Plasma electrolytic oxidation of Zircaloy-2 alloy in potassium hydroxide/sodium silicate electrolytes: The effect of silicate concentration

Abstract

This study mainly investigated the effects of silicate concentration at the range of (16 g/L to 56 g/L) on the plasma electrolytic oxidation of Zircaloy-2 in potassium hydroxide/sodium silicate electrolytes in detail, including the growth behavior, wear and corrosion resistance of as-obtained ZrO₂/SiO₂ alloyed coatings. It was found that the coating thickness increased continuously on increasing the silicate concentration in electrolyte. Besides, the amount of t-ZrO₂ in the coatings increased with increase of silicate concentration in electrolyte, while the amount of m-ZrO₂ decreased. Comparative studies have shown that the coatings formed in electrolyte with high silicate concentration possessed superior wear and corrosion performance, which could be ascribed to heavy silica deposition associated with the presence of t-ZrO₂ stabilized by SiO₂. The results may provide guidance for obtaining high performance of ZrO₂/SiO₂ alloyed coatings. Besides, it is believed that the presence of Si species in zirconia endows the coatings with enhanced bioactivity like bioactive glasses and ceramics coatings and we envision that the as-prepared ZrO₂/SiO₂ alloyed coatings have great potential for biological applications.

Resumen

Este estudio investigó en detalle los efectos de la concentración de silicato en el rango de (16 a 56 g/l), en la oxidación electrolítica por plasma de Zircaloy-2 en electrolitos de hidróxido de potasio/silicato de sodio, incluido el modo de crecimiento y la resistencia al desgaste y a la corrosión de los recubrimientos de aleaciones de ZrO₂/SiO₂ obtenidos. Se descubrió que el espesor del recubrimiento aumentaba continuamente al aumentar la concentración de silicato en el electrolito. Además, la cantidad de t-ZrO₂ en los recubrimientos aumentó con el aumento de la concentración de silicato en el electrolito, mientras que el contenido de m-ZrO₂ disminuía. Estudios comparativos han demostrado que los recubrimientos formados en electrolito con alta concentración de silicato poseían mayor resistencia al desgaste y a la corrosión, que podría atribuirse a una fuerte deposición de sílice asociada con la presencia de t-ZrO₂ estabilizado por SiO₂. Los resultados pueden proporcionar guía para obtener un alto rendimiento de los recubrimientos con aleaciones ZrO₂/SiO₂. Además, se cree que la presencia de especies de Si en la circona dota a los recubrimientos con una bioactividad mejorada como recubrimientos cerámicos o vítreos bioactivos, y prevemos que los recubrimientos de aleaciones ZrO₂/SiO₂ preparados tienen un gran potencial para aplicaciones biológicas.

Introduction

Surface treatment is a well-adopted method for preparation of advanced functional materials [1–8]. Recently, plasma electrolytic oxidation (PEO) as an effective surface treatment process has attracted much attention by scientists working in the fields of surface engineering and corrosion protection [9–12]. This technique evolved from conventional anodizing, but demands much higher working voltages to generate plasma discharge on the surface of treated work pieces. Under the high temperature of the plasma discharge, the functional ceramic coatings will be sintered and formed on the surface of the so-called valve metals (Mg, Al, Ti, Zr, etc.) or their alloys [13–18]. The technique is largely electrochemical in nature, however, plasma-enhanced physico-chemical processes are also known to participate concurrently in the coating formation process.

Zirconium alloys are commonly used as structural materials for water-cooled nuclear power reactors, where they are subjected to severe environmental conditions. Corrosion and wear are the main factors leading to the degradation of such materials [19–24]. Recently, many studies have shown that PEO treatments can be a feasible way to improve the corrosion resistance and wear resistance of these alloys [25–27]. More importantly, due to their excellent biocompatibility, high mechanical strength and fracture toughness, reasonably good corrosion resistance, low thermal conductivity, together with low elastic modulus (92 GPa) and low magnetic susceptibility [28–31], zirconium alloys are also seen as potential biomaterials. However, appropriate surface treatment is also required in such cases since the native oxide (ZrO₂) film on the alloys is bio-inert [28,32] and the formation of chemical bonds with bone tissue is difficult during implantation, which could be a drawback because an early integration between biomaterial and bone is advantageous for most implant applications [33].

