E— (49—1) — 3800—

7

Distribution Category UC-90g

. FILE CG -

! DO NOT HEMP

DEVELOPMENT, TESTING AND EVALUATION OF

MHD-MATERIALS

4

Quarterly Report

for the period January - March 1977

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

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

Date Published: March 31, 1977

PREPARED FOR THE UNITED STATES

ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION

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

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

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

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

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

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

Table of Contents Page

ABSTRACT 1

OBJECTIVES AND SCOPE OF WORK 2

SUMMARY OF ACHIEVEMENTS 3

Task G. Program Management and Coordination 4

Program Review and Consultation Activities

US-USSR Cooperative Program

Task I. Operational Design Properties 5

A. Viscosity of Coal Slags

B. Electrical Conductivity

C. Vaporization

Task J. Corrosion and Diffusion 14

A. Seed-Slag Interactions

B. Diffusion in Slag-Insulator and Insulator - Electrode Couples

Task K. Materials Testing and Characterization 21

A. U-02 , ANL , MIT and AVCO PROGRAMS

B. Joint US-USSR Test of Soviet Materials at UTSI (Phase II)

C. SEM Examination of Specimens Tested at Fluidyne Eng. CorPD. Thermal Evaluation of a 3MgAl^0^ : Fe^O^ Arc Plasma Sprayed Composite

Electrode

E. Chemical Properties of SMgAl^O^ : Fe^O^ (MAFF 31) Spinel Solid Solution

Task L. Assessment of Steam Plant Components 38

CONCLUSIONS 46

1

Abstract:

Efforts during this reporting period were directed primarily toward

the design, fabrication, testing and pre- and post-test evaluation of

real MUD electrode materials. Close collaboration was maintained with a number of MHD contractors and subcontractors with respect to procurement

of specialized materials and their design. Additionally, NBS is involved

with a number of MHD tests including the accelerated program of electrode

testing in the MIT rig, the U-02 experiments, the AVCO tests utilizing

slag and the UTSI/USSR tests. The evaluation/characterization of electrodes/

insulators from five tests were completed and final reports were submitted.

Several preheater refractories were also analyzed for Fluidyne Engineering

Corp.

Progress in other areas included:

1. Viscosities of synthetic slags based on analyses of USSR coal

( "Krushnitz" ) and "average" Montana Rosebud and Illinois #6 were measured.

2. The electrical conductivity of several oxide spinels was measured.

3. New measurements involving K-pressure over K^O-SiO^ solutions and

a crystalline potassium zirconate were completed.

4. Chemical reactions in the K^O-Al^O^-SiO^-FeO^ system were investigated.

5. Mg-A.l-Fe-oxide spinels were evaluated in terms of bonding to a metal

substrate. Important chemical aspects of these oxides are discussed.

6.

Evaluation of alloys for downstream components continued.

2

* ’ 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 MUD power systems. Major problem areas in which

investigations will be concentrated are:

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

and performance at prototype temperatures.

2. Development of electrode materials which provide adequate performance

over extended periods of time.

3. Insulating materials which limit thermal losses and are resistant

to prolonged thermal and erosion effects.

4. Preheater materials which can withstand the operating modes of

separately and directly fired operation.

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

feasible

.

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

action of combination on system components and materials.

7. Durability of prototype MHD sub-systems.

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

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

The activities are grcqped under six tasks:

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

and review of the various MHD-Materials Development Programs)

.

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

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

I. Operational Design Properties (viscosity, electrical conductivity,

vaporization)

.

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

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

post-test analysis)

.

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

and alloys)

.

Summary of Achievements-Jan , -March 1977

(Completed Milestones - See Work Statement)

3

Progress of research and development in terms of the projected milestones

of the NBS-ERDA contract are listed below. New priorities, particularly

involving MHD materials testing and characterization, have resulted in the

shifting of manpower as well as the modification of certain previous objectives.

Many of the milestones also are continual in scope.

la,b. Conducted negotiations with USSR concerning,

a. Phase II, U-02 test

b. A Phase III, U-02 test (Oct. -Nov. 1977)

c. Materials Test, U-25 high B field

d. USSR testing in US MHD facilities

e. Fourth version of US/USSR MHD Status Report (Materials Division)

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

4. Measure viscosity of real slags. A USSR plus two US slags (synthetic,

based on ash analyses) were measured.

5. Effect of oxide additions on viscosity. a) Effect of K^O completed.

6. Effect of dissolved H^O on viscosity. A high temperature-pressure viscometer

has been constructed.

10a. Determined first version of the phase diagram for the "quaternary" system

K»0-Al„0 -SiO -FeO (seed-slag)

.

11a. Concluded measurements to determine the K-pressure over K O-SiO -A1 O

solutions.

19. Measured the electrical conductivity of several spinels contain ng iron

oxide and of arc plasma sprayed LaCrO^ (MgO)

.

23. Measured seed vapor pressures for a portion of the K^O-ZrO^ system.

39a, 40a. Coordinated testing and/or conducted pre- and post-test analysis of

electrode/insulator assemblies tested in various MHD facilities ( ANL/Reynolds

,

AVCO , MIT, UTSl/USSR)

.

41,42. Assembled, updated and assessed data pertinent to corrosion of alloys

in a downstream MHD environment.

Talks and Papers

F. A. Mauer, T. Negas, A. Perloff, E.N. Farabaugh, C. R. Robbins, "Analysis of

MHD Electrodes and Ccal Gasification Reaceor Liners by Diffraction Methods",

Session on Application of Diffraction to Fossil Fuel Energy-Related Materials,

American Crystallographic Association, Feb. 23, Asilomar, Calif., Invited Talk.

T. Negas, "Chemistry of Ceramic Oxide MHD Electrodes", American Chemical Society

Symp. , Materials From a Chemical Viewpoint, New Orleans, March 22, Invited Talk.

H.P.R. Frederikse, "Electrical Conductivity of Mixed Spinels", American

Physical Society Mtg . , San Diego, Calif., March 1977.
.
4

Task G. PROGRAM MANAGEMENT AND COORD IN/vTION (S.J. Schneider)

Program Review and Consultation Activities

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

assorted program review/coordination meetings and briefings. The conclusions

and recommendations resulting from these meetings arc 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 MBS provided 8 such evaluations.

Part of the NBS experimental program involves characterization of materials/

component elements tested in various MHD test rigs and facilities throughout

the United States (and USSR) . In order to expedite the dessem.ination of this

information, NBS has instituted a separate reporting system in addition to the

regular Quarterly submissions. The system consists of the following elements:

1. Telephone report to submittor after initial examination, 2. Follow-up

letter report, and 3. Detailed report (if warranted) to submittor and ERDA.

The detailed report will be distributed to general contractors on an as needed

basis as determined by ERDA; however highlights of the detailed report will be

abstracted and published in the NBS Quarterly reports. In addition, general

overall summary reports will be compiled from time to time and published in

appropriate journals and special editions.

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

A. Perloff 301-921-2900

(alternates)

T. Negas 301-921-2843

W. Hosier 301-921-2940

US-USSR Cooperative Program

1. U-25 Electrodb Program

The major bulk of this activity has been completed by the participating

contractors. Henceforth, certain aspects of the program will be continued

although at a somewhat less accelerated pace. NBS responsibilities and

accomplishments are reported elsewhere in this report.

2. US-USSR Materials Program Activities

During this reporting period, the US-USSR Ad Hoc Materials Working Group

held its first 1977 meeting in Washington, D. C. Agenda items included:

1. Phase II U-02 materials test (final draft report now in preparation),

2. Phase III materials test on U-02 (agreement reached to conduct test in

Oct. -Nov. 1977), 3. Materials test on U-25 high B field facility (preliminary

discussion only with no firm commitments made), 4. USSR materials in US MHD

facilities (Phases I and II at UTSI completed; tentative plans laid for similar

testing at Westinghouse in the latter part of 1977), and 5. Joint US-USSR

chapter on materials (Status Report) (fourth revision agreed to by US-USSR

authors)

.

Task I . OPERATIONAL DESIGN PROPERTIES

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

Introduction

Part of the program of characterizing MHD materials is the preparation

of synthetic slags and the determination of viscosity of these and any real

slags furnished to NBS by other contractors.

In the past, six real combustor slags and one M11D channel slag have been

measured and seven . artificial slags based on real ash analyses have been made

and measured. This service to the MHD community will be continued.

Slag Preparation and Viscosity Measurements

1. A synthetic slag based on a chemical analysis of a "standard" USSR coal

slag (called Krushnitz) was prepared. It is relatively high in SiC>2 and

and low in an ^ CaO. Its viscosity is correspondingly high

(K-864). The analysis was obtained from Dr. Crawford of UTS I. The viscosity

and analysis are shown in Fig. 1.

2. A list of approximately 160 analyses of Montana Rosebud coal together

with an average analysis for the whole lot was obtained from Dr. Poste lthwai te

of ERDA-Fossil Energy. The analyses were made on core samples from a large area of the mine in Colstrip, Montana. An "average" Montana Rosebud slag was

made and measured (K-875) . This made a very stable melt which did not crystallize at temperatures 'as low as 1142 °C. See Fig. 1.

3. A series of analyses of ashes from Illinois #6 coal was also "averaged".

These were obtained by ERDA from .PERC. Melt number K-879 simulates this

average Illinois #6 slag. See Fig. 1.

4. The above mentioned average Rosebud and Illinois #6 analyses are quite

similar except with regard to CaO and Fe 203 concentrations. For systematic

laboratory investigations, it is desirable to minimize the number of melts

while realizing a maximum amount of data. Therefore with increased emphasis

on working with "standard" slags a new standard or "base" composition was devised which is intermediate between the Rosebud and the Illinois #6. This

base is identified as K-884 in Fig. 2. Systematic variations will be made

by increasing or decreasing each constituent oxide in turn in discrete increments .

The data will be very useful in developing a nomogram type of calculation

method for predicting viscosity using temperatures and composition as independent variables. Increments of SiC>2 of 5, 10 and -5% have already been made.

