ELECTROCHEMISTRY OF INTERMETALLIC PHASES
OF NI-CR-FE ALLOYS IN
LUEOUS ENVIRONMENT
by
JUN MATSUMOTO
B.S., Waseda University, Tokyo, Japan
(1982)
Submitted to the Departmet of Nuclear Engineering
in partial fulfillmet of the requirements
for the degree of
MASTER OF SCIENCE IN NUCLEAR ENGINEERING
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
May, 1991
© Jun Matsumoto, 1991
The author hereby grants to MIT permision to reproduce and to
distribute copies of this thesis document in whole or in part.
Signature of Author
'~-
Department of Nuclear Engineering
May,1991
Certified by.
Ronald G. Ballinger
Professor of Nuclear Engineering
Thesis Supervisor
Accepted 1by
.
I /
'"
~
1
Allan F. Henry
Chairman, Departmental Committee
DCpartment of Nuclear Engineering
ELECTROCHEMISTRY OF INTERMETALLIC PHASES
OF NI-CR-FE ALLOYS IN AQUEOUS ENVIRONMENT
by
Jun Matsumoto
Submitted to the Department of Nuclear Engineering on May 16, 1991, in partial fulfillment
of the requirements for the degree of Master of Science in Nuclear Engineering.
ABSTRACT
Precipitation-strengthened Ni-Cr-Fe alloys are widely used as light water reactor
(LWR) core internal structural materials because of their high strength, excellent creep
resistance, and general corrosion resistance. In spite of generally good performance,
failures of the alloys due to fatigue, corrosion fatigue and stress corrosion cracking (SCC)
have been reported. One of the key variables in the susceptibility to SCC has been the
formation of? calized galvanic couples between intermetallic precipitate phases and the NiCr-Fe matrix. However, investigation of extremely localized galvanic effects is normally
difficult because of the extremely localized nature of the reactions. When bulk materials
representing the intermetallic phases can be made, it is possible to assess electrochemical
characteristics of each intermetallic phases in the alloys by examining the bulk materials
representing the intermetallic phases.
Ten alloys representing Laves phases and delta/eta phase chemistries were
fabricated by the arc melting, and characterized by optical microscopy, scanning electron
microscopy (SEM), electron dispersion X-ray analysis (EDX), and X-ray diffraction
analysis. After the characterization, these ten alloys along with seven pure metal elements
(Ni, Cr, Fe, Ti, Nb, Mo, Al).were electrochemically examined. Corrosion potential
measurements were conducted in 0.05 M Na2SO4 solution at pH 1, pH 3, and pH 5.
Potentiodynamic polarization measurement was conducted in 0.05 M Na2SO4 solution at
pH 3 at room temperature.
Most of the alloys representing the Laves phase chemistries (except Fe2Ti) showed
a corrosion potential between -220 and -240 mV with respect to standard hydrogen
electrode (SHE) while the alloys representing the delta/eta phases indicated -150 mV to 215 mV at pH 3. Regarding pure metal elements, nickel showed relatively high corrosion
potential (-148 mV) while Cr and Al showed relatively low potential (-598 mV, -666mV) at
pH 3. Ti and Nb showed much higher corrosion potential than the reversible potential.
Based on these electrochemical data of pure metal elements, electrochemical
parameters of fabricated ten alloys were estimated by the mixed potential theory, and
compared with the measured values. The mixed potential theory predicted the corrosion
potential of the alloys representing the Laves phases (except Fe2Ti and Ni2Nb) with errors
of less than 10 mV while the prediction for the delta and eta phases showed approximately
50 mV higher corrosion potential than the measured value.
Thesis Supervisor: Ronald 0. Ballinger
Title: Associate Professor of Materials Science and Engineering and Nuclear Engineering
2
TABLE OF CONTENTS
ABSTRACT
2
TABLE OF CONTENTS
3
LIST OF FIGURES
5
LIST OF TABLES
9
ACKNOWLEDGEMENT
10
1. INTRODUCTION
11
2. LITERATURE REVIEW
2.1 Background of Ni-Cr-Fe Alloys
2.1.1 Development of Ni-Cr-Fe Alloys
2.1.2 Description of the Phases
2.1.3 Microstructure of Typical Ni-base Alloys
2.2 Electrochemistry of Corrosion
2.2.1 Anodic and Cathodic Reactions
2.2.2 Reversible Potential
2.2.3 Irreversible Potential of Metals
2.2.4 Polarization
2.2.5 Pourvaix Diagrams
2.2.6 Electrochemical Heterogeneity
2.2.7 Electrochemical Potential of Alloys in the Steady State
2.2.8 Potentials of Alloys in the Non-equilibrium Condition
2.2.9 Application to Multielectrode Systems
2.3 Galvanic Coupling in Ni-Cr-Fe Alloys
16
16
16
17
20
21
21
22
27
27
29
37
38
39
45
45
3. MATERIALS
3.1 Production of Ten Alloys
3.2 Microstructural Characterization of Ten Alloys
3.3 Pure Elements
49
49
51
68
4. EXPERIMENTAL PROCEDURE
4.1 Specimen Preparation
4.2 Corrosion Potential Measurement
4.3 Cathodic Anodic Potentiodynamic Polarization
70
70
70
76
5. RESULTS
5.1 Potential Measurement
5.2 Potentiodynamic Polarization Measurement
78
78
89
6. DISCUSSION
110
7. CONCLUSION
130
8. SUGGESTION FOR FUTURE WORK
132
APPENDIX A
Ni-Cr-Fe components used in the U.S. commercial LWRs
134
APPENDIX B
X-ray diffraction analysis for lattice parameter determination
141
REFERENCES
143
LIST OF FIGURES
2.1
Reversible condition and irreversible condition of electrode processes
on the surface of metal
24
2.2
Potential-pH equilibrium diagram for the nickel-water, at 25 0C
30
2.3
Potential-pH equilibrium diagram for the chromium-water, at 25 0C
31
2.4
Potential-pH equilibrium diagram for the iron-water, at 25 0C
32
2.5
Potential-pH equilibrium diagram for the titanium-water, at 25 *C
33
2.6
Potential-pH equilibrium diagram for the niobium-water, at 25 0C
34
2.7
Potential-pH equilibrium diagram for the molybdenum-water, at 25 0C
35
2.8
Potential-pH equilibrium diagram for the aluminum-water, at 25 *C
36
2.9
Corrosion potential of binary-phase alloy
41
2.10
Possible corrosion potential change due to chemical composition of
binary-phase alloy
42
2.11
Phase diagrams and possible potential diagrams of two-phase alloys
43
2.12
Graphic solution of a multielectrode system on the basis of ideal
polarization.curves
44
2.13
Galvanic cell model for intergranular crack tip in multiphase materials
48
3.1
Microstructure of alloy #1 (by optical microscopy, x 100)
55
3.2
Microstructure of alloy #1 (by SEM, x 2000)
55
3.3
Microstructure of alloy #2 (by cptical microscopy, x 100)
56
3.4
Microstructure of alloy #2 (by SEM, x 4000)
56
3.5
Microstructure of alloy #3 (by optical microscopy, x 100)
57
3.6
Microstructure of alloy #3 (by SEM, x 4200)
57
3.7
Microstructure of alloy #4 (by optical microscopy, x 100)
58
3.8
Microstructure of alloy #4 (by SEM, x 4300)
58
3.9
Microstructure of alloy #5 (by optical microscopy, x 100)
59
3.10
Microstructure of alloy #6 (by optical microscopy, x 100)
59
3.11
Microstructure of alloy #7 (by optical microscopy, x 100)
60
3.12
Microstructure of alloy #8 (by optical microscopy, x 100)
60
3.13
Microstructure of alloy #9 (by optical microscopy, x 100)
61
3.14
Microstructure of alloy #10 (by optical microscopy, x 100)
61
3.15
Results of EDX chemical composition analysis for alloy #1
62
3.16
Results of EDX chemical composition analysis for alloy #2
62
3.17
Results of EDX chemical composition analysis for alloy #3
63
3.18
Results of EDX chemical composition analysis for alloy #4
63
3.19
Results of EDX chemical composition analysis for alloy #5
64
3.20
Results of EDX chemical composition analysis for alloy #6
64
3.21
Results of EDX chemical composition analysis for alloy #7
65
3.22
Results of EDX chemical composition analysis for alloy #8
65
3.23
Results of EDX chemical composition analysis for alloy #9
66
3.24
Results of EDX chemical composition analysis for alloy #10
66
4.1
A specimen for electrochemical experiment at room temperature
72
4.2
Schematic system layout for corrosion potential measurement
and potentiodynamic polarization measurement
73
4.3
Saturated calomel Electrode (SCE)
74
5.1
Corrosion potentials as a function of nickel content
82
5.2
Corrosion potentials as a function of chromium content
83
5.3
Corrosion potentials as a function of iron content
84
5.4
Corrosion potentials as a function of titanium content
85
5.5
Corrosion potentials as a funiction of niohium content
86
5.6
Corrosion potentials as a function of molybdenum content
87
5.7
Activity dependence of nickel potential in pH 3 nickel sulfate solution
88
5.8
Anodic/carhodic potentiodynamic polarization curves of alloy #1
in 0.05M Na2SO4 with pH 3 at room temperature
92
Anodic/cathodic potentiodynamic polarization curves of alloy #2
in 0.O5M Na2SO4 with pH 3 at room temperature
93
5.9
Anodic/cathodic potentiodynamic polarization curves of alloy #3
in 0.05M Na2SO4 with pH 3 at room temperature
94
Anodic/cathodic potentiodynamic polarization curves of alloy #4
in 0.05M Na2SO4 with pH 3 at room temperature
95
Anodic/cathodic potentiodynamic polarization curves of alloy #5
in 0.05M Na2SO4 with pH 3 at room temperature
96
Anodic/cathodic potentiodynamic polarization curves of alloy #6
in 0.05M Na2SO4 with pH 3 at room temperature
97
Anodic/cathodic potentiodynamic polarization curves of alloy #7
in 0.05M Na2SO4 with pH 3 at room temperature
98
Anodic/cathodic potentiodynamic polarization curves of alloy #8
in 0.05M Na2SO4 with pH 3 at room temperature
99
Anodic/cathodic potentiodynamic polarization curves of alloy #9
in 0.05M Na2SO4 with pH 3 at room temperature
100
Anodic/cathodic potentiodynamic polarization curves of alloy #10
in 0.05M Na2SO4 with pH 3 at room temperature
101
Anodic/cathodic potentiodynamic polarization curves of nickel
in 0.05M Na2SO4 with pH 3 at room temperature
102
Anodic/cathodic potentiodynamic polarization curves of chromium
in 0.05M Na2SO4 with pH 3 at room temperature
103
Anodic/cathodic potentiodynamic polarization curves of iron
in 0.05M Na2SO4 with pH 3 at room temperature
104
Anodic/cathodic potentiodynamic polarization curves of titanium
in 0.05M Na2SO4 with pH 3 at room temperature
105
Anodic/cathodic potentiodynamic polarization curves of niobium
in 0.05M Na2SO4 with pH 3 at room temperature
106
Anodic/cathodic potentiodynamic polarization curves of molybdenum
in 0.05M Na2SO4 with pH 3 at room temperature
107
Anodic/cathodic potentiodynamic polarization curves of aluminum
in 0.05M Na2SO4 with pH 3 at room temperature
108
5.25
The schematic polarization curve showing unstable passivation behavior
109
6.1
Application of mixed potential theory to alloy #1
116
6.2
Application of mixed potential theoiy to alloy #2
117
6.3
Application of mixed potential theory to alloy #3
118
6.4
Application of mixed potential theory to alloy #4
119
5.10
5.11
5.12
5.13
5.14
5.15
5.16
5.17
5.18
5.19
5.20
5.21
5.22
5.23
5.24
6.5
Application of mixed potential theory to alloy #5
120
6.6
Application of mixed potential theory to alloy #6
121
6.7
Application of mixed potential theory to alloy #7
122
6.8
Application of mixed potential theory to alloy #8
123
6.9
Application of mixed potential theory to alloy #9
124
6.10
Application of mixed potential theory to alloy #10
125
6.11
Comparison of predicted corrosion potentials in 0.05 M Na2SO4 (pH 3)
solution at room temperature
129
LIST OF TABLES
1.1
Nominal chemical compositions of Ni-Cr-Fe alloys used in Light Water Reacters 12
1.2
Outline of tasks and procedures
15
2.1
Summary of electrochemical parameters in 0.05 M Na2SO4 (pH 3) solution
measured at 93 *C
47
3.1
Target chemical composition of the replicated alloys (wt% / at%)
50
3.2
Results of EDX chemical composition analysis (at%)
67
3.3
Lattice parameters and associated errors of Laves phases in 6 replicated alloys
calculated from X-ray diffraction data
69
4.1
Matrix of experimental conditions
71
5.1
Corrosion potential of 10 replicated alloys at room temperature
79
5.2
Corrosion potentials and reversible potentials of seven pure metals at room
temperature
79
5.3
5.4
6.1
6.2
6.3
Summary of electrochemical parameters of ten alloys in 0.05 M Na2SO4
(pH 3) solution measured at room temperature
110
Summary of electrochemical parameters of pure metals in 0.05 M Na2SO4
(pH 3) solution measured at room temperature
111
Comparison of predicted electrochemical parameters of alloys by mixed
potential theory and measured parameters in 0.05 M Na2SO4 (pH 3) solution
at room temperature (based on the atomic ratio of target composition)
126
Comparison of predicted electrochemical parameters of alloys by mixed
potential theory and measured parameters in 0.05 M Na2SO4 (pH 3) solution
at room temperature (basewd on the weight ratio of target composition)
127
Comparison of predicted electrochemical parameters of alloys by mixed
potential theory and measured parameters in 0.05 M Na2SO4 (pH 3) solution
at room temperature (based on the atomic ratio obtained by EDX analysis)
128
ACKNOWLEDGEMENTS
At first, I would like to express my deep appreciation to my thesis adviser,
Professor Ronald Ballinger. He has enormously contributed to my experience at MIT. He
has constantly encouraged and guided me throughout my MIT study.
I would like to specially thank to Dr. Il Soon Hwang for his generous advice and
valuable discussions. His instruction for the experiment and scholastic suggestion were
crucial for my research.
I would also like to thank to my colleagues in the Materials Laboratory for their
technical assistance and encouragement. The opportunity to interact with other graduate
students and researchers at MIT have made my experience more significant and enjoyable.
Their friendship and guidance have much contributed to my experience at MIT.
Tokyo Electric Power Company (TEPCO) gave me this wonderful opportunity to
study at one of the best schools in the U.S. TEPCO has financially supported my study at
MIT. I would like to thank to many TEPCO staff members who helped and encouraged
me.
Finally, I would like to thank to my wife, Shoko, for her patience and emotional
support. Her assistance was invaluable.
10
CHAPTER 1
INTRODUCTION
Structural materials used in nuclear power plants require special properties, in
addition to general mechanical properties required in most engineering designs. The
requirement are related to radiation exposure and severe corrosive environment of the
nuclear reactor systems. Considering potentially high impact of the failure on safety and
plant economics, the materials need to possess much higher reliability than those used in
other industries.
The core internal components for nuclear reactor are exposed to severe mechanical
loading, radiation exposure, high temperature, and aqueous environment [1-1].
Precipitation-strengthened Ni-Cr-Fe alloys have been widely used in these light water
reactor (LWR) core internal structural materials because of their high strength, good creep
resistance, and good general corrosion resistance[1 -2]. Typical alloys in this
classemployed in the LWRs include alloy 718, X-750, and A-286. The nominal chemical
compositions of these alloys are shown in Table 1.1.
Ni-Cr-Fe alloys are used as bolts, springs, beams, and guide tube pins in LWR
cores. The identified components in commercial LWRs in the U. S. are summarized by
Hayner and Ballinger [1-31. The lists of the components with the heat treatment condition
and the in service environment are shown in Appendix A.
