/7- (3i S [ pL

EA-77-A-01 -6010-10

Distribution Category UC-90g

DEVELOPMENT, TESTING AND EVALUATION OF

MHD-MATERIALS

Quarterly Report

for the period October - December 1 977

H. P. R. Frederikse, T. Negas and S. J. Schneider

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

Date Published: December 31, 1977

PREPARED FOR THE UNITED STATES

DEPARTMENT OF ENERGY

Under Contract No. EA-77-A-01 -601

0

'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, make 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 private owned rights.”

TABLE OF CONTENTS

PAGE

ABSTRACT 1 OBJECTIVES AND SCOPE OF WORK 2 SUMMARY OF ACHIEVEMENTS 3 TALKS AND PUBLICATIONS 4 SUMMARY OF PROGRESS 5 TASK G. PROGRAM MANAGEMENT AND COORDINATION 5 TASK I. OPERATIONAL DESIGN PROPERTIES 6

1.

2.

3.

Viscosity of Coal Slags

Electrical Conductivity

Vaporization Studies

TASK J. CORROSION AND DIFFUSION 25

1. The System CaO-K^O-Al^O^-SiO^

a. The System CaO-K^O-FeO^-MgO-Al^O^-SiO^

2. Diffusion in Insulator-Electrode Couples

TASK K. MATERIALS TESTING AND CHARACTERIZATION 34

1. Structural Analysis of Powdered and Solid Ceramics

a. X-ray Diffraction of MHD Materials

2.

3.

Thermochemical Effects in Laboratory and in situ Tests

Electrochemistry at the Pt/Slag Boundary (no seed,

P = 10" 3 atm)

°2

4. Laboratory Experiments Concerning the LaCrO^/Slag

Interface (no seed, P

Q

= 10~0*68 atm) u

2 5.

6.

7.

8.

9.

MIT Electrodes

AVCO/NBS Slagging Test 1 NBS/AVCO Slagging Test 2 NBS/APS Electrodes

Proof-Test I for U-02 Experiment Phase III

A. Electrode Systems and Component Materials

B. Post Test Handling of the Electrode Modules

C. Analytical Methods

D. General Electric Electrodes

E. Argonne Electrodes

F. Brunswick Anodes and Arc Plasma Sprayed Materials, Inc.

Cathodes

G. Conclusions

TASK L. ASSESSMENT OF STEAM PLANT COMPONENTS 88

TASK M. INFORMATION AND DATA ON MATERIALS FOR MHD-POWER SYSTEMS. . 91
'
ABSTRACT 1

During this quarter work continued on preparation for the U-02 Phase III

materials test to be held in Moscow early in 1978. Westinghouse Research

and Development Laboratories held two Proof tests as precursors for this

experiment. Two monitors from NBS were present for each test. Pretest electrical conductivities and x-ray diffraction data on all candidate

materials has been completed.

The post test characterization of the materials tested in the first proof

test for U-02 Phase III has been completed and the final report issued.

Analysis for materials in the second proof test is nearly completed.

A comprehensive program has started to determine the chemical and electrochemical reactions involved at the boundaries between molten (and solid)

slag and various electrode materials at different temperatures.

Electrodes of design based partly on the results of these experiments

are being constructed for future tests at AVCO.

Further progress has been made on the seed-slag interaction problem

including especially the relation between melt composition and temperature

in partially crystallized slags.

The effect on viscosity caused by changes in concentration of each oxide

of a five component slag at 1600 and 1300 °C are shown. Viscosity vs. temperature relationships are shown for slags from fly ash of a power plant

at Bow, New Hampshire and slag from coal from the Montana Rosebud seam.

Vapor pressure measurements of K over a K^O-SiO^-Al^O^ glass having an SiO^/Al 0 weight ratio of 5 indicate an enhanced K^O activity down to

20 w/o K^O; measurements over a glass having an SiO^/Al^O^ ratio of 2 show reduced activity as compared to K^O-SiO^ solution.

Diffusion of iron from iron containing electrode materials appears to be

less in the insulator magnesium aluminate than in pure MgO under identical

laboratory conditions.

Some 60 materials were analyzed by XRD. These materials included those

submitted by industrial firms and other governmental laboratories as well as materials under study in the NBS materials program.

. Two new activities were started in the first quarter of the new Fiscal

Year (1978).

a. A beginning was made with the establishment of a center which will

provide information on MHD materials and readily available data on their

properties for design and operational purposes. Reports from various

MHD contractors have been reviewed for the purpose of defining the

requirements for a computer based data system to be used in the Data

Center for MHD Materials. The assessment of downstream materials is

part of the center activities. In the evaluation studies, the use of

the unified numbering system (UNS) for metal alloy is described and an update of the requirement, test results and material recommendations

for steam superheater tubing is given.

b. We have also started the construction of a hot corrosion test rig

for the evaluation of candidate materials for the steam superheater

tubing under conditions that simulate the environment in the downstream section.
2

OBJECTIVES AND SCOPE OF WORK

The overall objective of this program is to obtain chemical and physical

definition of high temperature materials which have shown promise for use

in coal-fired open-cycle MHD power systems. Major problem areas in which

investigations will be concentrated are:

1. Characterization of coal slag and its effects on system components

and performance at prototype temperatures.

2. Development of electrode materials which provide adequate performance

over extended periods of time.

3. Insulating materials which limit thermal losses and are resistant

to prolonged thermal and erosion effects.

4. Preheater materials which can withstand the operating modes of

separately and directly fired operation.

5. Seed recovery methods from slag which are technically and economically

feasible.

6. Phase equilibria and diffusion rates of seed in slag and the corrosive

action of combination on system components and materials.

7. Durability of prototype MHD sub-systems.

The program is designed to contribute to the solution of these problems by

providing much needed data on candidate materials and by evaluating test sam- ples and structures that have been subjected to real or simulated MHD conditio

The activities are grouped under six tasks:

G. Program Management Coordination (Assisting ERDA in coordination, planning

and review of the various MHD-Materials Development Programs)

H. U-02 Materials Testing and Characterization (Coordination of U-02 Test

Activities, Phase I). (Terminated June 30, 1976).

I. Operational Design Properties (viscosity, electrical conductivity,

vaporization)

J. Corrosion by Seed and Slag (phase equilibria, diffusion)

K. Materials Testing and Characterization (test coordination, pre- and

post-test analysis)

L. Assessment of Steam Plant Components (corrosion resistance of metals

and alloys)
3

SUMMARY OF ACCOMPLISHMENTS (OCTOBER-DECEMBER 1977)

(Completed Milestones-See Work Plan FY 1978)

The results obtained during this first quarter (FY 78) are presented

in terms of the projected milestones of the NBS-ERDA (DoE) Contract.

TASK I. 6. Measured viscosities of several slags furnished by MHD contractors.

9. Measured electrical conductivities of new promising electrode materials

12. Measured K pressure over melts in the K

2 0-Si02 ~Al2 0 system in the

temperature range 1473-1873 K.

TASK J. 18. Determined electrochemical reaction processes of electrode materials

(LaCrO^, Pt) with seed/slag.

22. Conducted diffusion studies of iron containing spinel electrodeMgA^O^ insulator sandwiches without electric field.

TASK k. 26. Monitored and proof tests of Phase III-U-02 electrode materials at

Westinghouse R&D Laboratory, October 10, 11 and Nov. 17, 18.

27a. Phase III-U-02 proof test 1 pre- and post-test materials characterization completed.

b. Post-test analysis on MIT slagging electrode test completed

TASK L. 30. Updated assessment of materials for the steam heat exchanger tubes

to include new and promising alloys.

i
4

TALKS AND PUBLICATIONS

Phase Equilibria and Crystal Chemistry Related to Seed-Slag Reactions

Under MHD Conditions. The System K

2 0:Al20

3 :Fe O^iSiO , R. S. Roth,

University of Wyoming, Laramie, Wyoming, Nov. 10, 1977. (Paper will

be published later)

.

The System K O-Al O^-Fe^O -SiO^. Part II, Crystal Chemistry of Phases

on Joins KFeO^-SiO^ and KAlSiO -KFeSiO^, R. S. Roth. Annual meeting

of Geological Soc. of Am., Seattle, WA, Nov. 9, 1977.

Magnetic Susceptibility of Cerium Tantalate Compounds, G. A. Candela,

A. H. Kahn, T. Negas, C. L. McDaniel, 13th Rare Earth Research Conf .

,

Oglebay Park, Wheeling, WV, Oct. 1977.

Phase Relationships and Crystal Chemistry of Compounds Containing Cerium

Oxide, R. S. Roth, T. Negas, H. S. Parker, D. B. Minor, C. D. Olson,

C. Skarda, 13th Rare Earth Research Conf., Oglebay Park, Wheeling, WV,

Oct. 1977.

Crystal Chemistry and Oxidation- Reduction of Phases in Rare Earth

Tantalate-Niobate Systems., R. J. Cava, T. Negas, R.S. Roth, H. S. Parker,

D. B. Minor and C. D. Olson, 13th Rare Earth Research Conf., Oglebay Park,

Wheeling, WV, Oct. 1977.

Rare Earth Ceramics for MHD Power Generators, H. P. R. Frederikse,

W. R. Hosier, and T. Negas, 13th Rare Earch Research Conf., Oglebay Park,

Wheeling, WV, Oct. 1977.
5

SUMMARY OF PROGRESS

Task G. Program Management and Coordination (S. J. Schneider)

S. J. Schneider and other NBS staff participated in assorted DoE

arranged or sanctioned program review/coordination meetings, briefings

and technical conferences. These included:

1. U.S.-U.S.S.R. Technology Working group meeting,

November 1977, Washington, D. C. r S. J. Schneider.

2. U.S.-U.S.S.R. Steering Committee meeting, December 1977,

Moscow. S. J. Schneider.

3. Montana Tech and MSU Program Review, December 1977, Butte and

Bozeman, Montana. S. J. Schneider, T. Negas.

4. First International Meeting on Coal Conversion and

Utilization, October 1977, at NBS, Gaithersburg, MD.

(NBS was co-sponsor and host for conference; this

involved meeting arrangements, program planning and

lecture presentation)

5. 13th Rare Earth Research Conf . , Oglebay Park, Wheeling, WV,

Oct. 1977. H. Frederikse, T. Negas, R. Roth, G. Candela, D. Minor.

In addition, NBS hosts a number of visitors from MHD contractors and

others interested in MHD. A noteworthy visit to NBS was made by

Dr. J. P. Coutures, C.N.R.S., France. Dr. Coutures described his

materials work especially that pertaining to his interaction with

the IVTAN , USSR.

The conclusions and recommendations resulting from these meetings and

other DoE assigned activities are reflected in reports to DoE, through

direct consultation with DoE staff or through documents published

elsewhere

As part of NBS involvement in Phase III U-02 materials test, NBS

monitored two proof tests at Westinghouse and performed post test

analysis of exposed samples. Also NBS planned and performed an electrode test at AVCO (Mark VI, slagging). Results of these tests

are documented elsewhere in this report.

T. Negas attended meeting at Westinghouse Research and Development

Center to decide on electrodes to be utilized in Phase III U-02 test.
6

Task I . OPERATIONAL DESIGN PROPERTIES

1. Viscosity of Coal Slags (W. Capps and D. A. Kauffman, Inorganic Glass Secti

Introduction

Because of the increasing number of contractors involved in the development

of open cycle, coal-fired MHD, and because of the increasing emphasis on two

selected coals, a part of this task has concentrated on developing a means of

predicting viscosity of coal slags as functions of ash analyses and temperature.

Special attention has been given to the two selected coals, Montana Rosebud

Seam and Illinois #6 Seam coals.