See Fig. 2. These are K-887, K-901 and K-913, respectively. Future melts

will be made varying Al^O^, Fe 3^4' anc*
' I ' I ; I ' '
6

<3

* CD

CO

LO

N

CO

i

0> fcx>

I

/

/

/

o0

CO

co

1 X

h- CD vD

CD CM in

ro m

cm O O O cm cm O

•>H rH CD rd W ^ U

CVJ

o

o

ro

o

CD

O

o

LO

o

o

o

o

ro

o

o

CVJ

CD

iZ

o

(S8SI0C1

0 ! 60 | ) A 1 1 S 0 0 S I

A

TEMPERATURE

(C)

7

!200

1300

1400

1500

1600

1700

TEMPERATURE

(C)

8

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

We have measured the electrical properties of nickel aluminate

ferrous ferrite. These particular samples were composed of 17 NiAl 204 - 3 Fe 304

and are designated at NAFF 173. The original billet was pressed and sintered

at 1548 °C for 7 hours at which time it was cooled slowly in N2

. The sample

taken from this billet is designated 7756. The electrical conductivity shown

in Fig. 3 is low and oxygen pressure dependent. Apparently, oxidation of

the iron at the lower temperatures and hiqher O 2 pressures causes the considerable decrease in conductivity. This sample was 74.5% of theoretical

density and displayed many cracks after the completion of the measurement.

These cracks were probably caused by the iron oxidation toward an Fe^O^ phase.

A second billet was pressed and filed at 1560 °C for 6 hours and cooled

in N 2

- This billet had a density of 86 . 6 % theoretical. Electrical data on a sample from this billet is shown in Fig. 3 also and designated as 7760. The

electrical conductivity is higher than the less dense sample as expected and

the oxygen pressure dependence is less. NAFF 173 or some other proportion of

nickel aluminate and iron oxide might be a promising electrode material especially in a slagging environment where SiO^ would be a problem with most materials.

Fig. 4 gives the electrical conductivity as a function of reciprocal

temperature of magnesium aluminate ferrous ferrite (MgAl^C^-FegC^ ) cal] ed

MAFF 11. The data for reducing conditions (10

-

^ 03 ) was taken first. As can

be seen from the data the conductivity is excellent and the slope is relatively

flat. A room temperature value of conductivity after the run was 5 x 10“2

(ohm cm) -

-*-. No oxidation occurred at the lower temperatures. However, at

— 7 10 ' partial pressure rapid oxidation occurs even at 1100 °C and in air

the oxidation starts as high as 1400 °C. Indications are from this data that

the material would not be suitable for an MHD electrode except as a graded

component at very low temperatures, i.e., less than 500 °C. Even in this

case the conductivity at room temperature of 5 x 10

- 2 (ohm cm)"-*- is not

considered excellent for a joule heating face MHD electrode. This sample was

74.4% theoretical density. Further sintering and densi f ication would probably

increase the conductivity of this material and its resistance to oxidation.

Preliminary measurements have been made on arc plasma sprayed LaCrO -.05Mg.

While the conductivity is adequate for MHD power generation >1.0 (ohm cm) - l,

it is considerably less than that of the sintered LaCrO^-Mg samples measured

and reported in the July-Sept. 1976 quarterly report. This particular

sample was cut from a demonstration piece made to show the feasibility of arc plasma spraying LaCrO^ and its MgA^O^ insulator to a considerable thickness.

The MgAl^O^ insulator side was sprayed to a depth of approximately 3 mm. The

LaCrO^ was attached (sorayed) directly to the insulator to a depth of another

7 or 8 mm and finally the last insulator side was sprayed to the LaCrC>3 to

another 3 mm so that the total sandwich is 13 to 14 mm thick. The sample was

cut from this sandwich and had the insulator side in tact during measurement.

No density measurements have been made on this particular piece but some increase in conductivity as a function of time at high temperature was evident

which probably indicates that sintering was occurring. Additional details will

be given in the next report.

CONDUCTIVITY

(ohm-cmr

1

9

Fig. 3. Electrical conductivity vs. reciprocal temperature for 17NiAl-,0 -3Fe 0

(NAFF 173) for several partial pressures of oxygen. 7756 is 74?5% J 4 dense - 7760 is 86.6% dense,

10

O

mc4j

Fig.

4.

Electrical

conductivity

vs.

reciprocal

temperature

of

MgAl

O.-Fe

O,

(MAFF

11)

11

c. Vaporization Studies (E. R. Plante)

This section describes measurements of the potassium gas

pressure over K^O-containi'ng compounds or phases. Such data is of

critical importance in predicting the extent of interaction of the

seeded MUD plasma with construction materials or coal ash. During

this quarter measurements were completed on the vaporization of K 2 O

dissolved in silica, new vaporization data were obtained on K. 4 Zr 50 ]_ 2 ' and measurements on a glass composition which contains K 2 O, SiC^r and

A

1

2 O 3 were begun. All data were obtained by mass spectrometry using

the observed weight loss of K 2 O for calibration purposes when possible.

K20 -SiC>2 Measurements

In the last quarterly report (1) vapor pressure data for K were reported over the composition range 4 3.9 to 11.9 wt% K2 O in SiC>2 solutions. During this reporting period these measurements were

concluded. Vaporization measurements were terminated at a calculated

composition of about 6 . 8 % K 2 O when non- saturation effects became

apparent. During the final period of the experiments stress relief

in the furnace caused buckling to occur which resulted in local

hot spots in the Rnudsen cell. As a result, some platinum was vaporized from the. cell and a valid calibration constant for these

measurements could not be determined. However, calibration constants

previously determined in this work were sufficiently reproducible

that an average value based on the previous calibrations is probably

correct to within 10-20% so an average value was used. The data for

this composition range (11.9 - 6.8 wt% K2 O) was fit by least squares

to give the equation,

v logPK

(atm) =(3.841 ± .076)- (14654 ± 116J/T + (52.76 ± 4.26)N

where N is the mole fraction of KpO in the solution. The coefficients

in this equation differ somewhat from the data obtained previously in

this series especially with respect to the coefficients for the 1/T and

terms. It has not yet been determined how well this data fits the

equation covering the composition range from 43.9 to 11.9 wt% K^O. As

noted in the last quarterly report these- measurements do not pertain to

equilibrium as represented on an equilibrium phase diagram. In addition,

it must be noted that the treatment of the data assumes a homogeneous

glass or melt is present in the effusion cell for the determination of

the mole fraction of K^O in the solution. These points require further

investigation. However, it should be noted that the amount of seed capture possible at these low concentrations is far less important to MHD

than the higher concentration ranges.

Comparison of the current data with previous mass spectrometr ic

data obtained using the TOF mass spectrometer shows excellent agreement

at 20 wt% K 9 O with poorer agreement at higher K 2 O content. The ratio

of the potassium pressure observed using the TOF spectrometer to that

observed using the quadrupole mass spectrometer is 1.10, 1.25, 1.5, 1.8,

2.3 and 3.2 at 1725K and 20, 25, 30, 35, 40 and 45 wt% K 2 O respectively.

The cause of the difference between the TOF data and quadrupole data at

12

45 wt% KpO is surprisingly large and is not understood. It is believed to

result, However, from unknown differences in the samples or temperature

measurement rather than differences in the mass spectrometers used.

"K.ZrrC> " Measurements

4 5 12

Because of the importance of zirconia as an MUD construction material,

measurements were carried out of the potassium pressure produced by the

reaction

:

1/2K Zr 0 (c) = 2K(g) + 1/20 (g) + ZrO fc).

4 5 12 2 2 These measurements help to define the temperature and overpressures of K and 0^

which can be' tolerated by zirconia without compound formation. The compound

K^Zr^O -^2 (composition nominal until proven by a structure analysis) is the most

stable, easiest to prepare phase in the K20-Zn;c>2 system.

Pressures were measured using the quadrapole mass spectrometer and Pt

effusion cells with the temperature monitored by a Pt"Pt (10% Ph) thermocouple

.

Several sets of data were obtained but the results are not entirely unambiguous

because of possible interference from residuiil K2 CO 3 at the beginning of the

experiments and the possibility that the reaction is slow because of complicated

crystal chemistry. Data were obtained with crucibles having orifice diameters

of .020" and .0135". If the mass spectrometer constant did not change appreciably the product kac, where k is the observed mass spectrometer constant, a is the

orifice area, and c is the Clausing factor for the orifice, would be constant.

Failure to obtain this consistency makes it ajjpear likely that leakage of vapor

from around the cell lid took place during the measurements made with the smaller

orifice diameter. The potassium pressure based on what appears to be the most

realiable set of data is:

logP (atm) = -14133 + 106/T + 6.248 + .106

K

Further analysis and the implications of these results with respect to compound

formation because of seed interaction with zirconia will be differred to a future report.

K O - SiC) - Al C> Measurements

_2 2 2 3 Measurements of the K pressure over the three component system are in

progress. Preliminary data have been obtained on a sample designated K210 which

was obtained from the Geophysical Laboratories, Washington, D. C. and is among

the well characterized K2 0-Si02 -Al 2 C>3 glasses used by Schairer and Bowen (2) . Previous attempts to measure the K pressure over several of these glasses at

NBS were made using a microbalance system but without success.

The starting composition of K210 was 39.6% wt% K2 O, 10.0% AI 2 O 3 and 50.4%

Si. 02 - Thus far a number of vaporization experiments have been run using a Pt

effusion cell with a 1/2 mm diameter effusion hole. A number of preliminary

results can be briefly stated. The initial K pressures observed at about 39%

K

2 0 content are 3 to 4 times higher than the pressures of K currently

accepted as "best" for K^O-SiC^ solutions at 40 wt% K-,0, i.e., the results

using the quadrapole spectrometer reported in the last quarterly report ( 1 )

.

The slopes of logP K versus 1/T are less steep for the alumina containing

solutions than for the K

2 0-SiC> 2 solutions and appear to become less steep as the K2 0 evaporation proceeds. The potassium pressure decreases faster with

decreasing mole fraction of K2 0 than for the solutions containing only K

20 and

13

SiO^. An equation of the type logPK

= BO + Bl/T + B2-N2 with N the mole

fraction of K^O in the melt, which fit the K^O-SiO^ solution data over the

range from 44 wt% to 11.9 wt% K

2 O very well , gives a poor fit to the present

data over the range from 39% K^O to 28% K^O.

Future Work :

During the next quarter, measurements on the K pressure over K O-SiO -Al^CL

solutions will be continued. The amount and reliability of the data for the

K20-SiC>2 solution case is sufficiently good so that the implications of this

data to seed capture for a pure solica-K^O "slag" will be examined and reported.

References

1. NBS Quarterly Report, December 1976.

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

14

Task J. Corrosion and Diffusion

A. Seed-Slag Interactions

a. The System K2 0-Al ^0 - SiO ^, (L. P. Cook, J. L. Waring, R. S. Roth and

C. A. Harding)

During the past quarter a graphite furnace has been installed and

tested whi ch will allow experiments to be conducted at temperatures up to

2700 °C in an inert atmosphere. PtgQRh^g tubing has been found to be

useable up to ^1920 °C if care is used in annealing prior to welding.

