U6S|te 7‘B'-fS'Go

EA-77 A-01 -6010 - 13

Distribution Category UC 90g

DEVELOPMENT, TESTING AND EVALUATION OF

MHD-MATERIALS

Quarterly Report

for the period July - September 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: September 30, 1978

PREPARED FOR THE UNITED STATES

DEPARTMENT OF ENERGY

Under Contract No. EA-77-A-01 -6010

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

States Government. Neither the United States nor the United States

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

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

express or implied, or assumes any legal liability or responsibility

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

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

TABLE OF CONTENTS PAGE

ABSTRACT

OBJECTIVES AND SCOPE OF WORK 2

SUMMARY OF ACHIEVEMENTS 3

TALKS AND PUBLICATIONS 5

SUMMARY OF PROGRESS LISTED BY TASKS

TASK G. PROGRAM MANAGEMENT AND COORDINATION 6

TASK I. OPERATIONAL DESIGN PROPERTIES 7

1. Viscosity of Coal Slags 7 2. Electrical Conductivity 12

3. Vaporization Studies 26

TASK J. CORROSION AND DIFFUSION 29

1. Diffusion in Insulator Electrode Couples 29

2. High Temperature Charge Transport in Coal Slags 32

3. Electrochemical Effects in Simulated Slag Flow 33

4. Seed/Slag Interactions and Slag Corrosion 34

TASK K. MATERIALS TESTING AND CHARACTERIZATION 50

1. Structural Analysis of Powdered and Solid Ceramics 50

2. Electrode Systems and Component Materials Testing 55

3. Test Analysis (Pre and Post Test Materials Characterization. . 56

4. Materials Development and Analysis . 64

TASK L. ASSESSMENT OF STEAM PLANT COMPONENTS 67

1. Laboratory Hot Corrosion Screening Program 67

2. Material Assessment 68

TASK M. INFORMATION AND DATA ON MATERIALS FOR MHD POWER SYSTEMS. ... 72

1. Data Center for Materials Properties and Performance 72
'

.

.

,
ABSTRACT 1

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

A study was completed showing consistent increases in viscosity of several

slags due to seeding with K^O. The maximum increase was observed at 15 to

20 wt % K^O . At 30% the viscosity was only slightly higher than unseeded slags

A FluiDyne Corporation heater matrix effluent of seed-slag mixture (very

high seed to slag ratio) was shown to be extremely fluid and to exhibit

slight immiscibility

.

The electrical conductivity of a number of potential MHD electrode materials

has been measured.

At 2000 K the pressure of Fe (g) over MAFF-31 is about 10

_J atm which is

about 10 times the Mg pressure. The upper temperature limit is 1600 °C.

Interdiffusion of iron between MgO insulator material and MAFF-31

(3MgAl C> 4:Fe C> 4

) was measured at 1400 °C. The results indicate a strong

temperature dependence, since diffusion coefficients at 1400 °C were smaller by a factor 4 to 7 than those measured previously at 1600 °C.

Initial measurements on charge transport in slag indicate that the ionic

conductivity in an "Eastern" coal slag is much less than the electronic

conductivity at temperatures not much higher than the softening point of

the slag.

The phase compositions of six component "Eastern" and "Western" synthetic

coal slags has been measured and analyzed.

Approximately 50 materials have been analyzed structurally by X-ray diffraction methods including some of those tested in the U-02 Phase III experiment.

A brief analysis of the electrodes from the recent 250 hour AVCO test' is

presented. Electrochemical effects apparently played a large role in the

observed electrode changes but the tested electrodes were in exceptionally

good conditions considering the rigors imposed by the AVCO test facility.

Several samples from a unique deposit on the E wall - B wall joint in the

USSR-U-25 RM channel have been analyzed and found to contain unaltered Al^O^

and A1P0

4 on the plasma side with some 3-alumina at the center and near

the wall side.

Electrical conductivity measurements on a flowing layer of material joining

the electrode and insulator wall of the Phase III U-02 experiment have been

made. They do not prove conclusively that the steady deterioration of the

interelectrode insulation during the experiment was a result of this layer

which contained an inordinate amount of Al^O^. T^ e or:i-9 ;'- n °f the A -*- 2°3

remains unexplained.

The procedures for laboratory . testing of the hot corrosion of boiler tube

and other steam plant components have been established. Initial tests

on several materials are in progress.

Review and cataloging of contractors reports remain a continuing activity.

A data base structure has been designed and loaded into a computer for test

purposes. A proposal for bids from vendors of Data Base Management

Systems has been submitted for Department of Commerce approval.
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 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.
3

SUMMARY OF ACHIEVEMENTS (July-Sept. 1978)

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

TASK G.

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

documents in the area of MHD development (continuing)

.

TASK I.

6. The final series of viscosity measurements was completed which reveals

the effect of K 0 seeding on the viscosity of several different coal

slags (completed August 31, 1978).

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

(new) electrode materials (continuing)

.

14. Determine FeO losses over several spinel compositions which are promising electrode materials (completed September 15, 1978).

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 (continuing)

.

e. Determine melting relations for the CaSiO^-KAlSi O^-CaAl system

(pertinent to western coal ashes) (completed Sept. 15, 1978).

TASK J.

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

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

23. Conduct diffusion studies in slag-insulator (spinel, MgO, A1 0 ) systems

to study penetration of Fe, Si, Ca, etc. into the insulator materials

(continuing)

.

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 (continuing)

.

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)

.
4

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

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

tube applications (completed August 1, 1978)

32. Complete initial test measurements on 4-6 different materials using

the above corrosion apparatus (completed September 15, 1978).
5

Talks and Publications

Vapor Pressure Measurements of Potassium over K O-SiO^ Solutions by a Knudsen Effusion, Mass Spectrometric Method

E. R. Plante

10th Materials Research Symposium, NBS , Gaithersburg, MD

September 18-22, 1978.

Joint NBS-UTSI Cooperative Program Review

S. J. Schneider, T. Negas , J. Smith, L. P. Cook, E. N. Farabaugh , H. M. Ondik

,

A. Perloff from NBS and J. B. Dicks, S. Strom, J. Lanier, S. Wu, M. Beaton,

N. Egan, W. Long from UTSI at the University of Tennessee Space 'Institute,

Tullahoma, TN, July 18, 1978.
6

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

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

arranged or sanctioned program review/coordination meetings, briefings,

and technical conferences. These included:

1. U.S. -U.S.S.R. Cooperative Program on MHD ; Steering

Committee Meeting June 18-23, 1978 (in Moscow).

2. U . S

. -U . S . S . R. Steering Committee (U.S. side) planning

session August 31, 1978 (in Washington).

3. Materials Meeting for Cooperative Program (U.S. -U.S.S.R. )

,

August 23, 1978 (in Chicago).

4. UTSI visit and planning session on cooperative NBS-UTSI

work, July 18, 1978 (in Tullahoma) and September 13, 1978

(at NBS)

The conclusions and recommendations resulting from these meetings and

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

direct consultation with DoE staff or through documents published

elsewhere

In addition to the above, NBS is joint sponsor with DoE-Fossil Energy

and others of the 3rd Annual Meeting on "Materials for Coal Conversion

and Utilization", October 10-12 , 1978. S. J. Schneider (NBS) and

Wate Bakker, DoE, have organized the meeting; a session is devoted to

MHD.
7

Task I . OPERATIONAL DESIGN PROPERTIES

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

1.1 Introduction

The principal thrust of this part of this task for several years has

been an effort to find a means of predicting the viscosity of slags formed

from the combustion of coal in the open cycle MHD process.

One approach has been to make a series of five component slags by

melting mixtures of oxides and carbonates in the laboratory and measuring

the viscosity of the resulting liquids at temperatures ranging from about

1620 to about 1200 C. Systematic changes were made to a base or starting

composition by adding or subtracting one oxide at a time in 5% increments.

These melts contained only the five major constituents of real coal slags;

these are oxides of silicon, aluminum, iron, calcium, and magnesium.

