
normal force applied on AFM probe.


Schematic of lateral movement of
AFM tip

Trace

Retrace

Averaged: 53
±
13
nN

Five
measurements 0.5
cmc
, 1.1
±
0.2
nN


Cantilever is sensitive in z direction,
corresponding to normal force


The lateral spring constant is usually 1~2
magnitude higher than the normal spring
constant.


AFM instrument can directly measure normal
spring constant. But the lateral spring
constant
needs
to be calculated.


Lateral force measurements

Imidazoline

0.5
CMC
on Mica

Lateral force = [F(trace)


F(retrace)]/2


Average lateral
force = 53
nN

Normal force loaded on
AFM cantilever: ~ 30nN


Trace

Retrace

Lateral force on mica

Average lateral force = 16
nN

Normal force on AFM cantilever

30
nN

Net lateral force to remove inhibitor film on mica at 0. 5 CMC

Lateral force on mica with
inhibitors:
≈53
nN


Lateral force on
mica

16
nN

53


16


37
nN

force
needed to remove
inhibitor molecules
laterally. These forces are
several orders of magnitude above forces that
can be achieved by flow alone.

Imidazoline

2 CMC on Mica

Averaged lateral
force = 55
nN

Concentration

Pentration force

Lateral force

Film Thickness


0.5
CMC

1.1
nN

53
-
16 = 37
nN

2.4
nM


2.0
CMC

2.1
nN

55
-
16 = 39
nN

4.4
nM

Imidazoline
, on Mica

Lateral
force
appears to be related
to how
strong inhibitor molecules can bind to the
substrate. Therefore, different substrates,
different lateral removal force.


Penetration
force (vertical direction)
appear to be related
to
how strong
inhibitor
molecules can repel outside impact
.

Summary, Inhibitors on Mica

Imidazoline

Quaternary amine

(
Quat
)

Imidazoline

+

thiosulphate

Quat

+
thiosulphate

0.5CMC

2CMC

0.5CMC

2CMC

2CMC

2 CMC

Net lateral force
(
nN
)

37

39

27

33

45

48

Penetration force
(
nN
)

1.1

2.1

0.9

----

2.2

----

Lateral
force is related to the binding of inhibitor molecules to the substrates

Penetration
force is related to the
structure of adsorbed inhibitor molecules.


Net Lateral Force on Mica and Au

Imidazoline

(2CMC)

Quat

2CMC)

Imidazoline

+

Thiosulphate


(2CMC)

Quat

+

Thiosulphate

(2CMC)

Mica (
nN
)

39

33

45

48

Au (
nN
)

23

27

85

68

The sodium
thiosulphate

helps inhibitors bind
stronger to Au.


possibly due to
the formation of a sulfide film. Results support the hypothesis that the
measured lateral
force is related to the binding between inhibitor molecules
and substrate.


Significant difference

Na
2
S
2
O
3


47

Conclusions


AFM can be used to image surfactant molecules on
surfaces, measure film thickness and the forces to
penetrate and scratch the inhibitor films


The force to penetrate an inhibitor film is related to
the structure of the assembled molecules, which in
-
turn can be related to the surface roughness of the
substrate


The force to scratch or remove inhibitor molecules
from a surface is related to the surface adhesive force
and appears to have little relationship to film structure


Inhibitor molecules cannot be removed from a surface
by the force of fluid flow alone and this is supported by
thermodynamic data


Kinetics of inhibitor adsorption can also be possibly
studied using AFM




The End