S lag Preparation and Viscosity Measurements

a. A series of synthetic slags was prepared, based on a large number of

ash analyses available from Rosebud and Illinois #6 coals. Viscosity was measured on each composition from approximately 1600 C down to the onset of

crystallization. These data are being used to develop a mathematical model

suitable for the prediction of viscosity of slags from these two coals as functions of composition and temperature. Some of the results have been

presented in previous reports as they have become available. Figure 1 shows

at a glance the effect that each oxide has on the viscosity at 1600 C as the

oxide is added to or subtracted from the reference or base slag composition.

Figure 2 shows the viscosity-composition relationships for these same melts

at 1300 C. There is noticeably less data at 1300 C than at 1600. The reason

is that crystallization had commenced at 1300 C for many of these compositions

and therefore no viscosity data was taken.

b. Viscosity of a power plant flyash from a New Hampshire coal was measured at the request of AVCO-Everett Labs. See Fig. 3, Curve 1.

c. Viscosity of a power plant flyash from a Montana Rosebud Seam coal was measured at the request of AVCO-Everett Labs. See Fig. 3, Curve 2.

d. A sample of the New Hampshire flyash was added to K^CO^ an<^ remelted.

The resulting slag composition was 80% flyash and 20% by weight of K 0

representing a seeded slag. This was prepared for W. R. Hosier and

A. J. Armstrong for electrical conductivity measurements at NBS.

Future Plans

Viscosity data of the

slags will be used to find

of viscosity from coal ash

systematically varied series of synthetic coal

the best of several mathematical models for predictic

analyses

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2. Electrical Conductivity (W. R. Hosier and A. J. Armstrong)

I Introduction

The electrical conductivity of materials for both electrodes and

insulators is an important parameter in designing electrode systems

for MHD power ^generation. Generally, conductivities less than

0.1 (ohm cm) will cause a considerable amount of joule heating in

those materials used for electrodes. Insulating materials must have

low conductivity at high temperatures to prevent current leakage down

the channel due to the induced Hall voltage.

A number of samples have been sent to NBS from outside firms for

conductivity measurements. In addition, NBS has designed and procured

from outside vendors some materials of possible use as MHD electrodes.

All measurements reported here are done using a four probe d.c.

technique. They are done isothermally at temperatures between room temperatures and 1700 °C, and under several partial pressures of oxygen.

At each temperature an attempt is made to establish equilibrium

between the surrounding gas and the sample (oxidation-reduction)

.

SrZrO^ . Four samples have been received from H. U. Anderson of AT Research,

Rolla, Missouri in which various amounts of La and Cr were substituted into

SrZrO . The electrical conductivities of two of these samples were reported in our previous Quarterly Report (see pp. 17 and 18) . The

electrical conductivities of the remaining two samples in this series

have been measured (see Figs. 1 and 2). In general, the result of

increasing the amount of La and Cr was to increase the overall electrical conductivity and to increase the effect of the partial pressure

of oxygen on the electrical conductivity.

Two composite (two phase) samples were received from H. U. Anderson

of AT research (see Figs. 3 and 4)

.

Of all the samples based on SrZrO^ one composition shows promise as an MHD electrode. The composite sample of composition 50%

Sr Zr La Cr 3 + 50% Sr Zr La Cr 0 (see Fig. 4) displayed

• 9 • 9 1 • J. J • J. • X • 9 • 9 j

sufficient electrical conductivity for MHD use over a wide range of

temperature with little oxygen dependency.

NAFF 41. A sample of composition 4 NiAl^O . + 1 Fe^CL Was prepared by Trans Tec)

Gaithersburg, MD. This sample has been measured (see Fig. 5) and found to

possess sufficient electrical conductivity for MHD use only at elevated

temperatures (above 800 °c) .

LaCrO

^ + .05 Mg . A sample of magnesia stabilized lanthanum chromite was received from General Refractories. Its electrical conductivity is shown

in Fig. 6. This material appears to be useful for MHD purposes down to

very low temperatures (below 100 °C)

.

YFeO

^ + 5% CaZrO

^

• A sample of YeFeO^ doped with CaZrO^ was prepared by

Trans Tech. The electrical conductivity of this material has been

measured as a function of temperature and oxygen partial pressure (see
Figs. 7 and 8) . XRD measurements show a foreign peak in the pattern

due to unstabilized phase of ZrO . Other work involving nonstoichiometric

YFeO^(Ca) materials and YFeO^ (Caf + CaZrO^ is in progress.

Bow, New Hampshire Coal Slag . Fly ash from the Bow, New Hampshire

power plant was obtained from Mr. s * Petty of AVCO. The electrical

conductivity is shown in Figure 9. This slag is unseeded. Measurements

on the seeded material will be done in the future. This data was taken using a new four probe technique developed for obtaining four probe

conductivity data on slags above the melting temperature. In this

technique, powdered slag is placed in an Al 0^ closed end tube which has

four platinum probes, sealed by peening platinum sponge around the

wires, spaced along the side of tube near the closed end. When the

melting temperature of the slag is exceeded, the liquid slag flows and

fills the bottom of the Al^O tube. From the data shown in Figure 9,

it is apparent that the electrical conductivity (without K) is low even at 1300 °C. It should also be pointed out that these data were

taken using instantaneous current voltage application to avoid electrochemical reactions between platinum and slag described elsewhere in the

report. Polarization voltages begin immediately when current is drawn

during the measurement; therefore, this data should be taken as somewhat

qualitative. New methods of measurement are under investigation.
12

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of

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at

AT

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details

on

preparation

were

submitted.
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(ohm-cm )

1

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Fig. 2.

Electrical conductivity of Sr .La Zr Cr O as a function of temperature

• y • x • y * l 3

at several partial presures of oxygen. Sample prepared by A T Research by

H. Anderson. No preparation data supplied. Measurements made at NBS
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19

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Fig. 8.
CONDUCTIVITY

(ohm-cm)'

20

io

4

TOO

liq. 9. Electrical conductivity of coal slag unseeded (no added potassium)

ar. a function of temperature at several partial pressures of oxygen. The

slag melt was derived from the Bow, New Hampshire power plant fly ash used

in the AVCO/NBS slagging test #1.
21

Scanning Electron Microscopy Examination of LaCrO

^ Plus Stainless Steel

Wool (A. J. Armstrong)

Attachment of monolithic ceramics to a metal lead-out is a continuing

problem in MHD electrode design. Recently, metal meshes especially

prepared for a number of materials have been successfully used as electrode attachments either by directly brazing the ceramics to the

mesh or by arc plasma spraying the ceramic into the mesh. Dr. Graves

of the University of Dayton has attempted to hot press LaCrO^ integral

with a stainless steel wool (430) which in turn could be directly

brazed to a copper electrode base. Unfortunately, LaCrO^ will not

pressure sinter into a dense monolithic body at temperatures below the

melting point of the 430 stainless steel. In initial experiments a temperature of 1650 °C was necessary which is well above the melting

point of this alloy. SEM examination showed the diffusion of

Fe (main constituent) into the LaCrO^ and areas where the stainless

steel was intimately bonded to the LaCrO^. This technique appears

to have little promise as an attachment mechanism due to the high

sintering temperature required for LaCrO^. Use of this technique

using other ceramics is under investigation.
22

3. Vaporization Studies (E. R. Plante)

This section describes measurements of the potassium gas pressure

over K^O-containing compounds or phases. Such data are of critical

importance in predicting the interaction of a seeded MHD plasma

with construction materials or coal ash using equilibrium thermodynamics. For example, Spencer (1) noted that no activity data existed

for interior composition points in the K^O-Ai^O^-SiO liquid "slag"

system so that his ternary Margules model for predicting K^O

activities had to be based on data extrapolated from the binary

systems. The vaporization studies at NBS have been planned to

provide some of the lacking data for simple systems as well as to

measure I^O activities in solutions having up to 6 components which

will be more closely representative of slags in real MHD systems.

In this quarterly report the results of measurements on several

K^O-AJ^O^-SiO^-containing phases will be summarized.

K20-A^2°2 -S:*- 0

2

( Gl ass ) Vapor Pressure Measurements

These measurements were carried out using glasses obtained from

the Geophysical Laboratories, Washington, DC. They were among the well

characterized materials used by Schairer and Bowen (2) for their study

of condensed phase relations. Composition data was reported in a previous NBS report (3). Preliminary measurements of the K pressure

over several glass compositions were not successful because of

insufficient sensitivity of the microbalance system and a tendency

for the low melting glasses to creep from the Pt Knudsen cells.

Neither of these problems have been apparent in the mass spectrometer

work

.

Pressure measurements were generally restricted to the temperature

range of 1200 - 1600°C but depend to some extent on the pressure

generated by the particular sample. Potassium pressures below 10

atm are more difficult to measure because of a ubiquitous background

at mass 39 while pressures above 10“ 4 atm lead to rapid changes in

sample composition. In addition, experiments run at temperatures

in excess of 1600°C will result in significant loss of Pt through

vaporization so that independent measurements of the mass spectrometer

calibration constant are necessary. The above temperature range leads

to only a short extrapolation into the MHD region where seed slag

interaction is expected to be most important.

In this temperature range, the only appreciable pressure is

that due to the K

2 0 dissolved in the glass. The vaporization reaction

is

K

2 0 (dissolved in glass) = 2K(g) + 1/2 0^ (g )

As vaporization proceeds the K

2 0 content of the glass gradually

decreases while the ratio remains constant. The principal

source of error in the pressure is the temperature measurement while

the mass spectrometer constant is insensitive to errors in temperature.
As a pessimistic estimate, an error of ±50K at 1700K would lead to an error of ±100 percent (a factor of 2) in pressure while the mass spectrometer constant would be in error by only ±1 percent.

However, the mass spectrometer constant is also sensitive to the error in the weight loss used in its determination which can be

effected by sample impurities as well as weight loss in the Pt

effusion cells at temperatures of 1900K and above. For the

pressures summarized here, errors in pressure are thought to be

less than ±100 percent and errors in composition are less that ±10 percent

of the change in composition. Initial compositions are assumed to be

exact. Most of the samples have a tendency to absorb moisture and were dried prior to use in air at temperatures from 200 - 500°C for

periods of several hours. This may have lead to absorption of CO^

as evidence by small mass 44 signals during the initial vaporization

measurements. It is also possible that small amounts of CO^ as unreacted carbonate remain in the original glass preparations.

This would have a tendency to make the initial K pressures higher

than they should be at the starting compositions.

K210 Measurements

The starting composition of K210 was 39.6 wt percent K_0,

10.0 percent A^O^ and 50.4 percent SiO^ . The effusion cell had

a .5mm diameter effusion hole. Vaporization measurements on this

sample were carried out until the K^O composition had been reduced

to 15 percent K^O. Initially, the K pressures were about 3 times as high as the K pressures over 40 wt percent K^O in silica solutions

but were about the same as those over 20 wt percent K^O in silica

by the time the K^O content of K210 was reduced to 20 wt percent K^O.

Between 20 wt percent K 0 and 15 wt percent K^O, the pressures

decreased by a factor or about 6. This is in sharp contrast to the

behavior of K^O-SiO^ solutions which showed minor decrease in pressure

in this concentration range. It was not determined whether this was

due to a rapid decrease in pressure with composition in this concentration

range or whether non-equilibrium effects were becoming prominent.

This type of behavior would be expected if the rate controlling step for

the evaporation is diffusion of the K^O through the glass and a concentration gradient is established. Below 15 wt percent K^O

other non-equilibrium effects were observed. Here, the K pressure

fluctuated by roughly a factor of two at constant temperature. This

effect has previously been observed and is thought to result from

accumulation of gas in the slag sample with the resultant formation of

bubbles which burst through the surface exposing higher concentration

regions of K

2 0.

