The degradation mechanism of MmNi(3.55)Co(0.75)Mn(0.4)Al(0.3) hydrogen storage alloy in nickel-metal hydride (Ni-MH) sealed batteries and effects of corrosion on cell performance during cycling are investigated. A general equation for the corrosion of AB, alloy is derived from analyses of corrosion products in cycled electrodes and measurements of negative discharge reserve. Water consumption due to alloy corrosion explains the increase of impedance during cycling while internal pressure evolution is directly linked to the loss of charge reserve. The parameters controlling the corrosion rate of AB, alloys are also examined. It is shown that the pulverization mode of the alloy particles rather than the surface reactivity to electrolyte explains the differences in cycle-life performance of alloys of various compositions.