ORNL-2048.txt

RESEARCH LIBRARY.
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UNCLASSIFIED

ORNL.-2048

L

COPY NO. &7

Controct No. W-7405-eng-25

METALLURGY DIVISION
CORROSION OF MATERIALS IN FUSED HYDROXIDES

G. P. 5mith

DATE ISSUED

oo

OAK RIDGE NATIONAL LABORATORY
Operated by
UNION CARBIDE MUCLEAR COMPANY
A Division of Union Carbide and Carbon Corporation
Post Office Box P

Oak Ridge, Tennessee
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ORNL-2048

Metallurgy and Ceramics

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CONTENTS
A S T R A T e e e ettt 1
1o INTRODUCTION Lot ettt ee et ee et ee e em e e s e e s et 1
2. ATOMIC NATURE OF FUSED SODIUM HYDROXIDE oo 2
3. CORROSION OF CERAMICS ot 2
Solubility Relations in Fused Hydroxides oo, 2
Oxide-lon Donor-Acceptor REACHIONS . oot 3
Ceramics with Saturated Cations and Saturated ARIONS .ooveeeoveeoe oo 3
Ceramics with Acceptor Cations and Saturated AnionS ..oooveeoeeeeeeeeee oo, 4
Ceramics with Saturated Cations and Acceptor ANTONS oo eev oo 6
Oxidation-Reduction Reactions oo, 7
Corrosion of Other Kinds of Ceramics oo 7
Secondary Corrosion PRENOMENT ..o iciiieeee ettt en e e 7
SUMIMIGEY ot ettt ettt et et et e e ee e e ee s 2t eaee e e et e emee et e e en e e e e s e e s e e te e ee et ens e e e e et es e eeess oo 8
4. CORROSION OF METALS Lo ettt evae et er e e e s e e et ees e e 8
Corrosion of Metals by Oxidation ...t 8
Corrosion by Hydroxyl Tons oottt ee e 8
Corrosion by Alkali Metal lons ..o, et sttt ner e ane s aben e et eeeane s 11
Corrosion by Oxidizing SolUtes . ..o es oot e oo eee e (A
Corrosion of A0y S oo ettt e et sener et (R
5. MASS TRANSEER .ottt et ee e e ee e e er e e 16
TR | ettt bt e e et s et e e e e e e e et e et s oo es et n e e ee e rans 17
Other Elemental Metals ...ttt est e ee e ee s ers e 20
ALOY S ottt ettt et a et n e ee et e te e e et et e et ee s e e e e 20
Bimetal e B eets oottt 20
Mechanism of Mass Transfer oottt er e e e 21
Differential Solubility (Mechanism 1) ................................................................................... 21
Oxidation-Reduction Processes (Mechanisms V1) ..o 21
Local Cell Action (Mechanism Y o oo e 22
Reduction of Hydroxyl lohs(MechqniSm T e e e e 22
Reduction of Alkali Metal lons (Mechanism 1V) oo, 23
Reduction of Solutes (Mechanism V) e e e e 23
Disproportionation (Mechanism V1), ettt et as 23
Summary of MechaniSms ..ot er e nn. 24
SUMIMIAIY ettt e e asase s he s oaaa s s ase s e e et e ae s e et enesee et s e st e e estetsesaeseseebesseeeteesseses oo 25
REFERENCES ettt e et e ee e ee e s e e sannee s s et esens 27

UNGLASSIFIED
UNGLASSIFIED

CORRi)SION OF MATERIALS IN FUSED HYDROXIDES

G. P. Smith

ABSTRACT

Some of the fused alkali-metsl hydroxides are of potential interest in reactor technology both

as coolants and as moderators. The property. which most discourages the use of these substances

is their corrosiveness,

The corrosion of ceramics and ceramic-reloted substances by fused hydroxides occurs both

by solution and by chemical reaction.

A few ceramics rea¢t with hydroxides by oxidation-

reduction reactions, but most have been found to be attacked by oxide-ion donor-acceptor re-

actions,

Magnesium oxide was the most corrosion-resistant of the ceramics which have been

tested, although several other ceramics are sufficiently resistont to be useful.

The corrosion of metc}ls and alleys by fused hydroxides takes place primarily by oxidation of

the metal, accompanied by reduction of hydroxyl ions to form hydrogen and oxide iens. Three side

reactions are known, but two of them are not important in most corrosion tests. The corrosion

of alloys involves either the formation of subsurface veoids within the alloy or the formation of

complex, twa-phase corrosion products at the surface of the alioy. Nickel is the most corrosion-

resistant metal which has been studied in fused sodium hydroxide.

Mass transfer, inducéd by temperature differenticls, has been found to be the primary factor

limiting the use of metals which, under suitable conditions, do not corrode seriously.

 

I. INTRODUCTION

Some of the fused alkali-metal hydroxides are of
potential interest in reactor technology as moder-
ators, moderator-coolants, and, possibly, moderator-
fuel vehicles.! The heat-transfer properties of
these substances are sufficiently good for them
to have been recommended as heat-
transfer media.? For use as a high-temperature
moderator without a cooling function, sodium hy-
droxide possesses the virtues of being cheap and
available as contrasted with its competitors, either
liquid or solid. Furthermore, an essential sim-
plicity in reactor design is achieved by combining
in one material the two functions of slowing
neutrons and cooling.? '

industrial

There are only a few
substances which will do both at high tempera-
tures,

Of the high-temperature moderator-coolants, the
fused alkali-metal hydroxides are among the most
stable with respect to both thermal dissociotion
and radiation domage. |t has been known for
some time that the alkali metal hydroxides should
be stable toward thermal dissociation. However,
not until recently have techniques been developed
whereby this dissociation could be guantitatively

Steidlitz and Smith? have shown that
this dissociation is relatively small, For example,
at 750°C the partial pressure of water in equi-
librium with fused sodium hydroxide is of the order
of magnitude of 0.1 mm Hg.

The radiation stability of fused hydroxides' has
been reported by Hochanadel® with regard to
electron bombardment and by Keilholtz et ul. ® with
regard to neutron bombardment. The radigtion
levels reported were very substantial, and ne
evidence was found for radiation damage.

measured.

The property of fused alkali-metal hydroxides
which most discourages their application at high
temperatures is their corrosiveness. It is the
purpose of this discussion to review the current
scientific status of studies of corrosion by fused
hydroxides. , fi

During the past five years, knowledge of the
cotrosive properties of fused hydroxides at high
temperatures has increased considerably. Never-
theless, this is a new field of research and one
in which there has been found a remarkable variety
of corrosion phenomena, so that the areas of
ignorance are more impressive than the areas

of knowledge. For this reason, the writer has

UNCLASSIFIED _ 1
chosen to ploce emphasis on some of the problems
which are most in need of solution., This review
will, accordingly, be concermned with the kinds of
corrosion phenomena which have been observed
rather than with the compilation of engineering
reference data, Moreover, with a view toward
stimulating further research, the writer has taken
the liberty of presenting a number of quite specu-
lative ideas on the origins of some of the corrosion
phenomena, Only in this way con many importani
possible accomplishments of further research be
indicated.

In this report, the term ‘‘fused hydroxides’ is
taken to mean the fused alkali-metal hydroxides;
the exception is lithium hydroxide, which, like
the fused hydroxides of the alkaline-garth metals,
is less stable with regard to thermal dissociotion
than the other alkali metal hydroxides. Most of
the available corrosion data are for fused sodium
hydroxide. Information on corrosion in fused po-
tassium hydroxide indicates that this substance
behaves qualitatively like sodium hydroxide. There
are very little dofo available on the hydroxides
of rubidium and cesium.

2. ATOMIC NATURE OF FUSED 50DiUM
HYDROXIDE

Studies of electrical conductivity? and of the
freezing-point depression by electrolyte solutes®
indicate that fused sodium hydroxide is o typical
fused electrolyte which obeys, at lzast approxi-
mately, the model of ideal behavior proposed for
such substances by Temkin.? Thus, sodium hy-
droxide consists of soedium ions and hydroxyl ions
held together by coulombic attraction such that
no cotion may be considered bound to a particular
anion, although, on the avercge, each cation has
only anion neighbors and vice versa. When an
ionic sodium compound is dissolved in fused
sodium hydroxide, the cations of the solute become
indistinguishable from those of the solvent. The
hydroxyl anion is not a simple particle of unit
negative chorge but has a dipole siructure and,
under suitable conditions, u polypole structure.10

3. CORROSION OF CERAMICS

Studies which have been made of the behavior
of ceramic materials in fused hydroxides are
limited in scope both as regords the variety of
substances tested and the information obtained on
any one substance. The use of petrography and

x-ray diffraction, which have been largely ignored

in the past, together with o proper regard for the
reactivity of many corrosion products in contact
with moisfure and carbon dioxide, is essential for
further progress.

The corrosion of ceramics and of ceramic-ralated
substances by fused hydroxides has been observed
to take place by solution and by chemical reaction,
A few of the chemical reactions which have been
observed were found to be oxidation-reduction re-
actions, These will be described briefly near the
end of this section. The majority of the known
reactions of ceramics and ceramic-related sub-
stances with fused hydroxides have been found
to be oxide-ion donor-acceptor reactions. This
kind of reaction in fused hydroxides may be treated
in a quantitative although formal way by appli-
cation of acid-base analog theory. Such a ireat-
ment is beyond the scope of this report. However,
some of the concepts derived from the theory of
oxide-ion donor-acceptor reactions are very useful
in describing the reactions between ceramics and
hydroxides and will be considered.

Solubility Relations in Fused Hydroxides, — A
ceramic material which is chemically inert toward
fused hydroxides might be limited in usefulness
because of appreciable solubility in these media.
There have besn very few quantitative measure-
ments of solubility relations in fused hydroxides,
and none of these measurements have been made
for substances which are of importance as ceramics,
However, in a discussion of the corrosion of
ceramics, such solubility relations must inevitably
be menticned if only in a qualitative way.

There is a fundamental difference between solu-
bility relations in nonelectrolytes such as water
ond in fused electrolytes such as hydroxides.
Solutions of a simple ionic substance in water may
be described in terms of a binary system, while
such solutions in fused hydroxides are reciprocal
salt systems which require the specification of
four components provided that the solute does not
have an ion in common with the solvent, Under
special conditions reciprocal salt systems may
be treated as quasi-binary systems, but it is
unwise to assume such behavior in the absence
of confirmatory experimental evidence.

