Corrosion behaviour of Zr1−xTixV0.6Ni1.2M0.2 (M=Ni, Cr, Mn) AB2-type metal hydride alloys in alkaline solution

An examination is made of the discharge and cycle life of Zr0.5Ti0.5V0.6Ni1.4 alloys when a fraction (0.2 at.%) of the Ni-component is substituted by Cr or Mn. In addition, the Zr:Ti component ratios are varied to extend the cycle life of high capacity, Mn-substituted Zr1−xTixV0.6Ni1.2Mn0.2 (x=0.0, 0.25, 0.5, 0.75) alloys. The metallurgical microstructure is observed by X-ray diffraction analysis, scanning electron microscopy, and energy dispersive X-ray analysis. Active–passive potentiodynamic behaviour, as well as charge–discharge cycle characteristics, is evaluated, and dissolved V-species in the electrolytic solution is analyzed by inductively coupled plasma spectroscopy. The corrosion behaviour of the V–Cr or the V–Mn phase in the alkaline electrolyte solution is found to determine the cycle life of an AB2 alloy. Cr-substituted (Zr0.5Ti0.5Ni1.2Cr0.2) alloy, containing a V–Cr phase, is estimated to involve a dissolution rate of 0.028 wt.% vanadium per cycle in an alkaline electrolytic solution, while Mn-substituted (Zr0.5Ti0.5V0.6Ni1.2Mn0.2) alloy, containing a V–Mn phase, is estimated to have a dissolution rate of 0.138 wt.% vanadium per cycle. For Mn-substituted alloys, an optimum Zr:Ti ratio of 3:1, i.e., Zr0.75Ti0.25V0.6Ni1.2Mn0.2, is found to have the most stable cycle life. The improvement in cycle life caused by increasing the Zr content in the alloy is attributed to increase in the corrosion resistance of the alloy due to less formation of the corrosive V–Mn phase.