Formation Of α-Al2O3 Scale Research Articles

Microstructure evolution and degradation mechanisms of amorphous FeCrAlMoSiY coated Zr in 1200 °C steam

Herein, structural evolution and degradation mechanisms of the amorphous FeCrAlMoSiY coating during oxidation up to 16 h in 1200 °C steam was studied, using scanning electron microscopy, transmission electron microscopy, x-ray diffractometer, and Raman spectroscopy. The results revealed that no significant oxidation of the Zr substrate occurred within 12 h, which was attributed to the retarded atomic diffusion processes. These processes were achieved through the Al outward diffusion and formation of α-Al2O3 scale, along with the formation of an in-situ Zr2Si-rich diffusion barrier at the coating-substrate interface and the Y2O3 particles segregated at grain boundaries within the coating. Degradation of the coating was primarily due to the destruction of the α-Al2O3 scale by cracks caused by internal oxidation and voids accumulation. These results have implications for understanding the degradation mechanisms and achieving long-term stable protection of the coatings for Zr-based nuclear fuel claddings.

Effect of Zr on Initial Oxidation Behavior of FeCrAl Alloys

The initial oxidation behavior of Fe–24Cr–10Al (in at%) alloys with and without 0.006Zr or 0.06Zr during heating followed by isothermal oxidation for up to 30 min at 1000 °C was investigated by in situ high-temperature X-ray diffraction using synchrotron radiation and by STEM–EDS analysis. No apparent effect of Zr on the oxidation behavior, including the oxidation mass gain and scale microstructure, was observed during heating to approximately 1000 °C. The effect of Zr addition on the microstructure of oxide scale was first observed after formation of α-Al2O3 scale and inward growth of the inner Al2O3 layer. The effect of Zr was found to appear after Zr incorporation into the inner Al2O3 scale during its inward growth.

Electrochemical Corrosion of HVOF-Sprayed NiCoCrAlY Coatings in CO2-Saturated Brine

The effect of pre-oxidation treatment and surface preparation of optimized NiCoCrAlY coatings deposited by high-velocity oxygen fuel (HVOF) spraying and exposed to a low-temperature corrosive environment is reported herein. Coatings with two surface finish conditions (as-sprayed and ground) were heat treated under two different oxygen partial pressures (air and argon). The electrochemical corrosion behavior was evaluated in CO2-saturated brine via potentiodynamic polarization, polarization resistance, and electrochemical impedance measurements. The results show that the grinding process and pre-oxidation treatment in argon enhanced growth and formation of α-Al2O3 scale. The potentiodynamic polarization results show that both pre-oxidation and surface treatment had a positive influence on the corrosion resistance of the coating. The reduction of the porosity and the formation of a dense, uniform, and adherent oxide scale through pre-oxidation treatment led to an increase of the corrosion resistance due to a decrease in active sites and blocking of diffusion of reactive species into the coating. However, according to the results, complete transformation from metastable alumina phases to α-Al2O3 in addition to formation and growth of dense α-Al2O3 is required to ensure full protection of the coating and base material over long periods.

Oxidation Behavior of an Al-Modified Silicide Coating on an Nb-Silicide-Based Ultrahigh-Temperature Alloy

Abstract The oxidation behavior of an Al-modified silicide coating prepared through a siliconization and then an aluminization pack cementation process on an Nb-silicide-based ultrahigh-temperature alloy was investigated in air at 1,250°C. The coating consisted of a 50-μm-thick (Nb,X)Si2 (X represents Ti, Cr, and Hf elements) outer layer with (Nb,Ti)3Si5Al2 distributed discontinuously in its superficial part and a 90-μm-thick (Nb,Ti)Al3 inner layer containing Si-rich precipitates. The oxidation of this coating was controlled primarily by the preferential oxidation of Al with the formation of α-Al2O3 scale. The (Nb,Ti)Al3 inner layer could act as an Al reservoir for forming scale and maintaining the existence of (Nb,Ti)3Si5Al2 phase in the zone just beneath the scale. The parabolic oxidation rate constant of this coating was about 1.72 μm2/h, lower than that of the bare alloy by about 4 orders of magnitude.