Microstructure and Corrosion Behaviour of Aerospace Aluminium Alloys

369 Pages·2016·29.2 MB·English

by  Xinxin Zhang

Microstructure and Corrosion Behaviour
of Aerospace Aluminium Alloys

A thesis submitted to The University of Manchester for the
Degree of Doctor of Philosophy
in the Faculty of Engineering and Physical Sciences

2015

XINXIN ZHANG

School of Materials
Corrosion and Protection Centre
Table of Contents

Table of Contents ........................................................................................ 2
List of Figures .............................................................................................. 5
List of Tables ............................................................................................. 16
Abstract ...................................................................................................... 17
Declaration ................................................................................................. 18
Copyright Statement ................................................................................. 19
Acknowledgements .................................................................................... 20
1 Introduction ......................................................................................... 22
2 Literature Review ............................................................................... 25
2.1 Introduction ...................................................................................................... 25
2.2 Microstructure of aluminium alloys ................................................................. 26
2.2.1 Microstructure of Al-Cu-Mg alloys .......................................................... 27
2.2.2 Microstructure of Al-Cu-Li alloys ............................................................ 32
2.3 Corrosion behaviour of aluminium alloys ........................................................ 36
2.3.1 Corrosion behaviour of Al-Cu-Mg alloys ................................................. 36
2.3.2 Corrosion behaviour of Al-Cu-Li alloys ................................................... 56
3 Experimental Methods ....................................................................... 62
3.1 Materials ........................................................................................................... 62
3.1.1 AA2024-T351 aluminium alloy ................................................................ 62
3.1.2 2A97 aluminium alloys ............................................................................. 62
3.2 Specimen preparation ....................................................................................... 62
3.2.1 Mechanical polishing ................................................................................ 62
3.2.2 Electropolishing ........................................................................................ 63
3.2.3 Alkaline etching ........................................................................................ 63
3.2.4 Ultramicrotomy ......................................................................................... 64
3.2.5 GDOS etching ........................................................................................... 64
3.2.6 Focused ion beam ...................................................................................... 65
3.3 Testing .............................................................................................................. 65
2
3.3.1 Immersion testing ...................................................................................... 65
3.3.2 Electrochemical measurement .................................................................. 66
3.4 Characterization ................................................................................................ 67
3.4.1 Optical microscopy ................................................................................... 67
3.4.2 Scanning electron microscopy .................................................................. 67
3.4.3 Transmission electron microscopy ............................................................ 68
4 Microstructure Characterization of AA2024-T351 and 2A97
Aluminium Alloys...................................................................................... 74
4.1 Microstructure of AA2024-T351 aluminium alloy .......................................... 74
4.1.1 Constituent intermetallic particles............................................................. 74
4.1.2 Dispersoids and precipitates ...................................................................... 81
4.1.3 Crystallographic structure and stored energy ............................................ 83
4.1.4 Discussion ................................................................................................. 86
4.2 Microstructure of 2A97 aluminium alloys ....................................................... 90
4.2.1 Constituent intermetallic particles............................................................. 90
4.2.2 Crystallographic structure ......................................................................... 95
4.2.3 Dispersoids ................................................................................................ 99
4.2.4 The distribution of precipitates ............................................................... 100
4.2.5 Discussion ............................................................................................... 104
4.3 Conclusions .................................................................................................... 110
5 Corrosion Behaviour of AA2024-T351 Aluminium Alloy ............ 157
5.1 Corrosion behaviour of constituent intermetallic particles ............................ 157
5.1.1 General observation ................................................................................ 158