Further experimentation has been conducted to determine the melting point

of KA10

2

. The melting point appears, on the basis of experiments in an inductively heated iridium crucible, to be considerably higher than previously

estimated (Ref. 1). A series of experiments at 1750 °C showed no melting for

compositions richer in KA10 2 than 58KA102 :42 SiO^ in agreement with our preliminar

experiments, as given in a previous quarterly report (Ref. 2).

Molybdenum and tantalum tubing have been ordered and received. These

materials will be tested for vise in sealed quench experiments above 2000 °C.

A controlled atmosphere facility for loading and welding experiments is being

assembled

.

b. The Slag System CaO-K^O-MgO-A] O - SiO ^-"FeO" (L. P. Cook, C. L. McDaniel and T. Negas)

The six component system described above provides a working model

for "real" MHD slag (Ref. 3). The experimental approach to be used has been

outlined in a previous quarterly report (Ref. 4).

A controlled atmosphere facility with analytical balance has been installed

and placed in operation. Using this facility, starting materials are being

prepared as shown in Fig. 1. During the next quarter experiments will be

begun to determine liquidus temperatures in the indicated binary systems.

Analytical expressions relating composition to liquidus temperatvire will be

derived from these data. These expressions will be used to guide experimentation in more complex, multicomponent systems.

References

1. F. F. Spencer, J. C. Hendri.e, Jr. and D. Bienstock, Sixth Inti. Conf.

on MHD Electrical Power Generation, Vol . IT, Washington, D. C. 1975, p. 202.

2. H. P. R. Frederil.se, T. Negas and S. J. Schneider, NBS Quarterly Report,

October - December 1976, p. 21.

3.

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

April - June 1976, p. 30.

15

CaO, MgO , FeO

x

O CM •H -H

CO CO •H iH s s

Fig. 1. Synthetic Slag Compositions for Experimental Investigation

/

16

c. The System KAlC^-KAlSiO^- "FeOx

" (Han 111 Yoo, guest worker from

the Korean Standards Research Institute, R. S. Roth and T. Negas)

As part of our program to elucidate chemical processes involving

slag components the "quaternary", K20-Al 203 -"Fe0x "-Si 02 , is now under investigation. Ten specimens were prepared as indicated in lig. 2. These were equilibrated in sealed Pt- tubes, quenched, and examined by X-ray diffraction.

It was determined that the subsystem KAlSiO .- "FeOx

" is "binary" as indicated

by Fig. 3. However, the KA 102 ~"Fe 0x

" tie-line is not "binary". The

KA102 -KAlSi0 ^-"Fe0x

" compositional triangle, therefore, is not ternary.

Specimens react to form a KAl^_ x Fey 02 -type solid solution plus a 3-alumina

type compound containing a high percentage of iron oxide.

These preliminary data demonstrate several types of reactions to be

expected among, for example, Fe/Al-oxide construction materials and slag

components plus seed. Simultaneously the chemical behavior of slag plus seed

combinations can be elucidated. For example, consider the KAlSiC>4 -"FeOx

" system

in Fig. 3. Starting Fe/Al ratios and K/Si-oxide contents are varied by the

two components KAlSiO^ (a common crystalline phase in MHD slags) and FeOx

. Noteworthy are the subsolidus equilibrium assemblages KAlSiO^ + Fe 2 C>3 (hematite)

and KAlSiO^ plus Fe 30^ (magnetite) . The latter exists only above ^1425 °C

under oxidizing conditions. This temperature (Fe 2 C>3 -«-> Fe 3 C>4 equilibrium)

will be displaced toward lower temperature with decreasing Pn . The presence

u

2 of Fe 3 C>4 (which is a highly conductive spinel) as an equilibrium product in

s]ag may contribute to axial leakage in a slagging MHD generator. The more

extensive (compositional ly) system KAlSi04 -KA10 -,-"Fe0 " suggests that

K-3-alumina phases (containing iron oxide as well as AI 2 O3 ) are expected

corrosion products of Fe/Al-oxide construction materials. These 3-alumina

phases probably are mixed ionic/electronic conductors and are under investigation as part of a project independent of this MHD program.

B. Diffusion in Slag-Insulator and Insulator-Electrode Couples (A. J. Armstrong, E. N. Farabaugh and J. R. Manning

Because of the continuing interest in spinel materials for MHD insulator

and electrode applications, research has been planned which will lead to the

determination of diffusion profiles resulting from slag-insulator and insulatorelectrode reactions at various temperatures and times. Changes in composition

resulting from diffision may strongly affect the electrical, insulating and

conducting properties of spinel materials.

We have chosen Illinois #6 as the slag to be used in this work, since Illinois #6 has been selected as the representative eastern coal. A specimen of

the slag has been cut, polished and examined by SEM and EDX techniques to

document the elemental constitution and microstructure of an untested slag

sample. The slag was found to be composed of Mg, Al , Si, K, Ca , Ti and Fe as

is shown in Fig. 4, an EDX spectrum taken from the slag. This elemental

analysis is identical with that found for larger specimens of slag, thus

indicating a reasonably homogeneous slag material. We have characterized

the slag this reporting period and will fabricate slag-insulator couples and

study them next reporting period.
'
17

Fig. 2. Specimens Investigated in the KAlC^-KAlSiC^-FeC^ (plotted as Fe ? 0 ) compositional triangle.

18

JP

*©

n o

Pis)

4-

rn roj?

o

G

\

G

t

(/•>

1 o

,u r

sn

o>o-fc.

\

O

~n

L>

•JCtfs.'frEansaj.i rtstx& ;«3wwua: 8

i-; 8

n .

I

. |*j'» «y*Lfr/set f32j

Lf5 8 8 8

ji8swsrK3^s!3rara«x^<^«iM:*?.s«nBzsswiaj»w«iw^;T^^

& 1- e

Fig. 3. Preliminary phase diagram for the KAlSiO^-FeO system, in air

19

Various insulator materials, MgA^O^, MgO and A^O^ have been used

in the formation of insulator-electrode couples. The electrode material

used was MAFF-31 (3HgAl-,0^ + Fe^O^). These couples were assembled by

first wrapping the insulator specimen with 0.001" platinum wire, pressing

the insulator between two larger pieces of material (either slag or electrode samples) and clamping the diffusion couple between two AJ^O^

plates. The A^O^ plates are held together with thick platinum wire and

sufficient tension is induced to provide some compression of the diffusion

couple. The thin platinum wire, which is wrapped around each insulator

specimen, provides a boundary marker for the insulator edge. This construction is shown in Fig. 5.

In the next reporting period, it is planned to heat these insulatorelectrode couples at 1G00 °C for 1, 10 and 100 hrs. With the use of

diffusion boundary markers, it is anticipated that the problem of determining the position of the insulator-electrode interface, described in an earlier report, will be eliminated, thus allowing good Fe diffusion

profiles in the insulator to be determined.

Slag-insulator couples will be made and treated at 1300 ° - 800 °C for

1, 10 and 100 hrs. during the next period. Penetration of different slag

constituents into the three insulator materials will be .investigated

.

20

Fig. 4. tiM crom Illinois #6 slag.

Fig. 5. Typical Diffusion Couple Experiment Rig.

21

Task K. Materials Testi n g and Characterization

A. U-02, AN.L, MIT and AVCO Programs

I. U-02, Phase II (T. Negas)

As indicated in the previous Quarterly Report, analyses of U-02,

Phase II electrodes were completed and a final report was under preparation

for Westinghouse Corp. This report is rather extensive and will not be

reproduced here. However, the summary below illustrates the scope of the

work

.

Radiography, powder X-ray diffraction and high magnification SEM combined

with EDX analysis served as the dominant analytical methods. The report is

divided into the following sections,

I. Introduction

A. Handling of the Module

B. Scope of this work

II. General Features of the Module

A. Environmental Contaminants

B. Mobility of Iron and Chromium

C. Metal Leadout Zones and MgO Insulators

III. Post-Test Chemistry of Materials

A. Mg-Al-Fe Oxide Solid-Solution Spinels

B. "Hercynite" -Based Spinel Electrodes

C. LaCrO^ (Mg) and Cerium Oxide-ZrO^

IV. Conclusions

Eighty-seven figures are included. The following conclusions, not necessarily

in order of importance, can be summarized based on our analyses.

1. Excessive but interrelated thermal-mechanical-electrical stresses were

the most important factors influencing the performance of the module.

a. These combined to cause degradation of side insulators, leadout

materials and leadout zones including underlying MgO blocks.

b. In turn, original thermal parameters deviated toward higher temperatures

from original design. Overheating at leadout zones is apparent for many electrodes, heat being dissipitated probably via the MgO insulation.

c. The chemical integrity of materials, therefore, was compromised by

excessive, localized heating.

d. Intrusion of seed enhanced further mechanical and chemical deterioration

via reactions including solution and probable electrochemical action.

2. MgO insulation for electrodes containing high iron-oxide contents is not

suitable for temperatures designed to exceed 1700 °C.

3. The iron oxide-containing materials should be limited to applications below

^1700 °C. The "hercynite"-based spinels should be utilized probably below

1600 °C.
'
22

a. The " her cynite" -based electrodes should remain in high density

form to prevent,

1. exsolution reactions and reactions with seed.

b. The Mg-Al-Fc- oxide electrodes should have better conductivity at

leadout temperatures wherein oxidation and exsolution to resistive phase can

occur. (See extended discussion of the chemistry of MAFF-31 in this Quarterly

Report.

)

4. LaCrO. -based electrodes are encouraging although evidence for vaporization

of Cr-ox.ide(s) and solution by seed is abundant,

perovskites could be an effective leadout material

LaCrO^ or other related

5. Alternate methods of fabricating the Mg-Al-Fe-oxide spinel should be

explored to prevent segregation of impurities at grain boundaries.

2. ANL/MIT Testing

a. NBS/Trans-Tech Tests (W. R. Hosier)

During this reporting period several tests were carried out in

the MIT test rig. The first test, was made on January 14, 1977 and the

electrodes tested consisted primarily of magnesium aluminate ferrous

ferrite (MAFF 31) fastened to a Hastelloy B base through a cermet grade.

The MAFF 31 had a yttria stabilized zirconia cap. The Hastelloy B was brazed directly to the standard MIT test rig base.

