/£ f6 - EA- 601 0-23

Dist. Category UC-90C

MATERIALS RESEARCH FOR THE CLEAN UTILIZATION OF COAL

QUARTERLY PROGRESS REPORT

January - March 1980

Samuel J. Schneider

Project Manager

Center for Materials Science

National Bureau of Standards

U. S. Department of Commerce

Washington, D. C. 20234

PREPARED FOR THE UNITED STATES

DEPARTMENT OF ENERGY

Office of Advanced Research and Technology

Under Contract No. EA-77-01 -601

"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

1. Materials Performance and Properties 3

2. Creep of MHD Refractories 5

3. Electrical Transport Mechanisms in Slag 7

4. Corrosion of Downstream MHD Components 15
I. SUMMARY OF PROGRESS TO DATE

Brief Summary

1. Materials Performance and Properties

An interpretive report, summarizing erosive wear failures in coal

conversion pilot plants was completed, reviewed by DOE and submitted for

NBS editorial review. The Failure Information Center transmitted 198

abstracts and 8 hard-copy reports to industry. Benchmark testing of

candidate computer software vendors was completed during the quarter and

additional progress was made in completing the first draft of sections

of the Construction Materials Handbook for Coal Gasification.

2. Creep of MHD Refractories

Compressive yield measurement at 1400 °C, 1500 °C, and 1580 °C are presented for loading at 0.0002 in/min. The status of the 12 station

creep assembly is presented.

3. Electrical Transport Mechanisms in Slag

The electrical conductivity of YCr03 doped with five percent Ca has been

measured in a highly reducing atmosphere at temperatures from 1300 to

1600 °C. The samples are not single phase in composition, however, and

this may have caused some errors in the bulk conductivity values due to

hair-line cracks or phase boundaries.

Several polarization experiments have been completed using slag

(Bow, New Hampshire) as an electrolyte and using platinum electrodes.

These were made in a specially designed cell incorporating four probes to

allow polarization to be measured at any probe as a function of time at

a given temperature and for a preset condition regarding charge transfer

(constant current or constant voltage)

The electrical conductivity of a Bow, New Hampshire, slag sample containing

an added amount of iron has been measured. At low temperatures (1200 °C)

the conductivity increased nearly proportional to the added iron but at

higher temperatures (above 1350 °C) the conductivity increased by a factor of five when the iron content was doubled. This increase in con- ductivity may be due to the decrease in viscosity associated with iron

addition and/or equivalent silica decrease.

1
4. Corrosion of Downstream MHD Components

Type 316 stainless steel tubes, cooled to 400 °C, 500 °C, and 590 °C, were exposed to oxygen-rich hot gas streams seeded with K2 S04

. Optical

SEM, and EDX analysis indicate the geometric and chemical nature of the

deposits and reaction zones to be similar to those of Type 304 stainless

steel exposed previously under the same conditions. As in those cases,

Fe appears to have penetrated the farthest into the salt deposit region

while the dispersion of Ni in the reaction zone area remains the most

difficult to follow. The extent of wastage (weight loss) of Type 316 and

Type 304 stainless steel exposed at 590 °C and 400 °C to K2 SO4 seeded

oxygen-rich hot gas streams was also determined.

In the coming quarter further exposure tests will be conducted on

Type 304 and Type 316 stainless steels to determine wastage under oxygenrich conditions with a seed of 80 percent by weight K2 CO 3 and 20 percent

by weight K2 S0 4

.

2
II. DETAILED DESCRIPTON OF TECHNICAL PROGRESS

1. Materials Performance and Properties (H. M. Ondik, R. C. Dobbyn,

A. Perloff, W. S. Brower, and

W. A. Willard)

Progress : Collection and evaluation of materials performance and properties data continued during the quarter.

An interpretive report, analyzing erosive wear failures of pilot

plant components was completed and submitted to DOE for comment and

approval. The report entered the NBS editorial review process in March

after receipt of the sponsor's approval.

The Failure Information Center also handled six requests for information; a total of 198 abstracts and eight hard-copy reports were transmitted in response to these requests. The Center also received several

visitors during this period. A current failure mode analysis by component

type is given in Table 1.

Project staff have spent most of this quarter extracting, evaluating,

and tabulating data for Section B (Materials Evaluation Test Data) of

the Construction Materials Handbook. Approximately seventy-five percent

of this section has been completed. The compilation and evaluation of

some data has been purposely postponed in anticipation of final reports.

Benchmark evaluation of candidate suppliers of the software package

for the new Materials Performance and Properties Data Base was completed

during the quarter. The Department of Commerce ADP procurement office

will soon set a date for contractors' submission of "best and final"

co^t proposals; selection of a software supplier will follow.

