(A) kinetic study on hydride formation in hafnium

Abstract

The topochemical form of the reaction between hafnium and hydrogen gas was investigated by metallographic examinations of partially hydrided specimens and the kinetics of hafnium hydride formation was studied by measurement of overall reaction rates in the temperature range of 168-$344\,^\circ\!C$ under 600torr H$_2$ gas pressure. The H$_2$-Hf reaction resulted in the formation of a continuous protective hydride layer that progresses into the hafnium matrix and appeared to follow a paralinear behavior under these conditions. The activation energy was calculated from the dependence on temperature of the evaluated intrinsic kinetic parameter when the paralinear rate equations were fitted to the reaction kinetic curves. The reaction rate appears to be controlled by the diffusion of hydrogen through the protective thickening hydride layer that finally reached a constant thickness as a result of cracking of the outer surface of the hydride layer. The constant hydride layer thickness appeared to be dependent of temperature. The behavior of hydrogen during the evolution of hafnium hydrides was also studied by the thermal analysis technique using a gas chromatograph. Three hydrogen evolution peaks of hafnium hydrides appeared at the heating rate of $2\,^\circ\!C/\min$. It can be inferred that three thermally activated processes take part in the evolution reaction of hafnium hydrides.