//£ ?0 - ^ *//

EA-601 0-22

Dist. Category UC-90C

MATERIALS RESEARCH FOR THE CLEAN UTILIZATION OF COAL

QUARTERLY PROGRESS REPORT

October - December 1979

Samuel J. Schneider

Project Manager

Center for Materials Science

National Bureau of Standards

Washington, D. C. 20234

PREPARED FOR THE UNITED STATES

DEPARTMENT OF ENERGY

Office of Advanced Research and Technology

Under Contract No. EA-77-A-01 -601

0

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

States Government. Neither the United States nor the United States

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

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

expressed 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 privately owned rights."

TABLE OF CONTENTS

PAGE

I. SUMMARY OF PROGRESS TO DATE 1

II. DETAILED DESCRIPTION OF TECHNICAL PROGRESS 3

4. Failure Prevention 3

5. Creep of MHD Refractories 3

6. Electrical Transport Mechanisms in Slag 8

7. Corrosion of Downstream MHD Components 17

I. SUMMARY OF PROGRESS TO DATE

Brief Summary

4. Failure Prevention

b. Materials Properties Data Center

A. test set of MHD data has been assembled for use in benchmarking the six remaining computer-services vendors. Some eighty data sets were

prepared, keyboarded, corrected and transferred to magnetic tape. The tapes, each containing over 5000 lines of information, were delivered to the vendors so that they all would be working with the same data base for comparison testing. Additional amendments to the solicitation for vendor's services have been required and issued.

Routine procedures for handling the contractors' research reports have

continued as usual

.

5. Creep of MHD refractories

Work was concentrated on completing the evaluation of the chrome-spinel

refractory (Corhart RFG); this includes creep and swelling measurements at 1400 °C, 1500 °C and 1600 °C. The results indicated considerable deformation at all testing temperatures. The specimen tested

at 91 psi and 1600 °C failed catastrophically.

A technical report on these measurements will be prepared during the

next quarter. In addition, four new furnaces will be installed for

the planned expansion of the creep facilities.

6. Electrical Transport Mechanisms in Slag

The dependence of the electrical conductivity of Yq 5 Ca 05 CrO 3 on oxygen

pressure in the range of pressures from 10° to 10"’® atmospheres have

been completed for temperatures from 1300° to 1600 °C. The apparatus

has been rebuilt to allow measurements in the high temperature range

at oxygen pressure from 10-6 to 10 -15 atmospheres.

A three probe electrochemical cell has been constructed and the

electrochemical effects measured at a temperature of 1254 °C. This

cell allows the measurement of slag-electrode voltages as a function of

time and ultimate analyses of the electrochemical effects.

The first series of experiments on the electrical conductivity of Bow

NH slag has been completed. The conductivity at low temperatures

(below 1100 °C) depends on the crystalline state of the slag. The

conductivity at high temperature (above 1400 °C) is independent of the

oxygen pressure and of the amount of K present at least up to 6% by

weight.

7. Corrosion of Downstream MHD Components

Type 304 stainless steel tubes, cooled to 400 °C, 500 °C and 590 °C,

were exposed to fuel rich and oxygen rich hot gas streams seeded with

a mixture of 80% by weight K2 C0 3 and 20% by weight K2 S04

. Optical and

SEM and EDX analysis revealed differences, geometrical and chemical,

1
in the deposit formed on the tubing on a result of the tube temperature

and the gas stream stae. Tubes at the higher temperatures had thinner

deposits as did tubes exposed to the fuel rich gas streams. Information

with respect to the distribution of Fe, Cr and Ni in the reaction zone (stainless steel- deposit interface) was also obtained with Fe appearing to be the species penetrating most deeply into the salt deposit.

In the coming quarter exposure tests, similar to those conducted on type 304 stainless steel, will be initiated on type 316 stainless steel.

2
II DETAILED DESCRIPTION OF TECHNICAL PROGRESS

4. Failure Prevention

b. Materials Properties Data Center (MHD materials effort) H. M. Ondik

and A. Perloff, 565

Progress : Work on the MHD state-of-the-art has had to be set aside

temporarily because the Data Center staff has had to be involved in

the preparation of test data for the use of the computer vendors benchmark test sessions. Although some eighty data sets had been delivered

to the vendors on typed sheets, it became necessary to provide these

data on magnetic tape. The drafting of the requirements for the amendment and the keyboarding, proofreading, correcting of tape cassettes,

and the transfer of the data to computer magnetic tape reels have

occupied much of the time and effort of the personnel during the early

part of the quarter. Legal procedures also required the issuance of

an amendment to the original solicitation for the vendors' services,

as well as information for their use. Benchmark sessions were scheduled

and then postponed because of questions submitted by the vendors. A

good bit of time was required to draft the answers to the vendors'

technical questions and to provide supplemental information which we hope will give the vendors sufficient information to perform. The

technical data which we have is rather more complicated than the

material with which most commercial vendors deal. Although there has

been some time for work on the MHD manuscript, these periods have not

been long enough to be efficiently productive. Much time has been

consumed in communicating with Main Commerce, and through that office,

with the vendors. Since no direct communication may be made with

vendors, everything must go through the Commerce contract officer.