Metathesis reactions in aqueous media are, of
course, the manifestation of reciprocal salt solu-
The corresponding type of re-
action in a fused hydroxide solution is complicated
by the fact that six composition variables would
in general be needed to describe the possible
solid phases which could precipitate from solution.

bility relations,
Oxide-lon Donor-Acceptor Reactions. — Many
chemical substances have a significant affinity for
oxide ions. Two well-known examples are carbon
dioxide and water, which react with oxide ions to
form, respectively, carbonate ions and hydroxyl
ions. Such substances are referrad to as “‘oxide-
ion acceptors.” Once an acceptor has reacted
with an oxide ion, it becomes a potential oxide-ion
donor and will give up its oxide ion to a stronger
acceptor. Thus, the hydroxy! ion is an oxide-ion
donor which is conjugate fo, or derived from, the
oxide-ion acceptor, water. Carbon dioxide is «
stronger acceptor than water and hence will react
with hydroxyl ions to form carbonate ions and
water, Such a reaction may be viewed as a compe-
tition for oxide ions between the two acceptors,
carbon dioxide and water:

(3.1)  CO, + 20H-

it

Co,~~ + H,0
acceptor | + donor Il = donor | + acceptor [}

The reaction goes in that direction which produces
the weaokest donor-acceptor pair, '

Many substances will behave toward hydroxyl
ions like carbon dioxide in Eq. 3.1; that is, they
will capture oxide ions and liberate water. This
is not necessarily because the acceptor in question
is intrinsically stronger than water. An acceptor
which is stronger than water must successfully
compete with water for oxide ions when water and
the acceptor in question are at the same concen-
tration. Frequently reactions like Eg. 3.1 take
place because water is easily volatilized from
fused hydroxides at high temperatures, and conse-
quently its concentration is very low compared
with the concentration of the competing oxide-ion
acceptor. However, the water concentration in a
fused hydroxide may be maintained constant by
fixing the partial pressure of water over the melt,
Under this condition o comparison may be made
of the relative abilities of two oxide-ion acceptors
to capture oxide ions from hydroxyl ions in the
presence of the same fixed concentration of water.
Thus it is meaningful to speck of the relative
oxide-ion acceptor sirengths of two substances in
fused hydroxide solutions.

As was pointed out in the preceding section,
when an ionic substance is dissolved in a fused
hydroxide, dissociation, in the sense of separation
of ions, occurs, and the cations and anions of the
dissolved substance behave as chemically separate
entities, This does not mean that complex ions

may not form. It means that an ionic compound
such as sodium chloride dissolves as sodium ions
and chloride ions rather than as sodium chloride
molecules.  This concept of ionization is' the
foundation of most of the modern chemistry of fused
electrolytes. On the basis of this concept it is
postulated that, when an ionizable solute is dis-
solved to form a dilute solution in a fused
hydroxide medium, separate oxide-ion affinities
may be ascribed to the cations and to the anions
of the solute., Although there are definite limi-
tations to the application of this postulate, it is
very useful for two reascns. First, it provides a
satisfactory qualitative description of the known
reactions between solutes and hydroxyl ions in
fused hydroxides. Second, to the extent that this
postulate is true, it allows inferences to be made
about the reactivity of an untested ionic compound
composed of cations C* and anions A~, provided
that the behavior of C* and A~ is known separately
trom the reactions of compounds which have as
cations only C* and other compounds which have
as anions only A—.

Considerations of the origin of the oxide-ion
affinity of cations, such as has been presented
by Dietzel'! and by Flood and Férland,'? indicate
that cations of very low ionic potential should
have low oxide-ion affinities, The cations to be
found in this category are the alkali-metal ond
alkaline-earth These ions have ac-
cordingly been found to show only o weak-to-
negligible tendency to react with hydroxyl ions in
tused hydroxide solution, _

Likewise, certain kinds of anions have such
small oxide-ion affinities that they have been
found to show only a weak-to-negligible tendency
to capture oxide ions from hydroxyl ions in fused
hydroxide solution. The onions in this category
include the oxide ion, the halide ions, and some
oxysalt anions such as carbonate and sulfate.
Obviously, ortho-oxysalt anions belong in this
category inasmuch as they are all saturated with
oxide ions. lons which show little or no tendency
to accept oxide ions from hydroxyl ions in fused
hydroxides will be referred to, for convenience,
os "‘saturated’’ ions. ;

Ceramics with Saturoted Cotions and Soturated
Anions, — lonic compounds with soturated cations
and anions have thus far all proved to be too
soluble for use as solid components in fused
hydroxide media. The alkali metal oxides!3 gnd
halides®:14 have been found to be quite soluble.

cations.
Barium chloride!3 showed significant solubility
in sodium hydroxide ot 350°C. A sample of
calcium oxide fired at 1900°C to give an apparent
porosity of 3% was reported!® to be severely
attacked by fused sodium hydroxide at 538°C,
although other tests!3 indicate that its solubility
is small at 350°C.

Ceramics with Accepter Cotions and Soturated

Anions, — A substantial proportion of the ceramics.

and ceramic-related substances which have been
tested in fused hydroxides is composed of cations
with appreciable oxide-ion acceptor strengths and
anions which are saturated. Cations which accept
oxide ions from hydroxyl ions should do so by the
formation of either oxides or oxysalt anions,

The formation of an oxide by the reaction of an
acceptor cation with a fused hydroxide has been
observed in studies!3 of the reactions of mag-
nesium chloride and nicke! chloride with fused
sodium hydroxide. These compounds reacted very
rapidly at 400°C to form insoluble magnesium oxide
and insoluble nickel oxide. The reaction of mag-

nesium chloride is given by the equation

(3.2) MgClz(solid) + 20H~ (melt)

= MgO(sclid) + 2CI~(melt) + HQO(gos)

Considerakble data are available on the reaction
of oxides with hydroxyl ions. Three oxides, those
of magnesium, zinc, and thorium, have been tested
up to temperatures without giving
evidence of Four oxides, those of
cerium(lV), nickel, zirconium, and aluminum, have
been shown under some conditions to have a sig-
nificant resistance toward reaction with hydroxyl
ions, although they are all known to be capable
of reacting completely. Two oxides, those of
niobium(V) and titanium, have been found to react
relatively rapidly at lower temperatures.

substantial
reaction,

Further
details on the above oxide-hydroxide reactions will
be given below.

The sction of fused sodium hydroxide on mag-
nesium oxide was studied by D'Ans and Loffler!¢
at temperotures up to 800°C, but they were unable
to detect any woter evolution,  Steidlitz and
Smith,? who made mass spectrometric determi-
nations of the gases evolved on heating sodium
hydroxide to 800°C in o vessel cut from a mag-
nesium oxide single crystal, found water vapor,
but its presence was accounted for quantitatively
in terms of the thermal dissociation of hydroxyl
ions in the presence of sodium ions. The chemical

stability of magnesium oxide in fused sodium
hydroxide is not surprising, since magnesium is

not known to occur in an oxysalt anion. 7

Magnesium oxide is not only chemically stable
in fused sodium hydroxide but is also quite in-
soluble. In corrosion tests, Steidlitz and Smith4
found that single-crystal specimens of magnesium
oxide in a large excess of anhydrous sodium
hydroxide at 800°C. decreased in thickness by less
than 0.001 in. in 117 hr.

Water in fused sodium hydroxide has been found
to attack magnesium oxide. Boston!® reported that
anhydrous sodium hydroxide has no visible effect
on cleavage planes and polished surfaces of mag-
nesium oxide crystals up to 700°C but thut the
presence of water vapor over the melt caused rapid
etching of the crystal surfaces. This effect of
water is enfirely consistent with the oxide-ion
donor-acceptor concept.

The equilibrium between magnesium oxide and
water in the presence of sodium hydroxide may be
expressed as

(3.3)  MgO(s) + H,O(melt)

= 20H"(melt) + Mg++(me|f)

The activity of solid magnesium oxide, MgQ(s), is
a constant, the hydroxy! ions,
OH=(melt), are present in great excess and should
have approximately constant activity. Conse-
quently, the application of the law of mass action
to Eq. 3.3 shows that the activity of magnesium
ions in the melt should be proportional to the
activity of water in the melt; that is, an increase
in the conceniration of water in the melt should
increase the apparznt solubility of magnesium
oxide.

Furthermore,

The practical application of magnesium oxide as
a ceramic material for service in fused anhydrous
hydroxides presents difficulties. Most sintered
compacts of pure magnesium oxide ore sufficiently
porous to absorb appreciable quantities of sodium
hydroxide. l.arge magnesium oxide single crystals
can be machined into fairly complicated shapes
when a special need for a particularly corrosion-
resistant part justifies the expense. At the Ook
Ridge Nationa! Laboratory, reaction vessels have
been machined from massive magnesium oxide
crystals, and in every instance such vessels have
proved to be very satisfactory containers for fused
sodium hydroxide.
Qualitatively, zinc oxide behaves much like
magnesium oxide in the presence of fused sodium
hydroxide. D'Ans and Loffler’® were unable to
find evidence of a reaction between zinc oxide
and anhydrous sodium hydroxide at 600°C. How-
ever, oxysalt anions of zinc are known to exist, 17
and it is possible that a reaction occurs at temper-
atures above 600°C. D’Ans and Léffler report
that water greatly increased the apparent solubility
of zinc oxide in sodium hydroxide. This behavior
may be analogous to that of magnesium oxide, or
it may result from the formation of complex zinc
anions involving water.'? In a corrosion test!?
compressed and sintered zinc oxide was found to
have lost 8% in weight at 538°C.

Chemical studies'® on thorium oxide indicate
that it does not react with fused sodium hydroxide
up to 1000°C. However, no corrosion or solubility
data are available.

As pointed out above, cerium(lV) oxide, nickel
oxide, zirconium oxide, and aluminum oxide are
all known to react completely with sodium hy-
droxide under suitable conditions, However, they
_all have been reported to show considerable re-
sistance toward reaction under other conditions.

D*Ans and L&ffler!® found that cerium(lV) oxide
reacted very slowly with an excess of sodium
hydroxide, the interaction beginning first between
950 and 1000°C, but, surprisingly enough, an
excess of cerium(lV) oxide reacted more easily
at 900°C to give N«JIZCeC)3 and water. '

Nickel oxide was found!3 to be unreactive in
fused sodium hydroxide at 400°C, at least for short
periods of time. Williams and Miller?? did not find
any reaction even at 800°C for a period of 2 hr.
However, Mathews, Nauman, and Kruh2? found that
at B00°C nickel oxide reacted rapidly with sodium
hydroxide to give water and Na,NiQ, according to
the equation

(3.4) NiO(s) + 2NaOH(l)
: = quNioz(s) + Hzo(g)

