Corrosion of Nickel-Based Superalloys in Molten Chloroaluminates
Authors: Polovov Ilya B. , Abramov Aleksandr V. , Karpov Vyacheslav V. , Gibadullina Alfiya F. , Zhilyakov Arkadiy Yu. , Belikov Sergey V. , Volkovich Vladimir A. , Rebrin Oleg I. 
Published: February 2020

Molten alkali chloroaluminates can be used as working media for molten salt nuclear fast reactors (MSNFR). One of the most important and difficult to solve problems that hinder practical realization of MSNFR is finding suitable durable construction materials. These can be selected from the existing ones or, in some cases, developing entirely new materials might be required.

In the present work the corrosion behavior of a number of nickel alloys was studied at 450–650 oC in fused KCl–AlCl3 mixtures with the initial AlCl3-to-KCl ratio of 1.1. The alloys were selected from those manufactured by Haynes International, Inc., VDM Metals, and Special Metals Co. and included high-temperature alloys Haynes 230, Hastelloy types S and X, VDM Alloy 600 (previously known as Nicrofer 7216); corrosion-resistant alloys Hastelloy types N, B-3, G-35, VDM Alloy C-4 (Nicrofer 6616), VDM Alloy 825 (Nicrofer 6020); and corrosion and heat resistant alloys Inconel 600 and 625. Samples of the alloys were kept in the melt for 6 to 1000 h. Alloys were tested in «as received» state, as well as after various typical technological operations (bending, welding, heat treatment, etc.).

The results of the tests showed that high-temperature alloys could not be used in contact with molten electrolytes at relatively high temperatures due to intergranular corrosion. The corrosion rates of the corrosion resistant nickel-based alloys were determined by red-ox reactions resulting in dissolution of the most electronegative alloy, i.e.Cr, Fe, and Mn. The corrosion of nickel-based superalloys in molten chloroaluminates has therefore electrochemical nature.

Increasing temperature led to a noticeable increase of determined by red-ox reactions resulting in dissolution of the most electronegative alloy and also changed the corrosion processes nature. Transmission electron microscopy showed that a prolonged high-temperature exposure could result in the formation of intermetallic phases, i.e.sigma-phase in Hastelloy G-35 or Ni2(Cr,Mo) secondary phase in VDM Alloy C-4. Such phenomena can accelerate intergranular corrosion and stress corrosion cracking of the materials studied under industrial conditions. The results obtained agree well with the thermodynamics analysis, mechanical and thermophysical properties of the alloys, and "time-temperature-precipitation" diagrams constructed. 

Excessive sigma phases were also formed in the alloys’ surface layer along the grain boundaries after prolonged exposure at 650 ºC. Formation of the secondary phases in the surface layer was caused by selective chromium leaching and degradation of the nickel-based solid solution.

Tests performed on welded and bent samples showed that their corrosion rates were higher than could be explained by changes in the structure of the alloy caused by high temperatures (in case of welds) and increased number of defects. Heat treatment of the alloys after welding and/or bending increased their corrosion resistance.

The studies performed showed that physical and mechanical properties of the alloys studied were influenced by changes of their structure and composition. The results obtained here allowed determining maximum working temperatures and exposure time of the alloys thus providing the information on the limitation of their application in MSNFR technologies.