c
2014
AUSTIN GREGORY SMITH
ALL RIGHTS RESERVED
CHARACTERIZATION AND QUANTIFICATION OF EARLY STAGES FOR
ORGANIC COATINGS APPLIED ON AA2024/AA7075 BY CORRELATING
FREQUENCY DOMAIN APPROACH IN REAL TIME
A Thesis
Presented to
The Graduate Faculty of The University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Austin Gregory Smith
May, 2014
CHARACTERIZATION AND QUANTIFICATION OF EARLY STAGES FOR
ORGANIC COATINGS APPLIED ON AA2024/AA7075 BY CORRELATING
FREQUENCY DOMAIN APPROACH IN REAL TIME
Austin Gregory Smith
Thesis
Approved:
Accepted:
Advisor
Dr. Homero Castaneda-Lopez
Dean of the College
Dr. George K. Haritos
Faculty Reader
Dr. Gang Cheng
Dean of the Graduate School
Dr. George Newkome
Faculty Reader
Dr. Jie Zheng
Date
Department Chair
Dr. Harry Michael Cheung
ii
ABSTRACT
The successful performance of some aircraft structures are dependent on the corrosion control properties. One effective action to control the electrochemical process
is to introduce a physical barrier (coating) between the environment and the substrate. Understanding the early stage degradation mechanisms due to environmental
surroundings effects during corrosive exposure is a key factor to develop preventive actions and to extend the life and increase the reliability of the structure. Hex
chrome-free coatings can be used to control and mitigate potential corrosion risks during operation conditions. This work aims to show the effects of coating based primers
(Deft 02-Y-40) versus a pretreatment layer (Alodine 1600) varying corresponding dry
film thickness at different pH conditions. Electrochemical Impedance Spectroscopy
(EIS) is used to characterize meaningful parameters that quantify the performance
for each characteristic during 120 days of exposure. Time domain modeling will support the EIS results (frequency domain) that describe the early stage mechanisms
for each layering coating condition and surface analysis, such as Atomic Force Microscopy (AFM) and Infinite Focus Microscopy (IFM) will be used to validate the
experimental and theoretical results developed describing the transport mechanisms
occurring within the coating during early stages.
iii
ACKNOWLEDGEMENTS
I would like to acknowledge my advisers on this endeavor from the Department of
Biological and Chemical Engineering including Dr. Homero Castaneda-Lopez, Dr.
Gang Cheng, and Dr. Jie Zheng for all of their time, effort, and guidance that resulted in this research work. This could not be done without the support of the
CERL-U.S. Department of Defense Office of Corrosion Policy and Oversight.
I would also like to thank University of Akron Ph.D. Chemical Engineering students
Jonathon Fouts, Omar Rosas Camacho, Enrique Maya Visuet, and Ivan Karayan, for
their help and support with editing this thesis as well as work in the lab. I want
to thank the Mann Family and Fouts Family for keeping me in check, whether they
know it or not, they have allowed me to continue strong throughout this adventure.
Lastly, I would like to have this paper be in memory of my late father Gregory L.
Smith who passed away during April, 2012 at the age of 52.
One good solid hope is worth a cartload of certainties.
- The Fourth Doctor, Steven Moffat
iv
TABLE OF CONTENTS
Page
LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vii
LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ix
CHAPTER
I.
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1.1 Motivation of Research . . . . . . . . . . . . . . . . . . . . . . . . .
2
1.2 Corrosion Terminology . . . . . . . . . . . . . . . . . . . . . . . . . .
4
1.3 Corrosion Protection . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
1.4 Experimental Techniques . . . . . . . . . . . . . . . . . . . . . . . .
6
1.5 Damage Evolution Concept . . . . . . . . . . . . . . . . . . . . . . .
7
1.6 Experimental Assumptions for Early Stages of Damage Evolution . .
9
1.7 Approach Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . .
10
1.8 Work Highlights . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
II. MATHEMATICAL BACKGROUND AND PROPOSED MODEL . . . .
12
2.1 Proposed System Model . . . . . . . . . . . . . . . . . . . . . . . . .
12
2.2 Current Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
2.3 Time Dependent Reaction Product Model . . . . . . . . . . . . . . .
20
v
2.4 Impedance Equations . . . . . . . . . . . . . . . . . . . . . . . . . .
23
2.5 Frequency Domain Analogs . . . . . . . . . . . . . . . . . . . . . . .
24
III. EXPERIMENTAL PROCEDURE . . . . . . . . . . . . . . . . . . . . . .
27
3.1 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
3.2 Experimental Design Matrix . . . . . . . . . . . . . . . . . . . . . .
32
3.3 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
IV. RESULTS OF EXPERIMENTS AND ANAYLSIS . . . . . . . . . . . . .
39
4.1 Electrochemical Impedance Spectroscopy Results . . . . . . . . . . .
39
4.2 Equivalent Electrical Circuit Analog . . . . . . . . . . . . . . . . . .
54
4.3 Exposed Panel Photos . . . . . . . . . . . . . . . . . . . . . . . . . .
68
4.4 AFM High Resolution Results . . . . . . . . . . . . . . . . . . . . . .
82
4.5 IFM High Resolution Results . . . . . . . . . . . . . . . . . . . . . .
83
V. CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
5.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
5.2 Future Implementations . . . . . . . . . . . . . . . . . . . . . . . . . 104
BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
vi
LIST OF TABLES
Table
Page
3.1
Substrate Composition by Elemental Weight Percent . . . . . . . . . .
28
3.2
Components in pH Buffer Solutions . . . . . . . . . . . . . . . . . . . .
29
3.3
Molecular Weights and Masses needed for Chemicals . . . . . . . . . . .
30
3.4
Experimental Design Matrix for Pretreatment Samples . . . . . . . . .
37
3.5
Experimental Design Matrix for Primer Samples . . . . . . . . . . . . .
38
4.1
Experimental Open Circuit Potential (V) Values . . . . . . . . . . . . .
41
4.2
Equivalent Circuit Modeling Results for Cell 39 . . . . . . . . . . . . .
57
4.3
Equivalent Circuit Modeling Results for Cell 57 . . . . . . . . . . . . .
58
4.4
Equivalent Circuit Modeling Results for Cell 43 . . . . . . . . . . . . .
59
4.5
Equivalent Circuit Modeling Results for Cell 47 . . . . . . . . . . . . .
60
4.6
Equivalent Circuit Modeling Results for Cell 15 . . . . . . . . . . . . .
62
4.7
Equivalent Circuit Modeling Results for Cell 51 . . . . . . . . . . . . .
63
4.8
Equivalent Circuit Modeling Results for Cell 19 . . . . . . . . . . . . .
64
4.9
Equivalent Circuit Modeling Results for Cell 31 . . . . . . . . . . . . .
65
4.10 Equivalent Circuit Modeling Results for Cell 55 . . . . . . . . . . . . .
66
4.11 Equivalent Circuit Modeling Results for Cell 67 . . . . . . . . . . . . .
67
vii
4.12 IFM Profile Results from Pretreatment Samples . . . . . . . . . . . . . 101
4.13 IFM Profile Results from Primer Samples . . . . . . . . . . . . . . . . . 101
viii
LIST OF FIGURES
Figure
1.1
Page
Damage evolution concept considering different stages, each stage
includes different transport mechanism and interfacial processes. . . . .
8
2.1
Proposed Physical Experimental Setup . . . . . . . . . . . . . . . . . .
13
2.2
Flow Chart showing derived formulas from Fick’s Second Law. . . . . .
19
2.3
Theoretical Sensitivity Analysis . . . . . . . . . . . . . . . . . . . . . .
21
2.4
Theoretical Profiles for Concentration, Current, and Impedance versus Position through Time Progression . . . . . . . . . . . . . . . . . .
22
2.5
Uniform Graphical Format for EIS Results . . . . . . . . . . . . . . . .
25
2.6
Equivalent Circuit Examples . . . . . . . . . . . . . . . . . . . . . . . .
26
3.1
Experimental Design Setup . . . . . . . . . . . . . . . . . . . . . . . . .
31
3.2
3D Panel Setup Schematic . . . . . . . . . . . . . . . . . . . . . . . . .
32
3.3
Actual Photo of the Experimental Design Setup showing Plate 018 . . .
32
3.4
Schematic of the Electrochemical Cell Setup . . . . . . . . . . . . . . .
34
4.1
Experimental EIS results over 120 days for an AA2024 substrate
with a primer application of 0.3 − 0.5 mils being exposed to a 3.5
wt.% NaCl environment at pH = 7. (Left) Nyquist Plot and (Right)
Bode Plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
42
ix
4.2
Experimental EIS results over 120 days for an AA7075 substrate
with a primer application of 0.3 − 0.5 mils being exposed to a 3.5
wt.% NaCl environment at pH = 7. (Left) Nyquist Plot and (Right)
Bode Plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
43
Experimental EIS results over 120 days for an AA2024 substrate
with a primer application of 0.7 − 0.9 mils being exposed to a 3.5
wt.% NaCl environment at pH = 4. (Left) Nyquist Plot and (Right)
Bode Plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
Experimental EIS results over 120 days for an AA2024 substrate
with a primer application of 0.7 − 0.9 mils being exposed to a 3.5
wt.% NaCl environment at pH = 10. (Left) Nyquist Plot and
(Right) Bode Plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
46
Experimental EIS results over 120 days for an AA2024 substrate
with a pretreatment application of 15 mils being exposed to a 3.5
wt.% NaCl environment at pH = 7. (Left) Nyquist Plot and (Right)
Bode Plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
Experimental EIS results over 120 days for an AA2024 substrate
with a primer application of 1.2 − 1.5 mils being exposed to a 3.5
wt.% NaCl environment at pH = 7. (Left) Nyquist Plot and (Right)
Bode Plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
48
Experimental EIS results over 120 days for an AA7075 substrate
with a pretreatment application of 5 mils being exposed to a 3.5
wt.% NaCl environment at pH = 4. (Left) Nyquist Plot and (Right)
Bode Plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
50
Experimental EIS results over 120 days for an AA7075 substrate
with a pretreatment application of 15 mils being exposed to a 3.5
wt.% NaCl environment at pH = 4. (Left) Nyquist Plot and (Right)
Bode Plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
51
Experimental EIS results over 120 days for an AA7075 substrate
with a primer application of 0.3 − 0.5 mils being exposed to a 3.5
wt.% NaCl environment at pH = 4. (Left) Nyquist Plot and (Right)
Bode Plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
52
4.10 Experimental EIS results over 120 days for an AA7075 substrate
with a pretreatment application of 1.2 − 1.5 mils being exposed to
a 3.5 wt.% NaCl environment at pH = 4. (Left) Nyquist Plot and
(Right) Bode Plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
53
4.11 Equivalent Circuit Models used for the analysis of the EIS Results . . .
55
4.3
4.4
4.5
4.6
4.7
4.8
4.9
x
4.12 Actual Photos of Panels 001, 002, and 003 after 120 days of electrolyte exposure. Panels 001 - 003 includes a Pretreatment application of 5 mils on a AA2024 substrate with a buffered electrolyte
pH level of 4, 7, 10 respectively. . . . . . . . . . . . . . . . . . . . . . .
69
4.13 Actual Photos of Panels 004, 005, and 006 after 120 days of electrolyte exposure. Panels 004 - 006 includes a Pretreatment application of 10 mils on a AA2024 substrate with a buffered electrolyte
pH level of 4, 7, 10 respectively. . . . . . . . . . . . . . . . . . . . . . .
70
4.14 Actual Photos of Panels 007, 008, and 009 after 120 days of electrolyte exposure. Panels 007 - 009 includes a Pretreatment application of 15 mils on a AA2024 substrate with a buffered electrolyte
pH level of 4, 7, 10 respectively. . . . . . . . . . . . . . . . . . . . . . .
71
4.15 Actual Photos of Panels 010, 011, and 012 after 120 days of electrolyte exposure. Panels 010 - 012 includes a Pretreatment application of 5 mils on a AA7075 substrate with a buffered electrolyte
pH level of 4, 7, 10 respectively. . . . . . . . . . . . . . . . . . . . . . .
72
4.16 Actual Photos of Panels 013, 014, and 015 after 120 days of electrolyte exposure. Panels 013 - 015 includes a Pretreatment application of 10 mils on a AA7075 substrate with a buffered electrolyte
pH level of 4, 7, 10 respectively. . . . . . . . . . . . . . . . . . . . . . .
73
4.17 Actual Photos of Panels 016, 017, and 018 after 120 days of electrolyte exposure. Panels 016 - 018 includes a Pretreatment application of 15 mils on a AA7075 substrate with a buffered electrolyte
pH level of 4, 7, 10 respectively. . . . . . . . . . . . . . . . . . . . . . .
74
4.18 Actual Photos of Panels 019, 020, and 021 after 120 days of electrolyte exposure. Panels 019 - 021 includes a Primer application of
0.3 − 0.5 mils on a AA2024 substrate with a buffered electrolyte
pH level of 4, 7, 10 respectively. . . . . . . . . . . . . . . . . . . . . . .
76
4.19 Actual Photos of Panels 022, 023, and 024 after 120 days of electrolyte exposure. Panels 022 - 024 includes a Primer application of
0.7 − 0.9 mils on a AA2024 substrate with a buffered electrolyte
pH level of 4, 7, 10 respectively. . . . . . . . . . . . . . . . . . . . . . .
77
4.20 Actual Photos of Panels 025, 026, and 027 after 120 days of electrolyte exposure. Panels 025 - 027 includes a Primer application of
1.2 − 1.5 mils on a AA2024 substrate with a buffered electrolyte
pH level of 4, 7, 10 respectively. . . . . . . . . . . . . . . . . . . . . . .
78
xi
4.21 Actual Photos of Panels 028, 029, and 030 after 120 days of electrolyte exposure. Panels 028 - 030 includes a Primer application of
0.3 − 0.5 mils on a AA7075 substrate with a buffered electrolyte
pH level of 4, 7, 10 respectively. . . . . . . . . . . . . . . . . . . . . . .
79
4.22 Actual Photos of Panels 031, 032, and 033 after 120 days of electrolyte exposure. Panels 031 - 033 includes a Primer application of
0.7 − 0.9 mils on a AA7075 substrate with a buffered electrolyte
pH level of 4, 7, 10 respectively. . . . . . . . . . . . . . . . . . . . . . .
80
4.23 Actual Photos of Panels 034, 035, and 036 after 120 days of electrolyte exposure. Panels 034 - 036 includes a Primer application of
1.2 − 1.5 mils on a AA7075 substrate with a buffered electrolyte
pH level of 4, 7, 10 respectively. . . . . . . . . . . . . . . . . . . . . . .
