EA-77-A-01-6010-9

Distribution Category UC-90g

DEVELOPMENT, TESTING AND EVALUATION OF

MHD-MATERIALS

Quarterly Report

for the period July - September 1977

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

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

Date Published: September 30, 1977

PREPARED FOR THE UNITED STATES

ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION

Under Contract No. E(49-1)-3800

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

States Government. Neither the United States nor the United States ERDA, nor any of their employees, nor any of their contractors, subcontractors,

or their employees, make any warranty, express or implied, or assumes

any legal liability or responsibility for the accuracy, completeness, or usefulness of any Information, apparatus, product or process disclosed,

or represents that its use would not Infringe private owned rights."

TABLE OF CONTENTS

PAGE

ABSTRACT 1

OBJECTIVES AND SCOPE OF WORK 2

SUMMARY OF ACHIEVEMENTS 3

TALKS AND PUBLICATIONS 4

TASK G. PROGRAM MANAGEMENT AND COORDINATION 5

1.

Program Review and Consultation Activities

TASK I. OPERATIONAL DESIGN PROPERTIES 7

1. Viscosity of Coal Slags

2. Electrical Conductivity

3. Vaporization of K^SO^ and xK^O-yFe^O^

TASK J. CORROSION AND DIFFUSION 31

1. Seed-slag Interactions

a. Thermochemistry of the ^O-CaO-Al^O^-SiO^ System

2. Diffusion of Fe in MAFF-MgO Couples

3. Phases of New Electrode Materials

TASK K. MATERIALS TESTING AND CHARACTERIZATION 43

1. X-ray Diffraction of MHD Materials

2. Electrode Design and Testing

3. Characterization of Electrode and Preheater Materials

TASK L. ASSESSMENT OF STEAM PLANT COMPONENTS 69

1

ABSTRACT

During this quarter preparations went forward for the testing of an electrode/insulator assembly in the U-02 facility under auspices of the

US-USSR cooperative program on MHD power generation (Phase III) . Samples

of various electrode materials to be used in this test were received from

the Westinghouse Research Laboratories and from other organizations for

determination of their electrical conductivity.

Some 60 materials were analyzed by X-ray diffraction with regard to

crystallinity and phase composition.

Further progress was made on the seed-slag interaction problem by

mapping major sections of the phase diagram for the system K

2 0-Ca0-Al

2 0^-Si02

.

A set of 11 cathodes and anodes was delivered to AVCO and tested in

t.heir Mark VI MHD channel under slagging conditions on September 14, 1977.

Characterization of six electrodes tested in the MIT rig in May of

this year was completed. It appeared that the HfC^- and ZrC^-based caps

had separated completely from the underlying spinel on the cathode side.

Samples of chrome-magnesia, tested for 30 hrs. in a seed environment, were received from the Fluidyne Engineering Corporation, and examined by

means of X-ray diffraction and Scanning Electron Microscopy.

A list was compiled of references to property data for some 20 stainless

steels and alloys which show promise for use in downstream components of

MHD systems.

2

Objectives and Scope of Work

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

definition of high temperature materials which have shown promise for use

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

investigations will be concentrated are:

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

and performance at prototype temperatures.

2. Development of electrode materials which provide adequate performance

over extended periods of time.

3. Insulating materials which limit thermal losses and are resistant

to prolonged thermal and erosion effects.

4. Preheater materials which can withstand the operating modes of

separately and directly fired operation.

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

feasible

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

action of combination on system components and materials.

7. Durability of prototype MHD sub-systems.

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

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

The activities are grouped under six tasks:

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

and review of the various MHD-Materials Development Programs)

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

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

I. Operational Design Properties (viscosity, electrical conductivity,

vaporization)

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

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

post-test analysis)

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

and alloys)

3

Summary of Accomplishments (July-Sept. 1977)

(Completed Milestones - see Work Statement for FY 1977)

The results obtained during the 4th Quarter are presented in terms of

the projected milestones of the NBS-ERDA Contract.

lc. Reviewed together with participating groups the status of preparations

made by the Westinghouse staff for the Phase III, U-02 materials test planned

for early 1978.

2a. Participated in a number of program coordination meetings and technical

conferences including the ERDA Workshop on Materials (Germantown, MD, 7/13/77).

b. S. J. Schneider and H. P. R. Frederikse attended as observers a 12-hour

test of electrode/insulator assemblies in the Mark VI at the AVCO Everett

Research Laboratories.

5b. Determined the effects of CaO and MgO additions and deletions on the

viscosity of coal slag.

19b. Measured the electrical conductivity of a number of electrode materials

(prepared by Westinghouse, G.E. and ANL) for the Phase III-U-02 test.

39c. Analyzed by means of X-ray diffraction and Scanning Electron Microscopy

several electrode and preheater materials that were tested in various

test rigs (MIT, Fluidyne)

.

43. Updated a list of references to properties of promising wrought alloys

(stainless steels, Ni- and Co-based alloys) , including cost.

4

Talks and Publications

MHD-Mate rials

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

Proc. of the Colloquium on Refractory Oxides for High Temperature

Energy Sources, Odeillo, France, July 1977.

Spinels as MHD-Generator Wall Materials

W. R. Hosier

American Ceramic Society (Electronics Division)

Montreal, Quebec, Canada, September 1977.

Preparation and Properties of CeO -HfO Electrodes for MHD Applications

J. W. Hafstrom and J. T. Durek (ANL) , J. L. Bates (Battelle NW) , and

T. Negas (NBS

American Ceramic Society (Basic Science and Nuclear Division)

Hyannis, Mass., September 1977.

5

TASK G. PROGRAM MANAGEMENT AND COORDINATION (S. J. Schneider)

1

Program Review and Consultation Activities

S. J. Schneider participated in assorted ERDA-arranged or sanctioned

program review/coordination meetings, briefings and technical conferences.

These included:

1. Alcoa Laboratories, Pittsburgh, July 18-19, 1977 (Lecture plus

consultation on MHD materials applications)

2. Colloquium on Refractories for Energy, Odeillo, France,

June 30 - July 3, 1977. (Invited lecture on MHD Materials.)

3. Materials Ad Hoc Working Group Meeting, Westinghouse , Pittsburgh,

July 14, 1977 (U-02 Phase III).

4. ERDA Workshop on Materials, Germantown, July 13, 1977. (Organized

by Division of Physical Research, ERDA.)

In addition, NBS hosted a number of visitors from MED contractors

(and others interested in MEED) to consult on a variety of specific topics.

During this quarter, visitors included representatives frcm Stanford, MET,

Montana TECH, MSU, CEGB (England) , and Petten Establishment (The Netherlands)

The conclusions and recommendations resulting from these meetings and other

EIRDA-assigned activities are reflected in reports to ERDA, through direct

consultation with EI3DA staff or through documents published elsewhere.

NBS receives on a regular basis, MEED technical reports and proposals

for review. During this period NBS provided 1 such evaluation.

Part of the NBS experimental program involves characterization of

materials component elements tested in various ET© test rigs and facilities

throughout the United States (and USSR) . In order to expedite the dessimination of this information, NBS has instituted a separate reporting system in

addition to the regular Quarterly submissions. The system consists of the

following elements: 1. Telephone report to submittor after initial examination,

2. Follow-up letter report, and 3. Detailed report (if warranted) to submittor

and ERDA. The detailed report will be distributed to general contractors on

an as-needed-basis as determined by EEOA; however, highlights of the detailed

report will be abstracted and published in the NBS Quarterly reports. In

addition, general overall summary reports will be compiled from time to time

and published in appropriate journals and special editions.

Inquiries regarding the status of any particular characterization or requests for additional assistance should be directed to:

A. Perloff , 301-921-2900

alternates

T. Negas, 301-921-2843

W. Hosier, 301-921-2940
.
6

During this period, a detailed technical review of the Phase III, U-02

activity was held at the Westinghouse Research Laboratory. The overall effort

is being managed by Westinghouse; other participating groups include MBS,

Battelle-NW, ANL and GE. The minutes and recommendations resulting from, this

meeting are on file at ERDA. As a follow-up, NBS requested a complete review

of the anticipated electrode loading scheme for both the proof tests and the

actual U-02 experiment; this has been done (by Westinghouse) and the current

thinking will be incorporated in appropriate work and test plans.

In addition to the above, the final draft of the Joint US-USSR chapter

on materials (Status Report) has been proof-read and final corrections

inserted. Publication is expected during the next quarter.

7

Task I. OPERATIONAL DESIGN PROPERTIES

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

Introduction

As the number of laboratories investigating various aspects of coal-fired,

open-cycle MHD has increased so has the tendency to experiment with two specific

sources of coal. These are Montana Rosebud Seam Coal, typical of western

sub-bituminous coals and Illinois # 6 , typical of mid-western sub-bituminous

coals

.

The NBS program, therefore, has concentrated recently on these two types

of coal for its slag experiments.

