Corrosion of Nickel-Based Superalloys in Molten Chloroaluminates

Abstract

The corrosion behavior of the corrosion-resistant alloy Hastelloy G-35 (manufactured by Haynes International, Inc.), corrosion and heat resistant alloy VDM Alloy 600 or Nicrofer 7216 and corrosion-resistant alloys VDM Alloy C-4 or Nicrofer 6616 and VDM Alloy 625 or Nicrofer 6020 (all produced by VDM Metals) was studied at 450-650 °C in fused KCl-AlCl3 mixture with the initial AlCl3-to-KCl ratio of 1.1. Time of exposure varied from 6 to over 1000 h. The corrosion rates of all the nickel-based alloys studied were determined by the red-ox processes resulting in dissolving the most electronegative alloy components (Cr, Fe and Mn) indicating that the processes taking place had electrochemical nature. Increasing temperature led to a noticeable increase of corrosion rates and a change of the corrosion process nature. Transmission electron microscopy revealed that intermetallic phases (such as sigma-phase in case of Hastelloy G-35 and Alloy 625 or Ni2(Cr,Mo) secondary phase in VDM Alloy C-4) can be formed during prolonged high-temperature exposure. These phenomena can accelerate the processes of intergranular corrosion and stress corrosion cracking of studied materials in industrial conditions. The results obtained agreed well with thermodynamic analysis, mechanical and thermophysical properties of the alloys and constructed "time-temperature-precipitation" diagrams.

Corrosion of Metallic Materials in 3LiCl-2KCl and 3LiCl-2KCl-UCl3

“…It was shown that both types of alloys undergo intergranular corrosion below the critical conditions. Later we also found that VDM® Alloy C-4 and Hastelloy® G-35® had the highest phase stability and can be used in molten chloroaluminates under 650 °C (7). VDM® Alloy C-4 in as-received state demonstrated high resistance against intergranular corrosion in 3LiCl-2KCl below 650 °C (8).…”

“…When temperature increased to 650 ºC and 750 ºC, the corrosion rates of all alloys increased rapidly (Table II). Our experience shows that the temperature of 650 ºC constitutes the limit for the studied nickel-based alloys (7). Above this temperature, the corrosion rate increases significantly, and intergranular corrosion is also activated due to the rapid growth of secondary phases of intermetallides along the grain boundaries of the alloys.…”

“…4). Appearance of the secondary phases along the grain boundaries in the bulk of the nickel alloy samples was due to the thermal effect, which caused the decomposition of the metastable structure of the γ-solid solution based on nickel (7). Formation of these phases in the surface layer of the samples was also due to a change in the chemical composition of the surface layer of the alloy as a result of continuous corrosion.…”

Corrosive Resistance of Nickel Hastelloy G-35 Superalloy in Various Aggressive Media

“…The rate of corrosion of the alloy samples in AlCl3-KCl melt was high because of the increased oxidizing ability of aluminum ions in the salt mixture (15,16). In the air atmosphere, the rate of corrosion was determined by the presence of oxygen, which diffused through the melt to the alloy samples and caused their high-temperature oxidation.…”

Structure and Corrosion Resistance Against Intergranular Corrosion of Nickel-based Superalloy VDM ® Alloy C-4

“…Previously we demonstrated that VDM ®Alloy C-4 is one of the prospective materials for molten salt application at high temperatures [4][5][6]. However, this alloy has a The current work was focused on nickel-based superalloy VDM ® Alloy C-4 (UNS 06455) structure and its susceptibility to intergranular corrosion (IGC).…”