The difference between the results of these two
groups of experimenters is not understood.
Zirconium oxide was found by D’Ans and Loffler!®
to react with excess sodium hydroxide to form
Na,ZrO,. However, corrosion tests on stabilized
zirconium oxide have shown this substance to be
resistant to attack. Craighead, Smith, Phillips,
and Jaffee'’ found that stabilized zirconium oxide,
fired at 1700°C in either air or argon to give an
apparent density of less than 0.5%, was unaffected

by exposure to fused sodium hydroxide for 25 hr
at 538°C. Stabilized zirconium oxide differs from
the pure substance in having o different crystal
structure at the temperatures of interest here and
in having a few weight per cent of calcium or
magnesium oxide in solid solution. However, it
is difficult to see how these differences could
significantly alter the thermochemistry of the zir-

conium oxide—sodium hydroxide reaction, although

they may have an effect on the rate of reaction.
Aluminum oxide was found by D'Ans and Léffler!®
to react with sodium hydroxide to give the oxysalt
NuzAlzod. Craighead, Smith, Phillips, and Jaffee!?
exposed a single crystal of aluminum oxide to
sodium hydroxide at 538°C and reported that the
crystal decreased in weight by 0.40% after 24 hr.
Simons, Stang, and Logedrost?? used hot-pressed
A|203 as a pump bearing submerged in fused
sodium hydroxide ot 538°C. After 8 hr of pump
operation they were unable to detect any signs of
bearing wear or other damage. Tests have also
been reported!® for hot-pressed and for sintered
aluminum oxide. Weight changes which were
observed were less than those for single-crystal
material under the same conditions. This sur-
prising result may have been caused by abserption
of hydroxide into pores of the hot-pressed sample,
which compensated for a weight loss. :
The relatively good corrosion resistance of
aluminum oxide found in these corrosion tests is
confirmed by day-to-day experience at ORNL.
Vessels made of pure, binder-free, recrystallized
aluminum oxide without poresity have been found
to be excellent containers for fused sodium hy-
droxide under a wide variety of service conditions
where a small aluminate contamination could be
tolerated. It should be remembered, however, that
most commercial alumina and some so-called
““corundum ware’’ have a silica or fluoride binder
which is readily ottacked by fused hydroxides.
Niobium(V) oxide has been found to react very
readily with fused sodium hydroxide. Spitsyn and
Lapitskii?3 heated niobium(V) oxide with fused
sodium hydroxide for 1-hr periods at temperatures

of 350 to 650°C and found the reaction :
(3.5) Nb205 + T0NaCH = QNGSNE)OS + 5!’120

Titanium dioxide reacts readily with fused sodium
hydroxide at 800°C. D’Ans and Loffler'® studied
this reaction and concluded that when an excess of
hydroxide was present the reaction was not quanti-
tative but that an equilibrium existed, probably
between sodium orthotitanaie and a metatitanate.
Such an equilibrium might be as follows:

(3.6) Na,Ti, 0, + 2NaOH = 2Na,TiO; + H,O

In corrosion tests,!? titanium dioxide ceramics
sintered at 760°C were found to be very severely
attacked by sodium hydroxide at 538°C.

Inasmuch os sodium aluminate appears to be
stable in fused sodium hydroxide, it might be
supposed that the aluminate anion is of itself inert.
It has been shown!3 that ceramics with compo-
sitions in the neighborhood of MgAl, O, are quite
corrosion~resistant in fused sodium hydroxide.
This result is not surprising, bacause sodium
aluminate has a low solubility and the magnesium
ion would tend fo react fo give magnesium oxide,
which is very corrosion-resistant,

Ceramics with Soturcted Caticns and Accepter
Anions. — The saturated cations, as pointed out
above, are those of the alkali and alkaline-earth
metals. Only a few tests have been made on
compounds with cations which are saturated and
anions which will accept oxide ions from hydroxyl
ions, and they have been limited to the sodium
oxysalts with high oxide-ion affinities. Obviously,
such substances should not be corresion-resistant.
However, an exomination of their behavior will
serve to illustrate the kinds of reactions which
anions undergo.

The chemical composition of a sodium oxysalt
can alwsys be expressed in terms of the number
of equivalent weights of sodium oxide per equivalent
weight of the other oxide of which the oxysalt is
staichiometrically composed. Feor convenience,
this quantity will be designated as 3.

In general, the greater the 2 value of an oxysalt,
the less will be the oxide-ion affinity of its anion
as compared with other cnions composed of the
same elements. Therefore, if an oxide is found
to react with fused sodium hydroxide to yield at
equilibrium an oxysalt with a value of 2 equal to
o and if there are other oxysalts of the oxide in
question with values of X less than o, it may be
presumed that the other oxysalts will also react
with fused sodium hydroxide. This well-known
postulate, herein called the 'Y postulate,’’ under-
lies all phase-diagram work in which an oxide-ion
donor such as sodium carbonate is used as a
source of oxide ions.

In agreement with this postulate Spitsyn and
Lapitskii?® showed that sodium metaniobate(V),

like niobium(V) oxide, reacts with sodium hy-
droxide to give sodium orthoniobate(VY). This
reaction is as follows:

(3.7) NaNbO, + 4NaOH = NagNbQ, + 2H,0

Some information is available on two nonmetallic
systems of sodium oxysalts, [t has been known
for a long time that silicon dioxide, boron oxide,
the silicates with smoll & values, the borates of
boron{lil) with small 2 values, and the glasses
based on these substances nearly all react readily
with fused hydroxides to form water and the more
alkaline silicates and borates. This reactivity has
been utilized for many years in the solubilization
of minerals by a process known as alkali fusion,
On the other hand, very little information is
available on the action of fused hydroxides on
the more alkaline silicates and borates, many of
which should be relatively unreactive toward fused
hydroxides provided that they contain unreactive
cations.

From a knowledge of the end products of alkali
fusions it would be possible to estimate the
reactivity of the silicates and borates with high
3 values by application of the 3 postulate. Un-
fortunately, there are very few data of this kind.
Usually the products of alkali fusions are not
examined until ofter they have been hydrolyzed.

The results of two experiments are available,
however. First, Morey and MerwinZ4 mention,
without giving experimental conditions, that sodium
pyroborate is stoble in fused sodium hydroxide.
Second, D'Ans and Loffler1® found that silicon
dioxide reacts with an excess of sodium hydroxide
to yield an equilibrium between the pyrosilicate
and the orthosilicate as follows:

(3.8) Na $i,0, + 2NaOH = 2Na,5i0, + H,0

From these dota it is concluded thot borates with
Y less than 2 and silicates with 2 less than 1.5
will react in a similar fashion. This rather terse
conclusion can be interpreted in terms of the
structural chemistry of the borates and the sili-
cates.

Oxygen atoms in simple borates and silicates are
of twe kinds: those which are bound to two boron
or silicon atoms and form so-called ‘‘oxygen
bridges" and those which are bound to only cne
boron or silicon atom and carry o negative chorge.
Thus, the pyroborate ion has one oxygen bridge
and may be structurally represented as

“O\B/O\B/O“
1L

When an oxide ion reacts with a borate or sili-
cate, an oxygen bridge is broken and is replaced
by two nonbridge oxygen atoms. Thus, the pyro-
borate ion shown above can react with one oxide
ion to form two orthoborate ions each with the
following structure:

-0 o-
N
I

The X postulate implies that the greater the number
of oxygen bridges attached to a given boron or
silicon atom, the greater the oxide-ion affinity of
that atem. Since simple borates and silicates with
Y greater than 2 and 1.5, respectively, react with
sodium hydroxide, it is concluded that for these
compounds a boron or silicon atom with more than
one oxygen bridge has sufficient oxide-ion affinity
to accept an oxide ion from a hydroxyl ion. |

There are ‘other alternatives to the structural
interpretation of silicate and borate reactions in
fused hydroxides. For example, equilibrium 3.8
is thermodynamically consistent with the as-
sumption that, rather than the oxygen bridge in
the melt being a physical entity, the pyrosilicate
ions (known to exist in solids) dissociafe into
orthosilicate and SiG_ 2~ ions. Another alternative
is that there is an equilibrium in the melt between
5i,0 6~ on the one hand and :SiOA‘*" and Si03:2“
on the other without either one or the other pre-
dominating. The present technique of studying
fused electrolytes does not make it possible to
determine which reaction takes place, ‘

Oxidation-Reduction Reactions. — Only a few
examples have been found of oxidation-reduction
reactions of ceramics and ceramic-related sub-
stances in fused anhydrous hydroxides. Further-
more, all oxides and oxysalts which contain metals
capable of existing in more than one valence state
in fused hydroxides should undergo oxidation or
reduction reqctions, There are very few studies
of oxidationreduction reactions of oxides or oxy-
salts in fused hydroxides., In the former category
the behavior of silicon and carbon, two elements
which might be considered ceramics, has been
studied. :

As shown by LeBlanc and Wey!,?% silicon reacts
violently when displacing hydrogen from fused
potassium hydroxide at 400°C. The over-all re-
action was not determined, but an obvious guess,
on the basis of the information in the preceding
section, is that the orthosilicote is formed ac-
cording to the equation

(3.9)  Si + 4NoOH = Na,Si0, + 2H,

Carbon in the form of massive graphite shows
very fittle tendency tfo react :o‘r temperatures up to
500°C but does absorb some hydroxide because
of its porosity. However, it has been demonstrated
that oxidation of graphite does occur, although net
rapidly, at temperatures as low as 500°C. Ketchen
and Overholser?® made a careful study of: the
action of a large excess of sodium hydroxide on
graphite (possible oxidants other than the hy-
droxide were excluded) and found that, above
500°C, graphite is oxidized to form sodium car-
bonate. At 815°C, graphite has been shown to
react quite rapidly.!3 '

D'Ans and L&ffler'6 report that under an atmos-
phere of nitrogen, iron(lil) oxide reacts with fused
sodium hydroxide to give mostly sodium ferrate(ll1),
with a small amount of an oxysalt in a higher
valence state, which they suggest may have been
sodium ferrate(V1).

Corrosion of Other Kinds of Ceramics. — The
action of sodium hydroxide on a number of other
kinds of ceramics such as carbides has been
studied13 in corrosion tests, However, no experi-
mental information was obtained which would
identify the chemical reactions.

Secondary Corrosion Phenomena. — The three
causes of the deterioration of ceramic bodies,
which will be considered secondary in the sense
that they do not reflect the infrinsic chemical
reactivity of the primary ceramic constituents, are
spalling, binder attack, and pore penetration.

It is well known that many ceramics spall when
subjected to severe thermal shock. Occasionally,
spalling which resulted from the im-
mersion of a cold specimen into a hot liquid has
been confused with corrosion,

however,

Many ceramic bodies contain binders which have
different chemical properties from those of the
principal constituent. Some of the more common
binders are among the most reactive materials in
fused hydroxides, so that aitack on the binder
cavses disintegration of the ceramic body. An
example of this is the ottack of fused hydroxides
on magnesia ond olumina. The pure oxides MgO
and Al,O, are resistont to corrosion by fused
hydroxides. However, these substances are fre-
quently fabricated with the aid of a high-silica
binder, which is strongly attacked.

Fused hydroxides show a pronounced capillary
activity, ond they very readily penetrate small
There is
no evidence that such pore penetration will of
itself cause disintegration of the ceramic,

pores and cracks in ceramic wmaterials,

How-
ever, ceramic spscimens ars frequently washed
with waoter after test to remove adhering hydroxide
Under

these conditions ahydroxide-impregnated specimen

and are exposed to air during examination.

will disintegrate, since the hydroxide rapidly forms
hydrates or hydrated carbonates with appreciable
volume expansien. Such behavior has been ob-
served for porous specimens made of pure mag-
nesium oxide and pure aluminum oxide.

The pronounced capillary activity of fused hy-
droxides lenods to other difficulties in corrosion
tests, Fused hydroxides wet o very wide variety
of substonces, both metallic and nonmetallic, and
show a strong tendency to creep. This property
has not been observed to be serious near the
melting point of the hydroxide, bui at higher
temperatures it presenis significant experimental
difficulties.

