Corrosion resistance inversion of Ni-Mo-Cr and Ni-W-Cr alloys under isothermal oxidation and cyclic NaCl-deposit hot corrosion

The degradation behaviors of Ni–Mo–Cr GH3535 (7.01 wt.% Cr) and Ni–W–Cr GH3539 (5.70 wt.% Cr) were compared at 700 °C under isothermal oxidation and cyclic NaCl-deposit hot corrosion. The alloys exhibited a performance inversion between the two conditions. During isothermal oxidation, GH3535 showed a lower oxidation rate and a thinner affected region than GH3539. The higher oxidation rate of GH3539 was associated with CrWO 4 formation, which may compete with continued NiCr 2 O 4 formation, along with a W 6+ -related increase in nickel-vacancy concentration that could accelerate outward cation transport. During cyclic hot corrosion, both alloys underwent severe degradation; however, GH3539 showed a lower cumulative material loss under the wash–recoat protocol: the average thickness reductions after 100 h were approximately 42% for GH3535 and 25% for GH3539. Thermodynamic calculations indicate that possible Mo-bearing chlorides and oxychlorides have substantially higher equilibrium vapor pressures than the corresponding W-bearing species. These volatile Mo-bearing intermediates could hinder the retention of a coherent scale on GH3535, while the lower volatility of W-bearing intermediates is consistent with the more adherent scale on GH3539. Nevertheless, GH3539 developed a subsurface Cr- and W-depleted zone. These results demonstrate the environment-dependent roles of Mo and W in low-Cr Ni-based alloys.