Influence of a CaNa₂(EDTA) Additive on Plasma Electrolytic Oxidation of Zirlo Alloy and the Properties of the Resulting Coatings

Highlights

• PEO of Zirlo alloy in phosphate electrolyte with CaNa₂(EDTA) additive.
• Chelated Ca ions exist in the electrolyte free from precipitation.
• Significant incorporation of Ca and P elements into PEO coatings on the alloy.
• Incorporated Ca effectively stabilized tetragonal ZrO₂ within the PEO coatings.
• Coatings with Ca incorporation show improved corrosion and wear performance.

Abstract

The plasma electrolytic oxidation (PEO) of Zirlo alloy was carried out in a phosphate electrolyte with CaNa₂(EDTA) as an additive (0–15 g/L) to improve its corrosion and wear resistance. The PEO behavior, microstructure, phase composition, and performance of coatings were characterized as a function of the concentration of the additive. The results indicate that the addition of CaNa₂(EDTA) promotes coating growth and improves the coating structure and phase composition. When the additive concentration is 5–10 g/L, the coating shows an improved thickness, and denser microstructure. The coatings consist of m-ZrO₂ and t-ZrO₂ as the main crystalline phases, as well as amorphous materials with Ca and P. The t-ZrO₂ phase content rises sharply when CaNa₂(EDTA) is added into the electrolyte (81.3% t-ZrO₂ is obtained under the condition with 10 g/L CaNa₂(EDTA)). Potentiodynamic polarization tests demonstrate that PEO treatment significantly enhances the corrosion resistance of Zirlo alloy. Under the condition of 5 g/L CaNa₂(EDTA), the corrosion current density of the coating decreases by two orders of magnitude compared to the substrate, achieving the best corrosion resistance. Friction and wear tests also show that the coating obtained at 5 g/L CaNa₂(EDTA) exhibits the shallowest wear scar and the lowest wear rate, demonstrating optimal wear resistance. This study shows the novelty of obtaining high-quality PEO coatings on Zirlo alloy based on Ca and P incorporation.

1. Introduction

Zirconium has a low neutron absorption cross section, excellent mechanical properties, and corrosion resistance, and is widely used in the nuclear industry. However, the long-term stability of zirconium alloys still faces challenges under extreme conditions, such as neutron irradiation and exposure to high-temperature, high-pressure water in reactor environments [1]. In particular, during a loss-of-coolant accident (LOCA) in the reactor cooling system, the zirconium alloy cladding reacts with water vapor, releasing a large amount of heat and hydrogen gas, which may cause hydrogen explosion accidents and seriously threatens the safety of nuclear power plants [2,3,4]. To improve the corrosion resistance, mechanical properties, and thermal stability of zirconium alloys, various surface treatment methods have been adopted, such as physical/chemical vapor deposition, plasma spraying, and plasma electrolytic oxidation (PEO) [5,6,7,8]. Among them, PEO technology has the advantages of using environmentally friendly alkaline electrolytes and having a fast coating growth rate. By generating ceramic oxide coatings, it significantly enhances the corrosion protection performance of zirconium alloys.

PEO is an advanced surface treatment technique for the in situ generation of oxide ceramic films on valve metals and their alloys such as Al [9], Mg [10], and Zr [11]. The coating formation involves not only the matrix metal and alloy elements via plasma electrochemical reactions, but also the participation of anionic and cationic components in the electrolyte. The electrolyte system is one of the most critical process parameters in PEO technology, and different electrolytes can lead to significant differences in coating composition, structure, and properties [12,13]. The commonly used PEO electrolytic systems for forming ceramic coatings on zirconium alloy include silicates, aluminates, phosphates, and various composite systems. Cheng et al. [14] treated Zircaloy-2 with PEO in an alkaline silicate electrolyte containing Na₂SiO₃·9H₂O and KOH. When the silicate concentration increased to 30 g/L Na₂SiO₃·9H₂O, the content of t-ZrO₂ (tetragonal phase) in the coating significantly increased. Sandhyarani et al. [15] prepared PEO coatings on Zr in phosphate-containing electrolytes, the phases in all PEO coatings are the dominant m-ZrO₂ (monoclinic phase) and trace amounts of t-ZrO₂. Yan et al. used electrolytes with different concentrations of NaAlO₂ to prepare Al₂O₃/ZrO₂ composite PEO coatings [16]. As the concentration of NaAlO₂ increased from 0.2 M to 0.3 M, the coating became thicker and denser, with fewer discharge holes and cracks, and a significant increase in the content of α-Al₂O₃ and t-ZrO₂ phases.

Corrosion is critical in industry which causes economic problems due to the deterioration of infrastructure, equipment, and machinery [17]. In the case of Zr alloys, PEO has been demonstrated to be an effective method to improve their corrosion resistance. Chen et al. [18] demonstrated that the PEO coatings made in a dilute silicate electrolyte on Zr-2.5Nb alloy have a much better corrosion resistance and lower weight gains than the Zr-2.5Nb substrate after 30 days in an autoclave. Wei et al. [19] studied the electrochemical behavior of PEO films on Zirlo alloy in lithium borate solution and found that the PEO films hardly changed in terms of morphology and microstructure after in situ electrochemical impedance spectroscopy (EIS) tests at 25–300 °C, showing excellent corrosion resistance.

