LABORATORY EVALUATIONS OF CORROSION
PREVENTION COMPOUNDS FOR AIRCRAFT
Feng Gui1, Keith Furrow2, Jackie Williams2, Kevin Cooper2, Robert G. Kelly1
1
Department of Materials Science and Engineering
University of Virginia
P. O. Box 400475
Charlottesville, VA 22904
2
Luna Innovations
701 Charlton Ave.
Charlottesville, VA 22903
Submitted to:
Garth Cooke
Program Manager
NCI Information Systems
Fairborn, OH
Index
1
2
3
4
5
Introduction ......................................................................................................................................... 4
1.1 Fundamentals of Corrosion Prevention Compounds .................................................................. 4
1.2 Objectives of the Current Work..................................................................................................... 5
1.3 CPC Tested..................................................................................................................................... 5
Design of Test Protocols ...................................................................................................................... 6
2.1 Test Selection ................................................................................................................................. 6
2.1.1
Solution Composition and Exposure Modes ......................................................................... 6
2.1.2
Protective Film Formation..................................................................................................... 7
2.1.3
Inhibitor Leaching and Electrolyte Modification ................................................................. 7
2.1.4
Surface Chemistry .................................................................................................................. 7
2.1.5
CPC Wicking Ability .............................................................................................................. 8
Sample Preparation and Test Methods ............................................................................................. 9
3.1 Sample Preparation ....................................................................................................................... 9
3.1.1
Aluminum Alloy Sample Preparation ................................................................................... 9
3.1.2
CPC Application Method ....................................................................................................... 9
3.2 Test Methods................................................................................................................................ 10
3.2.1
Exposure Methods................................................................................................................ 10
3.2.2
Electrochemical Impedance Spectroscopy .......................................................................... 10
3.2.3
Cyclic Potentiodynamic Polarization................................................................................... 10
3.2.4
FTIR ..................................................................................................................................... 10
3.3 Surface Property Measurements................................................................................................. 10
3.4 Wicking Tests............................................................................................................................... 12
3.4.1
Assembly Of Wicking Specimens......................................................................................... 12
3.4.2
Wicking Into Wet And Dry Simulated Lap Joints............................................................... 13
3.4.3
Fiber Optic Sensor Instrumentation and Data Reduction.................................................. 14
3.4.4
Wicking Into CPC-Treated Lap Joints ................................................................................ 16
Results................................................................................................................................................. 17
4.1 Constant Immersion .................................................................................................................... 17
4.1.1
AA7075-T6............................................................................................................................ 17
4.1.2
Clad AA2024-T3................................................................................................................... 18
4.1.3
Precorroded AA7075-T6 ...................................................................................................... 19
4.1.4
Precorroded Clad AA2024-T3 ............................................................................................. 20
4.2 Alternate immersion .................................................................................................................... 20
4.2.1
AA7075-T6............................................................................................................................ 20
4.2.2
Clad AA2024-T3................................................................................................................... 21
4.2.3
Precorroded AA7075-T6 ...................................................................................................... 22
4.2.4
Precorroded Clad AA2024-T3 ............................................................................................. 22
4.3 Proof of Remnant CPC After Apparent Loss of Film ................................................................ 23
4.3.1
FTIR ..................................................................................................................................... 23
4.3.2
Cyclic Polarization Scan ...................................................................................................... 24
4.4 Leaching ...................................................................................................................................... 25
4.5 Surface Properties of CPC, AA2024-T3 and Al2O3 .................................................................... 26
4.6 Water Displacement..................................................................................................................... 27
4.7 Water Emulsification................................................................................................................... 29
4.8 Wicking into Dry Lap Joints ....................................................................................................... 31
4.9 Wicking into Wet Lap Joints ....................................................................................................... 35
4.10 Wicking into CPC-Treated Lap Joints........................................................................................ 42
Discussion ........................................................................................................................................... 44
5.1 Testing Protocol Development For Boldly Exposed Surface..................................................... 44
5.1.1
Assessment Vs. Prediction of CPC Performance by Test Protocol .................................... 44
2
6
7
8
9
5.1.2
Relative Corrosivity of Exposure Protocols......................................................................... 47
5.1.3
Ranked Performance of CPC............................................................................................... 48
5.1.4
Failure Mode ........................................................................................................................ 48
5.2 Precorrosion Effects .................................................................................................................... 49
5.2.1
Effect of Prior Corrosion on Effectiveness of CPC for Boldly Exposed Aluminum ......... 49
5.3 Wicking ........................................................................................................................................ 50
5.3.1
CPC Surface Chemistry ....................................................................................................... 50
5.3.2
Wicking Into Dry and Wet Lap Joints................................................................................. 54
5.3.3
Wicking Into a Pretreated Lap Joint ................................................................................... 56
Conclusions......................................................................................................................................... 57
6.1 Development of Quantitative Testing Protocol........................................................................... 58
6.2 Relative Effectiveness of Several Commercial CPC................................................................... 59
6.3 Surface Thermodynamic Properties and Wicking into Occluded Regions ............................... 59
Future Work ...................................................................................................................................... 60
Acknowledgements ............................................................................................................................ 60
Appendix—Kruss Report ................................................................................................................. 61
3
1
1.1
Introduction
Fundamentals of Corrosion Prevention Compounds
Corrosion management of aircraft is beginning to replace “find-it-fix-it” philosophies in
both the commercial and defense sectors. Successful implementation of corrosion management
requires a range of technologies. Corrosion prevention compounds (CPC) are materials that can
both prevent new corrosion sites from forming and, more importantly, suppress any corrosion
that has initiated.
It is important to recognize that CPC are applied as a post-production
treatment to provide cost-effective, temporary corrosion protection and to control existing
corrosion.
CPC have been used on aircraft for many years as a relatively inexpensive method of
combating corrosion. [1] One of the main advantages of using CPC is that little or no preparation
of the affected site is required before application. Consequently, these CPC can be used at the
field maintenance level instead of requiring application at the depot. Thus, corrosion can be
suppressed early on, before substantial structural damage can occur. CPC are not meant to
replace high-performance coating systems, but they can be effective for on-site repair of coated
regions that may have been damaged or degraded, for extending the service life of a coating, and
for protecting regions of aircraft that did not receive corrosion prevention treatments during
original manufacture. CPC can, in principle, serve a key function as a component of a corrosion
management strategy.
Effective CPC function via one or more of four mechanisms: (a) blocking film formation,
(b) kinetic inhibition of surface reactions, (c) water displacement, or (d) local electrolyte
modification. Film formers produce a covering on the surface to be protected that prevents the
formation of an aqueous phase at the metal surface. Without such a phase, the electrochemical
reactions required for corrosion cannot occur. Inhibitors that directly impede the electrochemical
kinetics can be added to CPC. Some CPC seek to actively remove moisture from a metal surface,
with this water displacement serving to stop any corrosion that could occur. Finally, CPC can
function by altering the chemical conditions of any aqueous phase that does form at a surface by,
for example, buffering the pH to a less aggressive range or reacting with aggressive species that
my be present.
4
In general, CPC development and deployment has been ad hoc.
Most commercially
available CPC were in fact developed for other uses, such as lubrication. Due to the cost
involved, corrosion inhibitors have been only sporadically used in CPC, although they are more
widely applied to primers. Unfortunately, many effective corrosion inhibitors are
environmentally damaging and/or have health risks for worker exposure. In addition, recent
work [2] has shown that the currently available CPC are not effective at inhibiting corrosion
inside occluded regions on aircraft (e.g., lap joints). These types of sites are very often the
location of some of the more severe corrosion on aircraft, affecting repair costs and time, fleet
readiness, and potentially safety of flight. To be effective in occluded regions, the CPC requires
the ability to wick into the occluded region and perform some corrosion inhibition.
1.2
Objectives of the Current Work
The current work has two primary objectives: (a) to develop a quantitative testing protocol
that allows both discrimination of CPC performance and guidance on selection of the most
effective CPC for a particular component, and (b) to test the effectiveness of several commercial
CPC on both boldly exposed and occluded regions on aircraft.
1.3
CPC Tested
There are several different types of CPC. These CPC were selected not only to cover the
range of CPC types, but also to allow comparison with exposure work being performed by S&K
Technologies in Dayton, OH. The classification of CPC given in the following table is based
upon the manufacturer self-reporting.
5
Table 1 CPC selected for testing in Year 1.
*Water Displacing Hard Film (WDHF), **Water Displacing Soft Film (WDSF), ***Non-Water
Displacing Soft Film (NWDSF)
CPC
Specifications
Color of Liquid
Description
Film Type
General purpose, heavy
Amlguard
Mil-C-85054
duty, durable, hard film
Dark Blue/Green
forming CPC
Dinitrol ®
AV30
Dinitrol®
AV8
None
Light Brown
BMS 3-23
Light Brown
Penetration,
inhibiting
High penetration,
corrosion inhibiting
BMS 3-23
LPS3
Self-healing,
Dark Yellow
dry
film)
WDSF**(waxy)
WDSF (non tacky)
anti-sling NWDSF***(self
lubricant, high VOC
BMS 3-29
2
corrosion
WDHF*(hard,
healing waxy film)
Design of Test Protocols
2.1
2.1.1
Test Selection
Solution Composition and Exposure Modes
As mentioned above, one of the project objectives is to develop an effective test for
corrosion prevention and abatement compounds used for both boldly exposed and occluded
regions on aircraft. In the past several years, work at the University of Virginia on the chemical
conditions inside occluded regions has led to the development of the Lap Joint Simulation
Solution (LJSS)[3]. This solution has been shown to mimic the occluded solution inside lap
joints. The corrosion morphology observed after exposure corresponded to the attack observed
in the field. Therefore, in this project, LJSS was used as the sample exposure environment.
6
The two most widely used aerospace alloys, AA7075-T6 and clad AA2024-T3, were the
materials investigated. Both constant immersion (CI) and alternate immersion (AI) were used to
study the degradation of the CPC protection.
Although AI is generally found to be more
aggressive, CI is a more straightforward exposure method.
2.1.2
Protective Film Formation
As a non-destructive inspection method, electrochemical impedance spectroscopy (EIS)
has been widely used to study organic coatings on metals to get complementary information to
that obtained by traditional techniques for characterizing the behavior of organic coatings in
corrosive environments [4]. In this work, EIS was used to investigate the ability of CPC to form
and maintain a protective film on aluminum alloys in LJSS. The interfacial impedance was used
to characterize the protective film performance.
2.1.3
Inhibitor Leaching and Electrolyte Modification
One means by which CPC may function is via the release of corrosion inhibitors. Any
inhibitors that may be present in CPC film could leach into the LJSS solution to enhance the
protection of metal from corrosion. To study the ability of the CPC tested to leach inhibitors,
each CPC was applied onto Pt- coated Nb mesh and then exposed to a small volume of LJSS.
The mesh was exposed to the LJSS, and the effects of the leachate solution on the interfacial
impedance of clad AA2024-T3 were investigated by EIS.
As with inhibitor leaching, electrolyte modification from CPC could be another factor to
influence the protection of metal in corrosion environment. The release of species that could
change the pH or the buffering capacity could act to make the local solution more benign. Thus,
for each of leaching solutions, the pH values were monitored periodically.
2.1.4
Surface Chemistry
The purpose of this portion of the project was to develop test methods that will quantify
the ability of CPC to penetrate lap joints and displace water. The testing evaluated the surface
properties of the CPC and their ability to form water-in-oil emulsions. Accompanying the
surface property testing, the ability of the CPC to penetrate simulated lap joints was measured
using embedded fiber optic sensors. The penetration tests were conducted on pristine specimens,
with and without previously entrained water.
7
The surface free energy and contact angle are thermodynamic properties describing, in
part, the ability of a fluid to wet a surface. Metals and metal oxides have high-energy surfaces,
whereas oil presents a low energy surface. Therefore the oil-based CPC would be expected to
easily wet the dry lap joint surfaces based on thermodynamic considerations. The presence of
entrained water in the lap joint complicates the wetting process, but thermodynamics can again
be used to assess the feasibility of water displacement. Thermodynamic surface properties do
not indicate the rate of penetration or the rate of water displacement, only whether or not these
processes will occur spontaneously. The rates need to be measured in a wicking test. The
challenge is to relate macroscopic wicking effects to the interfacial properties of the materials
involved and to learn how to manipulate the latter to achieve the desired effects. [5]
The surface thermodynamic property testing was contracted to Kruss—a manufacturer of
surface chemistry instrumentation. The data and discussion are condensed from the Kruss
report, which is included in the appendix. The testing specifically evaluated:
1. The surface properties of the CPC, and the surfaces to which they are applied.
2. The ability of various CPC to displace water from surfaces to which they are applied
(both in an equilibrium sense, and in a dynamic sense).