Questions concerning non-equilibrium can be answered if the K

pressure of glasses having lower K O concentrations but the same SiC>

2 /

A^2°3 ra^ ;*- 0 anc^ known to be homogeneous are measured. Two attempts

to do this were made using sample K1113 which had an initial composition
24

of 21.7 wt percent K^O, 13 wt percent A£ 0^, and 65.3 percent SiO

This composition corresponds closely to the composition K210 wouL

have when its K^O content was decreased to 21.7 wt percent K^O.

Unfortunately, none of these measurements led to valid data.

K220 Measurements

The starting composition of K220 was 35.2 wt percent K^O, 20. percent

an<^ ^4.8 percent SiO^. The SiO^/Ai^O^ ratio of 2.2 approximates

that for a "typical" Eastern coal and represents a signigicant change

from the SiO^/AJ,^®, ratio of 5.0 of K210. A Pt Knudsen cell having

a .34mm diameter effusion orifice was used. The measured pressures

over this sample were much below those for the K210 at similar K^O

concentrations as well as below those of K^O-SiO^ solutions having

the same K^O concentration. At a concentration of 19 wt percent

the K pressure is close to an order of magnitude below the K pressure

of a K^O-SiO^ solution having 10 wt percent K^O. This data appears

to be confirmed from preliminary measurements on K728 which has an initial composition of 14.9 wt percent K^O, 28 percent and 57.1 percent

Conclusions

K^O-SiO^-AJl 0^ glasses with an SiO /A£ 0^ wt ratio of 5 appear to

have an increased activity of K^0 down to anout 20 wt percent K^0 at

which concentration the K^O activity is the same as a K^O-SiO^ solution.

Glasses with an SiO^/Atf. 0 wt ratio of 2 show reduced activity of K^O

from about 32 wt percent K^O to 19 wt percent K^O as compared with

K 0-SiC) solutions.

2 2

Future Work

Future work will include K pressure measurements over selected,

compositions in the K^O-SiO^-AS, 0 system to complement the current

results. Extension to several K^O-SiO^-Ai 0^-Ca0 compositions is

planned. Comparison of experimental results on the K^O-SiO^-AS.^0

^ system with Spencers (1) ternary model will be made.

References

1. F. E. Spencer, Jr., J. C. Hendries, Jr., and D. Bienstock, Vlth

International Conference on MHD Power Generation, Washington, DC

(1975)

.

2. J. F. Schairer and N. L. Bowen, Am. J. Sci., 253 , 681 (1955).

3.

H. P. R. Frederikse, t. Negas and S. J. Schneider, MBS Quarterly

Report, September (1975).
25

Task J. Corrosion and Diffusion

1. The System Ca0-K

2 0-Al

20

3

-SiC>

2 (L . P. Cook)

Further experimentation has been directed toward determining the

temperature of the invariant melts first appearing within four phase

subvolumes in the Al, Si-rich portion of the system Ca0-K

2 0-Al

2 0

3

~Si0

2

. Results are summarized in Table 1, below. These experiments were

performed using mixtures of the crystalline end-members indicated.

The temperatures given have not been reversibly bracketed by equivalent

glass crystallization experiments. However temperatures are believed

to be not far from equilibrium, as extensive superheating does not seem likely. Possible exceptions are melts involving KAlSi_0_ - these

are undergoing further study, and results will be given in the next

quarterly report.

Table 1. Temperatures at Which Melt First Appears Upon Heating Four-Phase

Mixtures in System Ca0-K

20-Al

2 0

3

-Si0

2

.

, Temperature (°C)

Four Phase Volume — Phases of Melting

2 KAlSiO -CaAl

20 - Ca

2Al2 Si07 -CaAl

2 Si20

g

1275±20

3 KAlSiO -KAlSi

206- Ca

2Al2

SiC>

7 -CaAl

2 Si2 0g

1280±20

4 KAlSiO -KAlSi

206- Ca

2Al2 Si0

7 -CaSi0

3

1260115

5 KAlSiO -KAlSi 0 - Al

20

3

-CaAl

2 Si20g

1335115

6 KAlSi 0 -Al 0 - 3A1

20

3

. 2Si0

2 -CaAl

2 Si20g

1370115

7 KAlSi 0 -Ca

2Al SiO - CaAl

2 Si20g -CaSI0

3

1235115

1 / Refer to previous two quarterly reports.
26

a. The System CaO-K^O-FeO^-MgO-Al ^O^-SiC^ (L. P. Cook)

With the aid of a statistical analysis of coal ash data in U.S.

Bur. Mines Bull. 567, and available channel slag analyses, two com- positions, representing hypothetical "Eastern" and "Western" seeded

channel slags, have been chosen for experimental study (Table 2).

Experiments made under oxidizing conditions (P 0 0.21 atm) show that

the melting temperatures of these slags are somewhat higher than the

equivalent unseeded slag, due to the presence of refractory potassium

aluminosilicates (Figure 1) . Initial melting of "Eastern" channel

slag occurs in the range 1350-1400°C; liquidus temperature is in the

range 1600-1650°C. Equivalent temperatures for "Western" channel slag are 1400-1450°C and 1650-1700°C, respectively. The volume of melt does

not become more than 50% until temperatures above 1550°C are reached.

Under oxidizing conditions, "Eastern" channel slag melt shows a marked

enrichment in Fe at lower temperatures; "Western" channel slag melt

shows a similar enrichment in Ca (see Figure 2) . Analogous experiments

are planned for the same compositions under reducing conditions, more

applicable to the cathode side.

In operating MHD channels, there are at least two competing effects

which may serve to locally modify the composition of the accumulating

slag layer, in addition to the temperature gradient: (1) diffusion of K

inward toward the cooler walls along chemical potential gradients, (2)

electrochemical migration of mobile species. Limited test data available (e.g. AVCO 1976 100 h test; G.E./AVCO test on 9/27/77) suggest that

the electrochemical effect is most important in a zone a few hundred

microns wide adjacent to the electrode. Both calcium and potassium

appear to diffuse away from the anode. However in the outer zones of

the slag layer away from the electrode interface, temperature may exert

an important control on melt compositions, if low enough (less than

'\'1600°C) that solid/liquid interaction occurs.

Table 2. Suggested Average Compositions for Modeling MHD Channel Slag

(wt %). 1/

Western Coal Eastern Coal

Si0

2 34.78 36.19

A12°3 24.58 25.36

FeO as Fe o 0. 5.32 12.13

x 3 4 CaO 9.27 1.77

MgO 3.33 0.63

k

2o 22.71 23.92

99.9 100.0

Refer to April-June 1977 Quarterly Report.
27

ft

Figure 1. SEM micrographs of experiments on synthetic "Western" channel slags.

Granular outlines are produced by contact with Pt.

A) Eutectic intergrowth (1600 °C) , B) Crystal/liquid mixture (1500 °C)

Crystals are potassium aluminosilicates, presumably KAlSiO^.
28

o.

O

s 5-H

d

P

1500 1600

Fig. 2 -A

1700 T,°C

Figure 2. Composition of melt as a function of temperature for six oxides

(dashed - Western; solid - Eastern) . Vertical axes indicates wt% oxide in melt,

as estimated from comparison with completely melted "Eastern" and "Western"

glasses. Horizontal axis shows temperature of melting.
OTS %

29

Fig. 2-D
FeO

as

Fe

30

Fig. 2-F
Diffusion in Insulator - Electrode Couples (A. J. Armstrong,

E. N. Farabaugh and J. R. Manning)

Investigation of the diffusion of Fe from MAFF31 (3MgAl 0 rlFe^O ) into various insulator materials is continuing. This investigation

should serve to study the possible degradation of the insulating

characteristics of the insulator material after exposure to high

temperature while in contact with iron spinel electrode material.

During this reporting period electrode - insulator-electrode couples

of MAFF31 and SP71 (MgAl^O ) were made and tested at 1600 °C for 1,

10 and 100 hrs. These diffused couples are shown in Fig. la (1 hr.),

Fig. lb (10 hr.) and Fig. lc (100 hr.). Notice the difference in Fe

penetration in the SP71 (insulator) indicated by the dark bands in

the SP71 at the insulator-electrode interface. It is seen that the

bands become wider with longer time at 1600 °C.

A part of Fe concentration profile from the 10 hr. test is shown in

Fig. 2. (This profile is from one electrode-insulator interface. The

Fe concentration profile taken across the electrode and through the

insulator and into the other electrode material showed very nearly the same shape at each electrode-insulator interface) . The profile in

Fig. 2 is not as smooth as those profiles obtained from MAFF31-MgO

couples (Quarterly Report 9/77) . In fact, a small plateau is possibly

present in the MAFF31 about 60 ym from the interface. When this 10 hr

test profile is compared to the 10 hr. test profile for the MAFF31-MgO

couple (Quarterly Report 9/77) , it is seen that the Fe penetration is

markedly less in SP71 than in the MgO. The Fe concentration in the

SP71 goes to zero approximately 250 ym from the MAFF31-SP71 interface,

whereas the Fe concentration goes to zero in the MgO approximately

450 ym from the MAFF31-MgO interface. Thus, it appears that for equal

10 hr. test conditions there was deeper Fe penetration in MgO than in

SP71

Concentration profiles will be obtained from the 1 and 100 hr. tests

and during the next reporting period compared to the 1 and 100 hr.

MAFF31-MgO tests.
<1

Fig. la MAFF31-SP71 Diffusion couple, tested at

l600°C for 1 hr.

Fig. lb MAFF31-SP71 Diffusion couple tested at

1600°C for 10 hr.

Fig. lc MAFF31-SP71 Diffusion couple, tested at

l600°C for 100 hr.
1600° C

for

>4000

MAFF31-SP71

couple

33

SJUOOD 0j

concentration

profile

for

10

hr

tested

MAFF

31-SP71

couple.
34

Task K. Materials Testing and Characterization

1. Structural Analysis of Powdered and Solid Ceramics

a. X-Ray Diffraction of MHD Materials (C.L. McDaniel)

During this quarter we received about 60 materials (powders, dense

ceramics, etc.) from various external sources. These were analyzed by

x-ray diffraction methods. Data, transmitted to the appropriate source, are shown in Table 1.
X)o

>n o X o C O -—

X X) XI x X X

X '—

04

O o o o o o R o X

(0 o O o o o o o 3 o X 4-1 o O o o CL, CL, CL X o 43 (0 00 Cl, X X 4-i s- 43 (0 04 33 Q u 3X

4-1 a

tox

X

X >4-1 3 to tO

>4-4 o 04

X

X

X

XX

Q 4-> 43 3 4) 3 3 X O X O X X 4) 43 3 43 3 43 O X 43 43 O X 43 43 +

c0 o 4-1 4-IOO 4-IOO 4-1

X

X

XX +1

XX

X

a: 4-> 03

X XXX XXX

X 0 3 XX e 3 3 X

X

X 1 c X X X LI X X 4-1 X O X X X O X O X O X 0) X 03 03 3 3 03 3 3 03 X O 03 03 X O X 03 03 X

03

X > > O

H > O rH > 04 CO > > X 03X > > >-> 0) rH o O O o o o O O 3 O o

04 o 04 04-1-03 04+03 04 + X

X

X + X

X

X

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cl 03 4) 43 43 43

X 43 43X o 43 43 Cl C a s x) CL 3 X CL C X CL 3 CL 3 X CO CL 3 CL 3

0) a>

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H -H rH •—1

X i—CJ

X 03X

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<0

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O to 4-1O

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X C0

XX

XX

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X 04 04 3 04 3 03 04 3 03

X 3 43

X 3 X 3 43 1—

X 3 -0 3 0

CL o X)

X X) rH

X r-l

B

X

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x o B

x S «—< CN 3 a) o 43 O 43 43 O 43 O O X 43 O o o x CO o o OOQJ

03 43 i—

X 03

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a >4-1 o O X O X 04 03 O X CO X X X

<0 £ XiXX X) 04X

X 04

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X > XX

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tO C_> Xxo •» 0k * 0k 0k

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r—

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40

2. Thermochemical Effects in Laboratory and in situ Tests (T . Negas, w. HOsler)

a. Introduction

For several years, this laboratory has described features of tested,

MHD electrode materials that could be interpreted only by considerations

involving electrochemistry (see references 1, 2, for example). More

recently, Westinghouse Corp., Stanford Univ. and apparently Battelle

Northwest researchers have undertaken laboratory studies which have suggested the potential importance of electrochemically induced phenomena

on electrode performance and durability. Although we have been quite aware of these phenomena, this provides us the opportunity to review

some of our previous data pertinent to static, electrochemical tests,

as well as, to relate some recent data obtained from dynamic, "in-channel"

testing of electrodes. These particular data are limited to a slagging

system although anionic conductors in a clean-fuel system also reflect

well-known and obvious electrochemically-induced features.