Routine procedures for handling the contractors' research reports have

continued as usual

.

Plans: Work on the MHD paper will continue as time permits, allowing

for interruptions due to the computer services procurement procedure.

It is expected that most of January will be occupied with the procurement since benchmark sessions are currently expected to begin about the

middle of January. This next quarter should see the awarding of a contract for computer services.

5. Creep of MHD Refractories (N. J. Tighe, C. L. McDaniel and S. M.

Wiederhorn, 562.00)

Proqress: Work during the quarter was concentrated on completing the^

evaluation of the chrome-spinel refractory designated Corhart RFG. This

evaluation included creep measurements and swelling measurements at

1400° C 1500° C and 1600° C and identification of structural and com- positional changes after exposure. For the magnesia spinel designated

Corhart X-317 creep measurements and compressive yield measurements

were continued.

Experimental Procedure: Creep measurements were made on cored samples

"Hfnch reduced cross-section using a dead-weight

results were obtained on reduced cross- machine with a cross head speed of

inches long with a 1 machine. Compressive yield

section samples in an Instron

3
2 x 10"4 in/min. The structural and compositional data were obtained

using x-ray powder diffraction, energy dispersive x-ray analysis (EPAX)

in a scanning electron microscope.

Results : The RFG refractory exhibited considerable creep at 1400 °C,

1500 °C and 1600 °C. Some of the data obtained from specimen measurements

during loading are shown in Fig. 1. The plots show data obtained from

the dialometric measurements and from dimensional measurements of the

specimen. The specimen tested at 91 psi at 1600 °C failed catastrophically

in approximately 2 hours. The actual deformation of the specimens is

shown in the light micrographs shown in Fig. 2. In these pictures, the

cracking and swelling are seen clearly.

From the x-ray diffraction patterns, of as-received and exposed specimens;

it was found that the RFG undergoes a phase transformation during heating

at >_ 1400 °C. The as-received refractory has MgO plus two spinels in

solid solution: a chromium rich spinel (MgCr Al^_ 0.) and an iron rich

spinel (Mg Fe Alp 0,). This material is magnetic x because of the iron

rich spinel. ^After annealing at 1400 °C the RFG transforms so that only

one spinel (Mg Cr Al~ 0*) is found and the material is non-magnetic

indicating a change in Valence of the iron. This phase transformation

and structural change can account for the swelling discussed in the last

quarterly report. The structure and composition of RFG refractory brick

samples taken from the failed run in the Montana State air-preheater

test rig changed considerably from the top to the bottom. These samples were designated 2-3, 5-3, 14-3, 23-3 and 49-3, where the first number

indicates the brick number from the top of the stack. The changes

result from both the thermal treatment and the interaction with coal

slag. Although the analyses are not complete, it is clear from the

examples shown in Fig. 3 that slag penetration occurred throughout the

stack. The slag adhering to the top bricks was identified as Mg^SiO,

(olivine) plus a trace of Ca.Al.Si.Ox plus amorphous material whereat

the slag found on the lower bricks was identified as 2 CaO.AlpCL.SiOp

gehlenite with a trace of Ca.Al.Si0x. These two phases have different

melting temperatures. The RFG brick exhibited compositional changes

from two spinels plus MgO and Mg

? SiOd to a single spinel plus MgO and

Mg

2 Sio

4

.

The complete analysis of the data on the RFG refractory is too lengthy

for this report and will be compiled and published as a separate report.

Failure of the RFG in service can result from a combination of structural

and compositional changes which take place during exposure at high

temperatures and under load in a slagging environment. The presence of

slag during the exposure can increase the degredation rate of the material,

because it can infiltrate the microstructure and cause chemical and

physical changes.

The magnesia spinel refractory designated Corhart X-317 exhibits greater

creep resistance than the RFG. The initial compressive yield tests

indicate a yield drop at about 2% strain at a stress of 8.8 Ksi (62 MPa)

at 1400 °C. A test run at 1400 °C for 725 hours showed total creep of

approximately 0.1% at a stress of 200 psi.

Plans : During the next quarter a report on the RFG refractory will be

prepared. The creep tests on the X-317 will be continued. The four new furnaces, which arrived the end of December, will be set up and made

ready for the delivery of the furnace control system.

4
STR*

l

A/>

Fig • 1 Creep curves obtained under constant load conditions for the

chrome spinel RFG refractory.

5
i 16 mm a b< * C

Fig. 2. Refractory RFG specimens from creep tests (a)

363 hr 7.5% deformation (b) 1500°C, 92 psi, 133 hr, 8.

(c) 1600°C, 51 psi, 63 hr 9.1% deformation.

1400°C , 150 psi,

4% deformation

6
Fig. 3. Top and bottom surfaces of 1 in thick brick sections from

Montana State test rig (a) brick 5-3 near top (b) brick 49-3, near bottom.

7
6. Electrical Transport Mechanisms in Slag (W. R. Hosier)

Progress

a. Electrical Conductivity of YCr0 3 .

