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EA-77-A-01 -6010 - 12

Distribution Category UC 90g

DEVELOPMENT, TESTING AND EVALUATION OF

MHD-MATERIALS

Quarterly Report

for the period April - June 1978

W. R. Hosier, T. Negas and S. J. Schneider

Ceramics, Glass and Solid State Science Division

Center for Materials Science

National Bureau of Standards

Washington, D.C. 20234

Date Published: June 30, 1978

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.”

ABSTRACT

1

NBS involvement in MHD related program review and coordination has continued.

Initial results on a coal slag intermediate in composition to Illinois #6

and Montana Rosebud slags show that the log viscosity is raised about 20% by a 15% concentration of K^O seed.

Slag exposed to pressurized steam undergoes a change that increases the

log viscosity by about 5%.

The pre-test electrical conductivity of the U-02 Phase III materials has

been completed. Electrical conductivity of a possible lead out material for

a slagging generator is given.

The K pressure over KFeO^-K^O • TFe^O^" is about an order of magnitude

greater than the K pressure over KA10

2 -K2

<D • 7A1 2 C> 3 showing that K

? 0 interacts

more strongly with AI 2 O3 than with Fe 2

<03 . Preliminary measurements show

preferential loss of Fe from a MAFF-31 composition.

Analysis of diffusion between MAFF 31 ( 3 MgAl 204 *Fe 304 ) electrode

material and MgAl^O^ insulator material showed that the diffusion coefficients

at 1600 °C for Mg were at least twice as large as those for Al, although Fe

diffusion was still the dominant metal diffusion process.

The electrochemical effects in a simulated slag flow using platinum

electrodes indicate substantial attack on the cathode electrode and minimal

effect on the iron depleted layer on the anode.

A thermodynamic analysis of the seed/slag system using the most recently

available data has been completed.

Approximately 50 materials from external sources have been analyzed

structurally by X-ray diffraction methods.

NBS/AVCO slagging test 2 electrodes will be constructed by the end of

June and testing should proceed when accommodations in the AVCO channel can

be obtained.

The electrode modules from the Phase III U-02 test carried out in Moscow

May 13-19, 1978 have been sectioned. Post test analysis will be made at

Battelle NW Laboratories and NBS.

_ The post test analysis of the electrodes tested at MIT Feb. 28, 1978

(MIT 223-NBS F) has been completed. An analysis is given concerning the K

penetration into the electrode body as well as the effectiveness of the

various electrode grades and bond lines with respect to the channel conditions.

A brief summary of the pre and post test analysis of the USSR materials

tested at UTSI in October 1976 and February 1977 is given.

Stainless steel 304 was tested in the hot corrosion apparatus for 4 hours

at 550 °C using K2 SO4

. Extensive corrosion was not detected although discontinuous and localized oxidaticn/sulfidaticn was ncted.
2

Physical property data and prices are given on two new alloys that mav

prove to be more cost effective than 310 SS where sulfidizing is a problem

such as the seed recovery system. Also property data and prices are given

on new composite materials for extreme abrasion and wear.
3

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

feas ible

.

6. Phase equilibria and diffusion rates of seed in slag and 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 samples

and structure that have been subjected to real or simulated MHD conditions. The

'activities are grouped under six tasks:

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

and review of the various MHD-Materials Development Programs)

.

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)

.

M. Information and Data on Materials for MHD Power Systems.
4

SUMMARY OF ACHIEVEMENTS (April-June 1978)

(Completed or Continuing Milestones-See Work Plan FY 1978)

TASK G.

1. Plan, prepare, guide and conduct negotiations of US-USSR Working Group

on Testing of MHD Materials, Phase III (completed May 30, 1973).

2. Participate in reviews, planning sessions, preparation of working

documents in the area of MHD development (continuing)

.

TASK I.

6. Measure the viscosities of coal slags made synthetically or furnished

by other MHD contractors (continuing)

.

9. Measure the electrical conductivity of established as well as promising

(new) electrode materials (continuing)

.

11. Measure K pressure over selected K^O-FeO compounds to ascertain

stability of Fe^O^ containing electrode materials (completed May 30, 1978).

14. Determine FeO losses over several spinel compositions which are promising

electrode materials (continuing)

.

17. Determine thermochemistry in a number of critical multicomponent systems

to assess slag/seed interaction, provide potassium activity data (in conjunction

with vapor pressure measurements) and to define reactions leading to corrosion

of construction materials.

a. Determine invariant solid-liquid reactions and potassium activities in

the SiO^/Al^O^ - rich portion of the CaO-K^O-Al^O^-SiO^ system.

b. Same as (a) but the MgO-K^O-Al^O^-SiO^ system.

c. Perform exploratory studies in the K

2 0-rich part of the K0-A1 0 -SiO system.

d. Investigate the effect of divalent iron on solid-liquid reactions and on potassium activity in the FeO -K Q-Al 0 -SiO system.

X 4 X J

e. Determine melting relations for the CaSiO^-KAlSi^Q -CaAl„Si

2 Og system

(pertinent to western coal ashes) . (continuing)

TASK J.

18. Determine chemical reaction processes of construction material (e.g.,

LaCrO^ zirconia-based, etc.) with seed/slag (continuing).

22. Conduct diffusion studies in spinel electrode (MAFF 31) spinel insulator

(MgAl^O^) sandwiches without electric field to determine the degree of degradation

of both materials and diffusion constant for Fe into MgAl O . (completed March 1978

2 4
4a

TASK K.

26. Coordinate testing and pre- and post- test analysis of specimens

exposed to simulated or real MHD conditions in a variety of test rigs, arc heaters, small- and large-scale MHD-generators , etc. Stimulate,

expedite and organize materials procurement, electrode design and preparation,

test scheduling, materials evaluation, etc. Encourage and promote

collaborative efforts aimed at gathering maximum information on MHD-materials

and components in the minimum amount of time. Several tests are anticipated:

U-02 Phase III proof test 1, 2 and 3. (completed June 1978)

27. Use a variety of chemical and physical techniques to assess the stability

(or degradation) of materials and structures tested for specific time periods

of real or simulated MHD-facilities . Pre- and post-characterization of tested

samples from experiments, performed in the MIT test rig, AVCO and U-02 generators,

etc. (Continuing)

28. A system of efficient communication between MHD-workers at various

institutions concerning MHD-materials and testing will be extended and

formalized. This includes also the establishment of a data base management

system which will be available to designers, facility operators and materials

experts in the field of MHD power development (continuing)

TASK L.

29. Assess potentially promising materials for other critical components in

addition to the steam heat exchanger tubes, such as (1) valve surfaces subject

to a combination of abrasion and hot corrosion, and (2) seed recovery system

component subject to chemical attack. Service requirement will be based on

the "Baseline Plant Design Description", in terms of the chemical, physical

and mechanical property requirement involved. Updates will be taken into

account where feasible, (continuing)

30. Update assessment of materials for the steam heat exchanger tubes to

include new promising alloys (metal alloys that are just entering the

commercial market) . (Continuing)

31. Design and construct apparatus to provide screening of metals for

corrosion-resistance to potassium and/or slag attack for MHD boiler tube

applications. (continuing).
5

TALKS AND PUBLICATIONS

The U-25B Facility for Studies in Strong Magnetohydrodynamic Interaction,

V. A. Kirillin, A. E. Scheindlin, A. V. Karpvthin, V. I. Maksimenko,

S. A. Pashtov, D. S. Pinkhasik, N. P. Privalov, V. D. Semenov, V. S. Sidorov,

Yu. D. Sokirko, Yu. N. Sokolov, E. M. Shelkov, Institute of High Temperatures,

USSR Academy of Sciences; R. V. Shanklin, A. L. Liccardi, G. Rudins

,

W. D. Jackson, E. Levi, U. S. Department of Energy; M. Petrick, R. Neimann,

B. Wang, R. P. Smith, Argonne National Laboratory; S. Schneider, National

Bureau of Standards; J. Louis, D. Teare, Massachusetts Institute of Technology;

17th Symposium on Engineering Aspects of MHD , Stanford University, Stanford, CA,

March 27-29, 197S.

Invariant Melting in the System CaO-^O-Al-jOg-SiC^ , L. P. Cook, American

Ceramic Society, Annual Meeting, May 8, 1978, Detroit, Michigan.

Phase Equilibria and Crystal Chemistry Related to Seed-Slag Reactions

under MHD Conditions. The System K^O-MgO-Al^O^-Fe^O^-SiO^ , R. S. Roth,

H. S. Parker, T. Negas, et.al, American Ceramic Society Annual Meeting,

May 8, 1978, Detroit, Michigan.
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. Fluidyne Corporation program review

April 25, 1978 (in Minneapolis)

2. Fluidyne consultation (slag simulation and behavior)

(at NBS, May 23 & 24)

3. U.S.-U.S.S.R. , U-02, Phase III post-test activities, NBS

hosted three U.S.S.R. representatives May 25 - June 1 to

participate in general discussion, report preparation

and module characterization

4. U.S. Policy Group - U.S.-U.S.S.R. Cooperative Program on MHD

The conclusion 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
/

Task I. OPERATIONAL DESIGN PROPERTIES

1. Viscosity of Coal Slags (W. Capps and D. A. Kauffman)

1 . 1 Introduction

Previous reports have contained extensive viscosity data as functions

of temperature and slag composition. Most of this work has been on systemati

variations to a synthetic "base" slag. This base composition is a compromise

between the high-iron Illinois #6 slags and the high-calcium Montana Rosebud

slags . The first two figures in the quarterly report for the period ending

in December 1977, gave a summary of the effects of varying the constituent

oxides one at a time at 1600 and at 1300 C. These slags contained no K^G

to represent seed material. As an extension of this whole series K^O

is being added to this same base slag to determine the effect of seeding

material on the viscosity.

These synthetic or artificial slags under study are somewhat simplified

compared with natural slags. The unseeded slags contain five oxides, the

seeded slags contain six oxides, whereas the usual coal ash analyses report

about nine oxides. The minor constituents, such as K^O, Na^O, TiC^ / P 2^5'

and SO were omitted from the "base" and other compositions in the series.

The only exception is the current "seeded" series being reported on here

in which K^O is the sixth oxide.

Although it has been of interest no previous attempt has been made to

determine the effect of water vapor on coal slag viscosity. This is now

being explored.

1.2 Slag Preparation and Measurement

a. Seeded Series:

During this quarter three out of a series of six slags were prepared

by adding K^O to the base composition (K-884) in 5% increments. The compositions are shown in Table 1 and the viscosity vs. temperature relationships

are shown in Fig. 1.

Some previous data on seeded slags were reported in the last three

quarterly reports for 1975, but the base to which the K 0 was added (K-488)

was different from the base presently under study (K-884)

.

Because these two base slags are quite different from each other

the quantitative effects of the K^O might be different. It is desirable

to compare the effects of a given amount of seed in different bases. From

the previous work one would conclude that first additions of seed will

raise the viscosity which will reach a maximum at about 15 weight % Ko 0
8

and further additions will cause a lowering. At about 30% Ko 0 the viscosity

will return to the unseeded level. Figure 1 bears-out this expectation for

the first additions of K^O. There is an increase in viscosity with increasing K^O. The next report will have the completed series for comparison.

b. In the atmosphere of a real MHD channel water vapor will be present

and this is expected to affect the flow properties of molten slags. As was

promised in the early stages of this viscosity program, an effort is being

made to measure and understand the influence that water might exert. A

steam viscometer is under development that will permit molten slags to be

exposed to steam under pressure. This is expected to cause water to

penetrate the slags and change the viscosity.

In the report for the period ending in March 1978, page 8, the

viscosity of K-1090, a synthetic slag, measured in air at ambient pressure

was shown. This slag was then placed in the steam viscometer, viscosity

measurements were made under pressure and then rerun in the ambient pressure

viscometer. Figure 2 shows both the pre-steam data and the post-steam data,

the latter collected to see if there is a residual effect of the steam

treatment. The viscosity increased slightly as a result of the "pressurecooker" treatment. The high-pressure, high- temperature data is not presented

here because the data is tentative. The temperature measuring system in

the steam viscometer is being improved and new data will be reported later.

The experiment was successful in showing two things. The viscosity increased

with steam pressure and there was a permanent increase shown after remelting.

More detailed study is being undertaken. The slag used in this preliminary

experiment was not a typical coal slag composition. It contained 15% Na„0

in order to produce a sufficiently low viscosity to obtain measurements

with the new viscometer. This instrument is a new concept and its temperature and pressure limits were being explored for the first time, but results

show that further study is warranted. It also furnished needed information

to assist in further improvement in the apparatus. More realistic coal slag

compositions will be used as the improvements are made in the design.

Future Plans

1. The slag-seed-viscosity study will be completed.

2. The study of the influence of water vapor on slag viscosity will

be continued.
9

Table 1. Composition of Slags in Seeded Series

wt %

Melt No. K-S84 K-1190 K-1196 K-1205

Si0

2

47.77 45.62 42.99 40.60

A1 2°3 21.76 20.78 19.58 18.50

Fe 2°3 16.58 15.31 14.92 14.09

CaO 10.57 10.09 9.51 8.98

MgO 3.32 3.17 2.99 2.82

k

2 ° — 5.03 10.00 15.00
10

O

O

Iso

o

CD

O

in

o

o

O

O

ro

O

o

CM

TEMPERATURE

(C)

Fig.