Slag Preparation and Viscosity Measurements

A series of synthetic slags was prepared in order to assess the contribution of each oxide component to the viscosity of the slag. A composition

for the starting point or the "base" composition was chosen that is between

the Rosebud and the Illinois #6 slags. Each of these types varies over a wide range; a large number of ash analyses from each coal type was averaged.

The NBS "base" composition was the average of these two averages. This means that the Si0

2 content of the base was averaged, the A^O^ content

was averaged, etc.

To this base composition SiC> 2 was added in 5% increments or removed

in 5% increments keeping the other oxides in constant proportion. Next

AI

2 O3

, Fe

2°3

' Ca0 and Mg0 were altered incrementally. The report for the

period ending on December 31, 1976, showed the results of the Si0

2

, A^O^ and

Fe

2 0 _ changes.

This report gives the results of the changes of CaO and MgO on viscosity.

Figure 1 shows the effect of the CaO and Figure 2 the effect of the

MgO variations.

Table 1 shows the compositions of these melts.

8

Two observations can be made at this time. CaO has a strong fluxing

action, reducing the viscosity markedly but not quite as much as the cor- responding wt % additions of Fe^O^. MgO is the strongest agent in reducing

viscosity on a wt % basis but is usually present in smaller amounts than

the other oxides.

The data is being analyzed in order to select that mathematical model

which will best describe the viscosity of slags from these two coals as functions of ash composition and melt temperature.

Future Plans

A number of methods will be compared to select the best method for

estimation of coal slag viscosity.

9

Table 1

Coal Slag Compositions (wt %)

NBS

Melt No. sio

2 A1 2°3 FS 2°3 CaO MgO

K-884 47.77 21.76 16.58 10.57 3.32 Base

K-960 45.10 20.54 15.65 15.57 3.13 + 5% CaO

K-965 42.43 19.33 14.73 20.57 2.95 +10% CaO

K-969 50.44 22.97 17.51 5.57 3.50 - 5% CaO

K-971 53.42 24.33 18.54 — 3.71 -10.57% CaO

K-973 39.76 18.11 13.80 25.57 2.76 +15% CaO

K-976 45.30 20.63 15.72 10.02 8.32 +5% MgO

K-980 42.82 19.51 14.87 9.48 13.32 +10% MgO

K-985 49.41 22.51 17.15 10.93 — - 3.32% MgO

10

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EFFECT

OF

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ADDITIONS

11

c\i

cr> iZ

O

1200

1300

1400

1500

1600

1700

TEMPERATURE

(C)

EFFECT

OF

Mg

0

ADDITIONS

12

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

K 0 • 7 [Fe n Ti ._0_]. Several samples of potassium ferrites were made

2. 1 . 9b .Ob 3 but T. Negas of NBS . During the conductivity measurements, drifting of voltages

and currents was a problem and an attempt to reproduce the data obtained was not successful. Work is underway to construct a test cell in which we can measure the conductivity of this and other materials under different pressures

of potassium.

SrZrO^. A sample of SrZrO^ designated SZ-UMO101 was received from

Westinghouse Research and Development Laboratory, Pittsburgh, PA. This was

a high purity sintered piece mentioned in our previous Quarterly Report

(April- June 1977) . The electrical conductivity of this material (see Fig. 3)

appears to be higher than desired at elevated temperatures for it to be used

as an effective insulator for MHD electrodes.

An attempt was made to lower the electrical conductivity of SrZrC>3 by

substituting small amounts of La for Sr in a 2 to 3 ratio. A sample of

Sr g-yLa

q

g ZrO 3 was made by H. U. Anderson of A T Research and its electrical

conductivity was measured (see Fig. 4). By comparing Figures 3 and 4, it can

be seen that the electrical conductivity was indeed lowered but not sufficiently to make the material viable as an MHD insulator.

Four samples have been received from H. U. Anderson of A T Research in

which various amounts of La and Cr were substituted into SrZrOg. Two of

these samples have been measured (see Figs. 5 and 6 ) and the effect of the

substitutions ia a slight increase in electrical conductivity at high

temperatures with a substantially greater increase in conductivity at lower

temperatures. Measurements are in progress on the two remaining samples

(Sr La Zr Cr 0 and Sr La Zr Cr 0 ) and will be reported in our

t 1/ • X • _/ ® _L • _L •/ • J J next Quarterly Report.

85 m/o ZrO^ - 12 m/o CeO^ - 3 m/o . Two samples of composition

85 m/o ZrC>2 - 12 m/o CeC^ - 3 m/o Y

2 O 3 were received from Westinghouse

Research and Development Laboratory, Pittsburgh, PA. The sample designated

CZ-CE0101 was prepared by sintering and was found to have a lower electrical

conductivity (see Fig. 7) than did sample CZ-CE0201 (see Fig. 8 ) which had

been hot-pressed.

LaCrO^(:Mg) 4- [ZrO^ (12% Y^O^)]. Tw° samP-*- es °f lanthanum chromite plus

yttria stabilized zirconia were received from Westinghouse Research and

Development Laboratory, Pittsburgh, PA. The zirconia is interdispersed in

the LaCrC>3 and is intended to make the material more refractory and wear

resistant in an MHD environment and yet to maintain the electrical conductivity to low temperatures where attachment to a metal lead-out is simplified.

Figure 9 shows the measurement on this material with 30% yttria stabilized

ZrC>2 added. The electrical conductivity is reduced by about a factor of four

from that of La^gjMg Q 5 Cr03 over the temperature range measured. The data

for La^g^Mg Q^CrOg is shown in Figure 8 of Quarterly Report - July-Sept. , 1976.

The electrical conductivity of this material will be measured down to room temperature at a later date.

13

Figure 10 shows the electrical conductivity of LaCr03 containing 40 w/o

of yttria stabilized ZrC^ down to about 550 °C. The data down to near room temperature is shown in Figure 11. It is evident that the conductivity is

substantially reduced from that of La_ gsMg_ 05 CrO2 and any lead-out connection

should be made to operate at 300 °C or above for the material.

Layered Electrodes . Two separate samples composed of 3 different layers

each but formed into one composite were received from Westinghouse Research

and Development Laboratory, Pittsburgh, PA. In each case the top layer was

La QC Mg Cr cnAl cnO_, the middle layer La _ c Mg A[_Cr , 0A1 _ o 0_ and the lower

.95 .05 .50 .50 3 .95 .05 .68 .32 3 layer La Mg Cr Al 0 . One of the composite samples was hot pressed at

• y Id • U o • o o • 1. d j

the Westinghouse Laboratory while the other was prepared by sintering by

H. U. Anderson of A T Research. These layers were separated by cutting with

a diamond saw for the electrical measurements shown in Figures 12 and 13.

In all cases but one, the effect of the Al additions was a reduction in

electrical conductivity. The hot-pressed sample, in all cases but one, had

a higher electrical conductivity than the sintered sample. At the request

of Barry Rossing of Westinghouse the sample of La AC Mg n .Cr _ aA1 ,-.0. (hot-

.95 .05 .50 . 50 3 pressed) was heated in air at 1600 °C for 16 hours and at 800 °C for 80 hours

to assure complete oxidation. No changes in electrical conductivity was seen after this treatment.

Spinel . A sample of MgAl2 04 + Fe 3 C>4 spinel was received from General

Electric. The precise composition is not known. The electrical conductivity

of this sample (see Fig. 14) was slightly lower but very similar to MAFF-31

(see Fig. 1 in April - June, 1976 Quarterly Report).

Hf ( : CeO^

)

• Three samples of HfO^ (:CeC>

2

) and HfO^^CeO^) plus Y

2 °3 were received from Argonne National Laboratory. A sample of composition

82 m/o Hf02 - 18 m/o Ce02 (see Fig. 15) was measured electrically with some swelling, deformation, and void formation. When attempts were made to

measure the electrical conductivity of a sample of composition 74 m/o Hf02 - 16.3 m/o CeO^ “ 9.7 m/o 9ross swelling, deformation and void formation

occurred.

It is a possibility that these effects were due to the formation of a low melting eutectic when the sample came in contact with the Al 2 02 conductivity apparatus. Both samples were stuck to the Al 2 02 but the sample of

composition 74 m/o Hf02 - 16.3 m/o Ce02 - 9.7 m/o Y202 was too firmly

attached to be removed. Further investigation of this reaction and further

attempts to measure the electrical conductivities of these samples are in

progress

.

Scanning Electron Microscope Examination of Arc-Plasma Sprayed Materials

(A. J. Armstrong)

Seven samples of arc-plasma sprayed materials were received from APS

Materials, Inc., Dayton, OH. These samples were prepared during the

investigation of optimizing spraying parameters to obtain sprayed materials

of highest density single phase compositions in conjunction with the

Phase III-US-USSR materials test. Three of the samples were of MAFF-31,

14

three more were of LaCrO., + MgO and the last was of S-71 (pure MgAl^O^) . These samples were sectioned and polished for SEM evaluation by the“Solid

State Physics Section of NBS . Samples were designated as 71107-1, 71107-2,

etc. and will be referred to as sample 1 , 2 , etc.

