LABORATORY EVALUATIONS OF CORROSIONPREVENTION COMPOUNDS FOR AIRCRAFT

Feng Gui1, Keith Furrow2, Jackie Williams2, Kevin Cooper2, Robert G. Kelly1

1Department of Materials Science and Engineering

University of Virginia

P. O. Box 400475

Charlottesville, VA 22904

2Luna Innovations

701 Charlton Ave.

Charlottesville, VA 22903

Submitted to:

Garth Cooke

Program Manager

NCI Information Systems

Fairborn, OH

2

Index1 Introduction ......................................................................................................................................... 4

1.1 Fundamentals of Corrosion Prevention Compounds .................................................................. 41.2 Objectives of the Current Work..................................................................................................... 51.3 CPC Tested..................................................................................................................................... 5

2 Design of Test Protocols ...................................................................................................................... 62.1 Test Selection ................................................................................................................................. 6

2.1.1 Solution Composition and Exposure Modes ......................................................................... 62.1.2 Protective Film Formation..................................................................................................... 72.1.3 Inhibitor Leaching and Electrolyte Modification ................................................................. 72.1.4 Surface Chemistry .................................................................................................................. 72.1.5 CPC Wicking Ability .............................................................................................................. 8

3 Sample Preparation and Test Methods ............................................................................................. 93.1 Sample Preparation ....................................................................................................................... 9

3.1.1 Aluminum Alloy Sample Preparation ................................................................................... 93.1.2 CPC Application Method ....................................................................................................... 9

3.2 Test Methods ................................................................................................................................ 103.2.1 Exposure Methods................................................................................................................ 103.2.2 Electrochemical Impedance Spectroscopy .......................................................................... 103.2.3 Cyclic Potentiodynamic Polarization................................................................................... 103.2.4 FTIR ..................................................................................................................................... 10

3.3 Surface Property Measurements................................................................................................. 103.4 Wicking Tests............................................................................................................................... 12

3.4.1 Assembly Of Wicking Specimens......................................................................................... 123.4.2 Wicking Into Wet And Dry Simulated Lap Joints............................................................... 133.4.3 Fiber Optic Sensor Instrumentation and Data Reduction.................................................. 143.4.4 Wicking Into CPC-Treated Lap Joints ................................................................................ 16

4 Results................................................................................................................................................. 174.1 Constant Immersion .................................................................................................................... 17

4.1.1 AA7075-T6............................................................................................................................ 174.1.2 Clad AA2024-T3 ................................................................................................................... 184.1.3 Precorroded AA7075-T6 ...................................................................................................... 194.1.4 Precorroded Clad AA2024-T3 ............................................................................................. 20

4.2 Alternate immersion .................................................................................................................... 204.2.1 AA7075-T6............................................................................................................................ 204.2.2 Clad AA2024-T3 ................................................................................................................... 214.2.3 Precorroded AA7075-T6 ...................................................................................................... 224.2.4 Precorroded Clad AA2024-T3 ............................................................................................. 22

4.3 Proof of Remnant CPC After Apparent Loss of Film ................................................................ 234.3.1 FTIR ..................................................................................................................................... 234.3.2 Cyclic Polarization Scan ...................................................................................................... 24

4.4 Leaching ...................................................................................................................................... 254.5 Surface Properties of CPC, AA2024-T3 and Al2O3.................................................................... 264.6 Water Displacement..................................................................................................................... 274.7 Water Emulsification................................................................................................................... 294.8 Wicking into Dry Lap Joints ....................................................................................................... 314.9 Wicking into Wet Lap Joints ....................................................................................................... 354.10 Wicking into CPC-Treated Lap Joints........................................................................................ 42

5 Discussion ........................................................................................................................................... 445.1 Testing Protocol Development For Boldly Exposed Surface..................................................... 44

5.1.1 Assessment Vs. Prediction of CPC Performance by Test Protocol .................................... 44

3

5.1.2 Relative Corrosivity of Exposure Protocols......................................................................... 475.1.3 Ranked Performance of CPC............................................................................................... 485.1.4 Failure Mode ........................................................................................................................ 48