3. The propensity of various CPC to form emulsions with water (which might be found
to be an advantage, in that it gives the displaced water somewhere to reside, away
from the protected surface).
2.1.5
CPC Wicking Ability
The CPC ability to penetrate simulated lap joints was measured using embedded fiber optic
sensors that could detect the presence of water or CPC. The penetration tests were conducted on
pristine joints, with and without entrained water. Two types of simulated joints were used. The
first consisted of an aluminum face sheet and a Plexiglas face sheet that allowed visual
observation of the wicking process. The second simulated lap joint consisted of two AA2024-T3
face sheets. CPC was dropped at the edge of the faying surfaces and allowed to wick into the
joint. The response of the fiber optic sensor was monitored as a function of time. When the
Plexiglas face sheet was used, the sensor response was verified with visual observations. The
last set of wicking tests involved treating the joint with CPC and then soaking the joint in water.
Embedded sensors monitored the ingress of any water into the joint.
8
3
Sample Preparation and Test Methods
3.1
Sample Preparation
3.1.1
Aluminum Alloy Sample Preparation
The materials used in this project are shown in the table below.
Materials
Sample Thickness
Size
Clad AA 2024-T3
0.072”
3”×3”
AA 7075-T6
0.090”
3” ×3”
All panel samples were degreased in an ultrasonic bath with acetone for 5 minutes,
methanol for 5 minutes, rinsed in high purity water, and then dried in the air. All lap joint
coupons were treated as panels before they are assembled into lap joints.
3.1.2
CPC Application Method
All CPC were applied on the samples by spraying. Figure 1 is the schematic drawing for
spraying:
Release spraying
button here
Figure 1. Schematic drawing for CPC applied on samples
After CPC applied, all samples were allowed to dry for overnight before expose to the solution.
9
3.2
3.2.1
Test Methods
Exposure Methods
Constant immersion samples were immersed in the LJSS by hanging the fully exposed
samples via a nylon line. Alternate immersion cycles involved immersion in the LJSS solution
for 12 hours followed by drying in air for 12 hours. Both pristine and precorroded samples were
used. The precorroded samples were exposed in either NaCl or LJSS for one week before CPC
application, while the pristine samples were not exposed immediately after the washing process.
3.2.2
Electrochemical Impedance Spectroscopy
Electrochemical Impedance Spectroscopy (EIS) was used to measure CPC performance
with the increase of exposure time. Every 5 days, the samples were taken EIS test after one-hour
open circuit potential (OCP) monitoring. The frequency range is 100KHz~10mHz and voltage
amplitude is 7mV rms. EIS was performed on the AI samples at the end of the wet portion of the
cycle.
3.2.3
Cyclic Potentiodynamic Polarization
Cyclic polarization scans were generated in 0.5M borate solution (0.42M H3BO3/0.08M
Na2B4O7) + 10-5 M NaCl at pH=8 at a scan rate of 5mv/s.
3.2.4
FTIR
FTIR spectra were generated on areas of exposed AA7075-T6 that had appeared to have
lost its CPC film during exposure and compared to a surface of pristine AA7075 coated with
CPC. A FTS 6000 Spectrometer was used for FTIR and the wavenumber range used was from
4500 to 700.
3.3
Surface Property Measurements
Dinotrol AV8, Dinotrol AV30, LPS-3, and Amlguard (So Sure) were characterized for
overall surface tension (energy) by the pendant drop technique. The overall surface tension was
then divided into polar and dispersive components according to the Fowkes approach.[6] In the
pendant drop technique, a drop of liquid is suspended on the end of a downward-pointing
capillary tip. The drop is typically formed to about 90% of its detachment volume. The drop is
then digitally imaged. The drop’s image is mathematically fit to determine the drop’s mean
curvature over its surface. The curvature of a drop at any given point on its surface is dependent
10
on two opposing forces—gravity and surface tension. Gravity works to elongate the drop while
surface tension works to keep the drop spherical. Pendant drop surface tension evaluation
involves observing the balance between these two forces, in the form of the drop’s mean
curvature at various points along its surface. Lower surface tension leads to a more “drip-like”
shape, whereas higher surface tension leads to a more spherical drop shape. The surface energy
components of Al2O3 powder were characterized using ethylene glycol and water as the probe
liquids and applying Fowkes theory to the resultant contact angle data. However, a different
contact angle determination method is applied to powders. The necessary contact angle values
were obtained (in duplicate) by the Washburn adsorption method using a Kruss Processor
Tensiometer K12, with an FL12 powder cell accessory. Hexane was used as the material
constant determination liquid.
Interfacial tensions at the CPC/water interface were also measured by the pendant drop
method. The measurement procedure for this work was equivalent to that used for the pendant
drop surface tension experiments described above except that instead of a drop of CPC being
formed in air, it is formed on the end of an upward facing capillary tip submerged in water
A key purpose of this project was to consider water displacement in lap joints. To measure,
in part, the ability of a CPC to penetrate a lap joint, the contact angle of CPC on an aluminum
surface submerged in water was measured as shown in Figure 2.
Al
CPC θ
Water
Figure 2: Contact angle measurement of CPC against aluminum submerged in water
A test was also performed to determine if the CPC were capable of forming stable
emulsions. Three mL of water and about six mL of CPC were slowly added to a glass vial.
Then the vial was shaken to form an emulsion. The stability of the emulsion was monitored
visually and recorded with photographs.
11
3.4
3.4.1
Wicking Tests
Assembly Of Wicking Specimens
The same simulated lap joint was used for the first set of wicking tests and consisted of
two sheets, one of Plexiglas, the other of AA2024-T3. As illustrated in Figure 3, grooves were
machined into the aluminum to secure the fiber optic sensor while minimizing the sensors
influence on the flow front. The grove was machined to precisely hold the fiber whose diameter
is 250 µm. The empty space above the optical fiber was filled with 400-mesh silica. The
powder ensured that when the CPC flow front reached the grove, the CPC wicked into the
groove and wet the sensor. The bulb at the end of the sensor groove housed a reflector for the
fiber optic system. The sensor was ½” long and was located about ¼” behind the bulb. The 3”
by 5” plates were assembled with hilok fasteners to insure consistent torque and fit up between
tests. Three plates were fastened together—a Plexiglas plate followed by and aluminum plate
followed by another offset aluminum backing plate.
Before each test, the aluminum plates were initially cleaned with acetone, alcohol or
toluene to ensure a clean, organic-free surface. The AV8 and AV30 products were best cleaned
with toluene. LPS3 and Amlguard were best removed with alcohol and acetone respectively.
After solvent cleaning the plates were washed with Alconox detergent until water broke off the
surface with a streak free finish. The Plexiglas sheets were cleaned with Alconox to remove
CPC from the previous tests. Toluene was occasionally used when the CPC became difficult to
remove. The face sheets were dried with a heat gun before testing.
12
Sensor 3
1.00 in.
Sensor 2
0.50 in.
Sensor 1
1.50 in.
250 µm
250 µm
Silica Filler
Optical Fiber
Sensor Groove
Pool of CPC
Optical Fiber Leads
Flow front under plexiglas,
sensor two is activated
Figure 3. Sketches of simulated lap joints and optical fiber placement from power point wicking
of CPC
3.4.2
Wicking Into Wet And Dry Simulated Lap Joints
Immediately before each test, about 4 mL of CPC were sprayed into a container. CPC
was placed on to the aluminum backing plate with a dropper and allowed to wick between the
aluminum and the Plexiglas. By adding additional CPC, the pool of CPC was maintained during
the test. When the wicking test was conducted on a wet joint, several drops of water were placed
at the edge of the faying surface. The water was allowed to wick in and the excess was wiped
with a chemwipe. Then the CPC was added to the backing plate. The sides were not masked as
masking the edges seemed to cause problems with preferential wicking. The sensors were
placed near the center of the joint to avoid edge effects and the need for masking. The flow front
was monitored visually and with the sensors. The time required for the CPC to reach the sensors
was recorded along with the sensor response. When the Plexiglas sheet was replaced with an
aluminum sheet only the sensor data were recorded. If the flow front advanced uniformly across
the joint, then the sensors would wet out in the following order– sensor 2, sensor 3 and sensor 1.
The test was allowed to continue until all of the sensors wetted out or until the flow front stopped
moving.
13
3.4.3
Fiber Optic Sensor Instrumentation and Data Reduction
The fiber optic sensor measures refractive index, which changes from that of air to that of
CPC or that of water as the flow front passes the sensor. Infrared light is directed through the
fiber. When the light reaches the sensor light of a specific wavelength is coupled out of the
optical fiber. The remaining light is reflected back to the instrument. The wavelength of the
light that was coupled out of the fiber is determined. That wavelength is a function of the
refractive index of the environment surrounding the sensor. Figure 4 contains typical spectra
returned from the sensor when the sensor was dry, wet with water, wet with CPC or a
combination of water and CPC. The instrumentation collects the spectra and the software finds
the minima in the spectra. The graphs in Figure 4 consist of light intensity on the y-axis and
wavelength on the x-axis. Light coupled out of the sensor appears as a dip in the intensity
known as a loss peak. During the test the instrumentation record the wavelength of the loss peak
as a function of time as illustrated in Figure 5. When the sensor is wet out completely with CPC,
the loss peak moves off the scale and the instrumentation returns a loss peak of 1492 nm or 1570
nm. The moment the water or CPC reaches the start of the sensor, the wavelength starts rising
and that time is recorded as the wet out time. The vertical lines in Figure 5 indicate when the
CPC reached the CPC as determined from visual observation.
14
Air has a refractive index of one. For these sensors, light of
Water has a refractive index of 1.33. For these sensors, light of
wavelength of 1510 nm is coupled out of the fiber when the
wavelength of 1530 nm is coupled out of the fiber when the
environment surrounding the sensor has a refractive index of one.
environment surrounding the sensor has a refractive index of 1.33.
CPC has a refractive index of 1.4 to 1.5.
When CPC
When water and CPC are present or if only a portion of the
completely wets the sensor, the refractive index is out of range of
1/2:” long sensor is surrounded with CPC, a loss will occur
the instrumentation. As a result no loss peak is detected and the
between 1530 and 1560 cm.
software returns a value of 1492 nm or 1570 nm.
Figure 4: Typical spectra encountered during the wicking process
15
Wave Length, nm
1525
Sensor 1
Sensor 2
Sensor 3
1520
1515
1510
1505
1500
1495
1490
100
150
200
250
300
350
400
450
500
Time, sec.
Figure 5. Wicking of Protector 100 into a dry simulated lap joint. (Protector 100 is a
CPC that was used early in the project to develop the technique). The vertical lines
mark when the CPC first touches the sensor. When the sensor is fully wetted the
response drops off the scale and reads 1492 nm.
3.4.4
Wicking Into CPC-Treated Lap Joints
In this set of tests, dry lap joints instrumented with fiber optic sensors were
treated with CPC. Four simulated lap joints were assembled with two sensors located at
either 0.5”, 1.0” or 1.5” from the edge of the faying surface. AV8, AV30, LPS3 and
Amlguard were each applied to one simulated lap joint. The CPC were sprayed directly
on the simulated lap joints and allowed to dry over night. Special effort was taken to
ensure the spray reached the interface between the faceplates. The amount of CPC
applied exceeded that which one would expect to be applied in the field. Wetting of the
sensor with CPC was verified before immersing the joint in water.
The joint was
immersed in tap water to the position at which the fiber optic leads exited the joint
(Figure 6). The water level was limited to this point to avoid having to seal the fiber
ingress points. The water was not agitated or stirred except to replace water lost through
evaporation. The sensor response was recorded with time to determine if water had
compromised the inside of the joint.
16
Two baseline measurements were taken. The first was of the dry, untreated lap
joint and is labeled untreated in the results. This baseline verified that the sensor was
functioning properly and allowed a comparison for a wetted sensor. The second baseline,
called treated baseline, was taken after the joint was treated with CPC and dried. This
baseline determines if CPC wicked into the inside of the joint when CPC was sprayed on
the outside of the joint.
Fiber egress point
connectors
Immersion level
Potential water ingress
Figure 6 Treated lap joint showing immersion level and potential water ingress
areas. For scale the joint is 3” wide.
4
Results
4.1
4.1.1
Constant Immersion
AA7075-T6
Figure 7 shows the interfacial impedance change with exposure time. For all CPC,
interfacial impedance decreased with increasing exposure time. Among the four CPC
tested in constant immersion, Amlguard failed first at about 110 days as indicated by a
sharp drop in interfacial impedance. Compared with untreated AA7075-T6, the interfacial
impedance for CPC-treated samples is at least 2 orders of magnitude higher, indicating
protection against corrosion.
