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MHD MATERIALS - SEED/SLAG INTERACTIONS AND EFFECTS

QUARTERLY PROGRESS REPORT

October 1 - December 31, 1980

Samuel J. Schneider

Project Manager

Center for Materials Science

U. S. Department of Commerce

National Bureau of Standards

Washington, D. C. 20234

PREPARED FOR THE UNITED STATES

DEPARTMENT OF ENERGY

MHD OFFICE

“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,

express or implied, or assumes any legal liability or responsibil ity

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 2

1. Thermochemistry of Seed and Slag. . 2

2. Electrical Conductivity and Polarization. . 11

3. Corrosion of Downstream MHD Coirrjonents 18

I. SUMMARY OF PROGRESS TO DATE

1. Thermochemistry of Seed and Slag

New K pressure data on a Western-type slag initially containing

20 vit% K2 O are presented. The decrease in K pressure with decreasing

K2 O composition is less than in previous- measurements . Analysis of

previous data shows a decrease in K pressure of 20 percent for unit

decrease in wt% K2 O and an increase in K pressure of 8 percent for unit

increase in wt% CaO.

A s'eries of 3 types of experiments for conclusive test of subsolidus

equilibria involving 12 compounds in the system K20 -Ca0-Al 2 O 3 -Si 02 has

been devised and is nearly complete. Initial indications are that the

previously postulated equilibrium diagram is essentially correct.

However, an important modification must be made to allow for the .presence

of potassium-deficient KAlSi 206 (ss). Existence of this previpusly . unreported solid solution may have significant effect on the thermodynamics

of K2 O vaporization or absorption involving this phase.

2.

Electrical Conductivity and Polarization

Both AC and DC conductivity measurements have been initiated on a Western

slag derived from coal from the Rosebud seam in Montana. These measurements are designed to investigate the conductivity mechanism in this

slag which contains a minimal amount of iron (approximately 4 w/o)

compared to a typical Eastern slag (approximately 12-16%). In this

report, primary emphasis is placed on measurements in the temperature

range between 1200 and 1500 °C.

Measurements were made on a sample of yttria stabilized zirconia in an effort to verify the measurement technique on a purely ionic conductor

of a given single phase composition.

3.

Corrosion of Downstream MHD Components

Optical and SEM and EDX analysis were undertaken on sections of Type 316

stainless steel tubing exposed to fuel rich and oxygen rich hot gas

streams seeded with 80% by weight K2 CO 3 and 20% by weight K2 SO 4. Analysis

revealed only slight differences as a result of the hot gas stream state.

Distribution of species, Fe, Ni and Cr in the reaction zone, (stainless

steel - deposit interface) paralleled observations made on Type 304 stainless steel and detailed in previous reports.

1
II. DETAILED DESCRIPTION OF TECHNICAL PROGRESS

1. Thermochemistry of Seed and Slag (E. R. Plante and L. P. Cook)

A. Vaporization Studies (E. R. Plante)

Progress : Vapor pressure measurements were continued on selected compositions

in the KaO-CaO-AlaOa-SiOa system. As in prior work, vapor pressures

were determined using a Knudsen-effusion , modulated beam mass spectrometric

method.

Previous measurements have shown v/ithi-n the limited composition

ranges studied that the K2 O activity as determined from measurements of

the K and O 2 pressures is insensitive to the individual AI 2 O 3 and Si 02

concentrations, but is decreased as the sum of ’the AI2O3 and Si 02

concentrations is increased, increased by an increasing CaO concentration

and increased by an increasing K2 O concentration.

One of the objectives of the current research is to attempt to

construct a simplified model to predict interaction of seeded MHD combustion

gases with Western-type coal ash. Thus far, measurements have been

-completed on three compositions and are in progress on a fourth composition.

Table 1 lists the sample desianation, starting mineral constituents and

wt% of each of the four components for the compositions being studied,

as well as the final concentration of components.

