liu dissertation

7/29/2019 Liu Dissertation

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Copyright

by

Guangjuan Liu

2010


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The Dissertation Committee for Guangjuan Liucertifies that this is the approved version of the following dissertation:

Fluorescent Coatings for Corrosion Detection in Steel

and Aluminum Alloys

Committee:

Harovel G Wheat, Supervisor

John B Goodenough

Desiderio Kovar

Eric M Taleff

Maria Juenger


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and Aluminum Alloys

by

Guangjuan Liu, B.E., M.E.

DISSERTATION

Presented to the Faculty of the Graduate School of

The University of Texas at Austin

in Partial Fulfillment

of the Requirements

for the Degree of

DOCTOR OF PHILOSOPHY

THE UNIVERSITY OF TEXAS AT AUSTIN

May 2010


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Dedicated to my parents and my husband.


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Acknowledgments

It is a pleasure to thank those who made this dissertaion possible.

Foremost, I am heartily thankful to my advisor, Dr. Harovel Wheat,

whose encouragement, guidance and support from the initial to the final level

enabled me to develop an understanding of the subject.

My sincere thanks also goes to all other committee members: Dr. John

Goodenough, Dr. Desiderio Kovar, Dr. Eric Taleff, and Dr. Maria Juenger.

They have made available their support in a number of ways, and helped me

to keep making progress during my doctoral study.

I would also like to acknowledge System & Processes Engineering Cor-

poration (SPEC) for providing partially funding and sample preparation.

I owe my deepest gratitude to my family, especially my parents.Their

unconditional love and long-lasting support have always been my driving force

for my development. Finally, I express my unique love and appreciation to

my husband, Dr. Lin Xu. His encouragement, care and understanding are

definitely the bedrock on which my past and future life builds on.

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and Aluminum Alloys

Publication No.

Guangjuan Liu, Ph.D.

The University of Texas at Austin, 2010

Supervisor: Harovel G Wheat

Coatings are often used as a means of protecting aluminum alloy and

steel structures in industry. The assessment of corrosion under these coatings

can be challenging. Corrosion sensing coatings can exhibit properties that

allow undercoating corrosion to be identified before it can be seen with thenaked eye. This would be very advantageous and could potentially result

in tremendous savings in time and money when structures undergo routine

maintenance.

Our work involved the study of corrosion sensing coatings with incor-

porated fluorescent indicators that can be used to sense the undercoating cor-

rosion on metal substrates. The fluorescent indicator in the coated-aluminum

system was a negative indicator, i.e. the indicator in the coating was ini-

tially fluorescent and subsequently non-fluorescent due to the reduced pH at

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the anodic sites of corrosion. The fluorescent indicator in coated-steel system

was positive, in the sense that the coating changed from non-fluorescent tofluorescent over the cathodic areas due to increased pH.

The corrosion sensing coating was composed of commercial epoxy-

polyamide and the indicator: 7-amino-4-methylcoumarin (7-AMC) for the

coated-aluminum alloy system and 7-diethylamino-4-methylcoumarin (7-DMC)

for the coated-steel system.

The feasibility of using 7-AMC for sensing early undercoating cor-

rosion was demonstrated by using fluorescent observations, Electrochemical

Impedance Spectroscopy (EIS), Scanning Electron Microscope (SEM) and En-

ergy Dispersive Spectroscopy (EDS) tests. EIS results estimated that with

continuous immersion the undercoating corrosion occurred within 24 hours af-

ter immersion in the salt solution. When corrosion occurred, the corrosion was

invisible under natural light. However, small spots appeared in the fluorescent

image, changing from initially fluorescent to non-fluorescent where the anodic

sites were identified by SEM and EDS. In other words, the fluorescent indica-

tor could sense the early undercoating corrosion, although blistering can be a

competing mechanism associated with corrosion under some conditions.

The sensitivity of the 7-AMC corrosion detection system was tested

by applying anodic current to the metal and measuring the charge at which

fluorescence quenching was detected. The critical charge for a detectable pitunder the coating was approximately 2x105 C, which implied a critical radius

of a single corrosion spot or set of spots of approximately 10 m.

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The fluorescent properties of 7-AMC, its effect on the protectiveness,

its sensitivity to pH and its concentration in the coating are explored as well.Fourier transform spectroscopy (FTIR) was used to characterize the structure

of the coating with and without 7-AMC. The results suggested that there is

no structure change occurring after adding 7-AMC into the coating.

Fluorescence behavior, electrochemical behavior, microscopic evidence,

and visual observations of coated steel specimens with 7-DMC are compared

based on exposure to saltwater conditions. Some of the challenges associated

with the use of these types of coatings will be presented. This includes the

interference from the increased production of ferrous and ferric ions. All of

this information is aimed at the development of corrosion sensing coatings that

can reveal undercoating corrosion before it is visible to the naked eye.

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Table of Contents

Acknowledgments v

Abstract vi

List of Tables xiii

List of Figures xiv

Chapter 1. Introduction 1

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Coating as a means of corrosion protection . . . . . . . . . . . 2

1.3 Corrosion of the coated-metal system . . . . . . . . . . . . . . 4

1.3.1 Blistering . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3.2 Cathodic delamination . . . . . . . . . . . . . . . . . . . 6

1.4 Undercoating corrosion detection . . . . . . . . . . . . . . . . . 7

1.4.1 General techniques . . . . . . . . . . . . . . . . . . . . . 7

1.4.2 Indicators . . . . . . . . . . . . . . . . . . . . . . . . . . 81.4.2.1 Color pH change indicator . . . . . . . . . . . . 9

1.4.2.2 Fluorescent indicator . . . . . . . . . . . . . . . 11

1.5 Coating estimation method- Electrochemical Impedance Spec-troscopy (EIS) . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

1.5.1 EIS theory . . . . . . . . . . . . . . . . . . . . . . . . . 21

1.5.2 Data analysis . . . . . . . . . . . . . . . . . . . . . . . . 22

1.6 Objectives and overview of dissertation . . . . . . . . . . . . . 27

Chapter 2. Experimental and Materials 29

2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.2 Test specimen preparation . . . . . . . . . . . . . . . . . . . . 31

2.3 Test procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

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2.3.1 Electrochemical tests . . . . . . . . . . . . . . . . . . . 34

2.3.2 Scanning Electron Microscopy (SEM) & Energy Disper-

sive Spectrometry (EDS) . . . . . . . . . . . . . . . . . 352.3.3 Fourier transform spectroscopy (FTIR) test . . . . . . . 35

2.3.4 The fluorescent absorption and emission spectrum tests 37

Chapter 3. Preliminary Results and Discussion 38

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.2 Experimental procedure . . . . . . . . . . . . . . . . . . . . . . 39

3.2.1 Specimen preparation . . . . . . . . . . . . . . . . . . . 39

3.2.2 Prototype FCI paint scanner system . . . . . . . . . . . 40

3.2.3 Saltwater spray tests . . . . . . . . . . . . . . . . . . . . 413.2.4 EIS test . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . 42

3.3.1 Aluminum coupons examined using the FCI scanner system 42

3.3.2 Steel coupons examined using the FCI scanner system . 44

3.3.3 Small coupons examined using EIS . . . . . . . . . . . . 48

3.3.3.1 Aluminum coupons examined using EIS . . . . 48

3.3.3.2 Steel coupons examined using EIS . . . . . . . . 51

Chapter 4. Characteristics of Epoxy-polyamide Based Aluminum

Alloy System 554.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

4.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

4.2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . 60

4.2.2 Test procedure . . . . . . . . . . . . . . . . . . . . . . . 60


4.3.1 Basic corrosion sensing characteristics of the coating . . 64

4.3.1.1 Influence of fluorescent indicator on fluorescenceintensity of the coating system . . . . . . . . . . 64

4.3.1.2 Response of fluorescent indicator to undercoatingcorrosion . . . . . . . . . . . . . . . . . . . . . . 66

4.3.2 The sensitivity of 7-AMC in the coating to corrosion . . 73

4.3.3 Basic compositional analysis of the coating system . . . 78

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4.3.3.1 Elemental distribution . . . . . . . . . . . . . . 78

4.3.3.2 Chemical structures of different layers . . . . . 83

4.3.4 Characteristics of fluorescent indicator . . . . . . . . . . 88

4.3.4.1 The absorption and emission spectra of 7-AMCin different solutions . . . . . . . . . . . . . . . 88

4.3.4.2 The sensing of the fluorescent indicator to pH . 89

4.3.4.3 Reversible effect of 7-AMC . . . . . . . . . . . 92

4.3.4.4 The sensing of 7-AMC to ions . . . . . . . . . . 94

4.3.4.5 Effect of air and UV light on fluorescence degra-dation . . . . . . . . . . . . . . . . . . . . . . . 98

4.3.4.6 Determination of the indicator concentration . . 101

4.3.5 Protectiveness of the coating . . . . . . . . . . . . . . . 1034.3.5.1 Effect of coating thickness . . . . . . . . . . . . 103

