lit:

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MHD MATERIALS - SEED/SLAG INTERACTIONS AND EFFECTS

QUARTERLY PROGRESS REPORT

October - December 1979

Samuel J. Schneider

Project Manager

Center for Materials Science

National Bureau of Standards

Washington, D. C. 20234

PREPARED FOR THE UNITED STATES

DEPARTMENT OF ENERGY

MHD OFFICE

"This report was prepared as an account of work sponsored by the United

States Government. Neither the United States nor the United States

Department of Energy, nor any of their employees, nor any of their

contractors, subcontractors, or their employees, makes any warranty,

express or implied, or assumes any legal liability or responsibility

for the accuracy, completeness, or usefulness of any information,

apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights."

TABLE OF CONTENTS

Page

I. SUMMARY OF PROGRESS TO DATE 1

II. DETAILED DESCRIPTION OF TECHNICAL PROGRESS 2

1. Thermochemistry of Seed and Slag 2

2. Electrical Conductivity and Polarization 13

3. Corrosion of Downstream MHD Components 22

I. SUMMARY OF PROGRESS TO DATE

1. Thermochemistry of Seed and Slag

Measurements of the K pressure over the phases orthorhombic KAlSi 04 - leucite-beta alumina were begun. The vapor pressure measured with

approximately 1/2 the KAlSi 04 phase remaining is about four times higher

than the vapor pressure over the K2 O- 9AI 2 O 3 -AI 2 O 3 two phase mixture.

Equilibrium relations among solid phases in the more K20 -rich parts

of the systems K2 O-AI 203 -Si

O

2 and K20 -Ca 0-Al 203 -Si 02 have been explored.

Evidence has been found for the existence of previously unreported com- pounds of significance in seed/slag reaction.

2. Electrical Conductivity and Polarization

The dependence of the electrical conductivity of Y , 9 sCa , 05 Cr 03 on oxygen pressure in the range of pressures from 10 ° to 10 ”^ atmospheres have been completed for temperatures from 1300° to 1600° C. The

apparatus has been rebuilt to allow measurements in the high temperature

range at oxygen pressure from 10 "^ to 10 "^^ atmospheres.

A three probe electrochemical cell has ben constructed and the

electrochemical effects measured at a temperature of 1254° C. This cell

allows the measurement of slag-electrode voltages as a function of

time and ultimate analyses of the electrochemical effects.

The first series of experiments on the electrical conductivity of

Bow NH slag has been completed. The conductivity at low temperatures

(below 110° C) depends on the crystalline state of the slag. The con- ductivity at high temperature (above 1400° C) is independent of the

oxygen pressure and of the amount of K present at least up to 6 % by

weight.

3. Corrosion of Downstream Components

Type 304 stainless tell tubes, cooled to 400° C, 500° C and 590° C,

were exposed to fuel rich and oxygen rich hot gas streams seeded with

a mixture of 80% by weight K2 CO 3 and 20% by weight K2 SO4 . Optical and

SEM and EDX analysis revealed differences, geometrical and chemical,

in the deposit formed on the tubing on a result of the tube temperature

and the gas stream state. Tubes at the higher temperatures had thinner

deposits as did tubes exposed to the fuel rich gas streams. Information

with respect to the distribution of Fe, Cr and Ni in the reaction zone (stainless steel-deposit interface) was also obtained with Fe appearing

to be the species penetrating most deeply into the salt deposit.

In the coming quarter exposure tests, similar to those conducted on type 304 stainless steel, will be initiated on type 316 stainless steel.

1
II. DETAILED DESCRIPTION OF TECHNICAL PROGRESS

1. Thermochemistry of Seed and Slag (E, Plante and L. Cook)

A. Vaporization Studies (E. Plante)

Progress : The vaporization section of the thermochemistry of seed and

slag task involves the determination of the KpO activity in simplified

slag solutions. These studies will initially be limited to selected

subsections of the four component system K20 -Ca 0 -Al 203 -Si 02 while later

studies will include Fe203 as a component. The overall aim of the

vaporization section is to determine the K2 O activity as a function of

temperature and selected limited composition ranges. The limits on composition will be generally dictated by the compositions which are

likely to be obtained in the combustion of seeded coal.

Determination of the K2 O activity is made by measurement of the

K(g) and 02 (g) pressure from slag samples using the Knudsen effusion

method coupled with modulated beam mass spectrometric analysis of the

vaporization products. The typical vaporization reaction will be,

K2 O (dissolved in slag) = 2 K(g) + 1/2 02 (g) (

1 )

In general, the activity of K2 O with respect to the gaseous phase can be

defined as.

( 2 )

or with respect to a reference state such as

D? n 1/2

'K2O 1/2

'•9

pure 1 iquid K2 O as

over slag

over pure K., 0 ( 1 )

L. (3)

The compound, orthorhombic K.AlSi 04 is of great importance in the study

of seed-slag interaction as it is a major crystalline product in seeded

coal slags. The phase diagram as well as previous vaporization studies

at NBS indicate that vaporization of K and O 2 from KAlSi 04 should result

in formation of leucite (K 2 O • AI 2 O 3

• 4S 102 ) and beta alumina. Beta

alumina has a wide composition range which extends from the composition

K 2 O • 5 AI 2 O 3 to K 2 O • 9A 12 D 3

. Over this comoosition range the pressure

of K varies by a factor of more than 100. Assuming that the beta

alumina formed is at the <2 ^ rich phase boundary (which is not possible

for thermodynamic reasons) the reaction for decomposition of KAlSi 04 would be, r H

5KAlSi04 = 5/4 K2 O • AI 2 O 3

• 4Si02 + 1/4 K2 O • 5A1203 + 2K + 1/2 O2 (4)

If the beta alumina formed has the composition at the K2 O poor phase

boundary, the reaction for decomposition of KAlSi 04 would be.

