Soft Sparking Discharge Mechanism of Micro-Arc Oxidation Occurring on Titanium Alloys in Different Electrolytes

Abstract

Soft sparking during micro-arc oxidation can form a ceramic coating with high hardness and high bond strength on titanium alloy while avoiding the continuous strong micro-arc that can damage the substrate properties and the integrity of the coating. Existing studies have reported that the soft spark discharge is significantly influenced by the electrolyte anions, and the detailed mechanism of its influence remains unclear. Therefore, we considered four monolithic electrolytes, namely Na₂B₄O₇, NaF, Na₃PO₄, and Na₂SiO₃, for the bipolar pulsed micro-arc oxidation (MAO) treatment of the Ti6Al4V alloy to investigate the mechanism of the soft sparking discharge and the affections of different electrolytes on the soft sparking discharge. The results showed that soft spark discharges were observed in both Na₂SiO₃ and Na₃PO₄ electrolytes while not in Na₂B₄O₇ and NaF electrolytes. We attributed this situation to the fact that the deposition of Si and P elements in the coating changed the structure and passivation ability of the coating and affected the rate of ion transport and electron tunneling in the coating, resulting in forming a thick and dense, soft spark MAO inner layer. Additionally, the soft sparking discharge facilitated particle deposition and did not destroy the structure of the initial film layer, and also had no significant effect on the corrosion resistance.

1. Introduction

The Ti6Al4V alloy is widely used in the electronics, aerospace, automotive, and biomedical industries owing to its high specific strength, low modulus, high yield strength, high toughness, excellent corrosion resistance, and adequate biocompatibility [1]. Surface modification techniques, such as micro-arc oxidation (MAO), are often used to improve the surface properties of Ti6Al4V and further expand its applications in different fields [2,3,4,5,6,7,8,9,10,11].

Soft sparking is a unique phenomenon in the MAO process. When the ratio of the anode current to the cathode current is 0.8–1.0, a soft sparking transition can be created during the micro-arc discharge to achieve a voltage drop [12]. This voltage drop weakens the spark intensity and prevents the negative effect of the increasing spark intensity on the coating. Consequently, a thicker, more uniform, and smaller hole-diameter amorphous oxide coating can be generated on the metal surface. Moreover, this process does not require any manual changes in the electrical parameters. Soft sparking discharge was initially observed in Al-based alloys. Tsai et al. [13], Melhem et al. [14], and Bahador et al. [15] used the method of increasing the cathode charge to overcome the discharge channels and defects that are formed when the anode charge is greater than the cathode charge during the MAO of aluminum alloys. The results indicated that after a period of MAO processing, the anode voltage decreases and enters the stage where an extremely small spark is observed. This soft-spark MAO resulted in thicker coatings, increasing the thickness ratio of the dense to porous layers and reducing the defects in the coating in comparison with normal MAO. Martin et al. [16] and Tjiang et al. [17] further investigated the mechanism of a soft-spark formation on the surfaces of aluminum alloy and determined that the soft sparking transition began with the formation of flake nanocomposites, which gradually filled the coating’s cavities. The cavities being filled can effectively reduce the porosity of the coating, thus enhancing the adhesion of the coating to the substrate. This has been confirmed in the study by Tsuekawa [18]. Chen et al. [19] attributed the wear resistance of the coating also to the different sizes and distribution of the cavities in the coating. Thus, the filling of cavities also contributes to the wear resistance of the coating. Typically, the dense inner layer formed by the soft sparking of aluminum alloys is of significant interest to researchers. Dense oxides protect the underlying metal from corrosion and moisture penetration, which is particularly essential for applications requiring high reliability. Several researchers identified soft sparking phenomena in the MAO treatment of magnesium-based alloys, including an abrupt reduction in the photoemission, acoustic emission, and voltage in a constant current mode [20]. The coating obtained under the soft-spark discharge of magnesium alloys includes a porous outer layer and an inner layer with low porosity; however, the microstructure of the coating was inferior to that observed in the coating obtained via the soft-spark discharge of aluminum alloys. Additionally, residual pores were present in the inner layer [20]. Hussein et al. [21,22] performed the MAO of magnesium alloys in electrolytic solutions of NaAlO₂ and KOH. They determined that when only the anodic monopolar current is used, the microstructure of the coating prepared at the positive-to-negative current ratios of 0.74 and 0.63 is superior to that of the coating prepared at a positive-to-negative current ratio of 1.0. The cathodic current in the mixed-current mode repaired the defects of the previous monopolar current treatment and achieved the best corrosion resistance. Tjiang et al. [17] investigated the soft sparking discharge phenomenon during the MAO of pure magnesium and reported that the phenomenon occurred at an anodic to cathodic charge ratio of 0.85. The soft sparking discharge significantly reduced the harmful strong arc in the oxide layer, and a thick uniform oxide layer was successfully developed on a pure magnesium surface without large discharge channels and porosity. Although the coating structure was improved compared to the non-soft sparking treatment, a few cracks and pores existed.