81
4.24 Atomic Force Microscope analysis for the (a) Pretreatment and (b)
Primer applications before electrolyte exposure. . . . . . . . . . . . . .
82
4.25 Infinite Focus Microscope 2D and 3D Surface Profiles for the Pretreatment application of 5 mils on a AA2024 substrate with a
buffered electrolyte level of pH 4 after 120 days of exposure. . . . . . .
84
4.26 Infinite Focus Microscope 2D and 3D Surface Profiles for the Pretreatment application of 10 mils on a AA2024 substrate with a
buffered electrolyte level of pH 4 after 120 days of exposure. . . . . . .
85
4.27 Infinite Focus Microscope 2D and 3D Surface Profiles for the Pretreatment application of 15 mils on a AA2024 substrate with a
buffered electrolyte level of pH 4 after 120 days of exposure. . . . . . .
86
4.28 Infinite Focus Microscope 2D and 3D Surface Profiles for the Pretreatment application of 5 mils on a AA7075 substrate with a
buffered electrolyte level of pH 4 after 120 days of exposure. . . . . . .
88
4.29 Infinite Focus Microscope 2D and 3D Surface Profiles for the Pretreatment application of 10 mils on a AA7075 substrate with a
buffered electrolyte level of pH 4 after 120 days of exposure. . . . . . .
89
4.30 Infinite Focus Microscope 2D and 3D Surface Profiles for the Pretreatment application of 15 mils on a AA7075 substrate with a
buffered electrolyte level of pH 4 after 120 days of exposure. . . . . . .
90
4.31 Infinite Focus Microscope 2D and 3D Surface Profiles for the Primer
application of 0.3 − 0.5 mils on a AA2024 substrate with a buffered
electrolyte level of pH 4 after 120 days of exposure. . . . . . . . . . . .
92
xii
4.32 Infinite Focus Microscope 2D and 3D Surface Profiles for the Primer
application of 0.7 − 0.9 mils on a AA2024 substrate with a buffered
electrolyte level of pH 4 after 120 days of exposure. . . . . . . . . . . .
93
4.33 Infinite Focus Microscope 2D and 3D Surface Profiles for the Primer
application of 1.2 − 1.5 mils on a AA2024 substrate with a buffered
electrolyte level of pH 4 after 120 days of exposure. . . . . . . . . . . .
94
4.34 Infinite Focus Microscope 2D and 3D Surface Profiles for the Primer
application of 0.3 − 0.5 mils on a AA7075 substrate with a buffered
electrolyte level of pH 4 after 120 days of exposure. . . . . . . . . . . .
96
4.35 Infinite Focus Microscope 2D and 3D Surface Profiles for the Primer
application of 0.7 − 0.9 mils on a AA7075 substrate with a buffered
electrolyte level of pH 4 after 120 days of exposure. . . . . . . . . . . .
97
4.36 Infinite Focus Microscope 2D and 3D Surface Profiles for the Primer
application of 1.2 − 1.5 mils on a AA7075 substrate with a buffered
electrolyte level of pH 4 after 120 days of exposure. . . . . . . . . . . .
98
4.37 IFM Profile Defined Values . . . . . . . . . . . . . . . . . . . . . . . . . 100
xiii
CHAPTER I
INTRODUCTION
The purpose of this work is to characterize and quantify the behavior of the coating
layer(s)/substrate interfaces when exposed to a corrosive environment. The damage
evolution concept has been introduced lately to explain different stages based on the
performance of each protective layer. This latter, in principle can be achieved by validating experimental results with a mathematical model to quantify each stage. The
first stage considers the electrolyte transport of the corrosion species (ions) within
or through each layer and the second stage considers the activation of the metallic substrate. The first stage is critical for the development and validation of the
damage evolution, this latter will help further understanding of corrosion behavior
and reliability of the substrate in such conditions. A simple model will represent
and characterize the first stage or early stage processes by using time domain theoretical approach, the experimental characterization and quantification of measurable
parameters are determined with impedance in the frequency domain. The unification
of both approaches will lead the path to elucidate and postulate early stage processes
and validate the quantitative analysis.
1
1.1 Motivation of Research
Advancements in corrosion prevention have been shown to further the life of products
and infrastructure that would corrode away at their normal pace otherwise. Currently,
the United States has gone from spending eighty billion dollars in 1975, and a quarter
billion dollars in 2000, to over four hundred billion in 2014 annually over all industries
on the repair and replacement of materials affected by corrosion effects [1, 2, 3]. This
increasing cost of replacing, restructuring, or salvaging crucial systems have forced
scientists to work together to extend useful life times [4]. Industry is currently looking
for new ways to describe and tackle the damage evolution of different structures to
help protect systems. One of the methods is the use of coatings.
Three-dimensional architecture of organic/inorganic coatings has different
physical properties considering crystalline state, amorphous, semiconductor, dielectric, hydrophobic and hydrophilic zones, affecting the charge transport within the
organic/inorganic bulk and interfacial dissolution processes. Past studies have mostly
focused on modeling water uptake and water-enabled migration of ions in single or
multilayers by deterministic path [5]. The proposed objective is to quantify the transport mechanisms by considering the water activity, infiltration, and water-enabled
migration of ions; the electrochemical processes of organic/inorganic coatings and
corrosion products by the spatial current, capacitance, and impedance distribution
within coating layers. Three approaches will be used to accomplish this objective:
(1) a defined substrate/layer(s) physical prototype that enables a theoretical deter-
2
ministic approach, (2) different resolution laboratory validation approach, providing
current and impedance spatial distribution, and (3) a mathematical model involving
coatings properties vs. performance approach based on computer based analysis. All
three approaches, ending the research with a simple model validated with experimental testing will be developed and evaluated during the one hundred and twenty day
experimental duration.
One of the grand challenges of coatings design and development is the quantification and control of transport processes within the coating volume and at the
interface level. A simpler, but comprehensive characterization and quantification is
included: the spatial impedance distribution due to water uptake within the coating and the electrochemical/chemical reactions at the substrate/coating level, where
spatial current, impedance or voltage offers sensitive control variables. However, the
best control parameter will only be successful if it is validated on a well-designed laboratory system. The coating/substrate combination that is envisioned is an organic
layer on top of a metallic alloys (Aluminum 2024 and 7076) designed by combining
modeling and advanced diagnostics for multiscale characterization. In addition, the
electrolyte used as a corrosion environment is assumed to have continuous conditions.
A continuous immersed environment is proposed to set up to simulate constant immersion to show basic principles development under a more controlled corrosive environment. A proposal is arrived at to characterize and quantify performance of
the barrier layers by considering the stochastic nature of the transport influencing the
behavior of the organic-inorganic layer(s) for intact condition and under electrochem3
ical activity where the substrate or inorganic coating bridges to the electrolyte. This
approach complements two developments: the initial coating/layer three-dimensional
architecture properties that are multiscale in nature and the material performance
under corrosion conditions at the coating/substrate interface level.
1.2 Corrosion Terminology
Corrosion is defined by the deterioration or breakdown of a substance due to internal properties with the external environment [2]. The substances that are usually
under investigation are metals and/or alloys. Corrosion can be divided up into different types including electrochemical and chemical corrosion. Metallic degradation
is mainly caused by the electrochemical corrosion process. Electrochemical corrosion
occurs when there are oxidation and reduction reactions, a chemical solution, and a
metallic pathway.
Galvanic, pitting, and crevice corrosion are various types of electrochemical
corrosion [6, 7, 8, 9, 10]. Electrochemical processes involve the transportation of
electrical currents from anode to cathode through an electrolyte. These reactions
created free electrons which flow through metals transport to the cathodic electrolyte
[11]. Galvanic corrosion begins with two different metals in contact with or close
enough for induce electrical fields. These electrical fields normally occur at (or near)
a surface boundary and in physical contact with a conductive environment, such as
an electrolyte solution [12]. Unless one of these four parts needed for corrosion is
blocked, corrosion will continue.
4
Thermodynamic related effects play a role in the aggressiveness of the corrosion over time of electrochemical corrosion. Studies have shown that there is a direct
relationship between temperature and rate of corrosion [13]. Thermodynamics is the
study of changes in energy that are caused by a physical or chemical change [14]. If
a physical system is thermodynamically unstable, energy levels will change to force
the system to become thermodynamically stable, i.e., steady state.
An electrolyte solution is a compound that ionizes when dissolved in solvents.
In many cases, this is a specified concentration of sodium chloride (NaCl) which simulates seawater. Since this electrolyte solution will be on the surface boundary of the
metal surface, the electrolyte will react with the metal, through anodic metals. This
environment allows for current flow, therefore an electronic pathway [12]. To protect
the material of interest from corroding, numerous methods of corrosion protection
can be used ranging from introducing inhibitors, implementing methods of cathodic
protection, and using external coatings.
1.3 Corrosion Protection
Certain methods can be taken to prevent or slow down the corrosion process. One of
the most common methods to prevent corrosion is to apply a protective coating.
Coatings that are applied to the surface of a metal are designed to decrease the
rate of corrosion of the metal beneath. There are three types of coatings: inorganic,
organic, and a hybrid mixture between inorganic and organic. Organic coatings are
mainly used to control the corrosion of steel-based structures. The main disadvantage
5
of organic coatings is the ability to uptake water into the polymeric coating, which
will lead to a less effective coating, and eventually failure of the coating [15]. Different
types of coatings as well as coating thickness can delay water reaching the surface of
the metal substrate.
The coating application is used to interfere with one of the parts of the corrosion cell. Even though there are cosmetic reasons for coating applications, protective
coatings are have been applied to systems in corrosive environments such as off-shore
drilling rigs, power plants, ships, pipelines, and military aircraft [16].
1.4 Experimental Techniques
Electrolyte uptake through the coating can be measured using a few different techniques. In this work, we will use Electrochemical Impedance Spectroscopy (EIS) to
characterize the electrochemical signal of the interface when a coating acts as a barrier
of aggressive environment. The Atomic Force Microscopy (AFM) and Infinite Focus
Microscopy (IFM) measurement techniques will be used to validate and characterize
the surface.
Water uptake through organic coatings takes place due to the transport mechanisms. The electrolyte transport mechanism or water uptake is quantified by EIS
(Electrochemical Impedance Spectroscopy) [17]. The EIS technique is used to gather
experimental data via transfer function values which, when presented, creates Nyquist
and Bode plots. Since this is a non-destructive experimental technique, time depen-
6
dent measurements can be taken and analyzed without changing the electrochemical
cell conditions.
One method to analyze the EIS results is by equivalent circuit analog models. Several cell elements including the electrode double layer capacitance, electrode
kinetics, diffusion layer, and the solution resistance can be shown [18]. Actual EIS
results determine the equivalent circuit analogs that need to be used along with the
modeling variable values.
Atomic Force Microscopy (AFM) and Infinite Focus Microscopy (IFM) are
used throughout corrosion research to be able to analyze the surface at different scales;
AFM considers atomic level and IFM considers submicro scales. AFM techniques are
commonly used to produce two and three dimensional surface profiles [19]. AFM
measurements produce topography images that can show a details as curvature in a
sample. AFM can show shape evolution at the nanoscale level during corrosion [20].
These figures are taken when the microscope is in focus in the zero line is apparent
[21]. IFM techniques produce topography images through scanning techniques which
allow for positional profiles in a similar manner to the AFM process.
1.5 Damage Evolution Concept
Figure 1.1 shows a schematic describing the conventional stages of the cumulative
damage function of layer(s)/substrate system and the proposed concept for damage
evolution concept. The damage is based on the transport, activation, and degradation mechanisms existing during exposure time; the physical characteristics of the
7
Figure 1.1: Damage evolution concept considering different stages, each stage includes
different transport mechanism and interfacial processes.
coating are quantified by the transfer function (or impedance technique). The coatings/substrate interface is analyzed with deterministic modeling; the phase angle and
impedance modulus are used to quantify the physical characteristics of the organic
and inorganic coatings, such as porosity, water uptake, corrosion products evolution,
and low impedance regions, among others [22].
The damage evolution concept includes four stages for barrier layers describing the layer/electrode/electrolyte interfaces as a result of transport mechanisms and
the degradation process [23]. The stages can be classified according to transport
and chemical and electrochemical mechanisms. Initiation, active, active-passive, and
growth are the four stages that have been reported for the coating/substrate system.
8
Each stage can be characterized by the mechanism existing within the layer and at
the layer/substrate interface. The initiation stage (Stage I) is a characteristic of mass
transport where the water/electrolyte uptake occurs. The active stage (Stage II) can
be a combination of mass and charge transfer when electrochemical reactions activate the solid surface layer. The third stage (Stage III) is associated with mechanisms
and processes that involve active-passive surfaces and interfaces and the fourth stage
(Stage IV) is the growth of corrosion or the active metal state as a charge transfer
process considering metal dissolution and metal wall loss, as illustrated in Figure 1.1
.
1.6 Experimental Assumptions for Early Stages of Damage Evolution
An electrochemical cell by definition contains two electrodes that allow the transportation of electrons and ion species. The cell will be contained above and secured
to the sample for EIS experiments [24, 25]. To simplify this three-dimensional model,
assumptions will have to be made. First, the electrolyte solution is assumed to be
“well-mixed” in the electrochemical cell. This means that there are no ionic species
concentration effects and that species transport is neglected. Second, an assumption
can be made that the oxygen concentration is constant in the electrolyte solution,
there are no metal hydrolysis precipitate of either metal, and there is no hydrolysis
forming metal hydroxide in the electrolyte. The geometry of the electrode considers
the Cartesian coordinates [26, 27].
9
1.7 Approach Hypothesis
The critical features of this concept are the following: (1) the coating selection and
synthesis is strongly related to the balance and architect of the inorganic/organic
components; designing and characterizing the changes that occur in the coating
and during substrate dissolution can be quantified with experimental and theoretical tools including: EIS, macro/micro surface characterization via AFM and IFM,
and equivalent circuit analysis. (2) The inorganic coating pretreatment in the substrate will have influence on the corrosion resistance properties of the system when
the organic coating no longer acts as a natural physical barrier from the environment; the passivation mechanisms in the active substrate will be quantified and
measured using EIS/theoretical analysis. (3) The degradation mechanisms for the
formed layers will be characterized and monitored to feedback the performance of the
electrochemical system. (4) An appropriate electrochemical model accounting the
feasible mechanisms for the system electrolyte/coating/substrate to introduce the
performance/degradation state, and organic/inorganic compounds that controls the
reliability of the system in the first stage.
1.8 Work Highlights
The proposed concept is the integration of deterministic modeling and experimental
procedure to account for the spatial distribution of water infiltration, water sorption,
and water uptake within the organic coating and interfacial mechanisms over time.