2 The electrode was designed for a heat flux of 100 watts/cm and with

allowance for temperature excursions of 200 °C at the plasma electrode

interface. The design temperatures were:

1830 °C surface

1300 °C Zr0 2

(

Y

2 ° 3

) -MAFF 31 bond line

550 °C cermet bond line

50 °C copper-IIastelloy B bond line

Using the appropriate thermal conductivities of the materials involved,

the thicknesses were:

ZrC^

(

Y

2 °

3

^ 0. 70 mm

MAFF 31 2.04 mm Hastelloy B 6.75 nun

The test lasted a total time of 1 hour, 47 minutes and was terminated because

it became apparent that no more useful information could be obtained.

Almost immediately upon ignition, the magnesium aluminate plate insulators

popped out into the plasma stream about 3 to 8 mm. From the start, the ZrO^

caps began coming off in areas and were almost entirely gone at 15 minutes

into the run. During the electrode set-up no particular precautions were

taken to keep the ceramics dry. In addition, the test date was an extremely

cold day in Boston and the gas stream scrubber on the roof froze. At startup a malfunction occurred and water streamed down over the electrodes. While

heat gun v/as trained on the system for a short while before ignition, the
.
23

ceramics may have still contained some moisture. Even with the above difficulties, the electrode functioned well and application of current continued

according to a pre-arranged plan. The electrodes were returned to NBS where

they are being examined in detail. A more detailed report will be made later.

A second test was carried out on the MIT test rig on February 18. These

electrodes were made of lanthanum chromite which was arc plasma sprayed on the standard MIT test rig base. A yttria stabilized ZrC^ cap v/as sprayed on

the LaCrO^ . A schematic design is shown in Fig. 1. Before ignition extreme

care was taken to keep .the electrodes dry. They were kept under an infrared

heat lamp until the chamber was closed and cooling water was not run through

the electrodes until 15 to 30 seconds before ignition of the pilot burner.

Ignition of the main burner was accomplished with a good cold start which

minimized shock to the electrodes. The caps and electrode insulators which

were magnesium aluminate sprayed directly on the electrode side functioned

well. All caps remained in place except a small central portion of the lower

cathode which oblated away during the run. The run lasted for 3 hours and

52 minutes with a total time under power of 2 hours, 41 minutes. No arcing

was observed on any electrode at any time and the current voltage relationships indicated a soft electrode condition, i.e., voltages were not high to

maintain the current densities of 1.5 a/cm^. During the test we observed a loosening of the bond between the copper electrode base and the LaCrO^ at the

outer electrode edges. After the run the anode and cathode sections were clamped in their positions for transport to NBS for analysis. The loosening

of the LaCrO^-Cu bond appeared to be the most serious difficulty. It is impossible to determine how much loosening was done during the run and how

much happened during and after cool down. Generally, the electrodes functioned

well, and an electrode incorporating this design appears promising.

A detailed analysis is being carried out now and a report will be

available

.

b. SEM and EDX Examination of ANL" Dummy" Electrodes (E.N. Farabaugh)

Sections were cut from the dummy electrodes which revealed typical

cross sections from relatively undamaged areas and from severely eroded areas

of the electrode. A detailed report, summarized below, is available.

The electrodes consisted of a ZrC>2 cap on top of a Ni-Cr-Al alloy layer

which, in turn, was on top of stainless steel. The stainless steel was joined

to a copper cooling strip. It was noted that the copper cooling strip came free

from the stainless steel in most cases. There was no evidence, however, of

separation at either the ZrO^ -alloy or alloy-stainless steel interfaces.

The electrode, #101, with the thickest ZrC^ cap (^1 mm) withstood the

testing better than those with thinner caps. There was some Zr02 left in the

severely eroded areas of 101 while 102, 201 and 202 retained little or no ZrO^

in the severely eroded areas. All the electrodes with ZrC>2 caps <1 mm thick

suffered damage to a depth of .5 to .6 mm below the bottom of the ZrC>2 cap.

New Fe-Si-K phases were formed in the damaged areas.

The Al in the alloy layer (the layer was ^.1 mm thick) often appeared in

the form of elongated islands in the Ni-Cr-Al phase. In some cases with thin

ZrC>2 caps, the thickness of the alloy layer was reduced in the heavily eroded

areas.
-

'
24

ALL CERAMIC MATERIALS ARE

ARC PLASMA SPRAYED

0.88mm

2.30mrn

25

c. Technetics Spinel Elec erodes (T. Negas)

Introduction

Spinel-based electrodes fabricated by Technetics are shown schematically

in Fig. 2. Construction materials include,

a. Spine] solid-solution, 75MgAl^O^ : 25Fe^O^ as the conducting ceramic,

arc-plasma sprayed.

b. Fe/Cr alloy mesh (plus smaller amounts of A1 , Si and Ni ) brazed to

Cu as the leadout complex.

c. Sintered MgA^O^ insulators not bonded directly to the electrode complex.

These electrodes were tested by ANL at the Reynolds Research Laboratories

on October 27, 1976. Test conditions are summarized below.

a. 5:09 pm, 10/27/76, start-up, after 5-10 min. surface temperature

below 800-900 °C;

b. Temperature raised in 4 hrs. from ^900 °C to 1300 °C , Q z 55-60 V,

1 /cm ; c. Maintained overnight;

d. 11:45 am, 10/28/76, seed flow started, voltage applied, J up to 1.5

A/cm ;e. Arcs on cathode, temperature increased by 'ulOO °C;

f. Test condition maintained 1 hr. whereupon arcing became more intense

and concentrated at upstream edge, temperature locally up to 1660-1700 °C;

g. Field and seed shut-off at 1:15 pm, cool-down began at 2:45 pm.

In early November 1976, the electrodes were received from ANL whereupon they were

"potted" in epoxy and sliced for analytical work. Representative sections were

chosen for detailed analysis from slices in the vicinity of the middle portion

(see Fig. 2) of anode and cathode. A detailed report (26 figures) is available

but will not be reproduced here. The summary below illustrates the scope of

the work.

1. The mechanical integrity of the arc-plasma sprayed spinel was compromised

during the test.

a. Spalling of the material is particularly evident at the anode.

b. Arcing at the cathode probably enhanced the mechanical degradation.

2. Mechanical degradation combined with poor contact between electrode and

insulator (not bonded initially) contributed to considerable penetration of

seed, particularly at the cathode.

a. The original spinel and Fe/Cr/Al-wire mesh suffered extensive chemical

alteration at the leadout area. The spinel also degraded adjacent to the

insulators

.

3. Evidence for exsolution of iron oxide from the spinel at the leadout

zone was noted. This should be avoided b> preventing oxidation (maintaining

high density and/or using suitable oxides as dopants.)

d. GE Electrode Test at MIT (T. Negas)

We received (December 1976) an electrode pair from a test conducted

at MIT by GE and ANL. Test conditions obtained from ANL include, 11:24 am - start, 12:00 at test temperature 12: 08-15: 08 at test condition, gas = 2200 K,

1600 °C surface temp., measured Q = 110-150 W/cm^. Cathode and anode were not

labelled' nor was a pre-test specimen furnished. Consultation with ANL later

established the identity of cathode and anode. GE provides the following

declaration

:

ARC-PLASMA

SPRAYED

ELECTRODE

26

I

l

DW

I *

-><{• 25

i i ^

jj

Cl

Fig.

2.

Design

of

spinel

electrodes

for

A.NL-Reynolds

te

27

PROPRIETARY NOTICE

The General Electric Co. considers its FLEXBED Electrode Design to be

proprietary and the General Electric Co. is providing Electrodes for test

and evaluation purposes only. The FLEXBED Design concept, physical structure

and characteristics shall not be willfully disclosed or duplicated without

the express written authorization of the General Electric Co. unless the

design is available from another source without restriction.

In view of the above, specific compositions of construction materials and

special characteristics of engineering design will not be disclosed apart from

details promulgated in weekly report #1 to ERDA from ANL, 12/1/76. This report

suggests that, " electrodes were of 25 m/o Fe 3

C> 4

: 75MgA].2C>4 . . . . insulators

were MgA^O^ . " X-ray diffraction data for relatively undistrubed materials

show that the conductor consists of either a poorly crystalline spinel or of

a mixture of spinel solid-solutions having slight variability in composition.

Regardless, the material is not a 25Fe30^-75MgAl20^ (solid-solution) but one having a bulk composition lower in iron oxide content. The insulators are richer in the MgO component than MgAl^O^.

A detailed report (15 figures) is available but will not be reproduced

here. The summary below illustrates the scope of the work. Conclusions based

on the available specimens and evidence are summarized by:

a. every attachment involving unlike materials detached during the test,

b. this compromised the original thermal design parameters and led to

localized excessive heating,

c. in turn, the spinel conductor melted locally, perhaps fluxed by

oxidized Mo and intrusion of potassium,

d. the use of Mo-containing construction materials (or, in fact, Cumetal) for temperature regimes certainly above 500 °C, in our opinion,

becomes overly optimistic; oxidation of these components, even

locally, will cause detachment of the electrode and its ultimate

destruction

,

e. the spinel electrode either exsolved iron oxide during operation or during fabrication; for either case, a more resistive phase assemblage would be localized at the critical metal-ceramic leadout zone.

3a. AVCO , Copper-Boron Nitride -Slag Cathode (T. Negas)

We received from AVCO an electrode system (cathode wall) consisting

of two grooved copper frames insulated by boron nitride (BN) . Grooves were

filled with an "alum: na"-based castable. The electrodes were operated with seed

plus slag for ^20 hours. Low resistance developed between copper frames during

operation but remained high after cool-down. Possible reasons for axial

leakage, therefore, were investigated. A final report (39 figures) is available

but will not be reproduced here. The summary below illustrates the scope of

the work.

Figure 3 illustrates the Cu-BN-castable-slag electrode complex. The

following conclusions can be derived from the analysis:

1. Slag is differentiated into three primary zones, perhaps somewhat modified

during cool-down.

a. The basal, slag is glassy and probably did not crystallize at operating

temperatures during the test duration. It probably represents the "coldest"

slag, being associated only with the Cu-ridges and BN. Alternatively, it may

28

be argued that this slag was cold but, nevertheless, entirely fluid during

operation. The high K-content presumably could lower crystallization temperatures especially if the underlying BN oxidized and was incorporated.

2. The middle slag zone is partly crystalline, containing an Fe-rich phase.

3. The uppermost slag zone is porous and glassy (molten during the test)

as expected.

4. The A] /Si-rich castable degrades via reaction of K primarily with Si.

a. This suggests that Si is selectively leached or fluxed by K, a phenomenon borne out by past laboratory experience. K/Si-liquid phases can form at

temperatures <700 °C. The materials are ionic conductors as shown by numerous

studies in the past.

