2.1 Reagents and materials

Analytical grade LiCl (99.8 %), KCl (99.97 %), K₂TiF₆ (99.99 %), tungsten wire (99.5 %, Φ1 mm), and graphite rod (99.5 %, Φ2 mm) were purchased from Sinopharm Chemical Reagent Co., Ltd., and Ti sponge (99.7 %, Φ<25.4 mm) was purchased from Alfa Industrial Inc., K₂TiF₆ (99.99 %) and Ti sponge (99.7 %, Φ<25.4 mm) were used directly without further purification.

2.2 Electrochemical measurements and electrodes

The eutectic LiCl-KCl molten salt was prepared by mixing LiCl and KCl in the molar ratio of 59:41, and the mixture was placed inside an alumina crucible. This crucible was loaded in a chamber of a resistance furnace (Fig. 1.), that was connected to a glove box charged with argon (99.999 %) to maintain the oxygen/water concentration of less than 10 ppm. The LiCl-KCl mixture was stepwise heated to 573 K for 3 h and was maintained at this temperature for 5 h to remove the absorbed humidity. Later, the temperature was raised to 823 K and was maintained for 1 h to melt the eutectic. Different concentrations of LiCl-KCl-Ti^x+ salts were prepared by adding certain amounts of K₂TiF₆ (0.1-1.0 mol%), and an excess of Ti sponge into the LiCl-KCl molten salt at 823 K, and was maintained at this temperature for 1 h to complete the reaction between Ti⁴⁺ and Ti. The concentration of dissolved Ti ions in the melts was primarily determined using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) measurement.

Fig. 1.Full-screen View

(Color online) Schematic illustration of the layout of molten salt electrolytic cell.

All electrochemical experiments were performed by Autolab PGSTAT302N with Nova 1.9 software. Three-electrode system was used for the electrochemical measurements. We used pure tungsten (W) wire (Φ1 mm) as the working electrode (WE) to investigate the electrochemical behavior of Ti^x+ because W is regarded as an inert material. 316L stainless steel wire (Φ1 mm) was used as the working electrode for electroplating experiments. It was immersed in dilute nitric acid (0.1 mol%) for 30 min followed by ultrasonic cleaning with deionized water to remove the oxide layer from the surface. Graphite rod (Φ2 mm) or titanium wire (Φ1 mm) was used as counter electrode (CE). A homemade Ag/AgCl was used as the reference electrode (RE). Detailed description of the Ag/AgCl reference electrode has been published previously. In summary, we placed the mixture of LiCl-KCl and AgCl (5.0 mol%) in a BN tube, an Ag wire was immersed into the salt to serve as the electrode, and a tiny hole in the side of BN tube served as the ion tunnel.

2.3 Characterization and analysis

After electroplating, the WE was taken out from the molten salt and cooled in the glove box. Later, it underwent stepwise washing using deionized water and acetone to remove the adhesive salts. The washed samples were dried in a vacuum dry box overnight before further characterization. Energy dispersive spectrometer (SEM-EDS, Zeless Merlin Compact LED 1530 VP) was used to investigate the micro-morphology, and analyze the composition of the film. The XRD instrument model used was D8 Advance (Bruck, Germany). Samples of XRD were used to detect the elemental analysis of the coating. The contents of molten salt were characterized by ICP-AES.

3.1 Electrochemical behavior of Ti

Synthesis of Ti^x+ (2<x<3) by the reaction of Ti⁴⁺ and Ti metal in various molten salts has been extensively confirmed. The Ti^x+ ions obtained by this reaction vary from case to case because the stability of Ti²⁺ or Ti³⁺ depends on both the composition of the molten salt and experimental conditions (e.g. temperatures). In order to identify the state of Ti^x+ in our case, the melts from LiCl-KCl-K₂TiF₆-Ti were sampled at particular time intervals, and the samples were characterized using various techniques to deduce the evolution of Ti^x+ (2<x<3) ions in this system.