5.1.2 Corrosion behaviour of individual intermetallic particle ........................ 160
5.1.3 Corrosion behaviour of clustered intermetallic particles ........................ 168
5.1.4 Discussion ............................................................................................... 170
5.2 Stable localized corrosion............................................................................... 179
5.2.1 Electrochemical behaviour ...................................................................... 179
5.2.2 Intergranular corrosion ............................................................................ 181
5.2.3 Crystallographic corrosion ...................................................................... 194
5.2.4 Discussion ............................................................................................... 200
5.3 Conclusions .................................................................................................... 215
5.3.1 Corrosion behaviour of constituent intermetallic particles ..................... 215
3
5.3.2 Intergranular corrosion ............................................................................ 216
5.3.3 Crystallographic corrosion ...................................................................... 216
6 Corrosion Behaviour of 2A97 Aluminium Alloys .......................... 258
6.1 Corrosion behaviour of constituent intermetallic particles ............................ 258
6.1.1 General observation after 5 h immersion ................................................ 258
6.1.2 General observation after 20 h immersion .............................................. 260
6.2 Localized corrosion in 2A97 Al-Cu-Li alloys ................................................ 263
6.2.1 Electrochemical behaviour ...................................................................... 263
6.2.2 Localized corrosion in cold worked alloy ............................................... 264
6.2.3 Localized corrosion in naturally aged alloy ............................................ 269
6.2.4 Localized corrosion in artificially aged alloy.......................................... 276
6.2.5 Localized corrosion in cold worked and artificially aged alloy .............. 283
6.3 Discussion ...................................................................................................... 290
6.3.1 The influence of thermomechanical history on localized corrosion ....... 290
6.3.2 Corrosion propagation ............................................................................. 298
6.3.3 The influence of grain-stored energy on localized corrosion .................. 305
6.4 Conclusions .................................................................................................... 311
7 Conclusions and Suggestions for Further Work ........................... 351
7.1 Conclusions .................................................................................................... 351
7.1.1 AA2024-T351 aluminium alloy .............................................................. 351
7.1.2 2A97 aluminium alloys ........................................................................... 353
7.2 Suggestions for further work .......................................................................... 356
References ................................................................................................ 358

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List of Figures
Figure 3.1: Schematic diagram of the equipment set-up during electropolishing process.
......................................................................................................................................... 70
Figure 3.2: Schematic diagrams showing the sectioning process of ultramicrotomy (a)
Trimming process by glass knives, (b) Sectioning process by the diamond knife. ........ 71
Figure 3.3: Schematic diagram of the electrochemical cell for potentiodynamic
polarization measurement in a de-aerated testing solution. ............................................ 72
Figure 4.1: Scanning electron micrographs of AA2024-T351 aluminium alloy surface (a)
Mechanical polishing with 1 µm diamond paste, (b) OPS polishing. ........................... 112
Figure 4.2: Typical EDX spectra of constituent intermetallic particles (a) S-phase, (b) θ-
phase, (c) α-phase without silicon, (d) α-phase with silicon. ........................................ 113
Figure 4.3: Typical Kikuchi patterns along with the corresponding indexed results of
constituent intermetallic particles (a)-(b) S-phase, (c)-(d) θ-phase, (e) α-phase. .......... 114
Figure 4.4: Backscattered electron micrographs of different existence forms of S-phase
and θ-phase intermetallic particles in AA2024-T351 aluminium alloy. ....................... 115
Figure 4.5: Typical S-phase and θ-phase intermetallic particle cluster (a) Backscattered
electron micrograph, (b) Secondary electron micrograph, (c)-(d) Kikuchi pattern with its
corresponding indexed result of θ-phase at point 1, (e)-(f) Kikuchi pattern with its
corresponding indexed result of S-phase at point 2. ..................................................... 116
Figure 4.6: Backscattered electron micrographs of typical intermetallic particle clusters
of S-phase and θ-phase particles in AA2024-T351 aluminium alloy. .......................... 117
Figure 4.7: (a) Scanning electron micrograph of a typical intermetallic particle cluster,
(b) EDX maps, in the sequence of Al, Cu, Fe, Mn, Si and Mg..................................... 118
Figure 4.8: (a) Scanning electron micrograph of a typical intermetallic particle cluster,
(b) EDX maps, in the sequence of Al, Cu, Mn, Si, Mg and Fe..................................... 119
Figure 4.9: Backscattered electron micrographs of typical α-phase intermetallic particles
on the surface of AA2024-T351 aluminium alloy, existing individually and in cluster.