A soft-spark MAO treatment can produce dense soft-spark layers on the surfaces of aluminum and magnesium alloys; however, this phenomenon is currently less successful on titanium alloy [20]. Martin et al. [16] attributed this to the significant difference in the surface plasma discharge characteristics of titanium and its alloys compared to those of magnesium and aluminum; however, the mechanism involved remains unclear. Yao et al. [23] and Hussein et al. [24] used double-pulse currents in a NaAlO₂-Na₃PO₄ electrolyte for the MAO treatment of the Ti6Al4V alloy. Herein, although no soft sparking discharge was observed, the internal densities of the coating were slightly increased. Furthermore, an increase in the cathode component of the bipolar current increases the amount of Al₂TiO₅ in the coating, which in turn reduces the number of residual discharge channels and increases the coating thickness and density [25]. Aliasghari et al. [26] performed the MAO treatment of titanium in silicate and phosphate electrolytes using electrical parameters with a positive-to-negative current ratio of 1 in a constant current mode. During the MAO process of titanium, a voltage drop similar to that occurring in the soft sparking transition phenomenon of aluminum was observed. Additionally, the acoustic and optical emissions were reduced, which is common in the soft sparking discharge phenomena; however, the coating did not produce a densification effect.

The aforementioned analysis indicates that the soft-spark discharge characteristics of titanium alloys can be observed during a bipolar pulsed MAO treatment in electrolytes containing SiO₃²⁻ anions. However, electrolytes containing AlO₂⁻ do not exhibit these characteristics, indicating that the composition of the micro discharge plasma is significantly influenced by the electrolyte anions. Nevertheless, the detailed mechanism of its influence remains unclear. To investigate the mechanism of the soft-spark MAO process and reveal the growth mechanism of the coating of titanium alloy, the following experiments were designed: Four monolithic electrolytes, namely the Na₂B₄O₇ electrolyte, NaF electrolyte, Na₃PO₄ electrolyte, and Na₂SiO₃ electrolyte were selected for the MAO treatment on the Ti6Al4V alloy in a bipolar pulsed MAO device [17,26,27,28]. Then the anode voltage, the morphology of the spark, the microstructure evolution of the MAO coatings, and the corrosion resistance of the MAO coatings during the MAO were observed [29,30,31,32] to analyze the formation mechanism of soft sparking MAO on titanium alloys.

2. Materials and Methods

The experiments were conducted using Ti6Al4V alloy rods with a composition content distribution of 0.3% Fe, 0.015% H, 0.2% O, 6.1% Al, 4.1% V, and the rest Ti. The Ti6Al4V alloy rods were wire-cut into specimens with a diameter of 18 mm and a thickness of 5 mm, and a threaded hole of M3 × 0.5 × 5 mm was machined on the cylindrical curved surface for connecting to the anode titanium rods on the MAO device. Then, the machined cylindrical specimens were sanded and polished, ultrasonically cleaned with anhydrous ethanol, and dried.

