Effect of cathodic polarization on oxide layers formed on zirconium alloys

J. O. ZERBINO; A. VISINTIN

Tesalónica, Grecia

Congreso; 55rd ISE Annual Meeting; 2004

ISE

Zirconium metal alloys are extensively used in metal hydride batteries due to their high hydrogen absorption capacity. The electrodes are prepared with the sieved alloy powder cold pressed. Two processes rule the hydrogen storage rate: a) the charge transfer reaction on the metal/ electrolyte interface and b) the hydrogen diffusion in the metal with hydride formation. Moreover, the growth of a thin but dense and passive oxide film on the alloy surface plays a critical role in the electrode activation process applied previously to the battery charge [1-3]. The passive behaviour of Zr0.36 Ni0.64 alloy electrodes, mechanically polished, has been studied in aqueous 1 M KOH as a function of different cathodic and anodic polarization programmes using “in situ” voltammetry, ellipsometry and microscopy techniques. Comparative optical studies were made with pure zirconium in the 0.4 V ≤ E ≤ 1.2 V and the –1.0 V ≤ E ≤ 0.4 V regions, anodic and cathodic to the open circuit potential. In the case of Zr0.36 Ni0.64 the oxide layer formed at anodic potentials related to the rest potential, Eoc, shows optical indices n= 2.25 and k 0. On the other side during the sweeps applied in the activation process, in a region cathodic to Eoc, different passive layers grow with optical indices n2.45 and k 0.14 indicating the formation of more conducting and protonate oxide layers related to that obtained at more anodic potentials [4-5]. Microscopic observation shows an increase in the inter-granular crevice after cathodic sweep cycling has been applied. This indicates fragmentation of the grains and oxide growth during the activation process.