Summory, — Studies which have been made of the
behavior of ceramic materials in fused hydroxides
are limited in scope both as regards the variety of
substances tested and the information abtained on
any one substance. The corresion of ceramics and
ceramic-related materials hos been found to occur
both by solution and by chemical reaction. A few
substances, among them graphite and silicon, were
found to undergo oxidation-reduction reactions, but
for the majority of compounds the attack proceeded
by an oxide-ion donor-acceptor reaction,

Compounds of the alkali and alkaline-earth
metals such as oxides, halides, and saturated
oxysalts have not been found to react with fused
sodium hydroxide but have usually shown ap-
preciable solubility, Compounds of other metals
with the above anions were found to react by an
oxide-ion acceptor-donor mechonism to form an
oxide of the metal or to form an oxysalt.

Oxides of magnesium, zinc, and thorium have
been tested at substantial temperatures without
giving evidence of chemical reaction.
oxide

Magnesium
in anhydrous
sodium hydroxide but is susceptible to mild attack

is very corrosion-resistant

by small amounts of water. Zinc oxide is not so
corrosion-rgsistant as magnesium oxide. No cor-
rosion data are available on thorium oxide,

Oxides of cerium(lV), nickel, zirconium, and
aluminum have been shown under some conditions
to have a significant resistance toward reaction
with hydroxyl ions, although they are all known to
be capable of reacting by an oxide-ien donor-
acceptor mechanism. Under many conditions alu-
minum oxide is sufficiently inert to serve as a
useful container material for fused hydroxides.

Oxides of niobium({V) and titanium react fairly
readily with fused sodium hydroxide by an oxide-
ion donor-acceptor mechanism at relatively low
temperatures.,

There have been very few studies of compounds
containing reactive anions but unreactive cations,
In the few examples known, the reactions all
proceeded by oxide-ion donor-acceptor mechanisms.
lndications are that the oxide-ion affinity of simple
borates in fused hydroxides is not satisfied until
the pyroborate anion is formed., Simple silicates
react by a mechanism like that of the borates, but
the end product seems to be an equilibrium between
the pyrosilicate and orthosilicate anions.

The corrosion resistance of a fabricated ceramic
body is dependent on the intrinsic solubility and
chemical stability of the primary constituents and
is also determined by porosity and the resistance
to attack of any binder material it may contain,

4, CORROSION OF METALS

Studies have been made of the action of fused
hydroxides on at least 31 elemental metals and
65 alloy compositions. The results of these
studies show a wide variety of corrosion phe-
In the following discussion, only the
more common types of corrosion will be treated.
These types will be illustrated by the better known
examples.

Comrosion of Metols by Oxidation. — The oxi-

nomend.

dation of metal ctoms to form ions is a basic step
in the corrosion of metals by fused hydroxides.
Oxidation may be coused by the action of hydroxyl
ions, alkali metal ions, or foreign substances
dissolved in the hydroxide.

Corresien by Hydroxyl lons, - Oxidation by
hydroxy! ions is the most common form of corrosion
of metals in fused hydroxides.
the hydroxyl ions are reduced to hydrogen and
oxide ions, while the meta! is oxidized to form
metal ions. These metal ions may dissolve as

In such reactions
such or may act as oxide-ion acceptors and form
on oxide or an oxysalt, Oxide-ion acceptor re-
actions were discussed in the preceding section
on ceramics,

For a metal M of valence v, the basic step in
corrosion by hydroxyl ions may be represented as

(4.1) M + OH- = M & LO-= 4 li H,

It has been demonstrated that this reaction occurs
for a wide variety of metals from the most reactive
to the most inert.

In a few instances this hydroxy! ion reaction is
accompanied by a side reaction which produces a
hydride of the metal. If the hydride is volatile, it
may escape; thus arsine and stibine are evolved
from the reactions of arsenic and antimony, re-
spectively, with fused sodium hydroxide.?” If the
hydride is not volatife, it may react with the
hydroxide as do the alkali metal hydrides. This
side reaction does not seem to be important in the
corrosion of most metals. Water is formed in a
second side reaction which is sometimes observed,
The possible processes involved here will be
discussed later.

Attempts to account for the relative corrosion
resistance of different metals foward fused hy-
droxides have had only limited success. There
is some correlation between the standard free
energies of formation of the oxides and corrosion
resistance, as has been pointed out by Brasunas,?8
The metals whose oxides are most stable are
frequently the least corrosion-resistant and vice
versa, Thus calcium with a very stable oxide
reacts vigorously with fused hydroxides, while
silver with a relatively unstable oxide is fairly
corrosion-resistant. This correlation, however, is
no more than a rough, general guide. For example,
nickel is more corrosion-resistant to fused hy-
droxides than gold is, although gold oxide is mueh
less stable than nickel oxide.

For present purposes, elemental metals will be
classified in two groups according to corrosion
resistance toward fused sodium hydroxide. The
first group consists of those metals which seem
to be strongly and irrepressibly attocked by hy-
droxyl ions at 500°C under all conditions which
have been studied. This group includes alkaline-
earth metals, antimony, arsenic, beryllium, cerium,
niobium, magnesium, manganese, molybdenum, tan-
talum, titanium, tungsten, and vanadium. The
second group consists of those metals which show
a measure of corrosion resistance toward fused

sodium hydroxide under suitable conditions at
500°C. This group includes aluminum, bismuth,
chromium, cobolt, copper, gold, indium, iron, lead,
nickel, palladium, platinum, silver, and zirconium,

The second group is, of course, of more interest,
Some of the metals appear to be corrosion-resistant
because of protective film formation, while others
appear to be corrosion-resistant under suitable
conditions because the thermodynamic driving force
is small. Aluminum will serve as an example of
a metal which seems to be protected by film
formation, and nickel is an example of a metal
which can be maintained almost in thermodynamic
equilibrium with the melt under isothermal con-
ditions. Film formation will be considered first.

There is no thermodynamic barrier to the reaction
with sodium hydroxide no matter
whether the reacfion product is aluminum oxide
or sodium aluminate or whether the product occurs
as a solid phase or is dissolved in the melt. Free-
energy chonges which accompany the possible
reactions between aluminum and sodium hydroxide
have been estimated and were shown to be quite
negative.  The hydrogen pressure required to
reverse these reactions was likewise shown to be
much greater than any hydrogen pressures thus far
encounfered in corrosion tests. Mevertheless, at
[ower temperatures the reaction between aluminum
and fused hydroxides proceeds slowly. Craighead,
Smith, and Jaffee?? tested 2S5 and high-purity
aluminum in fused sodium hydroxide for 24 hr at
538°C. The specimen of 25 aluminum lost 0.337
mg/em? in weight and pitted slightly, while the
high-purity. aluminum lost 1.68 mg/cm? but showed
no other signs of corrosion on metallographic
examination. The origin of the corrosion re-
sistance of aluminum is not known, but in view
of the corrosion resistance of aluminum oxide (see
Sec, 3, ‘‘Corrosion of Ceramics’’) it seems probable
that the reaction of hydroxyl ions with aluminum
produces at first a thin, pré’fective film of oxide,
which then very slowly reacts to form the slightly
soluble aluminate.

of aluminum

Nickel represents a metal which can be held,
even at high temperatures, almost at thermodynamic
equilibrium with the melt. The corrosion rate of
nickel at high temperotures has been found to be
dependent on the partial pressure of hydrogen over
the melt. ‘Under 1 atm of hydrogen, nickel shows
only negligible signs of corresion in fused sodium
hydroxide for 100 hr up to 815°C, provided that

thermal gradients are absent, as shown by Smith,
Steidlitz, and Hoffman.?? However, if the hydrogen
atmosphere is not maintained, the corrosion rate
of nickel in fused sodium hydroxide may become
very rapid at high temperatures. Williams and
Miller?® gnd Petersen and Smith!'3 have studied
this reaction undeir such conditions that the hy-
drogen which formed was removed by high-speed
vocuum pumping. It was found that below 400°C
the reaction is exceedingly slow but that above
800°C it is exceedingly rapid. The only gaseous
product found was hydrogen. The metal formed a
sodium nickelate(ll) of uncertain composition but
was probably Na,NiO,. This reaction might be
represented by

(4.2) Ni(s) + 2NaOH(I) = Na,NiO,(s) + H,(g)

Williams and Miller2? suggested that the product
of the above reaction might be either a compound
or a mixture of sodium oxide and nickel oxide.
Petersen and Smith'? isolated single crystals of
the reaction product and were able to obtain the
unit ce!! dimensions by x-ray methods. The results
left no doubt that the reaction product which they
obtained was a sodium nickelate(l!) compound.

In other studies at 950°C the hydrogen which
formed was not rapidly evacuated from the system
but was removed slowly so that a significant
partia! pressure existed over the melt, Under these
conditions, and Hannan®! and
Mathews, Nauman, and Kruh?! report that con-

Peoples, Miller,

siderable water vapor was evolved. Mathews et al.
found that water and hydrogen were evolved ap-
proximately in an equal molar ratio and suggested
that the corrosion process consisted in twe steps:
the formation of nickel oxide with the evolution of
hydrogen,

(4.3) Ni + 2NaOH = NiO + Na,0 + H,

followed by
(4.4)  NiO + 2NaOH = Na,NiQ, + H,0

The latter reaction was discussed in Sec. 3. The
over-all reaction would be

(4.5) Ni + 4NaOH
= N\:12Ni02 + NaZO + HZO + H2

Peoples, Miller, and Hannan3! did not check the
hydrogen-water correlation, but they, also, sug-
gested equations similar to those above as a
source of water.

An additiona! source of woter was also postu-
lated by both Williams and Miller20 and by Peoples,

10

Miller, and Hannan,®! namely, the reduciion of

oxide ions by hydrogen when the latter is present
Such a reduction of
oxide ions would have to be accompanied by a
reduciion of positive ions such as nicke! ions
either in NiO or in Na2Ni02.

There are several

in sufficient concentration.

kinds of evidence which
support the plausibility of this second postulate.
It is known from the work of Pray and Miller3?
that the sodium nickelate(!!) which is formed from
the hydroxide-nickel reaction can be reduced to
metallic nickel by a partial pressure of hydrogen
of 0.1 atm ot 950°C. In reactions between nickel
and sodium hydroxide, particulate nickel is fre-
quently found in the hydroxide phase, Mathews,
Nauman, and Kruh?! found particulate nickel in
the experiments from which they deduced Egs. 4.3
and 4.4, Williams and Miller?® reported that in
their experiments there was a definite correlation
between the occurrence of particulate nickel in
the melt and the occurrence of water in the gas
phase and that neither particulate nicke! nor water
occurred if the hydrogen evolved was removed
rapidly enough for its partial pressure over the
melt to remain low,

Data on the action of hydroxy! ions on the metals
bismuth, chromium, cobalt, copper, gold, indium,
iron, lead, palladium, platinum, and silver are very
fragmentary, and no attempt at a review will be
made here except to point out two facts which are
important in  evaluating corrosion
generally.

mechanisms

First, Lad3?® has shown thai, when chromium
reacts with fused sodium hydroxide, it is oxidized
first to a lower valence state and is followed by
a very slow oxidation to a higher valence state.
Similar data are not available for other metals
which have more than one oxidation state in fused
hydroxides, but the work of Lad suggests that the
reaction kingtics of such metals in fused hy-
droxides moy be quite complicated.