The dependence of the electrical conductivity of V . 95 Ca . 05 Cr0 3 on oxygen pressure in the range of pressures from 10° to 10" 6 atmospheres have been completed for temperatures from 1300 °C to 1600 °C. In order to obtain high density YCr0 3 , it is necessary to sinter it in a highly reducing atmosphere at about 1700 °C. This atmosphere may be

forming gas_(5% H 2- 95% N 2 ) which results in a low pressure of 0 2 of

10~ 14 to 10" 15 atmospheres. Material produced under these conditions

has a high density (98% theoretical) and is very hard. Subsequent

oxidation renders the material much softer and it sometimes contains

hair line cracks. The material with 5% Ca added is still single phase

when oxidized but the oxygen take up must cause enough change in the

lattice parameters in certain areas to cause a stress build up with the

resultant physical sample degradation. This aspect of the problem is

being investigated by the Crystal Chemistry Section of CMS at NBS. The

dimensional changes associated with this effect are very small and

cannot account for the increase in conductivity noted in the last quarterly report for the the 1600 °C temperature point in an atmosphere of

air. The continual drift upward in conductivity noted for the 1600 °C

point in air was not evident at any temperature in 900 and 2 ppm oxygen

in nitrogen.

The sample holder has been revised using 0.25 mm molybdenum wire

instead of platinum. The wire leads are anchored in the sample with

platinum. The phase diagram of Pt-Mo shows a eutectic mixture at about

25 atomic % platinum and at a temperature of 2080 °C. The conductivity

vs. pressure measurements will be made using a maximum temperature

(1600 °C) well below this eutectic point. No reaction is expected. _In

the previous measurements in the oxygen pressure range of 10 7 to 10 15

ppm 0 2 , the platinum wire leads failed. The platinum contacts in the

sample always remained intact, however. These measurements at the low

oxygen pressure will be made as soon as the oxygen sensor necessary for

measuring these low pressures is repaired.

b. Corrosion of YCr0 3 in Slag

In the last quarterly report, the results of a test on an electrochemical cell composed of YCr0 3 electrodes and Bow NH slag with 20%

K2 S 04 added were reported. This test was carried out at 1343 °C and

resulted in considerable cathode reaction but with not much reaction at

the anode. A new cell has been constructed and tested using the same materials (Bow NH slag with 20% K2 S04 electrolyte and Y ,g 5 Ca >05 Cr0 3 electrodes) but with a slightly different configuration. This cell

contains 3 electrodes, two of which are active (anode and cathode at

each end of the cell) and one which is passive (center of cell) which

carries no current. This allows for direct measurement of the potential

distribution along the length of the cell i.e. for potentials between

8
a n°de and reference and between reference and cathode. No polarization should result on the reference electrode since no current enters or leaves at this point. There may be degradative chemical reactions, however, at the reference electrode not associated with the transport of

charge. These measurements were carried out at 1254 °C and with a power supply providing a constant DC voltage source of 118 volts. The results

are shown in Figure 1. Initial current was about 75 mA which would

correspond to approximately 0.5 A/cm2 current density at the electrode

slag interface. This current drops off to about 20 % of its original

value or 15 mA in the first 20 minutes of cell operation and finally

down to about 8 mA after the total test time of 3 hours and 50 minutes.

In the initial 20 minute period, the anode to reference voltage increased

to 11 0V i.e. within 8 volts of the total applied potential while the

reference to cathode voltage dropped to about 8 volts. It is obvious

that a large polarization potential has built up at the anode. This is

most likely due to the zone near the anode that is silica rich or depleted in movable positive ions such as iron and potassium. On the

other hand, positive ion transport toward the cathode lowers the potential barrier there, as well as the bulk conductivity of the slag. This

effect has been observed before using other materials ( 1 , 2 , 3 ) and is

certainly a contributing factor in the cathode electrode segment voltage

nonuniformities observed in some of the AVC0 tests reported in the 16th

Symposium on Engineering Aspects of MHD held at the University of Pittsburgh in May 1977.

Electrical conductivity measurements on Bow NH slag discussed later

in this section of the report under Slag Electrical Conductivity (Part

C) indicates that a sizeable fraction of the charge carried in slag is

transported by ions. The basic question in MHD electrode materials is:

How can this mass transport and its associated electrochemical effects

be handled? One interesting aspect of the situation is to ascertain if

any of the resulting electrochemical effects are reversible and under

what conditions. Preliminary indications from the AVC0 generator experiments show that the effects causing cathode voltage nonuniformities can

be reduced by running the channel at no load (open circuit) for a period of time. An electrochemical cell for investigating the reversibility of these effects is under construction.

