STRUCTURE AND KINETICS OF GLASS CORROSION
BY
DAVID M. SANDERS
A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL
OF THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1973
ACKNOWLEDGEMENTS
The author thanks R. Condrate for sparking an interest
in materials spectroscopy while at Alfred. At the Univer-
sity of Florida, the author became indebted to W. B. Person
for the use of his spectrometers and for aid in the inter-
pretation of many of the results of this work related to
infrared spectroscopy. The author is especially indebted
to his advisor, L. L. Hench, who provided invaluable
encouragement, advice, and assistance throughout the entire
study. Finally, the author wishes to thank his wife,
Marilyn, without whose help and encouragement this research
would not have been possible.
This work was supported in part by the Office of Naval
Research and in part by the Glass Container Corporation.
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS . . . . . . ... iii
LIST OF TABLES. .. . . . . . .. vi
LIST OF FIGURES . . . . . . . ... vii
ABSTRACT . . . . . . . .... . xii
Chapter
I INTRODUCTION . . . . . . . 1
II METHODS FOR STUDYING GLASS CORROSION
KINETICS . . . . . . . . 10
Infrared Reflection . . . . .. 10
Continuous Atomic Emission . . .. 12
The Optical System . . . . .. 12
The Minirig System . . . ... 17
The Automated System . . . ... 24
Continuous Atomic Emission . . .. 33
III QUANTITATIVE ANALYSIS OF GLASS
STRUCTURE USING INFRARED REFLECTION
SPECTRA . . . . . . . .. 37
Methods . . . ... . . . . 38
Results . . . . . . ... 40
Discussion of Results . . . .. 49
Conclusions . ... . . . . 68
IV ANALYSIS OF SILICA-RICH GELS FORMED
DURING GLASS CORROSION . . . . .. 70
Methods . . . . . . . .. 76
Results . . . . . . . . 78
Discussion . . . . . ... 93
Conclusions . . . . . . . 101
TABLE OF CONTENTS (continued)
Chapter Page
V CORROSION OF A LTTHIA-SILICA GLASS
IN VARIOUS AQUEOUS ENVIRONMENTS ... .103
Methods . . .. . . . . 104
Results . . .... . . . 105
Discussion . . .... . . . 117
Conclusions ...... .. . . . 131
VI SURFACE ROUGHNESS EFFECTS ON GLASS
CORROSION . ... . . . . . 133
Results . . . . . . . . 135
Discussion of Results . . . .. 152
Conclusions ..... . . . 154
VII CONCLUSIONS AND NEW STUDIES . . .. 156
BIBLIOGRAPHY . ... . . . . . . 162
BIOGRAPHICAL SKETCH . . . .... . . 167
LIST OF TABLES
Table Page
1 Examples of General Corrosion Conditions
and Sample States Which Might Influence
Corrosion Behavior . . . . . . 5
2 X-ray Analysis of Corroded and Auto-
claved 33L Glass . . . . . . 114
LIST OF FIGURES
Figure Page
1 The ray diagram for the optical system
used in reflection studies made using the
expanded sample compartment of the Perkin-
Elmer model 621 spectrometer . . . 14
2 Sample-holding device ("minirig") shown
mounted in aluminum holder, which is in
turn permanently fixed to the Plexiglas
mounting block . . . . . . .. 16
3 Minirig sample holder showing a sample
mountedd under stress being applied
through the loading bars . . . . 19
4 Infrared reflection spectra of a cor-
roded and of a freshly abraded glass of
a composition corresponding to lithium
disilicate . . . . . . ... 21
5 Corrosion cells used in this study as
described in the text . . . ... 23
6 "Automated system" showing one sample
rotated from the corrosion bath position
into the position for reflectance
measurement . . .... . . 26
7 Reflectance at 985 cm-I of a lithium
disilicate glass (A) as a [unction of
time, compared to that from a standard
composed of non-corroding vitreous
silica .. . . . . . . . 30
8 Schematic drawing of atomic emission
sampling techniques used in this study . 34
9 Compositional dependent changes in
infrared reflection spectra in the
lithia-silica system .. . . . .. 42
10 Compositional dependent changes in
infrared reflection spectra in the
soda-silica system . . . . ... 44
LIST OF FIGURES (continued)
Figure Page
11 Compositional dependent changes in
infrared reflection spectra in the
potash-silica system . . . . ... 46
12 A comparison of infrared spectra of
50 mole % Na2O-Si02 glasses . . .. 48
13 The reflectance of the silicon-oxygen
stretching maximum as a function of
composition for the three systems
studied . . . . . . . ... 51
14 The reflectance of the silicon-oxygen
rocking maximum as a function of compo-
sition for the three systems studied . 52
15 The reflectance of the silicon non-bridging
oxygen vibration associated with alkali
ion as a function of composition .... 53
16 The comparison between the recorded and
calculated spectra of phase separation
in a 25 mole % Li20-SiO2 glass as
described in the text ......... 56
17 Spectra of the silicon-oxygen rocking
peak for the three systems studied
showing reflectance due to lithium vibra-
tion (L) as described in the text . . 62
18 Plot of the difference in reflectance at
520 cm-1 between the spectrum of vitreous
silica and those of lithia-silica glasses. 64
19 Infrared spectrum of a 50 mole % SiO2
glass . . . . . . . ... 67
20 Two simplified models of the composition
profile of the silica-rich corrosion
layer. . . . . . . . .. 74
21 An example of data obtained from solu-
tion analysis of glass corrosion and
its relation to the calculated quantities
a, E, and B. . . . . . . ... 80
viii
LIST OF FIGURES (continued)
Figure Page
22 Changes in infrared reflection spectra
of 33L glass on exposure to static
water at 40C . . . . . ... 82
23 A comparison of the corroded and non-
corroded glass spectra in the soda-
silica system . . . . . ... 84
24 A summary of the plots of E and a cal-
culated from all the solution analysis
data reported in this work . . ... 86
25 An S.E.M. micrograph of the edge of the
corroded layer of 33L glass exposed to
static water for 350 minutes at 79.50C 88
26 A composition profile of the corroded
33L glass film obtained with an electron
microprobe . . . . . . ... 91
27 A comparison of normalized infrared
reflectance plots with normalized e
behavior for corrosion of 33L at 400C 92
28 A comparison of normalized infrared
reflectance plots with normalized c
behavior for corrosion of 33L at 79.50C 94
29 Plots of the various reaction coordi-
nates described in this work to obtain
apparent activation energies . . ... 96
30 Changes in the infrared spectrum of
33L glass with exposure to 100% relative
humidity at room temperature . . ... .107
31 Comparison of the extent of possible
reaction determined by the reflectance
of the stretching (S) and non-bridging
oxygen (NSL) peaks of 33L glass
exposed to both 0.9 ml of static
liquid water and 100% relative humidity 110
32 Infrared spectra of 33L glass given
the crystallization treatments shown . 112
LIST OF FIGURES Continued)
Figure Page
33 Changes in the infrared reflection
spectra of 33L glass on exposure to
85% relative humidity .. . .... 116
34 Comparison of the extent of reaction,
determined by the magnitude of the
stretching peak reflectance (S), for
33L glass exposed to water under static
and flowing conditions . . . ...119
35 Changes in infrared reflection spectra
upon exposure of 33L to water contain-
ing corrosion cells of different volumes
and to the residual water present in a
desiccator filled with Drierite . .. .121
36 Changes in infrared reflection spectra
upon exposure of 33L glass to hydro-
chloric acid, to a basic solution used
to corrode 33L powder, and to hydro-
fluoric acid 123
37 S.E.M. micrographs of 33L glass having
the following corrosion histories:
A. freshly abraded with dry 600 grit SiC
B. 4.5 hours with 0.9 ml static water
C. 120 hours with 0.9 ml static water
D. 216 hours with 0.9 ml static water
E. 195 hours with 100% R.H.
F. 239 hours with 100% R.H.
The corrosion temperature was 23.5C
and all surfaces were initially abraded
with dry 600 grit SiC . . . ... 125
38 Variation of B and concentration of
dissolved SiO2 with corrosion time for
33L glass abraded with dry 120, 320,
and 600 grit SiC . . . .. . . 137
39 Variation of E with corrosion time for
33L glass abraded with dry 120, 320,
and 600 grit SiC . . . . . . 140
40 Variation of a with corrosion time for
33L glass abraded with dry 120, 320,
and 600 grit SiC . . . . . . 142
LIST OF FIGURES (continued)
Figure Page
41 Variation of pH with corrosion time
for 33L glass abraded with dry 120,
320, and 600 grit SiC . . . ... 144
42 S.E.M. micrographs of freshly abraded
and corroded 33L glass. The initial
surface was abraded with dry 600 grit
SiC and the corrosion treatment was
with static water at 79.50C in a
0.9 ml cell . . . . . . ... 147
43 S.E.M. micrographs of freshly abraded
and corroded 33L. The initial surface
was abraded with dry 320 grit SiC and
the corrosion treatment was with static
water at 79.5C in a 0.9 ml cell ... 149
44 S.E.M. micrographs of freshly abraded
and corroded 33L glass. The initial
surface was abraded with dry 120 grit
SiC and the corrosion treatment was
with static water at 79.5C in a
0.9 ml cell . . . . . . . 151
Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
STRUCTURE AND KINETICS OF GLASS CORROSION
By
David M. Sanders
June 1973
Chairman: L. L. Hench
Major Department: Materials Science and Engineering
A general procedure is established for studying the
effects of chemical reactions on the structure of glass
surfaces. This work is designed to serve as a basis for
future studies relating corroded glass structures with
mechanical behavior, biological applications, and the
design of amorphous materials having greater chemical resis-
tance.
