Corrosion of Ceramic and Composite Materials Part 5 pps

106 Chapter 2
9. The spontaneity of a reaction depends upon more
than just the heat of reaction. To predict stability,
one must consider also the entropy.
10. If the reaction is spontaneous, the change in free
energy is negative, whereas if the reaction is in
equilibrium, the free energy change is equal to zero.
11. The real problem with predicting whether a reaction
may take place or not is in selecting the proper
reaction to evaluate. Care must be taken not to
overlook some possible reactions.
12. Since the corrosion of ceramics in service may never
reach an equilibrium state, thermodynamic calculations
cannot be strictly applied because these calculations are
for systems in equilibrium. Many reactions, however,
closely approach equilibrium, and thus the condition
of equilibrium should be considered only as a limitation,
not as a barrier to interpretation of the data.
13. There is a general tendency for oxides to be reduced
at higher temperatures at constant oxygen partial
pressures. One should be aware that any metal will
reduce any oxide above it in the Ellingham diagram.
14. Unit activity should be applied only to species in the
pure state.
15. The most important parameter of corrosion from the
engineering viewpoint is the reaction rate.
16. Diffusion coefficients depend upon the composition and
structure of the material through which diffusion occurs.
17. The rate of the reaction expressed as the rate of
change of concentration, dc/dt, depends upon the
concentration of the reactants.

18. The discrepancies between the experimental data and the
theoretical models are often due to nonspherical particles,
a range in sizes, poor contact between reactants,
formation of multiple products, and the dependency of
the diffusion coefficient upon composition.
19. Arnold et al. [2.141] concluded that dynamic
thermogravimetric studies provide insufficient data
Copyright © 2004 by Marcel Dekker, Inc.
Fundamentals 107
for calculation of reaction kinetics, that the data are
influenced by the experimental procedures, and that
the results are uncertain.
20. The enthalpy of the reaction is often sufficient to raise
or lower the sample temperature by as much as 1000°C.
21. The flow of material by diffusion is proportional to the
concentration gradient and is directed from the region
of high concentration to one of low concentration.
22. In isometric crystals, the diffusion coefficient is
isotropic, as it is in polycrystalline materials as long
as no preferred orientation exists.
23. In most real cases, the diffusion coefficient can vary
with time, temperature, composition, or position
along the sample, or any combination of these.
24. Silica-forming reactions are the most desirable for
protection against oxygen diffusion.
25. A diffusion flux will set up a thermal gradient in an
isothermal system.
2.11 ADDITIONAL RELATED READING
Vetter, K.J. Electrochemical Kinetics; Academic Press, New York, 1967.
Bockris, O.M.; Reddy, A.K.N. Modern Electrochemistry; Plenum Press,

New York, 1970; Vol. 2.
Shaw, D.J. Charged Interfaces. Introduction to Colloid and Surface
Chemistry, 3rd Ed.; Butterworths, London, 1980; 148–182.
Chp. 7.
Marshall, C.E. The Physical Chemistry and Mineralogy of Soils: Soils
in Place; Wiley & Sons: New York, 1977; Vol. II.
Reviews in Mineralogy: Mineral-Water Interface Geochemistry;
Hochella, M.F. Jr., White A.F., Eds.; Mineral. Soc. Am.,
Washington, DC, 1990; Vol. 23.
Burns, R.G. Mineralogical Applications of Crystal Field Theory;
Cambridge University Press: Cambridge, 1970.
Shackelford J.F., Ed.; Bioceramics, Applications of Ceramic and Glass
Materials in Medicine; Trans Tech Publications: Switzerland, 1999.
Copyright © 2004 by Marcel Dekker, Inc.
108 Chapter 2
Reviews in Mineralogy: Health Effects of Mineral Dusts. Guthrie,
G.D. Jr.; Mossman B.T., Eds.; Mineral. Soc. Am., Washington,
DC, 1993; Vol. 28.
P.G.Shewmon. Diffusion in Solids. J.Williams Book Co., Jenks, OK, 1983.
2.12 EXERCISES, QUESTIONS, AND
PROBLEMS
1. Discuss the reaction products that may form and how
they may relate to any interfacial reaction layer formed.
2. If a “unified theory of corrosion of ceramics” were
to be developed, what structural characteristic would
be included and why?
3. Look up the vapor pressure of several materials to
confirm the concept that covalent materials vaporize
more quickly than ionic materials due to their higher
vapor pressure.

