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DEVELOPMENT, TESTING AND EVALUATION OF

MUD-MATERIALS

Quarterly Report

for the period October - December 1976

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

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington. D. C. 20234

Date Published: December 31, 1976

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 " 2

SUMMARY OF ACHIEVEMENTS 3

TASK G. Materials Program Coordination 5

Program Review and Consultation

US/USSR Cooperative Program

TASK H. U-02 Phase I - Terminated June 30, 1976

TASK I. Operational Design 7

a. Viscosity of Coal Slags

b. Electrical Conductivity

c. Vaporization Studies

TASK J. Corrosion and Diffusion 19

a. Interaction of Seed with Si02/ etc.

b. Diffusion of Potassium into Alumina

TASK K. Materials Testing and Characterization . . . 24

a. U-02, ANL and AVCO programs

• b. Preheater Materials

• c. USSR Test at UTSI

TASK L. Assessment of Downstream Components 53

CONCLUSIONS 60

. \ #
.
1

« ABSTRACT

•

• Much time and effort during this first Quarter of FY 77 has been

devoted to the coordination of electrode/insulator tests in various MHDfacilities. This involved close collaboration with a number of MHD

contractors and sub-contractors with respect to the procurement of materials,

the design of electrode/insulatc r assemblies, as well as the planning and

information of actual running conditions. NBS is participating in a number

of test series: the U-02 experiments, the ANL/Reynolds screening tests for

U-25 electrodes, the recently initiated accelerated program of electrode

testing in the MIT rig, as well as the AVCO-program on materia] s for slagging

conditions. A considerable number of tested electrode assemblies were

received by NBS; these are being analyzed and evaluated using optical microscopy, x-ray diffraction and SEM techniques. Valuable information was obtained concerning the durability of various electrode materials and the

possible causes of degradation.

Two major problems addressed during this reporting period were:

a. The fabrication of relatively thin layers (1-4 mm) of spinel, zirconia

and chromite by means of arc plasma spraying.

b. The joining of ceramics to metal-alloy lead-outs.

Because of the heavy emphasis on Materials Testing and Characterization

(TASK K) , work in other areas was somewhat reduced. Progress on other tasks

included:

1. . Viscosity was determined of some slag .(Illinois #6) obtained from the

DTSI MHD channel. ' - ' ~ " --

2 . Additional data were obtained relevant to the interaction of seed and

slag. *

* :

' .1' "

3 . A. thorough investigation was made of reaction layers on Corhart X-317

Ceramics exposed to flyash and seed in the test rig of the Fluiayne Engineering

Cprp. ..

4 . A number of new and modified spinels were prepared and electrical

conductivities of these materials were measured.

#
“

.
2

t » Objectives and Scope of Work

The overall objectives of this program are 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.

g 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)

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

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

2, 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)

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

and alloys)
.

.

. *-
Summary of Achievements - Oct. -Dec. 1976

(Completed Milestones - See Work Statement)

Progress of research and development in terms of the projected milestones

of the NBS-ERDA contract are listed below. Because of new priorities and

changes in the overall MHD program, certain objectives have been modified

or replaced by more urgent goals:

la. Conducted negotiation with USSR concerning materials testing U-02

(Phase I and II) and preliminary discussions concerning the Bypass U-25

experiments.

lb. Reviewed progress on US/USSR MHD Status Report.

2. Participated in ERDA-MHD reviews, planning sessions, etc.

A. Extended viscosity measurements on various coal slags.

16,19a. Determined the electrical conductivity of different coal slags

and of various electrode materials (spinels, LaCrO^)

30. Obtained preliminary data on diffusion of Fe into pure spinel.

32. Prepared new spinels (also by means, of arc-plasma spraying)

39a. Coordinated testing and pre- and post-test analysis of electrode/

insulator assemblies tested in various MHD facilities (ANL/Reynolds , AVCO, MIT).

'40a. Performed pre- and post-test characterization of various electrodes

(Mg-Al-Fe spinel, ZrOp, LaCrO^, etc.) by means of optical microscopy, SEM

techniques, X-ray diffraction, etc.

42b.' Assessed the degree of degradation of certain alloys (due to corrosion

and oxidation) in a downstream MHD- environment

An overview of the NBS-ERDA program ' schedule and accomplishments is

presented in the form of a PERT-chart (Table A).
'
TABLE A. Program Schedule and Accomplishments

Jul Oct

FY 76

Jan

TASK G-Program Coord .

*-

1 US/USSR Working Group on

• Materials

Phase I

Phase II

Phase III, Bypass U-25

2 Planning, Reviews, etc.

TASK I-Oper. Design Parameters

5,7 Viscosity of Slags, devitr.

11 Vaporization

16,19 Electrical Conductivity,

Slags, Spinels — -

TASK J-Corrosion, etc .

20 Diffusion . _ . ... 29,30 Corrosion (seed, slag,

insulator)

32 Preparation Spinels .

TASK K-Materials Characterization

39a Charact. U-02

. b " ANL-Reynolds

c AVCO

d " _ MIT --

TASK L-Downstream Components

41 Data on K-Corrosion

42 Alloy Assessment

Tartest

Apr Jul

—j- 4a FY 77 - Oct Jan Aor Jul

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’ • Task 'G. MATERIALS PROGRAM COORDINATION (S. J. Schneider)

• « Program Review and Consultation Activities

S. J. Schneider participated in the regular ERDA-MHD staff meetings

and in program review meetings (e.g., ANL U-25 program). The conclusions

and recommendations resulting from these meetings are reflected in reports

to ERDA or through direct consultation with ERDA staff.

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

for review. During this period, NBS staff provided 36 such evaluations.

US-USSR Cooperative Program

1. U-25 Electrode Program —

ERDA-MHD Division in early November 1976 initiated an accelerated

program to identify viable electrode systems for possible application to

.

- the U-25 generator being developed through ANL. Participating organizations

• in the program (managed by ERDA) include NBS, Westinghouse , ANL, MIT,

r ' ' Battelle NW, Gilbert Associates. These groups act as the central focus

for accomplishing specified programmatic functions.

The electrode systems receiving initial attention are reflected by

.-..the following Table (not all inclusive).

1 -- 1 - 1 ~ ELECTRODE/ INSULATOR/LEADOUTrSYSTEMS*

Base Electrode -Electrode- Insulator Attachment

Electrode Caps Frame Interface

' Material (oxides)

-MIT spinel ZR-Y Mesh MgAl O Mechanical or ZR-Ce Cermet v

2 Solder

Hf-Ce Pin (cooled/hot)

NBS Spinel ZR-Y Mesh MgA! ° Mechanical or

- ZR-Ce Cermet -*2°3 Solder

Hf-Ce Pin (cooled/hot)

LaCrCL ZR-Y Mesh MgAl O Mechanical or

J

ZR-Ce Cermet Y.O,

2 4 Solder — ^Hf-Ce Pin (cooled/hot)

LaCrO^ - —.—-MgAl °

-- Y 0_

(cooled/hot)

r,, —Various ____ -rVU

*

*Cerainic base systems designed for 2100 K electrode surface temperature and a range of operation of 1900 K - 2300 K.

NBS has the responsibility for overseeing the electrode development efforts

involving cermet application. In addition to this, NBS will perform the

attendant characterization of pre- and post-electrode systems from key

screening tests.
.

'

'
6

1

* 2. Materials Tests in USSR and US

*

• NBS coordinated the various activities related to the Phase II U-02 test

conducted in Moscow during late September 1976. In addition to participating

in the test, NBS staff are cooperating with Westinghouse and Battelle NW in

the various post-test characterization efforts. The first draft of the

final report is due in January 1977.

• NBS also participated in the USSR materials tests conducted in October 1976

. ..at UTSI. NBS staff will also aid in the characterization of tested samples.

3. US-USSR Status Report on MHD Materials

NBS .working with the USSR counterparts and ANL has reached a joint

consensus on the initial draft of the report. It is expected that all

details will be finalized by February 1977.

4. Steering Committee and Materials Ad-Hoc Working Group Meetings

S. J. Schneider participated in the October 1976 meetings in Moscow

fas chairman of the materials group and member of the steering committee)

The specific results of the meeting are reflected in the Protocol and trip

reports. Basically, initial plans were formulated for testing activities

to be carried out in the facilities of both countries during CY 1977 and

subsequent years (tentative). --

7

% Task I . OPERATIONAL DESIGN PROPERTIES

a. VISCOSITY OF COAL SLAGS (W. Capps and D. A. Kauffman, Inorganic Glass Sec.)

Introduction

1 In an effort to furnish engineering data on the flow behavior of coal

slags, NBS has been requested to measure viscosity-temperature relationships

of slags furnished by several MHD contractors. These slags were from the

combustion of Montana "Rosebud" and Illinois #6 coals. Some of this data

has appeared in previous reports.

Slag Preparation and Viscosity Measurement

Viscosities were determined of a real slag and of a synthetic slag whose

composition was chosen from the analysis of a real slag.

(1) Viscosity was determined from some slag obtained from the channel

wall of the University of Tennessee Space Institute MHD test facility. This

slag was from Illinois #6 coal. See Fig. 1. This slag is labelled N3S-K-983.

