Inhibiting effect of oxidized zirconium on parasitic cathodic reactions in the sodium chlorate process Part I: Hypochlorite reduction

To clarify the effect of oxidized zirconium on parasitic cathodic reactions in the chlorate process, electrochemical studies were carried out at laboratory scale. The techniques used were cyclic voltammetry and recording of polarization curves. In this paper the reduction of hypochlorite ions to chloride ions was studied. It is shown that oxidized zirconium cathodes reduces the rate of hypochlorite reduction, although not entirely inhibiting it, which is mainly related to a lowered active area due to the porous layer of zirconium dioxide. Further, it has also been shown that the oxidized samples are partly passivated, giving high overvoltages for the hydrogen evolution reaction. These overvoltages gradually decrease during cathodic polarization due to the simultaneous reduction of the zirconium oxide. Studies of the selectivity indicate that hypochlorite reduction occurs on the oxidised zirconium cathodes to a high extent, the thermal oxide being somewhat better. This further proves that zirconium oxide is not a suitable cathode material for the sodium chlorate process.