The composition of KCAS-VP-4 was selected in order to increase the

range of K2 O concentrations stiJclied. There is evidence from engineering

studies on seeded MHD combustion products that K2 O absorption by Westerntype slags might lead to a final K 2 O concentration of as much as 20 wt"

K2 O. The composition for KCAS-VP -4 listed in Table 1 was arrived at by

neglecting the minor components in a Rosebud ash analysis and replacing

the MgO and Fe203 components with the same amounts of CaO and AI2O3

respectively on a wt basis. The composition data for the Rosebud ash

and synthetic ash composition are shown in Table 2. This composition

was closely aoproximated using a mixture of the minerals, 66.9 wt%

KAlSi 04 and 33.1 vit% Ca 2 Si 04 .

Some data selected from the KCAS-VP-4 experiment are shown in

figure 1 and compared with a line representing data obtained from the

KCAS-VP-1 data in the 14.3-12.4 wt" composition range. Qualitatively,

the agreement is satisfactory but the decrease in K pressure with K 2 O

composition is not as great as v/as observed in the KCAS-VP-1 experiments.

Thus, K pressures shov/n with a K 2 O concentration in the 7-10 wt% range

are about a factor of 2 greater than those from the KCAS-VP-1 experiment

when the condensed phase contains 14.8-12.4 wt% K2 O. It does not appear

likely that this difference is due to variations in the concentrations

of the other constituents in the melt because the CaO concentration is

actually greater for the KCAS-VP-1 melt by a few percent while the AI 2 O 3 and Si 02 concentrations vary by only a few percent. Thus, there must be

2
other differences in the experimental measurements responsible for the

increase in pressure and its slower decrease in K pressure with decreasing

K2 O content. Additional study of this problem is required.

In the previous report, it was noted that measurements in the KCASVP-2 and KCAS-VP-3 series appeared to be consistent with each other if

the K pressure was dependent at fixed temperature, only on the K2 O and

CaO, concentration. These data have now been fit to the equation,

log P|^ = 3.492 - 16153/T + .0903(wt?^ K

2

O) + .0351(wt% CaO).

The standard deviation in the fit is 30 percent with maximum deviations

in individual points of about 100 percent. It is interesting to note

that the above equation predicts an increase in K pressure of 20 percent

for an increase of 1 wt% K2 O and an increase in K pressure of 8 percent

for 1 wt% increase in CaO.

Table 1. Mineral Constituents and Composition of Vaporization Samples

Initial /Final

Composition \lt%

Sample Mineral Constituents (Vlt/j) K

2

O CaO AI

2

O

3

Si 02

KCAS-VP-1 KAlSiO^ (49.8), Ca^SiO^ (16.5) 14.8 27.5 22.2 35.5

Ca2Al2Si07 (16.5), Ca3Si20y (17.2) 1.6 31 .8 25.7 40.9

KCAS-VP-2 KAlSiO^ (28.6), KAlSi20g (13.8) 11.5 17.6 33.7 37.2

Ca2Al2Si0y (28.9), CaAl2Si20g (28.7) 6.1 18.7 35.7 39.5

KCAS-VP-3 KAlSiO^ (42.0), CaSiOg (58.0) 12.5 28.0 13.5 46.0

1.8 31.4 15.2 51.6

KCAS-VP-4 KAlSiO^ (66.9), Ca

2

SiO^ (33.1) 19.9 21.6 21.6 37.0

7.1 25.0 25.0 42.9

3
Table 2. Synthetic Composition Approximating Rosebud Ash with 20 wt% K

2

O

Rosebud Ash^^^ Simplified KAlSiO^ (66.9 wt%)

Component V/t% Composition Ca^SiO^ (33.1 vit%)

Si 02 40.34 34.32 36.96

AI

2

O

3

24.60 24.67 21.56

CaO 18.90

>1

21.02 21.56

MgO 5.80 —

^®2^3 4.40 —

0C\J .53 20.00 19.92

Na 20 .46 —

.28 —

Pol 1 ina and R. Larsen, ANL-77-21 (1977)