According to the experimental requirements, a small, dedicated MAO device was designed and built, and the schematic diagram of the device is shown in Figure 1. The device consisted of a high-frequency double-pulse DC power supply (AN-7505DM, Wuxi, China), an electrolyte bath, a cooling system, an anode speed control system, and an auxiliary system. The test was conducted in the constant current mode for double-pulse MAO experiments, with an anode current of 0.4 A and a cathode current of 0.7 A, and a pulse duty cycle of 25%. The electrolytes were 0.075 mol/L Na₂B₄O₇·10H₂O, 0.075 mol/L NaF, 0.075 mol/L Na₃PO₄·12H₂O, and 0.075 mol/L Na₂SiO₃·9H₂O solutions, respectively. A liquid cooling circulation system was used to control the electrolyte temperature at 10~15 °C during the MAO. The processing time of the samples in each electrolyte and the corresponding labeling method are shown in

Table 1. The processing time in each electrolyte and the corresponding labeling method of each sample.

Electrolyte	Abb.	Treatment Time	Sample No.
Na2B4O7·10H2O	B	1000 s	B 1000
Na2B4O7·10H2O	B	12,000 s	B 12,000
NaF	F	600 s	F 600
NaF	F	12,000 s	F 12,000
Na3PO4·12H2O	P	1800 s	P 1800
Na3PO4·12H2O	P	12,000 s	P 12,000
Na2SiO3·9H2O	Si	1200 s	Si 1200
Na2SiO3·9H2O	Si	12,000 s	Si 12,000

.

The specimens were observed by a focused ion beam field emission scanning double beam electron microscope (FIB/SEM, Auriga, Zeiss, Germany) and its equipped X-ray energy spectroscopy probe (EDS) for a microstructure and composition analysis. A multifunctional high-resolution X-ray diffractometer (XRD, Empyrean, PANalytical, The Netherlands) with a diffraction angle measurement range of 10°–90° and a scanning speed of 1° min⁻¹ was used for the XRD analysis of the MAO coatings. The phase content was then quantified by the adiabatic method [33]. According to the theory that the peak intensity of the phase in the mixture is proportional to the mass fraction, if there are n phases in the system, the mass fraction of the phase can be calculated by Equation (1):

$$W_i=\frac{I_i}{K_j^i{\displaystyle {\sum }_{i=1}^N\left(\frac{I_i}{K_j^i}\right)}}$$

(1)

where $W_i$ is the mass percentage of phase $I$, $I_i$ is the diffraction intensity of the strongest diffraction peak of phase $I$ and $K_j^i$ is the diffraction intensity of the strongest diffraction peak after mixing the phase I and reference phase $J$ at 1:1. The value $K_j^i$ is from the PDF card. The MAO coating thickness was measured using a Fischer DUALSCOPE^®MPO film thickness gauge. In this study, the MAO coating thickness was measured at 15 points in the middle of each specimen, and the average value was taken as the valid value of the MAO coating thickness for that specimen. The RST5000 electrochemical workstation (Dufu instruments, Zhengzhou, China) was used to measure the polarization curves of the MAO specimens treated with different parameters in a 3.5 wt% NaCl solution. In the three-electrode system of the electrochemical workstation, the saturated glycerol electrode was the reference electrode, the platinum electrode was the auxiliary electrode, and the micro-arc oxidation specimen was the working electrode (anode). The exposure area of the test specimens was 1 cm², the test temperature was 25 °C, the scanning potential range was −1–1 V, and the scanning speed was 1 mV/s. Before the polarization resistance test, all specimens were immersed in a 3.5 wt% NaCl solution for 2 h to obtain a stable open circuit potential. Based on the measured polarization curves, the Tafel constants (${\beta }_{\alpha }$ and ${\beta }_c$), corrosion potential ($E_{corr}$), and corrosion current density ($i_{corr}$) were calculated by fitting using origin software (version 8.0). Then, the polarization resistance ($R_p$) was calculated according to the Stern–Geary equation [34]:

$$R_p=\frac{{\beta }_{\alpha }\times {\beta }_c}{2.303i_{corr}\left({\beta }_{\alpha }+{\beta }_c\right)}$$

(2)