ICP-AES was used to determine the concentration of dissolved Ti ions, and was found to be 5.2 × 10^-3 mol/L for LiCl-KCl-Ti (5.0 mol%) after 16 h, 2.0 mol/L for LiCl-KCl-K₂TiF₆ (1.0 mol%)-Ti after 11 h, and 0.24 mol/L for LiCl-KCl-K₂TiF₆ (0.5 mol%) after 15 h, respectively (Table 1). The results showed that the dissolving concentration of Ti⁰ metal in LiCl-KCl was very low (approximately 170 ppm) while the dissolution of Ti⁴⁺ (K₂TiF₆) reached saturation easily in LiCl-KCl-K₂TiF₆ (0.5 mol%) system. In contrast, the concentration of Ti^x+ significantly increased as K₂TiF₆ and Ti metal were present together in the melts, and implied that the reaction between Ti⁴⁺ and Ti occurring in this case, introduced extra Ti ions into the melts as the dissolved states. It should be noted that the concentration of Ti^x+ in LiCl-KCl-K₂TiF₆ (1.0 mol%)-Ti was 10 times of that in LiCl-KCl-K₂TiF₆ (0.5 mol%), which further indicated the contribution of other reactions in introducing the Ti ions into the melts rather than reaction (5) because stoichiometric calculation from this reaction indicated that the concentrations of Ti^x+ increased only by 67 %. However, previous literatures indicated that not only Ti⁴⁺, but also Ti³⁺ reacted with Ti metal to form low-valent Ti ions.

Fig. 2 show the cyclic voltammetry (CV) curves of LiCl-KCl-K₂TiF₆ (0.5 mol%)-Ti that were recorded after particular time intervals. The results obtained in LiCl-KCl-K₂TiF₆ (0.5 mol%) are also shown for comparison. Three couples of redox peaks were observed in LiCl-KCl-K₂TiF₆ (0.5 mol%) melts, which are marked as A/A’, B/B’, and C/C’ respectively. According to the previous works, one can easily assign A/A’ to Ti⁴⁺→Ti³⁺, B/B’ to Ti³⁺→Ti²⁺, and C/C’ to Ti²⁺→Ti⁰. The CV curve of LiCl-KCl-K₂TiF₆ (0.5 mol%)-Ti evolve with time. Firstly, the disappearance of Ti⁴⁺ (A/A’) signal, and the simultaneous promotion of Ti³⁺ (B/B’) and Ti²⁺ (C/C’) peaks implies the reaction between Ti⁴⁺ and Ti⁰ occurring in this system, which further depletes the Ti⁴⁺ ion while producing Ti³⁺, Ti²⁺ in the melts. As the time increased, the signal of Ti²⁺ peak increased continuously accompanied with coinstantaneous suppressing of the signal of Ti³⁺, indicating a shift from Ti³⁺ to Ti²⁺. In fact, the dominance of reaction (6) in the system promotes the reduction of Ti⁴⁺ and Ti⁴⁺ ions were exhausted.

Fig. 2.Full-screen View

(Color online) Cyclic voltammetry curves of (a) LiCl-KCl-K₂TiF₆ (0.5 mol%)-Ti, and (b) LiCl-KCl-K₂TiF₆ (0.5 mol%) at 823 K by using W as the working electrode. Scanning rate: 0.1 V/s, CE: graphite rod, RE: Ag/AgCl.

Figure 3 shows the time-depedent evolutional XPS patterns obtained for LiCl-KCl-K₂TiF₆ (0.5 mol%)-Ti. It can be inferred that Ti⁴⁺ and Ti³⁺ are the dominant species at 15 h. As the time elapsed, the signal of Ti⁴⁺ gradually decreased with increase in the generated pattern of Ti²⁺, implying the reaction between Ti⁴⁺ and sponge Ti to consume the Ti⁴⁺, and consequently, form Ti²⁺. Eventually, Ti⁴⁺ was exhausted and the only Ti species present in the system were Ti³⁺ and Ti²⁺, in which the Ti²⁺ was the dominant one. The observations further confirmed the results of the CV experiments.

