the application of embedded sensors for in-situ monitoring of protective coatings on metal...

8/10/2019 THE APPLICATION OF EMBEDDED SENSORS FOR IN-SITU MONITORING OF PROTECTIVE COATINGS ON METAL SUBST

1/259

T HE APPL ICAT ION O F E MBE DDE D SE NSORS

F O R

IN-SITU

M O N I T O R I N G O F P R O T E C T I V E C O A T I N G S

ON ME T AL SUBST RAT ES

A Dissertation

Submit ted to the Grad uate Facul ty

of the

North Dakota State University

of Ag riculture and Applied Science

By

Quan Su

In Partial Fulfi l lment of the Requirements

for the Degree of

D O C T O R O F P H I L O S O P H Y

Major Depar tment :

Coatings and Polymeric Materials

May 2008

Fargo, North Dakota


2/259

UMI Number: 3322479

INFORMATION TO USERS

The quality of this reproduction is dependent upon the quality of the copy

submitted. Broken or indistinct print, colored or poor quality illustrations and

photographs, print bleed-through, substandard margins, and improper

alignment can adversely affect reproduction.

In the unlikely event that the author did not send a complete manuscript

and there are missing pages, these will be noted. Also, if unauthorized

copyright material had to be removed, a note will indicate the deletion.

UMI

UMI Microform 3322479

Copyright 2008 by ProQuest LLC.

All rights reserved. This microform edition is protected against

unauthorized copying under Title 17, United States Code.

ProQuest LLC

789 E. Eisenhower Parkway

PO Box 1346

Ann Arbor, Ml 48106-1346


3/259

North Dakota State University

Graduate School

Title

I n - s i t u m o n i t o r i n g of thec o r r o s i o n p r o t e c t i o n p e r f o r m a n c e o f

organi c coatings using embedded s ensors

By

Quan

Su

The Supervisory Committee certifies that thisdisquisition com plies with North Dakota

State University's regulations and meets the accepted standards for the degree of

DOCTOR OF PHILOSOPHY

SUPERVISORY COMMITTEE:

i.(Z_(2^jJL

Approved by Department Chair:

v Date


4/259

ABSTRACT

Su, Quan, Ph.D., Department of Coatings and Polymeric Materials, College of Science

and Mathematics, North Dakota State University, May 2008. The Application of

Embedded Sensors for in-situ Monitoring of Protective Coatings on Metal Substrates.

Major Professor: Dr. Gordon P. Bierwagen.

Platinum sensors embedded into coating films were coupled with electrochemical

measurements, such as electrochemical impedance spectroscopy (EIS) and

electrochemical noise measurements (ENM ), to give continuous,in-situmonitoring of the

corrosion protection performance of coatings.

Both EIS and ENM measurements via embedded sensors were conducted to

examine the coated panels when exposed to the AC-DC-AC accelerating condition. The

results demonstrated that the EIS measurements from embedded sensors could detect the

coating changes whenever there were changes detected by the standard three-electrode

EIS measurements. It was found that the ENM method was consistent with the EIS

method in monitoring the coating's corrosion protection performance. The shot noise

method was also used in the ENM data analysis to differentiate the corrosion mechanisms

between aluminum alloy and steel substrates.

The quantitative influence of the environmental humidity and temperature on the

corrosion protection performance of coatings was monitored using EIS via embedded

sensors. The obtained results were useful in the determination of the proper application

environmental conditions for the given coating system. Both EIS and ENM were used for

in-situ monitoring of the coating performance as coated panels exposed to

Prohesion /QUV accelerating condition. Unique in-situexposure data were obtained w ith

the application of embedded sensors.

i i i


5/259

The feasibility of using the sensor-sensor two-electrode EIS method via

embedded sensors in monitoring the coating's corrosion protection performance was also

investigated. The obtained results were promising, and more efforts are still needed to

fully exploit this technique.

Finally, embedded sensors coupled with EIS measurements were used to study

changes w ith temperature of each coating layer in two-layer coating system s. The coating

systems and their coating layers were shown to be reversible in the temperature range

studied, and the Arrehnius temperature dependence of their low frequency modulus was

observed.

iv


6/259

ACKNOWLEDGEMENTS

I wish to express my sincere appreciation to my major adviser, Dr. Gordon

Bierwagen, for his support and guidance throughout this study. I would also like to thank

Dr. Kerry Allahar for his generous help and suggestions and my comm ittee m emb ers, Dr.

Dennis Tallman, Dr. Stuart Croll and Dr. Chao You, for their support.

In addition, I would like to acknowledge the staff members in Department of

Coatings and Polymeric Materials and my fellow graduate students and colleagues in the

corrosion group for their kindly help and friendship during the study.

This research work was supported by the Air Force Office of Scientific Research

under Grant No. FA9550-04-1-0368 and the Army Research Laboratory under Contract

N o.

W911NF-04-2-0029.

I dedicate this thesis to my parents, Shijun Su and Guixian Wu, my wife, Fengqin

Huang, and my son, Kevin Su, for their support and sacrifice during my graduate study.

v


7/259

TABLE OF CONTENTS

ABSTRACT iii

ACKNOWLEDGEMENTS v

LIST OF TABL ES xii

LIST OF FIGURE S xiii

CHAPTER

1.

INTRODUCTION TO CORROSION, CORROSION CONTROL

BY COATINGS AND ELECTROCHEMICAL MEASUREMENT MET HODS 1

1.1. Introduction to M etal Corrosion 1

1.2. M etal Corrosion Protection M ethods 3

1.3. Corro sion Controlled by Coa tings 5

1.4. Eva luation of the Corrosion Protection Performan ce of Coatings 8

1.5. Electroc hem ical M ethods in Coating Eva luation 11

1.5.1. Electrochemical impedance spectroscopy 11

1.5.2.

Electrochemical noise measurem ents (ENM) 16

1.5.2.1. ENM theories, test methods and noise sources 16

1.5.2.2. ENM data analysis methods 20

1.5.2.2.1. Time domain analysis 20

1.5.2.2.2. Frequency domain analysis 21

1.5.2.2.3. Shot noise me thod analys is 24

1.6. Referen ces 25

CHAPTER 2. SENSING TECHNIQUES FOR THE MON ITORING OF METAL

CORROSION AND CORROSION PROTECTION OF COATINGS 33

2.1.

Sensing Techniques for Metals 34

2.1.1. Sensing technologies detecting corrosion products and corrosive species 34

VI


8/259

2.2.1.1.

Weight loss corrosion sensing technologies 34

2.1.1.2. Optical fiber corrosion sensors 34

2.1.1.3.

Acoustic corrosion sensors ....3 6

2.1.1.4. Sensors detecting cathodic reaction products 36

2.1.1.5.

Sensing metho ds detecting corrosive species 37

2.1.2. Electrical and electrochemical corrosion sensors 37

2.1.2.1.

Electrical resistance and polarization resistance corrosion sensors 37

2.1.2.2. Galvanic corrosion sensors 38

2.1.2.3. EIS and ENM corrosion sensors 39

2.2.

Sensing Techn ologies for Coated Me tals 40

2.2.1.

Sensing technologies to detect corrosive species and corrosion produ cts 41

2.2.2. Electrochem ical sensors 43

2.2.2.1.

Electroche mical surface sensors 43

2.2.2.2. Electrochemical embedded sensors 45

2.3.

References 47

CHAPTER

3.

MONITORING THE CORROSION PROTECTION OF ORGANIC

COATINGS UNDER AC-DC-AC ACCELERATION CONDITION USING

EMBEDDED SENSORS 57

3.1.

Introduction 57

3.2. Experiment 60

3.2.1.

Samp le preparation 61

3.2.2. Exp erimental procedure 63

3.3.

Experimental Results and Discussions 66

3.3.1.

EIS studies '. 66

vii


9/259

3.3.1.1. EIS studies of the aircraft coating 66

3.3.1.2. EIS studies of the industrial coating 73

3.3.2. EN M studies 78

3.3.2.1. Statistical method: noise resistance and localized index 82

3.3.2.2. Shot noise method 88

3.4. Conclusions 95

3.5. References 96

CHAPTER 4.

IN-SITU

EMBEDDED SENSOR MONITORING OF THE INFLUENCE

OF ATMOSPHERIC HUM IDITY AND TEMPERATU RE ON AN AIR FORCE

PRIMER/SUBSTRATE BENEATH A TOPCOAT 99

4.1. Introduction 99

4.2. Experime nt 101

4.2.1. Sample preparation 101

4.2.2. Experimental procedure 103

4.3. Experime ntal Results 105

4.4.

Data Analysis and Discussion 106

4.4.1. Influence of hum idity 110

4.4.2. Influence of temp erature 119

4.4.3. Coupled humidity and temperature influence 124

4.5.

Conclusions 127

4.6. References 128

CHAPTER 5.