Silicate-based electrolytes are widely used in the PEO of valve metals [11,12,14,34], and they also have been used for the treatment of zirconium and its alloys recently [25–27]. The use of silicate electrolytes can incorporate a large amount of Si species into the obtained coatings, which is an efficient way to enhance the biocompatibility of the underlying metals. Silicon (Si) is known to be an essential element for the normal growth and development of bone and connective tissues [35–37]. In recent years, an increasing number of evidence has supported the hypothesis that the presence of Si can contribute to the enhanced bioactivity of some bioactive glasses or ceramics, and significantly increase the up-regulation of osteoblast proliferation and gene expression [38].

PEO coatings on zirconium alloys are mainly consisted of zirconium oxide, which, as a promising engineering material, is known for its good chemical and thermal stability, wear resistance and mechanical strength [39]. The performances of the coatings are strongly correlated with the phase composition of present zirconia, which could be significantly determined by the processing conditions of PEO such as the electrolyte composition, electrical regime and so on. Among the various electrolytes that are suitable for PEO, silicate-, aluminate- and phosphate-based ones had been widely used. Cheng et al. [40,41] studied the phase compositions and microstructures of PEO coatings formed on zirconium alloys in an silicate-based electrolyte, and found that the wear resistance of the coatings was significantly improved because the silicon species that were incorporated into the coating had caused the stabilization of tetragonal zirconia (t-ZrO₂). Although a few researches about investigating the effect of electrolyte composition on PEO process have been reported, a high concentration of electrolyte, especially for Na₂SiO₃, is still rarely applied for PEO studies. In addition, studies on the content of Si in PEO coatings also play an important role in the preparation of biocompatible coatings. Finally, it was also reported that the use of a high concentration of aluminate electrolyte can benefited the wear resistance of the resultant PEO coatings significantly [42–44,40]. In this paper, the PEO behaviors of Zircaloy-2 in potassium hydroxide/sodium silicate electrolytes were investigated. The effects of silicate content in electrolytes on the wear and corrosion properties of the resultant coatings were compared for the first time, and our research may provide guidance for obtaining high performance of ZrO₂/SiO₂ alloyed coatings.

Experimental

Zircaloy-2 alloy was cut into the shape of rolled plate, and then mounted in resin to prepare specimens with a working area of 20 mm × 10 mm. The experimental setup was same as the one described in our previous paper [30]. The compositions of the electrolytes were 1 g/L KOH + c g/L Na₂SiO₃·9H₂O (c = 16, 32, 48 or 56). The electrical regime was a pulsed bipolar waveform, using a duty cycle of 20% at 1000 Hz [30], with the positive and negative current densities kept at 150 and 100 mA/cm² (rms), respectively.

The thicknesses of the coatings were determined by an eddy current thickness gauge (TT260, Time Group, Beijing). The surfaces and cross-sections of the PEO coatings were characterized by scanning electron microscopy (SEM, QUANTA 2000, FEI, USA, or a JEOL JSM6700F Instrument), and the elemental composition was analyzed by energy-dispersive X-ray spectroscopy (EDS). Phase compositions of the coatings were examined by using a Rigaku D/MAX 2500 X-ray diffractometer (Cu-Kα radiation).

The tribological performance of the PEO coatings was evaluated using a CETR UMT-3 tribometer. The test method utilized a Cr steel ball (diameter 9.5 mm, hardness 62 HRC) that slides against a flat PEO-treated specimen in a linear, reciprocating motion. A load of 20 N or 30 N was applied, with a stroke length of 7.5 mm and frequency of oscillation of 5 Hz.