4.3.5.2 Influence of 7-AMC on the protectiveness of thecoating system . . . . . . . . . . . . . . . . . . 111

4.3.6 Corrosion mechanism . . . . . . . . . . . . . . . . . . . 116

Chapter 5. Characteristics of Epoxy-polyamide Based Steel Sys-tem 128

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

5.2 Experimental procedure . . . . . . . . . . . . . . . . . . . . . . 130

5.2.1 Characteristics of corrosion-sensing coating . . . . . . . 1305.2.2 Absorption and emission spectra of 7-DMC in different

polarity solution . . . . . . . . . . . . . . . . . . . . . . 131

5.2.3 Absorption and emission spectra of 7-DMC in differentpH solution . . . . . . . . . . . . . . . . . . . . . . . . . 131

5.2.4 Reversible effect of 7-DMC to pH . . . . . . . . . . . . . 131

5.2.5 The sensing of 7-DMC to F e3+ and F e2+ ions . . . . . . 132


5.3.1 Element analysis of the coating . . . . . . . . . . . . . . 132

5.3.2 7-DMC characteristics . . . . . . . . . . . . . . . . . . . 133

5.3.2.1 Determination of absorption and emission spec-tra of 7-DMC . . . . . . . . . . . . . . . . . . . 133

5.3.2.2 Determination of pH reaction range of 7-DMC . 133

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5.3.3 Reversible effect of 7-DMC to pH . . . . . . . . . . . . . 136

5.3.4 Sensing of 7-DMC in the coating to corrosion . . . . . . 138

5.3.5 Parameters that affect the fluorescent indicator . . . . . 146

5.3.5.1 Ferrous and ferric ions . . . . . . . . . . . . . . 146

5.3.5.2 Surface modification . . . . . . . . . . . . . . . 149

Chapter 6. Conclusions and future work 152

6.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

6.1.1 Coated-aluminum corrosion sensing system . . . . . . . 153

6.1.2 Coated-steel corrosion sensing system . . . . . . . . . . 156

6.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

6.2.1 Coated-aluminum alloy system . . . . . . . . . . . . . . 1586.2.2 Coated-steel system . . . . . . . . . . . . . . . . . . . . 158

Bibliography 160

Vita 172

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List of Tables

1.1 Common circuit elements . . . . . . . . . . . . . . . . . . . . . 23

2.1 The specification of primer and topcoat . . . . . . . . . . . . . 30

2.2 Composition limits for 2024 aluminum alloys, referring to [1] . 32

2.3 Composition limits for 1018 steel alloys, referring to [2] . . . . 32

4.1 Element percentage of labeled site . . . . . . . . . . . . . . . . 73

4.2 Natural color, and fluorescent color of the coatings with theindicator under UV light. . . . . . . . . . . . . . . . . . . . . . 79

4.3 Element percentage of coating . . . . . . . . . . . . . . . . . . 83

4.4 The excitation and emission wavelength of fluorescent indicatorin different solvent . . . . . . . . . . . . . . . . . . . . . . . . 89

4.5 The excitation and emission wavelength of 7-AMC in differentpH solution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

4.6 The initial time for blistering appearance with different testmethods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

5.1 Element percentage of primer and topcoat layer. . . . . . . . . 1335.2 Element analysis of corrosion spots on steel substrate after primer

plus indicator layer is peeled off. . . . . . . . . . . . . . . . . . 145

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List of Figures

1.1 Cathodic blistering and delamination of a scribed coated steelpanel exposed to NaCl solution, taken from [60]. . . . . . . . . 6

1.2 Photograph of color change of phenolphthalein paint on Al 5454substrate following immersion in 1.0 M NaCl solution after 8days, taken from [87]. . . . . . . . . . . . . . . . . . . . . . . . 10

1.3 Accelerated corrosion testing in salt fog spray chamber (twolayer structures-tape protected exposed area) of scribed alu-

minum panels. From left to right: 0, 96, 432 h, taken from[26]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.4 Schematic energy diagram showing light absorption and fluo-rescence emission, referring to [48,67]. . . . . . . . . . . . . . 13

1.5 Absorption and emission spectrum of a fluorescent material,taken from [48]. . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.6 Structure of oxidized and reduced Fluorescein, taken from [6]. 16

1.7 Conceptual drawing of fluorescent indicator in the coating forcorrosion sensing, taken from [3]. . . . . . . . . . . . . . . . . 18

1.8 Chemical structure of oxine, taken from [25]. . . . . . . . . . . 19

1.9 The chemical structure and numbering scheme of coumarin,taken from [73]. . . . . . . . . . . . . . . . . . . . . . . . . . . 20

1.10 Equivalent circuit for coated metal system, taken from [57]. . . 24

1.11 a: Bode impedance plot, b: Bode angle plot, taken from [57]. . 26

2.1 a. Chemical structure of 7-amino-4-methylcoumarin. b. Chem-ical structure of 7-diethylamino-4-methylcomarin, taken from[11, 61]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.2 Schematic drawing of coating system used on the Al and steelalloy substrates. . . . . . . . . . . . . . . . . . . . . . . . . . . 33

2.3 Setup for electrochemical testing. . . . . . . . . . . . . . . . . 35

2.4 Exploded view of K105 flat specimen holder, taken from [32]. . 36

3.1 left: Prototype FCI Paint Scanner and PC, right: FCI PaintScanner Internal View, taken from [19]. . . . . . . . . . . . . 41

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3.2 a. Aluminum coupons in the salt spray tank, b. The updatedsalt spray tank for steel coupons, taken from [19]. . . . . . . . 42

3.3 Coupon 10 (metal plus primer with indicator) at start of expo-sure, taken from [19]. . . . . . . . . . . . . . . . . . . . . . . . 43

3.4 Scanner signal for coupon 11 shows localized darkening on theleft edge after 456 hours salt spray exposure. (Darkening onright may be artifacts), taken from [19]. . . . . . . . . . . . . 45

3.5 Difference signal for coupon 13 does not show any obvious dark-ening after 456 hours. The stripped coupon also showed littlecorrosion, taken from [19]. . . . . . . . . . . . . . . . . . . . . 45

3.6 Coupons 28, 29, and 30 showing the pre-rusted patterns cre-ated by placing water drops on the surface of the bare steelmetal before painting the coupons with topcoat with indicator,

taken from [19]. . . . . . . . . . . . . . . . . . . . . . . . . . 473.7 Coupon 29 pre-rusted pattern with topcoat paint layer with

indicator showing strong fluorescence response, taken from [19]. 47

3.8 Bode impedance plots for the coated aluminum (metal plus top-coat) immersed in 3.5% NaCl. . . . . . . . . . . . . . . . . . . 50

3.9 Bode impedance plot for the coated aluminum (metal plus primer)immersed in 3.5% NaCl. . . . . . . . . . . . . . . . . . . . . . 50

3.10 Bode impedance plot of coated-aluminum with different coatingcombinations immersed in 3.5% NaCl solution after 5 days. . . 51

3.11 Bode impedance plot for the coated-steel (metal plus topcoat)immersed in 3.5% NaCl. . . . . . . . . . . . . . . . . . . . . . 52

3.12 Bode impedance plot for the coated-steel (metal plus primer)immersed in 3.5% NaCl . . . . . . . . . . . . . . . . . . . . . . 52

3.13 Bode impedance plot of the coated-steel with different coatingcombinations immersed in 3.5% NaCl solution after 9 days. . . 53

4.1 Left: Side view of corrosion wheel and test chamber, Right:Front view of corrosion wheel and test chamber. . . . . . . . . 62

4.2 The sample setup for spray test. . . . . . . . . . . . . . . . . . 62

4.3 left: Coated aluminum alloy with primer plus indicator, right:Coated aluminum alloy with primer without indicator. . . . . 65

4.4 left: Coated aluminum alloy with primer plus indicator plustopcoat, right: Coated aluminum alloy with primer plus topcoatwithout indicator. . . . . . . . . . . . . . . . . . . . . . . . . . 65

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4.5 Bode impedance plots for coated 2024 aluminum with primerplus indicator and topcoat system immersed in 3.5% NaCl through

48 hrs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684.6 Bode impedance of bare aluminum alloy immersing in 3.5%

NaCl solution. . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

4.7 Coated-2024 aluminum with primer plus indicator and topcoatsystem immersed in 3.5% NaCl solution at 0 hrs. The left figureis 10X fluorescent image with 5V UV light The right figure isthe 10X image with natural light. . . . . . . . . . . . . . . . . 69

4.8 Coated-2024 aluminum with primer plus indicator and topcoatsystem immersed in 3.5% NaCl solution at 4 hrs. The left figureis 10X fluorescent image with 5V UV light The right figure isthe 10X image with natural light. . . . . . . . . . . . . . . . . 69