IN-SITU

MON ITORING OF PROTECTIVE COATINGS IN

PROHESION/QUV EXPOSURE CONDITIONS BY EMBEDDED SENSORS . . . .131

5.1. Introdu ction 131

5.2. Experim ent 133

viii


10/259

5.2.1. Panel preparation 133

5.2.2. Exp erime ntal configurations 135

5.2.3.

Testing procedure 137

5.3.

Results and Discussions 138

5.3.1. In-situ

mon itoring under Prohesion condition s 138

5.3.1.1.

OCP and coating capacitance studies 138

5.3.1.2. EIS results and discussions 144

5.3.1.3. ENM results and discussions 147

5.3.2. In-situ monitoring under QUV conditions 151

5.3.2.1. OC P and coating capacitance studies 151

5.3.2.2. EIS results and discussions 154

5.3.2.3.

EN M results and discussions 156

5.4. Con clusions 161

5.5. References 162

CHAPTER 6. THE USE OF THE SENSOR-SENSOR EIS MONITORING OF THE

CORROSION PROTECTION OF COATINGS FROM EMBEDDED SENSORS 164

6.1. Introduction 164

6.2. Experiment 168

6.2.1.

Sam ple preparation 168

6.2.2. Expe rimen tal procedure 168

6.3.

Results and Discussions 171

6.4. Con clusions 182

6.5.

References 183

IX


11/259

CHAPTER 7. APPLICATION OF EMBEDDED SENSORS IN THE THERM AL

CYCLING OF ORGAN IC COATINGS 186

7.1.

Introduction 186

7.2. Experimen t 187

7.2.1.

Samp le preparation 187

7.2.2. Expe rimental procedu re: therm al cycling testing method 190

7.3.

Exp erimen tal Results 192

7.3.1.

AF primer-topcoat coating system 192

7.3.2. AF primer-primer coating system 194

7.3.3. AV primer-to pcoat coating system 196

7.3.4. AV primer-p rimer coating system 197

7.4. Data Analy sis and Discussion 199

7.4.1.

Reversib ility study 199

7.4.1.1.

AF primer-topcoat coating system 200

7.4.1.2. AF primer-prim er coating system 203

7.4.1.3.

AV primer-topcoat coating system 208

7.4.1.4. AV primer-primer coating system 210

7.4.2. Temperature dependence of EIS impedance 213

7.4.3. Equivalent circuit model analysis 218

7.4.3.1. Resistance parameters 220

7.4.3.2. Capacitance parameters 223

7.5 Con clusions 225

7.6. References 226

CHAPTER 8. SUMMARY AND CONCLUSIONS 229

x


12/259

CHAPTER 9. FUTURE WORK 234

9.1.

References 236

APPEN DIX. PUBLICATIONS 237

X I


13/259

LIST OF TABLES

Table Page

6.1.

The designation of the EIS tests 170

7.1.

The coating systems used in this study 188

7.2. The activation energies of different coating systems derived from Figures 7.21

and 7.22 215

7.3.

The activation energies of R

p

and R

ct

for different coatin gs 221

xii


14/259

LIST OF FIGURES

Figure Page

1.1. The diagram of the three-electrode EIS m ethod used in the test of coated

metals 13

1.2. The example ofEIS(a) Nyq uqist and (b) Bode plots 14

1.3. Ra ndies mo del for intact, high-resista nce coatings in EIS data fitting 15

1.4. Electrical model used for weathered or medium- to low-resistance coatings in

EIS data fitting 16

1.5. Diagram of the traditional three-electrode, two-metal substrate EN M

configuration 17

1.6. Diag ram of the reverse three-elec trode EN M configuration 18

1.7. Diag ram of the three-elec trode, substrate-less EN M configuration 19

1.8. Exam ples of the power spectra density plots from (a) FFT and (b) ME M

transforms of the same original noise data associated with intact and failed

coatings 22

3.1.A picture of embedding the platinum sensor: (a) the platinum sensor applied

on the primer and (b) sensor embedded between the primer and the topcoat

.. .

.62

3.2. The schematic potential changes in the AC-DC-A C accelerating process used

in this study 63

3.3.Sc hematic diagram of the configurations used to perform the EIS experim ents:

(a) inner test from sensor and (b) total EIS test 65

3.4. Schematic diagram of the configuration used to perform the EN M

experiments 66

3.5.

The EIS Bode impedance m odulus plots of the aircraft test sam ple: (a) total

coating system and (b) inner part 67

3.6. The EIS Bode impedance modulus plots of the aircraft control sample: (a) total

coating system and (b) inner part 68

3.7. The comparison of the low-frequency impedance m odulus of the aircraft

test sample 71

xiii


15/259

3.8. The comparison of the low-frequency impedance modulus of the aircraft

control sample 71

3.9. The EIS B ode impedance m odulus plots of the industrial test sam ple: (a) total

coating film and (b) inner part 74

3.10. The EIS Bode impedance m odulus plots of the industrial control sam ple:

(a) total coating film and (b) inner part 76

3.11.The comparison of the low-frequency impedance m odulus of the industrial

test sam ple 77

3.12. The com parison of the low-frequency impedance m odulus of the industrial

control sample 77

3.13.

Exam ples of the measured potential and current noise associated w ith the

intact and failed states of(a)the industrial and (b) aircraft coatings 79

3.14. Examples of the power spectral density of the potential and current noise

associated with the intact and failed states of(a)the aircraft and (b) indus trial

coatings 81

3.15. Examples of the spectral noise resistance associated with the intact and failed

states of(a)the aircraft and (b) industrial coatings 82

3.16. No ise resistance, R

n

, of the industrial (a) test sample and (b) control sample ...84

3.17. Noise resistance, R

n

, of the aircraft (a) test sample and (b) control sample ....85

3.18. The localization index,L I, of aircraft test sample (o) and control sample (x) ..87

3.19. The localization index,

LI,

of industrial test sample (o) and control

sample (x) 87

3.20. Calculated thermal noise and the total potential noise of the aircraft coating

that was under AC-D C-AC conditions 89

3.21.

Calculated thermal noise and the total potential noise of the industrial coating

that was under AC-DC -AC conditions 90

3.22. The average charge per event,q,as calculated by a shot noise analysis

of data from the aircraft test sam ple (o) and control sam ple (x) 92

3.23. The event frequency,/,, as calculated by a shot noise analysis of data from the

aircraft test samp le (o) and control sam ple (x) 92

xiv


16/259

3.24. The average charge per event,

q,

as calculated by a shot noise analysis

of data from the industrial test sample (o) and control sample (x) 93

3.25. The event frequ ency ,/,, as calculated by a shot noise analysis of the data

from the industrial test sam ple (o) and control sam ple (x) 94

4.1.A picture of embedding the platinum sensor: (a) the platinum sensor applied

on the primer and (b) sensor embe dded between the primer and the topcoat 102

4.2.

The e volution of the coating capacitance m easured with single frequency E IS

method at 10 kHz (a) when hum idity was changed from 70% to 80% at

different tem peratures at zero hour and (b) when humidity was increased by

10% each time at 25C 104

4.3.

The test procedure of the coated panels under different environmental

hum idity and temperature conditions 105

4.4. The EIS Bode surface m aps for the A F coating system at (a) 25C

and(b)65C 107

4.5.The EIS Bode surface maps for the AF coating system under the humidity

of(a)40% and (b) 90% 108

4.6.

The equivalen t electrical circuit mo del used in fitting the EIS data 109

4.7.

The evolution ofC79kHz values under different temperature and humidity

conditions 110

4.8. The evolution of coating water content under different temperature and

humidity conditions I l l

4.9. |Z|O.OIHZ

as a function of relative humidity with exposure temperature as a

parameter 112

4.10. (a) The coating capacitance parameter, C

c

, and (b) the resistance parameter,

R

p

, associated with the bulk primer as functions of relative humidity with

temperature as a parameter 114

4.11.(a) The capacitance parameter, C

d

i, and (b) the resistance parameter, R

ct

,

associated with the metal/coating interface as functions of relative humidity

with temperature as a parameter 114

4.12. |Z|O.OIHZas a function of absolute humidity with temperature a s a parameter ...117

xv


17/259

4.13.Resistance values associated with the fitting of the equivalent circuit model

to the EIS data as functions of absolute humidity with temperature as

a param eter 118

4.14. Slope values associated w ith the linear relationships between the logarithm

of the |Z|o.oiHz> Ret and R

p

parameters and absolute humidity as functions of

temperature 118

4.15.Low -frequency impe dance m odulu s, |Z|o.omz, as functions of the reciproc al

of temperature with relative hum idity as a parameter 120

4.16. Resistance values associated w ith the equivalent circuit mode l of the EIS

data as functions of the reciprocal of temperature with relative humidity

as a parameter, (a) R

p

and (b) R

ct

120

4.17.

