Electrochemical Evaluation of Ag–CaP–ZrO₂ Composite Coatings on Ti6Al4V for Enhanced Corrosion Resistance in Dental Implants

Abstract

Objective: The Ti6Al4V titanium alloy is widely used for dental implants because of its excellent mechanical properties, corrosion resistance, and biocompatibility. However, its bioinert surface limits both osseointegration and resistance to bacterial colonization. Methods: To address these challenges, this study develops a composite coating based on calcium phosphate (CaP) and silver (Ag), reinforced with zirconium oxide (ZrO₂). The coating was deposited on Ti6Al4V using an immersion technique to improve the surface properties of the alloy. Electrochemical analyses (OCP, EIS, and potentiodynamic polarization) were performed in simulated physiological conditions to evaluate the corrosion behavior, while SEM/EDS was used to characterize the surface morphology and composition. Results: The Ag- and Zr-containing CaP coatings significantly improved the corrosion resistance of Ti6Al4V compared with uncoated and CaP-coated samples. Conclusions: This approach provides a promising strategy to enhance the electrochemical stability and long-term durability of titanium dental implants, thereby contributing to their reliable performance in the oral environment.

1. Introduction

Dental implants are predominantly fabricated from commercially pure titanium (CP-Ti) or Ti-6Al-4V alloys due to their excellent physicochemical and biological properties [1,2]. These materials exhibit high biocompatibility, strong corrosion resistance in the oral environment, and mechanical properties ideally suited for implantology, including a low elastic modulus, high tensile strength, and the ability to promote osseointegration [1,2,3,4,5].

Titanium has long been considered the gold standard for dental implants because of its high clinical success rate and long-term reliability.

Despite their widespread success—with reported 5- to 10-year survival rates ranging from 93% to 99% [6,7,8], biological and mechanical complications remain a clinical challenge. Peri-implant diseases, including mucositis and peri-implantitis, affect approximately 20–30% of implants, leading to marginal bone loss and implant instability [9,10,11].

Among the main contributing factors, corrosion and tribocorrosion at the implant–abutment interface have been identified as critical mechanisms that compromise long-term performance and induce the release of metallic ions [12,13,14].

The surface of titanium implants is continuously exposed to wear, fatigue, and electrochemical attack in the oral cavity. These processes often act simultaneously, producing a synergistic tribocorrosion mechanism, in which mechanical wear accelerates corrosion reactions and metallic ion release from the substrate [15,16,17].

The release of metallic ions such as Al³⁺ and V⁵⁺ from Ti-6Al-4V under chemical or mechanical degradation can trigger local immune responses, chronic inflammation, and ultimately implant failure [1,2,18]. To mitigate these effects, surface modification strategies have become a major focus for improving corrosion resistance, bioactivity, and osseointegration of titanium implants [19]. The quality of the material, the precision of the connection, and the manufacturing process are critical to ensure stability under occlusal forces and minimize mechanical or biological failures [18,20,21,22].

Although titanium is biocompatible, it remains bioinert—it does not actively promote bone formation and lacks antibacterial properties, making it vulnerable to peri-implant infections, a major cause of post-surgical complications. To overcome this limitation, calcium phosphate (CaP)-based coatings, particularly hydroxyapatite (HAp, Ca/P ≈ 1.67), have been widely used to enhance osseointegration and bone–implant bonding [23,24]. These coatings promote protein adsorption and osteoblast attachment and proliferation, but their effectiveness depends strongly on the deposition technique, which must ensure strong adhesion and phase stability [25,26].

Silver (Ag) is well known for its broad-spectrum antibacterial activity. When incorporated into CaP coatings at controlled concentrations, it can inhibit bacterial colonization and improve corrosion resistance without impairing coating integrity or cytocompatibility [23,27,28,29], Silver ions (Ag⁺) act by binding to bacterial cell membranes, proteins, and DNA, leading to structural and functional disruption. However, excessive Ag⁺ concentrations can reduce osteoblast viability and hinder osseointegration; therefore, maintaining Ag⁺ levels below approximately 1 mmol·L⁻¹ ensures antibacterial efficacy while preserving biocompatibility [30,31,32]. Zirconium dioxide (ZrO₂) has also been investigated for its chemical stability, mechanical reinforcement, and resistance to wear and corrosion. When used as a reinforcement phase or dopant, ZrO₂ improves coating density and adhesion to the titanium substrate, enhancing long-term durability [33]. Although zirconia-based implants have shown certain limitations, such as increased failure rates and bone resorption in specific clinical conditions, ZrO₂ remains an attractive candidate for coating reinforcement because of its bio-inertness and stability. Moreover, zirconia interacts with biological systems through protein adsorption and cell migration modulation, influencing cell adhesion and differentiation during bone regeneration [34,35].

