Introduction 1) Molten salt technology has been widely applied in the industrial world because of its physical and chemical characteristics, especially its high electrical conductivity, high processing rate, fluid features, etc. And recently, it has attracted much attention in the fields of jet engines, fuel cells, catalysts, solar energy, and a metal refinement. Therefore, studies on the corrosion of the equipment and the structure materials for handling high temperature molten salts have also been continuously carried out. For example, the corrosion of a gas turbine by Na 2 SO 4 depos- it [1-3] and the corrosion related to a molten carbonate fuel cell [4,5] have been extensively investigated. Also there are many reports on an accelerated oxidation in chloride molten salts [6-10]. However, they are mainly short time electrochemical experiments and few reports are found on long term experiments at a high temperature for the corrosion of commercial alloys in a chloride mol- ten salt by considering their corrosion products, rate and behaviors. Because of a chloride molten salt without oxygen and a low solubility of oxygen in a molten salt [6], the corrosion rate measured by the electrochemical † To whom all correspondence should be addressed. (e-mail: [email protected].) methods in a LiCl-KCl mixed molten salt was propor- tional to the concentration of the oxidative impurities, NO 3 - . Also a corrosion was not observed at a very low impurity level in a molten salt. It has also been reported that an oxidation reaction was a primary corrosion mech- anism in a chloride molten salt [7,8]. The electrochemical reduction process for a spent oxide nuclear fuel is carried out in a LiCl-Li 2 O molten salt at 650 o C. The liberation of oxygen on an anode and the high temperature molten salts of the process cause a chemically aggressive environment that is too corrosive for ordinary structural materials. Therefore, for an im- plementation of the electrochemical reduction technol- ogy, corrosion resistance materials should be developed. However, few reports are found on the corrosion resist- ance of the structural materials for handling a hot molten salt. Guided by their excellent mechanical property and corrosion resistance at a high temperature atmosphere, we selected Haynes 263, Inconel 718, Nimonic 80 A, and Incoloy 800 H as the candidate materials and inves- tigated their corrosion behaviors under simulated electro- chemical reduction conditions. Experimental The chemical compositions of the superalloys used are Soo-Haeng Cho † , Chung-Seok Seo, Ji-Sup Yoon, Hyun-Soo Park, and Seong-Won Park Korea Atomic Energy Research Institute, 305-353 Daejeon, Korea Received December 6, 2006; Accepted July 16, 2007 Abstract: The electrolytic reduction of a spent oxide fuel involves a liberation of the oxygen in a molten LiCl electrolyte, which is a chemically aggressive environment that is too corrosive for typical structural materials. So, it is essential to choose the optimum material for the process equipment for handling a molten salt. In this study, corrosion behaviors of Haynes 263, Inconel 718, Nimonic 80 A and Incoloy 800 H in a molten LiCl-Li 2 O salt under an oxidizing atmosphere were investigated at 650 o C for 72 216 h. Haynes 263 alloy showed the ∼ highest corrosion resistance among the examined alloys. Corrosion products of Haynes 263 were Li(Ni,Co)O 2 and Li((Cr,Al)TiO 4 ), and those of Inconel 718 were Cr 2 O 3 , NiFe 2 O 4 , and CrNbO 4 , while Cr 2 O 3 , LiFeO 2 , (Cr,Ti) 2 O 3 , and Li 2 Ni 8 O 10 were identified as the corrosion products of Nimonic 80 A. Incoloy 800 H showed Cr 2 O 3 and FeCr 2 O 4 as its corrosion products. Haynes 263 showed a localized corrosion behavior while Inconel 718, Nimonic 80 A and Incoloy 800 H showed a uniform corrosion behavior. Keywords: Hot corrosion, molten salt, electrolytic reduction, structural material
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Introduction1)
Molten salt technology has been widely applied in theindustrial world because of its physical and chemicalcharacteristics, especially its high electrical conductivity,high processing rate, fluid features, etc. And recently, ithas attracted much attention in the fields of jet engines,fuel cells, catalysts, solar energy, and a metal refinement.Therefore, studies on the corrosion of the equipment andthe structure materials for handling high temperaturemolten salts have also been continuously carried out. Forexample, the corrosion of a gas turbine by Na2SO4 depos-it [1-3] and the corrosion related to a molten carbonatefuel cell [4,5] have been extensively investigated. Alsothere are many reports on an accelerated oxidation inchloride molten salts [6-10]. However, they are mainlyshort time electrochemical experiments and few reportsare found on long term experiments at a high temperaturefor the corrosion of commercial alloys in a chloride mol-ten salt by considering their corrosion products, rate andbehaviors. Because of a chloride molten salt withoutoxygen and a low solubility of oxygen in a molten salt[6], the corrosion rate measured by the electrochemical
methods in a LiCl-KCl mixed molten salt was propor-tional to the concentration of the oxidative impurities,NO3
-. Also a corrosion was not observed at a very low
impurity level in a molten salt. It has also been reportedthat an oxidation reaction was a primary corrosion mech-anism in a chloride molten salt [7,8].The electrochemical reduction process for a spent oxidenuclear fuel is carried out in a LiCl-Li2O molten salt at650
oC. The liberation of oxygen on an anode and the
high temperature molten salts of the process cause achemically aggressive environment that is too corrosivefor ordinary structural materials. Therefore, for an im-plementation of the electrochemical reduction technol-ogy, corrosion resistance materials should be developed.However, few reports are found on the corrosion resist-ance of the structural materials for handling a hot moltensalt. Guided by their excellent mechanical property andcorrosion resistance at a high temperature atmosphere,we selected Haynes 263, Inconel 718, Nimonic 80 A, andIncoloy 800 H as the candidate materials and inves-tigated their corrosion behaviors under simulated electro-chemical reduction conditions.
Experimental
The chemical compositions of the superalloys used are
Soo-Haeng Cho†, Chung-Seok Seo, Ji-Sup Yoon, Hyun-Soo Park, and Seong-Won Park
Korea Atomic Energy Research Institute, 305-353 Daejeon, Korea
Received December 6, 2006; Accepted July 16, 2007
Abstract:The electrolytic reduction of a spent oxide fuel involves a liberation of the oxygen in a molten LiCl
electrolyte, which is a chemically aggressive environment that is too corrosive for typical structural materials.
So, it is essential to choose the optimum material for the process equipment for handling a molten salt. In this
study, corrosion behaviors of Haynes 263, Inconel 718, Nimonic 80 A and Incoloy 800 H in a molten LiCl-Li2O
salt under an oxidizing atmosphere were investigated at 650oC for 72 216 h. Haynes 263 alloy showed the∼
highest corrosion resistance among the examined alloys. Corrosion products of Haynes 263 were Li(Ni,Co)O2
and Li((Cr,Al)TiO4), and those of Inconel 718 were Cr2O3, NiFe2O4, and CrNbO4, while Cr2O3, LiFeO2,
(Cr,Ti)2O3, and Li2Ni8O10 were identified as the corrosion products of Nimonic 80 A. Incoloy 800 H showed
Cr2O3 and FeCr2O4 as its corrosion products. Haynes 263 showed a localized corrosion behavior while Inconel
718, Nimonic 80 A and Incoloy 800 H showed a uniform corrosion behavior.
Keywords: Hot corrosion, molten salt, electrolytic reduction, structural material
Soo-Haeng Cho, Chung-Seok Seo, Ji-Sup Yoon, Hyun-Soo Park, and Seong-Won Park730
Table 1. The Chemical Compositions of the Tested Alloys (wt%)