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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
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UMI Number: 3322479
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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
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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.
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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.
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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.
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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
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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
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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
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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
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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
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CHAPTER 9. FUTURE WORK 234
9.1.
References 236
APPEN DIX. PUBLICATIONS 237
X I
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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
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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
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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
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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
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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
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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
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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
function of cycle at room temperature and set temperatures 204
7.14. The
|Z|O.IHZ
values associated with the A F primer-primer system as a
function of cycle at room temperature and set temperatures 205
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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
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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
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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].
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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.
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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.
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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
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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
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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
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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,
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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.
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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
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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)
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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
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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.
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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
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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^
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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
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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)
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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
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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
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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
*
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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
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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
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56 G.D. Davis, C M . Dacres, US 58595 37,1999
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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
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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.
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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
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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