2
( 5 ) 36/8 KAlSi 04 = 9/8 K2O • AI2O3 • 4Si 02 + 1/8 K2O • 9AI2O3 +

2 K(q) + 1/2 02(g)

Reaction 5 has the property that the K pressure should depend only on temperature and be Independent of bulk composition since it is a four

phase three component system. The vapor pressure curve in this region

of the phase diagram will, therefore, serve as a useful reference curve characteristic of a system containing KAlSi04. Measurements at lower

potassia concentrations may be useful because they will be carried out

with the system partially in the liquid phase which will be true of many

slag systems. Finally, these data will be useful as references for more

complex slag solutions containing additional oxides such as CaO or

Fe203.

In previous attempts to measure the K pressure over the

KAlSi04-K20 • AI2O3 • 4Si02- beta alumina system it was found that a steady state K pressure could not be obtained. This was attributed to

the complexity of the crystalline transformation which might limit the

vaporization rate. The major difference between the previous work and

that currently undertaken is that the ratio of evaporating to escape

area is about 2 X greater in the current work.

Although the measurements are not yet complete, the results obtained thus far are plotted in Figure 1 . The upper reference curve

corresponds to the K pressure observed for decomposition of KAIO2 to

the K2O rich beta-alumina and vapor products. Since KAlSi04 is formed

by reaction of KAIO2 with Si02, values of the K pressure are expected to

fall below this line. The lower reference curve is the K pressure

observed for the decomposition of K2O poor beta alumina to form AI2O3

and vapor products. If decomposition of KAlSi04 takes place according

to reaction ( 5 ), the observed K pressure data should fall above this

line. Data from a series of heatings are shown. Some of the initial

data actually exceeds the upper reference line but decreases as K and O2 are evaporated from the sample. These points are believed to be due to

evaporation of residual impurities (unreacted K2CO3) from the sample.

As the evaporation process proceeds, there is a tendency for the

pressure to decrease with time at constant temperature which is con- sistent with the traverse of the beta alumina composition toward the

K2O poor phase boundary. In the last series of data (indicated by a dashed curve), it was observed that the K pressure was essentially

constant with time even though a large fraction of K2O has been removed

from the sample during this series. At the current stage of the ex- periment, the K pressure is about a factor of four higher than the K

pressure over the K2O • 9AI2O3-AI2O3 reference curve. About fifty

percent of the K2O content which must be removed before the phase

KAlSi04 disappears from the sample, is still present.

3
B. Phase Equilibrium Studies (L. Cook)

1 . Quench Experiments in the System K20-Al203-Si02

Determination of subsolidus relations involving KAlSi04, KAIO2,

K4Al2Si2ng, K2Si20s, and K2Si03 was begun, in preparation for study of

the K20-rich portion of the quaternary system K20-Ca0-Al203-Si02

(see Figure 2 ). While K4Al2Si20g was readily synthesized in the

solid state and gave an x-ray powder pattern which agreed well with

that reported in the literature (see Ref. 1 ), considerable difficulty

was encountered in synthesizing l<2Si03, perhaps explaining the fact

that there are apparently no published x-ray data. Removal of carbonate

from compositions of 1:1 K2C03:Si02 appears to be impossible without

melting, which further complicates synthesis in view of the pronounced

tendency for wetting which these melts have when in contact with

platinum. In addition, there appears to be a phase transition in

"K2Si03" in the range 600-700 °C. Further attempts will involve complete melting and recrystallization, as the solid state method is

seemingly inappropriate. Several combinations of starting materials

will then be reacted in order to verify, or perhaps modify, the subsolidus relations depicted in Figure 1 , so that detailed thermodynamic

analysis of seed/slag interaction may rest on a firm footing.

During the attempt to synthesize K3AIO3 (see Ref. 2 ) for use as an alternative reactant, some unexpected reactions of significance were

observed. At temperatures above 1000 °C, potassium carbonate reacts

with potassium aluminate to form what is possibly a series of solid

solutions (see Figure 2 ) having rather unusual properties. First, in

spite of the fact that both K2CO3 and KAIO2 lines are observed in the

x-ray powder pattern of 1:1 KC03:KA102, it does not show signs of

melting at temperatures up to 1400 °C. A gray-green color is characteristic, suggesting unusual electronic properties. Additional lines

in the powder pattern indicate that the material may have a superlattice if indeed it is single-phase. Further investigation will

attempt to determine the nature of this complex seed/aluminate interaction under carefully controlled atmospheric conditions.

2 . Quench Experiments in the System K20-Ca0-Al203-Si02

Experiments were conducted to verify the ternary nature of the

plane: Ca0-KA102-Si02 , which is thought to be a compositional divide

of major chemical significance in the quaternary system K20-Ca0-Al203-Si02

(see M'qure 3). Specifically, experiments were conducted to determine

if the following five reactions proceed to the right as indicated noting

that KAIO2 and Si02 will combine to form the appropriate phases.

4
a. 2K2CaSi04 + 2Ca2Al2Si0y = 3Ca2Si04 + 4KAIO2 + Si02

b. K2CaSi04 + CaAl 2 Si 20 s ~ Ca2Si04 + 2KAIO2 2 Si 02

c. 6K2CaSi04 + 2(3Al203 • 2AI2O3) = 3Ca2Si04 + 12KA102 + 75102

d. 12K2CaSi04 + 2CaAli2^i9 ~ 7 Ca2 Si 04 + 24 KA102 55102

G. 2K2Ca5104 + 2A1203 Ca25104 + 4 KAIO2 + 5 IO2

Results of the experiments are Indicated In Table 1 below. In

general, allowing for the possible loss of K2O, It appears that the

plane Is ternary, as evidenced by formation of (KAIO2-SIO2) phases

and the presence of Ca2S104 or related phases. Phase "X" probably

contains K2O. The results of experiment #3 are somewhat In conflict

with other results; however, the Identity of phase "X"' Is not known,

and so a judgement cannot be made. Additional experiments will center

on the definition of these unknown phases.