To accomplish these goals, a systems approach is used
to establish the variables required to completely specify
a corrosion reaction. New techniques including infrared
reflection spectroscopy are introduced and a theoretical
basis provided for interpretation of results obtained from
these techniques. In contrast to most previous work in
glass corrosion, a variety of techniques are used to observe
both morphological and chemical changes in the glass surface
during exposure. Corrosion conditions systematically varied
include pH, solution composition, relative humidity, solu-
tion replenishment, temperature, and pressure. The effect
of surface roughness is also studied. Details of glass
corrosion mechanisms are discussed and related to the
effects of sample state and corrosion condition on result-
ing corroded structures.
The infrared reflection techniques developed are sensi-
tive measures of both silica and alkali concentrations of
the gel formed on the glass surface by the corrosion process.
These concentrations, used in conjunction with new parameters
obtained from solution data, define a composition profile
which determines future corrosion behavior of glasses. It
is shown that infrared reflectance is proportional to the
concentration of the reflection species. This result has
general application in the use of reflection spectroscopy
for analytical as well as for structural studies. Both
glass surface roughness and the corrosion condition used
are seen to strongly influence the chemical and morphological
nature of the corroded surface in addition to affecting the
kinetics of the corrosion process.
xiii
CHAPTER I
INTRODUCTION
Reactions of silicate glass surfaces with aqueous solu-
tions are important as an engineering design parameter for
many applications. The mechanical strength,(1) optical
properties,(2) surface electrical conductivity, and
resistance to structural deterioration are all influenced
by the nature of these reactions. In addition, liquids
whose beneficial properties are destroyed by the release
of ions are frequently stored in glass containers. One
example of a problem area which has occurred in the past is
the leaching of certain lead glass glazes by acidic fruit
juices causing lead poisoning; another is the destruction
of human blood by defective glass containers. (2
Reactions of glasses with aqueous solutions can also
have beneficial effects. Recently a calcium and phosphate
containing glass(4) was discovered which becomes chemically
bonded to living tissues, promising to make possible the
design of a new generation of rejection-free prosthetic
devices. This behavior is intimately connected with chemical
reactions between the glass surface and its surroundings.
In addition, certain corrosion reactions increase resistance
to further attack of the glass. (2) Related reactions have
been proposed to lead to the formation of low index of
refraction antireflection films,(3) although other methods
are normally used commercially to produce these films.
Finally, the soluble silicate cement industry and the deter-
gent industry depend on an intimate knowledge of the condi-
tions under which binary silicates dissolve.(5)
The traditional approaches used to study the corrosion
kinetics of glasses have been based either on the rate at
which ions from the glass are taken into solution(6-8) or
on the rate of change of corrosion film thickness, surface
roughness, or some other property associated with the bulk
material. 9-13) The first approach is exemplified by the
work of Rana and Douglas, (13) who chose to follow the corro-
sion reactions of a series of binary glass compositions with
water by measuring changes in both the alkali and the silicon
concentrations in the corrosion media. These workers fre-
quently used powdered samples in order to increase the sur-
face area exposed to the corrosion media, thereby increasing
the sensitivity of their measurements. One of the most
informative studies using the second approach was made by
Charles,(14) who found that superheated stean and water
cause layers of corroded glass to form on soda lime glass.
These layers are of sufficient thickness that they could
easily be measured on sectioned samples.
Some of the difficulties encountered by workers using
the first approach are related to the need for sufficient
ion concentration in the corrosion solution for accurate
measurement. The use of powders and grains to increase the
surface area of the corroding glass complicates the inter-
pretation of the results obtained when applying these results
to the bulk systems frequently encountered in normal use.
Also, using powders and grains as the starting material
makes it difficult to take surface roughness of the freshly
prepared glass into account. In addition, the need for a
solution for analysis eliminates the possibility of studying
the corrosion of glass surfaces by gas phase reactants such
as water vapor.
Both conventional approaches share additional drawbacks.
One of the most serious is the lack of information concern-
ing the chemical makeup of the corroded glass surface. The
techniques used to measure film thickness based on optical
interference or microscopic inspection do not yield this
information at all and the solution analysis experiments
only supply indirect evidence. As a result, both the dis-
tribution and bonding of the atoms at the corroded glass
surface have remained open to speculation.(2) Another diffi-
culty with all the approaches used previously is the use of
single techniques for corrosion studies. It is unusual in
the corrosion literature for more than one tool to be
applied to a corrosion problem. As a result, much of the
careful solution analysis data was carried out using simple
model glasses, (15) most of the film thickness measurement
using a wide variety of commercial glasses, 3) and the
B.E.T. pore structure analysis using phase separated boro-
silicate glasses.16) In addition, there is scant informa-
tion about the effect of the corrosion condition adopted
and the sample state chosen on the resulting corrosion
reactions. Finally, no workers, to the author's knowledge,
have applied a systems approach to determine the variables
which must be controlled and explored to completely under-
stand the corrosion process of even one glass composition.
An ideal corrosion study would involve the use of
multiple experimental approaches to obtain complete infor-
mation about the effect of all possible corrosion variables
on all possible resulting corrosion structures and kinetics.
Some of the corrosion conditions requiring specification are
listed in Table 1. The size, shape and distribution of
microstructures and chemical species produced in the corroded
glass surface, as well as the concentration of the ions
released into the corrosion solution, are all needed to
specify the resulting corrosion structures and kinetics.
At the initiation of this research it was found that
there was no satisfactory experimental tool in current use
to obtain direct information about the concentration and
bonding of atoms present in the corroded glass surface.
Early results by Pfund(17) and later results by Anderson(18)
Table 1
Examples of General Corrosion Conditions and
Sample States Which Might Influence Corrosion Behavior
Corrosion Conditions
Ratio of glass surface to corrosion solution volume
Replenishment rate
Solution volume
Flow patterns at glass surface
Initial solution composition
Initial solution pH
Relative humidity exposure
SO2 exposure
SnC14 exposure
Temperature
Pressure
Sample States
Phase separation
Cords
Initial composition of glass
Roughness
Stress
Prior corrosion
suggested that infrared reflection spectroscopy might pro-
vide such a tool. Since the infrared absorbtion spectra
contain information about the concentration, position, and
strength of chemical bonds,119) and since reflectance spectra
can be related to absorbtion spectra,(20) it follows that it
should be possible to obtain the same information from
reflection spectroscopy. Consequently, suitable techniques
were developed in this research to enable infrared spectros-
copy to be applied to kinetic and structural studies of
glass surfaces. These techniques are discussed in detail
in Chapter II.
Also of interest is the initial stage of reaction of
glass surfaces with water. Since corrosion reactions can be
very rapid in the first few minutes of exposure compared to
later corrosion times, it was necessary to develop automated
techniques to monitor these early stages to see if there was
any unusual behavior. One of the techniques developed was
based on atomic emission spectroscopy, while the other was
based on infrared reElection spectroscopy. These techniques
are also discussed in Chapter II.
Chapter III describes the relationships between infra-
red reflection spectra and glass structure. Reflection
spectra were recorded for a series of glasses to determine
spectral trends as a function of glass composition. Several
observations are made about the structure of silicate based
glasses, independent of corrosion responses. As with all
succeeding chapters, the literature pertinent to this chapter
is reviewed there. The effect of glass corrosion is only
mentioned in passing in Chapter II[, but that work forms the
basis for all the infrared related corrosion studies which
follow. In Chapter IV the structural relations developed
in Chapter III are applied to the glass corrosion problem in
a systematic manner.
In addition to the new techniques developed to supply
specific information about the corrosion process, the more
conventional solution and microscopy techniques were also
applied where appropriate. At the onset it was clear that
it would not be possible to explore all of the variables
possible and several of these variables were held constant
throughout the entire study in order to explore the others.
One of these variables was the composition of the starting
glass which was chosen to be 33 mole % lithia and 67 mole %
silica (33L). This particular composition was chosen because
it was found to have an intermediate chemical resistance
compared to a variety of glass compositions prepared in an
exploratory study. Also, the glass was not phase separated
and its glassy structure has been studied extensively.(21)
One additional glass composition, 22 mole % soda and 78
mole % silica, was chosen for a preliminary comparison and
its behavior was found to be qualitatively similar as dis-
cussed in Chapter IV.
Another variable held constant throughout the study
was the bulk planar surface of the glass to be corroded as
discussed in Chapter II. This surface was prepared by
abrading the glass with dry silicon carbide paper, a treat-
ment which is thought to produce scratches similar to those
produced during normal use.(22) With these constraints,
some of the other possible corrosion parameters listed in
Table 1 were explored and the results discussed in Chapters
V and VI.
In Chapter V various modes of corrosion are explored
using the techniques developed earlier. Various corrosion
conditions are chosen to be models for actual exposure
environments and, while the Full analysis described in Chap-
ter IV is not possible, it is possible to project results
From that analysis to the more realistic cases found in
Chapter V. This trend toward applications is continued in
Chapter VI, where the effect of surface roughness on the
corrosion behavior is explored. This is of practical inter-
est because glass in most uses becomes scratched. Therefore
surface roughness must be studied to relate experimental
results to actual corrosion problems.