Samples 1, 2 and 3 were of MAFF-31. The EDX patterns of all three

samples were compared and found to be identical indicating chemical

uniformity. The EDX patterns were not compared to any standard so they

should be considered indicative only. An examination of the physical

bodies was done at 100X and 500X. Sample #3 was seen to have the lowest

total porosity with sample #1 second, and sample #2 being highest in total

porosity. Sample #1 appears to be the most homogeneous from the standpoint

of pore size and pore distribution with samples #2 and #3 both showing

regions of high density interdispersed with larger pores. In my opinion,

sample #1 is the optimum material.

Samples 4, 5 and 6 were of LaCrC^ + MgO. The EDX patterns of all

three samples were examined and small amounts of Al were found in samples

#5 and #6 . The amounts of La, Cr, and Mg appear to be the same in all

three samples. A much greater difference can be seen between these samples

than was seen between the samples of MAFF-31. Sample #4 has a higher density

than sample #6 which has a higher density than sample #5. All three samples

appear to be of similar homogeneity. The optimum choice here is the procedure resulting in sample #4.

Sample 7 was of S-71 (MgA^O^ . Although light and dark areas may be seen on the photomicrograph, an EDX examination of both areas showed no elemental differences between them. The concentration of Mg and Al were seen

to be the same for both areas. The physical body is quite dense, despite its

mottled appearance, and has a high degree of homogeneity.

Comparing all the samples with each other leads to the conclusion that

sample #7 (S-71) has the lowest porosity with samples #4, 5 and 6 (LaCrO^+MgO)

a close second, and samples #1, 2 and 3 (MAFF-31) a distant third.

X-ray diffraction results on these seven materials were obtained from

C. L. McDaniel of NBS. Samples 1, 2 and 3 were found to be poorly crystalline

spinel. Samples 4, 5 and 6 were found to be LaCrC>3 of medium to good crystallinity with from 1 to 5% of an unknown phase present. The unknown phase,

although unidentified, does not appear to be Al, AI2O3 of MgO. Sample 7 was

found to be MgAl^O^ of medium to good crystallinity.

CONDOCTIWYfohm-cm)'

15

IQ

4

TfK)

Figure 3. Electrical conductivity of a sample of SrZrO^ from Westinghouse

(SZ-UMOlOl) sintered four hours at 1650 °C in air.

Jiuo-iuip)

All

AllOnQNOO

Electrical conductivity of Sr ' 9r~n ^ . • _ .97 .02 3 as ' function of temperature at

ITAlA'i • T.

1 preEsures of °=yg=n. Sample prepared by H. Anderson Research. Measurements made at NBS.

'

l

vuuv^

i

TV

t

r

r

omr

17

TOO.

Figure 5. Electrical conductivity of 0.9 (SrZrO_ ) + 0.1 (La Sr CrO ) as a function

3 .84 .16 3

of temperature at several partial pressures of oxygen. Sample prepared by

H. Anderson, AT Research. Measurements made at NBS.

CONDUCTIVITY(ohm-cm)'

,1 18

10

'

10r2

'

T(K)

Figure 8 . Electrical conductivity of Sr ^La ^Zr ^Cr 2^2 aS a ^unc^::'- on temperature at several partial pressures of oxygen. Sample

prepared at A T Research. No details on preparation were given.

TTTTT

19

TOO

Figure 7.

Electrical conductivity of sintered 85Zr02 _ 12Ce02-3Y203 (mole percent) as a function of. temperature at several partial pressures of oxygen. Sample

CZ--CE0101 from Westinghouse . Measurements made at NBS

(uuo-iitp)

All

ALLODQNOO

20

T(K)

Figure 8. Electrical conductivity of 85 m/o Zr0

2 12 m/o Ce0

2

3 m/o Y

2 0

3

as a function of temperature at several partial pressures of oxygen.

^ This

sample (CZ-CE0201) was made at Westinghouse and hot pressed at 1620 C

and 4000 psi.

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Fig.

lo.

Electrical

conductivity

of

La^Mg

Q5

Cr03

‘with

40

w/o

Zr02

-12

Y

2

03

added.

Westinghouse

sample

LC-GR0603,

hot

pressed

at

3000

psi

and

1620

°C.

Measurements

Fig. 11. Electrical conductivity of La. 9 5 Mg^ Q5 Cr0

3 with 40 w/o Zr02 -12Y2 03 added.

Westinghouse sample LC-GR0603, hot pressed at 3000 psi and 1620 °C.

Measurements made at NBS The dashed line is data taken after the sample

_ O had been heated a second time to 1600 °C in 10 CL in N

2

'

CONDUCTIVITY

(chm-cmj

24

T(K)

Figure 12. Electrical conductivity of hot pressed La

alumina.

„ c Mg CrO with added

yb .Ob J

La

.95Mg.05

Cr

.85

A1 .15°3

2 ' La

.95

M9

.05

Cr

.68

A1 .32°3

3 - La

.95Mg.05

Cr

.50

A1 .50°3

The samples were made at Westinghouse (pressed at 2000 psi and 1620 °C)

Measurements made at NBS.

CONDUCTIVITY

(chm-cmr

25

1

i

TOO

Figure 13. Electrical conductivity of sintered La

alumina. ,95

Mg

.05

CrO

3 with added

1 .

2 .

3.

La Cr Al 0

.95 .05 .85 .153

La Mg Cr Al O

.95 .05 .68 .32 3 La

.95

M9

.05

Cr

.50

A1 .50°3

This sample was made by H. U. Anderson, A T Research. Measurements were made at NBS

26

<Dy

<T' •H

fa

Electrical

conductivity

of

General

Electric

magnesium

alurainate

ferrous

ferri

Sample

prepared

by

G.E.

Measurements

made

at

NBS.

ma aaaaa

Fig. 15. Electrical conductivity of 82Hf0

2 - 18ce0

2 (m/o) . Sample T-l

hot pressed at 3500 psi and 1290 °C in argon. Sample made at

ANL and measured at NBS.

28

3. Vaporization Studies (E. R. Plante)

This section describes measurements of the potassium gas pressure

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

importance in predicting the interaction of a seeded MHD plasma with

construction materials or coal ash using equilibrium thermodynamics.

During this quarter measurements were completed on K SO (s) using

the Mettler Microbalance System (in collaboration with C. Olson and

T. Negas) . Vaporization measurements were also made using the mass

spectrometric technique on "KFe0

2

" and several compositions in the

K

2 0-A£

20

2 -Si0

2 system.

K^SO^ Measurements

In the last quarterly report (1) details of results of the vaporization

of K-SO^ were reported. During this quarter additional measurements

on tnis system were completed. The purpose of this work was to confirm

our previous measurements and to extend the temperature range over which

data had been obtained. To accomplish this, measurements were made

using a Knudsen cell having a 1mm diameter orifice in contrast to the

l/2mm diameter orifice used in the previous work. Other things being

equal, this will increase the rate of mass loss from the effusion cell

by a factor of 4, thus increasing the pressure range over which measurements can be made.

Analysis of the new data shows that it agrees very well with the

results obtained previously. A summary of the second law heat of sublimation, third law heat of sublimation and entropy of sublimation at 298K

using the l/2mm and 1mm diameter effusion orifice is listed in table 2.

TABLE 2

Heats and Entropy of Sublimation of K^SO^Cs)

1mm effusion orifice l/2mm effusion orifice

AH 0 s (2nd) (cal/mol) 80150 + 930 81140 + 1960

AH 0 s (3rd) (ca 1/mol) 81750 + 40 81780 + 60

AS 0 s (2nd) (gibbs/mol) 46.60 + .90 45.82 + 1.55

AS 0 s (3rd) (gibbs/mol) 45.313 45.313

The calculated third law heat of sublimation is from the work of

Gurvich et at (2). Uncertainties are standard errors. This confirms

our previous results which showed that the generally accepted saturation

pressure of K

2 S0

4 reported by Halstead (3) is too high by a factor of

29

"KFeO^" Measurements

These measurements were undertaken because iron is an important slag

phase constituent as well as a possible electrode dopant. A sample of

KFeO„ provided by C. McDaniel was studied by mass spectrometry. The

sample was contained in a Pt effusion cell having a .0135" diameter orifice

and measurements of the ^K+ and 3 2q^+ -j_ on currents were made in the

temperature range 900 - 1300° C. In air (Pq

2 = *20 the ^O-Fe^O

system is reasonably well characterized as a binary system up to about

1500° C but under the conditions of our experiment the C> 2 pressure

generated by the sample may be sufficient to cause changes in the sample

composition. One can imagine the vaporization reaction to take place

stepwise so that phases analogous to 3 alumina could be formed according

to the following reaction sequence (compositions approximate)

7 K

2 0 • Fe20

3 = 2K(g) + l/20 2

(g) + 6/7 K

20 • 7 Fe^ (1),

K

20 • 7Fe

20

3 = 2K(g) + l/2o

2 (g) + 7Fe

203 (s) (2),

3Fe

203 (s) = 2Fe

3 0

4

(s) + l/20

2

(g) (3).

