Effects of electrolyte parameters on the iron/steel cathode potential in the chlorate process

Abstract

This study focuses on how different electrolyte parameters of the chlorate process affect the cathode potential for hydrogen evolution on iron in a wide currentdensity range. The varied parameters were pH, temperature, mass transport conditions and the ionic concentrations of chloride, chlorate, chromate and hypochlorite. At lower current densities, where cathodic protection of the electrode material is important, the pH buffering capacity of the electrolyte influenced the potential to a large extent. It could be concluded that none of the electrolyte parameters had any major effects (\50 mV) on the chlorate-cathode potential at industrially relevant current densities (around 3 kA m -2 ). Certainly, there is more voltage to gain from changing the cathode material than from modifying the electrolyte composition. This is exemplified by experiments on steel corroded from operation in a chlorate plant, which exhibits significantly higher activity for hydrogen evolution than polished steel or iron.Keywords Chlorate cathode Á Chlorate process Á Hydrogen evolution Á Iron Á Steel List of symbolsCathode potential vs reference electrode (Ag/AgCl) (V) F Faraday constant (As mol -1 ) j k Current density for reaction k (A m -2 ) k 0 2 Coefficient in the Tafel expression of Eq. 2, which is given in Eq. 15 (mol m -2 s -1 ) k 14 Coefficient in the Tafel expression of Eq. 14, which is given in equation 16 (m s -1 ) N i Molar flux of species i (mol m -2 s -1 ) R Universal gas constant (J mol -1 K -1 ) R i Homogeneous production rate of species i (mol m -3 s -1 ) T Temperature (K) u z Convective velocity perpendicular to the electrode surface, i.e. in the direction of the z-axis (m s -1 ) zAxial coordinate (m) aTransfer coefficient d D Diffusion layer (m) d R Reaction layer (m)

Current efficiency of individual electrodes in the sodium chlorate process: a pilot plant study

“…The remaining potential dependence can be explained by effects at the cathode and are in the range of what has been reported earlier, 1.6-1.9 mV°C -1 [10].…”

“…The influence of different parameters, such as the addition of chromate, changing the temperature, the electrolyte concentration, the pH and the presence of additives, on the performance of the chlorate process has been reported [7][8][9][10][11]. The current efficiency has been studied in some publications, but the most common approach is to study the cell or electrode potential and oxygen production [9,10,[12][13][14][15][16][17].…”

“…The current efficiency has been studied in some publications, but the most common approach is to study the cell or electrode potential and oxygen production [9,10,[12][13][14][15][16][17]. It has also been reported that the time it takes to reach stable conditions in a chlorate cell or at the surface of a chlorate cathode is significantly long [8].…”

Energy loss in electrochemical diaphragm process of chlorine and alkali industry – A collateral effect of the undesirable generation of chlorate

“…Therefore, in the presence of chlorate ion, the HER must occur through the other possible process, a mechanism involving two electron transfer steps (the Volmer-Heyrovsky sequence) that is only operative when the electrode is at more cathodic potentials than À1.3 V. Tilak et al [17] extensively studied the activity of the cathode for the chlorate reduction that occurs in competition with H 2 generation, which lowers the total current efficiency. An equivalent result was obtained by Nylé n and Cornell [18] for the polarization curves in the presence of chlorate in the À0.8 to À1.2 V vs. Ag/AgCl region.…”

Understanding Selectivity in the Chlorate Process: A Step towards Efficient Hydrogen Production

“…This is also supported by results obtained for polished titanium and steel substrates, where it was shown that μM levels of chromate efficiently suppress hypochlorite reduction . However, in the commercial chlorate process, concentrations between 2–8 gL −1[48–49] are required to obtain 90–95% cathodic current efficiency . In a recent pilot study, it was shown that the cathodic current efficiency increased with increasing chromate concentration up to 5 gL −1 and then levelled off .…”