4
B. Phase Equilibrium Studies- The System KaO-CaO-Al aOa-SiOz

(L. P. Cook)

During the past quarter, experiments have concentrated on finalizing

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

system. The portion of the system under consideration (Fig. 2) involves

12 crystalline solids and 13 four-phase volumes. As noted in previous

quarterly reports, the first melt to appear upon heating each of the

four-phase combinations bears a definite relationship to the thermodynamic

properties of the solids involved, allov/ing calculation of an equilibrium

Py Q. Therein lies the basis for construction of a quantitative fourcoiiiponent model for calculation of potassium, activities as a function of

composition for KaO-CaO-AlaOs-SiOa melts simulating Western slags. Such

a phase- equilibrium-derived model is useful for supplementing models

based on direct measurement of potassium pressures, with the resultant

feedback allowing refinement of the thermodynamic properties of the

solids. For such a model to be accurate, subsolidus relations must be

well known. Unfortunately, 1200-1250 °C appears to be the limit for

comprehensive subsolidus studies, and so kinetic factors must be considered

in the experimental approach. For this reason, it was thought essential

to conclusively verify the results of earlier preliminary work with a new series of experiments. This is being done with three types of

experiments, as follows:

1 . 1000-Hour Subsolidus Equilibrations

These experiments are being conducted at 1200 °C for 9 of the 13

four-phase volumes and at lov;er temperatures for the assemblages containing

KAlS'iaOs. The procedure is to seal carefully mixed equal weights of the

four solids thought to be in equilibrium in Pt capsules, heat them,

quench, examine by x-ray, regrind, reseal and reheat. In this way

thermodynamically important solid solutions can be detected by subtle

shifts in the x-ray parameters. So far no shifts have been detected

after 764 hours. However, evidence has been found for potassium-deficient

KAlSiaOs (ss) showing no change in unit cell parameters (see discussion

of second type of experiment below). The 1000-hour equilibrations are

'^^^90 percent complete at the time of writing and results will be fully

discussed in the next quarterly report. It is hoped that a formal

publication describing the results can also be prepared.

2. Solid-State Tie Line Reactions Using Unstable Combinations of Phases

The purpose of these experiments was to verify as many of the tie

lines in Fig. 2 as possible by starting with alternative solid assemblages.

Reaction to produce the postulated tie line is proof of its relative

stability. Eleven tie lines can be investigated in this way as follows

(the tie line believed to be stable is on the right):

(1) llCa2SiO^ + IKAlSi^Og + 2CaAl-,20-,g iKAlSiO^ + 12Ca2Al2Si0y

5
(2) 2CaO + lKAlSi 20 g IKAlSiO^ + lCa

2

Si 04

(3) 3Ca2SiO^ + lKAlSi20g IKAlSiO^ + 2Ca3Si20y

(4) lCa3Si207 + lKAlSi20g -> IKAlSiO^ + 3CaSi03

(5) 3Ca2Al2Si0y + 1 (3A1 203

* 25102 ) + 7KAlSi20g yKAlSiO^ + 6CaAl2Si203

o

(6) 3Ca2Al2Si07 + 11 (3Al203*2Si02) + 25 KAIO

2

25KAlSiO^ + 6CaAl-,20-,g

(7) 2 KAlSi 04 + ICaAl 251*203 + CaSi 03 2 KAlSi* 20 g + 1 Ca

2

Al

2

S-nr

7

( 8 ) 3Ca2Al2Si0y + 1 (3A1 203

* 251

* 02 ) + 7KA15i*303 7KA15i20g + 6CaAl2Si*203

(9) 2Ca35i*207 + 2 KA 1 S 1303 2KA15i20g + 6Ca5i03

(10) 5K25i*20g + CaAl-j20-jg + 225102 —^ 10KA15i30g + lCaAl2Si203

( 11 ) 6K25i20g + lCaA1

^ 2 °l 9 t 255102 I 2 KAI 5 I 3

O

2 + lCa5103

Results of these experiments are summarized In Table 3 below. Tie line

reactions 8 and 9 were not Investigated because of the current unavailability

of KAISI 3 O 8 starting materials. 5tudy of tie line reactions 10 and 11 was not successful due to the low-melting nature of these compositions.