While no attempt was made to explore the silicate glass
corrosion problem in its entirety, an attempt was made to
define what would be required to make such a study. Then
specific areas of the total problem were chosen to form a
foundation upon which a more complete study could be based.
9
Rather than varying glass composition as many previous
(23)
workers have done,23) composition was held constant and
both the corrosion conditions and surface roughness were
varied in ways designed to model actual systems. The use
of the new techniques developed in this work in combination
with techniques used more conventionally in glass corrosion
studies is quite general and should be applicable to other
glass compositions and corrosion conditions.
CHAPTER II
METHODS FOR STUDYING GLASS CORROSION KINETICS
Infrared Reflection
Actual reflection can be described in terms of two
idealized components. In the first (specular reflection)
all the reflected light leaves an ideally smooth surface
at an angle equal to the angle of incidence; in the second
(diffuse reflection) the light is reflected from an ideally
(24)
matte surface with equal intensity in all directions.
Because of the manner in which samples were prepared (to be
described in a later section), the spectra obtained in this
study are largely specular in nature with a very small dif-
fuse component. This can be demonstrated experimentally
by comparing the reflection spectrum of a glass sample
polished to 600 grit with the reflection spectrum of the
same sample polished to 1/4v diamond paste. The resulting
spectra have peak positions that are identical, with the
reflectance of the maxima for the two spectra differing by
less than 0.8% because of light scattering in the 600 grit
sample.
General discussions of the reasons for resorting to
infrared specular reflection as opposed to infrared
absorption were given by Harrick 25 and Simon. (26)
Briefly, there are three primary reasons why the reflection
method is especially useful in corrosion studies. The most
obvious is that with strongly absorbing materials the
reflection method eliminates the necessity of preparing very
thin samples. In addition, the area of investigation is
limited to a depth of penetration determined by the large
extinction coefficient of silica-based glasses in the wave-
length region studied. Using an equation given by Born and
Wolf,(27) this depth at normal incidence is estimated to be
on the order of 0.5p at 1,000 cm-. As one increases this
angle, approaching grazing incidence, the penetration depth
decreases to a very small fraction of this value. A logical
extension of the present work is to use a variable angle of
incidence in order to vary this depth of penetration and to
increase the specular component reflected from rougher sur-
faces. 28) Thirdly, the spectra, as in the case of absorp-
tion spectra, are characteristic of the vibrations oE cer-
tain bonds. Thus the reflection spectra can be used both
to determine the rate of reactions taking place in the
reacting layer and, at the same tine, to provide information
about the structural changes that take place as a result of
these reactions. Also, whenever there may be an advantage
in interpretation, reflection spectra can be converted to
absorption spectra via methods discussed in detail by Simon
and others. (29,30)
Continuous Atomic Emission
Atomic emission spectroscopy has been used by many
investigators to study the amount of alkali ion corroded
from powdered glass samples as a function of time. A
recent review of these investigations was given by Das.(31)
In contrast to most of the work done in this area, the
samples used in this study were in bulk form. The planar
surfaces were ground with silicon carbide paper to simulate
the abrading effect of normal use on the glass surfaces.
Atomic emission measures the concentration of alkali
ion taken into solution, which is a direct reaction coordi-
nate for one phase of the corrosion process. Understanding
the alkali ion loss rate aids in the interpretation of the
changes which occur in the infrared reflection spectra due
to corrosion. Also, the use of continuous or semicontinuous
atomic emission sampling, described later in this chapter,
allows one to see subtle changes in the early stages of the
corrosion process that are characteristic of the state of
the surface before it begins to corrode to be compared
with the corresponding changes in the infrared reflection.
The Optical System
The optical system used for the infrared reflection
study in a Perkin-Elmer 621 spectrometer with an expanded
sample compartment is shown in Fig. 1. It consists of a
Fig. 1. The ray diagram for the optical system used in reflection studies made
using the expanded sanple compartment of the Perkin-Elmer model 621
spectrometer.
reference
f pj,^^cavity
E C..
S.----'oI of 4 toroid
E m sample
Mirrors
double beam configuration of mirrors that focus a source
image onto the sample and reference surfaces with a reduc-
tion in image size by approximately four times. The angle
of reflection, 6, was fixed at 250 in this system.
It was soon discovered that special means were needed
to permit repositioning of the front surfaces of the glass
samples at the exact point of focus after each corrosion
treatment. Two alternative experimental designs have been
employed for this purpose -- one providing greater precision
(especially in rapid reactions), the other providing more
versatility. This positioning was accomplished by milling
a cavity into the plate supporting the sample beam optics.
This cavity accepts identical Plexiglas mounting blocks
(Fig. 2) for the different positioning systems.
The reference beam has a holder similar to the one in
Fig. 2 to accept interchangeable sample-holding devices,
called "minirigs." In most of the experiments, a plane
front-surfaced aluminum mirror was placed in a minirig
which in turn was placed in the reference beam position
(Fig. 1). However, it was at times desirable to place a
glass in the minirig in the reference beam in place of the
mirror in order to take "difference" spectra.
.Fi. 2. Samnle-holding device ("iniirig") shown mounted
in aluminum holder, vwhicn is in turn permanently
fixed to the PLexiglas mounting block.
The Minirig System
Further detail of the minirig sample holder is shown
in Fig. 3. The holder is designed to hold a glass plate
under various loads to determine the effects of strain on
the corrosion process. The load is applied using the set
screw located in the back of the minirig, and the level of
strain in the glass is determined using standard stress-
optical methods. In the experiments described in this
work, howev-r, the minirig was used without the loading
roids and therefore served simply as a means of holding the
sample glass and reference glass or mirror in the infrared
b -.; between corrosion treatments. The results of the study
of the effects of stress on corrosion will be reported in
a waterr wor.. The minirigs, in conjunction with the
"'liniiig holder" show-n in Fig. 2, comprise what will be
termed the "minirig system."
As an example of the application of this minirig system
we shoe in Fig. 4 two sample reflection spectra obtained
from a glass initially of lithium disilicate composition
(33 mole % Li20-67 mole % Si02). The detailed assignment
of the peaks will be discussed in a later work, but one can
see the extent of spectral change which can be expected.(32)
Briefly, the sharp reflection peak Forming at the higher
frequency (1,090 cm- ) upon corrosion is attributed to a
transformation oF a part of the reactive layer to a
Fig. 3. Minirig sample holder showing a sample mounted under stress being
applied through the loading bars.
FRONT VIEW OF MINIRIG
WITHOUT LOADING FIXTURE
li
o I
4
Fig. 4. Infrared reflection spectra of a corroded and of a freshly abraded glass
of a composition corresponding to lithium disilicate. The freshly
abraded sample was prepared by grinding the glass surface with dry sili-
con carbide paper. The corroded sample was prepared by subjecting the
surface of a freshly abraded glass sample to distilled water in the
corrosion cell described in Fig. 5A for 24 hours at 230C.
0
0
L? O
0 0 0 0 0
0 0 0 0 0
j -..freshiy
[ .Q abraded
WV 0 0E en co
WAVE NUMBER (CM-')
configuration more closely resembling vitreous silica,
while the peak at 910 cm-1 is attributed to a lithia-rich
compound. The spectrum of a sample subjected to corrosion
for a short time can be changed to that of the freshly
polished sample by several 6-inch strokes over 600 grit
silicon carbide paper, confirming the statement above that
the reflection spectrum is characteristic of a surface
layer less than a few microns thick. For the over-all study,
many pairs of spectra similar to those in Fig. 4 were re-
corded under differing experimental conditions. Each pair
consisted of a spectrum of a corroded glass sample and
reference spectrum of the same glass sample before corro-
sion, recorded consecutively.
The corrosion cells employed in the minirig system are
shown in Fig. 5. The cell labeled A, used with corrosive
still media, is a Teflon block with a cavity containing the
medium. Figure SB shows how this cell is clamped against
the glass sample during the corrosion treatment. Only a
known and reproducible portion of the glass surface (deter-
mined by cavity shape) is exposed to the corrosive medium,
which can be replenished at predetermined time increments
if desired.
The continuous flow cell, shown in Fig. SC, consists
of a Teflon block with two perpendicular holes. Two mini-
rigs containing glass samples that have had identical pre-
vious treatments are pressed against the cell as shown in
A.
I I-
I I
I I
teflon still cell
B.
C.
-II
from to collection
flow-e- system
meter
teflon flow cell
D.
teflon finger
teflon finger
Fig. 5. Corrosion cells used in this study as
described in the text.
Fig. 5D. A controlled flow of water or other corrosive
fluid is then introduced through the tube to the sample
surfaces as indicated. The fluid can be collected from the
outflow tube for studies in atomic emission spectroscopy
using procedures described below. After a specified length
of corrosion time, the minirig is removed from the cell and
mounted in the spectrometer to record the spectrum.
The Automated System
It is sometimes desirable to compare the spectral
changes with time occurring upon corrosion for several sam-
ples subjected to different applied stresses, or subjected
to different surface treatments. Also, the kinetics of the
rapid corrosion reactions that occur at elevated tempera-
tures are of interest. A system that permits such studies
is pictured in Fig. 6 and is termed the "automated system"
in the following discussion.
The automated system consists of a "Ferris-wheel" type
of arrangement (Fig. 6) where the glasses under study are
alternately subjected to a corrosion treatment, dried, and
placed in the sample beam of the spectrometer automatically.