5.2 Precorrosion Effects .................................................................................................................... 495.2.1 Effect of Prior Corrosion on Effectiveness of CPC for Boldly Exposed Aluminum ......... 49

5.3 Wicking ........................................................................................................................................ 505.3.1 CPC Surface Chemistry ....................................................................................................... 505.3.2 Wicking Into Dry and Wet Lap Joints ................................................................................. 545.3.3 Wicking Into a Pretreated Lap Joint ................................................................................... 56

6 Conclusions......................................................................................................................................... 576.1 Development of Quantitative Testing Protocol........................................................................... 586.2 Relative Effectiveness of Several Commercial CPC................................................................... 596.3 Surface Thermodynamic Properties and Wicking into Occluded Regions ............................... 59

7 Future Work ...................................................................................................................................... 608 Acknowledgements ............................................................................................................................ 609 Appendix—Kruss Report ................................................................................................................. 61

4

1 Introduction

1.1 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.

5

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.

6

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

Amlguard Mil-C-85054 Dark Blue/Green

General purpose, heavy

duty, durable, hard film

forming CPC

WDHF*(hard, dry

film)

Dinitrol ®

AV30None Light Brown

Penetration, corrosion

inhibitingWDSF**(waxy)

Dinitrol®

AV8BMS 3-23 Light Brown

High penetration,

corrosion inhibitingWDSF (non tacky)

LPS3BMS 3-23

BMS 3-29Dark Yellow

Self-healing, anti-sling

lubricant, high VOC

NWDSF***(self

healing waxy film)

2 Design of Test Protocols

2.1 Test Selection

2.1.1 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.

7

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.

8

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.

9

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:

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.

Release sprayingbutton here

10

3.2 Test Methods

3.2.1 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

11

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.

θθθθ

Al

WaterCPC

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.

12

3.4 Wicking Tests

3.4.1 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.

13

Sensor 11.50 in.

Sensor 31.00 in.

Sensor 20.50 in.

Optical Fiber

250 µm

250 µmSilica Filler

Sensor Groove

Pool of CPC

Optical Fiber Leads Flow front under plexiglas,sensor two is activated

Sensor 11.50 in.

Sensor 31.00 in.

Sensor 20.50 in.

Optical Fiber

250 µm

250 µmSilica Filler

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.

14

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.

15

Air has a refractive index of one. For these sensors, light of

wavelength of 1510 nm is coupled out of the fiber when the

environment surrounding the sensor has a refractive index of one.

Water has a refractive index of 1.33. For these sensors, light of

wavelength of 1530 nm is coupled out of the fiber when the

environment surrounding the sensor has a refractive index of 1.33.

CPC has a refractive index of 1.4 to 1.5. When CPC

completely wets the sensor, the refractive index is out of range of

the instrumentation. As a result no loss peak is detected and the

software returns a value of 1492 nm or 1570 nm.

When water and CPC are present or if only a portion of the

1/2:� long sensor is surrounded with CPC, a loss will occur

between 1530 and 1560 cm.

Figure 4: Typical spectra encountered during the wicking process

16

1490

1495

1500

1505

1510

1515

1520

1525

100 150 200 250 300 350 400 450 500

Time, sec.

Wav

e Le

ngth

, nm

Sensor 1Sensor 2Sensor 3

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.

17

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.

Immersion level

Fiber egress pointconnectors

Potential water ingress

Immersion level

Fiber egress pointconnectors

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 Constant Immersion

4.1.1 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.

18

0 50 100 150 200 250 30010

-3

10-2

10-1

100

101

102

Exposure Time (Days)

Inte

rfac

ial I

mpe

danc

e (M

ohm

s.cm

2 )AmlguardAV30AV8LPS3Bare AA7075

Figure 7 Interfacial impedance vs. exposure time for AA7075-T6 constantimmersion

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.