(2) A synthetic slag was prepared and the viscosity was measured. The

composition was determined by an analysis of a real slag from Montana "Rosebud"

coal. This slag is labelled NBS-K-795. See Fig. 2. The composition is as follows

Si02 42.JO

ai2o3 19.50

Fe 203 7.10

CaO 24.48

MgO 5.50

Na90 0.21

k2o 0.10

Ti0

2

0.90

weight %

Viscometer Calibration

Due to the interest shown by a number of MHD contractors in viscosity

measurement methods and viscosity data on both real and synthetic coal slags,

it may be of use to have a brief description of the NBS Standard Reference

Materials (SRM) which are used in calibration of the NBS high temperature

viscometer and which are available to the public for this purpose.

NBS Standard Reference Materials 710 and 711 are the two SRM's used (out

of the three available) for calibration of the NBS viscometer used on the coal

slag program. SRM 710 is a soda-lime-silica glass and SRM 711 is a leadsilica glass. These are certified as to their viscosity-temperature
,
* relationships (the log^Q viscosity of 710 from 2.00 to 12.00 over a temperature range from 1435 to 577 C; 711 from 1.90 to 12.00 over a temperature range from 1360 to 465 C)

8

These do not cover either the entire temperature range or viscosity

range of interest to MHD studies but they cover broad enough ranges to

determine the relationships between viscosity and the actual parameters

being measured.

These two SRM's were used to recalibrate the NBS

platinum crucible and two different platinum spindles

viscometer with a new

during this period.

Liquidus Temperatures

Liquidus temperatures have not received much attention in reports

characterizing different coal slags. Such data may prove to be very important

because they will determine the lowest temperature at which the slags may be

held without having them solidify because of crystallization. Most coal slags

tend to crystalline rapidly once the temperature is low enough to permit

crystal growth.

For purposes of discussion here the liquidus temperature is defined as that temperature above which no crystals can form or below which crystals

Will form if given sufficient time. Time, therefore, is an important consideration. Because different slags will crystalline at different rates any

tabulation of liquidus data should indicate the time interval allowed during

measurement.

Approximate liquidus temperatures can be inferred from NBS coal slag

viscosity curves by referring to the lowest temperatures at which viscosities

have been reported (actual data points) . Crystallization was observed in

each case at temperatures 10 to 25 degrees below the viscosity data point

representing the lowest temperature reported.

If knowledge of more precise liquidus temperatures becomes important

to the MHD program these could be determined from viscosity samples still

on hand

Plans

1) Conclude experiments to determine the influence of SO^ on the

viscosity of coal slags.

2) Attempt to incorporate factors for SO^ and K^O into equations for

predicting viscosity from slag composition. }

3) Measure viscosity-temperature relationships of real coal slags for

other MHD contractors on request.
'
9

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TEMPERATURE

(C)

11

* b. Electrical Conductivity (A.J. Armstrong, W.R. Hosier, H.P.R. Frederikse,

T. Negas and C. Olson)

Spinels

. Until now the Mg-Al-Fe-spinel , used as an electrode material, has been a solid solution between MgA^O^ and Fe 3 04 in the ratio 3:1 (MAFF-31) . The

U-02 (Phase II) test has shown that at the high temperature end, 1720 °C

(or perhaps as high as 1820 °C) , thismaterial looses some iron oxide. Another

feature of this spinel is its adequate conductivity at low temperatures

(^500 °C) . It might be preferable, however, to attach the spinel to the

metal base at a still lower temperature.

Both aspects make it worthwhile to consider modifications of the MAFF-31

spinel: first, less Fe^O^ in order to boost the melting point at the top

surface of the electrode, and second, more Fe^O^ to increase the conductivity

of the bottom part in theXXX temperature range below 500 °C.

Moderately dense ('''80-85% of theoretical) slabs of Mg-Al-Fe-oxide spinels

were prepared, in house, by conventional ceramic processing methods. Materials

include the solid-solutions SMgA^O^ : Fe^O^ and MgA^O^ : Fe^O^ (mole %) . The

former should be more refractory but less conductive than SMgAl^O^ : Fe^O^ . The

latter will be less refractory but more conductive. Samples were forwarded to

Armstrong for electrical conductivity measurements. Our laboratory-scale

preparations were intended to define potential problems prior to possible

commercial preparations on a larger scale. In cooperation with an independent

firm, billets of 9MgAl

7 0 :Fe 0 were developed. However, billet density was

low (^80-85%) suggesting 'that rinal sintering temperature should be increased.

This factor also was apparent in the laboratory-scale experiments. Laboratory

scale experiments and attempts at commercial production suggest that the

MgAl^O^rFe 0^ material will be more difficult to control due to excessive

grain growth even at relatively low final sintering temperatures.

A review of the pertinent chemical literature revealed that NiAl^O -based

spinels may be promising materials in contact with silicate solutions. See,

for example. Figures 2544-2546 in Phase Diagrams for Ceramists, The American

Ceramic Society, 1969 Supplement. NiAl^O^ has an unusually extensive primary

phase field in the NiO-Al 0 -SiO system and is as refractory as MgAl^O^. The

electrical properties of the material can be enhanced by additions of^ironoxides similar to the case of MgAl^O^ . Anticipating processing requirements

similar to MgA^O^-Fe^O spinels, we have ordered ceramic billets of two selected

compositions for electrical measurements and slag corrosion studies.

^Measurements of electrical conductivity were performed on 3 samples out

of the four materials discussed above. The preparation of the samples has

not yet been optimized as far as density and grain size is concerned. Fig. 3

shows—tho-electrica 1- conductivity -of —sample- MAFF-91' '(SMgAT'O^TlFe 0 , density ^83 %) as a function of temperature at 3 different -partial pressures of oxygen.

These results indicate that the conductivity is about two orders of magnitude

lower than that of MAFF-31 (3MgAl

2 04

:lFe

304

, see NBS-ERDA Quarterly Report #5,

Sept. 30, 1976). Of course, this is to be expected because of the much lower

iron content and the lower density. However, the conductivity at 2000 K

('V'O.OS ohm_1cm~l, slightly extrapolated) is probably adequate for an MHD electrode

at this temperature. Measurements on higher density material will be made in

the near future.
.

'X

.
12

The conductivity, of a sample of MAFF-61 (bMgA^O^-lFe-^O^ ) , presented in

Fig. 4 are somewhat better in this respect. Again, the density of this

particular material is only around 80%.* Less porous samples may show an appreciably higher conductivity.

The third sample is a solid solution of 11 Ni(FeAl)0^ and 1 MnAl^O^

(according to the manufacturer). The conductivity of this material (NAFF- 11

(see Fig. 5 ) is very similar to that of MAFF 31.

#

I 13

sF

o H

Fig. 3. Electrical conductivity of MAFF 91 (90MgAl 204 -10Fe..0j

as a function of temperature at several partial ^ ^ pressures of 0 2

. Material prepared by Trans Tech Inc.

and is 82.7% dense.

14

SJ-Q

Fig. '4 . 'Electrical conductivity of MAFF 61 (85MgAl

? 0.-15Fe? 0

d

) as

ia function of temperature at several partia Vpressures

Of oxygen. Material prepared at NBS and is dense.

15

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to

•T—u

to QJ 4->

Figure

5.

Electrical

conductivity

of

approximately

11

N1(FeAl)0

4

-l

MnA^Cty

as

function

of

temperature

for

several

partial

pressures

of

oxygen.

Ma

prepared

by

Trans

Tech

Inc.

16

Vaporization Studies (E. R. Plante)

During this quarter vaporization measurements were continued on the K(g) pressure of K 2 O dissolved in Si02- This work is considered

important because of the role that silica plays in potassia-s lag

interaction. Efforts to study this problem in this laboratory have

been outlined previously (1) and some results were presented at MHD

meetings (2,3). Briefly, K(g) pressures were determined by a weight

loss method using our Mettler TGA system and by mass spectrometry

using a Bendix Time-of-Flight Mass Spectrometer. Differences in

observed pressures in these measurements were attributed to hole size

effects resulting from the use of different size effusion orifices.

Data from the Mettler TGA work had been used (3) to estimate the

interaction of potassia with silica. The implications of the mass

spectrometric results were twofold. First, the Mettler TGA work

yielded a thermodynamic estimate for the equilibrium concentration

of K2 0 in a pure "silica slag" which is too high at lower temperatures

and secondly, the failure to achieve equilibrium under relatively

ideal Knudsen cell conditions with a sample area/escape area ratio of

about 60 can probably be taken as fairly strong evidence that the

equilibrium concentration of K2 O in slag will not be attained except

possibly for slag which has a long life on channel walls.

The measurements undertaken during this quarter were made in part

to confirm the previous mass spectrometer measurements and in part to

extend the composition range over which the measurements were made.

They were made using a newly available Extranuclear mass spectrometer

which employs a modulated molecular beam.

Since the K(g) pressure depends on the K^O concentration in the

single phase melt, it is necessary to determine the amount of

vaporized as a function of time and temperature. This can be done if

the initial or final composition of the sample is known by determining

the weight loss of K2 O which corresponds to a total integrated ion

current II+/T*At where I+ is the ion current due to K^, T is the

absolute temperature and At is a time interval during which the ion

current was observed. The mass spectrometer instrument constant can be determined from these measurements provided the following assumptions

are valid:

(1) The mass spectrometer instrument constant is in fact constant;

(2) The total weight loss observed is due to vaporization of K2 O

only

(3) The only K containing species being~lost is K(g) .