Plans: Completion of Sections A and B of the Construction Materials Handbook is scheduled for the next quarter. Normal data acquisition activities

will continue. The interpretive report on erosive failures in pilot

plants will be submitted for publication as an NBS Internal Report;

publication as a DOE report is recommended. Selection of a software

contractor for the data base is also expected to be completed during the

next quarter.

3
TABLE 1

NATIONAL BUREAU OF STANDARDS

FAILURE INFORMATION CENTER

FAILURE MODE ANALYSIS

FAILURE MODE COMPONENT

AUX

EQUIP(l)

COAL

HAND(2)

PIPING PUMPS VALVES

CORROSION 14 1 56 4 8

DESIGN 9 0 3 9 7

EROSION 6 0 28 19 19

EROSION/CORROSION 3 0 4 5 0

FABRICATION 0 0 6 0 1

PROCESS CONTROL 18 6 22 9 9

QUALITY CONTROL 4 1 1 0 3

see 0 0 21 0 2

SCC-CL 0 0 22 0 0

VESSELS(3)

13

7

3

1

0

7

0

6

2

STRESS/TEMP 6 1 12 4 1

UNKNOWN 7 2 10 7 5

WEAR __3_ __

1 _ __

0 _ 5_ 6

70 12 185 62 61

(1) INCLUDES COMPRESSORS

,

CYCLONES

,

THERMOCOUPLES

,

AND OTHERS

(2) INCLUDES BAGHOUSE, FILTERS, CONVEYORS, ELEVATORS, AND OTHERS

(3) INCLUDES GASIFIERS, REGENERATORS, DISSOLVERS, SEPARATORS, AND OTHERS

12

1

52

TOTAL NUMBER OF INCIDENTS = 442

4

IO
2 . Creep and Related Properties of MHD Refractories (N. J. Tighe and

C. L. McDaniel, 562)

Progress : Work during the quarter was directed toward 1) assembling the

12 creep stations, and 2) acquiring data on the compressive yield of the

magnesia-rich spinel refractory X-317.

Experimental Procedure : Compressive yield measurements were made on core samples obtained from R. Smyth of Fluidyne. The cores were machined to

3 in long with a 1 in long by 1/2 in diameter reduced cross-section.

The compressive yield experiments were carried out using a constant loading

rate of 0.0002 in/min at temperatures of 1400 °C, 1500 °C, and 1600 °C.

Results

:

Creep Station : The four furnaces for the creep station were delivered

in late December. The furnaces have been sited and the load cells and

pneumatic loading fixtures for the 12 stations were mounted on the frames.

The furnace assemblies are shown in Figures la and lb. Drive rods and

water-cooled rod holders are being machined.

Compressive Yield : The X-317 refractory was shown to be relatively

creep-resistant in our earlier reports and in the reports from Fluidyne.

From the compressive yield measurements we can obtain the limiting conditions

for application of dead-weight creep. The measurements are planned for three

loading rates with three specimens at each loading rate. The first results

at 0.0002 in/min are as follows:

1400 °C 1500 °C 1580 °C

Stress at yield drop, ksi 8.5 8.0 6.0

Total Deformation, percent 2.3 3.9 1.7

Time at constant load, hr. 2.0 2.5 1.0

Plans : The creep stations should be completed during the next quarter and

long-term creep tests can be started. The relevance of the present choice

of materials for the creep tests will be discussed with members of the MHD

materials group. A paper covering the results of our creep tests and micro

structural analysis of the chrome-magnesite refractory designated RFG will

be presented at the Annual Meeting of the American Ceramic Society in April

1980

.

5
Figure 1. The 12 station creep assembly showing: (a) arrangement of the

four furnaces, (b) arrangement of the loading furnace,

and load cells.

6
3. Electrical Conductivity and Polarization (W. R. Hosier)

Progress :

a. Electrical Conductivity of YCr03

Initial measurements of the pressure dependence of the electrical

conductivity of YCr03 doped with five percent calcium on the yttrium site

have been completed. There has been some difficulty in establishing the

oxygen partial pressure in the sample chamber that corresponds to the

oxygen partial pressure in the flowing gas where oxygen/nitrogen gas

mixtures are used. This difficulty arises because of the large sample

chamber (1200 cm3

) originally filled with air which must be swept out

with the flowing environment gas of chosen composition. True oxygen

pressure dependence of the conductivity can only be measured after the

environment gas has been completely changed in the sample chamber. This

requires times in excess of four hours with a flow rate of 450 cm3 /minute.

In contrast, however, for those atmospheres where very low oxygen pressures

are obtained by the reaction 2H2 + 02

-» 2H2 0 such as is obtained using

forming gas (95 nitrogen to 5 hydrogen), the low oxygen pressures are obtained quickly (1 hour or less) in the environmental chamber. The

measurements have been completed in atmospheres containing about 5 x 10 6

atmosphere 0 2 in one atmosphere N2 and in 10 25 atmosphere 0 2 as indicated

on a zirconia tube oxygen sensor gauge based on the Nernst-Einstein relation.