The activation energies (E

a

) for

|Z|O.OIHZ,

Rp and R

ct

as a function of relative

humidity 123

4.18.

(a) Three-dimensional surface map and (b) contour map relating the values

of

|Z|O.OIHZ

with the values of humidity and temperature 125

4.19. Contour m aps relating the resistance values of the equivalent circuit model,

R

p

and R

c

t, with the values of humidity and temperature 126

4.20. Contour m ap relating theR

c

t/R

p

values with the values of temperature

and hum idity 127

5.1.

A picture of embedding the platinum sensor: (a) the platinum sensor applied

on the primer and (b) sensor embedded betw een the primer and the top co at .. ..134

5.2. Schematic diagram of the configuration used to perform the EIS

experiments 136

5.3.Schem atic diagram of the configuration used to perform the ENM

experiments 137

5.4. The temperature profile at the surface of the coated panel under Prohesion

conditions 139

5.5. The open circuit potential evolution of the coated panel under Prohesion"

conditions 140

5.6. The evolution of coating capa citance under Prohesion conditions 142

5.7. The evolution of coating water absorp tion under Prohesion conditions 142

xvi


18/259

5.8. The examples of the EIS Bode plots of the coated samples under Prohesion

dry step after 20 minutes of drying: (a) impedance modulus and (b) phase

angle 145

5.9. The evolution of the

Z|O.IHZ

values of EIS data measured at Prohesion dry

step as a function of acceleration tim e 146

5.10. Examples of the changes of noise potential and current with the accelerating

cond ition under Prohesion dry and we t steps at (a) early day and (b) late

day of acceleration 148

5.11.

Exam ples of the change of R

n

values with the accelerating condition under

Prohesion dry and wet steps at (a) early day (day45) and (b) late day

(day 174) of acceleration 149

5.12. The evolution of the average R

n

values obtained at the Prohesion dry

and fog steps with the accelerating time 150

5.13.

The temperature profile at the coating surface of the coated panel under

QUV conditions 151

5.14. The open circuit potential evolution of the coated panel under QUV

conditions 152

5.15. The evolution of coating capacitanc e under QUV condition s 153

5.16. The examples of the EIS Bode plots of the coated samples under QUV

condensate step after 2 hours of water condensation: (a) impedance modulus

and (b) phase angle 155

5.17. The evolution of the

Z|O.IHZ

values of EIS data measured at QU V dry step

as a function of acceleration time 156

5.18. An example of the change of noise potential and current with the accelerating

condition at QUV condensate and UV steps 157

5.19. An exam ple of the change of R

n

values with the accelerating condition at

QUV condensate and UV steps 159

5.20. The evolution of the average Rnvalues obtained at QUV condensate and UV

steps with the accelerating tim e 160

6.1.

The schematic diagram of the sensor-sensor EIS set for

1-panel

sample 169

6.2. The schem atic diagram of the sensor-sensor EIS set for 2-panel sample 169

xvn


19/259


20/259

6.16. Diagrams of the three possible current flow paths for the sensor-sensor EIS

measurement of the1-panelsample: (a) through the m etal, (b) through the

coating and (c) through the electrolyte 181

7.1.

A picture of em bedding the platinum sensor: (a) the platinum sensor applied

on the primer and (b) sensor embedded between the primer and the topcoat ..189

7.2. The temperature profile used in the thermal cycling experiment 190

7.3.Schematic diagrams of the EIS configurations for the (a) inner, (b) outer and

(c) total coating system tests 192

7.4. Bode plots of the (a) total coating system and (b) outer layer of the AF

primer-topcoat coating system with temperature as a parameter 193

7.5.

Bode plots of the inner layer of the A F primer-topcoat system w ith

tempe rature as a parameter 194

7.6. Bode mod ulus plot of the total coating system of the AF primer-primer

system with temperature as a parameter 195

7.7. EIS Bode modulus plots of the (a) inner layer and (b) outer layer of the AF

primer-primer system with temperature as a parameter 196

7.8.

EIS B ode m odulus plots of the (a) inner and (b) total layers of the AV

primer-topcoat system with temperature as a parameter 197

7.9. EIS Bode modulus plots of the (a) inner and (b) total layers of the AV

primer-primer system with temperature as a parameter 197

7.10. Bode plots of the outer layer of the AV primer-primer system with

temperature as a parameter 198

7.11.

The capacitance values associated w ith the AF primer-topcoat system as a

function of cycle at room temperature and set temperatures 201

7.12. The

|Z|O.IHZ

values associated with the A F primer-topcoat system as a

function of cycle at room temp erature and set temp eratures 202

7.13.

Th e CiokHz values associated w ith the AF p rimer-prim er system as a


7.14. The

|Z|O.IHZ

values associated with the A F primer-primer system as a


xix


21/259

7.15.

The |Z|IOOHZvalues associated with the AF primer-primer system as a

function of cycle at room temp erature and set temp eratures 207

7.16. The Z'IOOHzvalues associated with the AF primer-primer system as a

function of cycle at room tempera ture and set temperatures 207

7.17. T he CiokHz values associated w ith the AV primer-top coat system as a

function of cycle at room temp erature and set tempe ratures 208

7.18. The |Z|O.IH~Zvalues associated with the A V prim er-topcoat system as a

function of cycle at room temp erature and set tempe ratures 209

7.19. Th e QokHz values associated w ith the AV primer-prim er system as a

function of cycle at room temp erature and set temperatures 211

7.20. The |Z|O.IH~Zvalues associated with the AV primer-primer system as a

function of cycle at room temp erature and set tempera tures 211

7.21.

|Z|O.IHZ(or |Z|IOOHZ)values as functions of the reciprocal of the absolute

temperature for (a) AF primer-topcoat system and (b) AF prim er-primer

system 214

7.22.

|Z|O.IH~Z

values as functions of the reciprocal of the absolute temperatu re for

(a) AV primer-topcoat system and (b) AV primer-primer system 214

7.23.The equivalent circuit used for the fitting of the EIS data of the inner layer

of the coating systems 219

7.24. R

p

and Rc

t

values as functions of the reciprocal of the absolute tem perature

for the inner layer of(a)AF primer-topcoat system and (b) AF

primer-primer system 221

7.25.

R

p

and R

ct

values as functions of the reciprocal of the absolute temperature

for the inner layer of (a) AV primer-topcoat system and (b) AV

primer-primer system 221

7.26. The coating capacitances and the double layer capacitances of the AF coating

systems at different set temp eratures: (a) AF primer-top coat system and

(b) AF primer-primer system 223

7.27. The coating capacitances and the double layer capacitances of the AV coating

systems at different set temperatures: (a) AV primer-topcoat system and

(b) AV primer-primer system 223

xx


22/259

CHAPTER

1.

INTRODUCTION TO CORROSION,

CORROSION CONT ROL BY COATINGS

AND ELECTROCHEMICAL MEASUREMENT MET HODS

1.1. In trodu ct ion to M etal Corrosion

Metal corrosion refers to the degradation of metals by chemical processes in an

environment [1]. Except for a few very noble metals, most metals originally exist in

nature as complex minerals, and the extraction of metals from minerals involves

consuming much energy. When metal is used in the environment, it has the tendency to

return to its natural existing state of metal oxide, and the theoretical reaction energy

released is equal to the energy input to extract the metal from the minerals. The metal

corrosion process is a thermal-dynamically preferred process, and the rate of the

corrosion is an issue of thermal-kinetics [2]. For bare metals directly exposed to the

environment, even if there is no water or other pollutant impurities, metal surface can still

be oxidized by the oxygen in the air through the equation [3]

x M + y 0

2

= M

x

0

2 y

(1.1)

If the environment is humid, a thin layer of moisture will form on the metal

surface, or when metals are used under water immersion, the more common corrosion

processes involve an electrochemical reaction process in which a local anode, a local

cathode and a conductive pathway for electronic charges to flow between them are

1


23/259

involved [2]. In the anode, which is the more active local site on the metal surface, the

metal is remov ed by the anodic/oxidation reaction:

M = M

XT

+ x e " (1.2)

Electrons that are released in the anodic reaction flow to the local cathode, where the

electron-consuming cathodic/reduction reactions occur. When oxygen is available, the

cathodic reaction involves the reduction of the dissolved oxygen:

2 H

2

0 + 0

2

+ 4e

_

= 40H

-

(1.3)

This reaction is the most favored cathodic reaction, and it is very common in corrosion

when the pH is above 7. Other reactions are the cathodic hydrogen evolution reactions:

2H

+

+ 2e" = H

2

(in acid media) (1.4)

2 H

2

0 +

2e"

= H

2

+ 2 0H ' (in basic media) (1.5)

The hydroxide anions produced at the cathode and the metal ions released at the anode

can react with each other to produce metal oxides, metal hydroxides and their hydrous

products. Metal ions may also react with water and dissolved carbon dioxide to produce

precipitated metal carbonates. All these corrosion products can build up on the metal

surface. Dissolved ions in water (also called aqueous electrolyte) can increase the

environmental conductivity and promote the transportation of ionic charges between the

cathode and anode. Then the corrosion process will be accelerated. Some ions also

participate in the corrosion reactions and catalyze the corrosion process. For example,

chloride anions can help to dissolve metal oxides and then accelerate the metal corrosion

process [4].