This is accomplished by means of the rotor which holds up
to four minirigs. The rotor is mounted in a Plexiglas block
of dimensions allowing it to be placed snugly in the cavity
of the optical bench in Fig. 1. The rotor is driven at
1 rpm by a Hurst synchronous motor.
Fig. 6. "Automated system" showing one sample rotated from the corrosion bath
position into the position for reflectance measurement.
POSITION
RECORDING
FOR
SPECTRA
DRYING
POSITION
CORROSION
POSITION
1
motor
driven
axle
plexiglas
mounting
block
'ihen a glass sample in the minirig is covered by the
corrosive medium in the "down" (or corrosion) position, the
corrosion reaction occurs for a small increment of time,
controlled by the angle t in Fig. 6. This angle can be
varied with the different modes of corrosion to be dis-
cussed shortly. As the minirig rotates to the drying posi-
tion, the corrosion reaction stops as the glass is dried
(and cooled) by blowing a stream of dry nitrogen or air
onto the surface. When the minirig reaches the "up" (or
recording) position, the intensity of reflection at a par-
ticular wavenumber is recorded by the spectrometer. Blinds
or flags on the minirigs block the infrared beam when the
rotor is not positioned so that one of the ninirigs is
exactly vertical, in order to eliminate stray reflections
from the ninirig holders.
The wavenumbers chosen for the reflectance study are
held constant during the entire kineLic study, so that there
is a maximum change in reflectance from the sample for the
particular reaction under study. This wavenumber is chosen
prior to any kinetic study after examining the spectra from
a corroded specimen and from a freshly polished sample oF
the same material (as shown in Fig. 4). The spectrometer
gain is adjusted so that when the minirig reaches the "up"
position the pen rises to its maximum with an "overshoot"
(refer to PE manual) and then Calls to zero when the minirig
goes out of the recording position. The chart speed is
adjusted so that these maxima occur next to one another to
result in a record of intensity as a function of time.
-I
Results are shown in Fig. 7 for the reflectance at 985 cm-
from an experimental lithium disilicate glass in a minirig
mounted on the rotor opposite a minirig containing a stan-
dard glass, in this case vitreous silica. This particular
wavenumber (985 cm-1) was selected because the variation in
the reflectance was relatively large after very short per-
iods of time, as seen in Fig. 4.
The relatively constant intensity spikes in Fig. 7
indicate the reflectance from vitreous silica, the standard
chosen because it does not corrode on exposure to water.
This set serves the twofold purpose of marking the exact
time of recording and of indicating the noise level of the
spectrometer. The spikes which monotonically decrease in
height form the corrosion envelope for the experimental
sample of lithium disilicate glass. The changes in reflec-
tance are amplified by using a scale expansion of five
times, and, as shown in Fig. 7, the noise is still at an
acceptable level. For the spectrum shown in Fig. 7, the
sample was corroded by water at approximately 900C flowing
through a Teflon tube, as described later. The actual cor-
rosion time added on each revolution was 10 seconds, indi-
cating that the corrosion causing the changes occurred in
less than 5 minutes, causing a drop in absolute reflectance
-1
Fig. 7. Reflectance at 985 cm of a lithium disilicate glass (A) as a function
of time, compared to that from a standard composed of non-corroding
vitreous silica. This corrosion curve was obtained using the automated
system described in the text and shown in Fig. 6. The reflectance
scale is expanded five times with respect to that shown in Fig. 4.
LU
(.)
0 7
F-
LUI VITREOUS
J (6 SILICA STANDARD
CL A
LU
5 10 15 20 25 30
MEASUREMENT TIME
IN MINUTES
intensity of roughly 1%. The fluctuation in the absolute
reflectance of the standard is seen to be less than 0.15%.
This illustration shows both the advantages and dis-
advantages of the technique. It is clearly most useful for
studies of the detailed structures of kinetic curves of
very rapid reactions. The fact that it took 30 minutes to
obtain 5 minutes of reaction time makes the method less
desirable for studying reactions that take greater than
1 h. By increasing the angle of the arc (C in Fig. 6),
the length of time the sample is exposed to the corroding
fluid can be increased to about 20 seconds of each revolu-
tion. However, other methods to be described shortly are
more appropriate for kinetic studies of slower reactions.
If all of the positions in the rotor of Fig. 6 are
filled, reflectance measurements from four different sam-
ples are recorded each revolution. One of the four minirigs
always contains a standard glass for reference, chosen so
that its reflectance at the wavenumber in question is nearly
identical with that of the glasses of interest and so that
its reflectance does not change when subjected to the cor-
rosive medium.
Several possible modes of corrosion of the glass sam-
ples can be chosen depending on the conditions required.
In the example cited in Fig. 7, a Teflon tube shown in
Fig. 5E was held with its end just touching the glass sur-
face of the minirig in the "down" position. As the rotor
revolved, the glass surface was exposed for roughly 10
seconds to water from the tube and then to the blast of air
that dried the surface prior to the reflectance measurement.
This configuration is useful for several reasons. It allows
the study of the effect of flow rates of the corrosive media
from 0.05 ml/min to more than 10 ml/min. The cell area is
identical with the other corrosion cells described in Fig.
5 Small increments of corrosion are possible because the
corrosion time for each cycle of the rotor is small.
One of the disadvantages of the automated system is
that the spectrum is monitored at only one wavenumber for
the entire experiment. This disadvantage is not serious if
peak frequencies do not shift during the corrosion process,
or if the extent of shift is known. Another disadvantage
is that the automated system ties up the infrared spectrom-
eter for the duration of the corrosion experiment.
For slightly slower reactions, the cell shown in Fig.
6 is used in a somewhat different mode. The entire spec-
trum of the untreated glass sample is recorded first. The
rotor is then turned manually so that the sample dips into
the corrosion solution. After an accurately determined
corrosion time, the rotor is again turned quickly to place
the sample in the infrared beam, the glass is wiped dry,
and the reflection spectrum of the corroded sample is then
recorded. This procedure is repeated at appropriate inter-
vals to establish a set of reflection spectra as a function
of corrosion time. From these spectra one can plot reflec-
tances at individual wavenumbers as a function of time.
Continuous Atomic Emission
The last two procedures for studying corrosion to be
described here involve modifications of standard atomic
emission techniques, using the Teflon cell shown in Fig.
5B A schematic drawing of both techniques is shown in
Fig. 8. In the first procedure (option 1), the corrosion
fluid is drawn by the suction produced by the atomizer of
the atomic emission machine. The fluid is pulled succes-
sively through a flow meter, a metering valve, and finally
through the corrosion cell before reaching the instrument.
The whole sample assembly is submerged in a water bath with
temperature controlled to 0.10C. The concentration of
analyte is recorded continuously on a chart recorder con-
nected to the photomultiplier output.
In this experiment it is necessary to monitor and con-
trol the flow rate carefully since it controls the amount
of dilution experienced by a given quantity of lithium ion
leached from those surfaces, and the flow into the atomizer
influences the intensity of emission due to a given concen-
tration of analyte. It has been found experimentally that
fluctuations must be controlled to about 1% of the flow
rate to avoid these difficulties. A continuously variable
to collection
system
Corrosion cell
shown in detail
in figure 5 D.
TEMPERATURE BATH
COLLECTION
SYSTEM
option I
VARIABLE FLOW
NEBULIZER (AUTOMATED
r OPTION)
from corrosion cell
option
from corrosion
cell
2 (semi-aulomated)
0 microbeoker
S CH holder
SYNCHRONOUS MOTOR
Fig. 8. Schematic drawing of atomic emission sampling
techniques used in this study.
atomizer and a micrometer valve provide the necessary con-
trol. Since all of the corrosion solution is analyzed
continuously, this method is particularly useful for rapid
reactions in their initial stages.
For longer periods of time and slower reaction rates a
second technique is used. This involves a positive pressure
head established by an elevated fluid supply with automatic
collection of the corrosion microbeakers (Fig. 8, option 2).
The microbeakers are rotated by a synchronous motor under
the outlet from the corrosion cell at a rate determined by
the flow rate chosen for the corrosion treatment. Each
beaker collects a sample of the analyte that is an average
of the solution concentrations over the time required for
collection. If the concentration is changing very rapidly,
the previous technique is to be preferred; if not, the
technique involving microbeakers provides certain advan-
tages.
The use of this second option in Fig. 8 allows a
blank, in most cases distilled water, to be aspirated
between the measurement of each microbeaker solution, in
order to help eliminate the errors due to machine drift.
Also, solutions flowing at rates too slow to be measured
directly using the technique involving option 1 of Fig. 8
can be collected in microbeakers, thus alleviating some-
what the problem of flow rate sensitivity discussed earlier.
Finally, this second option permits one to analyze for more
36
than one element, and to measure pHl and electrical conduc-
tivity of each solution. Thus, as is the case with the
infrared techniques, the automatic measurement of corrosion
is best suited to short experiments while the semiautomatic
method is more versatile.
CHAPTER III
QUANTITATIVE ANALYSIS OF GLASS STRUCTURE
USING INFRARED REFLECTION SPECTRA
Infrared spectroscopy has been used extensively in the
study of glass largely because it is sensitive to the local
positioning of atoms and the strength of the chemical bonds
between them.(19) An excellent review article by Simon(29)
shows that traditional emphasis has been on the study of
hydroxyl groups in glass. However, there is considerable
current interest in quantitative studies of the vibrations
of the atoms making up the glassy network, as reviewed by
Wong.(33) This shift in emphasis has been encouraged by the
introduction of infrared spectrometers having greater spec-
tral wavelength ranges(34) and by the widespread use of
laser Raman spectrometers permitting the study of vibrations
not seen in infrared. (35) Recent theoretical studies by
Bell and Dean,(36) and Gaskell(37) greatly facilitate the
interpretation of spectra from simple silicate glasses having
compositions corresponding to stoichiometric crystalline
compounds.