A summary of the data for this series was presented in the report

for the period ending in June 1978, in Figures 1 and 2. Figure 1 showed

the data obtained at 1600 and Figure 2 the data for 1300 C. Figure 2 con- tains an error of omission. The curve for A^O^ was out °f this

figure even though the label for it was present. The corrected figure

appears in this present report as Fig. 1.

1.2 Slag Preparation and Measurement

a. Seeded Series

As an extension of the previously reported series of artificial

slags, K^O is being added in order to simulate seeded-slag behavior.

K^O was added in 5% increments up to and including 30% by weight

to the slag, K-884, which is the base or starting point for the entire

series. This base composition was chosen to fall between the two coal

slags selected by most MHD contractors for study. They are slags from

Montana Rosebud Seam coal and Illinois #6 coal. The composition of this

base slag is shown in Fig. 2, which also shows the viscosity data at 1600,

1500, 1400 and 1300 C for the K^O-base slag series.

The above series shows the same trends as those shown by several

earlier series where K^O was added to different slags. All of these indicate

that K^O causes the viscosity to increase at all concentrations up to and

including 25 weight percent K2 O. Only the low silica series starting with K-48R

shows a lower viscosity than the base slag due to K2 0 at a concentration of 30%.

These studies show that the increase reaches a maximum at 15 to 20% K 0

and that the effect of the K^O is greater in magnitude for the low silica,

low viscosity series than for the high silica, high viscosity series. Figure 3 shows this effect for several K^O slag series, all at 1600 C.
8

b. Seed-Slag Heater Effluent

Viscosity was measured of a seed-slag mixture which was the effluent

from the FluiDyne MHD heater matrix. The starting materials were K

2 S°4

which was injected at the rate of 19 pounds per hour and coal which was injected at the rate of 10 pounds per hour.

The viscosity was so low that it was below the recommended limit

for the viscometer, but a rough figure would be 0.16 poises or -0.8 log

1Q poises. This means that this material is many times more fluid than

unseeded slag.

Care was taken to observe any immiscibility or separation of

seed and slag while in the liquid state. Some immiscibility was in fact

observed but the heavy darker layer was present in such a small amount that

its viscosity could not be measured at all. A rough estimate is that the

lower layer was less than 0.1% of the total mass of the mixture.

1.3 Future Plans

In order to more closely simulate natural coal slags small amounts of

the usual minor constituents will be added to the base slag to indicate the

extent of their influence on viscosity. These additions will include oxides

of phosphorous, titanium, sodium, and sulphur.
9

-10%

-5%

BASE

5%

lOJfc

15%

SUBTRACTIONS

ADDITIONS

CHANGE

IN

COMPOSITION

(wt.%)

Fig.

1.

The

Effect

of

Change

of

Concentration

of

Each

Oxide

on

the

Viscosity

of

Base

Slag

K-884

at

1300

C.

(47.3%

SiO_,

21.8%

Al_0,

;

16.6%

Fe„0„,

10.6%

CaO,

3.3%

MoO)
BASE

COMPOSITION

10

(s-Dd°'&0|) AilSOOSIA

K

0

CONCENTRATION

(wt.

%)
u

01 , (s.Dd Boi) A1IS00SIA

K

2

0

CONCENTRATION

(wt.

%)

Fig.

3.

The

Effect

ofK

0

Seed

Concentration

on

the

Viscosity

of

Several

Different

Slags

at

1600
12

2. Electrical Conductivity (W. R. Hosier, J. Langley)

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 Q) compared

to the sample resistance.

2 . 1 U-02 Phase III Materials

The electrical conductivity of most materials used in the Phase III

U-02 experiment has been measured. This data is reported in the Quarterly

Report (April-June 1978, page 12). However, two additional materials used

in Group 2 of the test were sent to us by Westinghouse Research and

Development Center. These materials were used in place of some Group 2 Materials due to some last minute fabrication difficulties. They are from a two component electrode containing mixtures of LaCrO^-LaAlO^

The electrical data is shown in Figures 4 and 5. The conductivity of

both compositions appears to be oxygen pressure dependent with the higher

conductivity associated with that material containing less LaAlO as expected. However, even this material in an air atmosphere would

experience some joule heating when used as a leadout near room temperature.

In the Phase III experiment they were attached to a nickel mesh and thus

designed to have the lowest temperature well above that of the copper

cooling block, however, at the cathode where oxygen pressure may be low,

the electrode function could be impaired by inadequate conductivity.

2.2 MgO-Cr^O^ Mixtures

A series of seven samples were sent to us by Westinghouse Research

and Development Center for evaluation of the electrical conductivity and

phase composition. Table 1 lists the nominal compositions of these

materials. Chemical analysis of the fabricated samples is in progress.

Table 1.

Sample MgO Cr„0_

2 3 Figure

MC-AT-0501 60m/o 40m/o 6

MC-AT-0601 56 44 7

MC-AT-0401 52 48 8

MC-AT-0101 50 50 9

MC-AT-0301 48 52 10

MC-AT-0701 2 98 11

CO-AT-0101 0 100 12

All these materials were hot pressed at 3000 psi and 1650 °C except

for the Cr

2 °3 sample CO-AT-0101 which was pressed at 3000 psi and 1450 °C.

One would expect the conductivity of the material just off the 50-50

composition on the Mg rich side to be higher than the 50-50 composition.

This is not the case; however, the 56 MgO-44 Cr^O^ mixture has the
13

highest conductivity of all compositions measured. X-ray diffraction

data, Task K, Section 1, page 52 of this report indicates that all

materials except the Cr^O^ an<^ MC-AT-0101 and MC-AT-0401 contain

second phases. The pure stoichiometric material 50 MgO-50 Cr^O^ or MgCr^O^ should have a high electrical resistivity.

2.3 YCrO^ Doped with Ca and Mg

In addition to the above series of seven samples, Westinghouse R&D

has sent a sample of YMg Cr --0 which was hot pressed at 3000 psi

. 05 • 95 3

and 1650 °C (YC-AT-0101) . The conductivity data is shown in Figure 13.

This sample was very dense and there appears to be little oxygen pressure

dependence except above 1600 °C. In this sample the magnesium is

substituted on the chromium site.

NBS has fabricated several samples of YCrO^ doped with calcium where

the calcium has been substituted on the yttrium site. These samples

Y Ca CrO (TT-MS-8416) and Y Ca CrO (TT-MS-8417) were thermally

• 9o • 0 3 • 9 5 • 0 5 3 sintered at 1746 °C for 6 hours in an atmosphere of forming gas. The

former was 90.2% dense ^while the latter was 95.2%. Theoretical density

was taken as 5.75 g/cm . Data is shown in Figures 14 and 15. Both samples

indicate a good electrical conductivity. One advantage of YCrO over

LaCrO^ is the possibility of avoiding the hydration problem particularly

in post test conditions when excess lanthana caused by evaporation of chromium,

at high temperatures in an MHD duct, goes to lanthanum hydroxide

resulting in a crumbling of the electrode material after a test.

Samples of this material doped with Ca and Mg substituted on both

the yttrium and chromium sites are being hot pressed at the University

of Dayton, Dayton, OH.
14

! (luo-ujl|0) AllAllDnQNOO

Figure

4.

T(K)

Electrical

conductivity

of

LaMg

2

f'

x

75^3

as

af

unct

l°n

of

temperature

at

several

partial

pressures

ofO2

.

This

sample

was

tested

as

part

of

Group

II,

Phase

III

U

-02

Materials;

it

being

the

lower

half

of

a

two

component

electrode.