2


24/259

Metal corrosion has a very big influence on our lives. It is so common that it can

be seen almost everywhere, from the stains in the cooking utensils to rust in our cars, and

the rusty stains in the civil infrastructures [2]. Corrosion also has a big impa ct on the

economy. The total annual estimated direct cost of corrosion in the U.S. was estimated to

be $276 billion in 1998, which accounted for approximately 3 .1% of the total na tion's

Gross Domestic Product (GDP) [5]. Other indirect costs such as the plant downtime,

structure over-design and performance testing etc. were not counted. Corrosion is not just

an economic issue; it is also related to human life and safety. The most recent disaster

from corrosion is the collapse of the Minneapolis Interstate 35W bridge into the

Mississippi River during rush hour on August 1, 2007, due to the corrosion of the

supporting steels, which resulted in lost lives and many injuries. Another unforgettable

accident is the Aloha Flight 243 disaster, which involved the tearing away of the whole

upper fuselage of a Boeing 737 airplane during its Hawaii inter-island flight from Hilo

Airport to Honolulu International Airport on April 28, 1988.

1.2. Metal Corrosion Protection Methods

Corrosion is so important and it influences our lives so much. The role of metals

in our society still cannot be totally replaced by other materials. Methods have to be

introduced to prevent (or at least slow down) the corrosion of metals. One of the first

things we can do is to frequently examine metal structures. If corrosion becomes very

servere and disasters are imminent, the corroded metal structures should be shut down

and repaired or replaced immediately. Good design can reduce metal corrosion

significantly. Other methods have also been used in the corrosion protection of metals.

3


25/259

These methods can be classified into three major categories: cathodic/sacrificial

protection, passivation/inhibition, and protection by coatings [1,2].

The cathodic/sacrificial protection method is based on the idea of providing

electrons to the targeted metal object, such that the metal substrate is forced to become a

cathode. Therefore, corrosion is stopped at the metal surface. There are two major

methods for imposing metal cathodic/sacrificial protection [6]. One is to directly apply a

DC current to provide electrons to the metal surface. The other is to connect the protected

metal object to a more active metal, and then the active metal performs as an anode and is

sarcrified, while the metal object acts as a cathode and is protected. The

cathodic/sacrificial protection method is widely used in pipeline protection by applying

DC current and the protection of ship hulls by connecting to an active metal, or the

protection of steels with a galvanized zinc surface layer.

For some metals such as aluminum, stainless steel, and chromiun, when they are

exposed to the environment, the fresh metal surface can be oxidized to form a layer of

dense insoluble metal oxides. This insoluble metal oxide layer can prevent the direct

contact of corrosive species with the underlying metal. Therefore, further corrosion of the

metal is ceased. This process is called passivation. For metals which cannot form a

passivation layer on the surface by themselves, some special chemical treatment methods

for creating such layers are available

[7,8].

For example, steel objects can be dipped into

phosphate or chromate solutions, so a ferrous phosphate or chromium oxide passivation

barrier layer can be formed on the steel surface to prevent further corrosion of the steel

substrate.

4


26/259

The third method for the prevention of metal corrosion is to apply a thin layer of a

protective coating on the metal surface. This coating layer can separate the active metal

from the corrosive environment and performs as a barrier for corrosion protection. Active

additives can also be included in the coating formulation, such that the coating can

provide other types of protection such as sacrificial protection and passivation protection

besides the barrier protection [1,9].The details of corrosion protection by coatings will be

discussed later.

These three corrosion protection methods are not independent and they can be

used together to provide better corrosion protection. For example, if the DC current is

applied directly to the bare metal to provide cathodic protection, especially when the

protected metal object is buried underground or imm ersed under water, the current can be

dissipated directly by electrical conductivity. This method almost becomes impossible

because of the enormous cost, or the requirement for such a huge power supply. After a

coating layer has been applied onto the metal surface, the current required for cathodic

protection drops dramatically since coatings are not conductive. This makes this

cathodic/sacrificial method more realistic. Also, after a passivation layer has been formed

on the me tal surface by chemical treatments, the passivation film can be destroyed either

mechanically or chemically. Therefore, it is still better to apply a layer of protective

coatings onto the pretreated metal surface to provide additional protection to the

passivation layer and the metal substrate.

1 . 3 . C o r r o s i o n C o n t r o l b y C o a t i n g s

There are three ways in which protective coatings provide corrosion protection to

the metal substrate. First, the coating can act as a simple barrier between the corrosive

5


27/259

environment and the metal substrate. This barrier cannot totally stop, but can significantly

slow dow n the transport of corrosive species to the metal substrate. The concentrations of

corrosive species in contact with the metal surface can also be significantly reduced. So,

the corrosion of the metal substrate can be significantly reduced.

The pigment volume concentration (PVC) of protective coatings should be

carefully controlled to optimize their barrier protection. Because inorganic pigments are

generally impermeable for corrosive species, such as ions, oxygen and water, the

diffusion or transmission of these species can be reduced if more inorganic pigments are

added. Therefore, protective coatings are normally formulated with high PVC. However,

their PVCs cannot be too high. Otherwise, voids will start to appear in the coating and

they can significantly improve the transportation of corrosive species. The PVC of barrier

protective coatings is normally close to but lower than 85% of the critical pigment

volume concentration (CP VC ), the concentration where there is just enough resins to fill

the interstices between the pigm ent particles.

The adhesion of protective coatings to the m etal substrate, especially the adhesion

under water or aqueous electrolyte solution (wet adhesion), is also very important for

corrosion control. Even after water, oxygen and electrolytes have penetrated the coating

film and reached the coating/metal interface, if the coating's wet adhesion is very good,

those corrosive species cannot displace the coating from the metal surface. Thus, the

direct access of the corrosive species to the metal surface can be prevented, and the

corrosion can be slowed down.

To optim ize the barrier performance of coatings, careful m etal surface preparation

and pretreatment before coating application are also very important since they are the

6


28/259

prerequisites of good adhesion. During the coating application process, methods should

be used to minimize the imperfections in the coating film as much as possible, which

requires strictly following the instructions of the used coatings. If it is allowed, a multi

layer coating is preferred instead of a layer since it can reduce the chance of the direct

penetration of imperfections from the coating surface to the substrate [10].

Even after perfect barrier protective coatings have been applied on the metal

surface, when coated metal objects are used, it is more likely that some local areas may

be accidently damaged, such as the common chip damage of the coating at the lower part

of a car. In this case, the metal substrate is exposed to the corrosive env ironment and the

corrosion often occurs. Desired protective coatings are expected to provide protections

for these damaged spots. Therefore, besides passive barrier protective coatings, many

active protective coatings were also developped to provide the desired damage protection

for metal substrates.

The ideas of sacrificial and passivation protection for metals were used in the

design of active protective coatings. Metal rich coatings were developed to provide

sacrificial protection by m ixing the coating formulation with m etallic pigm ents wh ich are

more active than the metal substrate. The PVC of metal rich coatings should be equal to

or higher than the CPV C to ensure the electrical conducting of the metallic pigments w ith

each other and with the metal substrate. One of the widely used metal rich coatings is

zinc-rich primer, which is used in the protection of steel substrates [11]. Recently,

another type of metal rich coating, magnesium-rich primer, was also developed by

Bierwagen and used in the protection of aluminum alloy substrates [12]. If metal rich

coatings are used alone, the active metal pigments in the coatings can be consumed fast

7


29/259

and lose protection for the metal substrate in a short time. Because of this, they are

always used with a topcoat, which can provide extra barrier protection for the metal rich

primers, thus lengthening the service-life of metal rich coatings.

Another active protective coating is the inhibitor-pigmented coating [13]. One of

the examples is the chromate primer for aircrafts, which is highly pigmented with

strontium chromate. Chromates are slightly soluble in water. When there is a small

damage spot, the dissolved chromate can migrate to the exposed area and oxidize the

metal surface. Therefore, a passivation metal oxide layer can be formed on the exposed

metal surface, and then the further corrosion of the exposed metal substrate in the damage

area is prevented. These active primers are also used together with a topcoat, with the

topcoat providing barrier protection to the primer. O therwise, the inhibitive pigme nts can

be washed away by rain or other exposed water; or they may absorb too much water by

osmotic proc ess, which can result in blisters on the coating film. M any of these inhibitive

pigments are toxic, and there is thus an urgency to replace them, such as the replacement

of the chromate in primers with nontoxic inhibitors or other nontoxic c oatings.