17
Interfacial Impedance (Mohms.cm 2)
10
10
10
10
10
10
2
1
0
-1
Amlguard
AV30
AV8
LPS3
Bare AA7075
-2
-3
0
50
100
150
200
250
300
Exposure Time (Days)
Figure 7 Interfacial impedance vs. exposure time for AA7075-T6 constant
immersion
4.1.2
Clad AA2024-T3
Figure 8 is the interfacial impedance vs. exposure time for clad AA2024-T3 under
constant immersion. AV30 failed at about 60 days whereas the other CPC still work well
after > 200 days exposure. The interfacial impedance for CPC treated samples is one or
two order magnitude higher than that for untreated AA2024-T3.
Interfacial Impedance (Mohms.cm 2)
10
10
10
10
10
2
1
0
Amlguard
AV30
AV8
LPS3
Bare AA2024
-1
-2
0
50
100
150
200
250
Exposure Time (Days)
Figure 8 Interfacial impedance vs. exposure time for clad AA2024-T3 constant
immersion
18
4.1.3
Precorroded AA7075-T6
The interfacial impedance changes with exposure time for AA7075-T6 samples
precorroded before being treated with CPC are shown in Figure 9. All CPC applied
samples had higher interfacial impedance than CPC untreated sample. Among them, the
interfacial impedances on AV30, AV8 and LPS3 applied samples were about 2 orders of
magnitude higher, but Amlguard provided only one order magnitude higher protection. In
general the interfacial impedance decreased with the increase of exposure time and were
lower than for samples treated with CPC before corrosion (section 4.1.1).
Figure 9 Interfacial impedance vs. exposure time for Precorroded AA7075-T6,
constant immersion
19
4.1.4
Precorroded Clad AA2024-T3
Interfacial Impedance (Mohms.cm 2)
10
10
10
10
10
2
Amlguard
AV30
AV8
LPS3
1
0
-1
-2
0
20
40
60
80
100
120
Exposure Time (Days)
Figure 10 Interfacial impedance vs. exposure time for precorroded clad AA2024T3, constant immersion
The interfacial impedance for precorroded clad AA2024-T3 is shown in Figure 10.
At the beginning of the test, there are some differences of the interfacial impedance on all
samples. With the exposure time, however, this difference became negligible.
4.2
4.2.1
Alternate immersion
AA7075-T6
Figure 11 shows the change on interfacial impedance for AA7075-T6 during
alternate immersion exposure. For all samples, the interfacial impedance decreases with
the increase of exposure time. Amlguard failed after 55 days exposure and AV8 failed
after exposed 120 days. For LPS3 and AV30, after the observation of a portion of the
film floating in solution, the tests were stopped.
20
Interfacial Impedance (Mohms.cm2)
Exposure Time (Days)
Figure 11 Interfacial impedance vs. exposure time for
AA7075-T6, alternate immersion
4.2.2
Clad AA2024-T3
Figure 12 shows the relation between the interfacial impedance and the exposure
time for AI-exposed, precorroded clad AA2024-T3. AV30 and AV8 failed in 100 days
and Amlguard failed at about 240 days. LPS3 is still performing well after 260 days. The
general tendency was for the interfacial impedance to decrease for all CPC. The LPS3treated sample had the highest interfacial impedance for exposures longer than 20 days.
2
2
Interfacial Impedance (Mohms.cm )
10
10
10
10
10
1
0
-1
-2
0
Amlguard
AV30
AV8
LPS3
50
100
150
200
250
Exposure Time (Days)
Figure 12 Interfacial impedance vs. exposure time for
clad AA2024-T3, alternate immersion
21
4.2.3
Precorroded AA7075-T6
Figure 13 Interfacial impedance vs. exposure time for precorroded AA7075-T6,
alternate immersion
The interfacial impedance for precorroded AA7075-T6 alternate immersion is
shown in Figure 13. At the beginning of exposure, all CPC treated samples had higher
interfacial impedance than untreated sample. The impedance decreased with time. After
100 days, the interfacial impedances for Amlguard and AV8 were slightly lower than that
of the untreated sample.
4.2.4
Precorroded Clad AA2024-T3
Figure 14 shows the interfacial impedance for precorroded clad AA2024-T3
exposed to alternate immersion. At most times, AV30 had the highest interfacial
impedance and Amlguard had the lowest interfacial impedance. The interfacial
impedance changed little with time.
22
Interfacial Impedance (Mohms.cm 2)
10
10
10
10
2
1
0
Amlguard
AV30
AV8
LPS3
-1
0
20
40
60
80
100
120
Exposure Time (Days)
Figure 14 Interfacial impedance vs. exposure time for precorroded clad AA2024T3 alternate immersion
4.3
Proof of Remnant CPC After Apparent Loss of Film
For some samples, large pieces of CPC film appeared to detach from the surface
and were found floating in the solution. Thinking that this meant total failure of the CPC,
the samples were removed from testing. However, inspection of the areas from which the
film was lost showed no initiation of corrosion. Further investigations demonstrated that
although the majority of the CPC film was detached, protection of the surface continued.
4.3.1
FTIR
The FTIR measurement was taken on two types of surfaces: a bare area on
exposed AA7075-T6 on which portion of the film has peeled off, and an area on pristine
AA7075-T6 with the same CPC (AV30) applied and cured. The results are shown in
Figure 15. Solid and dot line represents bare area on exposed AA7075-T6 and pristine
AA7075-T6 respectively. Both lines are the result after background absorbance due to the
AA7075-T6 substrate has been subtracted. The two lines have very similar
characteristics, showing peaks relating to bond stretching from organic species on the
surface.
23
0.35
Bare Area on Exposed AA7075
CPC on Pristine AA7075
0.3
Absorbence
0.25
0.2
0.15
0.1
0.05
0
500
1000
1500
2000
2500
3000
3500
4000
4500
Wavenumber
4.3.2
Figure 15 FTIR for bare area on exposed AA7075-T6 (red line) and CPC on
pristine AA7075-T6 (Blue line)
Cyclic Polarization Scan
To determine the extent to which the remnant CPC provided protection to the
AA7075-T6 surface, cyclic polarization scans were generated for both pristine AA7075T6 and AA7075-T6 that had lost the majority of its CPC film during exposure. Tests
were conducted in borate solution with 10-5 M Cl-. The results are shown in Figure 16.
The solid line is for pristine AA7075-T6 and the dotted line is for exposed AA7075-T6.
From the figure, the pitting potential for exposed AA7075-T6 is much higher than
pristine AA7075-T6 and also, the passive current density for exposed samples is lower
than for the pristine sample.
24
0
Potential (V vs. SCE)
-0.2
-0.4
-0.6
-0.8
-1
-1.2
-1.4
-9
AA7075 Pristine
AA7075 Exposed (AV30)
-8
-7
-6
-5
logi (A/cm 2)
-4
-3
-2
Figure 16 Cyclic polarization scan for pristine AA7075-T6 and exposed AA7075-T6 in
borate solution.
4.4 Leaching
Figure 17. Interfacial impedance vs. leaching time for different CPC.
Figure 17 shows the interfacial impedance of clad AA2024-T3 exposed to solutions
produced after different times of exposure of the CPC-coated Pt-Nb meshes to the LJSS.
Within the first two weeks of leaching, the inhibitors leached from the AV30, AV8 and
Amlguard all increased the interfacial impedance of the AA2024-T3. The LPS3 leachate
had no effect. After two weeks of leaching, the AV30 continued to leach inhibiting
species, leading to an interfacial resistance of more than an order of magnitude higher
25
than that of the baseline sample.
The other CPC had a more limited effect. It should be
noted that these results are for clad AA2024-T3 and repetition of the experiments are
planned for unclad AA2024-T3 and AA7075-T6.
4.5
Surface Properties of CPC, AA2024-T3 and Al2O3
Using the pendant drop technique, the overall surface tensions of the CPC were
obtained and the results are shown in Table 2. Contact angle measurements were also
taken for each CPC against poly(tetrafluoroethylene), PTFE, as required by the Fowkes
theory. Substituting these contact angles and the overall surface tensions into the Fowkes
theory results in the component surface tension data shown Table 2.
Table 2 Overall and component surface tension data for CPC obtain from the pendant
drop in technique and the Fowkes, respectively.
Surface Tension
Amlguard
LPS 3
AV30
AV8
(mN/m)
(mN/m)
(mN/m)
(mN/m)
Overall
22.86
22.55
24.15
24.22
Polar
2.48
5.70
8.05
3.83
Dispersive
20.38
16.85
16.10
20.39
% Surface Polarity
10.9
25.3
33.3
15.8
The overall surface tensions are similar for the Amlguard and LPS 3 products, but
the surface polarities are quite different (10.9% for Amlguard versus 25.3% for LPS 3).
The same type of situation exists for the two AV CPC. They have very similar overall
surface tensions, which are higher than those of the other two CPC. However, their
surface polarities differ widely (33.3% for AV30 and 15.8% for AV8).
A clean AA2024-T3 aluminum alloy surface was characterized for overall surface
energy by the Fowkes method using water and ethylene glycol as the probe liquids.
Applying Fowkes method to the average contact angle data yielded the following surface
energy information for the AA2024-T3 surface.
26
Table 3. Overall surface energy for AA2024-T3 and Al2O3 obtained with the Fowkes
method and with Washburn adsorption method[need reference]
Surface Energy
Al (mJ/m2)
Al2O3 (mJ/m2)
Overall
34.04
44.95
Polar
3.43
27.79
Dispersive
30.61
17.16
% Surface Polarity
10.07
61.81
Al has a relatively low surface energy (34.04 mJ/m2) and surface polarity
(10.07%) compared to Al2O3, which has a surface energy of 44.95 mJ/m2 and a surface
polarity of 61.81%.
For the Al2O3 powder the necessary contact angle values were obtained (in
duplicate) by the Washburn adsorption method using a Tensiometer which measure the
weigh to liquid wicking into a powder packed column. Hexane was used as to determine
the void volume of the column.
4.6
Water Displacement
From the surface characterization data of the aluminum and aluminum oxide, one
can calculate interfacial tension values between each of the CPC and water, against the
two solid surfaces using Good’s equation.[7]
σ
SL
=σ
S
+σ
L
− 2 (σ
D
L
σ
D
S
) 1 / 2 − 2 (σ
P
L
σ
P
S
)1 / 2
where σ is the over all surface energy and the letters L, S, D, P refer to liquid, solid,
dispersive and polar respectively. The results of these calculations are given in Table 4.
Table 4 Liquid/Solid Interfacial Tensions, σSL, between CPC and Water and Al
and Al2O3 surfaces
Amlguard
LPS3
AV30
AV8
Water
(mN/m)
(mN/m)
(mN/m)
(mN/m)
(mN/m)
AA2024-T3
1.11
2.32
3.28
1.04
24.76
Al2O3
13.80
8.32
5.95
11.13
3.36
27
The interfacial tensions for all of the CPC against the aluminum surface were
calculated to be less than the interfacial tension for water against the surface (i.e., σSL for
CPC against AA2024-T3 is less than σSL of water against AA2024-T3). This relationship
was found for every CPC on a clean, non-corroded AA2024-T3 surface, but for none of
CPC on an Al2O3 surface (model oxidized Al surface).
The CPC with the lowest
interfacial tension on the Al surface were the low surface polarity CPC (Amlguard and
AV8). On the Al2O3 surface, the leading CPC, in terms of low interfacial tension, were
the ones with higher surface polarity (LPS3 and AV30). However, none of the CPC
matched the surface polarity of the Al2O3 as well as water (63.7% surface polarity).
When a CPC interacts with water on an aluminum surface three interfaces are
formed—CPC/aluminum, Water/aluminum and CPC/water.
The surface energies of
Water/aluminum and CPC/aluminum interfaces are given in Table 4 while the surface
energy between water and CPC needs to be measured.
Interfacial tensions were
measured between CPC and water by the pendant drop method and the results are shown
in Table 5
Table 5 Interfacial Tensions for CPC Against Water Measured by the Pendant Drop
Technique
Amlguard
LPS 3
AV30
AV8
(mN/m)
(mN/m)
(mN/m)
(mN/m)
Average
7.80
3.42
3.03
4.76
Std. Dev.
0.05
0.07
0.05
0.04
Finally, with the CPC/water interfacial tension, the free energies for water
displacement were calculated. These free energies were calculated by adding the
CPC/Water interfacial tension (Table 5) and the CPC/surface interfacial tension (Table 4)
to give the final condition, following the water displacement process. Then the relevant
water/surface interfacial tension, the initial condition was subtracted from the final
condition. Free energy of water displacement results for each of the CPC for Al and
Al2O3 are listed in Table 6:
28
Table 6 Free Energies for Water Displacement by CPC
Amlguard
LPS 3
AV30
AV8
(mJ/m2)
(mJ/m2)
(mJ/m2)
(mJ/m2)
AA2024-T3
-15.85
-19.02
-18.45
-18.96
Al2O3
18.24
8.38
5.62
12.53
All four CPC have negative free energies for displacing water from Al7075, but
the free energy for displacing water from Al2O3 are all positive.