The

sample

was

prepared

at

Wes

tinghouse

Research

and

Development

Center,
15

o

CS| O

4-1o O

3

CO 4-1 • 0)u to CO 3

<0 44 O

to o m

oi i-4 u C 1 CL O H

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4J CO w

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in o E 01 •H cO rH

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14 4-1 3 CJ CO 4-1 O' 0) tH >4

-r-4 H JO tO

1X4 W H CL
CONDUCTIVITY

(ohm-cm)

C 16

I

Figure 6. TOO

Electrical conductivity of 60 ra/o MgO-AO m/o ^203 as a function of temperature at

several partial pressures of 0_. This sample, prepared at Westinghouse R & D Centex was hot pressed at 3000 psi and 1650°C. (MC-AT-0501)

..... •f.s. .
17

sr

o T(°K)

Fig.

7.

Electrical

conductivity

of

56

m/o

MgO-44m/o

C^O^

as

a

function

of

temperatures

at

several

partial

pressures

of

0_

.

This

sample,

prepared

at

Westinghouse

R

&

D

Center,

was

hot

pressed

at

3000

psi

and

1650°C.

MC-AT-0601
CONDUCTIVITY

(ohm-cm)

18

Figure 8.

Electrical conductivity of 52 m/o MgO-48 m/o as a function of temperature

at several partial pressures of 0 . This sample, prepared at Westinghouse R & D

Center was hot pressed at 3000 psi and 1650 °C (MC-AT-0401)
CONDUCTIVITY

(ohm-cm)

19

Figure 9.

Electrical conductivity at 50 m/o MgO - 50 m/o Cr 0 as a function of

temperature of several partial pressure of O . This sample, prepared

at Westinghouse R & D Center was hot pressed at 3000 psi and 1650 °C.

(MC-AT-0101)

.
CONDUCTIVITY

(ohm-cm)

20

I04

Figure 10. -ryOiy'N

Electrical conductivity of 48 m/o MgO - 52 m/o Cr 2°3 as a function of temperature

at several partial pressures of 0 . This sample, prepared at Westinghouse R & D

Center was hot pressed at 3000 psi and 1650 °C. (MC-AT-0301)
21

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r—irz a

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£

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CONDUCTIVITY

(ohm-cm)

22

Fig. 12. Electrical conductivity of Cr^O^ as a function of temperature

at several partial pressures of 0^. This sample, prepared by

Westinghouse R & D Center, was hot pressed at 3000 psi and 1450°c.

CO - A1 - 0101
23

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O C3 —

V) W

4-J c/) •H •ri • > C—• wi •H rH 72 •UU

=3 • ~

T3 r-j C C c\> G

U of 0

»—

LTl

03 C3> u 0) •H >-i 03

>- a o: U 03 li U 03

cu <u -a

r-H 3-1 C

U4 cl o:

in

r-H

O' •H

Cl,
26

3 . Vaporization Studies (E. R. Plante)

Previous reports of vaporization studies have dealt extensively

with measurements of the potassium gas pressure over K^O-contaming

compounds or phases. These data are necessary for prediction and

correlation of the interaction of a seeded MHD plasma with construction

materials or cold ash using equilibrium thermodynamices . For example,

Spencer (1) noted the severe lack of thermodynamic or vapor pressure

daha 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 0 activities in solutions having up to 6 components which will be

more representative of slags in real MHD systems.

An additional objective of vaporization measurements is to

determine the stability of potential MHD construction materials with

respect to degradation due to loss of doping of materials by vaporization processes. In this quarter, results of measurements on the

potential electrode material MAFF-31 are reported.

3 # 3 Vapor Pressure Measurements on MAFF-31

MAFF-31 has a spinel structure in which Fe

+2 and Fe ions randomly

replace Mg+2 and A1 + 3 ions in spinel. It has the nominal composition

3MgAl

2 0

4

-Fe

3 04

.

The sample was prepared by Trans-Tech and was fired at 1400 °C in air

at NBS for 1/2 hour to remove binders. Since the MAFF-31 composition is

a solid solution the partial pressures will depend on composition. During

the measurements reported here less than 4 wt % of the sample was vaporize

so that the pressures are representative of the 3:1 molar ratio composition.

The sample was vaporized from an Ir Knudsen cell having an orifice

diameter of 1 mm and a Clausing factor of .41. The principal vaporization

reactions are

1/3 Fe_ 0

4

(soln)

MgO

1/3 Fe

30

4

(soln)

(soln)

Fe(g) + 2/3 O^fg)

Mg(g) + 1/2 C> 2 (g) (2)

FeO (g) + 1/6 0

2

(g) (3)

The measurements were made in our modulated beam, quadrupole

mass spectrometer. The mass spectrometer constant for Fe was determined assuming that the entire weight loss was due to evaporation of "Fe,<V ^ich is a good approximation in this case Mass

spectrometer constants for 0,. Mg and FeO were then estimated from

relative ionization cross sections obtained from literature.
27

The data are shown in Figure 16. In order of decreasing pressure

the lines represent the Fe(g), 0(g), Mg(g) and FeO(g) pressures. It is interesting to note that measured Fe(g) and Mg(g) pressure are only

20 to 50% below the values that would be expected if Fe 0 and MgO were

at unit activity. This observation is consistent with iihe relatively

low heat of formation of spinel from its component oxides ('i -10 kcal) The slope of the O^g) pressure curve is significantly less than the slopes of the other 3 curves. This is consistent with the fact that spinels tend to become oxygen deficient at higher temperatures.

At a temperature of 2000K the Fe pressure over the MAFF-31 composition is close to 10 atm. Under clean electrode conditions the rate of loss

o Fe at 2000 K would be appreciable. However, electrode lifetime would be enhanced by a significant factor if electrode surface temperatures could be held in 1800-1900 K range.

Reference

1. F. E. Spencer, Jr., J. C. Hendrie, Jr., and D. Bienstock, 6th Inti.

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

Future Work

In the next quarter, measurements on the K(g) pressure over K O containing compositions will be continued. 2
VHPDR

PRESSURE

DP

MRFF

31

200

[

a

1300

1000

1700

I

000

28

on

l pLl hi‘a

uj

‘s fa

a

fa

( ( Hid >d )901
29

TASK J. CORROSION AND DIFFUSION

Insulator-Electrode Couples (E. N. J. R. Manning Farabaugh and

Interdiffusion of iron between MgO insulator material and MAFF 31 (3MgAl

2 0

4

:lFe

3 0

4

) at 1400 °C has been measured and analyzed. Combining these results with previous measurements of interdiffusion at 1600 °C (Quarterly Report Sept. -Dec. 1977, page 31) provides an estimate of the temperature dependence of diffusion in these materials. The concentra- tion profile obtained from the 100 hr. test of 1400 °C for the MAFF 31-MgO couple is shown in Figure 1. In the plot, the complete profile is not reproduced because of the long tails in the MAFF and MgO. The figure shows the tail in the MAFF, the profile near the interface and the tail in the MgO.

Analysis of the interdiffusion results was done by plotting the concen- tration vs. distance points on probability paper. On such a plot a single straight line would be obtained if the diffusion coefficient

were constant. The probability plot indicated that there are three distinct diffusion regimes in the couple, each of which is characterized

by a more-or-less linear segment on the probability plot. Near the original interface a sharp concentration gradient was found in iron

concentration; whereas, by contrast, the gradients in the neighboring regions were noticeably much less sharp. Iron diffusion in the MgO

was found to proceed about 3 times as fast as iron diffusion in the MAFF 31. The value of the iron interdiffusion coefficient in the MgO

is approximately 12 x 10 10 cm2/sec and that in the MAFF 31 is approxi- mately 4 x 10-iU cm2 /sec

.

The sharp concentration gradient in the middle region corresponded to an effective diffusion coefficient of about 0.5 x 10~ 10cm2 /sec . This

regime covered the composition range from about 30% to 75% of maximum

iron concentration in the MAFF 31 and extended in distance from approxi- mately the final position of the interface after diffusion to near the

Matano interface position, or roughly the original interface location.

Thus, this interface region may represent a transition regime between the

original MAFF 31 and MgO sintered samples.

