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

Abstract

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.

An Electrochemical Route for Hot Alkaline Blackening of Steel: A Nitrite Free Approach

“…According to the Pourbaix diagram of iron [17,24] and the well-known electrochemical behavior of iron in alkaline solution [11,[25][26][27][28][29][30], the anodic peak A1 at −0.08 V vs. Pt for steel in 23 M NaOH (Figure 2c,d) was identified as the oxidation of Fe to Fe(II) [28][29][30]. This is in agreement with the reaction that is given in Equation (1).…”

“…This is in agreement with the reaction that is given in Equation (1). The peak A2 at around 0.1 V vs. Pt in blackening salt solution and in the range of 0.06 to 0.16 V vs. Pt in sodium hydroxide corresponds to the oxidation of iron hydroxide to magnetite that is in correspondence to the Schikorr reaction (Equation 3) [11,[27][28][29]. Magnetite is finally oxidized to iron(III) oxide at peak A3 [11,27].…”

“…FOR PEERREVIEW 5 of C2 and C3 are observed for nitrite-free solutions(Figure 2c,d). They correspond to the related oxidation peaks A2 and A3[29]. Measurements that were obtained with blackening salt solution(Figure 2a,b)only exhibit the anodic peaks A2, A3, and the hydrogen evolution regime.…”

The History and the Current Revival of the Oxo Chemistry of Iron in its Highest Oxidation States: FeVI – FeVIII

“…Various anode materials have been probed, including pure iron, silicon‐rich steel, or white cast iron (“silver steel”, rich in carbide). Current efficiencies did not exceed a 60 % threshold …”

Degradation of Lignosulfonate to Vanillic Acid Using Ferrate

“…The maximum ferrate concentration after 2 h of 0.99 ± 0.07 g L −1 was achieved in 40 wt% (14 m ) NaOH, followed by 0.76 ± 0.08 g L −1 in 50 wt% NaOH (19 m ) with current efficiencies of 13.3 ± 0.1% and 10.8 ± 0.1%, respectively (Figure 2b). Considering that the electrochemical formation of ferrate in alkaline media from sacrificial anodes consumes vast amounts of OH − , [ 67 ] one would expect the highest ferrate concentration in 50 wt% NaOH with decreasing performance in lower base concentrations. However, this is not the case with 50 wt% NaOH being almost twice as efficient as 30 wt% but about 20% less efficient than 40 wt% NaOH (Figure 2b).…”