1

Effect

ofK

0

(Seed)

Additions

on

Slag

Viscosity
11

>

1 ]

0]O

ow

•H>

O'

<u

i—t cnc

o

u

o

a,

ttj >

u

o

4J

<a 3:

4-1O

-Uo

aj

4-1 4-i

<N

Cn 'H
12

2- Electrical Conductivity (W. R. Hosier)

During this reporting period work has continued on measurement of the

electrical conductivity of promising electrode materials developed at NBS or sent to us by other MHD contractors. The electrical measurements

reported in this section are all made using the well known DC four probe

technique thus basically eliminating difficulties due to contact emf's or resistances. The current is supplied by a constant current supply of

variable voltage and the potential drop is measured across the conductivity

probes by a DC electrometer with a high input impedance (>10 14

ft) compared

to the sample resistance. In order to increase the efficiency of the

measurement process a sample holder was fabricated which will accommodate

two samples for measurement at the same time. Since certain samples take

long periods of time (several hours) to equilibrate at each temperature

point and at the several 0^ partial pressures used this double sample

holder has proven extremely useful.

2.1 K

2

0 - 7 Fe

2

0

3

A sample of potassium ferrite (K^O - 7 Fe 2C> 2

) was fabricated and the

electrical properties are shown in Figure 3. The data shows the conductivity

values for both increasing and decreasing temperatures in an atmosphere of

10“ ^ oxygen in nitrogen. These measurements are preliminary but the data

does show that the electrical conductivity of the material is well suited

for possible use as an MHD electrode material particularly in the area of

current lead-out to room temperature. More details concerning the preparation

and phase composition will be given at a later date.

2.2 U 02 Phase III Materials

The electrical conductivity of the materials used in Phase III - U-02

has been completed. The data on several compositions has been reported

previously and are listed below.

Group 1-70 w/o La Mg CrO + 30 w/o (88 m/o ZrOq + 12 Y 0 ) (a mixture) ,

« 7J • Uj j w Quarterly Report, January - March 1978, page 12, Figure 3.

Group II - see below.

Group III - see below.

Group IV - La Mg CrO , Quarterly Report, January-March 1978, page 11,

^ 5 • 0 3 3 Figure 2.

Group V - La Mg Cr 0 , Quarterly Report, January - March 1978, page 13,

. . 1*00 *02 *98 j Figure 4.

Group VI - same as Group I.

The material in group II was LaCrO^ graded with various amounts of LaAlO,

in layers of equal thickness. The layers consisted of:

top La

.95Mg

.05

A1

.32

Cr .68°3

middle La

>95Mg

-05

Al

-24

Cr

-76

O

3 La

.95Mg

.05

A1

.15

Cr .85°3 bottom
and each section was sliced in such a manner to obtain a sample of single

composition material for electrical measurements. The data is shewn in

Figure 4. The conductivity decreases as the LaAlO^ content increases as expected but the material containing the smaller amount still has an adequate conductivity for use as a lead-out with a minimum amount of Joule

heating.

Group III materials were composed of LaCrO with four different mixtur

yttria stabilized Zr02 in four layers of equal thickness. The Y^O^ conteil't: was 9% of the ZrO^. The layers consisted of

top 30% ZrO

second 20% ZrO

third 10% ZrO

bottom 0% ZrO

These electrode sections were sawed again in such a manner to obtain

samples for electrical conductivity and the data is shown in Figure 5.

Here again the conductivity of the LaCrO^ decreases as the ZrO^ content

increases ,as expected ,but the material containing only 10% ZrO^ has an adequate conductivity to be used at several degrees centigrade “without

appreciably joule heating for current densities of the order of 1.5 a/cm .

A brief description of the Phase III - U-02 experiment is presented

in Task K, section 2 of this report.
14
oL1

T(K)

Figure

4.

Electrical

conductivity

of

Group 2

U-r02

Phase

II

materials

asa

function

of

temperature

at

several

partial

pressures

ofo.
16

_(WD-uJuo)A±|AllonaNOO

T(l<)

Fig.

5.

Electrical

conductivity

Of

Group

3

U-02

Phase

III

materials

as

a

function

of

temperature

at

several

partial

pressures

of

o

0

.
17

3. Vaporization Studies (E. R. Plante)

This section describes measurements of the potassium gas pressure over

K20-containing compounds or phases. These data are necessary for prediction

and correlation of the interaction of a seeded MHD plasma with construction

materials or coal ash using equilibrium thermodynamics. For example, Spencer (1)

noted the severe lack of thermodynamic or vapor pressure data needed to model

seed-slag interaction parameters in ternary or higher order systems. The

vaporization studies at NBS have been planned to provide some of the required

data as well as to measure K 2O activities in solutions having up to 6 components

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

Because of the importance of ferrites as electrode materials measurements

of the K and O 2 pressure over KFe02 were carried out. In addition, observations of the pressures of several vapor species over MAFF-31 were initiated.

This report will discuss the data on KFeO 2 in some detail and will summarize

the preliminary work on MAFF-31.

3 . 1 Vapor Pressure Measurements on n KFeQ„ "

Results of mass spectrometric measurements of the vapor pressure of

KFe02 (series 1) were summarized in a previous report (2) . In the series 1

data an abnormally high Ij'j/Ij'g (0‘£/K+) ratio was observed which was thought

to indicate that significant reduction of the KFeC>2 took place during the

vaporization experiments. However, this observation was contrary to results

obtained by x-ray diffraction studies of the residual sample which indicated

that the evaporation process was consistent with the reaction,

7/6 (K20*Fe 20 3 ) = 2K (g) + 1/2 0 2 (g) + 1/6 (K 2 0 - 7Fe 2 03.) .

The possibility that a high O 2 pressure was due to reaction of FeO x with Pt

allowing release of O 2 was considered but SEM examination of the Pt Knudsen

cell indicated only minor interaction.

Because of these conflicting observations the mass spectrometer constant

could only be determined within a factor of 3. If it was assumed that the

x-ray diffraction data were correct the mass spectrometer calibration factor

for K would be a factor of 3 greater than that obtained assuming the itz/lts

ratio was correct.

More r ecent

+observations (3) on unrelated K2 O -containing slag samples have

shown that the l3 2/I"3 9 £ata were probably incorrect because of misalignment

of the molecular beam with the ion source. These observations showed that the

Ig^/ 1 ^ ratio could be varied over a factor of 3 depending on the ion optics

adjustments of the source. This effect is possible because O 2 is non-condensible

and can bounce off surfaces into the active volume of the ion source while K

is condensible and will be effectively removed from the molecular beam.

New measurements on the vaporization of KFe02 were therefore undertaken.

The measurements were made in our modulated beam, quadrupole mass spectrometer

using a platinum effusion cell having a 0.343 mm diameter orifice. The

experimental arrangement was essentially the same as previously used except

that the thermocouple used previously had been replaced. As before, measurements of both the K

4- and 0^ ion currents were made as a check on the

vaporization stoichiometry. Within experimental error, the vaporization

reaction is that previously noted.
18

Certain non- equilibrium effects were observed during the experiments.

Measurements of the ion currents are made at constant temperature but the

initial ratio of I^ 2 /I^ g depended on whether the temperature was increased

or decreased in going from one constant temperature to another. When the

temperature was increased, after the initial increase in the K+ intensity, there

was a tendency for the K

4- ion current to decrease with time and the 0 2 ion

intensity to increase with time until a steady state was reached. The

opposite effects were noted when the temperature was decreased but the same

steady state was eventually achieved. It appeared, however, that a shorter

period of time was required to achieve steady state pressures if the temperature

was increased from point to point and the data presented was taken in that

direction. Above 1300 K, equilibration rates were quite rapid and steady state

pressures were observed within 10-15 minutes while at the lower temperatures,

periods of about an hour were required.

These effects could be due to a phase boundary dependence on temperature.

For example, the "K20*7Fe203" composition might become O2 deficient at higher

temperatures. Other possibilities include instrumental problems with the mass

spectrometer or kinetic factors in the diffusion of K and O2 from the evaporating

phase (insufficient equilibration time) . At steady state conditions, the

I^ 2 /I 39 ratio indicated a deficiency of O2 at low temperatures but at higher

temperatures the O2/K pressure ratio appeared consistent with the proposed

decomposition reaction.

Some representative data obtained during the measurements is shown in

Figure 6 . The upper curve is the K pressure and the lower curve is the O2

pressure. The ratio of the K pressure to the O2 pressure is based on previous

experience which indicates that the mass spectrometer sensitivity constant

is about 5.6 times that for O2.

The K pressure over KFe02 -1' K20* 7Fe20 3" is about an order of magnitude

greater than the K pressure over KAIO2 - K20-7A1203 and roughly a factor of

5 greater than the K pressure over K2SO4. This confirms the general notion

that K2O interacts more strongly with AI2O3 than with Fe2C>3. It also shows

that IC2O • 7F e 2O 3" will not react with K2SO4 to form KFe02*

3 .

MAFF-31 Vaporization Measurements

These measurements are in a preliminary stage and will be summarized.

MAFF-31 is a spinel structure in which Fe+2 and Fe+3 ions replace Mg+2 and

Al+3 ions in the spinel. It has the nominal composition 3MgAl2<34 • FesOi*

Observations of ion currents due to Fe(g), 02(g), Mg(g) and FeO(g) have been

made. At 1700 °C the pressure of Fe is about 10

-3

atm which is about 10 times

as high as Mg pressure. These preliminary measurements indicate that the

upper temperature limit of MAFF-31 under clean electrode conditions should

be limited to about 1600 °C.

Conclusions

The K pressure over KFe02 K^O* 7Fe203" is about an order of magnitude

greater than' the K pressure over KAIO2 - showing that K2O interacts

more strongly with AI2O3 than with Fe203. Preliminary measurements showpreferential loss of Fe from a MAFF-31 composition.
19

References

1. F. E. Spencer, Jr., J. C. Hendrie, Jr., and D. Bienstcck, Vlth

International Conference on MHD Power Generation, Washington, D.C. (1975)

2. Development, Testing and Evaluation of MHD Materials, H.P.R. Frederikse,

T. Negas and S.J. Schneider, (Sept. 1977)'

3. E.R. Plante and L.P. Cook, 17th MHD Symposium on Engineering Aspects

of MHD, Stanford, CA (1978)
i

02

PRESSURE

UVER

KlEU2 I

500 I

H00 I

300 I

200

20

X 1/1 LD ‘s t —f —? —

E-H I/TK

Figure

6-

Representative K

(upper

curve)

and O2

pressures

over

KFeC

>2-

K20*7Fe2C>3
21

TASK J. CORROSION AND DIFFUSION

1. Diffusion in Insulator-Electrode Couples

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

Concentration profiles for Fe, Mg and A1 have been obtained from MAFF31-MgO

diffusion couples tested at 1400° C for 1 and 10 hours. Here MAFF31 has the

nominal composition 3 MgAl 2 04 * lFe 3 0 4. These profiles show a marked decrease

of Fe penetration into the MgO insulator when compared to equal testing times

at 1600°C. The 1 hr. test at 1600°C yields a Fe concentration profile similar

to the 10 hr. test at 1400°C. These tests all showed significant discontinuities

in the Fe concentrations at the couple interface. The 100 hr. test at 1400°C

has yet to be examined. The rate of Fe diffusion into MgO or MgAl 2 0 4 insulators

determines the rate at which the insulating properties will degrade if used

in MHD channels. This rate is smaller in MgAl 2 0 4 than in MgO.

The A1 and Mg concentration profiles in MAFF31-S71 couples diffused at 16C0°C

for 10 hrs. and 100 hrs. have been analyzed. Here S71 has the composition

MgAl 2 04 . The Fe profiles in these couples were analyzed and reported previously

(Quarterly Report 3-78). The data were plotted on probability paper and

normalized for diffusion time assuming a /t" dependence. Smooth curves then

were drawn through these points, which fell in a single composition-distance

band as expected for a simple diffusion process, with the scatter covering

about 10% of the total concentration variation. The Matano interface, which

is the plane across which equal amounts of material flowed in both directions,

was located 20 to 25ym into the S71 specimen after 10 hours. Calculations

for all three metal concentration profiles (Mg, A1 and Fe) agreed on the

location of this Matano interface.

From the composition-distance profiles, the total amount of Mg and A1 that

had passed from the S71 specimen into the MAFF31 specimens was calculated.

This was found to be very nearly equal for Mg and Al; also the amount of Fe

that had passed from the MAFF31 specimens to the S71 specimens was determined.

From these values, intrinsic diffusion coefficients for Mg, Al and Fe were

calculated for the composition at the couple interface, Mg^ Al^ ^

Fe^

The intrinsic diffusion coefficient D. is given by the equation

D. = -A . [ 2 1 ( 3c . / 3x) ] where t is thk diffusion time, (3c^/3x) is the concentration gradient of element i at the interface at time t and A. is the net

amount of i which has flowed across the interface. At 1600°C these measurements

yielded

D = 4x10 10 cm2 /sec Mg

= 1%x10 10 cm“/sec

D = 7x10 i0 cm2 /sec

Fe

It may be noted that D,,Mg

is appreciably larger than D.,. Al

Calculations by a Matano analysis from the individual Mg and Al profiles also

showed that the interdiffusion coefficient D. (which is related to the _total

penetration and mixing of species) is significan.'tly larger for Mg than for Al

in these materials. ranged from 6x10 * °cm2 /secri in the MAFF31 side of the

couple to 14xl0_ 1

G

cm-/slc on the S71 side, whereas D ranged from 2x10 10

cm^/sec to 6x10 10 cm2 /sec with again the larger diffusion coefficients and

penetrations being in the S71 specimen.
22

2. High Temperature Charge Transport in Coal Slags (K. Young, A. Franklin,

N. Adams, L. Hsu)

The current problem under this subtask is to measure, separately, the

low-current electronic and various ionic conductivities of coal slags.