This reaction sequence will take place only if the 0

2 pressure is

sufficiently high to stabilize subsequent phases formed, for example,

K20 * 7Fe

20

3 and Fe„0

3

. X-ray analysis of the residue from the vaporization

experiment showed the presence of a small amount of KFeC>

2 and primarily

the 3" phase; a slightly reduced phase having the approximate composition

K20 * 7Fe

2 0

3

- This data indicates that reaction (1) is the primary

vaporization process. However, the mass spectrometric measurements were

not entirely in accord with this interpretation because the oxygen

pressure estimated from the measured I +

32 was about twice the potassium

pressure. This would not be possible if the vaporization process is

represented by reaction (1) . It appears possible that the oxygen pressure

may have been increased because of solution of Fe in the Pt cell from the

KFeC>

2 or K

2 0 • 7Fe

2

<9 3 phases.

Because of this problem, the mass spectrometer calibration factor is

uncertain by a factor of 3. Accepting the lower limit as the calibration

factor, the calculated K pressures vary roughly from 10 times to twice

as high as the K pressures over KA£C>

2 at temperatures from 925 to 1300° C

respectively.

These results are consistent with the idea that K

2

<D interacts less

strongly with Fe C> 3 than with A^

2°3 or Si0

2 even though the K pressures

may have been reduced because of excessive oxygen pressures.

K

2 0-A&

2 03 -Si0

2 Measurements

These measurements are still in progress and will be reported on

in more detail in forthcoming reports.

Future Work

During the next quarter measurements on the K pressure over K

20-Si0

2 ~

30

A& 2 O3 solutions will be continued. Measurements on KFeO^ in which the sample

is not in contact with Pt will be attempted.

References

1. H. P. R. Frederik.se, T. Negas, and S. J. Schneider, NBS Quarterly Report,

June 1977.

2. L. V. Gurvich, 0. V. Dorofeeva, and V. S. Yungman, Proceedings of the

Conference on High Temperature Science Related to Open-Cycle, Coal-Fired

MHD Systems, ANL-77-21 (1977).

3. W. D. Hafstead, Trans. Farad. Soc. , 66, 1966 (1970).

Summary

Vapor pressures of K^SO^ (c) using a 1 mm effusion orifice agreed with

previous NBS measurements showing that the heretofore accepted measurements

of Halstead are high by a factor of 5. Measurements of the K pressure over

KFeO^ are consistent with reduced stability of KFeO^ as compared to KA^O^.

TASK J. Corrosion and Diffusion 31

1. Seed-Slag Interactions

a. Thermochemistry of the System CaO-K^O-Al^O^-SiO^ (L. P. Cook)

In the report for the preceding quarter, a working model for

equilibrium subsolidus relations in the (Ca, Al , Si) -rich part of the

seed/slag model system was given, based upon experimental data available at that time. Since then a series of long-duration crystallization experiments on seven synthetic calcium - containing slags

has been completed (Table 1) . These experiments were conducted as follows: roughly equal parts of well-characterized crystalline phases

were weighed according to the proposed model and thoroughly mixed.

These were then melted at 1610°C, quenched and subjected to 3 days,

1200°C crystallizations, followed by grinding, x-ray, and a two-week,

1200°C crystallization. Final products are shown in Table 1. On

this basis, it appears that the equilibrium relations proposed are verified for the four-phase volumes: Ca

2 Al„Si0^-KAlSi2 06

-KAlSi0^-

CaAl si-Og , Ca

2 Al

2 Si0?-KAlSi

2 06

-KAlSi0 -CaSiOg, Ca

2 Al

2 Si0

7

~CaSi0 - Ca

2 Al

2 Si 0g-KAlSi

206

, and Al

2 0g-CaAl2 Si2 0 -KAlSiC^-KAlS^Og at 1200°C

(Figure I) . This temperature is not far from the temperature of

minimum melting for these volumes and the use of these assemblages

in calculating potassium pressures is warranted. Owing to the difficulty

in crystallizing the incongruently-melting compounds CaAl^o^g and

3Al

2 03 * 2 Si02 complete reversibility of melting experiments was not

obtained at 1200°C for the four-phase volumes CaAl^

2 0

4 „-Ca Al2 S:'-2 08 _A^20 3 _ KAlSiO , CaAl

12

0

ig-Ca

2Al

2 Si07 -CaAl2 Si2 0g-KAlSi0

4

, and3Al

2 0 -2SiC>

2 - Al 0

2 -CaAl

2 Si

2 0g-KAlSi2 0g. However no reactions occurred which would

indicate disequilibrium within these four phase volumes as proposed.

Additional slag crystallization experiments , at higher temperatures

,

are planned for definitive verification of assemblages.

As noted in the previous quarterly report, thermochemical calculations gave log (P^*Pq 1/2) vs T curves for invariant melts from

four-phase volumes. Thesl curves plotted in relative positions at

variance with those predicted from equilibrium topologies. Preliminary

calculations indicate that approximately 20 mole % substitution of

KCaAlSi C> 7 in Ca

2 Al2 SiC>

7 (see reactions 2-b, 3-b, 4-b, 7-b in preceding

quarterly report, Table 1, p. 43) would alter the positions of these

curves so that they fall nearly in the order predicted, within the

uncertainties of the thermochemical data. The first step in the experimental approach to this problem is determination of the solubility of

K O in gehlenite and relation of this to its x-ray properties. Unfortunately

the results of Nurse and Midgeley [1] have proved impossible to duplicate.

A similar problem has been encountered by Yoder [2] in his work on sodium solubility in Ca

2 Al2 Si0

7

. There are indications that low

temperature decomposition may be the explanation, and further experimentation must utilize high-temperature x-ray apparatus. This has caused

an unexpected, though hopefully not serious, delay in the completion of

milestone 17-a.

32

Additional experiments have been conducted in the system to determine

the compatibility of K^CaSiO^ with KA10 -SiO^ compounds (Table 2) . This

compound (mp ^1610°) has been postulated as structurally related to KA10

2 [3]

,

possible only if a relationship of the form CaSi X A1A1 holds; this

is not likely as it would require Ca in tetrahedral coordination. X-ray

analysis of K^CaSiO /KA10

2 mixtures equilibrated at 1200°C for two weeks

suggests intermediate phases. On the other hand, as data in Table 2 show, K^CaSiO^ and KAIS^O^. react extensively to form melt at 1200°C.

K CaSiO^ and KAlSiO react in an undetermined way with extensive formation

of tetragonal K, , Al, , Si, 0 -type solid solution.

1+x 1+x 1-x 4

J

Various substitutional possibilities involving calcium will be further

investigated under Milestone 21.

References

1. R.W. Nurse and H.G. Midgley, 1953, J. Iron and Steel Inst. 174 , 121-131. ““

2. H.S. Yoder, 1973, Fortschr. Miner. 50_, 140-173.

3. H. Hughes, 1966, Trans. Brit. Ceram. Soc . 65, 661-679.

33

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35

CaO

fil2°3

Figure 1. Four-phase equilibrium volumes in the system CaO-K O-Al 0 -SiO , as verified by synthetic slag crystallization experimen^s^at ^ 1200°C . Abbreviations as in Table 1.

36

Thermochemistry of the System MgO-K^O-Al^O^-SiO^ (L. P. Cook)

A series of long duration crystallization experiments on four

synthetic magnesium-containing slags has been completed (Table 3)

These experiments were conducted in a manner analogous to those con- ducted in the calcium containing system. Final products are shown in

Table 3. On this basis, it appears that the equilibrium relations are verified for the four-phase volumes: A1 0 -MgAl 0 -KAlSiO^-KAlSi^O^

and Mg SiO^-MgAl^O^-KAlSiO -KAlSi 0

& at 1200°C (Figure 2) . Owing to

the difficulty in crystallizing tne incongruently melting compounds

3Al

2 0^*2Si0

2 and Mg^Al Si^O , complete reversibility of melting

experiments was not obtained at 1200°C for the four-phase volumes:

3A12°3* 2Sl0

2 -MgAl

20

4

-KA1S;L

206”Al

2 0

3

and Mg

2A1

4

Sl 5°187^1Sl 2°6~MgAl

2 0

47,

Mg SiO^. However no reactions occurred which would indicate disequilibrium

within these four-phase volumes as proposed. Additional slag crystallization

experiments, at higher temperatures, are planned for definitive verification

of assemblages.
'
37

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38

MgO

ai 20

3

Figure 2. Four-phase equilibrium volumes in the system MgO-K 0-A1 0 -SiO

as verified by synthetic slag crystallization experiments ^at

1200°C. Abbreviations as in Table 3.

