Electrodeposition of Aluminum Coatings from Alcl3-Nacl-Kcl Molten Salts with Tmacl and Nai Additives

materials

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

Tianyu Yao 1,2 , Haiyan Yang 1,*, Kui Wang 1,*, Weiping Wu 3, Haiyan Jiang 1, Hezhou Liu 2, Qudong Wang 1 and Wenjiang Ding 1

1 National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, Shanghai 200240, China; [email protected] (T.Y.); [email protected] (H.J.); [email protected] (Q.W.); [email protected] (W.D.) 2 The State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China; [email protected] 3 Department of Electrical and Electronic Engineering, School of Mathematics, Computer Science and Engineering, City, University of London, Northampton Square, London EC1V 0HB, UK; [email protected] * Correspondence: [email protected] (H.Y.); [email protected] (K.W.)

Received: 28 October 2020; Accepted: 29 November 2020; Published: 2 December 2020

Abstract: The Al coatings achieved via electrodeposition on a Cu electrode from AlCl3-NaCl-KCl (80–10–10 wt.%) molten salts electrolyte with Tetramethylammonium Chloride (TMACl) and Sodium Iodide (NaI) additives is reported. The effect of the two additives on electrodeposition were investigated by cyclic voltammetry (CV), chronopotentiometry (CP), scanning electron microscopy (SEM) and X-ray diffraction (XRD). Results reveal that compact and smooth Al coatings are obtained at 150 ◦C by the electrodeposition process from the electrolyte with 1% TMACl and 10% NaI. The Al coatings exhibit great corrosion resistance close to that of pure Al plate, with a corrosion current of 3.625 µA. The average particle size is approximately 2 1 µm and the average thickness ± of the Al layer is approximately 7 2 µm. The nucleation/growth process exhibits irrelevance ± with TMACl or NaI during the electrodeposition of Al. TMACl cannot affect and improve the electrodeposition effectively. However, the addition of TMACl and NaI can intensify the cathodic polarization, producing an inhibition of Al deposition, and contribute to form uniform Al deposits. This can increase the conductivity and facilitate in refining the size of Al particles, contributing to forming a continuous, dense and uniform layer of Al coating, which can be used as effective additives in molten salts electrolyte.

Keywords: TMACl; NaI; electrodeposition; molten salts; Al coating; additive

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

Materials 2020, 13, 5506; doi:10.3390/ma13235506 www.mdpi.com/journal/materials Materials 2020, 13, 5506 2 of 16

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 [6–10], 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. 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 [12–15], some binary or ternary alloys were successfully synthesized by incorporating different metal ions [16–20] 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]. 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. Materials 2020, 13, 5506 3 of 16

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).

2. Materials and Methods

2.1. Electrolytes Preparation Sodium chloride (NaCl, AR, 99.9% purity, Sinopharm, Beijing, China) and potassium chloride (KCl, AR, 99.9% purity, Sinopharm) were initially dried in a vacuum oven for 72 h at 300 ◦C. Anhydrous Al chloride (AlCl3, AR, 99.8% pure, Sinopharm), NaI (AR, 99.5%, Macklin, Shanghai, China) and TMACl (AR, 99%, Macklin) were used as received. AlCl3, NaCl and KCl (80–10–10 wt.%) were mixed and melted at 150 ◦C in an electrolytic cell on a heating device with temperature control system. The mixture was purged with high-purity argon (Ar) gas. All of the electrodeposition and electrochemical experiments were performed in the glove box (Mikrouna Upure 1220/750/900, Shanghai, China). In order to investigate the eﬀect of NaI and TMACl on the electrodeposition of Al, NaI, TMACl or their mixture were added into the electrolytes and the electrolyte continuously stirred for 4 h to yield a homogeneous yellowish liquid.

2.2. Electrolyte Volatility Tests The electrolyte volatility tests were performed after the electrolyte had been well prepared in the glove box. The temperature-controlled heater maintained the electrolytic system at 150 ◦C. When we opened the lid of the electrolytic cell, the AlCl3 started to spill out of the electrolytic cell. Then, we weighed the mass of the electrolytic cell system every 30 min, where the mass loss over the ﬁxed period of time could be calculated as the volatile mass of AlCl3. The electrolyte volatility tests lasted for 4 h.

2.3. Electrochemical Tests An electrochemical workstation (CHI655D, Chenhua, Shanghai, China) was adopted in all electrochemical experiments. A typical three-electrode cell was used in the experiments, a glassy carbon (GC, 0.07 cm2) was adopted using as a working electrode and an Al plate (99.99%, 350 mm2) was used as a counter electrode. A pure Al bar (99.99%, diameter Φ = 0.5 mm) was adopted as the reference electrode, and it was ﬁxed in a small glassy tube which was full of the AlCl3-NaCl-KCl (weight ratio 8:1:1) molten salts. The Al plate and the pure Al bar were burnished with abrasive paper, cleaned in an ultrasonic bath for 3 min, rinsed with distilled water, then dried in a vacuum oven before Materials 2020, 13, 5506 4 of 16 all the experiments and characterizations. Cyclic voltammogram (CV) tests were carried out with a scan rate of 50 mV/s.

2.4. Electrodeposition In the electrodeposition process, the Al bar described above was adopted as the reference electrode. The Al plate described in the last section was used as anode. A pure Cu foil (the area exposed to the solution is 5 mm 5 mm, 99.9%) was adopted as the cathode. The Cu foil, Al plate and Al bar were × all cleaned with distilled water and then dried in a vacuum oven. The electrodeposition of Al was 2 performed at 150 ◦C under a constant current mode. Current density was kept at 50 mA/cm , and the electrodeposition process lasted for 10 min. The samples were cleaned after the electrodeposition using an ultrasound bath for 3 min, rinsed with distilled water and then dried in a vacuum oven.

2.5. Characterization of the Al Coatings A ﬁeld emission scanning electron microscope (FE-SEM, SIRION200, FEI, Shanghai, China) was used to observe the surface and cross-sectional morphologies of the Al coatings. The samples were examined by X-ray diﬀraction (XRD, D/MAX2000V, Rigaku, Tokyo, Japan) to identify the constituents and the crystal structure by the 2θ/θ scanning mode. The data was collected in a 2θ = 30◦ 90◦ range 1 − with a scanning speed of 5◦ min− with 0.01◦ (2θ) step size. Electrochemical behavior was studied in the 3.5 wt.% NaCl solution at room temperature by potentiodynamic polarization to evaluate the corrosion resistance. A graphite electrode and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. All the specimens were held at open circuit for 30 min to reach the steady value prior to potentiodynamic polarization. Scans were obtained from 100 mV below open circuit potential (OCP) to and scanned upwards at a rate of 0.5 mV/s. A PARSTAT 2273 advanced electrochemical system from Princeton Applied Research (New York City, State of New York, USA) was adopted to perform the electrochemical impedance spectroscopy (EIS) tests. The EIS tests conducted at OCP with a sinusoidal signal amplitude of 10 mV. The frequency ranged from 105 Hz down to 1 Hz.