0 50 100 150 200 25010

-2

10-1

100

101

102

Exposure Time (Days)

Inte

rfac

ial I

mpe

danc

e (M

ohm

s.cm

2 )

AmlguardAV30AV8LPS3Bare AA2024

Figure 8 Interfacial impedance vs. exposure time for clad AA2024-T3 constant

immersion

19

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

20

4.1.4 Precorroded Clad AA2024-T3

0 20 40 60 80 100 12010

-2

10-1

100

101

102

Exposure Time (Days)

Inte

rfac

ial I

mpe

danc

e (M

ohm

s.cm

2 ) AmlguardAV30AV8LPS3

Figure 10 Interfacial impedance vs. exposure time for precorroded clad AA2024-

T3, 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 Alternate immersion

4.2.1 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.

21

Figure 11 Interfacial impedance vs. exposure time forAA7075-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 LPS3-

treated sample had the highest interfacial impedance for exposures longer than 20 days.

0 50 100 150 200 25010

-2

10-1

100

101

102

Exposure Time (Days)

Inte

rfac

ial I

mpe

danc

e (M

ohm

s.cm

2 )

AmlguardAV30AV8LPS3

Figure 12 Interfacial impedance vs. exposure time for

clad AA2024-T3, alternate immersion

Inte

rfac

ial I

mpe

danc

e (M

ohm

s.cm

2 )

Exposure Time (Days)

22

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.

23

0 20 40 60 80 100 12010

-1

100

101

102

Exposure Time (Days)

Inte

rfac

ial I

mpe

danc

e (M

ohm

s.cm

2 )

AmlguardAV30AV8LPS3

Figure 14 Interfacial impedance vs. exposure time for precorroded clad AA2024-

T3 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.

24

500 1000 1500 2000 2500 3000 3500 4000 45000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Wavenumber

Abs

orbe

nce

Bare Area on Exposed AA7075CPC on Pristine AA7075

Figure 15 FTIR for bare area on exposed AA7075-T6 (red line) and CPC onpristine AA7075-T6 (Blue line)

4.3.2 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 AA7075-

T6 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.

25

-9 -8 -7 -6 -5 -4 -3 -2-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

logi (A/cm2)

Pote

ntia

l (V

vs. S

CE)

AA7075 PristineAA7075 Exposed (AV30)

Figure 16 Cyclic polarization scan for pristine AA7075-T6 and exposed AA7075-T6 inborate 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

26

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 pendantdrop in technique and the Fowkes, respectively.

Surface Tension Amlguard

(mN/m)

LPS 3

(mN/m)

AV30

(mN/m)

AV8

(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.

27

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]

2/12/1 )(2)(2 P

S

P

L

D

S

D

LLSSL σσσσσσσ −−+=

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

(mN/m)

LPS3

(mN/m)

AV30

(mN/m)

AV8

(mN/m)

Water

(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

28

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

(mN/m)

LPS 3

(mN/m)

AV30

(mN/m)

AV8

(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:

29

Table 6 Free Energies for Water Displacement by CPC

Amlguard

(mJ/m2)

LPS 3

(mJ/m2)

AV30

(mJ/m2)

AV8

(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

(degrees)

LPS 3

(degrees)

AV30

(degrees)

AV8

(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

30

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

31

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 formemulsion 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 standarddeviation 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

32

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.

0

100

200

300

400

500

Amlguard LPS AV8 AV30

Tim

e to

Sen

sor,

sec.

0.5 in. 1.0 in. 1.5 in.

Figure 19 Wicking into a dry lap joint. The error bars represent one standard deviationtaken 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.

33

Table 9: Photographs of AV8 wicking into a simulated lap joint. The orange lineindicates 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

34

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.

1500

1510

1520

1530

1540

1550

1560

1570

0 100 200 300 400Time, sec

loss

pea

k, n

m

0.5"1.0"1.5"

Figure 20 Sensor trace of Amlguard wicking into a dry aluminum lap joint.

1500

1510

1520

1530

1540

1550

1560

1570

0 100 200 300 400

Time, sec

loss

pea

k, n

m

0.5"1.0"1.5"

Figure 21 Sensor trace of LPS-3 wicking into a dry aluminum lap joint.