In spite of generally good performance of the alloys in the LWR environment, failures of
some components fabricated from these alloys have been reported [1-4]. The main causes
of the failures are fatigue, corrosion fatigue, and stress corrosion cracking (8CC). Fatigue
is produced by cyclic stress which continuously rise and fall in magnitude. In aqueous
environment, it is frequently found that crack growth rate is higher than in the
II
Table 1.1 Nominal chemical compositions of precipitation-strengthened NiCr-Fe alloys used in Light Water Reactors (from ref.[2-2])
Ni
Cr
Fe
Nb
Ti
Mo
Al
Mn
Si
C
Alloy718
52.5
19.0
18.5
5.1
0.9
3.0
0.5
0.2
0.2
0.04
X-750
73.0
15.5
7.0
1.0
2.5
0.0
0.7
0.5
0.2
0.04
A-286
26.0
15.0
54.0
0.0
2.0
1.3
0.2
1.3
0.5
0.05
12
gaseous environment due to the effect of corrOsion. This is called corrosion fatigue. While
there are many similarities between corrosion fatigue and SCC, SCC is a term given to the
cracking of metal by the con joint presence of a static tensile stress, susceptible material, and
a specific environment.
A significant amount of effort to understand the mechanism of these failures has
been expended for many years [1-3][1-41. According to Hwang et al.[1-6], grain
boundary precipitates and matrix of Ni-based alloys can form extremely localized galvanic
couples, and the resulting anodic and cathodic process accelerates SCC and/or Hydrogen
Assisted Cracking (HAC).
Although it is possible to estimate the possibility of galvanic effects by comparing
electrochemical corrosion potential and polarization behavior of constituent elements,
electrochemical characterization of each phase is necessary for accurate assessment.
Normally, it is very difficult to investigate the separate anodic and cathodic behavior of
constituents using alloys because the coupling is often too local. Therefore,
electrochemical experiments such as current and potential measurement can not be
performed for each individual phase. Hence, it is necessary to use bulk materials
representing these phases in separate effects studies.
Prybylowski [1-7] investigated the chemical composition and crystal structure of
the intermetallic phases at the grain boundary of Alloy 718. Similar work [1-8][1-91 has
been reported on the chemical composition and the crystal structure of the phases in other
Ni-Cr-Fe alloys. Based on the information, it is possible to make alloys whose chemical
compositions are the same as the phases seen in Ni-Cr-Fe alloys. Furthermore, with
proper heat treatment, it is possible to make the alloys which have the same crystal structure
as the intermetallic phases of Ni-Cr-Fe alloys.
When an alloy has the same chemical composition and crystal structure as a
intermetallic phase in the grain boundary of the alloy, its electrochemical properties can be
13
assumed to be identified to those of the intermetallic phases. Using these alloys, basic
electrochemical experiments can be performed.
The main objectives of this study are to acquire basic electrochemical data of
various intermetallic phases and matrices, and to estimate their galvanic coupling behavior.
The study involves successful production and verification of alloys which have the same
chemical composition and crystal structure as the intermetallic phases in the grain boundary
and in the matrices of Ni-Cr-Fe alloys.
Table 1.2 outlines the tasks required to achieve these goals. When an alloys is
found to deviate from intended microstructure, it is reprocessed with different heat
treatment and verified again.
14
nowt ----
-
-- .
Table 1.2 Outline of tasks and procedures
Task
1. Production of alloys
Goals
Procedure
a) design of alloys
Investigation of intenietallic
phases
Replication of intermetallic
b) processing
phases
2. Verification of alloys
a) Optical microscopy
Verification of general
microstructure
b) Scanning Electron
Microchemical verification
Microscopy (SEM)
3. Electrochemical
characterization of alloys
c) X-ray diffraction analysis
Phase identification
a) Corrosion potential
Corrosion potential of pure
measurement
b) Anodic/cathodic
metal and alloys
Kinetic characterization
potentiodynamic
polarization
4. Analysis
Mixed potential theory
Estimation of galvanic
coupling behavior
15
--
CHAPTER 2
LITERATURE REVIEW
In this chapter, the literature on microstructures of Ni-Cr-Fe alloys and the
electrochemical theory for aqueous corrosion are reviewed. Electrochemical relationships
between pure elements and their alloys are examined. Its physical metallurgy with the
application to the prediction of electrochemical behavior of multiphase alloys is discussed.
2.1 Background of Ni-Cr-Fe Alloys
In this section, the background of Ni-Cr-Fe alloys are characterized by the history
of their development, the microstructure of intermetallic phases, and the structure of the
representative alloys.
2.1.1 Development of Ni-Cr-Fe Alloys
Originally, Ni-Cr-Fe alloys were developed from austenitic stainless steels for the
aircraft jet engines and other gas turbines[2- 1]. Therefore, the improvement of creep and
fatigue resistance at high temperature has been the most important issue for the alloy
development. Through additions of small quantities (less than 2 wt. %)of titanium, it was
found that nickel-chromium alloys develop high strength levels and retained their properties
to remarkably high temperatures because of age-hardening [2-2]. The development from
the early Ni-base alloys such as A-286 in 1940s to the contemporary Ni-base alloys such as
Incoloy 901 followed a general trend toward higher strength by increasing age-hardening
elements such as niobium, titanium, and aluminum to provide higher volume fractions of
the intermetallic precipitates. The addition of niobium for precipitation strengthening has
16
led to the development of contemporary alloys such as alloy 718 and Inconnel 706 with
mechanical property levels that far exceed those of their predecessors.
2.1.2 Description of the Phases
Most of Ni-Cr-Fe alloys have complex microstructures, and consist of at least
several phases. Following is descriptions of the major phases seen in Ni-Cr-Fe alloys.
Gamma phase (matrix)
Most of Ni-Cr-Fe alloys have an austenitic matrix. The matrix is a face-centered
cubic (FCC) nickel-base phase. The minimum level of nickel required to maintain a stable
austenitic matix is about 25 wt.%. The matrix is solid-solution strengthened mainly by
chromium, molybdenum, and iron. Chromium has another major role increasing
oxidation-corrosion resistance by the formation of protective oxide scale including Cr203
and NiCr2O4 that allows their use in high temperature environments [2-3]. Normally, NiCr-Fe alloys contain chromium in the range of 10 - 25%, and molybdenum in the range of
0 -9%.
Gamma prime phase
Gamma prime is the FCC ordered intermetallic Ni3(Al,Ti) phase which precipitates
coherently within the matrix. Gamma prime is a metastable precipitate which nucleates and
grows quite readily. It forms very fine dots in spherical shape. This phase stabilizes
Cr203, and improves high temperature oxidation resistance significantly. Gamma prime
plays a dominant role in strengthening of the alloys containing aluminum and titanium such
as A-286 and X-750.
17
Gamma double prime phase
Gamma double prime is a body-centered tetragonal (BCT) intermetallic Ni3Nb
phase. Although this phase is also precipitates coherently within the matrix, the size
difference between the BCT unit cell and FCC lattice and the limitation of available slip
directions of BCT are believed to attribute to the exceptionally high strength exhibited by
these alloys. Ni-base alloys forming gamma double prime phase contain niobium as the
principal age-hardening constituent. The representative alloys in this category, alloy 718
and 706, contain niobium 5 and 3 wt.%, respectively.
Eta p~hase
Eta is a stable hexagonal close-packed (HCP) Ni3Ti phase. Eta phase is
transformed from gamma prime phase during elevated-temperature exposure. This
transformation can occur through forging/heat treatment or long-time service exposure.
Since it has a needle shape and extents from grain boundaries into matrix, fracture
toughness is lowered by its presence.
Delta phase
The formation of delta phase in alloys strengthened by gamma double prime is
closely similar to the formation of eta phase in alloys strengthened by gamma prime. Delta
is an orthorhombic Ni3Nb phase, and is the thermodynamically stable form of the
metastable gamma double prime. Delta phase forms over the 650 - 980 0C (1200 - 1800 OF)
temperature range with a needle shape in the case of alloy 718. In the temperature range of
700 - 950 0C, the formation rate of delta increases significantly as the temperature
increases.
The formation of large amounts of delta under long-time exposure degrades the
mechanical properties in most cases. The degradation have been considered to be caused
by the decrease of the niobium in the matrix and the formation of coarse delta phase.
18
Laves phase
Laves phase is an A2B-type hexagonal compound, which forms in both Ni-base
and Fe-base alloys during high temperature processing. In Ni-base alloys, A-element is
usually nickel, iron, and chromium; B-element is niobium, molybdenum, and titanium. In
the case of alloy 718, about 40 to 45% Ni, 25% Nb, 10% Fe, 10% Cr, 10% Mo, and
possibly a little Ti are indicated as an example [2-4]. This phase is considered as one of the
most brittle phases, and is avoided by proper control of chemistry or heat treatment. It is
possible to minimize Laves phase by adding boron and zirconium. However, the
formation of the phase escalates as niobium, titanium, and silicon increases.
Laves phase appears in heavily cored or segregated areas, and tends to deplete the
matrix of the elements required for precipitation hardening. According to Michel et al. [2-
5], Laves phase was found to be the most prevalent grain boundary phase of alloy 718
while several other workers [2-6] have identified delta phase as the primary grain boundary
precipitates. Prybylowski [2-7] suggested that Laves phase precipitation during the low
temperature anneal is responsible for susceptibility of Intergranular Stress Corrosion
Cracking (IGSCC).
Carbides
Carbon is primary present as MC-type carbides, where M represents a metallic
constituent. Alloys strengthened by niobium mainly form NbC, and alloys added with
titanium contain TiC. Since these carbides take strengthening elements away from the
precipitation strengthening phases, such as gamma prime and gamma double prime, the
presence of carbon decreases the strength of the alloy. However, a small amount of carbon
present as NbC is considered to help retain a fine grain size during high temperature
processes.
19
M6C and M23C6-type carbides are also formed on solidification and/ or heat
treatment in some alloys [2-81. Metal atoms in M2 3C6 are usually chromium while metal
atoms in M6C are mainly molybdenum. M23 C6 is forned at grain boundaries during
processing, heat treatment, or service. In oxidizing environments, chromium depletion at
grain boundaries can promote SCC. However, Yonezawa et al.[2-91 introduced the
opposite effect that the formation of M2 3C6 in the specific manner at grain boundaries
improves SCC resistance of alloy X-750.
2.1.3 Microstructure of Typical Ni-base Alloys
Alloy X-750
The chemical composition of alloy X-750 is similar to that of alloy 600 while more
Ti and Al are added for gamma prime precipitation hardening(see Table 1-1). Generally,
gamma prime precipitates in the intragranular region, and is coherent with the gamma
matrix although the lattice parameter is not identical to that of the matrix. Carbides are MCtype and Cr23C 6 , where M is normally Ti and Nb.
Standard two-step aging of alloy X-750, called AH, results in coarse gamma prime
precipitates at grain boundaries. Annealing and aging, called HTH, results in formation of
M2 3 C6 carbide and sensitization at grain boundaries.
Alloy 718
This alloy is strengthened mainly by gamma double prime precipitation while
gamma prime are also seen in the structure. Gamma double prime is metastable, and
transforms into delta phase by overaging. MC carbide and Laves phase form during
solidification and/or heat treatment and degrade the material. Laves phase can be dissolved
into matrix by solution annealing, but MC carbide can not be removed by typical solution
annealing.
20
2.2 Electrochemistry of Corrosion
Corrosion can be defined as a deleterious chemical reaction of a solid material with
its environment. The chemical reaction involves electron transfer and charge in valency in
case of metallic material. Therefore, a review of the basic theory of electrochemistry for
corrosion is needed. First, the fundamental theory is reviewed, and then application to
multiphase alloys is discussed.
2.2 1 Anodic and Cathodic Reactions
Corrosion of metals in an aqueous environment (electrolytes) is divided into two
distinct processes: the anodic process and the cathodic process.
The anode involves corrosion of metal and loss of electrons. The metal, which is
otherwise electrically neutral, dissolves and remains in the solution or forms insoluble
corrosion products. These anodic corrosion reaction can be expressed by the following
equation;
M = Mz+ + ze-
(2.1)
where M is a symbol for metal, and z is the valency of the metal.
There are several important reactions which may occur at the cathode in a water
environment. Normally, the cathode does not corrode. These reactions vary as the pH and
oxygen content of the solution change [2-10]. Only low oxygen environments will be
considered here for the sake of simplicity. In the lower pH range (pH<7), the following
reaction is preferred;
2H+2e-= H2(atom) .
(2.2)
In the higher pH condition,(pH>7), the representative reaction may be;
21
2H20 + 2e- = H2 + 20H-. .
(2.3)
2.2.2 Reversible Potential
Thermodynamic driving force of a particular electrochemical reaction is expressed
by the reversible potential. The reversible metal potential shows the tendency of the metal
to react, and is proportional to the decrease of free energy of the reaction. Therefore, the
reversible potential can be calculated based on the thermodynamic data. The electrical
potential difference of an electrode from a standard state when it is brought into an
equilibrium with a non-standard electrolyte can be generally expressed by the Nernst
Equation;
E - E0= RT In [products](2.4)
zF
[reactants]
where EO is standard reversible potential, R is the universal gas constant, F is the Faraday
constant, and T is temperature.
For example, the equations for the anodic/cathodic reactions in a simple metalhydrogen system are:
M = M2++ 2e-
(2.5)
2H+ + 2e- = H2 (gas),
(2,6)
where the valency of the metal is assumed two as a typical value. Then, the overall reaction
is written as
M + 2H+ = M2 + + H2 (gas).
(2.7)
Then, the Nernst equation is
E
= F0
-
zF
0
[M][H+]
(2.8)
22
where [H+] and [H21 are activity of H+ and pressure of H2 gas. These should be unity in
the standard condition. Therefore, substituting the standard temperature, 298 'K, and the
universal gas constant R= 8.3143 J/mol K, the equation becomes
0.059
E= E.0-02
log[M 2 +].
(2.9)
If the activity of metal ions is made equal to 1, the potential is reduced to E0.
When electric current at metal/electrolyte interface is represented with forward and
backward reactions made by a single carrier as in this example, the potential can be used for
a quantitative description of the equilibrium between two electrode processes (see Figure
2. Ia). In this case, the electric charges transferred from the metal into the solution are
equal to the charges from the solution to the metal, and this establishes a reversible metal
potential.
Extensive effort has made to determine the reversible potential. Latimer [2-11]
summarized the available potential data of metals in aqueous environment:
Nickel
The most important oxidation state for nickel is the +2 state although there are
evidences of existence of +4, +6, and +8 oxides. An accurate value for the reversible
potential of the Ni - Ni2 + couple cannot be given, because it is impossible to attain a
precisely reversible equilibrium condition due to the multiplicity of the ground states.
Several different values are reported for this reversible potential, 0.227 volt by Colombier
[2-12], 0.231 volt by Haring and Bosche [2-13], and 0.248 volt by Murata [2-141. The
Bureau of standards free energy corresponds to 0.241 volt. However, Latimer
recommends the following value from the viewpoint of consistency to the value for the
entropy of Ni 2 +, -38.1 cal/deg;
Ni =Ni 2 ++2e-,
Eo= 0.250 V
23
(2.10)
2+
Me-01,Me
s....
MeM2++
2+
Me-Me
ea
Electrolyt
""""" """"""
Me+HMe
""""""K
HH-
H
"00I
,111'
,111,
1110
011111,11111
4
Metal
(a)
(b)
Figure 2.1 Reversible condition and irreversible condition of electrode
processes on the surface of the metal, (a) reversible condition; exchange
current is transfered by a single carrier, (b) irreversible condition;
precess of electron aquisition at the electrode is different from the
process of electron loss
24
The corresponding free energy of formation of Ni 2 + is -11.5Kcal.
Chromium
The basic oxidation state for chromium is the +2 state. There are other oxidation
states including +3 and +6. Some investigators measured the potential of the Cr - Cr 3+
couple through the intermediate chromous ion. They found that E0 values for the metalchromic couple were near 0.51 volt, but this value was not in agreement with the
thermodynamic data. Latimer calculated reversible potentials assuming that the chromium
electrode is not reversible and data interpretation should be relied entirely on the
thermodynamic third-law calculations. His calculation gives
Cr = Cr 3 + + 3e-,
E9= 0.74V,
Cr = Cr 2 ++ 2e-,
9 = 0.91 V .