The PEO coating of zirconium alloy is mainly composed of a monoclinic phase (m-ZrO₂) and tetragonal phase (t-ZrO₂) [20,21]. The tetragonal phase is a high-temperature phase, which transforms into m-ZrO₂ at 950 °C, accompanied by a volume expansion of 7%–9%, resulting in shear strain and cracking in zirconia ceramics [22,23]. Stabilizers such as MgO, Y₂O₃, CaO, and CeO₂ can maintain t-ZrO₂ stability at room temperature [24,25]. These stabilizers suppress or delay the martensitic transformation from t → m by introducing interfacial stress, thermal expansion mismatch, or nucleation barriers [26]. For example, Arun et al. prepared 3 mol% CaO doped ZrO2 nano powder by reverse co-precipitation method, and then sintered to obtain high-density t-ZrO₂ ceramics [27]. After CaO is added as a stabilizer into the ZrO₂ lattice, the stability of t-ZrO₂ can be maintained at room temperature by refining the grain size and increasing the concentration of oxygen vacancies. Savushkina et al. added submicron Y₂O₃ powder to the electrolyte and prepared a coating on the surface of the Zr-1Nb alloy using PEO technology [28]. The addition of Y₂O₃ promotes the stabilization of high-temperature phases (t-ZrO₂ and c-ZrO₂) in zirconium oxide, thereby improving the high temperature resistance and corrosion resistance of the coating. Apelfeld et al. also found that after the addition of 6 g/L yttria nanopowder in the PEO electrolyte, only the t-ZrO₂ phase was found in the coating surface layer [25]. Among various stabilizers, Ca as an additive element not only stabilizes the t-ZrO₂ structure but also has good biological functional properties. Ca and P are important elements in human bone tissue, and Ca-P coatings have been widely proven to significantly improve the biocompatibility, bone conductivity, and cell adhesion behavior of metal implants [29,30]. Therefore, the Ca-P coating has become a research hot spot in the field of the surface modification of metal implants.

Although it is possible to prepare a Ca-P coating using electrolytes containing Ca²⁺ and phosphate anions, cations of Ca²⁺ are prone to react with phosphate or hydroxide ions to form calcium phosphate precipitates under alkaline conditions, significantly reducing the concentration of Ca²⁺ in the solution [31,32]. Zhu et al. [32] conducted the PEO of AZ31B magnesium alloy in an electrolyte containing NaOH, sodium phytate (Na₁₂Phy) and Ca(H₂PO₄)₂·H₂O. Their results indicate that an increase in NaOH concentration raises the pH of the electrolyte and reduces the effective solubility of Ca(H₂PO₄)₂·H₂O, inducing the formation of calcium-related precipitates in the electrolyte, thereby weakening the source of Ca²⁺ available for coating formation. Therefore, determining how to increase the concentration of calcium ions in phosphate solution to obtain coatings with a high Ca-P content has become a key issue in the PEO field. It has been reported that Ca²⁺ ions can exist stably in solution through chelation, such as CaNa₂(EDTA) [33,34]. The chelated ions are so stable that calcium phosphate precipitates will not be formed in this case. Moreover, the chelated ions of [CaEDTA]²⁻ are negatively charged, which can be easily attracted into the discharge channels during PEO, thereby facilitating the acquisition of PEO coatings with a high calcium content and controllable composition. Shi et al. [35] used AZ31B magnesium alloy as the substrate and performed a PEO treatment in an electrolyte containing CaNa₂(EDTA), phytic acid, and phosphoric acid. The study showed that increasing the concentration of CaNa₂(EDTA) can significantly increase the Ca content of the coating. However, to the knowledge of the present authors, the PEO of zirconium alloys in similar electrolytes has not been reported yet. In addition, CaNa₂(EDTA) is safe and non-toxic, and can be used as an antidote for heavy metal poisoning [33]. Moreover, compared with the rare earth powder of Y₂O₃, Ca is more available and has the advantage of being available at a low price. Therefore, the use of a CaNa₂(EDTA) electrolyte will be promising with its ability to stabilize t-ZrO₂, and improving the Ca and P contents withing the PEO coatings.

In this study, a CaNa₂(EDTA) additive is introduced into a Na₃PO₄-based electrolyte for the PEO treatment of a Zirlo alloy. The effect of the additive concentration on the PEO behavior, phase composition, corrosion resistance, and wear resistance of the coatings has been systematically studied. The novelty of this study lies in that using chelated Ca as an additive enables the simultaneous incorporation of a large amount of Ca and P into the coating, which is demonstrated to be crucial for stabilizing the tetragonal phase and improving the wear and corrosion resistance of the resulting coatings.