39

2. Diffusion in Insulator-Electrode Couples (A. J. Armstrong, E. N. Farabaugh

and J. R. Manning

Work has continued on the study of insulator-electrode diffusion couples.

Fe concentration profiles for the MAFF-MgO type couples have been determined

for the case of 100 hr., 10 hr. and 1 hr. tests conducted at 1600 °C. These

profiles are shown in Fig. 3.

A profile for the 10 hr. test was reported in the June 1977 report. The

profile for the 10 hr. test shown in Fig. 3 is another set of data points

taken from the same specimen which does essentially reproduce the previously

reported profile.

There are several points that the profiles of Fig. 3 illustrate. Although

there was a small gap between the MAFF and MgO (caused by the platinum wire

boundary marker) , the Fe profile follows rather smoothly from the MAFF to MgO.

The gap hasn't caused any obvious changes in the profile. This was also noted

in an earlier report. It is observed that a hump in the Fe profile appears

only for the 10 hr. test. There is no explanation for this at present. The

100 hr. test diffused so much Fe into the MgO that the tails from the profile

from opposite interfaces overlapped, prohibiting the Fe profile to go to zero

in the MgO. The 1 hr. and 10 hr. runs show maximum penetrations proportional

to t^/ 2

, as expected for diffusion processes. All the couples were scanned

across the complete couple sandwich (MAFF-MgO-MAFF) , i.e., data was taken in

the MAFF, across the MgO and into the opposite slice of MAFF. This data showed

that the profiles at the two MgO-MAFF interfaces for each sandwich were essentially the same. Only one interface is shown in the accompanying figure

because of this reproducibility. Data for Mg and A1 concentration profiles

were recorded simultaneously with the Fe data for these couples. Although not

shown in this report, these profiles indicated little diffusion of Al from the

MAFF into the MgO and similarly little movement of Mg between the MAFF and MgO.

The profiles shown in Fig. 3 also indicate that the diffusion of Fe is

faster in the MgO than the MAFF as evidenced by the larger penetration of

Fe into the MgO as compared to depth of depletion of Fe in the MAFF for a specific test time. Preliminary analysis of these data indicate that diffusion

coefficients can be determined for the 1 hr. and 10 hr. tests. In the case of the 100 hr. test, the overlapping of the profiles from opposite edges introduces some question to the usefulness of the test in obtaining a diffusion

coefficient.

These data will be treated in the same manner as the case for K diffusion

into AI 2 O3

. We will assume a constant diffusion coefficient, D, in an infinite

linear system obeying Fick's second law for a one-dimensional case. The data

will be plotted on probability paper to yield D. These results will be given

at the end of the next reporting period.

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41

3. Electrode Materials (C. Skarda, T. Negas and R. S. Roth)

a. The Ta^On -CeO -FeO System

2 5 x x The Ta205 ~CeOx-FeOx system was explored, 1300-1550 °C in air (see Figure 4)

,

to determine whether new mixed oxide phases having potentially desirable

electrical properties can be prepared. Under oxidizing conditions two phases

were found within the composition triangle (Fig. 4) . Phase b has a homogeneity

region only partly delimited thus far. The material is particularly interesting

as it has a "pyrochlore-like" structure similar to many conducting rare-earth

oxide/zirconia materials. Phase relations involving the b phase have not been

sorted completely as oxidation-reduction (probably involving both Fe^ +/Fe^ + and

Ce 4 +/Ce 3+

) complicate equilibria. Electrical measurements for this material

will be initiated once an optimum composition is selected.

b. Potassium Ferrite ( 3-alumina-type)

As suggested in the previous Quarterly Report, K-ferrite having the

3 -alumina structure type have more than adequate electrical conductivity

(mainly electronic) to be considered for usage as low temperature (<1400 °C)

construction materials. Reactive powders (^100 g) were prepared to provide

starting materials for fabrication of specimens via hot-pressing. Several

runs under varying conditions were conducted for us by Naval Research Labs.

(NRL) . The starting bulk composition was K20 : 7 (Fej_ gTi Q

^3

) ; Ti02 added

to enhance the electrical conductivity. Single-phase, homogeneous products

were not obtained from these preliminary hot-pressings. Rather than utilizing

reactive starting powders , single phase K-ferrite powders will be used in

future work. This requires some special handling of starting materials, and

initially calcined powders together with appropriate control of the gaseous

environment to maintain stoichiometry. In cooperation with a private firm

these powders, hopefully suitable for hot-pressing, are being prepared.

42

FeO x

Figure 4. Projection of system Ta O^-cerium oxide-iron oxide in air.

1300-1400°C. Open circles are bulk compositions investigated.

FT = FeTaO^ (rutile-type) ; CT^ = CeTa^O (perovskite-type)

CT = CeTaO^; a = compound, 2CeTaO^ :FeTaO^ and b = homogeneity

region, of 'a mixed oxide having a pyrochlore-like structure.

Compositions plotted in terms of mol % of components as indicated without specific oxygen content (e.g. FeO = Fe0

1 5 (Fe

2 0

3

) / FeO^

33

(Fe

3 04

), etc.; CeO

x

= Ce0

2

, Ce0

1 5

, etc.)

—

43

TASK K. Materials Testing and Characterization

1. Structural Analysis of Powdered and Solid Ceramics

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

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

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

x-ray diffraction methods. Data, transmitted to the appropriate source,

are shown in Table 1.

.tst ;

1300°C

10

hrs

LaCrOtype

phase -

good

crystallinity

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48

2 . Electrode Design and Testing

a. Phase III-U-02 (W. R. Hosier)

It is necessary to obtain the best possible arc plasma sprayed materials

for the Phase III electrode module. A program was worked out with APS

Materials, Inc., Dayton, OH for determining the optimum spray parameters in

order to obtain the highest density, single phase materials. While there

are many variables to consider, those given systematic study in the program

were (1) type of torch used, (2) arc gas flow, velocity and constituent,

(3) arc current, and (4) torch to substrate distance. A report on the

experimental details has been received from APS Materials and is available

for anyone interested. The samples resulting from the optimization program

have been analyzed and are reported on elsewhere in the report.

The first of several Phase III proof tests will be carried out at the

Westinghouse Research and Development Center, Pittsburgh, PA sometime during

the week of October 3, 1977. A representative from NBS will be present at

each test. Samples from the tests will be returned for post-test analysis.

Pre-test samples from the various electrode suppliers have been received.

The XRD and electrical conductivity of these materials are reported elsewhere

in this report.

b. AVCO Slagging Test "A"

The anode and cathode sections for this test were delivered to AVCO

Everett Research Center on August 10, 1977. The purpose of the experiment

is to test several electrode configurations consisting of several materials

designed in such a way as to withstand the conditions of a slagging MHD

generator under load. The material just under the slag must be compatible

with slag at about 1200 °C where the slag is solid, tightly bonded to the

material and yet has a good electrical conductivity.

The lay-out of the electrodes and the identification system is shown

in Fig. 1. Schematic diagrams of the electrodes showing the materials, thicknesses, bond line design temperatures, and material thermal conductivities

are shown in Figs. 2 through 6.

All pre-test data on the materials will be given in the final report.

This set of electrodes was tested for 10 hours at AVCO on September 14, 1977.

Post-test analysis and materials characterization is in progress and will be

reported at a later date.

c. MIT Test 185-186

Six electrodes (3 anodes and 3 cathodes) were fabricated by APS Materials,

Inc., Dayton, OH according to NBS design and specifications. They were

mounted on the standard MIT test rig base. A schematic design is shown in

Fig. 7. Option 2 was used. The top electrode in each stack used the

Hf02 _ 5Y203 cap. The other electrode used the Zr02 ~ 5 Y 203 cap. This test

was completed May 26, with representatives from Westinghouse, APS Materials,

and NBS present. The samples were returned to NBS for analysis. This analysis

is completed and reported elsewhere in this report. It appears likely from

the initial observations on this and previous tests that large mechanical

stresses are placed on the electrode ceramics (especially the cathode) because

of the thermal expansion and large temperature gradient transverse to the surface.

49

Another set of electrodes has been constructed but with the electrodes segmented at 20 mm intervals along the surface in an effort to relieve these

stresses. These electrodes are composed of LaCrC>3 with a 85Zr02“3Y2C>3-12 CeC>2 cap. The LaCrO

^ is bonded to nickel on plated copper through a cermet grade

using METCO 447 powder. The test will be completed in early October.