To measure, in part, the ability of a CPC to penetrate a lap joint, the contact angle
of CPC on an aluminum surface submerged in water was measured as shown in Figure 1.
Such contact angles for CPC displacing water from an aluminum surface were measured
with the results given in Table 7.
Table 7 Contact Angles for CPC on Aluminum Submerged in Water
Amlguard
LPS 3
AV30
AV8
(degrees)
(degrees)
(degrees)
(degrees)
Average
32.6
13.6
23.6
17.3
Std. Dev.
0.5
0.7
0.7
0.5
All of the contact angles are fairly low (less than 90û). Furthermore the contact
angle trend does follow the free energy trend:
Amlguard (-15.85, 32.6o)> AV30 (-18.45, 23.6o)> AV8 (-18.96, 17.3o)> LPS 3 (19.02, 13.6o)
None of the angles was found to be particularly dynamic in nature. They all
developed an equilibrium values as soon as the CPC was applied. So the dynamic aspect
of this study was not completed. The same trend on the Al2O3 cannot be checked because
there is no method to measure the contact angle for CPC displacing water from a powder.
4.7
Water Emulsification
Each of the four CPC was shaken with water to form an emulsion. The LPS3
adhered to the glass and a portion of the Amlguard appeared to have been extracted into
29
the water. Photographs of the experiment are shown Figure 18. The Amlguard/water
mixture separated immediately. After ten minutes all of the emulsions were gone and the
water and CPC had separated.
CPC and Water before being shaken
CPC immediately after shaking The AV8, AV30 and LPS3 form an emulsion
30
All of the emulsions are unstable. After 10 minutes the CPC and water have
all separated
Figure 18: CPC from left to right AV8, AV30, LPS3 and Amlguard were shaken to form
emulsion then allowed to stand to determine if the CPC formed stable emulsions. None
of the CPC formed stable emulsions.
4.8
Wicking into Dry Lap Joints
The CPC were wicked into a simulated lap joint with two dry AA 2024 face sheets.
The time to reach sensors located 0.5”, 1.0” and 1.5” inside the joint was recorded. The
Table 8 Average wicking time to sensors 0.5”, 1.0” and 1.5” from the edge and standard
deviation of wicking times for three replicates
Time to sensor, sec
Std. Dev., sec
0.5 in.
1.0 in
1.5 in
0.5 in.
1.0 in.
1.5 in.
Amlguard
20
63
87
Amlguard
3.9
18.5
17.3
LPS
105
214
259
LPS
70.9
99.4
146.9
AV8
57
186
166
AV8
2.8
109.2
47.3
AV30
124
341
332
AV30
95.8
116.9
46.7
results of three replicates are tabulated in Table 8 Average wicking time to sensors 0.5”,
1.0” and 1.5” from the edge and standard deviation of wicking times for three replicates
and illustrated in Figure 19. The CPC were not observed to selectively wick along the
31
machined groove well ahead of the local flow front.
The average wicking time is
different for each CPC. The standard deviations, however, are quite large making it
difficult to quantify differences between the CPC. In general the sensor wet out in the
expected order, however, this was not always the case. Overall Amlguard wicks the
fastest.
Time to Sensor, sec.
500
400
0.5 in.
300
1.0 in.
1.5 in.
200
100
0
Amlguard
LPS
AV8
AV30
Figure 19 Wicking into a dry lap joint. The error bars represent one standard deviation
taken from three replicates.
Table 7 contains pictures of AV8 wicking into a dry lap joint. AV8 is used
because it is very dark and thus provides excellent contrast. The orange line highlights
the flow front. The flow front was not uniform, but did not preferentially wick along the
sensor groove. Bare spots or non-wetted areas remained after the flow front passed. This
phenomenon was not unique to AV8. If given sufficient time, the CPC would wick into
the entire faying surface of a dry joint.
32
Table 9: Photographs of AV8 wicking into a simulated lap joint. The orange line
indicates the location of the flow front
AV8 wicking into a dry
lap joint 5 second after
the CPC was applied
AV8 wicking into a dry
lap joint 1 minute after
the CPC was applied
AV8 wicking into a dry
lap joint 3 minutes after
the CPC was applied
AV8 wicking into a dry
lap joint 4 minutes after
the CPC was applied
33
Figure 20 through Figure 23 are representative data used to obtain the wicking times
in Figure 19. Typically the moment the sensor first wet out there was a slight increase in
the wavelength.
Shortly afterward when the sensor was completely wet out the
wavelength would move to either 1492 nm or 1570 nm. The initial increase in the
wavelength was marked as the wet out time.
1570
loss peak, nm
1560
1550
1540
0.5"
1530
1.0"
1520
1.5"
1510
1500
0
100
200
300
400
Time, sec
Figure 20 Sensor trace of Amlguard wicking into a dry aluminum lap joint.
1570
loss peak, nm
1560
1550
1540
0.5"
1530
1.0"
1520
1.5"
1510
1500
0
100
200
300
400
Time, sec
Figure 21 Sensor trace of LPS-3 wicking into a dry aluminum lap joint.
34
1550
loss peak, nm
1540
1530
1520
0.5"
1510
1.0"
1500
1.5"
1490
1480
0
100
200
300
400
Time, sec
Figure 22 Sensor trace of AV8 wicking into a dry aluminum lap joint.
1560
loss peak, nm
1550
1540
1530
1520
0.5"
1510
1.0"
1500
1.5"
1490
1480
0
100
200
300
400
Time, sec
Figure 23 Sensor trace of AV30 wicking into a dry aluminum lap joint.
4.9
Wicking into Wet Lap Joints
The CPC were wicked into a simulated lap joint that had water entrained between
the two face sheets. To allow visual observation, one face sheet was Plexiglas and the
other was AA2024-T3. The time for the CPC to reach sensors located 0.5”, 1.0” and 1.5”
inside the joint was recorded visually and with imbedded fiber optic sensors. The results
of four replicates are tabulated in Table 10 and illustrated in Figure 29 and Figure 30.
The data in Figure 29 are from sensors behind the Plexiglas. The data in Figure 30 are
35
from visual observation through the Plexiglas. Figure 31 shows the wicking times for
CPC wicking into a simulated lap joint with two AA2024-T3 face sheets for which only
the sensor data are available. Whenever a wicking time is not recorded, either the CPC
did not reach the sensor or the sensor reading was spurious because of a broken sensor.
The average wicking time was different for each CPC. Amlguard wicked the fastest and
most thoroughly followed (in order of decreasing speed) by AV8, LPS3 and AV30 under
the Plexiglas. The standard deviations, however, were quite large making it difficult to
quantify differences between the CPC. The wicking time for the wet lap joint was an
order of magnitude slower than the wicking times for a dry lap joint as report above.
When two aluminum face sheets are used, the Amlguard does not appear to be the fastest.
All of the wicking rates are roughly the same.
Wicking of CPC into a wet lap joint appears to be an inherently variable process.
Visual observations of wicking into the Al-Plexiglas joint, shown in Figure 24, indicated
very non-uniform flow fronts. In some cases the flow front at one location advanced two
to three times farther than at other locations within the same amount of time. The CPC
were not observed to selectively wick along the machined groove well ahead of the local
flow front. Edge effects and fastener location do influence the flow front as shown in the
series of photos in Figure 24. Some pockets of water remained after the CPC flow front
had passed through the faying surfaces
The fiber optic sensor data were consistent with the visual observations. The
variability associated with sensor data was higher than the visual observations. Figure 25
through Figure 28 are representative of the output from the sensors. Amlguard and LPS3
gave a definitive response whereas the AV8 and AV30 gave less obvious responses when
they reach the sensor. In each case the sensors started out dry with the loss peak at ~1510
nm. When the water reached the sensor, the sensor response goes to ~1530 nm. The
water wicked very quickly compared to CPC in a dry lap joint. Finally, if the CPC
reaches the sensor the wavelength again increased.
The wicking time for the joint with two aluminum face sheets is shown in Figure
31. The data for these tests have more scatter and the wicking times do not agree with the
wicking times determined from the test using a Plexiglas face sheet, indicating the
importance of the embeddable fiber optic sensor.
36
Table 10 Average wicking time to each sensors 0.5”, 1.0” and 1.5” from the edge. Times
were recorded visually and with the sensors during the experiment
Time to sensor (visual reading), sec
0.5 in.
1.0 in.
1.5 in.
Amlguard 473
667
833
AV8
620
1853
LPS
1200
AV30
1583
Time to sensor (sensor reading), sec
0.5 in.
1.0 in.
1.5 in.
Amlguard 825
683
1300
2830
AV8
870
2400
2405
1900
3400
LPS
2213
3783
4450
7000
4700
AV30
5038
5367
1950
Table 11 Standard deviation of wicking times. Four tests were conducted. Zero to four
observations were made at each sensor location.
Std. Dev. (visual reading), sec
0.5 in.
Std. Dev., (sensor reading), sec
1.0 in.
1.5 in.
Amlguard 197
125
125
AV8
275
486
LPS
216
AV30
388
0.5 in.
1.0 in.
1.5 in.
Amlguard 460
437
505
130
AV8
410
0
955
-
-
LPS
1500
1710
2299
2777
-
AV30
4466
3668
0
AV8 wicking into a wet
lap joint 4 minutes after
the CPC was applied
37
AV8 wicking into a wet
lap joint 15 minutes after
the CPC was applied
AV8 wicking into a wet
lap joint 40 minutes after
the CPC was applied
Figure 24: AV8 wicking into a wet simulated lap joint
Loss Peak wavelength, nm
1570
1560
1550
1.5 inches
1540
0.5 inches
1530
1.0 inches
1520
1510
1500
0
500
1000
1500
2000
2500
3000
time, seconds
Figure 25. Wicking of Amlguard into a wet lap joint. At time 0 the sensor is dry. The
first jump in wavelength is water wicking into the joint. The second increase in
wavelength indicates the presence of CPC.
38
Loss Peak Wavelength, nm
1570
1560
1550
1.5 inches
1540
0.5 inches
1530
1.0 inches
1520
1510
1500
0
2000
4000
6000
8000
time, seconds
Loss peak Wave Length, nm
Figure 26. Wicking of LPS3 into a wet lap joint. At time 0 the sensor is dry. The first
jump in wavelength is water wicking into the joint. The second increase in wavelength
indicates the presence of CPC.
1550
1545
1540
1535
1.5 inches
1530
0.5 inches
1525
1.0 inches
1520
1515
1510
0
2000
4000
6000
8000
10000
Time, seconds
Figure 27. Wicking of AV8 into the lap joint. Note that the response to the CPC is
weaker than LPS3 and Amlguard.
39
Loss Peak Wavelength, nm
1545
1540
1535
1530
1.5 inches
1525
0.5 inches
1520
1.0 inches
1515
1510
1505
0
2000
4000
6000
8000
10000 12000
Time, seconds
Figure 28 Wicking of AV30 into the lap joint. Note that the response to the CPC is
weaker than LPS3 and Amlguard. CPC never reached the 1.0 inch sensor.
Figure 29. Wicking into a wet lap joint. Data is from sensors imbedded in a Plexiglas
lap joint.
40
Figure 30: Wicking into a wet lap joint. Data is from visual observations through
Plexiglas.
Figure 31 Wicking into a wet aluminum lap joint. All of the data were collected from
the sensors.
41
4.10 Wicking into CPC-Treated Lap Joints
Lap joints were treated with CPC and then immersed in water.
The sensor
responses over time are shown in Figure 32 through Figure 35. For the joint treated with
LPS3 (Figure 32), the sensor located 0.5” from the edge immediately gave a loss peak of
1570 nm after treatment. The other senor responded the same way after one day. After
13 days the response dropped to 1560 nm on both sensors. An oil film appeared on the
surface of the immersion solution.
For the joint treated with AV8 (Figure 33) the treated baseline is missing. After
one day, however, both sensors gave a loss peak of 1570. After 13 days the response of
the 0.5” sensor remained at 1570 nm while the 1.0” sensor dropped 1555nm.
For the joints treated with AV30 (Figure 34) and Amlguard (Figure 35) the sensors
do not respond after treatment.
Both sensors remain at 1510 nm.
