CORROSION ASSOCIATED WITH FLUORINATION IN THE OAK RIDGE NATIONAL LABORATORY FLUORIDE VOLATILITY PROCESS

Technical Report · Mon Jun 19 00:00:00 EDT 1961

DOI:https://doi.org/10.2172/4051734· OSTI ID:4051734

Litman, A. P.; Goldman, A. E.

: : 9 7 7 8 6 9 : = 7 9 9used during the fluorination of fused-salt fuels and subsequent associated operations in the Oak Ridge National Laboratory (ORNL) Fluoride Volatility Process was evaluated. Corrosive attack is reported as mils per month based on molten salt residence time or mils per hour based on fluorine exposure time. Two fluorinators were used in the VPP to carry out the fluorination reactions. These vessels, Mark I and Mark II, were fabricated into right cylinders, approx 4 1/2 ft in height, from the same heat of L (low carbon nickel. The first vessel contained equimolar NaF- ZrF/sub 4/ or NaF-ZrF/sub 4/-UF/sub 4/ (48-48-4 mole%) for approx 1250 hr at 600 to 725 deg C. Over a period of 61 hr, 57,500 standard liters of F/sub 2/ were sparged into the slats. This constituted a F/sub 2/:U mole ratio of 3:1 beyond theoretical requirements. The Mark II fluorinator contained fluoride salts of approxi-mately the same compositions plus small additions of PuF/sub 4/ during three runs. The salts were kept molten at 540 to 730 deg C for approx 1950 hr and about sixty 500 standard liters of F/sub 2/ were sparged into the Mark II melts in 92 hr. Both fluorinators sustained large corrosion losses consisting of extensive wall thinning, severe interior inter- granular attack, and a moderate exterior oxidation attack. Maximum deterioration on the Mark I vessel occurred in the middle vapor region at a calculated rate of 1.2 mils/hr, based on fluorine sparge time, or 46 mils/month, based on time of exposure to molten salts. The second vessel showed maximum attack in the salt-containing region at similarly calculated rates of 1.1 mils/hr and 60 mils/month. Some evidence was found to indicate that the intergranular attack may have resulted from sulfur in the systems. Bulk metal losses from the vessel's walls were believed to be the result of cyclic losses of NiF/sub 2/ ""protective'' films. The shift in maximum corrosion attack geometry in the two fluorinators is believed to have resulted from differences in operating conditions. The Mark II vessel experienced higher temperatures, longer fluorine exposure times, and uranium residence times in its salt baths. Specimens removed from the wall of the first fluorinator showed a variation in aversge ASTM grain-size number of 5 or 6 to >1, the largest grains being found in the middle vapor region. The second vessel had a more uniform grain-size pattern, average ASTM grain-size numbers varying from 3 to 5 to 2 to 4. The variations in grain sizes are believed to have resulted from variable heating rates during initial usage. Examinations of bench-scale reactors, where simulated fluorination environments were provided to study process variables and corrosion, showed that A nickel had the highest degree of corrosion resistance as a fluorinator materiai of construction when compared with Inconel and INOR-8. Intergranular penetration and subsequent sloughing of whole grains seemed to be the predominant mode of corrosive attack on the Inconel vessel. At the higher test temperatures, 600 deg C, INOR-8 miniature fluorinators showed large bulk metal losses plus selective losses of chromium, molybdenum, and iron from the exposed alloy surfaces. Evidence of a marked reduction in attack on nickel and INOR-8 was found during lower temperature studies at 450 to 525 deg C. Scouting corrosion tests were performed in the VPP's fluorinators using rod, sheet, or wire specimens of commercial and developmental alloys. These tests were subjected to serious limitations due to the lack of control over operating conditions and thus considerable variation in the corrosion of L nickel control specimens resulted. Those nickel-rich alloys containing iron and cobalt showed some superiority in corrosion resistsnce when com- pared with L nickel specimens. Nickel-rich alloys containing molybdenum additions showed variable behavior in the fluorination environment. Additional experimental nickelbase alloy corrosion specimens, containing magnesium,

• Research Organization: Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
• DOE Contract Number: W-7405-ENG-26
• NSA Number: NSA-15-021095
• OSTI ID: 4051734
• Report Number(s): ORNL-2832
• Resource Relation: Other Information: Orig. Receipt Date: 31-DEC-61
• Country of Publication: United States
• Language: English

Related Subjects

METALS, CERAMICS, AND OTHER MATERIALS
ALUMINUM ALLOYS
CHROMIUM ALLOYS
COBALT ALLOYS
CONTROL ELEMENTS
COPPER ALLOYS
CORROSION
FILMS
FLUORINATION
FUSED SALT FUEL
HIGH TEMPERATURE
IRON ALLOYS
MAGNESIUM ALLOYS
MANGANESE ALLOYS
MATERIALS TESTING
METALLOGRAPHY
MOLYBDENUM ALLOYS
NICKEL ALLOYS
NICKEL FLUORIDES
RODS
ROLLING
SAMPLING
SODIUM FLUORIDES
SULFUR
SURFACES
TENSILE PROPERTIES
ULTRASONICS
URANIUM TETRAFLUORIDE
VESSELS
WIRES
ZIRCONIUM FLUORIDES