1. NBS Quarterly Report, June (1976)

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

Crystallization and Vaporization Studies on Synthetic Coal Slag

Compositions, 15th MHD Symp. , Philadelphia, PA, May (1976).

3. E.R. Plante, C.D. Olson and T. Negas, Interaction of K

2 0 With

Slag'irT Open “Cycle 7~ CdaTTlrecTTlHD , Sixth “Tritl. Conf. on MHD

Electrical Power Generation, II, 211 (1975)

17

Assumption (3) is borne out by the mass spectrometric observations and

thermodynamic considerations while assumptions ( 1 ) and (3) can be

checked by measurement of the mass spectrometer constants in different

experimental series and confirmation that the total K2 O loss is close to

that expected. These assumptions were made for both the present

measurements and those previously carried out using the Bendix spectrometer. For the sake of brevity, we will present here only a summary

of the results obtained during this quarter using the Extranuclear

mass spectrometer.

The data were fit to the equation

log PK

= B

q

+ B /T + B

2

«N

2

where N is the mole fraction of K2 O in the melt and T is the absolute

temperature. The separation of points into various series is somewhat

arbitrary but was done to try to determine if the heat term would show

a variation with composition. Points were separated into the same series if they had a common mass spectrometer calibration constant

and if the range of composition was thought to be wide enough to give

a reasonable determination of B . _

-

The data are summarized in Table 1 which lists the series identification, the number of points, the composition range, the values of

standard deviations and the standard deviation in log units of log P^.

Examination of the values for BO and B1 does not appear to reveal any

Systematic trend with composition and fit of all the data to a single

equation in which the heat term is independent of composition appears

justified. Over the composition range from .44 to .12 wt fraction of

K2 O in Si02 the equation, - - -

log PK

= 4.721 - 15624 /T + 9.318 -N

2

adaquately represents the K(g) pressure over liquid solutions of Ko 0

in Si02 * These weight fractions correspond to a range of mole fractions

of .33 to .08. The temperature range for the measurements varied from

1300 - 1900 K. These measurements have a higher precision than any of

those previously obtained in our laboratory.

At weight fractions below .20 and temperatures below about 1100°C

crystalline tridymite would precipitate from the melt if equilibrium

were established and the solution phase would become richer in K2 O.

Thus far, we have not seen any evidence that this process is actually

taking place. This is probably not too surprising since transformations

in-this system are notoriously slow.

Future Work: ~

;The current measurements will be continued to test the K pressure

dependence to as low a K2 O composition as practical and as a check on

the K2O composition variable. Work will be extended to more complicated

slags'. Of especial interest are slags containing both Si02 and AI2 O

3

.

18

Table 1

Least Squares Fit of K(g) Pressures

over ^O-SiC^ (£) Solutions

Series No. Pts. K£0 wt . fraction B

o S.D. B

i

S.D. B

2 S.D. S.D.

1104 28 .439 - .404 4.482 .131 -15637 192 12.02 1.29 .041

1110 38 .403 - .344 4.790 .092 -15843 115 10.17 .66 .038

1115 26 .344 - .294 4.559 .076 -15485 98 10.42 .83 .034

1122 18 .293 - .265 5.349 .141 -15958 218 - 2.89 3.53 .047

1123 31 .265 - .211 4.491 .067 -15305 93 10.35 1.05 .029

1126 23 .211 - .166 4.784 .081 -15706 113 8.65 1.76 .030

1129

" . 20 .166 - .119 4.735 .092 -15226 182 - 9.77 5.46 0.42

All

' ' 184 .439 _- .119 4.721 .047 -15624 72 9.32 .13 0.57

- ;

f

19

Task J. Corrosion and Diffusion

a. Interactions of Seed with SiO^ and Other Oxides (L.P. Cook and C.L. McDaniel)

Work is continuing on a systematic study of phase relations in the system

K2O-Al203~Si02 . Compositions investigated are indicated in Fig. 1. A

synthesis diagram is given in Fig. 2.

Experiments along the join KFe02 -Si02 also continued. KFeSiO^ undergoes

an undetermined reaction at 945 °C and complete melting occurs at 1215 °C.

The tetragonal bodyJ centered phase K, Fe. Si, 0. melts at 1230 °C. 1+x 1+x 1-x 4

Another addition to the 3 component seed-slag system (K-Al-Si-O) that

has been investigated is CaO. Data for subsolidus Ca0-K2 0-Al 2 0

2 -Si02 demonstrate that the K^O-activity (-actually it is better to refer to the

"chemical potential" of ICO-) increases in the ternary system KAlO^-SiO^-CaO

with increasing amounts of CaO. The occurrence of this effect in slags is

suggested by K-pressures over synthetic slags and by actual MHD slag

compositions.

t

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Sa 1!

x; c col s #

o

SS 3110031

CM O

in

0oco 0oco oo

0oCNJ o

COo

CO

CO

CO <N

CP •H

Cn

Schematic

phase

diagram

of

the

system

SiO

-KA10

(<30

m/o

SiO

22

• # b. Diffusion of K into Alumina (J. R. Manning and E. N. Farabaugh)

To treat the problem of potassium diffusing into AI 2 O3 ceramic we must

make some basic assumptions. We assume a constant diffusion coefficient, D,

in an infinite linear system which obeys Fick's second law for a one dimensional case: 3c/ct = D(3 2 c/3t 2 ). Upon application of the proper boundary

conditions, which essentially are C = CQ at x<0 at t = o and C = 0 for

X>0 at t = 0 and a plane source, the solution to the diffusion equation is

given in terms of an error function: C = [1 - erf (x/2>/dt) ] •

Thus, by determining C, the concentration as a function of X, the

penetration into the ceramic, the value of D can be determined for known

diffusion times at a specific temperature. The concentration profile of

potassium in AI 2 O 3 (90% dense) diffused at 1400 °C for 90 hrs. was determined using an electron probe. Because of the porosity of the AI 2 O3 , data

were collected by scanning along a strip parallel to the edge of specimen

and then moving into the interior of the specimen and scanning again. This

method has been described in earlier reports. A 10 ym beam was used and

data were collected at intervals of 30ym from the edge. The data, as plotted

on probability paper, is shown in Fig. 3. Data which fit the error function

expression given above yield a straight line when plotted on probability paper.

In Fig. 3, we see there are essentially two straight line segments. One which

ends ^200y from the surface and the second which begins there and continues

to the end of the collected data. The first straight line segment yields a diffusion coefficient of 5.7 x lO

- ^

cm2/sec. The second yields a diffusion

coefficient of ^1 x 10“ ® cm2 /sec (at 1400 °C)

The first diffusion coefficient

would seem to be characteristic of a volume diffusion through the AI 2 O 3 grains

while the second region could well represent a predominately grain boundary

diffusion at a much faster rate. The data representing the volume diffusion

has much less scatter than that which represents the faster rate which suggests

that the K is not penetrating as uniformly in the faster diffusion region.

The measurement of these diffusion coefficients of potassium into A^O^ ceramic

represents achievement of a milestone under Task J (Corrosion and Diffusion)
'
23

I

Figure 3. K concentration plotted on probability paper.

X = distance from the surface in 30y units.

C = concentration at X.

C *= concentration of K at surface.

o t

24

Task K. Materials Testing and Characterization

a. U-02, ANL and AVCO programs

During calendar year 1976, NBS has become involved in a large number of

tests of electrode/insulator assemblies and in the cha racterization of these

assemblies. The conditions and objectives vary considerably from one test

to the next. The materials tests in the Soviet U-02 facility are performed

with clean fuel, heat fluxes of 15-20 W/cm^ and for periods of 100 hrs. The

ANL/Reynolds screening tests for U-25 electrodes are conducted in a small

test rig for shorter periods of time, again with the clean fuel and heat

fluxes of the order of 60-100 W/cm^. Experiments in the AVCO Mark VI are being run either with clean fuel or with simulated coal, large heat fluxes

100-300 W/cm^, and periods from 5 to 100 hrs. Because this report will

discuss a number of characterizations of materials tested in different rigs

and generators, it might be helpful to list the various tests for which NBS

has performed the materials evaluation starting with the U-02 (Phase I) test

of September 1975 (Table 1, Test 1 through 13)

1. U-02 Phase I (W.R. Hosier)

The Final Report describing the materials evaluation and the conclusions

derived from this test has just been published by ERDA (December 1976)

2. U-02 Phase II, Analysis of Electrodes (T. Negas , W. R. Hosier, C. L.

McDaniel, E. N. Farabaugh and A. Perloff)

La-Chromite and Spinel Electrodes (Test No. 2) . The U-02, Phase II module

was received, photographed, sectioned for analysis and re-photographed at NBS.

Complete electrode/insulator assembly sections were forwarded to the Soviet

side, Westinghouse Corp. , and Battele NW. We have completed our analyses,

concentrating on the materials new to the U-02 program. These include LaCrO^

("doped" with MgO) and the iron oxide-containing spinels. Power X-ray diffraction

combined with high magnification scanning electron microscopy (SEM) and energy

dispersive X-ray analysis (EDX) were utilized as the dominant analytical methods.

A final report to be transmitted to Westinghouse Corp is under preparation. A

summary will be provided in our next Quarterly Report.

