Impact of surface oxide on hydrogen permeability of chromium membranes
Vincenc Nemanič, Janez Kovač, Marko Žumer, Janez Zavašnik
Abstract
Hydrogen permeation through pure and oxidised bulk chromium membranes was measured by the classical gas technique to get insight into oxide as a hydrogen permeation barrier (HPB). An additional palladium-coated reference chromium membrane was tested to avoid the influence of native Cr oxide. Key parameters for Cr permeability: P0 = 3.23 × 10−7 mol H2/s/m/Pa0.5 and Ea = 0.68 eV and Cr diffusivity D0 = 9.0 × 10−5 m2/s and Ea = 0.59 eV. In the sample preparation stage, a thin ∼2 nm thick oxide was formed. Additional oxidation in pure oxygen at 400 °C increased the thickness from 20 to 50 nm. At this temperature, its efficiency as HPB was evaluated by comparing permeation rates to the reference chromium membrane. The highest permeation reduction factor of ∼3900 corresponded to only a ∼28 nm thick Cr oxide layer. Surface morphology and oxide thickness were investigated by SEM, while the thickness and type of chromium oxide by XPS.