10
The uniqueness of the proposed concept will allow an estimate for expected locations
of corrosion product formation at the organic/inorganic/substrate interface.
The deterministic modeling of the electrolyte uptake model can characterize
the transport and current carrier species due to the interactions of charge species
with metal and metal oxide substrates. The model will account for intrinsic parameters in the system structure affecting the impedance distribution, such as deposited
inorganic pretreatments on the substrate, electrochemical stability of the formed corrosion products, as well as the morphology and porosity on the solid state layers. The
material distribution in the designed structure influences the protection mechanism
when exposed to a corrosive environment.
This proposal includes the development of a comprehensive multiscale mathematical model and experimental method. The mathematical model unifies the transport expressions with physical and chemical properties considering the electrolyte
characteristics and transport within the coating. The experimental algorithm is based
on multiscale resolution electrochemical and surface techniques when different layer(s)
cover the metallic substrate while exposed to corrosion environments.
This thesis will present important results that validate early time damage
evolution stages. This thesis work is organized as follows. In Chapter II, the theory
and mathematical modeling information for the transport of electrolyte is developed.
The experimental methods for the EIS, AFM, and IFM can be found in Chapter
III. The presentation of results and analysis can be found in Chapter IV, while a
summary, conclusion, and future work follows in Chapter V.
11
CHAPTER II
MATHEMATICAL BACKGROUND AND PROPOSED MODEL
This chapter explains the mathematical background. First, a proposed system model
that will be used in a mathematical setup to show reason for the experimental setup.
Once the system model is understood, then the mathematical system will be shown
and assumptions made. Electrochemical Impedance Spectroscopy is a common analog
for frequency domain analysis and the methodology is presented. The theory of taking
EIS results and modeling them around equivalent circuits is given and the correlation
with the damage evolution will then be explained. A mathematical time dependent
solution is presented with a sensitivity analysis. A correlation is presented afterwards
to pair these theoretical results together.
2.1 Proposed System Model
The electrochemical cell used with the coated substrate can be assumed to have
distinct layers and boundaries. This setup shows that initially there is no electrolyte
transport through the coating, therefore, no corrosion products. There are a few main
chemical half reactions to remember in regards to corrosion and are given below as
Ox + e− = R and 2H + + 2e− = H2 .
12
Assumptions are needed to reduce the three-dimensional electrochemical cell
to a one-dimensional problem to decrease difficulty when modeling. Since there is
symmetry around the radius of the electrochemical cell, the three-dimensional problem has been reduced to a two dimensional problem. A three-dimensional and reduced
two-dimensional representative diagram of the physical layout is shown in Figure 2.1.
(a) Three Dimensional Diagram
(b) Side View Diagram
Figure 2.1: Proposed Physical Experimental Setup
An assumption can be made as the transport in the other directions are negligible
in comparison to the direction toward the application layer. This shows that the
electrochemical cell exists above the application layer, which in turn exists above
the substrate. Shown in Figure 2.1, L is defined as the height of electrolyte plus
application layer away from the substrate and δ is the height of the application layer.
13
2.1.1 Governing Equation
The Nernst-Planck equation is given by equation (2.1) and is shown by
∂Ci
= −∇ · Ji ,
∂t
(2.1)
where Ci is the concentration of the ith species in the electrolyte and Ji is the flux
transport of the concentration. The Nernst-Planck equation shows the conservation
of mass of a chemical species under the influence of an electrical field [28]. The flux
equation is written as
Ji = −Di ∇Ci − Zi
Di F
Ci ∇φ + uCi ,
RT
(2.2)
where Di is the mass diffusion coefficient of the ith species, Zi is the valence number
of the ith species, F is Faraday’s constant, R is the universal gas constant, T is the
temperature of the electrolyte, φ is the potential, and u is the velocity vector of the
electrolyte medium. Note that equation (2.2) has three distinct parts on the right
hand side, which represent the diffusion, migration, and convection terms, respectively. Nor density or charge gradients are considered, the only gradient considered
is the concentration. These assumptions can be validated and performed experimentally, this model will only the diffusion term will considered, equation (2.2) is reduced
to Fick’s Second Law, which is given as
∂C
= D∇2 C.
∂t
14
(2.3)
Figure 2.1 shows that the system can be analyzed from a cylindrical coordinate system
which allows equation (2.3) to now be written as
∂Ci
Di ∂
∂Ci
∂ 1 ∂Ci
∂
∂Ci
=
r
+
+
r
∂t
r ∂r
∂r
∂θ r ∂θ
∂z
∂z
due to symmetry around the center axis,
constant,
∂Ci
∂r
∂ 2 Ci
∂θ2
(2.4)
= 0, and that the radial effects are
= 0. These assumptions will allow the three-dimensional model to
arrive at a one-dimensional diffusion equation, which is given in equation (2.5) as
∂Ci
∂ 2 Ci
= Di 2 .
∂t
∂z
(2.5)
Equation (2.5) can be used to describe the electrolyte transport through the coating
application in one cartesian coordinate.
2.1.2 1-D Initial & Boundary Conditions
Now that the governing equation is established, the initial and boundary conditions
need to be introduced. At initial time, the electrolyte concentration away from the
wall, z ≥ 0, is equal to the concentration in the bulk, CiBulk . This leads to Ci∗ = 0.
Since the system is theoretically bounded between [0, ∞ = L], the boundary conditions are only available at or near z = 0. After initial time, the electrolyte concentration as z → 0 is the concentration in the bulk, CiBulk which also gives Ci∗ → 0. The
electrolyte concentration at z = 0 is related to the governing differential equation,
15
equation (2.17). These formally shown below in equations (2.6 - 2.9) as
Ci (t = 0, z ≥ 0) = CiBulk
(2.6)
Ci (t > 0, z → 0) = CiBulk
(2.7)
∗
Ci∗ (t > 0, z = 0) = Ci exp{−αz}
(2.8)
z [0, ∞)
(2.9)
2.1.3 Laplace Transform
The Laplace Transform on the one-dimensional Fick’s Second Law will lead toward
a solution for the concentration over time and position. To solve equation (2.5), the
Laplace Transform is applied on both sides of the equation giving
L
∂Ci
∂t
∂ 2 Ci
= L Di 2 .
∂z
(2.10)
Solving equation (2.10) gives
sCi (s) − c (0) = D
d2 C i
,
dz 2
(2.11)
and by assuming that c (0) = CiBulk and dividing through by the diffusion constant,
D, and setting equal to zero, equation (2.11) can now be written as,
d2 C i
1
1
− s Ci (s) + CiBulk = D.
2
dz
D
D
(2.12)
To simplify equation (2.12), C ∗ is defined in equation (2.13) as
Ci∗ = CiBulk − Ci (s) .
(2.13)
Again, the Laplace Transform is taken on both sides of equation (2.13) and given as
∗
Ci =
CiBulk
− Ci (s) .
s
16
(2.14)
Now that all variables are defined in the Laplace domain, combining equations
(2.12) and (2.14) gives
d2 C i
s
1
+ CiBulk − Ci (s) = 0.
2
dz
D
D
(2.15)
Rearranging equation (2.15) and substituting in equation (2.13) will allow a simple
Laplace differential equation, given as
d2 C i
s
+ Ci (s) = 0.
2
dz
D
(2.16)
The differential equation given in equation (2.16) has a solution and is given as
Ci (z, s) = A exp{−αz} + B exp{αz} +
CiBulk
,
s
(2.17)
where α is defined as
r
α=
s
.
D
(2.18)
2.1.4 Solving the Governing Equation with Conditions
Since the domain of z is bounded on the higher end by ∞, exp{−αz} → ∞. Therefore, B in equation (2.17) is equal to 0. Since the domain of z also bounded by 0,
exp{−αz} → 0. Therefore, exp{−αz} → 1. Then, applying the boundary conditions
from equation (2.7) and (2.9) into the governing differential equation, equation (2.17)
the constant A is solved for as
A = Ci (0, s) +
CiBulk
.
s
(2.19)
Substituting equation (2.19) into equation (2.17) gives
Ci (z, s) =
C Bulk
Ciz=0 − i
s
17
exp{−αz} +
CiBulk
.
s
(2.20)
Remembering that the second boundary condition, Ciz=0 = 0, equation (2.20) can be
reduced to
Ci (z, s) = −
C Bulk
C Bulk
exp{−αz} + i
.
s
s
(2.21)
Since all variables are in terms of position and in the Laplace domain, the Inverse
Laplacian Transform is taken
L
−1
CiBulk
CiBulk
−1
−
Ci (z, s) = L
exp{−αz} +
,
s
s
(2.22)
and solved for as
Ci (z, t) = −CiBulk erf {
z
} + CiBulk .
4Dt
(2.23)
Substituting Ci∗ from equation (2.13) in reducing equation (2.23) gives
Ci (z, t) =
Ci∗ erf
z
,
4Dt
(2.24)
which is the governing equation for electrolyte concentration profiles dependent on
time and position.
Since the time and position dependent concentration profile has been derived
from Fick’s Second Law. Figure 2.2 shows all of the equations that can be easily
derived from Fick’s Second Law and the order of which they are solved.
2.2 Current Equations
To derive the equation that describes current versus position and time, the time
derivative of concentration is needed and is given as
i
∂C0 (z, t)
−J0 (0, t) =
= D0
.
nF A
∂z
x=0
18
(2.25)
Figure 2.2: Flow Chart showing derived formulas from Fick’s Second Law.
where i is defined as current in terms of position and time, n is defined as the number of electrons being transferred, and A is defined as the area that is exposed to
the electrolyte solution. Since equation (2.25) is based on the flux equation, but
could also be defined for current based on position and time, equation (2.25) can be
solved by taking the derivative of the concentration equation from equation (2.24)
and substituted into equation (2.25) giving,
∂C0 (z, t)
.
i (t) = nF AD0
∂z
z=0
Taking the Laplace transform is throughout equation (2.26) gives
∂C0 (z, t)
L [i (z, t)] = L nF AD0
,
∂z
z=0
(2.26)
(2.27)
and would then give the Current equation in the Laplace domain
1
i (z, s) =
nF AD02 C0∗
1
s2
.
Since equation (2.28) is given, applying the Laplace inverse
"
#
1
∗
2
nF
AD
C
0 0
L−1 [i (z, s)] = L−1
1
s2
19
(2.28)
(2.29)
will give the current equation based on the time domain and position, shown as
equation (2.30),
r
i (z, t) = nF AC0∗
D0
.
πt
(2.30)
2.3 Time Dependent Reaction Product Model
The Department of Theoretical & and Applied Mathematics at The University of
Akron [29] have come to understand the electrolyte uptake into a coating at early time
as well as “corrosion time”, which takes into account the corrosion product reactions
at the surface interface. Equation (2.31) describes the early time concentration of
electrolyte uptake is shown as
∂ [Ci ]
K(pH)([CiBulk ] − [Ci ]) + d(BR)
=
,
∂t
d
(2.31)
where C Bulk is the sodium chloride concentration in the bulk, K is the rate of reaction,
and d is the coating thickness (in microns). The equations that describe the species
concentration and the corrosion product concentration during “corrosion time” is
given by
∂ [CCP ]
KCP ([Ci ] − [CCP ]) + (w + H)(B) + (MSR)[i(t)]
=
∂t
(w + H)
Bulk
∂ [Ci ]
K(pH)([Ci ] − [Ci ]) − KCP ([Ci ] − [CCP ]) + d(BR)
=
∂t
d
(2.32)
(2.33)
where BR is defined as the Bulk Reactions, MSR is defined as the Metal Surface
Reactions, z = w(t) is defined as the height of the corrosion product/coating interface, and z = −H(t) is defined as the metal/corrosion product interface. Since the
20
corrosion product/coating interface will move over time, w(t) is given in equation
(2.34) as
w(t) =
ρmetal − ρCorrP rod
.
ρCorrP rod
(2.34)
Theoretical profiles that describe the percent volume fraction of electrolyte uptake
into the applied coating is predicted over time is shown in Figure 2.3. For the sensitivity study, application thicknesses (100, 500, 1000 microns) and pH levels (2, 4,
10, 12) were varied. The electrolyte saturation level was defined at 0.013 (1.3%), and
Aluminum, Iron, and Aluminum Hydroxide product were present. Figure 2.3 shows
that to slow down the volume of electrolyte uptake, cover the substrate with a thicker
application and try to lower the pH of the electrolyte if possible.
Figure 2.3: Theoretical Sensitivity Analysis
21
Figure 2.4 shows the time profiles that model the one-dimensional electrochemical system described above, which were created by equations (2.24), (2.30), and
an Ohm’s Law conversion, respectively.
Figure 2.4: Theoretical Profiles for Concentration, Current, and Impedance versus
Position through Time Progression
22
2.4 Impedance Equations
Ohm’s Law states that the voltage is equal to the current times the resistance at a
certain point in space and time and is shown as
E = IR,
(2.35)
where E is defined as the potential voltage, I is defined as current, and R is the
resistance. In electrical diagram models, impedance has different definitions and are
given in equations (2.36), (2.37), and (2.38) as
ZR = R
(2.36)
−j
ωC
(2.37)
ZL = jωL
(2.38)
ZC =
for a resistor, capacitor, and inductor, respectively. When the electrical phase angle
is equal to zero, Ohm’s Law is given. From this, impedance can be simply defined as
the ratio of the potential voltage applied versus the current at a point in space and
time, which can be shows as
Z (z, jw) =
E(jw)
,
I(jw)
(2.39)
remembering that this would require only resistors in the electrical circuit model and
the voltage applied to the system is known. Experimental design will allow the value
of the applied voltage to be defined and held constant throughout. The potential at
any time can be represented by equation (2.40) as
Et = Eo sin(wt) = Eo sin(2πf ),
23
(2.40)
where Eo and Et are the potential values at initial time, t0 , and time, t, and ω is
the radial frequency. A conversion of ω = 2πf is needed to work in the frequency
domain. In a similar manner, the current response signal, It , is defined in equation
(2.41) as
It = Io sin(wt + θ).
(2.41)
exp(iφ) = cos(φ) + isin(φ),
(2.42)
The Euler relationship,
allows the impedance to be represented as a complex number. Substituting equation
(2.42) into equations (2.40) and (2.41), impedance can be calculated at any time.
An analogous equation to Ohm’s law uses both equations. Equation (2.43) gives the
impedance at any time in the system is given as
Z(ω) =
E
= Zo exp(iφ) = Zo exp(cosφ + isinφ).
I
(2.43)
2.5 Frequency Domain Analogs
Electrochemical Impedance Spectroscopy is an analog into analyzing the frequency
domain. To analyze the results for the experimental multi-layer measurements, analysis of the EIS data and equivalent circuit modeling is introduced.