Recent studies combining CaP, ZrO₂, and Ag have demonstrated that sol–gel and wet-spray methods can yield adherent composite films with rough, bioactive surfaces that favor apatite nucleation and biological mineralization in simulated body fluid (SBF) [32]. However, these techniques often require complex equipment or high processing temperatures, which can induce microcracks or alter the substrate properties.

In this context, the present study aims to develop an innovative composite coating based on CaP and Ag, reinforced with ZrO₂, and deposited on Ti-6Al-4V by a simple low-temperature immersion process [36,37].

This method enables the simultaneous incorporation of Ag⁺ and Zr⁴⁺ ions under mild conditions, leading to homogeneous, adherent, and chemically stable coatings. The study focuses primarily on the electrochemical behavior and corrosion resistance of the developed coatings as a first step toward multifunctional surface design. Structural, compositional, and biological characterizations, including EDS, FTIR, and antibacterial testing, will be addressed in forthcoming work to complete the multifunctional evaluation of the proposed Ag–CaP–Zr system.

2. Materials and Methods

2.1. Specimen Preparation

Rectangular specimens of Ti6Al4V titanium alloy (2 mm × 20 mm × 20 mm) were cut from metallic plates and subjected to a standardized cleaning protocol to ensure a contaminant-free surface. The procedure consisted of sequential ultrasonic cleaning for 10 min each in distilled water, acetone, distilled water again, and ethanol.

Prior to coating, the Ti6Al4V substrates were mechanically polished to a mirror finish (up to 2500 grit followed by 0.05 µm alumina). The polished alloy exhibited the typical α + β dual-phase microstructure of wrought Ti6Al4V, and the average surface roughness (Ra) was 0.12 ± 0.02 µm, as determined by contact profilometry.

After cleaning, the specimens were divided into four groups according to the applied surface treatment:

Group 1 (Ti6Al4V): uncoated substrates, used as the control.

Group 2 (Ti6Al4V–CaP): substrates coated with a calcium phosphate (CaP) layer.

Group 3 (Ti6Al4V–CaP/Ag): substrates coated with a CaP layer containing silver ions (Ag⁺).

Group 4 (Ti6Al4V–CaP/Ag,Zr): substrates coated with an equimolar mixture (50:50) of CaP/Ag and zirconium ions (Zr⁴⁺) derived from zirconia dissolution.

This experimental design aimed to evaluate the specific contributions of Ag⁺ and Zr⁴⁺ incorporation to the chemical stability, corrosion resistance, and interfacial performance of Ti6Al4V implants.

2.2. Preparation of CaP and CaP/Ag Coatings

Calcium phosphate (CaP)–based coatings were prepared using an immersion phosphating process. The CaP and CaP/Ag solutions were developed by the authors as an original formulation designed to produce stable and homogeneous coating baths. The formulation parameters—Ca²⁺/PO₄³⁻ ratio, pH (2.8–3.0), and Ag⁺ concentration—were optimized based on recent studies on calcium phosphate solution chemistry and silver incorporation [38,39].

The Ca²⁺ ion concentration was maintained at 0.29 mol·L⁻¹, while the phosphate ion (PO₄³⁻) concentration was adjusted to 0.10 mol·L⁻¹ (Ca/P = 2) or 0.29 mol·L⁻¹ (Ca/P = 1).

The use of two Ca/P ratios (2 and 1) aimed to investigate the effect of ionic balance on coating morphology and interfacial stability, rather than to form stoichiometric hydroxyapatite (Ca/P ≈ 1.67). A higher Ca/P ratio promotes Ca²⁺-rich amorphous calcium phosphate (ACP) nucleation and stronger adhesion to the substrate, whereas a lower ratio favors PO₄³⁻-rich layers with greater ionic reactivity. Under the acidic (pH 2.8–3.0) and low-temperature (≈60 °C) conditions applied, amorphous CaP phases are expected to dominate, in agreement with previous studies [40,41].

Calcium nitrate tetrahydrate [Ca(NO₃)₂·4H₂O] and concentrated phosphoric acid (H₃PO₄, 85% v/v) were used as the sources of Ca²⁺ and PO₄³⁻ ions, respectively. The pH of the solution was adjusted to 2.8–3.0 using sodium hydroxide (NaOH).

This acidic environment (pH ≈ 2.8–3.0) was intentionally selected to slow down the precipitation kinetics and to favor the formation of amorphous calcium phosphate (ACP) rather than crystalline hydroxyapatite, in agreement with recent studies on pH-driven CaP nucleation [40,41].

The Ti6Al4V specimens were immersed in the phosphating bath at 60 ± 2 °C for 5 min to promote the nucleation and deposition of the CaP layer.

