SURFACE ACTIVATION AND HYDROGENATION KINETICS OF TI SPONGE

The influence of vacuum thermal treatments on the hydrogenation kineti cs of titanium sponge has been studied. The samples were annealed at 4 73 K in a rough vacuum (10(-1) Pa) for different time periods in order to clean and activate their surfaces. After this step they were hydro genated at 623 K, 773 K and 1073 K. It has been found that the sample pretreatment has no influence on the Ti hydrogenation kinetics at 773 K and 1073 K where the process is controlled by hydrogen diffusion in the bulk Ti. On the other hand, samples hydrogenated at 623 K present an incubation time (no change in the chamber hydrogen pressure) that i s followed by a linear pressure drop, corresponding to a surface contr olled process, and a parabolic stage (hydrogen diffusion in the bulk). The incubation time at 623 K has been correlated to the activation ti me at 473 K. This correlation has been explained by considering two me chanisms operating during the cleaning stage: one in which the occlude d gases are released from the Ti cavities leaving fresh titanium surfa ces, and another one in which the fresh surfaces are oxidized. Copyrig ht (C) 1996 International Association for Hydrogen Energy