AVCO/NBS SLAGGING ELECTRODE

ANODE 1

flow

CATHODE

APS slag on MAFF-31 attached to

Brunsbond pad

Slag 1 . 1 mm

MAFF-31 1.5

Pad 1.0

APS Slag on MAFF-31 attached

to Cermet (MAFF-31 40% Cr)

Slag 1.1 mm

MAFF-31 1.5

Cermet 1.0

APS MAFF-31 on Brunsbond pad

MAFF-31 2.5 mm

Pad 1.0

APS MAFF-31 attached to Cermet

(MAFF-31 40% Cr)

MAFF-31 2.5 mm

Cermet 2.5

APS Slag on LaCrO.

Slag 1. 1 mm

LaCrO. 2.6

! 1101 2101

1102 2102

i 1103 2103

1204 2204

1205 2205

: 1306 2306

| 1307 2307

: 1408 2408

1409 2409

1510 2510

1511 2511

FIGURE 1

Fig. 2•

SCHEMATIC

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56

Mechanical Attachment (W. J. McKean, S. J. Schneider and W. R. Hosier)

In most concepts of a commercial generating plant involving MHD, it

is conceded that repairs to the electrode walls will have to be made at

certain times either on an emergency or routine basis. In order to

accomplish this, it would seem advantageous to have an electrode system

that could be easily removed and replaced without disassembling the

channel. We have designed and built a "snap-on" electrode segment that

might be used on a channel frame section. This electrode segment can be

removed by mechanical force applied perpendicular to and away from the

base. Conversely, it is attached or "snapped on" the base simply by

applying force perpendicular to the base. This electrode piece is made

of copper and is held to the channel frame element by bullet latches

appropriately spaced. The ball end of the latch contacts the electrode

in such a way as to place constant pressure downward to assure intimate

contact between the copper electrode and the channel frame. In addition,

the two copper surfaces are tinned with a solder that can be melted at

the temperature of boiling water. This is done to assure good thermal

and electrical contact. Electrode removal, of course, can only be done

by heating the electrode above the melting point of the solder. Figures

8 , 9, and 10 show drawings of the design.

This concept is in the preliminary stage of development but an electrode

using this "snap-on" configuration will be tested soon at Westinghouse

Research and Development Center. It should be noted that any material

ceramic or appropriate metal can be attached to the electrode copper on

the plasma side. For test rigs and small experimental generators this

could allow greater flexibility and increased frequency in testing. If

the preliminary test at Westinghouse appears encouraging, further testing

of the concept will be carried out in other test rigs using various

electrode materials attached to the copper "snap on".



Figure



60

3. Characterization

a. Electrodes Tested at MIT (T. Negas , W. R. Hosier)

On May 25-26, 1977, a set of NBS electrodes were tested at MIT.

Figure 11 illustrates the general layout of electrodes including

representative cross sections of anode and cathode (potted in

epoxy) after testing. Electrodes were fabricated by APS Materials

Dayton, OH according to NBS design and specifications. They were mounted on the standard MIT test rig base but were 3/8" wide to

conform to the proposed U-25 window frame width. A cermet (40% Cr - 60% MAFF-31) was sprayed on the Cu blocks after these were "tinned"

with a high melting Sn-based solder. The cermet is graded to pure

MAFF-31 which, in turn, is graded to a "capping" material, either

88Zr0

2 -12Y

2 0

3 or 95Hf07 -5Y 0 . Insulation is MgA^O^ . Most materials

were plasma sprayed. ^Electrode thicknesses were calculated using a heat flux of 100 W/cm and the best available values for thermal

conductivity

The main test events were as follows,

a. 10:40 - 10:45

b. 11:00 a.m.

c. 11:21 - 11:43

d. 12:36 - 12:47

e . 1:06 p.m.

f . 1:12 p.m.

g. 1:21 p.m.

h. 1:30 p.m.

a.m. - cooling H^O on and ignition,

flame out, electrodes reached ^1400°C,

a.m. - restart and reignition

p.m. - temperature ^1675°C, seed on,

HfO -based cap beginning to spall,

2 0.5 A/cm on all electrodes

slight bulging at cathode,

sudden collapse of cathode center, bulging

of entire cathode, apparent melting,

temperature increasing, anode in perfect

shape,

i. 1:34 - 1:45 p.m. - shut-down.

As suggested by the visual observations noted above, the cathodic

side sustained considerably more thermomechanical-chemical degradation.

Capping materials were almost entirely absent and localized melting was

apparent. The cathode cross section in Figure 11B, therefore, is not

representative but includes several features visually noted during

operation. The ZrO^-based caps are detaching and appear to be "uplifting"

with the detached insulators which have sintered together and have been

intruded (reacted) by iron oxide. The vesicular nature of the ZrO^-

based caps also suggests partial melting while the dark discoloration

is due to penetration by iron. At points where unlike materials, even when graded, are juxtaposed, the MgAl

2

c> 4

insulation is developing

fractures.

61

It remains unclear whether detachment of "caps" results from a thermal expansion mismatch between the cap and underlying spinel or whether a combination of chemical and thermomechanical effects

dominated. Significantly, the "caps" on the anodic side remained

intact and fracture of the insulators is far less evident. Inspection

of the "intact" interface between the cap and underlying spinel at

the anode reveals one significant chemical feature. Figure 12A (50X)

illustrates a typical region. The darker material (bottom) is MAFF-31

followed by a lighter material (top) via a graded zone which appears

to be undergoing alteration. Hand specimens clearly reveal a thin

band, reddishly discolored, for this zone. For cathodes, this band

is particularly conspicuous and thick in many areas beneath the

remaining caps. Fig. 12B, maps potassium (for Fig. 12A) and clearly

shows that the cap-spinel interface is penetrated, and, probably,

being chemically altered by condensed seed (note, this interface was designed for ,v-1200 o C) . Fig.- 13A (1000X) again shows the cap-spinel

interface but clearly reveals that alteration predominates within

the zone wherein ZrO^ (Y^O-) and MAFF-31 previously were graded.

The brighter phase is the former, the darker is the latter material.

Within the upper half of Figure 13A, "islands" of unaltered ZrO^ (y

2 0^) are prominent (Figure 13B,map for Zr)

The surrounding matrix, once

spinel, now lacks cohesion, is friable and is precisely the region

wherein seed is concentrated. It is tempting to conclude that the

spinel portion of the graded interface is suffering from reactions

with condensed seed and that this process was accelerated on the

cathodic side. Development of resistive phases and poor thermal

contact would accelerate excessive heating and lead to enhanced thermomechanical and chemical (melting) degradation.

Noted also is the highly irregular nature of the Cu/Sn-based

solder/Cr-metal bond line. Figs. 14A,B (50X) shows, for example,

the following sequence of materials from bottom to top, Cu/Sn-solder/

Cr-metal/cermet . Fig. 14B, , far right, shows that the Cu/Sn-solder

band is practically absent.

62

LEFT n RIGHT

(ANODE) H (CATHODE)

Figure n . Surface view (A) of electrode arrangement in MIT test-rig with pertinent

cross sections (B) of electrodes after testing. Plasma flow is from

bottom to top.

63

Figure 12. SE.M microgr

ZrC^-based

EDX map in

aph (50X) in A illustrating bonding area (anode) between a cap (light material) and MAFF-31 spinel (darker material)

B shows concentration of potassium at the interface.

!

i

I

SEM micrograph (1000X) of the Zr0 2 cap-spinel interface (A) shown in

Fig. 12- This region is the graded zone between spinel and the cap.

EDX map in B shows distribution of Zr. See text for details.

Figure 13

65

Figure 14. SEM micrographs (50X) illustrating discontinuous nature of bond line

between Cu (lowermost material) and Sn-based solder. In A solder is

continuous while in B is practically absent.

66

b. X-ray Examination of Fluidyne RFG Samples (A. Perloff)

Two samples of a chrome-magnesia refractory material were examined by

X-ray powder diffraction techniques. The samples were FN-36 which had been

cycled between 1800 - 1600 °C for 30 hours with exposure to K2 CO3 seed (but no ash) , and FN-81 which was cycled between 1800 - 1600 °C for an unspecified

time period with exposure to both K2 CO 3 seed and ash. Material was chipped

from the reacted layer and ground for examination. Original (untested) 0 material, previously examined, is composed of well crystallized MgO (a = 4.212A)

and a single phase spinel (a = 8.278A) which is probably a magnesium iron

chromium aluminum oxide spinel plus traces of forsterite (Mg^SiO^) and

monticellite-like (CaMgSiO^) material.

FN-81 (K^CO^ an<^ as h)

O O This sample is predominantly MgO (a = 4.218A) and spinel (a = 8.260A).

The proportion of MgO relative to the spinel is clearly less than in FN-86

and the crystallinity is poorer. Some substitution of larger ions has taken

place in the MgO lattice as indicated by the slight increase in the cell size.

The spinel phase has also suffered more loss of the larger ions as its cell

size is somewhat smaller than the previous sample. Small, but clearly identifiable, amounts of forsterite Mg 2 SiO^ and tetragonal (K-rich) KAlSiO^ are present. Traces of what is probably monticellite (CaMgSiO^) and hexagonal

or orthorhombic KAlSiO„ are also visible.