2.5.1 EIS Data Presentation
As EIS experiments are run, the data is presented in a uniform format [30]. Figure
2.5 presents the format for experimental data. The Nyquist Plot on the left shows
24
the complex values for the real and imaginary impedance dependent on the frequency
ran. The Bode plots on the right show the absolute impedance and phase angle of
the system, which is also dependent on the frequency .
(a) Nyquist Plot Example
(b) Bode Plot Example
Figure 2.5: Uniform Graphical Format for EIS Results
2.5.2 Equivalent Circuit Modeling
Due to the effects at the near application/substrate interface, various effects and
magnitudes occur and need to be analyzed to figure out how the electrochemical cell
is acting [31]. Equivalent Circuit Model analogs are used and two examples that
could be used are shown in Figure 2.6.
25
Figure 2.6: Equivalent Circuit Examples
The equivalent circuit on the left of Figure 2.6 is usually used for initial
stages of electrolyte uptake where the equivalent circuit on the right is one used
for Stage II, where Ri ’s are defined as resistors, the CP Ei ’s are defined as constant
phase elements (CPE), and W oi ’s are defined as a Warburg Constant. R1 is used
to describe the resistance given in the electrolyte and the rest of the circuit is used
to describe different layers in the electrochemical cell. When a resistor is introduced
into an electrical circuit, the resistor is not dependent on frequency. As shown in
equations (2.36), (2.37), and (2.38), a value for impedance is given in every part of
the circuit analog and when capacitors and inductors are introduced, the circuit will
now be dependent on frequency.
26
CHAPTER III
EXPERIMENTAL PROCEDURE
This chapter will give in great detail the methodology and steps taken in the experimental procedure. First, the experimental parameters and a construction description
are given. Secondly, the experimental steps for the EIS, AFM, and IFM, are presented
respectively. Afterwords, an explanation of the experimental setup is covered.
3.1 Sample Preparation
For the experiments to be constant and reliable, the planning is critical. First, the
substrates are described in detail. Second, the process of which the electrolytic pH
buffers are created for the electrochemical cells are given. Third, the design and
creation of the electrochemical cell design is shown.
3.1.1 Substrate / Working Electrode
The substrates that are used throughout are the AA2024-T3 and AA7075-T6 alloys
and are in the form of a 3 × 6 inch panel. These are aluminum alloys with main
secondary components of copper and zinc, respectively [32, 33, 34, 35, 36]. Table 3.1
shows the breakdown of elements that compose each substrate. For these substrates,
one half of them were treated with a primer or a pretreatment application layers. The
27
Table 3.1: Substrate Composition by Elemental Weight Percent
Element
˜ AA2024-T3
AA7075-T6
Aluminium
(Al)
90.7 − 94.7
87.1 − 91.4
Chromium
(Cr)
< 0.10
0.18 − 0.28
Copper
(Co)
3.80 − 4.90
1.20 − 2.00
Iron
(Fe)
< 0.50
< 0.50
Magnesium
(Mg)
1.20 − 1.80
2.10 − 2.90
Manganese
(Mn)
0.30 − 0.90
< 0.30
Silicon
(Si)
< 0.50
< 0.40
Titanium
(Ti)
< 0.15
< 0.20
Zinc
(Zn)
< 0.25
5.10 − 6.10
< 0.15
< 0.15
Other
primer used was Deft 02-Y-40 and the thicknesses are roughly 0.3 − 0.5, 0.7 − 0.9,
and 1.2 − 1.5 mils respectively [37]. The pretreatment used was conversion coating,
Alodine 1600, at a mass per area of 20, 40, and 60
the
mg
f t2
mg
,
f t2
respectively [38]. Converting
to mils will the thicknesses are roughly 5, 10, or 15 mils respectively. The
application of the pretreatment and primer was applied by The University of Dayton
Research Institute [39].
28
Table 3.2: Components in pH Buffer Solutions
pH Level
Chemical #1
Chemical #2
4.00
Acetic Acid
Sodium Acetate
7.00
Sodium Phosphate (Dibasic Anhydrous) Hydrochloric Acid
10.00
Sodium Phosphate (Dibasic Anhydrous) Sodium Hydroxide
3.1.2 Electrolyte & pH Buffers
A 3.5 wt.% sodium chloride solution is created since the experimental setting is to
simulate sea water equivalent concentrations. For each of these conditions, the pH of
the electrolyte is set at 4.00, 7.00, and 10.00, respectively with a buffer. The following
components that comprise the created buffers are listed in Table 3.2. Proper amounts
are needed from each chemical to prepare the 0.1 M pH buffers. To calculate the
volume required to make a batch of pH buffers equation (3.1) is given as
0.1 × 0.05 × M Wi = M assi .
(3.1)
Once these are measured and placed into a 250 mL beaker, 50 mL of 3.5 wt% of
NaCl solution is added and mixed. The chemical molecular weights and calculated
masses are given in Table 3.3.
After dissolving the needed chemicals, 10 mL of the buffer solution is added
to 200 mL of the 3.5 wt% of NaCl solution. A pH meter is used to measure the
29
Table 3.3: Molecular Weights and Masses needed for Chemicals
g
M.W. [ mol
] Calc. Mass [g]
Chemical
Formula
Acetic Acid
CH3 COOH
60.05
0.3003
Sodium Acetate
CH3 COON a
82.00
0.4100
Sodium Phosphate
N a2 HP O4
142.00
0.7100
Sodium Hydroxide
N aOH
40.00
0.2000
Hydrochloric Acid
HCl
36.50
0.1830
acidity of the solution the second chemical is being added, drops at a time until pH
level is reached.
3.1.3 Electrochemical Cell Design
For each panel that is selected, two corners opposite of each other were scratched (with
using a metal scratching tool) to create a working contact location. The etched surface
is three-fourths inch from the corner in both directions. The area with the circle
represents the area surrounded by the glass cell. Once all plates under investigation
have been scratched, check plates once over for experimental work.
Once all plates are finished, find the glass cells that are 2.25 cm in diameter
which will include a rubber O-ring in the box. The O-ring included with the cell has
a nominal inner and outer diameter of 1
3
16
inch and 1
7
16
inch, respectively. Next,
find the cut acrylic plates and cell clamps. Carefully, place the O-ring in the slot
30
(a) Unmodified Panel
(b) Etched Corners
(c) Cell Placement
Figure 3.1: Experimental Design Setup
underneath the glass cell and place together (green) on the coated cell. Afterwards,
place one of the 3 × 6 inch acrylic plates underneath the coated plate. Then, attach
the cell clamp onto the cell while clamping onto the acrylic plate, coated metal, and
the cell (circle) and when completed should look similar to Figure 3.1.
Take a 100 mL graduated cylinder and measure out 20 mL of the 3.5 wt%
NaCl buffered solution and pour into each of the glass cells. Make sure that there
are no leaks when pouring onto the coating in the cell as the O-ring should be tight.
When done pouring the 20 mL of 3.5 wt% NaCl buffered solution into the cell, cap
the top of the cell with a #4 stopper and place off to the side until all are complete.
Figure 3.2a and 3.2b schematically shows what each of the panels look like when
finished with construction of the electrochemical cells. There will be a duplicate cell
on the plate to validate data and estimate the certainty for each condition. Figure
3.3 shows an actual photo of the electrochemical cells setup.
31
(a) Pretreatment Plate 3D Setup
(b) Primer Plate 3D Setup
Figure 3.2: 3D Panel Setup Schematic
Figure 3.3: Actual Photo of the Experimental Design Setup showing Plate 018
3.2 Experimental Design Matrix
Tables 3.4 and 3.5 (presented at the end of the chapter) show the experimental design
matrix for all of the combinations of experiments investigated for the EIS study along
with the replicates and cell numbering system.
32
3.3 Experimental Setup
Three experiments will be run throughout the research being conducted. Electrochemical Impedance Spectroscopy (EIS) will be conducted for a 120 day duration.
Then, Atomic Force Microscopy (AFM) and Infinite Focus Microscopy (IFM) measurements will be performed.
3.3.1 EIS Experimental Setup
To initially set up this experiment, the electrochemical cells were constructed on top
of the assigned substrates. The following subsections will present the procedures
needed to conduct the experiments.
We use the following procedure for each sample in order, and repeated each
time, for updating the cell analysis over time. To be able to take EIS measurements,
a stopper harness is constructed to hold the reference and counter electrode inside of
the electrochemical cell. A #4 stopper is used and holes are created throughout, one
in the center for the reference electrode and one off-center to hold the counter/counter
reference platinum disk. The reference electrode and counter/counter reference platinum disk need to be washed with dioinized water before use in the electrochemical
cells. Place the stopper with the counter/counter sense platinum disk on the top of
the electrochemical cell slowly to avoid damaging the soft platinum disk. Then, place
the reference solution probe through the center of the stopper, which should now
mirror Figure 3.4. (Make sure that the reference solution probe and the platinum
counter/counter sense disk are not touching inside of the electrochemical cell.)
33
Figure 3.4: Schematic of the Electrochemical Cell Setup
Once the stopper (with the counter/counter sense platinum disk and reference
electrode) is secured in the solution of the electrochemical cell, place the cell in the
Faraday cage. Inside the Faraday cage, there will be six wires (red, orange, green,
blue, white, and black). Connect the orange clip to the red clip and connect the red
clip to the top of the platinum counter/counter sense disk/rod. Connect the blue
clip to the green clip and connect the green clip to the exposed area of the working
electrode. Connect the male end of the reference solution probe into the white wire
and the black wire should already be grounded to the Faraday cage. Check that
none of the metal clamps are touching that should not be. Once everything is wired
correctly, close and secure close the Faraday cage door.
To run the EIS, a GAMRY Reference 600 is used along with the GAMRY
Framework software package . The pre-installed program called Experiment - EIS300
Electrochemical Impedance - Potentiostatic will be used throughout all experiments.
34
The parameters set for the EIS experiments used here is an frequency range
from 100, 000 to 0.01 Hz, with 10 points per decade. The AC voltage is 10 mV r.m.s.
and the DC voltage is 0 V vs. Eoc . The default area is set to 1 cm2 . An initial
delay of 3 minutes to let the open circuit potential values reach an equilibrium.
The experiment will take about 35 (± 10) minutes to run. Initially (first 10 days),
experiments are run every day; afterwards, then a three day gap is allowed from day
10 through day 40. After day 40 is completed, experiments can be run every fifth
day through day 100. Once day 100 is reached measurements will take place on day
110 and day 120.
3.3.2 AFM Experimental Setup
Atomic Force Microscopy (AFM) analysis pictures are taken using a Bruker Micromode 8 microscope in the ScanAssyst mode. The AFM procedure is used to investigate the surface profiles of the samples before any exposure to the electrolyte. The
scan size is 500 nm, scan rate of 0.710 Hz, measuring 640 lines. When scanning there
was a aspect ratio of 1.00, an amplitude setpoint of 250.00 mV, and a drive amplitude
of 122.38 mV.
3.3.3 IFM Experimental Setup
Infinite Focus Microscopy (IFM) analysis pictures are taken using a Bruker microscope along with Vision64 software. The IFM procedure is used to investigate the
surface profiles of the samples after the 120 days of electrolyte exposure. The 5x lens
was used with 1.00x magnification. Supporting the panel is an acrylic plate to protect
35
the microscope surface. When running the IFM, various dials will be changed to focus the image until clear. Once a clear image is achieved, decrease the light intensity
as much as possible while holding the “fringe line” centered. The IFM measurement
settings during use are as follows: Speed 1x, Backscan 150 um, Length 150 um,
Threshold 1%, and the processing method used is VSI. Each measurement is taken
three times and are averaged together, to complete the physical profile analysis.
36
Table 3.4: Experimental Design Matrix for Pretreatment Samples
Type
Substrate
Thickness (mil)
pH level
Panel ID
Pretreatment
AA2024
5.00
4.00
250 − 001
1
2
Pretreatment
AA2024
5.00
7.00
250 − 002
3
4
Pretreatment
AA2024
5.00
10.0
250 − 003
5
6
Pretreatment
AA2024
10.0
4.00
250 − 004
7
8
Pretreatment
AA2024
10.0
7.00
250 − 005
9
10
Pretreatment
AA2024
10.0
10.0
250 − 006
11
12
Pretreatment
AA2024
15.0
4.00
250 − 007
13
14
Pretreatment
AA2024
15.0
7.00
250 − 008
15
16
Pretreatment
AA2024
15.0
10.0
250 − 009
17
18
Pretreatment
AA7075
5.00
4.00
250 − 010
19
20
Pretreatment
AA7075
5.00
7.00
250 − 011
21
22
Pretreatment
AA7075
5.00
10.0
250 − 012
23
24
Pretreatment
AA7075
10.0
4.00
250 − 013
25
26
Pretreatment
AA7075
10.0
7.00
250 − 014
27
28
Pretreatment
AA7075
10.0
10.0
250 − 015
29
30
Pretreatment
AA7075
15.0
4.00
250 − 016
31
32
Pretreatment
AA7075
15.0
7.00
250 − 017
33
34
Pretreatment
AA7075
15.0
10.0
250 − 018
35
36
37
Cell Num.
Table 3.5: Experimental Design Matrix for Primer Samples
Type
Substrate
Thickness (mil)
pH level
Primer
AA2024
0.3 − 0.5
4.00
250 − 019
37
38
Primer
AA2024
0.3 − 0.5
7.00
250 − 020
39
40
Primer
AA2024
0.3 − 0.5
10.0
250 − 021
41
42
Primer
AA2024
0.7 − 0.9
4.00
250 − 022
43
44
Primer
AA2024
0.7 − 0.9
7.00
250 − 023
45
46
Primer
AA2024
0.7 − 0.9
10.0
250 − 024
47
48
Primer
AA2024
1.2 − 1.5
4.00
250 − 025
49
50
Primer
AA2024
1.2 − 1.5
7.00
250 − 026
51
52
Primer
AA2024
1.2 − 1.5
10.0
250 − 027
53
54
Primer
AA7075
0.3 − 0.5
4.00
250 − 028
55
56
Primer
AA7075
0.3 − 0.5
7.00
250 − 029
57
58
Primer
AA7075
0.3 − 0.5
10.0
250 − 030
59
60
Primer
AA7075
0.7 − 0.9
4.00
250 − 031
61
62
Primer
AA7075
0.7 − 0.9
7.00
250 − 032
63
64
Primer
AA7075
0.7 − 0.9
10.0
250 − 033
65
66
Primer
AA7075
1.2 − 1.5
4.00
250 − 034
67
68
Primer
AA7075
1.2 − 1.5
7.00
250 − 035
69
70
Primer
AA7075
1.2 − 1.5
10.0
250 − 036
71
72
38
Panel ID Cell Num.