4.9 Coated-2024 aluminum with primer plus indicator and topcoatsystem immersed in 3.5% NaCl solution at 24 hrs. The leftfigure is a 10X fluorescent image with 5V UV light The rightfigure is the 10X image with natural light. . . . . . . . . . . . 70

4.10 Coated-2024 aluminum with primer plus indicator and topcoatsystem immersed in 3.5% NaCl solution at 48 hrs. The leftfigure is a 10X fluorescent image with 5V UV light. The rightfigure is the 10X image with natural light. . . . . . . . . . . . 70

4.11 a. SEM image of pH-induced fluorescent quenched site, b.Chlorine element mapping of quenched site, c. Aluminum ele-ment mapping of quenched site, d. Oxygen element mapping ofquenched site. . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

4.12 Detection time for fluorescent intensity change v.s. applied cur-rent density for Al alloy coated with primer plus indicator coat-ing in 3.5 wt% NaCl solution. . . . . . . . . . . . . . . . . . . 75

4.13 Modified detection sensitivity for fluorescent intensity changevs. applied current density for Al alloy coated with primer plusindicator coating in 3.5 wt% NaCl solution. . . . . . . . . . . 77

4.14 SEM and EDS image of primer plus indicator layer (Part I) : A:SEM image of primer plus indicator layer (100X), B: SEM imageof primer plus indicator layer(500X), C: Mapping of carbon inimage B, D: Mapping of oxygen in image B. . . . . . . . . . . 80

4.15 SEM and EDS image of primer plus indicator layer (Part II) :

E: Mapping of magnesium in image B, F: Mapping of calciumin image B, G: Mapping of titanium in image B, H: Mapping ofsilicon in image B. . . . . . . . . . . . . . . . . . . . . . . . . 81

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4.16 SEM and EDS image of primer plus indicator plus topcoat layer.A: SEM image of primer plus indicator plus topcoat layer, B:

Mapping of carbon, C: Mapping of oxygen , D: Mapping ofsilicon, E: Mapping of calcium, F: Mapping of titanium. . . . . 82

4.17 FTIR spectra of primer and topcoat layer. . . . . . . . . . . . 84

4.18 FTIR structure of primer and primer plus 0.05 wt% indicatorlayers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

4.19 FTIR structure of primer and primer plus 0.5 wt% indicator layer 86

4.20 The excited state protonation equilibrium of 7-AMC and 7-DMC in acid solution, referring to [58] . . . . . . . . . . . . . 90

4.21 The ground state protonation equilibrium of 7-AMC and 7-DMC in acid solution, referring to [58] . . . . . . . . . . . . . 90

4.22 The chemical structures of excited 7-AMC and 7-DMC in strongalkali, referring to [58] . . . . . . . . . . . . . . . . . . . . . . 91

4.23 Absorption and fluorescence spectra of 7-AMC in solutions ofdifferent pH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

4.24 The relative fluorescent intensity of 7-AMC in different pH so-lution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

4.25 Fluorescent images at different steps, step 1: Before exposure,step 2: After immersion in 0.5 M HCl, step 3: After immersionin 0.5 M NaOH, step 4: after immersion in 0.5 M HCl, step 5:After immersion in 0.5 M NaOH . . . . . . . . . . . . . . . . 95

4.26 The absorption and emission spectra ofAl2(SO4)3 solution with7-AMC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

4.27 The absorption and emission spectra of MgCl2 solution with7-AMC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

4.28 A: Fluorescent images of primer plus indicator before exposureto air, B: Fluorescent images comparison of primer plus indica-tor after 3 years exposure to air . . . . . . . . . . . . . . . . . 98

4.29 A: Fluorescent image of coated aluminum alloy with primer plusindicator before UV exposure, B: Fluorescent image of coatedaluminum alloy with primer plus indicator after 10 hours UVexposure, C: Fluorescent image of coated aluminum alloy withprimer plus indicator after 15 hours UV exposure. . . . . . . . 99

4.30 Absorption and emission intensity of fluorescent indicator in

ethanol solution at different UV exposure time. . . . . . . . . 1004.31 A: Fluorescent observation of the primer layer with 0.05 wt%

indicator, B: Fluorescent observation of the primer layer with0.5 wt% indicator . . . . . . . . . . . . . . . . . . . . . . . . . 102

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4.32 Impedance comparison of primer plus different amount of indi-cator system on aluminum alloy immersed in 3.5% NaCl through

168 hrs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1024.33 A. Before test. B. Blister appears after 168 hours immersion

(the blistering is small and is not so obvious under the UV light)C. Blister after 30-day immersion. D. The metal corrosion underthe coating after 30-day immersion. . . . . . . . . . . . . . . . 104

4.34 Fluorescent image with different layers and their thickness, A:primer plus indicator (30 microns), B: primer plus indicator(30 microns) covered with topcoat (20 microns), C: primer plusindicator (30 microns) covered with topcoat (50 microns), D:primer plus indicator (30 microns) covered with topcoat (95microns). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

4.35 The quantitative fluorescent intensity of coating with different

layers and their thickness. . . . . . . . . . . . . . . . . . . . . 1074.36 Bode impedance plots for coated aluminum with primer plus

indicator plus topcoat immersing in 3.5% NaCl over 192 hours. 109

4.37 Bode angle plots for coated aluminum with primer plus indica-tor plus topcoat immersing in 3.5% NaCl over 192 hours. . . . 109

4.38 Equivalent circuit for coated metal system, taken from [57]. . . 110

4.39 Modified electrical circuit used to simulate the EIS results. Here,Rs=solution resistance; Rpore=pore resistance of the coating;Rp=polarization resistance; CP Ec and CP E=constant phaseelement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

4.40 Impedance Rpore plots of coated aluminum with different thick-ness coating vs. immersion time in 3.5% NaCl solution for coat-ings of thickness of 30-80 m . . . . . . . . . . . . . . . . . . 112

4.41 Left: Bode impedance plots for coated aluminum with primerimmersed in 3.5% NaCl for up to 356 hrs, Right: Bode angleplots for coated aluminum with primer immersing in 3.5% NaClfor up to 356 hrs. . . . . . . . . . . . . . . . . . . . . . . . . . 114

4.42 Left: Bode impedance plots for coated aluminum with primerplus indicator immersed in 3.5% NaCl for up to 144 hrs, Right:Bode angle plots for coated aluminum with primer plus indica-tor immersed in 3.5% NaCl for up to 144 hrs. . . . . . . . . . 114

4.43 Modified electrical circuit used to simulate the EIS results. Here,

Rs=solution resistance; Rpore=pore resistance of the coating;Rp=polarisation resistance; CP Ec and CP E=constant phaseelement; Ws=Warburg impedance. . . . . . . . . . . . . . . . 115

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4.44 Left: Bode simulated and tested impedance plots for coatedaluminum with primer plus indicator and pure primer immersed

in 3.5% NaCl for 0 hrs, Right: Bode simulated and tested angleplots for coated aluminum with primer plus indicator and pureprimer immersed in 3.5% NaCl for 0 hrs. . . . . . . . . . . . . 115

4.45 Impedance Rpore plots of coated aluminum with primer plusindicator and pure prime system primer plus indicator and pureprimer VS. immersion time in 3.5% NaCl solution . . . . . . . 116

4.46 Coated-2024 aluminum with primer plus indicator system im-mersed in 3.5% NaCl solution at 0 hrs. The left figure is thefluorescent image with 5V UV light. The right figure is theimage with natural light. . . . . . . . . . . . . . . . . . . . . . 118

4.47 Coated-2024 aluminum with primer plus indicator system im-mersed in 3.5% NaCl solution at 23 hrs. The left figure is the

fluorescent image with 5V UV light. The right figure is theimage with natural light. . . . . . . . . . . . . . . . . . . . . . 118

4.48 Coated-2024 aluminum with primer plus indicator system im-mersed in 3.5% NaCl solution at 67 hrs. The left figure is thefluorescent image with 5V UV light. The right figure is theimage with natural light. . . . . . . . . . . . . . . . . . . . . . 118

4.49 Aluminum 2024 surface after peeling the coating off, red arrowpoints the corroded circle. . . . . . . . . . . . . . . . . . . . . 119

4.50 Left: The blistering after 4 days immersion, Right:The blister-ing after 5 days immersion. . . . . . . . . . . . . . . . . . . . . 121

4.51 Coated-Al 2024 immersed in 3.5% NaCl solution on 1st day. a

is 10X fluorescent image with 5V UV light. b is the 10X imagewith natural light. . . . . . . . . . . . . . . . . . . . . . . . . 122

4.52 Coated-Al 2024 immersed in 3.5% NaCl solution on 4th day. ais 10X fluorescent image with 5V UV light. b is the 10X imagewith natural light. . . . . . . . . . . . . . . . . . . . . . . . . 123