4

FN-86 (K^CO^ , no ash)

O O This sample is predominantly MgO (a = 4.213A) and a spinel (a = 8.268A).

The proportion of MgO relative to the spinel is approximately the same as the

original material but the crystallinity is poorer (wider peaks, little

resolution). There is, apparently, no substitution of larger ions in the MgO

lattice since there is no significant change in the unit cell dimension from

that found in the original material. The spinel phase shows a slight reducO tion in cell size (^O.OIA) suggesting a small loss of larger Fe or Cr ions.

This change is much less than found in previous samples FN-91 and FN-96 and

there is not the clear splitting into two distinct spinel phases found in

the earlier samples. Very small amounts of monticelitte (CaMgSiO^) and K^MgSiO^

are visible. No visible amount of forsterite is present.
.

.
67

c. SEM and EDX Examination of FluiDyne Engineering Corporation tested

samples

(E. N. Farabaugh)

Four samples of a rebonded magnesia chrome spinel-iron spinel refractory

(RFG) preheater material were examined. They were: FN81, cycled between

1600-1800°C in the presence of seed (K2 CO 3 ) and ash; FN86 , cycled between

1600-1800°C for 30 hours in the presence of seed (K2 CO3 ) but no ash; FN91

cycled between 1600-1800°C for 30 hours in the presence of seed (K2 SO4 ) and ash; FN96, cycled between 1600-1800°C for 30 hours while exposed to

ash but no seed.

The results of the examination are summarized and briefly stated as follows

FN81

There was a rather well defined reacted layer (^.4mm) thick which

was depleted in Mg, probably at the expense of MgO. The reacted

layer appeared to be a little more dense than the rest of the

material. K was found to have penetrated at least 1 to 1.5 mm

where it was found in material, probably KAlSiC>4 , between the

larger Cr and Fe spinel phases. Si, Ca, and Ti were also detected

in amounts somewhat larger than were present in the untested material.

The presence of these elements above that found in the untested

material is probably traceable to some downstream source. The

concentration of Mg is greatly reduced, in general, when compared

to untested RFG material.

FN86

There didn't appear to be any well defined Mg depleted layer, even though the material at and near the reacted surface posses less Mg

than the bulk. The drop in the concentration of Mg toward the

reacted surface is much more gradual than for FN81. K has penetrated

a little deeper, 3 to 4 mm, than for FN81. Again, the K was found

in material surrounding the Cr and Fe phases. Si, Ca, and Ti were

detected in the specimen at levels exceeding those for untested

material. Si, Ca, and K were seen in intergranular material possibly

forming CaMgSiC>4 and K^MgSiC^ phases. There was a densified layer

(^lmm) but there was no unique elemental distribution associated

with it.

68

FN91

No well defined Mg depleted zone was observed, just a gradual

decrease of Mg as the reacted surface was approached. The Mg, Si,

K, and Ca phases were detected surrounding the spinel grains.

There was a larger penetration of K, about 4 mm, in this specimen

than was observed in the two previously described samples. No

obvious microstructural changes were observed in going from the

reacted regions to the bulk, only elemental changes in the phases

of the intergranual material.

FN96

There was Mg loss observed again towards the outer edge of the

specimen. However, no well defined layer could be detected. Ca

and Si, coming from a downstream source was observed to have penetrated deep (^6mm) into the specimen. The Mg, Ca, and Si phases

are again present in the material surrounding the spinel grains.

Once again no striking microstructural changes were seen.

In summary, all of the tested RFG material whose results are reported

here, exhibit a loss of Mg (probably MgO) during testing when compared

to the untested material. The loss was most pronounced in FN81 where a well defined layer Mg depleted layer could be demonstrated by mapping

techniques. Possibly the action of seed and ash together accelerate the

Mg depletion reaction when compared to ash-free or seed-free testing.

The spinel phases in the RFG were apparently only slightly reacted,

since the concentrations of Cr and Fe varied only slightly from the

exposed edge to the interior ( rV'-6mm) of the specimen. Changes in the

phases of the material constituting the intergranual material were

observed in all specimens with CaMgSi04 and K2MgSiC> 4 , among others

probably being formed. Ca and Ti, as well as some Si were picked up by

the RFG material from a downstream source.

69

Task L. Assessment of Steam Plant Components - ( J . R. Cuthill)

This is the 9th quarterly progress report since the initiation of

Task L. In the course of this Task L a number of wrought alloys have

been selected for further consideration as construction materials for

the steam heat exchanger tubes and similar components. This list to

date of selected alloys with their pertinent mechanical properties, and

with their nominal chemical compositions were given in Tables 1 and 2,

respectively, under Task L in the previous Quarterly Progress Report

(April 1 to June 30, 1977) . Also, in the same report the ASME Boiler

Code designation, maximum allowable working stresses and temperatures

are given in Table 3, for those alloys that have been given code

designations.

The majority of alloys included in the selected list would appear

promising to varying degrees in MHD type service, on the basis of the

hot corrosion results reported in the literature. Other alloys have

been included because they have been considered or even specified as the

construction material in key MHD studies such as the Baseline plant

design and the ECAS studies but they should be carefully reconsidered in

light of known performance data; or just that there are steam plant

performance data and/or simulated MHD performance data on them that

makes them of interest as a basis of comparison. In the 7th Quarterly

report (Jan. 1 thru Mar. 30, 1977) an attempt was made to qualitatively

rank the alloys in respect to their ability to resist hot corrosion

attack using the data in the literature from a variety of hot corrosion

tests and conditions. In the future course of this program (Task L) it

is proposed to obtain experimental hot corrosion data on some of these,

as well as other selected alloys. A particular effort will be made to

simulate the conditions for the precipitation of a K^SO^ deposit on the

steam heat exchange tubes. This will necessitate simulating

temperatures and geometry to see if a hard deposit of K^SO^ builds up on the leading edge of the tube as predicted.

In this current Quarterly Report the list of "selected wrought

alloys" has been reproduced in Table 1 together with references to

comprehensive and readily available data compilations (Column 2) for the

design engineer who needs more detailed property data then what has been

included in these quarterly reports for preliminary screening purposes.

It would be an unnecessarily duplication of effort to reproduce all of

the readily obtainable data that may be of use in the design of a particular MHD componet. However, an attempt will be made to reproduce

any difficult to obtain data that are particularly pertinent to the MHD

application. One of the most comprehensive and readily available data

compilations is the 5 volume Aerospace Structural Metals Handbook (1)

which is published in loose-leaf form and periodically updated. The

volume and Code numbers in column 2 refer to specific entries in this

data compilation. Generally of equal importance with these master data

compilations are the alloy producers own property data bulletins.

References to these are given in Column 3. In the case of the Cr-Mo low

alloy heat exchanger steels supplementary creep-rupture data have been

recently compiled for the Metals Properties Council, N.Y.C. and

published by ASTM (2). Also for the following steels:

70

(Stress Rupture Properties of:)

Mild steel

1/2% Mo steel

1% Cr 1/2 % Mo steel

2 1/4% Cr 1% Mo steel

9% Cr steel

12% Cr steel

A1S1 304 stainless steel

A1S1 321 stainless steel

A1S1 347 stainless steel

A1S1 316 stainless steel

the Central Electricity Generating Board, London has included a particularly detailed set of stress-rupture curves in a recent Bulletin

(3).

Salient characteristics are given in the last column of Table 1.

Table 2 is the corresponding list of nominal compositions

duplicated from the previous quarterly report.

Some newly revealed experience by the power industry in the UK on the spalling of stainless steel superheater tubes on the steam side is

summarized here.

Steamside Spalling of Boiler Tube Alloys

It has recently been reported (4) that in steam plants in the UK

spalling of type 316 stainless steel in superheated steam at ^ 565° is a normal occurance sometime after 7000 hours and has resulted in the

complete blocking of tubes, overheating, and complete failure. Type 321

stainless steel, which is a titanium stabilized stainless steel, is

apparently similar in behavior to type 316. However, type 347 which is

a niobium stabilized stainless steel apparently oxidizes at a slower

rate with smaller size flakes. The same phenonemon is observed with the

ferritic stainless steels. However, apparently Incoloy 800 is much more

resistant to spalling than any of the austenitic or ferritic stainless

steels.

The spalling is claimed to be due to a F 62 (1)3 l a Yer that forms on

top of the Cr oxide film. It is claimed that maintaing 10 ppm or more

of free hydrogen in the steam will prevent this outer layer of Fe203

from forming.

An update on the prices of many of the alloys in Table 1 has been

compiled and is presented as Table 3. The prices are for material in

tubular form unless otherwise marked. The prices of the Hastelloys and

Haynes alloys are from the Stellite Division of the Cabot Corp. The

prices of the INCO alloys are from the International Nickel Co.

