f& <r / - a 3

MHD MATERIALS - SEED/SLAG INTERACTIONS AND EFFECTS

QUARTERLY PROGRESS REPORT

April 1 - June 30, 1981

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,

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

SUMMARY OF PROGRESS TO DATE 1

DETAILED DESCRIPTION OF TECHNICAL PROGRESS 2

1. Thermochemistry of Seed and Slag 2

2. Electrical Conductivity and Polarization 24

3. Corrosion of Downstream MHD Components 30

i

I. Summary of Progress to Date

1. Thermochemistry of Seed and Slag

Development of a model equation based on four component simplified Western

channel slags continued. Additional data were i ncl uded to modify the constants

previously obtained. The equation uses three linear terms in the concentration

variables, a constant term, and a term in 1/T to relate log P K to the concentration variables and temperature. The modified equation predicts changes in

the K pressure of 16, 2, and -3 percent for increases in the concentration of

K2 0, CaO, and A1 2 0 3 of 1 percent respectively. The model equation is found to

predict K pressures within a factor of 2 of those measured for other more

complicated slags if the Fe 2 0 3 and MgO components are assumed to be chemically

equivalent to A1 2 0 3 and CaO, respectively. The model equation is less successful

in predicting K pressures over slags containing less than 10 wt.% CaO or simpler slag solutions.

The formulation of a phase equilibrium derived K^O-CaO-A^O -SiO^ model for

prediction of potassia activities has been presented. This is largely an empirical model and has not been fully developed and tested. In a more

theoretical effort, the quasi chemical melt polymeri zation model is extended

to calculation of liquidus surfaces in the system K-O-CaO-Al 20q-Si O-. The

practical goal of this effort is an algorithm for prediction^? slag melting

behavior as a function of composition. So far the fit to six subsystems is

reasonably good, but an independent test on a seventh points up the need for

considerable refinement. The model allows consideration of polyatomic cations

as well as polymeric anions.

2. Electrical Conductivity and Polarization

An average anode and cathode function has been calculated which

shows that in a low-iron ,high -calcia slag, there is considerable ionic

conductivity above 1400 °C. SEM and EDAX coupled with previous XRD

analyses show that iron-alumina spinel is the only detected conducting

crystalline material but that bridging of this material through the main

component (leucite or potash feldspar) is not sufficient to explain the

observed conductivity below the slag solidification point.

3. Corrosion of Downstream Components

Specimens of NiCrAlY, MgAl^O^ (spinel) and ZrO (doped with MgO) arc plasma sprayed on A I S I 1015 steel were examined by SEM/EDX techniques

for species distribution and structural integrity. Analysis was conducted

on metal lographically prepared specimens taken from sections of material in

the as -received condition, after cycling in air to 700 °C and after cycling

to 700 °C in forming gas. The ramp rate was approximately 3 °C/min. and

the peak temperature was maintained for about 36 hours. Metal lographically

prepared sections of AISI 1015 steel tubing, the material used as a substrate for the arc plasma sprayed coatings, were analyzed also by

SEM/EDX techniques after exposure to fuel rich and oxygen rich K^SO^ seeded

hot gas streams. Tube wall temperature for these exposure tests was held

at 590 °C. The extent and some aspects of the chemical nature of the

reaction zone between the metal and the deposited salt coating was determined.

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)

An objective of the current work is to obtain K-pressure data for selected

compositions of the four component system, K2 0-Ca0-Al 2 0 3 -Si0 2

. These selected

compositions correspond to the channel slag which would be produced by the

interaction of seeded combustion gases with simplified "Western" coal ash. A

second objective is to predict the K-pressure or K2 0 activity in melts with

compositions not too different from those studied by using the data base

generated to develop an initially empirical model. Finally, it is planned to

extend the model to other coal ash compositions, including additional components,

such as Fe 2 0 3 and MgO. The basic use of such a model is to aid in determining

the magnitude of seed loss to slag in MHD systems and to suggest the best

method to minimize this effect.

Thus far, vapor pressure data have been determined for four different

samples. Table 1 lists the initial mineral constituents and the starting and

final composition in wt.% of each of the four components. All vapor pressure

measurements have been obtained using a Knudsen effusion, modulated beam mass spectrometric method.

Table 1

Mineral Constituents and Composition of Vaporization Samples

Sample Mineral Constituents (wt.%) k

2o

Initial /F i nal

Composition wt.%

CaO A

2 0 3

Si0

2

KCAS-VP-1 KA1 Si0

4

(49. 8) , Ca

2

Si0

4

(16. 5) 14.8 27.5 22.2 35.5

Ca

2

Al

2

Si0

7 (16. 5) , Ca

3

Si

2

0

7

(17. 2) 1.6 31.8 25.7 40.9

KCAS-VP-2 KAlSi0

4 (28.6), KA1 Si

2

0

6

(13 . 8) 11.5 17.6 33.7 37.2

Ca

2

Al

2

SiO

? (28. 9) , CaAl

2

Si

2 0g(28. 7) 6.1 18.7 35.7 39.5

KCAS-VP-3 KAlSi0

4 (42.0), CaSi0

3

(58.0) 12.5 28.0 13.5 46.0

1.8 31.4 15.2 51.6

KCAS-VP-4 KAlSi0

4 (66.9), Ca

2

Si0

4

(33. 1) 19.9 21.6 21.6 37.0

2.7 26.2 26.2 44.9

2
Progress : During this guarter analysis of the KCAS-VP-4 data was completed

and comparison of the model results with experimental data from other more

complicated slag compositions was made. As in previous measurements, the data

were separated into groups to limit the change in the K2 0 composition and

least sguares analysis of the data were made to determine the slope and intercept

in the eguation,

log P K = A/T + B.

These data are tabulated in Table 2 along with similar results obtained from

results of the KCAS-VP-1, KCAS-VP-2, and KCAS-VP-3 series. The basic data in

the four series of measurements includes 389 K pressures at varying T and

compositions of K2 0, CaO, A1 2 0 3 , and Si02

. The slope terms in Table 2 generally

do not deviate greatly from -17000 ± 1000 except for the 3/13/80 and 3/14/80

experiments from the KCAS-VP-1 series and the 12/15/80 and 12/16/80 experiments

from the KCAS-VP-4 series. These data were obtained at compositions at which

the calculated K2 0 concentrations were relatively high. The relatively high

slopes could be explained by the presence of a crystalline KA 1 S i

0

4 phase in

the solution at lower temperatures which gradually dissolved as the temperature

was increased leading to enhanced K2 Q concentration at the higher temperatures.

Slopes lower than 17,000 may be due to impurity evaporation (unreacted K2 C0 3 ) at the beginning of the series or to inclusion of too wide a K2 0 composition

in the data analyzed.

Because the variation of K2 0 concentration is expected to have the greatest

effect on the K pressure each set of data was also fit to the equation

log P K = A/T + B + C (wt.% K

2 0).

The results of this calculation are shown in Table 3. Again, the average slope

is close to -17,000 ± 1000. The change in K pressure per percent change in K2 0

concentration varies from a high of 22 percent to a low of 17 percent. Thus,

the variation in K pressure is close to the same value for all four series.