(Intermediate oxygen pressures between 5 x 10 6 and 10 25 atmospheres

will be obtained in future experiments by mixing the 5 ppm gas with

forming gas.)

Table 1 gives values of conductivity (a) and soak temperatures. The

conductivity values are those taken at the end of the soak period, but

data were taken at five to ten minute intervals during the soak period, while

equilibrium was being established. The order in the Table is the sequence

in which the data were taken. As can be seen, the scatter in the conductivity

data is quite large.

Table 1

02 pressure Elec. Conduct Total flow time Soak Tii

Temp. °C (atmos .

)

(ohm-cm) 1 before meas. (hr) at T. (]

1300 5 x

10‘ 6 2.9 18 18

1500 It 3.12 20 2.5

1400 ft 3.05 23.5 3.0

1400 5 x 10" 25 1.05 11.5 11.5

1600 tf 1.30 14.5 3.0

1500 tt 1.05 16 1.5

1400 It 0.92 17 1.0

1300 tt 1.04 36.5 19.5

1600 tt 1.02 45.0 8.5

1500 tt 0.88 56.5 11.5

7
There has been some difficulty in preparing good single phase

material. Possibly as a result of this multiphase condition samples sometimes exhibited small hairline cracks when they were oxidized after being

prepared in a highly reduced state. These cracks would cause erratic results

in the conductivity values.

YCr03 doped with Ca or Mg in a highly reduced state is very hard

and it is difficult to saw or drill with diamond tools. In the oxidized

state, however, it is relatively soft and sometimes fragile and breaks

easily upon handling. More work must be done on phase composition and

microstructure and their relation to oxidation/reduction conditions

before final work can continue on the associated electrical conductivity.

b. Electrochemical Effects of Slag-Electrode Interfaces

An electrochemical cell consisting of four platinum probes immersed

in Bow, New Hampshire, slag containing 20 percent added K2SO4 was constructed.

This cell was heated to 1250 °C and subjected to a constant applied voltage

mode test where the voltage was reversed periodically during the course of a 2 \ hour run. Voltages were monitored on all probes during the test.

Figure 1 is a radiograph of the electrochemical cell, while Figure 2

shows the voltage on the various probe sets as a function of time. A

constant 31 volts was maintained between the top and bottom probes and

this resulted in an initial current after each current reversal of 60 to

100 mA which drops off with time as shown in the figure. The current

was reversed at five minute intervals (after an initial interval of 2\

minutes) over a period of 2\ hours. Only 23^ minutes of the data

are shown, but the remaining data over the entire experiment period are repetitive. It can be readily seen, for example, that when the top

probe is positive (anode) as in periods A, C, and E, nearly all the potential

drop in the cell is across the 1^ set of probes after a few minutes

operation, while the voltage across I_ p_ (I_ cathode) is low. Current

D D D

reversal then as in periods B and D, (I- (anode), I_ (cathode)) shows

D I the potential drop large at the Ig Pg probes and small at the 1^

probes. In previous experiments (see QR Sept. -Dec. 1977), some iron

depletion was observed in the anode area just outside the platinum

electrode in an electrochemical cell run for a period of 20 hours at

1400 °C. At the same time, definite enrichment of iron was observed at

the cathode slag-electrode interface. Cadoff^ in electrochemical work

on slag-electrode reactions shows that in cells designed for static slag

(non-flowing), the potential buildup of the cell is due to the accumulation

( 2 ) of electrically resistive ions at the slag/anode interface. Koester et al , have shown that the critical potential for arcing or breakdown is

higher for 1018 steel than for nickel or 304 stainless steel. This

may be due to the supply of iron from the anode electrode alleviating the

usually iron poor (or silica rich) area just at the anode/slag boundary

usually associated with an electrode material not containing iron. This

experiment demonstrates that this potential buildup is relatively rapid

8
and is associated with the slag-anode interface. At the same time, the

conductivity of the bulk slag remains unchanged, since the potential on

the probes Pg drops off in proportion to the drop off in the primary

current while the applied voltage is constant.

A second experiment on this same cell was performed using a constant

current (30 mA) mode but reversing the current periodically (five minute

intervals). Figure 3 shows that data obtained over a 20 minute period

in the experiment. Although the experiment was run for 1^ hours,

only slight changes were observed with time on reversal of the current.