Table 1 . Experiments In the Plane Ca 0-KA 102 - 5102 ^

Temperature

(°C)

Duration

(hours)

5 tart1 ng

Materials

Products^

1. 1305 118 (1:1)

K2Ca51 O4/

Ca2 A1 2 51 07

KAl O2 ( ss )+Ki^^Al i^.^S 1 i_j.^04

Ca2Al2S107 + Phase "X"

2. 1305 118 (1:1)

K2 Ca 5104 /

CaAl2Sl208

KAI5IO4 + "Ca25104"

3 . 1287 119 ( 3 : 1 ) K2Ca51 O4/

( 3Al 203 * 25102 )

KAl O2 ( ss )+Ki^^Al 1 _j^04+

Phase "X" + Phase "X"'

4 . 1305 118 (6:1)

K2 Ca5104 /

CaAl 12^19

KA102(ss)+Ca2Al2S107 +

Phase "X"

5 . 1305 118 (1:1)

K2Ca51 O4/

A1 2O3

KAl O2 ( ss )+Ca2

1 2SI O7+

Phase "X"

1 . 5 pec 1 mens were sealed In Pt capsules and quenched In liquid N2.

Invariably capsules with K2Ca5104 swelled extensively during the

experiments, and so some loss of K2O may have occurred.

2 . Phase "X" Is an unidentified phase, probably related to Ca25104

with major lines at 1 . 98 , 2 . 19 , 2 . 60 , 2 . 74 , and 2.78 A.

Phase "X"' has major lines at 3.05 and 3.11 A.

5
In Table 2 are given results of experiments designed to explore

the phase equilibrium properties of the more K20-rich portion of the

quaternary system K20-Ca0-Al203-Si02 • In these experiments, as well

as those in Table 1 , the compound K2CaSi04 has been employed extensively as a starting material: its reported melting point (Ref. 3 ) is relatively high, in excess of 1600 °C, and hence its region of

stability is of interest. As Table 2 shows, however, K2CaSi04 is

present only in results of experiment # 7 . This experiment suggests

existence of a compound between KAIO2 and K2CaSi04. The identity and

stability of this compound will be investigated further. Phase "Z"

in experiment #8 may be a compound intermediate between K2CaSi04 and

Ca2Si04. The results of experiment #9 reaffirm compatibility of

KAIO2 and CaO (Ref. 4 ).

Table 2 . Experiments in' the K20-rich

Part of the System K20-Ca0-Al203-Si02^

Temperature

(°C)

Duration

(hours

)

Starting

Materials

Products^

6. 1287 119 (1:1)

K2 CaS i O4

/

KAlSi04

Ca2Al2Si07 + KAlSi04 +

Phase "Y"

7 . 1287 119 (1:1)

K2 CaSi O4/

KAIO2

KAIO2 + K2CaSi04 + Intermediate phase

8. 1305 118 (1:1)

K2CaSi O4/

Ca2Si O4

Glass + Phase "Z"

9 . 1287 119 (1:1)

KAl 02/Can

KAIO2 + CaO

1. See Footnote 1 , Table 1 .

2. Phase "Y" has major lines at 2 . 14 , 3 . 42 , and 4.27 A.

Phase "Z" has a major line at 2.97 A and numerous weaker lines.

Figure 4 has been constructed to show the rather extensive stability

of the compound Ca2Al2Si0y. It coexists with compounds under both

relatively high K2O pressures (with KAIO2 ss) and at relatively low K2O

pressures and high silica contents (with KAlSi206).

6
Plans

:

Vaporization measurements on the K20-Al203-Si02 system will be

completed to as low a K2O composition as practical.

Vaporization measurements on K2O containing compositions in selected

sections of the K20-Al203-Si02-Ca0 system will be started.

Verify existence of K2CO3/KAIO2 solid solutions and determine

their nature.

Confirm proposed subsolidus relations in K20-Al203-Si02

.

Identify unknown phases in K20-Ca0-Al203-Si02 system and determine

their stability.

Prepare quaternary melts for vaporization studies.

7
References

:

1. L. K. Yakovlev, Russ. J. Inorg. Chem. , 10[12], 1459 (1965).

2. A. Bon, C. Gleitzen, A. Courtois and J. Protas, C. R. Acad. Sci.,

Ser. C., ^[11], 785 (1974).

3. G. W. Morey, F. C. Kracek and N. L. Bowen, J. Soc. Glass Tech.,

T4, 149 (1930).

4. L. T. Brownmiller, Am. J. Sci., 5th Ser., 268 (1935).

8
Potassium

Pressure

(Atm.

Temperature, K

1800 1600 1400

Selected K pressures over orthorhombic KAlSi04 , leucite and beta alumina

showing decrase in pressure possibly because of shift in composition of

beta alumina.

9
SiO,

Al^Os

Figure 2

*

.

*

kTa^o i< 20 -rich part of 'the system "

s Y 3fter H. P. R. Frederickse, T.' Negas and . J. bcnneider, Development; Testing and Evaluation' of MtlD-Material s NBS

Adminir Research and Develo menJ

’ Administration under Contract No. £(49-189-123(1). July-September 197B, p. 30)

10
CaO

Si02

Figure 3

Compositional divide formed by the tornary plane KAl 02-Si02Ca0

in the quaternary system K2O-AI 203-Si 02-Ca0.

*

I

I

11
Ca 0

RilOL %

f- Figure 4

Compatibility of gehlenite ("ge"

phases in the quaternary system.

