Optimization of Heat Treatment Cycles in Sub-atmospheric LiF-NaF-KF Based Fluoride Ion Cleaning for Removing Oxide Layers in Cracks of IN738-LC

Document Type : Original Research Article


1 Department of Ceramics, Materials and Energy Research Center (MERC), Meshkindasht, Alborz, Iran

2 Department of Semiconductors, Materials and Energy Research Center (MERC), Meshkin Dasht, Alborz, Iran

3 Department of Materials Engineering, Bu-Ali Sina University, Hamedan, Hamedan, Iran


An improved Fluoride Ion Cleaning (FIC) process required for removing all oxide layers by a molten mixture of alkaline fluoride salts under sub-atmospheric pressure was developed and applied to oxide layers on the cracks formed on the surface of Inconel 738-LC samples. This method is directly characterized by filling up the cracks with a molten mixture of alkaline fluoride salts (LiF-NaF-KF) overheated under sub-atmospheric pressure and subsequently, by injecting hot hydrogen gas into the process chamber. The effect of cleaning time on the microstructure of the finished surface was studied in time durations up to 120 min in intervals of 30 min using cross-sectional micrographs and elemental distribution maps. In accordance with the amount of mass loss and microstructural studies during the cleaning process, the optimum cleaning time was suggested to be 90-120 min. Perquisite microstructural outcome shows that in the suggested cleaning condition, all oxide scales in the cracks would be removed without any extra damage to the gamma prime depleted layer, which is a necessary layer for preventing sample oxidation before repair. In this regard, subsequent brazing operations need an oxide-free surface.


