Contract No. W-7405-eng-26

ORNL-2495

Metallurgy and CeramicsTID-4500 (13th ed.. Rev.)

February 15, 1958

METALLURGY DIVISION

PROGRESS REPORT ON VOLATILITY PILOT PLANT CORROSION PROBLEMS

TO APRIL 21, 1957

L. R. Trotter and E. E. Hoffman

DATE ISSUED

SEP 3Q1958

OAK RIDGE NATIONAL LABORATORYOak Ridge, Tennessee

operated byUNION CARBIDE CORPORATION

for the

U.S. ATOMIC ENERGY COMMISSION

MARTIN MARIETTA ENERGY SYSTEMS LIBRARIES

3 ^^Sh D35Dboa =i

CONTENTS

Introduction 1

Summary 2

Early Fluorination Tests on "A" Nickel and "L" Nickel 2

Embrittlement of Nickel Encountered During Intermediate-Level Activity Studies 5

Embrittlement of Nickel by Fused Salts Nos. 30 and 31 Containing Sulfur 7

Freeze Valve Tests 14

Charge Melt Tank Corrosion Problem 16

Early Corrosion Tests in Fluorinator 19

Corrosion of Monel in Scrubber System for Disposal of Excess Fluorine inVolatility Pilot Plant 29

PROGRESS REPORT ON VOLATILITY PILOT PLANT CORROSION PROBLEMSTO APRIL 21, 1957

L. R. Trotter

INTRODUCTION

In July 1955 the Metallurgy Division receiveda request from the Chemical Technology Divisionfor assistance in evaluating some of the corrosionproblems associated with the Volatility Processfor recovering uranium from fused salts. Sincethat time, seven different corrosion problemshave been encountered, and some experimentalwork was done on each. By April 21, 1957, themetallurgical problems involved in the processseemed to warrant the assignment of two full-time metallurgists. This report brings up todate the progress made on each of the variousproblems as of April 21, 1957.

The major components of the Volatility PilotPlant are shown in Fig. 1, a simplified schematic

ICHARGE MELT TANK

(347 STAINLESS STEEL,600°C)

PUMP

JUMONEL

E. E. Hoffman

of the actual plant. This figure will be referredto in the discussion of the corrosion problemsencountered in the various locations of the system.Additional information about the Volatility Processmay be found in reports issued by the ChemicalTechnology Division.1-3

1G. I. Cathers, "Fluoride Volatility Process forHigh Alloy Fuels," p 560-573 in Symposium on theReprocessing of Irradiated Fuels, Held at Brussels,Belgium, May 20-25, 1957, Book 2, TID-7534.

G. I. Cathers and R. E. Leuze, A VolatilizationProcess for Uranium Recovery, Preprint 278 of paperpresented at Nuclear Engineering and Science Congress, Cleveland, Ohio, Dec. 12-16, 1955; also printedin "Selected Papers," Reactor Operational Problems,vol 2, Pergamon Press, London, 1957.

3G. I. Cathers, M. R. Bennett, and R. L. Jolley,ORNL CF-56-9-21 (Sept. 4, 1956) (classified).

MONEL

CAUSTIC SURGE

TANK

MONEL

SPRAY

NOZZLES-

UNCLASSIFIED

ORNL-LR-DWG 30402

SCRUBBER

CHEMICAL

WASTE

FREEZE C_JVALVE

FREEZE

VALVE

COLD TRAP MONEL SHELL,COPPER BAFFELS •

01

FLU0RINAT0R (NICKEL)(650°C)

WASTE CAN

(MILD STEEL)

ABSORBERS

(INCONEL)

TO PRODUCT

RECEIVER

Fig. 1. Volatility Pilot Plant Flowsheet.

CHEMICAL

TRAP

FURNACEMUFFLE

CLOSED END

FOUR EQUALLY SPACED CHARGEP0TS(7-in.-DIA., V8-in.-WALL)

EIGHT EQUALLY SPACED RIBS(Ix V4in.)