6
Table 3. Tie Line Reactions

Starting Temp. Duration — Reaction Materials (°C) (Hr.) Products

1 Y-C«S, CA^, (Lc) 1250 23

L 0 1250 64 6 “C

2

S , Ge , CAg

2 CaO, Lc 1250 16

1250 24 Kls, 3 -C

2

S

3

• C«S, Lc 1250 16

c. 1250 24 Ra, Kls

4 Ra, Lc 1250 16

1250 24 Kls, Wo

5 Ge, Lc, Mu 1250 16

1250 24 Ge, Lc

6 Ge, Mu, KAIO^ 1250 23

1250 64 K1 s , CAg , Ge

7 Kls, An, Wo 1250 23

1250 64 Lc, Ge, (An), Wo, Kls

10 K2Si20g, CAg, Si02 1050 22

950 64 Melt, (unident, phase)

11 l<2Si20g, CAg, Si02 1050 22

950 64 Melt, (unident, phase)

C

2

S

= Ca

2

SiO^ Cor = AI

2

O

2

Mu = 3A1203- 2Si02

Lc = KAlSi 20 g

Ksp = KAlSi 303 An = CaAl2Si 2°8

CAg =' CaAli20i9 Kls = KAlSiO^ Wo = CaSi 03

Ge = Ca2AlpSi0y Ra = Ca3Si20y == CoSp (

)

= phase barely detectable by — x-ray diffraction

7
Reactions 1, 2, 3, 4, 6 and 7 have unquestionably gone to the right.

The existence of extra phases among the products of reactions 1, 6 and 7

is probably a matter of incomplete reaction. In the case of reaction 5,

the mullite peak detectable in the starting material disappears with

concomitant increase in intensity of the leucite lines. This observation

has been made in numerous other experiments, and all results taken

together constitute very strong evidence for extension of a leucite (ss)

field toward potassium-deficient compositions. However, the long-term

equilibrations discussed above indicate that the Kls + An tie line is

probably stable; therefore, the existence of Lc (ss) + Ge at the expense

of Kls + An in tie line reaction 5 is possibly a kinetic phenomenon.

Alternatively, there may be a Kls (ss) field such that Kls coexisting

with An*is potassium deficient. Hopefully, this problem will be resolved

during the ensuing quarters.

3. Glass Crystallization Experiments

A third type of experiment designed to test phase relations proposed

in Fig. 2 involves melting mixtures from the postulated four-phase

volumes, quenching to a glass, crushing and crystallizing. Previously,

this type of experiment was not successful owing to incomplete crystal 1 izati

The effect of periodic grinding and slight hydration will possibly prove

beneficial in this respect. These experiments are not yet complete and

results will be discussed in the next Quarterly report.

P lans : Continue analysis of K data in an attempt to determine simple

relations hi p relating K pressure to composition. Determine new data or verify existing data as required.

Finalize subsolidus equilibria in (Si, Al)-rich K2LQ-_Ca.Q-Al a-Qs-SiO-a

and prepare results for formal publication. Determine accurately the

composition of 13 minimum melts in the quarternary system. Continue

efforts at modeling quarternary liquidus surfaces. Continue study of

subsolidus equilibria in KaO-rich portion of system involving KaCaSi04 and

Ka S i a 0 5

.