c. Slag Electrical Conductivity

The electrical conductivity of Bow NH slag with 20% K2 SO 4 added to

the melt has been completed. Figure 2a and b show radiographs of the

crucible containing the slag before and after the experiment. No observable physical change has taken place during the experiment. Figure 3

shows the conductivity data as a function of temperature. All data was taken at a partial pressure of oxygen of 2 x 10 6 atmospheres. The

temperature from which the slag was furnace quenched to 900°C is indicated

on the graph. The conductivity data at reduced oxygen pressure is not

quite so quench temperature dependent as that data for the conductivity

in air shown in the last quarterly report (July-Sept. 1979). The magnitude

and the general trend is the same, however. There appears to be two

9
crystalline phases contributing to the conductivity. In the reduced 02 pressure experiment, a break in the conductivity curve occurs near

1160 °C for all data; the break becoming more pronounced for the runs where the sample is quenched to 900 °C from soak temperatures above

1400 °C. All data shown on the graph is taken with increasing temperature

after being quenched to 900 °C and cooled slowly to the lower temperature

(^500 °C). The data below 800 °C may be affected by cracks in the slag

body formed by quenching to 900 °C and subsequent cooling to lower temperatures. Cracking appears to be more severe in those samples with

the K2SO4 added. It would be interesting to see how the coefficients of

thermal expansion for these seeded and unseeded slags differ. X-Ray

diffraction measurements (courtesy of H. Parker, Center for Materials

Science, Solid State Chemistry Group) show that the phase composition

below 1180 °C is a kalsilite like phase (probably KA1 Si

0

4 ) and spinel

(Fe 3 04) for that material quenched from 1426 °C. This kalsilite like

phase was not detected in the slag measured in an air atmosphere and

quenched from nearly the same temperature (1420 °C). The spinel (Fe 3 04 ) was detected in the sample measured in air. The interesting and unusual

aspect is that the slag conductivities for Bow NH slag with and without

seed and measured in air and in an atmosphere of 2 x 10” 6 02 partial

pressure are the same as a function of temperature above 1400 °C. If the

potassium ion conductivity is the predominant mechanism for charge

transport, above 1400 °C, the samples containing added K2 S0 4 should have

a higher conductivity. Chemical analysis for K and S of the Bow NH slag

with 20 wt.% K2 S04 added to the melt before and after the electrical

measurement (2 x 10" 6 atmos 02 ) are given in Table 1.

Table 1

% K % S

Before conductivity measurement 6.34 .022

After conductivity measurement 5.99 0.12

The magnitude of sulfur detected in each case was below the accurate

limits of detection. It apparently leaves during initial melting of the

slag and K2 S04 mix. The reason for the discrepancy between the amount of

K added (9%) and that detected in the analysis is unknown. If the conduction

mechanism is+

pred<j>minantly electronic above 1400 °C and due to the iron

ion ratio Fe 2 /Fe 3 one would expect a higher conductivity in that

sample measured in a partial pressure of oxygen of 2 x 10 6 atmospheres.

Johnson ^4) has reported an Fe 2 /Fe 3 ratio at 1450 °C of 1.6 for an oxygen pressure of 10’ 6 atmospheres and of 0.13 for an air atmosphere.

This gives a ratio change of near 13 which should be reflected in the

conductivity if there is exchange between the iron ions of different

valence. The measured conductivity of the slag above 1400 °C is not

likely to be due to oxygen ions since the conductivity is not oxygen

pressure independent. The slag sample at high temperature is a disordered structure (molten) arjid it is tjiot surprising that the electron

exchange is small between Fe 2 and Fe 3. Simnad, Derge, and George

(5) in transport measurements on iron in silicate slags conclude that

10
the transport number of the iron ions Fe 2 is one and therefore that the

current is being carried almost completely by.these ions. The ionic

radiys of K is large (1.33A) while that of Fe 2 and Fe 3 is 0.74A and

0.64A, respectively. This difference in ionic radii coupled with the

difference in ionic charge may make the iron ion the predominant charge

carrier in the molten slag.

The relative amounts of Fe 2 to Fe 3 then would have a secondary

effect on the electrical conductivity. The total iron content, however,

would affect the magnitude of the conductivity. This correlation was pointed out earlier by Bates (6) in his analysis of early slag conductivity work done by Frederikse and Hosier (7).

d. Other Activities

1. During the course of channel proof tests in preparation for the

3rd Joint US-USSR Materials Test in the U.02 MHD facility in Moscow, the

electrical conductivity of Sr 0 . 9 La 0> iZr 0 7O0.3O3 was measured as a function of temperatue and oxygen pressure, this material was prepared

by A. T. Research and no details were given on its preparation. The

data was presented in quarterly report Oct-Dec. 1977 to DoE. An inquiry was received from Dr. H. Shapiro from TRW concerning this material

with regard to its large range showing a positive temperature coefficient

of resistance in an air atmosphere. Another sample was sent from TRW

and the conductivity measured during this reporting period. The resulting

data is shown in Figure 4. The range showing a positive temperature

coefficient of resistance is reduced considerably and shows a hysteresis

effect with increasing or decreasing temperature indicative of a reversible

phase change.

2. During the week of October 8th, W. R. Hosier attended the joint

16th International Thermal Conductivity Conference and the 7th International Thermal Expansion Symposium held in Chicago at the I IT Research

Institute on Nov. 7-9.