To incorporate silver, silver nitrate (AgNO₃) was added directly to the phosphating solution at two concentrations—0.2 mmol·L⁻¹ and 0.5 mmol·L⁻¹. These values were selected based on previous studies showing that Ag⁺ ion concentrations between 0.1 and 1 mmol·L⁻¹ provide effective antibacterial activity while preserving the structural integrity and biocompatibility of CaP coatings [27,30].

This allowed the co-deposition of Ag and CaP on the substrate surface, forming a composite CaP/Ag coating. After deposition, the specimens were rinsed with distilled water and dried at room temperature to stabilize the coating.

The reproducibility of the coatings was assessed by measuring the layer thickness from cross-sectional SEM micrographs of three representative samples for each condition. The average thickness was approximately 2.5 ± 0.3 µm for CaP and 2.7 ± 0.4 µm for CaP/Ag coatings, confirming the good uniformity and consistency of the immersion process.

The immersion phosphating method used for coating deposition is schematically illustrated in Figure 1. This figure shows the experimental setup used to immerse Ti6Al4V specimens in the CaP-containing solution at controlled temperature and pH. The image highlights the uniform distribution of the bath and the arrangement of the samples during coating formation, rather than comparing surface conditions.

2.3. Preparation of an Aqueous Zirconium Chloride Solution from ZrO₂

Because zirconium oxide (ZrO₂) is highly refractory and only sparingly soluble in water, it was first converted into a soluble ionic form through acid digestion. A 6 M hydrochloric acid (HCl) solution was prepared from concentrated commercial acid (37–38% w/w). Then, 10 g of ZrO₂ powder were gradually added under continuous mechanical stirring to the hot acidic solution to promote dissolution and prevent local precipitation.

The reaction between ZrO₂ and HCl generates soluble zirconium (IV) chloride (ZrCl₄) species, according to the following equation:

ZrO₂ (s) + 4H⁺ (aq) + 4Cl⁻ (aq) → Zr⁴⁺ (aq) + 4Cl⁻ (aq) + 2H₂O (l)

(1)

After the reaction was complete, the solution was diluted with 100 mL of distilled water preheated to 80 °C under gentle stirring to obtain a clear and stable Zr⁴⁺/Cl⁻ ionic solution.

The pH of the final solution was maintained at 1.2 ± 0.1 by adding small aliquots of concentrated HCl during dissolution. This acidic environment suppressed Zr(IV) hydrolysis and prevented precipitation, ensuring a stable ionic solution throughout the coating process.

Ti6Al4V specimens previously coated with CaP/Ag were then immersed in this ZrCl₄ solution at 60 °C for 30 min to promote the uniform incorporation of ZrO₂ within the composite coating. The specimens were subsequently rinsed with distilled water and dried at room temperature.

2.4. Electrochemical Characterization

2.4.1. Preparation of Test Solutions

To evaluate the corrosion behavior of the coated and uncoated Ti6Al4V specimens, tests were performed in Fusayama artificial saliva (AS). The chemical composition of this solution is presented in

Table 1. Ionic composition of artificial saliva, comparable to that of physiological saliva.

Compounds	Quantity in g/L
NaCl → Na+, Cl−	0.4
KCl → K+, Cl−	0.4
CaCl2 → Ca2+, Cl−	0.906
NaH2PO4·2H2O → H2PO4−, Na+	0.69
Na2S·9H2O → Na+, S2−	0.005
urea	1
lactic acid	For adjust pH à 5.5

, and its pH was adjusted to 5.5 prior to use.

2.4.2. Electrochemical Test

For each condition, all electrochemical measurements were performed in triplicate (n = 3) using independently prepared specimens to ensure data reproducibility and statistical reliability.

Electrochemical impedance spectroscopy (EIS) measurements were performed using a CH Instruments Electrochemical Workstation (model CHI660E, CH Instruments, Inc., Austin, TX, USA) controlled by CHI proprietary software (version 22.24). A conventional three-electrode cell was employed, with the Ti6Al4V specimen as the working electrode (WE), a platinum wire as the counter electrode (CE), and a saturated calomel electrode (SCE) as the reference electrode (all potentials are referred to SCE). The experimental setup is illustrated in Figure 2.

The working electrode was embedded in epoxy resin, leaving an exposed surface area of 1 cm² in contact with the artificial saliva (pH ≈ 5.5), maintained at 37 ± 1 °C to simulate oral conditions.

Prior to each measurement, the specimens were immersed in the electrolyte for approximately 30 min to allow natural stabilization at the corrosion potential. The EIS spectra were then recorded using a 10 mV RMS sinusoidal perturbation over a frequency range of 100 kHz to 10 mHz.

The impedance data were analyzed using equivalent electrical circuit models to extract polarization resistance and capacitive parameters, as summarized in

Table 2. Electrochemical test parameters.