35

1480

1490

1500

1510

1520

1530

1540

1550

0 100 200 300 400

Time, sec

loss

pea

k, n

m

0.5"1.0"1.5"

Figure 22 Sensor trace of AV8 wicking into a dry aluminum lap joint.

148014901500151015201530154015501560

0 100 200 300 400

Time, sec

loss

pea

k, n

m

0.5"1.0"1.5"

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

36

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.

37

Table 10 Average wicking time to each sensors 0.5�, 1.0� and 1.5� from the edge. Timeswere recorded visually and with the sensors during the experiment

Time to sensor (visual reading), sec Time to sensor (sensor reading), sec

0.5 in. 1.0 in. 1.5 in. 0.5 in. 1.0 in. 1.5 in.

Amlguard 473 667 833 Amlguard 825 683 1300

AV8 620 1853 2830 AV8 870 2400 2405

LPS 1200 1900 3400 LPS 2213 3783 4450

AV30 1583 7000 4700 AV30 5038 5367 1950

Table 11 Standard deviation of wicking times. Four tests were conducted. Zero to fourobservations were made at each sensor location.

Std. Dev. (visual reading), sec Std. Dev., (sensor reading), sec

0.5 in. 1.0 in. 1.5 in. 0.5 in. 1.0 in. 1.5 in.

Amlguard 197 125 125 Amlguard 460 437 505

AV8 275 486 130 AV8 410 0 955

LPS 216 - - LPS 1500 1710 2299

AV30 388 2777 - AV30 4466 3668 0

AV8 wicking into a wet

lap joint 4 minutes after

the CPC was applied

38

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

1500

1510

1520

1530

1540

1550

1560

1570

0 500 1000 1500 2000 2500 3000

time, seconds

Loss

Pea

k w

avel

engt

h, n

m

1.5 inches0.5 inches1.0 inches

Figure 25. Wicking of Amlguard into a wet lap joint. At time 0 the sensor is dry. Thefirst jump in wavelength is water wicking into the joint. The second increase in

wavelength indicates the presence of CPC.

39

1500

1510

1520

1530

1540

1550

1560

1570

0 2000 4000 6000 8000

time, seconds

Loss

Pea

k W

avel

engt

h, n

m

1.5 inches0.5 inches1.0 inches

Figure 26. Wicking of LPS3 into a wet lap joint. At time 0 the sensor is dry. The firstjump in wavelength is water wicking into the joint. The second increase in wavelength

indicates the presence of CPC.

1510

1515

1520

1525

1530

1535

1540

1545

1550

0 2000 4000 6000 8000 10000

Time, seconds

Loss

pea

k W

ave

Leng

th, n

m

1.5 inches0.5 inches1.0 inches

Figure 27. Wicking of AV8 into the lap joint. Note that the response to the CPC isweaker than LPS3 and Amlguard.

40

1505

1510

1515

1520

1525

1530

1535

1540

1545

0 2000 4000 6000 8000 10000 12000

Time, seconds

Loss

Pea

k W

avel

engt

h, n

m

1.5 inches0.5 inches1.0 inches

Figure 28 Wicking of AV30 into the lap joint. Note that the response to the CPC isweaker 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 Plexiglaslap joint.

41

Figure 30: Wicking into a wet lap joint. Data is from visual observations throughPlexiglas.

Figure 31 Wicking into a wet aluminum lap joint. All of the data were collected fromthe sensors.

42

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.

1400

1450

1500

1550

1600

1650

1700

untreated treatedbaseline

day 1 day 13 day 33

wav

elen

gth,

nm

0.5"1.0"

Figure 32. Sensor response in a simulated lap joint treated with LPS3 and immersed inwater.

43

1400

1450

1500

1550

1600

1650

1700

untreated treatedbaseline

day 1 day 13 day 33

Wav

elen

gth,

nm

0.5"1.0"

Figure 33. Sensor response in a simulated lap joint treated with AV8 and immersed inwater.

1400

1450

1500

1550

1600

1650

1700

untreated treatedbaseline

day 1 day 13 day 33

wav

elen

gth,

nm 0.5"

1.0"

Figure 34. Sensor response in a simulated lap joint treated with AV30 and immersed inwater.