5. Debris between the slag and Cu-BN-castable complex consists primarily of

K/Al/Si-oxide phases apparently derived from the castable and, perhaps, the

slag. Slag components such as Fe and Ti are not present.

6. The BN was penetrated by K and, possibly, products from reactions among

K, Al and Si-oxides.

a. Some of thcAl and Si may have been derived from castable incompletely

cleaned from above the BN during fabrication of electrodes.

7. A zone of acid-soluble materials is located at the slag/Cu-BN castable

interface

.

Based on these observations, possible causes for axial leakage can be

attributed to:

1. Condensed seed plus conductive fluids rich in K/Si-oxides derived

primarily from the castable and possibly from the slag.

2. The upper two slag zones (Fe-rich crystals plus liquid and liquid) may

be connected electrically by the castable.

3. Within the castable K-rich and K-Si-oxide rich fluids could complete the

circuit when in contact with the adjacent Cu.

Possible design changes to remedy the problem include:

1. Use a castable without SiO^ as a dominant component.

2. Completely leave out the castable in the electrode design (also the

Cu-grooves) in which case slag will not "stick" to Cu or BN.

!

b. Testing at AVCO (W. R. Hosier)

A program for testing materials in slagg ing environments has been

initiated with AVCO Everett. The goal' is to find a material or combination of

materials that will withstand the stresses under MHD conditions. The principal

concerns are the erosion and corrosion of materials due to arcing and chemical

interaction with coal slag as well as the problems of shorting the Hall voltage

across the insulators along the length of the channel.
-
29

The initial designs incorporate generally the use of magnesium aluminate

ferrous ferrite (MAFF 31) bonded to the steindard AVCO Mark VI copper channel

base. Bond types using both metal mesh and cermets will be used to attach

the MAFF 31 to the metal base. In some cases, the electrodes will be capped

with coal slag applied by arc plasma spraying before run initiation.

The copper base electrodes (22) have already been received from AVCO.

Specific designs have been made and will be fabricated as soon as funding

details can be arranged.

B. Jo int US-USSR Test of Soviet Materials at UTS

I

(Phase II) (A.J. Armstrong)

During the period February 14-18, 1977, five Soviet scientists together

with their colleagues at the University of Tennessee Space Institute performed

a series of experiments to explore the effect of coal slag on various high

temperature electrode materials under electric field conditions. Four runs were made in the UTSI II MHD test channel. A. J. Armstrong of NBS participated

in the tests. These tests were very similar to the corrosion tests (Phase I)

reported in NBS-ERDA Quarterly Report it 6 (Dec. 31, 1976) with the addition of

an externally applied current. The tests consisted of two runs with acid coal

and two runs simulating basic coal. All four runs used K^SO^ seed and an externally applied electric field of 1.0 A/cm^ across each pair of electrodes.

The duration of the tests was between 43 and 60 minutes burning coal.

The same two Soviet materials tested previously in Phase I (SiC + 7 wt .

%

Ti and 60 wt. % LaCrC^ + 40 wt.% Cr) were tested under current in Phase II.

Preliminary post-test evaluation indicates that the LaCrC>3 + Cr cermet is more

resistant to attack by both acid and basic slags. Observation of the voltagecurrent relationship during and after each run indicates a large current flow

(shorting) through the slag when a stable slag layer is obtained.

C. SEM Examination of Specimens Tested at FluiDyne Eng. Corp . (E.N. Farabaugh)

Characterization of materials tested at Fluidyne Eng. Corp. is continuing.

Special attention was paid to rebonded Corhart material designated RX-317. This

is fused cast material which has been ground and rebonded using other two phase

(spinel-magnesia) material. These materials were tested for 30 hours in the

presence of seed and ash and without seed or without ash. They were tested

typically by cycling between 1800°/1600 °C. A detailed report, summarized

below, is available.

Analysis of the rebonded material revealed varying depths of penetration

of Ca, K, and Si. The paths for the penetration being around the larger grains

of the original X-317 material and through the less dense spinel-magnesia bonding

material. Some magnesia phase was found close to the exposed surface of the

tested materials. Magnesia had not been found so close to a tested exposed

surface before. Thus, for the rebonded material, the K, Si, and Ca did not

give rise to a uniformly thick reacted layer found typically in the fused cast

material

.

One fused cast specimen was examined, FN-95. It was tested at 1800°/

1600 °C for 30 hours in the presence of ash out no seed. This specimen

showed a densified reacted layer. The densification could have been caused

by new Si-Ca-Mg phases forming. This was the first specimen to exhibit such

densif ication

.

30

Fig.

3.

SEM

micrograph

(20X)

showing

Cu-EN-Cu-castable

plus

slag,

AVCO

cathode.

S

=

slag

(differentiated

into

three

zones,

dashed

lines);

E

=

epoxy;

Cast.

=

castable.

31

The next materials to be examined will be a series of magnesia chromite

samples tested under conditions similar to those conditions which the X-317

and RX-317 materials were subjected to.

D. Thermal Evaluation of a 3MgAl 2 04 ; Fe 3 0 4 Arc P lasma Sprayed Composite

Elec trode (W. R. Hosier and A. J. Armstrong)

Samples of 3MgAl 2 04 : Fc^O^ spinel bonded to Hastelloy B were obtained from

Trans Tech. Four samples were heat treated for from 20 to 72 hrs. at 600 °

,

700 °, 800 0 and 900 °C i.n a furnace in an air atmosphere. After heating

,

the spinel portion of the sampi.es was seen to have changed color from dark

grey to a mixture of areas of red and dark grey. The 600 °C sample was ^50%

red, the 700 °C sample was more than 75% red, and the 800 ° and 900 °C samples

were more than 90% red.

Under the electron microscope four distinct layers were seen in each

sample. Figure 4 shows all four layers which consisted of a layer of

spinel (1) above a layer of spinel - Hastelloy B graded cement (2) which

was bonded to the Hastelloy B base (4) with a thin r'^20lJ layer (3) of Ni-Ti

bonding agent. All layers but the Ni-Ti bonding agent layer were physically

uneffected (except the spinel as noted above) by the heat treatments. The

Ni-Ti layer appeared uneffected on the 600 ° and 700 °C samples. The 800 ° and 900 °C samples showed a large amount of porosity in the Ni-Ti 3ayer.

Figure 5 is a photomicrograph of the Ni-Ti layer of the 600 °C sample which

is comparable to Figure 6 of the same area of the 900 °C sample. By EDX

mapping it was seen that Ni had remained J.argely in its original position

while Ti had diffused throughout the bonding agent layer leaving many small

and several large voids behind.

Although none of these samples failed due to fracture through this

bonding agent layer; it seems prudent to avoid exceeding a temperature of

700 °C for extended periods of time in this area.

A subsequent test of electrodes of this type, during which a temperature

of 500 °C was maintained over the bonding agent layer, showed no apparent

weakening of the bond and no cracking along the bonding agent layer.

E. Chemical Properties of 3MgAl

2 0 : Fe (MAFF 31) Spinel Solid Solution

(T. Negas and C. D. Olson)

In view of the flurry of activities (past and current) involving the

utilization of so-called MAFF 31 as a conducting ceramic, this section is

devoted tc elucidating the chemistry of this (and related) spinel material.

Chemical properties are exceedingly important to fabrication methods and to

the electrical properties of MAFF-31.

Firstly, it must be appreciated that the composition 3MgAl 2 0^ : Fe 3 04 (solid solution) is only nominal. Here we are not considering site-occupancy

of cations vs. temperature, a feature which is largely irrelevant. Fig. 7 illustrates the composition triangle MgAl^O^ (spinel) -Fe^O^ (magnetite

spinel) -fe 2 03 (hematite). Under relatively oxidizing conditions (e.g., in

air, PQ

s 6.2 atm) the bulk composition 3MgAl

2 04

:Fe

3 0

4 is attained only

above 1400 °C. At lower partial pressures of oxygen this temperature, of o course, will decrease. The spinel solid solution is cubic with a = 8.195 A. The rmogravime trie (TGA) and quenching data indicate that below ^3.400 °C (in

i2

Figure 4 . A 100X scanning electron

micrograph of the 800 °C sample showing

all four layers.

Figure 5 . a 1600X scanning electron

micrograph of the 600 °C sample showing

the unaffected bonding agent layer.

Figure 6 . A 1600K scanning electron

micrograph of the 900 °C sample showing

the large amount of porosity produced

in the bonding agent layer.

33

air), the iron oxide (Fo 3O4 ) component oxidizes rapidly and reversibly as a function of decreasing temperature. As the material (in loose, fine powder

form with access to available oxygen) oxidizes, the bulk composition (Fig. 7)

shifts toward the MgAl^O^-FepO^ tie-line. Maximum oxidation is attained below

''-'1000 °C and the material can be maintained in oxidized form at lower tern- peratures (red coloration). At maximum oxidation, in air, the Fe ' /Fe ratio

decreases to 0.21. 0 The material remains as a cubic spinel but the a-dimension

contracts to 8.178 A. These phenomena related to oxidation are not unusual

for most spinels and have been reported in the literature frequently. Table 1 summarizes the observed reactions. The conducting sj^inel reacts with available

oxygen below M400 °C (in air) to form oxidized spinels based on solid solutions

mong MgA^O^, Fe^O^, and ®“^2®3 ( a l so a spinel) components. Solid solutions

of this type necessarily contain cation vacancies ( [vj ) rather than excess

oxygen. These vacancies provide convenient paths for Sif fusion of Fe

.

When the oxidized spinel is annealed for extended durations at low

temperatures (<1000°, in air), iron oxide as Fe2C>3 ultimately exsolvcs. The

resulting assemblage consists of a phase rich in the F&203 component and

MgAl2C>4 containing less iron oxide than the original material. Exsolution

thus produces a phase assemblage more resistive than the original single phase

spinel solid solution. Prior to exsolution an oxidized spinel with metal

vacancies can exist at low temperatures. This oxidized spinel based on

Y-Fe2C>3 may have a significant conductivity based on Fe-oations migrating

through available vacancies. For the 3MgAl2C>4 : Fe3C>4 spinel, the reactions

related above can be avoided by retaining the ceramic body in dense crackfree form (i.e., minimize diffusion of oxygen). Similar reasoning is applicable

to "hercynite"~based spinels. TGA data derived from dense MAFF 31 prepared by

Trans-Tech, Inc. indicate that oxidation iq effectively negated.

