Abstract
Single crystal superalloys were screened in Type II molten (Na,K)-sulfate hot corrosion re-coat tests in air +300 ppm SO2 at 700 °C. They exhibited large 20–40 mg/cm2 weight changes, repeated spallation, and non-protective, 25–50 μm thick corrosion layers after 300 h of testing. Scale cross sections revealed dual outer Ni(Co)O and inner Al(Cr)S-rich corrosion layers. This chemical differentiation was partially consistent with previous models of oxide fluxing, alloy sulfidation, NiO micro-channel diffusion, and synergistic dissolution mechanisms. Broad shallow pits or uniform attack morphologies were consistent with prior studies performed in high >100 ppm pSO2 environments. Higher Mo experimental alloys trended toward more degradation, producing 100 μm thick scales with distinct Al(Cr)S-rich inner layers or 500 μm thick NiO. The aggressive behavior in these environments supports the need for LTHC-resistant coatings for single crystal superalloys.
Highlights
Low temperature hot corrosion (LTHC) is a recurring concern for Ni-base superalloy components, especially for advanced disk applications envisioned up to 815 ◦ C (1500 ◦ F)
It can be expected that Ni(Co)SO4 -Na2 SO4 eutectic salts may be formed as low as 660 ◦ C (585 ◦ C) with the potential for aggressive corrosion as well as strength-limiting corrosion pits, where low cycle fatigue debits up to 92% have been reported due to 760 ◦ C LTHC [1]
The use of a corrosion resistant Cr2 AlC MAX phase compound has been explored for this application [2,3,4]
Summary
Low temperature hot corrosion (LTHC) is a recurring concern for Ni-base superalloy components, especially for advanced disk applications envisioned up to 815 ◦ C (1500 ◦ F). It can be expected that Ni(Co)SO4 -Na2 SO4 eutectic salts may be formed as low as 660 ◦ C (585 ◦ C) with the potential for aggressive corrosion as well as strength-limiting corrosion pits, where low cycle fatigue debits up to 92% have been reported due to 760 ◦ C LTHC [1]. The use of a corrosion resistant Cr2 AlC MAX phase compound has been explored for this application [2,3,4]. In a recent LTHC screening test, Cr2 AlC MAX phase samples and the NASA LSHR (low solvus high refractory) disk alloy were exposed to repeated salt coatings and exposed to 700 ◦ C air with 300 ppm SO2 , using 25 h cycles [4]. Elemental rasters were obtained for polished cross sections of samples exposed for 300 h
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