3. Results and Discussion

3.1. Electrolyte Volatility

To study the effect of TMACl on the volatility of AlCl3-NaCl-KCl molten salts electrolyte, the volatilization experiments were performed and the results are shown in Figure1. It can be seen that the AlCl3-NaCl-KCl molten salts electrolyte volatilized continuously at 150 ◦C. Figure1a shows the mass of volatile AlCl3 could be as much as 2.180 g after a 4 h exposure. However, when TMACl was added into the electrolyte, the volatility of the electrolyte declined dramatically. As 1 wt.% TMACl, 5 wt.% TMACl, 10 wt.% TMACl was brought into the electrolyte, the mass of volatile AlCl3 was 0.716, 0.818 and 0.765 g, respectively. The addition of TMACl could effectively inhibit the volatilization of AlCl3, and more TMACl in the electrolyte could not further reduce the volatility. Different contents of NaI were added to test whether TMACl could inhibit the volatilization of AlCl3 in the electrolyte with NaI. As shown in Figure1b–d, the mass of volatile AlCl 3 effectively declined after the addition of TMACl, proving that the effect of inhibiting the volatilization of TMACl could not be impaired by NaI. Different content of TMACl had the similar effect on the volatilization of + AlCl3. A reasonable explanation of the effect of TMACl is that the TMA ionized from TMACl formed complex ions with Al2Cl7−, preventing Al2Cl7− from forming AlCl3.

3.2. Voltammetric Behavior The cyclic voltammograms (CV) of Al electrodeposition on the glassy carbon electrode in the AlCl3-NaCl-KCl (80–10–10 wt.%) molten salts electrolyte at 150 ◦C has been studied, as shown Materials 2020, 13, 5506 5 of 16 in Equation (1), only one electrodeposition reaction occurred according to the composition of the electrolyte in which the main type of ions is Al2Cl7−:

4Al Cl − + 3e− Al + 7AlCl − (1) 2 7 ↓ 4 The crossing current loop and the oxidation/reduction peaks demonstrate that the electrodeposition of AlMaterials in AlCl 20203-NaCl-KCl, 13, x FOR PEER molten REVIEW salts is irreversible. 5 of 16

FigureFigure 1.1. TheThe mass-timemass-time transientstransients ofof thetheelectrolyte electrolyte volatility volatility experiments experiments in in the the AlCl AlCl3-NaCl-KCl3-NaCl-KCl moltenmolten salts salts electrolyte, electrolyte, ( a(a)) no no additive, additive, ( b(b)) 1 1 wt.% wt.% NaI, NaI, ( c()c) 5 5 wt.% wt.% NaI, NaI, ( d(d)) 10 10 wt.% wt.% NaI, NaI at, at 150 150◦ C.°C.

TheDifferent CV test content was performeds of NaI were in AlCladded3-NaCl-KCl to test whether (80–10–10 TMACl wt.%) could molten inhibit salts the electrolytevolatilization with of variousAlCl3 in amount the electrolyte of the two with additives NaI. Ason shown the glassy in Figure carbon 1b– electroded, the mass in orderof volatile to study AlCl the3 effectively effect of NaIdeclined and TMACl after the on addition the voltammetric of TMACl, behavior proving that of Al. the The effect CV of curves inhibiting in AlCl the3 -NaCl-KClvolatilization molten of TMACl salts electrolytecould not be without impaired additive, by NaI. with Different 1% TMACl, content 5% of TMACl, TMACl 10% had TMACl, the similar 1% effect TMACl on+ the5% volatilization NaI and 1% TMAClof AlCl+3. 10%A reasonable NaI are presented explanation in Figure of the2 .effect of TMACl is that the TMA+ ionized from TMACl formedEvery complex cyclic voltammogramions with Al2Cl was7−, preventing recorded on Al a2Cl fresh7− from glassy forming carbon AlCl electrode3. surface. It shows that the cathode deposition current density increased with the addition of TMACl, indicating that TMACl cannot3.2. Voltammetric give rise to B anehavior inhibition of Al deposition. However, as both NaI and TMACl were added into the electrolyte, the deposition current density decreased dramatically. The related experiments [31] The cyclic voltammograms (CV) of Al electrodeposition on the glassy carbon electrode in the proved that the addition of NaI decreased the cathodic current density and gave rise to an inhibition of AlCl3-NaCl-KCl (80–10–10 wt.%) molten salts electrolyte at 150 °C has been studied, as shown in Al deposition. A possible reason is that the introduction of two additives leads to plenty conductive Equation (1), only one electrodeposition reaction occurred according to the composition of the ions in the molten salts, and the TMACl does not impair the effect of NaI. As shown in the inset electrolyte in which the main type of ions is Al2Cl7−: of Figure2, the reduction potential of cathodic was stable at approximately 0.01 V with or without the increase of TMACl, which indicates4Al2Cl7− the + 3e addition− ⇆ Al↓ of+ 7AlCl TMACl4− has no prominent effect on changing(1) the reduction potential of cathodic. While, the reduction potential of cathodic shifted obviously The crossing current loop and the oxidation/reduction peaks demonstrate that the electrodeposition in a negative direction after the NaI was added into the electrolyte. The results showed that the of Al in AlCl3-NaCl-KCl molten salts is irreversible. addition of both TMACl and NaI intensified the cathodic polarization, while the addition of TMACl The CV test was performed in AlCl3-NaCl-KCl (80–10–10 wt.%) molten salts electrolyte with could not, implying the polarization effect might be caused by NaI. Generally, appropriate polarization various amount of the two additives on the glassy carbon electrode in order to study the effect of NaI is necessary for electrodeposition as high overpotential is needed for the nucleation. The addition of and TMACl on the voltammetric behavior of Al. The CV curves in AlCl3-NaCl-KCl molten salts electrolyte without additive, with 1% TMACl, 5% TMACl, 10% TMACl, 1% TMACl + 5% NaI and 1% TMACl + 10% NaI are presented in Figure 2.

