The influence of electrolyte composition on electrochemical ferrate(VI) synthesis. Part III: anodic dissolution kinetics of a white cast iron anode rich in iron carbide

The anolyte composition and process temperature could improve the kinetics of iron anode dissolution and subsequent ferrate(VI) production significantly. This also holds for the anode composition. Following pure iron and silicon-rich steel (SRS), white cast iron (WCI) was the last representative of anode material tested that is typically used to produce ferrate(VI). Using anolytes 14 M NaOH, 14 M KOH, and mixtures thereof, the systems were studied by potentiodynamic methods, electrochemical impedance spectroscopy, and batch electrolysis experiments. Additionally, metallographic analysis of the material was performed. The dissolution kinetics increases with increasing temperature and also, at 60 °C, with increasing K⁺ content in the anolyte, but less progressively than in the case of SRS. Similar to SRS, WCI also easily dissolves into ferrate(VI) even at 20 °C in pure NaOH, thus indicating the inferior protective properties of oxo-hydroxide surface layers. In general, a maximum current efficiency of approx. 60 % was obtained at 60 °C in pure KOH solution. The authors conclude that, at 60 °C, the high efficiency of the synthesis is caused by the low protective properties of the oxo-hydroxide surface layer caused by the preferential dissolution of cementite and at the same time by the precipitation of the potassium salt of the product in the electrolyte immediately after its formation. This minimizes the effect of its decomposition.