progress report on volatility pilot plant corrosion
TRANSCRIPT
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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