Investigation of Failure Mechanisms in Ti Containing Brazing Alloys Used in SOFC/SOEC Environments

The two braze alloys TiCuNi® and Silver-ABA® were tested as possible candidates for seals in SOFC or SOEC stacks. The different Ti amount in the samples allowed an evaluation of Ti as a matrix filler and as an active metal/wetting agent in brazing alloys. The sealing ability towards ferritic steel and yttria-stabilized-zirconia (YSZ) was investigated. After the sealing process steel/braze/YSZ joints were further annealed in oxidizing (Air) or reducing (9% H₂ in Ar) atmospheres at 750 °C for 150 h. The boundaries of the braze and the joining partners for as-sealed and annealed samples were examined with SEM/EDS. XRD and EXAFS were used to characterize reaction products after the annealing process. Based on these results and taking before reported data into consideration, the advantages and disadvantages of Ti containing seals and the role of Ti in failure mechanism are discussed. Even if the use of Ti in active brazing alloys has its benefits, the disadvantages caused by Ti and its reaction products strongly suggest the use of Ti free brazes in SOFC/SOEC applications.