8
Potassium

Pressure

(atm)

10**/T, K-i

Figure 1. Selected vapor pressures of K above KCAS-VP-4 sample at noted bulk composition of K2 O vs IOVT. The straight

line represents the K pressure over kcAS-VP-T sample in

the K2 O bulk composition range 14.8-12.4 vjt% K 2 O. Upward

or downward tics on points indicate whether temperatures

were increased or decreased during the experimental

sequence. 9
Ca© P/iOL %

Figure 2. relations for synthetic CaO-KaO-Al 2O3 -SiO,

ss;ns,rs'sf.'; s;a;s:<s!;

10
f 2. Electrical Conductivity and Polarization (W. R. Hosier)

Progress :

Introduction

In the last quarterly report (July-Sept. 1980), four probe AC electrical

conductivity measurements were compared with four probe DC measurements

on slag from a coal burning steam plant in Bow, NH. AC and DC data agree

quite well with some differences existing near 1100 °C. These measurements

supported the suggestion made in previous reports that above 1385 °C,

ionic conductivity is the predominant transport mechanism in this particular

slag which contains a relatively large amount of iron ('^-14%).

During thi^ quarter, work has continued on the investigation of the

conductivity mechanisms in slag containing a lesser amount of iron (3-4^)

than has been studied previously. This slag is derived from coal mined

from the Rosebud seam in Montana.

A conductivity measurement has also been made on a sample of yttria

stabilized zirconia. The purpose of this measurement was not necessarily

to determine the conductivity of Zr02 , but to test the measurement

technique using the comparison between four probe AC and DC methods as discussed in the previous quarterly report.

Slag Conductivity

Four probe AC and DC measurements have been made on several samples of

slag derived from coal from the Rosebud seam in Montana. An elemental

analysis of this slag as received from Montana State University is given

in Table 1

Table I

Analysis of Rosebud slag

Element

Si

Fe

A1

Mg

Ca

K

Na

(independent laboratory)

w/o

20.3

3.9

10.9

3.8

13.3

0.3

0.4

11
This slag is considerably lower in iron content than the Eastern slag

measured previously, but has a larger amount of calcium. 20 w/o

K2SO4 has been added to the base slag. In the process of adding the

potassium, phase separation occurs and some of the potassium is rejected

(see QR July-Sept. and October, 1973) so that the final material contains

less potassium than is indicated by the amount of K2SO4 added. This

potassium rejection is enhanced with slags containing large amounts of

calcium. An analysis for final potassium will be obtained and the

electrical properties over the temperature range from 500 to 1500 °C

will be given in a later report. It became evident during initial

measurements, that long times were required at certain temperatures to

reach a state in the sample where the conductivity did not change with

time at a given temperature. If one sets an arbitrary rate of change of

conductivity below which the absolute conductivity magnitude change is

negl igible ,for a very long time, then it is possible to complete a set

of measurements in some finite time (for example, one week) and approach

closely an equilibrated value of conductivity. This approach is not

valid, however, at low temperatures where initial conductivity changes

with time are small when a new temperature point is established and

thermal kinetics prevent an equilibrated process in a reasonable time.

The dividing line in this material below which an equilibrated value

(Aa/Ax < 50 ppm/hour) of conductivity cannot be obtained easily is near

1000 °C. Therefore, it seems prudent to discuss only the data above

1000 °C as characteristic of the material at a given temperature. Data

at lower temperatures is dependent on the past treatment and cannot be

treated as characteristic of the material without qualifications.

In the Seventh International Conference on MHD Electrical Power Generation

held at MIT, Cambridge, Massachusetts, June 16-20, 1980, in session 07,

the conductivity of Eastern slags is discussed. Two distinct slopes in

the conductivity vs 1/T curves are shown for those slags (containing no added potassium) above 1100 °C. The conductivity of all Bow, NH slag

samples containing iron in the amount as received (14 w/o) as well as those with added iron show a two slope region in this high temperature

range. The slope at the highest temperatures gives an activation energy

of approximately 1.8 eV while the steeper slope at lower temperatures

varies giving an apparent activation energy up to near 4.0 eV. The

slope in the lower temperature region is very sensitive to the equilibration time allowed. In earlier measurements, careful enough attention

was not paid to this time effect. In the Rosebud slag, the high temperature

slope again appears to be the same as that for the Eastern slag but the

magnitude of the conductivity in that temperature range is reduced by

approximately a factor of three. The slope at the highest temperatures

is shown in Figure 1 for slags with different iron content and for

Eastern and Western slags which have different base compositions. It

demonstrates that the conductivity in this region is some type of activated

process with an activation energy independent of the iron content, but

that the magnitude of the conductivity is dependent on the iron content.