In the previous quarterly report, data from the KCAS-VP-1, KCAS-VP-2, and

KCAS-VP-3 data were used to determine coefficients for the equation

log P^Catm) = A/T + B + C(wt.% K^O) + D(wt.% CaO) + E(wt.% Al^O^).

Inclusion of the KCAS-VP-4 data in the data pool leads to slight changes

in the coefficients. A comparison of the coefficients is given in Table 4.

The first set of coefficients based on the KCAS-VP-1, 2, and 3 experiments

indicates that each percent increase in the K2 0 concentration increases the

K pressure by 20 percent and each percent increase in the A1 2 0 3 concentration

decreases the K pressure by 4 percent. The effect of an increase of a percent

in the CaO concentration is essentially negligible but leads to an increase in

the K pressure of 0.4 percent. The second set of coefficients based on the

inclusion also of the KCAS-VP-4 data shows an increase in the K pressure of

16 percent for each percent increase in the K2 0 concentration, an increase of

2 percent for a percent increase in the CaO concentration and a 3 percent

decrease in the K pressure for a percent increase in the A1 2 0 3 concentration.

The agreement of these model equations with each other is good but we will use that based on all of the experimental data rather than the first three series.

3
Table 2

Slope, Intercept, and Standard Deviations for K-Pressure Data

Expt. Id Wt.% K

2

0 A SD , A B SD ,B

KCAS-VP-1

3/13/80 14.8-12.4 -20,047 391 8.062 .255

3/14/80 12.4- 9.3 -19,071 524 7.196 .334

3/18/80 9.3- 7.2 -17,429 452 5.976 .289

3/18/80 6.2- 4.8 -16,372 210 4.986 .129

3/21/80 4.8- 4.0 -16,896 279 5.207 .178

KCAS-VP-2

5/14/80 11.4-10.7 -14,632 434 4.200 .278

5/15/80 10.7- 8.6 -16,366 180 5.097 .112

5/16/80 8.6- 6.5 -17,571 257 5.656 .158

5/19/80 6.5- 6.2 -16,606 526 4.992 .321

KCAS-VP-3

8/13/80 12.5-10.8 -15,052 775 4.983 .512

8/14/80 10.8-10.3 -17,512 453 6.282 .298

8/14/80 10.3- 7.4 -15,441 612 4.732 .374

8/15/80 7.4- 6.0 -16,006 518 5.178 .362

8/15/80 6.0- 3.9 -15,390 320 4.544 .197

8/18/80 3.9- 3.2 -16,948 801 5.417 .505

KCAS-VP-4

12/15/80 19.5-18.7 -18,153 893 6.994 .618

12/16/80 18.6-17.8 -19,616 423 7.978 .293

12/17/80 17.8-15.7 -17,611 308 6.457 .213

12/29/80 15.7-14.9 -16,375 722 5.588 .501

12/30/80 14.9-13.2 -17,620 204 6.392 .139

1/13/81 13.2-10.8 -17,740 285 6.301 .191

1/14/81 10.8- 6.8 -16,567 697 5.294 .457

1/15/81 6.8- 2.7 -14,906 720 3.938 .449
Table 3

Slope, Intercept, and K^O Coefficient for KCAS-VP-Seri es

Series A SD , A B SD ,B C SD ,C

KCAS-VP-1 -17,940 326 5.491 .198 .0876 .0047

KCAS-VP-2 -16,528 241 4.412 .147 .0844 .0063

KCAS-VP-3 -15,811 283 4.405 .174 .0786 .0049

KCAS-VP-4 -17,445 190 5.248 .119 .0677 .0026

Table 4

Coefficients and Standard Deviations for Model Equation.

First set of coefficients are for KCAS-VP-1, KCAS-VP-2, and

KCAS-VP-3 data. Second set of coefficients includes

KCAS-VP-4 data in addition.

Term Val ue Standard Deviation

A -16,794 176

B 5.288 0.173

C 7.887E-2 0.309E-2

D 1.741E-3 0.318E-3

E -1.825E-2 0.209E-2

A -17,271 140

B 5.281 .172

C 6.516E-2 .236E-2

D 1.002E-2 .345E-2

E -1.318E-2 . 217E-2

5
Both model equations show a standard deviation in the fit of 35 percent while

maximum deviations in individual points are less than 100 percent except for a relatively few cases.

Comparison with Slag Data : Some comparisons of the model equation with data

obtained on more complicated slags as well as with simpler slags have been

made. Because the model equation is based on data obtained for four component

slags in which the slag phase is mainly liquid it would be expected to be

better suited to predict the K pressure over slags which are primarily in the

liquid state.

Figure 1 shows a plot of (P 0.4 over a synthetic slag (K2) having

an initial composition of 47.3 wt.% Si

0

2 , 11.1 A1 2 0 3 , 12.0 Fe 2 0 3 , 13.9 CaO,

7.0 MgO, and 8.7 K2 0. This slag was specially formulated to have a low melting

point and was pourable from a container at 1250 °C. The term (P|<2 'Pq

2 ) 0 ' 4 is

equal to the K pressure which would be found if the 0 2 pressure over the slag

had not been effected by an enhanced 0 2 pressure because of the presence of

Fe 2 0 3 in the slag. In order to use the model equation to compare the K pressure

predicted with that measured we assume that A1 2 0 3 and Fe2 0 3 are chemically

similar and constitute 23.1 wt.% of the slag. Likewise, we assume that CaO

and MgO are chemically alike and constitute 20.9 wt.% of the slag. Using these

modifications, that is, a slag consisting of 47.3 wt.% Si0 2 , 23.1 A1 2 0 3 , 20.9 CaO, and 8.7 K2 0, and the coefficients based on all four series of the

KCAS data the resulting line predicted by the model is plotted in Figure 1.

It can be seen that the agreement between the model line and the experimental

points is extraordinarily good.

As a second test of the model a comparison with data for a Western model

slag was made. Some of this data was previously reported by Plante and Cook (1).

The initial composition of this slag was 34.7 wt.% Si0 2 , 24.5 A1 2 0 3 , 5.5 Fe 2 0 3 , 9.3 CaO, 3.3 MgO, and 22.7 K2 0. Using the same approximations as previously,

the data in Figure 2 covering the K2 0 concentration range from 17.6 to 18.9 wt.%

K2 0 can be compared with an average slag concentration of 18.2 wt.% K2 0,

13.3 CaO, and 31.8 A1 2 0 3 while the data shown in Figure 3 which covers the K2 0

concentration range from 12.9 to 10.8 wt.% K2 0 can be compared with the average

slag composition 11.8 wt.% K2 0, 14.4 CaO, and 34.2 A1 2 0 3. In the comparison

of the model line in Figure 2 with the experimental data the greatest deviation

occurs at the lower temperatures. This could result because of precipitation

of KAlSi04 from the melt as the temperature is decreased. Cook (2) has made

estimates of the K2 0 concentration in the liquid phase for the Western slag

composition. For the lowest temperature point in Figure 2 the K2 0 concentration

would be 17.6 wt.% if all the slag was in the liquid phase while Cook's data

indicate that the K2 0 concentration at this temperature (1530 K) would be only

11.7 wt.%; the remaining K2 0 being present as KAlSi04 (c). For this point the

observed pressure is 3.4 x 10" 6 atm while the model predicts a pressure of

7.5 x 10" 6 atm. The difference between the actual K2 0 concentration and that

used in the model calculation is about 6.5 percent so that this difference

alone would cause the calculated line at 1530 K to be high by 104 percent.