4.55 The schematic of the main stress in the blistering, taken from[13]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

5.1 The absorption and emission spectra of 7-DMC in different so-lutions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

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5.2 The relative fluorescent intensity of 7-DMC in different pH so-lution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

5.3 The reversible sensing of 7-DMC to high-low pH cyclic test. . 1375.4 Fluorescent observation of steel plus primer with indicator, be-

fore immersion. . . . . . . . . . . . . . . . . . . . . . . . . . . 139

5.5 Fluorescent observation of steel plus primer with indicator, after7 days immersion in 3.5% NaCl solution. . . . . . . . . . . . . 139

5.6 Fluorescent observation of steel plus primer with indicator, after14 days immersion in 3.5% NaCl solution. . . . . . . . . . . . 140



5.9 The steel substrate after peeling the coating off. . . . . . . . . 141

5.10 Bode plot of coated-steel (primer plus indicator) immersed in3.5% NaCl. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

5.11 Fluorescent observation of coated-steel (primer plus indicatorwith thickness of 30 microns) a. After 30 minutes of exposureto 3.5% NaCl, b. After 62 hours of exposure to 3.5% NaCl. . . 143

5.12 SEM topography of 1018 steel substrate after peeling primerplus indicator layer off. . . . . . . . . . . . . . . . . . . . . . . 145

5.13 Fluorescence response of 7-DMC to ferrous ions tested by adding0.2 ml 0.05 wt% 7-DMC into 3 ml ferrous chloride solution. . . 147

5.14 Fluorescence response of 7-DMC to ferric ions tested by adding0.2 ml 0.05 wt% 7-DMC into 3 ml ferric sulfate solution. . . . 148

5.15 Fluorescent image of 7-DMC without surface modification inthe coating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

5.16 Fluorescent image of 7-DMC with surface modification in thecoating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

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Chapter 1

Introduction

1.1 Introduction

Steel and aluminum alloys are two of the most widely used metal mate-

rials in industry. Steel is most often the first choice due to its excellent balance

of strength, formability, abundance and low cost. Aluminum alloys are not as

strong as steel, but are approximately 50% lighter. Thus, aluminum alloys are

mainly used in weight-constrained circumstances, such as in ships and aircraft

[45,63,76].

One serious concern about using metallic materials is corrosion, the

destructive attack from a chemical or electrochemical reaction. No metals

are completely corrosion resistant under all circumstances. Stainless steels

are considered very corrosion resistant, but still suffer pitting corrosion in

certain environments. Aluminum alloys also have excellent corrosion resistant

properties because of a passive protective oxide film. However, this film is only

formed in a certain pH range. Outside this pH range, aluminum alloys can

suffer serious corrosion problems [21, 24, 44, 78].

Corrosion is a big threat to steel and aluminum alloy structures in

industry. For example, in 1998, corrosion costs an economic loss of 3.1%

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of the US gross domestic product (GDP) as reported by CC Technologies

in a cooperative agreement with the Department of Transportation FederalHighway Administration and NACE International in 2001 [81]. The direct

costs of corrosion for military systems and infrastructure is approximately $20

billion annually, one of the largest components of life-cycle costs for military

weapon systems [46].

Corrosion can take various forms, such as uniform, galvanic, crevice,

pitting, and environmentally induced cracking. The development and rate

of corrosion depend on the material, the environment (water, minerals, and

salts), the availability of oxygen, the ambient temperature, and the exposure

time to the environment.

1.2 Coating as a means of corrosion protection

Widely used corrosion control methods include selecting high intrin-

sic corrosion resistant materials, deploying catholic protection, and applying

protective coatings. Protective coatings are widely applicable under various

corrosive conditions, and are cost effective. They can loosely be categorized

into three types: metallic coatings, inorganic non-metallic coatings and organic

coatings.

Metallic coatings can be divided into two types, noble coatings and sac-

rificial coatings. Noble coatings are noble in the galvanic series with

respect to the metal substrate. They can act as a protective barrier to

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protect the metal substrate from attack by corrosive agents. Sacrificial

coatings, like Zn coatings on a steel substrate, are sacrificed as the anodewhile the substrate is protected as the cathode. Compared with noble

coatings, sacrificial coatings can provide cathodic protection in addition

to acting as a protective barrier for the metal substrate [22, 85].

Inorganic non-metallic coatings can be formed by chemical reaction.

Chemical conversion coatings (one type of inorganic coatings) are used

very often. The coatings are produced in-situ by chemical reactions withthe metal surface to form a film of metallic oxide or a compound which

has better corrosion resistance than the natural oxide film. Conversion

coatings include chromate and phosphate coatings [22, 85].

Organic coatings have been widely used for corrosion protection in vari-

ous industries. They account for about 50% of the corrosion protection

systems for steel structures. Organic coatings are complex mixtures of

chemical components including certain resins, volatile components, pig-

ments and additives. Volatile components help to adjust coating viscos-

ity for application and evaporate during and after application. Pigments

provide the color, opacity and specified characteristics for the coating.

Additives are a small part of the coating and can act as a stabilizer, flow

modifier, etc.. Organic coatings provide protection by acting as a bar-

rier to separate oxygen, water and other aggressive ions from the metal

substrate [17, 69, 84].

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1.3 Corrosion of the coated-metal system

In real applications, the coatings are porous and/or permeable to water,

oxygen and other ions. With increased exposure time, water or other aggres-

sive ions can penetrate through the coating to attack the metal substrate.

Undercoating corrosion could occur in some form during the service life of the

coating. This kind of corrosion has the potential to do serious damage because

it is under the coating and may not be detected in time [69, 84].

There are several types of corrosion that can occur on the metal surfaceunder organic coatings including: anodic undermining, blistering, cathodic de-

lamination, filiform corrosion, early rusting and wet adhesion [50, 69]. Blister-

ing and cathodic delamination are the most common corrosion types, and will

be discussed in detail next.

1.3.1 Blistering

A blister is a raised area, often dome shaped and is one of the first

signs of protective coating failure. Blistering can be caused by various factors

such as volume expansion due to swelling during water uptake, gas inclusion

or gas formation, osmotic pressure, phase separation during film formation,

cathodic blistering, etc. Osmotic and cathodic blistering are typically consid-

ered as dominant causes of blistering. Blistering by water uptake is one type

of corrosion in our system. Thus the details of osmotic blistering, cathodic

blistering and blistering due to water swelling will be discussed next.

Osmotic blistering: Blistering is initiated from the permeation of salt

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solution to the interface of the metal-coating. The concentrated salt solution

at the interface attracts more water because of the salt solution concentrationgradient. The solution under the coating builds up pressure and causes the

blistering. The osmotic pressure (2500 to 3000 kPa) is estimated to be far

greater than adhesive strength at the metal/coating interface (6 to 40 kPa).

This amount of pressure will provide the continued expansion of the blister

[27, 29].

Cathodic blistering: The chemical reaction of cathodic blistering

is illustrated in Fig. 1.1. As shown in Fig. 1.1, cathodic reaction under the

coating will produce (OH): O2 + 2H2O + 4e 4OH.

Cations in the salt solution can diffuse into the coating or the coat-

ing/metal interface, through coating defects. The defects could be caused by

mechanical damage or inherent faults in the coating, i.e. pores. These cations

and the (OH) produced in the cathodic reaction form an alkaline environ-

ment under the coating.

Na+ + OH NaOH

The alkaline environment under the coating reduces the adhesion be-

tween the metal and the coating. When the adhesion is not strong enough to

hold the metal and the coating together, blistering will form [34, 60, 70].

Blistering by swelling: The coating absorbs water when exposed

to salt solution. The water uptake of the coating results in the swelling and

volume expansion. When the adhesion of the coating cant accommodate the

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Figure 1.1: Cathodic blistering and delamination of a scribed coated steelpanel exposed to NaCl solution, taken from [60].

swelling of the film from hygroscopic stress, the coating is lifted and the blisters

form [13, 34, 50].

1.3.2 Cathodic delamination

Cathodic delaminantion is caused by the same chemical reactions as

cathodic blistering. The difference is that cathodic delamination happens ad-

jacent to the defects and results in delamination from the substrate. The

schematic of cathodic delamination formation is shown in Fig.1.1. The defects

include scratches, cracks or abrasions in the coating formed during manufac-

turing or due to other mechanical damage. The defects are preferred sites for

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corrosive agents, and can cause anodic dissolution. As the anodic dissolution

starts at the defects, the cathodic reaction occurs under the adjacent organiccoating (Fig. 1.1). Cathodic reactions form an alkaline environment under the

coating. The alkaline environment attacks the air-formed oxide layer or the

coating itself and this attack results in the loss of adhesion and subsequent

delamination [29, 33, 47, 64, 65, 69].