Because of the effective low diffusion coefficient, the interface region

may provide a barrier to Fe diffusion from the MAFF 31 into the MgO; but

within the MgO itself iron diffuses more readily than in the MAFF 31

spinel (3MgAl

2 0^- lFe

3 0

4

) structure. This difference occurs despite the

MgO specimen being closer in density to the theoretical density value than

the MAFF specimen (98% of theoretical density for the MgO vs. 95% for the

MAFF) . It is possible that in both the MgO and MAFF regions grain boundary

diffusion is strongly influencing the results that are observed.

When the present results are compared with previous results on interdiffusion

in MAFF 31-MgO samples at 1600 °C, it is found that the increase in temperature

from 1400 °C to 1600 °C increases the diffusion coefficient in all three

regions of the couple by a factor of approximately 4 to 7. Since the

penetration distance of the iron into the MgO and the extent of the iron

depletion region in the MAFF are proportional to the square root of the

diffusion coefficient, the penetration distance for iron diffusing out of

the MAFF electrode material and into the MgO insulator materials is

increased by approximately a factor of 2 to 2-1/2 when the temperature is

increased from 1400 °C to 1600 °C.
30

Similar factors of increase in diffusion coefficient and penetration distance

would be expected if the temperature were increased for example to 1800°C, with

the penetration doubling about every 200C deg., presuming no change in diffusion

mechanism. Large extrapolations become increasingly doubtful, however, both

because some change in diffusion mechanism probably will occur as temperature is

increased, thus changing the temperature dependence, and because the temperature

dependence even in the region 1400°C to 1600°C still is not known with great

accuracy.

At the end of 100 hours at 1400°C, penetration of Fe into the MgO is about 600ym

and from the MAFF about 300pm. According to theory, diffusion penetration is

expected to be proportional to the square root of the time. Thus, penetration

after 400 hours would be double those shown in the figure. This dependence on time was confirmed for the present materials in the measurement made at 1600°C, and

similar results are presumed to be valid at other nearby temperatures, again

presuming no major change in the reactions that are occurring. Thus, present

results allow estimates to be made of total penetration expected for not only

other temperatures but also other times. Further measurements underway on a similar couple diffused at 1500°C will provide more detailed information on the

temperature dependence.

MAFF31-MgO and MAFF31-S71 (MgA^Op) diffusion couples tested at 1500°C for 30

hrs. each have been prepared. Profiles for these couples and their analysis

will be presented in the next quarterly report.
5000

Figure

1.

Fe

concentration

profile

in

MAFF31-Mg0

diffusion

couple

tested

at

1400
32

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

A. D. Franklin, N. K. Adams, L. Hsu and C. K. Chiang)

The furnace described in our Quarterly Report for April-June 1978

(EA-77-A-01-6010-12) describes a platinum-wound furnace for electrical

measurements on slags at high temperatures. This furnace has been

completed, tested and put into operation. The maximum operational

temperature is about 1550 °C. The temperature variation over the region

of the furnace occupied by the specimen is less than 1 °C.

Preliminary measurements have begun on a sample of synthetic slag

K302 (the composition of this slag is given in the Quarterly Report

cited above) at a relatively low temperature, around 1250 °C. The

slag was broken up with a hammer and placed as a powder around the

electrodes. As described in our last Quarterly Report, one electrode

was Pt and the other Fe.

Under open circuit conditions a voltage is observed, with the Fe

negative, whose magnitude drops slowly with time from more than 0.4 V

to around 0.1 V. Under short circuit conditions a current of 0.1 to

2 mA flows. Evidently, the electrochemical reaction of iron at the

Pt electrode

2 + +

Fe + 2e Fe (Pt)

is sufficiently rapid that the cell can act as a battery. The Pt in

parenthesis at the right side indicates that the Fe dissolves into or alloys with the Pt.

The drop off in open-circuit potential with time suggests that

at least the surface of the Pt becomes saturated with Fe , even under

open-circuit conditions. This can only occur if transport in the slag

by both Fe ions and by electrons is possible.

When a potential difference was applied across the electrodes,

zero current occurred when the applied potential exactly matched this

open circuit potential. With the Fe more negative than this, ohmic

behavior was observed for over potentials of up to several volts.

This current should be purely electronic, although conceivably

dischargeable anions could also carry current. However, oxygen ions

appear to be the only viable candidate, and these experiments are being done under tank Ar (P <10~^ atm.) so that oxygen transport

2 seems unlikely. At higher voltages the current increased more rapidly

than linearly with voltage, suggesting that additional charge transport

or transfer mechanisms are activated at higher voltages.

With the Fe positive with respect to the Pt the behavior again was ohmic for small over voltages, with a slope indistinguishable from that

for the opposite polarity. Since this current should contain both

electronic and ionic currents, it may mean that the ionic conductivity is

much less than the electronic, a not unexpected state of affairs at

temperatures not much higher than the melting temperature of the slag.

A systematic study of this behavior is planned for the next Quarter.
3. Electrochemical Effects in Simulated Slag Flow (E . N. Farabaugh, W. R. Hosier

W. Capps)

Previous work on "hot" platinum electrodes in a simulated slagging

generator have been described in the last Quarterly Report (April - June 1978,

page 31) . Several of these electrodes have now been examined by SEM and EDX

techniques. The results presented here cover the examination of the

first two (E5 and E6) runs which were made using fly ash but no seed at

a temperature of 1400°C. A brief summary of these results is given

below.

E-5; rotor—cathode, stator—anode. (See April - June 1978, page 33 for

photos of stator and roter) .

1) The Fe had alloyed to some degree with the cathode. There was

a high Fe concentration in the platinum at the cathode-slag

interface. The concentration of Fe drops off towards the

interior of the cathode. No Fe was detected in the anode.

2) No Fe-rich layer was found next to either the cathode or anode

.

3) The cathode had a very porous microstructure at the cathode - slag interface. The anode exhibited the same type porous

layer at the anode - slag interface, only thinner.

4) The slag was, in general, richer in Fe around the cathode than

the anode.

E-6; rotor—anode, stator —cathode.

1) No Fe was detected in either the cathode or anode.

2) A thin Al-rich slag phase layer was found at the cathodeslag interface. Clumps of an Al-Fe-Cr phase were observed

at the cathode-slag interface also.

3) No layer or clumps were seen at the anode-slag interface.

4) A porous microstructure layer was seen in the electrode at

both the cathode-slag and anode-slag interfaces, thinner in

the anode.

Runs 7 and 8 will be studied by SEM and EDX techniques and a complete analytical

discussion of the results of runs 5,6,7 and 8 will be made in the next Quarterly

Report

.
34

4 . Seed/Slag Interaction and Slag Corrosion (L. P. Cook)

Six-component synthetic "Eastern" and "Western" channel slags^

with the compositions given in Table 1 were prepared according to the

following treatment. Crystalline KAIO^, Mg^SiO and Ca SiO were

prepared by calcining the necessary reagents, with repeated homogenization, until x-ray powder patterns indicated complete reaction to

produce the desired phase. These materials, with Fe 0

3

and a-SiC>

2

, were weighed in the appropriate proportions in a drybox, homogenized

under acetone, dried at 160°C under vacuum, and dry-homogenized

repeatedly while hot. Pellets of 1/2" diameter were pressed, wrapped

in Au foil, and fired at 950°C in air for 24 hrs. The products were

brick red, indicating very little reduction. These pellets were

crushed, re-pressed, wrapped in Pt foil, dried briefly at 950°C and

weighed. They were then fired in air at 1300# C for 24 hrs, with

periodic weighings. The model Eastern channel slag lost 17 mg,

almost entirely within the first 3 hrs - in view of the high Fe content, this may represent predominately reduction. The model Western

channel slag lost 9 mg continuously over the 24 hrs of heat treatment - this probably represents K 0 loss. Such weight losses indicate negligible

K

20 vaporization in view or the 10 g total weight. Following this the

products (now dark brown) were crushed, re-pressed, wrapped in Pt foil

and refired at 1300°C in air for 36 hrs. Both compositions showed slight

evidence of melting during the final heat treatment. X-ray powder patterns

indicate that KAlSiO^ is by far the dominant phase in both the Eastern and

Western compositions as prepared. The Eastern composition contains as well small amounts of KAlSi

20g and at least one unidentified phase. The

Western composition also contains at least one other unidentified phase,

presumably a calcium (iron, magnesium) silicate.