This would appear to indicate that if the actual K2 0 concentration in the slag

solution were known, significantly better agreement between the observed

pressure and the pressure predicted by the model would be found.

6
However, there is a significant increase in slope of the data obtained on lowering the temperature as compared to the data obtained when the temperature

was increased. The slope obtained on lowering the temperature is more consistent with the precipitation of KA1 Si

0

4 from the melt suggesting that the data

obtained when the temperature was increased may have been influenced by the

evaporation of residual impurities (probably K2 C0 3 ) from the melt. The

data shown in Figure 3 also shows quite reasonable agreement with the model

prediction. This is quite surprising because the slope of the data in Figure 3

(-20,500) is much steeper than the model slope (-17,300). The agreement of

the western slag data with the model equation is not significantly poorer than

some of the data used to derive the model equation.

Comparison of the K pressure predicted by the model has also been made

with a few other slag compositions. For low calcia slags the model predicts

too high a K pressure by a factor of 5 to 10 or more. Further work on comparison with multicomponent (5 or more components) slags is in progress.

Although the term in the model governing the K pressure dependence on the CaO

concentration is not large, the presence of CaO in the slag solution at concentration in excess of 10 percent appears to be paramount to obtaining good

agreement (within a factor of 2) between the measured K pressure over the slag

and the K pressure predicted by the model.

Comparison of the predicted model pressure with K pressures of simple

slag solutions such as K2 0-Si02 or compositions in the K2 0-Si0 2 -Al 2 0 3 system

shows that the model pressure is always higher than the measured pressure by

factors of 2 (best agreement at low K2 0 concentration) to 10 or greater.

Although the model based on the four component K2 0-Ca0-Al 2 0 3 -Si02 slags would

not be expected to apply to the simpler slags, the direction of disagreement

probably indicates the importance of CaO in increasing the K pressure.

PI ans : Continue comparison of K pressure predicted by four component model

with those over other slag compositions. Supplement model data with data using

lower CaO concentration.

References

1. E. R. Plante and L. P. Cook, Seventeenth Symposium on Engineering Aspects

of Magnetohydrodynamics, Stanford, CA (1978).

2. L. P. Cook, Seventh International Conference on MHD Electrical Power

Generation, Cambridge, MA (1980).
Figure 1

TEMPERATURE K

• O

i 1 CM - O

CL

CM ^

QQO

cc

CL

CO

CO

LL! CC

CL

o

H

<

O

o

CO

CO

Q

1666 1428

10 4 / T (K)

A

. Comparison of (P^ 2 ‘Pn^)°* 4 vs 10 /T with line predicted by model equation.

, U - 2

(P^Pq^) 0 ' 4 is equivalent to P^ for system in which Pq = P^/4. Slag

composition is 47.3 wt.% Si02 , 11.1 A1 2 0 3 , 12.0 Fe 2 0 3 , 13.9 CaO, 7.0 MgO,

and 8.7 K2 0. Modified slag composition for model line is 47.3 wt.% Si0 2 , 23.1 A1 2 0 3 , 20.9 CaO, and 8.7 K2 0.
Potassium

Pressure

(atm)

Temperature, K

2000

s

h

5.0

1

1667

1 ] 1 T— 1429

i T [ r

_L

7.0

10

4

/T (K)

Figure 2. Comparison over Western channel slag having a K

? 0 concentration

varying between 18.9 ana 17.6 wt.% K

? Q compared with the model prediction with

18.2 wt.% K^O, 13.3 CaO ,31.8 A1 0 , and 36.7 SiO •

9
Potassium

Pressure

(atm)

Figure 3. Comparison P K over Western channel slag having a KgO concentration

varying between 12.9 and 10.8 wt.% K

? 0 compared, wi th the model prediction with

with 11.8 wt.% K

20, 14.4 CaO, 34.2 AT^, and 39.6 Si0

2

.

Temperature, K

2000 1667 1429

10
B. Phase Equilibria (L. P. Cook)

1 . Phase Equilibrium-derived Model for the Calculation of Activity Coefficients

As noted in previous quarterly reports [1], knowledge of subsolidus

equilibria can be used to derive an independent estimate of potassia

activities in the melt phase in the system ICO-CaO-Al ^CL-SiO^. This is

done by fitting a function to the activities calculated^frotTi thermodynamic

properties of solids involved in invariant melting reactions. By combining

topological constraints with the Gibbs-Duhem solutions for the system, it

is,in principle ,possible to derive a model which is capable of predicting K

2 0

activities to within an order of magnitude or better over a large compositional range. The principal assumptions are heat capacity expressions for

the melt phase, and in some cases, Henry's Law coefficients.

The Gibbs-Duhem equations can be summarized as a series of differential

equations of the form

-SdT + VdP - 2X

i dy

i = 0.

These equations apply of course only at equilibrium, and specifying

constant temperature and pressure leads to:

£ X .j dy . = 0.

These equations include a non-ideal term usually expressed by way of an activity coefficient, y^ . Thus the differential equations of interest are:

X CaO

d ^ n Y CaO

= 0

XAl

2 03

dln Y

A1

2

0

3

= 0

X

Si0

2

dln y Si0

2

= 0

X

K

2 0

dln y

K

2

0

= 0

A solution to equations of this type is customarily presented in power series

form [2], For quaternary ICO-CaO-Al

2 03

-Si0

2 melts, an expression for the

activity coefficient of potassia is:

4 D D

9 D D . lny,, n

= £ ((21 n-y -In y )X • + 2(lny -In y )X .) + j=l j-K

2

0 K

2 0-j J K

2 0 - j j-K

2 0

J 44

£ £ (l/2(31n y

D + 31 n y

DDDDD

-In y -In y -In y -In y )X -X, + j=l k=l k-K

2 0 j-K

2

0 K

2 0-j K

2 0-k j-k k-j J

D D D D D D 2

( 31 n y -31 n y +ln y -In y +ln y -In y )X. X, ) +

K

2 0-j j-K

2

0 K

2 0-k k-K

2 0 j-k k-j J

4 4 4 D D D D D D

£ £ £ (In y

+ In y + In y -In y -In y -In y )X.X X j=l k=l m=l j-K

2 0 k-K

2 0 m-K

2 0 K

2 0-j K

2 0-k K2 0-m

11
where 1 = K«0, 2 = CaO, 3 = A1

? CL, 4 = SiCL, and y? is the activity coefficient

at infinite^dilution.