How does this discussion correlate with the electrical conductivity

measurements published for MAFF- 31? In the first place the four-probe

method was utilized combined wi.th high density ceramic bodies. Response to

P variation was minimal. This, however, must be expected as the material

°2 • was maintained in dense, crack-free form and, therefore, not permitted to

undergo equilibrium oxidation. Although the conductivity of MAFF-31 was measured the conductivity of oxidized variants of MAFF-31 cannot be determined

by the imposed experimental methods. However, oxidation phenomena are clearly

evident in the less dense "NAFF" and "MAFF-11" materials given in this report

(Task I. Electrical Conductivity).

It also should be appreciated that for MAFF-31, exsoiution to more resistive

phases requires durations exceeding ^1 week at temperatures <1000 °C (air).

The U-02, Phase II test, 75 hours, however, clearly showed that the MAFF-31

electrodes, primarily cathodes at high current density, exsolved Fe^O^ during

the short test duration. This suggests that the exsolution process is enhanced

clectrochemically . Again, an oxidized MAFF-31 containing cation vacancies

would be prone to accelerated exsolution via passage of current.

Let us now focus on the methods recently employed to fabricate MAFF-31

electrodes. Figure 8 illustrates features of electrodes fabricated by L. Bates

at BNW for U-02, Phase II. Starting MAFF-31 powders were purchased from

Trans Tech, Inc. The upper SEM micrograph illustrates the relatively dense
'

-
34

microstructure of the upper portion of a typical electrode. The corresponding

line (at right) represents a portion of the X-ray diffraction powder pattern

for the dense MAFF-31. It also corresponds with the original Trans-Tech

powder which consi sts of IMgAt^O^ : FC 3 O4 solid solution, hereafter referred to

as reduced MAFF-31. To provide a surface for bonding of a metal leadout

(^600 °C interface design, done by Wcstinghouse Corp.) the porosity of MAFF-31

was enhanced by L. Bates at the lower portion of the electrode, Fig. 8 , bottom

SEM micrograph. The X-ray lines, at right, show that the lower, porous zone now consists of at least two spinels, the fully oxidized MAFF-31 (based on Y-Fe 2

C> 3 , plus cation vacancies, see Table 1) plus reduced conducting MAFF-31.

U-02, Phase II, MAFF-31 electrodes, therefore, contained an oxidized, probably

more resistive spinel at the leadout zone prior to the test. Clearly, the

increased, controlled porosity at the electrode base enhanced oxidation

probably during the cool-down segment of the fabrication process. Exsolution

of F^C^, enhanced electrochemically , necessarily followed during the U-02

test. Significantly, the MAFF-31 cathode that performed for the longest

duration (V56 hrs.) was fabricated by Trans Tech, Inc. It was a dense,

sintered ceramic without a porous leadout zone.

Another method of fabricating electrodes based on MAFF-31 involves arc- plasma spraying. One design utilizes a porous metal mesh at the leadout zone. MAFF-31 is sprayed onto this mesh. This configuration, in our opinion, invites

oxidation of the MAFF-31 at the critical spinel-porous mesh interface. Indeed,

the exsolution process (see above) has been observed at this interface for

electrodes tested for limited time durations. A second design involves spraying

MAFF-31 onto a metal such as Hastalloy B. Specimens of these electrodes were heated overnight at 600, 700, 800 and 900 °C in air. This was done to investigat

the metal-MAFF 31 bond line. We obtained these materials to analyze the MAFF-31.

Each sample yielded an x-ray powder pattern illustrated by a typical portion in

Fig. 9. The pattern consists of a spinel (S) containing less iron oxide than

MAFF-31 plus Fe^O^ (F) . This pattern also is typical for some of the U-02

cathodes (post-test) and for powders permitted to oxidize and exsolve iron

oxide under laboratory conditions. Here we have a dilemna summarized below.

1. Powdered MAFF-31 exsolves Fe^O^ only sluggishly, requiring 'vl week at

temperatures <1000 °C.

2. Dense, sintered MAFF-31 is not susceptible to oxidation and exsolution

processes

.

3. Arc-plasma sprayed "MAFF-31" oxidizes and exsolves Fe 2 C>3 within 20 hours

(600-900 °C) , no current.

4. Identical starting powders (Trans-Tech) were utilized for the materials

noted above

.

The above observatiors clearly point to the arc-plasma spraying process as the medium by which MAFF-31 is left prone to oxidation/exsolution. Two factors

can be isolated. X-ray analysis of several., virgin, sprayed specimens of

MAFF-31 reveal that the material is characterized by broad diffraction lines.

This suggests poor crystallinity and/or poor homogeneity. Sintered

ceramics yield x-ray patterns containing sharp x-ray lines indicative of a well-crystallized, ordered material. As the spraying process introduces

temperatures far exceeding the melting point of MAFF-31 and as iron oxide is

highly susceptible to reduction at these elevated temperatures, it appears

hardly likely that the MAFF-31 product would be as chemically and "crystallographically" homogeneous as a carefully sintered MAFF-31. Indeed, the sprayed

35

MAFF-31, having rapidly crystallized from the liquid phase (and disordered?)

may c quilibriate more rapidly to the phase assemblage (exsolution products)

stable at low temperatures. Alternatively, the sprayed material, although

poorly crystalline/homogeneous may contain a second phase such as Fe metal,

Fe^O^, and/or Fc^_ x O ( wustite) which serves as the precursor for oxidation

to Fe 2C>3 (Fig. 9). Tne precursor phase need not be entirely crystalline

and, hence, not detectable by x-ray diffraction. Evidence for a precursor

was obtained from one specimen of MAFF-31 sprayed by Techneti.es Corp. The

X-ray pattern consisted of broad x-ray lines attributed to spinel prus weak,

This discussion illustrates the importance of thoroughly understanding

the chemical properties of a material prior to developing fabrication methods

and engineering designs. Chemical alteration, particularly via oxidation,

is often subtle and difficult to deduce by electron microprobc and SEM/EDX

methods (unless exsolution occurs) . However, this alteration can lead to the

formation of resistive phases (for electrodes) and consequent damage if

ignored. For MAFF-31 and related spinels, it is suggested that suitable oxide

dopants be utilized to effectively block oxidation, formation of vacancies,

and exsolution at low temperatures. These approaches are under investigation.

Table 1. Summary of Generalized Reaction Sequences of MAFF-31 Spinel

MgAl

2

0/j + a- Fe 2

03

Exsolution to

RESISTIVE PHASES

Exsolution without current -- sluggish, e.g,,

500°-1000 °c, in weeks

broad lines of a secondary pjhase

.

wustite

.

The secondary phase appears to be Fc^ ^0,

Spinel solid-solution

conductor

Spinel solid-solution

with Y'Fe/Oj

Exsolution with current -- enhanced, within hours

36

Figure 7. MgAl 204 -Fe 30^-Fe 20 ^ composition triangle illustrating the oxidation,

exsolution behavior of jllgA^O^ : Fe 30^ spinel solid- solution (MAFF-31). Data

refer to thermogravimetric analysis from 800-1400 °C, in air.

i

V

I ri;

v J

8

r.i

f h

u

( 5 A.... i

/

t A

J,

A

(4 4 0) R

(44 0) + K

(440)

Q

G4 65

20, Cu K«

Figure 8. SEM micrographs (original 1000X, reduced by 2/3) and X-ray spectra

for MAFF-31 spinel fabricated for U-02 , ’T’hase II. Upper micrograph is for the

upper dense portion of the electrode while the lower illustrates the more poroi

leadout area. The selected x-ray lines correspond with the material in each

micrograph. The (440) spinel line is shown; R = reduced MAFF-31; O = oxidized

MAFF-31.

37

CO

4-1 O

Prao

m

Dt ^

m

cd i o Cm

p ^

CO

O T5

CO <D X >1

CD cdM

<4-( CL| O W

0) CO > G

•r-t CO 4J d!

rtf f-t -P Q< C I cu u

CO P

0) <0 M

-p

<U M

M "H

cd

CD v

CN O

•H O

O

CU CTi H I cd o

u o

CO lO

iH

cd X3

4J cu C '-j o fd N CD •H £

m c

o <d

<44 - o

aX T5 a c U cd

M

4J

CO

0)cH

CU • U CO °o

CU O

a h

CO rn CU

I CM >1 Cm

Cd 1^ II M c

I Cm X ^ •' cm cu

*—

i

O M <U

<U C C r O -M o ^ a, •MOW

-O CU M II 0 cm

CU O CO

CT>

cuMP

C

n

•H

Cm

.. 38

Task L. ASSESSMENT OF STEAM PLANT COMPONENT (j . R. Cuth ill)

This is the 7th quarterly report since the initiation of Task L. Included in this report is (1) an update of the Table 8-1, "Mechanical

Properties" that was presented in the quarterly report for Oct-Dec. 1975; (2)

attempt at a ranking of selected alloys in the order of their relative

resistance to hot corrosion attack; (3) conclusions to date on the assessment

of alloys for the steam heat-exchanger tubes; and (b) coatings for protection

against hot corrosion and high-temperature oxidation.

Update of Table of Mechanical Properties

Table 1 is an update of Table' 8-1, "Mechanical Properties" in the

quarterly report for Oct. -Dec. 1975; the property values have been obtained

from various sources (l), (2), (3), (4) , (5) and all values have been

converted to S.l. Units with appropriate rounding. All of the alloys in the

Table are wrought alloys selected with the steam superheater tubing

application in mind, with the exception of Wl 52 and FSX-418 which are cast

alloys. The latter two alloys had been included in Table 8-1 in the earlier

report only because in that same report they were cited in a discussion of

alloy composition vs. hot corrosion and high temperature oxidation behavior.

Table 2 lists the compositions of the alloys in Table 1 and is also an update of the earlier table.

Relative Resistance of Selected Alloys to Hot Corrosion

A considerable effort was spent this quarter in trying to establish a relative ranking of selected alloys, in respect to their resistance to

hot-corrosion attack with the steam superheater application in mind.

However, the data is such that a very satisfying relative ranking cannot be

made. The data in the literature have been obtained under v/idely varying

conditions of hot corrosion attack under different test conditions, including

marine gas turbine test rigs, simulated MHD ducts, crucible immersion tests,

and potent i omet r i c measurements, etc. There are even diametrically opposite

results frequently reported in hot corrosion literature. Of course, most of

the hot corrosion data in the literature are in connection with marine gas

turbine research. [A review of a number of comparisons of superalloys for

gas turbines, in regard to their hot corrosion resistance, is contained in a report by John Stringer ( 1 1 ) ] . However, the gas turbine literature does not

include the austenitic stainless steels. This makes it particularly

difficult to mesh the stainless steel data from the MHD simulation tests

(.12), ( 13 ) with the superalloy data.