1 .4 . E v a l u a t i o n o f t h e C o r r o s i o n P r o t e c t i o n P e r f o r m a n c e o f C o a t i n g s

Coatings need to be ranked based on their corrosion protection performance

during the development of protective coatings. The known protection abilities of coatings

will assist in material selection and construction design. The best approach to rank a

coating is to evaluate it under the actual service conditions. This often takes too long,

making this approach impractical for the coating manufacturers and customers.

Accelerating test methods have been developed to simulate coating service env ironments,

8


30/259

resulting in coating failure in a much shorter time as compared to actual service

conditions [14].

ASTM B 117 salt spray standard is a widely used accelerating test method [15].

In this method, coated panels are place in a weathering chamber where 3.5% sodium

chloride aqueous solution is constantly sprayed at 35C to produce a salt fog in the

chambe r. The coated panels are taken out routinely to evaluate the corrosion protection

performance. The Prohesion accelerat ing exposure method was introduced for

simulating cyclic fog/dry conditions (ASTM G85) [16,17]. It consists of alternative

cycling of lhr of salt fog spray at 25C and lhr of drying at 35C. The salt solution used

in Prohesion test is dilute Harrison's solution (DHS), a lightly acidic salt solution, which

is composed of 0.35% ammonium sulfate and 0.05% sodium chloride. The pH of this

solution is around 4-5, which is close to the pH of acid rains. This method is closer to the

actual environment since dew condenses onto outdoor objects at night and in the morning,

and it can be dried off as the temperature rises in the daytime. The drying process can

increase the conce ntrations of corrosive species and accelerate the corrosion process. T he

Prohesion m ethod can also be coupled w ith the QUV accelerating m ethod to include the

UV light degradation process into the evaluation of coating's corrosion protection

performance (ASTM D5894) [18,19]. The QUV method is composed of alternative

cycling of 4 hr of UV light radiation at 60C and 4 hr of water condensation at 50C

(ASTM G53) [20]. During the Prohesion/QUV accelerating exposure test, the samples

are al ternately placed in Prohesion and QUV chambers weekly and periodical

exam inations are carried out to evaluate the corrosion protection performance of coatings.

9


31/259

Besides these three methods mentioned above, other accelerating methods are

also used in laboratory studies of the corrosion protection performance of coatings.

Constant immersion of coated panels under aqueous electrolyte solutions can be used to

evaluate protective coatings [21-23]. The salt solutions used in constant immersion can

be DHS, sodium chloride solution, or other salt solutions. The thermal cycling method

can be used to study the hydrothermal influence o n the corrosion protection properties of

coatings, and it was found to be a good m ethod in the evaluation of very durable coatings

[24-29].

The cathodic polarization method and AC-DC-AC accelerating method were

also used to accelerate coating failure by applying a negative potential across the coating

film. C athodic reactions were forced to occur on the m etal surface. The c athodic reaction

byproducts promoted coating degradation and delamination [30-34]. There are many

other accelerating methods used in coating studies and some new methods may be

developed in the future [35].

The corrosion protection properties of coatings should be ranked after the coated

panels have been exposed to the accelerating conditions. The original ranking was done

by visual evaluation. In the evaluation, either the distance of the coating delamination

from the scribed area (ASTM D 1654-92) [36], or the total blistered or rusted area on the

coating surface is measured (ASTM D610-01 ) [37], and then a ranking number from 0 to

10 is assigned to represent the corrosion protection provided by the coating. This method

can only give qualitative ranking of coatings and it is only useful after visible defects

have been observed. The development of electrochemical instruments allows us to

quantitatively evaluate the corrosion protection performance of coatings based on

electrochemical measurements [38-41]. The electrochemical measurement methods are

10


32/259

able to detect coating failure and give coating ranking before any visible defects start to

appear on the coating surface. Two of the electrochemical measurement methods,

electrochemical impedance spectroscopy and electrochemical noise measurem ents, are in

wide use in the performance studies of protective coatings. The following part of this

chapter will focus on the introduction of these two methods and their applications in the

studies of the corrosion protection performance of coatings.

1 .5 . E l e c t r o c h e m i c a l M e t h o d s i n C o a t i n g E v a l u a t i o n

1.5.1. Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) has been used by many

researchers in the study of the corrosion resistance of coated metals [24,42-46]. In the

EIS test, a small AC potential signal is applied over a wide range of frequency and the

electric current changes are detected, then the impedance can be obtained.

The applied AC potential in EIS tests can be expressed as the following equation:

V(t) = V

0

exp(/) (1.8)

11


33/259

Similar to the resistance from DC measurement in the equation R = V/I, the

corresponding resistance-like parameter obtained from AC measurement is called

impedance and is given by

Z = V(t) / I(t) = V

0

exp(/'cot) /IoexpO'cot -

)

= (V

0

/ Io)expO^) (1.9)

As shown in the above equation, the impedance has a complex form. The

modulus of the impedance |Z| is equal to |V

0

/Io|, and the phase angle is (j). So, the above

equation can be rew ritten as

Z - |Z| exp(/)

= |Z| (cos^ +ysin


34/259

connected to the w orking electrode lead (W E), a platinum m esh or other noble material is

used as the counter electrode (CE), and a reference electrode (RE), such as saturated

calomel electrode (SCE), silver/silver chloride and mercury/mercury sulfate, is used

during the test as the tested sample and the electrodes were under electrolyte immersion

[47]. In situations where a traditional standard reference electrode cannot be used, such as

in very corrosive or high-temperature environment or in some cases where electrolyte

immersion cannot be used in the test, a two-electrode configuration can be used in the

EIS measurement [47-50]. In the two-electrode configuration, the coated metal substrate

is still used as the working electrode and the inert material such as platinum or gold

serves as both the reference and counter electrodes. The two-electrode configuration is

suitable for field application and has been used in the development of sensors for the

corrosion protection monitoring of protective coatings [51-59]. A four-electrode EIS

configuration wa s also used in the investigation of the corrosion protection performance

of the outer and inner layers of a two-layer coating system [60,61].

RE

W E

Electrolyte

Topcoat

"^ Primer

" Su bstrate

Figure 1.1. The diagram of the three-electrode EIS method used in

the test of coated m etals.

13


35/259

EIS data

can be

expressed

in a

Nyquist plot,

in

which

the

real part

of the

impedance is plotted against the imaginary part of the impedance.Itcan also be shownin

Bode plo ts, in which the phase angle and logarithm of the impedance mo dulus are plotted

against the frequency (Figure

1.2).

a

-2400

-1600

-800

0

"

I

i

~

1

1 ' 1

o

i

6

ghigh f requency

I

. I

1

1 ' 1 _

-o

o

o

h

b

b

low frequency -- -

1

, I

10 '

a 10

3

N"

800 1600 2400 3200

Z ' / n

10

I HIH, IIIIM.

MUM

IIMM

MUM,

Mll iq IIMM I HUH

^

I I I I I J J J

i t i n J

J J J

feb

-30

-60

I

10~

2

10 10

2

10

4

10

6

Frequency/Hz

(a) (b)

Figure 1.2. The example of EIS (a) Nyquist and (b) Bode plots.

The low-frequency impedance m odulus is frequently used in the EIS data analysis

asitis com parable to the polarization resistance of coated m etals and can be related to the

barrier protection performance of the coating [43-46].

Apart from the graphic analysis of EIS results byobtaining useful param eters

from the EIS spectrum, equivalent electrical circuit models have also been used to

fit

the

EIS spectrum [47]for further analysisofthe coating performance. An equivalent circuit

model

is

composed

of

passive electric elements. T hese elements should have physical

meanings corresponding tothe studied coating system. Normally, m ore detailed m odels

can have

a

better

fit to the

data than

a

simpler model.

The

explanation

of

the electric

elementsin detailed models can be very difficult. Onlya few simple circuit models that

14


36/259

can give good physical explanations are suitable to be used in the analysis of EIS data

associated w ith coated metals [44,45,62,63].

Defect-free coatings have very high resistance, and alm ost no corrosion related

reaction occurs on the coating/metal interface. A R andies model can be used in the fitting

of defect-free coating's EIS data. The Randies model is shown in Figure 1.3 and consists

of a resistor, R

s

, which corresponds to electrolyte solution resistance; a capacitor, C

c

,

which corresponds to the coating capacitance; and a resistor, R

p

, which represents the

coating resistance.

Cc

AM

Electroly te Coating Metal

Figure 1.3. Randies model for intact, high-

resistance coatings in EIS data fitting.

For weathered coatings, or for less protective coatings, small pores may exist in

the coating film and corrosion related reactions can occur on the coating/metal interface.