Our static tests concentrated first on Pt electrodes in slag at

temperatures above the softening point. These provided data for the

electrochemical behavior strictly of slag without intervening reactions

among slag and other constituents of a more complex ceramic oxide electrode.

LaCrO -based electrodes (arc plasma sprayed) were tested next and other

materials, including hot pressed and sintered LaCrO^, are under

investigation as time permits. Test parameters and materials include,

a) fly ash from Bow, New Hampshire power plant (AVCO-type)

,

seeded (20 w/o K^O) and unseeded,

b) isothermal conditions, 1300-1400°C (tests at lower temperatures

are reserved for future work)

,

2 c) initial current at 250 ma (current density, ^1 A/cm ),

d) P varied from 10 - 10 ^ atm.

°2

The test-cell temperature was raised without current to the test condition.

When thermal equilibrium was attained, a voltage was applied to drive a maximum current of 250 ma in the constant current mode. Data (voltage,

current, temperature) were recorded every ten minutes or less. As the

test continued, voltage increased (to maintain 250 ma current) to 50 V

at which point the power supply was out of compliance and the current

decreased over the total test period of ^20 hours. After testing, the

electrode/slag assembly was cooled, sectioned, potted in epoxy, polished

and examined by SEM/EDX.
41

For this report, we describe features of Pt- and LaCrO -slag

electrodes without seed and under static conditions. A MIT cathode

with flowinq slaq also is presented. Our n^xt report will

include results from static, seeded slag tests plus features of

electrodes tested under MHD conditions at AVCO

b. Electrochemistry at the Pt/Slag Boundary (no seed, atm)

Figure 1 (43X) illustrates the post-test condition of a Pt anode,

20 hours in unseeded slag, 1400°C, P Q

= 1C) atm. The microchemical/microstructural details for ?his anode correlate well with

the macroscopic examination of a Pt/iron oxide-silica "slag" anode

described as early as 1954 (3)

The spectrum in Figure 2 illustrates

the elemental content of our New Hampshire-type slag. Dominant constituents

include Al, Si and Fe while K, Ca and Ti are characteristic minor com- ponents. This spectrum provides a frame of reference to be compared

with ensuing spectra (see below) showing elemental partitioning con- trolled by thermochemical/electrochemical processes.

Fig. 3A (860X) illustrates the "slag" in contact with the Pt anode within

porous zone C, Figure 1. The brightest phase, Pt metal, occurs as domains

in intimate contact with "slag" but segregated from the main anode wire.

The lack of a sharp Pt/slag interface suggests evolution of gas, probably

oxygen, and, possibly, slight oxidation-solution of the Pt wire. The

source of oxygen is clearly from the slag. In reference (3), it was demonstrated that molten iron oxide-silica "slags" are primarily ionic con- ductors. It also was suggested that at the anode, silicate groups (of undefined

degree of polymerization) are "discharged" according to,

[Si0

4

] 4 '

-*• Si0

2 + 0

2 + 4e~.

Figures in series 3 and 4 demonstrate that the above anodic process is

viable. Figure 3B magnifies (4300X) a portion of Figure 3A. The bright

phase is Pt in contact with "phases" 1 and 2. Phase 1 consists predominately of Al and Si, and an appreciable amount of Ti plus traces of

Pt, Rh and Fe (Figure 3C)

Calcium and K are entirely absent. Phase 2 is similar but contains somewhat more Fe. Figure 4A (4300X) magnifies a typical area in zone C (Figure 1) closer to the undisturbed Pt metal.

Pt is associated with partly filled pores (1) and dark domains (2) having

a microchemistry depicted by Figure 4B. Very little, if any, Al is

present with Si (SiO ) dominating. The presence of Pt may suggest

some oxidation/solution, fine Pt-particles , or interference from

adjacent Pt during the analysis at high magnification.

Within the slag (B)

,

Figure 1, dendrites, illustrated in Figure 5 (1720X) , are common. These were not observed for cathodic slag. X-ray

mapping of these dendrites at high magnification is not precise due

to their small size. However, they appear to be enriched in Fe. The
42

microstructure may develop on cool down of the slag and/or may represent

in situ enrichment of the Fe-component in the slag away from the Pt/slag

interface

.

Figure 6 (43X) contrasts the post-test condition of the pertinent

Pt/slag cathode. Considerable damage to the Pt-metal remains obvious.

Domains and streamers of Pt are distributed well within the "slag" while

dark streamers of "slag" have intruded the remaining Pt cathode. Throughout Figure 6, the Pt is significantly "impure" (in contrast with anodic

Pt) having been intruded by and alloyed with Fe. This probably accounts

for the apparent degradation of the Pt. Figure 7A (4300X) shows a typical

Pt domain (1) in contact with darker "slag" (2) . Figure 7B indicates that

the metal is enriched with substantial Fe derived from the immediately

adjacent slag (2) which is depleted of this component (enriched in Al, Si).

c Laboratory Experiments Concerning the LaCrO-/Slag Interface (no seed,

P0

2 - lO" 6 - 6 ® atm)

LaCrO^-based electrodes in slag provide a veritable smorgasbord for

the microscopist . Figure 8 (90X) illustrates several well developed, sharply

defined interfaces and zones involving arc plasma sprayed LaCrO^ (zone 1)

and unseeded slag, without current (1400°C) . Chemical partitioning,

significantly unlike LaCrO /slag electrodes with current, occurs within

and between each zone. Figure 9A (9100X) shows a blown-up section of

the Mg-doped, LaCrO^. The material is somewhat porous and consists

of at least two phases (1, 2 Figures 9A, B) . The sharp interface between

zones 1 and 2 (Figure 8) is magnified in Figure 10 (4500X) . Zone 2 marks

the first appearance of slag components Al , Si, and Fe. Although similar

in microstructure to zone 3 (Figures 8, 11A) , zone 2 differs in being

enriched in La/Cr-components . Zone 3, Figure 11A (4500X) , has apparently

densified via reaction between slag and LaCrO (Figure 11B) . This zone

(plus zone 2), probably partly crystalline, constitutes a reactiondiffusion barrier providing the LaCrO "protection" from further reaction

with slag. Zone 4 (Figure 8), the outermost and thickest portion of this

barrier, is magnified in Figure 12A (4600X) to illustrate its "glassy"

nature. Chromium is practically absent but La-oxide (plus other rare earths) has dissolved preferentially in the slag (Figure 12B) . Adjacent

to zone 4 (now occupied by epoxy in Figure 8) typical New Hampshire-type

slag (see above) was present but detached with considerable ease.

A LaCrO anode in slag is shown in Figure 13. The material (corroded

considerably; and surrounding "slag" display noteworthy microstructural

and microchemical features. Areas (1), Figure 13, are structureless

(glassy ?) and are enriched in the La-component similar to zone 4 (Figure

8, see also Figures 12A, B) . Darker areas (2) adjacent to areas (1)

consist of irregularly shaped domains within a featureless (glassy ?)

matrix, Figure 14A. This matrix has a bulk composition similar to areas (1), Figure 13, while the domains consist of Cr, Fe, Al-oxides

(Figure 14B) , possibly a spinel or a sesquioxide solid solution depending
on the oxidation state of iron. Similar but better developed crystallites

are observed at the cathodic side (see below) . The bulk iron content

and bulk Al/Si ratio of areas (2) , qualitatively, are enhanced in comparison

with normal New Hampshire- type slag. Elemental partitioning of this

type (not observed without current) suggests solution of LaCrO^ in slag

followed by electrochemical migration of the more mobile network

modifying cations (such as Cr and Fe) away from the positive anode.

Simultaneously, the slag near the anode can be depleted of networkforming, polymerized (n) , [Al,Si] groups which accumulate and discharge oxygen at the anode/slag interface. Similar to Pt/slag anodes

but in contrast to cathodes and the "no current" configuration, this

interface for LaCrO./slag is particularly porous suggesting simultaneous

solution and gas evolution. Zone A, Figure 13, magnified in Figure 15,

is typical of the anode interface. Figure 16A (4500X) includes area 4 of zone A, Figure 13, while Figure 17A (1800X) depicts adjacent areas 3.

In Figure 16A, reaction/solution features dominate while partitioning of

elements is demonstrated by Figures 16 B, C, D. Dark areas (1) rich in SiO^

are adjacent to domains of Al, Cr, Fe-rich material (2) similar (but

with much less Fe) in chemistry but not morphology to the domains observed

in the outer slag, areas 2, Figures 13, 14. The groundmass (3), containing

the La-component from the pre-existing LaCrO^, also is similar chemically

(but with far less Fe) to area 1 (Figure 13) and the matrix in area 2 (Figures 13, 14). Notably absent in appreciable amounts are K and Ca.

Figure 17A of adjacent areas 3 (Figure 13), illustrates the resistive layer

developed near the former LaCrO^/slag interface. Hicrochemistry varies

but is dominated by Al/Si-oxides with traces of La, Cr, and Fe (e.g., see Figure 17B) . Powder x-ray diffraction data reveal that these deposits

consist of silica (tridymite and cristobalite) plus mullite.

Area 5, Figure 13 reflects the processes which degrade this particular

LaCrO ceramic. Slag apparently intrudes via the available porosity and

dissolves the material to produce rounded LaCrO- islands (1) . Figure 18

(4600X) . The groundmass consists of dark, Al/Si-rich areas (2) and

brighter, Si/La-rich areas (3). Very little Fe, K, or Ca is present.

Except for microchemical details, cathodic LaCrO

^ degrades in similar

fashion (see below)

Figure 19 illustrates a LaCrO /slag cathode. Areas 1 and 2 summarize

the most pertinent features. Figure 20 (48X) shows area 1 which is

subdivided into three distinct zones. In contrast with the anode, a porous resistive layer, rich in Si/Al-oxide is clearly absent. Instead,

rather sharply delimited reaction zones have developed. Zone A, Figure

20, is magnified in Figure 21A (920X) . This zone is typical of the

"slag" adjacent to the altered LaCrO^ electrode (zone C, Figure 20).

Isolated islands or crystallites (2) plus "stringers" or dendrites (3) are contained within a featureless matrix (1) . The crystallites are rich in

Cr, derived from the LaCrO^, and Fe, derived from the slag. These may be

a Fe/Cr-oxide spinel (plus traces of Mg and Al, Figure 21B) . Powder
44

< x-ray diffraction data, indeed, reveal the presence of a spinel phase.