....................................................................................................................................... 120
Figure 4.10: (a) Typical scanning electron micrograph of individual α-phase
intermetallic particles, (b) Corresponding EDX spectrum from point 1 in (a). ............ 121
Figure 4.11: Backscattered electron micrographs of typical α-phase intermetallic
particle clusters in AA2024-T351 aluminium alloy...................................................... 122
Figure 4.12: (a) Scanning electron micrograph of a typical α-phase intermetallic particle
cluster with well-defined shell-core structure, (b) EDX maps, in the sequence of Al, Cu,
Mn, Si, Fe and Mg. ....................................................................................................... 123
Figure 4.13: (a) Scanning electron micrograph of a typical α-phase intermetallic particle
cluster without well-defined shell-core structure, (b) EDX maps, in the sequence of Al,
Cu, Mg, Fe, Mn and Si. ................................................................................................. 124
Figure 4.14: (a) HAADF micrograph of a typical region in the twin-jet electropolished
AA2024-T351 aluminium alloy, (b)-(d) EDX spectra from points 1-3 in (a). ............. 125
Figure 4.15: Transmission electron micrographs of the twin-jet electropolished AA2024
aluminium alloy (a) Intermetallic particle cluster of S-phase and θ-phase, (b) Framed
5
area in (a) with increased magnification, (c) Corresponding diffraction pattern, (d)
Individual θ-phase intermetallic particle, (e) Framed area in (d) at increased
magnification, (f) Corresponding diffraction pattern, (g)-(h) Other typical linear
crystallographic defects in θ-phase intermetallic particles. .......................................... 127
Figure 4.16: (a) Transmission electron micrograph of a typical α-phase intermetallic
particle, (b) Increased magnification of framed area in (a). .......................................... 128
Figure 4.17: (a)-(b) Bright field TEM micrographs of AA2024-T351 aluminium alloy,
(c) EDX spectrum of rod-like dispersoid, (d) EDX spectrum of round-shaped dispersoid.
....................................................................................................................................... 129
Figure 4.18: Typical scanning electron micrographs of dispersoid free zone around (a)-
(b) Individual S-phase / θ-phase intermetallic particles, (c)-(d) Multi-phase particle and
intermetallic particle cluster of S-phase and θ-phase. ................................................... 130
Figure 4.19: (a)-(b) Bright field and dark field STEM micrographs of a triple grain
boundary joint, (c)-(d) Typical EDX line-scan results along line 1 and line 2 in (a), (e)-
(f) Bright field TEM micrograph of a relatively clean grain boundary and the
corresponding EDX line-scan result, (g)-(h) HAADF micrograph of grain boundary and
the corresponding EDX line-scan result, revealing the presence of Mg segregation, (i)-(j)
HAADF micrograph of grain boundary and the corresponding EDX line-scan result,
revealing the presence of Cu and Mg segregations. ...................................................... 132
Figure 4.20: (a) Crystallographic orientation distribution map in Euler’s colours, (b)
Misorientation distribution map, (c) Grain boundary misorientation distribution map, (d)
Histogram of grain boundary misorientation distribution, (e) Grain-stored energy
distribution map in grey scale, with yellow lines applied to represent HAGBs, (f) The
distribution of Schmidt factors in AA2024-T351 aluminium alloy. ............................. 133
Figure 4.21: Scanning electron micrographs of (a) 2A97-T3, (b) 2A97-T4, (c) 2A97-T6
and (d) 2A97-T8 aluminium alloys after mechanical polishing to 1 µm. ..................... 134