Breakdown in the current conducting mechanisms after a current reversal

appears to occur earlier when the Ig probe is an anode as in period B

and D (1.5 to 2.0 minutes) than when the 1^ probe is an anode as in

periods A and C (four to five minutes). Since the experiment was run at

1250 9 C, there could be a difference in the oxidation state of the iron

between the top and bottom of the cell and this might cause a difference

in the breakdown potential when the anode is at the bottom of the cell

(away from ambient atmospheres). Also with the top of the cell as anode

for a five minute period followed by a five minute period with the current

off (these periods not shown in figure) the cell returns to its original

state, i.e., the potentials are distributed evenly along the length of

the cell showing that any depletion layer has been repaired. However,

with the bottom of the cell as anode, and the same procedure followed,

the depletion layer is not repaired completely even after a period of 50

minutes. A measurement of all probe voltages with current off yields

zero potential indicating that there is no measurable battery effect

present. In this constant current mode, the high potential develops

between the anode and its adjacent conductivity probe as was also observed

in the constant voltage mode. Breakdown occurs also at the anode. The

corresponding cathode potential decreases with time after a slight

initial increase after reversal. The slag bulk conductivity decreased

(potential on p probes increased) slightly during the course of the

experiment which may be due to iron diffusion into the platinum electrodes.

Both of these experiments were done at a temperature (1250 °C) where

the slag is very viscous and where there may be a combination of ionic

and electronic (semiconductive) conductivity. If time permits, this

experiment should be repeated at higher temperatures where the conductivity is believed to be largely ionic and due primarily to the movement

*

. . (3) of iron ions

.

c. Slag Electrical Conductivity

A sample of slag was made from the Bow, New Hampshire, slag which con- tained an added 20 percent K2 SO 4 , but now with an additional 20 percent of

Fe 203 . Figure 4 shows the conductivity data as a function of temperature in an air atmosphere. The sample was heated to 1500 °C and held at the temperature until the conductivity became constant as a function of time (< ^-hour)

.

It was then cooled as rapidly as possible in the furnace to 900 °C which

required a period of 24 minutes. The temperature was then lowered slowly

to 500 °C. The data shown in Figure 4 were taken with increasing temperature

9
and at each temperature point, conductivity vs. time was monitored to obtain

maximum or minimum conductivity values. The conductivity is higher in this

sample than in its companion sample without the added iron (shown in QR

July - Sept., 1979). The conductivity near the maximum at 1200 °C is

approximately a factor of two higher in this sample with the added iron

when long times are allowed for crystallization of the slag in this temperature range. X-ray diffraction measurements indicate that at these

conductivity maxima, the crystalline components of the slag are feldspar,

sesquioxide, and spinel solid solutions of undefined bulk compositions. It

is not possible to deduce whether the conductivity is due to these crystalline phases or to an iron-rich glassy matrix likely to be surrounding the

crystalline components. In the latter case, the conductivity would be

predominantly ionic. However, polarization effects were not observed in

this temperature range during the electrical measurement.

The conductivity in the higher temperature range above 1350 °C is a factor of five greater in this sample with added iron (24.2 w/o) than in

(4) the sample with no added iron (14.7 w/o). Capps has shown that there

is a decrease in slag viscosity with increased iron content and with

decreased silica content. The relative silica content necessarily

decreases with any added component other than silica itself. This

disproportionate increase in conductivity may be due to the viscosity

decrease reflecting the validity of the Walden rule^^ i.e., r| • <J = const,

where r| is viscosity and a is the electrical conductivity for conducting

ionic species (iron ions) in a molten glass.

Plans : The electrochemical experiments will continue with the experiments

described in the report repeated at higher temperatures in order to

determine the polarization and relaxation times associated with any

depletion layer formed while the cell is carrying current.

Work on the slag electrical conductivity will continue also.

Preliminary investigations indicate that adding iron as Fe 2

C>3 or Fe 304

to Bow, New Hampshire, slag without potassium causes excessive frothing

and that these additions can only be made in a highly reducing atmosphere.

Additional transition element dopings may be investigated in order to

better understand the conductivity.

References

(1) Westinghouse Electric Corporation--Quarterly Report for the Period

Oct. 1 to Dec. 31, 1977, Contract No. EX-76-C-01-2248

(2) Electrical Behavior of Slag Coatings in Coal Fired MHD Concentrations,

J. K. Koester and R. M. Nelson, 16th Symposium on Engineering Aspects

of Magnetohydrodynamics, University of Pittsburgh, Pittsburgh,

Pennsylvania, May 16-18, 1977.

(3) Conductivity Mechanisms in Iron-Containing Slag, W. R. Hosier,

G. S. WTiite, and T. Negas - 7th International Symposium on MHD, M.I.T.,

Cambridge, Massachusetts, June 16-20, 1980.

10
Figure 1. Radiograph of the electrochemical cell

showing the arrangement of the platinum

electrodes

.

11
Figure 2. Probe set voltages and current as a function of time with periodic

applied voltage polarity reversal. The applied voltage magnitude

was constant during the run. Cell temperature 1250 °C.

12
Figure 3. Probe set voltages as a function of time with periodic applied

voltage polarity reversal. The current magnitude was constant

during the run. During breakdown, recorded voltage magnitudes

do not represent systematic cell behavior. Period A and C, I T was anode. Period B and D, Ig was anode. Cell temperature 1250 °C.