The "stringers" (3) are Fe-rich (Figure 21C) and may represent

crystallization during cool-down. These were not observed, however,

in anodic slag. Microchemistry of the groundmass (1), given in Figure

21B, is similar to area 1, Figure 13 (anode) and area 1, zone 4, Figures 8,

12A (no current). This material, containing the La-oxide component, occurs within cracks and fissures obvious in Figure 20. Noteworthy is the

rather high concentration of Ca and K.

Zone B, Figure 20, is magnified in Figure 22 (1900X) . Interconnected domains (2) , rich in Fe/Cr-oxide (Figure 21B) , now enclose a featureless material (1) having "slag" components similar to Figure 21B.

Zone C, Figure 20, includes the volume previously occupied by the LaCrO

electrode. Figure 23A (1840X) is a magnification of this zone. Slag has

intruded the ceramic, similar to the anode, to produce a phase assemblage

composed of rounded domains of LaCrO

^ (2) enclosed within a complex matrix.

In contrast to the anode, the matrix consists of several materials showing

partitioning of Si, La, Cr and Fe (Figure 23B, C) . Phase 4 (Figure

23A, C) appears to be the precursor for the Fe/Cr-rich domains in area

B, Figures 20, 22. The bulk Ca, K and Fe contents are significantly

higher than for corresponding anodic areas, Figure 18. Within the "neck" (2)

of the LaCrO

^ in Figure 19, slag again has penetrated, probably via the

available porosity, Figure 24. Although LaCrO particles are more

numerous the phase assemblage is identical witn that observed in Figure 23.

(3 ^ MIT Electrodes

We received from W. R. Cannon, MIT, two electrodes (designed, fabricated

and tested in their laboratories) for microstructural examination. Both

are identical in terms of initial materials, design and fabrication. One

was subjected to current (number S4048) while the other (S4055) apparently

was tested only for thermal behavior. Test conditions and materials included,

a) 304 stainless steel base, grooved,

b) 36% Fe^O,-64 FeAl^O, , nominal composition (plasma-sprayed?)

originally 0.030" above grooves followed by,

c) hercynite (FeAl^O^, 0.20" thick),

I QC d) surface temperature 1360°-1390°C before introduction of

slag,

I

e) ^ 1650 C slag surface temperature,

f) ^2 hrs slag time with 1 hr current as cathode (40 min at

1 1/2 A/cm 2 ),

g) ^2 1/4 hrs slag plus seed with no current, and

h) the upper hercynite layer was completely dissolved off.
45

SEM/EDX examination revealed that insulation is MgAl 0, while the

specified "304 stainless steel" base is nearly pure Fe (mild steel ?)

with traces of Mn. Figure 25 (15X) illustrates the basic features of the

MIT electrodes. Pertinent microstructure and microchemistry to be

documented for the electrode tested without current should be compared

with corresponding areas of the cathode (see below) . A rather sharp

interface exists between ceramic and vesicular slag as suggested in

Figure 26A. The pre-existing hercynite layer was sacrificial, having

been dissolved by the slag. The slag (S) layer is characterized by

domains (e.g. 1 and 2, Figure 26) within a structureless matrix. The

spectra in Figure 26B indicate that these domains are enriched with

respect to Si, K, Ca and Fe components while the groundmass is predominately a vitrified potassium aluminosilicate plus some iron oxide.

This partitioning of elements, not evident closer to the slag surface,

may have developed during cool-down ojr during testing in response to

crystallization controlled by the inherent thermal gradients. If the

latter situation prevailed, the segregation of Fe within discontinuous

domains could diminish the electrical conductivity of the slag significantly.

The uppermost portion of ceramic (C) , Figures 25, 26 is illustrated by

Figure 27 (5000X) . A two-phase mixture clearly is evident. The brighter

domains are rich in Fe-oxide, low in Al-oxide (a spinel ?) while the

darker matrix is rich in Al-oxide, low in Fe-oxide (plus potassium).

This layer may be the remains of the altered hercynite layer (see above)

intruded by K or may be recrystallized ceramic similar in bulk com- position to that existing below the original hercynite layer. The bulk

of the ceramic (C) , Figure 25, is shown in Figure 28A (2000X) . Variation

in the microchemistry is reflected by the varigated nature of the ceramic.

Bright areas correlate with Fe-rich zones which grade into darker bands

containing variable Al/Fe contents (Figures 28B, C) . This microchemistry

developed either during fabrication (plasma spraying of Fe/Al-oxide ?) or via oxidation of the ceramic during testing.

Features related to the slag/ceramic interface of the electrode tested without current are well defined and not particularly complex. The corresponding

cathode, having similar materials and thermal conditions but with current,

differs substantially. Figure 29 (200X) illustrates the slag-ceramic

interface for much of this cathode. Within the slag (3) and in interface

(2), bright domains (A), magnified in Figure 3QA, are evident. These are

not abundant near and at the surface of the slag. In contrast to the

domains in the slag of the electrode without current (see above) , these

have different morphology and are particularly enriched in Fe. The

featureless groundmass (B)

,

however, has a similar composition (Figure

30B, with apologies). Interface (2), Figure 29, extends but widens toward

the MgAl-0^ insulator. Figure 31 (90X) illustrates interface (2) within

several type-areas (5, 6, 7) are evident. Areas (6) "intrude" the

insulator (4) and contain domains (5) of a Ca-rich material. Area 7
46

includes domains of the Fe/Al-oxide ceramic undergoing reaction with slag

components. Figure 32A (2000X) magnifies a typical portion of areas (6).

Bright, Fe-rich domains (1), are contained within a matrix (2) of Al,

Si, K and Fe components (Figure 32B) . Chlorine and Zn from unknown

sources are also present within areas (6) as well as in porous zones

of insulator (4) adjacent to area (6), Figure 31.

The microstructure within interface (2), Figures 29 and 31, are illustrated in Figure 33A (900X) . Fe-rich segregations (1), Figure 33B, are confined within a K/Al/Si-rich (2) matrix (Zn often present).

Cathodic reduction and segregation of iron (as reduced oxide or even metal) mixed with "slag" has been observed previously (reference

3) . For a Pt cathode (see above) available Fe reacts with the noble metal

provided temperature is high enough. In this Fe/Al-oxide cathode,

dissolution of the ceramic by slag probably enhances the concentration

and segregation of Fe-rich material. LaCrO^-based cathodes (see above)

provide chrome to the slag and this component reacts with the available

Fe to precipitate crystalline Fe/Cr-oxide, probably a spinel containing

reduced iron. Thus, the chemistry at a slag/metal or slag/ceramic interface is controlled not only by cathodic reduction of iron in the slag

electrolyte but, obviously, by temperature and bulk chemistry of the

electrode

.

Microstructural and microchemical features of the Fe/Al-oxide ceramic

below the slag are of two types. Figure 34A (2000X) illustrates the

ceramic immediately adjacent to interface (2), Figure 29. A two-phase

assemblage (Figure 34B) is characterized by bright domains rich in Fe

and Al (a spinel ?) plus darker intergrowths rich in Al, K, and Fe

(K/Al/Fe-oxide , 0-alumina ?). The bulk of the ceramic below this zone

of apparent interaction with K remains relatively unaltered and is

identical with Figure 28A (tested without current).

e . AVCO/NBS Slagging Test 1

Post test analysis and materials characterization of these electrodes

is in progress and will be reported at a later date. Some delay has been

caused by the urgency for characterization of electrode systems from the

proof tests carried out at Westinghouse Research and Development Labs,

in preparation for the U-02 Phase III test in Moscow early next year.

f • NBS/AVCO Slagging Test 2

Another set of electrodes has been designed and is partially constructed

for eventual testing at AVCO. This set will consist of LaCrO^ and/or

platinum as the electrode materials. Careful attention has been paid to

the bond line temperature between platinum and slag or LaCrO and slag.

Experiments reported elsewhere in this report were designed to give information concerning the electrochemical effects expected at these boundaries.

These experiments are continuing.

Details of the electrode construction will be given at a later date.
47

g . NBS/APS Electrodes

A set of six electrodes (3 anodes and 3 cathodes) have been constructed.

The set is composed of arc plasma sprayed LaCrO^ (Mg) attached through a cermet to a nickel ion plated copper base. A cap of 85ZrO -3Y 0 -12CeO

(aps) is attached through a grade to the LaCrO^ . The insulator consists

of MgA^O^ arc plasma sprayed over the entire side of each electrode and

ground flat.

Two thermocouples are built into the electrode set; one in a cathode and

one i n an anode. The electrodes are designed for a heat flux of 100 w/cm2 and a surface temperature of 1650 °C. Stress relief slots are cut into

the surface at 20 mm intervals. Fig. 35 (p. 77) shows a schematic diagram of

the electrode. Testing of this electrode has been delayed due to the

extensive renovation of the MIT test bed facility. The proposed test

program has been submitted to Prof. J. Cordero of MIT and the test

should be conducted during the next reporting period.

h. Conclusions

Several conclusions and speculations can be derived from the electrodes

examined for this report and from those (including seed, static and

dynamic slagging conditions) to be reported for the next quarter,

1. NDlten slag (certainly, New Hampshire-type) has a significant

ionic component to its bulk electrical conductivity.

a) This probably includes both anion (oxygen) and cation

(Fe, Ca, K) current carriers which are manifested by,

b) Partitioning of elements particularly at the slag/

electrode interface.

c) Resistive A^O^/SiC^- or SiO -rich deposits develop

at the anode interface regardless of the electrode

material (e.g. Pt or LaCrO^) , and these are accompanied

by enhanced porosity due to evolution of gas, presumably oxygen; network-forming , polymerized [(Al,Si)0^]

n groups appear to be one source of available oxygen

discharged.

d) Mobile network-modifying cations, especially Fe

n+

, K+, and Ca^+ are displaced away from the anode and enhanced

at the cathode.

e) Over and above these phenomena, several related effects

have been observed:
48

1) Pt cathodes react (alloy) with the available iron

while,

2) LaCrO^ and the MIT Fe/Al-oxide (spinel) cathodes

develop Fe-rich deposits at the ceramic/slag interface, the chemistry being controlled by the cathodic

material itself plus thermal and electrochemical

factors

.

For MHD electrodes, the following recommendations can be formulated:

1. Pt appears to be a reasonable anodic material although build-up

of resistive layers could inhibit current transfer and contribute to

accelerated arcing and degradation.

2. Pt at the cathodic side probably should be operated cold or with an appropriate design to prevent degradation via Fe-diffusion.

3. Porous LaCrO -based electrodes are subject to considerable

solution/reaction at both anode and cathode.

a) LaCrO^, nevertheless, develops resistive oxide layers at

the anode.

b) Fe/Cr-oxide (probably spinel) develops adjacent to LaCrO^-

cathodes to preclude deposition of highly conductive

Fe-rich zones,

4. When slag is molten at the cathodic side, Fe-enrichment near

or at the electrode/slag interface could contribute to axial shorting

in an MHD channel

.

a) Pt prevents this enrichment in short-term experiments

by alloying with Fe.

b) LaCrO^ provides Cr-oxide which interacts with the iron

to precipitate spinel.

c) Fe/Al-oxide may aggravate the formation of axially

conductive layers by providing additional Fe to the

cathode/slag interface.

5. Spinels containing Fe-oxide may be desirable anodic materials;

dissolved Fe could diminish the build-up of Al/Si-oxide resistive layers.

Problems (build-up of resistive or conductive layers) associated with

the electrochemical behavior of slag could be diminished by:
49

1. Operating "cold" metal or oxide cathodes at temperatures such

that thin, crystalline and, hopefully, conductive (electronic) deposits

develop at the electrode/slag interface.