After one day
immersed in water, the sensors gave a loss peak of 1530nm, which remained roughly
constant through 33 days. AV30 got paler in color. The Amlguard specimen began
corroding with black spots on boldly exposed surfaces and foamy bubbles coming from
one of the fasteners. An oily film also appeared on the surface of the immersion solution.
1700
wavelength, nm
1650
1600
1550
1500
0.5"
1450
1.0"
1400
untreated
treated
baseline
day 1
day 13
day 33
Figure 32. Sensor response in a simulated lap joint treated with LPS3 and immersed in
water.
42
1700
Wavelength, nm
1650
1600
1550
1500
0.5"
1450
1.0"
1400
untreated
treated
baseline
day 1
day 13
day 33
Figure 33. Sensor response in a simulated lap joint treated with AV8 and immersed in
water.
1700
wavelength, nm
1650
0.5"
1600
1.0"
1550
1500
1450
1400
untreated
treated
baseline
day 1
day 13
day 33
Figure 34. Sensor response in a simulated lap joint treated with AV30 and immersed in
water.
43
1700
wavelength, nm
1650
1.5"
1600
0.5"
1550
1500
1450
1400
untreated
treated
baseline
day 1
day 13
day 33
Figure 35. Sensor response in a simulated lap joint treated with Amlgurad and immersed
in water.
5
Discussion
5.1
5.1.1
Testing Protocol Development For Boldly Exposed Surface
Assessment Vs. Prediction of CPC Performance by Test Protocol
All of the commercial CPC tested do provide some protection for aluminum
alloys in LJSS as demonstrated by the higher interfacial impedance for treated samples
relative to an untreated sample. This protection degraded with time. During the EIS
testing, it was found that a decrease in interfacial impedance was correlated to the
observation of visible corrosion on the sample surface for most CPC. However samples
treated with AV30 or LPS3 were also likely to lose a portion of the CPC film after a long
exposure times. In AA2024-T3 alternate immersion (Figure 11), the observation of a
portion of the film floating in solution was initially assumed to indicate imminent failure
although no corrosion was observable. On the other hand, the observation of good
performance CPC was always correlated to higher interfacial impedance. Therefore,
interfacial impedance can be used as one parameter to assess CPC performance.
As mentioned above, samples treated with AV30 or LPS3 tended to lose a portion
of the film after exposure for a long time. One example picture is shown below in Figure
36 An example of film loss sample (Substrate: AA7075-T6; CPC: AV30). It is a sample
of AA7075-T6 with AV30 after exposure ten months.
44
No visible corrosion was observed on those film loss areas, however, if these
samples were continued to expose to LJSS. There are two possibilities for this
phenomenon: either protection developed from modification of the solution by species
leached from CPC, or; there was still some CPC left on the film loss area. To decide if
the protection was from the solution, impedance spectra on both fresh LJSS and CPCexposed LJSS were generated (Figure 37). It was found the impedance results in both
solutions were very similar, which indicated that the protection on film loss area was not
from the solution. In Figure 15, the FTIR results show similarities on pristine AA7075T6 with CPC and the bare area on exposed AA7075-T6, which indicates that there was
still CPC remain on the aluminum alloy surface after the bulk of the film appeared to fall
off.
Intact CPC Film (AV30)
Area of Film Loss
Figure 36 An example of film loss sample (Substrate: AA7075-T6; CPC: AV30)
45
10
4
LJSS (Without Exposed)
CPC Exposed LJSS
3
|z|
10
10
2
1
10 -2
10
10
0
10
2
Frequency (Hz)
10
4
10
6
Figure 37 results on both fresh LJSS and CPC exposed LJSS using AA7075-T6
The cyclic polarization results on pristine AA7075-T6 and exposed AA7075-T6
also support the assertion that the protection on film loss area is from the remaining CPC
(Figure 16). From Figure 16, the corrosion on exposed AA7075-T6 was inhibited due to
remaining CPC relative to pristine AA7075-T6. In addition, after cyclic polarization
scan, pits were observed on the surface of the pristine AA7075-T6, but not on the
exposed AA7075-T6.
Table 12. A CPC ranking method
Time to Failure (Days)
Approximate percentage of
corrosion area when failure
Failure mode
No. of failed samples to
date/total exposed
Ranking when consider all
factors
Amlguard
AV30
AV8
LPS3
55
55
120
85
25%
3%
5%
0
Blistering
Breaching of film
Blistering
3/3
3/3
0/3
0/3
1
2
3
Breaching
of
film
46
4 (the best
one)
Figure 38 A correlation between CPC ranking and short-term interfacial impedance.
A ranking method for CPC performance was developed and applied to the CPC
tested as shown in Table 12. This ranking method was based on the impedance test
results of AA7075-T6 alternate immersion as well as the exposure performance of CPC
treated samples. A relationship between CPC ranking after 120 days exposure and
interfacial impedance was found as shown in Figure 38, which shows that at 40 days, a
CPC with higher interfacial impedance will have better performance after 120 days
exposure. Using data from before 40 days, however, no such correlation was found
between ranking and interfacial impedance.
A correlation between short-term test data and long-term performance was not
found on CPC applied to clad AA2024-T3. The reason for that is due to that the clad
layer is sufficiently corrosion resistant that the gains by the CPC application are minimal.
5.1.2
Relative Corrosivity of Exposure Protocols
Alternate immersion and constant immersion were used in this work for both
alloys. Comparing CPC performances in both environments, it was found that alternate
immersion is more aggressive than constant immersion. For example, for AA7075-T6
alternate immersion, Amlguard failed in 55 days and for AA7075-T6 constant
immersion, Amlguard failed after 100 days exposure. Again, for clad AA2024-T3
47
alternate immersion, Amlguard failed at around 200 days exposure but in constant
immersion, it still goes well after 240 days.
5.1.3
Ranked Performance of CPC
a. Ranked performance of CPC and alloys
CPC performance is different for different alloys and different exposure
environments. Based on measurements in this work, the following ranking was obtained:
For AA7075-T6:
1) AA7075-T6 constant immersion: AV8>LPS3>AV30>Amlguard
2) AA7075-T6 alternate immersion: LPS3>AV8>AV30>Amlguard
3) Precorroded AA7075-T6 constant immersion: AV30>LPS3>AV8>Amlguard
4) Precorroded AA7075-T6 alternate immersion: LPS3>AV30>AV8≈Amlguard
Total ranking for pristine aluminum: LPS3, AV8>AV30>Amlguard; Total ranking for
corroded aluminum: AV30, LPS3>AV8≈Amlguard; Total ranking for AA7075-T6:
LPS3>AV30, AV8>Amlguard.
For clad AA2024-T3:
1) Clad AA2024-T3 constant immersion: AV8>Amguard>LPS3>AV30
2) Clad AA2024-T3 alternate immersion: LPS3>Amlguard>AV30>AV8
3) Precorroded
clad
AA2024-T3
constant
immersion:
alternate
immersion:
AV8>Amlguard≈LPS3≈AV30
4) Precorroded
clad
AA2024-T3
AV30>LPS3>AV8>Amlguard
Total ranking for pristine clad AA2024-T3: Amlguard, LPS3>AV30, AV8; Total ranking
for corroded clad AA2024-T3: AV8, LPS3>AV8, Amlguard; Total ranking for clad
AA2024-T3: LPS3>AV8, AV30, Amlguard.
5.1.4
Failure Mode
Two different CPC failure modes were found in this work: blistering and
breaching of film. The pictures in Figure 39 show these two failure modes. Breaching of
film is local region where corrosion occur due to losing of film protection and Blistering
48
means failure of film by blisters formation, which are local regions where adherence
between the film and the substrate has been lost.
Breaching of film
Blistering of film
Figure 39 CPC failure modes.
Failure modes were controlled by CPC (e.g., Amlguard vs. AV30) not by type of
immersion (e.g., Alternate vs. Constant).
5.2
5.2.1
Precorrosion Effects
Effect of Prior Corrosion on Effectiveness of CPC for Boldly Exposed Aluminum
As shown by the interfacial impedance measurements for the precorroded samples,
the CPC can slow or inhibit the initiated corrosion sites on both clad AA2024-T3 and
AA7075-T6. The interfacial impedance values for precorroded samples were lower than
49
those for pristine samples. For example, the interfacial impedance for precorroded
AA7075-T6 under constant immersion was 2 order magnitudes lower than that for
pristine AA7075-T6 constant immersion. In addition, the relative poor CPC adherence
ability on precorroded aluminum was another factor to make CPC perform worse on
precorroded samples. For AV30, the film started to peel off from the precorroded sample
in the first week of exposure whereas the film started to peel off from the pristine sample
after 6 weeks.
5.3
Wicking
The discussion of the wicking studies will show that fundamental surface chemistry
measurements can predict if a CPC will wick in between two pristine aluminum plates.
The surface chemistry shows that the CPC would wick into a dry simulated lap joint and
a simulated lap joint containing entrained water. The surface chemistry also suggests that
CPC would not wet out and displace water from a completely oxidized aluminum surface
(as modeled by Al2O3). Wicking test on pristine aluminum plates verified the former but
experiments were not available to verify the latter.
5.3.1
CPC Surface Chemistry
CPC and AA2024-T3 Surface Energy. The differences in the overall surface tension of
the CPC and the differences in their polar and dispersive components will affect their
interaction with pristine aluminum surfaces and heavily corroded surfaces. What this is
likely to mean to their relative interactions with surfaces, as well as to water, is the
following:
1. The two higher surface polarity CPC—LPS-3 and AV30—will have greater
adhesion to highly polar surfaces, like the Al2O3 powder when compared to
Amlguard and AV8. When two materials adhere to one another the interfacial
tension between them is low. The high surface polarity CPC will likely have
lower interfacial tensions with water, since water is very polar (63.7% surface
polarity). When two liquids easily form an emulsion the interfacial tension
between them is low.
2. The two lower surface polarity CPC, Amlguard and AV8, may have the
advantage of stronger adhesion to (lower interfacial tensions against) Al.
50
3. The fully oxidized aluminum oxide had a higher surface polarity than the
uncorroded alloy. This trend represents the expected effect of oxidizing a
surface. It should be noted that the alloy used in service does have a thick
oxide coating and that two different methods were used to obtain the contact
angle of the AA2024-T3 and the Al2O3 powder. The next experiment would
be to corrode the alloy surface under different conditions such as high and low
pH and then measure the surface properties.
The first two hypotheses described above were verified by measuring the
CPC/water interfacial tension and the contact angle of CPC on aluminum submerged in
water.
Water displacement. The best adhesion (lowest interfacial tensions) occurs when the
liquid and solid in question have very similar surface polarities. Thus, Amlguard (10.9%
surface polarity) and AV8 (15.8% surface polarity) are more compatible with the low
surface polarity AA2024-T3 (10.1% surface polarity) while LPS3 (25.3% surface
polarity) and AV30 (33.3% surface polarity) are more compatible with high surface
polarity Al2O3 (61.8% surface polarity). These trends are illustrated in Figure 40. The
lower the surface polarity of the CPC the lower its surface energy with the less polar
AA2024-T3 surface. The higher the surface polarity of the CPC, the lower its surface
energy with the more polar Al2O3 surface would be. The lower the energy between the
CPC and the surface the more tightly bound they are.
51
35
14
30
12
10
8
25
CPC/Al
CPC/Al2O3
% polarity
20
15
6
10
4
Surface polarity, %
Surface energy, J/m2
16
5
2
0
0
So Sure
Amlguard
AV8
LPS3
AV30
Figure 40: Surface polarity of CPC and surface energy between CPC and AA2024-T3
and Al2O3 surfaces
The affinity water has for the aluminum surface affects the CPC ability to
displace the water from that surface. The water had a strong affinity for the polar Al2O3
surface. Demonstrated by the low surface tension between water and the Al2O3. In cases
where the interfacial tension for a CPC against a surface is calculated to be less than the
interfacial tension between water and that same surface, it is thermodynamically
favorable to replace a water/surface interface with a CPC/surface interface.
When a CPC interacts with water on an aluminum surface, three interfaces are
formed—CPC/aluminum, water/aluminum and CPC/water. The surface energy between
water and CPC was measured to determine if the CPC could displace water from
aluminum. From high to low, the trend in the interfacial tension with water, is:
Amlguard > AV8 > LPS 3 > AV30
These results were expected, based on the relative surface polarities of the CPC, which
follow the inverse trend as illustrated in Figure 41
52
35
30
8
25
6
20
4
15
10
CPC/WATER
2
% polarity
Surface Polarity, %
Surface Tension, J/m2
10
5
0
0
So Sure
Amlguard
AV8
LPS3
AV30
Figure 41. Surface polarity of CPC and surface tension between CPC and water. The
higher the surface polarity of the CPC the surface tension between it and water.
The more surface polarity a CPC has, the more compatible with water (surface
polarity of 63.7%) it would be expected to be.