4. Why does the corrosion rate decrease when a thermal
gradient is present?
5. The Arrhenius equation has been used to represent
the temperature dependence of corrosion. Discuss
when this equation is most appropriate and why.
6. Discuss the difference between direct and indirect
dissolution. What other terms are used to describe
these types of dissolution?
7. What is the most predominant parameter in the
equation for corrosion rate under free convection?
Why is this parameter more predominant than the
others?
8. Discuss the various problems relating to the
experimental verification of the galvanic corrosion
of ceramics.
9. Describe how the cross-linking of silica tetrahedra
affect corrosion in silicates by aqueous solutions.
10. How does pH affect the corrosion of crystalline ceramics
and how does this relate to isoelectric point (IEP)?
Copyright © 2004 by Marcel Dekker, Inc.
Fundamentals 109
11. Discuss the difference between electrochemical and
chemical dissolution. What material parameters are
important in each type?
12. Describe how one tells whether solid-solid corrosion
occurs by bulk, grain boundary, or surface diffusion.
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Fundamentals 121
thermogravimetric curves at increasing temperature. Talanta
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122 Chapter 2

radioactive lead in lead metasilicate glass. Z. Phys. Chem.
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Copyright © 2004 by Marcel Dekker, Inc.
123
3
Methods of Corrosion Analysis
One must learn by doing the thing; for though you think
you know it, you have no certainty until you try.
SOPHOCLES
3.1 INTRODUCTION
According to Weisser and Bange [3.1] it was Lavoisier who in
1770 first recorded the aqueous corrosion of a silicate glass
predominantly by use of an analytical balance. The analysis of
corrosion has been changing over the years with the greatest
changes probably taking place within the last 25 years. These
changes have been due mostly to the availability of
sophisticated computerized analytical tools. It has taken many
years for investigators to become familiar with the results
obtained and how to interpret them. In some cases, special
Copyright © 2004 by Marcel Dekker, Inc.
124 Chapter 3
sample preparation techniques had to be perfected. Although
one could conceivably employ all the various characterization
methods described below, in most cases, only a few are needed
to obtain sufficient information to solve a particular problem.
The determination of the overall mechanism of corrosion

requires a thorough detailed investigation using several
characterization methods. Many times, though, the investigator
has a limited amount of time and/or funds to obtain data and
thus must rely on a few well-chosen tools. It should be obvious
that considerable thought should be given to the selection of
samples, test conditions, characterization methods, and
interpretation of the results, especially if the data are to be
used for prediction of lifetimes in actual service conditions.
The reader is referred to the book by Wachtman [3.2] for a
review of the principles involved in the various characterization
techniques.
3.2 LABORATORY TEST VS. FIELD TRIALS
There are two general ways to approach a corrosion problem:
either to conduct some laboratory tests to obtain information
as to how a particular material will behave under certain
conditions, or to perform a postmortem examination of field
trial samples. It is best to perform the laboratory test first to
aid in making the proper selection of materials for a particular
environment and then perform the field trial. Laboratory
tests, however, do not always yield the most accurate
information since they rely on the investigator for proper
setup; however, they are easier to control. The investigator
must have a thorough understanding of the environment
where the ceramic is to be used and must select the portions of
the environment that may cause corrosion. For example, it is
not sufficient to know that a furnace for firing ceramicware is
heated by fuel oil to a temperature of 1200°C. One must also
know what grade fuel oil is used and the various impurities
contained in the oil and at what levels. In addition,
parameters such as partial pressure of oxygen, moisture

Copyright © 2004 by Marcel Dekker, Inc.
Methods of Corrosion Analysis 125
content, etc. may be important. Once all these various
parameters are known, the investigator can set up an
appropriate laboratory test.
One must also understand all the various things that cannot
be scaled down to a laboratory test, such as viscosity of liquids,
time, temperature, etc. Care must be exercised when
attempting to perform an accelerated laboratory test, which is
usually accomplished by raising the temperature or increasing
the concentration of the corrosive medium or both. Since the
mechanism of corrosion in the accelerated test may not be the
same (generally, it is not the same) as that under actual service
conditions, erroneous conclusions and inaccurate predictions
may be obtained. The mechanisms must be the same for
accurate application of laboratory test results to actual service
conditions. Sample size is one parameter that is easily scaled;
however, this can also cause problems. For example, when
testing the corrosion of a ceramic by a liquid, the ratio of liquid
volume present to the surface area of the exposed ceramic is
very important. The investigator must remember that
corrosion is controlled predominantly by thermodynamics and
kinetics. Assuming that the proper laboratory tests have been
conducted, the probability that any problems will arise is
minimal.
The only way to analyze corrosion accurately is to conduct
a field trial. This entails placing selected materials in actual
service conditions, generally for an abbreviated time, and then
collecting samples for analysis along with all the operational
data of the particular environment. The size and amount of