C3S2 = Ca3Si2ny; "WO" = CaSi 03 ; "kls" = KAlSin^^; "ic" = KAlSi2ng

- Ca2Al2Si07) with other

"c2s" - Ca2Si0i^; an — CaAl 2 i 2 ^ 7 >

> ca0 “ CaAl]^2C3g.

12
2. Electrical Conductivity and Polarization (W. R. Hosier)

Progress

a. Electrical Conductivity of YCrOci .

The dependence of the electrical conductivity of Y.gsCa jjsCrOg on oxygen pressure in the range of pressures from 10 ° to 10 "° atmospheres have been completed for temperatures from 1300°C to 1600°C. In

order to obtain high density YCrOg, it is necessary to sinter it in a highly reducing atmosphere at about 1700°C. This atmosphere may be

forming gas_(5% H 2 - 95% N 2 ) which results in a low pressure of O2 of

10”^^* to 10’^° atmospheres. Material produced under these conditions

has a high density (98% theoretical) and is very hard. Subsequent

oxidation renders the material much softer and it sometimes contains

hair line cracks. The material with 5% Ca added is still single phase

when oxidized but the oxygen take up must cause enough change in the

lattice parameters in certain areas to cause a stress build up with the

resultant physical sample degradation. This aspect of the problem is

being investigated by the Crystal Chemistry Section of CMR at NBS. The

dimensional changes associated with this effect are very small and

cannot account for the increase in conductivity noted in the last quarterly report for 1600°C temperature point in an atmosphere of air. The

continual drift upward in conductivity noted for the 1600°C point in air

was not evident at any temperature in 900 and 2 ppm oxygen in nitrogen.

The sample holder has been revised using 0.25 mm molybdenum wire

instead of platinum. The wire leads are anchored in the sample with

platinum. The phase diagram of Pt-Mo shows a eutectic mixture at about

25 atomic % platinum and at a temperature of 2080°C. The conductivity

vs. pressure measurements will be made using a maximum temperature

(1600°C) well below this eutectic point. No reaction is expected. In

the previous measurements in the oxygen pressure range of 10 ”^ to 10 '^° ppm O2 , the platinum wire leads failed. The platinum contacts in the

sample always remained intact, however. These measurements at the low

oxygen pressure will be made as soon as the oxygen sensor necessary for

measuring these low pressures is repaired.

b. Corrosion of YCrOg in Slag

In the last quarterly report, the results of a test on an electrochemical cell composed of YCrOs electrodes and Bow NH slag with 20%

K2 SO4 added were reported. This test was carried out at 1343°C and

resulted in considerable cathode reaction but with not much reaction at

the anode. A new cell has been constructed and tested using the same materials (Bow NH slag with 20% K2 S 04 electrolyte and Y .gsCa .osCrOs

electrodes) but with a slightly different configuration. This cell

contains 3 electrodes, two of which are active (anode and cathode at

each end of the cell) and one which is passive (center of cell) which

carries no current. This allows for direct measurement of the potential

distribution along the length of the cell i.e. for potentials between

13
anode and reference and between reference and cathode. No polarization

should result on the reference electrode since no current enters or leaves at this point. There may be degradative chemical reactions,

however, at the reference electrode not associated with the transport of

charge. These measurements were carried out at 1254°C and with a power

supply providing a constant DC voltage source of 118 volts. The results

are shown in Figure 1. Initial current was about 75 ma which would

correspond to approximately 0.5 amp/cm^ current density at the electrode

slag interface. This current drops off to about 20% of its original

value or 15 ma in the first 20 minutes of cell operation and finally

down to about 8 ma after the total test time of 3 hours and 50 minutes.

In the initial 20 minute period, the anode to reference voltage increased

to llOV i.e. within 8 volts of the total applied potential while the

reference to cathode voltage dropped to about 8 volts. It is obvious

that a large polarization potential has built up at the anode. This is

most likely due to the zone near the anode that is silica rich or depleted in movable positive ions such as iron and potassium. On the

other hand, positive ion transport toward the cathode lowers the potential barrier there, as well as the bulk conductivity of the slag. This

effect has been observed before using other materials (1,2,3) and is

certainly a contributing factor in the cathode electrddd segment voltage

nonuniformities observed in some of the AVCO tests reported in the 16th

Symposium on Engineering Aspects of MHD held at the University of Pittsburgh in May 1977.

Electrical conductivity measurements on Bow NH slag discussed later

in this section of the report under Slag Electrical Conductivity (Part

C) indicates that a sizeable fraction of the charge carried in slag is

transported by ions. The basic question in MHD electrode materials is:

How can this mass transport and its associated electrochemical effects

be handled? One interesting aspect of the situation is to ascertain if

any of the resulting electrochemical effects are reversible and under

what conditions. Preliminary indications from the AVCO generator experiments show that the effects causing cathode voltage nonuniformities can

be reduced by running the channel at no load (open circuit) for a period of time. An electrochemical cell for investigating the reversibility of these effects is under construction.

c. Slag Electrical Conductivity

The electrical conductivity of Bow NH slag with 20% K2 SO 4 added to

the melt has been completed. Figure 2a and b show radiographs of the

crucible containing the slag before and after the experiment. No observable physical change has taken place during the experiment. Figure 3

shows the conductivity data as a function of temperature. All data was

taken at a partial pressure of oxygen of 2 x 10"^ atmospheres. The

temperature from which the slag was furnace quenched to 900°C is indicated

on the graph. The conductivity data at reduced oxygen pressure is not

quite so quench temperature dependent as that data for the conductivity

in air shown in the last quarterly report (July-Sept. 1979). The magnitude

and the general trend is the same, however. There appears to be two

14
crystalline phases contributing to the conductivity. In the reduced O2 pressure experiment, a break in the conductivity curve occurs near

1160°C for all data; the break becoming more pronounced for the runs where the sample is quenched to 900°C from soak temperatures above

