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<p>Iron impurities are among the most detrimental impurities in magnesium electrolysis because they can promote cathode passivation, reduce current efficiency, and contribute to Mg loss. This study systematically investigated the electrochemical reduction of Fe(Ⅲ) and the corresponding cathode passivation mechanism in an anhydrous carnallite melt composed of MgCl₂ and KCl with a molar ratio of 1:1 at 973&#xa0;K. Based on cyclic voltammetry (CV), square wave voltammetry (SWV), chronoamperometry (CA), chronopotentiometry (CP), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS), the results show that Fe(Ⅲ) reduction is a single-step, diffusion-controlled, quasi-reversible process. Current reversal chronopotentiometry (CR-CP) measurements based on representative curves suggested a decrease in apparent current efficiency from 92.5% to 79.1% after 10&#xa0;min of electrolysis. Morphological and electrochemical analyses further suggest the underlying passivation mechanism. During the initial stage of electrolysis, Fe(Ⅲ) is preferentially reduced and deposited on the cathode surface as a porous, sponge-like structure. The newly formed porous Fe deposits may adsorb MgO-containing species from the molten salt, suggesting the possible presence of an Fe/Mg/O-rich passivation layer on the cathode surface. Such a surface layer may reduce the wettability of the cathode toward liquid Mg and hinder the spreading and coalescence of Mg droplets, thereby promoting the deposition of Mg as fine, discrete, caviar-like particles with diameters below 1&#xa0;mm. This process may contribute to Mg loss and the decrease in current efficiency during electrolysis. These findings provide fundamental insight into the role of Fe(Ⅲ) in cathode passivation during magnesium electrolysis.</p>

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Electrochemical behavior of Fe(Ⅲ) and mechanism of cathode passivation induced by Fe(Ⅲ) in MgCl₂-KCl molten salt

  • 期刊论文

  • 发表日期：

    2026年6月19日

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  • Zhuangzhuang Liu,
  • Xiaobo Yang,
  • Shengting Li,
  • Guimin Lu,
  • Ze Sun

摘要

Iron impurities are among the most detrimental impurities in magnesium electrolysis because they can promote cathode passivation, reduce current efficiency, and contribute to Mg loss. This study systematically investigated the electrochemical reduction of Fe(Ⅲ) and the corresponding cathode passivation mechanism in an anhydrous carnallite melt composed of MgCl₂ and KCl with a molar ratio of 1:1 at 973 K. Based on cyclic voltammetry (CV), square wave voltammetry (SWV), chronoamperometry (CA), chronopotentiometry (CP), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS), the results show that Fe(Ⅲ) reduction is a single-step, diffusion-controlled, quasi-reversible process. Current reversal chronopotentiometry (CR-CP) measurements based on representative curves suggested a decrease in apparent current efficiency from 92.5% to 79.1% after 10 min of electrolysis. Morphological and electrochemical analyses further suggest the underlying passivation mechanism. During the initial stage of electrolysis, Fe(Ⅲ) is preferentially reduced and deposited on the cathode surface as a porous, sponge-like structure. The newly formed porous Fe deposits may adsorb MgO-containing species from the molten salt, suggesting the possible presence of an Fe/Mg/O-rich passivation layer on the cathode surface. Such a surface layer may reduce the wettability of the cathode toward liquid Mg and hinder the spreading and coalescence of Mg droplets, thereby promoting the deposition of Mg as fine, discrete, caviar-like particles with diameters below 1 mm. This process may contribute to Mg loss and the decrease in current efficiency during electrolysis. These findings provide fundamental insight into the role of Fe(Ⅲ) in cathode passivation during magnesium electrolysis.