Effect of yttrium on the corrosion of AB 5-type alloys for nickel–metal hydride batteries

Abstract

The influence of Y 2O 3 powder addition (as well as of rare earth oxides of Gd, Ho, Dy, Nd, Sm or Yb) to the metal hydride electrode on AB 5 alloy corrosion rates and on the chemical composition, morphology and density of the corrosion products was studied by X-ray, SEM, TEM and HRTEM. Adding Y 2O 3 reduces the corrosion rate. This is particularly observed in the long-term behavior of the batteries and is more pronounced at higher temperature. When yttrium is added as an oxide to the electrode or electrolyte, it is dissolved in the electrolyte and is further incorporated as hydroxide in the corrosion scale. The decrease in the global alloy corrosion rate can be explained by a decrease of the driving force for diffusion of Mm 3+ and OH −. Yb is the rare earth element which is expected to produce the largest decrease in corrosion rate, since its solubility in the electrolyte and its content in the corrosion scale are higher than for any other rare earth element.