In these cases, the second model shown in Figure 1.4 is commonly used in the fitting of

the EIS data. In this model, besides the three electric elements used in Randies model,

two electric elem ents which represent the coating/metal interface are also included: R

ct

is

for charge transfer resistance and Cai for double layer capacitance. These two models can

fit most of the EIS spectra associated with coated metals. Normally, the solution

y-yy^s'yyyy^

15


37/259

resistance is negligible when it is compared to the resistances of coatings and the

interface, and can be omitted in the fitting process. Under some conditions, the coating

film and/or the interface may not perform as a perfect capacitor. In order to fit the EIS

spectrum more accurately, one or both of the capacitors can be replaced by a constant

phase element (CPE) [64,65]. The impedance of a CPE can be calculated from the

following equation: |Z| = l/[T(/co)

n

], where

f =

- 1 , T is a constant, co is the angular

frequency and n is the exponent with 0.5 < n < 1. As n = 1, CPE becomes an ideal

capacitor and constant T equals to the capacitance.

Cc

:

:

:

:

:

:

:Ks:;:;:

:

:

Rp

Cdl

Ret

Electrolyte Coating Metal

Figure 1.4. Electrical model used for weathered or medium- to

low-resistance coatings in EIS data fitting.

1.5.2. Electrochemical noise measurements (E NM )

1.5.2.1. ENM theories, test methods and noise sources

Electrochemical noise measurements have been used in the characterization of the

corrosion protection performance of coatings [66-71]. This technique measures the

electric noise fluctuations originated from the tested samples and no outside electrical

perturbation is applied during the test [72]. It is totally non-intrusive and can be used for

continuous monitoring of the corrosion protection performance of coatings. Similar to

16


38/259


39/259


40/259

1

W E 2

Figure 1.7. Diagram of the three-electrode, substrate-less ENM

configuration.

noise generated by the separation of charge across resistors caused by thermal motion

was given by the following equation [72]:

W

E

=4kTR

(1.15)

wh ere ffc is the therm al potential noise (the power spectral density of mea n-squar e noise

voltage),

k

is Boltzmann constant, 1.3807 x 10"

23

J.K"

1

, Tis the absolute temperature, and

R

is the resistance of the system. The thermal noise is frequency independent or white. It

is associated with the low-frequency plateau in the power spectral density (PSD) plot. In

systems such as passive metals and painted metals, the resistance of the coating/metal

interface or polymer may be very high, so the influence of thermal noise can be very

important.

Shot noise is generated by the random passing through a giving point in the circuit

of the quantized charge carriers. If the m ovement of the charges each time is independent,

the current noise from shot noise can be given by this equation [72]:

I

=2qI =2f

n

q

2

(1.16)

19


41/259

where

Wi

is the shot noise (the power spectral density of mean-square n oise current),

q

is

the charge on a charge carrier, I is the mean current, andf

n

is the mean frequency of

charge em ission. Shot noise does not change with the measured frequency and is a white

noise.

Unlike thermal noise and shot noise, flicker noise is frequency dependent and the

noise power spectral density (PSD) falls as the frequency increases [72]. The relationship

of PSD with frequenc y/can be expressed as PSD cc /", wheren


42/259

respectively. Previous works found that R

n

is close to the polarization resistance and the

low-frquency impedance modulus, and it is a good indicator of how much corrosion

protection the coating can provide to the metal substrate [67-71,81]. The introduction of

R

n

is based on the assumption that the current and potential noise data are Gaussian-like

distribution or normal distribution. In the ENM test, the base line of electrochemical

noise may drift, and then the original noise data may deviate from Gaussian-like or

normal distributions. Base line drift should be removed before the calculation of noise

resistance, and a linear detrending method is commonly used for the drift removal.

The other important parameter obtained from statistical analysis is the localization

index (LI) and it is given by the given equation:

U = cn/I

rms

(1.18)

where

I

rms

is the root mean square of the noise current. It was reported that the LI values

could give some insights into the corrosion mechanism of the metal substrate [72,74,82].

It was demonstrated that if LI is smaller than 10"

3

, the dominant corrosion mechanism is

uniform corrosion; with LI between 10"

3

and0.1, the major corrosion type is a mixture of

uniform and localized corrosion; and the corrosion type is localized corrosion if LI is

larger than 0.1. By examining the LI values, the dom inant corrosion type of the corroded

metals can be identified [73,83]. However, the application of LI in the characterization of

corrosion mechanism was questioned by some other researchers, and they found that LI

was not necessarily related to the corrosion mechanisms of metal substrates [84,85].

1.5.2.2.2. Frequen cy dom ain analysis

Besides the use of a statistical method in the analysis of ENM data in the time

domain, the original noise data can also be transformed into the frequency domain with

21


43/259

mathematical algorithm calculations [72,86-90]. One of the most widely used frequency

domain spectra is the power spectrum, w hich estimates the power present in the signals at

different frequencies over a period of

time.

The power at each given frequency is plotted

against the frequency and the resulting power spectrum plot is also called a power

spectral density plot (PSD). Two typical mathematical algorithms are commonly used in

the transform of the original noise data. One is the fast Fourier transform (FFT), which

can substantially reduce the total machine operations required to compute a discrete

Fourier transform, and is widely used in the analysis and manipulation of digital or

discrete data. Details of the FFT algorithm can be found elsewhere [91-93]. An example

of the PSD plots for coated metals after FFT is given in Figure 1.8(a).

1

j i n n i i l l

11IT;

1i i i i|

10

uiu

i i i i m i l l i '

10 10"

1

10"

10"

10

10"

0.01 0.1 1

/ /Hz

10

JTTTT]

r

U i i

" X . Fa i l e d

"^^v Intact

*


44/259

order of the MEM transform [72,94]. Smaller orders can give smoother spectrum, but it

may lose some imp ortant information. L arger orders give noisier spectrum and it is more

similar to the spectrum obtained from FFT. In the application of MEM, a proper order

should be chosen to keep most of the information and still yield a smoother curve than

FFT (Figure 1.8(b)).

The power spectral density of both the current Q{ and potential

Qi'v)

noise can

be obtained after the transform. The spectral noise impedance, R

sn

(/), can be obtained by

the equation [88,94-96]:

Rs(/)=

(Vy/Vf

2

(1.19)

R

sn

(/) can also be calculated by FFT as introduced by Mansfeld and coworkers [9 7]:

Rsn(/)=

\iy

FF T

(f)/lFFT(f)\,

(1-20)

whereV

FFT

(f) is the FFT of potential noise time records, and

I

FFT

(f) is the FFT of

current noise time records. After R

sn

(/) was calculated, a noise impedance spectrum can

be obtained by plotting R

sn

(/) versus frequency. Comparison of the noise impedance

spectrum with EIS impedance spectrum has been reported where the spectra were similar

[88,96,98]. However, the frequency range of the noise impedance spectrum is very

limited. The highest frequency is one half of the frequency used in the noise data

collection. The lowest frequency depends on the total time range of the original data used

in the transformation, and it is related to the total acquisition time

t

by the equation:

f

mm

-

lit.

The most useful parameter from noise impedance spectrum may be the low frequency

spectral noise impedance: R

sn

(/-_^), which has been found to be equal to the polarization

23


45/259

resistance of the measured system and comparable to the noise resistance R

n

and the EIS

low-frquency impedance modulus |Z|/-_^> [99].

1.5.2.2.3. Shot noise method analysis

The power spectral density can also be used in the analysis of ENM data by the

shot noise method to investigate the metal corrosion mechanism [72,84,100,101]. Shot

noise is one of the three major types of electric noise. If the electrochemical noise

generated by corrosion can be considered as shot noise and it is the dom inant form of the

noise, the shot noise method can be used in the analysis of the corrosion mechanism. In

the shot noise analysis method, it is assumed that the current noise is composed of

packets of charges passing through the circuit and each packet of charges is generated by

independent pulse [84]. In corrosion systems, it is also assumed that these pulses are

generated by independent corrosion related events. It has been found by Cottis that the

electrochemical noise in corrosion systems could be considered as shot noise under a

series of assum ptions

[100].