12
In the QR July-Sept. 1972 from this laboratory, there was a discussion

of the conduction mechanism in slag samples particularly with respect to

the iron content and the semiconducting electron exchange between the

Fe"*"^ and the Fe'^^ ions. The activation energies displayed by the high

temperature data in these slag samples is higher than would be expected

from a semiconducting transition involving the iron. Since a (conductivity)

= neu where n is the concentration of the current transport carrier which

remains nearly constant with T, and y is the mobility of that carrier,

the mobility may be the predominant activated process which itself would

then depend on the phase composition of the material in a given temperature

range. The phase composition of these slags must be studied in correlation

with the temperature ranges where the data shows these activated processes.

This can be done by equilibrating a sample. at a given temperature within

the range of interest and quenching to room temperature with subsequent

analyses by SEM and XRD techniques. Careful attention must be paid also

to the viscosity of the slag above the mel ting 'po-int and its possible

effect on the conductivity.

ZrO? Conductivity

The material used for this experiment was cut from a Zr02 tube of very high

density which was used as an oxygen sensor. In order to test the four

probe AC-DC comparison method of measurement, it is desirable to have a completely ionic conductor and to have the contacting electrodes completely

irreversible to the ionic species conducting. It is well known that

Zr02 is an oxygen ion conductor which makes it useful as an oxygen

sensor material. In a sensor, however, the contacting electrodes must

be reversible to oxygen. Contacts are usually porous platinum. In the

case for these measurements, platinum was also used but the leads were

attached in the same manner as described in the quarterly report [AprilJune 1980) and platinum in these contacts does not appear to be porous.

Figure 2 shows a plot of the DC and the AC (40Hz) conductivity measured

in air. The curves are identical as expected and furthermore, the

conductivity versus frequency (40Hz to 40KHz) plots show no conductivity

change with_frequency. Similar measurements were made in an atmosphere

of 1.2 X 10"^ ppm O2 in N2 and the bulk conductivity did not change.

For those measurements carried out in an air atmosphere, there is little

difference between the voltage values measured on the current carrying

probes i.e., V12 and V34 (see previous quarterly report) with respect to

AC and DC measurements. This indicates that no oxygen ion buildup or

depletion is evident at the electrodes and exchange takes place rapidly

•contrary to what was expected. For those measurements done at the low

oxygen pressure, the DC measurements did show some slight polarization

(increase in V12 and V34) compared to the AC data on the same probes at

40Hz. When the sa.mple was removed from the holder and examined, a darkened area located symmetrically around the current probes (probes 1 and 4) was evident. Apparently in a reduced oxygen pressure reduction

of the oxide to zirconium can occur rapidly at the cathode when an electric

field (1-2 v/cm) is applied, particularly at elevated temperatures (above

13
/ 1200 ®C), At the same time, reoxidation should take place at the

anode. During the DC portion of the measurement, current was left on the sample for approximately 60 seconds during each data acquisition at

each temperature point and reversed for the same length of time so no net reduction should show at a particular current probe. The net reduction

observed must be due to the lack of oxygen available in the atmosphere

for reoxidation when a current carrying probe becomes an anode. Therefore,

oxidation is a less rapid process than reduction in Zr02 under an ambient

partial pressure of oxygen of near one part per million. This causes a polarization effect near the current contacts as the data shows in

Figure 3 for the DC measurement. No such effect was observed for those

measurements in air. This material, it turns out, was not well suited to

demonstrate the viability of the analytical technique used in slag

conductivity measurements because electrode reversibility with the respect

to the transported ion (oxygen) is not zero .and the ion is also a component

in the atmosphere so that exchange can take place--in- near-electrode areas,

thus eliminating polarization (in an air atmosphere). Another known

ionically conducting material should be used where the conducting ion

does not react with the electrode material and does not readily exchange

with the ambient atmosphere up to approximately T500 °C.