Initial work will be done using a synthetic slag (K 302), selected

because although relatively simple in composition it is expected to

exhibit the major characteristics of charge transport in real slags,

and because the viscosity has already been measured over the temperature range of interest (data reported on fig. 36, p. 105 of Interim

Report for period July 1, 1972 to June 30, 1974, labeled NBSIR 74-543,

OCR-14-32-001-1230, High Temperature MHD Materials, S. J. Schneider,

et al., August 1974). This slag composition is:

Wt. % MoL %

SiC>2 50.79 60.00

AJ^O

3 21.55 15.00

Fe 30

4 16.31 5.00

MgO 11.36 20.00

The experiment to be performed to measure the electronic conductivity

is to measure the change in current with change in applied potential

in a simple conductivity cell containing the slag as electrolyte and

with the cathode reversible with respect to both electrons and Fe ions

(an Fe alloy, e.g., with Pt, chosen to be stable in the presence of the

test atmosphere, e.g., 10 -3 or 10 *"^ atm of 0 £) and the anode reversible

only to electrons (e.g., Pt)Following the discussion given by F. A.

Kroger in "The Chemistry of Imperfect Crystals", (Nor th-Holland

,

Amsterdam, 1964), Chapter 22, the expected behavior of the slag may

be analyzed as follows. The mobile species capable of changing their

average concentrations are expected to be:

Fe

2+

Fe

3+ J2-

e

where e represents an electron. Mg'" is expected to be mobile but

fixed in average composition. The following equilibria should hold

everywhere

2 e + hP2 Z 0

2

( 1 )

Fe 2+ Z Fe 3"*" + e ( 2 )

Fe Fe 2+ + Fe (virtual iron activity) (3)

The only external current that can flow is that carried by electrons

and oxygen ions

CeVq(e) +

> 2 --Vn(CK?->

J

2F

(4)
23

where Ce and are the (position-dependent) conductivities due to

electrons and C> 2 _ ions, F is the Faraday, r) denotes the electrochemical

potential, and V signifies the gradient. Using eq. (1),

Vn(02 ") = 2Vu(e) + Vn(^02 ) (5)

Since all of the specimen is exposed to the same oxygen partial pressure,

the last term is zero. For a simple cell with parallel electrodes L cm

apart, eq. (4) becomes

. /"Anode

J = - — /

(oe + a , )dn(e) ( 6 ) FL

cathode 0

Z ‘

Since oe and a o- are unknown functions of position at this stage, the

integral cannot be evaluated. However, the derivative with respect to

the potential across the electrodes is ~ = - nr- (CTe + G- 2 _) A , (7) dV FL Q

z Anode

where the zero of potential has been taken at the cathode. Thus the

slope of a plot of current vs voltage gives the sum of the electronic

and oxygen conductivities at the anode.

In order for this information to be useful, it is necessary to know

also the activities of the iron and oxygen ions at the anode. These

are found by noting that the total Fe flux is zero, or

aFe 2+ dn(Fe 2+

) g

Fe 3+ dn(Fe 3*

) (2F) 3 dx (3F) Z dx ( 8 )

Then using the equality of electrochemical potentials implied in the

equilibria expressed by eqs . (2) and (3), and integrating from the

cathode to the anode.

y(Fe) Anode y(Fe) Cathode 4- 6 /

Anode 2oFe 3+ 3a Fe 2+

Cathode

4o 0 , , 9o 0 , Fe 3+ + Fe 3-1- dn (e) (9)

For the snecaal case xn which i-i- >> O-, ?+ the integration can be done,

. , , . Fe z+ Fe z^ ° yielding

y(Fe). , = y(Fe) , + 3V Anode Cathode

or for 3+ << a_ 2+ Fe J Fe

y (Fe) . , = y(Fe) _ , + 2V Anode Cathode

( 10 )

(ID

The chemical potential of Fe is fixed by the iron-reversible electrode

at the cathode, and by eq. (10) or (11) at the anode. The oxygen

chemical potential is fixed by the atmosphere. Therefore, the controlling
24

activities at the anode are known, and eq. (7) can be used to obtain

(ae + a

0 2~) as a function of these activities. Under conditions such

that a 3+ ^ a

pe

2+» ecl- (9) cannot be simplified, and it will be

necessary to resort to models.

It should be noted that the presence of the Fe-reversible electrode is

essential since the concentrations of the various mobile Fe ions and

of mobile electrons, and therefore the corresponding conductivities,

vary with potential across the specimen. Only by fixing the Fe

activity somewhere in the specimen can meaningful measurements be made.

To obtain ionic conductivities both electrodes will be reversible to

Fe, and variation of the oxygen partial Pressure will be used to vary

the Fe2+/Fe 3+ ratio. Conductivity by Mg 2+ ions will be blocked, but

should be obtainable using ac techniques, as described in the last

report.

The specimen holder and furnace to be used in making these measurements

have been constructed and are illustrated in figs. 1 through 6 . The

furnace is shown in fig. 1. It is a resistance tube furnace with two

80 Pt 20 Rh windings. Figure 2 shows the power supply and control

set-up for the windings. They are controlled simultaneously, with the

relative currents set by feeding the outer winding through a variac.

The inner tube is a gas-tight alumina tube fitted with brass end-plates

(fig. 3) with fittings to allow a gas stream to pass in one end and out

the other, and to support the central alumina rod of the specimen

holder (fig. 4). This control rod is a 6 -hole thermocouple tube that

acts also as the feed-through and support for the electrical leads

and thermocouple brought in to the specimen boat. The alumina disks

of the specimen holder provide radiation shields, mechanical support

inside the furnace tube, and act as baffles for gas mixing and

thermal izat ion

The specimen boat (fig. 5) is an alumina boat into which blocks of

cold slag will be placed, fitted around the electrodes and then melted

in place. The oxygen partial pressure of the flowing gas atmosphere

will be controlled by mixing O 2 into a dried Ar stream using a variable

leak. The oxygen partial pressure will be monitored using a stabilized

zirconia Nernst^ cell. This system (f ig. 6 ) has been tested and found to

produce a gas stream of very stable and reproducible composition.
25

Figure

1

Furnace

for

Slag

Conductivity

Measurements
26

Figure

2

Furnace

Control

Network

for

Slag

Conductivity

Measurements
27

Figure

3

End-Plates

for

Furnace

for

Slag

Conductivity

Measurements
28

Figure

4

Snecir.cn

Holder

for

Slag

Conductivity

Measurements
K

<3

K-

--a

Ul

a.

figure

5

Specimen

float

for

Slag

Conductivity

Measurements
REGULATOR

30

Figure

6

Gas

Flow

Network

for

Slag

Conductivity

Measurements
31

3. Electrochemical Effects in Simulated Slag Flow (W. Capps, D. A. Kauffman,

and W. R. Hosier)

In the last Quarterly Report ( January-March 1978) (page 30) some preliminary measurements were reported using an apparatus to simulate the

slag flow on "hot" electrodes in a slagging generator. The problem of slag

flow and its relation to the electrochemical effects on the junction of

slag and the electrode material below is extremely important. At the same

time effective ion transport, particularly iron in coal slag alone can be

dependent on the slag flow velocity. Experiments to address these and

related problems are continuing.

During this reporting period at least four runs have been made using

the apparatus mentioned above and described in the previous report on page 30.

Each of these 20 hour runs were made using fly ash from the Bow New Hampshire

Power Plant. The first two (E5 and E6) runs were made with this flyash as received and contained no added seed material and were made at a temperature

of 1400 °C.

The second two runs (E7 and E8) were made with the slag containing 20%

by weight of K^O added in the form of and made at a temperature of

1500 °C. The reason for the different temperatures is due to the increased

viscosity of the slag containing seed material. Both temperatures were

chosen such that the slag would wipe or flow over the electrode surface

during rotation. After each run transverse sections were sawed off the

rotor (platinum tube approximately 8 mm diameter and 0.5 mm wall) and off

the stator (platinum sheet approximately 8 mm wide and 0.5 mm thick). The

removed circular cross section of the rotor in each case then was cut

transverse to the cross section plane and all pieces were potted for examination. Figure 7 is a schematic diagram of the sections sawed and potted.

Figure 8a is a photograph of the section from run E5 in which the rotor

was negative (cathode) and the stator was positive while figure 8b shows the

time dependence of the current as the run continued. Figures 9a and 9b is

the same information for run 6E but with the rotor as anode and stator as cathode. One conclusive result from this set of runs is that the platinum

electrode used as a cathode is attacked by the coal slag (iron) and that

the rotational effect or slag flow does not inhibit the attack. At the same

time the rotational effect did not prevent an apparent iron depleted layer

from forming on the rotating anode as can be noted by the decay in current

with time in figure 9b (run 6E)

Figures 10a and 10b show the potted cross sections of E7 and time dependence

of E7 respectively, while figures 11a and 11b give the same information for

run E8. In E7 the rotor was positive (anode) while in E8 it was negative

(cathode) . Here again it is quite obvious that the platinum cathode is badly

attacked by the molten slag either rotor or stator. There appears, however,

to be an effect on the time dependence of the iron depletion layer on the

rotating anode. The depletion layer has less tendency to form with a rotating

anode as seen from comparison of figures 10b and lib.

The results of these experiments are generally inconclusive and not what

we had anticipated. The potted sections will be examined (using scanning

electron microscope techniques) for iron depletion layers near anode-slag

interfaces as well as iron (or other slag components) attack on the cathode.
ROTOR

32

Figure 7. Schematic diagram showing form of rotor and stator, sectioning of

tested electrodes and potted sections.
33

Run E5

Fig. 8a. Potted electrodes from E5 rotor cathode, stator anode - 1400 °C and

no seed.

Fig. 9a. Potted electrodes from E6 - rotor anode,

stator cathode - 1400 °C and no seed.

h
34

o

Ld

(v)iN3aano

Fig.

'8b.

Time

dependence

of

current

for

r.un

E5
o

</j

LU

A

(T\

cn •H

CV) lN3ddflO

Time

dependence

of

current

for

run

E6.
JD

Run E7

Fig. 10a. Potted electrodes from E7 - rotor

anode, stator cathode - 1500 °C - 20 w/o K^O

seed added to slag.
37

o

CM

CO

CD

CM

O

CO

CD

CM

O

CO

sz

(V) lN3dd n

0

Fig.

10b.

Time

dependence

of

current

for

run

E7.
38

I Cv) iNBaano

Fig.

lib.

Time

dependence

of

current

for

run

E8.
39

4. Seed Slag Interactions

4.1 Thermochemistry of the System CaO-K 0-A1 O^ -SiO (L. P. Cook)

Subsolidus phase relations in the quaternary system CaO-K^O-Al^O^-SiC^

have been presented in earlier reports [1,2,3], and for convenience

are shown in Fig. 12, where tie lines involving Ca SiO^ and Ca^Si 0^

have been added [4,5]. Abbreviations for chemical formulae are given in

Table 1. As explained previously [1] , these data when combined with the

composition of invariant melts, can be used to calculate potassia activity

coefficients, in addition to giving more direct information cn the behavior

of Western MHD channel slags . Mixtures of each quaternary four phase

equilibrium assemblage have been prepared and held at 10 to 50 °C or

more above the temperature of minimum melting, to produce sufficient melt

for microchemical analysis (this was not possible for some of the compositions involving KAlSi 0 , due to kinetic factors). Weight percentages

of CaO, K 0, Al^O^ and SiO^ in the melt phase were estimated using an I/I

q approximation from comparison with a series of standard glasses. Results

are given in Table 2. It is hoped that compositions can be refined

by further experimentation and application of a Bence-Albee quantitative

analysis routine.

Using more recent thermochemical data [6] the calculations begun in a previous quarterly report [1] have been repeated and expanded for four-phase

volumes in Fig. 12, according to reactions in Table 3. As Figure 13 shows,

calculated potassia pressures are nearly in agreement with the order predicted from topology, at least at 1200 K (referring to Table 3, the predicted

order of decreasing P^-Pq/2 is 13, 12, 4, 3, 2, 1 [5,7], [6,8], 9, 11, 10).

This is in marked contrast to calculations performed in a previous quarterly

report [1] , made using earlier thermochemical data [7]

,

which, though of

high precision, have recently been shown to possess a systematic error

[8]

.

However in view of the revised calculations graphed in Figure 13, it

is no longer necessary to postulate entry of K in the gehlenite structure

[1] , in agreement with the failure of all efforts to synthesize K-containing

gehlenites experimentally. The phase diagram presented by Nurse and

Midgley [9] must therefore be regarded as open to question (Yoder [10]

has disproved the diagram proposed for the sodium analogue)

.

The dashed curves in Fig. 13 (with labels in parentheses) indicate

equilibria which do not fall in the correct order with respect to the

others, and have higher values of P^.Pq/^ than allowed by the topology,

suggesting metastability. It is significant that two of these involve

tridymite, and the other two involve CA^. Solution of KAIO^ in the tridymite

structure, expected cn crystal -chemical grounds, is thus also predicted

indirectly from the thermodynamics. Similarly, CA^. has a structure like

that of S-Al^O^, and substantial entry of potassium into the structure could

also be expected.
40

4.2 Thermochemistry of the System K^0-MgQ-Al o 0 n -Si0 ^ (L. P. Cook)

Subsolidus phase relations in the quaternary system K O-MgO-Al^O^-SiC,,,

derived from the experimental work of Schairer [11], Luth [12] and

experiments in this laboratory [2]

,

are shown in Fig. 14 for reference;

abbreviations for chemical formulae are given in Table 4. Calculations

begun in a previous quarterly report [1] have been carried out in more

complete form, according to the reaction schemes in Table 5 below, and

using the more recent thermochemical data [6]

.