Materials 2020, 13, 5506 6 of 16

TMACl and NaI intensiﬁes the polarization and contributes signiﬁcantly to the formation of better AlMaterials coatings. 2020, 13, x FOR PEER REVIEW 6 of 16

Figure 2. The cyclic voltammograms of the AlCl 3--NaCl-KClNaCl-KCl molten molten salts salts electrolyte electrolyte without additive, 1%1% TMACl,TMACl, 5%5% TMACl,TMACl, 10%10% TMACl,TMACl, 1%1% TMAClTMACl ++ 5% NaI and 1% TMACl + 10% NaI, at 150 °C.◦C.

Additionally, the addition of the additives made the anodic current density peak shift to the Every cyclic voltammogram was recorded on a fresh glassy carbon electrode surface. It shows positive direction in different extent, the potential difference (∆E )[33] between oxidation and reduction that the cathode deposition current density increased with theP addition of TMACl, indicating that became increasingly large. This means that the two additives could aggravate the irreversibility in TMACl cannot give rise to an inhibition of Al deposition. However, as both NaI and TMACl were the electrodeposition system [34]. The physical properties of the electrolyte do not change with the added into the electrolyte, the deposition current density decreased dramatically. The related addition of two additives, the SEM, EDXS and XRD characterizations proved that the two additives experiments [31] proved that the addition of NaI decreased the cathodic current density and gave could not be introduced in Al deposit as the impurities. rise to an inhibition of Al deposition. A possible reason is that the introduction of two additives leads 3.3.to plenty Chronoamperometric conductive ions Investigations in the molten salts, and the TMACl does not impair the effect of NaI. As shown in the inset of Figure 2, the reduction potential of cathodic was stable at approximately 0.01 V with Chronoamperometricor without the increase experiments of TMACl, werewhich conducted indicates the on aaddition glassy carbonof TMACl electrode has no on prominent purpose ofeffect further on changing studying the the reduction effect of potential TMACl and of cathodic. NaI on AlWhile, nucleation the reduction and growth potential mechanism of cathodic in AlClshifted3-NaCl-KCl obviously molten in a negative salts electrolyte. direction The after experiments the NaI was started added soon into after the the electrolyte. short induction The results times, andshowed the potentialthat the addition started at of the both value TMACl where and no reductionNaI intensified of Al(III) the took cathodic place, polarization, and then the while potential the increasedaddition of gradually, TMACl could to a more not, implying negative valuethe polarization which was effect much might more be negative caused than by NaI. the nucleationGenerally, andappropriate growth ofpolarization Al particles is occurred. necessary The for experiments electrodeposition were performed as high overpotential in the AlCl3-NaCl-KCl is needed moltenfor the saltsnucleation. electrolyte The addition with TMACl of TMACl and NaI and at NaI diff intensifieserent content the densities,polarization and and four contributes typical representative significantly current-timeto the formation transients of better are Al plotted coatings. in Figure 3. IrrespectiveAdditionally, of the the addition different of content the additives of additives, made thethe shapesanodic ofcurrent the peaks density in the peak potential shift to step the currentpositive densitydirection transients in different are closely extent, relatedthe potential to nucleation difference/growth (∆EP process.) [33] between An obvious oxidation augment and owingreduction to the became formation increasingly and growth large. of aluminum This means nuclei that in each the two current additives curve cancould beeasily aggravate observed. the Additionally,irreversibility thein the process electrodeposition of formation system and growth [34]. madeThe physical Al ions properties collect rapidly. of the As electrolyte the overlap do [not35] activates,change with the increasingthe addition current of two density additives, receives the theSEM, maximum, EDXS and im, XRD and the characterizations time the peak appears proved is that the time,the two tm .additives After that, could all of not the be current introduced density in transientsAl deposit tend as the to impurities. decrease because of the consumption and diffusion of Al2Cl7−. With the applied potential increases, the im becomes higher, while the tm becomes3.3. Chronoamperometric shorter at the same Investigations time. It indicates that the time of overlapping decreased as the nucleation Chronoamperometric experiments were conducted on a glassy carbon electrode on purpose of further studying the effect of TMACl and NaI on Al nucleation and growth mechanism in AlCl3- NaCl-KCl molten salts electrolyte. The experiments started soon after the short induction times, and the potential started at the value where no reduction of Al(III) took place, and then the potential

Materials 2020, 13, 5506 7 of 16 density rises. In addition, the current density transients are not going to converge to the same value, theMaterials stable 2020 values, 13, x FOR of currentPEER REVIEW density increase as the potential applied increases. This proves that7 of the 16 electrodeposition of Al in AlCl3-NaCl-KCl molten salts electrolyte is the electrochemical reaction controlled.increased gradually, The reactions to a determinemore negative the main value ions which in the was AlCl much3-NaCl-KCl more negative molten than salts the electrolyte nucleation are Aland2Cl growth7− ions, of and Al particles abundant occurred Al2Cl7.− Theions experiments count for the were electrochemical performed in reactionthe AlCl3 control.-NaCl-KCl Di ﬀmoltenusion controlsalts electrolyte is usually with observed TMACl in and most NaI ionic at different liquids due content to the densit lackies of, reductive and four ionstypical near representative the cathode electrodecurrent-time [36]. transients are plotted in Figure 3.

Figure 3. 3. TypicalTypical current current density density–time–time transients transients of the of electrodeposition the electrodeposition experiments experiments in the inAlCl the3-

AlClNaCl3--NaCl-KClKCl molten molten salts electrolyte, salts electrolyte, (a) no (additive,a) no additive, (b) 1% ( bTMACl,) 1% TMACl, (c) 1% (TMAClc) 1% TMACl + 5% NaI+ 5% and NaI (d) and 1% (TMACld) 1% TMACl + 10% NaI,+ 10% at NaI,150 °C. at 150 ◦C.