Finally, using the CPC/water interfacial tension, the free energies for water
displacement can be calculated. This free energy characterizes the ability of each CPC to
displace water from Al and Al2O3 surfaces. These free energies were calculated by
adding the relevant CPC/Water interfacial tension and the CPC/surface interfacial
tension.
This sum described the final condition following the water displacement
process. The initial condition, which is the relevant water/surface interfacial tension, was
subtracted from the final condition to give the free energy for water displacement.
Negative free energy results indicate that water displacement from the surface is
thermodynamically favored. Positive results indicate that displacement of water is
unfavorable. All four CPC are favored to displace water from Al, but not from Al2O3,
since water adheres strongly to Al2O3. This conclusion can be justified experimentally
and related to water displacement in lap joints. To have any chance of penetrating a lap
joint and displacing water in such a joint a CPC must exhibit, at the very least, a less than
90o contact angle on an aluminum surface submerged in water. A 90o contact angle, in
this situation, defines the condition under which the free energy for water displacement
by a CPC is equal to 0 mJ/m2 . In other words, in cases where the free energy for water
displacement by a CPC is negative, the contact angle should be less than 90o.
Furthermore, the actual contact angle measured in such an experiment, and the magnitude
53
of the free energy for water displacement, should have a trend-wise relationship. The
more negative (favorable) the free energy of the displacement – the lower the contact
angle should be. Such contact angles for CPC displacing water from an aluminum surface
are given in Table 7
All of the contact angles are fairly low, such that the displacement of water is very
favorable – which would portend well for lap joint penetration. The lower the angle, the
better capillary-based penetration into lap joints should be. The contact angle trend does
follow the free energy trend:
Amlguard (-15.85, 32.6o)> AV30 (-18.45, 23.6o)> AV8 (-18.96, 17.3o)> LPS 3 (-19.02,
13.6o)
Emulsification. A good CPC might very well also need to be a good emulsifier of water,
in addition to having good water displacement properties, and capillary spreading action.
The CPC/water interfacial tension values characterize the emulsification abilities. Lower
interfacial tension is better –since interfacial tension is a direct measure of the amount of
work necessary to create interface (emulsify). Listed from best to worst emulsifier the
results were as follows:
AV30 (3.03 mN/m) > LPS3 (3.42 mN/m) > AV8 (4.76 mN/m) > Amlguard (7.80 mN/m)
The interfacial tension trend follows inversely the surface polarity trend for the
CPC. More surface polarity = better compatibility with water = better emulsification
ability. The Amlguard did not emulsify water at all which fits the trend above. No real
distinction could be made between the other three CPC. When water is entrained in a lap
joint or crevice and the CPC removes the water from the surface, if the water is not
emulsified, then the question of where does the water go remains unanswered.
5.3.2 Wicking Into Dry and Wet Lap Joints
Wicking of CPC into a lap joint appears to be an inherently variable process.
Visual observations of wicking into an Al-Plexiglas joint indicated very non-uniform
flow fronts. In some cases the flow front at one location advanced two to three times
farther than at other locations within the same amount of time. The presence of the
machined grooves did not appear to influence the wicking. Because of the variability, it
is not surprising that for a given CPC, the distribution of measured penetration rates was
54
relatively large, as indicated by the relative standard deviation (RSD = standard deviation
/ mean x 100 %). RSD ranged from 5% for AV8 at 0.5“ to 78 % for AV30 at 0.5”.
Typical RSD values were ~ 40 %.
The fiber optic sensor data were consistent with the visual observations.
The
variability associated with sensor data was comparable with visual observations. All of
the CPC tested exhibited comparable wicking ability into a dry aluminum lap joint.
Differences in performance, if any, may come down to differences in other properties
such as water displacement and/or emulsification, film formation and self-healing ability.
The visual observation is subjective and the observer is inclined to mark the time
based on previous data consequently the variability appears smaller. Variability in the
sensor data is associated with variability in how the sensor is packed into the grooves and
judging when the sensor has responded.
Variation in the surface properties of the
aluminum and the Plexiglas may also contribute to the variability in the results.
Furthermore, the dimensions of the occluded region were probably not uniform between
the fasteners. As a result wicking near the fastener is different than wicking between
fasteners.
All of the CPC tested exhibited comparable wicking ability into a dry aluminum
lap joint. The contact angle of all of the CPC against aluminum in air was nearly zero,
and the overall surface energies of the CPC were nearly equal. (Amlguard 22.86 mN/m,
LPS3 22.55 mN/m, AV30 24.15 mN/m, AV8 24.22 mN/m) Therefore, the driving forces
for wicking were about the same. In the case of a wet joint, however, differences in
surface properties of the CPC/water/aluminum interface would potentially indicate
differences in wicking rates. Thermodynamically, the surface measurements did predict
that all of the CPC would wet the aluminum, which they did. As seen in Table 13, the
contact angles of the CPC against aluminum in water were not zero and the surface
energy of the water-CPC interface were different for each CPC. The resultant of the
surface energy and the contact angle could be considered a driving force for wicking and
if so Amlguard (resultant 6.57mN/m) would wick the fastest and AV30 (resultant 2.78)
would wick the slowest.
In the case of the CPC wicking through the joint containing a Plexiglas face sheet
this trend seemed to be the case. When two aluminum face sheets were used, however,
55
the surface property measurements were not a good indication of wicking rate, although
they were a good indicator of whether or not the CPC would wick into a wet joint.
Table 13: CPC surface properties and contact angles
CPC
surface energy against water,
mN/m
contact angle against
aluminum in water
degrees
Resultant,
mN/m
Amlguard
7.8
32.6
6.57
Av8
4.76
17.3
4.54
Lps3
3.42
13.6
3.32
Av30
3.03
23.6
2.78
The CPC wicked much faster into a dry joint than into a wet joint. This result
would be expected thermodynamically by comparing the overall surface tension of the
CPC to the resultants in Table 13. Because the contact angle on a dry aluminum surface
was nearly zero then the resultant on the dry surface would be equal to the overall surface
tension of the CPC which were about 24 mJ/m2 the resultant for the wet surface was
about an order of magnitude lower as was the wicking rate. By itself wicking rate is not
important until the wicking time become so long that the CPC on the outside of the
occluded region dries up and stops any wicking. These tests were very forgiving in that
the CPC was not allowed to dry. There are two significant issues related to CPC and
corrosion in occluded regions (1) the wicking is not uniform or complete which will leave
some areas exposed to corrosive environments and essentially untreated and (2) Although
the CPC removes the water from the surface it may only be trapped in the faying surfaces
or forced to another area where corrosion will continue to occur.
5.3.3
Wicking Into a Pretreated Lap Joint
Water wicking into a pretreated joint would be an indicator of how the CPC
would hold up to the environment once it is applied. Given that the wicking process is
inherently variable, it is not surprising that each of the CPC behave differently when
exposed to water after treatment. Only one lap joint was tested for each CPC, therefore,
56
the effects of variability in wicking rate or coverage are not assessed. These experiments
show that water will penetrate past the CPC by passing through fastener holes, by
wicking around CPC or by softening the CPC and diffusion through it.
These
experiments do not differentiate between the wicking paths or wicking mechanisms.
They only measure that water has penetrated the joint and affected the sensor.
Recall that a dry sensor has a loss peak of 1510 nm, a water wet sensor has a loss
peak of 1530 nm. A sensor wet by CPC has a loss peak off the scale and the instrument
returns a value of 1570 or 1492 nm. In the lap joint treated with LPS3, the 0.5” sensor
wetted out with CPC as expected. The second sensor did not wet out until one day later,
which may have occurred because the LPS3 is a soft waxy film and could potentially
continue to wick into the joint. The peak location dropping to 1560 nm may indicate that
water had diffused into the LPS3 or water had wicked into the joint and part of the sensor
was wetted with water while the other part remained wet with CPC. The former is more
likely because both sensors behaved the same way.
The AV8 appeared to wet out both sensors.
After 13 days the 1.0” sensor had
dropped to 1560 nm indicating that the water had in some way affected the sensor.
Presumably the water reached the sensor by diffusion or directly by wicking into regions
that the CPC did not reach. The AV8 kept the water away form the 0.5” sensor for at
least 33 days.
The AV30 and the Amlguard behaved in the same manner.
Neither CPC
appeared to wet out the sensors. After one day water had reached both sensors in each
lap joint. The water had definitely affected the integrity of the AV30 judging by the
color change of the boldly exposed surface.
The Amlguard became completely
ineffective since corrosion has appeared even on the boldly exposed surfaces and water
had infiltrated the occluded regions. If the CPC does not reach all of the occluded regions
it is safe to say that when exposed to a salt solution the occluded regions are likely to
corrode.
6
Conclusions
The objectives of this project are (a) to develop a quantitative testing protocol that
allows discrimination of CPC performance and allows selection of the most effective
CPC for a particular component, and (b) to test the effectiveness of several commercial
57
CPC on both boldly exposed and occluded regions on aircraft. In this annual report,
progress has been described on both objectives with a focus on boldly exposed surfaces
for the corrosion testing. An investigation of the surface thermodynamic properties and
wicking kinetics was also conducted.
6.1
Development of Quantitative Testing Protocol
•
Alternate immersion in LJSS is more aggressive than constant immersion leading
to failure of CPC at much earlier times (e.g., 55 days for Amlguard on AA7075T6 in alternate immersion vs. 111 days for Amlguard on AA7075-T6 in constant
immersion).
•
A means of CPC ranking was defined using the EIS results and visual assessment
of CPC performance.
•
Generally a substantial decrease in the interfacial impedance (ca. a factor of 5)
correlated to the observation of visible corrosion on the substrate, except for those
samples for which a portion of the CPC film was lost. In those cases, the decrease
in interfacial impedance did not presage a loss in protection.
•
CPC fail via two different modes: blistering and breaching of film.
•
The assessment/prediction methods that have been successful for organic coating
cannot be directly transferred for use in studying CPC. The barrier properties of
the CPC (as measured at high frequency) are low and do not predict corrosion
performance.
•
CPC can somewhat suppress the initiated corrosion on precorroded sample for
some time period as demonstrated by the increased interfacial impedance of
precorroded samples subsequently treated with CPC before re-exposure to
solution. The protection of CPC on precorroded sample is not as good as that of
CPC on pristine sample, with interfacial impedance being 2 orders of magnitude
lower than for pristine 7075 samples treated with CPC.
•
The CPC adherence ability on pristine sample is better than that on precorroded
sample (e.g. for AV30, the film started to peel off from the precorroded sample in
58
the first week of exposure and the film started to peel off from the pristine sample
after 6 weeks).
6.2
Relative Effectiveness of Several Commercial CPC
•
The CPC ranking for AA7075-T6 exposed to alternate immersion was found to be
LPS3 > AV8 > AV30 > Amlguard)
•
All of the CPC tested can provide substantial protection of AA7075-T6, albeit for
a limited time period.
In the constant immersion testing, the interfacial
impedance of the intact CPC-coated AA7075-T6 was approximately three to four
orders of magnitude higher than those for untreated, bare AA7075-T6.
•
AV30-treated samples were more likely to lose a portion of their film after a longtime exposure. Over these areas, it appeared that the vast majority of the CPC
film detached and was found floating in the solution. Nonetheless, there was
remnant CPC left on the apparently bared regions, and this remaining CPC still
protected the substrate for some (as yet undetermined) time period.
6.3
Surface Thermodynamic Properties and Wicking into Occluded Regions
•
The ability of the different CPC to wick into dry and wet simulated lap joints was
predictable by fundamental surface chemistry measurements.
•
Wicking is much slower in wet joints due to larger contact angles on the wet joint.
Wicking rates into wet joints requires the displacement of water and was found to
take as much as ten times longer to move to a given position.
•
The surface polarity of the CPC appeared to dominate the wetting and moisture
displacement capability of CPC.
A low surface tension promotes spreading
whereas high surface polarity promotes water displacement from oxidized
surfaces.
•
Wicking is a spatially and temporally variable process depending on local
geometry and surface properties. In both dry and wet joints, areas of surface
behind the CPC front were left unprotected, leaving behind air and water pockets,
respectively.
This observation could have importance on the initiation of
corrosion within occluded regions.
59
7
Future Work
•
CPC testing and analysis on unclad AA2024-T3 in order to assess their
effectiveness on the base alloy ands comparison to the effectiveness of the clad.
•
Exposure of samples with intentional scratches through a CPC film to accelerate
corrosion on constant immersion samples.
•
Further investigate CPC performance in occluded regions. We have simulated lap
joints of both AA2024-T3 and AA7075-T6 pretreated with CPC and being
exposed to LJSS. Samples are being removed every three months for analysis.