Second, although the reaction between nickel and
sodium hydroxide can be inhibited by hydrogen,
Williams and Miller?® have shown that hydrogen
has no inhibiting effect on the reaction of iron with
sodium hydroxide. Furthermore, they were unable
to find any metallic iron resulting from attempts
ai hydrogen reduction of iron corrosion products
in the presence of fused sodium hydroxide. This
result is important in a consideration of the
mechanisms of mass transfer in fused hydroxides
and will be discussed in Sec. 5.
Corrosion by Alkeli Metal lons, -~ The ability
of metals such as iron to reduce alkali meta! ions
in fused alkali-metal hydroxides has been known
for more than a century and was af one time used
in the laboratory preparation of the alkali metals.
LeBlanc and Weyi” reported that chromium, mo-
lybdenum, tungsten, magnesium, and sodium were
found to displace potassium metal, as well as
hydrogen, from fused potassium hydroxide. Villard®4
found that fused sedium hydroxide was reduced to
give sodium or sodium hydride by magnesium,
chromium, tungsten, iron, cobalt, nickel, and ferro-
manganese. Brasunas?® found that zirconium
reduced sodium hydroxide to give sodium metgl
and hydrogen. '

Williams and Miller?? reported that nickel acts
on fused sodium hydroxide in a two-step process
by which hydroxy! ions are reduced and, atter they
are consumed, sodium ions are reduced. This
sequence of events corresponds to the reaction
in which sodium nickelate(ll) is formed and then
the sodium ions are reduced in the nickelate,

From the standpoint of over-all reactions it might
be concluded that (1) mild reducing agents such
as nickel act first on hydroxyl ions and when they
are reduced to a low cencentration, sedium ions
are attacked; (2) very strong reducing agents like
zirconium act on both kinds of ions at the same
time, However, the experimental findings are also
consistent with the postulate that hydroxyl ond
sodium ions are simultaneously reduced by all

metals. Sodium-ion reduction may be represented
as
(4.6) Ni o+ 2Ma® = Ni*" 4+ 2Na

The sodium thus generated con react with hydroxyl
ions as follows: ‘

(4.7) 2Na + 20H- Na® + 207~ 4+ H,

It reaction 4.7 proceeds as rapidly as sodium is
generafed, the net result is as though only hydroxyl
ions hod been reduced. As the concentration of
hydroxy! ions decreases, reaction 4.7 will proceed
more slowly, untii finally sodium is generated
faster than it is consumed. In the case of the very
active metals such as zirconium, the rate of gener-
ation of sodium may be much more rapid than the
rate of reaction 4.7 even for a high concentration
of hydroxyl ions. Petersen and Smith'? studied
the reduction of sodium hydroxide by sodium and
found thot it was not by any meons so rapid as
reduction by such active metals as calcium,

The action of weok reducing agents such as
nickel on alkali metal ions in fused hydroxides
would be brought te equilibrivm by a very small
concenfration of the alkali metal. Hence, the
generation of appreciable quantities of alkali metal
is only possible under conditions in which the
alkali metal can escape from the reacting mixture.
Gold forms an alloy with sodium, so that in the
sodiuvm hydroxide—geld reaction some sodium may
be removed from the field of reaction by alloy
formation, as was pointed out by Williams and
Milier.20 |n corrosion tests at temperatures above
800°C smoll amounts of sodium metal have some-
times been found on cooler parts of the apparatus
above the liguid., Presumably, this metal had
distiilled out of the melt. However, under the
conditions of most corrosion tests on relatively
corrosionresistant metals in fused hydroxides
below 800°C, appreciable quantities of alkali metal
are never found, Therefore, from the standpoint of
the thermodynamics of corrosion, over-all reactions
in which alkali metals are produced would seem
to be of little importance. Mevertheless, such
reactions may be quite significant in the kinetics
of corrosion. _

Corrosion by Oxidizing Solutes. — Dissolved
oxygen and the peroxide ion are known to be
powerful oxidizing agents in fused hydroxides. By
using such solutions, Dyer, Borie, and Smith33
were able to cause very rapid corrosion of nickel
with the production of oxysalts in a valence state
greater than 2,

Water at sufficient concentrations seems to act
as a weak oxidizing agent. Peoples, Miller, and
Hannan®! reported that at 950°C small additions
of water to the blanketing atmosphere over fused
sodium hydroxide had little effect on the corrosion
rate of nickel but that substantial amounts of water
made the corrosion much more severe.

Comosion of Alloys. ~ Although nickel hos o
superior corrosion resistance in fused hydroxides
at high temperaiures, it has o low mechanical
strength. Consequentiy, studies have been made
of the effect of alloying constituents on corrosion
behavior with the ultimate aim of obtaining a ma-
terial which is both corrosion-resistant and use-
fully strong.

There is no evidence as yvet that the corrosion of
alieys by fused hydroxides involves any kind of
chemical reaction which is different from that en-
countered with elemental metals. However, an
examination of microstructures shows two corrosion

i1
phenomena distinctive of alloys: (1) the formation
of pores or voids benegath the surfuce of the cor-
roded alloy and (2) the fonmation of complex, iwo-
phase, corrosion products at the surface and along
the grain boundaries of the corroded alloy. Ex-
amples of each of these forms of coirosion will be
cited below from the work of Smith, Steidlitz, and
Hofman, 30

The formation of pores or subsurface voids was
found to be characteristic of the attack of fused
sodium hydroxide on high-purity nickel-iron alloys.
Nickel exposed to fused sodium hydroxide at 815°C
for 100 hr in an evecuated capsule was found to be
slightly etched, with an average loss in thickness
of 4.5 x 1073 in. as computed from weight-change
data. lvon under the same conditions was found
to lose about 100 times as much in thickness. A
high-purity nickel-iron alloy containing 20 wt %
iron not only suffered atiack by removal of surface

 

Fig. .

metal but also developed subsurface voids to a
depth £ 310 5x 1073 in,

Subsurface void formation is not an uncommon
phenomenon in high-temperature corrosion.
and Gront36

Manly
proposed a mechanism for this form of
coirasion based on the well-established theory of
void formation during interdiffusion of metals.
Brasunas®’ has discussed this theory and has
presented some pertinent experimental data. Quali.
tatively, the thecry con be applied to hydroxide
corrosion of nickel-iron alloys as follows. During
the eorly stages of corrosion, the iron atoms at the
surface of the metal specimen are oxidized much
more rapidly thon the nickel atoms. As a result,
the surface becomes depleted in iron. This deple-
tion establishes a concentration gradient in the
proper direction to couse a diffusion of iron from
the interior of the specimen to the surface, where
reaction continues.

The diffusion of iron is pre-

L Y.sgo1 B

 

 

 

 

 

Complex Corrosion Product Layer Formed on Surface of Corpenter Compensator 30 Alley

(Nomina! Composition 30% Ni, 70% Fe) by Action of Fused NoaOH for 100 hr ot 815°C. 250X. (From the

sfudies of Smith, Steidlitz, and Hoffman, ORNL.)}

12
sumed fo occur by a vacani-laitice-site mechanism
so that there is a flux of vacancies diffusing from
the surface into the interior of the specimen. Since
the crystal lattice cannot maintain more than a
certain concentration of vacant lattice sites, the
excess vacancies, which eccumulate from inward
diffusion, “‘precipitate’’ as wvoids. These voids
continue to grow as long as the inward Hux is
maintained. |

Subsurface void formation has been found not
only in high-purity nickel-iron alloys but also
in high-purity nickel-molybdenum and nickel-iron-
moly bdenum alloys with o high nickel content.
Pure molybdenum under the some test conditions
was severely attacked.

As an example of the second kind of cormosion
phenomenon characteristic of the corrosion of
alloys by fused hydroxides, Fig. 1 shows in cross
section a corrosion product layer which formed on

 

a specimen of Carpenter Compensator 30 golloy
{nominal composition 30% Ni, 70% Fe) exposed to
fused sodium hydroxide for 100 hr at 815°C.  The
corrosion product loyer consisted of three zanes:
(1) an irregular, gray layer of nonmetallic material
in contuct with the fused hydroxide; (2) beneath
this layer, a zone consisting of a mixture of me-
tallic ond nonmetaliic phases; {3) lost, a zone of
intergranular penetration. The formation of such
complex layers of corrosion products was charace
teristic of the corrosion of a number of alloys, in-
cluding the commercial nickel-iron-chromium afloys.
Very few alloys show all three zones seen in
Fig. 1. Usually the zone of massive nonmetallic
material is absent, and, under suitable conditi;ons,
one of the ather two zenes may be absent.

Figure 2 shows the corrasion preduct formed on
Timken 35-15 alley {neminal composition 35% Ni,
15% Cr, 50% Fe) after exposure to fused sodium

 

 

 

 

 

Fig. 2. Corrosion Product Layer Formed on Timken 35-15 Alloy (Mominal Composition 35% Ni, 15%
Cr, 50% Fe) by the Action of Fused NaOH for 100 hr ot 815°C. 100X. (From the studies of Smith,
Steidlitz, and Hoffman, ORNL) '

13
hydroxide for 100 hr at 815°C. Here, only the zone
consisting of a metallic and nonmetallic phase is
to be seen, with a rudimentary grein boundary at-
tack appearing at the comosion product-base alloy
interface.

Of the various metals which form complex cor-
rosion-product layers in fused hydroxides, only
Incone! has been extensively studied. Results
with this metal illusirate the structural complexity
of the layers. Most of the tests have been con-
ducied for various times up to 100 hi, at various
temperctures up to 800°C, and under blanketing
atmospheres of purified helium and of purified hy-
drogen at 1 atm. The results obtained for tests
under helium were quite different from those ob-
tained for tests under hydrogen with regard to both
corrosion rate ond microstructure of the corrosion
product,

Under

a blonketing atmosphere of hydrogen,

 

lncone! showed a slight amount of corrosion ofter
100 hr at 600°C. From 600 to 800°C the corosion
increased very ropidly. At all temperaivies cor-
rosion begon as grain boundory attack, an example
of which is shown in Fig., 3. When thess grain
boundary regions were examined at a high magnifi-
cation, it was found that a comrosion prodyct had
formed in these regions and that this corrosion
product consisted of a metallic matrix within which
were acicular particles of a nonmetallic phase.
After groin boundary attack had advonced a few
thousandths of an inch into the alloy, a massive,
twe-phase, corrosion product layer had begun to
form on the surface and to thicken with time, Fige-
ure 4 shows the microstructure of this two-phase
layer at a high magnification.