1.4 Undercoating corrosion detection

Undercoating corrosion should be detected as early as possible. Failing

to detect undercoating corrosion at an early stage may cause serious failures

of the metal structures [82]. However, undercoating corrosion is difficult to de-

tect, particularly in the early stages of attack. There are some techniques that

have been attempted to solve this problem. These techniques include: ther-

mal imaging, X-rays, magnetometers and dielectrometers, embedded metal

sensors, indicators that change color as a function of pH, and indicators that

change their fluorescence condition as a function of pH. The details of each

technique will be discussed next.

1.4.1 General techniques

In thermal imaging, corrosion sensing is realized by monitoring the

change of the sample surface temperature after heating by a rapid flash of light.

Corrosion at the subsurface influences the heating and the cooling behavior on

the surface. Thermal imaging can generate a macroview of the corrosion, but

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it cannot quantify corrosion damage, so the results are interpreted subjectively

by the operator [4, 68, 74].

X-rays can be used to monitor x-ray diffraction pattern changes of ad-

ditives in the coating to predict the initiation of corrosion. However, extended

or frequent x-ray usage is not safe for people, and this prevents x-rays from

being widely used in field applications [15, 17].

Magnetometers and dielectrometers can detect corrosion relatively ac-

curately with magnetic fields and inductive coupling. However, they can only

work on a very limited area [28].

Metal sensors such as gold (Au) and cadmium (Cd) have been embed-

ded in thin polymer films to detect corrosion. When the dissimilar metals are

short circuited, galvanic current is generated and the magnitude of this current

can be related to the corrosivity of the environment. Metal sensors generally

provide only an indirect correlation to the actual corrosion condition [5].

Indicators that change color or fluorescence as a function of pH have ad-

vantages such as low cost, straightforward results and easy operation. There-

fore, at the time we initiated our investigation, indicators showed potential for

corrosion sensing and use in field applications.

1.4.2 Indicators

The indicators include two types, the color pH change indicator andfluorescent indicator. They change their optical signature when subjected to

the effects of pH and/or oxidation-reduction reactions [39]. The following

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will review some of these indicators in terms of their application for corrosion

sensing of coated metal alloys.

1.4.2.1 Color pH change indicator

These indicators can change their color when exposed to different pH

environments. There are several researchers applying these indicators in cor-

rosion sensing. Some of the works are described next.

In 1974, G. M. Hoch added color change pH indicators into clear

polyurethane to evaluate the mechanism of filiform corrosion, a particular

type of corrosion that occurs under coatings [38].

In 1999, Zhang and Frankel [87] selected different color changing com-

pounds (phenolphthalein or bromothymol blue) to sense corrosion under coat-

ings at early stages. The indicators were pH sensitive and associated with

the cathodic reaction, i.e. oxygen reduction. The cathodic site was taken as

the sensing target because it was a more easily accessible location than the

accompanying anodic site for crevice corrosion. The coating system on an Al

alloy was composed of clear acrylic paint and color-changing indicators. After

certain immersion times in salt solution, pits initiated and developed, locating

at the color spots (Fig. 1.2).

W. Feng etc. also reported that nanoclay modified with pH sensitive

colorants could be used for sensing underlying corrosion at early stages. Theexfoliated nanoclay acted as a barrier for the metal substrate and carrier for

the ionizable color dyes. The color dye reflected color change at localized

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Figure 1.2: Photograph of color change of phenolphthalein paint on Al 5454substrate following immersion in 1.0 M NaCl solution after 8 days, taken from[87].

alkaline environments. Fig. 1.3 shows an example of a scribed coated-metal

system with nanoclay colorant after accelerated corrosion testing. The metal

substrate is an aluminum alloy while the coating is a two-layered structure

with a poor anticorrosive liquid acrylic as the bottom layer and an excellent

anticorrosive crosslinked acrylic as the top layer. The colorant is located in

the bottom layer. The blue color shown around the scribe indicated localized

increasing alkalinity as a result of corrosion at the scribed area [26].

Color-pH indicators have the advantage of being cheap and easily ap-

plicable to direct corrosion sensing. However, the coating system needs to be

transparent for a visible color change when corrosion takes place. This limitsits real application in commercial opaque coatings. Also, it is not desirable

to see coatings stained with colorful spots as this may cause concern about

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Figure 1.3: Accelerated corrosion testing in salt fog spray chamber (two layerstructures-tape protected exposed area) of scribed aluminum panels. Fromleft to right: 0, 96, 432 h, taken from [26].

appearance.

1.4.2.2 Fluorescent indicator

Fluorescence occurs when a molecule, atom or nanostructure is elec-

trically excited and relaxes to its ground state. Usually the emission light is

in the visible range and has longer wavelength than the excitation light in

the ultraviolet range. Fluorescence typically occurs from aromatic molecules.

There are many natural and synthetic compounds that exhibit fluorescence,

and they have a number of applications.

For corrosion sensing, fluorescent compounds have been investigated

for their sensing capabilities to temperature, pH, reduction and oxidation be-

havior, and the production of metal ions resulting from corrosion.

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Fluorescence mechanism: The fluorescence of molecules occurs as

the result of absorption of near-ultraviolet radiation. The fluorescence genera-tion process is illustrated in the Jablonski diagram(Fig. 1.4). After absorbing

light energy (hvA), the electrons are excited to electronically excited singlet

states, S1. Subsequent to the excited state, electrons can relax by various com-

peting pathways. They could undergo non-radiative emission (relaxation) to

a lower level in the excited state, in which the excitation energy is dissipated

as heat. Subsequently, if the excited electron in the lower level of the excited

singlet state has a different spin from that of the electron in the ground state

orbital, fluorescence is emitted when electrons return to the ground state. This

fluorescence emission light energy is equal to (hvF), where (h=6.626 x 1034

J.s) is Planks constant and (vF) is the fluorescence emission frequency. Ex-

cited molecules can also relax via phosphorescence or a secondary non-radiative

relaxation step [48, 67].

There are four important experimental parameters to describe the flu-

orescence materials. These are the wavelength of maximum fluorescence in-

tensity, F,max (alternatively the frequency of maximum intensity, vF,max), the

fluorescence emission intensity at a particular wavelength, the fluorescence

quantum yield and the fluorescence lifetime [48]. The value of F,max mea-

sures the energy difference between the excited and ground singlet states.

F,max of materials can shift to a shorter wavelength (blue-shifting) or a longer

wavelength (red-shifting) depending on the environment. Quantum yield is a

measure of the emission efficiency of a given fluorescent material. Quantum

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Figure 1.4: Schematic energy diagram showing light absorption and fluores-cence emission, referring to [48, 67].

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yield can be defined as the ratio of photons emitted over photons absorbed

through fluorescence. In other words, there are several relaxation pathwaysto deactivate the excited state; quantum yield describes the probability of

fluorescence relaxation relative to all relaxation pathways, including nonra-

diative decay. Fluorescent lifetime is a measure of the lifetime of the excited

state, meaning the average time the molecule stays in its excited state before

emitting a photon and returning back to ground state. Both quantum yield

and fluorescence are determined by the rate constants for radiative and non-

radiative decay. Fluorescent emission intensity, quantum yield and fluorescent

lifetime can be changed by the local environment [31, 48].

Fluorescence can be measured by the absorption and emission spec-

trum, which is the curve of intensity versus absorption and emission wave-

length. Fig. 1.5 shows one example spectrum of a fluorophore. There are

several important features that can be read from the spectrum [30, 31, 48].

The emission spectrum is typically a mirror image of the absorption

spectrum.

The emission wavelength is longer than the absorption wavelength. The

longer emission wavelength is because of less energy of the emitted pho-

ton than the absorbed photon. The energy difference is the Stokes shift.

Several common fluorescent indicators and their application:Ion sensing indicator: The ion sensing indicator is not designated as a spe-

cial indicator. It represents one type of indicator which can sense ions like

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Figure 1.5: Absorption and emission spectrum of a fluorescent material, takenfrom [48].

Al3+, Mg2+, F e3+, etc. Sibi [75] used three probes, lumogallion, N,N-bis-

(salicylidene)-2,3-dia- minobenzofuran (SABF) and Phen GreenTM to study

their response to Al3+, Mg2+ and Cu2+, respectively, in aqueous solution and

corrosion processes. The fluorescent probes were added into an epoxy/polyamide

resin on a Al 2024T3 surface.

D.E. Bryant and D. Greenfield [14] chose a fluorescent probe, 8-hydroxyq

uinoline-5-sulfonic acid (8-HQS), for an early warning of under-film corrosion

of aluminum and iron metal substrates. 8-HQS molecules are able to coordi-

nate to metal centers and form complexes with chelation-enhanced fluorescence

(CHEF) or chelation-enhanced quenching (CHEQ).

Li et al, [51] applied phenylfluorone (PF) to acrylic paint to sense cor-

rosion of aluminum alloys. PF could form a P F Al3+ compound with alu-

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Figure 1.6: Structure of oxidized and reduced Fluorescein, taken from [6].

minum ions from a corroded aluminum substrate. The P F Al3+compound

could quench fluorescence of PF in the coating. By checking the variation of

fluorescence over the coating, underlying corrosion could be detected.