2. Experimental

2.1. Material

The Zirlo alloy (manufacturer: State Nuclear Baoji Zirconium Industry Co., Ltd., Baoji, Shaanxi, China) consists of (wt%) Nb 1%, Sn 1%, and Fe 0.1%, with Zr making up the balance. The alloy is cut into samples with a size of 20 mm × 10 mm × 5 mm, and then sealed with epoxy resin, leaving a working area of 2 cm². Electric contact is provided by a copper wire. The working surface was polished by SiC sandpaper ranging from 120 mesh to 2000 mesh in sequence, then degreased with acetone, rinsed with distilled water, and dried with warm air flow.

2.2. PEO Treatment

PEO was conducted in a 1 L glass vessel, equipped with a stirring and water-cooling system. The power supply is a 5 kW pulse power supply, using a bipolar pulse mode. In the PEO treatment, the Zirlo alloy samples serve as the anode, and a large piece of stainless steel serves as the cathode. The electrolytes are prepared by analytical reagents and distilled water, with a composition of 30 g/L Na₃PO₄·12H₂O + 1 g/L KOH and 0, 5, 10, and 15 g/L CaNa₂(EDTA) as the additive, respectively. The pH values of the electrolytes are measured to be 12.78, 12.75, 12.70, and 12.73 in the order of increasing CaNa₂(EDTA) concentration. All chemicals were of analytical grade and obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The electrical parameters included the average anodic and cathodic current densities of 0.22 A·cm⁻² and 0.15 A·cm⁻², respectively, a duty cycle of 20%, and a frequency of 1000 Hz. Triplicated samples at each condition were prepared to ensure the reproducibility and statistical reliability of the results. During PEO tests, the electrolyte temperature is kept below 30 °C through the water-cooling system. The duration of PEO treatment is 300 s.

2.3. Characterization

After PEO treatment, the coating thickness was randomly measured 10 times using an eddy current thickness gauge (TT260, Time Group Inc., Beijing, China), and the average value and standard deviation were calculated. The surface morphology and composition of the coating were observed and analyzed using a field emission scanning electron microscope (QUANTA FEG 250, FEI, USA) and energy-dispersive X-ray spectroscopy (EDS, EDAX, USA). Before testing, the sample was sputtered with gold for 180 s. The phase composition of the coating was characterized using a Rigaku D/max 2500 X-ray diffractometer (Cu Kα, 40 kV, 250 mA, Rigaku Corporation, Japan) with a scanning range of 20° to 80° and a scanning speed of 4°/min. Polarization curves were tested in a 3.5 wt% NaCl solution using an electrochemical workstation (CHI660, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China), with a platinum sheet as the counter electrode and a saturated calomel electrode as the reference electrode. The open circuit potential (OCP) was recorded for 1800 s to achieve a stable state. The polarization curve was tested within a potential range of −0.5 V to +1.5 V (relative to the OCP) at a scanning rate of 1 mV/s. Dry sliding wear tests were conducted at room temperature using an HT-1000 friction and wear tester, with an Al₂O₃ ball (diameter 6 mm, hardness 85 HRC) as the counterpart material. The wear track radius was 2 mm, rotational speed was 300 r/min, load was 5 N, and sliding time was 1800 s. The wear track morphology was measured using an optical profilometer (Wyko NT 9100, Veeco Instruments Inc., Plainview, NY, USA), and the wear rate was calculated by dividing the wear volume loss by the total sliding distance and total load.

3. Results and Discussion

3.1. Cell Potential–Time Responses

Figure 1 shows the cell potential–time curves at different concentrations of CaNa₂(EDTA). During PEO treatment with different concentrations of additives, the variation trends in potential over time are similar. In the initial stage of PEO, the anodic potential rises rapidly and reaches the breakdown potential, after which it enters a stable stage with a significantly slower growth rate. Ultimately, the anodic potential stabilizes at approximately 485 V, while the cathodic potential remains around –150 V. In the absence of CaNa₂(EDTA) (0 g/L), the anodic potential exhibits a brief drop of about 90 V at around 60 s, followed by a quick rebound within approximately 20 s, after which it re-enters a steady growth phase, with the potential continuing to increase slowly thereafter. The cathodic potential, on the other hand, increases continuously. In the experiment with the addition of 5 g/L CaNa₂(EDTA), minor fluctuations in cell potential were also observed. However, when 10 g/L and 15 g/L CaNa₂(EDTA) were added, the potential curves became more stable without significant fluctuations.

Figure 1. Cell potential–time responses during PEO of Zirlo alloy in phosphate electrolyte with the addition of 0–15 g/L CaNa₂(EDTA).

3.2. Coating Thicknesses and Macroscopic View

Figure 2 presents the thickness (measured by a thickness gauge) and morphology of the coatings obtained after 300 s of PEO treatment at different concentrations of CaNa₂(EDTA). As the concentration of CaNa₂(EDTA) increased from 0 g/L to 5, 10, and 15 g/L, the coating thickness increased from 22.8 µm to 33.9, 38.9, and 36.4 µm, respectively. This overall increasing trend with higher additive concentration indicates that the introduction of CaNa₂(EDTA) effectively promotes the growth of the oxide film. In the electrolyte without the addition of CaNa₂(EDTA), the coating was relatively thin, with the smallest standard deviation in thickness, indicating a uniform coating. After adding CaNa₂(EDTA), the standard deviation of the coating thickness increased, suggesting that the coating became rougher. It is noted that the coating growth rates in this study are higher than those reported by Aubakirova et al. [36]. In their work, the PEO of a Zr-1%Nb alloy was carried out in a phosphate and calcium acetate electrolyte, forming a coating of 14.7 μm after a treatment time of 600 s. The sample prepared without CaNa₂(EDTA) addition appears light colored with dispersed local black areas, whereas the coatings prepared with the addition of CaNa₂(EDTA) exhibit a darker grayish-black color.

