Aluminizing and boroaluminizing treatments of Mar-M247 and their effect on hot corrosion resistance in Na2SO4−NaCl molten salt

Abstract

The effect of surface modifications of Mar-M247 superalloy on hot corrosion resistance was examined in Na 2 SO 4 -NaCl molten salt. The Mar-M247 was aluminized and boroaluminized by pack cementation in Ar and underwent a cyclic hot corrosion test in Na 2 SO 4 -NaCl molten salt. The XRD results showed that a Ni 2 Al 3 phase was formed between the aluminized layer and the substrate when the surface modification temperature was below 1273 K. However, a NiAl phase formed when the temperature was above 1273 K. The intensity of the XRD peak in the NiAl phase increased after post heat treatment. Hot corrosion resistance increased for the specimens containing NiAl rather than Ni 2 Al 3 phase. The ductile NiAl phase suppressed the potential for crack initiation during thermal cycling. Post heat treatment increased the corrosion resistance of the aluminized layer for Mar-M247, which underwent surface modification at 1273 K and above. In the boroaluminized Mar-M247 specimens, corrosion resistance decreased as a result of the blocking of outward diffusion of Cr by boron and decreased cohesion between the oxide scale and the aluminized layer during thermal cycling.

A Comparative Study on Characterization and High-Temperature Wear Behaviors of Thermochemical Coatings Applied to Cobalt-Based Haynes 25 Superalloys

“…Additionally, the boro-aluminized coating is considerably thinner compared to the aluminized and boronized samples, indicating limited solubility between aluminum and boron, which adversely affects coating growth kinetics. Only one study on boroaluminizing superalloys reported NiAl, Ni3Al, and a small amount of CrB phases as the phase structure in Ni-based Mar-M247 superalloys [26]. Therefore, the presence of both boride and aluminide phases in the boro-aluminized sample is agreeable with the findings in the literature.…”

“…The highest hardness values are found in the boride layer due to the presence of harder phases like Cr-B and W-B in the Co-B and Co 2 B phases [19][20][21]. Additionally, numerous studies have reported that phases containing aluminum exhibit lower hardness values compared to boride phases [18,24,26].…”