3. Results and Discussion

3.1. Voltage–Time Response

Figure 2a depicts the anode voltage–time response curves during MAO in the four electrolytes. In the case of electrolyte B, the anode voltage initially increased linearly and rapidly; this stage is represented as stage Ⅰ. Subsequently, the anodic oxide film dilapidated, gradually increasing the anode voltage parabolically (stage Ⅱ). When the anode voltage exceeded approximately 380 V, it oscillated in waves and then stabilized (stage Ⅲ); no significant reduction in the anode voltage was observed during the MAO. Figure 2b illustrates the discharge spark morphologies in different electrolytes at stages Ⅱ and Ⅲ. In electrolyte B, the intensity and color of the spark in stage Ⅲ did not change from those observed in stage Ⅱ. As a significant reduction and weakening in the anode voltage and spark, respectively, are the two characteristic effects that accompany soft sparking discharge [35], it can be concluded that the soft sparking discharge did not occur during MAO in electrolyte B. In the case of electrolyte F, the anode voltage increased slightly, followed by a small reduction (stage Ⅱ) as the oxide film dilapidated; this was immediately followed by the oscillation of the anode voltage (stage Ⅲ). As the morphology of the sparks remained unchanged and faint, no soft sparking discharge occurred during MAO in electrolyte F. In the case of electrolyte P, a gradual increase was observed in the anode voltage parabolically when the anodic oxide film was destroyed. Herein, the intensity of the spark was high, and the spark was bright orange (stage Ⅱ). Thereafter, the anode voltage decreased gradually. When the MAO treatment time exceeded approximately 1400 s, the anode voltage rapidly decreased to the breakdown voltage of the anodic oxide film and maintained a slow rising, wave-shaped oscillation (stage Ⅲ). The intensity of the discharge spark momentarily decreased in this stage (Figure 2b), which is a typical soft sparking phenomenon. The situation in electrolyte Si was similar to that observed in electrolyte P, except that the magnitude and rate of the anode voltage drop were relatively small, and the spark intensity was not reduced substantially.

In summary, electrolytes can be divided into two categories based on the variation pattern of the anode voltage. The first type includes Na₂B₄O₇ and NaF electrolytes. Here, no soft sparking discharge was observed. The second type includes Na₃PO₄ and Na₂SiO₃ electrolytes. Here is a typical soft sparking discharge phenomenon.

3.2. Microstructure Characteristics of MAO Coatings

Figure 3 depicts the surface morphology, cross-sectional morphology, and the corresponding EDS element distribution of the coating obtained by performing the MAO treatment in the Na₂B₄O₇ electrolyte for 1000 and 12,000 s. As indicated in Figure 3a, the MAO coating thickness was approximately 1.99 μm, and a nearly 0.5-μm-thick dense barrier layer was formed at the interface between the coating and substrate when MAO was performed for 1000 s. The MAO coating comprised numerous crater-shaped discharge pores sized 0.5–1 μm and a few cascades of discharge pores larger than 2 μm in size. A few molten oxides that were not completely dissolved due to the weakening of the micro-arc discharge strength by the cathodic current, accumulated around the pores. Although these molten oxides did not block the pores, they formed a mesh structure after interconnecting the pores, which significantly differed from the previously reported morphology of the coating obtained using a single pulse [29].

The EDS results of the surface and cross-section of the MAO coating indicated that nearly no B elements existed on the surface and inside the coating. This is because the B₄O₇²⁻ ions in the electrolyte combine with OH⁻ to form B(OH)₄⁻. At a high temperature generated by the micro-arc discharge, B(OH)₄⁻ combines with Ti⁴⁺ to form soluble [TiB(OH)₅]²⁺ (Equations (3) and (4)) [29]:

Na₂B₄O₇ + 3H₂O + 2OH⁻ → 2B(OH)₄⁻ + 2Na⁺ + 2BO₂⁻

(3)

Ti⁴⁺ + B(OH)₄− + 2OH⁻ → [TiB(OH)₅]²⁺

(4)

When the MAO treatment time was increased to 12,000 s, numerous worm-like cascade discharge pores uniformly appeared on the coating surface (Figure 3b). The cathode current eventually failed to suppress the formation of the cascading discharge. The thickness of the coating increased to 7.64 μm, while the coating/substrate interface remained nearly unchanged and continued to be approximately 0.5-μm-thick. Based on the local magnification, numerous nanoscale particles were scattered on the surface of the coating, including the smooth molten oxide surface and inner wall of the discharge pores. This indicated that the cathodic current facilitated the discharge of the plasma atmosphere on the coating surface. The EDS results indicate that the particles primarily comprised Ti and O.