(2.11)
(2.12)
There are two major oxidation states for iron, which are known as ferrous (Fe2+),
and ferric (Fe3+). There are also +6 and +8 states. Richards and Behr [2-15] obtained the
first reproducible results using finely divided iron. Randall and Frandsen [2-16]
determined the reversible potential from the free energy of Fe2+, -20310 cal.
Fe = Fe2 + + 2e-,
W= 0.440 V .
(2.13)
Latimer also calculated the potential of ferric ion as follows;
Fe = Fe3 + + 3e-,
E= 0.036 V.
(2.14)
Titanium
Titanium forms compounds of the +2, +3, and +4 oxidation states, but the dioxide,
TiO2, is stable in contact with water. The potential of the mixture of the titanium chlorides,
25
TiCl2 - TiCl3, was measured at 0 'C, and 0.37 volt was given as a E0 at 0 *C. This relation
can be also applied as an approximated value at 25 *C;
Ti2+= Ti3 + + e-,
(2.15)
E = ca0.37 V.
From the potentials relating TiO 2+ to the metal and to the lower oxidation states, following
potential of Ti2+ can be calculated;
Ti = Ti 2+ + 2e-,
E3= 1.63V.
(2.16)
Because of this very positive value, titanium cannot be predicted by cathodic reduction in
water solutions. However, the metal is very passive and is not readily oxidized by mild
oxidizing agents.
Niobium
Niobium has +5, +4 and +3 oxidation states. In the +4 state, NbO2 is known as
well as Nb205 of the +5 oxidation states. Solution containing the ion, Nb 3 +, or complexes
of this ion are made by cathodic reduction or by reduction of zinc, of solutions of niobic
oxide in sulfuric, hydrochloric acids. Based on the solubility data for niobic oxide in
sulfuric acid, Latimer estimated the potential of niobium as
Nb = Nb 3+ + 3e-,
(2.17)
E=ca 1.1 V .
No values can be given for niobium potentials in alkaline solution.
Aluminum
The only stable oxidation state of aluminum is the +3 oxidation state. The best
results of potential measurement of aluminum have been obtained with aluminum amalgam,
but these potentials have not been reproducible. Latimer calculated the potential by using
the entropy of cesium alum and aluminum ion as follows;
Al =Al3+ +3e-,
E9= l.66 V.
26
(2.18)
2.2.3 Irreversible Potential of Metals
Practically, it is difficult to measure the equilibrium potential in most metals because
of the complexity and irreversibility of the reactions. When the process of electron
acquisition at the electrode is not a reverse reaction of the process of electron loss, the
potential is called the irreversible potential. Such an irreversible reaction can continue at a
constant rate at a steady state potential. The steady state irreversible potential is a result of a
balance of charges, and the anodic/cathodic processes depend on the nature of the electrode
and the solution (see Figure 2.1 b). Therefore, irreversible potentials are characterized by
the kinetics of the electrode processes.
2.2.4 Polarization
Polarization is a very important concept on which the corrosion rate can be
quantified. In the equilibrium condition, the anodic reaction rate is the same as the cathodic
rate, and these are expressed as ia and ic respectively. Regarding the anodic reaction, for
which the activation free energy is AG**, ia is written as
is (at equilibrium) = io = AO exp ( RT)
(2.19)
where A0 is constant. When the oxidation reaction is faster than the reduction reaction, the
equilibrium is destroyed. This occurs when the difference in the free energies between
metal and its ions deviates from the equilibrium value.
A deviation from the equilibrium potential, polarization, is made by changes in the
environment. When the total polarization, E - Ee, is expressed as the overpotential Ti,
27
anodic polarization can be defined as arj and cathodic polarization as (1-a)l. By using
these relations, the anodic current can be written as,
-AG** + ail zF
ia=Aexp
RT
)
AOexp(-AG**exp( aUzF
=0
"NCRT
xP(RT
=ioexp(
(.0
-anzF
RT)(2.20)
Rearranging this equation in terms of 1
"1=
-
-
2.303l
ia
log (r-)
0
azF/RT
$a log (i)
10
a log
ia -
@a log
(2.21)
io,
where $a is constant. This equation shows that polarization will be linear on a logarithmic
plotting of current density, and is known as the Tafel equation [2-17]. The diagrams of
potential versus current density on a logarithmic scale are called Evans diagrams. The
slope of linear region in the diagram is expressed as the constant Pa in the equation (2.21).
The anodic Tafel slope $a is given by
(2.22)
a= 2.303RT
czzF
Similarly, the Tafel slope for the cathodic reaction is
p= -2.0R
(2.23)
(1-cz)zF
28
l
2.2.5 Pourbaix Diagrams
Pourbaix diagrams (potential/pH diagrams) are a graphical representation of the
Nernst equation, and correlate the dependence of pH and the potential of the electrode upon
the condition of the electrode. In Pourbaix diagrams, the regions of stability cf major
species are plotted in a domain of electrochemical potential and pH. Pourbaix diagr ns
normally have following three regions:
1. Immunity - the pure metal is thermodynamically stable and dissolution or
corrosion cannot o< cur;
2. Corrosion - ions if the metal are thermodynamically stable and dissolution or
corrosion will occur and
3. Passivity - a compc und reaction product of the metal with elements from the
environment is thermodynamically stable, and further reaction can be impeded by
passive layers.
Pourbaix diagrams for nickel, chromium, iron, titanium, niobium, molybdenum,and
aluminum are shown in Figures 2.2 to 2.8 [2-181.
While Pourbaix diagrams have been widely used as a useful means of assessing
metal-oxide systems in aqueous environment, several important limitations are pointed out.
Hubbel et al.[2-19] listed the limitations. These include difficulties in applying them to
alloys, for predicting kinetics of electrode reactions, and for assessing the effectiveness of
passive films which form on the electrode surfaces. Trethewey et al. also mentioned that
the domains have been calculated using thermodynamic data while the actual reactions are
kinetically as well as thermodynamically controlled. They also pointed out that the
presence of counter-ions may invalidate the theoretical predictions.
29
Figure 2.2 Potential-pH equilibrium diagram
for the nickel-water, at 25 0C (from ref.[2-18])
30
zcr 0
1,6
C
4
IIj
Cr.
HCrO
4
Cr(OM)
0,8
0,6
...
S
Cr&+++..
0
.0,4
-
-4
-6
-0,2
2s
r3
Cr++
-
-1,4
22
Figure 2.3 Potential-pH equilibrium diagram
for the chromium-water, at 25 *C (from ref.[12-18])
31
1,
Figure 2.4 Potential-pH equilibrium diagram
for the iron-water, at 25 *C (from ref.[2-181)
32
13
14
Figure 2.5 Potential-pH equilibrium diagram
for the titanium-water, at 25 *C (from ref.[2-18])
33
15
16
pH
Figure 2.6 Potential-pH equilibrium diagram
for the niobium-water, at 25 0C (from ref.[2-18J)
34
-I
0
1
2
3
4
Figure 2.7 Potential-pH equilibrium diagram
for the molybdenum-water, at 25 0C (from ref[2-181)
35
pH
Figure 2.8 Potential-pH equilibrium diagram
for the aluminum-water, at 25 *C (from ref.[2-18])
36
2.2.6 Electrochemical Heterogeneity
On the surface of the metal in the contact with electrolyte, there may be prefered
sites for simultaneous anodic and cathodic reactions. The characteristics of this
electrochemical heterogeneity differ widely in terms of the size of the anodic and cathodic
areas and of the cause of the heterogeneity. Sources of heterogeneity include variations in
metal chemistry, crystallographic orientation, dislocation density and the presence of
discrete second phase particles. From this viewpoint, the coupling due to electrochemical
heterogeneity are roughly categorized into three types: macrocorrosion cells,
microcorrosion cells, and submicrocouples.
There are many factors which cause electrochemical heterogeneity. One typical
factor is a nonuniform concentration of the solid solution(segregation). The region of the
alloy with more noble components usually act as cathodes. Accelerated corrosion of Al-Zn
alloys in aqueous solutions due to segregation is a good example of this case. Another
factor is the presence of crystalline grain boundaries. Generally, the boundaries act as
anodes. For instance, in 18Cr-8Ni stainless steel, it is known that it becomes sensitized to
the intergranular stress corrosion cracking (IGSCC) after aging between 600 and 700*C.
This type of attack is due to precepitation of chromium carbides at the grain boundaries. In
this temperature range, chromium diffusion from the grain bodies to the grain boundaries is
too slow to prevent localized depletion in chromium [2-20]. There are more factors such as
discontinuities in the protective film, unequal distribution of corrosion products, unequal
internal stresses, and so on. Also, heterogeneity in the aqueous environment, such as
differential concentration of solution, can cause electrochemical heterogeneity.
37
2.2.7 Electrochemical Potential of Alloys in the Steady State
For practical application, it is necessary to assess the behavior of a material which
consists of two or more elements. In those cases, further consideration is needed to
understand the relationship between measured potential and their electrochemical behavior.
At first, a binary alloy system, the M-N system, is considered for simplicity. The
ions of the metals are expressed as Mz+ and NY+. When the activities of the M and N, aN
and am, can be expressed, the Nernst equations for these two metals become
MF++ ze-= M ,
EM
Elm
n am+(2.24)
NY+ + yeNYe==N,,
aNy+
EN=EoN+ln
F InaN
(2.25)
This alloy is supposed to be in the equilibrium condition when the potentials of both
metals are equal. Therefore,
(2.26)
EM =EN,
EON - ElM=nRT
zyF
1
aMz+azN
ayM az(NY+
The reversible potential of the alloy, E, becomes
E = EMeq = ENeq = E0M+ F In aMzei
-
E0N +
ayIn.q
(2.28)
In most cases, the alloys exist in an unstable nonequilibrium condition and it is
impossible to obtain the reversible potential. The potential of the alloy in the unstable
condition is called the mixed potential.
38
2.2.8 Potentials of Alloys in the Nonequilibrium Condition
A simple example is considered for discussion. The polarization curvcs for both
cathode and anode are linear and have the same shape like EA-A and Eco-C lines in Figure
2.9a. In this case, the potential varies linearly with an increase in the area of the second
element (Figure 2.9b). When the fraction of area of the cathode fc and that of the anode fa
are the same, the sum of anodic current density should be the sum of 2athodic current
density. When fc = fa, the potential of the alloy is determined by the intersection of the
total anodic and cathodic lines. Since this rule, mixed potential theory, is based o., current
densities, total surface area does not have to be known.
When the slope of cathodic current density, Tafel slope, is relatively high (line EAoX') and the anodic one is lower as shown by line EC,-C, the potential Ex' approaches the
anode potential EAO. Ex' is less noble than Ex. On the other hand, when the cathodic Tafel
slope is lower than the anodic one in absolute magnitude, lines are EAo-A and EC0 -X". The
potential Ex" is closer to initial cathodic value, and is more noble than Ex. Thus it can be
seen that the mixed potential is determined by the weighted-average of potentials of two
dissimilar metals. In the Figure 2.9b, this behavior is illustrated by curves EAr-X'-ECO and
EAo-X'-ECo as a function of composition.
When the cathodic and anodic polarization curves have the same shape and the
potential has logarithmic dependence on current density, the potential dependence on the
composition in a binary alloy becomes like the curve EA(,MPNECr in Figure 2.10. The
phase of smaller area has higher current density, and the potential changes more rapidly as
the area increases. The equal-area potential (fc=fa) will fall exactly on the straight line
between pure metals value.
Practically, when it is applied to the actual alloys, there are more complicating
factors such as 1) Increase of the cathodic area with time due to secondary discharge of
atoms of the cathodic phase, 2) Relative increase of the cathodic phase due to the
39
preferential dissolution of the anodic phase, 3) Change of the passivity due mainly to two
reasons mentioned above, 4) Structural complexity of phases in the actual alloys, etc 12211.
The major factors that make the curve more positive than the EAO-ECO straight line in
Figure 2.10 are;
1) the anodic polarization has a steeper slope than the cathodic ; the polarization
is anodically controlled.
2) the polarization current varies exponentially as the potential increases
(logarithmic polarization curve shown as the curve EAoMPNECo in Figure 2.10).
3) the anode phase area decreases due to selective dissolution of anode
4) secondary separation of cathodic element from solution
5) increase of integrity of passive film
6) increase of chemical stability.of metallic system
Factors 1), 2), 3), and 4) cannot shift the potential of the alloy more positive than
that of pure cathodic component ECO, and the shape of the curve will be similar to EAlECO
in Figure 2.10. However, factors 5) and 6) can shift the potential of the alloy more
positive than that of pure cathodic component Eco, and the shape of the curve can be similar
to EAo3'ECo in Figure 2.10. Also, the potential shift of the alloy to the negative side can be
evaluated in the same manner, and the possibility of the curves EAo2 ECO and EAO3ECO can
be indicated. Based on the theory mentioned above, several examples of potential in binary
phase alloys are shown in Figure 2.11 [2-22]. It may be possible to estimate the potentials
of intermetallic phases by combining the mixed potential theory with experimental data.
40
Ex
--
------ XEx-x
Ex
Ex
-
Ix
I
a.
Ex'
-
EAo
Ex'
r
~x
C
Current (1)
100%A
0%C
(a)
50%A
50%C
Comosition (vol.%)
(b)
Figure 2.9. Corrosion potential of binary-phase alloy (from ref.[2-211), (a):
anodic/cathodic polarization curve of the binary-phase alloy, (b):
potential-composition diagram in volume %. When the polarization is
cathodically controlled (line EAo-X'), the potential Ex' approaches the initial
anodic value EAB; Ex'is less noble than Ex.
0
O/A
100%C
w
N
*d
M
a.
3
EA|
100%A
50%A
50%C
100%C
Figure 2.10. Possible corrosion potential change due to
chemical composition of binary-phase alloy (from
ref.[2-21]); Line EA-Ec for equal and linear polarization
of cathodic/anodic phases. Curve EA-M-P-N-Ec for equal
but logarithmic dependence of cathode/anode potential on
current density.
42
T -----------------------
----------T-
T
I -
A
B A
AmB
EBC
EAo
A
B
AmBn
EBC
EAo
B A
EAO
B
A
AmBn
B
A
AmBn
B
(assumption ;, A is nobler than B)
Figure 2.11 Phase diagrams and possible potential diagrams
of two-phase alloys (from ref.[2-22])
c-
09
>
Ex-
CL
-
-
~
EA3
9
-
-
*
91
*,
EA2
EAi EA*
I
', *
E'
9'+.
C5
C4
'
C1 C2
S
C3
Ix
Current density
Figure 2.12 Graphic solution of a multielectrode
system on the basis of ideal polarization curves
(from ref.[2-211), solid lines - anodic curves,
dashed lines
curve, ECoX
-
cathodic curves, EAoX
total cathodic curve.
44
-
total anodic
2.2.9 Application to Multielectrode Systems
Figure 2.12 shows an example of application of the mixed potential theory to a fiveelectrode system. The figure was constructed based on the knowledge of theoretical
anodic/cathodic polarization curves and the areas for each of the five electrodes.
In this figure, all cathodic polarization curves start from one common initial
cathodic potential ECo, and form five different curves, EcoCI, EC
0 C2, ECoC3, ECOC4, and
ECoC5. On the other hand, all anodic curves start from different potentials EAi, EA2, EA3 ,
EA4, and EA. Combined all cathodic curves and anodic curves separately, a total cathodic
curve ECOX and a total anodic curve EAoX would be obtained. The potential at the
intersection of the total anodic curve and the total cathodic curve defines the overall
potential Ex and the overall r .srent density of this five-electrode system.
2.3 Galvanic Coupling in Ni-Cr-Fe Alloys
Hwang et al. [2-23] evaluated the effect of galvanic coupling between precipitated
phases, matrix, and near grain boundary chemistries in nickel-base alloys. They
investigated corrosion potential, potentiodynamic polarization behavior, galvanic effects,
and passivation kinetics for grain boundary boundary gamma prime, Laves phase, and NiCr-Fe matrices in aqueous environment. Table 2.1 is summary of their experimental
results. The results revealed that electrochemical activity of gamma prime, Laves phase,
and sensitized chemistries is significantly higher than the matrix and varies with
composition and temperature. They theorized that the electrochemical relationship between
grain boundary phases and their couples with the matrix can provide for an extremely local
source of anodic and cathodic process in low conductivity environments such as water.