•
Determination of test method for determination of reapplication time.
•
Surface chemistry measurement of aluminum alloy surfaces that have been
corroded as part of lap joints in order to better simulate the relevant surface
chemistry.
•
Wicking measurements for water and CPC into precorroded, simulated lap joints
to better assess the effects of corrosion product surface chemistry and porosity on
water and CPC transport.
8
Acknowledgements
Technical interactions with L. B. Simon (S & K Technologies), J. Dante (Univ. of
Dayton Research Institute), and R. C. Kinzie (Air Force Corrosion Program Office) are
gratefully acknowledged.
Financial support provided through the Air Force Research
Lab (Dr. B. T. Peeler) via NCI Contract (NCI Subcontract Number: NCI-USAF-9191010), Garth Cooke, Program Manager is gratefully acknowledged.
60
9
Appendix—Kruss Report
January 29, 2002
Keith Furrow
Luna Innovations
P.O. Box 11704
Blacksburg, VA 24062-1704
Dear Keith,
We have completed our characterizations of your corrosion prevention compounds
(Dinotrol AV8, Dinotrol AV30, LPS-3, and SoSure Amulguard) with regard to their
interaction capabilities with Al, Al2O3, and water. The work was performed according to
our statement of work proposal of 12/10/01, with the objectives discussed, both in that
proposal and in this document, in mind.
Background Information from the Statement of Work
CPC’s are sprayed onto metal structures to slow corrosion. On boldly exposed surfaces
they are effective. However, their functionality inside occluded regions can be disputed.
The purpose of this investigation is to develop test methods that will quantify the ability a
CPC to penetrate lap joints and displace water. The testing will specifically evaluate:
4. The surface properties of the CPC’s, and the surfaces to which they are applied,
(specifically Al and Al2O3).
5. The ability of various CPC’s to displace water from surfaces to which they are
applied (both in an equilibrium sense, and in a dynamic sense).
6. The propensity of various CPC’s to form emulsions with water (which might be
found to be an advantage, in that it gives the displaced water somewhere to reside,
away from the protected surface).
Item 1 – Surface Properties
Each of the following four CPC’s (Dinotrol AV8, Dinotrol AV30, LPS-3, and So Sure
Amulguard) were characterized for overall surface tension (energy) by the pendant drop
technique. The overall surface tension was then divided into polar and dispersive
components according to the Fowkes approach, which is discussed in mathematical detail
on pages 11 through 14 of the enclosed technical note entitled, “So You Want to Measure
Surface Energy?” - with a special note on liquids appearing on page 9.
The pendant drop technique works as follows. A drop of liquid to be studied for surface
tension is formed on the end of a downward-pointing capillary tip. The drop is typically
formed to about 90% of its detachment volume (from the capillary). The drop is then
61
digitally imaged. The drop’s image is fit by a robust mathematical approach to determine
the drop’s mean curvature at over 300 points along its surface.
The curvature of a drop that is pendant to a capillary tip, at any given point on its surface,
is dependent on two opposing factors (or forces). Gravity works to make the drop
elongated or “drip-like”. The greater the difference in density between the liquid and the
outside gas (in this case air), the greater this force. Surface tension works to keep the drop
spherical- since a sphere has the lowest surface to volume ratio of any shape, and surface
tension is by definition the amount of work necessary to create a unit area of surface.
Pendant drop surface tension evaluation involves observing the balance that exists
between these two forces on a pendant drop, in the form of the drop’s mean curvature at
various points along its surface. Lower surface tension means a more “drip-like” drop
shape, higher surface tension means a more spherical drop shape.
The actual mathematics of pendant drop analysis are based on the Laplace equation
which says that pressure differences exist across curved surfaces. The pressure difference
at any given point on the surface (∆P) is equal to mean curvature of the surface at that
point ((1/r1 +1/r2), where r1 and r2 are the principal radii of curvature) multiplied by twice
the tension (σ) contained in the surface.
∆P = (1/r1 +1/r2) 2 σ
For a pendant drop, the pressure difference within the drop between any two vertical
positions is:
∆ρ g Z
where ∆ρ = the difference in density between the liquid that is forming the drop and the
bulk gas, g = gravity, and Z = the vertical distance between the two positions, as shown
below.
A
z
B
The measurement of surface tension is actually made by determining the mean curvature
on the drop at over 300 points (like those labeled A and B above). Those points are then
62
used in pairs, with the equations given above, to solve for surface tension. In the
following manner:
( (1/r1 +1/r2)at A - (1/r1 +1/r2)at B ) 2 σ = ∆ρ g Zbetween A and B
Therefore, from one drop image, surface tension is determined at least 150 times. These
surface tension values are averaged to give a single value for the overall surface tension
of the drop. This technique has been found to be extremely accurate for determining
surface tensions of liquids with known surface tension (typical errors of less than 0.1%).
Of course, the disadvantage of the pendant drop method is that the densities of all liquids
studied have to be predetermined to the same level of accuracy one expects for the
surface tension data being measured.
For your CPC’s, we determined densities by the most simple of approaches – weighing
controlled volumes of each CPC. The following results were obtained:
Sample
Density (g/cm3)
So Sure
0.8452
LPS 3
0.8079
AV30
0.9017
AV8
0.8768
Using these density values, and running pendant drop experiments, with five drops tested
for each condition, we obtained the following overall surface tension values for your
CPC’s:
Overall Surface Tension Results for CPC’s
So Sure
LPS 3
AV30
AV8
Drop #
(mN/m)
(mN/m)
(mN/m)
(mN/m)
1
22.68
22.52
24.18
24.19
2
22.65
22.50
24.17
24.23
3
22.69
22.58
24.14
24.24
4
22.67
22.53
24.15
24.22
5
22.69
22.63
24.12
24.22
Average
22.86
22.55
24.15
24.22
63
Std. Dev.
0.02
0.05
0.02
0.02
Contact angle measurements were also taken for each CPC against
poly(tetrafluoroethylene), PTFE, as described by the Fowkes theory. The contact angle
work on PTFE, was also performed with five drops being tested, and gave the following
results:
Contact Angles for CPC’s Against PTFE
So Sure
LPS 3
AV30
AV8
Drop #
(degrees)
(degrees)
(degrees)
(degrees)
1
47.6
57.1
65.4
54.5
2
47.7
57.2
65.8
54.3
3
47.3
57.1
66.2
54.7
4
47.9
57.2
65.8
54.4
5
47.2
56.3
65.8
54.2
Average
47.5
57.0
65.8
54.4
Std. Dev.
0.3
0.4
0.3
0.2
Combining these data with Fowkes theory then produced the following component
surface tension data for the CPC’s:
Component Surface Tension Data for CPC’s
Surface Tension
So Sure
LPS 3
AV30
AV8
(mN/m)
(mN/m)
(mN/m)
(mN/m)
Overall
22.86
22.55
24.15
24.22
Polar
2.48
5.70
8.05
3.83
Dispersive
20.38
16.85
16.10
20.39
% Surface Polarity
10.9
25.3
33.3
15.8
At this point, it is worthwhile to draw some preliminary conclusions concerning the
relative natures of the various CPC’s. You can see from the data directly above that the
overall surface tensions are similar for the So Sure and LPS 3 products, but the surface
polarities are quite different (10.9% for So Sure versus 25.3% for LPS 3). The same type
of situation exists for the two AV CPC’s. They have very similar overall surface tensions,
64
which are higher than those of the other two CPC’s. However, their surface polarities
differ widely (33.3% for AV30 and 15.8% for AV8).
What this is likely to mean to their relative interactions with surfaces, as well as to water,
is the following:
1. The two higher surface polarity CPC’s, LPS 3 and AV30, will have greater
adhesion to (lower interfacial tensions against) highly polar surfaces, like your
Al2O3 would be expected to be, versus their counterparts, SoSure and AV8.
They will also likely have lower interfacial tensions with water, since water is
very polar (63.7% surface polarity) – meaning that they would emulsify water
more easily. The emulsification of water is, of course, a possible water
displacement mechanism in lap joints – as we have discussed.
2. The two lower surface polarity CPC’s, So Sure and AV8, may have the
advantage of stronger adhesion to (lower interfacial tensions against) Al (the
surface of which is often found to be less than 20% polar). We will discuss
these aspects in more detail shortly. First, however, I will discuss
characterization of the Al and Al2O3 surfaces.
The aluminum surface you provided was characterized for overall surface energy by the
Fowkes method described in the enclosed application note, with water and ethylene
glycol used as the probe liquids. Five drops each of water and ethylene glycol were
placed on the surface of the Al, and the following contact angles were obtained, by the
goniometer method.
Contact Angles for Water and Ethylene Glycol Drops on Al
Water
Ethylene Glycol
Drop #
(degrees)
(degrees)
1
82.4
55.8
2
82.6
55.7
3
81.9
55.9
4
81.9
55.8
5
82.1
55.7
Average
82.2
55.8
Standard Deviation
0.3
0.1
Applying Fowkes method to the average contact angle data, given that water has an
overall surface tension of 72.8 mN/m, with 46.4 mN/m being polar component, and
ethylene glycol has an overall surface tension of 47.7 mN/m, with 21.3 mN/m being polar
65
component (as discussed on page 9 of the enclosed application note), yields the following
surface energy information for the Al surface.
Al - Surface Energy Components
Surface Energy
Al(mJ/m2)
Overall
34.04
Polar
3.43
Dispersive
30.61
% Surface Polarity
10.07
The Al2O3 powder you provided was likewise be characterized for surface energy
components using ethylene glycol and water, and applying Fowkes theory to the resultant
contact angle data. However, a different contact angle determination method is applied to
powders.
The necessary contact angle values were obtained (in duplicate) by the Washburn
adsorption method using a Kruss Processor Tensiometer K12, with an FL12 powder cell
accessory. Hexane was used as the material constant determination liquid, and all
experiments were performed exactly according to the protocol described on pages 9 and
10 of the enclosed Kruss application note #302, with 1.6 grams of Al2O3 used for each
experiment.
The following properties of hexane, and the probe liquids, were important for this work:
Liquid
Overall
Surface
Tension
(mN/m)
Polar
Component
Dispersive
Component
(mN/m)
(mN/m)
Density
Viscosity
(g/cm3)
(cp)
n-Hexane
18.4
0.0
18.4
0.661
0.33
Ethylene
Glycol
47.7
21.3
26.4
1.109
15.1
Water
72.8
46.4
26.4
0.998
1.00
And, the following material constant and contact angle values were obtained (raw mass2
versus time data from all experiments also attached in graphical form).
66
Test #
Material Constant and Contact Angle Values for Al2O3
1.6 gram packs in a Standard FL12
Contact Angle
Contact Angle
Material Constant
With
With
With Hexane
Water
Ethylene Glycol
(cm5)
(degrees)
(degrees)
1
6.9301 x 10-5
19.3
53.4
2
6.8575 x 10-5
18.7
53.6
Average
6.8938 x 10-5
19.0
53.5
Applying the Fowkes method to these data yields the following surface energy values for
the Al2O3 powder:
Al2O3 - Surface Energy Components
Surface Energy
Al2O3(mJ/m2)
Overall
44.95
Polar
27.79
Dispersive
17.16
% Surface Polarity
61.81
So, what we have learned from the surface energy characterizations on Al and Al2O3 is
that Al has a relatively low surface energy (34.04 mJ/m2) and surface polarity (10.07%)
compared to Al2O3, which has a surface energy of 44.95 mJ/m2 and a surface polarity of
61.81%. This trend represents the expected effect of oxidizing a surface.
Item 2 – Water Displacement by CPC’s
From the surface characterization data given above, it is now possible to calculate
interfacial tension values between each of the CPC’s, as well as water, and the two solid
surfaces we have studied. This is done using Good’s equation (first equation on page 7
of my “So You Want to Measure Surface Energy?” technical note), with the following
results:
Liquid/Solid Interfacial Tensions Between CPC’s and Water and Al and Al2O3
Al
So Sure
LPS3
AV30
AV8
Water
(mN/m)
(mN/m)
(mN/m)
(mN/m)
(mN/m)
1.11
2.32
3.28
1.04
67
24.76
Al2O3
13.80
8.32
5.95
11.13
3.36
In cases where the interfacial tension for a CPC against a surface is calculated to be less
than the interfacial tension between water and that same surface, it is thermodynamically
favorable to replace a water/surface interface with a CPC/surface interface.
Please note, that this is the case for every CPC on a fresh Al surface, but for none of
CPC’s on an Al2O3 surface (model oxidized Al surface). Also note that, the leading
CPC’s, in terms of low interfacial tension on the Al surface, are, of course, the low
surface polarity CPC’s (So Sure and AV8). And, on the Al2O3 surface, the leading
CPC’s, in terms of low interfacial tension, are the ones with higher surface polarity
(LPS3 and AV30).