AsIrrespective the different of the content different of additives content were of additives, taken into the consideration, shapes of the it waspeaks found in the that pot theential addition step 3+ ofcurrent TMACl density and NaI transient made thes are potential closely at related which the to nucleation/ reduction ofgrowth Al started process shift. An to negative obvious direction. augment Onowing the to contrary, the formation the addition and growth of TMACl of aluminum did not nuclei work in e eachffectively. current The curve result can illustratedbe easily observed. that the NaIAdditionally, could increase the process the overpotential of formation of and electrodeposition growth made Al and ions promotes collect rapidly. the cathodic As the polarization,overlap [35] butactivates TMACl, the could increasing not, which current agreed density well receives with those the maximum, of the above im,voltammograms. and the time the peak I− ions appears ionized is fromthe time, NaI mighttm. After act as that, some all surfactant of the current and the adsorptiondensity transients of I− on thetend surface to decrease of the electrode because benefits of the 3+ nucleationconsumption and and inhibits diffusion reduction of Al2 ofCl Al7−. W[ith34 ].the It alsoapplied revealed potential that additivesincreases, producedthe im becomes an increase higher, in responsewhile the currenttm becomes and shorter this effect at increasesthe same time. with theIt indicates concentration that the of time the additive,of overlapping whether decreased TMACl oras NaI.the nucleation It is probably density because rises the. In addition addition, of the the current two additives density increases transients the are number not going of charged to converge particles to andthe same activity value, of the the electrolyte, stable values so that of thecurrent electrical density conductivity increase as increases the potential and the applied response increases. current rises.This Iproves− ions would that the impede electrodeposition active sites on the of surface Al in of AlCl cathode,3-NaCl increasing-KCl molten the number salts electro of growinglyte isnuclei. the Theelectrochemical I− ions inhibit reaction the growth controlled of nuclei,. The which reactions lead todetermine an energetic the homogenizationmain ions in the of AlCl the Al3-NaCl coatings.-KCl While,molten the salts Cl − electrolyteions and ammonium are Al2Cl7− ionsions, ionized and abundant from TMACl Al2Cl could7− ions not, count which for implies the electrochemical TMACl is not areaction suitable control. additive Diffusion in the AlCl control3-NaCl-KCl is usually molten observed salts electrolyte. in most The ionic combined liquids dueaddition to the of TMACl lack of orreductive NaI would ions help near improve the cathode the Alelectrode coatings. [36]. DuringAs the different the electrodeposition content of additives of metals, were taken the three-dimensional into consideration, nucleation it was found and that hemispherical the addition growthof TMACl of theand initial NaI made nuclei the usually potential occur at which at the samethe reduction time. A of total Al3+ of start twoed di shiftfferent to modelsnegative are direction used to. analyzeOn the contrary, three-dimensional the addition nucleation of TMACl/growth did qualitatively,not work effectively. namely ‘instantaneous’ The result illustrated or ‘progressive’ that the [NaI37]. Thecould instantaneous increase the model overpotential depictsthe of statuselectrodeposition that all of the and nucleation promotes sites the are cathodic activated polarization, simultaneously. but TMACl could not, which agreed well with those of the above voltammograms. I− ions ionized from NaI might act as some surfactant and the adsorption of I− on the surface of the electrode benefits nucleation and inhibits reduction of Al3+ [34]. It also revealed that additives produced an increase in response current and this effect increases with the concentration of the additive, whether TMACl or NaI. It is probably because the addition of the two additives increases the number of charged particles

Materials 2020, 13, x FOR PEER REVIEW 8 of 16

and activity of the electrolyte, so that the electrical conductivity increases and the response current rises. I− ions would impede active sites on the surface of cathode, increasing the number of growing nuclei. The I− ions inhibit the growth of nuclei, which lead to an energetic homogenization of the Al coatings. While, the Cl− ions and ammonium ions ionized from TMACl could not, which implies TMACl is not a suitable additive in the AlCl3-NaCl-KCl molten salts electrolyte. The combined addition of TMACl or NaI would help improve the Al coatings. During the electrodeposition of metals, the three-dimensional nucleation and hemispherical growth of the initial nuclei usually occur at the same time. A total of two different models are used Materials 2020, 13, 5506 8 of 16 to analyze three-dimensional nucleation/growth qualitatively, namely ‘instantaneous’ or ‘progressive’ [37]. The instantaneous model depicts the status that all of the nucleation sites are In contrast,activated thesimu progressiveltaneously. model In contrast, depicts the the progressive status that model the nucleation depicts the sites status are gradually that the nucleation activated in thesites whole are process gradually of chronoamperometric activated in the whole experiment. process The of comparison chronoamperometric of the experimental experiment. transients The andcomparison the theoretical of the transients experimental is adopted transients for the and identification the theoretical of nucleation transients models. is adopted The theoretical for the transientsidentification for instantaneous of nucleation and models. progressive The theoretical nucleation transients equations for are instantaneous listed as Equations and progressive (2) and (3), respectively:nucleation equations are listed as Equations (2) and (3), respectively: ii 22 11.9542.9542 tt22 ( ) == {1 −expexp [−1.25641.2564 ( )]} (2) (2) imim tt/⁄tm − − tmtm 2 ( " #)2 i 1.2254 t 2 2 i 2= 1.2254 1 exp 2.3367 t 2 (3) (im ) = t/tm {1 − exp [−−2.3367 (tm ) ]} (3) im t⁄tm tm where i represents the current density, t represents the time and im represents the maximum of where i represents the current density, t represents the time and im represents the maximum of the the current density at tm time. To determine effect of TMACl and NaI on the nucleation/growth current density at tm time. To determine effect of TMACl and NaI on the nucleation/growth mechanism, the data obtained from the chronoamperometric tests were normalized and compared mechanism, the data obtained from the chronoamperometric tests were normalized and compared with the theoretical transients. Figure4 depicts the experimental and theoretical plots of (i /i )2 vs (t/t ) with the theoretical transients. Figure 4 depicts the experimental and theoretical plots ofm (i/im)2 vs m with different composition of additives. (t/tm) with different composition of additives.

2 Figure 4. The comparison of the experimental and theoretical plots of (i/im) vs (t/tm) in the AlCl3-NaCl-KCl molten salts electrolyte, (a) no additives, (b) 1% TMACl, (c) 1% TMACl + 5% NaI and (d) 1% TMACl + 10% NaI.

It is obvious that the experimental curves are mostly in accordance with the instantaneous nucleation model, which suggests that the process of Al electrodeposition could be referred to instantaneous nucleation. The addition of two additives does not aﬀect the mechanism of Al nucleation and growth, indicating that the process of nucleation/growth exhibits irrelevance with TMACl or NaI. Materials 2020, 13, x FOR PEER REVIEW 9 of 16

Figure 4. The comparison of the experimental and theoretical plots of (i/im)2 vs (t/tm) in the AlCl3-NaCl- KCl molten salts electrolyte, (a) no additives, (b) 1% TMACl, (c) 1% TMACl + 5% NaI and (d) 1% TMACl + 10% NaI.