Following this mode of preparation, small samples were sealed in

Pt tubes and equilibrated for varying lengths of time (see Table 2)

,

then quenched. Polished surfaces were prepared and examined with

SEM/EDX . By comparison with known standards, semiquantitative estimates

of elemental compositions of various phases could be made.

Results for the "Eastern" channel slags are shown in Figures 2-8.

As Figure 2 shows, there is evidence for two liquid phases in the

"Eastern" model channel slag under the relatively oxidizing conditions

in which these experiments were performed (10 to 10

u atm P

Q

)

. Small

globules of iron-rich melt can be seen adhering along the 2

interfaces between crystalline KAlSiO and the main melt phase, both

at 1600°C where there is no Fe

3 0^ (Figure 2 -A) and at 1475°C (Figure

2-B ) below the precipitation temperature of Fe

3

0

4

. These droplets

appeared to coalesce slightly in some of the experiments, but location

of droplets large enough for x-ray analysis was difficult. The semiquantitative analyses of the immiscible phase in Figures 3-8 must

therefore be regarded with due caution, as the possibility of some contribution from surrounding phases cannot be completely discounted.
35

For this model "Eastern" channel slag composition, the iron-rich

immiscible melt disappears below 1450°C and Fe^O^ spinel becomes

correspondingly more abundant. These experiments indicate the

relatively low amount (Figure 7 ; 15-20 wt % Fe as Fe 0

3

) of Fe0

x which

can be accommodated by the model "Eastern" channel slag under oxidizing

conditions before supersaturation with respect to an iron-rich phase

occurs. This has probable application to generator operation (anode

side) . Analogous experiments at low Pq

2 (cathode) are planned. Model

"Eastern" channel slag contained an appreciable volume % of crystals

at 1625°C. The liquidus temperature although not determined, is thought

to be above 1700°C. As KAlSiO^ crystallizes with falling temperature,

the main residual melt phase becomes depleted in K

20 (Figure 3 ) . A12

0

3 changes only slightly with falling temperature (Figure 4 ) , and SiO^

actually increases slightly in the residual melt (Figure 5 ) . Calcium

oxide shows a rather marked increase from ^3 wt % at 1575 °C to ^10 wt %

at 1325°C (Figure 6 ). Iron oxide in the main melt phase (Figure 7 ) increases slightly as the temperature falls from 1625°C to 1475°C,

where an inflection in the fractionation curve (possibly corresponding

to the precipitation of Fe^O^) results in decrease of iron content

with further drop in the temperature to 1325°C. Magnesium oxide is

present at low levels such that analytical uncertainty possibly accounts

for some of the scatter in Figure 8 . Compositions of the immiscibile

iron-rich melt phase are also plotted for the range 1550-1625°C.

Significant ^0, A1

2

0 , and SiC>

2 are present, but in considerably

smaller percentages than FeO . CaO and MgO are definitely enriched

in the immiscible melt phase relative to the main melt phase. The

upward temperature termination of the postulated immiscibility field

is not known; note the suggestion for a trend toward convergence of

the two melt compositions above 1575°C.

Analogous experiments for the "Western" model channel slag are reported in Figures 9 - 14 . in contrast to the "Eastern" model com- position, no liquid immiscibility was observed, and the fractionation

trends can be interpreted largely in terms of the crystallization of

KAlSiO , with resultant depletion of K

20 (Figure 9 ), A1

20^ (Figure 10 ),

and Si0

2 (Figure 11 ) in the residual melt. Concomitant enrichment in

CaO (Figure 12 ) , FeO (Figure 13 ) , and MgO (Figure 14 ) occurs within the

residual melt as theXtemperature falls.

E. R. Plante and L. P. Cook, "Compositional Modeling of MHD Channel

Slag, with Preliminary Vapor Pressure Data", Proc. 17th Symp. Eng.

Aspects MHD, Stanford, Calif., C.1.1 (1978).
Table 1* Suggested Compositions for Modeling Channel Slag.

Bituminous ("Eastern") Sub-bituminous and Lower Rank ("Western")

wt % wt %

(Fe as Fe

2

C> 3

) (Fe as Fe

20

3

)

Si0

2

36.0 34.7

A12°3 25.5 24.5

CaO 1.8 9.3

MgO 0.6 3.3

k

2° 23.6 22.7

100.0 100.0

Table 2. Quench Experiments on Model Channel Slags.

Composition T°C Duration (h) Composition T°C Duration (h)

Eastern 1200 48* Western 1200 48*

1225 46* " 1225 46*

1250 23.5* " 1250 23.5*

1275 24* " 1275 24*

1300 24* " 1300 24

1325 10.5 " 1325 10.5

1350 8 " 1350 8

1375 21 " 1375 21

1400 6 " 1400 8.5

1425 5 " 1425 5

1450 4 " 1450 4

1475 2 " 1475 2

1500 2.5 " 1500 2.5

1525 1 " 1525 1

1550 2.5 " 1550 2.5

1575 2.5 " 1575 2.5

1600 1 " 1600 1

1625 1

* Not enough melt phase present for analysis
37

Figure 2. High contrast secondary electron images of polished surfaces,

quenched model "Eastern" channel slag (^4500X) . K = KAlSiO

KAlSi^O ; M = main residual slag phase; I = immiscible iron

rich melt; F = iron spinel; R = raster mark, showing size of

EDX analysis area.
Wt%

K

20

38

Figure 3

Eastern Channel Slag

T°C

Semiquantitative analyses (I/I ) of wt % as a function of T(°C) in reside

slag phases for model "Eastern® channel slag composition.

'rc

I
Wt

%

AI2O3

V)

26

24

>9

29

18

16

14

Eastern Channel Slag

12

1300 1350 1400 1450 1500 1550 1600

Oi T”C

c 4. Semiquanti tative analyses (I/I ) of wt % A1 0 as a function of T(°C)

slag phases for model "Eastern^ channel slag composition.

—

1650

in residual
Wt

j

Si0

2

40

Eastern Channel Slag

T°C

Figure 5. Semiquantitative analyses (I/I ) of wt % SiO^ as a function of T(°C) in res. slag phases for model "Eastern^ channel slag composition.
41

(0 o

1300 1350 1400 1450 1500 1558 160C 1650

t, . Semiquantitative analyses (I/I ) of wt % CaO as a function of T(°C) in residual

slag phases for model "Eastern^ channel slag composition.
Wt

%

FeO

x

as

Fe

2

<>3

42

Figure 7* Semiquantitative analyses (I/I ) of wt % FeO^ as Fe

2°3

as a function of T(°C) in residual slag phases for

model "Eastern" channel slag composition.
Wt%

MgO

4 i

1300 1350 1400 1450 1500 1550 1600 1650

T°C

Figure 8. Semiquantitative analyses (I/I ) of wt % MgO as a function of T( C) m reside

slag phases for model "Eastern^ channel slag composition.
Wt%

K

20

44

Figure

T°C

bemiquantitative analyses (I/I ) of wt % K 0 as a function of T(°C) in residu.

slag phases for model "Western channel slag composition.
Wt%

Al

2

0

3

45

T°C

'igure 10. Semiquantitative analyses (I/I ) of vt % Al^O^ as a function of T(°C) in residual

slag phases for model "Western^ channel slag composition.
Wt%

Si0

2

46

Western Channel Slag

1380 1358 1480 1458 1588 1558 1688

T°C

Semiquantitative analyses (I/I ) of wt % SiO^ as a function of T(°C) in

slag phases for model "Western® channel slag composition.

Figure 11
Wt%

CaO

47

T°C

jure 12. Semiquantitative analyses (I/I ) of wt % CaO as a function of T(°C) in residual

slag phases for model "Western^ channel slag composition.