The assumption of no Mg

solid solution in tectosilicate phases appears to be justified for these

relatively MgO-pocr four-phase volumes , based upon experimental data for

crystal/liquid fractionation in K O-MgG-FeO^-CaO-Al^O^-SiO., synthetic slags.

This may not be true for melts and solids coexisting with magnesia.

Calculated reaction curves are shown in Figure 15, Agreement with the order

predicted from topology is reasonably good (referring to Table 5, the predicted order of decreasing Pk*Pq/

2 £2,3], [4,6], [5,8], [7,10], 9).

Again, dashed curves represent higher potassia pressures (metastable) than

allowed by the topology. Significantly, two of the three curves involve

tridymite and the third is a four-phase volume metastable with respect to

the occurrence of Mg A1 SiO . Subsequent calculations will therefore

include estimated data for Sa.

4.3 Thermochemistry of the System K 0-FeQ-A1 ^0., -Si0 (L. P. Cook)

Subsolidus phase relations for a part of the quaternary system K C-FeOAl^O^-SiO are shown in Figure 16, based upon data of Roedder [13 f and

Muan'and Osborn [14] . Abbreviations are given in Table 6. The iron

analogue of MgSiO is unstable at low pressure, and an iron analogue of

Mg^Al^SiO., 0 has not been reported in synthetic systems. Reaction schemes

used £o calculate curves in Figure 17 are giver, in ?able 7. The topology

of Figure 16 gives , in order of decreasing p K* ?oi ' the following sequence:

2,1,3, which agrees well with that in Figure I?.

4 . 4 Discussion of Thermochemistry, Quaternary Systems (Ca ,Mg ,Fe) O-K

^ O-Al ^O^ -SiO^

(L. P. Cook)

The thermochemical calculative approach has important limitations which must

be mentioned before any discussion:

1. Certain thermochemical data have had to be estimated

for FeAl^O^ ; S® „ and Cp(T) fcr Ca^SijD.^

orthorhombic KAlSiO. and CaAl, _0,_.

4 12 19

and AHi

f ,298'

, including Cp(T)

° Q and Cp (T) for

2. Unaccounted for solid solution may lead to erroneous results, especially

when entry of K into a structure is involved.
41

With these restrictions in mind, it may be noted that the compositional

volumes involving KAlSiO^ with Ca SiO^, Fe^SiO^ and Mg^SiO^ occupy

similar compositional space and thus may be compared with regard to order

of magnitude potassia pressures, as has been done in Figure 18. CKAS

pressures appear to fall close to the plasma equilibration curve for the

volume C^S/C^S^/Kls/Ge . The FKAS curve for He/Fa/Kls/Lc intersects the

plasma curve at the high temperature end, whereas the analogous MKAS curve

doesn't intersect it at all. On this basis, and from comparison of Figures

13 & 14, CaO appears on a mole basis to have a more pronounced effect on K 0

pressures than MgO (a spread of 10 orders of magnitude at 1600 K as opposed

to 5) . However, it may be significant that only for the more CaO-rich

compositions can it be said that K 0 pressures are significantly higher for

CKAS than MKAS (and possibly FKAS) 7 Thus this effect may be important in

the most calcium-rich of the Western-type channel slags [5]

.

References

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

and Evaluation of MHD Materials, NBS

p. 25-46.

Schneider, Development, Testing

Quarterly Report March-June 1977,

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

and Evaluation of MHD Materials, NBS

1977, p. 31-38.

Schneider, Development, Testing

Quarterly Report July-September

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

and Evaluation of MHD Materials , NBS

1977, p. 25.

Schneider, Development, Testing

Quarterly Report October-December

4.

E.M. Levin, C.R. Robbins and H.F. McMurdie , 1964, Phase Diagrams for

CeramistsColumbus, Am. Ceram. Soc . , Fig. 630.

5.

L.P. Cook, E.R. Plante, T. Negas, R.S. Roth and C.D. Olson, 1976,

Crystallization and Vaporization Studies on Synthetic Coal Slag

Compositions, Proc. 15th Symp. Eng. Aspects MHD, Philadelphia,

P.II.4.1-II.4.6.

6. R.A. Robie, 3.S. Hemingway, and J.R. Fischer, Thermodynamic Properties

of Minerals and Related Substances at 298.15 K and 1 bar (10^ pascal)

Pressure and at Higher Temperatures: U.S. Geol. Surv. Bull. 1452.

7. R.A. Robie and D.R. Waldbaum, 1968, Thermodynamic Properties of Minerals

and Related Substances at 298.15 K (25.0°C) and one atmosphere (1.013

bars) Pressure and at Higher Temperatures: U.S. Geol. Surv. Bull. 1259.

8. B.S. Hemingway and R.A. Robie, 1977, Enthalpies of Formation of Low

Albite (NaAlSi 0 ) , Gibbsite (Al(CH)^) and NaAlO ; Revised Values

for AH° ana AG° of Some Aluminosilicate Minerals: J. Res.

U.S. Gedl. Surv. , V. 5, p. 413-429.
42

9.

R.W. Nurse and H.G. Midgley, 1953, Studies on the Melihite Solid

Solutions, J. Iron and Steel Inst., 174 , 121-131.

10. H.S. Yoder, 1973, Melilite Stability and Paragenesis, Fortschm.

Miner. 50_, 140-173.

11. J.F . Schairer, 1954, J. Am. Ceram. Soc., V. 37, p. 526.

12. W.C. Luth, 1967, J. Am. Ceram. Soc., V. 50, p. 174.

13. E. Roedder, 1951, Am. Mineral., V. 36, p. 283.

14. E.M. Levin, C.R. Robbins and H.F. McMurdie, 1964, Phase Diagrams for

Ceramists

:

Columbus, Amer. Ceram. Soc., Fig. 696.
Table 1

Table 2

1 .

2 .

3.

5.

7.

Abbreviations, CaO-K^O-Al^O^-SiO

C

2

S Ca SiO

2 4

c s

3 2

Ca

3 Si 2°7

wo = CaSiO.

ge

an

Ca A1 SiO

2 2 7 CaAl Si,0

2 2 8

ca„ = CaAl 0

12 19

trd = SiO„

2 ksp = KAlSi 0

J O

1c = KAlSi 0

2 6

kls = KAlSiO

4

mu = SAl^O^^SiO^

cor = A1 0

Estimated Compositions of CaO-K O-Al 0 -SiO„ Invariant Melts.

Four Phase Volume

kls/cor/ca^/an

kls/ca./ge/an

6

kls/lc/ge/an

kls/lc/cor/an

lc/ge/an/wo

Approximate wt% K^,0

12.5

8.8

12.3

1.7

2.6
44

Table 3. Equilibria in the System CaO-K^O-Al^O^-SiO^

•

Four Phase Volume Vaporization Equilibrium

1 kls/cor/ca./an

6

2kls + ca. = lan + 6cor + 2K + 1/2 0_

6 2

2 kls/ca./ge/an

6

34kls + 12ge = lea. + 23an + 34K + 17/2 0_

6 2

3 kls/lc/ge/an lkls + 11c + Ige = 2an + 2K + 1/2 O n

4 kls/lc/ge/wo 5kls + 2wo = lge + 31c + 2K + 1/2 0„

5 kls/lc/cor/an 4kls = 21c + lcor + 2K + 1/2 0

2

6 Ic/cor/mu/an 21c + 5cor = 2mu + 2K + 1/2 0

7 Ic/ge/an/wo 3ge + 41c = 5an + lwo + 4K + 0

2

8 lc/ksp/an/wo lwo + 51c = 3ksp + lan + 2K + 1/2 0 0

9 lc/ksp/mu/an 161c = lmu + lOksp + 6K + 3/2 0

2

10 ksp/rau/an/trd 6ksp = lmu + 16trd + 6K + 3/2 C> 2

11 ksp/an/wo/trd lwo + 2ksp = 5trd + an + 2K + 1/2 0

2

12 kls/c

3 s2 /ge/wo 18kls + 27c

3 s

2 = 9ge + 63wo + 18K + 9/2 0

2

13 kls/c

2 s/c3 s 2

/ge 12kls + 42c

2

s = + ^^ C

3 S

2 + + 3

Table 4. Abbreviations, MgO- K2°-A12°3-Si°2

fo = Mg SiC)

2 4

trd = SiC>

2 en = MgSiO^ ksp = KAlSi 0

J O cd = Mg

2Al4

Si

5 0

la

lc = KAlSi 0

2 6

sp = MgAl

2 0^ kls = KAlSiO

4 sa = Mg

2

Al

4

SiO

10

mu = 3A1 0 «2Si0

2 cor = Al2°3
45

Table 5. Equilibria in the System MgO-K^O-Al^O^-SiC^

.

Four Phase Volume Vaporization Equilibrium

1 sp/fo/kls/lc fo + 9kls = 2sp + 51c + 4K +

2 sp/fo/cd/lc 8sp + 201c + 5fo = 9cd + 20K + 5 0.

2

3 sp/kls/lc/cor 4kls = 21c + lcor + 2K + 1/2

4 fo/lc/cd/en 5fo + 41c + cd + 8en + 4K + 0^

5 sp/cd/mu/lc 61c + 4sp = mu + 2cd + 6K + 3/2 0^

6 sp/cor/lc/mu 21c + 5cor = 2mu + 2K + 1/2 0.,

7 cd/mu/lc/ksp 161c = lmu + lOksp + 6K + 3/2 0

2

8 en/cd/lc/ksp 4en + 181c = 2cd + lOksp + 8K + 2 0^

9 cd/trd/mu/ksp 6ksp = lmu + 16 trd = 6K + 3/2 0^

10 en/ksp/cd/trd 2en + 4ksp = cd + 9trd + 4K + O o

Table 6. Abbreviations, FeO-K O-Al^O^--sio

2

fa = Fe SiO

„

2 4

lc = KALSi^O^

he = FeAl O kls = KAlSiO.

2 4 4

mu = 3A1 0.2Si0_

2 3 2 cor = A^O^

Table 7. Equilibria in the System FeO-K^O-Al^O^-SiO^

.

Four Phase Volume Vaporization Equilibrium

1 cor/he/kls/lc 4kls = 21c + lcor + 2K + 1/2 0^

2 fa/he/kls/lc fa + 9kls = 2he + 51c + 4K + 0

2

3 he/cor/mu/lc 21c + 5cor = 2mu + 2K + 1/2 0

2

3 he/cor/mu/lc
46

Figure 12. Subsolidus relations for synthetic CaO-K O-Al 0 -siO slags

(refer to Table 1 for abbreviations) 2 2 3
Log

(P

K

' Po

47

Figure 13. Calculated equilibrium potassia pressures for subsolidus

assemblages in Figure 12.
48

MOL %

Figure 14. Subsclidus relations for synthetic MgC-K^O-Al^O^-SiO^ slags

(refer to Table 4 for abbreviations)

.
49

Figure 15. Calculated equilibrium potassia pressures for subsolidus

assemblages in Figure 14.
16 . Subsolidus relations for synthetic FeO-K C-Al ( (refer to Table 6 for abbreviations) . 2 2

iO^ slags
Log

<P,(

' p

0

T,X

Figure l 7 - Calculated equilibrium potassia pressures for subsolidus

assemblages in Figure 16.
Log

<P

K

‘ PO

52

T ,K

Figure 13. Comparison of calculated pressures for volumes containing

KAlSiO^ + (Ca, Mg, FeJ^SiO^ with plasma. Plasma curve supplied

by E. Plante (MBS )

.
53

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 50 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.
54

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2.

Electrode Systems and Materials Test (W. R. Hosier and E. N. Farabaugh)

2.1 NBS/AVCO Slagging Test 2 .

A description in the design and materials incorporated in this test

has been given in the last Quarterly Report ( January-March 1978) , page 38.

These electrodes (six) are being fabricated in part by APS Materials, Inc.,

Dayton, OH. It is expected that they will be finished by the end of June

with testing to take place as soon as arrangements can be made for accommodations

in the AVCO channel. A brief description of the analysis of the pretest

electrode is given in the next section of Task K, this report.

2.2 N3S/APS Electrode Test (MIT 22S-NBS F)

This set of six electrodes described briefly in Task K, page 43 of

Quarterly Report (October-December 1977) was tested at the MIT test bed on February 28, 1978. A schematic diagram of the electrode shown in figure 35,

page 77 of that report. A description of the test procedure and immediate

test results was given in Quarterly Report (January-March 1978)

.

The post test materials characterization performed on these tested

electrodes is given in the next section of this report.

A complete report including details on design, pretest analysis, test

procedure and post test analysis will be available.

2 . 3 U-02 Phase III, US-USSR Cooperative Program

Phase III of the US-USSR cooperative program was carried out from

13th to 19th of May, 1978 in Moscow. An NBS representative was a member of

the test team. The test section consisted of electrode walls made by the US

and of insulating walls made by the USSR.