13
St

o

14

Figure

4.

Electrical

conductivity

of

Bow.

NH

slag

with

an

added

20

w/o K

2

S0

4

and

with

34.6

w/o

Fe20

3

(24.3

w/o

Fe)

as

a

function

of

temperature.

These

measurements

were

done
(A) Coal Slag Properties Related to MHD, W. Capps, Conference on High

Temperature Sciences Related to Open-Cycle Coal Fired MHD Systems,

Argonne National Laboratory, Argonne, Illinois, April 4-6, 1977.

(5) International Review of Science —Electrochemistry-Physical Chemistry

Series Two, Volume 6, J. Bocknis, Editor, p. 290 (1976).

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

Progress : Tubular specimens of Type 316 and Type 304 stainless steel

were exposed, under varied conditions, to K2 S0 4 seeded, oxygen-rich,

oxygen-propane hot gas streams. Following exposure, the specimens

were analyzed using SEM, EDX, and, in some cases, gravimetric techniques

for indications of both incipient and gross corrosion. Test exposure

times, excluding apparatus warm-up and cool down time, was four hours.

Seeding with 250 grams of K2 S04 was effected during the first 25

minutes of that interval. Six stainless steel specimens were exposed

under these general conditions with the tube temperature maintained

at one of three levels: 400 °C, 500 °C, or 590 °C. These temperatures were

fixed by internal air cooling and were monitored by a Pt/Pt 10 percent Rh

thermocouple welded into the specimen wall at the mid-length point. The

specimens were seated in the test rig with the thermocouple located in the

center of the hot zone and facing the gas stream. The gas stream temperature in the vicinity of the test specimen was held at approximately 1300 °C

as indicated by a Pt/Pt 10 percent Rh thermocouple.

a. SEM/EDX Analysis of the Metal-Deposit Interface

Metallographic specimens were prepared from three of the Type 316

stainless steel specimens, 1-6, 2-6, and 3-6, exposed at 590 °C, 500 °C,

and 400 °C, respectively. These sections were taken approximately

10 mm from the mid-point position in the hot zone. To reduce the

possibility of external contamination and hydration in the coatings

formed on the tubular specimens during seeding, the specimens were

removed from the test rig upon cool down to about 40 °C and encapsulated

immediately in epoxy. The jnetallographic sections taken from the

encapsulated tubular specimens were cut, ground, and polished using

non-aqueous media and were stored in evacuated desiccators prior to

SEM/EDX analysis.

On all the specimens, a compact, somewhat columnar deposit formed on the

surface facing the gas stream. The thickness of this deposit varied

slightly with tube temperature and is approximately 1.7 mm. A much

thinner, almost undetectable deposit, probably from fume, formed on the

trailing surface. These deposits are in the 0.2 mm range. At the

boundary, where the surface deposit and the fume deposit intersected

a thicker drip zone formed, see Figs. 1, 2 and 3. SEM micrograph,

Fig. 4, shows a K2 S0 4 deposit on the leading surface of a Type 316

stainless steel tube held at 590 °C. This type of deposit is typical

of that produced from K2 S04 seeding and was seen on specimens of Type

304 stainless steel tested previously under these conditions. EDX analysis

indicates that potassium and sulfur is uniformly distributed through

15
the bulk of the deposit. SEM micrograph, Fig. 5, shows the top of the

salt deposit with a fine whitish band on the surface. This band contains

a higher proportion of potassium than the bulk of the deposit possibly

due to vaporization of some sulfur at the deposit-gas stream interface.

The stainless steel deposit interface is seen in SEM micrograph, Fig. 6.

Note that the adhesion is rather poor as epoxy fills a gap in the

interface region. A reaction area at the interface is shown in SEM

micrograph, Fig. 7. At the metal edge, areas of low chromium and low

nickel concentration are found dotted with regions of high nickel

concentration, Figs. 7a - 7d. A loss of iron is also associated

with these areas. At the deposit edge, concentrations of iron, chromium,

potassium, and sulfur are found indicating some degradation of the metal

surface and formation of a reaction zone between the metal and the

deposit, Figs. 7e - 7h. The SEM/EDX analysis seems to indicate a preferential leaching of the metal cations from the bulk material

(Type 316 stainless steel) in a somewhat orderly fashion as in the case of Type 304 stainless steel. However, the extent of corrosion is

not as great as in the case of Type 304 stainless steel. This is

corroborated by wastage (weight loss) experiments also described in

this report.

As found in previous specimens, though nickel loss is readily detected,

its displacement to other areas is difficult to follow. Few areas are found where nickel has migrated even though its concentration is low.

Fig. 8 shows a reaction area where low chromium and low nickel con- centration are found at the metal edge, Figs. 8a and 8b, and metal

cations are found at the edge of the deposit, Figs. 8c - 8e.