2. Develop "hot" ceramic/metal electrodes with thin, passivated

(equilibrium) but conductive layers at the electrode/slag interface; this

entails detailed chemical/electrochemical studies of slags combined with

candidate oxides.

References

1. W. R. Hosier, T. Negas, S. W. Petty, 15th Symposium, Proceedings

on Engineering Aspects of MHD , Philadelphia, PA, May 24-26, 1976

2. Final Report on the Joint U . S . -U . S . S . R. Test of U.S. Electrode

System in the U-02 Facility-Phase I, ERDA 1976.

3. M. T. Simnad, G. Derge, I. George, Trans. AIME 200 , 1386 (1954).

*
50

i

I

\

i

I

Figure 1. SEM micrograph

(A3X) of a Pt anode (A) in

slag (B) ; E = epoxy, C = porous Pt/slag interface.

Figure 2. EDX spectrum for

New Hampshire-type slag.

Elements from left to right

are A1 , Si (long vertical

markers superimposed), K,

Ca, Ti and Fe.
51

Figure 3A. SEM micrograph (860X) of porous Pt/slag interface C, Figure 1.

Bright domains are Pt.

Figure 3B. Magnification (4300X) of porous Pt/slag interface C, Figures 1 and 3A.
52

Figure 3C . EDX spectrum

for "phase" 1, Figure 3B.

Al,. Si dominate with

smaller amounts of Pt,

Rh, Ti (superimposed

vertical markers) and

Fe.

Figure 4A. SEM micrograph

(4300X) of zone C, Figure 1, near the undisturbed Pt

metal.

Figure 4B. EDX spectrum

for "phases" in (1) and (2)

of Figure 4A. Si (superimposed vertical markers)

dominates with some Pt also

present

.
53

Figure 5

Figure 6 SEM micrograph (A3X) of Pt (A) /slag (S) cathode. E = epoxy; areas

1 are "slag" intrusions and/or Pt/"slag" domains.
54

I

Figure 7A. SEM micrograph (4300X) of a typical Pt domain (1) in contact

with "slag" (2)

.

Figure 7B. EDX spectrum for typical Pt in Figures 6, 7A. Pt dominates but

is alloyed with Fe (superimposed vertical markers)

.
55

Figure 8. SEM micrograph (90X) of LaCrO^/slag, 1400°C, without current.

E = epoxy; Zone 1 = LaCrO ; Zones 2, 3 and 4 develop from

slag/LaCrO^ interactions (see text)

Figure 9A. SEM micrograph (9100X) of LaCrO^ (doped with MgO) in zone (1)

,

Figure 8.
56 i

Figure 9B. Upper EDX spectrum, brighter irregularly shaped domains (1)

,

Figure 9A; LaCrO^ (plus a trace of Sc and other rare earths

judging from the diffuse nature of the last line, CrKB)

.

Lower spectrum, darker matrix (2), Figure 9A; Mg (short

vertical marker), La (weaker than in upper spectrum), Cr

(long vertical markers).

Figure 10. SEM micrograph (4500X) of interface area between zones 1 and 2,

Figure 8. Zone 1 is LaCrO^ while zone 2 contains slag components

plus LaCrO^ (see text)

.
57

EDX spectrum for entire area, Figure 11A; Al, Si, Ca, La (short

vertical markers), Cr (long vertical markers), Fe. Darker phase

1, Figure 11A, is enriched in Al/Si and depleted in La/Cr which

are concentrated in phase 2.

Figure 11B.
58

Figure 12A. SEM micrograph (4600X) of zone 4, Figure 8.

Figure 12B. EDX spectrum for entire area in Figure 12A. Very little, if any,

Cr is present as the CrK(3 line is absent (third long vertical

marker). The CrKa line superimposes with other rare earths; A1

(short vertical marker), Si, Ca, La + rare earths, Fe.
59

Figure 13. LaCrO^/slag (no seed)

anode (4-5X) . Numerals refer to

typical areas examined by SEM/EDX;

E = epoxy.
60

Figure 14B

Figure 15.

EDX spectrum for irregularly shaped domains in Figure 14A: A1 Cr (long vertical markers), Fe.

J

SEM micrograph (47X)

S = modified slag; E

3, 4 shown in Figure

of zone A, Figure 13. L = LaCr0

3 anode;

epoxy. Areas 3, 4 are typical of areas

I
61

Figure 16B. EDX spectrum for phase 1, Figure 16A* Si (long vertical) markers

plus traces of La, Cr.
62

Figure 16C. EDX spectrum for areas 2, Figure 16A; Al, Cr (long vertical

markers), Fe.

Figure 16D. EDX spectrum for matrix 3, Figure 16A; Al, Si (long vertical

markers), Ca (trace). La (plus rare earths), Fe (trace).
63

Figure 17A. SEM micrograph (1800X) of areas 3, Figure 13.

Figure 17B. EDX spectrum for Figure 17A; Al, Si, plus traces Cr (long

vertical markers), Fe.
64

Figure 18. SEM micrograph

(4600X, poor focus) of area

5, Figure 13.

Figure 19. LaCrC^/slag cathode

(^4-5X) , no seed. Numerals

refer to typical areas examined

by SEM/EDX. A = portion of

boat containing the slag;

E = epoxy.
65

Figure 20. SEM micrograph (48X)

of area 1, Figure 19. A, B, C zones subdivide this area (see

text and figures below)

Figure 21A. SEM micrograph

(920X) of zone A, Figure 20
66

Figure 21B. EDX spectra for phases in

Figure 21A. Upper spectrum for

groundmass (1); Al, Si, K, Ca (strong)

La (plus rare earths) , Fe (long vertic

markers) . Lower spectrum for crystallites (2); Mg, Al, Cr, Fe.

Figure 21C. EDX spectrum for "stringe

(3), Figure 21A; Al, Si, K, Ca (short

vertical marker) , La (plus rare earths

Fe (strong, long vertical markers).

Figure 22. SEM micrograph (1900X

of zone B, Figure 20 (see text).

Areas 1, 2 are identical with are

1, 2, Figure 21A.
67

Figure 23A. SEM micrograph

(1840X) of zone C, Figure 20.

Figure 23B. EDX spectra for

phases in Figure 23A. Upper

spectrum, phase 1; Si, Ca,

La (plus rare earths) . Lower

spectrum, phase 2, LaCrO-j

Ca, La (plus rare earths), Cr

(long vertical markers).

Figure 23C. EDX spectra for

phases in Figure 23A. Lower

spectrum, phase 3; Al, Si,

K, Ca, La (plus rare earths)

,

Fe (short vertical marker)

Upper spectrum, phase 4;

Si, Ca (trace), La, Cr (strong),

Fe, Zn (long vertical markers).

I
68

Figure 24. SEM micrograph (910X) of area 2, Figure 19, within the "neck" of

the LaCrC>3 cathode. Phases indicated are identical with those

in Figure 23.

M

Figure 25. SEM micrograph (15X) of a portion of the cross-section of MIT

electrode (S4055, tested without current). M = grooved Fe

metal; C = ceramic, Fe/Al-oxide, see text; S = slag; I = insulation; E = epoxy.
69

Figure 26A. SEM micrograph (500X) of slag (S) - ceramic (C) interface of

Figure 25.

Figure 26B . Lower EDX spectrum typical of domains 1, 2, Figure 26A; Mg, A1

(traces), Si (strong), K, Ca, Ti and Fe (superimposed vertical

markers). Upper spectrum is for the main slag groundmass,

Figure 26A; Al, Si, K (strong), Fe (weak).
70

Figure 27. SEM micrograph (5000X) of the uppermost portion of ceramic (C)

,

Figure 26 (see text)

.

Figure 28A. SEM micrograph (2000X) of Fe/Al-oxide ceramic (C) , Figure 25.

Metal substructure and slag are located, respectively, above

and below this figure.
71

Figure 28B. EDX spectrum for

brightest areas, Figure 28A;

Fe (iron oxide) only.

Figure 28C. EDX spectrum of

darker areas, Figure 28A;

A1 (strong) plus Fe (superimposed vertical markers)

Figure 29. SEM micrograph

(200X) of the ceramic (1) - slag (3) interface (2) for

an MIT cathode (see text)
72

Figure 30A. SEM micrograph (450X) of slag, Figure 29, showing domains (A)

within groundmass (B)

.

H.tsaife jiMi .MIL

Figure 30B. Lower EDX spectrum for domains A, Figure 30A; predominately Fe

plus traces of Mg, Al, K. Upper spectrum for groundmass (B)

,

Figure 30A; Al, Si, K (strong), Fe.
73

Figure 31. SEM micrograph (90X) of MIT cathode. M = Fe metal fin; 1 = slag

with inclusions (see Figures 29, 30A) ; 2 = slag/ceramic interface; 3 = Fe/Al-oxide ceramic; 4 = MgA^O^ insulation; 5 = Carich domains; 6 = Fe-rich zones and intrusions; 7 = zone of

ceramic within slag.

' i
74

Figure 32A. SEM micrograph (2000X) of areas (6), Figure 31.

Figure 32B. Lower EDX spectrum, phase (1), Figure 32A; traces of Al, Si,

Cl (?, short vertical marker ), Fe dominant. Upper spectrum,

phase (2), groundmass, Figure 32A; Al, Si, Cl (?), K, Fe

plus Zn (long vertical markers)

.
75

Figure 33A. SEM micrograph (900X) of slag/ceramic interface (2), Figures

29 and 31.

Figure 33B. EDX map, FeKa, for Figure 33A.
76

Figure 34A. SEM micrograph (2000X) of ceramic adjacent to interface (2),

Figure 29. Microscope difficult to focus here.

Figure 34B. Lower EDX spectrum, bright domains, Figure 34A; Al, trace K, Fe

(strong). Upper spectrum, darker intergrowths, Figure 34A; Al

(strong), (Si-trace ?), K (vertical markers), Fe.

78

3 Proof-Test I for U-02 Experiment Phase III (T. Negas, L. P. Cook

and W. R. Hosier)

A. Electrode Systems and Component Materials

The first proof test of candidate electrode systems for Phase III of

the U-02 experiment series was carried out at Westinghouse Electric

Systems Test Facility (WESTF) on October 10 and 11 , 1977. The experiment

was designed primarily to verify several electrode system designs incorporating several materials and attachment mechanisms. The heat fluxes

and temperature gradients through the electrode systems were similar to

those expected in the U-02 test facility.

Fig. 1 (p. 84) is a schematic diagram showing the nine electrode pairs

included in this test with one upstream guard and two downstream guards

composed of LaCrO^. These guard electrodes were supplied by Westinghouse.

The first three upstream pairs were composed of magnesium aluminate

ferrous ferrite attached to a "flex bed" lead-out. This attachment and

flex bed lead-out is propriatary and was supplied by General Electric

Company. The next three anode systems (downstream from the GE electrode

pairs) were composed of arc plasma sprayed magnesium aluminate ferrous

ferrite (MAFF-31) spray-attached to a Brunsbond pad which was, in turn,

brazed to the copper base. These were prepared by Technetics Division

of the Brunswick Company under the guidance of Westinghouse. The three

corresponding cathode pairs were again composed of arc plasma sprayed

MAFF-31 attached through a nickel-MAFF-31 cermet to a Brunsbond pad

which was brazed to the copper base. These systems were prepared by

Arc Plasma Spray Materials, Inc. under the guidance of Westinghouse.