CHAPTER IV
RESULTS OF EXPERIMENTS AND ANAYLSIS
This chapter will show the various results of the experimental design matrix studied
as well as all microscopy techniques used. First, the EIS analysis results will be presented while comparing different parameter combinations. Second, equivalent circuit
modeling will convert the EIS data into theoretical electrical circuit values that be
compared throughout time. Next, AFM and IFM measurements are presented and
physical properties analyzed.
4.1 Electrochemical Impedance Spectroscopy Results
Electrochemical Impedance Spectroscopy experiments were performed on all samples
described in experimental matrix shown previously in Tables 3.4 and 3.5. These
electrochemical samples were exposed to the various pH level electrolyte environments
through a 120 day duration. Before any EIS experimental measurement is taken, the
electrochemical cell is required to reach steady state for the open circuit potential.
For comparison, the effects of each variable will be characterized. First, the substrates
will be varied while keeping the pH level of the electrolyte, treatment, and thickness
constant. Second, the pH level of the electrolyte will be varied while keeping the
substrate, treatment, and thickness constant. Third, the treatment will be varied
39
while keeping the substrate, pH level of the electrolyte, and thickness constant. Forth,
the thickness will be varied while keeping the substrate, pH level of the electrolyte,
and treatment constant. For this last condition, a comparison will be looked at for
both the pretreatment and primer applications. Days 1, 10, 25, 60, 80, and 120 will
be shown in the figures below.
4.1.1 EIS - Open Circuit Potentials
The Open Circuit Potential (OCP) is measured before the EIS measurements were
performed so there is a basis for the potential. As stated in Chapter III, there is a
three minute delay that is defined where the allows the system’s OCP to reach steady
state. For consistency, the OCP values given in Table 4.1 will be the same presented
throughout the rest of the EIS, equivalent circuit models, and IFM analysis.
The conditions of the individual cells are given at the end of Chapter III.
Since the OCP values during day 1 are not considered steady state, day 2 OCP
values will be used instead.
4.1.2 EIS - Analysis
Figures 4.1 and 4.2 show the results from the EIS measurements over the 120 day
exposure to show the effects of the AA2024 and AA7075 substrates where the pH = 7,
both had a primer coating of thickness 0.3 − 0.5 mils.
Figure 4.1 shows the damage evolution through EIS analysis. The Nyquist
plot on the left shows that there is minimal difference throughout the 120 day electrolyte exposure and the only noticeable difference is at the very first day of mea40
Table 4.1: Experimental Open Circuit Potential (V) Values
Day
Day 2 *
Day 10
Day 25
Day 60
Day 80
Day 120
Cell 43
−0.708
−0.763
−0.743
−0.679
−0.786
−0.753
Cell 47
−0.752
−0.707
−0.666
−0.631
−0.800
−1.034
Cell 39
−0.742
−0.703
−0.593
−0.765
−0.771
−0.822
Cell 57
−0.951
−0.890
−0.899
−0.928
−0.935
−0.951
Cell 15
−0.700
−0.677
−0.728
−0.787
−0.789
−0.842
Cell 51
−0.574
−0.478
−0.487
−0.465
−0.478
−0.519
Cell 19
−0.880
−0.886
−0.873
−0.875
−0.889
−0.910
Cell 31
−0.871
−0.868
−0.870
−0.891
−0.873
−0.911
Cell 55
−0.852
−0.868
−0.894
−0.853
−0.892
−0.905
Cell 67
−0.685
−0.772
−0.802
−0.685
−0.687
−0.699
surements where it shows a higher impedance. Note the scale here on the plot ranges
from 0 to 300, 000 ohms. The Bode plot shows that the absolute impedance is around
106.5 ohms and drops to roughly 105.3 ohms over time at low frequency. The absolute
impedance is around 101.5 ohms at high frequency throughout all exposure time and
as well as in the case of the Nyquist plot, the noticeable difference is shown at the
very first day when the impedance is higher. The fact that the impedance is much
higher at the first day is because the solution is not present in the coating during
41
Figure 4.1: Experimental EIS results over 120 days for an AA2024 substrate with a
primer application of 0.3 − 0.5 mils being exposed to a 3.5 wt.% NaCl environment
at pH = 7. (Left) Nyquist Plot and (Right) Bode Plot.
this measurement, it takes less than 10 days for the solution to diffuse through the
coating with this substrate however, no further changes are observed which means
that the diffusion of solution does not affect the metal/coating interface. Another
difference is present at 120 days, where the impedance values drop showing a plateau
at high frequencies. The phase angle diagram here shows that the phase angle values
are similar throughout and stable over time with the exception of the 120-day measurement. This difference is probably due to a penetration of the solution through
the coating.
Figure 4.2 shows the damage evolution through EIS analysis. The Nyquist
plot on the left shows a noticeable difference throughout the 120 day electrolyte
42
Figure 4.2: Experimental EIS results over 120 days for an AA7075 substrate with a
primer application of 0.3 − 0.5 mils being exposed to a 3.5 wt.% NaCl environment
at pH = 7. (Left) Nyquist Plot and (Right) Bode Plot.
exposure for this formulation. After day 1 and day 10, both, the real and imaginary
impedance values dropped and began to form more of semi-circular shape. Note
the scale here on the plot ranges from 0 to 100, 000 ohms. The Bode plot shows
that the absolute impedance is around 106 ohms and decreases to roughly 104.5 ohms
over time at low frequency. The absolute impedance is around 101.5 ohms at high
frequency throughout all exposure time. The phase angle diagram here shows that
the phase angle values are changing throughout and decreases at both the low and
high frequencies. Different to the previous formulation, all impedance plots show
the presence of two time constants (implicating the presence of two maximum in the
phase angle or two semicircles in the Nyquist representation), this means that the
43
Figure 4.3: Experimental EIS results over 120 days for an AA2024 substrate with a
primer application of 0.7 − 0.9 mils being exposed to a 3.5 wt.% NaCl environment
at pH = 4. (Left) Nyquist Plot and (Right) Bode Plot.
AA7075 substrate allows the penetration of solution at a higher rate forming perhaps
corrosion products in the metal.
Figures 4.3 and 4.4 show the results from the EIS measurements over the
120 day exposure to show the effects of the pH levels. This comparison will compare
pH = 4 and pH = 10 on a AA2024 substrate where both had a primer coating of
thickness 0.7 − 0.9 mils.
Figure 4.3 shows the damage evolution through EIS analysis. The Nyquist
plot on the left shows that there is minimal difference throughout the 120 day electrolyte exposure with the exception of the first day. The first 25 days show clearly
the presence of charge transfer and diffusion by a semicircle followed by a extended
44
line; after this time, the charge transfer is less noticeable and the Nyquist plot shows
a straight line. Note the scale here on the plot ranges from 0 to 1, 000, 000 ohms. The
Bode plot shows that the absolute impedance is around 108.2 ohms and decreases to
roughly 105.8 ohms over time at low frequency. The absolute impedance is around
102.5 ohms at high frequency throughout all exposure time. The phase angle diagram
here shows that the phase angle values are similar throughout and becomes more
stable over time. Both plots show the presence of the two time constants at the first
25 days; after this time the time constant at low frequencies disappears forming a
plateau. This fact indicates that when pH = 4, the reaction at the interface and the
diffusion through the coating occur at the same time but after 25 days the corrosion
products probably slow down the reaction at the interface metal/coating.
Figure 4.4 shows the damage evolution through EIS analysis. The Nyquist
plot on the left shows that there is a noticeable difference throughout the 120 day electrolyte exposure. After day 1 and day 10, the both the real and imaginary impedance
values drop and begin to form more of semi-circular shape. Note the scale here on
the plot ranges from 0 to 30, 000 ohms. The Bode plot shows that the absolute
impedance is around 108.1 ohms and drops to roughly 105.1 ohms over time at low frequency. The absolute impedance is around 102.5 ohms at high frequency throughout
all exposure time. The phase angle diagram here shows that the phase angle values
are changing throughout and dropping at both the low and high frequencies. The
phase diagram shows the different time constants formed along the time. These several time constants are due to the formation of corrosion products and the diffusion
45
Figure 4.4: Experimental EIS results over 120 days for an AA2024 substrate with a
primer application of 0.7 − 0.9 mils being exposed to a 3.5 wt.% NaCl environment
at pH = 10. (Left) Nyquist Plot and (Right) Bode Plot.
of water through the coating and the corrosion products. The high pH induces the
formation of these corrosion products but they are not quite stable and allow further
dissolution of the metal. When compared to the pH = 4, the main difference is the
formation of corrosion products at pH = 10.
The experimental design presents two different substrate applications: pretreatment and primer. When compared side by side, there are no application thicknesses in common with one another. In this instance, we will look at the thickest
application for each being exposed to a pH = 7 electrolyte with a AA2024 substrate.
This comparison is shown below in Figures 4.5 and 4.6.
46
Figure 4.5 shows the damage evolution through EIS analysis. The Nyquist
plot on the left shows that there is a slight difference throughout the 120 day electrolyte exposure. There is not much difference between in the first 25 days, but after
day 60, the Nyquist plot begins to curve into more into a semi-circle. Note the scale
here on the plot ranges from 0 to 100, 000 ohms. The Bode plot shows that the absolute impedance is around 105.5 ohms throughout exposure time at low frequency. At
high frequencies the absolute impedance is around 10 ohms throughout the exposure
time. The phase angle diagram here shows that the phase angle values are similar
throughout and have reached point of equilibrium. After 60 days, the electrolyte has
reached the metallic surface to produce the electrochemical activation.
Figure 4.5: Experimental EIS results over 120 days for an AA2024 substrate with a
pretreatment application of 15 mils being exposed to a 3.5 wt.% NaCl environment
at pH = 7. (Left) Nyquist Plot and (Right) Bode Plot.
47
Figure 4.6: Experimental EIS results over 120 days for an AA2024 substrate with a
primer application of 1.2 − 1.5 mils being exposed to a 3.5 wt.% NaCl environment
at pH = 7. (Left) Nyquist Plot and (Right) Bode Plot.
Figure 4.6 shows the damage evolution through EIS analysis. The Nyquist
plot on the left shows that there is a steady difference throughout the 120 day electrolyte exposure for each time profile. There is not much difference between in the
first 10 days, but after day 25, the Nyquist plot slightly begins to curve into more
into a semi-circle. Note the scale here on the plot ranges from 0 to 1, 000, 000 ohms.
The Bode plot shows that the absolute impedance is around 108.0 ohms and decreases
to roughly 107.1 ohms over time at low frequency. The absolute impedance is around
104.0 ohms at high frequency throughout all exposure time. The phase angle diagram
here shows that the phase angle values are fairly constant throughout the frequency
range. The phase diagram here also shows that there are no time constants being
48
introduced with time. Both Bode plots show the decrease in impedance and the presence of a reaction at the interface; however, this reaction is slow enough to compete
with the diffusion through the coating. The main difference between Figure 4.5 and
Figure 4.6 is the shift of the “peak” in the phase angle towards higher frequencies in
the latter, indicating a lower capacitance, meaning that the substrate in Figure 4.6
is taking much less electrolyte than the substrate in Figure 4.5.
After analyzing the results due to the effects of pH level of the electrolyte, substrate, and application, the last comparison is the application thickness. This needs to
be investigated for both the primer and pretreatment applications, separately. First,
the EIS result comparison of the pretreatment applications are presented. Second,
the EIS result comparison of the primer applications are presented. For this, the
minimum and maximum thickness applications are compared for both the AA7075
substrate with a pH = 4 electrolyte solution over the 120 day exposure.
Figure 4.7 shows the damage evolution through EIS analysis. The Nyquist
plot on the left shows that there is evolving damage throughout the 120 day electrolyte
exposure. For the first 25 days the behavior is similar changing solely in magnitude.
Once day 60 is reached, the Nyquist curve becomes smaller through day 120, where
there is the presence of a time constant introduced. Note the scale here on the plot
ranges from 0 to 20, 000 ohms. The Bode plot shows that the absolute impedance
is around 104 ohms throughout exposure time at low frequency. At high frequencies
the absolute impedance is around 10 ohms throughout the exposure time. The phase
angle diagram here shows that the phase angle values are similar throughout and
49
have reached point of steady state. In the Bode plots is more notorious that during
the first 25 days the behavior is similar and after 60 days, this behavior changes by
reducing the values of impedance and shifting the peak in the phase angle towards
lower frequencies. This fact indicates that the interface is becoming affected by the
presence of electrolyte after 60 days.
Figure 4.8 shows the damage evolution through EIS analysis. The Nyquist
plot on the left shows that there is a steady difference throughout the 120 day electrolyte exposure for each time profile. The Nyquist plot begins to curve into more
into a semi-circle. Note the scale here on the plot ranges from 0 to 20, 000 ohms. The
Bode plot shows that the absolute impedance is around 103.7 ohms over time at low
Figure 4.7: Experimental EIS results over 120 days for an AA7075 substrate with a
pretreatment application of 5 mils being exposed to a 3.5 wt.% NaCl environment at
pH = 4. (Left) Nyquist Plot and (Right) Bode Plot.
50
frequency. The absolute impedance is around 10 ohms at high frequency throughout
all exposure time. The phase angle diagram here shows that the phase angle values
are fairly constant throughout the frequency range. The phase diagram here also
shows that there are no time constants being introduced with time. In the phase
angle plot, it is important to notice the evolution of the interface; opposite to the 5
mil thickness, there is no a breakthrough time for the electrolyte to reach the metal.
In the 15 mil case, the electrolyte uptake happens gradually during all the 120 days
of exposure. What is important to note here is the comparison of the phase angle
between the 5 mil and the 15 mil. In both cases, the peak in the phase angle is shifted
Figure 4.8: Experimental EIS results over 120 days for an AA7075 substrate with a
pretreatment application of 15 mils being exposed to a 3.5 wt.% NaCl environment
at pH = 4. (Left) Nyquist Plot and (Right) Bode Plot.
51
Figure 4.9: Experimental EIS results over 120 days for an AA7075 substrate with a
primer application of 0.3 − 0.5 mils being exposed to a 3.5 wt.% NaCl environment
at pH = 4. (Left) Nyquist Plot and (Right) Bode Plot.
towards low frequencies, however, in the case of 15 mil, the peak at the day 120 does
not reach the low frequencies than any peak for 5 mil. This reflects the fact that the
electrolyte is taking more time to diffuse through the 15 mil thickness.
