Investigation of corrosion mechanisms between molten eutectic FliNaK salt and 316L stainless steel

Crevelt, Logan

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https://hdl.handle.net/2142/117847 Copy

Description

Title

Investigation of corrosion mechanisms between molten eutectic FliNaK salt and 316L stainless steel

Author(s)

Crevelt, Logan

Issue Date

2022-12-09

Director of Research (if dissertation) or Advisor (if thesis)

Heuser, Brent

Committee Member(s)

Stubbins, James

Department of Study

Nuclear, Plasma, & Rad Engr

Discipline

Nuclear, Plasma, Radiolgc Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Materials Science
• Nuclear Engineering
• Stainless Steel
• Molten Salt
• Corrosion

Abstract

When designing next generation nuclear reactors, material compatibility is a key component for safety and performance. One of these designs, the Molten Salt Reactor, can use molten eutectic LaF-NaF-KF (FLiNaK) salt as the heat transfer fluid and fuel carrier. Although a nickel-based alloy, Inconel, was originally used during the original Molten Salt Reactor Experiment, to become more economical and simplify material regulatory issues, using stainless steel as the primary containment material has gained interest. Much research has been done to identify the corrosive mechanisms within the stainless steel and FLiNaK system. In this work, the low carbon concentration stainless steel, 316L, was subjected to a static corrosion test of eutectic FLiNaK with varying carbon concentrations, salt purities, and atmospheric conditions for 72 hours at 600C. The 316L samples were analyzed to determine severity of corrosive attack by measuring mass loss. ICP-OES was performed on each salt sample to identify the corrosion products within the salt and on the containment crucible. Optical Microscopy was used to illustrate differences in corrosive attack between each run. Due to the experimental conditions, many different corrosion mechanisms occurred concurrently. The corrosive mechanisms caused by salt purity, atmospheric purity, and galvanic coupling were observed and discussed in depth. During the experiment, coupons of 316L stainless steel were contained in nickel crucibles and submerged in FLiNaK salt. To observe the effect metallic impurities have on corrosive intensity, two differently sourced salts were used. One set of samples used salt from Oak Ridge National Laboratory (ORNL) which contains impurity concentrations of calcium (180 ppm) and iron (25 ppm). The second set of samples used salt from Argonne National Laboratory (ANL) which contains larger impurity concentrations of iron (117 ppm) and nickel (179 ppm). Four samples of each salt were made with increasing graphite concentrations to observe the effect graphite has on mass loss. This experiment was repeated with different furnace atmospheric conditions to establish the consequences of water and oxygen impurities within a fluoride salt system. Run 1 had an atmosphere with greater atmospheric impurities than Run 2. This was done by installing a bubbler on the tube furnace used in the static corrosion experiment for Run 2. Results showed the mass loss in Run 1 being significantly greater than Run 2 across all samples, with less of a disparity between the different purity salts. The no-carbon ORNL sample for Run 1 and Run 2 had a mass loss per square centimeter of 6.05 mg/cm2 and 3.60 mg/cm2 respectively. Within just the Run 2 samples, the 316L samples exposed to the more impure, ANL salt had an appreciably higher mass loss. The no-carbon ORNL sample and the no-carbon ANL sample had a mass loss per square centimeter of 3.6 mg/cm2 and 5.45 mg/cm2 respectively. These results provide a clear picture of impurity effects on 316L corrosion with water and oxygen impurities causing intense corrosive attack of 316L and metallic impurities contributing more within a pure atmosphere. Although no definitive conclusions could be drawn about carbon concentrations affecting corrosive attack, being able to see these mechanisms occurring during the same experiment had yet to be done in previous work. Seeing how these mechanisms behave together provides an important look into how the treatment of FLiNaK salt in the preparation and operation of Molten Salt Reactors can increase stainless steel lifetimes within these systems.

Graduation Semester

2022-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Logan Crevelt

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