Figure 4 depicts the surface morphology, cross-sectional morphology, and corresponding EDS element distribution of the coatings obtained in the NaF electrolyte at 600 and 12,000 s. As indicated in Figure 4a, the coating comprised numerous discharge pores with a diameter of approximately 0.5 μm when MAO was performed for 600 s. The thickness of the coating reached 0.92 μm, and a dense barrier layer with a thickness close to 0.5 μm was formed at the coating/substrate interface. A few cake-like molten oxides accumulated around the discharge pores, which was significantly different from the morphology of the coating obtained with a single-pulse, as reported in previous studies [36]. This can be attributed to the combination of F⁻ in the electrolyte with Ti⁴⁺ to form a soluble TiF₆²⁻ under high-temperature conditions generated by the micro-arc discharge (Equations (5) and (6)). However, the cathodic current reduced the micro-arc discharge intensity, resulting in a few molten titanium oxides not being dissolved and accumulating around the discharge pores, which formed lamellar or large cake-like discharge pores.

Ti + 2H₂O →TiO₂ + 4H⁺ + 4e⁻

(5)

TiO₂ + 6F⁻ + 4H⁺ → TiF₆²⁻ + 2H₂O

(6)

As indicated in Figure 4b, when the MAO treatment time was increased to 12,000 s, a dry and cracked riverbed morphology appeared on the coating surface with no significant change in the discharge pore size. The coating thickness increased to 2.73 μm, and the thickness of the coating/substrate interface layer remained nearly unchanged at approximately 0.5 μm. However, numerous crack channels appeared on the outer layer of the coating. The magnified view indicates that the coating surface, containing a smooth molten oxide surface and cracked inner wall, was scattered with numerous nano-scale particles. The EDS results verify that these particles primarily comprise Ti and O, with small amounts of F retained. This phenomenon validates that the cathode current facilitated the discharge of the plasma atmosphere on the surface of the coating.

Figure 5 depicts the surface morphology, cross-sectional morphology, and the corresponding EDS element distribution of the coating obtained in the Na₃PO₄ electrolyte at 1800 and 12,000 s. As indicated in Figure 5a, certain discharge pores with diameters of approximately 3~5 μm appeared on the coating surface, and large pores with diameters equal to or more than 10 μm in the porous layer outside the coating when the MAO was performed for 1800 s. The coating thickness reached 15.41 μm. More fine pores in the barrier layer were generated. Numerous molten oxides accumulated around the discharge pores in a typical crater morphology, which was similar to the morphology of the coatings obtained using a single-pulse reported in the literature [37]. The EDS results verify that more phosphate remained in the outer porous layer of the coating.

As depicted in Figure 5b, the coating thickness increased to 15.81 μm when the MAO treatment time was increased to 12,000 s. Herein, the pore morphology on the coating surface did not change significantly due to the weakness of the discharge intensity. Some nanometer-sized particles appeared inside the discharge pores of the coating and filled the pores. These particles were molten oxides resulting from the soft sparking discharges that appeared inside the coating. The EDS results indicate that the particles primarily comprised Ti and O.

Figure 6 shows the surface morphology, cross-sectional morphology, and corresponding EDS element distribution of the coating obtained in the Na₂SiO₃ electrolyte at 1200 and 12,000 s. As indicated in Figure 6a, micropores with a diameter of approximately 2 μm and several sub-micron pores formed by the escaped gases or soft sparking discharge appeared on the surface of the coating when MAO was performed for 1200 s. Additionally, numerous molten oxides were deposited around these pores, resulting in a typical crater morphology. The EDS indicates that the fused oxide comprised Ti, O, and Si, implying that it was a co-deposition of silica and titanium dioxide. This rough surface with a typical crater morphology can be attributed to the poor mobility of the fused titanium dioxide containing silica oxide [12]. Based on the cross-sectional morphology of the coating, the thickness of the coating reached 10.17 μm, and a dense barrier layer of 0.5–1 μm in thickness was formed at the coating/substrate interface.