As can be seen, according to this formulation only 12 constants, y? - are required. These are:

i D

'* Y

K9 0-Ca0

? D YCa0-K9 0

, D

L Y K9 0-A1 9 CL

y

A1 9 0.-K9 0

Q

^ J <- Y

K

9 0-Si09

,b ‘ YSi0 9 -K9 0

7 D

/m Y CaO-Al 9 CL

0 D a * Y

A1 9 0 9 -Ca0

q D

y ‘ YCa0-Si09

in D 10 - Y Si0 9 -Ca0

n D

1 '• Y

A1 o 0 9 -Si0 9 ,, D

2 3 2

YSi0

2 -Al2 0

3

Of these, reasonable estimates of coefficients #5, 7, 8, 9, 10, 11 and 12 can be made [3]. In the forthcoming quarter a least squares fit, probably

using a Henry's Law approximation with linear temperature dependence, will

be made to the remaining coefficients. If the fit is reasonable, further

refinements will be made and the model will be tested as far as possible.

2. Calculations of Liquidus Surfaces in the System K

2 0-Ca0-Al

2 03

-Si0

2 One of the goals of the phase equilibrium portion of the slag thermochemistry

project is the build-up of a body of experimental data describing the melting

behavior of carefully selected slags. Another goal is the development, from

this data base, of models capable of accurately predicting the crystallization

and melting behavior of complex multicomponent slags over a broad range of

composition and temperature. Detailed knowledge of mel ting/crystall ization

behavior as a function of composition is essential for understanding and

accurately predicting the behavior of slag in the combustor, channel, and

diffuser, where components are designed to operate under a specific range of

slag flow properties. The ratio of crystals to melt in any given slag

affects not only viscosity but also electrical conductivity, corrosiveness

and the ability to wet ceramic or metal surfaces.

12
Silicate slags of the type found in MHD systems are among the most

complex of solutions - a typical MHD slag contains more than seven oxide

components. It is generally accepted that slags can be regarded as ionic

solutions, and it is thought that they are composed of simple cations such

as K , Ca , Na , Mg , Fe , etc. and a considerable range of anions,

primarily polymers of the type [Si

N 0« N+ ]

-2

^. In spite of such complexity,

it has been recognized in recent years ^hat application of polymer solution

theory yields, in certain cases, a reasonably good fit to the experimental

data which is encouraging, because this model is developed primarily from

a well-defined structural theory, rather than from an empirically derived

function. Accurate prediction of phase equilibrium properties over a range

of compositions requires just such a structural model to provide the

understanding of the actual species distribution needed for thermodynamic

calculation. This is true because in the absence of a unifying structural

theory, one must employ strictly a empirical approach involving the use of

fitted excess functions and the results cannot be expected to be reliable

in the general case, for complex slags without the availability of an enormous amount of data, generally not available. Unlike the prediction of

some properties such as viscosity, where for some purposes precision of an order of magnitude will suffice, prediction of melting properties requires

precision of at least a few tens of degrees and at least %10 mole %. For

these reasons the polymer ionic solution model has been developed for MHD

slags as outlined below, in the hope that ultimately this model may become

reliable enough to provide estimates to be used in system operation.

The essential features of the quasichemical model of melt polymerization

CQCP ) have been outlined in previous quarterly reports [4 ], and as noted,

one of the prime difficulties in applying this model is the treatment of

aluminum. During the past quarter an attempt was made to find a reasonable

treatment of A1 p0^ in terms of the model, by considering phase equilibria

in the system K^Q-CaO-A^O^-SiC^. Six binary systems on the "faces" of the

quaternary were used an input: anorthite-si 1 ica, anorthi te-wol lastonite

,

anorthite-corundum, KAlSiO^-corundum, leucite-corundum, and wollastonites 1 1 ica. As a test of the model, an attempt was then made to fit data for the

system anorthite-leucite, a join lying "inside" the quaternary tetrahedron.

There are essentially two alternatives for the treatment of aluminum.

One involves coupled solution of calcium and aluminum in the melt, the

other considers aluminum to be present in the melt as an independent species.

Although data for the system anorthite-si 1 ica can be modeled reasonably

well using a calcium aluminate species, this does not appear to provide a reasonable fit for the other systems. Attention has thus been focused on the second alternative for solution of alumina. Possibilities include both

positively and negatively charged species. In keeping with the amphoteric

nature of alumina it has been assumed that in compositions with atomic

K : A1 <1, some of the alumina will be present as positively charged species.

Thus it is necessary to define an equilibrium constant for the quasichemical

reaction

:

A1

67

0 . 67(A1

+3

} + O'

2

Figure 1 shows the effect of different values of the dissociation equilibrium

constant for aluminum on the melt chemistry for the eutectic composition in

13
the system anorthite-wol lastoni te. A change from K = 4 to K = 4000 increases

the proportion of non-bridging oxygens in the melt substantially. A value

of K : 40 at 1500 °C appears to fit the liquidus data for the six binary

systems best. This is a reasonable value for a strongly associated oxide [ 5 ].

The following precipitation reactions have been used. These reactions

are also quasichemical in that they do not involve charge balance [ 6 ]. Charge

balance is important in determining the mixing behavior of the melt, but at the

precipitation sites, electrical neutrality must be maintained, presumably by

_?

free diffusion of 0 ions toward or away from the reaction site. Hence, the

activity of the precipitating phase is defined only in relation to the primary

species taking part.

( A1 ,67°^corundum ~ 1/M

(S10,) 1/N (Si 2'cristobal ite

' ,/M VW, MN W0,J2N‘

(CaAl

2 Si 2°

8 ) anorthi te ^ Ca0 + 3/M ^ A1 .67M°M^

+ 2/N ^ Sl

N°2N^

^ CaSl0 3^wollastonite ^ Ca0 + 1/N ^ Sl

N°2N^

(KAlSi^crystal J ] / P (KpAl

p 0 2p ) + 1/N (Si^)

( KA1 Si

2 0

6

)

1 euci te ^ 1/P (KpAl

p 0

2p ) + 2/N (Si

N 02N )

If an "ideal activity" is defined according to:

nfusion 1

-1

a = e"^“R " T

fuslon ^

these activities as a function of temperature can be compared - on the

assumption that the QCP model correctly accounts for solution nonideality - with those calculated according to the following equations which apply at

saturation:

a _ 0 /v \ d corundum Y corundum ' a A1

1/M

0 ,v J/N

a cristobalite Y cristobal ite ^ X

S

i

N 02 ^

a anorthite Y anorthite ^ X Ca0 ^ X Al ^Si

'n 3

2 N^

2/N

0 2

a wol lastonite ~ Ywol lastonite ^ X Ca(P ^Si^N^

/N

a KA 1 S i 0 4

= Y KAlSi0

4

^ X K

pAlp0

2p

) 1/P

(X

si 0

) 1/N

^'N

U

2N

a KAlSi

2

0

6

= y KAlSi

2 0

6

(X

K

pAlp0

2 p

)1/P

(X Si

N

0

2N

} 2/N

14
where y° is a constant defined by the requirement that the activity of the

crystal in its own melt is unity. Note that the model allows considerable

1 attitude in varying the multiplicity of the "clusters"* taking part in

the precipitation reactions, indicated by the variables M, N, P.