Migration of the iron, Fig. 9 , is farther back in the deposit and

generally parallels previous results in which iron was preferentially

leached out of the bulk stainless steel and into the deposit.

Fig. 10 shows a portion in the fume deposit area where the corrosion

process seems to be evident. On the metal edge low nickel areas are

found, Figs. 10a - 10c, while areas of chromium, iron, potassium, and

sulfur are found in the reaction zone. Again nickel is the elusive

element. Though it is found in low concentration in the reaction zone,

it is not really found to have migrated into the deposit region.

At a lower temperature, 500 °C, corrosion is still observed in Type 316

stainless steel. Fig. 11 shows a region of corrosion at the metaldeposit interface. EDX analysis of the region, Figs. 11a - llh,

indicate areas of low metal cation concentration at the metal edge

and metal cation migration into the deposit. The same can be said

at the still lower specimen temperature of 400 °C but to a much lesser

extent.

b. Metal Wastage Analysis

Weight loss determinations and tube surface examinations by SEM and EDX

techniques were also made on Type 316 and Type 304 stainless steels.

Three specimens, one of Type 316 stainless steel, specimen 4-6 and

two of Type 304 stainless steel, specimens 13-4 and 14-4, were weighed

16
prior to exposure at tube wall temperature of 590 °C, 590 °C, and 400 °C,

respectively, under the general conditions as described in the first

paragraph of this report. In these cases after extraction from the

test rig the entire specimen was rinsed in warm water and subsequently

cleaned in ten percent HN03 at 50 °C for 20 minute cycles in an ultrasonic

bath until constant weight was obtained. In each case the second cleaning

cycle resulted in no further detectable weight change. The differences

in weight (weight loss) are attributed to loss of metal (wastage) from

the specimen, see Table 1. Figs. 12, 13 and 14 show the stainless

steel tubes after exposure and cleaning. After weighing, sections were

taken from each tube for SEM/EDX analysis.

Fig. 15 is a micrograph of specimen 4-6 which shows areas of pitting and

wastage. Fig. 16 along with the EDX analysis indicate regions of cation

migration and concentration. There seems to be no general trend toward con- centration of any metal cation; however, there seems to be a general

trend toward lower nickel concentrations. To see if these conclusions

were reasonable, another area, shown in Fig. 17, was examined. Fig. 18

shows again areas of low nickel, Fig. 18a and 18b, but also regions of

high nickel, Fig. 18c, along with areas of other metal cation concentration. However, the high nickel regions are generally isolated and

not in sufficient number to account for all the low nickel regions.

Although the wastage (weight loss) of this sample was minimal, the nickel

concentration tends to be lower than that of the bulk stainless. Possible

explanations for this would be either that the nickel is being attacked

and lost through some volatile chemical reaction or that there is a formatibn of a higher chromium, iron layer that has migrated upward

covering the nickel. In either case, lower nickel would be observed.

In samples 13-4 and 14-4 a comparison evaluation can be made of the same

type of stainless steel exposed at two different temperatures. The

greatest change in weight (wastage) was observed in sample 13-4, while

a minimal amount was lost in sample 14-4, see Table 1. Fig. 19 exhibits

an area of sample 13-4 riddled with holes and depressions between grains.

Figs. 20, 20a - 20f show metal cation concentration or loss in this area.

In sample 14-4 grain boundaries and grain forms are evident, see Fig. 21;

however, there are not as many pores or holes showing. Black spot regions,

though, are prevalent on the grain surfaces. Fig. 22 shows these areas

rather vividly; SEM/EDX analysis indicates them to be areas of low chromium,

low nickel, and high iron concentration associated with potassium and sulfur.

These areas are probably the beginning of reaction sites on the stainless

steel surface. Fig. 23 is a high magnification micrograph of one such

spot. EDX analysis, Figs. 23a and 23b, indicate low chromium and low

nickel along wi'.h potassium and sulfur in that spot with high metal cation

concentration surrounding blackish area.

A random sample of 304 stainless steel was selected for SEM/EDX analysis

and used as a comparison to the specimens placed in the corrosive

environment. Prior to sectioning the sample was cleaned with 10 percent

HN03 at 50 °C for 20 minute cycles in an ultrasonic bath. A comparison

of this surface to the surfaces exposed to the corrosive environment

17
shows that while grain boundaries are evident in both cases, figures

24-26, intergranular and surface attack seems to be occurring in the

latter cases. EDX examination of the surface of the unexposed specimen

revealed no non-uniform distribution of nickel, chromium, or iron.

Plans : Specimens of Type 316 stainless steel will be evaluated under

oxygen-rich conditions with a mixture of 80 percent by weight K2 CO 3 with

20 percent by weight K2 S0 4

. These specimens will be analyzed by SEM/EDX

techniques. Further gravimetric and SEM/EDX analyses will be conducted on Type 304 stainless steels.