Galvanic couples, such as gamma prime/matrix and Laves/matrix, are suggested as
possible anodic/ cathodic processes.
45
Figure 2.13 shows their schematic model which describes the relationship between
embrittlement mechanisms and local electrochemisty at the crack tip. The crack tip area
consists of precipitates, grain boundary, bare fresh metal, the filmed region, and the
localized aqueous environment. Precipitates can become either anode or cathode, and the
grain boundary itself is often more active than the matrix due to higher degree of defects
and disorder and possible presence of segregates such as sulfur, phosphorous, and boron.
Also, some elements may be depleted at the grain boundary. Normally, the crack flank is
covered with a passivadon film, and a fresh bare metal is exposed to the aqueous
environment only at the crack tip. These factors form electrochemical heterogeneity in the
alloys.
At the same iime, electrochemical heterogeneity also occurs on the electrolyte side.
The anodic process mentioned above introduces metal ions into the solution at the
immediate crack tip. These metal ions would be gradually diffused from the crack tip, and
chemical reactions such as hydrolysis will reduce the pH. Reduction of pH eventually
causes more hydrogen production, and promotes hydrogen assisted cracking. As a
dominant mechanism, they suggest hydrogen embrittlement at low temperature, and both
hydrogen embrittlement and stress corrosion cracking at high temperature.
46
Table 2.1 Summary of electrochemical parameters in 0.05 M
Na2SO4 (pH3) solution measured at 93*C (from ref.[2-23])
Alloy
Ecor
icorr
Cathodic
Anodic
designation
(mVSHE)
(A/cm2)
@(V)
P(V)
Ni3A1(Ti=0)
-260
2.4 x 10-5
-0.084
0.047
1.7 x 10-8
Ni3AI(Ti=0. 15)
-200
9.2 x 10-6
-0.049
0.038
1.2 x 10-9
Fe2Ti
-350
2.3 x 10-6
-0.084
0.120
1.2 x 10-9
5051(6.3 w/oCr)
-210
3.9 x 10-5
-0.050
0.107
2.2 x 10-9
X-750 (SA)
-190
2.9 x 10-6
-0.043
0.089
1.5 x 10-10
718(SA)
-240
2.0 x 10-6
-0.053
0.113
7.0 x 10-10
47
ioH+/Ha2
(A/cn2)
mommummmmomor""
Oxidized Crack Flanks
Anodic Precipitate
Precipitate
Grain Boundary
Fresh Bare Surface
Region of Local Depletion
Figure 2.13 Galvanic cell model for intergranular crack tip in multiphase
materials (from ref.[2.231)
48
CHAPTER 3
MATERIALS
This chapter describes the materials procured to attain the research objectives
discussed in chapter one. The materials were characterized mainly by optical microscopy,
the scanning electron microscopy (SEM), and x-ray diffraction .
3.1 Production of Ten Alloys
Based on the previous studies, ten chemical compositions of intermetallic phases
were selected. These ten alloys were fabricated at General Electric Company by using arc
melting technique. High purity powder, which was adjusted to the target composition, was
mingled well and melted by arc beam. During melting and solidifdication, the alloys were
flipped over several times for homogenization. Table 3-1 shows target chemical
composition of these ten replicated alloys. Alloys #1 to #4 consist of nickel, chromium,
iron, titanium, niobium, and molybdenum. Chemical composition of these alloys are
similar to chemical composition of various types of Laves phases. Alloy #5 and #6 have
the same compositions as those of Ni2Nb and Fe2Ti respectively. Alloys #7 to #10 are NiTi-Nb ternary phase alloys. Compositions of alloys #7 and #10 are the same compositions
as those of eta phase and delta phase respectively, and alloys #8 and #9 have intermediate
compositions between eta phase and delta phase.
49
Table 3.1 Target chemical composition of the replicated alloys (wt% / at%)
Alloy
Phase
Ni
Cr
Fe
Ti
Nb
Mo
Alloy #1
Laves
34.0/39.6
10.3/13.5
11.3/13.8
0.8/1.14
35.3/26.0
8.3/5.9
Alloy #2
Laves
40.0/45.4 10.3/13.2
11.3/13.5
0.8/1.1
29.3/21.0
8.3/5.8
Alloy #3
Laves
46.4/51.3
10.3/12.8
11.3/13.1
0.8/1.1
22.9/16.0
8.3/5.6
Alloy #4
Laves
43.8/47.5 14.6/17.9
12.3/14.0
1.0/1.33
22.9/15.7
5.4/3.6
Alloy #5
Laves
55.8/66.6
0
0
0
41.2/33.3
0
Alloy #6
Laves
0
0
70.0/66.7
30.0/33.3
0
0
Alloy #7
Eta
77.8/74.1
0
0
22.2/25.9
0
0
Alloy #8
Eta/Delta
73.0/74.1
0
0
14.2/17.7
12.8/8.2
0
Alloy #9
Eta/Delta
68.3/74.2
0
0
6.2/8.2
25.5/17.5
0
Alloy #10
Delta
65.5/75.0
0
0
0
34.5/25.0
0
50
3.2 Microstructural Characteiization of Ten Alloys
Samples of these ten alloys were cut from the ingots and observed using optical
microscopy and SEM. Figures 3.1 to 3.14 show the microstructures observed by the
optical microscopy and the SEM. Chemical composition of the alloys were also confirmed
by energy dispersing X-ray (EDX) analysis. When several phases were seen in one
sample, the chemical composition of all phases found were investigated. The results of the
chemical analysis by EDX are compared with the target chemical composition in Table 3.2
and Figures 3.15 to 3.24. In table 3.2, the area written in bold letters is the most dominant
area for each alloy.
Alloy #1 (Figures 3.1 and 3.2')
The structure of alloy #1 using optical microscopy (x100) is shown in Figure 3.1.
The dendritic structure seen in the photo is fairly uniform. An SEM micrograph (x2000) is
also shown in Figure 3.2. The chemistries of the dark area (area 2 in Figure 3.15), which
looks like grain boundaries, are relatively close to the.target composition with slightly
higher concentration of chromium and titanium and a little less niobium and molybdenum.
The bright area (area I in Figure 3.15) in the photo, which looks like matrix, contains
much more nickel (about 60 %). Titanium is also richer than the intended composition.
There are only 7 % iron and 7 %chromium in this area. However, the degree of
heterogeneity is assumed to be acceptable for this work.
Alloy #2 (Figures 3.3 and 3.41
Figure 3.3 shows the photo taken by optical microscopy (x100). The structure is
finer and more complex than alloy #1. Figure 3.4 is a photo by SEM (x4000).
Chemistries of the bright area (area 3 in Figure 3.16) are roughly same as the target
composition. However, some of the dark areas (area 4 in Figure 3.16) contain no
51
chromium, niobium or titanium. Moreover, some calcium was found from this area. This
might be due to the zirconia crucible used for the original casting. The dark area with
needle-shape structure (area 5 in Figure 3.16) contains more nickel than area 4 and less
chromium, iron and niobium. Calcium was found also in this area, but the amount was
smaller than that in area 4.
Alloy #3 (Figures 3.5 and 3.6)
The structure of alloy #3 observed by optical microscopy (Figure 3.5, x100) is
similar to alloy #2. In the EDX analysis (Figure 3.17), the chemistries are measured in two
different areas at which the structures did not look similar, but the results were almost the
same and those were close to the original compositions. The SEM photo is shown in
Figure 3.6 (x4200).
Alloy #4 (Figures 3.7 and 3.8)
The microstructure of alloy #4 seen in Figure 3.7 (by optical microscopy, x100) is
similar to alloy #2 while alloy #4 has firer structure than alloy #2. An SEM micrograph is
shown in Figure 3.8 (x4300). Alloy #4 showed large inclusions (area 8 in Figure 3.18;
not in the photos). These large inclusions were shown to be pure niobium. Since the other
areas also contain niobium, the finer niobium particles were melted during fabrication and
some of the larger particles were not melted and remained in the matrix as pure niobium .
The SEM photo was taken at the area which does not include the pure niobium . The
eutectic area (area 9 in Figure 3.18) contains slightly more nickel and niobium, and slightly
less chromium and iron than the target composition. This result includes both dark and
bright parts in the area since the structure was too fine to analyze separately. The darker
area (area 10 in Figure 3.18) contains slightly more nickel, but the fraction of other
elements was similar to the target composition. The fourth area (area 11 in Figure 3.18)
has less chromium.
52
Alloy #5 (Figure 3.9)
Figure 3.9 shows the microstructure of alloy #5 by optical microscopy (xlOO). The
sample of alloy #5 for EDX analysis also had an inclusion (area 12 in Figure 3.19), which
was shown to be pure niobium particle. The other area (area 13 in Figure 3.19) is fairly
uniform and the chemistries were almost same as the target composition.
Alloy #6 (Figures 3.10)
Alloy #6 was remelted by electron beam welding facility because of too many
cracks and flaws. The surface of remelted sample was fairly uniform and non-cracked.
The optical micrograph is shown in Figures 3.10. However, even after remelting, the alloy
was very brittle. The uniform area (area 14 in Figure 3.20) contains only iron and titanium
and those ratio was similar to the expected one (2Fe: 1Ti).
Alloy #7 (Figures 3.11)
Alloy #7 has relatively uniform structure as seen in Figure 3.11 (optical
micrograph, x100). Chemistries investigated by EDX are almost same as the target
composition although nickel is a little enriched and titanium is a little depleted (see Figure
3.21).
Alloy #8 (Figures 3.12)
In the optical micrograph (Figure 3.12), the matrix the grain boundary are seen, but
those chemistries checked by EDX were almost the same (see Figure 3.22). Titanium and
niobium are slightly more and nickel is slightly less than the target composition.
Alloy #9 (Figures 3.13)
The material is well homogenized as seen in Figure 3.13 (by optical microscopy,
xlOO). The structure and the chemistries are almost uniform and only one chemical
53
composition was recorded during EDX analysis (see Figure 3.23). Nickel is slightly more,
and titanium and niobium are slightly less than the target composition.
Alloy #10 (Figures 3.14)
#10 is well homogenized during casting and it has almost only one phase. The
structure and the chemistries are almost uniform and only one chemical composition was
recorded during EDX analysis (see Figure 3.24). Nickel is slightly more, and niobium is
slightly less than the target composition.
54
~
/
*b
L
V
-
.1*
Figure 3.1 Microstructure of alloy #1 (by optical microscopy, x 100)
Figure 3.2 Microstructure of alloy #1 (by SEM, x 2000)
Figure 3.3 Microstructure of alloy #2 (by optical microscopy, x 100)
Figure 3.4 Microstructure of alloy #2 (by SEM, x 4000)
Figure 3.5 Microstructure of alloy #3 (by optical microscopy, x 100)
Figure 3.6 Microstructure of alloy #3 (by SEM, x 4200)
Figure 3.7 Microstructure of alloy #4 (by optical microscopy, x 100)
Figure 3.8 Microstructure of alloy #4 (by SEM, x 4300)
Figure 3.9 Microstructure of alloy #5 (by optical microscopy, x 100)
Figure 3.10 Microstructure of alloy #6
(by optical microscopy, x 100)
Figure 3.11 Microstructure of alloy #7
(by optical microscopy, x 100)
Figure 3.12 Microstructure of alloy #8 (by optical microscopy, x 100)
Figure 3.13 Microstructure of alloy #9
(by optical microscopy, x 100)
Figure 3.14 Microstructure of alloy #10 (by optical microscopy, x 100)
Ni
Cr
Fe
Ti
Nb
Mo
Others
Target
Area 1
Area 2
0
20
40
60
80
100
Chemical Composition(at%)
Figure 3.15 Results of EDX chemical
composition analysis for alloy #1
Target
U3
13
...
13
Area 3
Area 4
..............
...............
...........
..............
..
..
.
...
.
I.
-.
............
..........
..........
..............
...........
....
......
...............
..............
.
..........
..............
Area 5
0
20
40
60
80
Chemical Composition (at%)
Figure 3.16 Results of EDX chemical
composition analysis for alloy #2
100
Ni
Cr
Fe
Ti
Mo
Mo
Others
*..Ni
Target
Fe
OTi
Area 6
Mo
Others
o
Area 7
20
0
40
60
80
100
Chemical Composition (at%)
Figure 3.17 Results of EDX chemical
composition analysis for alloy #3
Target
*
Ni
Cr
Fe
E: Ti
O
Area 9
-
Area 10
Area 11
0
20
40
60
80
Chemical Composition (at%)
Figure 3.18 Results of EDX chemical
composition analysis for alloy #4
100
Ab
Target
......
e.....
...
........
. ...... ..-. ...
s ...-.. .. -. e e . . . . .*e.*. . . .- e.* * * *.*-
Area 12
,e..
....-.
.....
....
..
...
.......e...........
......
-.-
.-
.
.
Ni
Cr
Fe
Ti
Nb
.....
...
.. ............
.
Mo
Others
. -.
. .. .- . . .- .- e . .- ..
e.....*............*..*..e..e...........
U
..................i..................2.................."...................................
.-
.-
.
.-
.e
.
.*
.*
.*.*
.*
.
.
.'.--
.
. .'
Area 13
20
40
60
80
100
Chemical Composition (at%)
Figure 3.19 Results of EDX chemical
Composition analysis for alloy #5
Ni
Cr
Fe
Ti
Target
Nb
Mo
Others
Area 14
20
40
60
80
Chemical Composition (at%)
Figure 3.20 Results of EDX chemical
composition analysis for alloy #6
100
Target
U
.
Ni
Cr
Fe
Ti
Mo
Others
Area 15
0
20
40
60
80
100
Chemical Composition (at%)
Figure 3.21 Results of EDX chemical
composition analysis for alloy #7
Ni
Cr
Fe
Ti
.........
.
Target
..................
. ..............................
.................................
................................
.............................
Mo
Others
Area 16
0
20
40
60
80
Chemical Composition (at%)
Figure 3.22 Results of EDX chemical
composition analysis for alloy #8
100
Target
[3
Area 17
20
0
40
60
80
Ni
Cr
Fe
Ti
Nb
Mo
Others
100
Chemical Composition (at%)
Figure 3.23 Results of EDX chemical
composition analysis for alloy #9
Ni
Cr
Fe
Ti
Target
-
....
............................
Nb
Area 18
Mo
Others
0
20
40
60
80
Chemical Composition (at%)
Figure 3.24 Results of EDX chemical
composition analysis for alloy #10
100
Table 3.2 Results of EDX chemical composition analysis (at %), the area
written in bold letters is the most dominant area in each alloy
Alloy
#1
#2
#3
#4
15.5
33.2
8.9
13.5
0.8
0.0
1.9
1.1
20.0
0.0
13.3
21.0.
3.7
2.4
1.5
5.8
0.0
6.4
1.6
0.0
15.4
15.5
13 1
0.7
0.7
1.1
19.8
19.2
16.0.
3.4
4.3
5.6
0.0
0.1
0.0
0.0
17.2
7.1
17.5
15.7
0.0
39.8
33.3
0.0
3.1
1.7
2.9
3.6
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
14.5
0.0
7.9
13.2
14.4
14.5
12.8
39.6
45.5
58.0
64.9
45.4
46.3
45.7
51.3
area 8
area 9
area 10
area 11
target
area 12
areal3
target
0.0
10.2
15.3
9.5
17.9.
0.0
0.2
0.0
0.0
11.4
14.7
10.5
14.0
100.0
1.1
1.2
1.3
1.3
0.0
0.0
0.0
100.0
0.0
0.0.
0.0
0.0
69.2
66.7
0.1
0.0
0.1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.3
0.0
0.2
0.0
17.4
8.2
0.0
0.0
0.0
0.0
30.2
33.3.