As depicted in Figure 6b, numerous submicron pores appeared on the surface of the deposited molten oxides when MAO was performed for 12,000 s, with multiple powdered molten oxides depositing around them. No apparent change was observed in the size of the micropores and crater morphology. The coating thickness reached 24.85 μm, and the dense barrier layer at the coating/substrate interface was replaced by an inner layer with a thickness of about 10 μm, generated by the soft sparking discharge; several submicron discharge channels existed in this inner layer. The EDS indicates that the co-deposited Si contents in the coating decreased significantly.

In summary, Si and P can be deposited inside the coating during MAO, changing the structure and the passivation ability of the coating, which thus affects the rate of ion transfer and electron tunneling in the coating [12]. Although soft sparking discharge appeared on the surface of the Ti6Al4V alloy in both Na₂SiO₃ and Na₃PO₄ electrolytes during the MAO process, the discharge was higher and more uniformly distributed in the Na₂SiO₃ electrolyte. However, the B and F elements were difficult to co-deposit with molten titanium oxide and exhibited a minor effect on the passivation ability of the coating. Therefore, soft sparking discharge behavior was not observed on the surface of the Ti6Al4V alloy in both Na₂B₄O₇ and NaF electrolytes. However, the cathodic current suppressed the spark discharge intensity and facilitated the formation of plasma atmosphere discharge.

Figure 7 illustrates the XRD diffractograms of the coating obtained by performing the MAO treatment for 12,000 s using the four electrolytes. As shown in the XRD patterns, anatase, rutile, and matrix titanium phases were detected in all the coatings. The three-phase content was quantified using the adiabatic method;

Table 2. Phase content of the coating obtained at 12,000 s in the four electrolytes.

Electrolyte	Rutile (wt%)	Anatase (wt%)	Ti (wt%)
Na2B4O7	49.64	2.63	47.43
NaF	30.79	18.09	51.11
Na3PO4	57.62	24.78	17.60
Na2SiO3	75.07	7.23	17.70

lists the corresponding results. In the Na₂B₄O₇ electrolyte, the coating was primarily in the rutile phase, which indicated that the micro-arc discharge intensity in this electrolyte was relatively high. The high discharge temperature caused most of the unstable anatase phase to gradually transform into a stable rutile phase. The high Ti content detected within the coating in the NaF electrolyte can be attributed to the low thickness of the coating obtained in this electrolyte. In comparison with the rutile content of the Na₂B₄O₇ electrolyte that within the coating obtained in the NaF electrolyte was significantly lower, whereas the anatase phase content was higher because of the low micro-arc discharge intensity in this electrolyte. In both Na₃PO₄ and Na₂SiO₃ electrolytes, the Ti content detected in the coatings was extremely low, indicating a thicker coating. However, the coating formed in the Na₂SiO₃ electrolyte comprised a higher rutile content than the coating formed in the Na₃PO₄ electrolyte. This can be attributed to the higher number of soft sparking discharges in the former electrolyte and the formation of soft sparking discharges inside the coating, which facilitated the formation of the rutile phase.

3.3. Corrosion Resistance of the MAO Coating

Figure 8 depicts the graphs of the kinetic potential polarization curves measured in a 3.5 wt% NaCl solution for the coating obtained via the MAO treatment in the four electrolytes.

Table 3. Fitting results of the potentiodynamic polarization curves.

Sample No.	${\mathit{\beta }}_{\mathit{\alpha }} \left(\mathbf{V}\right)$	${\mathit{\beta }}_{\mathit{c}} \left(\mathbf{V}\right)$	${\mathit{i}}_{\mathit{c}\mathit{o}\mathit{r}\mathit{r}} (\mathsf{\mu }\mathbf{A}/{\mathbf{cm}}^2)$	${\mathit{E}}_{\mathit{c}\mathit{o}\mathit{r}\mathit{r}} \left(\mathbf{V}\right)$	${\mathit{R}}_{\mathit{p}} \left(\mathsf{\Omega }{\mathbf{cm}}^2\right)$
B 1000	0.1688	0.1624	0.607	−0.1055	59,164
B 12,000	0.1635	0.1734	2.514	−0.2067	14,538
F 600	0.1687	0.1643	0.410	−0.1785	88,084
F 12,000	0.1659	0.1692	1.648	−0.1639	22,076
P 1800	0.1633	0.1677	2.706	−0.2221	13,278
P 12,000	0.1636	0.1756	2.760	−0.2248	13,322
Si 1200	0.1762	0.1607	3.198	−0.322	11,412
Si 12,000	0.1680	0.1698	2.938	−0.264	12,481