Two important shot noise parameters, the average charge of each event,

q,

and the

average frequency of these eve nts ,/,, can be obtained from the equations [84]:

q=

{Wv^if

1

1B (1.21)

and

f

n

= B

2

I W

v

,

(1.22)

where i|/y and\\>i are the low frequency power spectral density values of the potential and

current noise, respectively. B is the Stern-Geary constant, which is not a universal

constant and depends on the corrosion system studied. B can be related to the corrosion

24


46/259


47/259

7 E.W. Flickers,Corrosion Inhibitors,Noyes, Park Ridge, NJ., 1987

8 M.G . Fontana,C orrosion Engineering, 3

r

edition,Mcgraw-Hill, New York,1986

9 G.P. Bierwagen, Organic Coatings for Corrosion Control (Acs Symposium Series),

American Chemical Society Publication, Washington, DC,

1998

10 G.P. Bierwagen,

Progress in Organic C oatings,

28,1996,43

11 J.E.O. Mayne, U.R. Evans,

Chemistry and

Industry,63,1944 , 109

12

M.E. Nanna, G.P. Bierwagen,

JCT Research,

1(2),

2004,

69

13

H. Leidheiser, Jr.,

Journal of Coatings Technology,

53(678),

1981,

29

14

G. Wranglen,

An Introduction to Corrosion and Protection of Metal,

Chapman and

Hall Ltd., London,

1985

15 ASTM B-117,

Method of Salt-Spray Fog (Fog) Testing,

ASTM International,

Philadelphia, PA,

1990

16

F.D. Timmins, Avoiding paint failures by Prohesion,

Journal of the Oil and Colour

Chemists' A ssociation, 62,1979, 131

17 ASTM G85-98, Standard Practice for Modified Salt Spray (Fog) Testing, ASTM

International, Ph iladelphia, PA ,

1998

18 J. Andews, F. Anwari, B. J. Carlozzo, M. Dilorenzo, R. Glover, S. Grossman, C. J.

Knauss, J. McCarthy, B. Mysza, R. Patterson, R. Raymond, B. Skerry, P. M. Slifko,

W. Stipkovich, J. C. Weaver, M. Wolfe, Journal of Coatings Technology, 66, 1994,

49

19

ASTM D5894-05,

Standard Practice for Cyclic Salt Fog/UV Exposure o f Painted

Metal,

ASTM International, Philadelphia, PA,

2005

20

A. G53-96,

Standard Practice for Operating Light- and Water- Exposure Apparatus

26


48/259

(Fluorescent UV Condensation Type) For Exposure of Nonmetallic Materials, ASTM

International, Philadelphia, PA , 1996

21

L. Gray, B. Appleman,

Journal of Protective Coatings and Linings,

Feb.

2003,

6 6

22 A.M. Monsada, Y. Sekiyama, M. Yuasa, I. Sekine,Shikizai Kyokaishi, 72(6), 1999,

355

23 L.M. Callow, J.D. Scantlebury,Journal of the Oil and Colour Chemists' Association,

64(4), 1981,

140

24

G.P. Bierwagen, L. He, J. Li, L. Ellingson, D.E. Tallman,

Progress in Organic

Coatings, 39,2000,67

25 A. Miszczyk, K. Darowicki,Corrion Science,43 ,2001,1337

26

A. Miszczyk, K. Darowicki,

Progress in Organic Coatings,

4 6,

2003,

49

27

L Valentinelli, J. Vogelsang, H. Ochs, L. Fedrizzi,


45,

2002 ,405

28

L. Fedrizzi, A. Bergo, F. Deflorian, L. Valentinelli,


48,

2003,

271

29

S. Touzain, Q. Le Thu, G. Bonnet,


52,

2005

311

30 J. Hollaender, E. Ludwig, S. Hillebrand, Proceedings of the Fifth International

Tinplate Conference,London, 1992, 300

31

J. Hollaender,Food Additives and Contaminants, 14(6/7), 1997, 617

32 M. Bethencourt, F.J. Botana, M.J. Cano, R.M. Osuna, M. Marcos, Progress in

Organic Coatings,

49,

2004,

275

33 M. Poelm an, M.G . Olivier, N. Gayarre, J.P. Petitjean, Progress in Organic Coatings,

54,2005,55

27


49/259

34 M .T. Rodrig uez, J.J. Gracenea, S.J. Garcia, J.J. Saura, J.J. Suay,

Progress in Organic

Coatings, 50 , 2005,123

35

B. R. Appleman,


62(787),

1990,

57

36 ASTM D1654-92, Standard Test Method for Evaluation of Painted or C oated

Specimens Subjected to Corrosive Environments,

ASTM International, Philadelphia,

PA,

1992

37 ASTM D610-01, Evaluation Test Method for Evaluating Degree of Rusting on

Painted Steel Surfaces,ASTM International, Philadelphia, PA,2001

38

H. Leidheiser Jr.,


7(1),

1979,

79

39

H. Leidh eiser Jr.,


63(802),

1991,

21

40 I. Sekine,Progress in Organic Coatings, 31(1-2),1997,73

41 S.R. Taylor,Progress in Organic Co atings,43(1-3),2001,141

42

G.P. Bierwagen, J. Li, L. He, C. Jeffcoate,


46,

2003,

148

43

J.A. Grandle, S.R. Tayler,

Corrosion,

50,

1994,

792

44 A. Amirudin, D. Thierry,Progress in Organic Coatings,2 6 ,1995 , 1

45

J.N. Murray,


31 ,

1997,

375

46

H.K. Y asuda, C M . Reddy , Q.S. Yu, J.E. Deffeyes, G.P. Bierwagen, L. He,

Corrosion,

57,2001,29

47

R. Cottis,

Electrochemical Impedance and Noise,

Huston, TX: NAC E

International,

1999

48 C. Andrade, L. Soler, C. Alonso, X.R. Novoa, M. Keddam, Corrosion Science,

37(12),

1995,2013

28


50/259

49

S. Haruyama, S. Sudo,

Electrochimica Acta,

38(14),

1993,

1857

50

F. Ma nsfeld, S.L. Jeanjaquet, M.W . Ken dig,

Co rrosion Science,

26(9),

1986,

735

51

T.C. Simpson, P.J. Moran, W.C. Moshier, G.D. Davis, B.A. Shaw, C O . Arah, K.L.

Zankel,

Journal of the Electrochemical Society,

136,

1989,

2761

52

T.C. Simpson, P.J. Moran, H. Hampel, G.D. Davis, B.A. Shaw, C O . Arah, T.L. Fritz,

K. Zankel,

Corrosion,

46,

1990,

331

53

T.C. Simpson, H. Hampel, G.D. Davis, C O . Arah, T.L. Fritz, P.J. Moran, B.A. Shaw ,

K.L. Zankel,

Progress in O rganic Coatings,

20 ,

1992,

199

54

A. Am irudin, D. Thierry,

British Corrosion Journal,

30(3),

1995,

214

55 A. Amirudin, D. Thierry, Materials Science Forum, 192-194(Pt. 1, Electroch emical

Method s in Corrosion Research V, Pt. 1),1995,317

56 G.D. Davis, C M . Dacres, US 58595 37,1999

57 G.D. Davis, L.A. Krebs, C M . Dacres, Journal of Coatings Technology, 74(935),

2002 ,69

58 G.D. Davis, C M . Dacres, US Patent: US 6054038 ,2000

59 G.D. Davis, C M . Dacres, L.A. Krebs, US Patent: US 6328878,2001

60 J. Kittel, N. Celati, M. Keddam, H. Takenouti, Progress in Organic Coatings, 41,

2001 ,93

61 J. Kittel, N. Celati, M. Keddam, H. Takenouti, Progress in Organic Coatings, 46,

2003 ,135

62 G. Grundmeier, W. Schmidt, M. Stratmann,Electrochimica Acta, 45,2000,2515

63 D. Loveday, P. Peterson, B. Rodgers,

JCT- Coatings Technology,

October,2004,88

64 L. Nyiko s, T. Pajkossy,

Electrochimica

acta,30,1985 , 1553

29


51/259

65

U. Rammelt, G. Reinhard,

C orrosion Science,

27 ,

1987 ,

373

66 P.C. Searson, J.L. Dawson,Journal oftheElectrochemical Society, 135,1988, 1908

67 D.A. Eden, M. Hoffman, B.S. Skerry, ACS symposium series, 322, Polymeric

Materials for Corrosion Control, American Chemical Society,

Washington, DC,

1986,

36

68

B.S. Skerry, D.A. Eden,

Progress in Organic Co atings,

15 ,

1987 ,

269

69 G.P. Bierwagen, V. Balbyshev, D.J. Mills, D.E. Tallman, Proceedings of the

Symposium on Advances in Corrosion Protection by Organic Coatings II, 95-13,

1995,

82

70 G.P. Bierwagen, D.J. Mills, D.E. Tallman, B.S. Skerry,

ASTM Special Technical

Publication STP 1277, Electrochemical Noise Measurement for Corrosion

Application,

American Society for Testing and Materials, Philadelphia, PA,

1996,

427

71 G.P. Bierwagen, C.J. Jeffcoate, J. Li, S. Balbyshev, D.E. Tallman, D.J. Mills,

Progress in O rganic Coatings,

29 ,

1996 ,

21-29

72

R.A. Cottis,

Corrosion, 57,2001,

265

73

D.A. Eden, in

Uhlig's Corrosion handbook, 2nd

ed. (Eds.: Herbert H. Uhlig, R. W.