Fluorescein and its derivatives: Fluorescein is one of most common

fluorophores used for corrosion detection. In water, the absorption maximum

occurs at 494 nm and the emission maximum at 521 nm [67].

The fluorescence of fluorescein is pH and redox state dependent. The

conjugation of the molecule can be altered in different pH solutions. Fluo-

rescein is fluorescent in basic solutions and non-fluorescent in acidic solutions

[42]. Fluorescein also has excellent fluorescence in the oxidized state and total

absence of fluorescence in the reduced state. The fluorescence change is due

to the opening of a five-numbered ring, which decreases the rigidity (Fig. 1.6)

[39].

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Fluorescein was used to detect early corrosion on gold-metallized IC

test devices under an applied electrical bias in a humid environment [83].Fluorescein was used to identify and locate the corrosion product, hydrous

aluminum oxide, on large-area samples by staining them and observing fluo-

rescence changes [18].

Fluorescein was also used to study the initiation of corrosion on alu-

minum alloy 2024 with high resolution confocal laser scanning microscopy.

Fluorescein is pH sensitive. It was entrapped in the corrosion products that

were deposited on the surface. A ring pattern with a strong fluorescence signal

was formed at the corrosion site [6]. A similar study was carried out on Al

6061 alloy to extend the application of fluorescein in corrosion sensing [7].

Besides the application mentioned above, fluorescein is also widely used

in coatings for corrosion sensing. A schematic of how the fluorescent indicator

worked in the coating to sense the hidden corrosion was reported by Agarwala

as Fig. 1.7, taking fluorescein and oxine as an example. It was a two-layer

coating with the indicator dispersed in the primer layer. When the undercoat-

ing corrosion happens, the fluorescent signal of the indicator at the corrosion

site will be changed [3, 25, 41, 42].

The feasibility of corrosion sensing using fluorescein in the coating was

also studied by adding it into epoxy resin and acrylic paint to detect hidden

corrosion of aluminum alloy 7075 [39, 40]. Fluorescein is not fluorescent in thereduced state under epoxy primer but fluorescent in the normal, oxidized state.

Initially fluorescein was reduced to lose its fluorescence and would be reoxidized

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Figure 1.7: Conceptual drawing of fluorescent indicator in the coating forcorrosion sensing, taken from [3].

to fluorescence after corrosion. However, the reversible speed was too fast to

be applied for corrosion detection. The suggestion was to encapsulate the

indicator in a polymer to reduce the reoxidation rate [40].

Oxine and its derivatives: Oxine, 8-hydroxyquinoline, is a derivative of

the heterocycle quinoline. Its structure is shown in Figure 1.8. The application

of oxine used as corrosion indicator is due to the formation of a fluorescent

complex of oxine reacting with loosely aluminum oxide [10, 25]. The following

case shows the response of oxine to aluminum. A solution of oxine was sprayed

on Al 7075 plates. After the plates dried, the plates were coated with acrylic or

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Figure 1.8: Chemical structure of oxine, taken from [25].

clear epoxy paint. There was clearly a fluorescent glow over the metal surfaces,

indicating that there was a fluorescent response of oxine to the aluminum

substrate [25].

Coumarin and its derivatives: Coumarin, with the structure of fused

pyrone and benzene rings, represents a very large and important family of

compounds. The structure of coumarin is shown in Fig. 1.9. Positions 4 and

7 are active sites for the derivative groups [73]. There are many fluorescent

coumarin derivatives with various photophysical mechanisms for the fluores-cence properties developed and reported. The coumarin derivates are sensitive

to polarity, viscosity, pH, etc.

7-hydroxycoumarin (where position 7 is replaced with the hydroxyl

group) is widely used and pH-dependent. It has several absorption wave-

lengths, which will be altered in alkaline solution because of de-protonation of

the phenolic hydroxyl group [23].

7-Aminocoumarins (position 7 is replaced with amino), is one of the

most important coumarin derivatives applied widely as fluorescent probes. The

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Figure 1.9: The chemical structure and numbering scheme of coumarin, takenfrom [73].

effects of solvents, hydrogen bonding, substituent and concentration on the flu-

orescence of 7-aminocoumarin are reported [8, 43, 79]. Two 7-aminocoumarins

were selected as the indicators in this investigation. Their properties will be

described in detail in the following chapter.

1.5 Coating estimation method- Electrochemical Impedance

Spectroscopy (EIS)

Electrochemical Impedance Spectroscopy (EIS) is a powerful non-destructive

method for the evaluation of coatings. It can be used to determine the corro-

sion protection performance of the coating. EIS uses a periodic small ampli-

tude AC signal of a wide frequency range to perturb the specimen and offers

distinct advantages over the traditional DC techniques. Because of the very

small signal amplitude, it results in minimal perturbation of the test spec-

imen and provides time-dependent information about the ongoing corrosion

processes [20, 36, 37, 5557, 80].

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1.5.1 EIS theory

EIS normally uses a small sinusoidal potential excitation signal to mea-

sure an AC current signal response. The current response is sinusoidal at the

same frequency with a shift in phase and it can be analyzed as a sum of

sinusoidal functions. An electrochemical cell can be represented by an electri-

cal circuit model. The electrochemical system is characterized by AC circuit

theory described below.

A sinusoidal AC potential is expressed as a function of time:

Et = E0sin(t)

where Et is instant potential at time t, E0 is amplitude of sinusoidal

potential, is the radial frequency.

For a pseudo-linear current response, the current signal could be ex-

pressed as:

It = I0sin(t + )

where is a phase shift, It is the instant current at time t, I0 is the

amplitude of sinusoidal current.

According to Ohms law, the impedance of the system could be ex-

pressed as:

Z =EtIt

=E0sin(t)

I0sin(t + )= Z0

sin(t)

sin(t + )

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where Z0 is the impedance magnitude and is the phase shift.

With Eulers relationship, the impedance is then represented as a com-

plex number:

Z() = Z0exp(j) = Z0(cos +jsin) = Z +jZ

where Z is the real part of the impedance, Z is the imaginary part of

the impedance.

The absolute magnitude of the impedance could be expressed as:

|Z| =

(Z)2 + (Z)2

EIS data can be plotted in two ways: Nyquist plot and Bode plot. In

the Nyquist plot, the imaginary component, on the Y-axis, is plotted versus

the real component, on the X-axis at each frequency. A Bode plot is composed

of two plots; Bode Impedance plot and Bode Angle plot. Both plots have log

frequency on the X-axis and either log absolute impedance or phase-shift on

the Y-axis, respectively. The Bode plot can sometimes provide more useful

information of the system behavior.

EIS data can then be analyzed by simulating the data with an equiva-

lent electrical circuit model based on the real physical electrochemistry of the

system.

1.5.2 Data analysis

A. Equivalent Circuits

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Equivalent Element Admittance ImpedanceResistor( R) 1/R R

Inductor( L) 1/jL LCapacitor(C) jC 1/jC

Infinite Warburg (W) Y0

(j) 1/Y0

(j)Constant Phase Element (CPE) (Q) Y0(j)

1/Y0(j)

Table 1.1: Common circuit elements

A corroded coated-metal system can be described in terms of an equiv-

alent circuit. The common elements used in the equivalent circuit and their

admittance and impedance equations are listed in Table 1.1. Usually, the EIS

model consists of both serial and parallel combinations of elements to represent

the system according to its physical electrochemistry.

If the coating has few defects, it can be simulated as the coating ca-

pacitance and a resistor (primarily due to the electrolyte) in series at an early

immersion stage. Typically, the data for coated metals in corrosive media

can be modeled using an equivalent circuit similar to the one in Fig. 1.10

[57], where Rs is the uncompensated solution resistance between the reference

electrode and test electrode, Cc is the coating capacitance, Rpo is the pore

resistance or coating resistance, Rp is the polarization resistance of the area at

the metal/coating interface and Cdl is the corresponding capacitance or double

layer capacitance.

The ideal protective coating is a barrier with very high impedance and

perfect insulation between metal substrate and corrosive environment. The

coating is simulated as a capacitor formed when two conducting plates are

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Figure 1.10: Equivalent circuit for coated metal system, taken from [57].

separated by a non-conducting media, called the dielectric. The coating ca-

pacitance can provide information on the corrosion process and electrolyte

uptake in the coating. The coating capacitance is given by

Cc =0A

d

Where, is the dielectric constant of the coating, 0 is the free space

electric permittivity, equal to 8.85 x 1012 F/m, A is the exposed surface area

of test electrode, d is the distance between two plates, or the thickness of

coating.