3. On December 5 and 6, W. R. Hosier participated as a panel

member for Materials in the review of the DoE sponsored work of MERDI at

Butte, Montana. The review was held at Montana State University, Bozeman,

Montana.

References

(1) Electrochemical Effects in Simulated Slag Flow

W. Capps, D. A. Kauffman, and W. R. Hosier, Quarterly Report for

the period April-June 1978 to DoE under contract No. EA-77-A-01 -601 0

.

(2) Discharge Characteristics of Slagging Metal Electrodes

J. K. Koester and R. M. Nelson, 17th Symposium on Engineering Aspects

of MHD. Stanford University, Stanford, California. March 27-29, 1978.

11
(3) Electrochemical Corrosion of MHD Electrodes in Slags

L. D. Cadoff, D. B. Rossing, and H. D. Smith, 17th Symposium on Engineering Aspects of MHD, Stanford University, Stanford, California.

March 27-29, 1978.

(4) Oxidation - Reduction Equilibria in Iron Containing Glass

W. D. Johnson, Journal of American Ceramic Society, Vol . £7, No. 4,

April 1964.

(5) Ionic Nature of Liquid Iron - Silicate Slags AIME Transactions

M. T. Simnad, G. Derge, and I. George, Journal of Metals, 200 , p. 1386, Dec. 1954.

(6) Private communication.

(7) Electrical Conductivity of Coal Slag

H. P. R. Frederikse and W. R. Hosier, Journal of American Ceramic

Society, Vol. 56, No. 8, August 1973.

Plans

The electrical conductivity of YCr0 3 doped with 5 % Ca on the Y site

in the oxygen pressure range of 10~ 6 to 10~ 15 atmospheres will be completed.

Experimental difficulties here have been largely overcome and the outcome

measurements may depend on the physical stability of the material

upon going from an oxidizing to a reducing atmosphere.

The work on the slag electrical conductivity will continue. A

decision must be made whether to continue with the Bow NH slag (doping

or increasing iron content, etc.) or begin to study extensively the

conductivity mechanisms in low iron high calcium slag (Rosebud).

The electrochemical experiments involving slag and electrode

materials will continue particularly with respect to the possible

reversibility of these effects or with respect to the time limits

involved when electrochemical reactions degrade conductivities far

enough to limit usefulness.

12
(Vm±N3ddDO

s.

1 OA1 300H1VD 01 3DN3d

1 1 1 1 1333<l

1d v

= O

o o o o ^

G) 00 N

O o O O O ^

lO ^ K) C\J o O

SnOA 30N3d3J3d 01 3Q0NV o

ro

(j)

oc

ZD

C\J o

ho

13

Figure

1

.

Time

vs.

voltage

relationships

in

an

electrochemical

cell

of

Y

»

95

Ca.

05 c

rO3

in

a

slag

electrolyte

with

added

K2SO4.
Figure 2. Radiograph of crucible showing slag fill and electrode con- figuration before (a) and after (bj electrical conductivity

measurements.

14
o i-

(uio-ujqo) AllALLOnaiMOD

15

Figure

3.

Electrical

conductivity

of

Bow

NH

slag

with

20% K

2

S04

added

to

the

melt.

All

data

was

taken

in2

x

10 6

O

2

atmospheres

in

N

2

and

with

increasing

temperature

after

beina

auenched

to

900°C

from

the

equilibration

temperatures

indicated

on

the

graph.
o

^(uuo-uuMOjAHAIlOnaNOO

16

Figure

4

.

The

electrical

conductivity

of

Sro.9Lao.1Zro.7Cro.3O3

as

a

function

of

temperature

for

increasing

and

decreasing

temperatures.
7. Corrosion of Downstream MHD Components (J. Smit, 561 and

C. D. Olson, 565)

Progress : Type 304 stainless steel tubular specimens were exposed to

a seeded oxygen-propane fueled hot gas stream under varied conditions

and then analyzed by SEM and EDX techniques. Total exposure time to

the hot gas stream was 4 hours with the first 25 minutes of this period

being used to seed the gas stream with 250 g of a mixture of 80% by

weight K^CO., with 20% by weight ICSO.. Six stainless steel 304 specimens were tested under these general conditions. Three of the tubular

specimens were exposed to an oxygen rich gas stream environment at

tube temperatures of 400 °C, 500 °C and 590 °C, respectively. The

remaining three samples were exposed to a fuel rich gas stream

environment at 400 °C, 500 °C and 590 °C. Specimen temperatures were maintained at the desired value by internal air cooling and were monitored through a Pt/Ptl 0%Rh thermocouple welded into the specimen

wall. In the vicinity of the test specimen the gas stream temperature

was maintained at approximately 1300 °C as monitored through a Pt/Ptl 0%Rh

thermocouple. The fuel rich and the oxygen rich states were accomplished

by varying oxygen to propane flow rate ratios.

To reduce external contamination of the samples and hydration of the

KpC0

3

in the coatings formed during seeding of the hot gas stream,

tne samples were removed from the test rig upon cool down to about 40 °C

and potted immediately in epoxy. Metallographic specimens were then

prepared from sections of the stainless steel tube taken approximately

10 mm from the midpoint position in the hot zone. The specimens were

then ground and polished using nonaqueous media. All samples were stored in evacuated desiccators until ready for SEM/EDX analysis.