I

l
Wt

%

FeO

x

as

Fe203

48

13. Semiquantitative analyses (1/1 ) of wt % PeO as Fe 0

"Weste r

0f

1 T( “C> “ reSi8ual sl*9 f°r mo^el Western channel slag composition.

Figure
Wt%

MgO

49

Semiquantitative analyses

slag phases for model 'Western

of wt % MgO as a function of T(°C)

channel slag composition

.

in residual

Figure 14 .
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.), which were analyzed by x-ray diffraction methods.

These materials include those developed at NBS as well as those

sent to us by other MHD contractors. Data, transmitted to the

appropriate source, are shown in Table 1.
51 >

4J 03 03 03 03 03

rl 0 0 9 o 0 0 c 0 Q H O Q 0

ria (3 03 CJ CJ CJ

rH 03 rH 0 0 2 0 0 0

rtf

p P p P p

4-) £ o

(/) E p

n

p & d

•H •H

d

•H P p •rH -H P •rl 03 -rl

u 03 03 03 0 0 03 03 0 03 0 03

03 03 03 0 o 03 Q3 0 03

Q 03

T1 2 2 d

d 2 2 d 2 c5 2

4- +

r-H 03

d

d 0)

d •H

O

d o o d 0) o rH Cn 0

•H •H -rl 03

p •H 0 2 -H

-p (N

O

p Cl P o P CN 03

p

d

O CN

d (33 •H rH

d o + d i—1 P 1 1 rH p p p p 0) rH 0 (SI < 03 0 n* (33 o 0 IP p 03 o

(31 CJI P (33 o > o (33 -H P 01

CJ 2 •H CN 0 p

O

p

O +

co 03

H P 03 rH

p >1 03 P 03 P 03 o •H P + (33 •rl < 03 Cl p p > P -r| CN & rH 3 > 1—1

tp Cl

p

d 0 o rH o n1 u d 0 u 03 0 0 2 r- O o p 0 0 p o

03 0 (3) P p (33 (NO (33 03

p 01 (S3 CN

PI •H + O 03 + o CN + d P P p 03 1 1 Cl 03 , P 03 d 03 O < c + d -P 03 p P Q3 H (S3 P 03 z P •H Cn

d) rH -H P o 00 •H P CN P P on -H

p 2 w 0) 0 p 0 0 o P O 03 P O O o p 3

a)

p (33 (33 rH

p (33 1 1 p P (33 P P CN 03 o + p 0 > p rH > P o > P u P >

0. rH 03 o o Cl < o U

— + i—1 o o 03 IS! o + CN

p •rH

p 0 p o 03 03 P 0 P CNp on o

<D o rH 03

p [''•p

d 0)

p

p

d

d 03 P -

03 03 i—1 O

p

03 0 0 Cl >1 o u 0 Cl >1 0 •H 03 Cl >i P d 03 CNN

<0 P (3) Cl CN

— •H Cl + -H Cl Cl

Od P p >1 m P 03 p 00 P (33 P on + mp <1

O

cu Cl 03

O r~

N XId o t'' CNd

O

O i'- P CN

O dj rH -rH X> O CN rH o o rH n1 — — O 03 O .—

0) rH P

(0 t^H rH (N 03 + o rH CN

p 0 o 03 P d

p P • d

a o <C P XI (33 < P (S3 (33 <N P <i P 03

O (S3 on o

CN03 «.

N

r—1 N i—1 - IS1 03 •H ••

O o

E-t

p > P CN + 03 03 P CN o 03 <c + P (N 03 P 0 P (N CM +

(S3 0 u 03

d •H CJ 03 *H -H CP

U 03 i—1 d

p

U rH 03 (NP — XI %

•H rH

p

p rH 2 O

— P P Xj

— P P o o

03 03 03 Cl 0 03 3 0 CP 03 P o 3 03 CJi 1 CJi CJI XI Cl XI + 2 (33 (33 p + u (33 + 2 P + <c (33 O P 2 2

HeP 03

03 03 1—1 1—1 C 03 •» P r-H 03 0 2 n p r-H • s rtf •* p

-H 0 P o 03

p P p P 0) Cl (33 p E (33 (33 03 Cl p d 03 d CP d P • s

•H

p

p i—1

>i

p

d P 03 P p rH rH o d i—i o -H O E O u ol * 0} * i—i

He He P * He He P o * He He

01

d •H 03 P d P 03 P 03 P 03 P 03 P d P aJ cnP P P

a) 03

p 03 03 *H 03 •H 03 03 03 rH 03 03 03 03 03 i—i P 03 03 03

Q -P 03 2 03 03 P 03 2 03

d 03 5 03

p 03

d < 03 03 03

rH P 0 3 o P 2 0 (33 £ 0 5 p 03 CJI 5 Sj S 0 03 «—1 0 CP 0 03 O rH 0 d 0 P O 0 0 d 2 0 0 0 £ E n Cl 2 Cl d Cl d Cl H d d Cl 2 d M d d d H H H H M H M M M M

H

H

H

H IH

M

HHHMHM

a)

M 03

M 03

M Q3H • 03

H Qj P 03

H 03H 03M 03M 0)

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55

2. Electrode Systems and Component Materials Test (W. R. Hosier,

E. N. Farabaugh and T. Negas)

2.1 NBS/AVCO Slagging Test 2 .

The six electrodes described in Quarterly Report, January-March 1978,

page 38, have been sent to AERL in Everett, Mass, and testing will take

place as soon as they can be accommodated in the MHD test channel there.

There was a delay in the fabrication due to some difficulties in trying

to incorporate the thermocouples just below the platinum cap during the

spraying process. It was hoped that these thermocouples would give an indication of the cap temperature (a critical point in this electrode

design) • They have been eliminated. The specification on the cap thickness was decreased from 0.5 mm to 0.25 mm but the roll over on the

upstream anode edge was specified at 0.5 mm or thicker. Several leaks

at the water cooling tubes and passage plugs solder joints were detected

and had to be repaired also.

While special arc plasma sprayable powder (nominally +10 to 40 p

)

is used in fabricating these electrodes, the spraying efficiency is

not good (<50%) . Fortunately, most of the overspray and dusting

powder can be retrieved.

2.2. U-02 Phase III, US-USSR Cooperative Program

The Phase III test carried out in Moscow from May 13-16, 1978, was described briefly in the last Quarterly Report (April-June, 1978 on page 57)

Some findings in the post test analysis are given in

section 3 of Task K of this report.
3 . Test Analysis (Pre and Post Test Materials Characterization

31 Analysis of AVCO Electrodes (T. Negas)

56

Representative anodes and cathodes from a recent long term test

at AVCO were forwarded via DoE to NBS. The test apparently was discontinuous but 250 hr were accumulated mostly with slag. Cathodes were

constructed from copper which was "capped" via a solder with a Cu/W

metal composite. A nickel tab was soldered laterally to the Cu of each

electrode. Boron nitride insulation was utilized between this Ni-tab

and the Cu of the adjacent electrode. The cathode assembly provided a continuous, flat surface exposed to the plasma. Castable apparently

was not utilized to promote "sticking" of slag and, indeed, little slag was noticeable on the post-test specimens. Anodes were constructed in a similar fashion. However, the copper was "capped" via a solder with

oxidation resistant Pt-metal. The copper also was recessed at one extremity to accommodate a castable. After testing, the original castable

was absent having interacted with slag. Figure 1 illustrates schematically

the post-test condition of the electrodes.

Anodes and cathodes are in remarkably excellent post-test condition

considering the rigors imposed by the AVCO test facility (this writer has

not seen better) . Both, however, display, in hand specimen, similar

degradation and loss of metal. This recession is confined to the plasma

side of regions originally encompassing (in cross section) the boron

nitride/Ni-tab/Cu interfaces. The associated Pt and Cu/W "caps" as well

as the recessed Cu areas remained relatively intact. It was speculated

that electrochemical processes could account for the observed degradation

and, indeed, a reasonable model probably can be formulated.