Algorithms have been devised to determine the values of M, N, and P for each of the six binary systems input which optimize the simultaneous

fit for both liquidus curves in each binary at the eutectic point. These

algorithms also perform calculations of structon probabilities in each

melt composition so that the precipitation reaction can be accurately

described in terms of the anionic polymeric species involved. The rule

which has been employed in defining precipitation reactions in terms of

structon proportions is that no Si-0 bonds be broken during the precipitation

process. This is consistent with the simple notion that precipitation is

essentially a bond-building process while solution is essentially a bondbreaking process. This in the case of wollastonite precipitation, all

structons with less than 3 bridging oxygens are involved, whereas in the case of cristobal ite, all structons participate.

Results of fitting the QCP model to the six binary eutectics are illustrated

in figures 2-7 for various clustering parameters. The conclusion is that

values of M 2 4.5, N ; 3 and P>4 fit the data best. The fit for the silica

liquidus seems to require very large values of N, however. This is inconsistent with the values required for anorthite and wollastonite in the same melts. This discrepancy is not surprising in view of the miscibility gap near

the silica end in many binary systems, in particular the system wollastonitesilica, and it is obvious that for very silica-rich melts the QCP model must

be modified. Apart from the silica liquidii, most of the surfaces in the

six binary systems are represented to within 50 °C and 10 mol °l by the QCP

model

.

The values of the parameters, M, N, P should be discussed. For aluminum

species, M = 4.5 corresponds to three A1 atoms. It has been assumed that

this species is positively charged. The tetrahedral cluster [Al^O^j

4^ is one possibility. If M = 6 is allowable, then trigonal dipyramid

+2

[Al^O^] is a possibility. Turning to silica, values of N = 3 might

correspond to [Si ^O-j q]

-

Again, if slight variation is allowed to N = 4

then [Si^O-^J”^ is a possibility. Note that a substantial range in structon

proportions is represented by combining these species with higher polymers

such as [Si

N

0 2N+4.r

8

.

While figures 2-7 show that the QCP model gives a reasonable firstorder fit to the six binary systems with relatively few assumptions, the

validity of these assumptions must be verified more rigorously. However, a preliminary test of the model can be made on the system anorthite-leucite,

a binary join within the system K^O-CaO-Al ^-SiC^. Results are shown in

figure 8. Cluster parameters were chosen on the basis of the six previous

binary fits as M = 4.5, N = 3, P = 4. The fit to the anorthite liquidus is

*Here used as a general term applied to charged or neutral species containing

more than one metal atom.

15
somewhat disappointing, although quite good for the leucite liquidus.

However, it may be significant the anorthite fit improves markedly if P = 8

is employed (not shown), corresponding perhaps to the cubic species

-8

[KgAlgO-|

2 ]~ . (Note the similarity in charge of this species to that of

the silicate anions suggested above, raising the conjecture that perhaps

equality of charge is of prime importance in mixing on the anionic sublattice).

Before the QCP model can be tested further, it will be necessary to

refine it somewhat to allow cluster parameters to vary as a function of

melt composition. In the present model, M, N and P are identical in melt

and standard state. The ultimate aim is of course to produce a theoretically

self-consistent model capable of giving liquidus surfaces to within 50 °C

and 10 mole % over the entire range of ^O-CaO-A^Og-Si^ aluminosilicate

rich melts without new input. A concerted theoretical effort aided by

experimental results could produce highly valuable results. However, it

is doubtful that this can be accomplished during the present fiscal year.

Plans

1. Attempt least squares fit to constants of phase equi 1 ibrium-derived

quaternary potassia activity model.

2. Develop theoretical basis for QCP model in order to improve accuracy

of liquidus predictions.

3. Gather and input additional experimental data to test QCP model.

References

1. S. J. Schneider, MHD Materials - Seed/Slag Interactions and Effects,

NBS Quarterly Progress Report, October 1 - December 31, 1980, p. 5.

2. K. L. Currie and L. W. Curtis, J. Geol . 84, 179 (1976).

3. a. E. R. Plante, NBS Spec. Publ. 561, p. 265-281 (1979).

b. D, A. R. Kay and J, Taylor, Trans. Far. Soc. 56_, 1372 (1960).

c. J, C, Fulton and J, Chipman, Trans, AIME J. Metals 6_, 1136 (1954).

d. F. C, Langenberg, H. Kaplan and J. Chipman, Conf. Phys. Chem.

Steelmaking, American Iron & Steel Inst., p. 65-67 (1956).

4. S. J. Schneider, MHD Materials - Seed/Slag Interactions and Effects,

NBS Quarterly Progress Report, January - March 1980.

5. C. K, Mason, J. Amer. Ceram. Soc. 51_, 134 (1968).

6. D. G. Fraser, Thermodynamic Properties of Silicate Melts, p. 301 i_n D. G. Fraser, ed. , Thermodynamics in Geology , Proc. Natl. Adv. Study Inst.,

Rei del 0977).

16
1

1

ANORTHITE - WOLLASTONITE

K<A1>=4

ANORTHITE - WOLLASTONITE

K<A1 ) =4000

NO. MOM-BRIDGING OXYGENS IN STRUCTON

HO. MON-BRIDGING OXYGENS IN STRUCTON

Figure 1. Variation of structon proportions in anorthi te-wol lastonite eutectic

for two different values of the A1 dissociation constant. K = 4 (left diagram),

vs. K = 4000 (right diagram). Note dramatic change in number of non-bridging

oxygens.

• - silica

<WO/-Q>

Figure 2. Comparison of ideal activity in system wollastonite-silica with QCP

activities at eutectic temperature for different silica polymerizations (N).

Wollastonite liquidus on left, silica liquidus on right. QCP activity at end

member mp's is unity. Eutetic data are from Bowen, Schairer and Poznak,

Amer. J s Sci . 26, 195 (1933).

17
:

I

i

Figure 3. Comparison of ideal activity in system anorthite-silica with QCP

activities at eutectic temperature for different aluminum species (M) and

silica polymerizations (N). Anorthite liquidus on left, silica liquidus on. right. QCP activity at end member mp's is unity. Data are from G. A. Rankin

and F. E. Wright, Amer. J. Sci. 39, 45 (1915).