Figure 4.9 shows the damage evolution through EIS analysis. The Nyquist
plot on the left shows that there is a difference throughout the 120 day electrolyte
exposure. The Nyquist plot decreases throughout the first 80 days and increases
for the day 120 time profile and the Nyquist plot begins to curve into more into
a semi-circle showing a decrease in the charge transfer resistance. Note the scale
here on the plot ranges from 0 to 200, 000 ohms. The Bode plot shows that the
absolute impedance is around 107 ohms and decreases to around 105 ohms throughout
52
Figure 4.10: Experimental EIS results over 120 days for an AA7075 substrate with
a pretreatment application of 1.2 − 1.5 mils being exposed to a 3.5 wt.% NaCl environment at pH = 4. (Left) Nyquist Plot and (Right) Bode Plot.
exposure time at low frequency. At high frequencies, the absolute impedance is around
100 ohms throughout the exposure time. The phase angle diagram here shows that
the system has reached a point of equilibrium over the 120 days and it shows the
presence of two time constants, meaning that the electrolyte may have reached the
metallic substrate and formed corrosion products.
Figure 4.10 shows the damage evolution through EIS analysis. The Nyquist
plot on the left shows that there is a steady difference throughout the 120 day electrolyte exposure for each time profile. From day 1 to day 10, the Nyquist curve decreases. The impedance after the day 25 shows the typical behavior of charge transfer
and diffusion, which is kept until the 120 day when the charge transfer resistance is
53
reduced considerably. Note the scale here on the plot ranges from 0 to 200, 000 ohms.
The Bode plot shows that the absolute impedance is around 1010 ohms and decreases
to roughly 107 ohms over time at low frequency. The absolute impedance is around
104 ohms at high frequency throughout all exposure time. The phase angle diagram
here shows transition between damage evolution phases throughout the frequency
range. In this case, as well as in the Nyquist plot, it is observed that from day 25 to
day 80 the signal does not change and it shows two time constants, at day 120 the
two time constants (two maximum points) are also shown but they are displaced to
high frequencies, indicating the damage on the coating.
4.2 Equivalent Electrical Circuit Analog
Now that the EIS results have been presented, an equivalent electrical circuit analog
can be used to determine the mechanisms and processes involved in the electrolyte
uptake. Due to the damage evolution model effects, various equivalent circuit models must be used throughout the duration of the exposure time. The four models
used are shown below in Figure 4.11 and the modeling results of the equivalent circuit analog are given in Tables 4.2 - 4.11. The selection of these circuits obey the
behavior observed in the experimental results. All the circuits have in common an
electrolyte resistance R1; circuit analog #1 presents a constant phase element (CPE1)
in parallel with the charge transfer resistance (R2), this circuit allows the representation of one-time constant signals. This EIS representation is characteristic of the
capacitive behavior, for this system considers the mechanism prevailing within the
54
coating (layer). Circuit analog #2 shows the same arrangement than the previous
one with the addition of another time constant in series with R2; this circuit represents the two-time constant signal used commonly for coated interfaces. This analog
includes the mechanisms occurring within the coating producing first time constant
(high frequencies). It also includes the second time constant characteristic of the
coating/substrate interface, when the electrolyte uptakes the coating and reaches the
substrate. Analogous to circuits 1 and 2, circuits 3 and 4 show the addition of an
element accounting for the diffusion of species (Wo) in series with the time constant
at low frequencies, latter indicating the dominance of the mass transport mechanism.
(a) Equivalent Circuit Analog #1
(b) Equivalent Circuit Analog #2
(c) Equivalent Circuit Analog #3
(d) Equivalent Circuit Analog #4
Figure 4.11: Equivalent Circuit Models used for the analysis of the EIS Results
55
For Tables 4.2-4.11, the equivalent circuit analog model number is given along
with the Chi-Squared and Sum-Squared Errors. The units for the resistors (R1, R2,
and R3) are given as Ω·cm2 . The units for the constant phase element value, CPE1T and CPE2-T, are given as F ·cm2 ·n−1 , and the constant phase element value for
CPE1-P and CPE2-P are arbitrary units.
56
Table 4.2: Equivalent Circuit Modeling Results for Cell 39
Day
Day 1
Day 10
Day 25
Day 60
Day 80
Day 120
Model No
#1
#2
#2
#2
#2
#4
Chi-Squ.
1.27 × 10−2 8.75 × 10−4 2.69 × 10−3 1.14 × 10−3 1.23 × 10−3 4.17 × 10−3
Sum-Squ. 1.54 × 100 1.11 × 10−1 3.57 × 10−1 1.52 × 10−1 1.63 × 10−1 5.54 × 10−1
R1
2.61 × 100
3.72 × 100
3.35 × 100
4.37 × 100
3.97 × 100 2.00 × 10−0
CPE1-T
4.51 × 10−7 2.76 × 10−6 2.44 × 10−6 2.43 × 10−6 1.47 × 10−6 2.07 × 10−5
CPE1-P
9.23 × 10−1 8.54 × 10−1 8.68 × 10−1 8.88 × 10−1 9.25 × 10−1 6.43 × 10−1
R2
3.82 × 106
2.35 × 102
1.44 × 102
CPE2-T
n/a
1.64 × 10−5 1.50 × 10−5 1.46 × 10−5 1.72 × 10−5 5.31 × 10−6
CPE2-P
n/a
5.25 × 10−1 5.57 × 10−1 5.72 × 10−1 5.93 × 10−1 9.78 × 10−1
R3
n/a
1.00 × 1020 1.59 × 1019 1.73 × 1019 1.00 × 1020 3.94 × 105
Wo1-R
n/a
n/a
n/a
n/a
n/a
1.22 × 104
Wo1-T
n/a
n/a
n/a
n/a
n/a
1.07 × 10−1
Wo1-P
n/a
n/a
n/a
n/a
n/a
5.11 × 10−1
57
1.27 × 102
1.36 × 101
5.40 × 100
Table 4.3: Equivalent Circuit Modeling Results for Cell 57
Day
Day 1
Day 10
Day 25
Day 60
Day 80
Day 120
Model No
#1
#2
#4
#2
#2
#2
Chi-Squ.
7.88 × 10−3 1.76 × 10−2 5.59 × 10−4 7.11 × 10−4 6.91 × 10−4 2.85 × 10−3
Sum-Squ. 6.94 × 10−1 2.39 × 100 7.38 × 10−2 9.03 × 10−2 8.77 × 10−2 3.44 × 10−1
R1
4.91 × 100
2.50 × 100
3.43 × 100
4.24 × 100
3.67 × 100
3.20 × 100
CPE1-T
4.19 × 10−7 3.60 × 10−6 2.37 × 10−6 2.79 × 10−6 3.65 × 10−6 9.05 × 10−6
CPE1-P
8.72 × 10−1 7.50 × 10−1 8.06 × 10−1 8.56 × 10−1 8.52 × 10−1 7.94 × 10−1
R2
9.78 × 105
7.27 × 102
4.21 × 102
CPE2-T
n/a
9.65 × 10−5 5.65 × 10−5 9.18 × 10−5 8.62 × 10−5 2.77 × 10−4
CPE2-P
n/a
5.35 × 10−1 5.53 × 10−1 4.58 × 10−1 4.68 × 10−1 5.72 × 10−1
R3
n/a
1.86 × 107
7.05 × 103
3.46 × 104
1.14 × 104
2.77 × 1016
Wo1-R
n/a
n/a
8.96 × 10−5
n/a
n/a
n/a
Wo1-T
n/a
n/a
8.00 × 10−1
n/a
n/a
n/a
Wo1-P
n/a
n/a
3.95 × 1012
n/a
n/a
n/a
58
2.88 × 102
2.90 × 102
1.66 × 103
Table 4.4: Equivalent Circuit Modeling Results for Cell 43
Day
Day 1
Day 10
Day 25
Day 60
Day 80
Day 120
Model No
#1
#2
#2
#2
#2
#3
Chi-Squ.
2.29 × 10−3 1.87 × 10−3 6.27 × 10−4 7.94 × 10−4 8.20 × 10−4 6.91 × 10−3
Sum-Squ. 2.70 × 10−1 2.54 × 10−1 8.53 × 10−2 1.08 × 10−1 1.12 × 10−1 9.74 × 10−1
R1
2.50 × 100
3.00 × 100 9.00 × 10−1 1.10 × 100 8.50 × 10−1 8.00 × 10−1
CPE1-T
2.35 × 10−8 7.22 × 10−7 7.12 × 10−7 7.99 × 10−7 1.05 × 10−6 6.10 × 10−8
CPE1-P
8.76 × 10−1 7.12 × 10−1 7.21 × 10−1 7.25 × 10−1 7.07 × 10−1 9.32 × 10−1
R2
1.20 × 109
1.30 × 103
6.16 × 102
CPE2-T
n/a
2.48 × 10−6 3.57 × 10−6 6.98 × 10−6 8.21 × 10−6
n/a
CPE2-P
n/a
6.60 × 10−1 6.55 × 10−1 6.25 × 10−1 6.17 × 10−1
n/a
R3
n/a
1.26 × 106
1.41 × 106
1.74 × 106
1.92 × 106
n/a
Wo1-R
n/a
n/a
n/a
n/a
n/a
9.63 × 105
Wo1-T
n/a
n/a
n/a
n/a
n/a
9.12 × 101
Wo1-P
n/a
n/a
n/a
n/a
n/a
6.20 × 10−1
59
4.17 × 102
4.14 × 102
1.57 × 102
Table 4.5: Equivalent Circuit Modeling Results for Cell 47
Day
Day 1
Day 10
Day 25
Day 60
Day 80
Day 120
Model No
#1
#3
#3
#4
#4
#4
Chi-Squ.
1.10 × 10−2 3.63 × 10−2 3.90 × 10−2 1.00 × 10−4 3.71 × 10−4 1.51 × 10−4
Sum-Squ. 1.42 × 100
R1
4.64 × 100
5.00 × 10−1 2.00 × 100
5.39 × 100 1.32 × 10−2 4.94 × 10−2 2.01 × 10−2
2.00 × 100
2.52 × 100
4.20 × 100
1.02 × 101
CPE1-T
3.47 × 10−8 1.65 × 10−6 3.76 × 10−6 5.03 × 10−7 2.23 × 10−6 1.49 × 10−6
CPE1-P
9.05 × 10−1 6.93 × 10−1 6.15 × 10−1 7.95 × 10−1 6.89 × 10−1 7.57 × 10−1
R2
2.07 × 108
7.15 × 104
8.39 × 104
CPE2-T
n/a
n/a
n/a
3.35 × 10−6 5.54 × 10−6 1.25 × 10−5
CPE2-P
n/a
n/a
n/a
6.13 × 10−1 6.80 × 10−1 5.78 × 10−1
R3
n/a
n/a
n/a
9.99 × 104
3.25 × 103
9.36 × 103
Wo1-R
n/a
4.33 × 109
1.46 × 107
2.92 × 105
2.92 × 104
1.94 × 105
Wo1-T
n/a
2.02 × 107
7.00 × 103
1.57 × 101
2.92 × 100
1.07 × 101
Wo1-P
n/a
7.22 × 10−1 6.80 × 10−1 3.74 × 10−1 2.92 × 10−1 4.21 × 10−1
60
3.74 × 103
3.17 × 103
1.08 × 103
For Cell 39, the two-time constant circuit was mainly used. This cell showed
a fairly fast electrolyte uptake, forming a layer of corrosion products underneath the
coating. At the very end of the testing duration, a diffusion element appears, this
is due to the growth of the oxide layer, producing diffusion through the corrosion
products. This behavior is similar to the cell in the previous table; since the high pH
level is similar, this was expected.
For Cell 57, the initial time is again represented by a single time constant
that evolves to a two-time constant for the rest of the test. Again, this cell develops
a layer of corrosion products from day 10, fairly early in the process. This layer of
corrosion products remains constant in thickness and compactness.
Table 4.4 shows the evolution of the interface for the Cell 43. It can be
observed that day 1 required a one-time constant circuit, but the following days
required a two-time constant. This means that the electrolyte reached the metallic
surface and there was a corrosion reaction that progressed to become diffusion through
the corrosion products noticeable in the EIS response.
As well as in Table 4.4, Table 4.5 shows that the initial time showed just a
single-time constant. The interface also evolved toward different mechanisms, such as
diffusion became important after day 25, still showing a single time constant, where
electrolyte diffusion and charge transfer competed for controlling the process. After
day 80 the second time constant coming from corrosion products had more effect on
the total interface. This is comparable to Table 4.4; the fact that Cell 47 is exposed
to a higher pH level is what produces these differences.
61
Table 4.6: Equivalent Circuit Modeling Results for Cell 15
Day
Day 1
Day 10
Day 25
Day 60
Day 80
Day 120
Model No
#1
#1
#1
#1
#1
#1
Chi-Squ.
2.57 × 10−2 1.72 × 10−2 2.52 × 10−2 1.68 × 10−2 3.29 × 10−2 5.36 × 10−2
Sum-Squ. 2.90 × 100
2.34 × 100
3.42 × 100
2.29 × 100
4.47 × 100
7.29 × 100
4.72 × 100
5.56 × 100
5.49 × 100
6.52 × 100
5.47 × 100
8.15 × 100
R1
CPE1-T
2.81 × 10−5 3.68 × 10−5 3.60 × 10−5 3.74 × 10−5 3.43 × 10−5 2.79 × 10−5
CPE1-P
9.32 × 10−1 9.23 × 10−1 9.20 × 10−1 9.09 × 10−1 9.13 × 10−1 9.41 × 10−1
R2
4.65 × 105
1.04 × 106
6.72 × 105
2.28 × 105
1.35 × 106
7.55 × 104
For the specific case of Cell 15, only a single time constant circuit was needed.
The little evolution of the interface is due to the fact that this is a 15 mil substrate
that does not allow many changes in time.
62
Table 4.7: Equivalent Circuit Modeling Results for Cell 51
Day
Day 1
Day 10
Day 25
Day 60
Day 80
Day 120
Model No
#3
#3
#1
#3
#3
#3
Chi-Squ.
2.19 × 10−3 1.33 × 10−3 5.47 × 10−2 2.50 × 10−3 2.68 × 10−3 3.19 × 10−3
Sum-Squ. 2.97 × 10−1 1.79 × 10−1 6.45 × 100 3.45 × 10−1 3.64 × 10−1 4.44 × 10−1
4.50 × 100
R1
1.76 × 101
4.18 × 100 5.65 × 10−1 1.70 × 100
2.40 × 100
CPE1-T
1.69 × 10−9 4.73 × 10−9 5.06 × 10−5 1.18 × 10−8 1.25 × 10−8 1.13 × 10−8
CPE1-P
9.79 × 10−1 9.26 × 10−1 9.67 × 10−1 8.62 × 10−1 8.61 × 10−1 8.72 × 10−1
R2
3.90 × 107 8.29 × 10−2 2.05 × 104 2.26 × 10−2 3.20 × 10−3 7.66 × 10−5
Wo1-R
5.16 × 1010 2.27 × 108
n/a
2.13 × 108
2.10 × 108
3.75 × 108
Wo1-T
8.43 × 101
n/a
1.75 × 103
1.83 × 103
7.85 × 103
Wo1-P
6.86 × 10−1 3.83 × 10−1
n/a
3.50 × 10−1 3.57 × 10−1 3.90 × 10−1
8.17 × 101
In the case of Cell 51, the thickness is the highest for this substrate. The
electrolyte uptake is shown to influence the diffusion and this is reflected on the circuit
used in Table 4.7, where the diffusion element is coupled with the time single time
constant.