Revie), John Wiley & Sons, Inc, Hoboken, NJ, 2000

74 G.P. Bierwagen, X. Wang, D.E. Tallman, Progress in Organic Coatings, 46,2003,

163

75

X. Wang,

Study o n novel electrode configurations for in situ corrosion mon itoring on

coated metal systems,

Ph.D. Thesis, Chapter 3, Faculty of Science and Mathematics,

North Dakota State University, Fargo, ND,2002

30


52/259

76 S.J. Mab butt, D.J. M ills, Surface Coatings International Part B: Coatings

Transactions, 84(B4),2001,277

77

S.J. Mabbutt, D.J. Mills,

British Corrosion Journal,

33(2),

1998,

158

78

S. Mabbu tt, D.J. Mills, C.P. Woodcock,


59(3),

2007,

192

79

U. Bertocci, F. Huet,

Corrosion,

51 ,

1995 ,

131

80

D.A. Eden, in

C orrosion /98,

paper no. 386, NACE, Houston, TX,

1998

81

H.A.A. Al-Mazeedi, R.A. Cottis,

Electrochimica

Acta,49,

2004,

2787

82

F. Mansfeld, Z. Sun,

corrosion,

55 ,

1999 ,

915

83 D.A. Eden,

Journal oj

the

Electrochemical

Society, 141,

1994,

1402

84

J.M. Sanchez-Am aya, R.A. Cottis, F.J. Botana ,

C orrosion Science, Al, 2005,

3280

85

Z. Sun, F. Mansfeld, Corrosion,55 ,

1999 ,

915

86

S. Turgoose, R.A. Cottis, Corrosion Testing Made Easy: Electrochemical Impedance

and Noise,NA CE, Houston, TX,2000

87

C. Gabrielli, M. Keddam,Corrosion, 48,

1992,

794

88

U. Bertocci, C. Garbrielli, F. Huet, M. Keddam, P. Rousseau,

Journal of the

Electrochemical Society, 144(1),1997,3 7

89 H. Greisiger, T. Schauer,Progress in Organ ic Coatings,39 ,2000,31036

90

T. Schauer, H. Greisiger, L. Dulog,

Electrochimica

Acta,

43 ,1998,

2423

91

E.O. Brigham, The fast Fourier transform and its applications, Prentice-Hall,

Englewood Cliffs, NJ, 1988

92 L. Auslander, J.R. Johnson, R.W. Johnson, Advances in Applied Mathematics, 17,

1996,

477

31


53/259

93 U. B ertocci, J. Frydman, C. Gabrielli, F. Huet, M . Keddam ,

Journal of the

Electrochemical Society, 145,1998, 2780

94 U. Bertocci, C. Gabrielli, F. Huet, M. Keddam,

Journal of the Electrochemical

Society, 144,1997,31

95 F. Mansfeld, L.T. Han, C.C. Lee,Journal of the Electrochemical Society, 143,1996,

L286

96 C.C. Lee, F. Mansfeld, Corrosion Science,40(6),1998,959

97 H. Xiao, F. Mansfeld,

Journal of the Electrochemical

Society, 141,

1994,

2332

98

U. Bertocci, F. Huet, Corrosion, 51 ,

1995 ,

131

99 T. Schauer, H. Greisiger, L. Dulo g,Electrochimica

Acta,

43(16-17),1998,2423

100

R.A. Co ttis,

Russian Journal of the Electrochemistry,

42,

2006,

497

101Kyung-Hw an Na, Su-Il Pyun,Journal of Electroanalatical Chem istry, 596,2006,7

32


54/259

CHAPTER 2. SENSING TECHNIQUES FOR THE

MONITORING OF METAL CORROSION

AND CORROSION PROTECTION OF CO ATINGS

Metals are widely used as structural materials, and metallic corrosion is thus an

important issue. The corrosion resistance of metals, the effectiveness of metal corrosion

prevention methods, and the corrosion protection performance of protective coatings can

be thoroughly studied in the laboratory. However, when metals are in service, the

environment can be very different from the studied conditions in the laboratory and

changes with time. Therefore, the corrosion resistivity and the service life of metals

estimated from laboratory studies can be very different from their performance in the

field. There is thus a need for

in-situ

monitoring the corrosion status of metal structures.

Corrosion sensing methods have been developed to detect the metal corrosion status

under different environmental con ditions, and they can be used to detect the corrosion of

bare metals, metals in concretes, or metals under protective coatings. Some of these

techniques were designed based on the detection of either the corrosion products, such as

metal oxides and metal hydroxides, or the corrosive species, such as chloride ions, water

and oxygen. Some others are based on electrical or electrochemical measurement

metho ds, which can measure either the cumulative or instant corrosion rate.

In this chapter, the corrosion sensing techniques will be reviewed in two sections.

The first section will focus on the methods that are mainly used to detect the corrosion

process of metals; the second section will focus on discussions of those sensing methods

33


55/259

for coated metals, which can be used for

in-situ

monitoring the corrosion protection of

protective coatings and the corrosion process of the metal substrate.

2 . 1 . S e n s i n g T e c h n i q u e s f o r M e t a l s

Many sensing methods were developed for monitoring the corrosion process of

metals. These methods can be classified into two categories. The first one is those

techniques that are designed to detect corrosion produ cts or corrosive species; the second

includes those methods based on electrical and/or electrochemical m easurement meth ods.

2.1.1.S ensing technologies detecting corrosion products and corrosive species

2.2.1.1.

W eight loss corrosion sensing technologies

One of the most reliable and oldest methods for corrosion monitoring is to put

small pieces (coupons) of the structural metal under the same environment and weigh

them regularly to get the weight loss caused by corrosion. This weight loss is then

converted to the cumulative corrosion rate of the metal (ASTM D 2688) [1]. However,

this method highly depends on the methods of the initial sample preparation and the

cleaning process of the weathered coupons. It also cannot give continuous and

simultaneous m ass loss information of the studied m etal. The quartz crystal m icrobalance

(QCM) technique was introduced for the continuous monitoring of very small metal

weight changes caused by corrosion

[2-4].

The metal under test is attached to the QCM

by gluing or deposition, and the changes of the resonance frequency of the QCM is

detected by electronic devices, and then the weight change caused by corrosion was

calculated and correlated to the corrosion rate of the metal.

2.1.1.2. Optical fiber corrosion sensors

Metal corrosion products have different optical properties from the original metal.

34


56/259

By detecting the optical changes, the metal corrosion process can also be monitored.

Optical fiber corrosion sensors were developed based on the monitoring of the optical

difference of metals and their corrosion products [5-24]. Although there were different

types of optical corrosion sensors, the general preparation processes were similar. At the

sensing en d of an o ptical glass fiber, the cladding w as removed and a thin layer ofametal

is deposited onto the exposed fiber core. Then this sensing end is put into the testing

environment. As the metal at the sensing end starts to corrode, corrosion products would

build up on the fiber core and cause changes in the transmission of light through the

optical fiber, which then changes light power output or refraction index. These optical

changes are detected by an instrument, which has been connected to the other end of the

optical fiber, such that the metal corrosion process can be monitored.

It was demonstrated that the corrosion rate obtained from the optical fiber

corrosion sensor was comparable to the corrosion rate obtained from the standard

electrochemical methods [21]. The metals which have been studied by the optical fibers

included aluminum [18,23], aluminum alloy [9], steel [18], nickel [18,20] and copper

[14].

The methods used in the deposition of the metal onto the fiber core varied and could

be electroplating [18], sputtering [18], physical vacuum deposition [18,21], chemical

deposition [13] and thermal deposition [16]. The optical fiber corrosion sensor

technology was improved by incorporating the function of the detection of the strain

changes of the metal caused by corrosion [25]. A bundle of optical fibers instead of one

single fiber were investigated and it was found that this approach could increase the

output light signal [12].

How ever, when the metal has been deposited on the fiber core, the m icrostructure

35


57/259

of the metal on the fiber is very different from the bulk metal that was used in the

structure even thoug h they share the same chem ical comp osition. So their corrosion rates

can be different under the same environmental conditions. Therefore, the optical fiber

corrosion sensor actually measures the corrosivity of the metal environment.

2.1.1.3.

Acoustic corrosion sensors

The mass and/or size changes caused by corrosion can cause changes in the

frequency and/or magnitude oftheacoustic waves w hich pass through or are reflected by

the corroding metal object. Acoustic corrosion sensors have been designed to detect

corrosions by measuring the acoustic changes from the formation of corrosion products

on the metal surface [26]. An ultrasonic wave was also used as a guided wave in the

corrosion detection, and it could provide real-time, three-dimensional imaging of the

metal corrosion [27]. It has been reported that the acoustic method not only detects the

corrosion product film thickness but also the hidden corrosion spots and corrosion

induced cracks in m etal structures [27-30].

2.1.1.4. Sensors detecting cathodic reaction products

During metal corrosion processes, the anodic reaction consumes the active metal

and releases ele

the application of embedded sensors for in-situ monitoring of protective coatings on metal...

Documents