14
CONDUCTIVITY

(ohm-cm)'

TCK)

Figure 1. Electrical conductivity vs. temperature of several slags containing

amounts of iron. A, B, and C are Eastern slags (Bow, NH) containing 29, 20,

and 14 weight percent Fe respectively , but with no added potassium. (See QR

April - June 1979, page 33, for base composition). D is a Western slag

(Rosebud seam) with 4% iron and with 20% K,S0. added to the melt. (See

Table 1, this report, for base composition;. ^

15
o

16

Figure

2.

Four

probe

DC

and

AC

(40

Hz)

conductivity

vs.

temperature

of

a

sample

of

high

density

yttri

a

stabilized

zirconia

measured

in

air.
>

<M

C O)

•f* L.(U

t/1 O) c. *a to o o

s- S M -M U (T3 OJ •— C

<U (Q O) C

cr -I-

•1“>>-a

i- <u

S_ -f-

(X3 I

U Q.

O.

-M 03 C

d) (/I ^ T— 3 -O U I

O)

OJ -I-u c •— O i- c u O O) -M O)

03

Nl

C

•r— (Q (/) S_ r—

03

C 03

<D 3 0^0"

a. 2 <u

CVJI c\j[-a

CO

[-a CO

cvj

u_ 03 0 > C '

+-> 0 CM u 0 1—

O) S- t|_ t(- •r— M- M CM

<U

ao; OJ c

JZN •<- 1— CVJ cn-f- O c ^ — 03 E 2 +J Q. O to CL

to fO CVJ -M S_ O +J

4-> MQ_ >) O

CO

<US-3

C7>

17

where V

|2

and V

23

are

the

voltages

measured

between

probes 1

and 2

and

between

probes 2

and 3

respectively. d

^2

and d

23

are

the

distances

between

these

probes.

(See

text

and

previous

quarterly

report

fora

description

of

the

measurement.)
3. Corrosion of Downstream MHD Components (J. Smit and C. D. Olson)

Progress : Tests were continued on 316 stainless steel for suitability

as a downstream MHD component material.

Two samples of 316 stainless steel were exposed to corrosive gas stream

environments, at wall temperatures of 500 °C. Sample 7-6 (Figure 1) was maintained in an oxygen rich gas stream environment while sample

10-6 (Figure 2) was exposed to a fuel rich gas stream environment.

The gas stream environment, as previously reported, is determined by

varying the oxygen-propane gas ratio. Both tubular specimens, 12.5 mm

in diameter, 250 mm in length, and with 0.8 mm wall thickness were

exposed for a four hour period of time, exclusive of heat-up and cool

down time.

The hot gas stream was seeded with a mixture of 20 percent by weight

K2 SO4 and 80 percent by weight K2 CO 3

. Seeding of the hot gas stream

occurred during the first 25 minutes of the test run at the rate of 10

grams per minute. The gas stream temperature was maintained at 1300 °C

and monitored with a Pt/Pt-10%Rh thermocouple placed in close proximity

to the center of the sample tubes, while the wall temperature was monitored

using a Pt/Pt-10%Rh thermocouple welded into the wall of the tube at the

midpoint position. Cooling air was used to maintain the tube wall

temperature of the samples.

SEM/EDX Analysis of the Metal -Salt Interface

After the four hour test run the samples were cooled to approximately 40 °C

and immediately encapsulated in epoxy resin to prevent possible contamination

and hydration to the sample coatings. Sections for analysis were taken

10 mm from the midpoint of the tube and prepared for SEM/EDX analysis.

The sections were ground and polished using standard metal lographic

techniques. Specimen preparation was accomplished using a non-aqueous

medium and specimens were stored in a vacuum dessicator to minimize

hydration and contamination.