*'3-5. ~ANL Screening Tests (for U-25 electrodes) in the Reynolds Test Facility

The Reynolds test rig (see Fig. 1) is a small 4" long channel fueled with

propyl alcohol. Both electrode walls contain 8 electrodes (3/8" wide) ; on the

anode side, the test electrode is No. 4 (T^) and on the cathode side No. 6 (T2)-

(See Fig. 1). The other electrodes are bare copper. The facility has no magnet.

The surface temperatures of each of the test electrodes is measured with pyrometers (P^ and P2)* This way the state of the surface can be observed directly

and continuously (visible area cross section ^3/16"). Other quantities

measured are the temperature of the cooling water (which indicates the heat

flux) as well as the applied cross voltages and interelectrode potentials.

A number of electrodes were tested in this facility between Aug. and Oct.

1976. Some details of these experiments are presented in Table 2. Rough

sketches of the three electrodes tested in tests No. 3,4 and 5 are shown in

Fig. 2.

1

Table 1 . Characterization of MHD-electrode/insulator Materials

Test and Specimen Test Date Rec’d at NBS Analysis Report

U-02 (Phase I) zirconia

(+ceria)-clean

9/25/75

127 hrs . 10/75 Oct. 75-Jan.76 1. Express Report

2. Final Report

3. 15th MHD Svmp.

U-02 (Phase II)

Zr0

2

(+Ce0

2

)-MAFFhercynite-LaCrO^

9/21/76

100 hrs.

10/1/76 Oct .-Dec .76

Hosler-Negas

Franti

clean -

.

ANL/Reynolds

hercynite- (MIT)

clean

8/17/76 10/4/76 Oct. -Nov. '76

Farabaugh

NBS Quarterly

Report 12/31/76

ANL/Reynolds

Cermet- (U. C.

)

clean

8/22/76 10/4/76 Oct. -Nov. ’ 76

Farabaugh

NBS Quarterly

Report 12/31/76

ANL/Reynolds

IN+Zr0

2

+Cu(AVC0)

9/10/76 10/4/76 Oct .-Nov’ 76

Farabaugh

NBS Quarterly

Report 12/31/76

clean __

. a

- • - - • -' ------ : AVCO Report

9/76

ANL/Reynolds

Zr02+st.st. - (ANL)

10/76

(at least

10 hrs.)

” 11/29/76 in process

- - - --

' : ------ . - - : : - : -

' —

clean -

ANL/Reynolds

felt spinel -- -- (TN-NBS) -

10/27/76

(HPRF)

25 hrs.

11/29/76 In process

clean total

ANL/MIT spinel (GE)

clean - - - 11/15/76 11/24/76 in process

Negas

ANL/MIT spinel-cermet(TT-NBS) 12/2/76

clean - - - (WRH)

12/3/76 in process

Negas

-- " - - -

AVCO

IN- Cu ( r ail+bu 1 1o ns ) ( aVCO

)

slag i. ...

2/9/76 2/17/76

- r -L.- - Feb. -April' 7

6

Hosier

Negas . . . -

15th MHD Symp

.

Petty

Hosier Negas

AVCO

wire mesh+spinel (A/TN)

+ZrO (A-TN)

st. st.+hercyn. (MIT)

slag

Aug. '76 8/23/76

9/30/76

Sept. -Oct. ' 76

Negas

NBS Quarterly

Report 12/31/76

AVCO -Cu+W- (AVCO)

clean

Oct. 76 10/12/76 Oct .-Nov. ' 76

Negas

NBS Quarterly

Report 12/31/76

AVCO grooved copper (AVCO)

slag

Dec. 76

20 hrs.

12/8/76 in process

Negas

26

27

TABLE 2. ANL- REYNOLDS TESTS

(supplied by ANL)

1ST So .

3

Date and

Running Time

8/17/76

Aug. - Oct. 1976

.1) 8/22/76

14:22-16:20

4.2) 8/23/76

15:10-17:50

8/23/76

20:04-24:00

8/24/76

00:00-03:40,

8/24/76

21:53-24:06

8/25/76

00:00-07:03

8/26/76

13:46-23:39

8/27/76

13:06-20:21

9/10/76

16:00-24:00

9/11/76

00:00-24:00

9/12/76

00:00-20:31

9/12/76

00:00-20:31

10/27-28/76

17:00-12:00

12:00-13:15

14:00-18:00

4 . 3)

4 . 4 )

4 . 5)

Ml

' 4 . 6)

ELECTRODES

top layer: Zr0i(Y 7 0 ) middle layer: Zr0~

bottom layer: on top of grooved

•tainless steel

TEST SUMMARY

83v/oZr0

2-17v/oY2 0

3 graded to nichrome

to INC0 Alloy 600

inconel buttons surrounded by castable Zr0

2 in

copper "frames"

AN

ZrOj on stainless steel

TN

Arc plasma sprayed spinel

on wire-mesh brazed to Hastelloy. (See N3S

Quarterly Report 9/30/76).

The test lasted 42 minuces and terminated in an emergency shotdovn

due to a blocked fuel nozzle. The fuel flow rate indicated that

the test conditions reached about 25% of their design values. A

restart was initiated and lasted approximately two hours and

thirty minutes. This test was terminated at the end of the shift

without applying seed . Fuel flow rates were about 50% of the design

values. Later, test was started with duration of three hours and

eleven minutes and terminated when the "anode” was observed to be badly eroded. The test section was removed and stored in the

dry box for observation by ANL.

This test reached a fuel flow rate corresponding to 252 of rate

value and was terminated at the end of a shift. No seed or voltages were applied.

Test was terminated using the emergency shutdown Mode due to failure of exit cooling system on segment #17. At shut down

the fuel flow rate had been at 100% for approximately 1 hour.

Test was terminated due to the improper voltage gradient down

the channel. Ceramic was improperly cured and short-out the peg

vail. Fuel flow was at 90% design values for approx. 5 hours

and 6 minutes.

Primarily a check-out test of the electrical system - 100% fuel

flow design values for 6 hours and 57 minutes.

Check out test of electrical system. Heat flux measurements

and conductors. Fuel flow at design values for 9 hrs., 16 min.

Diagnostic test. Fuel flow at design values. Test section was

removed and placed in Hz fluid box for inspection, 7 hrs.

Heat transfer and conductivity tests. Design condition reached

at 22:00.

24-hour at design condition - heat flux tests being conducted.

Transverse voltage applied at 00.34 hr., 1 to 3 electrode

Current varied from 1.7 amps t» 10 amps from

Over a 6-hours period Aug current approximately 7 amp during

this time. Fuel flow setting at design condition.

9 no details available

Temperature raised to about 1LCG *C, without applied voltage

conditions, stayed very constant until noon (10/23) . Heat flux

''«60 W/cm^. Voltage applied aad seed flow started J*=1.5 A/ cm-. After 1/2 hour some degradation visible. Arcing and hot spots.

Voltage shut off - cooled doua to room temperature.
'
28

Insulator

*st 3

M

^1 mm

(B)

Zr0 2-V

2°3

Zr0

2

~Ce

sprayed in 2 batche

est 4

U

Y

.*7

=1

Test 5

A

Figure 2. Sketch of electrodes tested in Tests 3,4, and 5 of the ANL/Reynolds

program.

29

. Examination of Electrodes Tested in the ANL/Reynolds Test Facility (Test

Nos. 3/ 4 and 5) (E. N. Farabaug’n)

Material from Tests 3, 4 and 5 (Table 1) conducted at Reynolds was examined

using SEM and EDX techniques. The samples examined were: Test 3 , 11-101

(upstream anode) , 11-201 (upstream cathode) and C-l (samples from 05-202) and

a piece of D-ll untested electrode material; Test 4 , 01-102 (downstream anode)

and 01-202 (downstream cathode); Test 5 , 09-102 (downstream anode) and 01-202

(downstream cathode).

The electrodes were cut from the copper cooling block. Cross sections

taken from near the middle of the electrode were used for specimens. In all

cases, there was very little material left on the metal base of the electrode.

The electrodes examined were chosen because they represented the most

preserved material.

A. Zirconia-Hercynite-Stainless Steel Electrode (Test No. 3)

.

In sample D-ll ( untested ) there are four well defined layers of material

on top of the 304 stainless steel base. In Fig. 3, the top layer -4- is the

Zr-Y coating which has no Ce. Layer marked -3- has Zr-Ce and no Y. The next

material is Fe-AlOoxide (spinel) . This appears to have been sprayed in two

stages as shown in layers -2- and -1-. In the SEM micrographs a distinct

boundary is evident and is the interface of layer 1 and 2. This interface

. appears to possess poor mechanical bonding and it has been noted that several

of the electrode specimens broke apart along this interface. This breaking

occurred both in the tested and untested material. Also observed in Fig. 3 are cracks in the spinel around the stainless steel stubs. These cracks were

seen surrounding all stubs in the untested specimen. _ This indicates poor

bonding of the spinel to metal in these areas. Cracks in the metal are also

seen in this micrograph. A 200X micrograph of the stub (Fig. 4) reveals a typical crack. Note also the roughness of the top of the stub. Either the

surface was eroded when the spinel was sprayed on, or the surface was roughened

in expectation of achieving a better mechanical bond between the spinel and

metal. (In some of these SEM micrographs some noise streaking is recorded on

the print. This was due to a phosphor deteriorating during the examination

time) . It appears from these micrographs that the bonding between the various

layers in the electrode could be better optimized. Each layer appears to be

fairly uniform in microstructure, however. The major problem are the

interfaces between two apparent spinel layers, the spinel metal interface and

to a lesser extent the interface between the Zr-Ce-oxide layer and spinel.