On all specimens a compact deposit formed on the surface facing the

gas stream. A powdery deposit, probably from the fume, formed on the

trailing surface. At the boundary between the two, a thicker drip zone formed.

On the specimens exposed to the oxygen rich gas stream the leading

surface deposit, which varied with temperature (thickest at 400 °C)

ranged from 1.7 mm to 0.8 mm. The trailing surface deposit was fairly

uniform at about 0.2 to 0.3 mm, Figures 1, 2 and 3.

EDX analysis shows high concentration of potassium with traces of

sulfur throughout the bulk of the salt deposit, however, the top

mens.

small potassium ricft n8de£ wire found in a matrix of potassium and

sulfur. Figure 4, specimen temperature 400 °C, shows these small

nodes. Again, Figure 5, specimen temperature 500 °C, shows the

potassium rich nodular formations. In the 590 °C sample. Figure 6,

the nodes are not predominant and a more even surface can be

observed. An explanation for the difference in the coating would

be that two different salts with melting points varying by 178 °C

layer of the salt deposit is high in sulfur in previous

tests using only K

? S0*, a layered deposit was formed on the speci

However, with the Lc0n/Ko S0, charge, layering did not occur but

17
would show areas of incongruent melting. However, at the edge

of the coating, it is possible that the hot gas stream volatilized

the K-CCL leaving behind a greater concentration of sulfur than

potassium in the form of I^SO^.

The corrosion band area on the 304 stainless steel increases with

increasing temperature. The variation in corrosion can be observed

in the comparison of Figure 7 (specimen temperature 500 °C) with

Figure 8) specimen temperature 590 °C). SEM/EDX analysis of both

samples. Figures 7a-7g and Figures 8a-8j, indicate metal corrosion

and metal cation migration, however, the 590 °C specimen shows the

corrosion picture more vividly.

SEM/EDX analysis seems to indicate that preferential leaching of the

metal cations from the bulk material (304 stainless steel) is found

in a somewhat orderly fashion. The analysis shows that, moving from

the stainless steel bulk material to the salt deposit, a high chrome

concentration area is found along the edge of the tube and below the

surface of the tube top. Areas of high nickel concentration are found

within the high chrome concentration matrix while areas of high iron

concentration are found next to the salt. Figures 9a-9f show that

the iron is found in areas of high potassium concentration a good

distance away from the reaction interface region of the stainless

steel/salt deposit. Figures lOa-lOc again show iron well away from

the reaction zone.

In a fuel rich hot gas stream, a thin, 0.6 to 0.8 mm, as compared to

the oxygen rich environment, salt deposit was formed. As noted in

the oxygen rich series these samples also exhibited decreasing

thickness with increasing temperature. In contrast to the oxygen

rich specimen the fume deposit, 0.4 to 0.7 mm (the layer on the

under side of the tube), was heavier. However, it also decreased

with temperature. Figures 11, 12 and 13.

As in the oxygen rich test samples the fuel rich samples also

exhibited reaction zone areas. Again, these areas showed increased

activity in corrosion with increased sample temperature. Figure 14,

specimen temperature 500 °C and Figure 15, specimen temperature 590 °C

show areas of corrosion at the stainless steel/salt interface with

areas of high metal cation concentration. However, the reaction

areas do not seem to be as great under the fuel rich conditions as compared to the oxygen rich conditions.

As noted by SEM/EDX analysis. Figures 14a-14g and Figures 15a-15g,

the edge of the stainless steel bulk material contains regions of

high chrome concentration followed by areas of high nickel concentration.

Areas of high iron concentration are found next, with iron and

potassium in the salt deposit adjacent to the stainless steel. The

salt deposit seems to contain more sulfur than in the oxygen rich

cases, but both have a crust of potassium and sulfur in high con- centration on the top of the salt layer.

18
Both systems, oxygen rich and fuel rich, show evidence of metal

corrosion. However, the fuel rich system seems to show less evidence

of damage to the bulk stainless steel. Both system also seems to

indicate the same metal cation movement from the bulk stainless steel

towards the salt deposit.

Stainless Steel Reference Specimen - As with most analytical techniques,

reference samples are used to check some stage of the analysis. At

the present time a randomly selected sample type 304 stainless steel

that has been cleaned, cut and polished has been used to check outer

edges or surfaces for areas where corrosion would most likely occur.

A general overview of the tube (Figure 16) would seem to indicate

that the 304 stainless is very clean and uniform on the outer edges.

However, Figures 17 and 18, shows that in the manufacturing, rough

surfaces, i.e., holes and pit areas, can occur. These areas would be

more susceptible to a corrosion process and promote deterioration of the

surface under hot plasma and seeding conditions. Therefore, not all

areas analyzed for pits and holes are necessarily due to the corrosion

process. EDX analysis shows that traces of sulfur are already present

in the matrix. No other inconsistences such as areas of high chrome

concentration or high iron concentration are found in the metal, but

rather a uniform distribution of chrome, iron and nickel is found

throughout the 304 stainless steel.

