Anisotropic corrosion of Ti2AlC and Ti3AlC2 in supercritical water at 500 °C

Textured and untextured M(n+1)AX(n) compounds, Ti2AlC and Ti3AlC2, namely MAX phases have been synthesized and examined with respect to their corrosion resistance in static supercritical water at 500 degrees C. The textured ceramics were obtained by hot forging process at high temperatures. Both X-ray diffraction and SEM analysis revealed well alignment of c-plane of MAX phases parallel to the hot-forging surface. Better corrosion resistance on the surface perpendicular to the hot-forged direction was verified by SEM. On the other hand, the side surfaces of the samples showed thick oxidation layers and abundant cracks. The (001) faces consist of strongly bonded Ti3C2 and Ti2C layers in Ti3AlC2 and Ti2AlC, respectively, hence exhibit higher resistance to water corrosion. On the contrary, the side surfaces where most of weakly bonded interlayers of these hexagonal phases were exposed tend to be easily corroded especially through Al-layers. The corrosion process involved a phase transition of oxidized product, i.e. TiO2 from anatase to rutile phase, which gave rise to the formation of cracks due to accompanied volume changes.