There is good apparent bonding between the Zr-Y-oxide and Zr-Ce-oxide layers

.

Ca, Si and Al were found as the dominant impurities in the electrode

material. To show the difference in layers 3 and 4 spectra was taken from

each layer. Fig. 5 shows A the spectra taken near the top of the electrode

showing strong Zr and Y lines. Spectra B was taken in layer 3 just above the

spinel layer shows strong Zr and Ce lines no Y. The marker indicates the

position of the Ce lines.

Higher magnification micrographs (1000X) were taken of the spinel, the

Zr-Ce layer (above the spinel) , and the Zr-Y layer near the top of the

electrode. The Zr-Ce section appeared to be somewhat more porous than the

rest of the electrode.
v

.

.
30

The first of the tested materials is shown in Fig. 6a . This is a 30X micrograph of the’ anode (11-101, test 3). Note that the electrode has

cracked at two of the three interfaces indicated in Fig. 3 as being present

in the untested material. Also we see the cracking around the metal peg

which was also observed in the untested material. The porosity of the Zr-Y-oxide

layer is greater than that of the spinel. Thus, the test has not changed the

relative microstructure of the component materials. Figs. 6b and 6c

are Fe and Zr maps of the same area; distributions of Y and Ce were measured

also. The interface of the two Zr- based materials held up best and gives the

appearance of a good solid bond. There does not seem to have been much movement of Zr, Y or Ce during the test since they are in rather well defined

zones as shown by the mapping.

Fig. 7a is a SOX micrograph of the peg area shown previously. Again,

we see the cracking at the corners of the peg and the roughness of the peg

surface. Fig. 7b and 7c are Fe and Cr maps of the same area. There has

not been much attack of the steel pegs since the Cr is well defined within

the peg and the Fe in the peg is uniformly distributed. The A1 map (not shown)

indicates that some A1 aggregation has occurred. The compounds formed have

not been identified. This electrode, 11-101 test 3, was the most durable of

all tested electrodes. It had by far, the most material left on top of the

metal pegs and was in the best overall condition.

Fig. 8a is a 50X micrograph of the matching cathode (11-201 test 3).

Figs. 8b, 8c, 8d , 8e , and 8f are Fe, Cr, Ni, A1 and Zr maps of the same

area. Obviously, the cathode has suffered much more in testing than the anode.

The spinel material has been severely eroded away. The steel peg has also been

attacked. The first three maps indicate movement of Fe, Cr and Ni. These effects

are not seen in the anode. Again, there is compound formation involving the A1

as seen by the A1 map. It is seen that there is a decrease of A1 in the region

directly above the peg and heavier concentrations to the right and left of it.

The Zr (and Y) maps show how little of the material was left on the electrode.

There was little Ce left, not enough to map. This was typical of the cathodes.

11-201 was the best of the lot to examine.

The final specimen from test 3 was C-l. It was a layer piece which came

off of 05-202. Fig. 9a is a micrograph of a cross section. Figs. 9b and 9c

are Fe and Zr maps of the same area. This piece broke off at the interface

lying in the flame sprayed spinel. This has been seen as a weakness in these

electrodes. The zones of Fe, Zr (Y and Ce )are still veil defined.

B. Cermet (zirconia-nichrome) electrode (Test No. 4).

In Test 4 the downstream anode (01-102) and downstream cathode (01-202)

were examined. Fig. 10a is a 30X micrograph of the remains of the anode.

Figs. 10b and 10c are Y and Cr maps of the same area. Only small amounts of

Zr and Y are left in the wire gauze after the test. Some Si is present at

the gauze-metal interface. Similar photographs were taken of the cathode.

Even smaller amounts of Zr are seen and no Y could be detected in this cathode.

About all that can be said about these electrodes is that nearly all the top

layer of Zr-oxide was removed from the electrode during the test.

C. Metal (+zirconia) electrodes (Test No. 5).

Fig. 11a is a SOX micrograph of part of anode 09-102, the downstream

anode in Test 5. In the micrograph the copper is on the right and the
.
31

center steel button is on the left. This is just one half of the electrode

assembly. This anode material was very hard to polish and the castable

around the center button tended to polish away. The cathode polished much

more easily. Again, note that most of the material covering the metal part

of the electrode is gone. Only relatively small amounts of material are left

on either side of the center button. Fig. lib is a Ni map of the same area.

(Zr, Y and K maps not shown) . Analysis of what looked like voids in the maps

showed that these are regions in which the mounting compound has been exposed.

Other EDX work showed no Fe or A1 concentrations in the castable material.

SEM and EDX examination on the other side of the center pin (the more attacked

side) yielded the same type of information, Zr, Y and K were found in the

castable and no movement of metal into the castable was detected.

Fig. 12 is a SOX micrograph of part of the cathode 01-202, the downstream

cathodes, of test 5. The copper is on the right and the center button on the

left. The distribution of Ni, Zr, Y, K and Ca were mapped.

Again, there appears to have been more reaction at the cathode. We see Ni

aggregated now in the castable. There does not appear to be as much Y or K as in the corresponding anode. Also, we see a Ca concentration which was not

seen in the anode. SEM and EDX examination of the other side of the center

button yielded little new information. It must be noted, however, that on the

other side only about 10 7, of the castable remained after the test. Ni, Zr, Y

and K were detected in that remaining material. XXX X

- - • #

; In a general evaluation of these tested electrodes it is difficult

to make a detailed study since, in most cases, some of the interesting

material is gone. In addition, very few untested electrodes were submitted,

so that no valid comparisons could be made. The electrodes generally

Suffered degradation during the test and as a result it was sometimes

difficult to prepare specimens without further damaging the electrode.

Observations on the untested material suggest that better spraying techniques are necessary to withstand the rigors of operation.

In two cases 01-202 (Test 5) and 01-202 (Test 4) the metal of the

electrode parted from the copper cooling block. The brazing failed in

these cases. Typical brazes between cathode metal and copper are shown

in Fig. 13, 09-102 (test 5), Fig. 14, 11-201 (test 3) and Fig. 15, 01-202

(test 5) . Only in Fig. 15 is there any indication of a problem in bonding.

Under higher magnification examination the good bonds showed good intimate

contact with no voids or holes. With the exception of the two electrodes

mentioned, all joining of electrode metal to copper held up well under testing.

6 . ANL-electrodes *

7: -' These electrodes were tested in the ANL/Reynolds rig in mid-October

and were used mainly to check out the new conditions of the test channel

vith propyl alcohol fuel. Records of exact running times, temperatures and

Applied voltages have not yet been received.

•
.
32

Figure 3. 30X SEM micrograph of

untested material (D-ll).

Figure A . 200X SEM micrograph of

stainless steel stub.

Figure 5. EDX Spectra from top two

layers of electrode material.

B

9

Vi -’•FT’-’

1
V
-

;

'

L

E

>" ~»- y^| i^ii»y^y, p iwwr.

' j^r<ci.'J i i ^^.;.^ i!iy | r

' i p m"" "

*" ‘ •

»

* v,_

V •

-•

... ‘A '

-. i

-!

.; - ,^j

fy . r

•' • -*>

".'fi

»> .

’

$ *4

,i A

T- 1

33

Figure 6a . 30X SEM micrograph of anode

(11-101, test 3).

Figure 6b . Fe map of the same area

Figure 6c . Zr map of the same area

'• Vr

.irm,m mr9W7 *t «; ir ' M* 3f1 i

i mwiyww.1^^

’-J
'
34

Figure 7a. 50X SEIM micrograph of 11-101,

test 3.

Figure 7b . Fe map of the same area

Figure 7c. Cr map of the same area

pw . 'T"" ’vrr—

j' gJijKinMa. Tmxraenff .yMV

l

Figure

11- 201 ,

Figure

Figure

8a.

test

8b.

8c.

#

50X SEM micrograph of cathode

3.

Fe map of the same area

Cr map of the same area



37

Figure 9a 30X SEM micrograph of a cross section of specimen C-l.

9b. Fe map of the same area

Figure 9c. Zr map of the same area

Figure 10a. 30X SEM micrograph of anode

01-102, test 4.

Figure 10b.

Figure 10c.

t

i -J

p i» '

Y map of the same area

Zr map of the same area

yj-if.tfjj'j yT uw 'ji. u v.»

v

i j/uihk iHJfc alfth 31

Fig. 11a . 50X SEM micrograph of anode

09-102, test 5. Fig. lib. Ni map of the same area as

Fig. 12 . 50X SEM micrograph of cathode 01-202 test 5.

^LtkA»

L/0

i

Figure 13. 50X SEM micrograph of brazed

joint to cooling block, 09-102 (test 5) .

i

.i

;

'

Figure 1A. 50X SEM micrograph of joint

in 11-201 (test 3).

Figure 15 . 50X SEM micrograph of joint

in 01-202 (test 5).