Figure 4.22: Scanning electron micrographs and EDX spectra in 2A97-T3 aluminium
alloy (a)-(d) Typical intermetallic particles in 2A97-T3 aluminium alloy in different
existing forms, (e)-(f) Typical EDX spectra of intermetallic particles in 2A97-T3
aluminium alloy. ........................................................................................................... 135
Figure 4.23: Scanning electron micrographs of intermetallic particles in 2A97-T3
aluminium alloy and corresponding EDX maps (a)-(b) Typical intermetallic particles in
2A97-T3 alloy along with the corresponding EDX maps, in the sequence of Al, Cu, Fe,
Mn, Ag, Mg and Zn, (c)-(d) Another typical intermetallic particles in 2A97-T3 alloy
along with the corresponding EDX maps, in the sequence of Al, Cu, Fe, Mn, Ag, Zn
and Mg. ......................................................................................................................... 137
Figure 4.24: Scanning electron micrographs and EDX spectra in 2A97-T4 aluminium
alloy (a)-(d) Typical intermetallic particles in 2A97-T4 aluminium alloy in different
existing forms, (e)-(f) Typical EDX spectra of intermetallic particles in 2A97-T4
aluminium alloy. ........................................................................................................... 138
Figure 4.25: EDX maps of intermetallic particles in 2A97-T4 aluminium alloy (a) EDX
maps of intermetallic particles shown in Figure 4.24 (c), in the sequence of Al, Cu, Fe,
Mn, Ag, Mg, Ba and Zn, (b) EDX maps of intermetallic particles shown in Figure 4.24
(d), in the sequence of Al, Cu, Fe, Mn, Ag, Mg, Ba and Zn. ........................................ 139
6
Figure 4.26: Scanning electron micrographs and EDX spectra in 2A97-T6 aluminium
alloy (a)-(b) Typical intermetallic particles in 2A97-T6 aluminium alloy in different
existing forms, (c)-(d) Typical EDX spectra of intermetallic particles in 2A97-T6
aluminium alloy. ........................................................................................................... 140
Figure 4.27: Scanning electron micrograph of intermetallic particles in 2A97-T6
aluminium alloy and EDX maps (a) EDX maps for intermetallic particles shown in
Figure 4.26 (b), in the sequence of Al, Cu, Fe, Mn, Zn, Ag, Ca and Mg, (b) Typical
intermetallic particles in 2A97-T6 aluminium alloy, (c) EDX maps for intermetallic
particles shown in (b), in the sequence of Al, Cu, Fe, Mn, Mg, Ag, Ca and Zn. .......... 141
Figure 4.28: Scanning electron micrographs and EDX spectra in 2A97-T8 aluminium
alloy (a)-(b) Typical intermetallic particles in 2A97-T8 aluminium alloy in different
existing forms, (c)-(d) Typical EDX spectra of intermetallic particles in 2A97-T8
aluminium alloy. ........................................................................................................... 142
Figure 4.29: EDX maps of intermetallic particles in 2A97-T8 aluminium alloy (a) EDX
maps for the intermetallic particles shown in Figure 4.28 (a), in the sequence of Al, Ag,
Cu, Mg, Mn, Fe and Zn, (b) EDX maps for the intermetallic particles shown in Figure
4.28 (b), in the sequence of Al, Ag, Cu, Mg, Mn, Fe and Zn. ...................................... 143
Figure 4.30: Backscattered electron micrographs, revealing the grain orientation
distribution in (a) 2A97-T3, (b) 2A97-T4, (c) 2A97-T6 and (d) 2A97-T8 aluminium
alloys. ............................................................................................................................ 144
Figure 4.31: EBSD analysis of 2A97-T3 aluminium alloy (a) IPFZ map, (b) Grain
boundary misorientation distribution map with black lines and grey lines representing
HAGBs and LAGBs, (c) Histogram of grain boundary misorientation distribution in
2A97-T3 aluminium alloy, (d) Grain-stored energy distribution in grey scale. ........... 145
Figure 4.32: EBSD analysis of 2A97-T8 aluminium alloy (a) IPFZ map, (b) Grain