Electrochemical behaviors of the samples were evaluated by potentiodynamic polarization curve measurements using a CHI660C electrochemical workstation in 3.5 wt% NaCl solution. The electrochemical tests were carried out at room temperature in a conventional three-electrode cell with a test area of 20 mm × 10 mm, in which the samples were used as the working electrode, a platinum electrode as the counter electrode and a saturated calomel electrode as the reference electrode. Before the electrochemical analysis, the samples were kept in the solution for 1 h to stabilize the open circuit potential (OCP). The voltage applied in the potentiodynamic polarization test was varied in the range of −1.2 V to 3 V at a scanning rate of 1 mV/s.

Results and discussion

Cell potential responses and kinetics of coating growth

The cell potential-time curves during the PEO treatment in the electrolytes with different silicate concentrations are presented in Fig. 1(a). As is shown in the picture, all positive potentials showed a similar rapidly rising trend before reaching up to breakdown potential, after which the increase rates were slowing down. The inflection point in voltage-time response is usually called breakdown point, at which the potential is defined as breakdown potential because plasma sparks could be observed obviously at this stage. It can also be seen that the breakdown potential increased from 310, 357, 379–418 V with the increase of electrolyte concentration (16, 32, 48 and 56 g/L), respectively. This is due to the fact that the breaking down potential is closely related to the specific resistance of electrolyte; specifically, an electrolyte with higher concentration usually has lower specific resistance, resulting in a lower breakdown potential. Depicted in Fig. 1(b, c) are the current and potential waveforms of PEO in electrolyte of 16 g/L Na₂SiO₃·9H₂O + 1 g/L KOH. As shown in Fig. 1(b), the current waveforms at different stages of PEO remained almost constant, which means the output of the power source is quite stable. Also, the potential of PEO increased gradually because the film thickness, and thus impedance, increased with time, as shown in Fig. 1(c). In addition, the relationship between coating thickness and time is shown in Fig. 1(d). It is obvious that the coating thickness increased almost linearly along with PEO processing time. After PEO treated for 30 min, the coating thickness reached 70.2 μm, 78.8 μm, 88.3 μm and 289.6 μm, and the average increasing rate is 2.3 μm/min, 2.6 μm/min, 2.9 μm/min and 9.7 μm/min under different Na₂SiO₃·9H₂O concentration of 16, 32, 48 and 56 g/L respectively. One possible reason for this phenomenon may be that more substance would participate in the coating formation process at a higher concentration of electrolyte, resulting in a higher coating growth rate. After treated for 30 min, the number of sparks shrank while they became more intense and tended to stay at a fixed point longer, giving rise to a higher standard deviation and a rougher surface.

Fig. 1.

(a) Positive and negative (absolute value) peak cell potential-time responses for the PEO of Zircaloy-2 alloy of different concentrated silicate electrolytes, (b) current and (c) voltage waveforms during PEO of Zircaloy-2 at different times in 16 g/L Na₂SiO₃ + 1 g/L KOH, (d) dependence of coating thickness on time of PEO for Zircaloy-2 in the different electrolytes. Error bars represent the standard deviations.

Phase composition of the coatings

ZrO₂ is the main composition of coating formed on Zircaloy-2 alloy by PEO treatment. There are three types of ZrO₂ at different temperature, i.e., monoclinic (m-ZrO₂), tetragonal (t-ZrO₂), and cubic phase (c-ZrO₂). Among them, m-ZrO₂ is a stable phase at room temperature, and t-ZrO₂ is also stable but usually formed at high temperature [45]. During PEO process, the substrate and oxide are melted due to the high temperature generated by plasma sparks, and different kinds of ZrO₂ phases are formed at different stage. Fig. 2 shows the XRD results of coatings formed in different concentrations of silicate electrolyte after 30 minutes’ PEO treatment. As presented in Fig. 2, the amount of t-ZrO₂ in the coatings increased with increase of silicate concentration in the electrolyte, while the amount of m-ZrO₂ decreased. This could be attributed to the fact that in higher concentration of silicate electrolyte more Si species, which are usually considered to be effective for the stabilization of t-ZrO₂, had incorporated in the resultant coatings, and thus leading to a larger amount of t-ZrO₂[46–50]. With the increase of Si content, more t-ZrO₂ would be formed in the coating, which is indicated by the weak diffraction peak of m-ZrO₂ and the dominant peak of t-ZrO₂ in the coating formed in 56 g/L Na₂SiO₃ + l g/L KOH electrolyte. It should be noted that a weak amorphous peak was also detected. Based on the EDS analysis, the amorphous should mainly be SiO₂, which is quite usual in PEO coatings formed in Si-containing electrolyte because of the good glass forming ability of the element Si. Further, the amorphous phase in PEO coatings is usually considered to exist in an extremely thin amorphous layer at the interface between coating and substrate and in the wall of the large inner pores inside the PEO coatings because of the quenching effect there [51–54,12,55–58].