44

1400

1450

1500

1550

1600

1650

1700

untreated treatedbaseline

day 1 day 13 day 33

wav

elen

gth,

nm 1.5"

0.5"

Figure 35. Sensor response in a simulated lap joint treated with Amlgurad and immersedin water.

5 Discussion

5.1 Testing Protocol Development For Boldly Exposed Surface

5.1.1 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.

45

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 CPC-

exposed 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 AA7075-

T6 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.

Area of Film Loss

Intact CPC Film (AV30)

Figure 36 An example of film loss sample (Substrate: AA7075-T6; CPC: AV30)

46

10-2

100

102

104

10610

1

102

103

104

Frequency (Hz)

|z|

LJSS (Without Exposed)CPC Exposed LJSS

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

Amlguard AV30 AV8 LPS3

Time to Failure (Days) 55 55 120 85

Approximate percentage of

corrosion area when failure25% 3% 5% 0

Failure modeBreaching of

filmBlistering Breaching of film Blistering

No. of failed samples to

date/total exposed3/3 3/3 0/3 0/3

Ranking when consider all

factors1 2 3

4 (the best

one)

47

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

48

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:

AV8>Amlguard≈LPS3≈AV30

4) Precorroded clad AA2024-T3 alternate immersion:

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

49

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 Precorrosion Effects

5.2.1 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

50

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.

51

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.

52

0

2

4

6

8

10

12

14

16

So Sure AV8 LPS3 AV30

Surf

ace

ener

gy, J

/m2

0

5

10

15

20

25

30

35

Surf

ace

pola

rity,

%

CPC/AlCPC/Al2O3% polarity

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

Amlguard

53

0

2

4

6

8

10

So Sure AV8 LPS3 AV30

Surf

ace

Tens

ion,

J/m

2

0

5

10

15

20

25

30

35

Surf

ace

Pola

rity,

%

CPC/WATER% polarity

Figure 41. Surface polarity of CPC and surface tension between CPC and water. Thehigher 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

Amlguard

54

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

55

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,

56

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

CPCsurface 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,

57

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

58

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 AA7075-

T6 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

59

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 long-

time 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.

60

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-9191-

010), Garth Cooke, Program Manager is gratefully acknowledged.

61

9 Appendix�Kruss Report

January 29, 2002

Keith FurrowLuna InnovationsP.O. Box 11704Blacksburg, 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 theirinteraction capabilities with Al, Al2O3, and water. The work was performed according toour statement of work proposal of 12/10/01, with the objectives discussed, both in thatproposal 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 surfacesthey 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 aCPC 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 areapplied (both in an equilibrium sense, and in a dynamic sense).

6. The propensity of various CPC�s to form emulsions with water (which might befound 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 SureAmulguard) were characterized for overall surface tension (energy) by the pendant droptechnique. The overall surface tension was then divided into polar and dispersivecomponents according to the Fowkes approach, which is discussed in mathematical detailon pages 11 through 14 of the enclosed technical note entitled, �So You Want to MeasureSurface 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 surfacetension is formed on the end of a downward-pointing capillary tip. The drop is typicallyformed to about 90% of its detachment volume (from the capillary). The drop is then

62

digitally imaged. The drop�s image is fit by a robust mathematical approach to determinethe 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 dropelongated or �drip-like�. The greater the difference in density between the liquid and theoutside gas (in this case air), the greater this force. Surface tension works to keep the dropspherical- since a sphere has the lowest surface to volume ratio of any shape, and surfacetension 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 existsbetween these two forces on a pendant drop, in the form of the drop�s mean curvature atvarious points along its surface. Lower surface tension means a more �drip-like� dropshape, higher surface tension means a more spherical drop shape.