Figure 2. Macro appearances and thickness of PEO coatings formed for 300 s on Zirlo alloy in 30 g/L Na₃PO₄·12H₂O + 1 g/L KOH with the addition of 0, 5, 10, and 15 g/L CaNa₂(EDTA). The size of the samples is 10 × 20 mm.

3.3. SEM and EDS Analyses

Figure 3 shows the surface SEM images of the PEO coatings formed with different concentrations of CaNa₂(EDTA). Figure 3a displays the secondary electron image of the PEO coating prepared without CaNa₂(EDTA). The surface is relatively flat with few undulations. The local magnified view reveals numerous fine pores on the surface. Figure 3b shows the SEM image of the PEO coating prepared with the addition of 5 g/L CaNa₂(EDTA). The coating surface exhibits some prominent nodular structures and a small number of pores. The coatings formed at higher additive concentrations (10–15 g/L CaNa₂(EDTA)) show similar surface morphologies, characterized by an increased number of pores.

Figure 3. SEM morphology of the PEO coatings on Zirlo alloy formed for 300 s in electrolytes with the addition of different CaNa₂(EDTA) concentrations: (a) 0 g/L; (b) 5 g/L; (c) 10 g/L; (d) 15 g/L. The capital letters indicate the positions (marked with star) for EDS analysis.

Table 1 lists the EDS results of the marked locations (Figure 3) on the coating surfaces formed under different conditions. As the content of the electrolyte additive increased, the Ca content on the coating surface increased, reaching a maximum of 27.03 at%.

Table 1. Elemental composition of the marked locations in Figure 3 (at%).

Sample (g/L)	Location	O	Na	P	Zr	Ca	Nb
0	A	58.90	0.43	6.05	34.13	-	0.48
5	B	62.41	7.23	9.52	8.66	12.18	-
10	C	58.82	1.65	8.06	15.81	15.66	-
15	D	50.63	-	13.00	9.07	27.03	0.85

Figure 4 shows the cross-sectional SEM images of the PEO coatings formed with different concentrations of CaNa₂(EDTA). Consistent with the thickness gauge results, the coating prepared without CaNa₂(EDTA) is thinner, with a uniform thickness and dense structure. The locally magnified image in Figure 4a reveals numerous transverse microcracks within the coating layer. After the addition of CaNa₂(EDTA), the overall coating thickness shows an increasing trend, with the maximum thickness achieved at a concentration of 10 g/L. Concurrently, microcracks within the coating are reduced. The coatings prepared with 10 g/L and 15 g/L CaNa₂(EDTA) can be roughly divided into two distinct layers: an inner layer that is dense, and an outer layer that exhibits greater porosity.

Figure 4. SEM morphology of cross section of PEO coatings on Zirlo alloy formed for 300 s in electrolytes with the addition of different CaNa₂(EDTA) concentrations: (a) 0 g/L; (b) 5 g/L; (c) 10 g/L; (d) 15 g/L.

Table 2 lists the EDS analysis results of the cross sections of coatings formed under different conditions. The coatings are composed of O, Na, P, Zr, Ca, and Nb elements. For the coating formed without CaNa₂(EDTA) addition, a relatively high content of P element is present, while Ca element is absent. For the coatings formed in electrolytes containing different amounts of CaNa₂(EDTA), the Ca content within the coatings is significantly improved. However, the Ca content is mainly distributed in the outer layer, with the inner layer showing a much lower Ca content. As the additive content increases, the Ca content in the outer layer of the coating increases, reaching a maximum of 21.40 at% in the condition with 15 g/L CaNa₂(EDTA) addition.

Table 2. EDS analyses showing the element composition at the marked locations in the cross sections in Figure 4 (at%).