Fig. 2.

XRD pattern for the coatings formed on Zircaloy-2 for 30 min in different concentrations of silicate electrolyte (from 8 g/L Na₂SiO₃ + l g/L KOH to 56 g/L Na₂SiO₃ + l g/L KOH).

Morphologies of the coatings

Fig. 3 shows the surface and cross-sectional morphologies of coatings formed after PEO treated for 30 min in electrolyte with different concentrations of silicate. The SEM images of the coating formed in electrolyte with 16 g/L Na₂SiO₃·9H₂O were shown in Fig. 3(a–c). As shown, the micro pores and pancake structures could be clearly observed. In addition, the stacked layered structure also exists in the coating, as indicated in the inset. This typical structure might be formed under the deposition of silicate which comes from the electrolyte. Notably, a white flower-like structure could be seen in Fig. 3(b), which displays a solidification structure and is a typical feature of the coatings formed in silicate-containing electrolyte on zirconium alloy; however, such structures are not encountered generally in PEO coatings because their formation requires the existence of relatively long-lived melt pools caused by persistent sparks [41]. As indicated in the cross-sectional images (Fig. 3(c)), the coating is consisted of a dense barrier layer which is close to the substrate and a porous outer layer. When increasing the concentration of Na₂SiO₃·9H₂O to 32 g/L, the surface and cross-sectional of the coating shows similar morphologies, as shown in Fig. 3(d–f). Interestingly, the amount of white typical structures is less than that of coating formed in electrolyte with 16 g/L Na₂SiO₃·9H₂O. Further increasing the silicate concentration to 48 g/L Na₂SiO₃·9H₂O, the Fig. 3(g–i) indicate that the surface morphologies are still similar to the above two cases. Nevertheless, there are less micro pores in the coating, as shown in the cross sectional images (Fig. 3(i)). Based on the results above, it is reasonable to conclude that with the increase of silicate concentration in the electrolyte, more silicate would take part in the coating formation process and then co-deposit with the ZrO₂, resulting in the formation of a dense layer which is rich in ZrO₂ and SiO₂.

Fig. 3.

Surface and cross-sectional morphologies of the coatings formed for 1800 s in the different concentration of silicate electrolyte: secondary (a, d, g) and backscattered (b–c, e–f, h–i) electron images; (a, b and c) 16 g/L Na₂SiO₃·9H₂O + 1 g/L KOH, (d, e and f) 32 g/L Na₂SiO₃·9H₂O + 1 g/L KOH, (g, h and i) 48 g/L Na₂SiO₃·9H₂O + 1 g/L KOH.

In order to verify the conclusion that the deposition of silicate increased with increase of electrolyte concentration, the images of coating formed after 30 min treatment in 56 g/L Na₂SiO₃·9H₂O electrolyte are showed in Fig. 4(a, b). Fig. 4(a) was obtained under the secondary electron mode and Fig. 4(b) was obtained under the backscattered mode. In addition, Fig. 4(c, d) are EDS analysis results of coatings formed in 56 g/L Na₂SiO₃·9H₂O + 1 g/L KOH electrolyte and 48 g/L Na₂SiO₃·9H₂O + 1 g/L KOH electrolyte, respectively. In Fig. 4(a), the surface of as-formed coating is quite rough, containing a number of humps. The rough surface area is deeper in color in the backscattered mode (Fig. 4(b)), which might be attributed to a higher deposition of silicon oxide. EDS analysis (Fig. 4(c, d)) indicated that the when increasing the concentration of Na₂SiO₃·9H₂O from 48 to 56 g/L, the Si content would increase significantly, which is indicative of the deposition of more silicate. To conclude, the increase of concentration of silicate in the electrolyte could lead to the increasing deposition of silicon oxide in the coating, thus hindering the formation of micro cracks and pores, and finally generating a denser coating.