It is obvious that the experimental curves are mostly in accordance with the instantaneous nucleation model, which suggests that the process of Al electrodeposition could be referred to instantaneous nucleation. The addition of two additives does not affect the mechanism of Al Materialsnucleation2020 ,and13, 5506 growth, indicating that the process of nucleation/growth exhibits irrelevance 9with of 16 TMACl or NaI.

3.4. M Morphologyorphology of Al C Coatingsoatings Al deposited samples obtained from AlCl -NaCl-KCl molten salts electrolyte with varied Al deposited samples obtained from AlCl33-NaCl-KCl molten salts electrolyte with varied concentration of of TMACl TMACl and and NaI NaI at at 150 150 °C◦ Cwere were examined examined by bySEM. SEM. The The typical typical SEM SEM images images of the of thesurface surface of Al of coatings Al coatings are areshown shown in Figure in Figure 5. 5.

Figure 5.5. TheThe microscopicmicroscopic morphologies morphologies of of the the aluminum aluminum electrodeposits electrodeposits obtained obtained in AlCl in AlCl3-NaCl-KCl3-NaCl- (80-10-10KCl (80-10 wt.%)-10 wt molten.%) molten salts salts electrolyte electrolyte with with di ﬀdifferenterent additives, additives, (a ()a no) no additives, additives, ((bb)) 1%1% TMACl, ((c)) 1%1% TMAClTMACl + 5% NaI and ( d) 1% TMACl + 10%10% NaI. NaI.

As no additive waswas introduced introduced,, shown in Figure 5 5a,a, thethe surfacesurface ofof thethe AlAl coatingcoating waswas stillstill rough,rough, and the films were nonuniform—the average particle size was about 7 3 µm. When 1 wt.% TMACl and the films were nonuniform—the average particle size was about 7 ±± 3 μm. When 1 wt.% TMACl was added into the electrolyte, as shown in Figure5 5b,b, thethe grainsgrains becamebecame moremore uniform,uniform, thethe averageaverage particle size size became became smaller, smaller, the the Al coating Al coating became became dense. dense. Extra experiments Extra experiments proved provedthat the additionthat the ofaddition more TMACl of more at TMACl 5%, 10%, at and5%, even10%, 15%,and hadeven a 15%, similar had eff aect similar on the effect Al coatings, on the Al but coatings, the morphology but the andmorphology quality of and the quality Al deposit of the did Al not deposit change did much. not change This means much. that This more means TMACl that more could TMACl not magnify could thenot emagnifyffect which the serveseffect which as an eserveffectives as additive an effective in molten additive salts in electrolytemolten salts or electrolyte improve the or qualityimprove of the Al coatingsquality of e ffAlectively. coatings However, effectively. when However, TMACl andwhen NaI TMACl were bothand introduced,NaI were both whether introduced 1% TMACl, whether and 5%1% NaITMACl or 1% and TMACl 5% NaI and or 1% 10% TMACl NaI (Figure and 10%5c,d), NaI the (Figure Al particles 5c,d), becamethe Al particles smaller became and homogeneous, smaller and thehomogeneous, surface of Al the coating surface became of Al flatcoating and consecutive,became flat and theconsecutive, size of Al and particles the size appeared of Al uniform.particles Theappeared Al particles uniform. grew The neatly, Al particles and a compact grew neatly, and uniform and a compact electrodeposition and uniform Al coating electrodeposition was obtained, Al where the average particle size was approximately 2 1 µm. The dendritic crystal and spongy are coating was obtained, where the average particle size± was approximately 2 ± 1 μm. The dendritic preventedcrystal and espongyffectively, are indicatingprevented thateffectively, NaI is helpfulindicating to that refine NaI the is particlehelpful to size. refine On the account particle of size. the experimentOn account resultsof the experiment of cyclic voltammetry results of cyclic in Section voltammetry 3.2, it is reasonable in Section to 3.2 speculate, it is reasonable that the theto speculate addition of TMACl and NaI, especially the addition of NaI, could cause electrochemical polarization. A similar trend was also found in the electrodeposition of Al with ionic liquid electrolytes containing similar ammonium halides as additives [38]. While, there were a few tiny pores on the surface of the Al coating. The gap between Al particles became narrower after TMACl and NaI were added, and finally appeared in the form of voids on the coating surface. In order to determine the elementary composition of the Al deposits, the EDXS tests were performed. The typical EDXS spectrums of the Al electrodeposits from AlCl3-NaCl-KCl molten salts electrolyte with additives are presented in Figure6. The strong peak of aluminum can be clearly Materials 2020, 13, x FOR PEER REVIEW 10 of 16

that the the addition of TMACl and NaI, especially the addition of NaI, could cause electrochemical polarization. A similar trend was also found in the electrodeposition of Al with ionic liquid electrolytes containing similar ammonium halides as additives [38]. While, there were a few tiny pores on the surface of the Al coating. The gap between Al particles became narrower after TMACl and NaI were added, and finally appeared in the form of voids on the coating surface. MaterialsIn2020 order, 13, 5506 to determine the elementary composition of the Al deposits, the EDXS tests were10 of 16 performed. The typical EDXS spectrums of the Al electrodeposits from AlCl3-NaCl-KCl molten salts electrolyte with additives are presented in Figure 6. The strong peak of aluminum can be clearly observed,observed, indicating indicating the the main maincomposition composition of the coating is is aluminum. aluminum. The The tiny tiny peak peak of ofK was K was also also detecteddetected since since there there could could probablyprobably bebe aa little KCl residue residue from from the the electrolyte electrolyte on on the the surface surface of ofAl Al coatings.coatings. No No peak peak ofof oxygenoxygen oror otherother element was was detected detected after after the the addition addition of ofTMACl, TMACl, or orboth both TMAClTMACl and and NaI NaI (Figure (Figure6a,b). 6a, Theb). EDXS The EDXS results results proved proved that the that electrodeposited the electrodeposited Al coatings Al coatings consisted ofconsis pure Alted andof pure were Al clear and were of the clear TMACl of the or TMACl NaI, conﬁrming or NaI, confirming that the that electrodeposition the electrodeposition Al coatings Al obtainedcoatings with obtained additives with were additives of high were quality. of high A layer quality. ofhigh-purity A layer of highAl coating-purity can Al becoating obtained can befrom AlClobtained3-NaCl-KCl from AlCl molten3-NaCl salts-KCl electrolyte molten salts with electrolyte TMACl and with NaI TMACl added and as NaI additive. added as additive.