The first objective of this chapter is to introduce the
use of plots of reflectance vs. composition for the analysis
of the concentrations of specific vibrational species present
in three series of binary silicate glasses. It will be
demonstrated experimentally that reflectance is proportional
to the concentration of the vibrational species causing it.
Breaks in this linear behavior can be explained in terms
of abrupt changes in the nature of the species involved.
A second objective is the re-examination of the spectra
of the binary lithia-silica, soda-silica, and potash-silica
glass systems to include the silicon-oxygen rocking vibra-
tions. The spectrometers used by earlier workers who inves-
tigated these systems did not have sufficient wavelength
range to observe these peaks.(33)
in addition, it has been found that infrared reflection
spectra from glass surfaces are extremely sensitive to the
attack of the surface by water vapor and aqueous solutions
as discussed later. Therefore, a third objective of this
study is to examine the influence of various preparation
conditions on the infrared spectra of binary alkali silicate
glasses.
Methods
The glasses, except for vitreous silica and the 10 mole %
lithia composition, were prepared in platinum crucibles as
described in an earlier work by one of the authors.(3) The
glasses were cast between graphite blocks, and given a
minimal anneal between 4000-500C for 4 hours. Glass
compositions of 45 and 50 mole % Na20 and 38 % Li20 were
quenched between steel blocks to prevent crystallization.
Infrared analysis of specimens with and without annealing
and with the different quench rates showed equivalent IR
spectra.
The vitreous silica samples, GE125, were obtained from
the General Electric Company. The 10 mole % lithia-silica
samples were prepared by presintering a compact of lithium
carbonate, methyl cellulose and 5 micron silica at 1250C
and fusing the compact using an oxyacetylene torch, a method
similar to that used by Kumar.(39)
The samples were wet ground with 600 grit silicon car-
bide paper. The resulting corroded layer was removed immedi-
ately before taking the reflection spectrum using dry silicon
carbide 600 grit paper. It was found that for the 50 mole %
Na20-Si02 glass this procedure did not produce an uncorroded
surface due to the presence of water vapor in the air. It
was therefore necessary to grind the samples of that compo-
sition in a glove box sealed to the spectrometer while con-
tinuously purging both with nitrogen obtained by boiling
liquid nitrogen. This method excluded even momentary expo-
sure to atmospheric water. Only in this way was it possible
to obtain the true spectrum for the 50 mole % Na20-SiO2 glass
composition.
Selected glass compositions sent to Sharp-Schurtz Com-
pany for wet chemical analysis produced measured compositions
within 1 mole % of the nominal composition. The 50 mole %
glass was analyzed twice in this lab using the well-known
ammonium molybdate colorimetric analysis for silicon.(40)
These determinations were also within 1 mole % of the
nominal composition.
Results
Infrared reflection spectra of glasses having selected
compositions in the lithia-silica, soda-silica, and potash-
silica systems are given in Figs. 9, 10, and 11. This
three-dimensional mode of representation has been chosen to
illustrate trends in spectral bond shapes with increasing
alkali addition. The grid network drawn on each spectrum
permits the determination of both reflectance and wavenumber
for any point on a given spectrum. In Fig. 9 the spectra of
the compositions 20, 25, 30, and 35 mole % lithia are shown
broken for increased clarity. In Fig. 10 the spectrum of
16 mole % is shown broken for the same reason.
The three figures reveal similarities in the changes
which take place on the addition of alkali ions to vitreous
silica. The vitreous silica peaks tend to broaden and to
decrease in intensity. In addition, a new peak between 900
-l
and 950 cm- forms on the low frequency side of the prominent
reflection peak near 1,100 cm-i
reflection peak near 1,100 cm
Fig. 9. Compositional dependent changes in infrared reflection spectra in the
lithia-silica system.
0
540
WU 30
-j
LL
L)20
2 -- - -. -- - -- 10
I i,,
I LI II- 'IL
12001100 1000 900 800 700 600 500 400
WAVENUMBER(CM-1 )
Fig. 10. Compositional dependent changes in infrared reflection spectra in the
soda-silica system.
---Y. I I I~-~~
I I I I I',
I5 I I I
_; I I i_ i..;_ I;
100X 8G03
WJAVE N~UrvBEP(CN'fl)
Fig. 11. Compositional dependent changes in infrared reflection spectra in the
potash-silica system.
800 600
400
)D 800 600 400
WAVE NUMBER(CM 1 )
1200
There are also subtle differences in the trends seen
in Figs. 9-11 depending upon the kind of alkali ion which
is introduced. It can be seen that the exact amount of
change from the spectrum of vitreous silica occurring for a
given mole % of alkali ion increases in the order Li
Two spectra are shown in Fig. 10 for the 50 mole % soda-
silica glass. The dotted line is the spectrum obtained from
a sample whose surface is ground with dry 600 grit silicon
carbide paper immediately before taking the spectrum. The
solid line is the spectrum obtained using the dry nitrogen
technique described in the methods section. It can be seen
that there are significant changes in both peak position and
peak intensity due to the brief two-minute exposure of the
first sample to air at 50% relative humidity, thus emphasiz-
ing the importance of careful control of the environment
during the study.
As can be seen in Fig. 12, the spectrum reported for
the 50 mole % soda-silica glass kept in the dry nitrogen
atmosphere differs from the two spectra reported in the
literature for glasses of that nominal composition.(7,41
We originally thought that this difference could be explained
in terms of reactions caused by trace amounts of water pres-
ent in the grinding media used by the other authors. A
sample of 50 mole % soda-silica glass was therefore ground
under wet acetone to see the nature of the change in the
resulting spectrum. As can be seen in Fig. 12, the reaction
45mol/o Na20
\ This work
S50molo oNa2
Gaskell
"50mrol o/" N20
Flo-inskaya etal
wet
50 mdl / ,0 (ACE TONE)
This work
II
0I -L I III
1200 10C 10CO 900 so 700 S00 500 400 300
WAVENUMBER (CM 1)
Fig. 12. A comparison of infrared spectra of 50 mole %
ai20-SiO2 glasses.
3 i
explains not only the disappearance of the third hump near
800 cm but it also causes a shift of the silicon-oxygen
stretching vibration to lower frequencies. A glass of
45 mole % soda composition was then prepared and its spectrum
(see Fig. 12) was found to be quite similar to the spectra
of the nominal 50 mole % soda samples presented by the other
authors. We suggest, therefore, that the procedure used by
the other workers(42) of melting the glass samples in quartz
crucibles produced glasses whose actual compositions were
richer in silica than the nominal compositions by as much as
5 mole %. This postulate is consistent with the findings of
Vogel(21) that binary glasses having alkali concentrations
greater than 33 mole % tend to dissolve quartz crucibles.
Discussion of Results
The assignment of each of the prominent features in the
alkali-silicate glass reflection spectra is possible using
results From recent investigations. The high frequency peak
in vitreous silica near 1,100 cm-1 (S) is attributed to the
silicon-oxygen stretching vibration.(43) The new peak that
forms between 900 and 950 cm on the low frequency side of
the S peak as the concentration of alkali ion increases is
assigned to the silicon non-bridging oxygen stretching
vibration (NSR). (44)
-I
The peak found between 400 and 600 cm1 is due to rock-
ing motions of the silicon-oxygen bonds and the peak near
800 cm- is due to bending motions of the same bonds. The
last two assignments are based upon the lattice dynamical
calculations of a vitreous silica model developed by Bell
and Dean.(36)
In previous work done on binary silicate glasses, the
frequencies of various peaks for a given binary system have
been plotted as a function of composition, yielding irregular
curves. The discontinuities in these plots have been inter-
preted as indicating heterogeneous structures.(42) In the
present work, the reflectance of an individual peak maximum
rather than its wavenumber has been emphasized because it
has been found to be directly proportional to composition.
However, when frequencies are plotted against composition
and compared to those obtained by Cherneva and Florinskaya,(45)
the agreement is excellent. Differences in the 50 mole %
soda glass are also present, as discussed earlier.
Plots of the maxima of the spectra found in Figs. 9, 10,
and 11 vs. composition are given in Figs. 13, 14, and 15.
It can be seen that in all cases the behavior of the reflec-
tance is linear with composition of bulk glass. In each
case breaks occur in the otherwise linear plots. It is
argued that IF reflectance can be shown to be proportional
to the concentration of the species causing it and that it
is additive, then these breaks imply abrupt changes in the
nature of the -pecies causing the reflectance.
Rmax (Si-O STRETCH)
workThis Florinskayai
work
Gaskell
Li I 1 ___
Na 1 0
K +
10 20
30 40
MOL 0/o ALKALI OXIDE
Fig. 13. The reflectance of the silicon-oxygen stretching
maximum as a function of composition for the
three systems studied.
0
0
u 50
z
U
, 40
LL
W :
r
u 50 O
S0 LITHIA-SlLICA
S40- SODA-SILICA
LLU
-J
LLJ
LU 0h
20L POTASH -SILICA
10"
0 10 20 30 40 5
MOL O/o R20
Fi_. 14. The reflectance of the silicon-oxygen rocking
maximum as a function of composition for the
three systems studied.