The main purpose of the experiment was to conduct a life test of

electrode and insulator materials under MHD conditions. The electrode materials

were LaCrO^ based ceramics and the insulators were MgO^ or MgAl^O^. Thirtysix pairs of electrodes were tested operating under varied temperature and

electrical conditions. The test itself was divided into four general parts.

1. Heating of the test train and selection of test parameters,

2. Electrophysical tests of the electrodes,

3. Life tests of electrodes ,. and

4. Cooling the train.

The total exposure time of the module to the specified thermal conditions

was 107 hours including 100 hours in which the electrodes were under electrical

load. After the test the electrode walls were carried to NBS where sectioning

and post test analysis is in progress. Westinghouse Research and Development

Center, Pittsburgh, PA, was responsible for the design, construction and

execution of the test in Moscow. NBS and Batrelle N.W. laboratories are primarily responsible for the post test analysis.

An Express Report stating the main details of the test was written just,

after the experiment and this report is available. A complete and final report

including design, fabrication, testing and post test analysis will be

assembled later by Westinghouse Research and Development Center, Pittsburgh, PA.
58

3 . Test Analysis (Pre and Post Test Materials Characterization)

3.1 Examination of Zirconia, Yttria and Ceria Capped Lanthanum Chromite

Electrodes Tested at the MIT Test Bed on February 28, 1978; MIT //228-NBS #F

(E. N. Farabaugh)

These 6 electrodes consisted of a 85 Zr0 9 -3 Y^O^-12 CeO ? cap on top of

APS LaCrO^. The LaCrO

^ was joined to a nickel plated copper spacer which

was, in turn, brazed to the copper cooling strip. The cap was joined to

the LaCrO^ by means of grading from the cap material to LaCrO . Similarly,

the LaCrO

^ was joined to the copper spacer through a cermet grade METCO 447

to LaCrO^. The insulators were MgA1^0 / (S71)

.

Anodes were numbered 2, 5 and 1 with #5 containing a thermocouple. Anode

#5 was the middle anode in the anode stack. The anode closest to the

combustor was //I with #2 on top of the anode stack. (The plasma flow was vertically upward in this rig)

.

Cathodes were numbered 3,6 and 4 with #6 containing the thermocouple

on the cathode side. The middle cathode in the stack was // 6 with #3

nearest the combustor and #4 on top of the stack.

All the electrodes drew current during the entire course of the test.

Visual observation of the electrodes, together with monitoring of the

voltage and current characteristics, indicated that the cap parted from

#6 and #3. The cap loosened from #3 first then from #6 about 3/4 of the

way through the test. These electrodes still drew current, however.

Figures la and lb are optical micrographs of slices taken from the cathode

side. The slices shown in Figure la were taken so as to show the embedded

thermocouple in #6. Slices displayed in Figure lb were taken in material

away from the thermocouple and nearer the center line. Note in Figure la

the cap is raised in #6 just above the thermocouple and sheath. Most of

the cap is missing from #6 in both slices. The cap is partially gone from

// 3 in the slice shown in Figure lb. The cap to #4, the most downstream

cathode was in the best post-test condition of the cathodes.

Figure 2 shows cathode #6 at a magnification of 19X. In it are identified

the copper spacer, 1; the METCO-LaCrO grade, 2; the LaCrO., body, 3; and the

cap and grade to LaCrO^, 4. Very high K concentrations as^well as La, Cr

and some Mg are seen in 5, near the thermocouple sheath. Close examination

of these various regions suggested that there were sharp boundaries between

the APS LaCrO and where the cap to LaCrO^ grade ended. The METCO to copper

joint was better than the METCO to LaCrO^ bond. The poorest of bonds was

the cap grade to APS LaCrO^.

The outermost plasma surface (Area 4 in Figure 2) is seen at 180X in

Figure 3. The more porous appearing zone is what remains of the cap.

Figure 4 displays the EDX spectra from this area. What remains of the

cap is mostly single phase material. It is noted that no K is present

and further EDX examinations has, in fact, shown K concentrations vary

from zero in this cap area to a maximum where the cap to LaCrO

^ grade ends.
59

As one moves in from the cap, into the grade, La-Cr phases are encountered

which are Cr-poor. The material in which these phases are seen is usually

porous. Going further into the electrode, the density increases as does

the Cr concentration in the La-Cr phases. Generally, the Zr-Ce-Y concentration

decreases while moving in this direction. However, there are isolated Zr-rich,

Zr-Ce-Y-La-Cr regions encountered. The difference between the denser interior

areas and the more porous areas nearer the plasma surface seems to be that

a Zr-La-Cr phase rich in Cr and Zr, which surrounded larger grains, has been

reacted away as one moves toward the plasma surface. Thus, the change in

microstructure seen in Figure 3 is accounted for by the presence of intergranular Cr, Zr-rich phases in the denser regions.

As the boundary between the grade and the LaCrO

^ body is approached, K

concentrations begin to appear. Figure 5 shows, at 180X, a crack with

different microstructures present on adjacent sides of the crack. The crack

is a boundary between the grade, 2, and APS LaCrO,, 3. In regions 1 and 2,

cap constituents, mainly Zr, are found, while in 3, no Zr, Y or Ce was

detected. In region 1, the material is La-rich and Zr-K-poor compared to

region 2. The EDX spectra from these regions is shown in Figure 6. In

region 3, no cap materials were seen, the phases present being nearly

normal La-Cr phases with appreciable K concentrations. In area 4, the K

concentration is quite high, and there are no cap constituents and the

concentrations of La and Cr are somewhat lower than in 3. EDX spectra

from areas 3 and 4 are given in Figure 7. Thus, there is a sharp boundary

between the cap phases and the APS LaCrO

^ body. The boundary is evidenced

by a crack and by a La-poor, K-rich phase on the plasma side of the crack.

It can also be seen, in Figure 5, that the areas of highest K concentrations,

2 and 4, show surface microstructure attributed to K-phases forming on the

specimen surface.

Figure 8 shows at 180X the thermocouple and sheath embedded in # 6 . The

material between the sheath and the thermocouple wire is MgO. However, EDX

analysis of this MgO insulator material showed large K peaks as well as seme

La and Cr. Remembering the high K concentrations and the presence of some Mg

in material adjacent to the sheath in the electrode (region 5, Figure 2) it

seems possible that the sheath broke down someplace and allowed reactions to

take place between the MgO in the thermocouple assembly and seed. Figure 9

gives the EDX spectra from the thermocouple wire (lower) and MgO (upper)

.

No obvious reaction has taken place with the thermocouple wire.

Figure 10 shows the lower part of cathode #6 including the grade from APS

LaCrO^ to the copper spacer. The material at the bottom of the micrograph

is the copper spacer. In the copper, near the bond to the METCO, some K

has condensed, forming a K-rich Cu-K phase. The grade to the LaCrO

^ shows

the Ni, Mo and A1 concentrations decreasing as the LaCrO^ boundary is

approached. However, just before the grade turns into LaCrO a band, 1, is

observed which is rich in K, Cu, Ni and Al. In 2 , the expected La-Cr phase

is present together with a small amount of K. In this lower area, it seems apparent that K has condensed on or near some kind of boundary. There was

a band of K-rich material just inside the Cu spacer, and a band of K-rich

phases just inside the grade-electrode boundary. Perhaps this is a result

of some temperature effect. In this area, it was also noted that the

METCO-copper bond seems a little better than the electrode-METCO bond.
60

In Figure lb, the remains of the cap on cathode #3 show several different

colored areas. Figure 11 is a 20X SEM micrograph showing the partial cap

remaining on the electrode. Roll-off on the electrode and filling in by

the insulating material is seen at 1. 3y comparing Figures lb and 11, areas of discoloration could be located in Figure 11. Extensive EDX

examination failed to show unambiguously evidence for compositional

changes associated with the discolored material. It is possible that

different oxidation states of Ce or slightly increased concentrations of

Ce can account for the darker colored regions in the cap.

The MgAl 9 0^ insulator showed K penetration and reaction (with MgO grains)

to some extent over the length of the insulator down to the base. The

insulator was, despite these reactions, still in good post-test condition.

In summary, the investigation of the cathode material has shown:

(1) K has penetrated into almost all areas of the cathode, frequently

condensing on or near the interface between different phase regions.

(2) Most of the cap was gone on #6, and partially eroded away on #3. These

electrodes still drew current. The cap suffered least on cathode //4.

(3) The thermocouple and sheath were at least partially responsible for

the lifting of the cap on cathode #6. Some swelling and movement of

material was observed around the thermocouple.

(4) The cap and grade to cap materials are attached through reactions

with intergranular phases.

(5) K was detected in the 'MgO material insulating the thermocouple from

its sheath. It probably entered through some crack in the inconel sheath.

Figures 12a and 12b are optical micrographs of slices taken from the anodes.

Figure 12a shows slices taken so as to include the thermocouple probed

anode, # 5. The slices in Figure 12b were cut from material away from the

thermocouple, and more towards the electrode center. These micrographs

show the lifting of the cap on anode #5, the one containing the thermocouple. The caps on the other anodes were essentially damage free,

showing at the worst cracking of cap, but no loss at the cap. In one

case, anode #1 broke free of the copper strip at the METCO-LaCrO^ bond.

This was the most severe effect of testing on 1 and 2.

Figure 13 displays the cap of anode #5 at 15X, including the lifting of

the cap from the electrode body and the cracking originating around the

thermocouple sheath. It appears that the sheath may have been the source

of the problem with the cap. However, the material immediately above the

sheath doesn’t show the degree of lifting that is shown by the cap. It

is possible that the cap cracked, then, the force of the plasma may have

lifted the cap from the electrode. Thus, directly, by causing swelling

or indirectly, by being a source of cracks, the sheath and thermocouple

played a role in the loss of the cap in anode #5.
61

Analysis of the material seen in Figure 13 showed that the large crack

running horizontally (indicated by the arrow) separated the graded

material from the APS. LaCrO^, there is none of the Zr-Y-Ce phase below

the crack. The remains of the cap at 1 are nearly single phase Zr-Y-Ce

with very little K present. Material in 2 is part of the graded material

containing Zr, Y, Ce, La, Cr and K. Higher concentrations of K are seen

in 2 than 1. Material such as 3 is still part of the grade. In this

particular area, the phases are a little poor in Y and Ce but richer in

Cr than either 1 or 2. Material 4 is the APS LaCrO^ here containing

substantial concentrations of K. The anode has seemingly separated at

the cap-grade and grade-LaCrO^ boundaries.

The microstructure, (close to the plasma surface) in the APS LaCrO^ , is

seen in Figure 14 at 1800X. The large grain, 1 is LaCrO^. It is

surrounded by K-La-Cr phases, the dark layer-like material, 2.

The microstructure 3 of Figure 13 is shown in Figure 15 at 1800X. In

this piece of graded electrode the larger grains are the Zr-Y-Ce phase,

1. La-Cr-Zr phases were detected at 2 and Cr-poor La-Cr-Zr were found

in the vein-like material at 3. Little K was found in these phases.

The microstructure seen in the remains of the cap, 1 in Figure 13, is

shown in Figure 16 at 1800X. This material is nearly single phase

Zr-Y-Ce material. There was no K found in this material. The dark

appearing areas in the micrograph are holes or pores in the structures.

These areas were probably the La-Cr-Zr phases and were reacted away

by the seeded plasma.

Figure 17 shows the base of the anode at 45X. There are smooth junctions

of the copper to the METCO grade and of the METCO to the APS LaCrCL. No

K-rich bands were seen at the anode base. In fact, no K was detected in

the base area of the anode at all. The dark islands in the METCO are

Mo, Al-rich Mo-Ni-Al-La-Cr phases. The microstructure of the phase is

different from that seen in the cathode base. This may be the result of

the K reactions in the cathode.

The inconel thermocouple sheath did not allow K penetration into the

thermocouple assembly in anode #5. The MgO insulation between the sheath

and the thermocouple wire was essentially single phase MgO. However, in

electrode material surrounding the sheath, areas of Cr-rich phases are detected. Figure 18 shows a Cr-rich Al-Mg-La-Cr phase, 1, in the electrode

material which is adjacent to the inconel thermocouple sheath, 2. It is

difficult to determine if this phase resulted from reactions with the

inconel sheath or not.

In summary, the investigation of the anode has shown:

(1) No K-Rich bands at interfaces, where different composition in graded

material meet, exist in the anode base. The highest K concentrations

are where the cap to APS LaCrO^ grade ends. The K concentration are not

as strong as those detected in the cathode.
62

(2) Anode #5 was the only anode in which the cap came free. It is possible

that the thermocouple sheath played a role in this deterioration.

(3) The cap seems to be attacked through reactions involving the intergranular material surrounding the Zr-Y-Ce phases. Generally, the corrosion

mechanism appears to involve intergranular reactions first.

(4) The grade from Cu cooling strip to APS LaCrO^ was free from cracks or segration of any phases. At the plasma surface some cracking appeared

at cap-grade and grade-APS LaCrO^ boundaries.
63

Figure la. Optical micrograph of cathode slices.

Figure lb. Optical micrograph of cathode slices.
64

Figure 2. 19X SEM micrograph of cathode #6

Figure
6 5

Figure 4. EDX spectra from the remains of the cap

180X SEM micrograph showing the boundarv

APP T o between the grade 2 Figure 5,
66

Figure 7. spectra from areas 3 and 4 in Figure 5.
67

Figure 8. 180X SEM micrograph of thermocouple wires, protective sheath,

and MgO insulating material.

Figure 9 EDX spectra from the thermocouple wire (lower) and MgO

insulator (upper)

.
68

Figure 10. 45X SEM micrograph of lower part of cathode #6.