The corrosion rate of Inconel at a given tempera-
ture was significantly greater under o blanketing
atmosphere of helium than under hydrogen. A mas-

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 3. Grain Boundary Attack at the Surfoce of an Incone! Specimen Exposed to Fused NoOH for 100
hr ot §00°C. 250X. (From the studies of Smith, Steidlitz, and Hoffman, ORNL)

14
 

 

 

 

Fig. 4. Microstructure Typi;ul of the Corrosion Produet Layer Formed on lncanellby Exposure to Fused
NaDH Under ¢ Blanketing Atmosphere of Hydrogen for Periods of up to 100 hr ot Temperatures above
650°C. 1000X. (From the studies. of Smith, Steidlitz, and Hoffman, ORNL)

sive, two-phase zone also formed under helium, but
the microstructure was quite different, as can be
seen in Fig. 5.

Other, quite different, two-phase, corrosion prod-
uvct microstructures were observed when Inconel
was corroded by fused sodium hydroxide to which
various inorgonic compounds had been added. - In
oll cases the layer of corrosion product consisted
of various geometric arrangements of nonmemlhc
particles within a metallic matrix.

A study is now being made to determine the
chemical nature of the phases which occur in the
corrosion product layers on Inconel. It is as yet
too early for any firm conclusions to be drawn other
than those that the metallic matrix is depleted in
chromium and that the nonmetallic particles have o
relatively high content of a sodium oxysalt. _

Summary. — Studies have been made of the action

of fused hydroxides on at least 31 elemental metals
and 65 alloy compositions. Most of the results
can be interpreted in terms of the oxidation of the
metal accompanied by the reduction of hydroxyl
ions to form hydrogen and oxide ions. The oxidized
metal may occur either as an oxide or ¢s an oxy-
salt. Three complicating side reactions are some-
times found: the formation of a metal hydride, the
reduction of alkali metal ions, and the produchon
of water.

Film formation is apporently important in the
corrosion resistance of niummum in fused sodium
hydroxide. _

The corrosion of nickel by fused sodium hydrox-
ide under isothermal conditions con be stopped
almost altogether at temperatures as high as 815°C
by the application of a pressure of hydrogen of
1 gtm over the melt,

15
 

 

 

o
B
A

 

 

 

 

Fig. 5. Micrestructure Typical of the Corrosion Product Layer Formed on Incone! by Exposure to Fused
NMaOH Under a Blanketing Atmosphere of Helium for Periods of up to 100 hr ot Temperatures above 650°C,
The Cu plate was applied after test to protect the edge of the specimen during metallographic polishing.
1000X. (From the studies of Smith, Steidlitz, and Hoffman, ORNL)

The chemistry of the reaction between nickel and
sodium hydroxide has been extensively investi-
gated but is still not well understood. For all
metals other than nicke!, details on the chemistry
of corrosion reactions in fused hydroxides are very
fragmentary.

The corrosion of alloys by fused sodium hydrox-
ide does not seem to involve any kind of reaciion
which is different from the reactions encountered
with elemental metals. However, the microstiuc-
tures of commoded alloys show the distinctive cor-
rosion phenomena of the formation of subsurface
voids and the formation of complex, two-phase cor-
rosion products at the surface of the alloy and
along grain boundaries near the surface.

16

5. MASS TRANSFER

When liquids are circulated through metal plumb-
ing systems at high temperatures, it is often found
that the hot parts of the plumbing are corroded and
that the metal thus removed is deposited in the
cool parts of the system. This corrosion-related
phenomencn, usually called “mass transfer,”” is a
particularly serious problem with fused hydroxides.
Mass transfer has been observed in sodium hydrox-
ide for nickel, iron, copper, silver, gold, and a
number of alloys. Moreover, nickel has been ob-
served to masseiransfer in all the fused alkali-
metal hydroxides and, it might be mentioned, in
all the fused alkaline-earth hydroxides.®® Mass
transfer is the primary factor limiting the use of
metals which, under suitable conditions, do not
corrode seriously.

The mechanisms of mass transfer in fused hy-
droxides are not known. However, several pos-
sibilities exist, and they will be discussed at the
end of this section. First, however, some of the
empirical facts of mass transfer will be presented.

Nickel. « Some of the characteristics of mass
transfer of nickel in fused hydroxides are illys-
trated with the results of thermal-convection loop
tests.3837 Figure 6 is a photograph of a nickel
thermal-convection loop which was cut open to
show the appearance of the inside wall surfaces
after test. During test the loop was filled with
fused sodium hydroxide and placed in an upright
position. One side of the loop was heated and the
other cooled to maintain the temperature distribuy-
tion shown in Fig. 6. The highest temperature
measured was 825°C, near the bottom of the hot
leg (left side in Fig. 6), and the lowest tempera-
ture measured was 535°C, near the bottom of the
cold leg (right side in Fig. 6). This temperature
distribution caused a flow of fused hydroxide
around the loop because of thermal convection.
The numbers in Fig. 6 from 5 to 45, at infervals of
5, identify positions around the loop.

Before the loop was exposed to the hydroxide,
the inside walls were somewhat rough and had a
dull appearance. After test, as seen in Fig. 6, the
hot leg from position 25 to position 45 became
polished, whereas the cold leg from position 5 to
position 20 became encrusted with a deposit of
dendritic nickel crystals. Grain boundary grooving
occurred in the polished zone, but the groove angle
was wide and there was no serious grain boundary
attack. The polishing observed in the hot leg is
characteristic of the corrasion step of mass trans-
fer at high temperatures in a number of different
kinds of systems, including some containing liquid
metals. :

Figure 7 shows a deposit of dendritic nickel
crystals from a cold-leg section of a loop with the
solidified hydroxide intact. In the initial stages of
deposition it was found that the crystals grew as a
dense deposit, forming an almost continuous plate
over the cold-leg surface. The crystal grains in
this plate frequently continued the orientation of
the grains in the base metal,: which necessitated
the use of special techniques for metallographic
detemminations of the place at which the base metal
ended and the deposit began. When this dense
plate became several thousandths of an inch thick,

LNCLASSIFIEDY
8 Y.5623

CHaN LA |

 

Fig. 6. Nickel Thermol-Convection Loop Cout
Open to Show Appearance of inside Wall Surfaces
After Test. The numbers 5 through 45 identify
positions around the loop. * (From the studies of

Smith, Cathcart, and Bridges, ORNL)

very long dendrites began to appear. Because of
the dendritic form of the thicker deposits, a rela-
tively small amount of nickel was found to have a
large effect in restricting flow through a pipe.
When the flow of liquid was sufficiently rapid,
dendrites become dislodged. and they collected in
irregular masses which formed plugs that effec.
tively stopped fluid flow altogether. Figure 8
shows a section of pipe from a large loop which
contained several such plugs, and Fig. 2 shows a

17
UNCL ASSIFIED
Y.5543

 

Fig. 7. Section of Tubing Removed from the Coo! Parts of a Nickel Thermal-Convection Loop and Cut
Open After Test with the Frozen NaOH Intact, Dendritic nickel crystals may be seen attached to the
inside walls of the tube. (From the studies of Smith, Cathcart, and Bridges, ORNL)

[ UNCLASSIFIEDR
Ev.4383 |

i
h

 

Fig. 8 Section of Nickel Pipe Cu? from the Cocl Portions of a Thermal-Convection Loop. Irregular
masses of nickel dendrites which stopped the flow of fused NaOH in the pipe may be seen. (From the
studies of G. M. Adamson, ORNL)

18
 

Fig. 9. Plug Formed of Nickel Dendrites Taken
from the Section of Nickel Pipe Shown in Fig. 8,
The plug was about 1 in. across. (From the studies

of G. M. Adamson, GRNL)

single plug removed from the loop.

Because the corrosion accompanying mass frans-
fer tended to occur uniformly, while deposition
produced long dendrites, the consiriction of fHlow in
the cool parts of the system usually reached seri-
ous proportions long before the corrosion in the
hot parts of the system became troublesome,

Nearly all studies of mass transfer in fused hy-
droxides have been carried out in thermal-convec-
tion systems, although some forced-circulation
work has been reported.?® Several different de-
signs of thermal-convection apparatus and several
different methods of measuring mass transfer have
been used. The only extensive studies in which
numerical valves of the amount of mass transfer
were obtained were the work of Lod and Simon49
on the mass tronsfer of nickel in sodium hydroxide
and the work of Smith, Steidlitz, and Hoffman3? on
several metals and alloys in sodium hydroxide.
These two groups used not only different designs
of thermal-convection apparatus but also different
means of measuring mass transfer, Lad and Simon
measured mass transfer by determining the weight
loss of a specimen in the hottest part of the sys-

tem, while Smith, Steidlitz, and Hoffman micro-
scopically measured the thickness of deposits
formed in the coldest parts of the system. Never-
theless, confirmatory resuits were obtained when
the two studies overlapped.

Nickel, because of its excellent corrosion re-
sistance, has received more attention than other
metals in mass-transfer research. Studies have
been made of the effect of several variables on the
rate of mass transfer of nickel in fused sodium
hydroxide. These variables were temperature level
of the system, temperature differential, time of op-
eration, composition of the atmosphere over the
melt, and composition of the melt itself. _

Lad and Simon?9 ond Smith, Steidlitz, and Hoff-
man3® found that the rate of mass transfer accel-
erated with linearly increasing temperature level,
Smith, Steidlitz, and Hoffman found that mass
transfer of nickel in fused sodium hydroxide under
helium could be detected by their techniques after
100 hr at a maximum system temperature of 600°C
and a temperature differential of 100°C but not at
a maximum system temperature of 550°C, other
conditions being constant, The lowest maximum
system temperature for which Lad and Simon re-
ported data was 538°C, at which temperature under
helium with a temperature differential of 46°C they
found a very small amount of mass transfer after
about 100 hr. ,

The studies by Lad and Simon showed that,
under helium, mass troansfer of nickel was de-
pendent upon the time of operation and the tempera-
ture differential. They found that the rate of trans-
fer decreased slightly over the first 50 hr and
thereafter remained constant, The initial tronsient
in the rote agreed with the change of nickel con-
centration ' in the melt; both rates built up to a
steady-state value after 100 hr. The rate of trans-
fer increased rapidly with temperature differential
and then became independent of the differential at
higher values. Lad ond Simon suggested that at
higher temperature differentials the rote of flow
in their apparatus was too great to permit complete
safuration of the fluid with nickel compounds.

Because of the ability of hydrogen to suppress
corrosion it might be expected that this element
would be effective in inhibiting mass transfer,
This inhibiting effect was demonstrated by Smith,
Cathcart, and Bridges?? in 1951, but only recently
has it been subjected to extensive quantitative
test. Lad and Simon found that at 815°C the

inhibiting action of hydrogen on the mass transfer

19
of nicke! was considerable, Smith, Steidlitz, ond
Hoffman confirmed this result for temperciures
below 815°C and found that the maximum system
temperature at which no mass transfer was meas.
urable was about 75°C above that found under
helium in comparable tests. In addition, Lod and
Simon found that 3% water vapor in the blanketing
atmosphere is beneficial with and without hydrogen
at 815°C,

L.ad and Simon?? studied the effect on mass
transfer at 815°C of additions to the melt of up to
5 wt % of 33 substances. Many of the materials
which they odded react with sodium hydroxide at
this temperature; some are soluble, while some
are insoluble ond relatively inert. Sodium car-
bonate, commercial
sodium hydroxide, was found to have no effect for
additions up to 1 wt %. Above this concentration,
sodium carbonate was found to accelerate mass
transfer. Sodium chloride and sodium orthophos-
phate, both soluble, had no effect. Sodium and
lithium hydrides had very detrimental effects. Lad

a  common  copntaminantd  in

and Simoen suggested that these compounds react to
form sodium oxide which is ‘‘very corrosive.”
FHowever, tests made by adding sodium oxide, as
such, caused only about half as much mass iransfer
as was caused by the addition of amounts of hy-
dride which would give roughly the some oxide
concentration after reaction.