Fig. 4.

Surface (a–b) morphologies of a coating formed for 1800 s in the 56 g/L Na₂SiO₃·9H₂O + 1 g/L KOH electrolyte; (c) and (d) are EDS analysis of coatings formed in 56 g/L Na₂SiO₃ 9H₂O + 1 g/L KOH electrolyte and 48 g/L Na₂SiO₃·9H₂O + 1 g/L KOH electrolyte, respectively.

Dry sliding wear behavior of the coatings formed in silicate electrolyte

Fig. 5(a) illustrates the relationship between the friction coefficient and friction time obtained by dry slide friction test of the coating after 30 min formed in electrolyte with 32 g/L Na₂SiO₃ + l g/L KOH and 48 g/L Na₂SiO₃ + l g/L KOH (the applied force is 20 or 30 N). For the curve corresponded to 32 g/L Na₂SiO₃ + l g/L KOH under 20 N, at the beginning of the test, the friction coefficient increased quickly, shifting from 0.56 to 0.68 at first 100 s. This might be attributed to the rough surface of the coating, in which the protuberance is easy to be worn down by the steel ball, resulting in a quick change of friction coefficient. During 100–600 s, the friction coefficient increased slightly from 0.68 to 0.79 and then kept constant at 0.79, which suggests that the transferring layer has been formed and the wear depth has reached the ultimate value. At this stage, only by increasing the load lead to the formation of a deeper wear scar. As for the curve corresponded to 32 g/L Na₂SiO₃ + l g/L KOH under 30 N, the friction coefficient increased quickly from 0.7 to 0.9 at the first 73 s, after which it dropped suddenly to 0.5, revealing that the coating has been worn out, then the coefficient stayed between 0.4 and 0.56. By contrast, regarding the coating formed in electrolyte with 48 g/L Na₂SiO₃ + l g/L KOH for 30 min, under a load of 30 N the friction coefficient initially increased from 0.67 to 0.78 in the first 156 s, then it dropped to 0.57, and finally fluctuates between 0.57 and 0.67 till the end of the test. Based on Fig. 1(d), the difference of coating thicknesses under 30 min treatment is slight, being 70 and 78 μm respectively; however, the former only lasted for 73 s before worn out, while the later was worn out until 156 s later under a load of 30 N. From Fig. 2, it is obvious that former coating has a lower amount of t-ZrO₂. Thus, the reason why the coating formed in the later (32 g/L Na₂SiO₃ + l g/L KOH) shows a better wear resistance can be reasonably attributed to its higher content of t-ZrO₂ than the former (32 g/L Na₂SiO₃ + l g/L KOH).

Fig. 5.

(a) Coefficient of friction as a function of sliding time under an applied load of 20 N or 30 N after the corresponding dry sliding tests for a coating formed for 30 min in 32 g/L Na₂SiO₃·9H₂O + 1 g/L KOH and 48 g/L Na₂SiO₃·9H₂O + 1 g/L KOH; (b) Coefficient of friction as a function of sliding time under an applied load of 30 N after the corresponding dry sliding tests for a coating formed for 20 min or 30 min in 56 g/L Na₂SiO₃·9H₂O + 1 g/L KOH.