24.4
25.9
20.1
17.7
0.4
0.0
0.0
0.0
0.3
0.0
0.0
0.2
7.5
13.7
0.2
0.0
0.0
0.0 -
8.2
17.5
0.0
0.0
0.0
0.0
0.1
0.0
0.0
0.0
19.3
25.0
0.0
0.0
0.5
0.0
areal4
target
#7
area15
75.4
#8
target
area16
target
areal7
target
74.1
61.5
74.1
78.4
74.2
areal8
target
80.1
75.0
#10
Others
0.0
0.0
0.0
target
area 3
area 4
area 5
target
area 6
area 7
target
#6
#9
Mo
0.7
4.8
5.9
Fe
7.4
13.8
13.8
area 1
area 2
0.0
57.0
60.0
58.3
47.5
0.0
60.0
66.7
0.4
0.0
#5
4.6
1.2
1.2
Nb
15.0
19.4
26.0
Cr
7.5
14.0
13.5
Ni
64.8
46.8
67
T1
FROM
The other samples were also cut from the same ingots in order to analyze crystal
structures by the X-Ray diffractometer. The diffraction peaks and the d spacings obtained
by the analysis were compared with the d spacings of Laves phases found by various
investigators [3-1]. Lattice parameters were calculated by the computer program developed
by Morra [3-2], and existence of Laves phases was confirmed. Lattice parameters and
associated errors of the Laves phases calculated by the program are listed in Table 3.3. The
peaks and indices used for the lattice parameter calculation are listed in Appendix B.
3.3 Pure Elements
In addition to these ten alloys, seven pure metals, which are nickel, chromium,
iron, titanium, niobium, molybdenum, and aluminum, were prepared for the same
electrochemical experiments for the comparison with ten replicated alloys. The high purity
commercial metals for chemical experiments, which have more than 99.99% purity, were
used.
68
Table 33 Lattice parameters and associated errors of Laves phases in 6
replicated alloys calculated from X-ray diffraction data
Phase
Lattice
parameter a
(Angstroms)
Lattice
parameter c
(Angstroms)
Standard error
in a
(Angstroms)
standard error
in c
(Angstroms)
AHoy#1
Laves
4.81
7.88
0.02
0.05
Alloy#2
Laves
4.80
7.76
0.01
0.07
Alloy#3
Laves
4.80
7.80
0.01
0.02
Alloy#4
Laves
4.79
7.83
0.03
0.07
Alloy#5
Laves
4.84
8.95
0.05
0.10
Alloy#6
Laves
4.91
8.09
0.10
0.24
Alloy
69
CHAPTER 4
EXPERIMENTAL
PROCEDURES
Corrosion potential and anodic/cathodic potentiodynamic polarization behavior was
evaluated for all samples. The conditions at which the experiments were conducted are
summarized in Table 4.1.
4.1 Specimen Preparation
For the electrochemical studies, materials were prepared each with approximately
100 mm 2 surface area, and mounted in epoxy resin (EPON 828 with TETA curing agent)
which was cured at 75 *C for four hours. Electrical connection between the specimen and a
lead wire (Inconnel 600 with about 0.8 mm diameter) was made by spot welding or by
silver painting before mounting. The wire was then covered with a Pyrex glass tube, and
the glass tube was attached to the resin by an epoxy bond. The surface of the specimen
was polished to #600 SiC paper. In order to avoid crevice corrosion at the boundary
between the sample and the resin, the edge of the material surface was covered by a
chlorine-free lacquer (MICRO, Super XP2000). Figure 4-1 illustrates the specimen used
in this series of experiments.,
4.2 Corrosion potential measurement
Corrosion potential was measured in a Pyrex glass cell (500 ml) by using a saturated
calomel reference electrode as shown in Figure 4.2. This system can also conduct
potentiodynamic polarization measurement. The reference electrode in the saturated
potassium chloride (KCI) solution container is connected with the cell through a salt bridge
70
Table 4.1 Matrix of experimental conditions: "E" indicates corrosion
potential measurement, "P" indicates potentiodynamic polarization
experiment, and "A" indicates corrosion potential measurement at various
activities.
Specimen
pH 1
pH 3
pH 5
alloy #1
E
E,,P
E
alloy #2
E
ElP
E
alloy #3
E
E, P
E
alloy #4
E
EP
E
alloy #5
E
EtP
E
alloy #6
E
E, P
E
alloy #7
E
E, P
E
alloy #8
E
E, P
E
alloy #9
E
E, P
E
alloy #10
E
E, P
E
nickel
E
El, P, A
E
chromium
E
E, P
E
iron
E
E,P
E
titanium
E
E, P
E
niobium
E
E, P
E
molybdenum
E
E, P
E
aluminum
E
E, P
E
71
Lacquer
Specimen
Epoxy resin (EPON 828)
Epoxy bond
Electrical lead wire (Inconel 600)
Pyrex glass tube
Spot welding or silver painting
Figure 4.1 A specimen for electrochemical
experiment at room temperature
72
Reference
Electrode
Compartment
Thennometer
500ml
Pyrex
glass cell
Calomel
Electrode
(SCE)
KCl
Luggin Capillary
Figure 4.2 Schematic system layout for
potential measurement and potentiodynamic
polarization measurement
73
Electrical Connection
Bung
Glass tube
Saturated KCI Solution
Mercury
Mercury Chloride
Porous plug
Porous plug
KCl crystals
Figure 4.3 Saturated Calomel Electrode (SCE)
74
and a Luggin capillary. The reference electrode and the specimen were connected to the
potential measuring devices, EG&G Princeton Applied Research Model 173
potensiostat/galvanostat with a Model 276 digital interface. The system was controlled
using a Hewlett Packard Model HP-85 computer. The potential difference between the
specimen and the reference electrode was measured by the potentiostat.
A saturated Calomel (Hg2CI2) Electrode (SCE) was used as reference electrode(see
Figure 4.3). The SCE electrode employs a saturated KC solution, and is commonly used
in laboratories as a reference electrode because of its stable and reproducible potential. This
electrode mainly consists of two glass tubes. The larger glass tube is filled with saturated
potassium chloride. The smaller diameter glass tube is located at tie center of the larger
glass tube. The smaller glass tube maintains mercury and mercury chloride inside, and is
plugged by porous material at the end of the tube which allows ion current without causing
cross contamination of the potassium chloride.
The potential measured by the SCE reference electrode, ESCE, was converted to the
Standard Hydrogen Electrode (SHE) scale, ESHE. The relation between ESHE and ESCE is
expressed as
ESHE = ESCE + 244 - 1.003AT + 1.745x10-4 (AT) 2 - 3.03x10-6 (AT) 3 ,
(4-1)
where ESHE and ESCE are in mV, and T is the temperature of the electrolyte minus 25'C 141]. While this equation indicates +244mV at room temperature (25*C), Piron [4-2] used
+240mV, and Evans suggested +242mV [4-31.
A O.O5mol/liter Na2SO4 solution was used as the electrolyte in order to compare
results with previous work done by Hwang et al 14-41. The pH of the solution was
adjusted to three by using H2SO4 to simulate the conditions that may exist at the tip of
fatigue or stress corrosion cracks [4-51. In addition, the pH of the solution was varied
from one to five to study the effect of the pH. The experiments were conducted while
75
bubbling the solutiop with nitrogen gas to minimize the effect of oxygen in the solution, In
order to remove the oxide layer and the impurities on the surface, the specimen was
cathodically charged at -1000 to -1500 mV for about one minute before the measurement.
The potential of pure nickel in nickel sulfate solution was also measured to study
the relation between the corrosion potential and the reversible potential. The concentration
of nickel sulfate was varied from 0.001 mol/liter to 1 mol/liter.
4.3 Cathodic/Anodic Potentiodynamic Polarization
Polarization experiments were conducted using a three-electrode system as shown
in Figure 4.2. The current between the specimen and a platinum counter electrode was
measured during a linear potential scan with respect to the reference electrode. Three
electrodes are a working electrode (specimen), a reference electrode, and an auxiliary
counter electrode. The reference electrode and the working electrode were the same as
those used in the potential measurement, and a platinum foil was used as an auxiliary
counter electrode. The potentiostat monitors the potential between the working electrode
and the reference electrode, and automatically changes the current between working
electrode and auxiliary electrode with a rate of 0.5 mV/sec. The same potentiostat as the
potential measurement was used. This potentiostat was connected to the HP 85 computer
and plotter through a model 276 digital interface, and the potential was controlled by the
computer program. Data were stored in a 3.5 inch floppy disk through HP 9121 disk
drive, and the polarization curves can be plotted by HP 7475A plotter. The software for
potential control, data acquisition and processing was developed by Hosoyal 4-6]1.
During the measurement, nitrogen gas was bubbled through the solution to exclude
oxygen from the electrolyte. The pH of 0.05 mol/liter Na2SO4 solution was adjusted to
3.00 +0.01 by adding lN H2S0 4 solution. For each specimen, the cathodic polarization
was conducted twice at the beginning, and the anodic polarization was carried on. The
76
cathodic polarization, as well as cathodic charge, has a effect to remove the passive film
and impurities on the surface of the specimen. This occurs by reduction of the passive
oxide or absorbed oxygen film, and by the hydrogen gas production on the surface of the
specimen. At the interval of each polarization, corrosion potential was measured, and the
corrosion potential measuced was used as an initial potential of next polarization. These
corrosion potentials were compared with the corrosion potential obtained from polarization
curve after the experiment.
77
CHAPTER 5
RESULTS
Corrosion potential measurement and anodic/cathodic potentiodynamic polarization
experiment were conducted in the conditions summarized in Table 4.1. The results of the
experiment are shown in this chapter.
5.1 Potential Measurement
Corrosion potentials for the replicated ten alloys were measured with respect to
saturated calomel electrode (SCE) at room temperature. The corrosion potentials were
converted to the values for standard hydrogen electrode (SHE) by the equation (4.1);
ESHE = ESCE + 244 - 1.003AT + 1.745 x 10-4 (AT)2 - 3.03 x 10-6 (AT) 3 ,
where ESHE and ESCE are in mV, and T is the temperature of the electrolyte minus 25'C.
In order to study the pH effect, the potentials were measured at three different pH vatlues;
pH 1, pH 3, and pH 5. The corrosion potentials of 10 alloys in three different pH are
shown in Table 5.1. Also, the corrosion potentials of seven pure metals were measured in
three different pH conditions. The results are shown in table 5.2.
Generally, as the pH decreases, the corrosion potential increases. This tendency is
clearly seen in the cases of the Laves phases (alloys #1 - #6), and some of the pure metals
such as nickel and iron. Also, the potential at pH 1 shows the highest value in almost of all
specimens. This result is consistent with the shift in hydrogen reversible potential
following the Nernst equation;
78
Corrosion potential of 10 replicated alloys at room temperature
Table 5.1
(mV, SHE)
pH
#1
#2
#3
#4
#5
#6
#7
#8
#9
#10
1
-60
4
-17
-16
-37
-267
68
-54
41
20
3
-240
-223
-219
-229
-221
-320
-152
-184
-204
-215
5
-282
-257
-252
-266
-246
-488
-115
-125
-179
-150
Table 5.2 Corrosion potentials and reversible potentials of seven pure
meals at room temperature (mV, SHE)
Ni
Cr
Fe
Ti
Nb
Mo
Al
pH 1
-26
-450
-252
-457
-23
-48
-342
pi 3
-148
-598
-393
-222
-223
-262
-666
pH 5
-238
-369
-523
-346
-175
-218
-543
EREV*
-250
-910
-440
-1630
-1100
-200
-1660
( * fromref. [2-11] )
79
E (H+/H)
= E0
RT
[H,]
(H+/H) - iw I[H+]2.
(5.1)
The decrease of pH means the increase of the concentration of hydrogen ion in the solution.
As the concentration of hydrogen ion increases, Equation (5.1) predicts increase of
E(H+/H). If the metal dissolution behavior is not affected by pH decrease, the anodic vid
cathodic reaction lines intersect at an increased corrosion potential.
In the case of the alloys representing eta/delta phases (alloys #7 - #10), however,
the corrosion potentials at pH 5 were higher than those at pH 3. Pure elements including
chromium, niobium, molybdenum, and aluminum showed the minimum potential at pH 3
similarly as the alloys represemting the eta/delta phases. This results may be explained
from the viewpoint of oxide film formation (passivity). Chromium and aluminum are
known as strong oxide formers. These oxide are stable at medium pH levels. At lower pH
(pH 1), even in the cases of these metals, which can form oxide film easily, oxide film
dissolved into the solution, and the metal surface is in more direct contact with the aqueous
environment. Similar tendency is shown with iron [5-1]; the ferrous oxide film is
dissolved when the pH of the solution is less than 4. At higher pH (pH 5), the metals react
with oxygen in the solution, and form oxide layer.
Metals covered with oxide film act like nobler metals, and shows higher corrosion
potentials than those of the metals with bare surfaces. The formation of oxide layer
eventually increases the corrosion potentials. The similar trend of the eta/delta phase alloys
(alloy #7 -#10) suggests that these alloys may have such a transition of passivity near pH
3.
Relations between the composition of the elements and the potentials are shown in
Figures 5.1 to 5.6. It should be noted that balance of each material consists of various
combinations of other elements. However, some interesting tendencies are observed.
80
As the nickel content increases, the corrosion potential of alloys slightly increases.
This relation is clearly seen in particularly pH 3 data (see Figure 5.1). This result coincides
with the findings of Latanision and Staehle [5-21. They studied the polarization behavior of
ternary Fe-Ni-Cr alloys in sulfuric acid potentiokinetically, and found that the corrosion
potential become more noble with increasing nickel. They suggest that increasing amount
of nickel permits film formation process to passivate the surface more readily.
On the other hand, as the chromium, iron, and niobium compositions increase, the
corrosion potential decreases. In the case of chromium (Figure 5.2), the potential at pH 5
decreases only a little as chromium content increases while the potential at lower pH is
strong function of chromium composition. This tendency is also attributed to oxide film
formation. At pH 5, the alloys containing chromium form oxide layer even if chromium
composition is small, and the composition of chromium itself does not affect corrosion
potential very much. However, at lower pH, the oxide film is not stable as that at pH 5 and
the alloy surface is exposed to the solution more directly. As a result of direct contact of
alloys with the solution, the corrosion potential is strongly affected by chromium
composition at lower pH. In the cases of iron and niobium, the dependence of composition
is clearer at higher pH.
The potential of pure nickel in nickel sulfate solution was also measured at pH 3.
The results are shown in Figure 5.7. Activity was changed by changing the concentration
of nickel salfate, and calculated based on the activity coefficient in ref.[5-1J. As the activity
increases, the potential of pure nickel increases. The corrosion potential increase with an
order of activity increase was approximately 15 mV while Pourbaix diagram (Figure 2.2)
shows about 25 mV increase of reversible potential.
81
100
0
Q
0
0
.00..
0
0 -300
-400
0 00
0
...
3 02
r~
* .. ....
....
_....
0..............
) .-
400
- 00
.............
........ ..........
.... .......
o
Ecorr(pHl)
[0
Ecorr(pH 3)
0
Ecorr(pH5)
-600
0
20
60
40
80
Nickel (%)
Figure 5.1 Corrosion potentials as a function
of nickel content
82
100
100
0
Q
Ecorr(pHl)
------2-u--c--rr-p-----
.-~~Ecorr(pH3)
06 0o0
o
0
-
Ecorr(pH5)
-100
>
-200
w
-300
E
0
-400
0.
-500
05.
...
...-
.... ............................ .......
-60 0
0
20
40
60
80
Chromium (%)
Figure 5.2 Corrosion potentials as a function of
chromium contents
83
100
100
GD
0
0
Ecorr (pH5)
-100
E
w
>
-200
.................
-300
0
-400
....
-500
.....
.
-600
0
20
60
40
80
Iron(%)
Figure 5.3 Corrosion potentials as a function of
iron contents
84
100
100
:0
0
0
O
-100
0
Ecorr
(pHi)
0
Ecorr
(pH3)
0
Ecorr
(pH5)
...
.C
-200
IJ
.0
0
H
C,
w
-300
-400
)O
.0
....
...
....
......
..
.....