Pore resistance (Rpo), also called coating resistance, is an important

measure of coating performance. The pore impedance range of excellent pro-

tective coatings is higher than 1010 ohm/cm2, and for good coatings it is

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higher than 108 ohm/cm2. The pore impedance of poor coatings is less than

106

ohm/cm2

. Usually the thicker coatings have higher impedance.

The polarization resistance, Rp, can be used for electrochemical reac-

tion rate calculations. When a potential is applied to an electrode at the

open-circuit potential, it can cause current at the electrode surface by an

electrochemical reaction. The applied potential starts below the open-circuit

potential and ends above the open-circuit potential. The polarization resis-

tance is defined as the slope of the potential-current density curve at the open

circuit potential.

An electrical double layer, called the double layer, appears on the sur-

face of the metal substrate as an electrode and its surrounding electrolyte.

This double layer consists of two parallel layers of ions. The first layer is sur-

face charge from the solution stuck on the electrode surface. The other layer

is in the fluid with opposite charge of the first layer. The charged electrode and

the ion layer in the electrolyte separated by an insulator and acting like two

electrode plates form a capacitor. The value of the double layer capacitance,

Cdl, depends on many variables like electrode potential, temperature, ionic

concentrations, types of ions, oxide layers, electrode roughness, and impurity

adsorption. The double layer capacitance can provide information about ad-

sorption and desorption phenomena, film formation processes at the electrode,

and the integrity of organic coatings.

B. Data Explanation

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Figure 1.11: a: Bode impedance plot, b: Bode angle plot, taken from [57].

Usually there are three degradation stages of coatings when exposed to

a corrosive environment. Ideally, at early stages, there are typical character-

istics of a pure capacitor with a very high impedance and small capacitance.

The curve of log|Z| vs log(f) is linear with slope -1 and a phase angle of90

throughout the entire frequency range.

As the coating absorbs more water, there is a plateau range at low

frequencies of the Bode plot, as shown in Fig. 1.11 curve (1) and (2). The

impedance of the coating from the Bode plot decreases but maintains a linear

relationship with frequency. The phase angle also decreases, meaning the

impedance of the coating drops and the capacitance increases.

The next significant step is a rapid decrease in pore resistance and the

appearance of two time constants, which means the electrolyte has penetrated

through the coating to cause corrosion to happen at the interface of metal

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and coating. The typical equivalent circuit model at this stage is shown in

Fig. 1.10. The corresponding Bode plot is curve (3) in Fig. 1.11.

Polarization resistance and the double layer capacitance start to appear

and impact the Bode plot and Nyquist plot to form more complex plots with

two time constants. The time constants imply a kinetic phenomenon. For a

Bode plot, the impedance at the plateau range at high frequency is related

to the solution and pore resistance. The second plateau value is the sum of

solution, pore and polarization resistance [49].

1.6 Objectives and overview of dissertation

Prior to the work starting in our laboratory, Systems & Processes Engi-

neering Corporation (SPEC) had developed a prototype fluorescent corrosion

indicator (FCI) paint scanner and paint systems for aluminum alloy and steel.

In addition, they had performed a preliminary study on the technical feasibility

of using fluorescent indicators for detecting corrosion and wanted to continue

that work in cooperation with our laboratory.

The work described in this dissertation is about the characterization,

mechanisms and continuing development of fluorescent coating systems. The

dissertation includes six chapters. Chapter 1 provides the background and

significance of this project and introduces supporting information and basic

mechanisms of fluorescent indicators in coatings. Details of each fluorescentindicator will be discussed in the later chapters. Chapter 2 describes the

experimental procedures and materials used in our work. Chapter 3 describes

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the preliminary results carried out in our lab and obtained in cooperation with

SPEC. Chapter 4 describes the characterization of the epoxy-polyamide basedaluminum system, including properties of the fluorescent indicator, effect of

the fluorescent indicator on the coating, and corrosion mechanisms under the

coating. Chapter 5 describes the characterization and testing of the epoxy-

polyamide based steel system. In this system, the sensing of the fluorescent

indicator to corrosion and other ions is discussed. Chapter 6 includes the

conclusions and opportunities for future work.

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Chapter 2

Experimental and Materials

In this chapter, the materials and experimental procedures for both the

epoxy-polyamide based aluminum alloy and steel alloy system are introduced.

The specific experiments for either epoxy-polyamide based aluminum alloy or

steel alloy system will be described in later chapters.

2.1 Materials

Polymer coating: The primer and topcoat in this project are both

commercial epoxy-polyamide with different formulae. The formula for the

primer conforms to military specification MIL-P-24441/1B(SH), formula 150,

type I. Per the specification from database ASSIST, the components of the

primer include two parts with the main components in Table 2.1. The green

color of the primer comes from the mixture of copper phthalocyanine blue

and yellow iron oxide. The characteristics of the primer have rust prevention,

good adhesion on steel, fiberglass and aluminum alloys, and water and weather

resistance. The formula for the topcoat conforms to military specification

MIL-P-2441/3B(SH), formula 152, type I (Table 2.1). Without the colorful

pigments that are in the primer, the topcoat appears white. The other details

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Formula Resin Additives and Pigmentscomponent A polyamide magnesium silicate

titanium dioxidePrimer copper phthalocyanine blueMIL-P-2441/1B yellow iron oxide

component B epoxy magnesium silicatediatomaceous silica

Topcoat component A polymide titanium dioxideMIL-P-2441/3B component B epoxy magnesium silicate

Table 2.1: The specification of primer and topcoat

of the formula are in the appendix.

Fluorescent indicators: Based on different pH sensing ranges, dif-

ferent coumarin derivatives were chosen as Fluorescent Indicators (FI) for alu-

minum alloy and steel alloy. The 2024 Al-FI is 7-Amino-4-methylcoumarin (7-

AMC) and the 1018 steel-FI is 7-Diethylamino-4-methylcoumarin (7-DMC).

Fig. 2.1a and Fig. 2.1b show their chemical structures [11, 61]. The materials

were obtained from Sigma-Aldrich Co. The FI was added to the primer and

the concentration of FI agent in the primer ranged from about 0.05 wt% to

1.5 wt%.

Metal: The metal substrates used were 2024T4 aluminum and 1018

steel. The composition of 2024 aluminum and 1018 steel are listed in Table 2.2

and Table 2.3 [1, 2]. 2024 aluminum has good fatigue resistance and is used

in aircraft for its high strength to weight ratio as well. However, it has poor

corrosion resistance compared with some other aluminum alloys.

1018 steel has 0.18 wt% carbon. Compared with other steel with higher

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Figure 2.1: a. Chemical structure of 7-amino-4-methylcoumarin. b. Chemical

structure of 7-diethylamino-4-methylcomarin, taken from [11, 61].

carbon percentage, it has higher ductility but lower strength and hardness.

The alloy is easily formed, machined, welded and fabricated and commonly

applied in shafts, spindles, pins and other component parts.

The metal substrates were in the form of sheet with size of 255mm

x 75mm x 1mm and circular specimens (16mm diameter) were cut from the

sheet.

2.2 Test specimen preparation

The required amount of fluorescent indicator was weighed out and dis-

solved into a small amount methyl ethyl ketone (MEK). The solution was

added into component B of the primer at a concentration of approximately

0.05 wt% to 1.5 wt%. Afterwards, component A and component B were mixed

1:1 by volume. After a few minutes for stirring and mixing, the mixture was

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AA No. 2024Si Max 0.5Fe Max 0.5Cu 3.84.9Mn 0.300.9

Composition Mg 1.21.8weight% Cr 0.10

Zn 0.25Ti 0.15

Unspecifiedotherelements

0.15

Al balanced

Table 2.2: Composition limits for 2024 aluminum alloys, referring to [1]

Composition, weight%AA No. C Mn P S FeSteel 1018 0.150.20 0.600.90 max 0.04 max 0.05 balanced

Table 2.3: Composition limits for 1018 steel alloys, referring to [2]

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Figure 2.2: Schematic drawing of coating system used on the Al and steel alloysubstrates.

airbrushed or sprayed onto the Al 2024 to form the primer layer. The prepara-

tion procedure for coated 1018 steel is very similar to that of coated-aluminum

except the fluorescent indicator was mixed with silica before mixing with the

primer. After allowing the primer layer to dry for 24 hours at ambient temper-

ature, the topcoat, formed by mixing component A and B without indicator,

was airbrushed or sprayed onto the primer layer. The schematic of the paint

system is shown in Fig. 2.2.

Specimens were prepared with different coating layer thicknesses:

primer layer with indicator (PI), 30 5 m

primer layer with indicator covered with one layer of topcoat (TPIL),

50 5 m

primer layer with indicator covered with one layer of topcoat (TPI),

85 5 m

primer layer with indicator covered with one layer of topcoat (TPIH),125

5 m.

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In addition, there were other variations of layers such as: topcoat layer

with indicator and primer layer containing pre-corroded regions plus one layer

of topcoat.