0
\
W A Lithium
z dSodium
< OPotassium
Ill -
U7 0.
09 0
7 I
w
04
0 3-
MOL /% R 0
Fig. 15. The reflectance of the silicon non-bridging
oxygen vibration associated with alkali ion
as a function of composition.
Figure 16 is a demonstration of an analysis which is
possible using the principles introduced above. The solid
line is the measured spectrum of 25 mole % lithia-silica
glass. This glass is phase-separated and known to contain,
using the lever principle on the miscibility dome, approxi-
mately 75% of the reflectance of 33 mole % lithia-silica
glass and 25% of the reflectance of vitreous silica. The
dotted curve was obtained by multiplying the spectrum
reflectance of vitreous silica by .25 while the dashed
line was obtained by multiplying the reflectances of 33
mole 2 lithia-silica glass (33L) by .75. The triangles are
obtained by adding, at each wavenumber, .25 times the
reflectance of SiO2 plus .75 times the reflectance of 33L.
It is seen that the agreement between the calculated and
measured spectra of 25 mole % lithia-silica glass is excel-
lent. The slight discrepancy that is seen may be due to the
fact that the end points of the miscibility gap at the
annealing temperature are not exactly those given. Also,
the errors in the calculated spectrum may be due to errors
in measured reflectances compounded by the addition process
used to obtain it.
By separating the 25 mole % lithia-silica glass spectrum
into its component parts, as described in the previous para-
graph, it is seen that the recorded spectrum is simply the
sum of two simpler spectra. The peaks in the simpler spectra
have already been assigned. Thus, the prominent peak in the
Fig. 16. The comparison between the recorded and calculated spectra of phase
separation in a 25 mole % Li20-SiO2 glass as described in the text.
---MEASURED 25L
A CALCULATED 25L
SLL5 -- 75%" of 33 L
U 25% ot Si02
<40-
-30
i- 30 I &.
LL.
r20 - "
S,. NS 1
10 ../.. o ' ,
/ ..
1200 1000 800
WAV ENUMBER(CM )
recorded spectrum is the combination of the stretching peaks
due to 33L and silica. The shoulder at 950 cm- is largely
due to the silicon non-bridging oxygen stretching vibration
of 33 mole % lithia-silica glass.
In Fig. 13 the reflectance of this combination band
comprised of two stretching peaks for the three systems
studied is plotted against composition. The two stretching
peaks are the S peak already discussed and a new LS peak
which is the silicon-oxygen stretching peak of lithium
disilicate glass. The data of Gaskell(37) and Cherneva and
Florinskaya(45) are also plotted for comparison. It can be
seen in Fig. 13 that the reflectance of the bridging silicon-
oxygen stretching band varies linearly with the addition of
alkali with an abrupt change in slope between 18 and 33
mole % alkali depending on the kind of alkali ion added.
This linear behavior makes the technique an excellent non-
destructive analytical tool For the determination of the
alkali concentration of high-silica glass; the basis of a
technique used to follow the progress of glass corrosion
reactions discussed in Chapter II.
The break in the lithia-silica plot in Fig. 13, denoted
by the dashed line, is due to the domination of the combina-
tion band by the LS vibration as seen in Fig. 16. At high
silica concentrations, this combination band is controlled
by the S peak because the proportional contribution from
vitreous silica is higher. The S and LS contributions to
the combination band are illustrated in Fig. 9 with straight
lines drawn through the portions of the combination band due
to each respective peak. In the lithia-silica system, the
break in the plot of the reflectance of this combination
band vs. composition, designated by a dashed line, occurs
when the reflectance of the hump due to the LS mode is
greater than that due to the S vibration. The break is an
artifact due to the way in which the reflectance of a com-
pound band was defined. If, instead of plotting the highest
reflectance of the combination band, the reflectances due to
the S vibration alone are plotted, the break then occurs at
33 mole % lithia. This is taken to be the true reflectance
plot for this vibration and is shown ii Fig. 13 as a solid
line. In the bands for the potassia-silica and soda-silica
svstens the two stretching peaks are not separated (see
Figs. 10 and 11) and therefore do not exhibit this compli-
cat on.
The breaks for the three systems occur at 18, 21, and
33 mole % alkali oxide for the potash-siLica, soda-silica,
and lithia-silica systems, respectively. At the transforma-
tion temperature the boundaries of the miscibility gaps of
the soda-silica and lithia-silica systems are known to be
(46) (46)
20 and 33 mole 7, respectively. ( Charles(41 arg"'es that
the consulate temperature of the potassia-silica miscibiLity
gap is below the glass transformation temperature and hence
not observable experimentally. But by the same argument,
such a gap is thermodynamically probable. The gap boundary
should be closer to vitreous silica than for the soda-
silica system. (46) The breaks in the plots of reflectance
vs. glass composition are thus seen to be due to the deple-
tion of a vibrational species, in this case, the stretching
vibration of the high silica phase. The linear decrease in
reflectance corresponds to the expected linear decrease in
the volume fraction of the vitreous silica matrix as the
total glass composition moves across the miscibility region.
The fact that miscibility does not take place in the potash
system for kinetic reasons does not prevent the local
segregation of non-bridging oxygen around the alkali ions
described by Weyl.(2) Thus, the plots of reflectance vs.
composition detect both phase separation on a macroscopic
scale and the tendency towards phase separation on a micro-
scopic scale.
Figure 14 shows plots of the maximum reflectances of
the silicon-oxygen rocking peaks shown in Figs. 9, 10, and
11. The breaks occur at the same compositions as were
observed for the stretching plots just discussed. The fact
that the break in clots of the R peaks is not as abrupt as
observed for the plots of the S peaks indicates that the
effect of alkali ions on rocking vibrations is not as great
as on stretching vibrations. In fact, the break in the
lithia-silica plot is barely detectable in Fig. 13. The
fact that the spectra of strongly phase separated 20 and 25
mole % lithia-silica glass (given in Fig. 9) do not have
bimodal rocking peaks further strengthens the argument that
the rocking vibration is quite similar in vitreous silica
and lithium disilicate glass.
Figure 17 shows a detailed comparison of the rocking
neaks for the three systems studied. It is seen that the
snectra of all the glasses in the potash-silica and soda-
-I
silica systems exhibit a minimum near 520 cm The spectra
of the lithia-silica glass, on the other hand, exhibit no
such ninima, having additional reflectance in this area
(designated by the letter L). The greatest difEcrence
between the spectra of vitreous silica and 33 mole 2 lithia-
silica glass occurs near 520 cii It is concluded that
this Tma.tiumln is d;e to the motion of lithium ions. The
corrcsoondi;ng sodium and potassium-oxygen vibrations are
not seen because lhey are found at lo;: 'r frequencies and
therefore buried in the continuum.l. This assignment is
in agreement IJi.Lh the work of Ri.sen, who has reported tThis
vibration in the same frequency, range for a variety of
1 1hiio.; on containing system:-;.(48-50
FiIurc iS is o plot of the differ-eice In reflectance
at 520 cn; betvseen the spectrum of vi.troous silica and the
snectra of various' 1 it.hia-silica o lasscr. It is seen thkt
the beh'ovior con be approxi maed by stIraight lines having
breaks at 1.0 and 33 iaole % ithLia. Thei breaks indicate
abrupt chan 'es ii the envi ronme nt aroudn the li thi. u ions
Fig. 17. Spnctra of the silicon-oxygen rocking peak for the three systems
studied showing reflectance due to lithium vibration (L) as described
in the text.
POTAS H-SILICA
600 500 4.00
VWV/ENL CB,,lvR
CMf'O
600 500 400
WAVtE NiUiMBER
CM1'
600 500 400
WAiV E N UMBER
CM,1
LITHiA-SILICA
SODA-S ILICA
-1
Fig. 18. Plot of the difference in reflectance at 520 cr1
between the spectrum of vitreous silica and those
of lithia-silica glasses.
20 30 40
MOL /o Li20
'2_
30,
25-
20
ljI
J15-
LL
Lj
10o1.
/
'0
10
50
1111~11~
and coincide with breaks found in plots of the activation
energy for electrical conduction as a function of composi-
tion.. () Also, Kumar and Maitra(39) have shown that glasses
having lithia concentrations less than 10 mole % do not
release lithium ions into aqueous solutions, indicating that
the silica-rich phase completely surrounds the small volume
fraction of lithia-rich phase present in these phase
separated glasses.
Again, this linear behavior of the reflectance of
lithia-silica glasses at 520 cm- with mole lithia permits
the neasuroment of the concentration of lithium ion in
lithia-silica glasses. Since lithium is not easily detected
using x-ray techniques because of its lo.,; atomic number, the
use of infrared reflectance spectroscopy provides a pco(lsiny
analytical tool for the study of lithia-based glasses.
A plot of the re.lectance o the: nron-bridging silicon-
oxygcen stretching reflectance (NSR) vs. composition is g.ven
in Fli. 13. It is seen that he behavior is fiuite similar
in all three systems with a ina.i'mum ticlectance occurrin.g
at roughly 36 nole %, R70. The explanatior [or this .i;u
illustrates the nature of the species cause i .;_ this band.