Both specimens exhibited similar salt deposit characteristics. The

surface facing the gas stream was thinner than the deposit on the side

portion of the tube (drip zone), and a thin fume deposit was found on the underside of the tube. The thickness of the top surface deposit was found to be consistent with previous samples, in that, the surface salt

deposit is thinner as the wall temperature increases. Also that under

fuel rich conditions the deposit is thinner than in the oxygen rich

environment.
Examination and comparisons can be made with 7-6 and 10-6. Both

samples were seeded with 80 percent by weight K2CO3 and 20 percent by

weight K2SO4, with wall temperature of 500 °C, however 7-6 was in an oxygen rich environment and 10-6 was in a fuel rich environment. As

seen in previous samples (8-6, 9-6) a banding can be observed in the

coating (Figure 3) showing a region of K + S near the surface of the

steel followed by a high K region with a thin band of K + S on the

surface of the coating. However, the bulk of the coating is generally

high in potassium (K) with moderate amounts of sulfur (S) found. The

corrosion process exhibited in both specimens seem to follow the same

general corrosion mechanism as found previously. The cations are found

in a general order of mobility where: a) bulk stainless steel,

b) higher nickel-high chromium-iron , c) chromium-iron-nickel -potassium,

d) chromium-higher iron-potassium, e) iron-potassium, f) high potassium.

Again sulfur is found in small quantities in the reaction zone area and

coating.

At the lower wall temperature of 500 °C, the corrosion process is not

as vivid as samples examined at 590 °C (8-6, 9-6) however, the process

was observed and noted to be similar in nature as described above.

Conclusion : As stated previously the variance betvyee_n jP^uel__rich and

oxygen rich environments in the presence of K2CO3 and K2SO4 are minim.al.

Corrosion of type 316 stainless steel seems to occur readily in both

instances. In general, therefore, both systems indicate the same metal

cation movement from the bulk stainless steel into the salt deposit.

Clad Materials for Application in Downstream MHD Components

Recently much interest has been generated in regard to the suitability of

clad materials for application in downstream MHD components. This interest

stems not only from a standpoint of corrosion reduction (durability) but

also from a standpoint of economics (reduction of consumption of scarce

resources). Attention therefore is now being given to a class of materials

that are arc plasma sprayed on metals. Three comnercial ly available

examples of arc plasma sprayed materials are presently under consideration

^for testing and evaluation. These coatings, NiCrAlY, MgAl204 (spinel),

‘and Zr02 (doped with MgO) have been sprayed on a mild steel tube AISI

1015) substrate. The coating thickness averages 0.5 mm. Short length

specimens of each, 75 mm in length and 0.9 mm v/all thickness, have been

obtained for temperature cycling, and thermal shock tests. Longer specimens,

0.25 m, have been obtained with Pt/Pt-10%Rh thermocouples embedded at

the midpoint for corrosion testing.

SEM/EDX examination of these specimens prior to testing is currently

being undertaken. Evaluation of the boundaries betv;een substrate (steel)

and the cermet are of importance for adhesion properties. Surface

examination will reveal areas susceptible to penetration of possible

corrosive materials (porosity).

19
SEM/EDX evaluation will also include heat treated specimens. These

samples have been cycled from 18 °C -m- 700 ®C in air and another set

of identical samples have been cycled from 18 °C 700 ®C in forming

gas (5% H 2 , 95% N 2 ). Analysis of these sets should reveal any tendency

toward delamination or deformity in the cermet bonded material.

Plans : (1) Continuation of SEM/EDX analysis of the arc plasma sprayed

coatings , to include specimens exposed to seeded hot gas stream, (2)

to initiate exposure studies on 2 1/4 - 1 Mo. Croloy tubing.
vmem

tmn

Fig. 1 Section of Type 316 stainless steel tubing after ex- posure to K2CO3/K2SO4 seeded oxygen rich hot gas

stream. Tube wall temperature 500°C. 6x.

Fig. 2 Section of Type 316 stainless steel tubing after ex- posure to K2CO3/K2SO4 seeded fuel rich hot aas stream

Tube wall temperature 500°C. 6x.

21
L_3.16 SS

Fig. 3 SEM micrograph, 425 x, of stainless steel-salt deposit

reaction zone of specimen shown in figure 1.

22