This is due to surface polarity matching. The best adhesion (lowest interfacial tensions)
occur when the liquid and solid in question have very similar surface polarities. Thus, So
Sure (10.9% surface polarity) and AV8 (15.8% surface polarity) are more compatible
with the Al surface (10.1% surface polarity). And, LPS3 (25.3% surface polarity) and
AV30 (33.3% surface polarity) are more compatible with Al2O3 (61.8% surface polarity).
However, none of the CPC’s matches the surface polarity of the Al2O3 as well as water
(63.7% surface polarity) does. What does this mean to the actual displacement of water
from these surfaces by the CPC’s?
To answer that question completely, we actually need one more piece of information,
which we will follow with a direct measurement of the displacement capabilities of each
CPC, as justification that all this theory still makes good sense!
The other piece of information necessary is the interfacial tension between water and
each of the CPC’s, since when water is displaced by a CPC, there is not just an exchange
of a water/surface interface for a CPC/surface interface. There is also a CPC/water
interface created, during displacement.
Interfacial tensions were measured between each of your CPC’s and water by the pendant
drop method. The measurement procedure for this work was equivalent to that used for
the pendant drop surface tension experiments described above – except that instead of a
drop of CPC being formed air (to measure surface tension), it is formed on the end of an
upward facing capillary tip submerged in water (so that interfacial tension is measured).
In five-drop experiments, the following data was collected:
68
Interfacial Tensions for CPC’s Against Water
Measured by the Pendant Drop Technique
Drop #
So Sure
LPS 3
AV30
AV8
(mN/m)
(mN/m)
(mN/m)
(mN/m)
1
7.78
3.35
3.00
4.77
2
7.85
3.51
3.08
4.80
3
7.73
3.38
3.07
4.69
4
7.83
3.38
2.96
4.77
5
7.79
3.47
3.06
4.75
Average
7.80
3.42
3.03
4.76
Std. Dev.
0.05
0.07
0.05
0.04
You will note that the trend in the interfacial tension values, from high to low, is:
So Sure > AV8 > LPS 3 > AV30
This is completely expected, based on the relative surface polarities of the CPC’s, which
follow the inverse trend:
So Sure (10.9%) < AV8 (15.8%) < LPS 3 (25.3%) < AV30 (33.3%)
The more surface polarity a CPC has, the more compatible with water (surface polarity of
63.7%) it would be expected to be.
Finally, with the CPC/water interfacial tension information in hand, we can calculate
what I believe is the definitive data characterizing how capable each CPC is of displacing
water from Al and Al2O3 surfaces. That is, free energies for water displacement.
These free energies can be calculated by adding together the relevant CPC/Water
interfacial tension and the CPC/surface interfacial tension given above (to give the final
condition, following the water displacement process) and subtracting the relevant
water/surface interfacial tension (initial condition). Negative free energy results indicate
that it is thermodynamically favorable for a CPC to displace water from the surface.
Positive results indicate that displacement of water is unfavorable. And, the magnitude of
the result indicates the magnitude of the favorability or unfavorability, relative to the
other values calculated.
69
Example: To calculate the free energy for So Sure displacing water from Al, we take the
interfacial tension between So Sure and water (7.80 mN/m), add the interfacial tension
between So Sure and Al (1.11 mN/m), and subtract the interfacial tension between water
and Al (24.76 mN/m) – to find that So Sure is favored to displace water from Al by (15.85 mN/m).
Free energy of water displacement results for each of the CPC’s for Al and Al2O3 are as
follows:
Free Energies for Water Displacement by CPC’s
So Sure
2
LPS 3
AV30
2
2
AV8
(mJ/m )
(mJ/m )
(mJ/m )
(mJ/m2)
Al
-15.85
-19.02
-18.45
-18.96
Al2O3
18.24
8.38
5.62
12.53
From this data, you see that all four CPC’s are favored to displace water from Al, but not
from Al2O3, since water adheres strongly to Al2O3, because Al2O3 has a surface polarity
very similar to that of water. You should also note that the magnitude of disfavorability
for water displacement from Al2O3 follows inversely the trend on CPC surface polarity.
So Sure (10.9%) > AV8 (15.8%) > LPS 3 (25.3%) > AV30 (33.3%)
Some statements about the relative utility of each CPC, with regard to how extensively
oxidized of an aluminum surface each CPC, should be able to coat (displace water from)
can be developed from this data - without a whole lot of imagination. And, I will develop
such a thought for you shortly. However, first let’s justify the values calculated above,
with some experimental data.
One of original purposes of this project was to consider water displacement in lap joints.
And, I said that, to have any chance of penetrating a lap joint (and of displacing water in
such a joint) a CPC must exhibit, at the very least, a less than 90o contact angle on an
aluminum surface submerged in water – as per the diagram below.
Al
CPC θ
Water
What may have gotten lost, in the mathematics of the free energy calculations that I
presented above, is another fact that a 90o contact angle, in this situation, is also the
70
condition under which the free energy for water displacement by a CPC is equal to 0
mJ/m2 . In other words, in cases where the free energy for water displacement by a CPC
is negative, the contact angle, measured as shown above, should be less than 90o. And,
furthermore, the actual contact angle measured in such an experiment, and the magnitude
of the free energy for water displacement, should have a trend-wise relationship. The
more negative (favorable) the free energy of the displacement – the lower the contact
angle should be.
We actually measured such contact angles for each of your CPC’s displacing water from
an aluminum surface, with the following results.
Contact Angles for CPC’s on Aluminum Submerged in Water
So Sure
LPS 3
AV30
AV8
Drop #
(degrees)
(degrees)
(degrees)
(degrees)
1
32.7
13.8
23.2
17.2
2
33.0
13.8
24.0
17.5
3
32.8
13.6
24.1
17.6
4
32.3
13.6
23.4
16.9
5
32.3
13.2
23.5
17.2
Average
32.6
13.6
23.6
17.3
Std. Dev.
0.5
0.7
0.7
0.5
I am pleased to note that all of the contact angles are fairly low, such that the
displacement of water is very favorable – which seems good for lap joint penetration,
with LPS 3 leading the pack with the lowest angle. The lower the angle, the better
capillary-based penetration into lap joints should be. (None of the angles was found to
be particularly dynamic in nature. They all developed an equilibrium values just as soon
as the CPC was applied. So the dynamic aspect of this study was not completed.)
However, I am maybe even more pleased to observe that the contact angle trend does
follow the free energy trend:
So Sure (-15.85, 32.6o)> AV30 (-18.45, 23.6o)> AV8 (-18.96, 17.3o)> LPS 3 (-19.02,
13.6o)
We cannot, of course, check the same trend on the Al2O3 , because there is no good way
to measure the contact angle for CPC’s displacing water from a powder – particularly
since the surface energy data tells us that the CPC’s will have contact angles of greater
than 90o, in this situation.
71
However, verifying the trend with Al gives me enough confidence to make some
statements about the relative utility of each CPC, with regard to how extensively oxidized
of an aluminum surface each CPC should be able to coat (displace water from).
Using the data above, and assuming that there is a linear progression from the surface
properties of unoxidized Al to Al2O3 (which I will assume to be representative of
complete oxidation) with % oxidation, one can easily develop the following table.
Free Energies for Water Displacement by CPC’s on Theoretical Surfaces
Oxidation
So Sure
LPS 3
AV30
AV8
%
(mJ/m2)
(mJ/m2)
(mJ/m2)
(mJ/m2)
Al (0%)
-15.85
-19.02
-18.45
-18.96
5%
-14.15
-17.65
-17.24
-17.38
10%
-12.44
-16.28
-16.04
-15.81
15%
-10.74
-14.91
-14.84
-14.24
20%
-9.03
-13.54
-13.63
-12.66
25%
-7.33
-12.17
-12.43
-11.09
30%
-5.62
-10.80
-11.23
-9.51
35%
-3.92
-9.43
-10.02
-7.94
40%
-2.22
-8.06
-8.82
-6.36
45%
-0.51
-6.69
-7.62
-4.79
50%
1.19
-5.32
-6.41
-3.21
55%
2.90
-3.95
-5.21
-1.64
60%
4.60
-2.58
-4.01
-0.07
65%
6.31
-1.21
-2.80
1.51
70%
8.01
0.16
-1.60
3.08
75%
9.71
1.53
-0.40
4.66
80%
11.42
2.90
0.81
6.23
72
85%
13.12
4.27
2.01
7.81
90%
14.83
5.64
3.21
9.38
95%
16.53
7.01
4.42
10.95
Al2O3 (100%)
18.24
8.38
5.62
12.53
This table implies that So Sure is capable of displacing water from aluminum that is up to
45% oxidized. LPS 3 is capable of displacing water from aluminum that is up to 65%
oxidized. AV30 is capable of displacing water from aluminum that is up to 75% oxidized.
And, AV8 is capable of displacing water from aluminum that is up to 60% oxidized.
However, penetration into lap joints will require free energy values lower than 0 mJ/m2.
The lower the better, to promote capillary joint penetration.
Item 3 – Water Emulsification in a CPC
We had talked about the idea that a good CPC might very well also need to be a good
emulsifier of water, in addition to having good water displacement properties, and
capillary spreading action. At this point, we actually have already collected data on the
emulsification abilities of CPC’s, in the process of looking at other aspects. This data
takes the form of the CPC/water interfacial tension values. And, lower interfacial tension
is to be considered better –since interfacial tension is a direct measure of the amount of
work necessary to create interface (emulsify).
The results were as follows (listed from best to worst emulsifier):
AV30 (3.03 mN/m) > LPS3 (3.42 mN/m) > AV8 (4.76 mN/m) > So Sure (7.80 mN/m)
And, of course, the interfacial tension trend follows inversely the surface polarity trend
for the CPC’s. More surface polarity = better compatibility with water = better
emulsification ability.
Conclusions
So, which is the best CPC?
Overall, I’d say the data predict that AV30 is the most versatile. It has the lowest
interfacial tension with water (3.03 mJ/m2) due having the highest surface polarity, so it
can emulsify water the best. It is predicted to be able to displace water on Al surfaces all
the way up to 75% oxidized. And, it has a fairly reasonable displacement angle for joint
penetration on water covered Al (23.6o).
However, in certain situations, LPS3 and AV8 also have their merits, based on the data.
The oxidation state chart, as well as the water displacement contact angle values
measured, indicate that LPS3 (13.6o contact angle for water displacement on Al) may
well be a superior joint penetrator to AV30, at least for Al surfaces that are oxidized less
than about 20%. AV8 (17.3o contact angle for water displacement on Al) may also be
preferable to AV30 up to about 10% surface oxidation, again for its superior joint
73
penetration. However, in both cases water emulsification ability is sacrificed, relative to
AV30.
In the end, however, if you were to design a new CPC, the bulk of this data suggests that
surface polarity of the CPC is the overriding aspect. You want a low surface tension (to
promote wetting and spreading), but high surface polarity (to promote water
emulsification and water displacement from oxidized surfaces) product.
I hope this data proves useful for you. If you have questions or comments about it, or if
you wish to contract for further analyses, please let me know.
The charge for this work is $2500 (4 liquid surface tension component experiments @
$200 each, 1 solid surface energy determination @ $200, 1 powder surface energy
determination @ $500, 4 three-phase contact angle experiments @ $150 each, and 4
liquid/liquid interfacial tension experiments @ $100 each). That sum will be invoiced
against your P.O. number P11212F.
Best Regards,
Christopher Rulison, Ph.D.
Laboratory Manager
1
B. Hinton, et al., 4th International Aerospace Corrosion and Control Symposium, Jakarta,
Indonesia, June 1996
2
L. B. Simon, J. L. Elster, R. G. Kelly, “Quantitative Studies of the Effectiveness of Corrosion
Protection Compounds Used in Lap Splice Joints, “ Proc. of 2000 ASIP Conference, San Antonio
(2000).
3
Karen S. Lewis, Determination of the Corrosion Conditions Within Aircraft Lap-Splice Joints,
Master Thesis, University of Virginia, 1999
4
J. A. Gonzalez, E. Otero, A. Bautista, E. Almeida, M. Morcillo, “Use of electrochemical
impedance spectroscopy for studying corrosion at overlapped joints”, Progress in Organic
Coatings, 33, 61, (1998)
5
Hermenz, C. Paul, Rajagopalan, Raj; Principles of Colloid and Surface Chemistry, 3rd edition;
1997
6
Fowkes, F.M.; Industrial and Engineering Chemistry, 56,12,40, (1964)
7
Good, R.J.; Firifalco, L.A.; J. Phys. Chem., 64, 561, (1960)
74