Main Subjects

1.     Tao, P., Li, H., Huang, B., Hu, Q., Gong, S., Xu, Q., "The crystal growth, intercellular spacing and microsegregation of selective laser melted Inconel 718 superalloy", Vacuum, Vol. 159, (2019), 382-390. https://doi.org/10.1016/j.vacuum.2018.10.074
2.     Naderi, M., Farvizi, M., Shirvani, K., Rahimipour, M. R., "Cyclic oxidation behavior of uncoated and aluminum-rich nickel aluminide coated Rene-80 superalloy", Advanced Ceramics Progress, Vol. 4, No. 3-4, (2018), 1-7. https://doi.org/ 10.30501/acp.2018.92925
3.     Xiao, J., Prud’Homme, N., Li, N., Ji, V., "Influence of humidity on high temperature oxidation of Inconel 600 alloy: Oxide layers and residual stress study", Applied Surface Science, Vol. 284, (2013), 446-452. https://doi.org/10.1016/j.apsusc.2013.07.117
4.     Osoba, L. O., Oladoye, A. M., Ogbonna, V. E., "Corrosion evaluation of superalloys Haynes 282 and Inconel 718 in Hydrochloric acid", Journal of Alloys and Compounds, Vol. 804, (2019), 376-384. https://doi.org/10.1016/j.jallcom.2019.06.196
5.     Seal, S., Kuiry, S. C., Bracho, L. A., "Studies on the surface chemistry of oxide films formed on IN-738LC superalloy at elevated temperatures in dry air", Oxidation of Metals, Vol. 56, No. 5, (2001), 583-603. https://doi.org/10.1023/a:1012569803467
6.     Stankowski, A., "Advanced thermochemical cleaning procedures for structural braze repair techniques", In Turbo Expo: Power for Land, Sea, and Air, Vol. 36088, (2002), 1181-1195. https://doi.org/10.1115/gt2002-30535
7.     Tarancon III, G., Midwest Inorganics LLC, "Method for the Preparation of Anhydrous Hydrogen Halides, Inorganic Substances and/or inorganic Hydrides by Using as Reactants Inorganic Halides and reducing Agents", U.S. Patent 8,834,830, (2014). https://patents.google.com/patent/US8834830B2/en
8.     Wang, H., Liu, S., Li, B., Zhao, Z., "Characterization and removal of oxygen ions in LiF-NaF-KF melt by electrochemical methods", Journal of Fluorine Chemistry, Vol. 175, (2015), 28-31. https://doi.org/10.1016/j.jfluchem.2015.01.018
9.     Kim, M. T., Chang, S. Y., Oh, O. Y., Won, J. B., "Fluoride ion cleaning of gas turbine components using PTFE grease", Surface and Coatings Technology, Vol. 200, No. 24, (2006), 6740-6748. https://doi.org/10.1016/j.surfcoat.2005.10.012
10.   Miglietti, W., Blum, F., "Advantages of fluoride ion cleaning at sub-atmospheric pressure", Engineering Failure Analysis, Vol, 5, No. 2, (1998), 149-169. https://doi.org/10.1016/s1350-6307(98)00013-2
11.   Sangeeta, D., General Electric Co, "Method for Cleaning Cracks and Surfaces of Airfoils", U.S. Patent 5,685,917, (1997). https://patents.google.com/patent/US5685917
12.   Yin, H., Zhang, P., An, X., Cheng, J., Li, X., Wu, S., Wu, X., Liu, W., Xie, L., "Thermodynamic modeling of LiF-NaF-KF-CrF3 system", Journal of Fluorine Chemistry, Vol. 209, (2018), 6-13. https://doi.org/10.1016/j.jfluchem.2018.02.005
13.   Nicolaus, M., Möhwald, K., Maier, H. J., "Regeneration of high pressure turbine blades. Development of a hybrid brazing and aluminizing process by means of thermal spraying", Procedia CIRP, Vol. 59, (2017), 72-76. https://doi.org/10.1016/j.procir.2016.09.041
14.   Hamidi, S., Rahimipour, M. R., Eshraghi, M. J., Hadavi, S. M. M., Esfahani, H., "Kinetics and Microstructural Investigation of High-Temperature Oxidation of IN-738LC Super Alloy", Journal of Materials Engineering and Performance, Vol. 26, No. 2, (2017), 563-570. https://doi.org/10.1007/s11665-016-2487-4
15.   Wendt, H., Reuhl, K., Schwarz, V., "Cathodic deposition of refractory intermetallic compounds from flinak-melts—I. Voltammetric investigation of Ti, Zr, B, TiB2 and ZrB2", Electrochimica Acta, Vol. 37, No. 2, (1992), 237-244. https://doi.org/10.1016/0013-4686(92)85009-a
16.   Ouyang, F. Y., Chang, C. H., You, B. C., Yeh, T. K., Kai, J. J., "Effect of moisture on corrosion of Ni-based alloys in molten alkali fluoride FLiNaK salt environments", Journal of Nuclear Materials, Vol. 437, No. 1-3, (2013), 201-207. https://doi.org/10.1016/j.jnucmat.2013.02.021
17.   Esmaeili, H., Mirsalehi, S. E., Farzadi, A., "Vacuum TLP bonding of Inconel 617 superalloy using Ni-Cr-Si-Fe-B filler metal: metallurgical structure and mechanical properties", Vacuum, Vol. 152, (2018), 305-311. https://doi.org/10.1016/j.vacuum.2018.03.048
18.   Singh, A. R. P., Nag, S., Hwang, J. Y., Viswanathan, G. B., Tiley, J., Srinivasan, R., Fraser, H. L., Banerjee, R., "Influence of cooling rate on the development of multiple generations of γ′ precipitates in a commercial nickel base superalloy", Materials Characterization, Vol. 62, No. 9, (2018), 878-886. https://doi.org/10.1016/j.matchar.2011.06.002
19.   Shahbazi, M., Tayebifard, S. A., Razavi, M., "Effect of Ni content on the reaction behaviors and microstructure of TiB2-TiC/Ni cermets synthesized by MASHS", Advanced Ceramics Progress, Vol. 2, No. 2, (2016), 22-26. https://doi.org/10.30501/acp.2016.70020
20.   Zheng, L., Zhang, M., Dong, J., "Oxidation behavior and mechanism of powder metallurgy Rene95 nickel based superalloy between 800 and 1000 C", Applied Surface Science, Vol. 256, No. 24, (2010), 7510-7515. https://doi.org/10.1016/j.apsusc.2010.05.098
21.   Doolabi, D. S., Rahimipour, M. R., Alizadeh, M., Pouladi, S., Hadavi, S. M. M., Vaezi, M. R., "Effect of high vacuum heat treatment on microstructure and cyclic oxidation resistance of HVOF-CoNiCrAlY coatings", Vacuum, Vol. 135, (2017), 22-33. https://doi.org/10.1016/j.vacuum.2016.10.014
22.   Janz, G. J., Tomkins, R. P. T., "Physical Properties Data Compilations Relevant to Energy Storage. IV. Molten Salts: Data on Additional Single and Multi-Component Salt Systems", National Standard Reference Data System, National Bureau of Standards Report NSRDS-NBS 61 Part IV, U.S. Government Printing Office, Washington D.C., (1981). https://nvlpubs.nist.gov/nistpubs/Legacy/NSRDS/nbsnsrds61p4.pdf
23.   Ye, X. X., Ai, H., Guo, Z., Huang, H., Jiang, L., Wang, J., Li, Z., Zhou, X., "The high-temperature corrosion of Hastelloy N alloy (UNS N10003) in molten fluoride salts analysed by STXM, XAS, XRD, SEM, EPMA, TEM/EDS", Corrosion Science, Vol. 106, (2016), 249-259. https://doi.org/10.1016/j.corsci.2016.02.010
24.   Mantkowski, T. E., General Electric Co, "Fluoride Ion Cleaning Method", U.S. Patent 8,206,488, (2012). https://patents.google.com/patent/US8206488B2/en
25.   Fritscher, K., "Life and FCT failure of yttria-and ceria-stabilized EBPVD TBC systems on Ni-base substrates", Oxidation of Metals, Vol. 91, No. 1, (2019), 131-157. https://doi.org/10.1007/s11085-018-9870-5
26.   Williams, D. F., "Assessment of Candidate Molten Salt Coolants for the NGNP/NHI Heat-Transfer Loop", (No. ORNL/TM-2006/69). Oak Ridge National Lab.(ORNL), Oak Ridge, TN (United States), (2006). https://doi.org/10.2172/1360677
27.   Acrivos, J., "Physical chemistry, Third Edition (Levine, Ira N.)", Journal of Chemical Education, Vol. 65, No. 12, (1988), A335. https://doi.org/10.1021/ed065pA335.3
28.   Fukada, S., Morisaki, A., "Hydrogen permeability through a mixed molten salt of LiF, NaF and KF (Flinak) as a heat-transfer fluid", Journal of Nuclear Materials, Vol. 358, No. 2-3, (2006), 235-242. https://doi.org/10.1016/j.jnucmat.2006.07.011
29.   Cruchley, S., Evans, H. E., Taylor, M. P., Hardy, M. C., Stekovic, S., "Chromia layer growth on a Ni-based superalloy: Sub-parabolic kinetics and the role of titanium", Corrosion Science, Vol. 75, (2013), 58-66. https://doi.org/10.1016/j.corsci.2013.05.016
30.   Kool, L. B., Ritter, A. M., Cretegny, L., Pezzutti, M. D., Beitz, S. W., General Electric Co, "Method for Removing Oxide from Cracks in Turbine Components", U.S. Patent 7,125,457, (2006). https://patents.google.com/patent/US7125457B2/en