The interaction of Zr2Fe surface with O2 and H2O at the temperature range 300-770 K

The surface of metallic Zr2Fe was exposed to oxygen and water vapor, in order to study the effects of these gases, being the main degradation agents in its operation as a getter for hydrogen. It was found that both gases oxidize the Zr component of the alloy surface, leaving the Fe metallic and strongly diluted. The oxidation is temperature and pressure dependent and under oxygen pressure of up to 5 x 10-6 Torr the oxide thickness increases with temperature up to ∼470 K. At higher temperatures, above 570 K, the oxide film growth becomes predominated by decomposition and diffusion of oxygen into the substrate and higher oxygen pressure is needed to compensate the oxygen dissolution. It was found that for oxygen pressure of 1 x 10-5 Torr and 620 K, which is the operating temperature of the getter, the oxidation is most effective, since both the adsorption and inward diffusion are efficient. The oxide film thickness, at these conditions, is about the XPS probing depth (∼5 nm). For 770 K, the dissolution and inward diffusion are very fast and the adsorption not efficient enough, leaving a dissolved and depleted oxygen layer at the surface. The initial sticking coefficients per oxygen atom, at RT, are similar for oxygen and water vapor, but the oxide thickness, obtained for H2O, is more than twice of that for O2, which points to a different oxidation mechanism. The presence of oxygen vacancies or incorporation of hydroxyl groups in the oxide layer may be the factors enabling further inward growth.