1400°C. All data shown on the graph is taken with increasing temperature

after being quenched to 900°C and cooled slowly to the lower temperature

('v500°C). The data below 800°C may be affected by cracks in the slag

body formed by quenching to 900°C and subsequent cooling to lower temperatures. Cracking appears to be more severe in those samples with

the K2 SO4 added. It would be interesting to see how the coefficients of

thermal expansion for these seeded and unseeded slags differ. X-Ray

diffraction measurements (courtesy of H. Parker, Center for Materials

Science, Solid State Chemistry Group) show that the phase composition

below 1180°C is a kalsilite like phase (probably KAlSi 04 ) and spinel

(Fe 304 ) for that material quenched from 1426°C. This kalsilite like

phase was not detected in the slag measured in an air atmosphere and

quenched from nearly the same temperature (1420°C). The spinel (Fe 304 ) was detected in the sample measured in air. The interesting and unusual

aspect is that the slag conductivities for Bow NH slag with and without

seed and measured in air and in an atmosphere of 2 x 10 ’^ O2 partial

pressure are the same as a function of temperature above 1400°C. If the

potassium ion conductivity is the predominant mechanism for charge

transport, above 1400°C, the samples containing added K2 SO 4 should have

a higher conductivity. Chemical analysis for K and S of the Bow NH slag

with 20 wt.% K2 SO4 added to the melt before and after the electrical

measurement (2 x 10'^ atmos O2 ) are given in Table 1.

Table 1

% K % S

Before conductivity measurement 6.34 .022

After conductivity measurement 5.99 0.12

The magnitude of sulfur detected in each case was below the accurate

limits of detection. It apparently leaves during initial melting of the

slag and K2 SO4 mix. The reason for the discrepancy between the amount of

K added (9I) and that detected in the analysis is unknown. If the conduction

mechanism is^pred^minantly electronic above 1400°C and due to the iron

ion ratio Fe f/Fe ^ one would expect a higher conductivity in that

sample measured in a partial pressure of oxygen of 2 x 10 ^ atmospheres.

Johnson (4) has reported an Fe ^/pe ^ ratio at 1450°C of 1.6 for an oxygen pressure of 10’^ atmospheres and of 0.13 for an air atmosphere.

This gives a ratio change of near 13 which should be reflected in the

conductivity if there is exchange between the iron ions of different

valence. The measured conductivity of the slag above 1400°C is not

likely to be due to oxygen ions since the conductivity is not oxygen

pressure independent. The slag sample at high temperature is a disordered structure (molten) aipd it is ijjot surprising that the electron

exchange is small between Fe ^ and Fe Simnad, Derge, and George

(5) in transport measurements on iron in silicate slags conclude that

15
the transport number of the iron ions Fe ^ is one and therefore that the

current is being carried almost completely by^these ioijis. The ionic

radius of K is large (1.33A) while that of Fe ^ and Fe ^ is 0.74A and

0.64A, respectively. This difference in ionic radii coupled with the

difference in ionic charge may make the iron ion the predominant charge

carrier in the molten slag.

The relative amounts of Fe'*’^ to Fe''’^ then would have a secondary

effect on the electrical conductivity. The total iron content, however,

would affect the magnitude of the conductivity. This coorelation was pointed out earlier by Bates ( 6 ) in his analysis of early slag conductivity work done by Frederikse and Hosier (7^

d. Other Activities

1. During the course of channel proof tests in preparation for the

3rd Joint US-USSR Materials Test in the U.02 MHD facility in Moscow, the

electrical conductivity of Sro, gLaQ, iZro^yCro, 3 O 3 was measured as a function of temperatue and oxygen pressure. This material was prepared

by A. T. Research and no details were given on its preparation. The

data was presented in quarterly report Oct-Dec. 1977 to DoE. An inquiry

was received from Dr. H. Shapiro from T. R. W. concerning this material

with regard to its large range showing a positive temperature coefficient

of resistance in an air atmosphere. Another sample was sent from T. R. W.

and the conductivity measured during this reporting period. The resulting

data is shown in Figure 4. The range showing a positive temperature

coefficient of resistance is reduced considerably and shows a hysteresis

effect with increasing or decreasing temperature indicative of a reversible

phase change.

2. During the week of October 8th, W. Hosier attended the joint

16th International Thermal Conductivity Conference and the 7th International Thermal Expansion Symposium held in Chicago at the IIT Research

Institute on Nov. 7-9.

3. On December 5 and 6 , W. R. Hosier participated as a panel

member for Materials in the review of the DoE sponsored work of MERDI at

Butte, Montana. The review was held at Montana State University, Bozeman,

Montana.

References

(1) Electrochemical Effects in Simulated Slag Flow

W. Capps, D. A. Kauffman, and W. R. Hosier, Quarterly Report for

the period April-June 1978 to DoE under contract No. EA-77-A-01 -6010.

(2) Discharge Characteristics of Slagging Metal Electrodes

J. K. Koester and R. M. Nelson, 17th Symposium on Engineering Aspects

of MHD. Stanford University, Stanford, California. March 27-29, 1978.

16
(3) Electrochemical Corrosion of MHD Electrodes in Slags

L. D. Cadoff, D. B. Rossing, and H. D. Smith, 17th Symposium on Engineering Aspects of MHD, Stanford University, Stanford, California.

March 27-29, 1978.

(4) Oxidation - Reduction Equilibria in Iron Containing Glass

W. D. Johnson, Journal of Am. Cer. Soc., Vol . 47, No. 4, April 1964.

(5) Ionic Nature of Liquid Iron - Silicate Slags AIME Transactions

M. T. Simnad, G. Derge, and I. George, Journal of Metals, 200,

p. 1386, Dec. 1954.

(6) Private communication.

(7) Electrical Conductivity of Coal Slag

H. P. R. Frederikse and W. R. Hosier, Journal of American Ceramic

Society, Vol. No. 8, August 1973.

