Modeling of Inhibitor Release from Epoxy
Coating with Hydrotalcites
Using Finite Element Method
Hongwei Wang,1 Hong Guan,2
Francisco J. Presuel-Moreno,1
Robert G. Kelly,1 and Rudolph G. Buchheit2
1Center
for Electrochemical Science and Engineering
Department of Materials Science and Engineering
University of Virginia, Charlottesville, VA 22904 USA
2Fontana
Corrosion Center
Department of Materials Science and Engineering
Ohio State University, Columbus, Ohio 43210 USA
Acknowledgment: AFOSR
Presented in the Corrosion Inhibitor Session of the 202 ECS Fall Meeting at Salt Lake City, Utah, USA
October 22, 2002
Inhibition by Ion Exchange Using
Hydrotalcites
Host layer: double metal hydroxide:
Al-Mg, Al-Li, Al-Zn, Al-Ni . . .
• high temperature thermal stability
• anion selectivity
• exchange kinetics
Anion interlayer: OH-, CO32-,
NO3-, VO3-, V10O286-,
CrO42-, Fe(CN)63-, S2O82-,
MoO42-, MnO42-, . . .
• inhibitors
• sensing ions, e.g. pH
• hydrophilic
aggressive
anions
inhibitor
reservoir
inhibiting
anions
aggressive anions
immobilized
Hydrotalcites compound released inhibiting
anions and immobilized aggressive anions
[M1-xM′x(OH)2]x+(X)x/m.nH2O, where M=Zn(II), M′=Al(III) and
X=[V10O28]6- (decavanadate)
Model the Scratch on Coating
1.6
3
(mol/m )
Inhibitor Concentration
2
Inhibitor is released and
transports horizontally
1.2
0.8
coated AA2024
scratch
0.4
0
0
0.002
0.004
0.006
0.008
Water
layer
Primer
coating
25 µm 500 µm
Distance (m)
Substrate
AA2024
0.01
Sample Geometry
J=0 (symmetry)
.01 cm
HT inhibitor particles
A
B
AA 2024 T3
J=0
2S
2 cm
High aluminum
dissolution rate in scratch
J = Electric Flux
Figure not to scale
(Figure not to scale)
Aluminum
Clad
Coated AA2024
Model and Assumptions
- Transport modes: diffusion and migration
- Complex reaction system
- Electrochemical reactions
Al Æ Al3+ +3 e-
anodic reaction
O2 +2H2O +4e- Æ4OH-
cathodic reaction
- Chemical reactions/processes
Al3+ + yH2O = Al(OH)y3-y +yH+
hydrolysis
V6O286-(HT) = V6O286-(sol)
inhibitor release
Cl-(sol) = Cl-(HT)
chloride gettering
- Mass balance (11 chemical species)
- Electrical charge balance (electrochemical reaction)
- Solution electroneutrality (Na+ to neutralize)
Electrochemical Boundary Conditions
0
2024/2024
2024/V-HT
-0.2
-400
Potential V/SCE
Potential mV/SCE
-200
-600
-800
-0.4
-0.6
-0.8
-1
-1000
1e-101e-9 1e-8 1e-7 1e-6 1e-5 1e-4 1e-3 1e-2
Current density (A/cm2)
AA2024-T3 with/without
exposure to V/HT in
simulated scratch cell,
0.1 M NaCl
-1.2
0.000001
0.0001
0.01
1
2
Current density (A/m )
Simulated kinetics model
100
Modeling and Simulation System
Development
Numerical calculation
• Finite element method (ANSYS)
Engineering software development
• C++ Object Oriented Programming
• Open source codes and executable files available on web (IT IS FREE)
http://www.virginia.edu/cese/research/crevicer/
• Encourage different researchers to use for the specific purposes
• 1994-present at University of Virginia lead by R.G. Kelly
Computing facility
• PC is enough
• Super computer might be needed for the long term simulation
14
0.02
12
0.1 s
100 s
0.01
10
300 s
500 s
0.00
pH
2
Net Current Density (A/m )
I(x) and pH Evolution
8
6
0.1 s
100 s
300 s
500 s
4
-0.01
2
0
-0.02
0
0.002
0.004
0.006
Distance (m)
0.008
0.01
0
0.002
0.004
0.006
Distance (m)
2500 µm scratch, 500 µm water layer,
0.1 M NaCl, release rate (A4)
0.008
0.01
Inhibitor Concentration Evolution and
Protection of the Scratch
2500 µm scratch
3
t
1.2
Scratch
inhibition
(48%)
0.8
0.4
0
0
0.002
0.004
0.006
Distance (m)
0.008
0.01
Point B
1.2
Vanadate Inhibitor
3
Concentration (mol/m )
0.1 s
100 s
300 s
500 s
1.6
(mol/m )
Inhibitor Concentration
2
Point A
0.8
Time to inhibit
0.4
Point B
0.0
0
200
400
Time (s)
Point A
2500 µm scratch, 500 µm water layer, 0.1 M NaCl, release rate (A4)
600
Effect of Scratch Size and Inhibitor
Release pH Dependencies
Y=A pH + C
100
1.0E-04
Scratch Protection Percent (%)
Vanadate Inhibitor Release
Rate (mol/m2/s)
1.0E-03
Y=2e -6pH+7e -11
1.0E-05
Y=2e -7pH+7e -11
1.0E-06
Y=2e -8pH+7e -11
1.0E-07
1.0E-08
Y=2e -10pH+7e -11
1.0E-09
1.0E-10
-11
Experimental: Y=2e pH+7e
-11
1.0E-11
A5=2×10-6
80
60
Increase
release pH
dependence
40
A4=2×10-7
20
A2=2×10-10
A3=2×10-8
0
0
2
4
6
8
pH
10
12
14
0
1000
2000
3000
-11
A1=2×10
Scratch Size (um)
0.1 M NaCl, 500 µm water layer, 500 seconds
4000
5000
Increased Water Layer Thickness
Slows Inhibition
Humid air: 100 µm water layer
1
0.8
12
0.6
100
500
0.4
5000
10
Water layer
thickness (micron)
32%
8
0.2
pH
Vanadate Inhibitor
3
Concentration (mol/m )
1.2
0
0
0.002
0.004
0.006
0.008
0.01
Distance (m)
[Cl-] decrease is <10% in 500 s
6
4
100
500
2
5000
Initial pH=7
Water layer
thickness (m icron)
0
0
0.002
0.004
0.006
0.008
0.01
Distance (m)
2500 µm scratch, 0.1 M NaCl solution, release rate (A3), 500 seconds
Conclusions
•
We have extended the occluded corrosion mass transport
model to atmospheric exposure of multifunctional coatings to
include:
– Anodic and cathodic reactions in a closed (open circuit) system
– Al3+ hydrolysis
– pH-dependent inhibitor release and Cl- gettering
– Provides a tool for design parameter evaluation
– http://www.virginia.edu/cese/research/crevicer/
•
The pH dependency of the inhibitor release is the primary
controlling factor for protection of a scratch.
– Larger pH dependencies are desirable
•
Increases in water layer thickness have two compounding
effects:
– Slow the pH increase over the coating
– Dilute the inhibitor concentration