18
Fig. 1. Section of Type 316 stainless steel tubing after exposure to

K2SO4 seeded oxygen rich hot gas stream. Note formation of

deposit on the upper surface. Tube temperature 590 °C.

Fig. 2. Section of Type 316 stainless steel tubing after exposure to

K

2 SO 4 seeded oxygen rich hot gas stream. .Note formation of

deposit on the upper surface. Tube temperature 500 °C.

19
Fig. 3

Fig 0 4.

. Section of Type 316 stainless steel tubing after exposure to

K2SO

4

seeded oxygen rich hot gas stream. Note formation of

coating on the upper surface. Tube temperature 400 °C.

r.

SEM micrograph, 20 X, of K2SO4 deposit on leading surface of Type 316

stainless steel. Specimen temperature 590 °C gas stream oxygen rich.

20
Fig. 5 . SEfi micrograph, 200 X, of gas stream-depost interface showing

potassium rich band.

Fig. 6 . S EM micrograph, 500 X of Type 316 stainless steel -deposi t interface.

Specimen temperature 590 °C gas stream, oxygen rich.

21
Fig. 7. SEM micrograph, 2000 X, of center region of Fig. 6 showing reaction

area at interface. Lettered regions correspond to labeled EDX

spectra. Figs. 7a-7h. Specimen temperature 590 °C, gas stream

oxygen rich.

Fig. 7a. EDX spectra of regions A and B of Fig. 7 showing low fli concentration

areas. Specie positions and designations on all spectra in this section

of this report are as indicated above.

22
Fig. 7b. FDX spectra of regions C and D of Fig. 7 showing low Cr and low

Ni concentrations in the reaction zone.

Fig. 7c. EDX spectra of regions E and F of Fig. 7 showing high Ni and low

Cr concentrations.

23
Fig. 7d. EDX spectra of regions G and H of Fig. 7 showing high Ni at the

metal edge.

Fig. 7e. EDX spectra of regions I and J of Fig. 7 showing Fe penetration into

the deposit.

24
: -

-

L

* *

r

rr

-A

.jot K/

-T > A

V; * .

' ' *V\-

1 i 1 i 1 | (|| 1 1 1 L.l l. 1 l 1 I 1 1 1

f}4 >;

Fig. 7f. EDX spectra of regions K and L of Fig. 7 showing Fe and Cr penetration

into the deposit.

Fig. 7g. EDX spectra of regions M and N of Fig. 7 showing again Fe and Cr

penetration into the potassium-sulfur deposit.

25
p

a A ..A . *- v/r- * *

.. V-' V'

I | l.U J._l 1 L- I i I i I i I i I

I I 1 I I 1

Fig. 7h. EDX spectra of regions 0 and P of Fig. 7 showing penetration of all

three species, Fe and Cr and Ni into the deposit..

Fig. 8. SEM micrograph, 2000 X, of Type 316 stainless steel-deposi t interface.

Lettered regions correspond to labeled EDX spectra. Figs. 8a-8e.

Specimen temperature 590 °C, gas stream oxygen rich.

26
Fig. 8a. EDX spectra of regions A and B of Fig. 8 showing low Ni and low Cr

concentrationsat the metal edge.

Fig. 8b. EDX spectra of regions C and D of Fig. 8 showing low Ni and low Cr

concentrations at the metal edge.

27
Fig. 8c. EDX spectra of regions E and F of Fig. 8 showing metal cations in

the reaction zone of the deposit.

Fig. 8d. EDX spectra of regions G and H of Fig. 8 showing metal cations in

the reaction zone of the deposit.

23
Fig. 8e. EDX spectra of regions I and J of Fig. 8 showing metal cations in

the reaction zone of the deposit.

Fig. 9. EDX spectra of regions K and L of Fig. 8 showing Fe farther back

in the deposit.

29
Fig. 10. SEM micrograph, 2000 X, of Type 316 stainless steel-fume deposit

interface. Lettered regions correspond to labeled EDX spectra

Figs. lOa-lOc.

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

Cr, Fe and Ni at the stainless steel edge.

30
Fig. 10b. EDX spectra of regions C and D of Fig. 10 showing distribution of

Cr, Fe and Ni at the stainless steel edge.

Fig. 10c. EDX spectra of regions E and F of Fig. 10 showing absence of Ni in

the reaction zone.

31
Fig. 11. SEM micrograph, 2000 X of Type 316 stainless steel -deposi t interface.

Lettered regions correspond to labeled EDX spectra Figs, lla-llh.

Specimen temperature 500 °C

> gas stream oxygen rich.

Fig. 11a. EDX spectra of regions A and B of Fig. 11 .showing slight loss of

Ni at stainless steel edge as compared to the bulk.

32
Fig. lib. EDX spectra of regions C and D of Fig. 11 showing high Fe and

low Cr and I! i concentrations in reaction zone.

Fig. lie. EDX spectra of regions E and F of Fig. 11 showing the penetration

of Fe in the deposit.