The last three downstream electrode system pairs were composed of

two different materials and attachment mechanisms. One design included

a nichrome mesh base while the other used a Brunsbond pad base. In each case the lower portion (cold side) of the mesh contained no ceramic but was attached to copper via a gold/nickel bond. Further toward the hot

side the ceramic was integral with the mesh and finally pure ceramic

material served on the plasma side. 9.7 Y

2 °3 + 16*3 CeO + ^4 H^°2 ceramic was used with the nichrome mesh while 18 CeO^ + 82 HfO^ ceramic

was used with the Brunsbond pad (mesh composed of Hoskins alloy 875)

.

Electrodes 108, 109, 208, 210 were composed of Y , Ce , Hf ceramics while

110, 209 were composed of the Ce , Hf ceramics only.
79

B. Post Test Handlincr of the Electrode Modules

Each module (anode and cathode) was cut through the center in a plane parallel to the plasma flow and perpendicular to the plasma-electrode

surface. Half of each module was given to Battelle NW and half was retained

by MBS for analysis. Subsequent sectioning was done as shown in Figure 2.

Disposition of the sectioned electrodes is also shown in the figure. All

cutting was done after the systems had been potted in an epoxy resin, using

a diamond saw of appropriate diamond concentration with water as a coolant.

Mot shown in Figure 2 is the sectioning of the individual slices after

polishing for examination using the scanning electron microscope. Figures

3 and 4 are representative anode and cathode wall cross sections, respectively.

These slices illustrate the arrangement of materials for purposes of

orientation. The photographs were taken of the surface as cut.

C . Analytical Methods

This proof-test was concerned with electrode design, performance and

endurance under conditions closely approximating those for U-02 . Implicit

were the availability and development of candidate materials which were selected and "woven" by various contractors into a "best-effort" con- figuration. Thus, the test did not strictly involve development and

optimization of materials. For this reason, and as specific pre-test

electrodes were not available for comparison purposes , our characterization efforts are limited to gross features rather than being detailed

and comprehensive. Furthermore, the LaCrO^-based "guard" electrodes (one

upstream and two downstream), although in relatively good condition, were

not scrutinized. Proof-test II for U-02, Phase III will emcompass these

materials whereupon they will be subject to analysis.

Electrodes were investigated using SEM/EDX and x-ray diffraction

methods. Low magnification (<500X) optical micrographs can be effectively

used for orientation purposes and for detecting gross features such as bond detachments. For this set of electrodes, most important chemical/

microstructural features could be observed only at magnification greater

than ^500X

Low magnification orientation-type photographs provided,

herein, are not intended to convey scientific evidence.
80

D. General Electric Electrodes

1. The overall integrity of both the anode and cathode assemblies

appears to have been fairly well preserved in spite of extensive corrosion.

An estimated 35 - 50% corrosion has occurred at the anode and 20 - 35%

at the cathode, using LaCrO^ electrodes 101, 201 for comparison and

assuming all electrodes were at the same level at start-up. Penetration

of seed has caused severe weakening of the interior of the ceramic at

the cathode; seed has penetrated the interior of the anode only along

cracks, some of which appear to have healed themselves. Cracking has

occurred in all electrodes near the base along the tips of the (Ni,Cu)

lead-outs

.

2. Recrystallization of a zone of spinel a few mm in from the surface

occurs to a certain extent in all electrodes and appears to be associated

with a significant K content -- possibly this represents the K

2C°3

condensation isotherm. Such recrystallization is locally much more

intense in the anode, where evidences of a K, Fe-rich intergranular

phase were noted.

3. The insulators were also subject to alteration by (K, Fe) but • primarily along the deeper parts where cracking has occurred. The anode

insulators underwent corrosion by melting in the upper parts and were bridged by a (Fe, Ni) -containing phase (a spinel?).

4. Untested specimens were not available for comparison, and certain

features seem best related to the initial fabrication process: narrow

zones of Ca and Mg-rich oxides at the junction between electrode and

insulator; interspersed (La, Zr) oxides within the electrode spinel in a regular zone several mm from the surface; Si in the electrode spinel.

5. Melting is difficult to estimate and interpret because of the

complexity of the interaction with upstream components; Zr has obviously

reacted with the spinel, possibly to produce a melt. The fact that

shut-down was gradual further complicates the interpretation of microstructure. It is estimated that melting has occurred at one time or another

in a surface zone of variable thickness ^1-5 mm. Melting appears to

have been much more extensive at the anode. As would be expected,

spinel immediately adjacent to the plasma shows significant Fe-depletion.
81

E. Argonne Electrodes

1. Pre-test, virgin electrodes were not available for purposes of

comparison (especially for the interpretation of thermochemical/electrochemical phenomena)

2. The microstructure, microchemistry and macro-scale phase distribution

particularly for the Hf0

2 ~Y O^-CeO^ materials varies considerably. This

was done either intentionally or unintentionally during fabrication.

Alumina is a significant contaminant for all of these materials. It,

generally, resides within a groundmass containing other elements of the

ceramic bulk composition. Within this matrix is coarser-grained ceramic.

This contaminant may compromise the very refractory nature of Hf0

2 ~based

materials, contribute to a lowering of the electrical conductivity and

provide reaction paths for seed. "Bloating" of these materials during

electrical measurements above 1600°C (see above) could, in part, reflect

formation of a fluid phase initiated by the presence of A^O^. Original

ceramic powders may include this contaminant or it might have been introduced during a "ball-milling" procedure prior to final fabrication.

3. Hf°

2 ~ based materials retain substantial anionic conductivity.

Electrochemical reduction of the cathodic ceramic near and above the

mesh was demonstrated. This does not appear to cause severe mechanical

degradation in the short duration test. The melting temperature of the

metal leadout and pertinent oxidation products, are particularly important

for anode metal meshes combined with anion conductors. Oxidation of the

metal is accelerated due to directed oxygen-electron transfer. Anodic

oxidation/dissolution of metal could lead to formation of a resistive

phase assemblage around’ the wires and loss of thermal/electrical contact.

These ultimately would result in destruction of the anode. Evidence for

these events, including melting, is suggested. Platinum, by far, has

been demonstrated to be superior for use with anionic, anode ceramics

wherein leadouts are required at relatively high temperatures.

4. The refractory nature of HfO -based ceramics also is compromised by

using MgAl 0 -based insulation. Melting can occur at temperatures near roughly 17D0^C (but much lower with Fe-contamination) . If Hf0

2 ~based

electrodes are to be operated above 1700°C (surface) , a margin of safety

would be provided by using MgO insulation.
82

F. Brunswick Anodes and Arc Plasma Sprayed Materials, Inc. Cathodes

1. Pre-test electrodes were not available for purposes of comparing

post-test features.

2. Metal-ceramic bonds basically retained their integrity.

3. This spinel is sensitive to contamination (internal or external

in origin). At elevated temperature, certainly above 1600°C, this

Fe-containing material may be subjected to plastic flow and, combined

with interactions with the insulation will bridge adjacent electrodes.

The material is sensitive to oxidation, alteration by seed and electrochemical activity particularly at the cathodic side. The G.E. spinel

is similar in many respects. However, of electrodes fashioned from

plasma sprayed spinel, this particular set out-performed all others

tested in other facilities utilizing clean fuel . This reflects the

excellent design considerations provided by Westinghouse

.

4. Microstructure and microchemistry at the anodic side reveal con- siderable evidence for melting/recrystallization.
83

G. CONCLUSIONS

An extensive report with 74 figures has been submitted to Westinghouse

Corp. but will not be reproduced herein. A summary, however, of pertinent

conclusions is provided below.

Bonding among various components associated with leading-out ceramics

to a metallic substructure survived this test remarkably well. Designs

were well-conceived, utilizing available thermal and electrical conductivity and thermal expansion data effectively. Only the downstream

electrodes (8 through 10) showed evidence for significant detachment

between metallic components. All ceramics revealed some fissuring above

the metallic leadouts.

During the test, heat fluxes ran consistently higher for the anode.

Post-test inspection of the anode wall surface suggested deep erosion

particularly of the last six materials including the terminating two,

LaCrO^-based guard electrodes. Although morphologies and microchemistry,

consistent with melting/recrystallization are obvious for both electrode

walls, "melting" phenomena are more extensive at the anode. Furthermore,

ZrO :Ca as a contaminant from upstream is much more evident at the anode

wall. Available evidence suggests that anodes 208, 209, and 210 were

subjected to extensive surface melting, perhaps accelerated by interaction

of the HfO^-based materials with Fe-containing upstream electrodes.

Conceivably, zirconia from upstream more easily could be entrapped,

digested and retained within anode areas containing an extensive, preexisting fluid phase. Hence, the apparent partitioning between electrode

walls. Other explanations for the phenomena observed at the anode must

reside in unusual plasma conditions which are beyond the realm of these

analysts

This brings up a question involving the general philosophy of

materials testing. Within this report, it is quite clear that in a test

involving a multitude of differing materials, the probability of degradation due to interactions among adjacent (but unlike) materials and

upstream constituents is maximized. Ideally, similar (and basically

compatible) materials should be incorporated within one test section.

Apparently, Proof-test II for U-02 , experiment (Phase III) will have

such a configuration.
84

CATHODE ANODE MATERIAL

201 W LaCr0

3 202

203 GE MAFF

204

205

TECHNETICS

MAFF- 31

206

207

208

ARGONNE

HAFNIA-CERIA-YTTRIA

209

210

211

W LaCr0

3

212

Figure 1 SCHEMATIC DIAGRAM SHOWING ELECTRODE PAIRS
Cathode -

Lower

Half

to

BNW

Lab.-

Upper

Half

to

NBS

85

O 0) <D O O U OH ’H *H TOH OH O <H •HO HW T) O

in T3 -H

4J

p in

d -h c o o o o O O -H <4-1 rH

fO scuoa&

0) T305 <4-1 rH

flj SCwo2a2 cn

OQ § 0 0

+J

Figure

2.

SCHEMATIC

DIAGRAM

SHOWING

NBS

SECTIONING

OF

ELECTRODES

FROM

PHASE

III-U-02 -

PROOF

TEST 1
86

Figure 3 . Representative anode wall cross sections.

88

TASK L. ASSESSMENT OF STEAM PLANT COMPONENTS (J. R. Cuthill and J. Smit)

This is the 10th quarterly progress report since the initiation of Task L.

The following two items will be taken up in the quarterly progress reoort:

[1] Construction of hot corrosion test rig for the evaluation of candidate

materials for the steam super heater tubing, and [2] Summary and update of

requirement, test results, and material recommendations for steam super

heater tubing.

Laboratory Hot Corrosion Screening Program

At the start of this quarter, as a part of Task L, a laboratory hot

corrosion screening program was undertaken. The stated goals of this

program are to develop a test for and to test candidate materials for

downstream MHD components under conditions closely simulating anticipated service conditions.

Under Task L much work has been done thus far on evaluating the suitability of various metals and alloys to function as steam boiler tube

materials in the MHD downstream super heaters. These evaluations are made on the basis of data available in the materials literature, e.g.,

research papers, handbooks and producer bulletins. It is from these

studies that specimens are to be selected for further evaluation in

the laboratory screening program. The laboratory test conditions are

to be as close as possible to the calculated MHD downstream state point

conditions as determined by Gilbert Associates and STD Research Corp.

To perform these tests and as a first step in the execution of the

screening program the design and construction of a hot corrosion burner

rig was undertaken.

The burner rig, which is about 80% complete, consists of a verticle

chamber housing a combustion tube, a pressurized seed or seed-slag

injector, a four jet propane-oxygen burner, a specimen support and an exhaust vent. An instrument panel is mounted externally on the

chamber. The chamber, constructed of steel framing and sheet aluminum,

in 2.44 M high and 0.61 M square. It contains, mounted vertically

along and about 1.5 M up the center axis, the 0.4 M long combustion

tube which is directly under the pressurized injector. .The four jet

burner is positioned over the combustion tube and under the injector.