Figure 11. 20X SEM micrograph of cathode #3

Figure lb)

.

(from the section shown in
69

Figure 12a. Optical micrograph showing other slices taken from the anodes.

Figure 12b. Optical micrograph showing slices taken from the anodes
70

Figure 13. 15X SEM micrograph of anode #5.

Figure 14. 1300X SEM micrograph of microstructure in APS i.aC.0^ nea^ <.ne plasma suriace.
71

Figure 15. 1800X SEM micrograph of microstructure of material 3 in Figure 13.

Figure 16 1300X SEM micrograph of microstructure of reacted cap material

seen in Figure 13.
72

Figure 17. 45X SEM micrograph of the area at the base of the anode #5.

Figure 18 450X SEM micrograph taken of the area surrounding the thermocouple

sheath in anode #5.
73

3 . 2 Examination of Soviet Electrode Materials (Titanium Doped Silicon Carbide

and Lanthanum Chromite -Chromium Cermet) Tested in the Joint US-USSR

Experiment in the UTSI Test Installation (e. n. Farabaugn, A. Perloff)

An extensive investigation of these materials was conducted and a brief

summary of the results is given below. The full report, as presented to

the USSR team is available.

The electrodes were tested with the combustion products of both acid and

basic coals containing seed (K2 SO4 ) . The basic coal was made by adding

5.5 parts of Ca( 0H )2 pet 85 parts Kentucky acid coal. The first series

of electrodes was tested without current. A second phase of the test

was conducted under similar conditions to the first with the addition of

an externally applied electrical field.

Specimens from electrodes tested in thermal, acid coal + seed, basic

coal + seed with and without current, as well as untested coupons of

each material were examined by EDX, SEM, and XRD techniques.

3.2.1 SiC + Ti (Untested Material)

From information supplied by the USSR team, the titanium doped SiC is

composed of hexagonal and cubic forms of silicon carbide with excess

silicon and titanium silicide. X-ray diffraction analysis of the

untested material showed it was primarily various forms of SiC. Also,

there were identifiable amounts of free Si and TiSi 2- There was no indication of metallic Ti or titanium silicides with higher Ti content

than TiSi2

- Some small unidentifiable peaks exist in the diffraction

pattern, but they would represent material present in very minor proportions .

The untested material can be characterized as being composed nearly

completely of three phases — TiSi 2 islands, a free Si matrix containing

the islands and SiC grains. No oxidation, other than that of the surface, of either SiC or Si was detected. The untested material is quite

uniform in density, microstructure and distribution of grains.

3.2.2 siC + Ti electrodes (Tested Material)

Most of the microstructures, reaction products, and phase changes

observed in the current-tested cathode were seen in one or another of

the electrodes tested without current. One exception was that in the

current tested cathode, some possible oxidation of a Si phase was

apparent. Redistribution of Ti, melting of Ti-Si phase islands, observation of Ti-Si phase 'clumps' outside of the electrode body all were
74

seen in electrodes tested without current as well as the electrodes

tested with an applied electric field. Temperatures in excess of the

reported values, for electrodes with redistributed Ti, may well have

caused the Ti-Si phase island melting.

In all the electrodes the SiC phase appeared to be most resistant to

attack, the reactions mainly taking place either via slag interaction or oxidation with the Si and Ti-Si intergranular phases. Breaking up of

the electrode material probably started by weakening the microstructure

through these Si and Ti-Si phase reactions allowing erosion to take

place at the plasma surface.

The question of oxidation of these electrode is difficult to answer. No

striking evidence was developed but still there is some indication that

both Si and SiC material were oxidized to some degree. It is possible

that the oxidized material, probably being friable pulled out during

polishing thus accounting for the absence of heavily oxidized Si or SiC

phases

.

The attack was most severe on current tested cathodes and anodes. It is

possible that whatever reactions take place to degrade the electrode in

currentless tests are simply speeded up by the application of the electric

field.

3.2.3 LaCrO^ + Cr cermet (Untested Material)

From information supplied by the test team this cermet is composed of

grains of LaCrC>3 surrounded by areas of Cr with an eutectic of LaCrC> 3 - Cr £03 at the phase boundaries. XRD analysis of an untested piece of

cermet showed that all the peaks in the pattern could be identified as belonging to the typical orthorhombic LaCrC>3 phase or to the normal

cubic Cr metal. The eutectic phase must be a minor part of the cermet.

The untested LaCrC>3 cermet is characterized by:

(1) LaCrO^ grains showing nearly equal concentration of La + Cr in

their spectrum.

(2) Non-uniform density.

(3) No oxidized Cr phases detected even at LaCrO^-Cr boundary.

(4) Large variation of grain size in LaCrO^.

3.2.4 LaCrO^ + Cr electrodes (Tested Material)

As was the case with the SiC electrodes it is apparent that nearly all

of the features seen in the current tested material were seen in one or another of the electrodes tested without an electric field. Similar
75

phases were developed in all the tested material and the general corrosion

mechanism of intergranular reaction seem common to all materials. The

anode had much more extensive fluid slag phase regions in it, but fluid

phase regions in itself were not unique to that electrode.

The oxidation of the Cr metallic phase is well established, some oxidized

phase appearing in nearly every electrode. The role the oxidized -phase

plays is less well established. Vaporization of the oxidized phase may

well occur or oxidized phase slag reactions may predominate, to yield

the microstructure characteristic of the tested specimen.

Generally it would appear that the effect of the current on these

LaCrOj electrodes seemed mainly to speed up reactions and phase changes

which occur in non-powered condition only at a slower rate.
76

3.3 Westinghouse Proof Test III for U-02 , Phase III (T. Negas, L.P. Cook,

E.N. Farabaugh, W.R. Hosier)

A test of four candidate electrode systems was conducted at the Westinghouse

Research and Development Center on January 4-5, 1978. Twelve electrode

pairs were operated for 21.8 hours. Surface temperatures were between

1600 and 1785®C while current density was maintained at 1.0 A/cm2

. Results

of this test were submitted by Westinghouse to DoE via an Express Report.

Anode and cathode cross sections from the tested module were delivered to

NBS for examination. These were analyzed by powder x-ray diffraction and

SEM/EDX methods. Although an extensive report was submitted to Westinghouse

a summary of conclusions is provided below.

. 3. 1 LaCrO^-SrZrQ ., Electrodes, A-T Research Co . (T. Negas)

This set of electrodes was fabricated by sintering (T° , Pq

2 = ?) a composition

with LaCrO /SrZrO = 1.00. A 100 micron film of Pt was sputtered onto the

ceramic ana joined to copper via Ag-filled epoxy (<0.25 mm) . Monolithic

MgAl^O^ was used for the side insulation.

2 All three electrodes pairs (04 through 06) apparently maintained 1.0 A/cm throughout the test. Post-test visual observations by test monitors include,

a) " .... most severely eroded-corroded are 204-206 .... (and) .... resulted in enhanced damage to 203 and 204,” and

b) ”.... most severely damaged cathodes were 104-106 .... more

uniformly eroded."

Based on the electrode cross sections provided us , it appears that anodes

operated much hotter than cathodes. Use of MgAl^O^ insulation between

anodes 203 and 204 probably altered thermal design parameters to cause mutual up- and downstream interaction and obvious melting. Cathodes appear

to have operated within a lower temperature regime although MgAl^O^ also was used between 103 and 104. Pertinent cathode leadouts are in fair condition.

Secondary phases related to fabrication were noted for both anodes and cathodes

suggesting poor homogeneity and densification of ceramics. Due most likely

to relatively high initial porosity, anodes and cathodes suffered from

corrosion by seed. Cathodes, especially, were corroded considerably. The

chrome-oxide component of cathode ceramics was leached/vaporized selectively

to yield phase assemblages composed of refractory La^O^ (La (OH) , after

cool-down) , La Zr^O-, and SrZrO^-rich materials whicn are considerably mere

resistive electrically. Denser LaCrC^-based ceramics have performed much

better in this and previous proof-tests for U-02, Phase III.
77

3.3.2 LaCrQ ., "Flex-Bed" Electrodes (General Electric Co.) (L. P. Cook)

Microchemical and micro structural examination concentrated primarily on

electrode pair 108/208, to avoid contamination from the pair immediately

upstream as much as possible and also because this electrode pair exhibited

intermediate voltages (constant current) among those of the three G.E. pairs

tested. Reportedly, voltages ranged from 22 to 84 for electrode pair

108/208 during the test. The anode side exhibited heat fluxes 20 to 40%

higher than the cathode side.

LaCrO ceramic survived the test well, in terms of total preservation (70%

or better) and lack of seed penetration. Principal mechanism cf LaCrO^

degradation is thought to be an increase in porosity in the outer regions,

with resultant loss of mechanical strength. This may be associated with

vaporization of the ceramic. The attachments apparently did not survive

well, as evidenced by corrosion of lead-out tabs, ceramic/metal bond areas,

and in the case of the anode, areas of the ceramic near the attachment.

MgAl o. insulators have undergone degradation suggestive of interelectrode

breakdown, especially between GE and other electrode assemblies on the

cathode side. Evidence for melting at the electrode insulator interface

is strong on the anode side, where it should be noted that heat fluxes were

considerably higher than on the cathode side.

3.3.3 MAFF-31/"Hercynite" Electrodes, APS Materials (T. Negas)

As indicated in the Express Report for Proof-Test III to DoE (January 1978) , these electrodes consist of 13 mm of MAFF-31 spinel (SMgAl^O^ :Fe^0^ solid

solution) on 7 mm of "hercynite" bonded to copper via an ion plated Ni layer

and a "hercynite"/METCO 447 composite (3 mm) . The bulk composition of

"hercynite" was not given but is presumed to be an Fe/Al-oxide spinel

(the term hercynite should be reserved only for the spinel, FeAl^O^) . All

materials, including MgAl^O^ insulators, were plasma sprayed.

2 Electrode pairs 110 (cathode) /210 (anode) and 111/211 maintained 1.0 A/cm throughout the test while 112/212 operated at ^0.36 A/cm . Surface

temperature calculated for anode 211 was 1590±75°C in the initial stages

of the test but thermocouples were either inoperable or unreliable thereafter. Post-test visual observations by test monitors include,

a) "electrodes 210-212 are uniformly eroded but not excessively

so" , and

b) ".... next most severely damaged was electrode(s) 110-112.

Part of the damage is due to bloating . . . bloated region

appears to contain the reddish colored hematite...." (as

noted below, hematite is not an alteration product within

our cathode sections)

.
78

Based specifically on analytical data for the electrode cross-sections

provided us by Westinghouse Corp. the following conclusions are apparent,

1) surface temperature probably did not exceed 1700°C,

2) cathodes showed extensive penetration and corrosion by seed which

ultimately may have been the primary conductor, and

3) anode spinels were subjected to phase transitions while corrosion

by seed was less devastating.

These phenomena result from the enhanced porosity of plasma sprayed spinels.

Denser materials have performed much better but are limited by other factors

for clean-fired MHD.

3.3.4 Graded LaCrO /LaAlO Solid Solutions, Westinghouse Corp . (E .N . Farabaugh)

Of the three electrode pairs, the only pair which didn't suffer severe damage was 101-201. This could possibly be attributed to the fact that

it was run at the lowest current density.

Of the other two pairs, either the cathode or the anode of each pair was

damaged. The 102-202 was in the next best shape after testing. 102

suffered a little more damage than 202. In the pair 103-203, 203 was very severely reacted especially on the downstream side (toward 204)

.

The pair 102-202 possessed a different micro structure from 103-203, lathlike islands of an Al-Cr rich Mg-Al-La-Cr phase present in 103-203 but

not in 102-202. The islands (irregularly shaped) were also smaller in

202 and 203. It doesn't seem that this has had any effect on the performance of the electrodes since it was difficult to attribute any specific

degradation to these lath-like islands.

The anodes possessed a rather high concentration of K compared to other

anodes which have been examined.

The reactions, in both cathodes and ancdes, seem to be initiated in the

Mg-Al-La-Cr phases islands. The K appears to react with the islands,

depletes the Mg and probably causes the Cr to be vaporized more easily.

As the reaction goes to its conclusion, the result, in the extreme, is

that the islands are gone leaving a porous structure. The material left

is Cr depleted. In 102, 202, 203, there was evidence of a fluid phase

being developed also which seems to begin as a joining of islands and

results, in the plasma surface, in occupying much of the surface area.

A Cr-pocr Zr-Ca-Al-La-Cr phase was developed at the plasma surface on all

of the electrodes. The composition of this surface layer varied both in

composition and thickness from electrode to electrode.

All the electrodes examined were parted from the copper cooling strip so little study could be made of this joint.
79

3-3.5 U-Q2, Phase III Pre-Test Ceramics (T. Negas)

Prior to the U-02, Phase III test scheduled for May 1978, we received

several representative electrode materials from Westinghouse Corp. for

characterization by SEM/EDX methods . These materials include

,

a) 70 w/o La

0 _ 95Mg

0 _ 05

CrO

3 with

dispersed;

30 w/o ZrO^ (Y-doped)

b) same as in (a) but graded to 100% La

0 _ 9sMg0 _ 05

CrO3;

c) La

0.95M9

0.05

CrO

3

;

d) LaM9

0.02CrC.98°3 i and

e) Graded LaCr, Al 0_; x = 0.32 1-x ^ 3 contain the component MgO as

, 0.24, 0.15, and 0.00; all stated to

in (a) .

