Reduced strength degradation of reactive air brazed Ba0.5Sr0.5Co0.8Fe0.2O3-δ membranes by pre-oxidation of metallic components

Abstract

Oxygen transport membranes can be joined leak-tight and high-temperature resistant to metal components by reactive air brazing. To overcome the typical strength degradation of these braze joints during aging at the application temperature of 850 °C, the diffusion-controlled growth of mixed oxide layers at the interface between metal and braze alloy has to be reduced. In this study, we investigated how a pre-oxidation of the chromia forming austenitic steel X15CrNiSi25-21 and alumina forming ferritic steel X8CrAl25-5 influences the bending strength of BSCF-Ag3CuO-steel joints. Brazed double joint specimens were aged for 1000 h at 850 °C in air prior to 4-point bending and subsequent macroscopic as well microscopic analyses.Pre-oxidation of steel X15CrNiSi25-21 reduced the strength degradation during aging from 75 to 59% and effectively suppressed chromium poisoning of BSCF. The minimum fracture stress was raised from 0 to 24 MPa which significantly increased the reliability. For the first time, it was possible to obtain mixed and ceramic fracture types in addition to poor-strength delamination fractures. The strength of pre-oxidized X8CrAl25-5 joints was affected by axial cracks and pores at the mixed oxide layer and lower compared to joints with pre-oxidized X15CrNiSi25-21. In contrast to pre-oxidized X15CrNiSi25-21, no acicular phases were identified in the BSCF. Nevertheless, the apparently dense Al2O3 layer could not avoid chromium diffusion, as Ba–Cr oxides were detected in the braze.