The specimen holder is located immediately beneath the combustion tube.

Flow meters for propane and oxygen are located on the instrument panel

along side the regulator for pressurizing the injector. The exhaust

vent is located in the lower third of the chamber. In operation,

after gas stream ignition, the solid seed will leave the injector,

follow a mid line down the four jet burner (the nozzles are pointed

down the combustion tube) , traverse the combustion tube in the hot

gas stream and exit with the gas stream onto the specimen being evaluated.

In the next quarter the burner rig will be completed, debugged and

characterized. Among other things gas temperature profiles for various

propane and oxygen flow rates will be obtained as will combustion tube

exit temperatures and gas velocities as a function injector pressure.

With this completed, the screening program proper will begin.
89

Summary and Update of Requirements, Test Results, and Material

Recommendation for Steam Superheater Tubing;

This is an appropriate time to summarize and update the requirements,

test results and material recommendations for the steam superheater

tubing because (1) there were new and encouraging test data revealed at

the 1st International Conference on Materials for Coal Conversion and

Utilization this past October (ref. 1) , and (2) the imminent availability

of a test rig to test the selected alloysunder simulated steam heatexchanger tube conditions.

The steam superheater unit in the Baseline Plant Design (Ref. 2) has a gross cross section of 45 x 30 ft. and is 32 ft. long. The 80 looped

heat exchanger tubes are 3.18" O.D. and hang vertical on 6.5 inches

center to center making the net cross section about two-thirds of the

gross cross section for the flow of flue gas through the unit. This

configuration of the superheater unit is shown schematically in Figure 1 together with the reported temperature, flue gas velocity and calculated

quantity of K^SO^/hr-in of gross cross section entering the unit. The

outside surface of the heat exchanger tubing is estimated to be about

650 °C, which is even below the melting point of the K SO^ so it is

expected to condense out as a solid*on the leading surface of the tubing.

An attempt was made, in a recent Task L quarterly progress report, (for

the period ending March, 1977) , at a relative ranking of materials from

best to worst on the basis absolute life expectancy based on fragmented

data in the literature. At the top of the list was the INCOCLAD 671/800H.

This INCO 671 (approx. 48 Ni, approx. 52 Cr) had not only been considered

by the investigator but by others to be among the best if not the best

alloy to resist the hot corrosion attack that would be experienced by

the steam superheater tubes. It was of particular interest therefore

to note the report from INCO, at the recent Materials Conf. for, Coal

Conversion at NBS , (Ref. 1) , that in laboratory tests simulating such

conditions INCO alloy 814 , which has the nominal composition,

32Cr , 15.6 Fe, 3.5 Al, .35 Ti , .6 Mn, .2 Si, bal Ni

showed significantly less attack than INCO 671.

INCO considers this alloy to be a semi-commercial alloy. It is in

limited production but if tubing can be obtained, it will be included

in our hot corrosion test program.

References

1. First International Conf. on Materials for Coal Conversion and

Utilization, (Abstract) , Oct. 11-13, 1977, Gaithersburg, MD, Conf-771025

UC-90h, NTIS , Dept, of Commerce, 5285 Port Royal Rd. , Springfield, VA 22161.

2. Baseline Plant System Design Description, issued June 30, 1977, a report

to ERDA by Gilbert/Commonwealth Co., and STD Research Corp.

^Providing, of course, that only pure K2 SO^ condenses; solutions between

K^SO^, K

2 C°3 and K0H wou l d freeze at a considerably lower temperature.
2250 °F

(1505 °K)

Figure 1. Schematic of Steam Superheater Unit (Data from Baseline

Plant Design, Ref. 2 )

.
91

TASK M. INFORMATION AND DATA ON MATERIALS FOR MHD-POWER SYSTEMS

1. Data Center for MHD Materials Properties and Performance (H. M. Ondik,

A. Perloff, and J. R. Cuthill)

The new MHD Materials Properties and Performance Data Center has the

responsibility for designing and maintaining a data bank containing properties of materials for use in MHD applications. The program of the Center

should provide for more rapid transfer of evaluated laboratory data and technological information, obtained by DOE-supported MHD materials research,

to designers of systems, materials researchers, and operating plants.

The initial sources of information to be entered in the data bank are DOE contractors' reports of test data. Data from other sources will be

added as appropriate. The data in the contractors’ reports will be

evaluated and summarized so as to provide the users with as much hard

data in as brief a form as is possible. The information in the MHD

materials properties data bank can be made available to DOE and the MHD

community in a variety of ways — reports, handbooks, and by direct

query of the Data Center. Designing suitable means of distributing the

information is listed among the duties of the Center.

During this first quarter, the effort of the Center has been con- cerned with reviewing some of the contractors' reports in order to plan

the Center library, become familiar with the types of information which

must be handled, and define the requirements for a suitable computer

system. A computer data management system is needed which is open-ended

in order to provide for the addition of new types of information as well

as possible multiple entries from different sources for similar information. The system must have a matrix-type storage to provide for combination searches on varying sets of items so as to be responsive to users'

needs and answer a variety of questions. There are package programs

available with the necessary capabilities offered by various commercial

groups . These groups are highly competitive and care must be taken to

choose the best system. The system must be capable of handling numerical

and textual information. Possibilities for storing and retrieving

information in graphical form have been investigated and the flexibility

available in terms of software and equipment is very encouraging.

Enough of the data are in graphical form to make this feature desirable, even necessary.

Plans: Means for distributing up-to-date MHD materials information to

the MHD community in the form of brief memos or bulletins are to be

investigated and a sample item submitted for DOE approval. The Center

library is to be established and review of reports to begin.
92

Unified Numbering System

Implementation of the U.N.S. designations this quarter in the designation

of MHD candidate alloys is particularly timely because the second edition of

the Unified Numbering System manual (Ref. 1) was published this quarter. This

second edition includes for the first time most of the alloys that are of

concern to the MHD program. In the two years since the first edition was published the numbering system has been receiving wide acceptance. It now appears likely to really become the universal system of alloy designation

that it was intended to be, even supplanting completely some other specification systems. The Unified Numbering System activity is a joint effort by

ASTM, SAE, various metal producer trade associations, and government agencies

involved in standards activities including the National Bureau of Standards.

The Unified "limbering System was introduced in some of the early Task L

quarterly reports but enough of the alloys had not been entered into the

system at that time to pursue its use in the MHD application. Now all of the

alloys in the list of Selected Wrought Alloys from the quarterly report for

September 30, 1977 have been assigned UNS designation. See Table 1, Chemical

Compositions of Selected Wrought Alloys, reproduced from the previous

quarterly progress report with the UNS alloy designations added.

It will be noted that each alloy designation consists of a capital letter

followed by five digits. The primary series of UNS designations are given in

Table 2 , and the secondary division of those series that cover the alloys and

grades of more than one primary metal and which are of' interest in the MHD

program are given in Table 3. For a complete description of the system see Reference 1.

Ref. 1. Unified Numbering System for Metals and Alloys, 2nd ed. SAE, HS1086a,

ASTM DS-56A, Soc. Automotive Engrs., Inc. 400 Commonwealth Drive,

Warrendale, PA 15096.
Table

3,

chemical

Compositions

of

Selected

Wrought

Alloys

c#

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r- r^

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to

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rH<©

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w

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t?

O 'D ^ Q r-4 ^ rr n no n o o o o o o O vO vO HH'sT

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M <N 4*1 ^ I I 3

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cn i o © <n

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(A I I I I I &u in in in 0 '—4 • • • rH

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in

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rHr- oo o OH '—I lO oo r- r- oo r- r- oo o o

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fNm 0 0 o o o u c c o o

r-l

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1-4

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cn cn © © © d

CL & O © © ©

*H

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©

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m in

fN « © © I 8 O ©* «N

*H fN 8 8 © ® © rH I I I I 8 I 8 I I ! I 8 8 1 8 8 33» © rH ^ CA CN CN CN

Q3 *H rH rH 1 8 8 ©

CN fN CN in <3N

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r»

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crC § 3

ffl S)

U \ a o > m ^

^4 ^4

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O 04 o 0 CN

©

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©

®

©©®

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© rH S rH £

D> 3 >i >• > 3

**H r 2 o 2 ^

iA ifl ^ ©

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>- >* n 3

3S

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LO CO O CO O rH o o m m & & ® a* 8 I I I I 8 I 8 I I I I O O X *3 ^ — > X 0 ®

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Alloy

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products

are

not

given a

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designation

+Foreign

alloys

are

not

included
94

Table 2» Primary Series of UNS Designations

UNS Series Metal

Non-ferrous metals and alloys

A00001 - A99999 A1 and A1 alloys

C00001 - C99999 Cu and Cu alloys

E00001 - E99999 Rare earth and R.E.-like metals

' and alloys

L00001 - L99999 Low melting metals and alloys

M00001 - M99999 Misc. metals and their alloys

N00001 - N99999 Ni and Ni alloys

P00001 - P99999 Precious metals and alloys

R00001 - R99999 Reactive and refractory metals

and alloys

Z00001 - Z99999 Zinc and zinc alloys

Ferrous metals and alloys

D00001 - D99999 Spec. mech. property steels

F00001 - F99999 Cast irons

G00001 - G99999 AISI & SAE low alloy steel

H00001 - H99999 AISI H-steels

J00001 - J99999 Cast steels

K00001 - K99999 Misc. steels

S00001 - S99999 Stainless steels

T00001 - T99999 Tool steels
95

Table 3. Secondary Division of Reactive and Refractory Metal Series

R00001 - R99999 Reactive and

refractory metals and alloys

Metals

R01001 - R01999 Boron

R02001 - R02999 Hafnium

R03001 - R03999 Molybdenium

R04001 - R04999 Rhenium

R05001 - R05999 Tantalum

R06001 - R06999 Thorium

R07001 - R07999 Tungsten

R08001 - R08999 Vanadium

R10001 - R19999 Beryllium

R20001 - R29999 Chromium

R30001 - R39999 Cobalt

R40001 - R49999 Niobium

R50001 - R59999 Titanium

R60001 - R69999 Zirconium
.

.
hb^-l I 4 A (REV. 7-73) )

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

FE-3800-13

2. Gov’t Accession

No.

4. TITLE AND SUBTITLE

"MATERIALS RESEARCH FOR THE CLEAN UTILIZATION OF COAL"

Quarterly Progress Report, July 1 - September 30, 1977

3. Recipient’s Accession No.

5. Publication Date October 1977

6. Performing Organization Cod

jOBSlfc 11-\l

7. AUTHOR(S)

Samuel J. Schneider

8. Performing Organ. Report Nc

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No. E-C49-D-3800

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Department of Energy

20 Massachusetts Avenue, N.W.

Washington, D. C. 20545

13. Type of Report & Period

Covered Quarterly

Progress Report

14. Sponsoring Agency Code

Dist. Cat. UC-9QC

15. SUPPLEMENTARY NOTES

16. A.BSTRACT (A 200-word or less factual summary of moht significant information. If document includes a significant

bibliography or literature survey, mention it here.)

This progress report covers work on metal corrosion, metal erosion, ceramic

deformation, fracture, erosion, and chemical degradation as related to coal,

gasification systems. This report also covers the failure avoidance program

for DOE coal conversion pilot plants.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Ceramic corrosion, ceramic erosion, ceramic fracture, chemica

degradation, coal gasification material, failure avoidance, metal corrosion, __ metal erosion, and vaporization processes.

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