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

In this work, Yb 4 Hf 3 O 12 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 Hf 4+ and steady-state corrosion after the formation of Al 5 Yb 3 O 12 . In the initial stage, Hf 4+ and the surrounding O 2- tend to be hydrated and then volatilized in the form of Hf(OH) 4 , causing a metastable state of Yb 4 Hf 3 O 12 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 Hf 4+ , more vacancies promote bonding and diffusion of Al 3+ to form corrosion product (Al 5 Yb 3 O 12 ), and during this stage, regrowth and shedding of the corrosion layer reach a dynamic balance.