#

t

41

• 7 . TN-NBS Electrodes

The construction of these electrodes were described in the NBS-ERDA

Quarterly Report of September 30, 1976. They consist of a 3.5 mm arc-plasma

sprayed layer of MAFF spinel ( 3 MgAl 20^+lFe 304

) on top of a 1.5 mm thick

wire-mesh pad, which in turn is soldered to the watercooled copper holder.

This electrode was tested in the ANL/Reynolds MHD facility in late October.

It ran for about 15 hours under thermal conditions with a surface temperature

of 1450 - 1500 °C, followed by 1-1/4 hours under current of 1.5 A/cm 2 . Characterization of these electrodes is in progress.

8 . GE Electrodes

Analysis of these electrodes is in process. Details about the test are forthcoming from MIT, GE and ANL.

9 . TT-NBS Electrodes

(The following is a short report of the test supplied by MIT)

These electrodes were tested in the MIT facility on December 2, 1976.

The test plan called for a test of about one hour at a surface temperature

of 1700 °C and a current density of up to 1.5 amps/cm2 . The primary objective

of the test was to test the cermet bond between the MAFF 31 spinel and the

Hastelloy B substrate under thermal and electrical conditions similar to those

expected in the U-25. The electrodes were designed for a heat flux of 100

watts/cm^ at a surface temperature of 1700 °C and with an expected bond line

temperature of 600 °C. The braze at the bond line was done with a relatively

low-temperature solder. The electrodes, had plasma sprayed inter-electrode

insulation. Besides, MBS supplied additional insulator plates. of pure MgAl^O^

spinel and asked MIT to supply A^O^ P^- ates f°r additional insulation on one

of the surfaces.

, Heating was done gradually, reaching a surface temperature of 1685 °C in

one hour. The temperature was raised to 1725 °C before applying seed. Seed

application lowered the surface temperature to 1700 °C. The surface was

observed for 10 minutes for possible seed effects before applying electric

. current. Current at 0.45 amp/cm2 was applied for 10 minutes. During this

time the surface temperature rose to 1775 °C with hot spots at 1825 °C on

cathode no. 2. The current was raised to 0.8 amp/cm^ for 20 minutues. During

this time incipient melting started to appear at a surface temperature of

-1735 °C. (?) The current was subsequently raised to 1.15 amp/cm 2 for 10 minutes.

During this time the coating on cathode no. 1 disappeared at a surface

temperature of 1725 °C. (?) Consequently, the test was promptly terminated to

save the other, partially damaged, cathode for analysis. The surface on the

anodes was hardly damaged.

For the current densities used, the voltages on electrode 1 was higher

than on electrode 2. This was likely due, in part, to the fact that the

electrodes had separated from the copper blocks at the braze interface,

resulting in different resistance across the junction.

Examination of the electrodes after the test revealed that they had

separated at the brazed interface, the plate insulation had cracked normal

to the electrode face, and the sprayed insulation had broken at the electrodecopper interface.
'

'
42

NBS Comment ; • [Much of the damage might have been caused by the fact that these eJectrodes

were not designed for the MIT rig. The length was 3" instead of 4-1/2" and

hence, the flame had an opportunity to heat the ends of the electrodes

excessively.

]

% 10 . AVCO Electrodes ("rail"- and "button"-type)

A report of this test and the materials characterization has appeared

in the Proceedings of the 15th Symp. on Engineering Aspects of MHD (May 1976) .

11 . AVCO-Ceramic Electrodes (a ,b . Zi rconi_a_&_M£-A12

_F e_-£Pi.n^_l (Test la and b)

In the previous Quarterly Report some discussion was devoted to an analysis of AVCO anodes consisting of:

-a. wire-mesh (Fe, Cr,Al-alloy) , sprayed ZrO^ (Y^O^) P^us s l a9» &nd

b. wire-mesh, sprayed spinel (3MgAl^0^ :Fe^0

^ solid solution) plus slag.

Figures 16 and 17 (100X) , repsectively , show typical cross-sections. The

upstream end (left) of each electrode was subjected to current concentration,

heating, and oxidation, in situ , of the wire-mesh. Although most of the ZrO^

*Y

2

0

3

) [discontinuous, white patches in Fig. 16] reacted/dissolved in the slag,

the wire at the downstream end remains practically intact suggesting minimal

current activity and/or lower temperatures. Furthermore, the upstream wiremesh in Fig. 16 also is characterized by "pockets" of "invading" slag components

In contrast, the spinel layer (dark grey zones above wire in Fig. 17) suffered

recession (original thickness, ^1 mm), yet forms a continuous layer above the

wire-mesh. Oxidation products (light grey) of the wire are retained within

this layer. Possible current channeling is apparent at the downstream end

fright) as suggested by the oxidation of the underlying wire.

Within each electrode several noteworthy interfaces developed during this

short term test. In Fig. 16, a continuous layer of crystalline slag covers the

lower zone of wire-mesh plus Zr02 (Y 2 O3

) and metal oxidation products. This

layer extends upward to a well-defined porous zone midway through the crosssection. Above this zone is a continuous layer of glassy (molten during operation) slag. • The porous interface between molten/crystalline slag may suggest

gas evolution (probably oxygen) at the anode. Also, a crystalline , presumably

resistive, layer of slag overlies the wire-mesh; current channeling is evident

only at the upstream end. As seen in Fig. 17, a sharp, continous contact

exists between slag and the lower zone of wire-mesh plus spinel and metal

oxidation products. At the upstream end, this interface is not porous and

consists of glassy (molten) slag directly on wire-plus-oxides. An intervening,

crystalline slag layer is not evident. Approximately half-way across the

electrode, a highly porous but poorly defined interface begins and extends to

the downstream end. Crystalline slag exists below while glassy slag occurs

above'this interface, similar to the situation in Fig. 16. It seems apparent

that the controlling factor in the development of these interfaces is the

chemical nature of the original, sprayed ceramic oxide. Assuming a fixed

initial slag composition and, initially, nearly identical temperatures at the

upstream portion of each electrode, incorporation (reaction, solution, etc.)

Of refractory ZrC>2 (Y 2O3

) hy the slag should not lower the liquidus temperature

of that slag, i.e., crystallization canoccur within the temperature regime

wherein the initial slag composition normally is molten. Incorporation of

spinel, and especially the iron-oxide component, operates in the opposite manner
.
43

The liquidus (or molten range) of slag actually is extended to lower

temperatures. In Fig. 17, it is precisely at the hotter upstream end where

most of the original spinel was incorporated that the slag layer remains

molten. A zone of the crystalline slag occurs only toward the downstream,

presumably cooler, end where less spinel has been incorporated.

The observations above are noted for several reasons. For, at least,

short term dura tions the properties of a slag can be altered, possibly beneficially, by semi-sacrificial ceramic oxide layers. Care must be extended,

however, in the choice and design of these materials. For example, ironcontaining oxide coatings when incorporated with slag plus seed could lower,

even temporarily, the molten range of slag plus seed to excessively low

temperatures. Thick, highly conductive, low temperature, molten slag could

bridge insulation and lead to interelectrode breakdown.

c. Herc^nit£ (Test 11c)

We also received a cathode operated for 30 hr with slag/seed. In consisted,

originally, of stainless steel having a ridge and groove surface onto which a coating of an Fe-Al-oxide spinel (composition unknown) was applied. Fig. 18

(100X) illustrates a typical cross-section. Higher magnification micrographs

and EDX data are available. A flat polished surface could not be maintained

(shown by the irregularities in Fig. 18) as the electrode was saturated by

potassium salts which hydrated and "bled" during handling.

The electrode apparently suffered from excessive heating particularly

at the downstream end (right side) wherein the stainless stell sufferred extensive recession via oxida tion. The remaining stainless steel pins (grey) on

the_ upstream side also are being subjected to oxidation but at a slower rate.

Oxidation products (Fe/Ni/Cr-oxides) surround these pins. Slag components

including K have replaced entirely the pre-existing spinel ceramic. Dark

grey areas between the stainless and slag and the zones containing "bubbles"

above the slag consist of epoxy. ..

12 . AVCO Electrodes (T. Negas) . 1 . ~~

-

- ^-^j^el_ec_tr_od_es_ (Test No. 12) '

1 ; During this quarter we received an anode fabricated from a metal composite

consisting of a mixture of Cu plus W metals (presumably sintered W impregnated

with molten, Cu, or W/Cu metals sintered above the ijielting point of Cu, nearly

A:1 proportions) . This material, operated for 20 hr. under clean fuel/seed

conditions, sustained extensive recession at the upstream side. The electrode

surface was examined by SEM/EDX methods in an undisturbed (not polished) state.

Figs. 19 and 20 (5000X) illustrate typical views of the upstream area. The

vesicular surface consists of a framework of Cu metal (possibly partly melted)

Surrounding voids originally containing grains of W or containing pitted and

recessed W. Although both metals obviously receeded, the W appears to receed

preferentially. Obviously, the upstream area was subjected to localized

current concentration (arcing) and excessive heating. Cu-metal with superior

jthermal conductivity dissipated hear more rapidly than the W and the latter

oxidized. The oxidation products either vaporized or, being poorly adherent,

.were swept or abraided off by the gas stream. The deep voids, sometimes

retaining the vague hexagonal outline of the pre-existing W grains, may suggest

pxidation/vaporization as the dominant process.