The actual mathematics of pendant drop analysis are based on the Laplace equationwhich says that pressure differences exist across curved surfaces. The pressure differenceat any given point on the surface (∆P) is equal to mean curvature of the surface at thatpoint ((1/r1 +1/r2), where r1 and r2 are the principal radii of curvature) multiplied by twicethe 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 verticalpositions is:

∆ρ g Z

where ∆ρ = the difference in density between the liquid that is forming the drop and thebulk gas, g = gravity, and Z = the vertical distance between the two positions, as shownbelow.

z

A

B

The measurement of surface tension is actually made by determining the mean curvatureon the drop at over 300 points (like those labeled A and B above). Those points are then

63

used in pairs, with the equations given above, to solve for surface tension. In thefollowing 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. Thesesurface tension values are averaged to give a single value for the overall surface tensionof the drop. This technique has been found to be extremely accurate for determiningsurface 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 liquidsstudied have to be predetermined to the same level of accuracy one expects for thesurface tension data being measured.

For your CPC�s, we determined densities by the most simple of approaches � weighingcontrolled 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 testedfor each condition, we obtained the following overall surface tension values for yourCPC�s:

Overall Surface Tension Results for CPC�s

Drop #

So Sure

(mN/m)

LPS 3

(mN/m)

AV30

(mN/m)

AV8

(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

64

Std. Dev. 0.02 0.05 0.02 0.02

Contact angle measurements were also taken for each CPC againstpoly(tetrafluoroethylene), PTFE, as described by the Fowkes theory. The contact anglework on PTFE, was also performed with five drops being tested, and gave the followingresults:

Contact Angles for CPC�s Against PTFE

Drop #

So Sure

(degrees)

LPS 3

(degrees)

AV30

(degrees)

AV8

(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 componentsurface tension data for the CPC�s:

Component Surface Tension Data for CPC�s

Surface Tension So Sure

(mN/m)

LPS 3

(mN/m)

AV30

(mN/m)

AV8

(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 therelative natures of the various CPC�s. You can see from the data directly above that theoverall surface tensions are similar for the So Sure and LPS 3 products, but the surfacepolarities are quite different (10.9% for So Sure versus 25.3% for LPS 3). The same typeof situation exists for the two AV CPC�s. They have very similar overall surface tensions,

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which are higher than those of the other two CPC�s. However, their surface polaritiesdiffer 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 greateradhesion to (lower interfacial tensions against) highly polar surfaces, like yourAl2O3 would be expected to be, versus their counterparts, SoSure and AV8.They will also likely have lower interfacial tensions with water, since water isvery polar (63.7% surface polarity) � meaning that they would emulsify watermore easily. The emulsification of water is, of course, a possible waterdisplacement mechanism in lap joints � as we have discussed.

2. The two lower surface polarity CPC�s, So Sure and AV8, may have theadvantage of stronger adhesion to (lower interfacial tensions against) Al (thesurface of which is often found to be less than 20% polar). We will discussthese aspects in more detail shortly. First, however, I will discusscharacterization of the Al and Al2O3 surfaces.

The aluminum surface you provided was characterized for overall surface energy by theFowkes method described in the enclosed application note, with water and ethyleneglycol used as the probe liquids. Five drops each of water and ethylene glycol wereplaced on the surface of the Al, and the following contact angles were obtained, by thegoniometer method.

Contact Angles for Water and Ethylene Glycol Drops on Al

Drop #

Water

(degrees)

Ethylene Glycol

(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 anoverall surface tension of 72.8 mN/m, with 46.4 mN/m being polar component, andethylene glycol has an overall surface tension of 47.7 mN/m, with 21.3 mN/m being polar

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component (as discussed on page 9 of the enclosed application note), yields the followingsurface 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 energycomponents using ethylene glycol and water, and applying Fowkes theory to the resultantcontact angle data. However, a different contact angle determination method is applied topowders.

The necessary contact angle values were obtained (in duplicate) by the Washburnadsorption method using a Kruss Processor Tensiometer K12, with an FL12 powder cellaccessory. Hexane was used as the material constant determination liquid, and allexperiments were performed exactly according to the protocol described on pages 9 and10 of the enclosed Kruss application note #302, with 1.6 grams of Al2O3 used for eachexperiment.