Fig. 3Full-screen View

(Color online) XPS spectrum of LiCl-KCl-K₂TiF₆ (0.5 mol%)-Ti molten salt with different reaction time under melting state at 823 K.

3.2 Electroplating Ti film on 316L matrix

After preparing Ti^x+ electrolyte, the electrochemical experiments and deposition processes were carried out using a three-electrode system, in which a Ti wire was used as the counter electrode, a 316L stainless steel wire as the cathode, and Ag/AgCl molten salt as the reference electrode. Serial experiments were conducted at various conditions to evaluate the influence of deposition parameters such as applied current density, deposition time, and the temperature on the obtained Ti plated film (Fig. 4, Fig. 5, and Fig. 6).

Figure 4.Full-screen View

SEM images of titanium films deposited for 300 s with various current densities (a) -74 mA∙cm^-2, (b) -93 mA∙cm^-2, in LiCl-KCl-K₂TiF₆ (1.0 mol%)-Ti at 773 K, WE: 316L SS, CE: Titanium wire, RE: Ag/AgCl.

Fig. 5.Full-screen View

SEM images of titanium films deposited by applying -74 mA∙cm^-2 current density with different deposited times (a) 300 s, (b) 600 s, (c)1200 s, (d)2400 s, in LiCl-KCl-K₂TiF₆ (1.0 mol%)-Ti at 773 K, WE: 316L SS, CE: Titanium wire, RE: Ag/AgCl.

Fig. 6.Full-screen View

SEM images of titanium films deposited on -74 mA∙cm^-2 for 300 s with different deposited temperatures (a)773 K, (b)823 K, in LiCl-KCl- K₂TiF₆ (1.0 mol%)-Ti, WE: 316L SS, CE: Titanium wire, RE: Ag/AgCl.

Fig. 4 shows the SEM images of Ti obtained by applying different current densities at 773 K with fixed concentration of Ti^x+ in the melts. A statistical study of these data revealed that the average sizes of the deposited Ti layer were equivalent (2.0 μm), although the applied current densities varied, revealing a weak correlation between the current densities and crystal sizes thus formed. Indeed, the morphology of the electrodeposited Ti has been reported to mainly depend on the nucleation rate. In given conditions, the nucleation rate depends on the concentrations of reactive Ti ions on the surface of the electrode rather than current density or temperatures. In turn, the concentrations of Ti ions in the bulk molten salt, and the applied current densities on the electrode resulted in the rate of consumption of Ti ions in the diffusion layer near the electrode during the coating process ^[29].

In contrast, the granular average sizes were found to grow with a prolonged time (Fig. 5). The sizes were found to be 1.8 µm for a duration of 300 s and 600 s, while they were found to be 6.0 µm for a duration of 1200 s and 2400 s. The results indicated that longer deposition time was favorable for a layer composed with larger Ti particles. However, the sizes of Ti particles were distributed in two regions, small particles during the initial period while large ones during the extended time. It has been previously reported that the orientation of deposited Ti depended on the initially formed nuclei ^[10]. The extended deposition time led to the orientational growth of initial nuclei into larger granular rather than new nuclei formation.

The influences of deposition temperatures on the obtained Ti crystal sizes were also evaluated (Fig. 6). The obtained particle size was 2.0 μm for 773 K while it was 6.0 μm for 823 K, thus indicating the benefits of an elevated temperature for large particle formation.

In contrast to the deposition time, the current densities and temperatures were correlated directly to the electrode reaction and/or the stability of Ti^x+ in the melts. More negative (higher) current densities exhibited a better reductive capability, and thus, higher density will easily cause the reduction of high-valent Ti ions in the melt, while lower density environments are more conductive to the reduction of low-valent Ti ions. Actually, the Ti^x+ in our study was in a metastable state. Therefore, a narrow range of applied current densities were available for tuning. In addition, the temperature has more complicated influences on the system. On the one hand, the temperature evaluation drives the reaction Ti⁴⁺→Ti³⁺→Ti²⁺→Ti⁰ to the right. On the other hand, higher temperature was considered more feasible for the diffusion of Ti ions, and further favorable for the electrode reaction. Unfortunately, the temperature change in our case was limited by the physicochemical properties of the melts (723–873 K). When the temperature was more than 823 K, significant vaporization of melt components was observed which changed the compositions of this system subsequently.