Sample (g/L)	Location	O	Na	P	Zr	Ca	Nb
0	A	54.40	0.24	12.34	31.20	-	1.82
5	B	49.06	1.25	4.89	37.40	5.21	2.18
10	C	44.20	0.51	5.30	34.57	12.62	2.79
	D	59.21	0.75	6.76	31.41	0.44	1.43
15	E	38.60	0.08	8.34	29.98	21.40	1.61
	F	54.02	0.37	8.74	33.81	1.66	1.51

3.4. XRD

Figure 5 shows the XRD patterns of the PEO coatings prepared with different concentrations of CaNa₂(EDTA). It can be observed that all coatings are primarily composed of m-ZrO₂ and t-ZrO₂. In the absence of CaNa₂(EDTA), the diffraction peak intensity of t-ZrO₂ in the coating is weak, indicating its relatively low content. As the concentration of CaNa₂(EDTA) increases, the characteristic peaks of t-ZrO₂ gradually become stronger. According to the literature, the relative contents of t-ZrO₂ and m-ZrO₂ in the coating can be calculated using the following formula [37]:

$${\mathrm{X}}_{\mathrm{m}}={\displaystyle \frac{I{\left(\overline{1}11\right)}_m+I{\left(111\right)}_m}{I{\left(\overline{1}11\right)}_m+I{\left(111\right)}_m+I{\left(101\right)}_t}}$$

(1)

where X_m is the mass fraction of m-ZrO₂; I($\overline{1}$11)_m, I(111)_m, and I(101)_t represent the XRD peak intensities at 2θ = 28.2° (m-ZrO₂), 31.5° (m-ZrO₂), and 30.2° (t-ZrO₂), respectively.

Figure 5. XRD patterns of the PEO coatings formed on Zirlo alloy in electrolyte with the addition of different CaNa₂(EDTA) concentrations.

The relative contents of t-ZrO₂ and m-ZrO₂ in the coatings formed at different CaNa₂(EDTA) concentrations were calculated according to Equation (1) and are plotted in Figure 6. At CaNa₂(EDTA) concentrations of 0, 5, 10, and 15 g/L, the t-ZrO₂ contents are 4.5%, 60.9%, 81.3%, and 85.6 wt%, respectively. Therefore, the t-ZrO₂ content continued to increase with the increase in additive concentration. Combined with the EDS analysis results, the Ca content in the coating also increased with increasing CaNa₂(EDTA) concentration, indicating that calcium ions from the electrolyte participated in coating formation and contributed to the stabilization of t-ZrO₂. The incorporation of cations into the zirconia matrix can lead to a reduction in grain size and stabilize the t-ZrO₂ phase [27]. Ca and P are likely present in an amorphous state, as no corresponding peaks were found in the X-ray diffraction patterns. Calcium may exist in the form of amorphous compounds such as calcium oxide or calcium phosphate [34].

Figure 6. Relative content of t-ZrO₂ and m-ZrO₂ in the PEO coatings formed on Zirlo alloy in electrolyte with the addition of different CaNa₂(EDTA) concentrations.

3.5. Corrosion Resistance

Figure 7 shows the Tafel polarization curves obtained in a 3.5 wt% NaCl solution for the PEO coatings formed on Zirlo alloy in an electrolyte with the addition of different concentrations of CaNa₂(EDTA). Compared to the untreated substrate, all of the PEO coatings show more positive free corrosion potentials and the curves shift to left hand side of that of the untreated metal, indicating improved corrosion resistance. The corrosion current density can be obtained by the Tafel extrapolation method, and is presented in Table 3. Other parameters such as polarization resistance (R_p), Tafel slopes and corrosion rate (C_R) are also included in Table 3. The untreated substrate metal exhibited a corrosion potential of −0.738 V (vs. SCE) and a corrosion current density (i_corr) of 2.08 × 10⁻⁷ A cm⁻². After PEO treatment, the corrosion potential of the samples increased, and the corrosion current density decreased. Compared with the samples prepared without CaNa₂(EDTA), the PEO coatings formed with the addition of CaNa₂(EDTA) showed significantly reduced corrosion current densities. Among these, the coating formed with the addition of 5 g/L CaNa₂(EDTA) exhibited the lowest i_corr (2.03 × 10⁻⁹ A cm⁻²), demonstrating an improvement in corrosion resistance by two orders of magnitude compared to the substrate. However, when the CaNa₂(EDTA) concentration was further increased to 10 and 15 g/L, the corrosion current density increased to 3.38 × 10⁻⁹ and 1.97 × 10⁻⁸ A·cm⁻², respectively, indicating a degradation in corrosion resistance.

Figure 7. Polarization curves of the PEO coatings formed on Zirlo alloy in electrolyte with the addition of different CaNa₂(EDTA) concentrations.

Table 3. Electrochemical parameters of the uncoated Zirlo and PEO coatings obtained with the addition of different CaNa₂(EDTA) concentrations.

Sample (g/L)	Ecorr (Vvs·SCE)	icorr (Acm−2)	βa (mV/dec)	βc (mV/dec)	Rp (kΩcm−2)	CR (mm Year−1)
Substrate	−0.738	2.08 × 10−7	123	−202	160	2.39 × 10−3
0	−0.003	7.19 × 10−8	155	−174	495	8.25 × 10−4
5	−0.137	2.03 × 10−9	174	−178	18,800	2.33 × 10−5
10	−0.039	3.38 × 10−9	196	−347	16,100	3.88 × 10−5
15	−0.017	1.97 × 10−8	195	−206	2210	2.26 × 10−4

The corrosion resistance can also be evaluated from the R_p values, which are determined using the Stern Geary equation [38]:

$$\begin{array}{l}{\mathrm{R}}_{\mathrm{p}}={\displaystyle \frac{{\mathsf{\beta }}_{\mathrm{a}}{\mathsf{\beta }}_{\mathrm{c}}}{2.303i_{corr}\left({\mathsf{\beta }}_{\mathrm{a}}+{\mathsf{\beta }}_{\mathrm{c}}\right)}}\end{array}$$

(2)

where β_a and β_c are the anodic and cathodic Tafel slopes, respectively.