The following properties of hexane, and the probe liquids, were important for this work:

Liquid

OverallSurfaceTension

(mN/m)

PolarComponent

(mN/m)

DispersiveComponent

(mN/m)

Density

(g/cm3)

Viscosity

(cp)

n-Hexane 18.4 0.0 18.4 0.661 0.33

EthyleneGlycol 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).

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Material Constant and Contact Angle Values for Al2O3 1.6 gram packs in a Standard FL12

Test #

Material ConstantWith Hexane

(cm5)

Contact AngleWith

Ethylene Glycol(degrees)

Contact AngleWithWater

(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 forthe 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 isthat 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 of61.81%. 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 calculateinterfacial tension values between each of the CPC�s, as well as water, and the two solidsurfaces we have studied. This is done using Good�s equation (first equation on page 7of my �So You Want to Measure Surface Energy?� technical note), with the followingresults:

Liquid/Solid Interfacial Tensions Between CPC�s and Water and Al and Al2O3

So Sure

(mN/m)

LPS3

(mN/m)

AV30

(mN/m)

AV8

(mN/m)

Water

(mN/m)

Al 1.11 2.32 3.28 1.04 24.76

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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 lessthan the interfacial tension between water and that same surface, it is thermodynamicallyfavorable 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 ofCPC�s on an Al2O3 surface (model oxidized Al surface). Also note that, the leadingCPC�s, in terms of low interfacial tension on the Al surface, are, of course, the lowsurface polarity CPC�s (So Sure and AV8). And, on the Al2O3 surface, the leadingCPC�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, SoSure (10.9% surface polarity) and AV8 (15.8% surface polarity) are more compatiblewith the Al surface (10.1% surface polarity). And, LPS3 (25.3% surface polarity) andAV30 (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 waterfrom 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 eachCPC, as justification that all this theory still makes good sense!

The other piece of information necessary is the interfacial tension between water andeach of the CPC�s, since when water is displaced by a CPC, there is not just an exchangeof a water/surface interface for a CPC/surface interface. There is also a CPC/waterinterface created, during displacement.

Interfacial tensions were measured between each of your CPC�s and water by the pendantdrop method. The measurement procedure for this work was equivalent to that used forthe pendant drop surface tension experiments described above � except that instead of adrop of CPC being formed air (to measure surface tension), it is formed on the end of anupward facing capillary tip submerged in water (so that interfacial tension is measured).

In five-drop experiments, the following data was collected:

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Interfacial Tensions for CPC�s Against WaterMeasured by the Pendant Drop Technique

Drop # So Sure

(mN/m)

LPS 3

(mN/m)

AV30

(mN/m)

AV8

(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, whichfollow 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 of63.7%) it would be expected to be.

Finally, with the CPC/water interfacial tension information in hand, we can calculatewhat I believe is the definitive data characterizing how capable each CPC is of displacingwater from Al and Al2O3 surfaces. That is, free energies for water displacement.

These free energies can be calculated by adding together the relevant CPC/Waterinterfacial tension and the CPC/surface interfacial tension given above (to give the finalcondition, following the water displacement process) and subtracting the relevantwater/surface interfacial tension (initial condition). Negative free energy results indicatethat 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 ofthe result indicates the magnitude of the favorability or unfavorability, relative to theother values calculated.

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Example: To calculate the free energy for So Sure displacing water from Al, we take theinterfacial tension between So Sure and water (7.80 mN/m), add the interfacial tensionbetween So Sure and Al (1.11 mN/m), and subtract the interfacial tension between waterand 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 asfollows:

Free Energies for Water Displacement by CPC�s

So Sure

(mJ/m2)

LPS 3

(mJ/m2)

AV30

(mJ/m2)

AV8

(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 notfrom Al2O3, since water adheres strongly to Al2O3, because Al2O3 has a surface polarityvery similar to that of water. You should also note that the magnitude of disfavorabilityfor 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 extensivelyoxidized 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 developsuch 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 insuch a joint) a CPC must exhibit, at the very least, a less than 90o contact angle on analuminum surface submerged in water � as per the diagram below.