Plans: Specimens of type 304 stainless steel will be evaluated at 100

hours exposure to a hot gas atmosphere, fuel rich and oxygen rich, with

seeding of a salt every 24 hours. Samples will then be analyzed using

SEM/EDX to establish corrosion and leaching of metal cations.

A program will be initiated using type 316 stainless steel tubular

specimens under the same general conditions that have been established

in running the type 304 stainless steel samples.

19
Fig. 1. Section of Type 304 stainless steel tubing after exposure to K2CO3/K2SO4

seeded oxygen rich hot gas stream. Note formation of thick deposit,

1.7 mm, on upper surface and thin powdary deposit on lower surface.

Tube temperature 400°C. 6 X.

Fig. 2. Section of Type 304 stainless steel tubing after exposure to K2CO3/K2SO4

seeded oxygen rich hot gas stream. Note formation of thick deposit,

1 mm on upper surface. Tube temperature 500°C. 6 X.

20
Fig. 3. Section of Type 304 stainless steel tubing after exposure to K0CO3/K2SO4

seeded oxygen rich hot gas stream. Note formation of thinner deposit

on upper surface. Tube temperature 590°C. 6 X.

Fig. 4. SEM micrograph, 200 X, of small, high potassium concentration nodes in

the salt deposit. Specimen temperature 400°C; gas stream oxygen rich.

21
Fig. 5. SEM micrograph, 200 X, of small, high potassium concentration nodes in

the salt deposit. Specimen temperature 500°C; gas stream oxygen rich.

Fig. 6. SEM micrograph, 200 X, of salt deposit showing absence of high potassium

concentration nodules. Specimen temperature 590°C; gas stream oxygen rich.

22
Fig. 7. SEM micrograph, 2000 X, of stainless steel -salt deposit reaction

zone. Lettered regions correspond to labeled EDX spectra, figures

7a through 7f. Specimen temperature 500°C; gas stream oxygen rich.

Fig. 7a. EDX spectra of regions A and B of figure 7 showing high Cr concentration

area.

23
Fig. 7b. EDX spectra of regions C and D of Fig. 7 showing region of high

potassium concentration associated with Fe in the salt region of

the corrosion area.

Fig. 7c. EDX spectra of regions E and F of Fig. 7 showing Fe penetration in the

salt.

24
Fig. 7d. EDX spectra of regions G and H of Fig. 7 showing Fe penetration in

the salt.

Fig. 7e. EDX spectra of regions I and J of Fig. 7 showing areas of high Cr

and low Ni concentration.

25
Fig. 7f. EDX spectra of regions K and L of Fig. 7 showing areas of high Cr

concentration formed on stainless steel edge.

Fig. 8. SEM micrograph, 2000 X, of stainless steel-salt deposit reaction zone. Lettered regions correspond to labeled EDX spectra. Figs. 8a-8j.

Specimen temperature 590°C; gas stream oxygen rich.

26
Fig. 8a. EDX spectra of regions A and B of Fig. 8 showing Fe penetration in

the salt deposit.

Fig. 8b. EDX spectra of regions C and D of Fig. 8 showing area of high Cr

concentration as compared to bulk stainless.

27
Fig. 8c. EDX spectra of regions E and F of Fig. 8 showing area of high Cr

and high Fe concentration with absence of Ni at the stainless steelsalt interface.

I

- -

-

A A A £

.yf'1’ JtoM* . ..

A '

|

•

’ * L

i H

'

-V* V-^v >«i/ 4k \T ^ . ^

| , I , 1 , 1 1 1 1 1 . 1 .i L i 1 i 1 « •

Fig. 8d. EDX spectra of regions G and H of Fig. 8 showing area of high Fe and

low Cr concentration with traces of Ni at the stainless steel-salt

i nterface

28
Fig. 8e. EDX

and

1

r

J ./ r * “•* *

_y/

L i *

v> J i :

1 I

I | 1 | I 1 I.L 1 !, 1 .1 1. . 1 . 1 . 1

t-i •: 8 {

spectra of regions I and

Cr in the salt deposit.

J of Fig. 8 showing pi

y v -J ,

L° t/**

" ,/v v

-

_K

, . | i | , | , 1 i 1 t 1 .i L i 1° i 1 t 1

•: t : c

Fe

Fig. 8f. EDX spectra of regions K and L of

low Cr concentration with absence

i nterface.

Fig. 8 showing area of high Fe and

of Ni at the stainless-steel salt

29
Fig. 8g . EDX spectra of regions M and N of Fig. 8 showing areas of high Cr and

high Ni concentration as compared to low Cr and low Ni below the

stainless steel-salt deposit interface but within the reaction band.

Fig. 8h. EDX spectra of regions 0 and P of Fig. 8 showing areas of high Cr

and high Ni concentration below the stainless steel-salt deposit

interface, but within the reaction band.