The values of R_p show the same trend as that of the corrosion current density. As the concentration of the additive increased from 0 to 5 g/L, the polarization resistance significantly increased from 495 to 18,800 kΩ·cm². After that, the R_p value then decreased to 16,100 and 2210 kΩ·cm² at 10 and 15 g/L, respectively.

Moreover, the corrosion rate for metallic samples can be calculated according to the following equation [39]:

$$\begin{array}{l}{\mathrm{C}}_{\mathrm{R}}={\displaystyle \frac{3.27\times {10}^3\times \mathrm{M}\times {\mathrm{i}}_{corr}}{n\times \rho }}\end{array}$$

(3)

where C_R is the corrosion rate in [mm year⁻¹], M is the atomic weight of the metal in [g], i_corr is the corrosion current density in [A cm⁻²], n is the valence of the metal cations that dissolve into solution, and ρ is the density of the metal in [g cm⁻³].

Assuming that only Zr⁴⁺ ions enters the solution and the density of Zirlo is close to that of pure Zr (6.52 g cm⁻³), the corrosion rates of the Zirlo alloy and PEO coatings are also calculated and included in Table 3. The PEO coating formed with the addition of 5 g/L CaNa₂(EDTA) shows the lowest corrosion rate of 2.33 × 10⁻⁵ mm year⁻¹.

3.6. Wear Performance

Figure 8 shows the friction coefficient curves and macroscopic wear track morphologies of the Zirlo alloy substrate and the coatings prepared with different concentrations of the CaNa₂(EDTA) additive. The friction coefficient of the untreated substrate fluctuated significantly, generally ranging between 0.4 and 0.5 after 200 s of sliding. After PEO treatment, the friction coefficients of the coatings decreased slightly, and the friction curves exhibited a relatively stable trend. For the PEO coating prepared without CaNa₂(EDTA), the friction coefficient showed a significant jump at 1272 s, followed by increased fluctuations, which might be attributed to the coating being worn through, leading to contact between the grinding ball and the substrate. None of the coatings obtained in the electrolyte containing CaNa₂(EDTA) exhibited such a sudden change in friction coefficient. For the coating prepared with 5 g/L CaNa₂(EDTA), the friction coefficient was lower than those of the coatings prepared with 10 and 15 g/L CaNa₂(EDTA) before 1000 s, with smaller fluctuations, and it ultimately reached ~0.43 at 1800 s. With a further increase in CaNa₂(EDTA) concentration, the friction coefficient increased, and the fluctuations became more pronounced. After the completion of an 1800 s friction test, the macroscopic morphology of the samples showed that the metal substrate and the coating prepared without CaNa₂(EDTA) were severely worn, exhibiting a metallic luster. The wear resistance of the coatings obtained in the electrolyte containing CaNa₂(EDTA) was significantly improved, with the coating prepared in the electrolyte containing 5 g/L CaNa₂(EDTA) showing the shallowest wear track.

Figure 8. Macroscopic wear track morphologies and friction coefficient curves of uncoated Zirlo alloy and its PEO coatings prepared with different concentrations of CaNa₂(EDTA).

The three-dimensional profiles of the wear tracks on the substrate and different PEO coatings were examined using an optical profilometer (Figure 9). The results showed that the substrate exhibited the widest wear track and the most severe wear damage. The PEO coating prepared in the electrolyte without CaNa₂(EDTA) had a wear track width and depth similar to those of the substrate, indicating the limited wear protection provided by this coating. In contrast, the wear tracks of the coatings prepared with the addition of CaNa₂(EDTA) were significantly shallower. Among these, the PEO coating obtained from the electrolyte containing 5 g/L CaNa₂(EDTA) displayed the narrowest wear track, demonstrating the optimal wear resistance.

Figure 9. D profiles of the wear tracks on the substrate and PEO coatings prepared with different concentrations of CaNa₂(EDTA): (a) substrate; (b) 0 g/L; (c) 5 g/L; (d) 10 g/L; (e) 15 g/L.

Figure 10 presents the corresponding cross-sectional profiles of the wear tracks on the samples after the wear tests. The wear track on the Zirlo alloy substrate had a maximum depth of ~38.61 µm and a width of ~1859 µm. The depth of the wear track on the PEO coating obtained from the electrolyte without the ₂ additive was 34.94 µm, which exceeded the coating thickness, indicating that the coating had been worn through. The PEO coating obtained from the electrolyte containing 5 g/L CaNa₂(EDTA) exhibited the narrowest wear track, with a width of ~506 µm and a depth of ~8.66 µm.

Figure 10. Cross-sectional profiles of the wear tracks on the Zirlo substrate and PEO coatings prepared with different concentrations of CaNa₂(EDTA): (a) substrate; (b) 0 g/L; (c) 5, 10, and 15 g/L.