It caon be shown(52) thiat a b:and due to a specific vibra-
tional species produced by the combination of two reactants
caln 1'e plotted against the conLccnt rat on oE orn of those
Creactarts to )ield ai maxi num. This naiaum corresponds to
th, stoichiometry of vibrational species causing the
reflectance. In the present case, the alkali ion reacts
sith the silicon-oxygen network to produce non-bridging
silicon-oxygen bonds. At 33 mole % R20, there is one non-
bridging oxygen per silicon-oxygen tetrahedron. Since the
maximum in the plot of the NSR peak vs. composition occurs
at compositions slightly greater than 33 mole % R20, it is
seen that the NSR peak is due to a vibrational species which,
on the average, has slightly more than one non-bridging
oxygen per tetrahedron.
At concentrations greater than the 36 mole %, the number
of totrahedra having more than one non-bridging oay'gen
becomes substntiatl Those tetrahedra are linked by the
remaining bridging : -iiicon-oxygen bonds forming chainl ik;
structures. The two non-bridging ox-ygen;is in a tctrahedron
can then cou-ie to vibrate in a synmmetric:al anL d asyn irnetri ca
mn;ner, forrting peaks on either side of the io, -b ridgj;in
oxygen peak. This is, in fact, seen iri the spectrum for
50 m;oe t soda-silica glass shoi-n in Fi 1. 9. Coup in. of
the two NSR vibrations leads to theb too scial. n aks diagran.ed
sch.m:I ic(.al] v en either side of 900 cm-. Th. e'; pe'ks 'ad
to bo';th 1i. tilinrg in of the valley between the S and iSR
rpcai,;s snr th:, now ; shoulder formed on the low freq'uency s: do
of tiL NS a at S -20 cm
ofr tK'. 1150 "-e"' at %820 en
4; -
A!.
/ CI
1000
N
800 60.
WAVE N UMBER (CM')
~4i. 19, Infrared snectrun of a 50 role % SiO2 glass. The dotted lines
represent possiblee peaks forming this spectrum as described in
the t--t,
LjJ ?
u I
- .
w* 20,
LL
L 10
0
I-
1200L;;;Y'"~
400
"IL II-~3_L~IYI-Y~iYI-~ L-LI~II~PUPLI~U*l ~Y
s-^~lr. ,,~imrr~-yrrrmi~~~;~rr -ur;uuar-. .rrur~ -rrlr~';uu-r~wrurar;u
---
Conclusions
It has been shown experimentally that infrared reflec-
tances are proportional to the concentrations of the vibra-
tional species causing them. Further, these reflectances
can be added w.avenumber by wavenumber to show more clearly
the origin of the broad bands frequently encountered in the
spectra of amorphous substances (see Fig. 16). These char-
acteristics allow the extension of assignments made on
stoichiometric glasses to more complex glass systems.
In addition, breaks in the linear plots of reflectance
vs. bulk composition in a series of glasses imply changes in
the nature of the vibrational species involved. These
changes can be a depletion of a vibrational species as was
seen in the stretching neak behavior near the miscibility
gao boundaries (see Fig. 13), a change in the local geometry
as was seen in the behavior of the lithium vibration near
]0 mole ,, or a partial coupling of several vibrations as
was seen in the behavior of the non-bridging silicon-oxygont
bond at hi gh soda concentrations (see Figs. 15 and 19).
In the comnos ition sequences the rocking peak follows
much the same changes as does the stretching peak. This is
expected because both neaks arc due to dificrent motions of
the same atomic group, the silicon--oxygen tetrahedron. The
phase separation seen in the lithiu-silica system in the
20 and 25 wole t glasses does not cause shoulders on the
rocking peak as it does on the stretching peak (see Fig. 9).
This implies that the rocking peak behavior in the lithia-
silica system is not as sensitive to local composition
fluctuations caused by phase separation.
The high frequency base of the rocking peak at 520 cm-
contains reflectance due to the motion of lithium ions in
the glassy network. This reflectance is not seen to the
same extent (see Fig. 17) in the potash and soda-silica
systems due to the increased mass of those cations, which
causes the peaks due to them to be buried in the continuum.
In searching for an optimal method of preparation for
the glass surfaces, it was found that grinding with 600 grit
dry silicon carbide paper produced a very reproducible sur-
face free of reaction products due to the interactions of
the glass surface with its environment. For 50 mole %
soda-silica glass, this process had to be carried out in a
totally dry environment without even momentary exposure of
the sample to the water vapor in the atmosphere. For this
reason, the author feels that the spectrum of 50 mole
soda-silica glass given in this work is the most accurate
to date.
CHAPTER IV
ANALYSIS OF SILICA-RICH GELS
FORMED DURING GLASS CORROSION
Charles described the destruction of alkali silicate
glass by water in terms of three chemical reactions.(54)
The first involved the penetration of a proton from the
water into the glassy network, replacing an alkali ion which
is, in turn, released into solution:
-Si-O-R+H 20-Si-O-H+R +OH- (1)
This reaction produces an OH ion and a non-bridging oxygen
bond attached to a hydrogen ion (NSII). A second reaction,
with the hydroxyl ions obtained in Equation (1), destroys
silicon-oxygen-silicon bonds to form non-bridging silicon-
oxygen bonds (NS) as:
-Si- -Si-+ OH --Si--OH+-Si- (2)
I I I I
The NS bond formed in Equation (2) interacts with the water
molecule forming another NSII bond and a hydroxyl ion, which
is free to repeat reaction 2 over again, e.g.,
-Si-O + I12 O-Si-O-H+O-0H (3)
I 2 1
The silicic acid formed in all three reactions is soluble
in water under the appropriate conditions of pH, time, tem-
perature and ion concentrations.
Following the discussion by Douglas et al.,(55) the
relationship between the activity of NS groups in the aqueous
solution is derived as:
[NS] [-Si-O 1 = 0-9.8 (4)
NSH [-Si-O-Hi]
assuming that the pH of the solution is less than nine and
hence the LiOH and -Si-O-R (NSR) are completely dissociated.
Equation (4) shows that in acid and in neutral solutions
I -
[-Si-O-H] >> [-Si- ]
Also, if one assumes equilibrium conditions, [NSH] decreases
rapidly with increasing pH. When [NSH] is very low, [NSL]
increases and Equation (4) no longer holds. One expects,
from equilibrium considerations, that the amount of R
release would decrease with increasing pll above pH 9.
Douglas argues(55) that as the pH exceeds 9.8, the number
oF NS groups increases, leading to a net charge and breakup
of the structure and increased rate oF silica release.
The above relationships do not specify the location of
the various corrosion species, e.g., the silanol (NSH)
groups mny either go into solution or form a Film on the
glass surfaces. Likewise, NSR groups may be present in
films on the glass surface that have been exposed to high
pH values. The extent to which a glass forms a film and
the coherence of that film has a strong influence on corro-
sion behavior.
Shnidt(56) introduced a parameter, a, defined as
Si2 1
m NNa2
where R20-mSiO2 is the composition of the glass and N is
the moles/cm2 of glass surface which dissolves into solution.
a is a measure of the extent of selective dissolution, when
a = 0, compared to total dissolution, when a = 1.0. At
intermediate a values, a mixture or selective and total dis-
solution occurs concurrently. Shmidt found in binary soda-
silica glasses that at a given temperature a remained unity
with increasing silica concentration to a critical concen-
tration where it decreased precipitously to zero. At higher
temperatures the critical composition occurred at higher
concentrations of silica. Thus for the ono-hour reat!Ment
used by Shnidt, at 100"C, only compositions having silica
concentrations >80 mole % formed films.
In the present study the dependence of c on exposure
tiie is determined for 33 mole % Li 20-SiO2 (33L) and 31
mole % Na20-SiO2 (31N) glasses. In addition, a new parameter
which is a measure of the amount of sitica available for film
formation is defined. This quant ity, referred to as "excess
silica," or c, is the difference between the amount of
silica which would go into solution if the ratio between
the alkali concentration and the silica concentration in
the original glass were maintained (balanced silica or 3)
and the amount of silica which actually does go into solu-
tion. The relationships between these quantities are:
Y solution 'Y glass (6)
X solution X glass
mPM Si02 i mwR iF Pm
1PPM R+ 2
and [ + .
a (balanced silica) = (---P 7- = RmwR J
and
c (excess silica, PPM) = PP;i SiO2 = PPM Si021 ]
(9)
where Y = moles SiO X = moles R20, Pm = mole fraction
iR20 in glass, mw = moleculare weight, PPM SiO02 = concentra-
tion of silica in solution, and PPM R = concentration of
R' in solution.
Figure 20 shows two hypoLthetical plots of silica con-
centration vs. its distance from the film surface, winch
can be calculated from the parranters described in this work.
The area under the composition proFile (S) shaded in Fig. 20
is equal to one-half times the product of the ordinate and
Z
O
7 A
0 D
I A B. --
DISTANCE
Fig. 20 Two simplified models of the coiposi tion profile
of the silica-rich corrosion layer. Part A is a
profile oF a more resistant layer while Part B
represents a less resistant layer.
abscissa of the shaded triangle. The ordinate is the difEer-
once between the measured surface silica concentration (Y)
and the silica concentration of the original glass, tIus
having units of grains of excess silica divided by cm of
Film volume. The abscissa is the profile thickness (X) in
cm. The product, therefore, has units of grams of excess
2
silica divided by the film surface area (A) given in cm .
The quantity S can also be obtained from the parameter
c if the solution volume (V) and V/A ratio is known.