The obtained Ti plated 316L samples were cut out and the EDS section model was used to analyze the thickness of the coating and the reltionship between Ti layer and the substrate (Fig. 7). Interestingly, the thicknesses of the Ti layers were not linearly related to the deposition times. It was determined to be 5 μm for 300 s and 600 s, 17 μm for 1200 s, and 10 μm for 2400 s. Based on the observations, it is reasonable to deduce the deposition process is composited of three stage. The nuclei formation and grain growth occurred at the initial stages (below 600 s). As discussed above, the nuclei formation depended on the reactive Ti ions and the applied current densities. In our case, equivalent thicknesses were observed in 300 s and 600 s because, although the formation of nuclei were equivalent, the prolonged time contributed to the growth of the granular for covering the substrate. After covering of the substrate was completed, new nuclei were formed on the initial Ti layer, and therefore, an increase in the Ti layer was observed with an increase in the time period (1200 s). At the extended stage, the possibility of consumption of Ti^x+ near the electrode and the generation of high-valent Ti ions at the anode changed the composition of the melts. The high-valent Ti ions reacted with the deposited Ti layer of the cathode, and consequently, the Ti layer decreased as the time period increased (2400 s). Additionally, there is a possibility of potential defects that could be generated during such reactions, which would lead to the loss of anti-corrosion properties, and is indicated by the following experiments.

Fig. 7Full-screen View

(Color online) Cross-sectional characterization and EDS map scanning of titanium film on 316L substrate that was obtained at various conditions. (a) -74 mA∙cm^-2 for 300 s, (b) -74 mA∙cm^-2 for 1200 s at 773 K in LiCl-KCl- K₂TiF₆ (1.0 mol%)-Ti.

Moreover, it was clearly shown that the Ti plated layer and the 316L substrate were connected by a Ti-Fe alloy inter-layer, and the thicknesses of the inter-layers changed with the deposition times (longer time indicated thicker connection layer).

3.3 Anti-corrosion behavior of Ti-coated 316L

The anti-corrosion behavior of the as-prepared Ti-coated 316L was evaluated using potential polarization dynamics method in a 3.5-wt% NaCl aqueous solution (Fig. 8). Table 2 shows the analytical results of the polarization curves. The corrosion potentials, which are regarded as an index for chemical stability, were observed to shift positively, indicating a significant improvement in the corrosion tolerance of Ti-coated 316L either for the 5 μm layer, 10 μm layer, or 17 μm layer. Meanwhile, the corrosion reaction was suppressed from the kinetics point of view. This could be inferred by the dramatical decrease of the corrosion current and the calculated corrosion rate of the Ti-coated samples.

Fig. 8Full-screen View

(Color online)Polarization curves of raw 316L and Ti-coated 316L in 3.5-wt% NaCl solution at scanning rate of 1 mV/s (raw 316L; 5 μm Ti;10 μm Ti; 17 μm Ti).

However, two observations were noted for the anti-corrosion test. The first one was the determined values of 5 μm and 17 μm Ti corrosion current, and the corrosion rates were almost equivalent despite the varying thickness of Ti film. The corrosion rate of the 10 μm Ti-coated layer sample was the smallest, implying that the initial protection property was contributed by the presence of Ti-plated film rather than the thickness of layers, and we can conclude that the sample with 10 μm Ti coating had the best thickness condition. The corrosion potentials were determined to be -1 mV for the 5 μm Ti layer, and -37 mV for the 10 μm Ti-coated sample while it was -61 mV for the 17 μm Ti layer. The negative shift of the corrosion potential, as the layer thickness increased, indicated the loss of chemical stability during the layer growth process, that attributed to the presence of possible defects in the layer. In fact, the anti-corrosion properties were reported to deteriorate with increase in the Ti-coated layer.