Plans

The electrical conductivity of YCr03 doped with 5 % Ca on the Y site

in the oxygen pressure range of 10"- to 10'^- atmospheres will be completed.

Experimental difficulties here have been largely overcome and the outcome

measurements may depend on the physical stability of the material

upon going from an oxidizing to a reducing atmosphere.

The work on the slag electrical conductivity will continue. A

decision must be made whether to continue with tfie Bow NH slag Cooping

or increasing iron content, etc.) or begin to study extensively the

conductivity mechanisms in low iron high calcium slag CRosebud).

The electrochemical experiments involving slag and electrode

materials will continue particularly with respect to the possible

reversibility of these effects of with respect to the time limits

involved when electrochemical reactions degrade conductivities far

enough to limit usefulness.

17
Figure 1 . Time vs. voltage relationships in an electrochemical cell of

Y.gsCa.osCrOa in a slag electrolyte with added K2SO4.

18

TIME(HOURS)
Figure 2. Radiograph of crucible showing slag fill and electrode con- figuration before (a) and after (b) electrical conductivity

measurements.

19
Figure 3. Electrical conductivity of Bow MH slag with 20% K2 SO4 added

to the melt. All data was taken in 2 x 10"^ O2 atmospheres in

N 2 and with increasing temperature after beinq ouenched to

900°C from the equilibration temperatures indicated on the

graph.

20
|juuo-uu40)AllAll3naNOO

o

Figure 4 . The electrical conductivity of Sro. gLao, iZro . yCro. 3O3 as a function of temperature for increasina and decreasinn temperatures.

21

T(°K)
3. Corrosion of Downstream MHD Components (J. Smit, and C. D. Olson)

Progress: Type 304 stainless steel tubular specimens were exposed to

a seeded” oxygen-propane fueled hot gas stream under varied conditions

and then analyzed by SEM and EDX techniques. Total exposure time to

the hot gas stream was 4 hours with the first 25 minutes of this period

being used to seed the gas stream with 250 g of a mixture of 80% by

weight K2CO3 with 20% by weight K2SO4. Six stainless steel 304 specimens

were tested under these general conditions. Three of the tubular

specimens were exposed to an oxygen rich gas stream environment at

tube temperatures of 400° C, 500° C and 590° C, respectively. The

remaining three samples were exposed to a fuel rich gas stream

environment at 400° C, 500° C and 590° C. Specimen temperatures were

maintained at the desired value by internal air cooling and were

monitored through a Pt/Pt 10% Rh thermocouple welded into the specimen

wall. In the vicinity of the test specimen the gas stream temperature

was maintained at approximately 1300° C as monitored through a Pt/Pt

10% Rh thermocouple. The fuel rich and the oxygen rich states were

accomplished by varying oxygen to propane flow rate ratios.

To reduce external contamination of the samples and hydration of the

K2CO3 in the coatings formed during seeding of the hot gas stream,

the samples were removed from the test rig upon cool down to about 40° C

and potted immediately in epoxy. Metal lographic specimens were then

prepared from sections of the stainless steel tube taken approximately

10 mm from the midpoint position in the hot zone. The speicmens were

then ground and polished using nonaqueous media. All samples were

stored in evacuated desiccators until ready for SEM/EDX analysis.

On all specimens a compact deposit formed on the surface facing the

gas stream. A powdery deposit, probably from the fume, formed on the

trailing surface. At the boundary between the two, a thicker drip zone

formed.

On the specimens exposed to the oxygen rich gas stream the leading

surface deposit, which varied with temperature (thickest at 400° C)

ranged from 1.7 mm to 0.8 mm. The trailing surface deposit was fairly

uniform at about 0.2 to 0.3 mm. Figures 1, 2 and 3.

EDX analysis shows high concentration of potassium with traces of

sulfur throughout the bulk of the salt deposit, however, the top

layer of the salt deposit is high in sulfur (K2SO4). In previous

tests using only K2SO4, a layered deposit was formed on the specimens.

However, with the K2CO3/K2SO4 charge, layering did not occur but

small potassium rich nodes were found in a matrix of potassium and

sulfur. Figure 4, specimen temperature 400° C shows these small

nodes. Again, Figure 5, specimen temperature 500° C, shows the

potassium rich nodular formations. In the 590° C sample. Figure 6,

the nodes are not predominate and a more even surface can be

observed. An explanation for the difference in the coating would

be that two different salts with melting points varying by 178° C

22
would show areas of incongruent melting. However, at the edge of the

coating, it is possible that the hot gas stream volatilized the K2CO3

leaving behind a greater concentration of sulfur than potassium in

the form of K2SO4.

The corrosion band area on the 304 stainless steel increases with

increasing temperature. The variation in corrosion can be observed

in the comparison of Figure 7, specimen temperature 500° C with Figure

8 specimen temperature 590° C. SEM/EDX analysis of both samples.

Figures 7a-7g and Figures 8a-8j, indicate metal corrosion and metal

cation migration, however, the 590° C specimen shows the corrosion

picture more vividly.

SEM/EDX analysis seems to indicate that preferential leaching of the

metal cations from the bulk materials (304 stainless steel) is found

in a somewhat orderly fashion. The analysis shows that, moving from

the stainless steel bulk material to the salt deposit, a high chrome

concentration area is found along the edge of the tube and below the

surface of the tube top. Areas of high nickel concentration are found

within the high chrome concentration matrix while areas of high iron

concentration are found next to the salt. Figures 9a-9f show that

the iron is found in areas of high potassium concentration a good

distance away from the reaction interface region of the stainless

steel /salt deposit. Figures lOa-lOc again show iron well away from

the reaction zone.