Other Elemental Metals. = Data on elemental
metals other than nickel are meager. The moss
transfer of iron in fused sodium hydroxide hes been
found to be wore severe than that of nickel under
comparable conditions. [or example, Smith, Steid-
litz, and Hoffman3? found that under hydrogen at
55%°C the noss transfer of iron was worse than
thai of nickel at B00°C, other conditions being
constant. However, iron was not observed to form
the long dendrites thot werz found for nickel.
Rather, the crystals in ihe iron deposits remained
somewhat equiaxed in habit.

Copper in sodium hydroxide under hydrogen was
found®® to mass-iransfer appreciably in 100 hr at
600°C with a temperature differential of 100°C.
Thus, copper mass-transfers more readily thon
nickel under these conditions but not so severely
as iroi.

The mass transfer of silver and of gold has not
been quantitatively studied, but it has been ob-
served repeotedly.

Alloys. — Hastelloy B ond the stainless steels
have been observed3? to undergo mass transfer in

20

fused sodium hydroxide under hydrogen, but at
elevaied temperatures coifosion is o serious prob-
lem with these metals whethar mass transfer occurs
or not. Monel also showed3? mass transfer under
both hydrogen and helivm.

The most extensive studies of mass transfer of
an alloy have been those made on Inconel,30 |t
was f{ound that Incone! showed corrosion in fused
sodium hydroxide at roughly the same rate whether
mass transfer took pluce or not and that mass
transfer took place at roughly the same rate as for
Mass-
transfer deposits formed in Inconel systems con-
taining fused sodium hydroxide under on atmosphere

pure nickel under the same conditions.

of helium were virtually deveid of chromium. Hy-
drogen suppressed mass transfer of Inconel with
about the same effectiveness as it did for pure
nickel.

Bimetallic EHects, — When two different solid
metals are immersed in o liquid — for example, a
liquid metal — under isothermal conditions at high
temperatures, it is frequently observed that one of
the solid metals contominates the other metal or
that the metals contaminaote each other. This
process of metal transport without a temperature
differential is usually referred to os “‘isothermal
mass transfer,”*'  |n the case of liguid-metal
systems, the driving force for isothermal mass
transfer seems to be the tendency of the two solid
metals to form alloys,

A number of experiments have been conducted in
which two different metallic elements were im-
mersed in the some hydroxide melt under pre-
sumably isothermal conditions. LeBlanc and
Bergmann4? performed o series of experiments in
which different metals were immersed in fused
sodium hydroxide contained in a gold crucible at
700°C wunder nitrogen., When copper was immersed
in the melt, they found that the gold crucible be-
came alloyed with copper; when silver waz im-
mersed, the silver specimen betcome alloyed with
gold; but when nicke!l was immersed, nothing was
observed to happen. Brasunas?® heated sodium
hydroxide in an evacuated nickel capsule together
with a copper specimen at 815°C, He reported
that nickel plated out on the copper.

[t would be tempting to conclude that the in-
stances of metal transport cited above are anale-
gous to isothermal mass transfer in liquid metal
systems if it were not for some studies by Williams
and Miller.?® They immersed strips of nickel in
fused sodium hydroxide contained in a gold vessel
under a hydrogen atmosphere and found that the
nickel became gold-plated. However, they gave
convincing evidence that this plating occurred anly
during cooling. This observation shows that the
transfer of gold to nickel which was observed was
not an example of isothermal mass transfer, al.
though Williams and Milles's techniques would not
have detected small amounts of alloying which
might have token place in addition to the much
grosser plating effect,

Craighead, Smith, and Jaffee?? report the trans-
fer of nickel from a container vessel onto o number
of metals immersed in sodium hydroxide at 677 and
815°C., However, they point out that thermal gradi-
ents, which were known to exist, could have ex-
plained their results,

Mechanism of Mass Transfer. ~ The mechanism
of mass transfer of metals in fused hydroxides is
not known nor do the proper kinds of data exist
which are necessary to distinguish omong several
possible mechanisms., The pumose of the foilows.
ing discussion isto point ouf the various processes
by which a chemically reactive fused electrolyte
such as sodium hydroxide might transport metals
under the influence of a temperature differential.
These mechanisms are all possible in the sense
that they do not violate known principles.
ever, they are all quite speculative.

It is assumed that the mode of metal transport
involves either solution of the container material
as metal atoms or its solution as metal ions plus
electrons.

How-

If the solute consisis of metal atoms,
mass tronsfer could be considered in tenns of dif-
ferential solubility. If the solute consists of metal
ions plus electrons, several possibilities exist,
depending on the mechonism of transport ot the
electrons.  These various possibilities will be
discussed in the following sections,

Difterential Solubility (Mechanism ). - In a
superficial way the mass transfer of metals in
tused hydroxides appears to obey a simple solu-
bility-temperature relation. Mass transfer in liquid-
metal systems usually follows such a relation, and
it can also, in principle, be applied to fused hy-
droxide systems. Every metal must have a finite
solubility in a fused hydroxide, just as every metadl
must have a finite vapor pressure at all tempera.
tures, although this solubility may be so small as
to have only a statistical meaning., Symbolically
this process may be represented as

(5.1) M(s) = M{melt)

It is postulated here, however, that metals have
a negligible solubility as metal atoms in fused
hydroxides, This postulate is difficult to justify
Nevertheless, the differences
which exist in the intemal pressures and in the
binding forces for metals and tused hydroxides
strongly suggest that the solubility of the metal
as atoms in the tused hydroxide will be exceed-
ingly small.

by formal argument,

There are no direct experimental studies of the
solubility of any metals in fused hydroxides,
There is indirect evidence!3+18 that sodium metal
has a significent solubility in fused sodium hy-
droxide and that it dissolves before reacting. This
phenomenon appears to be analogous to the solu-
tion of a metal in its fused halide.¥? This process,
however, is not simply o case of the solution of
metal otoms but is dependent on some intferaction
between the valence electrons of the metal atoms
ond the ions of the same metal in the melt.

The dissolving of sodium in a sodium halide and
possibly in sodium hydroxide can be schematically
represented by the equation

(5.2) Nalg) = Nat(melt) + O(melt)

where the electrons B(melt) may be thought of as
8 ¥ &8 sk "

excess’’ or "'solvated’’ electrons which are more
or less associated with the cations of the melt. In
some instances the dissolution of a metal in its
fused halide seems fo involve the formation of o

Cd2++ is formed

when cadmium is dissolved in cadmium chioride.

diatemic cation; for example,

It is possible, in principle, for any metal to dis-
solve to some extent in a fused electrolyte as metal
ions plus excess electrons which may be con-
sidered as more or less associated with the cations
of the electrolyte. This process will be discussed
further in the section “Mechanism IV."”

Oxidation-Reduction Processes {Mechanisms {i-
V¥1). ~ The mechanisms considered most probable
for mass transfer in fused hydroxides all involve
oxidation-reduction processes. The two basicsteps
for all such processes may be schematically repre-
sented for a metal M of valence v by the following
equations., At the hot metal-melt interfaces there
occurs the oxidation

h
(5.3) M(s) s M¥(melt) + 10

where 0 indicates an electron and where M¥(melt)
represents a metal in whatever form in which it may
be stable in the melt, that is, as an uncomplexed

21
cation or as a suitable oxysalt anion. At the cool
metal-melt interfaces thers accurs the reduction

ol

(5.4) M¥(melt) + v0 ——> M(s)

which is the reverse of Eq, 5.3, For all kinds of
oxidation-reduction mechanisms the M¥{(melt) ions
will be transported by diffusion through boundary
layers at the interfaces and by flyid flow within
the bulk of the melt,
move by two quite different routes.

However, the electrons may
They may be
conducted through the metallic ports of the system,
or they may be transported through the melt by
some chemical species which is reduced at the hot
metal surfaces and oxidized at the cool surfaces.
The first method of electron transport represents
The second method might be
effected by any of a number of chemical species.

local cell action.

It is convenient to classify these species into four
groups according to the oxidized form of the elec-
tron carrier: hydroxyl ions, sodium ions, foreign
solutes, and a higher valence state of the metal
undergoing transport. These five modes of electron
transport may be considered as representing five
additional mass-transfer mechanisms.

Local Celi Action {(Mechanism ), ~ !f the elec-
frons are transported fhrough the metallic parts of
the system, Fq. 5.3 represents the anodic dissolu-
tion of M, and £q. 5.4 the cathodic deposition of M.
The driving force for this process con be thought
of as o thermoelectric potential. Such potentials
have been measured in fused electrolytes.??
order for such a process to tcke place at all, the
transported ions M*{(melt) must be already present,
However, any oxidizing substonce, including the
hydroxide itself, could serve this purpose.

lnasmuch as Pray and Miller?2 have induced

In

nickel to deposit preferentially on on Alundum
ir @ sodium hydroxide melt, local

cell action connot be the exclusive mode of mass

ring immerse
transfer. However, there is no reason why local
cell action might not be the primary mechanism
under selected conditions.

Reduction of Hydroxyl lons {(Mechanism ). -~
In Sec. 4, *‘Corrosion of Metals,”’ it was pointed
out that the most common over-c!l corrosion reac-
tion was the reduction of hydroxyl ions represented

by £q. 5.5

(5.5)  M(s) + vOH~{ms!lt) = MY(melt)

+ 107" (mel) 4 = Hyfme)

/ e . —

P
e
e

22

and that, at least in the case of. nicke!, MY{melt)
ions may be reduced to the metal again by hydro-
gen. It would not be surprising, therefore, to find
that hydrogen molecules serve to transport elec-
trons, the hydrogen acting together with an oxide
ion as an electron donor conjugate to a hydroxyl
ion.” This mechanism was proposed independently
by Skinner?> in 1951 and by Williams and Miller in
1952. Since then it has received wide ccceptance
as the only mechanism for the mass transfer of,
nickel, |
There is no doubt that £q. 5,5 represents a likely
mechanism of mass transfer under many conditions.
However, the evidence in favor of this mechonism
is ambiguous, and mass transfer has been observed
to occur under conditions where the operation of

this mechanisn seems doubtful. The evidence most

frequently cited in its support is the effectiveness
of a blanketing atmos phere of hydrogen in suppress-
However, regardless of the
nature of the electron carriers, hydrogen should, to

ing mass transfer.

a greater or lesser extent, suppress all oxidation
processes in fused hydroxides and thereby depress
the concentration of nickel ions moving through the
fluid. Furthermore, Smith, Catheart, and Bridges3?
have conducted severa! kinds of experiments in
which nickel preferentially deposited on a surface
at which the hydroxide was saturated with atmos-
pheric oxygen. In these experiments mass transfer
cannot be accounted for by Eq. 5.5, According to
this equation, hydrogen must be present ot the
interface of deposition ot a concentration greater
thon the minimum necessary to reduce M¥(melt)
ions. In the experiments of Smith, Cathcart, ond
Bridges, this condition could not have heen ful-
filled.
only according to Eq. 5.5, the absesnce of hydrogen
at the hot surfaces would occelerate the solution
step, but an absence of hydiogen at the cool sur-
faces would likewise prevent the deposition step.