FigureFigure 6. 6. TheThe EDXS EDXS spectr spectraa of the of theAl deposits Al deposits obtained obtained on a copper on a foil copper in AlCl foil3-NaCl in AlCl-KCl 3(80-NaCl-KCl-10-10 (80-10-10wt.%) molten wt.%) salts molten electrolyte, salts electrolyte, (a) 1% TMACl, (a) 1% ( TMACl,b) 1% TMACl (b) 1% + 10% TMACl NaI.+ 10% NaI. The cross-sectional morphology of the Al coating obtained from AlCl -NaCl-KCl molten salts The cross-sectional morphology of the Al coating obtained from AlCl3-3NaCl-KCl molten salts withwith no no additives, additives, 1%1% TMACl, 1% 1% TMACl TMACl + 5%+ 5%NaI NaIand 1% and TMACl 1% TMACl + 10% NaI+ 10% are NaIpresented are presented in Figure in Figure7. It 7 shows. It shows in Figure in Figure 7a 7 thata that the the thickness thickness of of the the electrodeposition electrodeposition Al Al coating coating obtained obtained from from the the electrolyteelectrolyte without without additive additive varies varies fromfrom 2.52.5 to 10 μmµm,, a andnd the the average average thickness thickness of ofAl Al electrodeposits electrodeposits obtainedobtained was was 6 6 ±4 4µ μm.m. TheThe nonuniformitynonuniformity of the the thickness thickness means means the the Al Al coatings coatings are are not not dense dense and and ± compact.compact. In In contrast, contrast, after after 1% 1% TMACl TMACl ++ 10% NaI NaI added added into into the the electrolyte, electrolyte, the the average average thickness thickness of of thethe achieved achieved Al Al coating coating is is approximately approximately 77 ± 2 μm.µm. There There is is no no gap gap between between Al Al coatings coatings and and the the Cu Cu ± substratesubstrate indicating indicating the the electrodeposited electrodeposited Al coating Al coating and the and Cu the substrate Cu substrate adhere adhere tightly. tightly. The Faraday’s The lawFaraday’s was used law to calculate was used the to cathodic calculate current the cathodic eﬃciency current based efficiency on the mass based of the on electrodeposited the mass of the Al, aselectrodeposited 1% TMACl and Al, 10% as NaI 1% TMACl were added and 10% into NaI the were electrolyte, added into Al coatings the electrolyte, with a currentAl coatings eﬃ ciencywith a of current efficiency of 99% was obtained. 99% wasMaterials obtained. 2020, 13, x FOR PEER REVIEW 11 of 16

Figure 7. The cross-sectional morphology of the Al coating deposited from the AlCl3-NaCl-KCl Figure 7. The cross-sectional morphology of the Al coating deposited from the AlCl3-NaCl-KCl molten molten salts electrolyte with different additives, (a) no additives, (b) 1% TMACl, (c) 1% TMACl + 5% salts electrolyte with diﬀerent additives, (a) no additives, (b) 1% TMACl, (c) 1% TMACl + 5% NaI and NaI and (d) 1% TMACl + 10% NaI. (d) 1% TMACl + 10% NaI. To further characterize the microstructure of the Al coating, XRD analysis was performed and Figure 8 shows the XRD patterns of the electrodeposited Al coatings. All the strong diffraction peaks in the XRD patterns can be referred to the Al coatings and the Cu substrate. In total, four obvious diffraction peaks, (111), (200), (220), and (311), can be found from the obtained aluminum deposits. It means that the Al coatings are constituted of fcc phase of Al metal. In order to study the effect of the two additives on the orientation and grain size of Al coatings, the texture coefficient (TC) and the grain size were adopted and calculated via XRD reflections [39], and the results are listed in Table 1. As the additive was added, the grain size of the Al decreased significantly, which is in accordance with the variation tendency of particle size. The addition of TAMCl and NaI could contribute to the formation of a preferred crystallographic orientation along (220) plane and refine the grain size effectively.

Figure 8. The XRD patterns of the electrodeposition Al coating obtained on a copper foil in AlCl3- NaCl-KCl (80–10–10 wt.%) molten salts electrolyte, without additive, 1% TMACl, 1% TMACl + 5% NaI and 1% TMACl + 10% NaI, at 150 °C.

Table 1. The TC values and grain sizes calculated from XRD reflections, Al coating electrodeposited without additive, with 1% TMACl, and with 1% TMACl + 10% NaI.

Additive TC(111) TC(200) TC(220) TC(311) Grain Size (nm) No additives 17.89% 15.11% 45.73% 21.27% 417

Materials 2020, 13, x FOR PEER REVIEW 11 of 16

Figure 7. The cross-sectional morphology of the Al coating deposited from the AlCl3-NaCl-KCl molten salts electrolyte with different additives, (a) no additives, (b) 1% TMACl, (c) 1% TMACl + 5% NaI and (d) 1% TMACl + 10% NaI. Materials 2020, 13, 5506 11 of 16

To further characterize the microstructure of the Al coating, XRD analysis was performed and FigureTo 8 further shows characterizethe XRD patterns the microstructure of the electrodeposited of the Al A coating,l coatings. XRD All analysis the strong was diffraction performed peaks and Figurein the 8XRD shows patterns the XRD can patterns be referred of the to electrodepositedthe Al coatings and Al coatings. the Cu substrate. All the strong In total, di ff ractionfour obvious peaks indiffraction the XRD peaks, patterns (111), can (200), be referred (220), and to the (311), Al can coatings be found and from the Cu the substrate. obtained aluminum In total, four deposits. obvious It dimeansffraction that peaks,the Al (111),coatings (200), are (220),constit anduted (311), of fcc can phase be foundof Al metal. from theIn order obtained to study aluminum the effect deposits. of the Ittwo means additives that the on Al the coatings orientation are constituted and grain ofsize fcc of phase Al coatings, of Al metal. the Intexture order coefficient to study the (TC) effect and of the twograin additives size were on adopted the orientation and calculated and grain via size XRD of Alreflections coatings, [39], the textureand the coe resultsfficient are (TC) listed and in the Table grain 1. sizeAs the were additive adopted was and added, calculated the grain via XRD size reflectionsof the Al decreased [39], and thesignificantly, results are listedwhich in is Tablein accordance1. As the additivewith the wasvariation added, tendency the grain of sizeparticle of the size. Al Th decreasede addition significantly, of TAMCl whichand NaI is c inould accordance contribute with to the variationformation tendency of a preferred of particle crystallographic size. The addition orientation of TAMCl along and (220) NaI could planecontribute and refine to the the grainformation size ofeffectively. a preferred crystallographic orientation along (220) plane and refine the grain size effectively.