boundary misorientation distribution map with black lines and grey lines representing
HAGBs and LAGBs, (c) Histogram of grain boundary misorientation distribution in
2A97-T8 aluminium alloy, (d) Grain-stored energy distribution in grey scale. ........... 146
Figure 4.33: EBSD analysis of 2A97-T4 aluminium alloy (a) IPFZ map, (b) Grain
boundary misorientation distribution map with black lines and grey lines representing
HAGBs and LAGBs, (c) Histogram of grain boundary misorientation distribution in
2A97-T4 aluminium alloy, (d) Grain-stored energy distribution in grey scale. ........... 147
Figure 4.34: EBSD analysis of 2A97-T6 aluminium alloy (a) IPFZ map, (b) Grain
boundary misorientation distribution map with black lines and grey lines representing
HAGBs and LAGBs, (c) Histogram of grain boundary misorientation distribution in
2A97-T6 aluminium alloy, (d) Grain-stored energy distribution in grey scale. ........... 148
Figure 4.35: HAADF micrographs of 2A97 Al-Cu-Li alloys, revealing the distribution
of dispersoids in (a) T3 alloy, (b) T4 alloy, (c) T6 alloy, (d) T8 alloy and typical EDX
spectra of the dispersoids: (e) Al-Cu-Mn dispersoid and (f) Al-Zr dispersoid. ............ 149
Figure 4.36: (a) Ultra-high resolution SEM micrograph, (b)-(d) HAADF micrographs at
different magnifications of 2A97-T3 alloy. .................................................................. 150
Figure 4.37: (a) Ultra-high resolution SEM micrograph of a grain boundary junction in
2A97-T4 alloy, (b) Grain boundary A-A in (a) at higher magnification, (c)-(d) Bright
field TEM micrographs of typical grain boundaries in 2A97-T4 alloy. ....................... 151
7
Figure 4.38: HAADF micrographs of 2A97-T4 alloy, revealing the heterogeneous
distribution of precipitates in the alloy matrix. ............................................................. 152
Figure 4.39: (a) Bright field TEM micrograph of 2A97-T6 alloy, (b) HAADF
micrograph of 2A97-T6 alloy, (c) High resolution TEM micrograph of the needle-
shaped precipitate, with FFT result inset, (d) HAADF micrograph, revealing the
heterogeneous distribution of precipitates in the 2A97-T6 alloy matrix. ..................... 153
Figure 4.40: (a) HAADF micrograph of 2A97-T6 alloy, (b) Bright field TEM
micrograph of a typical grain boundary in 2A97-T6 alloy, (c)-(d) Ultra-high resolution
SEM micrographs of 2A97-T6 alloy. ........................................................................... 154
Figure 4.41: (a) Bright field TEM micrograph of 2A97-T8 alloy, (b) High resolution
TEM micrograph of the needle-shaped precipitate, with FFT result inset, (c) Ultra-high
resolution SEM micrograph, (d)-(e) Bright field TEM micrographs of a typical grain
boundary at different tilting angles, (f) HAADF micrograph of a triple joint of grain
boundaries in 2A97-T8 alloy. ....................................................................................... 155
Figure 5.1: Scanning electron micrographs of AA2024-T351 aluminium alloy surface (a)
General view before the immersion, (b) General view after the immersion for 30 min,
(c)-(d) Stable localized corrosion site of marked area in (b) at increased magnification.
....................................................................................................................................... 218
Figure 5.2: Scanning electron micrographs and corresponding EDX spectra (a) General
view before the immersion, (b) General view after the immersion for 30 min, (c) EDX
spectrum before the immersion of particle A, (d) EDX spectrum before the immersion
of particle B, (e) High resolution SEM micrograph after the immersion of particle A, (f)
High resolution SEM micrograph after the immersion of particle B, (g) EDX spectrum
after the immersion of particle A, (h) EDX spectrum after the immersion of particle B.
