In 2006 Sato et al. [1] discovered the existence of a ternary γ′-Co₃ (Al, W) intermetallic phase with L1₂ structure in the system Co-Al-W, a structure similar to that of Ni-base superalloys. The high melting point of Co-alloys makes this class of materials a promising alternative for high-temperature applications. Nevertheless, their poor corrosion resistance remains a challenging and still open problem.

In the environmental scanning electron microscope the start of oxidation / corrosion processes and the progress of scale formation can be continuously monitored at high magnification. This enables a rather easy determination of the temperature where oxidation starts. Corrosion in hot steam was realized by use of water vapor as reaction gas. Besides the usage of different other gases, such as air, the temperature ramp can be varied as well. With respect to the diffusion velocities of the different elements in the alloy as a function of temperature the latter could be a critical factor. Nevertheless, the relative humidity / oxygen activity that can be used during in situ experiments cannot exceed certain limits, since otherwise the signal/noise ratio strongly decreases, which causes likewise a deterioration of the image quality.

For the investigations Co-based alloys with a nominal composition of Co-9Al-9W were used. To get a stress-free surface, mechanical polishing with diamond paste was followed by polishing with colloidal silica. Finally OIM (orientation imaging microscopy) maps were recorded to get information about the crystal orientation of the grains. The high-temperature oxidation experiments were performed by use of a heating stage mounted in the specimen chamber of an environmental scanning electron microscope ESEM Quanta 600 FEG (FEI, Eindhoven, The Netherlands). The experiments were carried out at a pressure of 133 Pa, which corresponds to a relative humidity of approximately 5% at room temperature (24 °C). Temperature ramps of 2 °C/min and 20 °C/min were used; the maximum temperature was around 800 °C.

 All results reveal that at the start of scale formation lattice diffusion and not grain boundary diffusion dominates. Fig. 1 shows that oxidation starts with the evolution of nodules scattered across the grains. Both, the onset temperatures of nodule growth, as well as the density of nodules per unit area are dependent on grain orientation. Fig. 1 also demonstrates that some of the grain boundaries are completely free of such corrosion structures (see arrow in the image), whereas others are nicely decorated by them. However, a clear correlation between scale formation and grain orientation or grain boundary structure could not be found. In case of rough surfaces an orientation dependence could no longer be observed, roughness dominated the oxidation behavior. It became also apparent, that oxidation started earlier at the slower temperature ramp.

To effectively slow down oxidation, the formation of a dense, protective oxide layer like Al₂O₃ would be necessary. The formation of such a dense looking alumina layer could be found occasionally, but only at individual grains [2]. Also the parameters governing the formation of such layers remain still unknown.

References

[1] Sato, J., Omori, T., Oikawa, K., Ohnuma, I., Kainuma, R., Ishida, K. (2006), Cobalt-base high-temperature alloys, Science 312, 90-91

[2] Weiser, M., Reichmann, A., Albu, M., Virtanen, S., Poelt P. (2015), In situ investigation of the oxidation of Cobalt-base superalloys in the environmental scanning electron microscope, Adv. Eng. Mat. 17 (8), 1158-1167

Figures:

Fig. 1: Co-based alloy; SE images of a polished sample recorded during the heating process at the temperatures given below the images (image width: 130 µm). The temperature ramp was 20 °C/min up to 400 °C, followed by 2°C/min at higher temperatures. Time specifications refer to the time elapsed since the start of the heating process. The arrow marks a grain boundary free of corrosion products.

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