Moisture-Induced Delayed Alumina Scale Spallation on a Ni(Pt)Al Coating

Delayed interfacial scale failure takes place after cooling for samples of a Ni(Pt)Al-coated CMSX4 single crystal superalloy, cycled at 1150 °C for up to 2000 h. One sample exhibited premature coating grain boundary wrinkling, alumina scale spallation to bare metal, and a final weight loss of 3.3 mg/cm2. Spallation under ambient conditions was monitored with time after cooldown and was found to continue for 24 h. This produced up to 0.05 mg/cm2 additional loss for each hold, accumulating 0.7 mg/cm2 (20% of the total) over the course of the test. After test termination, water immersion produced an additional 0.15 mg/cm2 loss (a duplicate sample produced much less wrinkling and time dependent spalling, maintaining a net weight gain). The results are consistent with the general phenomena of moisture-induced delayed spallation (MIDS) of mature, distressed alumina scales formed on oxidation resistant M-Al alloys. Relative ambient humidity is discussed as the factor controlling adsorbed moisture, reaction with the substrate, and hydrogen effects on interface strength.