....
.....
....
........
...
.....
.....
....
....
.....
....
.....
..
..
...
..
.........
.....
....
.
....
....
....
....
... ...
....
....
0... ..
....
......
...
...
..
....
.
.
j..
-500
20
60
40
80
Titanium(%)
Figure 5.4 Corrosion potentials as a function of
titanium contents
85
100
100
0
-0
-200
w -300
--400
-500
1i____
-600
0
20
__
60
40
80
Niobium(%)
Figure 5.5 Corrosion potential as a function of
niobium contents
86
100
100
0
-100
>-200
E
400
o
Ecorr (pHi)
...............
~~~~~~~~.............. ..........---.--.
--- -- .-. ... -E
o r (pH3)
(pH )
1500110
Ecorr
o
-
Ecorr (pH5)
-600
0
20
60
40
80
Molvibdenum(%)
Figure 5.6 Corrosion potentials as a function of
molybdenum contents
87
100
* -f-I-.,-
S. I--
V
-0.13
-0.14 [
0
AO
O
0--%
A
CA)
Aol
-0.15
""
A
A.0
A 40
A0
C
-0.16
I
-0.17
10
.
I...i
j1
II.i
la...i
i
. i
i
aI
10 2
10
Activity
Figure 5.7 Activity dependence of nickel potential
in pH 3 nickel sulfate solution
88
I
Ii
i
1
1
5.2 Potentiodynamic polarization measurement
Potentiodynamic polarization was conducted for all ten replicated alloys in 0.05M
Na2SO4 solution whose pH was adjusted to three by H2SO4. All anodic measurements
were conducted after initial cathodic polarization scan. Anodic polarization curves are
shown in Figures 5.8 to 5.17, which plot the logarithm of the current versus the potential.
Alloys #1 to #6 (Laves phases, Figures 5.8 to 5.13) have similar typical shape of
cathodic/anodic polarization curves while there are some minor differences. All cathodic
lines start between -400 mV and -600 mV and the current increases constantly as the
potentials decrease, and the current densities are saturated between 1- 3 Ampere/cm 2 and
10-4 Ampere/cm 2. On the other hand, there is an increase in current density at the
beginning of more positive potential than corrosion potential, and the current density
reaches a maximum which is called critical current density. Then, the formation of the
passivation film gradually prevent the current flow, and reduces the current density as the
potentials increases. At very high potential (>800 mV), some of the alloys, such as alloys
#2 and #3 (Figures 5.9 and 5.10), show the increase of current density due to the breaking
of passivation film as the potential increases. This region of the curve is called transpassive
region. In the case of eta/delta phases alloys (alloy #7 to #10, Figures 5.14 to 17), the
peak of criticat current density is much smaller than those of the laves phases, and
sometimes the second peaks is seen in the anodic line. The cathodic polarization curves are
the same shape as those of the Laves phases.
From these figures, the electrochemical parameters related to the polarization
experiment , such as Tafel slope and exchange current density, were obtained. The linear
portion of the curves are the Tafel regions with the slopes given by equations (2.22) and
(2.23);
2 303R
azF
(2.22)
89
C=
(2.23)
-2.303RT
(1-a)zF
Tafel slopes were obtained by finding the linear portion in the polarization curves and
drawing the Tafel lines. Exchange current density for two reactions was obtained by
extrapolating cathodic/ anodic Tafel lines, and by finding the cross point of the two Tafel
lines. Theoretically, the potential of the cross point should be matched with tle corrosion
potential. The values were compared with the corrosion potential measured beforehand,
and used to verify the effectiveness of the experiment. Also, hydrogen overvoltage was
obtained by extrapolating the cathodic polarization line toward the positive potential side
and finding the cross point of this line with the 0 mV potential line (SHE scale). These
polarization parameters obtained from polarization experiments are summarized in Table
5.3.
The cathodic/anodic potentiodynamic polarization experiments for seven pure
metals were also conducted in the same manner as mentioned above. The polarization
curves are shown in Figures 5.18 to 24. There are much more variety of the shapes of the
anodic polarization curves in the pure metals in comparison with alloys. Nickel (Figure
5.18) has a few peaks around the critical current density. Also, nickel has no completely
passive region, and the current density keeps increasing after the passivation. In the case
of chromium (Figure 5.19), the plot shows typical unstable passivation behavior. This
unstable passivation behavior is explained by Figure 5.25, showing schematic polarization
curve. There are three cathodic processes: hydrogen evolution, oxygen reduction, and
metal cation reduction. When oxygen reduction and metal cation reduction is negligibly
small compared with hydrogen evolution, hydrogen reduction becomes a dominant
cathodic process. If hydrogen evolution in acid condition2 is represented by the line OH,
the line intersect the anodic curve at three points, I, J and K. Each point of intersection
represents equal anodic and cathodic current densities. At these three points, the current
90
density measured as a total of anodic and cathodic current density should be zero,
theoretically. Therefore, the middle peak in three peaks iii Figure 5.19 coincides with the
region between J and K in Figure 5.25, and it shows cathodic (negative) current. The
polarization curve of titanium (Figure 5.20) has the similar shape as those of the alloys, but
the current density is much lower than other cases. Niobium is relatively unstable (Figure
5.21). Molybdenum (Figure 5.22) shows the most active behavior of anodic polarization.
After the slight passivation between -400 mV to 0 mV, the current increases significantly
and reaches to 0. 1 A/cm 2 at +600 mV. After the anodic polarization to +1000 mV, the
surface of the molybdenum specimen was completely covered by dark oxide layer.
Aluminum (Figure 5.23) does not show the passive region clearly.
The electrochemical parameters were also obtained through the same procedure as
mentioned above. The parameters are listed in Table 5.4. These parameters also have
much more variety than those of the alloys listed in Table 5.3.
91
.6
U
.4)-.-
Cn .2
0
H -. 2
I-
z
.4
-
-
-. 6
-. 8
-
71'
-07
10
''''"11111
-6
10
1
11
1
1I1
~
11Ill
-5
1010
CURRENT
-4
DENSITY
1
'il
"':
-3
10
'
''''"I-
10
(A/sq-cm)
Figure 5.8 Anodic/cathodic potentiodynamic polarization curves
of alloy #1 in 0.05M Na2SO4 with pH 3 at room temperarure,
solid line: anodic polarization , dotted line: cathodic polarization
-
10
1
.8
I
,
I
,,,,,,
,
,
11 ,
,,,111,,
FIT-111
I
I 1111
III
I
1
1 11
I1
.6
U)
.4A
t0
%0
z
0-
-. 4
-. 6
N
1
-7
10
I
IaIaI1I11111
I1I1111111
10
-B
I6
**~~*****
-5
10
'''alaIII
1
~I
-4
10
CURRENT DENSITY
a
11111W
I
'-3
10
I
1
I
I
2
I I a aa
1111111
I
-2
10
(A/sq-cm)
Figure 5.9 Anodic/cathodic potentiodynamic polarization curves
of alloy #2 in 0.05M Na2SO4 with pH 3 at room temperarure
solid line: anodic polarization , dotted line: cathodic polarization
1
I
2
b mA 2 a j
11111 I
I0
-1
a.8
I
I
I
I
I I
I
Ill
I
1111151
I
I
1111111
I
1
111111
II
I
II
I
I
I
I I I II I
I IIII
I
II151 I
I
I 1 111
I
1
1111
.6
Uf)
.4
UI)
.2
0
-J
'C
z
LU
0
a_
-. 6
-.8
I
10
_7
I
I1
Itill,
I..A IiIi1 0I
I
1
t
I
1 11
I
1
10
I
I
-5
CURRENT
Ia
11
I
11
10
a
-4
DENSITY
-
I
a
a a a I I m
11I1I
-3
10
(A/sq-cm)
Figure 5.10 Anodic/cathodic potentiodynamic polarization curves
of alloy #3 in 0.05M Na2SO4 with pH 3 at room temperarure
solid line: anodic polarization , dotted line: cathodic polarization
1-2
a
A
a
1IIIlMlI
m
.
.
.
.
I
-1
10
.8
I
*
A IF
1111111
*
*
ujsrii..
I
I
I
I I I I I I
I
I
I
I M II I
1
I
I 111
IIh
,~...
IIa
, ,,,,ii
.6
Li
L~)
Li)
(I)
.4
.2
0
%0
tit
F-
z -.24
0
CL
-.6B
I I
I-----------------------------------11I
-.8
10
-7
I
I
I
III11
-5
I
I
I1I1I1I1II
-4
I
?....~.'
I
10
10
CURRENT DENSITY
I mliii
I
I
a
...-
I
-3
10
10 -2
(A/sq-cm)
Figure 5.11 Anodic/cathodic potentiodynamic polarization curves
of alloy #4 in 0.05M Na2SO4 with pH 3 at room temperarure
solid line: anodic polarization , dotted line: cathodic polarization
A
I
a
I
a
a aIAaI
11111
II
-A
10
.8
1
1
,,1
1 1, 1
,1
,n,1,11
1
1,111111
1
,
,
,1
,
1,
11,
.6
4
--
CL
-.2
0
o-7
10
-6
10
-5
10
-4
10
CURRENT DENSITY
-3-
10
10
(A/sq-cm)
Figure 5.12 Anodic/cathodic potentiodynamic polarization curves
of alloy #5 in 0.05M Na2SO4 with pH 3 at room temperarure
solid line: anodic polarization , dotted line: cathodic polarization
io
-1
.8
1
I
1 -- l
I
I
I
a 5 1 1 1a
a
I
A I lei 11
1
1
1
a
a A ma r 0
0
5
0 m 0 1 Nm
I 111
1
.
.
.
.
.
.
.
-
I I1I111[i
1
.
.
I1 11111
.6
U>
to)
.4
.2
1..
1..11..11..1
........
1
1
.
..
..
11..11..1
-J
H
-.4
0
z
0
-
-. 6
-. 8
'
10
-7
'
'''""
I"I
10
1 "II
11
-6
I1
-5
10
CURRENT
~
111111
-4
10
DENSITY
1 I fi
ll,
I
I
I
I
-3
10
(A/sq-cm)
Figure 5.13 Anodic/cathodic potentiodynamic polarization curves
of alloy #6 in 0.05M Na2SO4 with pH 3 at room temperarure
solid line: anodic polarization , dotted line: cathodic polarization
I
II"
I
I
3
3
10
-i
a.8
I
I
II
Itill
0
1
IS
1
.
1I
1 1 1 11
.. 111
I
1
III1I
I
1
-
I
I
1
I
0A.,1,
i
.
t-
I
I
I
-r--
I1I1fil
.6
U
U)
.4
t0
.2
0
Z
CL
-. 6
-. 8
I
-1
10
I
IIII
I
11 I
0iIa
I
I
II
I
I
1
I12
a
I
1
I
-b
10
-5
7
p
-4
111
I
10
10
CURRENT DENSITY
111111111
I4I11
-3
10
1
-2
10
(A/sq-cm)
Figure 5.14 Anodic/cathodic potentiodynamic polarization curves
of alloy #7 in 0.05M Na2SO4 with pH 3 at room temperarure
solid line: anodic polarization , dotted line: cathodic polarization
1
1111ff
-i
10
.8
Am
I ma
I
1
I1I1II111
I
I
II
I1111r111[
II
II
I
II iI II II II II
I
I
III
I I I i I
I 111
I
v
1
1
I
a
1
a
i
v a
1 U 11111
U -I
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l
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I
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ini
U)
U)
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a.2
0
-J
H
-.2
z
0L
-.4
a_
-.6
.8
I
-7
10
I
IIIIIII1
I
I
-Bt
10
Iil
I11 s,
I
I
I II I
I I11
I
A
I
-5
lIlSo
I
I
I
I
~
10
DENSITY
a
I I a I a
'
-3
10
CURRENT
UI
!*~I
-2
10
(A/sq-cm)
Figure 5.15 Anodic/cathodic potentiodynamic polarization curves
of alloy #8 in 0.05M Na2SO4 with pH 3 at room temperarure
solid line: anodic polarization , dotted line: cathodic polarization
I
i
U
a
I
a
a 0 m,
iLLiL
-1
10
a.8
mrrm-ni-- 1 1r1 l I I I I
F.
IlulIl...,f
liii
I
I
II
I I I I I
-~
I1
I
I
II I111111
I I I If
I
I
1 111111
I
I
II
I I I 1 11
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1iii
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mu
LI)
.4
to
.2
0
F-
0
.4
CL
.6
N
.8
I
I0
-
I
I111
a
I0
-5
aI a I
anI
a ~p. a
-..
I
-5
10
CURRENT
a
ama...
*II
10
1
£
ft
ft
-4
DENSITY
I
'
10
I
1111119
-3
10
(A/sq-cm)
Figure 5.16 Anodic/cathodic potentiodynamic polarization curves
of alloy #9 in 0.05M Na2SO4 with pH 3 at room temperarure
solid line: anodic polarization , dotted line: cathodic polarization
I
-2
1
111111
-1
I0
a8
I
I
I
I
I
a1 I a I11
11
9
11
1
~ ~
1
I
l 1 a I I I
1l
71
I
I I I II
I 1 I11111
-- T
I
1
I
1
I
1 11 1 .11
I I I I II
I
.I.I.
1
I
1.
I III1
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.
H1
I
I
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a
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a--V
I Ifill
.6
U
m)
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to
.2
Cf)
0
C
0
-. 4
EL
-. 6
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i
I
-7
10
III
I1111
I
m
m Km
-6
10
m
III
I
I
II1I1
I
1111111
J--L-JL-
I
I
I
-5
I
-3
10
CURRENT
I..r~~III
10
DENSITY
I
I
A
A I I I II
-2
10
(A/sq-cm)
Figure 5.17 Anodic/cathodic potentiodynamic polarization curves
of alloy #10 in 0.05M Na2SO4 with pH 3 at room temperarure
solid line: anodic polarization , dotted line: cathodic polarization
a
I
I
a a aI
i
I
-1
10
.8
.6
o)
to
.4-
U
.2
0-
F-
z
0
EL
.6 -
.
-7
10
-6
10
-54-3
10
10
CURRENT DENSITY
10
1
(A/sq-cm)
Figure 5.18 Anodic/cathodic potentiodynamic polarization curves
of nickel in 0.05M Na2SO4 with pH 3 at room temperarure
solid line: anodic polarization , dotted line: cathodic polarization
10
.6
.4
U
0
.j2
H.4
Z
0L
a_
.6-
-. 8|
....
-7
10
|
J
1
1119
1
-6
10
I
10
-5
II
I
I
|.11.,|
j|I
-4
10
CURRENT DENSITY
-3
10
-2
10
(A/sq-cm)
Figure 5.19 Anodic/cathodic potentiodynamic polarization curves
of chromium in 0.05M Na2SO4 with pH 3 at room temperarure
solid line: anodic polarization , dotted line: cathodic polarization
10
-1
8
Ip
0
1
I
0I 0I 1 1Iv1 lI
9I
if
1
I
I
IFM
1 1
I Ig II I
1
11 11
I1~
1111111
l
1 IIIlt
-
I-|1
ii
-nrI I I IT
.6
Li
U)
U)
Lo)
0
z
LUJ
F0
-.4
EL
-.6
-8
-
-7
10
'
-
I
L
aaiaa
a
I
_
I
-6
10
10
-5
f
I
I
a
%1
-4
10
CURRENT DENSITY
11
1
2
m a a m a aa
1
1
11
-3
10
1
-2
I0
(A/sq-cm)
Figure 5.20 Anodic/cathodic potentiodynamic polarization curves
of iron in 0.05M Na2SO4 with pH 3 at room temperarure
solid line: anodic polarization , dotted line: cathodic polarization
1
1
1
1L
f
-1
10
8
9I
1
1
aI
1111
ff 1 4111
1
1
I
1
Iif
-1 1
I I g
11111.1
l
I
I
I
11.I
1 1111
II
I
--
II
I
I
It1
1
III
A
5
0
I
I
221.
I 11 I I II
;
I
.
I
I
i1,1,1
.6
iii
u
.4
C!)