18
Figure 4. Comparison of ideal activity in system anorthite-wollastonite with

QCP activities at eutectic temperature for different aluminum species (M) and

silica polymerizations (N). Anorthite liquidus on left, wollastonite liquidus

on right. QCP activity at end member mp's is unity. Eutectic data are from

6. A. Rankin and F. E. Wright, Amer. J. Sci . 39_, 45 (1915).

19
1557

1556

1555

1554

<1 - ACTIVITY)

Figure 5. Comparison of ideal activity in system anorthite-corundum with QCP

activities at eutectic temperature for different aluminum species (M) and

silica polymerizations (N). Anorthite, liquidus on left, corundum liquidus

on right. QCP activity at end members mp's is unity. Eutectic data from

G. A. Rankin and F. E. Wright, Amer. J. Sci . 39_, 46 (1915).
Figure 6. Comparison of ideal activity in system KAlSiO^-corundum with QCP * activities at eutectic temperature for different aluminum species (M) and

silica polymerizations (N) and potassium aluminate species (P). QCP activity

at end member mp's is unity. Eutectic data are from J. F. Schairer and

N. L. Bowen, Amer. J. Sci . 253, 719 (1955).

21
O

<•*
Figure 8. Comparison of experimental data for system anorthi te-leuci te with

a priori QCP calculation (solid curves) assuming M = 4.5, N = 3, P = 6. Data are from J. F. Schairer and N. L. Bowen, Amer. J. Sci . 245, 193 (1947).
2. Electrical Conductivity and Polarization (W. R. Hosier)

Progress

Introduction

In the last quarterly report (January-March 1981) electrical conductivity

data on a typical Western slag (Rosebud seam, Montana) were presented with

an analysis related to the specific impedance measured between all the

probe sets for a number of temperatures. To properly present the DC data,

an averaging method must be used when conducting ion mobilities are high

enough to cause polarization at the electrodes during the 40 second period

of data acquisition for each polarity of applied voltage. This had not been

done previously and is presented in this report. Quenching experiments

were also described in the last report. These are necessary to correlate

the conductivity of a sample after being held at a temperature for a long

time and having reached a stable condition to the physical state of the

sample in that stable condition. Some SEM and XRD work on these quenched

samples has been completed.

A paper. Electrical Conductivity of Mixed Ionic/Electronic Conducting

Materials at High Temperatures , was presented at the International Joint

Conference on Thermophysical Properties, National Bureau of Standards,

Gaithersburg, MD, June 15-18, 1981.

Slag Conductivity

As pointed out in most previous quarterly reports, the absolute value

of a measured conductivity is dependent on the previous treatment of a slag

sample. For that reason, it is necessary to set a definite procedure of*

sample treatment before measurement in order to be able to characterize

the sample condition during measurement. In this case, measurements

were taken with decreasing temperature after soaking at a particular

temperature long enough to reach a stabilized conductivity. This

stabilization period allows the slag body to reach a nearly equilibrated

oxidation/reduction condition, as well as a crystalline/glassy composition

for that temperature and slag composition. The format for taking data was given in the previous two quarterly reports, along with the definitions

of F-^CV) and ^ 34 ^0 - * n tlie case, an averaging process should be used

to determine polarization effects when conducting ion mobilities are large

enough to allow polarization to take place during the 40 seconds required

for acquiring the data for each applied voltage direction. It is necessary

to reverse the current at each temperature point to account for any thermoelectric voltage that might be present due to a temperature gradient along

the sample, but, more importantly, to minimize any permanent polarization

due to an ion concentration build up at a given electrode. For this case,

an average anode and cathode F ( V ) function may be defined as

F+(V) + F" (V)

average anode F(V) = ^

'F ; P (V) + Ft4

(.V) average cathode F(V) = ~

24
Figure 1 shows a plot of these anode and cathode F(V) values as a function of temperature. At high temperature, where the viscosity is low

(and ion mobility high), there is a large difference between the anode and

cathode values, possibly due to polarization at the cathode where positive

ions should migrate. At low temperatures , where the slag is highly viscous

or solid, the difference between the anode and cathode values is small.

It has not been established, in this material, that the conducting ion is

iron, as was the case for the Bow, NH (Eastern) slag reported previously.

SEM, EDX and XRD Analysis

The last sample on which the conductivity was measured was sectioned

for analysis. This sample had been heated to near 1500 °C and then cooled

slowly while collecting stabilized conductivity points to 700 °C and

subsequently cooled to room temperature. It was sectioned into four pieces,

each containing a probe. All sections showed essentially the same topography

with no differences being observed near a current carrying electrode (1 and 4) or near a voltage probe (2 and 3).

Figure 2 shows a micrograph of a region of the sample containing the

lower current carrying probe (probe 4) at 20X. At this magnification and

in figure 3, which is area A of figure 2 at 100X, it can be seen that the

platinum is rather porous. Figure 4 is a micrograph of area B in Figure 3

at 512X. The very light area is platinum. Figure 5 is area C in Figure 4

at a magnification of 2000X and shows the platinum slag interface in detail.

Figures 6a and 6b are EDAX spectra of the areas designated 1, 2, 3 and

4 of figure 5. Area 1 is platinum with no diffused iron detected. This

was of some concern during the course of the measurements since platinurr^

is known to permit iron diffusion in some cases where it is used to contain

high iron content materials. Apparently it does not happen in this case even though the electrodes are relatively porous. Scan 2 of figure 6a shows

the content of a medium grey area. It contains essentially aluminum, silicon

and potassium and is most likely the potash feldspar (KAlSi^Oo) or leucite

( KA1 Si 2 O4 ) as determined by XRD and reported in the last quarterly report.

Scan 3 of figure 6b shows the content of a lighter grey area near the

electrode. It contains iron and aluminum with a small amount of magnesium

and could be the iron-alumina spinel detected by XRD. Finally, scan 4 of

figure 6b is essentially the same as scan 2 and represents the same material

as described previously. This material makes up the main part of the slag

and is not a conducting material in its crystalline state. The iron-alumina

spinel found in the area of scan 3 is a conducting material, the conductivity

depending on the oxidation/reduction state of the iron in the crystalline

material. This material is considerably less in amount than the leucite

or potash feldspar as can be seen from figure 4 where the light grey areas are spinel in an extensive background of leucite or potash feldspar.

Bridging of the conducting material does not appear evident here and the

conductivity path through the material is not known.

25
AVERAGE

F(V)

2.2

Figure

2.0

1.8

TO 4.57

A

1

AT 1455°C

i

I

—x-x- AVERAGE CATHOOE F(V)

0.4

0.2

1. Average anode and cathode functions (see text) vs up to near 1500 °C.

•

• - temperature

26
Figure 2. SEM micrograph of a region of sample containing a lower

current carrying probe (light material running from upper

left to lower right). The slag in this series of micrographs is

derived from coal from the Rosebud seam in Montana. 20X.

Figure 3. SEM micrograph showing area A of figure 2 at 100X.

27
Figure 4. SEM micrograph showing area B of figure 3 at 512X.

Figure 5. SEM micrograph showing area C of figure 4 at 2000X.

28
Figure 6a. EDAX spectra of areas 1 and 2 of figure 5.

Figure 6b. EDAX spectra of areas 3 and 4 of figure 5.

29
3. Corrosion of Downstream MHD Components (J. Smit, 561 and

C. D. Olson, 565)

Introduction : The efficient utilization of materials in a combined cycle

magnetohydrodynamic (MHD) steam power electrical generating system requires a knowledge of the behavior of these materials to some or all of the characteristics of the working environment. A major component of large tonnage, and

having a direct bearing on the efficiency of operation of the system is the

heat exchanger in the steam bottoming plant. The materials in this unit must

function for extended periods, years, in a corrosive and errosive milieu. A

first step, a screening step, to determine the suitability of materials for

this application entails the evaluation of these materials after brief exposure

to some of the characteristics of the harsh environment expected. Selected

specimens are therefore being exposed to fuel -rich and oxygen-rich, metal

salt-seeded, hot gas streams which reproduce some of these downstream

conditions. Data from these exposure tests should give an indication, to

a first approximation, of the potential suitability for service of these

materials.

