Gas Phase Hydrogen Permeation in Alpha Titanium and Carbon Steels

Abstract

Commercially pure titanium rods were machined into hollow cylinders and gas-phase permeation and diffusion measurements obtained with a quadrupole residual gas analyzer at temperatures between 400-800°C and pressures between 0.533 and 266.6 Nrn-2. Pressure dependence measurements established that phase boundary reactions as well as lattice diffusion control the transport process. Lag-time data yield excellent agreement with the diffusivity data of Wasilewski and Kehl and Papazoglou and Hepworth for the case where phase boundary reactions are taken into account. The best value of the diffusivity is D = 2.1 x 10-2 exp (-50,40O/RT). Analysis of lag-time data reveals that unsteady state transport tends toward phase boundary reaction control for thin membranes (low b/a ratio) and diffusion control for thick membranes (large b/a ratio). Permeation activation energy, Qp, is 74,300 Joules mole-1. Due to first power pressure dependency, P0 values are a complex function of pressure and geometry and are presented in tabular form. Oxide and nitride films on the interior membrane surface retard permeation whereas the same films on the exterior surface enhance the permeation somewhat. This is explained in terms of a discontinuous TiH2 layer