θθθθ

Al

WaterCPC

What may have gotten lost, in the mathematics of the free energy calculations that Ipresented above, is another fact that a 90o contact angle, in this situation, is also the

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condition under which the free energy for water displacement by a CPC is equal to 0mJ/m2 . In other words, in cases where the free energy for water displacement by a CPCis 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 magnitudeof the free energy for water displacement, should have a trend-wise relationship. Themore negative (favorable) the free energy of the displacement � the lower the contactangle should be.

We actually measured such contact angles for each of your CPC�s displacing water froman aluminum surface, with the following results.

Contact Angles for CPC�s on Aluminum Submerged in Water

Drop #

So Sure

(degrees)

LPS 3

(degrees)

AV30

(degrees)

AV8

(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 thedisplacement 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 bettercapillary-based penetration into lap joints should be. (None of the angles was found tobe particularly dynamic in nature. They all developed an equilibrium values just as soonas 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 doesfollow 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 wayto measure the contact angle for CPC�s displacing water from a powder � particularlysince the surface energy data tells us that the CPC�s will have contact angles of greaterthan 90o, in this situation.

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However, verifying the trend with Al gives me enough confidence to make somestatements about the relative utility of each CPC, with regard to how extensively oxidizedof 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 surfaceproperties of unoxidized Al to Al2O3 (which I will assume to be representative ofcomplete oxidation) with % oxidation, one can easily develop the following table.

Free Energies for Water Displacement by CPC�s on Theoretical Surfaces

Oxidation

%

So Sure

(mJ/m2)

LPS 3

(mJ/m2)

AV30

(mJ/m2)

AV8

(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

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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 to45% 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 goodemulsifier of water, in addition to having good water displacement properties, andcapillary spreading action. At this point, we actually have already collected data on theemulsification abilities of CPC�s, in the process of looking at other aspects. This datatakes the form of the CPC/water interfacial tension values. And, lower interfacial tensionis to be considered better �since interfacial tension is a direct measure of the amount ofwork 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 trendfor the CPC�s. More surface polarity = better compatibility with water = betteremulsification ability.

Conclusions

So, which is the best CPC?

Overall, I�d say the data predict that AV30 is the most versatile. It has the lowestinterfacial tension with water (3.03 mJ/m2) due having the highest surface polarity, so itcan emulsify water the best. It is predicted to be able to displace water on Al surfaces allthe way up to 75% oxidized. And, it has a fairly reasonable displacement angle for jointpenetration 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 valuesmeasured, indicate that LPS3 (13.6o contact angle for water displacement on Al) maywell be a superior joint penetrator to AV30, at least for Al surfaces that are oxidized lessthan about 20%. AV8 (17.3o contact angle for water displacement on Al) may also bepreferable to AV30 up to about 10% surface oxidation, again for its superior joint

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penetration. However, in both cases water emulsification ability is sacrificed, relative toAV30.

In the end, however, if you were to design a new CPC, the bulk of this data suggests thatsurface polarity of the CPC is the overriding aspect. You want a low surface tension (topromote wetting and spreading), but high surface polarity (to promote wateremulsification and water displacement from oxidized surfaces) product.

I hope this data proves useful for you. If you have questions or comments about it, or ifyou 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 energydetermination @ $500, 4 three-phase contact angle experiments @ $150 each, and 4liquid/liquid interfacial tension experiments @ $100 each). That sum will be invoicedagainst 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 19962L. B. Simon, J. L. Elster, R. G. Kelly, �Quantitative Studies of the Effectiveness of CorrosionProtection 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, 19994 J. A. Gonzalez, E. Otero, A. Bautista, E. Almeida, M. Morcillo, �Use of electrochemicalimpedance spectroscopy for studying corrosion at overlapped joints�, Progress in OrganicCoatings, 33, 61, (1998)5 Hermenz, C. Paul, Rajagopalan, Raj; Principles of Colloid and Surface Chemistry, 3rd edition;19976 Fowkes, F.M.; Industrial and Engineering Chemistry, 56,12,40, (1964)7 Good, R.J.; Firifalco, L.A.; J. Phys. Chem., 64, 561, (1960)