0
-J
H
H
z
uJ
H
0
0-
.4
-. 6
N
.8
I
I
I
III
I
I1
I
10
-7
10
I6
I
1
IhIllil
I
1
10
CURRENT
a I I a Aa a
1111111
10
0
A
1
a m i s 2 ma
1111111
-4
DENSITY
I
I
a
.
.
a
.
.
.
a
11111111
I0
1
I0
(A/sq-cm)
Figure 5.21 Anodic/cathodic potentiodynamic polarization curves
of titanium in 0.05M Na2SO4 with pH 3 at room temperarure
solid line: anodic polarization , dotted line: cathodic polarization
-2
I
111111 I
-I
10
08
r----I-
I I 1 1-1 fit
-,
I
I1I1I111f11
T
I---I
I
-
I
"
-
I
i
!
I I I I I
I
I
I
is
1
III
I
I
11
1!
I
I I I I I I
I
1
111
I
I
I I I I I I
I
I
I
I
aI aI IsaIm1
I
Ia
I1 I11 I1
I
I
1 11
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iii
C)
in
.4
0
-
z
0
x
aL
-. 8
L I
10
-7
I
I
II
'
11111
-6
10
l11
""
Ia
I
-5
10
CURRENT
I II
III
I
-4
10
DENSITY
III
-3
I0
I0
(A/sq-cm)
Figure 5.22 Anodic/cathodic potentiodynamic polarization curves
of niobium in 0.05M Na2SO4 with pH 3 at room temperarure
solid line: anodic polarization , dotted line: cathodic polarization
-2
Il
10
-1
.8
.6
LU
cn
.4-
to
.
O.2
0
-.
2
4i
.
z
Fv--. 4
0
CL
-. 6
-
--
-
L- iA 111111
3
10
7-6-5
10
--
1 11 i e i l i
10
CURRENT
J-4
10
DENSITY
la s
-3
10
1
il i
i Ii I1
-2
10
(A/sq-cm)
Figure 5.23 Anodic/cathodic potentiodynamic polarization curves
of molybdenum in 0.05M Na2SO4 with pH 3 at room temperarure
solid line: anodic polarization , dotted line: cathodic polarization
-I
10
.61T
.4
LU
U
.2
> 0
.2
-J
z
0
a.
.8
10
-7
1I
I I
lIttI
I
-6
10
I
I I
I
10
I~
-5-4
I
t
1111
1
I
I 1 111111
10
CURRENT DENSITY
10
11
-3
1
1
(A/sq-cm)
Figure 5.24 Anodic/cathodic potentiodynamic polarization curves
of aluminum in 0.05M Na2SO4 with pH 3 at room temperarure
solid line: anodic polarization , dotted line: cathodic polarization
-1
10
F
C
0
%%
.C
B
A
Current density (log i)
Figure 5.25 The schematic polarization curve showing
unstable passivation behavior (from ref.[2-10])
109
Table 5.3
Summary of electrochemical parameters in 0.05 M
Na2SO4 (pH3) solution measured at room temperature
Alloy
Ecoir
ICOIT
Cathodic
Anodic
iOH+/H
designation
(mVSHE)
(A/cm2)
[(V)
[(V)
(A/cn2)
#1
-240
8.6 x 10-6
-0.092
0.130
2.1 x 10-8
#2
-223
2.2 x 10-5
-0.114
0.118
2.4 x 10-7
#3
-219
9.4 x 10-6
-0.086
0.120
2.7 x 10-8
#4
-229
7.5 x 10-6
-0.077
0.150
8.0 x 10- 9
#5
-221
4.0 x 10-6
-0.062
0.105
1.1 x 1-7
#6
-320
3.1 x 10-6
-0.070
0.060
8.3 x 10-11
#7
-152
6.4 x 10-6
-0.063
0.266
2.1 x 10-8
#8
-184
6.3 x 10-6
-0.078
0.093
2.8 x 10-8
#9
-204
7.2 x 10-6
-0.080
0.173
2.0 x 10-8
#10
-215
5.0 x 10-6
-0.050
0.159
2.5 x 10-8
110
Table 5.4
Summary of electrochemical parameters of pure
metals in 0.05 M Na2SO4 (pH3) solution measured at room
temperature
Alloy
Ecorr
icorr
Cathodic
Anodic
iOH+/H
designation
(mVSHE)
(A/cm2)
@(V)
P(V)
(A/cm2)
Ni
-148
7.0 x 10-6
-0.174
0.063
9.9 x 10-7
Cr
-598
5.5 x 10-5
-0.041
0.069
1.4 x 10-19
Fe
-393
6.2 x 10-5
-0.110
0.038
1.7 x 10-8
Ti
-222
5.0 x 10-8
-0.083
0.169
1.1 x 10-10
Nb
-175
1.4 x 10-7
-0.259
0.154
3.0 x 10-8
Mo
-262
1.0 x 10-6
-0.075
0.205
3.2 x 10-10
Al
-666
5.0 x 10-6
-0.084
0.333
5.9 x 10-14
111
CHAPTER 6
DISCUSSION
It is valuable if the corrosion potentials of alloys can be determined by applying the
mixed potential theory from the corrosion potentials of pure metals obtained in the
experiment. By the comparison of the estimated and the measured corrosion potentials, a
proper method of applying the mixed potential theory to this specific alloys can be
suggested.
To illustrate the method, the simplest case is considered. In this case, the corrosion
potentials of alloys are assumed to change linearly as the composition of the alloys varies.
The relation between potential and current of pure elements is expressed in the same way as
equation (2.21);
Ea=
2.303
azF/PT
ia
log(±)
0
= a log (P)
10
= a log ia - @a log io
= a log ia + Eao,
where $a = 2.303RT/azF and Eao
(6.1)
= -
a log io. Rearranging equation (6.1) in terms of in,
.
Ea- EaO
).
ia=exp (
(6.2)
In the case of combination of two pure elements, M and N, the equations for anodic
reaction become
112
eXp(EMa - EMaO)(6.3)
PMa
iNa=exp (
ENa - ENaO
~Na
) -(6.4)
Similarly, the equations for cathode are
EMc - EMcO
iMc=exp(
PMc
ENc
-
iNc=exp-(
ENcO
PNc
),
(6.5)
).
(6.6)
When the fractions of elements M and N are expressed as fM and fN, respectively, the sum
of the fractions for binary-phase alloy is one.
fM +fN = 1.
(6.7)
The anodic Tafel equation for the combined alloy become
.EMa
iTa=fM exp (
= fM exp (
-EMao
SMa
EiMa
-
EMaO
$Ma
) + fN exp(
ENa - ENaO
)Na
PNa
)+(1 - fM) exp (
ENa - ENaO)
$Na
)-,
68
(6.8)
and similarly, the cathodic Tafel equation becomes
.EMc
iT = fM exp (
- EMcO
ENc - ENcO)
Mc
) + (J-fM )exp ().
Mc
Nc
69
(69
Equations (6.8) and (6.9) are mathematical expfession of the mixed potential theory. The
requirement for this approach to be valid is that all constituenta are solid solutioned
homogeneously and there is no significant order energy involved.
113
This method is assumed to be generalized to the multi-phase alloys in the same
way. Based on Equations (6.8) and (6.9), the behaviors of alloys #1 through #10 were
estimated. Figures 6.1to 6.10 show the results of the estimation. The thin lines from left
bottom to right top in the figures show anodic behaviors of pure metals, and the thin lines
from left top to right bottom show cathodic behavior of pure metals. While each figure is
drawn in a different scale, the thin lines show the same behavior for each pure metals.
Two thick lines indicate the total anodic/cathodic behavior of the alloy in each figure.
The electrochemical parameter obtained from the figures are compared with the
results of experiment in Table 6.1. There are about 10 to 60 mV error in corrosion
potential and about one order of magnitude error in corrosion current. The predicted
corrosion potential reproduces the tendency of the measured value fairly well. Therefore,
this method can be used as a rough estimation for electrochemical behavior of specific
multi-phase alloys. Once electrochemical parameters of pure metals in the specific aqueous
environment are obtained, behavior of a multi-phase alloy can be predicted. This
estimation is based on the atomic ratio of target composition of ten alloys (see Table 3.1).
The comparison based on the weight ratio of the target composition is shown in Table 6.2.
The estimation based on the weight ratio indicates closer value to the experimental results.
Instead of using the target composition, the estimation based on the EDX results of
most probable composition is also attempted. The area of each alloy shown in bold letters
in Table 3.2 is the most dominant area in each alloy, which is detenined by the SEM
microscopy. By using these data, corrosion potential and corrosion current are calculated.
Results are shown in Table 6.3. Although there are some deviations, the results show the
similar accuracy as.Table 6.1 and Table 6.2. These three estimations are compared in
Figure 6.11.
From these results, alloys can be divided into three groups. The first group is
alloys #1 to #4, which are made of six pure metal elements to replicate various types of the
Laves phases. The second group is alloys #5 and #6, which are also to replicate the laves
114
phases but contain only two pure metal elements respectively. The third group is alloys #7
to #10, which are to replicate eta and delta phases.
The estimation of corrosion potential for the alloys in the first group is the most
accurate among three groups. The estimations for the other two groups are not accurate as
the first group. The estimated corrosion potentials of the second and third groups are
approximately 50 mV higher than measured corrosion potential. This means that the alloys
of the second and third groups react more active than the estimation.
115
Ni;cathoc
Cr;cathoic
800..........................................-
-Fe;cathodi
Ti;cathoc
Nb;cathoic
----
----
60 . . . . . .............................................. ..............................
..............
oc to i
-
Ni;anodic
-
600
z
0
Q Q
Cr;anodi
................. .......... Fe;anodi
----------.000
-
-
-
-
-.--
5
200
.-...........Mo..anodic.
TI;anodic
Nb;anodi
Mixed;catoi
Mixed;anoi
-00
0
a.OO
- 20 0 a4-400
-
.........
1
6
0
0
io 2
Current density (A/cm2)
Figure 6.1 Application of mixed potential theory
to alloy #1 (SHE scale)
800
I-I
I-I
1 0 0 01-
Ni ;cathodi
Cr;cathoc
-...
..
...
..
..
...
..
..
...
Fe;cath
..-----
-
600
-
-
-
c
Ti;cathodi
Nb;cathodi
Mctd
Nianodic
--
Fe;anodic
w400.-......
C;anodic
Ti;anod
(/2
% 200
-*-
000200
-----------Mo..anodicPo
C
-
CA
a
.0Mixed;catoi
Mxed;anic
-200
-00
-
6 0 0-F-
1 0- 7 1 0
i o 6
1 0o5
1-- ---
0--3-r0
Current density (A/cm2)
Figure 6.2 Application of mixedNpotential theory
to alloy #2 (SHE scale)
1000
Ni ;cathodi
Cr ;cathoc
800
Fe;cathc
..................................
Ti;cathodi
Nb;cathoc
----..-
600
400
w
4000
...........-. Fe..anodic.
40 0
E
200
Mo.cathodic
Nianodic
Cr;anodic
-.....
----------------
[--"------1 [
|
J
CO---Ti;anodic
|
-
-------
-6-0....
o..
-P
--
Nb;anodic
Mo-anodic
Ni-cahodi
Mixed;ca
--- Mixed;anic
Ti-cathodic
-20
-400
-600
10-1061-1-1Nboat
Current density (A/cm2)
Figure 6.3 Application of mixed potential theory
to alloy #3 (SHE scale)
1
0
0
01
1 1 1 1 11111 1 1 1 1 11111
8 0
.
.
.
.
.....
................................
1
........................................
-
400
----
Ni ;cathoc
Cr;cathodi
ec to i
-
-Ti;cathoc
Nb;cathoic
Ni;anodic
S 400
Cr;anodc
-
....................
Feanodc
CO
Thanodic
--
...
200........
Ni-cathodi
-
Mo00..00anodic.....
Mixed;catoi
CUMixed;a
C
0
-400 ----
600-
10o-7
10-610io
5
10-
Nb;anodi
-.--
r...-
1 310-210
Current density (A/cm2)
Figure 6.4 Application of mixed potential theory
to alloy #4 (SHE scale)
1000
800
-----------..................... ...............
.
Ni ;cathodi
Nb;cathodi
N. anodic
....
..........-.
--
-
600-.
-
4........................................................................
..........
........
-rr
*to
.0.
0
0
-2
0-..
.
0 0.
-400-
110
6
10"10102
Current density (A/cm2)
Figure 6.5 Application of mixed potential theory
to alloy #5 (SHE scale)
Nb;anodic
Mixed;catdc
Medadi
i-d
a
100011111117
800
........
- -
6-0-0 ..........
x-
C4
.......................
4000
202 0
..........
.
.....
.............................................-
-..-4.
*a--------------------
10106
-5.-4.-3.-2.-1
6
10
Ti;anodic
Mixed;cathodic
ixed; ano ic
'4 0 0 ------------------
2. 0.0 . .................
Fe;cathodic
Ti;cathodic
Fe.anodic
1101021
Current density (A/cm2)
Figure 6.6 Application of mixed potential theory
to alloy #6 (SHE scale)
1000
11if
I
1111111
I II
11i
Ni ;cathodi
T*i;cathodi
800.-...........Ni;anodic
*
*
6 0 0 .
D
-
-
-
-Ti;anodic
~Mixed;catoi
ixed.anodi
M
.............-.
00
c 4400.....................................................
E
2 0 0 ....................
0
0
a.
--------------- -
- 2 0 0-------400-----...
.
.
.
.
.
.......
-6007106ij1102
1010-610-51010
10
Current density (A/cm2)
Figure 6.7 Allpication of mixed potential theory
to alloy #7 (SHE scale)
1
1000
i
t
I
I IIII
I
1I 1
Ni;cathodi
Ti ;cathodi
800.-.......
---
----
----------------' L
..
...
......
..
..
.....
..
......
.....
.....
....... ...
...
-------------600-.......-Nb;anodi
-
4 00
ianodic
Ti;anodic
-- -------
-
Mixedpcthdi
Mixed;anic
Tiaoi
-C"
" iedDto i
-- -o i
cieca
to--alloy-#---S-Eranolec
>E 200
Figur
ppliction
6.8
f mixdNpotntialtheor
-200........
- ------..........
400
-
-
1600110-1
------
......
..........................................
...........................
...............
-
10-110~
10-t1i0l2
Current density (A/cm2)
Figure 6.8 Application of mixed potential theory
to alloy #8a (SHE scale)
1
1000
;catho
Ti ;cathoic
800
-.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
..-----....-..................................
Nb;cathoc
-
Ni;anodic
---
Ti;anodic
-.
600
-Nb;anodi
-
S-
E
---
-
4 Q.. . .......................
200
Mixed;cathodic
Mixed;anodic
.-......................
CO
10
4
10---1---51-0--10-
-1
Current density (A/cm2)
Figure 6.9 Application of mixed potential theory
to alloy #9 (SHE scale)
-2-1-0
1000
800 --------
-I
Ni
-
dic
Nb;anodic
-
Mixed;cathodic.
--wMixed;anodic
200
- ------.--.