In future quarterly reports, materials that have been specified

for the respective components in benchmark type plant designs such as the Baseline Plant and the ECAS II designes will be listed together with

the materials that this reviewer would propose.

71

Ref erences

1. Aerospace Structural Metals Handbook, Vols. 1 through 5, 1976 revision.

Mechanical Properties Data Center, Belfour Stulen, Inc., Traverse City,

Michigan.

2. Evaluation of the Elevated Temperature Fireside and Creep-Rupture

Properties of 3 to 9% Cr-Mo Steels, prepared for the Metal Properties

Council by G. V. Smith, ASTM Data Series Pub. DS 58 ASTM, 1916 Race

Street, Philadelphia, PA 19103, (1975).

3. The Control of High Temperature Fireside Corrosion, 2nd ed., D.W.C.

Baker, et.al., pub. by Central Electricity Generating Board,

Sudbury House, Newgate Street, London EC1A 7AU., 20.

4. M. I. Manning and E. Metcalfe, Oxide Spalling in Stainless Steel

Superheater and Reheater Tubes and Possible Control Measures,

to be presented at the 6th European Congress on Corrosion, London,

- 19-23 Sept. 1977.

5. Handbook of Stainless Steels, 1977 edition, D. Peckner, ed . , McGraw

Hill, N.Y. 1977.

6. "Chromium-Nickel Stainless Steel Data", the International Nickel

Company, Inc. One New York Plaza, N.Y., N.Y. 10004

7. "Allegheny Stainless Steel Technical Information", Allegheny Ludlum

Steel Corporation, Pittsubrgh, PA 15222.

8. Properties Bulletins from Cabot Stellite Division, Kokomo, Ind. 46901.

9. "Huntington Alloys Handbook" and "Incoloy Alloys" of Huntington Alloy

Products Division, INCO, Huntington, W. Virginia 25720.

10. "Alloy Fact Sheet", Ibid.

11. Property Bulletins from Babcock & Wilcox Company, Tubular Products

Division, Beaver Falls, PA 15010.

Table 1 Selected Wrought Alloys

Stainless Steel

Compilations

Property Data

(References

)

Producer * s Property Data

( References

)

Salient Characteristics

1.

2. 304 310 2(1302) ** 2/1305

)

(5) , (6) , (7)

(5) , (6) , (7) Highest oxidation resistance of any stainless steel,

3 times better than 304. Low strength. Excellent

resistance to H^S (see Q.R. for 4/1-6/30/77.

3, 316 2/1308) (5) , (6) , (7) Has exhibited severe spalling in superheated steam

4 . 446 (5) , (7) Highest oxidation resistance of any ferritic stainless

steel

Ni&Fe-Ni base

5. Hastelloy C-276 (8) Superior resistance to sulfur attack. Use in desulfurization equipment

6. Hastelloy X (8) Resistance to stress corrosion cracking, good weldability

resists oxidation to 2200 °Fy high Ho may lower hot

corrosion resistance

7. Haynes 556 (8) Developmental alloy, equal to Haynes 188 in oxidation, but

may be better in hot corrosion because of low W & Mo

8, Inconel 601 (9) Resistant to sulfurdizing atmosphere. Used for radiant

furnace tubes

9. Inconel 617 (10) New alloy. Oxidation & carbonization resis. to 1100 °C.

10, Inconel 718 (9) Oxidation resistance to 1800 °F, good weldability

11. Inconel X750 (9) Good for springs and bellowsto 1200 °F

12, Incoclad 671/800 INCO 671 can be clad to either 800 or 800 H. No strength

at MHD temperature

13,

|

Inconel 671 (9) Best hot corrosion resistance of any alloy under MHD

conditions

14, Incolov 800H (9) H is for higher carbon and higher strength than

Incoloy 800

.5,

j

Incoloy 800 2(1613) (9) Very resistant to spalling in steam heat exchanger

service

6 . Incoloy 802 2(1610) (9) Higher strength than 800, same oxidation resistance

Udimet U500 5/4206) (8) Good hot corrosion resistance, but IN-738 is better.

See Q.R. for 7/1-9/30/76

3-Base Alloys

3. Haynes 25* 5/4302

)

(8) Good formability weldability strength, oxidation

resistance to 1800 °K.

*. Haynes 188 5/4310) (8) Better oxidation resistance than Hastelloy X. Good high temperature strength

Croloy 9M (2) (11)

ame as designation

Reference given thus, Vol. No. 2 , Code No. (1302), Metals Handbook, RefJ 1. refer to Aerospace Structural

Table 2.

Chemical

Compositions

of

Selected

Wrought

Alloys

73

cn CQ cn cn

cn r>

v0 CO CD cn cn

cn cn z « z z 0 O 0 OJ3 cn cn

m p->

m in 04 CN CN

U 0 O 0 O in

OO 0 0 O CN cn • N O • • rH • • 4) • • O O O O CN 0 O 1 1 • 0 O & • • O O 1 jJ a Or a, <d id • « « 1 •3 3 3 CQ 1 in a. a. O n n in a > wJ J O 3 1 3 3 1 0 0 0 CO l O cn cn

*r d*T in CN CN CJ 1 CJ CJ CO CO 0 « O

O 0 O 0 O cn O O in cn in cn cn <T 0 O • • • • • • • • • . • • O O 0 O O O O O O O O O O O d 0

in

rH * « « « • n CD « « « CN in O in r* CD CD in in n in O 1 rH O

CO in OD

n

O <H rH 0 O

OO 0 0 cn rH rH in • • 0 CN O m • • • 0 O O

CJ 0 O

O 6 O <H

O 0 0 O

O 0 O

O

OO 0 • « hi

O in

in « O in in « O 0 O 0 • m

*H 0 in

O

O 0 O TT CN cn in

m in TT 1 • 0 l O

cn • • • • • • • l • 1 • • 1 rH l in in rH rH rH rH O rH O O O 0 O d 0 CN CN • O O O in

(N (N CNH

till I I I l O

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03 CN CN U n n

CN fd m XX i r i i i i i i i t i i o o

cn

m

id <d -Q XX

rH

rH CN I I ON O

CN

in

O 0 so CD • rH r* 0 rH 01

u CN CN rH CN vOm

O 0 O CN rH CN CN CN rH

U 1 1 1 1 rH CN • • • 1 1 03 rH rH rH 1 1 1 01 CD •<T sO cn 1 1 CN cn CN C* ^3* CN CN CN in cn O HI rH CN rH CN in in CN CN CN rH rH rH rH CN +J • • cn

cn

O

>H rH CN 0 4-> rH s CP rH 0 c c < SO 0) 0 cn GO 01 V4 tH rH 01 CN \ z 0 > in JJ

•u 0) 01 1 O rH rH

O

O CN 0 0 CN CO cn

Id (1) •d CJ

X rH r» CO in r* r^*

OOO in rH cn 00 c JJ CQ sO O rH H r* so sO CD CD CO D iH x <H jq

O' cn is >1

m vO sOX < CP • • • • —

.0 0 m *H

01 cn cn cn cn cn z rH rH rH rH <H rH id rH > N 01 s 0 cn • • 1 pH rH 01 01 01 0) 01 ri 0) 0 0 0 4J 01 01 01 D 0 cn cn cn cn 01 01 CJ u 3 c 3 c U c rH rH rH (JJ <d 01 0 >

rH Ut JJ JJ c 0 0 0 0 0 0 0 0 0 £ 02 c c 0 » 3 TT 0 sO vO 01 01 >H O u U 0 O u 0 U 0 -n > >1 rH u •r< 0 rH rH TT '-3 <d <d id c c c c c c C C C T3 JJ 3 3 rH a id cn m m HT *7* •** —

M M w *-» M Kt w w H Z3 rH s 23 <

lui jj •H id 1 2u cn z X3 5

rH CN (H *7 00 ON o

•H 'H

rH (Nm H*n SO o o ( 1 ON CD I I I I I I I I I I I I I I I I I I I I

04

•d 2: n

ON> 2! 0 ON
(
74

Table 3. Prices of Candidate Alloys for Steam Heat Exchanger Tubes, etc.

(in lots over 5000 ft.

)

Alloy 2" O.D. Tubing

Price/ft. Price/lb

Incoclad 671/800H $28.30 (a) $11.13

Inconel 617 30.25

(a) 11.50

Incoloy 802 18.50

(a) 7.28

Incoloy 800 9.15

(a) 3.60

Hastelloy C-276 27.22

(b) 8.98

Hastelloy X 19.49

(b) 6.96

Haynes 25 37.43

(b) 12.00

Haynes 188 35.28

(b) 11.35

aPrice/ft. of 2" O.D. x 0.125" wall. Update of prices given in Quarterly

Report for period ending June 30, 1977*

Price/ft. of 2" O.D. x 0.134" wall. Prices from Stellite Division, Cabot

Corp. , Kokomo, Ind.
** - r