Electrode cross sections (polished) and untreated, perpendicularly

disposed, metal surfaces exposed to slag/plasma were examined comprehensively

by SEM/EDX methods. Categorical evidence indicative solely of electrochemically-induced corrosion, although not precluded, was not obtained.

Gross features were either simple and obvious (e.g. melting) or so chemically

complex that pinpointing of specific corrosion mechanisms and derivable

conclusions cannot be fully justified. Major observations, nevertheless,

are summarized below. Photographic evidence including elemental EDX

analyses are on file and available.

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Pt

ANODE

Figure 1. Post-test condition of AVCO electrodes.

CATHODE
61

U-25, RM Channel Deposits (T. Negas, C. L. McDaniel)

S. J. Schneider, during his recent visit in Moscow, obtained two

types of "deposits" derived from the U-25, RM channel. Figure 2A,B

illustrates material (designated as Z) obtained from near the generator

inlet while Figure 3A,b,c shows a material (designated as C) taken from

near the channel outlet. Material C apparently was located at the intersection of the side and upper generator walls.

Specimens from the above materials were collected for x-ray diffraction

analyses. Material Z consists of monoclinic plus cubic variants of

zirconia. The discolored, wall side portion (Figure 2A) is dominated

by the cubic phase while the plasma side material (Figure 2B) consists

of nearly equal amounts of both variants. Specimens will be analyzed by

EDX to deduce what oxides were utilized to achieve at least partial

stabilization of the monoclinic ZrC>2

. This, possibly, will then aid in

determining whether the material has origins from the generator combustor

sections.

Material C is much more complex. The main bulk with a pinkish discoloration (note Figure 3

) consists of unaltered Al

2°3 P^us A3- p°

4

* T^e pinkish coloration is due to chromium as determined by EDX analyses (Si,

Fe, Ni and even Ag were noted as frequent impurities) . The phase assemblage

including the presence of chromium strongly suggests that material C is a chrome-aluminum phosphate bonded, aluminum oxide-based castable or cement.

If considered necessary, microstructure and microchemistry also will be

pursued. The plasma side of material C is frequently intermixed and overlain by rivulets of white to buff material (note Figure 3A,C). These

consist of zirconia (monoclinic plus cubic) admixed with alumina and

probably represent a phase once fluid derived from upstream (combustor?)

materials. Part of the interior and wall side of material C (Figure 3B) are characterized by corrosion products having green to brown discoloration.

These products consist predominately of K-f3 alumina. However, other

corrosion products are present as indicated by distinct hydration and

swelling at room temperature. Associated x-ray patterns yield weak,

broad, maxima (not seed) which cannot be interpreted unambiguously.
• V . ..

;

•

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* •

. ft *

- Qt f.

• 'C tftKM . '{‘X&'It’ t*' iO *» • "c. :

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*

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:

.
62

Figure 2A. U-25 f RM channel Deposit.

Figure 2B. U-25, RM Channel Deposit
J 'i'.J §'l
63

Figure 3A

.

Figure 3B.

Figure 3C.

64

3.3 U-02, Phase III, Westinghouse Test (W. R. Hosier, T. Negas

,

L. P. Cook, E. N. Farabaugh, C. L. McDaniel, A. Perloff)

During this reporting period post-test analytical data derived from

the U-02, Phase III module, are limited mostly to x-ray diffraction

work and radiography. Portions of the USSR, MgO-based insulating

wall adjacent to the electrode walls also were received and subjected

to x-ray analysis. Power supply problems for over one month have

precluded additional work utilizing SEM/EDX methods. A final report

to DoE and Westinghouse is scheduled for the end of October 1978,

although our preliminary data have been discussed with Westinghouse

personnel. The completed report (or salient results and conclusions)

will be given in the next quarterly report.

During the course of the Phase III test a flowing layer of material of

relatively high viscosity formed near the perpendicular junction of the

U.S. electrode walls and the USSR insulating walls. This layer was discovered after cool down. During the run a steady decrease in the

effectiveness of the interelectrode insulation was noted and post test

observation of this fluid layer led to speculation that it might have

contributed to the insulation degradation observed. Figures 4 and 5

show the electrical conductivity of an upstream and downstream sample

of this flowing layer up to a temperature of about 1550 °C.

The results of the conductivity measurements shown in Figures 4 and

5 do not conclusively prove that this layer was solely responsible

for the interelectrode insulation degradation.

X-ray diffraction data (Task K - section 1, this report) indicated a large amount of 6-alumina present in this material. The origin of

the alumina is not know. The foreign deposit was most heavy on the

anode side and was beige in color while the lighter deposit on the

cathode was partially pink probably due to the presence of chromium.

4. Materials Development and Analysis

4.1 YCrO

^ -Based Ceramics (T. Negas, W. R. Hosier)

During this quarter several runs were conducted to prepare high density

ceramic bars based on YCrO^. Using precipitated and precalcined powders

prepared at Trans-Tech Inc. ingots were thermally sintered at 1750-1760°C

(6-8 hr) in a forming gas environment. Better than 95% of theoretical

density ultimately was obtained for Y

Q 98

m9q o2Cr°3 and members within

the series Y^^Ca^CrO^ x <_ 0.05. We are experiencing some difficulty

with the x =0.10 composition. This possibly can be traced back to the

chemistry of the initial precalcined powder.
' *<

.

•

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.

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• ; , _ V A • , < J -’-3)

• • •. ~ .
CONDUCTIVITY

(ohm-cm)

65

TOO

Figure 4.

Electrical conductivity as a function of temperature of the ceramic deposit on

the insulating wall of U-02 (Phase III). This sample taken from upstream portion

near anodes 2209 and 2213. (BNW M 129C)
CONDUCTIVITY

(ohm-cm)

66

T(K)

Fiqure 5.

Electrical conductivity as a function, of temperature of ceramic deposit on the insulating wall of U-02 (Phase iff) . This sample taken from duwnsLiuiiJIT

portion near anode electrodes.
67

TASK L. ASSESSMENT OF STEAM PLANT COMPONENTS

1. Laboratory Hot Corrosion Screening Program (j. Smit)

One of the initial goals of this screening program was to develop

procedures for detecting corrosion in its early stages. With appropriate

metallographic techniques, SEM-EDX analysis, using a modified instrument

capable of detecting light elements such as oxygen, appears to provide

this capability. To date incipient corrosion has been detected in specimens

that have been exposed to a hostile environment in our burner rig for

relatively short times, test durations of about 4 hours (see Task L in

the quarterly report for the period April-June 1978, "Development, Testing

and Evaluation of MHD Materials"). In these short duration tests, if

corrosion is detected, the suitability of the material for use in downstream components is questionable. If, however, no corrosion is detected,

the converse does not necessarily apply. One can only say that the material

showed no signs of corrosion for the time interval of the test. It then

becomes necessary to test for longer intervals.

At present all our gas flows and cooling air flows are manually controlled.

Regulator valves are trimmed by hand to maintain the hot zone temperature

and specimen temperature at the required levels. Partly because of this

and partly because of current gas supply limitations the rig cannot be

operated unattended, this in turn limits test duration. To provide for

unattended operation and hence long duration tests, when required, our

rig is being rebuilt along the following lines.

A modified two gas blender-controller connected to the oxygen and propane lines and interfaced with a thermocouple output amplifier will be

used to control automatically the hot zone temperature. The specimen

temperature will be controlled automatically by a modified mass flow

controller on the cooling air line and interfaced with another thermocouple output amplifier. The propane and oxygen supplies are being

expanded from the single cylinder units now used. A six station manifold with supply and check valves is being obtained for the oxygen supply

and a two station manifold also with supply and check valves is being

acquired for the propane supply. With these manifolds and this valving,

sufficient gas can be kept on hand to permit unlimited time duration

runs. The automatic temperature control units will allow unattended

operation. With the improved rig, those materials that survive the

initial screening can be further tested to determine the effects of long

time exposure.

