Reasons for the DSA Passivation during Chlorate Electrolysis and the Means for Extending the Anode Service Life

Abstract

Based on experiments and analysis of literature data, it is shown that a poor utilization of the active mass of the coating on metal oxide anodes of the DSA type during the electrochemical sodium chlorate production is due to pseudopassivation (an increase in the anodic potential due to a predominant corrosion dissolution of RuO2as compared with TiO2in the outer working zone of the coating), which develops with time. The true irreversible anode passivation, resulting from the formation of a stable barrier layer of higher titanium oxides at the coating/substrate interface, occurs when the DSA active mass wears out to a residual ruthenium content below 1.4 g/m2in it. The coating service life can be extended with no increase in the potential and with a minimum active mass consumption by using, concurrently with RuO2and TiO2, catalytically active additives with higher corrosion resistance than that of RuO2at pH 6 to 7. This prevents the anode pseudopassivation and, with an increased initial amount of the active mass, makes it possible to use the anodes until its residual content exceeds the limit below which the true passivation starts, and regenerate the coating by applying fresh mass on the residue of the existing mass.