Electrodeposition of Aluminum Coatings from AlCl3-NaCl-KCl Molten Salts with TMACl and NaI Additives

1. Introduction

High temperature electrolysis in molten salts is an important industrial process across many important areas. In industry, it has been widely used for metals extraction, materials processing and metallic thin film deposition. Over the years, the Hall-Héroult process has been used for the smelting of pure aluminum (Al) at industrial scale [1], the most widely used non-ferrous metal in the world. Al oxide (alumina, Al2O3) dissolved in molten cryolite (Na3AlF6) and consumable carbon are used as anodes in the process. The electrolysis in molten salts is not energy efficient, consuming huge amount of energy and carbon materials (11.5-13.5 kWh electricity and 0.4-0.5 kg carbon anodes for producing 1 kg Al [2]). Al production is highly energy-intensive, more than 3% of the world’s entire electrical supply has been used to extraction of Al every year [3]. The primary Al industry induces 0.4 billion tons (Gt) of carbon dioxide equivalent (CO2e) greenhouse gas emissions every year [4]. As the inorganic salts (fluorides, chlorides and carbonates melts) have high melting points, the process requires high temperature conditions and consumes large amount of thermal energy [5]. It is essential and important to develop a low temperature, energy efficient and low carbon footprint for sustainable electrolysis processes in molten salts, for economical, safety and environmental considerations.

Electrodeposition of Al coatings has drawn much attention because of their excellent properties, i.e., low density, resistance to corrosion and high-temperature oxidation. Compared with other methods of preparing Al coatings [6,7,8,9,10], electrodeposition shows a lot of advantages: (1) the convenient and easy operation, (2) higher cost performance, (3) suitable for substrates of various geometric shapes, (4) quality and thickness of the coatings could be tuned through adjusting the processing parameters of electrodeposition.

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Among all categories of nonaqueous electrolyte systems used for Al electrodeposition, AlCl3-NaCl-KCl molten salts is becoming a popular candidate for electroplating Al coatings and has been frequently adopted in the electro-deposition of Al and Al alloys on account of their low eutectic point [11]. A lot of researches have been developed on the electro-deposition of Al in AlCl3-NaCl-KCl molten salts [12,13,14,15], some binary or ternary alloys were successfully synthesized by incorporating different metal ions [16,17,18,19,20] into the electrolytes. Different substrate materials, i.e., graphite [21], Au [19], were also investigated. However, the poor quality of Al coatings electrodeposited from AlCl3-NaCl-KCl molten salts is still a pressing issue. The dendrites of Al and microholes forming during the electrodeposition constrains the uniformity of Al coatings severely. Particularly, it is the dendrites and microholes that make the Al coatings more vulnerable to rupture and detach off the substrate with the increasing thickness of Al deposition. This challenge severely impedes the further applications of Al or Al alloy coatings prepared by electro-deposition.

In order to prepare high-quality Al coatings, appropriate additives in the electrolytes are essential [22,23]. Additives are widely used in metal electrodeposition in aqueous solutions, while, only a few studies pay attention to additives in the electrolytes for Al electrodeposition in nonaqueous solutions, especially in inorganic molten salts [24,25]. Generally, different types of additives serve different electrolytes, and the solubility in the electrolyte has to be taken in consideration. Amines [26], pyridines, nicotinamides [27] and other small molecule compounds are the proper candidate additives in organic solvents and ionic liquids electrolytes. In contrast, the additives added in molten salts electrolytes could be some small ionic and inorganic molecules. Previous researches on additives showed that some halides [12], especially alkali halides [24,25] could be adopted to improve the quality of coherent and dendrite-free Al coatings. Hydrogen halides are too difficult to handle, since they are usually liquid and volatile. Alkali halides could be ideal alternatives when used as remarkable additives since they exhibit good solubility and low volatility. LiCl and KCl had been proven to be helpful to improve the quality of Al coatings [22,23]. KI had also been found an effective surfactant in electrodeposition of Al from AlCl3-NaCl-KCl molten salts [28,29]. The addition of NaI showed improvement in the quality of deposited Al layers [30]. The authors’ researches also demonstrated that NaI could intensify cathodic polarization, inhibit growth of Al deposits and facilitate the formation of uniform Al deposits [31].

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The selected additives have improved the quality of electrodeposition Al coatings remarkably. While, the destruction of the AlCl3-NaCl-KCl molten salts electrolyte due to the volatility of AlCl3 is still another severe problem, which restricts the efficiency of the electrodeposition. The temperature of Al electrodeposition in AlCl3-NaCl-KCl molten salts should be set at the range of 130~250 °C. While, the AlCl3 in the molten salt’s electrolyte volatile seriously, so that the composition of AlCl3-NaCl-KCl changes gradually, leading to an inevitable degradation of Al coating quality. In order to promoting the quality of the electrodeposition coatings, some additives are also adopted to improve the electrolyte [32]. According to the reactions in AlCl3-NaCl-KCl molten salts [31], as some macromolecular cations are introduced into the electrolyte, facilitating the equilibrium shift. It is expected to reduce the volatility of the electrolyte, improve electrolyte stability and further improve the coating quality.

As one important type of halides, ammonium halides have seldom been studied as additives in molten salts electrolytes. In industry, ammonium salts are widely used as effective cationic surfactants. It is supposed that the combination of two different types of additives, alkali halides and ammonium halides, could contribute to improving the coating quality and prolonging the electrolyte service life. In fact, in order to understand the mechanism comprehensively and achieve better Al coatings, electrochemical process and nucleation/growth mechanisms are of a great benefit. Since single additive is effective, it is reasonable to predict that the addition of multiple additives could improve the quality of Al coatings, as the sum is always greater than the parts. However, the effects of multiple additives have not been investigated yet.

In the present study, we have introduced the ammonium halides as the additives in molten salts electrolytes for the low temperature (150 °C) electrodeposition of Al. We are focusing on two specific halides—TMACl (Tetramethylammonium Chloride) was adopted to reduce the volatility of the electrolyte and NaI (Sodium Iodide) was adopted to improve the coating quality. The effects of TMACl and NaI on the deposition mechanisms, the morphology and microstructures of Al from AlCl3-NaCl-KCl (80-10-10 wt.%) molten salt electrolyte were studied systematically. Cyclic voltammetry (CV) and chronoamperometry (CP) have been employed to investigate the electrochemical process and the deposition mechanisms of Al. Electrodeposits were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The corrosion resistance of Al coatings was tested through potentiodynamic polarization (PD) and electrochemical impedance spectroscopy (EIS).

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