63
Table 4.8: Equivalent Circuit Modeling Results for Cell 19
Day
Day 1
Day 10
Day 25
Day 60
Day 80
Day 120
Model No
#1
#1
#1
#1
#1
#1
Chi-Squ.
2.47 × 10−3 2.02 × 10−2 2.00 × 10−2 2.22 × 10−1 3.52 × 10−2 2.43 × 10−2
Sum-Squ. 2.18 × 10−1 2.06 × 100
2.41 × 100
3.07 × 101
4.85 × 100
3.36 × 100
5.15 × 100
4.05 × 100
5.03 × 100
4.45 × 100
5.14 × 100
R1
4.46 × 100
CPE1-T
4.36 × 10−5 7.00 × 10−5 7.11 × 10−5 1.44 × 10−4 1.63 × 10−4 1.50 × 10−4
CPE1-P
9.41 × 10−1 9.55 × 10−1 9.58 × 10−1 9.31 × 10−1 9.03 × 10−1 9.15 × 10−1
R2
6.07 × 104
1.40 × 104
1.96 × 104
3.03 × 103
1.28 × 104
2.03 × 104
As well as the substrate in Table 4.6, only a single time constant was needed
for representing the signal of this system; however, in this case, this is due to the fast
penetration of the electrolyte which causes the corrosion of the metal underneath the
coating which can be observed in the values presented in the tables.
64
Table 4.9: Equivalent Circuit Modeling Results for Cell 31
Day
Day 1
Day 10
Day 25
Day 60
Day 80
Day 120
Model No
#1
#1
#1
#1
#1
#1
Chi-Squ.
6.05 × 10−2 2.49 × 10−2 4.16 × 10−2 3.03 × 10−2 5.87 × 10−3 9.16 × 10−3
Sum-Squ. 6.78 × 100
2.49 × 100
4.41 × 100
3.51 × 100 8.10 × 10−1 1.26 × 100
3.92 × 100
4.48 × 100
3.88 × 100
4.74 × 100
R1
3.89 × 100
5.10 × 100
CPE1-T
2.55 × 10−5 5.21 × 10−5 6.34 × 10−5 8.46 × 10−5 1.03 × 10−4 1.16 × 10−4
CPE1-P
9.53 × 10−1 9.29 × 10−1 9.23 × 10−1 9.00 × 10−1 8.78 × 10−1 9.11 × 10−1
R2
2.00 × 104
3.00 × 103
1.95 × 103
4.83 × 103
1.73 × 104
9.25 × 103
Comparing with Table 4.8, it is possible to conclude that the thickness has
little influence on this substrate with a low pH. Even though the cell in Figure 4.5
has a thicker substrate than the cell in Table 4.8, the results for both, the circuit and
the circuit values do not change. At pH = 4 there is no effect of the thickness.
65
Table 4.10: Equivalent Circuit Modeling Results for Cell 55
Day
Day 1
Day 10
Day 25
Day 60
Day 80
Day 120
Model No
#1
#1
#1
#3
#3
#3
Chi-Squ.
1.20 × 10−2 6.16 × 10−3 2.29 × 10−2 6.65 × 10−4 1.80 × 10−3 6.22 × 10−3
Sum-Squ. 1.31 × 100 8.50 × 10−1 3.06 × 100 6.45 × 10−2 2.25 × 10−1 8.52 × 10−1
8.61 × 100
R1
4.56 × 100
4.10 × 100
2.82 × 100
3.03 × 100
4.81 × 100
CPE1-T
5.73 × 10−7 3.15 × 10−5 3.01 × 10−5 3.10 × 10−5 3.59 × 10−5 2.33 × 10−5
CPE1-P
8.71 × 10−1 6.58 × 10−1 6.65 × 10−1 6.58 × 10−1 6.47 × 10−1 6.99 × 10−1
R2
2.97 × 106
3.42 × 105
3.51 × 105
2.83 × 102
1.98 × 102
7.76 × 101
Wo1-R
n/a
n/a
n/a
4.33 × 104
6.04 × 104
3.32 × 106
Wo1-T
n/a
n/a
n/a
4.72 × 10−2 1.50 × 10−1 5.28 × 102
Wo1-P
n/a
n/a
n/a
7.80 × 10−1 7.22 × 10−1 6.98 × 10−1
For the low thickness cell shown in Table 4.10, the use of a single time constant
is also needed. At times higher than 80 days, the use of a diffusion element and
a second time constant are needed which is normal considering the formation of
corrosion products that block the interface.
66
Table 4.11: Equivalent Circuit Modeling Results for Cell 67
Day
Day 1
Day 10
Day 25
Day 60
Day 80
Day 120
Model No
#2
#2
#2
#2
#2
#2
Chi-Squ.
2.54 × 10−3 2.39 × 10−4 2.81 × 10−3 1.19 × 10−3 1.20 × 10−3 6.55 × 10−3
Sum-Squ.
3.45 × 10−1 2.39 × 10−2 3.82 × 10−1 1.61 × 10−1 1.63 × 10−1 8.64 × 10−1
1.13 × 100
R1
9.60 × 10−1 1.14 × 100
1.07 × 100
1.05 × 100
4.70 × 100
CPE1-T
1.45 × 10−9 5.35 × 10−9 9.85 × 10−9 1.15 × 10−8 1.26 × 10−8 8.68 × 10−9
CPE1-P
9.85 × 10−1 9.21 × 10−1 8.78 × 10−1 8.73 × 10−1 8.69 × 10−1 9.06 × 10−1
5.05 × 104
R2
9.72 × 104
1.38 × 105
1.70 × 105
1.75 × 105
1.21 × 104
CPE2-T
5.50 × 10−10 2.05 × 10−7 1.91 × 10−7 2.16 × 10−7 2.20 × 10−7 2.50 × 10−7
CPE2-P
7.28 × 10−1 4.68 × 10−1 8.15 × 10−1 7.70 × 10−1 7.66 × 10−1 8.04 × 10−1
R3
1.00 × 1020 3.51 × 107
2.23 × 107
4.21 × 107
4.02 × 107
2.02 × 107
Cell 67, shown in Table 4.11, shows again a low pH but with a thick substrate;
this is the reason for the quick formation of an oxide layer that is represented by a
second time constant in the analog electric circuits.
67
4.3 Exposed Panel Photos
After the EIS experimental measurement was run on day 120, the electrochemical
cells were taken apart. Photos of the exposed panels are presented below in Figures
4.12a - 4.23c. Panels 001 through 009 are shown in Figures 4.12 and 4.14. Panels 010
through 018 are shown in Figures 4.15 and 4.17. Panels 019 through 027 are shown
in Figures 4.18 and 4.20. Panels 028 through 036 are shown in Figures 4.21 and 4.23.
Figures 4.12 - 4.14 show the pretreatment applications with various buffered
pH levels and application thicknesses for the AA2024 substrate. Through visual
comparison, more physical corrosion damage occurred when the application thickness
is decreased. Furthermore, as the buffered pH level of the electrolyte decreases, the
visual physical corrosion damage increases. Localized pitting attacks are prevalent
throughout high pH on AA2024.
Figures 4.15 - 4.17 show the pretreatment applications with various buffered
pH levels and application thicknesses for the AA7075 substrate. The same trends
hold for the previous conditions, but from a visual comparison, there is in increase in
corrosion damage due to the change in substrate. Therefore, the level of the buffered
pH electrolyte has more of an effect than that of the AA2024 substrate. Localized
pitting attacks are prevalent throughout low pH on AA7075.
68
(a) Panel 001
(b) Panel 002
(c) Panel 003
Figure 4.12: Actual Photos of Panels 001, 002, and 003 after 120 days of electrolyte
exposure. Panels 001 - 003 includes a Pretreatment application of 5 mils on a AA2024
substrate with a buffered electrolyte pH level of 4, 7, 10 respectively.
69
(a) Panel 004
(b) Panel 005
(c) Panel 006
Figure 4.13: Actual Photos of Panels 004, 005, and 006 after 120 days of electrolyte
exposure. Panels 004 - 006 includes a Pretreatment application of 10 mils on a
AA2024 substrate with a buffered electrolyte pH level of 4, 7, 10 respectively.
70
(a) Panel 007
(b) Panel 008
(c) Panel 009
Figure 4.14: Actual Photos of Panels 007, 008, and 009 after 120 days of electrolyte
exposure. Panels 007 - 009 includes a Pretreatment application of 15 mils on a
AA2024 substrate with a buffered electrolyte pH level of 4, 7, 10 respectively.
71
(a) Panel 010
(b) Panel 011
(c) Panel 012
Figure 4.15: Actual Photos of Panels 010, 011, and 012 after 120 days of electrolyte
exposure. Panels 010 - 012 includes a Pretreatment application of 5 mils on a AA7075
substrate with a buffered electrolyte pH level of 4, 7, 10 respectively.
72
(a) Panel 013
(b) Panel 014
(c) Panel 015
Figure 4.16: Actual Photos of Panels 013, 014, and 015 after 120 days of electrolyte
exposure. Panels 013 - 015 includes a Pretreatment application of 10 mils on a
AA7075 substrate with a buffered electrolyte pH level of 4, 7, 10 respectively.
73
(a) Panel 016
(b) Panel 017
(c) Panel 018
Figure 4.17: Actual Photos of Panels 016, 017, and 018 after 120 days of electrolyte
exposure. Panels 016 - 018 includes a Pretreatment application of 15 mils on a
AA7075 substrate with a buffered electrolyte pH level of 4, 7, 10 respectively.
74
Figures 4.18 - 4.20 show the primer applications with various buffered pH
levels and application thicknesses for the AA2024 substrate. Through visual comparison, an increase in physical corrosion damage occurred when a thinner application is
applied. Furthermore, as the buffered pH level decreases, the visual physical corrosion
damage increases.
Figures 4.21 - 4.23 show the primer applications with various buffered pH
levels and application thicknesses for the AA7075 substrate. The same trends hold
for the previous conditions, but from a visual comparison, an increase in corrosion
damage due to the change in substrate is present. Therefore, the level of the buffered
pH electrolyte has more of an effect than that of the AA2024 substrate. For example,
Figure 4.20c shows very minimal visible corrosion damage.
Note, when the buffered pH level is 4, the coating will change colors which
can be seen in Figure 4.18a, for example. Minimal visual damage is seen from the
buffered pH levels 7 and 10.
The white solids on top of the plates of the primer applied panels is dehydrated electrolyte. Since all construction was uniform, this phenomenon could be due
to poor coating application.
75
(a) Panel 019
(b) Panel 020
(c) Panel 021
Figure 4.18: Actual Photos of Panels 019, 020, and 021 after 120 days of electrolyte
exposure. Panels 019 - 021 includes a Primer application of 0.3 − 0.5 mils on a
AA2024 substrate with a buffered electrolyte pH level of 4, 7, 10 respectively.
76
(a) Panel 022
(b) Panel 023
(c) Panel 024
Figure 4.19: Actual Photos of Panels 022, 023, and 024 after 120 days of electrolyte
exposure. Panels 022 - 024 includes a Primer application of 0.7 − 0.9 mils on a
AA2024 substrate with a buffered electrolyte pH level of 4, 7, 10 respectively.
77
(a) Panel 025
(b) Panel 026
(c) Panel 027
Figure 4.20: Actual Photos of Panels 025, 026, and 027 after 120 days of electrolyte
exposure. Panels 025 - 027 includes a Primer application of 1.2 − 1.5 mils on a
AA2024 substrate with a buffered electrolyte pH level of 4, 7, 10 respectively.
78
(a) Panel 028
(b) Panel 029
(c) Panel 030
Figure 4.21: Actual Photos of Panels 028, 029, and 030 after 120 days of electrolyte
exposure. Panels 028 - 030 includes a Primer application of 0.3 − 0.5 mils on a
AA7075 substrate with a buffered electrolyte pH level of 4, 7, 10 respectively.
79
(a) Panel 031
(b) Panel 032
(c) Panel 033
Figure 4.22: Actual Photos of Panels 031, 032, and 033 after 120 days of electrolyte
exposure. Panels 031 - 033 includes a Primer application of 0.7 − 0.9 mils on a
AA7075 substrate with a buffered electrolyte pH level of 4, 7, 10 respectively.
80
(a) Panel 034
(b) Panel 035
(c) Panel 036
Figure 4.23: Actual Photos of Panels 034, 035, and 036 after 120 days of electrolyte
exposure. Panels 034 - 036 includes a Primer application of 1.2 − 1.5 mils on a
AA7075 substrate with a buffered electrolyte pH level of 4, 7, 10 respectively.
81
4.4 AFM High Resolution Results
To see the topography of the primer and pretreatment applications, the Atomic Force
Microscope (AFM) was used on unexposed samples. Figures 4.24a and 4.24b show a
sample area of the primer and pretreatment applications respectively.
(a) Pretreatment Application
(b) Primer Application
Figure 4.24: Atomic Force Microscope analysis for the (a) Pretreatment and (b)
Primer applications before electrolyte exposure.
The main differences between pretreatment and primer application shown
in Figure 4.24 are mainly in thickness. When observing the scales, it is possible to
realize this difference. It is assumed that the roughness is not different throughout
these samples and can be assumed that all other treatment profiles have similar profile
measurements,
82
4.5 IFM High Resolution Results
To see the topography of the primer and pretreatment applications, the Infinite Focus
Microscope (IFM) was used on the samples that where exposed to the electrolyte after
120 days.
Figures 4.25 - 4.27 show the 2D and 3D profiles for the AA2024 Pretreatment
coatings at the 5, 10, and 15 mils thicknesses, respectively. Figures 4.28 - 4.30 show the
2D and 3D profiles for the AA7075 Pretreatment coatings at the 5, 10, and 15 mils
thicknesses, respectively. Figures 4.31 - 4.33 show the 2D and 3D profiles for the
AA2024 Primer coatings at the 0.3 − 0.5, 0.7 − 0.9, and 1.2 − 1.5 mils thicknesses,
respectively. Figures 4.34 - 4.36 show the 2D and 3D profiles for the AA7075 Primer
coatings at the 0.3 − 0.5, 0.7 − 0.9, and 1.2 − 1.5 mils thicknesses, respectively.