With this background, a ranking of those alloys included in Table 1, is attempted in Table 3. The listing is in order of decreasing resistance to

hot corrosion attack.

It is quite evident that Inconel 67 1 as a cladding, or the equivalent,

would be a first choice, as far as resisting hot corrosion attack is concerned, and that the low chrome-moly steels that formerly were widely used

for boiler tubes woul 1 be at the bottom, but the problem is in the middle in

meshing the group of stainless steels relative to the group of superalloys

having intermediate resistance to hot corrosion. For this reason the listing
.
39

is divided into the two columns.

The stainless steels present another problem. Type 316 and to a lesser

extent type 310 are subject to carbide precipitation upon slow cooling after

prolonged service in the range in which the steam heat-exchanger tubing will

be operating ( 1 1 00- 1 200° F) . This tendency for carbide precipitate formation

is minimized or eliminated by using the low carbon version of these stainless

steels, i.e. type 316L or 3 1 0 L . (1).

Conclusions to date on Assessment of Alloys for Steam Heat-Exchanger Tubes

Review of relationships between alloy composition and resistance to hot

corrosion attack, as discussed in previous quarterly reports, has lead to the

following conclusions as to composition guidelines to follow in judiciously

selecting alloys expected to have good hot corrosion resistance:

1. Hot corrosion resistance increases with increase in chromium content,

and is very sensitive to chromium content (see Figure l). Alloys that

depend upon a chromium oxide protective film for their corrosion

resistance are the best for resisting hot corrosion.

2. Avoid alloys that depend upon an Al^O protective film for their good

temperature oxidation resistance. ' ^

3. Avoid alloys high in tungsten and molybdenum added as carbide formers.

Their carbides at the surface act as nucleation centers for hot cor ros ion pitting.

h. The combination of chloride and sulfate is d i sporpor t i onatel y worse

than sulfate a 1 one

.

5. Data on attack by sodium salts gives a first approximation to what

might be expected from molten potassium salts.

6. The ambient flue gas composition is sufficiently critical in respect

to the severity of the hot corrosion attack that a test, in a rig that

simulates the service condition as closely as possible, should be

conducted before the final choice of alloy is made.

7.

A material such as INC0 671/800H clad tubing would be a first choice of

material for the superheater tubing in respgct to the certainty of

resisting the hot corrosion attack, providing on oxidizing atmosphere is

maintained. A less ex)ensive material may prove to be adequate but a simulation test in a burner rig is recommended, when the anticipated gas

composition is better known, before such a choice is made.

Coatings for Protection Against Hot Corrosion and High-Temperature Oxidation

An alternate method of achieving adequate corrosion resistance is by coa ting.

Chatterji, DeVries, and Romeo in a recently published review (7) list

some, - in the authors words, - "popular commercial coatings" to resist high

temperature oxidation and hot corrosion. This list is reproduced in Table 4. These coatings fall generally into two classes (1) diffusion coating or surface case that is usually achieved by a pack cementation process similar

to pack carburizing, and (2) overlay cladding in which the alloy generally is

applied either as (a) a slurry, or "slip", dried and fired, or (b) as a powder by a plasma sp r ay gun.

Figure 2 shows results of Bartocci (8), reproduced by Chatterji,

DeVries, and Romeo in their Review. The data shows the beneficial effect of

the coatings marked with an asterisk in Table 4 in protecting IN-713 against

hat corrosion in burner rig tests. The coatings were applied by the pack

cementation process.

A method of coating application not discussed by Chatterji, DeVries,

and Romeo is the use of a high power laser beam to melt a thin surface

layer while adding the desired alloying element into the molten region.

Surface layers containing up to 50% Cr are reported to have been produced in steel by AVCO Everett Research Laboratories, Inc., using the 15kW, CO^ laser

equipment which they have -developed (9), (10).

References

1. Aerospace Structural Metals Handbook, Vols. 2, A, and 5, 1970 revision,

Mechanical Properties Data Center, Bel four Stulen, Inc., Traverse City,

Michigan.

2. Structural Alloys Handbook, Vol . 2, 1977 edition, Mechanical Properties

Data Center, Belfour Stulen, Inc., Traverse City, Michigan.

3. "Materials Selector 76", Materials Engineering 82_, A, (1975).

A. Properties Bulletins from Cabot Stellite Division, Kokomo, I nd . A69OI

.

5. Research reports from Huntington Alloy Products Division, INCO,

Huntington, W. Virginia 25720.

6. A. M. Beltran and D. A. Shores, Chapter 11, p. 332, The Superalloys,

C. T. Sims and W. C. Hagel, eds., John Wiley, cl 972.

7. D. Chatterji, R. C. DeVries, and G. Romeo, Protection of Superalloys

for Turbine Application, Advances in Corrosion Science and Technology,

Vol. 6, M. G. Fontana and R. W. Staehle, eds., Plenum Press, 1976.

8. R. S. Bartocci, Behavior of High Temperature Coatings for Gas Turbines,

Hot Corrosion Problems Associated with Gas Turbines, ASTM-STP-A2 1 , 1967.

9. F. D. Seaman and D. S. Gnanamuthu, Using the Industrial Laser to Surface

Harden and Alloy, Metal Progress, 1 08 , (3), 67 ~ 7 A , 1975-
,
P a, 0

rj E O 41 , H • PQ CO >•

GJ in tn o 1 <N x m* vr tn TT m COX X in

i-J o X

OX CN o o O rj o r~ i o • d o C • rj o E o’ o o OB o o P l/> O

CO o m O £ /

-2 <

rHmx co

x

X CM in

O cc o • cm nj o n cm nj rH n X CM o O • c • 0 • E »H d o O o O o d a O o X o m o O tn o o X d • • • • • o X V) X m C/0 B rH rH rH rH o • rj • d X E O r- o o tn X <NJ o o in «n O X in X r« c C3 1 • • • CM 1 • d • d • S E »H cm CM rH • rH E o e o X o rH ro O

:* 1 1 l l 1 1 rH O r- rH

rH

cm I CO

dH I I

QJ tjXXPdu

~~d vr H P

H- tn £ -3 W to TTT"

rH P

tsj “7T" m

_?sM

"lA*

vOI XO

Ci, E

QJ I m dx

cx

IM _Q—

GJcx HI

Fe

OX • d

rn P

rHd

Xl

bal.

rHdX

m

X • d

CM E O

X • rj <r B

O

CM •

10c0

-HP

46.0

L0 o o o r—(

CM

X rH o< CP d 1 1 I 1 1 d d 1 , X cn*H X X u

iH

IS tj X \ I O' o

»H

d r*i X

to

QJ np

to

X P n o

*H i_) p n o E

XJd xj E x

CO

XJ 12 V) o

sit

p o ^ mCMo&

cil oo

•«o o o o“

o£

5-.? VO

rH

r-> *v v ‘ 6

-x o 0 ) COo c xou

C! M

M

O t7» r.P

QJ CO

d Ifi P TJ

QJ P E d

•O - q 10 d C P O p o Wl H w H 0) P >» P 4H CO d tn d 0 p O N 0 ) -ri QJ -H n

VH ,CC

d j •H H cj CJ TJ 4 J tn p c u u ti

dJ • 0 rp c

'd CO

d

-H CJ CO r-< P TJ d >t >, d n c c 0 p d 0 d

rH LO t) •H E

rHC

p

dd5 d

d cn

p

IH 4 J 10

X 0 -H QJ P C tJ> X d E C P x

c> *H QJ POP Cn p

to

X c C P

>, P Q)

x

O

to toT5 U

n e d

co v<

X

x

r$

x

a

U 0

tO P Qi c E -H U

o»

U

X to

CO QJ tn S C CO Ql QJ 0 ) O tn -» J x B U O P U

r-t -H (4

d

d rH E a d fc cn co

to 0 d Q) •H T> U TJ c P x. d d c r> rH cj o

rH 10 >. E v< '— rH X ci Ct, d p m a o Cl

c> cj o > E p E P 0 o

toa

TJ P to CO QI to TJ >= U H C If) * 'J c d to c d h cn E -«of: - c QJ p p oi to -H P

Qi

d Vy v^ CO

QJ Q) E >i to U X to d H H P E Z x tj :J tn d T3 0 rC r: tj -h q *«H

Q) l> rj

a tj Vj

*r( P CJ QJ 1m Q) T3 >, X X x E QJ o H-* f- CEO.i X X POO x r: c u x d rp 0) 0) TJ X x > TJ QJ d

V*

X Q) c X to d tj d 1

-*

p

.O

n o

~i> 3 ' a o rc

"'Cast

alloy -

not

commercially

available

42

coi<1> rH•9H

p

u

u

U

n) i 5 oo P 0Oo 0oo uo

Pi

H C4 M* *!• OO

M

>i xv P tn CO f" Ei Ei MO ID rt + * ••r U P CJ CO co co '

'

V-d i* ID 'd OO * Vi (d CD rl ' —

—

H o Q) cd CM Vl P CO crj N V | X E P o ro 1—1

uo t- • • 'd' HI1 p o H CO t> vO CM o • o o •MT P r~i rH rl CN rl iH CN H rd

ViQJ • 1 0) 0,

Vi MO QJ E P V) t) U O O r-l Cl to a> CJ o o o i—( U P rH o El f| ;H OO MO cro >i

OJ o <N c< tk cd

T rH M‘ Vl o CD 3 >i CO lO CO £ •ft * MO • O l/l tn u CO X) X5 X •

— •4* it Ml CO a> .c •rl *

1 1 P CO o r- Oo M* OO 1—1

r- o F-i rj o CM CO rl MO CN O' CO o JQ r-l Cm rl £ CO CN <n CO rl co CN e X to P C • cd tn 0 Vi U P •H XI -—s .

. .-x ,—.. ,—, U , , Vl

0) P u

u

u • U

a (J 0 a ,P .C

p cdO o o O 0 o 0 rl 0 a

p tr>

O o CM Oo CO t" i—1

r- Oo o rd to P O •^ P‘ 00 CN O' co •MT o e O * VO vO UO uo OO CO < •

* MU o

O. rH o '—

-

-—

—r *—

'—

<vh +: •V . x-r rl c

CD

O '—1

rj

<d •rl

0) CM ro co T- Oo 00 co

X Jj

VI dP P CM o CO rl UO MO CO I'' rd QJ o 1—(

cd £ rH i—i

rl rl 1 P

rl (d i—1

cd> ) (D

XI ri P \ ««x Vl £ u u u o e * Co 0 0 o o p

CO o O ,—.. >

„—,

CN Cl mO Ml o rl VO H [I CO CO < rl cd •rl u CO — £ (t Ct OO oo CO P •r| P <d >

•

—

It

—

01

n P f—1

CM •K * Go Q) QJ (/) p co r—1 o co o co CN co to D: rd rU o CO o Oo o MT •Mi* r-l Q) o i-i O rH rH CJ rl CN rl rH XI M w It X P

CM P .—. — ,— ,—^ s— .—,

CD tH u u CJ u o u a u u Vl rd o o 0 o 0 0 o o 0 cd u X. r—1

co co CO CN CM 1—{

oo oo a.

