Anodic polarization curves on dimensionally stable anodes ͑DSAs͒ of RuO 2 /TiO 2 in chlorate electrolyte bend to a higher Tafel slope at the critical potential ͑E cr ͒ of approximately 1.2 V vs Ag/AgCl. Operating the chlorate process above E cr leads to increased oxygen evolution and higher potential losses. In this study the impact of different electrolyte parameters and electrolyte impurities on the risk of reaching/exceeding E cr was investigated. A dependency of Cl − concentration on E cr of about −90 mV/dec C Cl − was found at pH 2. An addition of Na 2 Cr 2 O 7 to chlorate electrolyte is necessary in order to keep a high current efficiency on the cathode but was found to increase the anode potential and thereby increase the risk of exceeding E cr at galvanostatic operation. Additions of impurities as 30 g/L Na 2 SO 4 or 100 ppm Si ͑added as SiO 2 ͒ resulted in increased anode potentials, but adding 1.4 g/L KH 2 PO 4 or 1 g/L HF did not have significant short-term impact on the potential. The anode potential as well as E cr decreased with increased temperature. A high temperature is beneficial in terms of the decreased anode potential, which outweighs the negative effect of a decrease in E cr .

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During electrolysis of a solution containing Y͑III͒ ions, a hydrous Y͑OH͒ 3 film forms in the alkaline layer close to a hydrogenevolving cathode. The film hinders the reduction of dissolved oxygen and activates the reduction of water, hydrogen evolution. The ability to hinder certain reactions while catalyzing hydrogen evolution may be useful in electrolysis applications. In this work the electrochemical properties of an in situ formed yttrium-hydroxide film were studied on an iron rotating disk electrode ͑RDE͒ in 0.5 M NaCl with addition of YCl 3 , NaClO, and of NaNO 3 . It was found that the film also hinders the reduction of protons, hypochlorite ions, and nitrate ions. At low concentration of Y͑III͒ or at high current density, when the hydrogen evolution was vigorous, no activation of hydrogen evolution was observed. Under these conditions the film still hindered the reduction of ions. The reactant in the catalyzed hydrogen evolution reaction is most likely water molecules within the hydrous film. Nitrate ions were easily reduced on an iron cathode when no Y͑III͒ ions were present in the solution. When studying effects of yttrium addition to a chloride solution the use of YCl 3 , rather than Y͑NO 3 ͒ 3 , as Y͑III͒ source is recommended.

Chromate is today added to industrial chlorate electrolyte, where it forms a thin cathode film of chromium hydroxide that hinders unwanted reduction of hypochlorite and chlorate. The aim of this study was to investigate rare earth metal (REM) ions as an environmentally friendly alternative to the toxic chromate addition. Potential sweeps and iR-corrected polarisation curves were recorded using rotating disc electrodes of iron and gold. Addition of Y(III), La(III) or Sm(III) to 5 M NaCl at 70°C suppressed hypochlorite reduction. Activation of hydrogen evolution by REM ion addition to 0.5 M NaCl was more significant at 25°C than at 50 and 70°C. Increasing the chloride concentration to 5 M had a detrimental effect on this activation. The major problem in replacing chromate with REM salts is the poor solubility of REM ions at normal chlorate process conditions, and therefore REM salts are not a realistic alternative to chromate addition.

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)