material or samples placed into actual service conditions for a
field trial can be as little as one small laboratory test bar, or,
for example, as large as a complete wall in a large industrial
furnace. The larger the installation for the field trial, the more
confidence one must have in the selection of materials. The
larger installations are generally preceded by several
laboratory tests and possibly a small-scale field trial.
Abbreviated times may be as long as several years or as short
as several days.
Copyright © 2004 by Marcel Dekker, Inc.
126 Chapter 3
Data such as temperature and time are the obvious ones to
collect, but there exists a large amount of other data that should
be examined. Many times, however, some of the more
important data do not exist for one reason or another. For
example, maybe the oxygen partial pressure was not determined
during the duration of the service life of the ceramic. In some
cases, it may be impossible to collect certain pieces of data
during the operation of the particular piece of equipment. At
these times, a knowledge of phase equilibria, thermodynamics,
and kinetics can help fill in the gaps or at least give an indication
as to what was present.
3.3 SAMPLE SELECTION AND
PREPARATION
It should be obvious that powders will present a greater surface
area to corrosion and thus will corrode more rapidly than a
solid sample. One may think this to be a good way to obtain a
rapid test, but saturation of the corroding solution may cause
corrosion to cease, or even cause a reverse reaction (i.e., crystal
growth), giving misleading results. This points to the extreme

importance of the surface area to volume ratio (SA/V) of the
ceramic to the corroding solution. Another factor related to
this is that during corrosion, the surface may change, altering
the SA/V ratio effect. Surface areas during dissolution have
been reported to increase presumably due to opening of etch
pits, microfissures, etc. [3.3].
Selecting samples for analysis provides another challenge
to the investigator. Foremost in the selection process is selecting
an area for analysis that is representative of the overall
corrosion process. If this cannot be done, then many samples
must be analyzed. Much of the modern analytical equipment
necessitates the analysis of very small samples, thus one must
be very sensitive to the selection of representative samples or
at least evaluate multiple samples.
Much care must be given to preparing samples that contain
an adherent reaction product surface layer. It is best to select a
Copyright © 2004 by Marcel Dekker, Inc.
Methods of Corrosion Analysis 127
sample that is many times larger than required by the final
technique and then mounting this in some metallurgical
mountant (e.g., epoxy). After the larger sample has been
encased, then smaller samples can be safely cut from the larger
piece.
Solid samples, when prepared for laboratory tests, should
be cleaned in a noncorrosive solution to remove any loose
particles adhering to the surface and any extraneous
contamination. Brady and House [3.3] have reported that
initially, an accelerated, nonlinear dissolution may occur from
high-energy sites caused by grinding and incomplete removal
of ultrafine particles. Best results are obtained if the cleaning

is done in an ultrasonic cleaner. These cleaning solutions can
be obtained from any of the metallographic supply companies.
If the sample is mounted into one of the epoxy-type
metallographic mountants, one must be aware that some
cleaning solutions will react with the mountant. It is best to
use supplies from one manufacturer to avoid these problems.
If as-manufactured samples are used for corrosion tests, one
should remove a thin surface layer by grinding and cleaning
before performing the corrosion tests. In this way, remnants
from such things as powder beds or encapsulation media used
in the production of the material can be eliminated and
therefore not interfere with the corrosion process.
Quite often, the as-manufactured surface of a ceramic will
have a different microstructure or even chemistry than the bulk.
This often manifests itself as a thin surface layer (as much as
several millimeters thick) that contains smaller grain sizes (more
grain boundaries) and possibly a lower porosity. If the corrosion
test corrodes only this thin surface layer, again, misleading
results will be obtained. One way to solve this problem is to
remove the surface layer by grinding. Grinding, however, must
be done with some thought to the final surface roughness since,
again, this will affect the SA/V ratio. Diamond-impregnated
metal grinding disks should be used rather than silicon carbide
paper disks or silicon carbide loose grit. Loose grit and the
grinding media from paper disks have a tendency to become
Copyright © 2004 by Marcel Dekker, Inc.
128 Chapter 3
lodged within the pores and cracks of the sample being prepared.
The final grinding media grit size should be no greater than 15
µm. It is best to clean samples after each grinding step in an