Figure 8. 8. TheThe XRD XRD patterns patterns of the of electrodeposition the electrodeposition Al coating Al coating obtained obtained on a copper on a foil copper in AlCl foil3- inNaCl AlCl-KCl3-NaCl-KCl (80–10–10 wt (80–10–10.%) molten wt.%) salts moltenelectrolyte, salts without electrolyte, additive, without 1% TMACl, additive, 1% TMACl 1% TMACl, + 5% 1%NaI TMACl and 1%+ TMACl5% NaI + and10% 1% NaI, TMACl at 150+ °C.10% NaI, at 150 ◦C. Table 1. The TC values and grain sizes calculated from XRD reﬂections, Al coating electrodeposited Table 1. The TC values and grain sizes calculated from XRD reflections, Al coating electrodeposited without additive, with 1% TMACl, and with 1% TMACl + 10% NaI. without additive, with 1% TMACl, and with 1% TMACl + 10% NaI.

Additive TC(111) TC(200) TC(220) TC(311) Grain Size (nm) Additive TC(111) TC(200) TC(220) TC(311) Grain Size (nm) No additives 17.89% 15.11% 45.73% 21.27% 417 No additives 17.89% 15.11% 45.73% 21.27% 417 1% TMACl 9.76% 13.49% 60.73% 16.02% 401 1% TMACl + 5% NaI 10.51% 14.04% 57.49% 17.46% 395 1% TMACl + 10% NaI 5.72% 7.77% 62.24% 24.27% 363

3.5. Corrosion Behaviors The potentiodynamic polarization was performed so as to test the corrosion resistance of Al coatings obtained from AlCl3-NaCl-KCl molten salts electrolyte with varied concentration of additives. A cast pure Al plate was also tested as a parallel control experiment. Figure9 presents a series of typical potentiodynamic polarization curves of the electrodeposited Al coatings with diﬀerent additives, and other detail parameters calculated by the Tafel extrapolation are shown in the Table2.

Table 2. Electrochemical parameters calculated from the potentiodynamic polarization curves in 3.5 wt.% NaCl solution.

2 Specimens Ecorr (vs. SCE)/V icorr/µA cm · − Al coatings electrodeposited with 1% TMACl + 10% NaI. 0.985 3.625 − Al coatings electrodeposited with 1% TMACl 1.211 38.768 − Al coatings electrodeposited with No additive 1.196 40.161 − Pure Al 0.978 35.140 − Materials 2020, 13, x FOR PEER REVIEW 12 of 16

1% TMACl 9.76% 13.49% 60.73% 16.02% 401 1% TMACl + 5% 10.51% 14.04% 57.49% 17.46% 395 NaI 1% TMACl + 10% 5.72% 7.77% 62.24% 24.27% 363 NaI

3.5. Corrosion Behaviors The potentiodynamic polarization was performed so as to test the corrosion resistance of Al coatings obtained from AlCl3-NaCl-KCl molten salts electrolyte with varied concentration of additives. A cast pure Al plate was also tested as a parallel control experiment. Figure 9 presents a series of typical potentiodynamicMaterials 2020, 13 ,polarization 5506 curves of the electrodeposited Al coatings with different additive12 of 16s, and other detail parameters calculated by the Tafel extrapolation are shown in the Table 2.

FigureFigure 9. The 9. The potentiodynamic potentiodynamic polarization polarization curves curves of purepure Al, Al, Al Al coating coating electrodeposited electrodeposited without without additive,additive, with with 1% 1%TMACl TMACl and and with with 1% 1% TMACl TMACl + 10% NaI.NaI. It can be seen that the Al coatings electrodeposited from AlCl3-NaCl-KCl molten salts electrolyte withoutIt can be additive seen that present the Al the coatings poor corrosion electrodeposited resistance withfrom the AlCl highest3-NaCl icorr-KClof 40.161moltenµ saltsA. After electrolyte 1% withoutTMACl additive was added present in to the the poor electrolyte, corrosion the potentiodynamic resistance with polarization the highest parameters icorr of 40.161 did not μA change. After 1% TMAClmuch. was It added is reasonable, in to the since electrolyte, the Al coatings the potentiodynamic were not dense and polarization compact enough, parameters the loose did Al particles not change much.made It is the reasonable coatings easily, since corroded. the Al Further, coatings this were also gives not an dense explanation and compact as to why enough, there is no the passive loose Al region found in these two curves. In contrast, as the 1% TMACl + 10% NaI was added, the corrosion particles made the coatings easily corroded. Further, this also gives an explanation as to why there is current i of the electrodeposited Al coatings declined dramatically to 3.625 µA, which is quite close no passive regioncorr found in these two curves. In contrast, as the 1% TMACl + 10% NaI was added, the to that of pure Al. A conspicuous passive region was observed in the potentiodynamic polarization corrosioncurve, current indicating icorr thatof the the electrodeposited Al coating exhibits Al acoatings great corrosion declined resistance. dramatically It also to proved3.625 μ thatA, which the is quiteaddition close to of that TMACl of pure could Al. not A a ﬀconspicuousect the quality passive of Al coatings, region whilewas observ the additioned in ofthe TMACl potentiodynamic and NaI polarizationcould improve curve, the indicating Al coatings that prominently. the Al coating exhibits a great corrosion resistance. It also proved that the additionThe EIS testsof TMACl were performed could not ataffect the OCPthe quality to further of Al evaluate coatings, the corrosionwhile the resistanceaddition of TMACl the and NaIelectrodeposition could improve Al coatingsthe Al coatings obtained prominently. from AlCl3-NaCl-KCl molten salts electrolyte with additive and the results of are presented in Figure 10. It is shown that the shapes of Nyquist plots of pure Al, AlTable coating 2. Electrochemical electrodeposited parameters without additive,calculated with from 1% the TMACl potentiodynamic and with 1% polarization TMACl + 10% curves NaI in were 3.5 verywt.% similar,NaCl solution revealing. that the corrosion process is mainly controlled by the charge-transfer [40]. In the Nyquist plots, the larger semicircular arc diameter means the greater resistance. Bode and −2 Bode-phase results have provedSpecimens that the Al coating achieved with 1%Ecorr TMACl (vs. SCE+ 10%)/V NaIicorr presented/μA·cm the Al coatings electrodeposited with 1% TMACl + 10% NaI. −0.985 3.625 highest /Z/. Hence, the Al coating electrodeposited from AlCl3-NaCl-KCl molten salts electrolyte with 1% TMAClAl+ coatings10% NaI electrodeposited exhibit a great corrosion with 1% TMACl resistance. −1.211 38.768 Al coatings electrodeposited with No additive −1.196 40.161 The equivalent circuit in accordance with the Nyquist curves is shown in Figure 10d. RL is the Pure Al −0.978 35.140 resistance of the AlCl3-NaCl-KCl molten salts electrolyte at a high frequency limit. R1 is the polarization resistance related to the charge transfer, and Z determines the double layer capacitance. The simulation valuesThe EIS obtained tests were by Z-view performed software at for the the OCP equivalent to further elements evaluate are shown the corrosion in Table 3resistance. Generally, of the electrodepositiona higher R1 reveals Al coatings a lower obtained corrosion resistance.from AlCl3 According-NaCl-KCl to molten Table3, itsalts is obvious electrolyte that thewith Al additive coating and the resultselectrodeposited of are presented from AlCl in3-NaCl-KCl Figure 10. molten It is shown salts electrolyte that the withshapes 1% of TMACl Nyquist+ 10% plots NaI of presented pure Al, Al the best corrosion resistance. Materials 2020, 13, x FOR PEER REVIEW 13 of 16