The quontitative specification of the hydrogen
concentrations or, via Henry's law, of the hydragen

In other words, if mass transfer occurred

pressures necessary to cause mass fransfer under
given circumstances is dependent, omong other
things, on the temperatures and nickel concentra-
tions at both hot and cold interfaces, Data are not
yet available with which to make such specifica-
tions, but outstanding progress is being made
toward this end for the case of nickel by Kertesz,
Knox, and Grimes. 48

Iron has been shown3? to undergo mass transfer

in sodium hydroxide much more rapidly than nicke!
did under the same conditions. MHowever, Williams
and Miller?® have pointed out that the reaction be-
tween iron and sodium hydroxide is not reversed or
even inhibited by hydrogen at pressures up to 1atm.
It is, therefore, doubtful that hydrogen is an im-
portant electron carrier in this instonce,

Reduction of Alkali Metal fons (Mechanism
iV), ~ |t does not seem likely thor aikaii metal
ions would be effective agents in mass transfer
under most conditions, because they are thermo-
dynamically inferior oxidizing agents compared
with other possible species such as hydroxyl ions.
On the other hand, alkali metal jons and alkali
metal atoms could, et least in principle, act os
conjugate electron acceptor-donor pairs for slectron
transport in fused hydroxides. Therefore, this
possible mechanism will be discussed briefly,

Every metai must be capable of eflecting the
reduction of a finite number of sodiym ions in fused
sodium hydroxide. Hence, if is possible for mass
transfer to occur by the following process:

(5.6) M(s) + vNa*(melt) = M¥(melt) + E/Na(melt)

where Na(melt) is to be censidered as sodium ions
plus excess electrons, as discussed under Mecha-
nism |. However, sedium dissolved in fused sodium
hydroxide will react with hydroxyl ions:

(57) Nalmelt) + OH™(melt) = Na*(melt)

+ O~ (melt) + ]i.ij, Hz(g)
The reverse of this reaction is also known,?’ so
that Eq. 5.7 may be tdaken as representing an
equilibrium. (For reasons of simplicity, the inter-
mediate equilibrium involving hydride ions is
omitted.}) Therefore, it local thermodynamic equili-
brium exists in the melt, beth Eqgs. 5.6 and 5.7
must be simultaneously satisfied, and the over-all
equilibrium {the summation of Egs. 5,8 and 57)
becomes identical with that represented by £q. 5.5.
Therefore, if {ocal thermodynamic equilibrium
exists, Mechanism IV is the same as Mechanism
lii; that is, the same end result is achieved if it is
assumed that either the sodium ions or the hydroxyl
ions ure the electron acceptors. On the cther hond,
the reaction between sodivm metal and sodium hy-
droxide has been found to be surprisingly slow,*®
and it is pessible that Na(melt) is not in local
thermodynamic equilibrium according to Eq. 5.7.
Under this latter circumstance, the kinetics of
Mechanism 1V could be quite ditferent from the

kinetics of Mechanism [,

Reduction of dolutes {Mechanism V). -« In some
instonces dissolved substances which are more
easily reduced than the fused hydroxide may serve
as electron goceptors in the mass-transfer mecha-
nism, Such a substonce mighi be the ions of
anothier metal, A, with two avoilable valence states
yand y — 1. Then a reaction such as '

(5.8) M(s) + vAY(melt) = MY(melt) + vAY" melt)

would be expected to ovcur, Displacement of this
equilibrium could couse mass transfer. This mech-
anism will not account for the moss transfer ob-
served in most of the systems investigoted, since
suitably high concentrations of anions A” do not
usunlly exist, However, if such ions are present,
it is possible that mass transfer would be acceler-
ated.

One possible source of ions of A is from impuri-
ties in the original sample of metal undergoing
mass fransfer. Willioms ond Miller?? have shown
that iron is selectively lenched from the surfaces
of samples of commercial nickel which originally
corntained 0.09 to 0.41 % iron as an impurity.

Pisproportionation {(Mechonism V1), - If the metal
M exists in more than one valence state, it is pos-
sible for disproportionation te cause mass transfer,
For example, with M having the valence states v
and v + 1, ‘;thez} .

(5.9) M(s) + ;/gfij;flfmeh) - (v + a}M':"(mem

For this reoction to occur, ions of M must be pres-
ent in the melt, os wos true for the local cell
mechanism, Any of the oxidation mechanisms just
described could generate them. The possibility
that disproportionation might serve as a mechanism
for moss tronsfer in fused hydroxides was first
nointed out by Grimes and Hill,*?

It was noted above thot hydrogen is an unlikely
electron carrier in the mass transfer of iron. How-
ever, <o disproportionation mechonism might be
wvery effective in this cose, Little is known about
the valence stotes of iron which are stable in fused
hydroxides, but the indications are that more thon
one valence state occurs. D'Ans and Léffler ¢
report that, wunder an atmosphere of nitrogen,
iron{l{} oxide veacts with sodium hydroxide 1o give
mostly sodium ferrate(lll), with a small emount of
oxysalt in g higher valence state, which they sug-
gest may be sodium ferrate{VIl), Thus it is pos-
sible that o disproportionation process tokes place

23
involving iron{lil) and iron{V!). Under reducing
conditions, such as under a blanketing atmosphere
of hydrogen, iron{ll) and iron(lll) might be the
active valence states.

Nickel has been shown®% to occur in both the
valence states || and 1l in fused sodium hydroxide
in the presence of oxidizing agents. It has alse
been found3® that under suitable conditions, the
ratio of nickel(ll) to nickel(lll} in the fused hy-
droxide changed with thermal cycling in the way
required by a mechenism based on Eq, 5.9. The
absence of hydrogen at the cool interfaces does not
interfere with mass transfer by this mechanism.
Consequently, it is possible that mass transfer of
nickel in o hydroxide saturated with atmospheric
oxygen may toke place by a disproportionation

 

Observations of mass transfer of
nicke! in sodium hydroxide soturated with atmos-
pheric oxygen were cited in Sec. 5, '"Mass Trans-
fer — Nickel."” From what little is known,35 it ap-
pears that the presence of nicke!(lll} ions in fused
hydroxides is favored by oxidizing conditions, and

mechanism.

in the absence of these conditions its concentration
is quite low.

Summary of Mechanisms, =~ Six mechanisms were
discussed as possible modes of metal transport in
fused hydroxide media. These mechanisms are
summarized, and some of their interrelations shown
by the following outline. The symbolic processes
and reactions presented in this summary are to be
considered as taking place from left to right at the
hot metal-melt interfaces. The interpretation of the
symbals was discussed in the preceding section.

A. Transport as metal atoms, that is, differential solubility — Mechanisin |

M(s) T M{melt)

B. Transport as metal atoms plus electrons

Ms) == M"(melt) + 10

(See Eq. 5.1)

(See Egs. 5.3, 5.4)

1. Electrons transported by metallic conduction, that is, local cell action — Mechanism 1l

tAhot metal) —> 1 cool metal)

2. Electrons transported by the reduced form of some species in the melt

a. Reduced form of hydroxy! ions — Mechanism |1}

24

10 + YOH  {melt) ';’f:voflm(meh) ’}'"21{' H2(me|t)

Over-all reaction

v
M(s) + POH ™ (melt) T== M*(melt) + O™ "(melr) + 5 Hylmelt)

Reduced form of sodium ions - Mechanism IV
10 + vNa'(melt) =" vNa(me!1)
Over-all reaction
M(s) + ¥Na (melt) T==M*(melt) + vNa(melt)
Reduced form of a solute A”(melt) — Mechanism V
v + vAY (melt) == vA” *(melt)
Over-all reaction
M(s) + vAY (melt) == M"(melt) + vAY " Nmel)
Disproportionation — Mechanism VI
10 + M7 melt) == vM¥(meln)
Over-a!l reaction

M(s) + M7 melt) == (v + 1) M¥(melt)

(See Eq.

(See Eq.

(Sze Eq.

(See Eq.

5.5)

5.6)

5.8)

5.9
Mechanism 1, transport of metal atoms as such,
was discounted inasmuch as the solubility of
metals as atoms in fused hydroxides would be ex-
pected to be very small.

The remaining mechanisms, {I through VI, are all
oxidation-reduction processes. No single cne of
these mechanisms will aceount for all observations
of metal transport in fused hydroxide media. It
seems that no one process is always the exclusive
mode of transport but rather that different mecha-
nisms predominate under different conditions. Much
further research will be needed before specific
modes of metal transport can be proposed as the
probable mechanisms of mass fransfer under stipu-
lated conditions.

Two substances, hydregen and water, have been
noted to suppress mass transfer of nickel in fused
This
cited in support of any particular one of the oxida-
tion-reduction mass-transfer mechanisms. Hydrogen
should suppressthe corresion step for all oxidation-
reduction mechanisms,

sodium hydroxide. information cannot be

The role of water in moss transfer is unknown,
It is possible that water functions to suppress the
oxide ion concentration in the melt and thereby
causes the corrosion steps to produce a film of
relatively insoluble nickel oxide. The action of
water might be expressed by an equilibrium such as

(5.10)  NaNiO,(meit) + H,O(melt) = NiO(s)

+ ZNaOH{melt)

This film could function as o diffusion barrier to
slow down the corrosion step. If water were fo act

in some way such os this, it would be effective in
suppressing mass transfer by olmost any mecha-
nism, _ '

Symmary. - Moss fronsfer has been observed in
fused sedium hydroxide for nicksel, iron, copper,
silver, and a number of alloys. Moreover, nickel
has been chserved fo mass-transfer in all the fused
alkali-metal ond olkaline-earth hydroxides. In the
majority of investigations undertaken, mass transfer
wos induced by a temperature differential in the
system,

The rate of the mass transfer which takes place
under the influence of a temperature differential
was found to be affected by the following variobles:
nature of the metal undergoing transfer, composition
of the melt, composition of the atmosphere over the
melt, temperoture level of the system, temperature
differential within the system, and geometry of the
Undoubtedly, the rate of fluid flow is @
very important factor, but existing dota are in-
adequate for evaluation of this variable.

system.

Under ¢ particular set of comparable conditions,
was found faster than
nickel, and iron to mass-iransfer faster than copoer.

copoper to mass-transter

With alloy systems, mass transfer and corrosion
were found to occur simultaneously. '

The mechonisms of mass transfer in fused hy-
droxides are not known. Some of the possible
modes of mass fransfer were exomined, and five
mechanisms were proposed as being possible. It
was suggested that no one process is always the
exclusive ‘mode of transport but that different
mechanisms predominate under different conditions.

25
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29