High-temperature low-activity vs. low-temperature high-activity vapor aluminized coatings on DS200+Hf superalloy: Microstructure, diffusion treatment, and hot corrosion performance

In this study, aluminide diffusion coatings were deposited on a directionally solidified DS200+Hf nickel-based superalloy (equivalent to Mar-M200+Hf) using the Out-of-Pack vapor aluminizing (VPA) method under two distinct regimes: High-Temperature Low-Activity (HTLA) and Low-Temperature High-Activity (LTHA). A post-coating diffusion treatment was subsequently performed in an argon atmosphere at 900 °C for 20 min followed by 1080 °C for 4 h. Cyclic hot corrosion resistance was evaluated using a Na2SO4–V2O5 salt mixture (3:1 wt ratio) at 900 °C for 20 h. Microstructural analysis revealed that HTLA coatings developed a two-zone structure consisting of an outer β-NiAl layer (thickness: 101–310 μm, containing 27.38 wt% Al) and an interdiffusion zone (IDZ) with a thickness of 23–61 μm. Refractory elements (W, Hf, Nb) accumulated within the IDZ, effectively limiting excessive coating growth. The diffusion treatment increased the overall coating thickness by 31% and completely transformed the brittle δ-Ni2Al3 phase into β-NiAl. The HTLA-coated specimens (1100 °C, 6 h) exhibited a corrosion rate of 0.085 mg cm−2 h−1/2, which corresponds to a 65% reduction compared to the uncoated substrate (0.245 mg cm−2 h−1/2). The LTHA coating (970 °C, 10 h) formed a diffusion layer of 110 μm and showed intermediate corrosion performance. The superior protective performance of the HTLA coating is attributed to the formation of a continuous and adherent α-Al2O3 scale, with hafnium playing a critical role in enhancing scale adhesion and preventing sulfur-induced degradation.