OPEN END

VENT

TO FREEZE VALVE

ASBESTOS RING SEALWITH RETAINING BOLTS

UNCLASSIFIED

ORNL-LR-DWG 304CS

o

c

<£

1\AkJm

RETORT BASKET

TYPE 347

STAINLESS STEEL

Fig. 25. Volatility Pilot Plant Charge Melt Tank. Constructed of type 347 stainless steel.

discharge tube became plugged. Removal of thetop of the charge melt tank revealed that extensivecorrosion had occurred on the walls and bottom

of the tank (Fig. 26), thereby plugging the discharge line. A portion of the scale was recoveredfor chemical and x-ray analyses to determinethe nature of the scale.

The chemical analysis of the scale and of asection cut from the retort is given below:

Corrosion-Product Scale

from Melt Tank

(wt %)

Retort Basket

(wt %)

Fe 78.99 71.37

Ni 7.07 11.23

Cr 11.30 17.25

Nb <0.2 <0.2

Mo 0.17 0.28

Ti 0.06 0.09

The analysis of the retort basket sample indicatedthat it was probably constructed of type 304stainless steel, not type 347. X-ray analysisof the scale revealed that there was a largeconcentration of Fe20_, with lesser amountsof NiO«Cr203, NiO, Nb, and fused salt.

The heavy attack observed in the melt tankis attributed to air having leaked into the melttank during the test run, since the principalcorrosion products were oxides. The air leakprobably occurred through an asbestos "seal"on the cover lid of the melt tank, although anattempt was made to keep a positive pressureof nitrogen inside the tank during the test.

Since the corrosion observed in the chargemelt tank appeared to be a serious problem, aseries of bench-type tests was started to evaluatethe variables thought to be associated with theattack observed in the charge tank. Tests Nos.

17

500°C

TYPE 347 STAINLESS STEEL

NICKEL

INCONEL

BATH LEVEL

(650°C)

UNCLASSIFIEDORNL-LR-DWG 30406

Fiq. 27. Low-Carbon Nickel Capsule Test Assembly.

2 and 6). The very heavy attack on the test rodis apparent in locations (a) and (b) of Fig. 31,as compared with (c), which was from the vaporzone and showed very little attack. The type347 stainless steel test rod sections shown in

Fig. 32 (test 6) show much less attack than thesame rod in test No. 2, indicating that furtherpurification of the nitrogen gas had greatly reduced the attack. Figure 33 is an enlarged viewof the area indicated by an arrow in Fig. 32aand shows that the attack was still quite heavyin some areas. Test M-1 (see Fig. 30) demonstrated clearly the importance of protectingthe fused salt from air contamination, both duringthe loading operation and during test. Thenickel and Inconel rods were quite bright following test. The Inconel rod did, however,suffer a slight amount of attack. The type347 stainless steel rod from this test, althoughattacked slightly (see Fig. 34), showed a greatreduction in attack as compared with tests Nos. 2and 6 (Fig. 33).

Summary

From these tests it was concluded that

1. austenitic stainless steels will serve as satis

factory charge-melt-tank construction materialfor the Volatility Pilot Plant, if great careis taken to protect the fused fluoride saltfrom air contamination, both before and duringthe melt operation,

2. either Inconel or nickel would be more satis

factory materials from a corrosion standpoint,if some air contamination were unavoidable

during the charging and melting of the fluoridesalt.

EARLY CORROSION TESTS IN FLUORINATOR

Two preliminary tests were conducted in thefluorinator of the Volatility Pilot Plant to determine whether fused salts can be processed asplanned and to check out the equipment. Thefluorinator vessel, which was fabricated from"L" nickel, was exposed to fluorine spargingof the NaF-ZrF4-UF4 (56-37.5-6.5 mole %) melt

19

Table 4. Corrosion Experiments To Study Variables Affecting Corrosion In Volatility Pilot Plant Melt Tank

All tests conducted at 650 C

Testi rv. i-. Type of

Metals Diameter Time ,r

No. Tested (in.) (hr) "2Flow Rate Zone Level Zon

Bath: NaF-ZrF4-UF4 (48-48-4 Mole %)