All materials were examined and details are available upon request. Results,

however, are summarized below.

1) La Mg^ Q^CrO^-based electrodes; dense perovskite containing scattered,

irregularly shaped, segregations (<3 pm) enriched in Mg and Cr (MgCr^O^)

;

segregations frequently yield a weaker La signal suggesting the phase

assemblage MgCr^O^ plus LaCrO^.

2) La

Q

QC_Mg ^CrO^ plus dispersed or graded ZrO^; anhedral , dense zirconia

grains (<0.75*mm) within dense LaCrO with segregations noted in (1) above;

Al, Si, Fe (±Ca)-rich materials notea at LaCrO -/ZrO^ interfaces; Al, Si, Y,

(±Ca)-rich segregations (<0.01 mm) distributed within ZrO^ grains.

3) LaMg^

Q 2 Cr

Q

ggO^-based electrodes; relatively dense perovskite with

La 0 -rich segregations which tend to hydrate and pull-out during polishing;

fracture surfaces better to analyze; no MgCr^O^ segregations.

4) LaCr^ Al^O^-based material; micro f is suring frequent within sample as received; clense perovskite containing frequent segregations (generally

<3 pm) enriched in Mg, Al , and Cr (spinel); lowermost (leadout) portion of

ceramic resembles the LaMg^ Cr Q8°3

-^ase<^ (see above) rather

than the La^ ^Mg^ ^CrO^ ceramic ’as originally indicated by the supplier.
80

3.4 NBS/AVCO Slagging Test 2 (T. Negas , C. Olson, W. R. Hosier)

A prototype electrode for this test, constructed by APS Materials, was described in the previous Quarterly Report. A typical cross section

(Figure 1) was examined by SEM/EDX, primarily to verify attachment among

dissimilar materials. Attachment of copper (C, Figure 19) to LaCrO^ (L)

via a graded METCO 447/LaCrO^ bond appears excellent. Bonding between

slag (S) and platinum (P) is poor, much of the slag being held by the

side insulation (MgAl^O^, M) . The LaCrO^ (L) to Pt bond remains fair but

could be optimized by grading.

Plasma sprayed LaCrO^ faintly shows a lamellar structure and occasional

intersecting fissures parallel with and perpendicular to this structure.

This particular cross section was exposed at room temperature to air for

several days prior to examination. When inserted into the SEM chamber

difficulty was experienced in pumping down the system. This suggested

that a hygroscopic material was present. The sample was removed, "bakedout.", and repositioned within the SEM. After normal pump-down, the LaCrO

was found to be relatively inhomogeneous on a microscale. Figure 20 (9000X)

is typical of the material. The brighter material in Figure 20 is richer in

lanthana than the darker LaCrO^ proper. This suggests a La^O^ plus LaCrO^

phase assemblage which accounts for the pump-down problem noted above.

Hydration of lanthana with consequent swelling may create storage problems

prior to testing.
81

Figure 20. SEM micrograph (9000X) of LaCr0

3 (see text).
82

3.5 USSR Magnesia (T. Negas and W. R. Hosier)

Figs. 21A and 21B illustrate a specimen of the MgO-based ceramic utilized

as a construction material for insulating walls by Soviet MHD researchers.

This material was received from DoE.

The following properties can be summarized,

a) bulk density, 3.16 g/cc; apparent density, 3.586,

b) open porosity, ^12%

c) electrical conductivity,

1) 1000°C, < 3x10 6

-5

(ohm cm) ^

2) 1200°C

,

< 2x10 3

-4

If If

3) 1400°C

,

< 5x10 If If t d) powder x-ray diffraction of the bulk (major phases) , MgO plus merwinite

(Ca^MgSi^Og) and/or 3- (Ca ,Mg) ^SiO^ solid solution,

e) dominant major and minor constituents (EDX analysis) , Mg, Ca, Si, and

Fe; Ti and Mn in traces.

Powder x-ray diffraction data reveal the presence of a significant quantity

of Ca, Mg-silicate (s) within this particular ceramic specimen. The phase (s)

apparently does not develop from natural impurities but rather is functional

for processing and for operation. Figure 22A ('MOX) illustrates the major

general features of the material observed from a lightly polished surface.

Large (up to M. mm) , anhedral, and relatively dense particles of magnesia

(many of which are transparent to translucent) are contained within a finer-grained, more porous matrix of magnesia. Figure 2 2B is a typical

EDX spectrum taken from the large magnesia particles. Calcium and ircn

frequently are detectable and are not partitioned. This suggests that the

magnesia contains these minor components in solid solution. Refractoriness

of the magnesia, however, is not compromised.

The large particles in Figure 23A frequently are agglomerates "cemented" by

Ca, Mg-silicate (s) . Note, for example, the bright regions (1) in Figures 2 3A

(1450X) , 233 (76C0X) and23C (375X) . Figure 23 D (150CX) shows similar,

bright rounded inclusions within and adjacent to the larger particles.

The spectra in Figs. 23E & 23F are typical for the "cement" phase, and

inclusions. Calcium and Si always dominate but the Mg-content varies.

This association of elements agrees with the powder x-ray diffraction data

noted above although the presence also of a glassy phase is not precluded.

The finer-grained matrix of the ceramic is well illustrated by a freshly

fractured surface. Figure 24A (1500X) is typical in revealing darker rounded

particles of magnesia effectively "cemented" by the brighter Ca, Mg-silicate.

Note, for example, the corresponding map in 24B for Si plus Ca distribution.
83

Chemical and physical evidence suggest that this particular ceramic is

composed of fused-grain magnesia "grog" carefully classified according

to particle size. The material probably was then rebonded by reactive

thermal sintering utilizing a finer-grained magnesia fraction together

with a calcium silicate reactive agent. Development of a fluid phase

and compound formation with MgO enhanced bonding of the particles and

densification. This ceramic body has been cleverly engineered for ease

of preparation and for operation under thermal stress. Bulk size and

shape (hexagonal prisms) together with controlled optimum porosity serve

to diminish thermomechanical stresses. During operation, certainly above

1500°C, a very small amount of a liquid phase probably develops associated

with the Ca, Mg-silicate phase. In trace amounts liquid phase formation

would not be destructive but rather would serve to further "relax" the

ceramic body to accommodate thermal stress.

Obviously this ceramic has been utilized successfully by Soviet researchers

in U-02 type generators. For "windowframe" , high heat flux channel designs

this ceramic would not be necessary. However, its use could be considered

for other elements of the MHD system.
Figure 21. Magnesia-based ceramic for MHD generator insulating walls, USSR
85

Figure

22A.

Micrograph

('MOX)

of

lightly

polished

surface

of

magnesia-based

ceramic.
36

co

0)

1-4 *J 0 u

•H

O

JZ

M CO

CQ ' 0.

CO

CO OJ •H

CO ^

01 r-s c u

00 01 co .M S n

<U 1

00U rH

CO co iH a u -u O >4

u-i a>> • i—I 0) co 00 fa a c

•h o •> Da I—I X )4 U 0) 00 ^ g S >-< COg

CN

CN

O

<U 1N cO D. <U O

CD «H 3 -U X 00 S-J Q -h o; W fa > a

CN

CN

01u3

00
87

Figure

23A

Figure

2 3B
88
39

Figure 23. Micrographs A (1450X) , B (7600X) , C (375X) , and D (1500X)

showing bonding of magnesia particles by and inclusions

of a Ca, Mg-silicate phase (1). Figures E and F, are EDX

spectra for phase (1); Mg (short vertical marker), Si, and

Ca (long vertical markers)

.
90

24A.
91

3.6 UTSI Slag (T. Negas)

The following specimens recently were received from UTSI,

a) several milligrams of a residue taken from the surface of a copper anode

,

b) a "chunk" of anode slag, and

c) a well-polished cross-section of slag extracted from above a copper anode.

Insulation used and upstream/downstream orientation associated with item

c) were not specified. These materials were examined and a report was submitted to UTSI. A summary of this report is provided below.

Item (a) was identified as Cu 0 by powder x-ray diffraction. Item (b)

was ground and also analyzed by x-ray diffraction to determine major

crystalline phases present. Low-KAlSiO^ and a "spinel" having a cell

parameter similar to Fe 0^ dominate the material (excluding amorphous

glass) . The "spinel" phase could be a series of spinel solid solutions

closely related in bulk composition and having similar cell parameters (e.g.

iron oxide-rich members within the CuFe^O^-Fe^O series perhaps including

MgFe^O^, MgAl 0 , and/or FeAl„0^ components). SEM/EDX analyses support

this interpretation.

Item (c) was examined using SEM and EDX methods. The anode slag layer illustrated in Figure 25 is subdivided for purposes of characterization by the

superimposed dashed lines. Evidently the protrusion apparent in the figure

defines the position of the insulator gap between copper frames. The "slag"

has "stuck" and partly interacted with the cooled insulator (A1 0 ?) to locally

thicken. A thinner slag layer develops above cold copper at either side of the

insulator gap. Within the lowermost part of the protrusion (zone A, Figure

25) and within zone B, Figure 25, the "slag" remains relatively homogeneous

except when adjacent to Cu and/or insulation (see below) . Crystallization

and/or partitioning of elements are mostly absent within zone A and the

lower part of zone B, apparently reflecting the combined effects of lower

temperature , higher viscosity, and short equilibration time. Slag within

the upper (plasma side) part of zone B was probably entirely molten and

quenched to a glass during cooldown. This homogeneity combined with a relatively flat surface permits, at least, a qualitative assessment of the

distribution of copper dissolved in slag. Typical spectra, therefore, are shown in Figure 25. As expected, Cu-content diminishes from base to surface.

At and near the slag surface, copper is barely detectable.
Slag zones C and D (Figure 25) are heterogeneous and their interface is

marked by well-developed porosity. Zone D is characterized by extensive

internal porosity and fissuring suggestive of evolution of gas and/or

localized turbulence and crystallization. Zone C appears less well-defined

within the thinner slag segment and is included as part of zone D.

Anode slag zones are further characterized by rather consistent microchemical and micro structural features which are documented in the final

report.

Apart from the relatively large concentration of Cu within this anode slag,

few conclusions can be derived without examining additional anode and

cathode samples. Copper metal apparently is oxidized and simultaneously

dissolved within the slag layer. Sulfur and, possibly, phosphorus may

participate in corrosion processes. Available iron from the slag acts as

a '’scavenger 1 ' to precipitate much of the dissolved copper as an oxide,

probably a spinel. Some of our observations could be related to electrochemical phenomena but these interpretations are reserved until additional

samples are examined.
93
v/J i 4. Materials Development and Analysis

94

Preparation of YCrO^-Based Ceramics (T. Negas)

Utilizing sinterable powders developed by L. Domingues of Trans-Tech,

several exploratory thermal sintering runs were performed to establish

conditions which could be used to produce high density ceramics based on

YCrO^. The first experiment was conducted at 1700-1725°C / Pq

2 = 10”^ atm

(established by mixing CO /CO) for four hours of soaking. Cored, ceramic

blocks less than 95% of theoretical density were obtained. From 175Q°C,

Pq

2 = 10“^ atm and a soaking period of eight hours, crack-free ceramic

rods, 96-97% of theoretical density, were produced. Finally, a shortduration experiment at 'tl750°C (induction heating) in forming gas yielded

>96% dense specimens. Internal microfissuring , however, was generated by

the rapid cooldown used. Nevertheless, a forming gas environment combined

with controlled heating and cooldown rates and thermal sintering in the

1700-1750°C range should prove suitable to prepare test specimens.
95

TASK L. ASSESSMENT OF STEAM PLANT COMPONENTS (J. Smit and T. Negas)

1 . Laboratory Hot Corrosion Screening Program

The apparatus constructed for this program has produced the required

1330 C hot zone temperature. In addition the temperature of the test

specimen in the hot zone can be maintained down to the desired 550 C

by air cooling. The temperature gradients and variations in the 2.5 cm working hot zone and in the corresponding specimen region are approximately + 40°C at 1330°C in the hot zone and + 30°C at 550°C for the

air cooled specimen in that hot zone.

For tests conducted to date a reagent grade crystalline K2 SO 4 has been

added to the hot gas stream at the burner. This results in a formation

downstream in the hot zone of two types of deposits on the 1.25 cm diameter tubing test specimens. The first type is a heavy adherent

columnar deposit on the surface facing the hot gas stream; the second

type is a fine powdery deposit and is found on the remaining surface.

Both deposits were analyzed by x-ray diffraction and found to be

K2 SO 4. It was noted and not unexpected that as the deposits built

up on the tubing the amount of cooling air required to maintain the

specimen at the desired temperature diminished. The deposits acted

as a thermal insulator. It takes but a short time, less than thirty

minutes, at a K2 SO 4 flow rate of about 10 grams per minute to form

deposits that appear not to increase in thickness beyond that time.

A specimen of 304 stainless steel tubing, 1.25 cm in diameter with a wall thickness of 0.089 cm was exposed in the test apparatus for an abreviated flow time and then analyzed. The test conditions were:

a. Hot zone temperature 1330°C

b

.

Specimen temperature 550°C

c

.

Total test time 4 hours

d. K2 SO 4 flow rate 10 gm/min

e

.

K2 SO 4 flow time 30 min.

Metallographic specimens were prepared from sections taken out of the

test piece at the mid point position in the hot zone and at approximately

0.6 cm away from that position. The specimens were prepared using

non-aqueous media for both cutting and polishing to minimize reaction

with the K2 SO 4 deposits.