Currently specimens of AISI 1015 steel, 310, 316 and 446 stainless

steel are being evaluated. These materials have been exposed under the

following conditions:

a. Hot zone temperature 1330°C

b. Specimen temperature 550°C

c. Total test time 4 hrs.

d. K

2 S°4 F ^ow rate 10 gro/roin.
68

e. k

2 S0

4

f1ow time 30 mi n *

Metallographic specimens have been prepared from these pieces and

are now being analyzed.

2.

Material Assessment (J. R. Cuthill and D. J. Kahan)

A follow-up on the "800" alloys

INCO alloys 800 and 800H with the following respective nominal

composi tions

Cr Ni Fe Al Ti C

800 21 32.5 46 0.38 .38 .05

800H 21 32.5 46 0.38 .38 .1

have been included in previous reports in the list of Selected Wrought

Alloys (see for instance the quarterly report for the period ending Dec.

31, 1977) as alloys to be included in simulated performance testing. The

"800" alloys, including the 801 variation with increased titanium to

increase corrosion resistance to sulfur dioxide and stress corrosion, and

800H and 802 with increased carbon to develop carbide precipitation for

maximum strength, respectively, have become widely used for many

components of power plants both in the U.S. and in Europe (Ref. 1).

Applications of 800 alloys in fossil-fuel fired power plants include steam

superheater heat exchanger tubing and economiser and sulfur dioxide

recovery units because of their generally all around superior properties,

including: -

1. Very good high- temperature creep strength.

2. Good resistance to H2 S attack under certain conditions*.

Resistance to H2S attack twice that of 310 stainless steel.

(see for instance previous quarterly report for period ending

June 30, 1978.

3. Good resistance to high- temperature oxidation, carburization,

and hot corrosion.

4. Good resistance to stress corrosion cracking.

5. Good cold forming and weldability.

The specific alloy modifications alluded to previously are as follows

Alloy

801

800H and 802

Modification Application

twice Ti content of

"800" to optimize re- sistance to stresscraking

heat exhangers for SO 2 recovery and hi-temp.

steam

twice and 7 times C

content of "800",

respectively, to optimize

high-temp, strength

hi-temp. steam heat

exchanger tubing

*See "Limitations" for further details.
69

The relative cyclic high temperature oxidation resistance, resistance to

scaling, of these alloys in air in respect to 304 and 309 stainless is

shown in Figure 1 from "Incoloy Alloys" (Ref. 2), which is a good source

for physical and mechanical properties. The relative allowable tensile

stress for alloy 800 in respect to other commonly used alloys for power

plant components, based on the ASME Boiler Code, is shown in Figure 2 from a review by Mesko (Ref. 3)

.

Papers at the Petten Conference, which was cited previously (Ref. 1),

in addition to discussing applications of alloy 800 and its variations,

also discussed the metallurgy and the limitations which one must be

cognizant of in using these alloys. The principal limitations discussed

can be summarized as follows: -

Limitations on Using Alloy 800

1. Incoloy 800 alloys are prone to the precipitation of sigma

phase accompanied by sharp loss in ductility after long

periods of service (30,000 to 40,000 hours) in the 600 - 750 °C

range (Ref. 4)

.

2. Boron - containing coatings or boron in any form should not

be applied to the 800 alloys because of its detrimental effect

on welding properties (Ref. 4).

3. Perkins reports (Ref. 5) that alloy 800 can be used in a wide range of gas compositions containing a few ppm to 100%

H 2 S without significant attack, at temperatures below 1000 °F

(583 °C) . Excessive rates of attack are likely to occur above

that temperature with 1% or more of H

2 S in the gas. In an oxidizing atmosphere a reasonable life expectancy should be

obtained at operating temperature up to 1200 °F (650 °C) unless

CaO, (or presumably other alkali or alkaline earth oxides) are present in the gas which react to form sulfates that are molten

on the alloy surface, resulting in hot corrosion and accelerated

attack

.

References

1. M. A. Cordani , Existing and Potential Applications for Alloy 800,

Proceedings of the Petten International Conference on Alloy 800,

held March 14-16, 1978, Petten, The Netherlands, North-Holland

Publishing Co. , Amsterdam (in press)

.

2. "Incoloy Alloys", published by Huntington Alloy Products Division

of INCO, Huntington, WV 25720.

3. J. E. Mesko, Materials for Direct Combustion Fluidized - Bed Steam

Generators, Metal Progress, 112 , p. 30, July 1977.

4. J. Orr, A Review of the Structural Characteristics of Alloy 800,

Proceedings of the Petten International Conference on Alloy 800,

ibid

.

5. R. A. Perkins, High Temperature Gaseous Corrosion of Incoloy 800

in Sulphidizing Environments, ibid.
WEIGHT

CHANGE.

MG,

CM

70

F ig . 1 — Scaling resistance at 980 C. of some nickel iron-chromium

alloys. Each cycle consisted of 15 min. exposure to temperature and

5 mm cooling in air. Note: the performance of alloy 800 itself

virtually coincides with 800H under

these conditions and alloy 801 in

only. slightly better than 802

(compare with Fiq. 21 in Ref. 2).
36

32

28

24

20 -

16 -

12 -

8 -

4

0 .

71

mm allowable stress, Ksi

200 400 600 800 1000 1200 1400 1600 1800

95) (205) (5)5) (425) (540) (650) (760) (870) (980) Metal temperature

, Fro

Fig. 2—Estimated relative mechanical strength (in terms of

temperature) for candidate heat exchanger

materials based on ASME Boiler Code

(Criteria for Design Allowable Stresses)

( General Electric Co.

)

1 ‘Carbon steel 2*Carbon steel

3 C-0.5Mo

4 1.25Cr-0.5Mo

5 2.25Cr-lMo

6 18Cr-10Ni-Ti

7 16Cr-13Ni-3Mo

8 Haynes alloy 188 9 Inconel 617

54Ni-22Cr-12.5Co

-9MO-1A1

10 Incoloy 800

46Fe-32Ni-20.5Cr

11 AIS1 type 304

stainless steel

• Not applicable for advanced steam cycles.
Task M. INFORMATION AND DATA ON MATERIALS FOR MHD-POWER SYSTEMS

1 ) Data Center for MHD Materials Properties and Performance

(H.M. Ondik, A. Perloff and J.R. Cuthill)

Progress: Regular operations of the Center have continued with regard

to handling and cataloguing of contractors' reports.

After investigating a number of possible structure designs for the data

base, one version has been deemed most suitable by the Center staff with

the concurrence of an expert knowledgeable with respect to the Data Base

Management System (DBMS) most likely to be used. This version has been

successfully loaded into a computer and will be subjected to detailed

testing with real data. It is to be expected that the structure will be

modified and evolve as the testing proceeds.

The material required to put out a bid to obtain computer vendors service

has been prepared with full specifications and has been delivered to the

Department of Commerce. The Department rules do not permit the specification of a specific DBMS and, with the wide choice of vendors available,

it is possible that we may not end up with the DBMS for which our data

base structure was designed. However, any system meeting our specifications should be able to accept our data structure with relatively

minor modifications. Therefore, any such problems should cause only

minor delays, if any.

Additional equipment for the computer-terminal facility, a hard copy

unit and two terminal stands, have been received. There have been

delays in getting the promised room and necessary telephone lines for our facility due to the non-availability of workmen arising from reorganization-caused reconstruction around the Bureau. We are still hopeful

of getting our facility done in the near future. In the meantime testing will proceed using other available facilities.

A meeting of University of Tennessee Space Institute personnel and NBS

personnel took place this quarter to discuss possible cooperative projects. We used this opportunity to present the Data Center plans and

progress to them and to get helpful feedback from them on user needs and

desires

.

Plans

The regular operations of the center, i.e. review and cataloguing

of contractors' reports and extraction of data from them, will continue.

Additional contact with the user community is anticipated to get additional useful feedback. Data base structure modifications and testing

should accelerate.