2 Type 316 SS 0.187

Type 347 SS 0.157 18 Dried N, 12

Nickel 0.192

Inconel 0.187

6 Type 316 SS 0.187 26 Purified N2a 25150 cc/min

Type 347 SS 0.187 26 Purified N2 25150 cc/min

Bath: NaF-ZrF4-UF4 (53.5-40.0-6.5 Mole %)

M-1 Inconel 0.255 24 Tank N2 1 0.5 050 cc/min

Type 347 SS 0.240 24 Tank N2 11050 cc/min

Nickel 0.242 24 Tank N2 0 0 050 cc/min

aTotal attack includes decrease in rod radius and maximum intergranular attack.

See Fig. 27 for location of specimens.

cNitrogen dried by passing through Drierite.

Nitrogen dried by passing through Anhydrone at room temperature and over copper foil at 750 C.

20

18 Dried N2C100 cc/min

18 Dried N2100 cc/min

18 Dried N2100 cc/min

18 Dried N2100 cc/min

26 Purified N2rf150 cc/min

26 Purified N2150 cc/min

Total Attack" (mils)

N, Gas and Bath* Bath Vapor

10 1

0 0

1 0

Table 5. Results of Tests in Which "A" Nickel Rods Were Exposed to Molten Fluoride Salt

During Runs Nos. 1 and 2 in the Volatility Pilot Plant Fluorination Vessel*

Test Rod

No.

Attack ** (mils) on Rod Spec men from Location

No. A B=

D

1 1 2.0 (12.0) 2.5(10.5) 4.5 (5.5) 0.5 (2.5)

2 1.5 (9.5) 2.5 (10.5) 4.0 (6.0) 0.5 (5.5)

3 1.5 (16.5) 2.5 (10.0) 3.5 (8.5) 0.5 (5.5)

4 2.5 (20.5) 3.5 (21.5) 6.0 (7.0) 1.0 (4.0)

5 1.0 (16.0) 3.5 (10.5) 3.5 (7.5) 0.5 (2.5)

2 1 9.5 (44.5) 10.5 (31.5) 17.5 (38.5) 0.5 (6.5)

2 7.5 (22.5) 8.0 (25.0) 18.0 (38.0) 2.0 (8.0)

5 7.5 (26.5) 9.0 (27.0) 20.0 (37.0) 2.0 (8.0)

*See Fig. 36 for locations.

**First number is decrease in rod radius as measured by micrometer; number in parentheses is total attack, whichis sum of decrease in rod radius plus intergranular attack.

BATH LEVEL-*-

UNCLASSIFIED

ORNL-LR-DWG 30407

I i/4-in. NICKELTESTRODS

:]D

:]c

:] Bn.

3 A

Fig. 36. Locations (A-D) of Nickel Test Rods and

Sections Examined for Corrosion (See Table 5).

of the nickel test rods before and after exposureis shown in Fig. 40. All rods from test No. 2showed much heavier surface removal and total

attack than did the rods in test No. 1. The

total attack in the bath-zone specimens (A, B,and C in Table 5) ranged from a minimum valueof 22.5 mils to a maximum of 38 mils. Figure41 shows several high-magnification views ofthe surface of test rod No. 2 from the bath zone

of test No. 2. A second phase may be clearlyseen in the enlarged view of the surface (Fig.41a). This phase penetrated the grain boundarieson the nickel to a depth of approximately 25 mils.This unidentified phase is quite similar inappearance to that formed during fluorinator testNo. 1 discussed above and during earlier experiments on sulfur embrittlement. Analysis ona 10-mil cut from the surface of the nickel rod

revealed that the sulfur content had increased

from an as-received content of 0.0037% to 0.02%

following test. The sulfur content of the fusedsalt charged was reported as approximately10 ppm.

Summary

The heavy attack suffered on the "L" nickelrods in test Nos. 1 and 2 indicates that under

27

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