2.3 Test procedure

2.3.1 Electrochemical tests

In this study, EIS was performed on small epoxy-polyamide coated

aluminum alloy and steel specimens. The coatings of interest included pure

primer or primer plus indicator with or without topcoat. The specimens were

immersed in 3.5% NaCl solution for prolonged times and tested periodically.

A traditional three-electrode system was used in the EIS tests. The reference

electrode was a saturated calomel electrode (SCE). The auxiliary electrode

was graphite. A typical setup for conducting electrochemical tests with the

five-mouth flask is shown in Fig. 2.3. The specimen was placed into the spec-

imen holder (Fig. 2.4). The periphery of the test specimen was held by the

sample holder and wasnt exposed to salt solution; the center area (one square

centimeter) was immersed into the salt solution. The test equipment includes

a model 273 potentiostat and a model 5301A lock-in amplifier made by EG&G

Princeton Applied Research. The EIS experimental parameters were as fol-

lows: frequency range 0.01 Hz to 100 kHz, 10 points per decade, and 10 mV

potential amplitude relative to the open circuit potential. The experimental

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Figure 2.3: Setup for electrochemical testing.

data were analyzed using the commercial software Zview developed by Scribner

Associates, Inc..

2.3.2 Scanning Electron Microscopy (SEM) & Energy DispersiveSpectrometry (EDS)

Scanning electron microscopy was used to examine topography and

microstructure at high magnifications. EDS was used to determine elemental

percentages semi-quantitatively and to observe elemental distributions over

the specimen. Coated aluminum alloy and steel specimens were deposited

with Au-Pd before observation in the SEM. The type of SEM was a JEOL

JSM-5610 used with an applied 20 kv voltage.

2.3.3 Fourier transform spectroscopy (FTIR) test

Fourier transform spectroscopy (FTIR) is a measurement technique for

collecting infrared spectra to identify molecular structures. The cured coating,

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Figure 2.4: Exploded view of K105 flat specimen holder, taken from [32].

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primer layer and primer plus indicator layer, were selected as the test samples

and were peeled off from the coated-Al or coated-steel systems for FTIR tests.A quantity of coating was ground with KBr. This powder mixture was then

pressed in a mechanical die press to form a translucent pellet. The beam of the

spectrometer passed through the pressed sample and its FTIR spectra were

collected on a Mattson Infinity Gold FTIR system. The test wave-number was

from 500 to 4000 cm1.

2.3.4 The fluorescent absorption and emission spectrum tests

Ultraviolet visible spectroscopy (UV-VIS) uses light in the visible, near

ultraviolet (UV) and near infrared (NIR) to measure the absorption wavelength

and intensity caused by the electron transition from the ground state to the

excited state. A Cary 5000 UV-VIS NIR Spectrometer from Varian, Inc was

used with a double beam and high performance spectrometer. The wavelengths

were scanned from 200 to 500 nm.

A fluorimeter was used to measure the intensity and the wavelength

distribution of the emitted fluorescence light from molecules excited at specific

wavelengths within the absorption wavelength band of a particular compound.

The wavelength at maximum intensity is an important parameter to estimate

fluorescent properties of the indicators. A fluorimeter used is FluoroLog from

HORIBA, Ltd.

The other specified tests for coated aluminum or steel will be discussed

in their chapters individually.

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Chapter 3

Preliminary Results and Discussion

3.1 Introduction

In recent years, there has been a tremendous amount of research on

corrosion-sensing coatings or smart coatings. SPEC initiated a project on

corrosion-sensing coatings for aluminum alloys applied in aerospace and air-

craft, and for steel alloys applied in marine environments and offshore struc-

tures. They were cooperating with our lab in an effort to characterize the

coating systems in corrosive environments. To that end, they had developed

two potential fluorescent coating systems and a technique for measuring fluo-

rescence responses.

Many techniques are available to detect corrosion in marine and offshore

structures. However, they often require complicated equipment and their use

may result in information that is difficult to interpret. The preliminary work

was an effort to develop and examine corrosion-sensing coatings that could be

used on marine and offshore structures as part of a comprehensive health moni-

toring system. If successful, the development of new corrosion-sensing coatings

would represent an important advantage in terms of corrosion reduction and

mitigation.

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Epoxy-polyamide was chosen as the coating system and 7-aminocoumarin

derivatives were chosen as the fluorescent indicators. The indicators were to bemixed with the coating (typically the primer) to build a coating system with

sensitivity to undercoating corrosion. In particular, the corrosion indicators

were said to be positive indicators for steel in that they should be initially

non-fluorescent upon application, but glow over areas of corrosion. The corro-

sion indicators were said to be negative indicators for aluminum in that they

should be normally fluorescent when applied initially and that fluorescence

should later be quenched in the presence of corrosion. It was reported that

the quenching was usually followed by the formation of dark pits.

In addition to the potential coating systems, SPEC had developed spe-

cial equipment for qualitatively detecting fluorescent intensity changes of the

coating. The equipment is called a Fluorescent Corrosion Indicator (FCI)

scanner system. Preliminary tests were performed by using the FCI scanner

in our lab.

3.2 Experimental procedure

3.2.1 Specimen preparation

Preliminary test efforts focused on monitoring qualitative changes in

fluorescence in the presence of corrosion when large steel and aluminum coupons

were subjected to immersion in salt water conditions over a period of weeks.

In order to describe the basic protective properties of the coating sys-

tems, small coupons were cut from the large coupons. They were exposed to

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saltwater conditions and examined using EIS during exposure. The metals

(steel and aluminum) served as the substrates and specimens were preparedin various conditions including: metal plus primer with indicator; metal plus

topcoat with indicator; metal plus primer with indicator plus topcoat; metal

(containing pre-corroded regions) plus primer with indicator plus topcoat; and

metal (containing pre-corroded regions) plus topcoat with indicator.

3.2.2 Prototype FCI paint scanner system

The prototype paint scanner developed by SPEC contained a high en-

ergy UV source for illumination of the paint on the coupon, a high speed digital

camera with a UV blocking filter to record the resulting fluorescent image, and

a coupon fixture to hold the coupon under test in the same position for each

image scan. The coupon fixture was set up in such a way that the region be-

tween 80 and 160 mm along the length of the large coupon was examined. The

PC operating the prototype paint scanner recorded the resulting fluorescent

image from the digital camera and created a unique data file for each scanned

coupon image. Each coupon had the coupon number incorporated into the

paint pattern for easy identification. The scanned images were downloaded to

a CD for later image processing at SPEC. Fourier methods were used to inves-

tigate simple correlation and filtering operations on the images. The prototype

FCI paint scanner and its internal view are shown in Fig. 3.1.

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Figure 3.1: left: Prototype FCI Paint Scanner and PC, right: FCI PaintScanner Internal View, taken from [19].

3.2.3 Saltwater spray tests

The large coupons were suspended into a saltwater environment in an

aquarium containing the saltwater and aerators. The air was forced by the

pump into the air bubbler in the bottom of the tank in the saltwater. Coupons

were suspended in the modified salt spray tank by metal wire hangers. Theexposure set up is shown in Fig. 3.2a and 3.2b. Fig. 3.2b shows an updated

tank that was used for the steel coupons to filter out the corrosion byproducts

from the saltwater. The detailed procedure of using the FCI scanner system

and the saltwater spray aquarium was as follows:

Several sets of large coupons were exposed to the saltwater environment.

They were removed daily and placed in the scanner, and scanner images were

taken. When corrosion activity was indicated, coupons were removed from

exposure, and the topcoat was stripped off to identify the corrosion state

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Figure 3.2: a. Aluminum coupons in the salt spray tank, b. The updated saltspray tank for steel coupons, taken from [19].

under the coating.

3.2.4 EIS test

The smaller coupons were examined with a 5-mouth flask system and

EIS tests were conducted as a function of time and saltwater exposure. Theresults were plotted in the Bode format.

3.3 Results and discussion

3.3.1 Aluminum coupons examined using the FCI scanner system

In the first sets of tests on the metal plus primer with indicator plus

topcoat, the continuous primer coating contained a continuous signal over the

black and white image. The low contrast of the image made it difficult to detect

the indicator signal. Therefore, it was necessary to change the primer coating

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Figure 3.3: Coupon 10 (metal plus primer with indicator) at start of exposure,taken from [19].

process to have areas of primer coating and areas without primer coating (i.e.

areas of bare metal under the topcoat) to improve the signal contrast over the

image. In a subsequent set of tests on the metal plus primer with indicator plus

topcoat, the corrosion indicator was detected by the scanner (Fig. 3.3). As

shown in Fig. 3.3, the area with the fluorescence signal has higher brightness.

The cross line from the top left corner to the right bottom corner is only coated

with topcoat. It provides a dark background for the other fluorescent area.

The processing of the images was performed by SPEC and involved the

steps listed below.

Acquire several independent images of a coupon

Averag

liu dissertation

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