Multiphase Nb–TiCo alloys: The significant impact of surface corrosion on the structural stability and hydrogen permeation behaviour.

Multiphase Nb x Ti (100-x)/2 Co (100-x)/2 (x = 30–60) alloys are a promising material for hydrogen separating membranes. These alloy membranes exhibit a rapid decline in hydrogen permeation flux within ∼12 h when operated at 773 K. To address this issue, a dense oxide (e.g. Nb 2 O 5 , TiO 2 and CoO) layer was prepared between a Pd coating layer and an Nb–TiCo substrate by surface corrosion for improving their thermal stability, and the corrosion resistance of Nb–TiCo alloys was investigated. An increase in the Nb content (x) lowers the corrosion resistance of these alloys, but makes it easier to form the above oxide layer. Substantial enhancement of hydrogen permeability and thermal stability at 773 K was observed for the alloys (x = 30 and 40) after corrosion, which can be ascribed to an increase in hydrogen diffusivity. This improved permeability and stability are closely related to the formation of the above surface oxide layer that impeded interdiffusion between the Pd film and Nb–TiCo substrates. This study demonstrates that insertion of a diffusion barrier between the Pd and Nb-based substrates by surface corrosion is a viable approach to enhance the high-temperature stability of Pd-coated Nb–TiCo alloys, an aspect not widely explored in Nb-based hydrogen separation and purification membranes. • Dense intermediate oxide layer was prepared between Pd coating and the substrate. • Durability/permeability can be optimized by placing an inter-layer. • Nb 30 Ti 35 Co 35 with an oxide layer exhibits an excellent durability of ∼1053 min. • A high Φ value of 4.56 × 10−8 mol H 2 m−1s−1Pa−0.5 is derived for Nb 30 Ti 35 Co 35 at 773 K. • The variation of Φ is due to the change in D rather than K. [ABSTRACT FROM AUTHOR]