Thermal-history-dependent cyclic oxidation of Ge-modified CVD aluminide coatings on Hastelloy X

Ge-modified CVD aluminide coatings on Hastelloy X were evaluated under a two-stage cyclic oxidation sequence of 100 h at 1050°C followed by 100 h at 1200°C. Two Ge precursor-forming routes, 600°C for 4 h and 900°C for 2 h, were applied before CVD aluminizing at 1020°C for 3 h. One specimen was tested for each coating condition; therefore, the mass-change curves are reported as controlled exploratory single-specimen responses rather than statistically averaged oxidation kinetics. The conventional aluminide coating gained mass at 1050°C but changed to net specimen-mass loss during the 1200°C stage. Within this dataset, Ge900-Al exhibited the smallest final net specimen-mass change (+0.028 mg cm⁻²), a value that should be interpreted as a small net change close to the normalized balance resolution rather than as statistically established zero-degradation behavior. By contrast, Ge600-Al underwent severe mass loss after the transition to 1200°C and reached −16.37 mg cm⁻². XRD, SEM/BSE, line-scan, and EDS results indicate that the oxidation response was associated with the Ge-derived precursor state and the resulting aluminide coating architecture, rather than with Ge addition alone.