ultrasonic cleaner with the appropriate cleaning solution.
Surface roughness of solid samples is an item that is often
overlooked. Not only does a rough surface increase the area
exposed to corrosion, but it may also lead to problems with
some analytical techniques. For example, when the surface
roughness is on the order of the reaction layer thickness caused
by corrosion, errors will be present in the depth profiles
obtained by secondary ion mass spectroscopy (SIMS). In those
cases when surface analyses are planned, one should prepare
solid samples to at least a 5-µm finish.
Grinding and polishing of samples that contain a reaction
product surface layer should be done so that the reaction layer
is not damaged, or the interface obscured. If part of the sample
is metal, then polishing should be done in the direction ceramic
toward metal to eliminate smearing the metal over the ceramic.
If very thin reaction layers are present, one can prepare taper
sections to increase the area that is examined. Sample preparation
of composites presents some additional problems since materials
of very different characteristics will be presented at the surface
being polished. Chanat [3.4] of Buehler Ltd. has offered some
good advice for mounting, sectioning, and polishing. The most
effective method involves the use of diamond abrasives with
low nap cloths for FRCMC*. High nap cloths can induce
excessive relief at boundaries of different materials.
Many tips on how to prepare samples can be obtained by
reading the various technical journals published by the
manufacturers of consumable grinding and polishing supplies.
One particular article that offers some new ideas was that of
Damgaard and Geels [3.5]. They emphasized the importance
of polishing disk diameter and velocity, indicating that both

are directly proportional to material removal rates. Although
* FRCMC=fiber-reinforced ceramic matrix composite.
Copyright © 2004 by Marcel Dekker, Inc.
Methods of Corrosion Analysis 129
this may be true, one must be very aware of the amount of
lubricant used, the pressures applied, and the area of the sample
being polished. If the lubricant supply rate is constant, which
is generally the case, the material removal rate will peak at
about 300–350 rpm. Thus if one is thinking of purchasing
automatic grinding and polishing equipment, he should look
for something that has automatic lubrication flow rate control.
Although many advances have been made in the grinding and
polishing of ceramics, this area still is very much an art. Subtle
changes in the procedure can make a major difference in the
final finish of the sample*.
3.4 SELECTION OF TEST CONDITIONS
Although the selection of appropriate samples can be a major
problem, the selection of the appropriate test conditions is an
even more difficult task. The goal of the industrial corrosion
engineer in selecting test conditions is to simulate actual service
conditions. Selection of test conditions is much easier for the
scientist, who is attempting to determine mechanisms. The
major problem in attempting to simulate service conditions is
the lack of detailed documentation. This is caused by not
knowing the importance of such data in the corrosion of
ceramics, the cost of collecting the data, or both. Thus if one
wants to perform meaningful laboratory corrosion studies, it
is imperative that the industrial environment of interest be
accurately characterized.
When conducting laboratory oxidation studies, a convenient

way to obtain a range of oxygen partial pressures is desirable.
Very low partial pressures are never attained in practice by the
use of a vacuum system. Instead, a mixture of gases in which
oxygen is a component is used to establish the low partial
pressure. The most important mixtures that are used are
* One should also remember that a perfectly polished surface, although excellent
for reports, is not necessary to obtain sufficient data to solve a corrosion problem.
Copyright © 2004 by Marcel Dekker, Inc.
130 Chapter 3
CO
2
+CO and H
2
O+H
2
. Since the oxygen pressures are obtained
through the equilibrium reactions:
(3.1)
and
(3.2)
the partial pressure of oxygen is given by:
(3.3)
where k
1
and k
2
are the equilibrium reaction constants. For
constant ratios, the partial pressure of oxygen is independent of
the total pressure. Thus these gas mixtures provide a means to
obtain a range of oxygen pressures. Several techniques to mix

these gases are discussed by Macchesney and Rosenberg [3.6].
In the study of corrosion in coal gasification atmospheres,
gas mixtures such as CH
4
+H
2
and H
2
S+H
2
become important
along with the ones listed above. As the gas mixture becomes
more complex, the number of equations that must be solved
to obtain the equilibrium gas composition at elevated
temperatures and pressures also increases, making it convenient
to use a program such as SOLGASMIX [3.7] for the
calculations. One should not make the erroneous assumption
that gas mixtures are the same at all temperatures since the
equilibrium mixture is dependent upon the equilibrium
constant, which is temperature-dependent.
3.5 CHARACTERIZATION METHODS
3.5.1 Microstructure and Phase Analysis
Visual Observation
The most obvious method of analysis is that of visual
observation. The human eye is excellent at determining
differences between a used and an unused ceramic. Such things
Copyright © 2004 by Marcel Dekker, Inc.