Progress : The utilization of clad materials for application in downstream MHD

components is an attractive alternative to conventional materials from both

the standpoint of durability (corrosion reduction) and economics (reduction of

consumption of scarce resources). Arc plasma-sprayed coatings, one such class

of clad materials, has recently generated much interest. As a consequence

attention is now being given to the testing and evaluation of three commercially

available examples of this type of coating. Specimens of these coatings,

NiCrAlY, MgAl 2 0 4 (spinel) and Zr02 (doped with MgO) have been obtained, and arc

pi asma -sprayed on mild steel (AISI 1015) tubing. The coating thickness

averages 0.5 mm. Short length specimens of each, 75 mm long, were obtained

for characterization prior to test and for thermal cycling tests. Longer

specimens, 0.25 m in length, with Pt/Pt-10% Rh thermocouples welded into the

mid points (prior to arc plasma spraying) were obtained for corrosion testing.

All tubing is 12.7 mm in diameter with a wall thickness of 1.6 mm.

Sections of the 75 mm long specimens were examined in the as-received condition,

after cycling to 700 °C in air and after cycling to 700 °C in forming gas

(5% H 2 , 95% N 2

). During these cycles the specimens were held at the peak

temperature for about 36 hours. Heat up and cool down rate was approximately

3°/min. Cross sections were prepared using standard metal! ographic techniques.

The examination was conducted using SEM-EDX techniques.

A. Characterization of NiCrAlY : The composition of the NiCrAlY coating is

nominally 20% Cr, 12% A1 , 1/10% Y with the remainder Ni (wt.%). Figures 1, 2,

and 3 are sections of the NiCrAlY in the as received condition showing the

bonding to the AISI 1015 mild steel tubing and the apparent lack of homogeneity

of the sprayed coating. This is confirmed by the species maps, Figs. 4, 5,

and 6, which show in detail the distribution of the species associated with

Fig. 3. Note what appear to be islands of chromium in a matrix of nickel.

Thermal cycling in air and in forming gas had no discernable effect on the

NiCrAlY specimens. Figures 7, 8, and 9 show the species distribution associated

with Fig. 10 (specimen cycled in air). Figures 11 and 12 show the species

distribution associated with Fig. 13 (specimen cycled in forming gas). It was noted that some iron contamination existed in all sections examined. The source of this is unknown at this time.

30
The NiCrAlY surface is shown in Figs. 14, 15, and 16. It appears at low

magnification to be quite pocked, Fig. 14. At higher magnification, Figs. 15

and 16, the surface appears to be covered with platelets. These platelets

also appear to contain micropores. This surface roughness accompanied with

inhomogeneity of the species may ultimately pose a corrosion problem as crevices

in the surface may promote corrosion and the increase in real surface area

with respect to the apparent area may e nhance it.

B. Characterization of MqA1?Q 4 : The MgAl 2 0 4 is bonded to the mild steel

through a graded NiCrAlY coating which is nominally 0.1 mm thick. The NiCrAlY

shows the same characteristic inhomogeneity as described earlier. In the

as received condition the bond between the MgAl 2 0 4 and the NiCrAlY appears

everywhere to be intact as does the bond between the NiCrAlY and the mild steel,

Fig. 17. MgAl 2 0 4 also appears to be uniform and continuous, Fig. 18. Thermal

cycling in forming gas had no effect on the coherency of the coating but did

result in the formation of voids, Figs. 19 and 20, while cycling in air resulted

in cracking parallel to the substrate surface, but, interestingly just above

the NiCrAlY bond coat and not at the interface itself, Figures 21 and 22.

Note also the porosity. Calcium contamination was also observed. The surface*

as received, is also pocked and appears to be covered with platelettype forms, Fig. 23 and 24.

C. Characterization of ZrO? (doped) : The MgO doped Zr02 is bonded to the

mild steel substrate in a manner similar to that described for the MgAl 2 0 4

. As received the Zr02 appears to be well bonded to the intermediate NiCrAlY

coat which in turn is well bonded to the mild steel substrate. The coating

itself however contains regions of large voids paralleling the substrate

surface, Figs. 25 and 26. No adverse effects resulted from thermal cycling in

forming gas. Regions of voids, Figs. 27 and 28 exist in this specimen similar

to that in the as received condition. Thermal cycling in air resulted in

cracking parallel to the substrate surface and cracking normal to the surface,

Figs. 29 and 30. Note that cracking here occurred at the Zr0 2 -NiCrAlY interface.

Some Fe contamination was observed throughout the Zr02 coating.

At low magnification, the surface of the as received material appears pocked,

Fig. 31. At higher magnification, microcracks are evident, Fig. 32.

D. Examination of AISI 1015 Steel : AISI 1015 steel tubing, the material

used as a substrate for the arc pi asma -sprayed coating, was, in addition,

exposed to oxygen rich and fuel rich K2 S0 4 seeded hot gas streams. The tubing,

12.7 mm in diameter with a wall thickness of 1.6 mm, was prepared in 0.25 m lengths with Pt/Pt 10% Rh thermocouples welded into the midpoint position.

Prior to exposure the specimens were cleaned to constant weight in an ultrasonic

bath at 50 °C in a 10% by weight ammonium citrate solution.

The specimens were exposed for four hours ,excl usive of heat up and cool down

time. In the vicinity of the specimen the gas stream temperature was maintained

at approximately 1300 °C. Tube wall temperature was held at 590 °C. The gas

stream was seeded with K2 S0 4 for 25 minutes at the start of the four hour

interval at a rate of 25 gm/min. Immediately after cool down the specimens

were encapsulated and subsequently sectioned, ground, and polished using

standard metal 1 ographic techniques with the exception that at all stages

31
nonaqueous cutting and cooling media were used to prevent the occurrence of

post exposure reactions.

The AISI 1015 specimen, exposed at 590 °C to the oxygen rich K2 S0 4 seeded hot

gas stream ,devel oped a very adherent K2 S0 4 corrosion product coating with no lifting of the entire coating evident, i.e., no indication of epoxy between

the tubing and the corrosion coating. Figure 33 shows this adhesion on the

top (leading surface) region of the tube. Higher magnification, Fig. 34,

reveals what appears to be cracking at the surface. Figure 35, at still

higher magnification shows this cracking to occur as scaling in the top portion

of the steel. EDX analysis, Figs. 36-40, show potassium penetration under the

scale. The potassium readily reacts with iron, forming K + Fe products in this

region. Sulfur on the other hand does not react as easily and is found in

high concentration at the upper surface of the scale where Fe + S products are being formed. The general formation zones in the corrosion reaction region,. from

tubing outward, is as follows: (a) AISI 1015 steel; (b) K + Fe reaction

zone; (c) scale layer, K + Fe; (d) K + Fe + high S reaction zone; and

(e) S + K + trace Fe, outer reaction band. This pattern is apparent over the

entire surface of the tube with the reaction being most severe on the leading

surface under the columnar salt deposit and least under the powdery fume

deposit. The corrosion band thickness ranges from about 35 mm at the leading

surface (under the columnar deposit) to about 20 pm under the fume deposit.

