To better understand the decrease in storage capacity of AB(5)-type alloy?, in rechargeable Ni/MH batteries undergoing repeated charge/discharge cycles, the corrosion of a MmNi(3.55)C(0.75)Mn(0.4)Al(0.3) alloy in aqueous KOH electrolyte was studied. The crystal structure, chemical composition, and distribution of corrosion products were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Hollow and filled needles of a mixed rare earth hydroxide Mm(OH)(3) were found to cover a continuous nanocrystalline corrosion scale composed of metal (Ni,Co) solid solution, oxide (Ni,Co)O solid solution and rare earth hydroxide, and a Mm-depleted alloy subscale. Corrosion kinetics were measured for three different temperatures. Growth kinetics of the continuous corrosion scale and of the Mm(OH)(3) needles obeyed linear and parabolic rate laws, respectively. Models for the corrosion mechanism were developed on the basis of diffusional transport of Aim and OI-I through the hydroxide needles and subsequent diffusion along grain boundaries through the nanocrystalline scale.