In a fuel rich hot gas stream, a thin, 0.6 to 0.8 mm, as compared to

the oxygen rich environment, salt deposit was formed. As noted in

the oxygen rich series these samples also exhibited decreasing thickness with increasing temperature. In contrast to the oxygen rich

specimen the fume deposit, 0.4 to 0.7 mm (the layer on the under side

of the tube), was heavier. However, it also decreased with temperature.

Figures 11 , 12 and 13.

As in the oxygen rich test samples the fuel rich samples also exhibited

reaction zone areas. Again, these areas showed increased activity

in corrosion with increased sample temperature. Figure 14, specimen

temperature 500° C and Figure 14, specimen temperature 590° C show

areas of corrosion at the stainless steel /salt interface with areas of

high metal cation concentration. However, the reaction areas do not

seem to be as great under the fuel rich conditions as compared to the

oxygen rich conditions.

As noted by SEM/EDX analysis. Figures 14a-14g and Figures 15a-15g,

the edge of the stainless steel bulk material contains regions of

high chrome concentration are found next, with iron and potassium

in the salt deposit adjacent to the stainless steel. The salt deposit

seems to contain more sulfur than in the oxygen rich cases, but both

have a crust of potassium and sulfur in high concentration on the top

of the salt layer.

23
Both systems, oxygen rich and fuel rich, show evidence of metal

corrosion. However, the fuel rich system seems to show less evidence

of damage to the bulk stainless steel. Both system also seems to

indicate the same metal cation movement from the bulk stainless steel

towards the salt deposit.

Stainless Steel Reference Specimen - As with most analytical techniques,

reference samples are used to check some stage of the analysis. At

the present time a randomly selected sample type 304 stainless steel

that has been cleaned, cut and polished has been used to check outer

edges or surfaces for areas where corrosion would most likely occur.

A general overview of the tube (Figure 16) would seem to indicate

that the 304 stainless is very clean and uniform on the outer edges.

However, Figures 17 and 18, shows that in the manufacturing, rough

surfaces, i.e., holes and put areas, can occur. These areas would be more susceptible to a corrosion process and promote deterioration of

the surface under hot plasma and seeding conditions. Therefore, not

all areas analyzed for pits and holes are necessarily due to the corrosion

process. EDX analysis shows that traces of sulfur are already present

in the matrix. No other inconsistences such as areas of high chrome

concentration or high iron concentration are found in the metal, but

rather a uniform distribution of chrome, iron and nickel is found

throughout the 304 stainless steel.

Plans : Specimens of type 304 stainless steel will be evaluated at 100

hours exposure to a hot gas atmosphere, fuel rich and oxygen rich, with

seeding of a salt every 24 hours. Samples will then be anlayzed using

SEM/EDX to establish corrosion and leaching of metal cations.

A program will be initiated using type 316 stainless steel tubular

specimens under the same general conditions that have been established

in running the type 304 stainless steel samples.

24
Fig. 1. Section of Type 304 stainless steel tubing after exposure to K

2 CO 3 /K2 SO4

seeded oxygen rich hot gas stream. Note formation of thick deposit,

1.7 mm, on upper surface and thin powdary deposit on lower surface.

Tube temperature 400®C. 6 X.

Fig. 2. Section of Type 304 stainless steel tubing after exposure to K2 CO 3 /K2 SO4

seeded oxygen rich hot gas stream. Note formation of thick deposit,

1 mm on upper surface. Tube temperature 500®C. 6 X.

25
Fig. 3. Section of Type 304 stainless steel tubing after exposure to K

2 CO3 /K

2 SO4

seeded oxygen rich hot gas stream. Mote formation of thinner deposit

on upper surface. Tube temperature 590®C. 6 X.

Fig. 4. SEM micrograph, 200 X, of small, high potassium concentration nodes in

the salt deposit. Specimen temperature 400®C; gas stream oxygen rich.

26
Fig. 5. SEM micrograph, 200 X, of small, high potassium concentration nodes in

the salt deposit. Specimen temperature 500®C; gas stream oxygen rich.

Fig. 6. SEM micrograph, 200 X, of salt deposit showing absence of high potassium

concentration nodules. Specimen temperature 590“C; gas stream oxygen rich.

(

27
Fig. 7. SEM micrograph, 2000 X, of stainless steel -salt deposit reaction

zone. Lettered regions correspond to labeled EDX spectra, figures

7a through 7f. Specimen temperature 500°C; gas stream oxygen rich.

Fig. 7a. EOX spectra of regions A and B of figure 7 showing high Cr concentration

area.

28
Fig. 7b. EDX spectra of regions C and D of Fig. 7 showing region of high

potassium concentration associated with Fe in the salt region of

the corrosion area.

Fig. 7c. EDX spectra of regions E and F of Fig. 7 showing Fe penetration in the

salt.

29
Fig. 7d. EDX spectra of regions G and H of Fig. 7 showing Fe penetration in

the salt.

Fig. 7e. EDX spectra of regions I and J of Fig. 7 showing areas of high Cr

and low Ni concentration.

30
ig. 7f. eDX spectra of regions K and L of Fig. 7 showing areas of high Cr

concentration formed on stainless steel edge.

ig. 8. SEM micrograph, 2000 X, of stainless steel-salt deposit reaction zone. Lettered regions correspond to labeled EDX spectra. Figs. 8a-8j.

Specimen temperature 590°C; gas stream oxygen rich.

31
Fig. 8a. EDX spectra of regions A and B of Fig. 8 showing Fe penetration in

the salt deposit.

Fig. 8b. EDX spectra of regions C and D of Fig. 8 showing area of high Cr

concentration as compared to bulk stainless.

32
Fig. 8c. EDX spectra of regions E and F of Fig. 8 showing area of high Cr

and high Fe concentration with absence of Ni at the stainless steelsalt interface.

Fig. 8d. EDX spectra of regions G and H of Fig. 8 showing area of high Fe and

low Cr concentration with traces of Ni at the stainless steel-salt

interface.