....................................................................................................................................... 220
Figure 5.3: Scanning electron micrographs of a typical S-phase particle after 30 min
immersion in a 3.5 wt. % NaCl solution (a) Secondary electron micrograph, (b)
Backscattered electron micrograph, (c)-(d) Typical areas in the S-phase particle at
higher magnifications. ................................................................................................... 222
Figure 5.4: Transmission electron micrographs and corresponding diffraction pattern of
an ultramicrotomed foil of AA2024-T351 alloy after the immersion in a 3.5 wt. % NaCl
solution for 1 minute (a) General view, (b)-(c) At increased magnifications, (d)
Diffraction pattern of the de-alloyed area in the S-phase particle. ............................... 223
Figure 5.5: (a)-(b) Scanning electron micrographs of a typical θ-phase particle after the
immersion in a 3.5 wt. % NaCl solution for 30 min followed by GDOS to clean the
alloy surface after the immersion, (c)-(d) Scanning electron micrographs of typical θ-
phase particles after the immersion for 2 hours. ........................................................... 224
Figure 5.6: Scanning electron micrographs of typical de-alloyed θ-phase particles with
banding structures (a)-(b) A severely de-alloyed θ-phase particle at different
magnifications, (c)-(d) Typical θ-phase particles with mild de-alloying, (e)-(f) Cross
sections of a typical de-alloyed θ-phase particle generated using focussed ion beam. . 225
Figure 5.7: Transmission electron micrographs and corresponding diffraction pattern of
a de-alloyed θ-phase particle (a) General view, (b) Corresponding diffraction pattern
with indexed results, (c) Increased magnification, (d) High resolution TEM micrograph
8
displaying the linear defects, inset: corresponding FFT result, in consistent with
diffraction pattern. ......................................................................................................... 226
Figure 5.8: (a) HAADF micrograph of a de-alloyed θ-phase particle, (b) HAADF
micrograph of typical ligaments in the de-alloyed θ-phase particle, with the
corresponding EDX copper map inset, (c) High resolution HAADF micrograph of the
linear crystallographic defects marked in (a), with the corresponding FFT results inset,
revealing the crystallographic orientation, (d) HAADF micrograph of the typical
ligaments in the de-alloyed θ-phase particle, with the orientations marked. ................ 227
Figure 5.9: Scanning electron micrographs of Al-Cu-Fe-Mn intermetallic particles after
the immersion in a 3.5 wt. % NaCl solution for (a) 2 hours, (b) 24 hours. ................... 228
Figure 5.10: Typical Al-Cu-Fe-Mn intermetallic particles after the immersion for 24
hours (a)-(b) SEM micrographs of a typical Al-Cu-Fe-Mn intermetallic particle, (c)-(d)
SEM micrographs of another typical Al-Cu-Fe-Mn intermetallic particle, (e) EDX maps
of the Al-Cu-Fe-Mn intermetallic particle shown in (c)-(d), in the sequence of Al, Cu,
Fe, Mn, Mg and O. ........................................................................................................ 229
Figure 5.11: Scanning electron micrographs of Al-Cu-Fe-Mn intermetallic particles
after 24 hours immersion with gentle polishing to remove the corrosion product layer
(a)-(b) Typical Al-Cu-Fe-Mn intermetallic particles with linear de-alloyed bands, (c)-(d)
A typical Al-Cu-Fe-Mn intermetallic particle with linear de-alloyed bands at different
magnifications. .............................................................................................................. 230
Figure 5.12: (a) Scanning electron micrograph of a typical intermetallic particle cluster
prior to the immersion testing, (b)-(e) EDX spectra corresponding to points 1-4 shown
in (a), (f)-(i) Kikuchi patterns along with the corresponding indexed results from point 1
and point 3, (j) Scanning electron micrograph of the intermetallic particle cluster after
the immersion for 30 min. ............................................................................................. 231
Figure 5.13: (a) Scanning electron micrograph of the intermetallic particle cluster prior
to the immersion testing, (b)-(d) EDX spectra corresponding to points 1-3 shown in (a),
(e)-(g) Kikuchi patterns along with the corresponding indexed results from points 1-3 in
shown in (a), (h) Scanning electron micrograph of the intermetallic particle cluster after
30 min immersion, (i)-(j) EDX spectra of point 1 and point 3 shown in (h). ............... 234
Figure 5.14: Scanning electron micrographs of intermetallic particle clusters after 2
hours immersion in a 3.5 wt. % NaCl solution (a) General view, (b)-(d) Typical clusters.
....................................................................................................................................... 235
Figure 5.15: (a) Scanning electron micrograph of an attacked intermetallic particle
cluster after 2 hours immersion in a 3.5 wt. % NaCl solution, (b) EDX maps of the
intermetallic particle cluster shown in (a), in the sequence of Al, Cu, O, Mn, Fe and Mg.