VT c
'

'
44

• Interestingly, the downstream (presumably cooler) portion of the

electrode surface consists of swarms of ridges and shallow valleys trending

parallel with the direction of plasma flow. These originate near the vicinity

of but not from within the upstream area of active metal recession. They

terminate near the downstream edge of the anode. Fig. 21 (1000X) illustrates

these features. The ridges (center and far left and right) consist of relatively undisturbed grains of W bonded by Cu. Within the valleys, the material

consists of poorly bonded, almost loose, grains of W. Cu clearly is absent.

These valleys may represent zones of lesser current concentration and heating,

relative to the upstream area. They probably develop via oxidation of the Cu

within a lower temperature regime. This would serve to weaken the cohesion

among W grains which then would be subjected to ablation by the gas stream.

The observations above suggest that Cu/W anodes probably are not superior

to pure Cu. The mechanical mixture of metals, indeed, may compromise the

excellent thermal conductivity of copper alone.
.
]4

;ji
'
/

mm v>» 'Ukwxw ^» . Jt Jl«s*i*wuijii »nn>i

— i *"' *"»''' *"•'*' '.

i ‘‘mi i 'r Atfnn 1 1 it i iT 1- »V tbkdhmmAjwUi^h .,'M*^ -ii3

</7

VW "(-V- \

i V

•»” ' -'V' .. N V

V . /I f

v^S"'

'<''&* **.\r

. 'SJbCv^- ' V Vvij'j'V . *. >.

i®*§a^

v£w£^\

-f-

!

, i /. S *• • "

i A * r: hm* m?

Fig. 18. SEM micrographs (100X) , colligated, of a cathode cross section.

Downstream is in the southeast corner direction. Grey ridges

and base are stainless steel. Dark grey areas are epoxy.

i I i *- ' ; *- ^A. j . -< ji'J

Figure 19. SEM micrograph (5000X)

,

surface (unpolished) of Cu/W metal

anode, upstream area.

Figure 20. SEM micrograph (5000X)

,

surface (unpolished) of Cu/W metal

anode, upstream area.

Figure 21. SHIM micrograph (1000X) , surface (unpolished) of Cu/W metal

anode, downstream area consisting of

a ridge and valley complex.
v
49

b.* SEM Examination of Corhart X-317 Ceramics (Magnesia Spinels) Tested at

the Fluidyne Engineering Corporation (E. N. Farabaugh)

Preheater material tested at FluiDyne was examined to determine

elemental constituents, microstructure and the thickness of the reacted

layers

Specimen FN-90 (Corhart X-317) was cycled between 1800-1700 °K for

30 hours in the presence of ash and seed (K2 SO4

) . Fig. 22a is a 100X

micrograph of part of this specimen. The change in microstructure defines

the extent of the reacted layer. In this specimen this layer is ^0.5 mm

thick. Elemental concentrations of Mg, Al, Si, Ca, K, Cr, and Fe were

found in the reacted layer. At a distance of ^0.8 mm from the edge of the

specimen, only Mg and Al could be observed in the EDX spectra. Fig. 22b

is a Mg map of the same area. Note the Mg depleted reacted area. Fig. 22c

is an Al map of the same area. Maps of Si, K, Ca, Cr, and Fe in the

same area show concentrations of these elements in the upper layer.

Fig. 23a is a 100X micrograph taken of specimen FN-85. This specimen

was cycled between 1800-1600 °K for 30 hours in the presence of seed (K^CO^)

but no ash. Figs. 23b and 23c are Mg and Al maps of the same area. Maps

of K, Si, Fe and Ca distributions were also recorded. It is difficult from

these maps and micrographs to determine precisely where the reacted area

ends. Other SEM and EDX work has indicated a layer thickness of ^0.2 mm. This was determined by scanning from the edge in and observing the position

where K and Fe concentrations fell to zero.

These two specimens are interesting. FN-90 was tested under conditions

not different from FN-85. The results from the examination of these two

specimens were quite similar. Both showed Mg deficient layers which could

be explained by suggestions of preferential reaction of the MgO with the

gas stream. Both had relatively thick reaction layers 0.4-0. 5 mm. Elemental

constituents were also the same in these two specimens.

FN-85 was the only X-317 magnesia spinel tested using K2 CO 3 for seed.

It is the only one, so far, which has had such a vague difference between

the reacted layer and the bulk material. There is not the striking difference

in Mg and Al maps as seen before. Also, the reacted layer is thinner ^0.2 mm. This layer thickness compares to that reacted layer of FN-70 (^0.2 mm) which

r- - was also tested in the absence of ash. It seems that the X-317 didn’t react

as much with the K_C0_ seeded gas stream as with the Ko S0. seeded streams.

2 3 2 4

f
•


si

.-%. > % V * 3

>m

.t

- i

-V v

-' z:, ?*

Fig. 23a. 100X SEM micrograph of reacted layer of FN-85.

Fig. 23c. A1 map of the same area as 23a,
.
52

c. Joint US-USSR Test of Soviet Materials at UTSI (A. J. Armstrong)

From October 8-14, 1976 six runs were made at UTSI in the UTSI II MHD

channel. A. J. Armstrong of NBS participated in the tests. Three runs yere made with western Kentucky coal + K2 SO4 seed to produce an acid slag.

Two additional runs were made with the same coal and seed but with added

Ca (OH ) 2 to produce a basic slag. Finally, one run was made with kerosene

alone and no seed. In general, a run would consist of up to 15 minutes of

warm-up with kerosene followed by up to 60 minutes of operation with coal

and seed. The coal containing runs subjected the materials to severe

erosion, thermal shock and chemical attack.

The two Soviet materials tested (SiC + 7 wt.% Ti and 60 wt.% LaCrO^t

40 wt.% Cr) displayed high thermal shock resistance. Both materials seem

to have suffered from erosion with the doped SiC apparently standing up

best. The doped SiC appeared to show a large increase in chemical attack

when subjected to the basic slag. These observations are from viewing the

electrodes after sectioning and are preliminary to a more intense posttest examination now in progress at UTSI and IVTAN. A post-test analysis

at NBS is planned upon receipt of samples from UTSI.

One U.S. (or NBS) material was included in two of these runs with acid

slag. The material was dense sintered spinel OMgA^O^ : Fe^O^) . Out of four

samples tested only one was recovered. Failure seems to be due to thermal

shock. The one sample recovered has many cracks and apparent erosion of

0.3-0. 6 mm. Very little chemical attack by seed or slag has been detected

to date.

\

t
.
53

* Task L. ASSESSMENT OF STEAM PLANT COMPONENTS (J. R. Cuthill)

This is the sixth quarterly report since the initiation of Task L.

Task L is concerned with the selection of cominer cially available alloys

that appear to be promising materials for critical components in the

downstream steam plant, and recommer dations for further in-service testing.

The first critical component, and probably the most important on the

basis of the quantity of material involved, is steam heat exchanger tubing.

The requirement that distinguishes this MKD application from a conventional

coal-fired steam plant is the severe hot corrosion anticipated due to the

1% potassium by weight in the flue gas which is currently predicted ( 1 ) to

be almost entirely in the form of KOH and K 2 SO4 with a small amount of KC1

,

(see Table 1 ). However, in assessing alloys in reference to this application,

of necessity the hot corrosion results in N^SO^/NaCl mixtures have been used

as a guide. The marine gas turbine literature concerned with attack by

Na2S 04 /NaCl mixtures is relatively extensive whereas there is a surprising

paucity of data on attack by K^SO^ except for the investigations by the U.S.

Bureau of Mines at Bruceton, PA (2) and by the Central Electricity Research

Laboratories in England (3). Both of the latter studies have been discussed

in previous quarterly progress reports. (See previous quarterly reports

also for a discussion of the nature of the "hot corrosion" type of attack

and the behavior of various alloy additions to attack by molten Na^SO^/NaCl

mixtures). This marine gas turbine hot corrosion research has been considered

a useful guide as a first approximation to attack by molten potassium salts

on the basis that K2 SO 4 /KCI mixtures are reported to behave much the same way

as the Na2 S 04 -NaCl mixtures in the crucible tests (4,5). However, there are differences reported also. In a review on Corrosion in Fused Salts, D. Inman

( 6 ) reports that in the presence of air, NaCl produces greater attack than KC1

but in the case of hydroxides just the reverse is observed; KOH produces

greater attack than NaOH. This greater severity of attack by KOH in comparison

with NaOH is shown in some data of the Gurloch (7) which is reproduced in

Figs . la,b. A comparison of the weight loss in KOH vs. NaOH, for pure nickel,

Khl8N9T* stainless steel, and Armco iron is shown. (Compare Fig. la

vs. lb). An interesting implication of the data is that pure nickel resists

this type of attack better than the stainless steel.

Fortunately, the predicted flue gas composition (see Table 1) has the KOH

being converted to K2 SO4 in the steam superheater. The situation in the sulfate

form looks appreciably better. Shirley (5) reports essentially the same

attack on Nimonic 80A and F.C.B.(T) steel+ in a K2 SO4 , 1% KC1 mixture as in

Na2

S 04 , 1% NaCl at 750 °C. However, Shirley's data showed the potassium salts,

under the same conditions, as producing a 33% greater weight loss than the sodium

salts, in the case of 337 stainless steel.