Fig. 5 (b) shows the relationship between the friction coefficient and friction time obtained by dry slide friction test for coating formed in 56 g/L Na₂SiO₃ + l g/L KOH for 20 min and 30 min (the applied force is 30 N). It can be seen that the friction coefficient of the coating formed for 30 min fluctuated between 0.55 and 0.7 till the end of the test, and the coating was not worn out. In contrast, the wear resistance of the coating formed for 20 min is weaker than the former with worn out under slide drying 20 min.

Electrochemical tests

Fig. 6 shows the potentiodynamic curves of coatings formed for 30 min in electrolyte of different silicate concentration, the relating polarization data derived from polarization curves is summarized in Table 1. Because of their excellent corrosion resistance, zirconium and its alloys only corrode and dissolve in certain strong acids such as HF. Therefore, the passivation reaction occurs first in the corrosive medium, and the specific reactions are as follows:

Anode: Zr_(S) + O_2(g) → ZrO_2(s)

Cathode: O_2(g) + 2H₂O_(l) + 4e⁻ → 4OH⁻_(l)

Fig. 6.

The potentiodynamic curves of coatings formed for 30 min in different concentration of silicate electrolyte.

Table 1.

Polarization data derived from polarization curves (Fig. 5).

Samples	Ecorr/V	Icorr/A cm^−2
Bare alloy	−0.580	1.63 × 10^−7
16 g/L Na2SiO3 + 1 g/L KOH 30 min	−0.085	1.42 × 10^−8
32 g/L Na2SiO3 + 1 g/L KOH 30 min	0.032	2.23 × 10^−8
48 g/L Na2SiO3 + 1 g/L KOH 30 min	−0.481	1.99 × 10^−8
56 g/L Na2SiO3 + 1 g/L KOH 30 min	−0.510	1.05 × 10^−8

After the oxide film is formed, zirconium alloy can be effectively protected. Subsequently, pitting corrosion occurs, and the passivation film is gradually destroyed. As shown in Fig. 5(c), the pitting corrosion of the bare alloy begins at 0.16 V, while the potential of pitting corrosion for the PEO treated samples increased positively (∼1.09 V). All the corrosion current (I_corr) are almost lowered by one order of magnitude compared to the one of the bare alloy. Among all the samples, the PEO coating formed in electrolyte of 56 g/L Na₂SiO₃ + l g/L KOH has the lowest corrosion current (I_corr), which means the best corrosion resistance. The reason may be attributed to the fact that the coating formed in this electrolyte has less micro cracks and a larger thickness which hinders the penetration of corrosive media into the barrier layer. For the coating formed in electrolyte with 32 g/L Na₂SiO₃ + l g/L KOH, since it contains obvious cracks in the coating, corrosive media could easily pass through the outer layer and reach the barrier layer. Also, corrosion processes of coatings formed under other conditions almost have the same process. Based on the above results, it is the coating formed in 56 g/L Na₂SiO₃ + l g/L KOH that has the best corrosion resistance.

Conclusions

In conclusion, we provided a study of plasma electrolytic oxidation of Zircaloy-2 in potassium hydroxide/sodium silicate electrolytes. The effect of the silicate content on the growth behavior, wear and corrosion resistance of resultant coatings were systematically revealed. It was demonstrated that the coating thickness increases continuously on increasing the silicate content in electrolyte. Besides, increasing the silicate content in electrolyte can promote the formation of t-ZrO₂ in the coatings, and thus resulted in the formed ZrO₂/SiO₂ alloyed coatings with enhanced wear resistance. Differently, the corrosion resistance of the resultant coatings was not positively correlated with the silicate concentration. The coatings formed in the electrolyte with Na₂SiO₃ concentration at 56 g/L exhibited the best corrosion resistance depending on their relatively dense structure and less of cracks. Our results provided guidance for obtaining high performance of ZrO₂/SiO₂ alloyed coatings, which have great potential biological applications.

Conflicts of interest

The authors declared no competing financial interest.

Acknowledgments

The financial support by National Natural Science Foundation of China (51874129), the Natural Science Foundation of Hunan Province (2019JJ60049) and the Scientific Research Foundation of Hunan Provincial Education Department (19B153 and 19B158) is greatly acknowledged.