33
Fig. lid. EDX spectra of regions G and H of Fig. 11 showing concentrations of

Fe, Cr and Ni in the reaction zone of the stainless steel -deposi

interface.

Fig. lie. EDX spectra of regions I and J of Fig. 11 showing penetration of Fe,

Cr and Ni in the deposit.

34
Fig. Ilf. EDX spectra of regions K and L of Fig. 11 showing concentration of

Fe, Cr and Ni at the metal edge.

Fig. 11 g . EDX spectra of regions M and N of Fig. 11 showing low concentration of

Fe in reaction zone.

35
Fig. llh. EDX spectra of regions 0 and P of Fig. 11 showing concentrations of

Fe, Cr and Ni in the reaction zone.

Fig. 12. Type 316 stainless steel after exposure for 4 hours at 590 °C

to K 2 SO4 seeded, oxygen rich hot gas stream and subsequent cleaning.

Approximately 2 X.

36
Fig. 13. Type 304 stainless steel after exposure for 4 hours at 590 e C to

K2SO4 seeded, oxygen rich hot gas stream and subsequent cleaning.

Approximately 2 X.

Fig. 14. Type 304 stainless steel after exposure for 4 hours at ^00

seeded, oxygen rich hot gas strea"". a^d subsequent cleaning.

2 X.

°C to K2$ 0i

Approximately

37
Fig. 15. SEM micrograph, 400 X, of Type 316 stainless steel exposed for 4

hours at 590 °C to K2 SO4 seeded, oxygen rich hot gas stream.

Fig. 16. SEM micrograph, 1600 X, of mid region of Fig. 15 showing surface

attack.

38
Fig. 17. SEM micrograph, 400 X, of Type 316 stainless steel exposed for

4 hours at 590 °C to K

2 SO 4 seeded, oxygen rich hot gas stream.

Fig. 18. SEM micrograph, 1600 X, of mid region of Fig. 17 showing surtace attack.

Labeled regions correspond to labeled EDX spectra, Figs. T Sa -13c.

39
Fig. 18a. EDX spectra of regions A and B of Fig. 18 showing low Ni and high

Cr concentrations.

Fig. 18b. EDX spectra of regions C and D of Fig. 18 showing high Ni and high

Cr concentrations.

40
Fig. 18c. EDX spectra of regions E and F of Fig. 18 showing low Ni and high

Fe concentrations.

Fig. 19. SEM micrograph, 450 X, of Type 304 stainless steel exposed'for 4

hours at 590 °C to a K2 SO4 seeded,' oxygen rich hot gas stream.

41
Fig. 20. SEM micrograph, 1900 X, of the mid region of Fig. 19 showing surface

attack. Labeled regions correspond to labeled EDX spectra, Figs.

20a-20f

Fig. 20a. EDX spectra of regions A and B of Fig. 20 showing Ni depleted regions.

42
Fig. 20b. EDX spectra of regions C and D of Fig. 20 showing another Ni

depleted region.

Fig. 20c. EDX spectra of regions E and F of Fig. 20 showing a region of high

Cr concentration.

43
Fig. 20d. EDX spectra of regions G and H of Fig. 20 showing another region

of high Cr concentration.

Fig. 20e. EDX spectra of regions I and J of Fig. 20- showing another region

of high Cr concentration.

44
Fig. 20f. EDX spectra of regions K and L of Fig. 20 showing regions of high

Cr and low Ni concentrations.

Fig. 21. SEM micrograph, 350 X, of Type 304 stainless steel specimen 14-4,

exposed for 4 hours to a K 2 SO4 seeded oxygen rich hot gas stream.

45
Fig. 22. SEM micrograph, 375 X, of another region specimen 14-4 showing

what appears to be the onset of corrosion.

Fig. 23. SEM micrograph, 1500 X, of the mid region of Fig. 22. Labeled

regions correspond to labeled EDX spectra, Figs. 23a and 9 3b.

46
Fig. 23a. EDX spectra of regions A and B of Fig. 23 showing nickel depletion in

a dark region.

Fig. 23b. EDX spectra of regions C and D of Fig. 23 showing a Ni depleted region

C and a region of high Ni and high Cr concentration, as well as low Cr.

47
Fig. 24. SEM micrograph of Type 304 stainless steel, unexposed but cleaned.

Note grains and grain boundaries , 1600 X.

Fig. 25. SEM micrograph, 1900 X, of Type 304 stainless steel after exposure

for 4 hours at 590 °C to a K 2SO4 seeded hot gas stream and subsequent

cleaning. Note grain boundaries and grain surface.

48
Fig. 26. SEW micrograph, 1400 X, of Type

for 4 hours at 400 C C to a K2SO4

cleaning. Note grain boundaries

304 stairless steel a seated not gas se^ea'

ard grain surface.

r exposure

nd s-tsecuent

49