....................................................................................................................................... 236
Figure 5.16: Electrochemical measurement of AA2024-T351 aluminium alloy in a 3.5
wt. % NaCl solution (a) Voltage-current density curve during the anodic polarization,
Scanning electron micrographs of the AA2024-T351 aluminium alloy after anodic
polarization, (b) Plan-view, (c)-(e) Cross section at various magnifications. ............... 237
Figure 5.17: Optical micrographs of AA2024-T351 aluminum alloy at different stages
during the immersion testing (a) At the beginning of the immersion in a 3.5 wt. % NaCl
solution, (b) After 30 min immersion in a 3.5 wt. % NaCl solution, (c) Moving to
9
deionized water after 30 min immersion in NaCl solution, (d) After 30 min immersion
in deionized water. ........................................................................................................ 238
Figure 5.18: Scanning electron micrographs of typical stable localized corrosion sites in
AA2024-T351 aluminium alloy after the immersion in a 3.5 wt. % NaCl solution for 30
min followed by the immersion in deionized water for another 30 min (a) Plan-view, (b)
Tilted 30°, (c) Cross section with an attacked intermetallic particle intersected the alloy
surface, (d) Attacked intermetallic particle in (c) at higher magnification; Other typical
cross sections after the identical immersion testing, (e) Cross section with an attacked
intermetallic particle buried beneath alloy surface, (f) Cross section with no attacked
intermetallic particle beneath alloy surface. .................................................................. 239
Figure 5.19: Scanning electron micrographs of typical cross sections at stable localized
corrosion sites in AA2024-T351 aluminium alloy after the immersion in a 3.5 wt. %
NaCl solution for 30 min. ............................................................................................. 240
Figure 5.20: Scanning electron micrographs of typical cross sections at stable localized
corrosion sites in AA2024-T351 aluminium alloy after the immersion in a 3.5 wt. %
NaCl solution for 60 min. ............................................................................................. 240
Figure 5.21: 3D volumetric reconstruction of a typical stable localized corrosion site of
AA2024-T351 aluminium alloy after the immersion in a 3.5 wt. % NaCl solution
followed by deionized water, with selective transparency applied to alloy matrix, and
red and yellow applied to represent intermetallic particles and corrosion product,
respectively. .................................................................................................................. 241
Figure 5.22: Scanning electron micrographs of AA2024 alloy surface (a)-(c) After the
immersion in a 3.5 wt. % NaCl solution for 40 min followed by the immersion in 3.5
wt. % NaCl solution with 10-4 M copper chloride as addition for another 40 min, (d)-(f)
After the immersion in a 3.5 wt. % NaCl solution for 40 min followed by the immersion
in deionized water with 10-4 M copper chloride as addition for another 40 min, (g)-(j)
After the immersion in a 3.5 wt. % NaCl solution for 40 min followed by the immersion
in deionized water for another 40 min, then the specimen was immersed in 0.01 M
copper chloride solution for 5 seconds. ........................................................................ 242
Figure 5.23: TEM analysis of an intergranular corrosion filament in AA2024-T351
alloy after the immersion in a 3.5 wt. % NaCl solution for 30 min followed by deionized
water for another 30 min (a) Bright field TEM micrograph, (b) HAADF micrograph,
(c)-(d) EDX spectra from point 1, point 2 and point 3 in (b), (e) EDX line-scan result
along line 1 in (b), (f) The copper EELS map of the corrosion front shown in (b), (g)
High resolution TEM micrograph. Inset: fast Fourier transform of the framed region,
indicating Al Cu phase. ................................................................................................. 244
2
Figure 5.24: Typical HAADF micrographs of attacked grain boundaries along with the
intergranular corrosion front in AA2024-T351 aluminium alloy after the immersion in a
3.5 wt. % NaCl solution for (a)-(b) 1 hour, (c)-(d) 8 hours. ......................................... 245
Figure 5.25: (a) HAADF micrograph of AA2024-T351 aluminium alloy after alkaline
etching, (b) Cathodic polarization curves for as-received and alkaline-etched AA2024-
T351 alloy in de-aerated 3.5 wt. % NaCl (with HCl to pH=3) testing solution with a
scanning rate of 1 mV/s. ............................................................................................... 246
10