summarizes the obtained results.

The corrosion currents of B1000 and F600 were small, whereas their high polarization resistances resulted in better corrosion resistance. However, as the treatment time increased, the coating in the Na₂B₄O₇ electrolyte thickened while the corrosion resistance decreased. This can be attributed to the thickening of the coating; nevertheless, the destruction of the dense barrier layer, the increase in the pore size of the coating, and the decrease in the density resulted in poor corrosion resistance. Similarly, the thickness of the coating obtained in electrolyte F increased. However, numerous deep cracks were generated on the coating, decreasing the density of the coating and resulting in lower corrosion resistance. In the Na₃PO₄ and Na₂SiO₃ electrolytes, the corrosion resistance of the coating obtained after the soft sparking discharge did not change significantly. This is because the coating obtained in the Na₃PO₄ electrolyte was not substantially thick. Moreover, soft sparking discharge appeared in the discharge pores, and the fillings of the molten oxides produced by the soft sparking discharge increased the density of the coating. However, large cracks were generated on the coating, which caused no significant change in its corrosion resistance. Soft sparking discharge in the Na₂SiO₃ electrolyte increased the filamentary channels inside the coating and decreased the density of the coating. Therefore, the corrosion resistance did not change significantly despite the substantial increase in the thickness.

3.4. Mechanisms of Soft Sparking Discharge

Figure 9 depicts the mechanisms of soft sparking discharge during MAO in the four electrolytes. For the deposition type of electrolytes, such as Na₂B₄O₇ and NaF (Figure 9a), the MAO treatment entered stage Ⅱ with the destruction of the anodic oxide film, wherein discharge sparks were generated on the surface of the coating. The coating comprised a dense barrier layer and oxides outside the coating. As the MAO entered stage Ⅲ, the intensity of the sparks continued to increase, leading to an increase in the coating thickness, whereas the thickness of the barrier layer remained constant. Simultaneously, the size of the discharge pores and coating defects (cracks) increased, which was not conducive to improving the corrosion resistance of the coating.

Figure 9b shows the mechanisms of soft sparking discharge during MAO in the dissolution-type electrolytes, such as Na₃PO₄ and Na₂SiO₃. The comparison of the spark intensities of the two types of electrolytes in stage Ⅱ indicated that the sparks of the dissolution-type electrolyte were larger and stronger than those of the deposition-type electrolyte. Therefore, the coating in the dissolution-type electrolyte comprised larger discharge pores. As maintaining the high-intensity spark was difficult when MAO entered stage Ⅲ, the spark was softened, and numerous soft sparks were generated with low intensities. During this stage, a thick and dense inner layer was generated inside the coating, caused by a reduction in the discharge spark intensity and the transfer of the discharge spark to the inner part of the coating.

4. Conclusions

When the Ti6Al4V alloy was MAO-treated in silicate and phosphate ion electrolytes, the deposition of Si and P in the coating changed the structure and passivation ability of the coating, which affected the rate of ion transport and electron tunneling in the coating. As a result, soft sparking discharges were observed in both Na₂SiO₃ and Na₃PO₄ electrolytes. This led to the formation of a thick and dense inner layer but did not change the structure of the initial film layer and exhibited no significant effect on the corrosion resistance. In contrast, when the titanium alloys were subjected to MAO in borate and fluoride ion electrolytes, elements B and F were difficult to co-deposit with molten titanium oxides, and they had no significant effect on the passivation ability of the coating; therefore, no soft sparking discharge behavior was observed in Na₂B₄O₇ and NaF electrolytes.