The influence of electrolyte composition on electrochemical ferrate(VI) synthesis. Part II: anodic dissolution kinetics of a steel anode rich in silicon
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. Silicon-rich steel (SRS) was employed as the anode material to produce ferrate(VI), and the characteristics observed were compared with those of the pure iron anode obtained during our previous study. Using anolytes 14 M NaOH, 14 M KOH and mixtures thereof, the systems were studied by means of potentiodynamic methods, electrochemical impedance spectroscopy and batch electrolysis experiments. In addition, scanning electron microscopy and metallographic images of the material surface were taken to identify changes in the phase composition of the material, caused by anodic polarization in strongly alkaline solutions. The dissolution kinetics increases with increasing temperature and, at 60 °C, also with increasing K+ content in the anolyte. Compared to iron, SRS easily dissolves into ferrate(VI), even at 20 °C in pure NaOH, indicating the lower inferior protective properties of oxy-hydroxide surface layers. The current efficiency achieved was almost 55% under these conditions. In the other anolytes, a maximum current efficiency of ca. 40% was obtained at 60 °C. The authors conclude that, at 60 °C, the efficiency is lowered by intensified oxygen evolution. This causes intensive solution convection, disturbing the surface conditions supporting ferrate(VI) formation.
KeywordsFerrate(VI) Dissolution kinetics Silicon content Electrolyte composition Electrode composition
The authors gratefully acknowledge the financial support of this research by the Ministry of Education, Youth and Sports within projects Nos. ME890 and MSM6046137301. The authors would also like to express their thanks to Ing. Zuzana Cílová, Ph.D. for the SEM images and the analysis thereof.