•r~i CD P r- CM Oo OO CO CO <C

m

V p* c i—1

tn 00 uo uo in OO OO £ CD •tt MO MO

p

id •H

p

r .—. •—

T

—

K -—

—

•H

X 10 CD u p Vi <d CM CO Vj> CN GO T" CN co P

<y 0) P CM CM rl MO rl rl tn V? Q) £ El CO £ CO •O’ CO •sT uo r) >H

tn

00 * ^—•* .

, ,„ .—^ (—, ,—. ^—.r-x 0) P CJ u u U u u o o U Vi CD o o 0 o o o o O O p

X CO »H CO co CO CN (N rl OO O' XI p Ml r- oo Oo cro CO CO r- NS* •V rd tn Ml CO in to in oo 00 £ co MD mD Vi 03 P 0 '—•• V-' — ^r 'r —

•S '—

*—

CD i

l QJ Dt

<D Vl c*0 rd CO CO cnm vr r-l MO o o E

•H P CN Cm O') co

m uo o vf r- Oo o a> t* CO • £ CM .H rH rl co CN CN CN rH p XP

tnP

Q) r.

VlCN rs P E u o o O

U

to \ o 0 o o O • £ MO co ro CO O

Vla mo rl ao Oo OO MO

X

p o CO LD uo in P- P r| x^r X-. '—

»•—t o p o CM cd CN rH CTv CO o CN r- P* ro Mf rl C4 £ rl rH CN

in

CN • c 0 CO CO CO

CO • » « Q to to CO

u P CO

rl o MO MO rl

rd rl rH vr rrc CO co •rj* r-r

a) xp0p

mv ^—x

^-X.

p

u

u

U o o C o o 0 a

rl r- cro CN CJ) m- CN rr co CD CO 00 * VO r^- Vl V ' £ x^* Vlo

in CO H- OO

u

•MT o 00 MD rl rl to ' aip

rl

MrUt cd rH CO ;>

I'' o # ’0 MO CO >. n Cd £ x» n rl CO r-l >1 Vl CO CD O o 0 ai rH CN co p 1—1 it

O

V in

O \ o 0 o O X i U -H o o Vl in CO M £ r- p p CM ro ft, II i«0 1 1 1 1

nave

been

selected

in

most

cases

because

they

are

either

the

maximum

temperature

reported

at

which

the

alloy

could

used

under

MHD

conditions,

or

the

highest

temperature

at

which

data

is

given.

**The

thickness

of

the

alloy

cOOH

substrate

only

should

be

used

in

determining

design

stresses.

***Cast

alloy -

not

commercially

available.

43

10. Private communication, D. S. Gnanamuthu, AVCO Everett Research Laboratory,

Inc., Everett, Mass. 021A9.

11. J. F. Stringer, High Temperature Corrosion of Aerospace Alloys, AGARDograph

No. 200, August 1975, NT I S Cat. No. AD/A-016095, Nat. Tech. Info. Service,

Springfield, VA. 2 2

1

6 1

.

12. D. Bienstock, R. J. Demski, and R. C. Corey, Corrosion of Hea t - Exchange

Tubes in a Simulated Coal-Fired MHD System, J. of Engineering for Power,

ASME Trans. 9J_, Series A, No. 2, 2 ^ 9

~ 2 5 6 .

13. J. B. Heyv/ood and G. J. Womack, Open Cycle M.ll. D. Power Generation,

pp. 716-721, cl969, Pergamon Press, Oxford.

Table 3. Relative ranking of some selected alloys in decreasing order of

resistance to hot corrosion attack.

Inconel 67

1

Haynes 25

FSX - A 1

0

310 S.S. U500

316 S.S. WI -52

AA 6 S.S. IN-713

Croloy 9 (9Cr-lMo steel)

Croloy 2 1/A (2 1/A Cr-lMo steel)

Croloy 5 (5Cr- .55Mo steel

)

44

Table 4 . Some Commercial High-Temperature Coatings for Superalloys

(from Ref, 7)

Designation Vendor Coating constituents 1*10 ;css

NC10I A Sylvania Al-rich Pack cementation ASC-HI-15 Alloy Surfaces Al-Cr Proprietary (‘ vapor deposition") Nicrocoat ) 10 Wall Colmonoy Ni-Cr TiB, Slurry

Kicroco.it 130 Wall Colmonoy Ni- Cr Si -TiSi 2 -TiN Slurry

Type 701 Lycoming Div.. AVCO Al-rich Vacuum pack cementation Type 606 B Lycoming Div.. AVCO Cr- Al- St Cr plate + hot dip Al-Si WL-J Union Carbide Al-rich Pack cementation C-9 Union Carbide Al-rich (AT Ee) Pack cementation C-12 Union Carbide Al-rich Pack cementation WL-8 Union Carbide Al-rich Pack cementation WL-14 Union Carbide Al-rich Pack cementation WL-4 Union Carbide Al-Si Cr Pack cementation c-20 Union Carbide Al- Cr Pack cementation C-3 Union Carbide Al— Ni Pack cementation WL-9 Union Carbide Al-Si Pack cementation

1 AD Union Carbide Al Hot dip

WL-6 Union Carbide Be Pack cementation

Calorizing Calorizing Al Pack cementation Codcp A General Electric Al—Ti Pack cementation Codcp C General Electric Al-Ti Pack cementation

Aldtp Allison Div., GMC Al Hot dip

Alpak Allison Div., GMC Al Pack cementation A-U Martin Metals Al Diffusion-annealed CVD

PWA-47 Pratt Whitney Al-Si Slurry * M DC1

Misco Precision Casting Al-rich Pack cementation

. MDC-1A Misco Precision Casting Al Pack cementation AMDC-6 Misco Precision Casting Cr-AI Pack cementation M DC-7 Misco Precision Casting Al-rich Pack cementation M DC-9 Misco Precision Casting Al-rich l’ack ccmentat on . *uc Chromalloy Al-Cr Pack cementation SUD Chromalloy Al-rich (Al- Fc) Pack cementation SAC Chromalloy Al-Cr-Si Pack cementation UDM Chromalloy Al-rich Pack cementation

RT-21 Chromalloy Al-Cr Proprietary RT-22 Chromalloy Al-Pt-Cr Proprietary (duplex)

'1 RW Al-rich Vacuum pack cementation TRW Al-Cr Slurry

TRW Al-Cr Vacuum pack cementation S13-53C Solar Div., Int. Harvester Al-Fc Slurry S5210-2C Solar Div., Int. Harvester Ba—silicate glass + additives Glassy refractory S6100M Solar Div., Int. Harvester Modified S5210-2C Glassy refractory NBS A -41

8

NBS Frit 332 + C

r

2 O 3 Glassy refractory MAL-2 Vitro Al-rich Electrophoresis

Scnnetcl J V'itro Al-rich Elcctrop ,'oresis BB Chromizing Rh-AI Pro; .clary LDC-2 DEW (Germany) Pi- Al Electroplate pack cementation
•

•
40 r45

30

20

10

IN-100

Q U-700

» U- 500

\p REN E - 4 I o hastelloy x

\

\ M 3 I 3 \ HW-30 N- _i i i 10 20 30 40

WEIGHT %Cr

50

Fig. 1 . Hot-Corrosion resistance vs. chromium

content of some nickel base balloys (from Ref. 6)

TIMC (HOURS)

Fig. 2 . Coated vs. uncoated IN-713 in resisting

hot corrosion in burner rig tests. Coatings include

those marked by an asterisk in Tabic 4. (from

Ref. 8)

46

SUMMARY AND CONCLUSIONS

Task I . - Operational Design Properties

Viscosi ty

The viscosity of Soviet "standard" (Krushnitz) coal slag is %100 poise

at 1600 °C compared with "average" Rosebud and "average" Illinois #6

(both synthetic) which attain 100 poise near 1400 °C.

Electrical Conductivi t

y

Nickel aluminate ferrous-ferrite spinels may be promising electrode materials

in a slagging MHD generator. Oxidation appears to be a problem for this class

of spinels as well as those based on MgAl O^'iFe^O^ solid solutions. Oxidation

can lead to formation of a more resistive £jhase assemblage at low temperatures

Vaporization

Preliminary measurements of K-pressures over K-^O-AlpO^-SiO^ solutions using mass spectrometer methods indicate the potential for some success. Previous

attempts to perform these measurements using a TGA- if fusion cell arrangement

proved troublesome.

Task J. Corrosion and Diffusion

Seed-Slag Interactions

Reactions among K.20-Al20

3 -SiC>2-"Fc0, r " (plus MgO, CaO) are under investigation

in a systematic manner. For the KAl02-KASi0^-"Fe0x

" subsystem corrosion

products of construction materials based on Al/Fe-oxides or crystallization

products from slag alone include Fe 30^ and K-f-alumina phases containing Iron

oxide. These constituents could contribute to axial leakage in a slagging

generator provided temperature-composition limits are realized. The electrica

(electronic and ioni.c) conductivity of the K-3-aluminas containing iron oxide

should be investigated for they may prove to be valuable materials.

Task K. Materials Testing and Characterization

a. Most of the tested electrode/insulator assemblies analyzed to date have

one common feature, failure of bonds and/or attachments among unlike materials

For many cases, however, it is difficult to isolate, chemical, thermal and

electrochemical effects. Collaboration with all involved laboratories must

be encouraged for only "educated guesses" generally can be given. Laboratory

and rig tests of the MAFF-type spinels clearly indicate that these materials

must be modified within cooler electrole regions to prevent formation of

resistive phases. LaCrO^ (MgO), capped with Zr02 e l ectr°des offer

promise as indicated by recent NBS/Trans-Tech tests at MIT. Development of

YCrO^-based materials should also proceed.

b. The AVCO Cu-BN-slag electrodes show considerable promise provided that

the axial leakage problem can be solved. Possible causes were suggested but

these remain to be pinpointed by Cidditional testing.

Task L. Assessment of S team Plant Components

Based on accumulated and updated data, selected alloys are ranked with respect

to hot corrosion resistance.