coating electrodeposited without additive, with 1% TMACl and with 1% TMACl + 10% NaI were very similar, revealing that the corrosion process is mainly controlled by the charge-transfer [40]. In the Nyquist plots, the larger semicircular arc diameter means the greater resistance. Bode and Bode-phase results have proved that the Al coating achieved with 1% TMACl + 10% NaI presented the highest Materials/Z/. Hence,2020, 13 ,the 5506 Al coating electrodeposited from AlCl3-NaCl-KCl molten salts electrolyte with13 1% of 16 TMACl + 10% NaI exhibit a great corrosion resistance.

FigureFigure 10. 10.The The EIS EIS plots plots of of pure pure Al, Al, Al Al coatingcoating electrodepositedelectrodeposited without without additive, additive, with with 1% 1% TMACl TMACl andand with with 1% 1% TMACl TMACl + + 10% NaI. NaI. (a (a) ) Nyquist Nyquist plots, plots, (b) ( b Bode) Bode plots, plots, (c) Bode (c) Bode-phase-phase plots plots and (d and) (d)equivalent equivalent circuit. circuit.

TableThe 3. equivalentEIS data for circuit pure Al,in accordance Al coating electrodeposited with the Nyquist without curves additive, is shown with in 1% Figure TMACl 10d and. RL with is the resistance1% TMACl of + the10% AlCl NaI,3- inNaCl 3.5%-KCl NaCl molten solutions. salts electrolyte at a high frequency limit. R1 is the polarization resistance related to the charge transfer, and Z determines the double layer capacitance. Specimens R (Ω cm2) Z (µF cm2) R (Ω cm2) The simulation values obtained by Z-view software for theL equivalent elements are shown in1 Table 3. AlGenerally, coatings electrodepositeda higher R1 reveals with a 1%lower TMACl corrosion+ 10% resistance NaI. . According55.48 to Table 10.97 3, it is obvious 3258 that the Al coatings electrodeposited with 1% TMACl 74.03 76.42 488.5 Al coating electrodeposited from AlCl3-NaCl-KCl molten salts electrolyte with 1% TMACl + 10% NaI Al coatings electrodeposited with No additive 44.47 10.66 52.32 presented the best corrosion resistance. Pure Al 41.13 46.12 1059

Table 3. EIS data for pure Al, Al coating electrodeposited without additive, with 1% TMACl and with 4. Conclusions1% TMACl + 10% NaI, in 3.5% NaCl solutions.

In order to effectivelySpecimens reduce the volatility of the AlClRL3 -NaCl-KCl(Ω cm2) (80–10–10Z (μF cm2) wt.%)R1 molten (Ω cm2) salts electrolyte,Al coatings increase electrodeposited electrodeposition with 1% TMACl effi ciency+ 10% NaI. and improve55.48 the quality10 of.97 electrodeposition3258 Al coatings,Al two coatings different electrodeposited additives, with TMACl 1% TMACl and NaI were introduced74.03 and investigated.76.42 The488.5 addition of TMAClAl coatings could inhibit electrodeposited the volatilization with No additive of AlCl , extending44.47 the service10 life.66 of electrolyte.52.32 High Pure Al 3 41.13 46.12 1059 quality Al coatings were obtained by electrodeposition from AlCl3-NaCl-KCl molten salts electrolytes at 150 ◦C with varied concentration of NaI and TMACl. A uniform, compact and smooth layer of electrodeposited Al films can be obtained by adding 1% TMACl + 5% NaI and 1% TMACl + 10% NaI. The electrodeposited Al coatings achieved great corrosion resistance, close to that of pure Al plate, with a corrosion current of 3.625 µA. The average particle size of the Al deposits was approximately 2 1 µm. The average thickness of the electrodeposition Al coatings was approximately 7 2 µm. ± ± The electrodeposition of Al on a Cu electrode in AlCl3-NaCl-KCl (80–10–10 wt.%) molten salts electrolyte proceeded via a three-dimensional instantaneous nucleation—the process of nucleation and growth exhibits irrelevance with NaI or TMACl. The addition of TMACl could not affect and Materials 2020, 13, 5506 14 of 16 improve the electrodeposition effectively. However, the addition of TMACl and NaI could intensify the cathodic polarization and inhibit the electrodeposition of Al. The combined effort of the two additives could increase the conductivity and facilitate to refine the particle size, contributing to the formation of a continuous, homogeneous and uniform Al coatings. They can be used as effective additives in molten salts electrolytes.

Author Contributions: Conceptualization, H.Y.; Data curation, T.Y.; Funding acquisition, H.Y.; Investigation, Q.W.; Methodology, H.Y.; Resources, H.J., H.L. and W.D.; Supervision, K.W. and H.L.; Visualization, W.W.; Writing—original draft, T.Y.; Writing—review & editing, T.Y., K.W. and W.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Natural Science Foundation of China, China Postdoctoral Science Foundation and Science and Technology Innovation Action Plan—International Enterprises Science and Technology Cooperation Program of Shanghai. Grant number are (51301110), (2016M600311) and (17230732700). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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