The wear rates, calculated from the cross-sectional profiles of the wear tracks and other parameters of the friction tests, are shown in Figure 11. The wear rate of the untreated substrate was ~1.10 × 10⁻³ mm³ N⁻¹ m⁻¹. The PEO coating obtained from the electrolyte with the 5 g/L CaNa₂(EDTA) additive exhibited the lowest wear rate, at approximately 5.71 × 10⁻⁵ mm³ N⁻¹ m⁻¹. Similar to the corrosion tests, wear performance of the coating deteriorated at an increased CaNa₂(EDTA) concentration of 10 and 15 g/L.

Figure 11. Wear rates of the Zirlo alloy and different PEO coatings.

3.7. Discussion

This study systematically investigates the effects of the addition of different concentrations of CaNa₂(EDTA) additive in a phosphate electrolyte on the PEO behavior and coating properties on Zirlo alloy. It can be observed that the addition of CaNa₂(EDTA) effectively increases the Ca content and the proportion of the tetragonal phase within the coating, leading to an increased coating thickness and enhanced corrosion and wear resistance.

CaNa₂(EDTA) is an efficient metal chelating agent, characterized by good water solubility and strong chelation. Its structure is shown in Figure 12. The molecule is based on an ethylenediaminetetraacetic acid (EDTA) backbone, which can simultaneously provide multiple coordination sites for binding metal ions. Ca²⁺ is chelated inside the EDTA molecule, forming a stable cyclic complex structure, while Na⁺ ions are distributed on the outside of the molecule in ionic form to maintain the overall charge balance.

Figure 12. Structural formula of CaNa₂(EDTA).

CaNa₂(EDTA) in solution can ionize to yield [CaEDTA]²⁻ and Ca²⁺, as shown in Equations (4) and (5):

$$\mathrm{C}\mathrm{a}\mathrm{N}{\mathrm{a}}_2\mathrm{E}\mathrm{D}\mathrm{T}\mathrm{A}\to {\left[\mathrm{C}\mathrm{a}\mathrm{E}\mathrm{D}\mathrm{T}\mathrm{A}\right]}^{2-}+2{\mathrm{N}\mathrm{a}}^{+}$$

(4)

$${\left[\mathrm{C}\mathrm{a}\mathrm{E}\mathrm{D}\mathrm{T}\mathrm{A}\right]}^{2-}\to \mathrm{E}\mathrm{D}\mathrm{T}{\mathrm{A}}^{4-}+\mathrm{C}{\mathrm{a}}^{2+}$$

(5)

According to Equation (4), negatively charged [CaEDTA]²⁻ complexes are formed. These complex ions migrate toward the anode under the electric field. They subsequently dissociate further in the high-temperature discharge channels, as shown in Equation (5), releasing Ca²⁺. This facilitates the formation of PEO coatings with a high and controllable calcium content. In this experiment, the electrolyte containing CaNa₂(EDTA) remained clear and transparent throughout both the preparation process and the PEO process, indicating that the [CaEDTA]²⁻ chelate ions are very stable. Although PO₄³⁻ ions were also present in the solution, no calcium phosphate precipitation occurred. Therefore, the chelation effect of CaNa₂(EDTA) effectively stabilizes Ca²⁺ ions, thereby overcoming the issue of the low solubility of calcium salts under alkaline conditions. After dissolution in the electrolyte, CaNa₂(EDTA) ionizes to form negatively charged [CaEDTA]²⁻ ions, which migrate towards the anode under the electric field and participate in coating formation. Under the high temperature of plasma discharges, the incorporated Ca²⁺ ions might have entered the oxide lattice, stabilizing the t-ZrO₂ phase and increasing the density of the coating. As the concentration of CaNa₂(EDTA) increases, the content of the t-ZrO₂ phase in the coating increases significantly.

As is well known, ZrO₂ can exhibit three thermodynamically stable crystal structures in different temperature ranges: the monoclinic phase (m-ZrO₂), the tetragonal phase (t-ZrO₂), and the cubic phase (c-ZrO₂). The transformation from the monoclinic to the tetragonal phase occurs at 1170 °C, while the transformation from the tetragonal to the cubic phase occurs at 2370 °C, and only m-ZrO₂ is stable at room temperature [40]. The grain size effect proposed by Garvie [41] and the constraint effect proposed by Heuer et al. [42] are two main mechanisms for achieving the room-temperature stabilization of t-ZrO₂. Typically, the t-ZrO₂ structure is stabilized at room temperature mainly by adding stabilizers such as Ce₂O₃, CaO, MgO, and Y₂O₃ [43]. The doped metal cations can reduce the particle or grain size within the zirconia matrix, ultimately stabilizing the tetragonal structure of zirconia nanoparticles/ceramics [27]. In PEO research, Zhang et al. [44] investigated the influence of electrical parameters on the phase transformation behavior of PEO coatings on Zirlo alloy in a silicate electrolyte. Under conditions of high frequency (1000 Hz) and applied cathodic current, the t-ZrO₂ phase in the PEO coating could be increased to 36.9 wt%. Their analysis suggested that under high-frequency conditions, more SiO₂ participates in coating formation, which can also contribute to stabilizing the tetragonal phase. In the present study, after Ca²⁺ from the electrolyte permeated the coating, the content of the t-ZrO₂ phase reached as high as 85.6 wt%, indicating that Ca²⁺ ions are more effective in inhibiting the zirconia phase transformation.