83
(a) Infinite Focus Microscope 2D Surface Profile
(b) Infinite Focus Microscope 3D Surface Profile
Figure 4.25: Infinite Focus Microscope 2D and 3D Surface Profiles for the Pretreatment application of 5 mils on a AA2024 substrate with a buffered electrolyte level of
pH 4 after 120 days of exposure.
84
(a) Infinite Focus Microscope 2D Surface Profile
(b) Infinite Focus Microscope 3D Surface Profile
Figure 4.26: Infinite Focus Microscope 2D and 3D Surface Profiles for the Pretreatment application of 10 mils on a AA2024 substrate with a buffered electrolyte level
of pH 4 after 120 days of exposure.
85
(a) Infinite Focus Microscope 2D Surface Profile
(b) Infinite Focus Microscope 3D Surface Profile
Figure 4.27: Infinite Focus Microscope 2D and 3D Surface Profiles for the Pretreatment application of 15 mils on a AA2024 substrate with a buffered electrolyte level
of pH 4 after 120 days of exposure.
86
Figures 4.25 - 4.27 above shows the 2D and 3D profiles for the AA2024 panels
treated with 5, 10, 15 mils of a pretreatment application after a 120 day, pH = 4
electrolyte exposure. The application lines can be seen in all of the 2D profiles as the
horizontal times going across.
Figures 4.25a and 4.25b are the measurements for the AA2024 panels treated
with 5 mils of a pretreatment application. These results show no sign of crevice
corrosion or the beginning of pitting corrosion. The 3D profile here shows the depth
of the entire profile and the absolute topographical distance is roughly 64 µm with a
maximum height of 35.2 µm and a maximum depth of 28.3 µm.
Figures 4.26a and 4.26b are the measurements for the AA2024 panels treated
with 10 mils of a pretreatment application. These results show evidence of the formation of crevice corrosion but no signs of pitting corrosion. The 3D profile here shows
the depth of the entire profile and the absolute topographical distance is roughly
60 µm with a maximum height of 29.2 µm and a maximum depth of 39.1 µm.
Figures 4.27a and 4.27b are the measurements for the AA2024 panels treated
with 15 mils of a pretreatment application. These results show evidence of the formation of crevice corrosion. There is an area shown in the 2D profile in the upper
left that could be the start of pitting corrosion. The 3D profile here shows the depth
of the entire profile and the absolute topographical distance is roughly 74 µm with a
maximum height of 37.2 µm and a maximum depth of 36.7 µm.
87
(a) Infinite Focus Microscope 2D Surface Profile
(b) Infinite Focus Microscope 3D Surface Profile
Figure 4.28: Infinite Focus Microscope 2D and 3D Surface Profiles for the Pretreatment application of 5 mils on a AA7075 substrate with a buffered electrolyte level of
pH 4 after 120 days of exposure.
88
(a) Infinite Focus Microscope 2D Surface Profile
(b) Infinite Focus Microscope 3D Surface Profile
Figure 4.29: Infinite Focus Microscope 2D and 3D Surface Profiles for the Pretreatment application of 10 mils on a AA7075 substrate with a buffered electrolyte level
of pH 4 after 120 days of exposure.
89
(a) Infinite Focus Microscope 2D Surface Profile
(b) Infinite Focus Microscope 3D Surface Profile
Figure 4.30: Infinite Focus Microscope 2D and 3D Surface Profiles for the Pretreatment application of 15 mils on a AA7075 substrate with a buffered electrolyte level
of pH 4 after 120 days of exposure.
90
Figures 4.28 - 4.30 above show the 2D and 3D profiles for the AA7075 panels
treated with 5, 10, 15 mils of a pretreatment application after a 120 day, pH = 4
electrolyte exposure. The application lines can be seen in all of the 2D profiles as the
horizontal times going across.
Figures 4.28a and 4.28b are the measurements for the AA7075 panels treated
with 5 mils of a pretreatment application. These results show no signs of crevice
corrosion but have evidence of pitting corrosion. The 3D profile here shows the depth
of the entire profile and the absolute topographical distance is roughly 100 µm with
a maximum height of 48.9 µm and a maximum depth of 51.7 µm.
Figures 4.29a and 4.29b are the measurements for the AA7075 panels treated
with 10 mils of a pretreatment application. These results show no evidence of the
formation of crevice corrosion and no signs of pitting corrosion. The 3D profile here
shows the depth of the entire profile and the absolute topographical distance is roughly
82 µm with a maximum height of 44.5 µm and a maximum depth of 38.2 µm.
Figures 4.30a and 4.30b are the measurements for the AA7075 panels treated
with 15 mils of a pretreatment application. These results show evidence of the no
formation of crevice corrosion or pitting corrosion. Application marks can be seen
clearly throughout the profiles. The 3D profile here shows the depth of the entire
profile and the absolute topographical distance is roughly 91 µm with a maximum
height of 41.0 µm and a maximum depth of 50.7 µm.
91
(a) Infinite Focus Microscope 2D Surface Profile
(b) Infinite Focus Microscope 3D Surface Profile
Figure 4.31: Infinite Focus Microscope 2D and 3D Surface Profiles for the Primer
application of 0.3 − 0.5 mils on a AA2024 substrate with a buffered electrolyte level
of pH 4 after 120 days of exposure.
92
(a) Infinite Focus Microscope 2D Surface Profile
(b) Infinite Focus Microscope 3D Surface Profile
Figure 4.32: Infinite Focus Microscope 2D and 3D Surface Profiles for the Primer
application of 0.7 − 0.9 mils on a AA2024 substrate with a buffered electrolyte level
of pH 4 after 120 days of exposure.
93
(a) Infinite Focus Microscope 2D Surface Profile
(b) Infinite Focus Microscope 3D Surface Profile
Figure 4.33: Infinite Focus Microscope 2D and 3D Surface Profiles for the Primer
application of 1.2 − 1.5 mils on a AA2024 substrate with a buffered electrolyte level
of pH 4 after 120 days of exposure.
94
Figures 4.31 - 4.33 above show the 2D and 3D profiles for the AA2024 panels
treated with 0.3 − 0.5, 0.7 − 0.9, and 1.2 − 1.5 mils of a primer application after
a 120 day, pH = 4 electrolyte exposure.
Figures 4.31a and 4.31b are the measurements for the AA2024 panels treated
with 0.3 − 0.5 mils of a primer application. The 3D profile here shows the depth of
the entire profile and the absolute topographical distance is roughly 210 µm with a
maximum height of 103.8 µm and a maximum depth of 106.6 µm.
Figures 4.32a and 4.32b are the measurements for the AA2024 panels treated
with 0.7 − 0.9 mils of a primer application. The 3D profile here shows the depth of
the entire profile and the absolute topographical distance is roughly 124 µm with a
maximum height of 59.9 µm and a maximum depth of 64.2 µm.
Figures 4.33a and 4.33b are the measurements for the AA2024 panels treated
with 1.2 − 1.5 mils of a primer application. The 3D profile here shows the depth of
the entire profile and the absolute topographical distance is roughly 180 µm with a
maximum height of 111.1 µm and a maximum depth of 77.6 µm.
Compared to the pretreatment application, the electrolyte exposure shows a
“spotty” effect due to the coating application. These results show no signs of crevice
or pitting corrosion but show non-homogeneous corrosion of the coating.
95
(a) Infinite Focus Microscope 2D Surface Profile
(b) Infinite Focus Microscope 3D Surface Profile
Figure 4.34: Infinite Focus Microscope 2D and 3D Surface Profiles for the Primer
application of 0.3 − 0.5 mils on a AA7075 substrate with a buffered electrolyte level
of pH 4 after 120 days of exposure.
96
(a) Infinite Focus Microscope 2D Surface Profile
(b) Infinite Focus Microscope 3D Surface Profile
Figure 4.35: Infinite Focus Microscope 2D and 3D Surface Profiles for the Primer
application of 0.7 − 0.9 mils on a AA7075 substrate with a buffered electrolyte level
of pH 4 after 120 days of exposure.
97
(a) Infinite Focus Microscope 2D Surface Profile
(b) Infinite Focus Microscope 3D Surface Profile
Figure 4.36: Infinite Focus Microscope 2D and 3D Surface Profiles for the Primer
application of 1.2 − 1.5 mils on a AA7075 substrate with a buffered electrolyte level
of pH 4 after 120 days of exposure.
98
Figures 4.34 - 4.36 above show the 2D and 3D profiles for the AA7075 panels
treated with 0.3 − 0.5, 0.7 − 0.9, and 1.2 − 1.5 mils of a primer application after
a 120 day, pH = 4 electrolyte exposure.
Figures 4.34a and 4.34b are the measurements for the AA7075 panels treated
with 0.3 − 0.5 mils of a primer application. The 3D profile here shows the depth of
the entire profile and the absolute topographical distance is roughly 206 µm with a
maximum height of 92.1 µm and a maximum depth of 113.7 µm.
Figures 4.35a and 4.35b are the measurements for the AA7075 panels treated
with 0.7 − 0.9 mils of a primer application. The 3D profile here shows the depth of
the entire profile and the absolute topographical distance is roughly 198 µm with a
maximum height of 97.9 µm and a maximum depth of 99.8 µm.
Figures 4.36a and 4.36b are the measurements for the AA7075 panels treated
with 1.2 − 1.5 mils of a primer application. The 3D profile here shows the depth of
the entire profile and the absolute topographical distance is roughly 132 µm with a
maximum height of 70.9 µm and a maximum depth of 62.2 µm.
As the primer application thickness increases, there is a decrease in absolute
topographical distance. Compared to the pretreatment application, the electrolyte
exposure shows a “spotty” effect due to the coating application. These results show
non-homogeneous corrosion of the coating application.
99
Figure 4.37: IFM Profile Defined Values
Figure 4.37 shows some of the defined constants that the IFM profile measurements reported. From the IFM measurements, three of the five values that are
defined to described the profiles of any sample are shown here.
The values define in Figure 4.37 show Rp, the highest physical point in the
profile, Rv, the lowest physical point in the profile, and Rt, the total between the
highest and lowest physical points in the profile. Ra, the arithmetic mean deviation of
the profile, and Rq, the root mean squared deviation of the profile are also calculated
and given. Completing the three repeated measurements and averaging out the profile
results, the supporting profile data information is given in Tables 4.12 and 4.13.
100
Table 4.12: IFM Profile Results from Pretreatment Samples
Substrate Thickness (mil) Rt (µm) Rp (µm) Rv (µm) Ra (µm) Rq (µm)
2024
5.00
148.36
74.857
−72.839
0.438
1.173
2024
10.0
75.022
36.266
−38.756
0.330
0.570
2024
15.0
82.822
46.740
−36.083
0.591
1.108
7075
5.00
98.973
54.430
−44.543
0.716
1.258
7075
10.0
87.299
44.490
−42.809
0.716
1.042
7075
15.0
107.42
53.921
−53.497
2.040
2.996
Table 4.13: IFM Profile Results from Primer Samples
Substrate Thickness (mil) Rt (µm) Rp (µm) Rv (µm) Ra (µm) Rq (µm)
2024
0.3 - 0.5
177.29
84.513
−92.776
7.308
9.637
2024
0.7 - 0.9
169.76
81.343
−88.422
7.464
10.17
2024
1.2 - 1.5
197.29
103.51
−93.785
7.879
10.92
7075
0.3 - 0.5
180.13
79.941
−100.18
7.689
10.07
7075
0.7 - 0.9
194.87
98.096
−96.771
8.959
12.35
7075
1.2 - 1.5
144.14
71.821
−72.319
6.664
8.862
101
CHAPTER V
CONCLUSION
5.1 Conclusions
The work that is presented in the thesis includes the analysis of a one-dimensional
transport model for electrolyte transport through a coating application for use in
corrosion research. Many different experimental variables were investigated such as
an electrolyte with an electrolyte at a buffered pH level of 4, 7, or 10 on various
thickness of a primer or pretreatment application on a AA2024 or AA7075 substrate.
The one-dimensional transport equation along with the time dependent models were used to show the steps of damage evolution throughout the investigated exposure time. Not only were models shown in the time domain, but derivations of the
frequency and Laplacian domain equations were presented. The sensitivity analysis
shows that from the theoretical models, the thickness of the coating has more of an
effect of that of the electrolyte pH level. Throughout the thesis, Electrochemical
Impedance Spectroscopy, Atomic Force Microscopy, and Infinite Focus Microscopy
results and analysis were presented. The damage evolution can be monitored by
means of using models for the interface; these models represent the behavior of the
metal/coating/electrolyte interaction. In addition to the representation of the inter-
102
face when the system is exposed to different environments, following the evolution of
the interface in function of time is one of the challenges that must be faced in understanding the behavior of coatings and metallic interfaces. As part of these models,
the use of equivalent circuits is an everyday task for the expertise in corrosion in
order to make a fair interpretation of the processes happening at the interface.
To allow for no species variation in the electrolyte solution, the electrolyte
is assumed to be well-mixed. Since the only electrolyte transport is through the
application toward the substrate, there is no electrolyte flux through the top, left,
and right electrolyte boundaries. In the early stages of immersion, the electrolyte
uptake process is the predominate process to control the damage evolution. Different
stages of the damage evolution concept are identified and associated with results of
experimental conditions. The damage evolution for the coating /substrate sample
at different pHs and different layers coating produced four stages characterized by
interfacial mechanisms: the mass transport as early and initial, charge-mass transport
mix, activation stage, and finally the activationpassivation stage. The samples with
thinner thicknesses of the pretreatment application produced three stages for the
damage evolution process; the initial or early stage mechanism included water uptake
or mass transport mechanism. When correlating the results from the theoretical
mathematics and the experimental results shown from the EIS, AFM, and IFM, the
trends match. These trends show that the coating thickness is more important than
the electrolyte pH level when looking at the time it takes for the coating to become
saturated with electrolyte.
103
5.2 Future Implementations
As the experimental matrix only worked with one type of each application, three
variations of the buffered electrolyte pH level, and three variations of the application
thickness, more parameter choices could be analyzed experimentally with a mathematical model built to describe the slight differences throughout and the effects of the
corrosion products. The assumptions in this paper allowed the mathematics to only
be concerned with the one-dimensional transport toward the coating layer, where in
the future, an all-encompassing formula needs to be derived to show all parts of the
flux equation and in all directions. More analysis could be done concerning a more
specific location of the electrolyte uptake into the coating, which could be done by
using Localized Electrochemical Impedance Spectroscopy (LEIS).
104
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