I In view of these deviations between attack by potassium salts and by

sodium salts a reasonably extensive search was made, therefore, to find just

what data there was in the literature on attack by molten potassium salts.

A search requested from the Smithsonian Science Information Exchange of reports

on current research in their files, which covers the periods, FY'75, FY'76,

and a portion of FY'77, revealed no on-going research on the attack of metals

or alloys by molten ^SO^, KC1, K

2

C03# KOH, or other potassium salts. Data

*Khl8N9T steel: 18Cr, 9Ni, lTi. +

F.C.B. (T) steel: 17.5 Cr, 12 Ni, 1 Nb. (
'

.

'
54

abstracted from references pertinent to a comparison of the corrosion

attack by potassium salts vs. sodium salts, which were found by searching

primarily the last ten years of chemical abstracts are summarized in Table 2 .

It is evident that the data in the literature on the attack of

commercial alloys by molten potassium sulfate, carbonate, chloride, or hydroxide is very incomplete. However, data from laboratory tests in any

one of these potassium salts would be only a first order screening test

because the interaction between the constituents is more than likely not only

important but controlling.

This appears to be the case in marine gas turbine hot corrosion where

pure Na

2

S0

^ does not produce serious attack, but combined with a small amount

of'NaCl, the rate of attack is increased many-fold. (See previous quarterly

progress report on Task L for data and a discussion of Na 2 SC>4 - NaCl hot

corrosion attack). The chemistry of the MHD system flue gas is much more

complex. The mol fractions of the principal constituents that STD Research

Corp. predicts to be in the flue gas in the steam superheater is given in

Table 1 . Depending upon the exact amount of excess oxygen present in the

flue gas, Reid ( 8 ) discusses how pyrosulfates and trisulfates could form and,

in turn, be directly responsible for the heat exchanger tube attack in

conventional coal-fired plants. Reid proposes (9) that the trisulfates

Na 3Fe(S04)2 and K^Fe (SO^^ are mainly responsible for the loss of metal

from the superheater and reheater tubes at 1100 °F (600 °C) and above (metal

wall temperatures) . Note that this is the temperature range anticipated in

the Baseline Plant design (see Fig. 2 reproduced from previous quarterly

report. Sept. 30, 1976). In lower temperature regions, 600-900 °F range,

(300-500 °C), Reid (10) discusses how the molten pyrosulfate, K2 S 2O

7 could

be present and likely to be responsible for hot corrosion attack. Note

that K2 SO4 has a melting point of 1076 °C so is unlikely, in pure form, to

be a factor in corrosion at these lowT wall temperatures. Carey, Cross and

Reid (11) report that maintenance of oxidizing conditions near the tube wall

reduced metal attack. They attributed this to the suppression of SO 3 formation. The SO3 is required for pyrosulfate formation. However, oxidizing

conditions must be maintained around the superheater tubes for another reason - to-maintain the integrity of the protective oxide films on the alloy. For

example, the 50 Ni - 50 Cr alloy (INCO 671) is probably one of the best alloys

for resisting the hot corrosion attack. However, the atmosphere must be

oxidizing to maintain the integrity of the Cr^O^ protective film.

Conclusions - - ----- -

- r The general conclusion to be drawn from the literature search is that

the use of marine gas turbine hot corrosion research results as a guide to

the selection of promising alloys for further in-service testing, as has been

done in this series of quarterly reports, is valid. However, there are enough uncertainties in the complex chemistry of the MHD system to make

in-service testing in a rig that simulates as closely as possible the MHD

conditions necessary if one wants to determine the lowest cost alloy that

will do the job. - - :
'

.

'
55

• Future Plans

•

•

(1) As part of the literature search for data on corrosion attack by

molten potassium salts, a computer search was made of the National Technical

Information Service files. Some work of particular interest was turned up

but the full reports have not been received as yet. These data will be

reviewed in the next quarterly report.

(2) Data on the hard fa c ing coatings and alloy overlaps, to resist wear,

as well as hot corrosion attack, will be compiled.

References

1. Baseline Plant System Design Description, Joint Report-Gilbert Associates

and STD Research Corp., June 1976.

2. D. Bienstock, R. J. Demski and R. C. Covey, Corrosion of Heat-Exchange

Tubes in a Simulated Coal-Fired MHD System, J. of Eng. for Power (ASME Trans.)

93, 249 (1971).

3. J. B. Heywood and C. J. Womack, "Open-Cycle MHD Power Generation", Chapter

10, Pergamon Press, Oxford, 1969.

4. John Stringer, Hot Corrosion in Gas Turbines, MCIC Report #7 2—08 , Battelle

Columbus, 1972.

5. H. T. Shirley, Effect of Sulfate-Chloride Mixtures in Fuel-Ash Corrosion

of Steels and High-Nickel Alloys, J. Iron and Steel Inst. 182

ij 6. D. Inman, Corrosion in Fused Salts, Chap. 2.10, Corrosion, Vol. I, L. L.

Shrier, ed. , Newnes-Butterworths , London, 2nd ed. , 1976.

7. E. I. Gurovich, Effects of Fused Lithium, Sodium and Potassium Hydroxides

on Nickel, Copper, Iron and Stainless Steel, Zh. Grikl. Khim. 33_, 836-40 (1959),

(English Translation)

.

8. W. T. Reid, External Corrosion and Deposits, Boilers and Gas Turbines,

American Elsevier Publising Co., 1971.

9. ibid, p. 125.

10. ibid, p. 104.

11. R. C. Carey, B. J. Cross and W. T. Reid, External Corrosion of FurnaceWall Tubes-II Significance of Sulfate Deposites and Sulfur Trioxide in

Corrosion Mechanism, ASME Trans. 6_7, 289 (1945).

12. H. Arup, N. Korsgard, D. Hansen, Stress Corrosion Cracking of Au

Stainless Steel in Potassium Hydroxide at Elevated Pressures and Temperatures,

Proceedings of the 6th Scand. Corros. Congr., Sweden Corros. Inst. Stockholm,

1971, p. 22

13. N. D. Tomashov and N. I. Tugarinav, Mechanism of Corrosion of Metals in

Molten Chlorides, p. 1681-7 (1956), English Translation.
.

.

-
56

Table 1

Principal Constituents in Flue Gas Through Steam Heat-Exchanger (from Ref. 1)

Molecular Mole Fraction

Specie entering* entering* leaving*

superheater reheater reheater

CO .00010 — —

C0

2

.17212 .17269 .17212

H

2

0 .08888 .09095 .09102

K .00005 — —

KC1 .00004 .00004 .00004

KOH .00366 .00011 —

k

2 so

4 (D .00184 .00365 —

K

2

S0

4

(s) — — .00371

N

2

.72077 .72285 .72293

°2 .00093 .00004 .00001

S0 o .00253 .00073 .00067

*See Fig. 2 , Schematic of Baseline Plant Design, reproduced from previous

(5th) Quarterly Progress Report.

#

57

Table 2

Data From Pertinent References on Attack of Metal and Alloys by Potassium

Salts vs. Sodium Salts.

Ko S0, -KCl vs. Na o S0, -NaCl

2 A 2 A

Ref. 5 :

Contact Mixture,

750 °C

99% Na SO , 1% NaCl

2 A

99% K SO , 1% KCl

2

Wt loss, mg/cm

337 steel* F . C . B. (T) steel * Nimonic 80A*

11, 12, A 14,13,12 3A,3A ,53

16 13 31

*337: 16Cr , 18Ni , 7Co , 3Mo , 3Cu , ITi , bal. Fe; *F.C.B.(T): 18Cr,12Ni, bal.Fe;

*Nimonic 80A: 75Ni,20Cr, bal. Fe.

KOH vs. NaOH

Ref . 7 : KOH may be up to 20 times as corrosive as NaOH in respect to

18-8^ type stainless steels. See Fig.

Ref. 12 : KOH pellets placed in the bottom of U-bond stress corrosion specimens

caused cracking in 18-8 stainless steels but not in 2.25 Cr, 1 Mo steel at

310-A00 °C. The moisture content appears to be critical and there is indication

that 0.05 mol% or more of CO

2

inhibits cracking in the presence of the KOH.

Ref. 13:

KCl vs. NaCl

A

Corrosion of Fe in different

chlorides at temperatures 10'C above

the melting point of the chloride.

A) Corrosion activity K (in g/ m*),

B) time r (in minutes).

Curves: 1) NaCl; 2) KCl 1 3) CaCl2

.

58

Isotherms for corrosion in fused KOH: A) corrosion (in g/m2

); B) duration of experiment (hours); nickel: 1) 500*; 2) 450*; 3) 400*;KblSN9T steel: 4) 400*; 5) 450*; 6) 500*; *Ar;nco* Lron: 7) 500 *; 8) 400*; 9) 450*.

(b) Isotherms for corrosion in fused NaOH: A) corrosion (in g/m 2

); B) duration of experiment (hours).

Nickel: 1) 350*; 2) 400*; 3) 450*; "Armco" iron; 4) 350*; 5) 400*; Khl8N9T steel; 7) 350*; S) 400*; 9) 450 *.

Fig. la,b. Corrosion attack in Molten KOH vs NaOH (from Ref. 7)

#

f

L

Figure

2.

Baseline

Reference

Design

for

an

Open

Cycle

MHD

Power

Plant.

FEED WATER 59