SEM/EDX analysis revealed non uniform attack in both specimens. In

specimen 1 , the mid point specimen, no apparent attack occurred under

the heavy K 2 SO 4 deposit. Figure 1 is typical SEM micrograph of the

K 2 SO 4 - stainless steel interface in this region. The light gray

material on the left is 304 stainless steel; the material on the right

is K2 SO 4

. The interface region was analyzed by EDX and found to contain no abnormalities. In the same specimen, but under the powdery

coating, discontinuous attack at the interface was observed. Figure 2 shows one such region. EDX analysis indicates that the light gray

material on the left is the 304 stainless steel. The dark gray region

immediately adjacent to it is a Cr rich region. The bright region

next to that is Fe rich and the dark grayporous region to its right

is the powdery K2 SO 4 deposit. Figures 3,4, 5 and 6 are the EDX spectra

associated with these regions. The lower trace of figure 3 is the
96

spectrum obtained for the 304 stainless steel and shows Cr, Fe and Ni.

The upper trace is from the dark gray region immediately adjacent to the

stainless steel. It indicates a Cr rich or Fe depleted region. The upper

trace of figure 4 is of that same. region while the lower trace is of the

bright region between it and the dark gray porous K

2 S0 4 region. This trace

shows an Fe rich region containing some Cr but no Ni . Figures 5 and 6 are traces of the Cr rich and Fe rich regions which were analyzed for

oxygen. The oxygen peak shows at the far left of the left of the marker

on both traces. This would indicate the following sequence of materials:

304 stainless steel, chromium rich oxide, iron rich oxide and K2 S0 4

.

In specimen 2, taken about 0.6 cm away from the mid point, no detectable

attack occurred under the powdery K2 S0 4 deposit. There was, however, nonuniform attack under the heavy K2S04 deposit. Figures 7 and 8 are SEM

micrographs that illustrate this. Figure 7 shows a 304 stainless steel - K2SO4 interface with no region of abnormality. Figure 8 shows a region

of attack. Figure 9 is the same region as figure 8 but at a higher

magnification. This figure can be compared to figure 5 (approximately

the same magnification) noting that the depth of attack is twice that

is shown in figure 5. Also note the greater degree of definition in

the attack region in figure 9. As in figure 5 the EDX analysis shows the

light gray material on the left to be 304 stainless steel and the dark

gray fissured material on the right to be K?S0 4

. To the left of the

K2S0 4 , the region between the two slender dark bands is an Fe rich

Ni free region. To the left of this is the Cr rich region. However,

in this case there is a narrow sulfur rich band between the 304

stainless steel and the Cr rich region. Figure 10 is the EDX spectra of

the region between the S and the K2 S0 4 region. The lower trace is

of the Fe rich region next to the K2 S0 4 ; the upper trace is of the Fe

depleted region between the Fe rich region and the S- stainless steel

region. The corrosion band is about 25 microns thick in this case as opposed to the 10 micron thick band under the powdery deposits. The

material sequence is the same as the in 10 micron band except for the

sulfur band between the 304 stainless steel and the Cr rich oxide

region.

There are two interesting findings from the analysis detailed above. The

first is the localized nature of the corrosion attack and the second is the

appearance of sulfur in the region of most severe attack. An unstated,

though implied goal of this program is the detection of corrosion in its

initial stages. It appears that the procedures and techniques described

in this section are capable of detecting incipient corrosion induced by

a salt laden hot gas stream. Other candidate materials for down stream

components, as detailed in the previous quarterly report will be treated

and analyzed in this way to verify and to determine the reasonableness of

short duration exposures.
97

Figure 1. SEM micrograph (900X) of interface region between 304 stainless

steel (left) and columnar K„S0 „ (right).

2 4

s

Figure 2. SEM micrograph (1800X) of interface region between 304 stainless

steel (A) and powdery K2 SC 4 (D) showing corrosion band of Cr rich

oxide layer (B) and iron rich oxide layer (C).
98 l

Figure 3. EDX spectra of corrosion region of figure 2. Lower spectrum

is of 304 stainless steel, the upper trace is of zone (B).

Markers on the right are set on the Fe peaks, the two peaks

to the right of Fe on the lower spectrum are Ni, the two to

the left are Cr.

Figure 4. EDX spectra of the corrosion region of figure 2. Upper spectrum

is of zone (B), shown to the Cr rich: The lower is of zone (C)

shown to be Ni depleted and Fe rich.
99

Figure 5. EDX spectra of zone (B) showing 0 peak at left of> left hand

marker

.

Figure 6. EDX spectra of zone (C

marker.

showing 0 peak at left of left hand
100

Figure 7. SEM micrograph (900X) of

steel and columnar K

2 S0 4

interface region between 304 stainless

(right) (specimen 2).

Figure 8 . SEM micrograph 850X of

steel (E) and columnar

Cr rich oxide layer (G)

band (F). (Specimen 2)

interface region between 304 stainless

K SO * (K) showing corrosion band of

, "iron rich oxide layer (H)

,

and sulfur
101

Figure 9. SEM micrograph (1700X) of upper region shown in figure 8.

Notation of figure 8 applies. Compare to figure 2.

Figure 10. EDX spectra of

is of zone (G) zone (H) shown

corrosion region of

shown to be Cr rich

to be Fe rich.

figure 8. Upper spectrum

The lower spectrum is of

K
102

2. New Alloy Products for Critical Applications (J. R. Cuthill)

In the previous quarterly report (for the period ending March 31, 1978)

alloys were cited that appeared to have the possibility of offering advantages

in certain critical component applications.

Additional data on the properties and prices of some of these alloys

obtained from the respective producers are given in this report.

Substitute for 310 stainless steel to resist sulfur attack

Type 310 stainless steel has been successfully used for components

of high corrosive units such as the seed recovery unit subject to H S

attack. However, Incoloy 800 may be better from the point of view of

resisting H attack; at least it was reported to show approximately half

the weight loss of type 304 in H^S. (See quarterly report for period

ending June 30, 1977). However, Incoloy 800 costs about twice the price

of type 310 stainless steel. However, another alloy with the following

designation and composition

SANDIVIK 253 MA (1)

Cr Mi N Si_ Mn P C RE

21 11 0.17 1.7 0.30 0.04 0.10 0.04

is reported to exhibit mechanical properties that are generally better than

310 S.S., and the oxidation resistance about the same as 310 SS but the price

is considerably less. The price is reported to be comparable with type 316 SS

Current prices on some stainless steel are as follows:

Stainless Steel Base Price* (bar stock)

304 SS

316

310

$0. 86/lb.

1.16

1.64

*From American Metals Market, N. Y. , N. Y.

Data on the creep-rupture strength of Sandvik 253 MA in comparison with

type 310 SS and Incoloy alloy S00H is shown in Figure 11. The oxidation

resistance in air appears to be a little better than 310 SS up to 1100 °C

(see figure 12) . The catastrophic breakdown of its protective film ar

1150 °C is apparently due to the high percentage of silicon in the protective

oxide, in comparison with 310 S.S. Data on the resistance of Sandvik 253 MA

to sulfurdizing atmospheres is currently being obtained by the producer and

is not yet available, but because of its lower nickel content, 11 vs 20% in

310 S.S., one would expect the Sandvik 253 MA to be less vulnerable to

sulfur attack than the 310 S.S.
103

Another alloy that is reported to have even better mechanical properties,

(compare the creep rapture values in Figure 13 with those in Figure 11) , is

the following:

SANDVIK Sanicro 32X (1/

Cr Ni N Ti A1 Si C Fe_

21 31 3.0 0.4 0.4 0.6 0.09 bal

.

Even with this high nickel content it is reported to have good resistance

to the following high temperature sulfurous atmospheres

1. Oxidizing atmosphere containing SO up to approx. 900 °C

(1650 °F)

2

2. Reducing atmosphere containing H S up to approx. 650 °C

(1200 °F)

2

The price of Sanicro 32X is reported to be just double the price of 304 SS or about 40% higher than 310 SS.

Piping to carry abrasive material

In the previous quarterly report mention was made of composite tubing

products consisting of various hard facing compositions centrifugally cast

onto the inside of SAE 4140 low alloy steel tubing by Xaloy, Inc. (2) These

linings apparently have superior wear and abrasion resistance but they are expensive. Some price and property data were obtained this quarter. The

mechanical properties of one of the composite materials,

HiChrome, HC250

Cr Si Mn Mo V C Fe

27 1.0 1.0 0.5 0.3 2.7 bal.

are given in Table 1.

It is claimed that this material as a lining on 4140 steel exhibits

twice the life, on the average, of the Stellite No. 6 hard facing alloy

which Xaloy applies to the feed screw rotating inside of it when one is

used.
104

The price for a length of 8" I.D. x 9" 0.D 4140 steel tubing

8*4" long lined with 1/16" layer of "HiChrome 250", and the surface

honed smooth and round, is $5,027. The prices for 4140 lined with their

other hard facing coatings range from $4,300 to $7,900 for the same size

pipe. The price decreases proportionally with size, so the same length

of 4" I.D. pipe lined with the HiChrome 250 would be about $2,500.

Ref erences

1. Sandvik, Inc., Scranton Works, P.0. Box 1220, Scranton, PA 18501.

2. Xaloy, Inc., 3 Terminal Road, New Brunswick, N.J.

Physical Properties

Density: 7.35 gms. /cu.cm. (RT)

0.268 Ibs./cu.in. (RT)

Melting Point: 12803C (2336C F) Thermal Expansion Coefficient:

70-1500 3 F 8.1 x 10*4 in/in/°F

1700-2000 3 F 14.3 x 10*° in/in/°F

( 2 ) Table 1. Properties of HiChrome HC25CP

Typical Mechanical Properties in Heat

Treated Condition

Hardness-Rockwell C

Ultimate Tensile Strength, psi

Yield Strength, psi

Elongation -%

Elastic Modulus, psi x 10 4 Average Compression

Properties:

Yield .05% offset

Range 58-64

70,000-95,000

Near UTS

0- 1.0 32-35

145.000 psi

400.000 psi

1 . 2%

2.5%

Ultimate

Elastic Deformation

Plastic Deformation
STRESS, N/mm2

1000 1050 1100 1150 1200

TEST TEMPERATURE, °C

Fig* 12. Oxidation in air of SANDVI.K

253MA, Alloy 800H and TP 310 resulting

from cyclic exposure for 5 x 24 h in the

temperature range 1 000-1 200°C
Stress, N/mmJ (ksi)

Time to rupture, h Fig. 13. Creep-rupture strength of SANDVIK

Sanicro 32X^
107

TASK M. DATA CENTER FOR MHD MATERIALS PROPERTIES (H. M. Ondik , A. Perloff

,

and J. R. Cuthill)

Progress : A decision has been made as to the computer Data Base Management

System (DBMS) to be used. The system of choice has the necessary system

flexibility, an appropriate data storage structure including an inverted

file system, an ad hoc query language necessary for ready searching, and

suitably large storage capacity for the projected contents of the data bank.

The DBMS is available from several GSA-approved commercial vendors.

Investigation and comparison of the services provided by these vendors is in

progress. Arrangements have been made for short-term (the remainder of the

fiscal year) use of the chosen DBMS through the services of two different

vendors. Benchmark testing will be performed by building a small portion

of the Data Base, storing the data on the computer, manipulating it, and

searching it. These operations will test the system controls provided by

the two vendors , and also compare the costs incurred during use of the two

vendors' systems. The appropriate documents are being prepared for the

justifications to obtain the various approvals required for contracting

with a vendor.

Members of the staff have taken a one week course on the chosen DBMS,

learning how data should be structured to make the best use of the capabilities

of the system. Since each DBMS has its own special features, and complicated

restrictions, it is necessary to have detailed instruction in its use. The

chosen system, because of its large capability and flexibility, especially

requires this special study and instruction so that it may be used efficiently.

Inefficient use can drastically affect the computer expenditures.

The design features of the data bank are being planned. The arrangement of

the data must take into account the varied types of information to be stored

and the estimated order and frequency of requests for information. The

optimum structure of the file for ready search capability must provide for

the use of the fastest, therefore least expensive, search patterns compatible

with flexibility, having as many items being searchable as possible for

reasonable use. Major search elements will, of course, include the material

names or designations and the properties. Searches must be possible on various combinations of these elements. The conditions under which the

properties were studied must be retrievable along with the property data

and should probably also be a searchable item as well. Many other items

which must be appropriately entered for retrieval with the property data

such as brief summaries of the test procedures, references, etc., must be

considered carefully as. possible candidates for keying as search categories.

Since search items are carried in storage as inverted files and, therefore,

add to computer storage costs, the question of optimum use of the DBMS and

its storage and query capabilities is an important one to answer carefully.

The twc computer terminals and the two acoustic couplers which have been

ordered have been delivered, but the telephone lines necessary for communication with a central computer have not yet been installed. Even though the

terminals are not connected to a large computer, there are useful operations

which may be performed, providing experience with the equipment and aiding

in the data design.
108

More equipment for the computer-terminal facility is now on order. A

hard-copy unit, compatible with the "intelligent" terminal having, a cathoderay display tube, will enable the operator to obtain a high-quality copy of

material from the cathode-ray tube. Copies of graphs produced by the terminal

and displayed on the tube will be available very simply with this copier.

The cataloging and review of contractors' reports is continuing.

Plans : The usual operations of the Center, review and cataloging of

contractors' reports, and listing of materials data contained in them will

continue. Communication and interaction with the MHD community will increase

to provide the Center staff with users' needs. Design of the Data Base will

move ahead.