Corrosion behavior of ytterbium hafnate exposed to water-vapor with Al(OH)3 impurities

In this work, Yb4Hf3O12 ceramic, a typical rare-earth hafnate, was exposed to water vapor with Al(OH)3 impurities at 1400 ℃, and microstructural evolution and corrosion mechanisms were investigated. The long-term corrosion can be divided into initial hydration and gasification of Hf4+ and steady-state corrosion after the formation of Al5Yb3O12. In the initial stage, Hf4+ and the surrounding O2- tend to be hydrated and then volatilized in the form of Hf(OH)4, causing a metastable state of Yb4Hf3O12 on the surface. Besides, the preferential corrosion of small grains, grain boundary, and specific crystal plane on the surface and sintered and densified grains result in varied corrosion behavior from the early linear law to the later parabolic law. In the steady-state corrosion stage, due to the consumption of Hf4+, more vacancies promote bonding and diffusion of Al3+ to form corrosion product (Al5Yb3O12), and during this stage, regrowth and shedding of the corrosion layer reach a dynamic balance.