Effect of water immersion on alumina scale spallation for René N5+Y that had been oxidized at 1150 °C for 1000 1-hr cycles. The right side was hydrogen annealed at 1250 °C for 100 hr to remove sulfur and carbon before oxidation; the left side was not.
Alumina scales are widely used in high-temperature aerospace material systems for their slow-growing and adherent protective behavior on MCrAl-based alloys.1 Although their performance is optimized for alloys with reactive element dopants (Y, Zr, Hf)2 and/or extremely low (<1 ppmw)3 levels of sulfur impurity, additional secondary spallation phenomena have been occasionally observed. Tremendous thermal expansion mismatch stresses up to 4 GPa are developed immediately upon cooldown from oxidation at 1100 °C. But delayed spallation under ambient constant stress conditions is often observed. The same is true for the technologically important thermal barrier coating (TBC). Here delayed failure is often referred to as the “weekend effect,” “cold spallation,” or “desktop spallation.” These effects are believed to be caused by exposure to moisture, or moisture-induced delayed spallation (MIDS), see the preceding figure. However, the atomistics of the process have remained elusive.
Initial theories have related this phenomenon to moisture-assisted crack growth in bulk ceramics, including alumina (Al2O3). Here H2O reacts chemically with the oxide at the highly stressed region of the crack tip, causing only slowcrack growth under a static load, referred to as “corrosion fatigue.” Another intriguing possibility is analogous to moisture-induced hydrogen embrittlement of intermetallic compounds, most notably Ni3Al and FeAl. Indeed, earlier work using standard cathodic hydrogen charging had embrittled Ni3Al. Accordingly, researchers at the NASA Glenn Research Center used the same electrolytic recipe for cathodic charging a preoxidized René N5+Y superalloy for interfacial embrittlement. We found that, by monitoring current for every 0.1-V increment, a relatively intact scale could be cathodically removed at -2.0 V and at less than 1 mA (see the following figure).
Exposed bare metal surface after cathodic descaling of René N5. Montage of scanning electron microscope, backscattered electron, and secondary electron imaging micrographs showing oxide grain imprints in metal, residual alumina particles and plates, internal tantalum carbide (TaC), and external halfnium dioxide (HfO2) (polarized at -2.0 V and <1 mA in 1N sulfuric acid (H2SO4) for 1 hr, preoxidized at 1150 °C for 1000 cycles).
We conclude that cathodic hydrogen embrittlement of the scale-metal interface occurred. By analogy to the intermetallics case, we propose that moisture-induced delayed spallation arises from hydrogen embrittlement of the scale metal interface (see the final figure). Here reaction of water with aluminum from the alloy produces aluminum hydroxide and frees hydrogen to diffuse down the oxide-metal interface, encouraged by the biaxial tensile stress state under the scale. This interfacial weakening is consistent with fundamental studies that predict decreased Al2O3-Ni bond strengths due to hydrogen, with a negative synergy in the presence of sulfur.
MIDS of alumina scales due to interfacial hydrogen embrittlement. Moisture from ambient air reacts with aluminum in the alloy to form Al(OH)3 and H, which is then attracted by biaxial tension to diffuse into the alloy. After sufficient incubation time for hydrogen diffusion, a scale segment (and TBC) may detach under the large counterbalanced biaxial compressive stress; σCTE, thermal expansion mismatch stress; YSZ, yttria stabilized zirconia; τ, event duration.
Smialek, James L.: Moisture-Induced Delayed Spallation and Interfacial Hydrogen Embrittlement of Alumina Scales. JOM (NASA/TM--2005-214030), vol. 58, no. 1, 2006, pp. 29-35. http://gltrs.grc.nasa.gov/Citations.aspx?id=841
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