33
Fig. Re. EDX spectra of regions I and J of Fig. 8 showing penetration of Fe

and Cr in the salt deposit.

Fig. 8f . EDX spectra of regions K and L of Fig. 8 showing area of high Fe and

low Cr concentration with absence of Ni at the stainless-steel salt

interface.

34
Fig. 8g. EDX spectra of regions M and N of Fig. 8 showing areas of high Cr and

high Ni concentration as compared to low Cr and low Ni below the

stainless steel-salt deposit interface but within the reaction band.

Fig. 8h. EDX spectra of regions 0 and P of Fig. 8 showing areas of high Cr

and high Ni concentration below the stainless steel-salt deposit

interface, but within the reaction band.

35
Fig. 8i. EDX spectra of regions Q and R of Fig. 8 indicating a region of very

high Ni concentration in a matrix of high Cr concentration as compared to the bulk stainless steel.

Fig. 8j . EOX spectra of regions S and T of Fig. 8 showing high Ni concentration

with the high Cr concentration matrix.

36
Fig. 9. SEM micrograph, 2000 X, of stainless steel-salt deposit reaction zone. Lettered regions correspond to labeled EDX spectra Figs. 9a-9e.

Specimen temperature 400°C; gas stream oxygen rich.

Fig. 9a. EDX spectra of regions A and B of Fig. 9 showing areas of low Ni

concentration on the stainless steel interface.

37
Fig. 9b. EDX spectra of regions C and D of Fig. 9 showing areas of low Ni

concentration on the stainless steel interface.

Fig. 9c. EDX spectra of regions E and F of Fig. 9 indicating Fe penetration into

the salt deposit.

38
Fig. 9d. EDX spectra of regions G and H of Fig, 9 showing areas of low Ni

concentration on the stainless steel interface.

Fig. 9e. EDX spectra of regions I and J of Fig. 9 indicating Fe and Cr penetration

into the salt deposit.

39
Fig. 10. SEM micrograph, 2000 X, of stainless steel-salt deposit reaction zone. Lettered regions correspond to labeled EDX spectra Figs. 10a and 10b.

Specimen temperature 400®C; gas stream oxygen rich.

Fig. 10a. EDX spectra of regions A and B of Fig. 10 showing areas of Fe and

Cr penetration into an area of potassium and sulfur.

40
Fig. 10b. EDX spectra of regions C and D of Fig. 10 showing farther penetration

of Fe and Or into the salt deposit region.

Fig. 11. Section of Type 304 stainless steel tubing after exposure to K2 CO 3 /K2 SO4

seeded fuel rich hot gas stream. Note formation of 0.8 mm deposit on upper surface. Tube temperature 400°C. 6 X.

41
Fig. 12. Section of Type 304 stainless steel tubing after exposure to

K2 CO3 /K2SO4 seeded fuel rich hot gas stream. Tube temperature 500°C.

Fig. 13. Section of Type 304 stainless steel tubing after exposure to K2 CO3 /K2 SO4 seeded fuel rich hot gas stream. Note formation of thin deposit on upper surface. Tube temperature 590°C. 6 X.

42
Fig. 14. SEM micrograph, 2000 X, of stainless steel-salt deposit reaction zone. Lettered regions correspond to labeled EDX spectra. Figs. 14a to 14f.

Specimen temperature 500°C; gas stream fuel rich.

Fig. 14a. EDX spectra of regions A and B of Fig. 14 pointing out areas of high

Cr concentration and low Ni concentration on the stainless steel interface.

43
Fig. 14b. EDX spectra of regions C and D of Fig. 14 indicating areas of low

Cr and low Ni concentration at the stainless steel interface.

Fig. 14c. EDX spectra of regions E and F of Fig. 14 showing areas of high

Ni and low Cr concentration at the stainless steel interface.

44
Fig. 14d. EDX spectra of regions G and H of Fig. 14 showing areas adjacent to

E and F of high Ni concentration and low Cr concentration.

Fig. 14e. EDX spectra of regions I and J of Fig. 14 showing areas of high Ni

and high Cr concentration at the interface.

45
Fig. 14f. EDX spectra of regions K and L of Fig. 14 of areas below I and J

showing low Cr and low Ni concentration.

Fig. 15. SEM micrograph 2000 X, of stainless steel-salt deposit reaction zone. Lettered regions correspond to labeled EDX spectra, Figs. 15a to 15f.

Specimen temperature 590°C gas stream fuel rich.

46
Fig. 15a, EDX spectra of regions A and B of Fig. 15 showing area of high Cr

and high Ni concentration as compared to bulk stainless steel.

Fig. 15b. EDX spectra of regions C and D of Fig. 15 showing areas of high Fe,

low Ni and low Cr concentration at the stainless steel-salt interface.

47
Fig. 15c. EDX spectra of region E of Fig. 15 indicating area of high Cr

concentration with Ni and region F of Fig. 15 showing area of low Cr

concentration with high Ni concentration.

Fig. 15d. EDX spectra of regions G and H of Fig. 15 showing areas of high Fe

and low Ni and low Cr concentrations at the stainless steel-deposit

interface.

48
Fig. 15e. EDX spectra of regionsi and J of Fig. 15 showing Fe and Cr penetration

into the salt deposit.

Fig. 15f. EDX spectra of regions K and L of Fig. 15 showing areas of high Ni and

low Cr concentration at the stainless steel-salt interface.

49
Fig. 16. Section of Type 304 stainless steel prior to exposure to the seeded

hot gas stream. 6 X.

Fig. 17. SEM micrograph 4500 X, of section of Fig. 16 showing stainless steel

edge.

50
Fig. 1 . SEM micrograph

inclusions and

(montage) 9000 X, of a section of Fig.

pits in the stainless steel.

6 showing

51
i