A similarly adherent salt-corrosion product coating formed on the AISI 1015

steel specimen exposed at 590 °C to a fuel rich K2 S0 4 seeded hot gas stream.

Again, no general lifting was observed. Figure 41 shows the corrosion zone on the leading surface (tube top). This area is not as extensive as seen under

oxygen rich conditions nor is there a separation in the scale. The area under

the drop zone shows a similar corrosion response to the K2 S0 4 seed. Potassium

is found throughout the corrosion zone as well as sulfur, Figs. 42-46. The

products here being some combination of K,Fe, + S and Fe + S. It should be

noted that the migration of Fe goes well into the salt coating beyond the

corrosion band. Corrosion appears in this case to be more uniform over the

entire ci rcumference of the tube and ranges around 10 pm in thickness. The

demonstrated susceptibility of AISI 1015 to attack from exposure to the conditions described emphasizes the importance of the structural integrity of

whatever class of clad materials is applied to this or similar inexpensive

substrates.

Plans : (1) Conduct further exposure tests on arc plasma sprayed mild steel

tubing. (2) Analyze and evaluate exposed arc plasma sprayed mild steel tubing

using SEM/EDX techniques.
Fig. 1 Optical micrograph of a section of arc pi asma-sprayed

NiCrAlY steel.

Fig. 2 SEM micrograph, 50x, showing in detail the bonding of the

NiCrAlY coating to the AISI 1015 mild steel.
Fig. 3 SEM micrograph, 504x, of the mid region of Fig. 2, showing

in detail the distribution of materials in the NiCrAlY

coating as received.

:

Fig. EDX specie map of region shown in SEM micrograph Fig. 3 showing distribution of chromium

7d

4
Fig. 5 EDX

Fig

specie map

3 showing

of region shown

distribution of

in SEM micrograph

nickel .

Fig. 6 EDX specie map of region shown in SEM micrograph Fig. 3 showing distribution of iron at NiCrAlY- AISI 1015 interface.

>
Fig. 7 EDX specie map of region shown in SEM micrograph Fig. 10

showing distribution of nickel.

Fig. 8 EDX specie map of region shown in SEM micrograph Fig. 10

showing distribution of chromium.
Fig. 9 EDX specie map of region shown in SEM micrograph Fig. 10

showing distribution of aluminum.

Fig. 10 SEM micrograph, 464x , showing in detail the distribution

of material in the NiCrAlY coating after cycling in air.

o

/
Fig. 11 EDX specie map of region shown in SEM micrograph Fig. 13

showing distribution of nickel.

EDX specie map of region shown in SEM micrograph Fig. 13

showing distribution of chromium.

Fig. 12
Fig. 13 SEM micrograph, 476x , showing in detail the distribution

of material in the NiCrAlY coating after cycling in

forming gas.

Fig. 14 SEM micrograph, 22x,of NiCrAlY surface.
Fig. 16 SEM micrograph, 2240x, of NiCrAlY surface.

Ju

A

V
Fig. 17 SEM micrograph, 112x of a section of arc plasmasprayed MgAloOa coating on AISI 1015 steel showing

graded bond.

Fig. 18 SEM micrograph, 112x,of arc-pl asma-sprayed

V

MgAl 2 04 coating
Fig. 19 SEM micrograph, 96x,of MgAl 204 cycled in forming gas to

700°C showing voids.

Fig. 20 SEM micrograph ,950x , of MgAl 204 of m1*d region of Fig. 19.

4ty
Fig. 21 SEM micrograph, 43x, of MgAl 204 cycled in air to 700°C

showing cracking within the coating.

Fig. 22 SEM micrograph, 480x, of mid region of Fig. 21.
Fig. 23 SEM micrograph, 27x, of MgAl 204 surface.
Fig. 25 SEM micrograph, 24x , of a section of arc pi asma-sprayed

doped Zr02 coating on AISI 1015 mild steel showing large

void areas.

Fig. 26 SEM micrograph. 120x,ofmid region of Fig. 25.
Fig. 27 SEM micrograph, 45x, of Zr0 2 after cycling in forming gas

to 700°C showing voids.

Fig. SEM micrograph, 180x, of mid region of Fig

4

28 27
Fig. 29 SEM micrograph, 50x, of Zr0 2

after cycling in air to

700°C showing cracks parallel and normal to the substrate

tubi ng

.

Fig. 30 SEM micrograph, 206x,of mid region of Fig. 29
Fig. 31 SEN! micrograph, 50x of ZrOz surface.
Fig. 33 SEM micrograph, 52x, of a section of AISI 1015 steel

tubing after exposure to an oxygen-rich K^SO^seeded

hot gas stream. Surface shown faced the gas stream.

Fig. 34 SEM micrograph, 528x, of mid region of Fig. 33 showing

scale.
Fig. 35 SEM micrograph, 1030x, of mid region of Fig. 33 showing

cracking in the scale. Lettered regions correspond to

labeled EDX spectra Figs. 36-43.

EDX spectra of regions A and B of Fig. 35 showing iron across the scale break. Specie positions and designations

of all spectra in this report are as indicated above.

'

Fig. 36
Fig. 37 EDX spectra of region C and D of Fig. 35 showing

potassium sulfur and iron at scale-salt deposit interface.

Fig. 38 EDX spectra of regions E and F of Fig. 35 showing

potassium penetration into the AISI 1015 bulk steel.

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Fig. 39 EDX spectra of regions G and H of Fig. 35 showing

iron at the scale crack.

Fig. 40 EDX spectra of regions I and J of Fig. 35 showing iron,

potassium and sulfur across scale-salt deposit interface.
Fig. 41 EDX spectra of regions K and L of Fig. 35 showing

iron penetration into the salt coating.

Fig. 42 EDX spectra of regions M and N of Fig. 35 showing

iron penetration into the salt coating.
Fig. 43 EDX spectra of regions 0 and P of Fig. 35 showing

farthest penetration of iron into the salt coating.

Fig. 44 SEM micrograph,|920x , of a section of AISI 1015 steel tubing

after exposure to a fuel- rich , l<2

S 04~ seeded hot gas stream.

Surface shown faced the gas stream.

£7/
Fig. 45 SEM micrograph, 920x, of surface of tube of Fig. 44

showing corrosion at side of tube. Lettered regions

correspond to labeled EDX spectra Figs. 46-49.

Fig. 46 EDX spectra of regions A and B of Fig. 45 showing

potassium and sulfur penetration at bulk AISI 1015-

corrosion band interface.
Fig. 47 EDX spectra of regions C and D of Fig. 45 showing

high iron peak comparableto the bulk at the corrosion

band-salt coating interface.

EDX spectra of regions E and F of Fig. 45 showing

iron penetration into the salt coating.

Fig. 48
Fig. 49 EDX spectra of regions G and H of Fig. 45 showing

potassium and sulfur penetration in another region

of the corrosion band.