30
Fig. 8i . EDX spectra of regions Q and R of Fig. 8 indicating a region of very

high Ni concentration in a matrix of high Cr concentration as compared to the bulk stainless steel.

Fig. 8j. EDX spectra of regions S and T of Fig. 8 showing high Ni concentration

with the high Cr concentration matrix.

31
Fig. 9. SEM micrograph, 2000 X, of stainless steel-salt deposit reaction zone. Lettered regions correspond to labeled EDX spectra Figs. 9a-9e.

Specimen temperature 400°C; gas stream oxygen rich.

Fig. 9a. EDX spectra of regions A and B of Fig. 9 showing areas of low Ni

concentration on the stainless steel interface.

32
Fig. 9b. EDX spectra of regions C and D of Fig. 9 showing areas of low Ni

concentration on the stainless steel interface.

Fig. 9c. EDX spectra of regions E and F of Fig. 9 indicating Fe penetration into

the salt deposit.

33
Fig. 9d. EDX spectra of regions G and H of Fig. 9 showing areas of low Ni

concentration on the stainless steel interface.

Fig. 9e. EDX spectra of regions I and J of Fig. 9 indicating Fe and Cr penetration

into the salt deposit.

34
304 SS

Fig. 10. SEM micrograph, 2000 X, of stainless steel-salt deposit reaction zone. Lettered regions correspond to labeled EDX spectra Figs. 10a and 10b.

Specimen temperature 400°C; gas stream oxygen rich.

Fig. 10a. EDX spectra of regions A and B of Fig. 10 showing areas of Fe and

Cr penetration into an area of potassium and sulfur.

35
Fig. 10b. EDX spectra of regions C and D of Fig. 10 showing farther penetration

of Fe and Cr into the salt deposit region.

Fig. 11. Section of Type 304 stainless steel tubing after exposure to K2 CO3 /K2 SO4

seeded fuel rich hot gas stream. Note formation of 0.8 mm deposit on upper surface. Tube temperature 400°C. 6 X.

36
Fig. 12. Section of Type 304 stainless steel tubing after exposure to

K

2 CO3 /K2SO4 seeded fuel rich hot gas stream. Tube temperature 500°C.

Fig. 13. Section of Type 304 stainless steel tubing after exposure to K2 CO 3 /K2 SO4

seeded fuel rich hot gas stream. Note formation of thin deposit on upper surface. Tube temperature 590°C. 6 X.

37
Fig. 14. SEM micrograph, 2000 X, of stainless steel-salt deposit reaction zone. Lettered regions correspond to labeled EDX spectra. Figs. 14a to 14f.

Specimen temperature 500°C; gas stream fuel rich.

Fig. 14a. EDX spectra of regions A and B of Fig. 14 pointing out areas of high

Cr concentration and low Ni concentration on the stainless steel interface.

38
Fig. 14b. EDX spectra of regions C and D of Fig. 14 indicating areas of low

Cr and low Ni concentration at the stainless steel interface.

Fig. 14c. EDX spectra of regions E and F of Fig. 14 showing areas of high

Ni and low Cr concentration at the stainless steel interface.

39
Fig. 14d. EDX spectra of regions G and H of Fig. 14 showing areas adjacent to

E and F of high Ni concentration and low Cr concentration.

Fig. 14e. EDX spectra of regions I and J of Fig. 14 showing areas of high Ni

and high Cr concentration at the interface.

40
Fig. 1 4f . EDX spectra of regions K and L of Fig. 14 of areas below I and J showing low Cr and low Ni concentration.

Fig. 15. SEM micrograph 2000 X, of stainless steel-salt deposit reaction zone. Lettered regions correspond to labeled EDX spectra, Figs. 15a to 1 5f

.

Specimen temperature 590°C gas stream fuel rich.

41
Fig. 15a. EDX spectra of regions A and B of Fig. 15 showing area of high Cr

and high Ni concentration as compared to bulk stainless steel.

Fig. 15b. EDX spectra of regions C and D of Fig. 15 showing areas of high Fe,

low Ni and low Cr concentration at the stainless steel-salt interface

42
Fig. 15c. EDX spectra of region E of Fig. 15 indicating area of high Cr

concentration with Ni and region F of Fig. 15 showing area of low Cr

concentration with high Ni concentration.

Fig. 15d. EDX spectra

and low Ni

i nterface.

of regions G and H of Fig. 15 showing areas of high Fe

and low Cr concentrations at the stainless steel -deposi

t

43
Fig. 15e. EDX spectra of regionsl and J of Fig. 15 showing Fe and Cr penetration

into the salt deposit.

Fig. 15f. EDX spectra of regions K and L of Fig. 15 showing areas of high Ni and

low Cr concentration at the stainless steel-salt interface.

44
Fig. 16. Section of Type 304 stainless steel prior to exposure to the seeded

hot gas stream. 6 X.

Fig. 17. SEM micrograph 4500 X, of section of Fig. 16 showing stainless steel

edge.

45
Fig. 18. SEM micrograph (montage) 9000 X, of a section of Fig. 16 showing

inclusions and pits in the stainless steel.

46

I 0 0 2