Based on the experimental results of this study, the influence of CaNa₂(EDTA) on the PEO coating of Zirlo alloy can be elucidated using the schematic diagram in Figure 13. For the coating prepared without CaNa₂(EDTA), the coating is thinner and contains more m-ZrO₂. In this case, since most of the high-temperature tetragonal phase transforms into the monoclinic phase, and due to the volume change accompanying this process, the coating exhibits more microcracks. For the coatings prepared with CaNa₂(EDTA), the coating thickness increases. This is because [CaEDTA]²⁻ anions, under the influence of the electric field, enter the PEO coating layer. Under the action of plasma sparks, the [CaEDTA]²⁻ anions are decomposed, and Ca²⁺ ions are deposited in the PEO coating in the form of CaO or Ca₃(PO₄)₂. Since Ca²⁺ ions can participate in coating formation, the coating thickness increases. The symbol of yellow dots in the diagram represents calcium cations within the crystal lattice, which act as stabilizers, enabling the coating to retain more t-ZrO₂. It should be pointed out that the presence of Ca²⁺ ions within the crystal lattice is merely a hypothesis at this stage. Direct evidence such as atomic-scale mapping or detailed lattice parameter analysis would be required to unambiguously confirm the lattice substitution. Evidently, the participation of Ca²⁺ ions in film formation can increase the coating thickness and reduce cracks within the coating by stabilizing the tetragonal phase; these factors are beneficial for enhancing the wear and corrosion resistance of the coating.

Figure 13. Schematic illustration of the coating growth mode for PEO of Zirlo alloy in phosphate electrolyte (a) without and (b) with the addition of CaNa₂(EDTA).

A notable phenomenon in this study is that although the t-ZrO₂ content continuously increases with the CaNa₂(EDTA) concentration, the optimal corrosion and wear resistance is achieved at 5 g/L rather than at higher concentrations. This phenomenon may be rationalized by considering the combined effects of the elemental and phase composition and the coating microstructure. One of the reasons for this may be associated with the increased porosity of the coatings obtained at 10 g/L and 15 g/L, as shown by the SEM cross sections in Figure 4. Such porous structures provide preferential pathways for corrosive media to penetrate through the coating and compromise the mechanical integrity under sliding contact. The other aspect may be associated with the lower performance of Ca species themselves. It is shown by EDS that the coatings obtained at 10 g/L and 15 g/L CaNa₂(EDTA) possess a much higher Ca content than that of the coating at 5 g/L CaNa₂(EDTA). The Ca element might exist in the forms of Ca₃PO₄ or CaO within the coatings. Ca₃PO₄ is a compound with low solubility in aqueous solution (K_sp ≈ 2.07 × 10⁻³³ [45]). CaO may react with water to form Ca(OH)₂. Therefore, these Ca-containing compounds are less stable than the main oxide of zirconia of the PEO coating, which is practically insoluble. As a result, the PEO coatings obtained at 10 g/L and 15 g/L CaNa₂(EDTA) would be less corrosion resistant than the coating obtained at 5 g/L CaNa₂(EDTA). Wear performance of the coatings may be affected by the same reasons since compounds of Ca show inferior mechanical properties compared with zirconia.

4. Conclusions

The PEO of a Zirlo alloy was carried out in a phosphate electrolyte containing CaNa₂(EDTA) as an additive. The following conclusions can be reached:

• The introduction of the CaNa₂(EDTA) additive into the Na₃PO₄-based electrolyte can promote coating growth and alter the structural characteristics of the coating layer. As the concentration of CaNa₂(EDTA) increases, the overall coating thickness exhibits an increasing trend; in addition to the elements of Zr and O, the PEO coatings also contain P and Ca elements, with higher Ca incorporation for the coatings being obtained in the electrolyte with a high CaNa₂(EDTA) concentration.
• The phase composition of PEO coatings are primarily composed of m-ZrO₂ and t-ZrO₂. The addition of CaNa₂(EDTA) significantly increases the content of the t-ZrO₂ phase within the coating, which indicates that Ca might have entered the oxide lattice, effectively inhibiting the phase transformation from t-ZrO₂ to m-ZrO₂.
• The coating formed with 5 g/L CaNa₂(EDTA) exhibits the most compact structure, the strongest barrier effect against corrosive media, and thus the best corrosion-resistance performance.
• The results of the friction and wear tests show that the PEO coating can effectively reduce the friction coefficient and wear rate of the substrate. The coating formed with 5 g/L CaNa₂(EDTA) presents the shallowest wear track, demonstrating the best wear resistance. However, an excessively high additive concentration can lead to decreased wear resistance, due to increased coating porosity or the poorer mechanical performance of the incorporated Ca species.