-3
S grams excess silica grams excess silica x 10
cm2 film surface 1,000 cm solution
V CIcm.3 solution
x-
xA 2
S cm film surface
In the present study, the V/A ratio is simply the depth of
the cylindrical corrosion cell cavity (d) and therefore the
area under the composition profile (S) is proportional to
the product of e and the corrosion cell depth (d) in cm,
e.g., S = edlO6. Both plots in Fig. 20 have the same area
and hence the same e values For the surface film, but the
excess silica in film A is distributed much closer to the
surface thin in film B. Thus, Film A is characteristic of
a resistant film. Film B, on the other hand, is thicker
because it has the same c value as filn A but a lower sur-
face SiO2 concentration, a characteristic of less resistant
films.
Methods
The glass samples were corroded using the static cor-
rosion cells as described in Chapter IL. After exposure,
the cell solution was immediately diluted for later analysis
for lithium or sodium concentration using atomic emission
spectroscopy, and for dissolved silica using the well-known
molybdenum blue calorintotric method.(40) At low tempera-
tures and/or short times, it was necessary to corrode a
number of samples in the same manner in order to obtain
sufEicient concentrations of silica for analysis. The pH
was determined for each run using short range plI paper
(10.5) calib rated with a pll electrode.
The exposed glass surfaces were analyzed using 1RRS as
described in Chaptor II. Lach corroded glass sample was
run with a freshly polished 55L glass for comparison. The
diCffrence in reflectance between specific Features of the
uncorrod-d glass and that of the corroded samples was
measured for each spectral feature of interest. These
fe..tiures wore the omaximiumo valuo of Lhe silicon-oxygen
stretching peak at L,100 cm- (S), Lho maximum value of the
silicon non-brlidling oxygen stretching peak at 425 cm (NS)
the reflectance at 520 cm I due to Ithe 1 thium ion (L), and
thIe maximum value of the rocking peak at 475 cm- (T) The
greatest extent of deviation in reclectaince fromn the uncor-
roded re rectc ancer was taken as 100'l ract-ion. 1Thus, the
reflectance values of corrosion events loss than this maxi-
mum value were divided by the maximum value and multiplied
by 100 to obtain the value of the percent reaction reported
herein.
An Acton Laboratories, Model MS-67 electron microprobe
was used to determine the composition profile of one of the
films. The glass sample was exposed to iatcr at 79.5C for
17 hours. Its in [rared spectra wore recorded and the sample
imbedded in epoxy. The imbedded sample was then cut perpen-
dicular to the corrosion film and the resulting exposed
edge polished using standard metallographic techniques to
0.25 pm diamond paste. Both the corroded section and a
standard silica sample were plated side-by-side with a gold
film.
The 33L sample w-as scanned for silicon at 10 microns
per inch from the interior portion out to the edge of the
samnle. Tn addition, the filu was scanned point by point
manual at 2-micron intervals from the edge of the sample
inward. This procedure was repeated five times and an aver-
age taken. A linear re Lationship between the number of
counts nor second and the concentration oF the silica was
assumed as a first approximation.
Results
Figure 21 is the solution data obtained for 33L at 400C.
It is representative of results obtained over a temperature
range of 24.5 to 79.50C. In Fig. 21 the solid line with
circles is the plot of balanced silica, 4, computed from the
lithium concentration of the corrosion solution as discussed
previously. Since g is proportional to the lithium concen-
tration, as with lithium release, its plot is a straight
line on the log-log scales as shown in this figure.
The concentration of silica in solution is best approx-
imated by three line segments with breaks corresponding to
pH values of 7 and 9. a values are shown as a dotted line.
As with the behavior of silica in solution, a has three
linear portions and two breaks at the same values of pH, 7
and 9. Figure 21 shows that, upon exposure of a freshly
abraded 33L sample to water, the initial a value decreases
with time, indicating an increasing tendency for film forma-
tion. The initially higher value of a, which implies a
greater tendency For total destruction of the glassy network,
is probably due to the presence of high energy silica network
sites produced by the abrasion process, as discussed later.
In Fig. 21 the first break in the a curve occurs at
about 170 minutes, after which the slope of the a curve
becomes positive, indicating a continually decreasing ten-
dency for Film formation. Finally, at the second break,
Fig. 21. An example of data obtained from solution analysis of glass corrosion
and its relation to the calculated quantities a, e, and B.
-....... alpha
----excess silica (2)
- --silica in solution
-0--balanced silica (P)
1
\ 1
\ 1
1 10 100 1000
TIME IN MINUTES
0.1
0.01
0.01
10,000
occurring at 3,000 minutes, the a-time slope becomes still
steeper and rapidly approaches unity. When unity is reached
there is no tendency for film formation and the whole reac-
tion is one of total dissolution of the silicon-oxygen net-
work. However, the value of ,a = 1 does not preclude the
possibility of a high silica film being present on the glass
if elsewhere in the system there are insoluble corrosion
products having higher alkali content than the glass.
The e curve, representing the silica that can be avail-
able for film formation, is shown in Fig. 21 as a dashed
line. Because the excess silica is the difference between
a relatively large quantity, e.g., balanced silica, and a
relatively small quantity, silica in solution, its behavior
closely parallels the former. Only when the balanced silica
and the solution silica approach the same order of magnitude
does deviate significantly from 6, as can be seen in Fig.
21. The E curve reaches a maximum when the value of a is
0.45, indicating that roughly half of the total reaction
has led to film formation.
In order to interpret the solution behavior, infrared
reflection spectra of the same corroded 33L glasses were
recorded and are summarized in Fig. 22. The silicon-oxygen
stretching peak of 33L (LS) is seen to sharpen and increase
in reflectance with corrosion time while the non-bridging
silicon-oxygen peak associated with alkali ion (NSL) is seen
NSL
a: zB
L
0.1
10
1,00
O, O.
10,0000
1200 1000 800 600
WAVENUMBER(CM"1)
Fig. 22. Changes in infrared reflection spectra of 33L glass on exposure to
static water at 400C. The letters are explained in the text.
to decrease. Both these changes show that the corrosion
process leads to a structure similar to that of vitreous
silica.
The similarity of the vitreous silica reflection spec-
tra and a 33L glass corroded in water for 22 hours at 50.50C
is shown in Fig. 23. Concurrently, the silicon-oxygen rock-
ing peak (R) in Fig. 22 increases in reflectance and
sharpens during corrosion. This change can also be attrib-
uted to an increase in the silica content of the structure
sampled by the reflectance method as discussed in Chapter
III. After longer periods of time, the peak B is seen to
form, which is attributed to the bending mode of a silicon-
oxygen bond. All of the above changes are expected in a
leaching process with the increase in the silica concentra-
tion resulting from the loss of alkali ions.
Direct evidence for the decrease in alkali concentra-
-I
tion can be seen as a decrease in reflectance at 520 cm
This reflectance, labeled L in Fig. 22, is due to the vibra-
tion of lithium ions and its decrease signifies a decrease
in the average concentration of the lithia. Another change,
the decrease in reflectance between peaks LS and NSL, due to
coupling between them, is also related to the loss of the
lithium ions responsible for this coupling.
The shoulder () in Fig. 22) formed at 1,010 cm- after
exposure to water For 3,000 minutes at 400C has not been
conclusively assigned. It may be due to an interference
gl2aS5
SiO2
-.-- 31N
treatment
as abraded
as abraded
60- ........ 33L 22Hrs. 50.50
S-- 31N 15Min. 50.5
50- i\ 31 N 17Hrs. 50.5
/ AC
z 40-i
- 0 -, c \
20 -
, 0u / / ,' V. c \f'
W # I \ '*" \
10 / /, ,; ... ..... : .- :- .. ,
," # //* ...... -*'"
1200 1000 800 600 400
WAV ENUMBER(CM )
Fig. 23. A comparison of the corroded and noncorroded glass spectra in the soda-
silica system. The siectra of vitreous silica and corroded 33L are
included for comparison.
fringe caused by the corroded layer, but more probably is
due to the silicon non-bridging oxygen vibration associated
with bonded hydrogen ions in a polymerized gel structure.
A similar assignment was given by Naudin for an absorption
-1 (57)
peak found at 952 cm1
Figure 24 is a compilation of the solution data
obtained at four temperatures. Part II of Fig. 24 contains
plots of a values for the 33L glass. Also included is a
dotted line showing the behavior of a for 31N at 50.50C.
In part I of Fig. 24, the behavior of excess silica is shown
for the same glass compositions and temperatures. For
clarity, letters coded for the appropriate temperature are
placed at the ends and break points of the solid lines for
the 33L curves.
Figure 25 is a micrograph of the corroded layer of 33L
treated for 350 minutes at 79.5C. The area seen is the
fractured edge of the corroded layer made by drawing a
diamond stylus through the film. The gage ball marked with
the arrow is 0.87 microns in diameter. The white, jagged
flakes are debris produced by the diamond stylus. The
corrosion layer in the foreground has grinding scratches
with pits roughly 0.5 microns in diameter running along them.
These pits appear to be the same depth as the corrosion film.
They apparently arc due to the total dissolution of the
glass in the scratch valleys.
TIME IN MINUTES
Fig. 24. A summary of the plots of e and a calculated
from all the solution analysis data reported
in this work. The letters are used to
designate the corrosion temperature involved.
Fig. 25. An S.E.M. micrograph of the edge of the corroded layer of 33L glass
exposed to static water for 350 minutes at 79.50C. The gage ball
is 0.87 microns in diameter.
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