Ni...nodic
w
i1.u4
0 - --------
-200
-400
-6007 -607
1010
-6-5--
10
10
10
Current density (A/cm2)
Figure 6.10 Application of mixed potential theory
to alloy #10 (SHE scale)
10
2
1
Table 6.1
Comparison of predicted electrochemical parameters of alloys by
mixed potential theory and measured parameters in 0.05 M Na2SO4 (pH3)
solution at room temperature
Alloy
Ecorr
(measured)
designation
[mV, SHE]
icon
Ecorr
icon
(measured)
[A/cm 2]
p:-edicted)
[mV, SHE]
(predicted)
[A/cm2J
#1
-240
8.6 x 10-6
-259
1.1 x 10-6
#2
-223
2.2 x 10-5
-243
1.9 x 10-6
#3
-219
9.4 x 10-6
-238
2.5 x 10-6
#4
-229
7.5 x 10-6
-264
2.7 x 10-6
#5
-221
4.Ox 10-6
-143
1.1 x 10-6
#6
-320
3.1 x 10-6
-264
1.6 x 10-6
#7
-152
6.4 x 10-6
-126
1.8 x 10-6
#8
-184
6.3 x 10-6
-132
1.7 x 10-6
#9
-204
7.2 x 10-6
-138
1.7 x 10-6
#10
-215
5.0 x 10-6
-143
1.5 x 10-6
126
Table 6.2
Comparison of predicted electrochemical parameters of alloys by
mixed potential theory and measured parameters in 0.05 M Na2SO 4 (pl3)
solution at room temperature (based on the weight ratio of target
composition of pure elements)
Alloy
EcOT
icorr
EcoT
designation
(measured)
[mV, SHE]
(measured)
[A/cm 2J
(predicted)
[mV, SHE]
(predicted)
IA/cm 2
#1
-240
8.6 x 10-6
-234
8.7 x 1-7
#2
-223
2.2 x 10-5
-229
1.2 x 10-6
#3
-219
9.4 x 10-6
-226
1.6 x 10-6
#4
-229
7.5 x 10-6
-230
2.7 x 10-6
#5
-221
4.Ox 10-6
-141
7.8 x 10-7
#6
-320
3.1 x 10-6
-271
2.2 x 10-6
#7
-152
6.4 x 10-6
-127
2.2 x 10-6
#8
-184
6.3 x 10-6
-134
1.6 x 10-6
#9
-204
7.2 x 10-6
-140
1.2 x 10-6
#10
-215
5.Ox 10-6
-144
1.1 x 10-6
127
icwO
Table 6.3
Comparison of predicted electrochemical parameters of alloys by
mixed potential theory and measured parameters in 0.05 M Na2SO 4 (pH3)
solution at room tempera4 :re (based on the atomic ratio of composition
analyzed by EDX)
icoIT
Alloy
Ecorr
icorr
designation
(measured)
[mV, SHE]
(measured)
[A/cm 2]
Ecorr
(predicted)
[mV, SH0E
(predicted)
A/cm 2I
#1
-240
8.6 x 10-6
-246
2.1 x 10-6
#2
-223
2.2 x 10-5
-251
2.2 x 10-6
#3
-219
9.4 x 10-6
-214
1.O)x 10-6
#4
-229
7.5 x 10-6
-190
1.3 x 10-6
#5
-221
4.0 x 10-6
-146
8.4 x 10-7
#6
-320
3.1 x 10-6
-268
2.1 x 10-6
#7
-152
6.4 x 10-6
-126
2.0 x 10-6
#8
-184
6.3 x 10-6
-135
9.6 x 10-7
#9
-204
7.2 x 10-6
-133
2.2 x 10-6
#10
-215
5.0 : 106
-142
2.2 x 10-6
128
0
Ecorr(predicted by target at%)
Ecorr(predicted by target wt%)
Ecorr(predicted by EDX at/o)
Ideal Prediction
-100
.
.......
......
-......
-......
..
... ....
...
-
-150
8
E-200
-..
.~...7
/
..........
-
-
-..
-..
...
..
... ...
00
LM-20
w
-300
-350 '
-350
------ - -
-/. .........
-I
-300
-250
-200
-150
-100
Ecorr(measured, mV)
Figure 6.11 Comparison of predicted corrosion
potentials in 0.05 M Na2SO4 (pH3) solution
at room temperature
129
CHAPTER 7
CONCLUSION
Potential measurement and anodic/cathodic polarization were conducted in 0.05 M
Na2SO4 solution for seven pure metal elements (nickel, chromium, iron, titanium,
niobium, molybdenum, and aluminum) and ten alloys those were made to replicate
intermetallic phases of Ni-Cr-Fe alloys. The following major conclusions were infered
from the results of this electrochemical study.
1) By using arc melting technique, alloys were successfully fabricated to replicate
intermetallic phases of Ni-Cr-F alloys. The microstructure of alloys was analyzed by
optical microscope, SEM, and X-ray diffractometer. Although there were some
heterogeneity in alloys, a fairly uniform surface for electrochemical study was obtained.
2) Most of the alloys representing the Laves phases (except Fe2Ti) indicated the
corrosion potential between -220 mV and -240 mV in the 0.05 M Na2SO4 solution at pH 3.
The corrosion potential of the alloys representing the delta/eta phases was -150 mV to -215
mV in the same solution.
3) The corrosion potential was measured at three different pH; pH 1, pH 3, and pH
5. Generally, as the pH increases, the corrosion potential showed lower value. However,
in the case of the alloys representing the delta/eta phases, the corrosion potential at pH 5
indicated higher value than pH 3. This result was explained by formation of oxide film.
4) Relation between the chemical composition and the corrosion potential was also
investigated. As the nickel content increases, the corrosion potential showed slightly
higher value. On the other hand, as the chromium, iron, and niobium contents increase,
the corrosion potentials showed lower value,
130
5) By applying the mixed potential theory to the results of polarization and potential
measurement of pure metal elements, the electrochemical behavior of alloys was estimated.
The estimation showed the similar tendency to the results of electrochemical experiment of
actual alloys. Especially, the corrosion potential estimation for the alloys consisting of six
elements (nickel, chromium, iron, titanium, niobium, molybdenum) was close to the
measured potential with error of less than 10 mV. However, the alloys representing the
delta/eta phases predicted approximately 50 mV higher corrosion potential than the
measured potential.
6) The possibility of the application of the mixed potential theory for prediction of
electrochemical behavior of alloys, as a proximate estimation method, was indicated from
these results.
131
CHAPTER 8
SUGGESTION FOR FUTURE WORK
1. This study was done at room temperature for basic understanding. In order to
understand the behavior in the actual nuclear reactor core, it is necessary to conduct the
experiment at high temperature. The experiment at high temperature (at 288 *C) will
provide more practical results. The specimen design will be the critical factor for
success of the experiment. Since these alloys are not commercial alloys, some of them
are very brittle. It will be difficult to fabricate a specimen with complicated shape such
as the specimen designed for the experiment of commercial alloys at high temperature.
2. The fabricated alloys had some heterogeneity in chemical composition and
microstructure. It is necessary to analyze the effect of heterogeniety to electrochemical
behavior of alloys in addition to further material identification. Also, development of
material processing technique may be necessary. One possibility is heat treatment.
Proper heat treatment will be able to homogenize the material. Another possibility is
application of chemical vapor deposition (CVD) for specimen fabrication. It is possible
to make more uniform specimen surface for electrochemical study by using CVD.
3. Corrosion potential of the alloys representing the delta/eta phases showed higher value
than the predicted corrosion potential by the mixed potential theory. Further
consideration and experiment will be necessary to explain this phenomenon.
4. Yonezawa et al. suggested that grain boundary M23C6 has an orientation relationship
with either of neighboring grains. They found that the presence of semicontinuous
Type B M23C6 at the grain boundaries was associated with resistance to IGSCC in
deaerated, high temperature water, while Type A M23C6 was associated with IGSCC
susceptibility. In the electrochemical study using bulk alloys fabricated to replicate
132
intermetallic phases, it is impossible to predict the orientation relationship. In order to
obtain further comprehension, it will be necessary to consider the difference of
electrochemical behavior between bulk alloys and precipitated intermetallic phases at the
grain boundaries.
133
APPENDIX A
Ni-Cr-Fe alloy components used in the U. S. commercial LWRs
Alloy X-750 Components
Component
Condition
Environment
Remarks
Fuel assembly
holddown spring
(BAW)
AMS 5698
Exposed to PWR
coolant flow,
medium to low
neutron flux
34 fatigue-initiated failures in
six different
plants
Fuel assembly
holddown spring
(Combustion
Engineering)
AMS 5699
Exposed to PWR
coolant flow
Four springs per
assembly; over
10,000 made.
A
few failures in
early 1970s; cause
unknown
Fuel assembly
holddown spring
(General Electric)
AMS 5698C or AMS
56998
BWR coolant
No reported
failures in 15
years of
experience
CRDN springs:
Buffer
Segment arm
Belleville
AMS 5598,
Non-flowing PWR
coolant, 400'F
(204*C), medium to
Stresses normally
AMS 5698
low neutron flux
are low but become
high during a
reactor trip; no
reported failures
in over 10 years
experience
Control component
retainer spring
AMS 56998
Control component
plunger
AMS 5698C
Non-flowing PWR
coolant 600F
( 316*C) medium to
low neutron flux
No reported
Non-flowing PWR
coolant, 600F
( 316'C) medium to
No reported
low neutron flux
134
failures
failures
Alloy X-750 Components (Continued)
Condition
Component
Environment
Remarks
Power-operated
relief valve disc
spring
As supplied
PWR primary steam
at approximately
640'F (338*C),
alternate wetting
and drying
No failures, some
pitting reported
after 8 years of
service
Secondary-side
valve spring
As supplied
PWR secondary-side
coolant at 300* to
No reported
failures in over
10 years of
service
400*F (1490 to
204*C)
Baffle-to-former
bolt
Annealed at 1700'F
(927*C), machined
head, rolled
thread, aged at
1345*F (729'C) for
8 hours + 1150'F
(621*C) for 8
hours
PWR primary
coolant at 650'F
(343'C)
Highly stressed;
about 10% failure
rate (IGSCC) in
several Biblistype reactors
(KWU); 4 years of
service.
Reactor vessel
upper core barrel
bolt (B&W)
ASTM A 637, Grade
688, Type 2
PWR primary
coolant, 550* to
600'F (288* to
316'C), moderate
neutron flux
Stressed approximately 70% of the
material yield
strength; no
reported failures
in 7 years of
service
Reactor vessel
lower thermal
shield bolt (B&W)
HTH
PWR primary
coolant, 550* to
600*F (288* to
316*C), moderate
neutron flux
No reported
failures in 3
years of service
Reactor vessel
upper core barrel
bolt (foreign)
Unavailable
PWR coolant, 670*F
(354*C)
No failures in
more than 12 years
of service
135
Alloy X-750 Components (Continued)
Condi tion
Component
Environment
Remarks
Fuel assembly bolt
(foreign)
Unavailable
BWR coolant
Used since 1977 in
ASEA-ATOM BWR fuel
assemblies, four
per assembly; high
static stress;
seven failures due
to IGSCC since
1982
Control rod drive
guide tube support
pins (foreign and
domestic)
Various treatments
PWR primary
coolant; low to
moderate neutron
flux
One hundred per
plant; highly
stressed; SCC
failures in Japan,
France, USA since
1978
Jet pump beam
AH
BWR coolant
High-stressed,
IGSCC failures in
three BWR
reactors, both
foreign and
domestic, since
1979
136
Alloy A-286 Components
Remarks
Component
Condi ti on
Reactor vessel
internal bolting
(B&W)
ASTM A 453, Grade
660, Conditions A
and B
PWR primary
coolant,
moderately high
neutron flux
Highly-stressed,
0.5 to over 50%
failure depending
on the application; extensive
use; over 10 years
of service
Reactor vessel
external bolting
ASTM A 453, Grade
660, Conditions A
and B
Air
Highly-stressed;
no failures in
more than 10 years
service
Guide bar bolt
cover beams
Approximately ASTM
A 453, Grade 660,
Condition A
BWR coolant
IGSCC failures
have occurred in
ASEA-ATOM plants
since 1982; very
high stresses;
four cover beams
and 30% of the
guide bar bolts
have experienced
cracking in four
plants
Tie rod
Approximately ASTM
A 453, Grade 660,
Condition A
BWR coolant
Low stress, no
failures in
approximately 13
years of service
Fuel rod lower
spring
ASTM A 638, Grade
660, Type 1
Helium atmosphere
inside of a fuel
rod; high neutron
flux, temperature
660'F ( 316*C)
Low stresses;
about 700,000 used
to date, no
failures reported
137
Environment
Allov A-286 Components (Continued)
Component
Condition
CDRM:
Motor tube bolt
Nut closure
Screw
Bearing plate
ASTM A 453, Grade
660, Conditions A
and B
Vent valve jack
screw
AMS 5737C
Environment
Not exposed to PWR
primary coolant,
temperature
greater than 400'F
204*C), low to
moderate neutron
flux
Flowing PWR
primary coolant,
> 600'F (316'C),
low to moderate
neutron flux
138
Remarks
No failures in
over 10 years of
service; bolts
stressed to 2/3 of
material yield
strength, very low
stresses on nut
closure and
bearing plate
Stresses compressive; no reported
failures in over
10 years of
service
Alloy 718 Components
Component
Condition
Environment
Remarks
Fuel assembly
spacer grid
AMS 5596
In contact with
Zircaloy-4 and
Type 304 stainless
steel exposed to
PWR coolant flow,
high neutron flux
Approximately
30,000 used to
date; no reported
service failures
Fuel assembly
holddown spring
(coiled) (B&W)
Solution-annealed
at 1900'F
(1038'C),colddrawn, coiled and
aged at 1325'F
(718'C) for 8
Exposed to PWR
coolant flow;
medium to low
neutron flux
Exposure duration
2 years; 66 ksi
(455 MPa) torsional stress; 3
reported failures
hours + 11501F
(621*C) for 10
hours
Fuel assembly
holddown spring
(leaf)
(Westinghouse)
AMS 5596 or 5597
PWR coolant, about
660'F (316C),
medium to low
neutron flux
Medium to high
bending stresses,
no reported
failures
Component holddown
spring
Annealed, colddrawn, coiled, and
aged at 1325'F
(718C) for 8
Exposed to PWR
coolant flow
No reported
failures of
straight design
springs; 49
failures out of
396 springs
inspected of the
barrel design
springs; flowinduced vibration
(FIV) identified
as the cause of
the failures)
hours + 1150SF
(621*C) for 10
hours
139
Alloy 718 Components(Continued)
-Component
Condition
Environment
Remarks
0-ring
Solution-annealed
at 1950F (1066*C)
for 1/2 hour
maximum, aged for
3 to 16 hor.- at
1350'F (732%';
Non-flowing PWR
coolant; 570* to
650'F (299* to
343*C)
Outside surface is
si 1ver-pla ted;
plastically
deformed in
service; no
reported failures
in nearly 10 years
of operational
experience
Secondary-side
valve:
As supplied
PWR secondary-side
coolant, <500F
(260*C)
No reported
failures in over
10 years of
operational
experience
Diaphragm
Load ring
Pilot plug spring
140
APPENDIX B
X-ray diffraction analysis for lattice parameter determination
Alloy #1
Two theta
h
k
I
d spacing
37.50
1
1
0
(Angstroms)
2.399
40.84
44.24
45.00
46.46
49.66
69.88
72.12
79,64
93.56
1
1
0
0
2
1
0
2
3
0
1
0
0
0
2
0
2
1
3
2
4
4
2
3
6
0
3
2.210
2.048
2.015
1.955
1.836
1.346
1.310
1.204
1.058
h
k
I
1
2
2
2
3
0
0
0
0
1
3
0
1
2
3
d spacing
(Angstroms)
2.204
2.092
2.006
1.833
1.054
h
k
I
1
1
2
3
2
2
0
1
0
0
2
1
0
0
0
2
0
5
Alloy #2
Two theta
40.96
43.26
45.20
49.74
94.08
Alloy #3
Two theta
21.18
37.40
43.52
72.48
79.82
88.22
141
d spacing
(Angstroms)
4.195
2.405
2.080
1.304
1.202
1.108
Alloy #4
Two theta
40.90
43.38
44.34
45.12
46.06
49.70
67.62
72.36
79.98
90.66
95.66
Alloy #5
Two theta
38.48
42.18
42.46
48.28
78.10
93.24
Alloy #6
Two theta
29.40
44.30
69.92
84..50
h
k
I
1
2
1
2
0
2
3
3
1
1
4
0
0
1
2
0
2
3
3
1
1
4
3
0
2
1
4
2
0
2
4
7
0
h
k
I
1
2
1
2
1
2
0
0
1
0
0
2
3
0
2
2
7
4
h
k
I
1
0
3
2
0
0
0
1
2
4
2
5
142
d spacing
(Angstroms)
2.207
2.086
2.043
2.010
1.971
1.835
1.386
1.306
1.200
1.084
1.040
d spacing
(Angstroms)
2.340
2.143
2.129
1.885
1.224
1.062
d spacing
(Angstroms)
3.038
2.045
1.346
1.147
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