Open Circuit Potential (OCP)	24 h Stabilization Time
Electrochemical impedance spectroscopie	Frequency range 100 KHz–10 mHz, amplitude 10 mV (RMS).
Potentiodynamic polarization curves	Potential range: −1.0 V to +2.0 V vs. SCE; scan rate: 1 mV·s−1 (0.001 V·s−1)
Cyclic potentiodynamic polarization	Potential range: −1.0 V to +2.0 V vs. SCE; return potential: −1.0 V vs. SCE; scan rate: 1 mV·s−1 (0.001 V·s−1)
Reproducibility	Three tests (n = 3) per condition

.

3. Results

3.1. Electrochemical Evaluation

Electrochemical Impedance Spectroscopy (EIS)

EIS analysis provides essential information on the corrosion resistance of the different specimens and coatings when exposed to aggressive electrolytic environments,as illustrated in Figure 3.

The EIS plots correspond to the Ti6Al4V substrate and the Ti6Al4V-CaP, Ti6Al4V-CaP/Ag, and Ti6Al4V-CaP/Zr coatings, obtained at open circuit potential (OCP) after 24 h of immersion in AS adjusted to 37 °C. The dotted lines represent the experimental data, while the solid lines correspond to the data fitted to the equivalent circuit model.

Prior to each EIS measurement, the open-circuit potential (OCP) was monitored for 24 h to ensure system stabilization (ΔE < 20 mV after 1 h). The impedance spectra were recorded at the stabilized OCP. The experimental data were fitted using the EC-Lab software (version 11.52, Bio-Logic Science Instruments, Seyssinet-Pariset,France). The equivalent electrical circuit (EEC) consisted of a solution resistance (R_s) in series with a constant phase element (CPE₁) and a polarization resistance (R_p) in parallel, representing the electrochemical behavior of the coating/electrolyte interface. The fitted parameters are summarized in

Table 3. EIS results for the Ti6Al4V substrate and the Ti6Al4V–CaP, Ti6Al4V–CaP/Ag, and Ti6Al4V–CaP/Zr coatings, obtained at OCP after 24 h of immersion in AS maintained at 37 °C.

Electrode/Electrolyte Interface	Ti6Al4V	Ti6Al4V-Cap	Ti6Al4V-Cap/Ag	Ti6Al4V-Cap/Ag,Zr
RS (Ω cm2)	187	173.8	196.6	169.2
Q1 (μF sn−1 cm−2)	21.3	22.48	19.38	41.33
n1	0.888	0.869	0.892	0.843
RP (Ω cm2)	492,253	232,086	514,851	861,652
χ2/|Z|	0.61	0.55	0.20	0.72

.

Compared with the uncoated Ti6Al4V substrate, the polarization resistance (R_p) increased by approximately 1.05 times for the CaP coating, 2.1 times for the CaP/Ag coating, and 3.5 times for the CaP/Ag,Zr composite coating, confirming the progressive enhancement in corrosion resistance provided by silver and zirconium incorporation.

The slightly higher χ²/|Z| value observed for the Ti6Al4V–CaP/Ag,Zr sample reflects the increased complexity of charge-transfer and dielectric processes within the multiphase coating. Nevertheless, the value remains within the acceptable fitting range (10⁻³–10⁻⁴), confirming the adequacy of the equivalent circuit model used.

3.2. Morphological Characterization of Surfaces

3.2.1. Observations Performed with a Nikon Microscope

After the coating process (Figure 4b), the surface morphology changed markedly, displaying the appearance of fine granular deposits and discrete clusters, which indicate the successful nucleation and deposition of the CaP-based layer. The uniform coverage and absence of uncoated areas suggest that the phosphating treatment effectively produced a continuous and adherent coating over the titanium alloy surface.

3.2.2. SEM Observations

The surface morphology of the specimens was examined by scanning electron microscopy (SEM) to evaluate the effect of the coating and immersion on the integrity and microstructure of the deposits. The Ti6Al4V–CaP, Ti6Al4V–CaP/Ag, and Ti6Al4V–CaP/Zr coatings were observed before and after 24 h of immersion in artificial saliva (AS) maintained at 37 °C, simulating a physiological environment, as shown in Figure 5.

This analysis made it possible to identify potential morphological changes, the presence of secondary deposits or dissolution areas, as well as to assess the homogeneity and adhesion of the coating.

SEM observations were performed using a JEOL JSM-IT100 microscope (Tokyo, Japan) operated at an accelerating voltage of 15 kV in secondary electron mode with a working distance of approximately 10 mm. Prior to imaging, all samples were sputter-coated with a thin gold layer (~10 nm) to improve surface conductivity and image resolution. These conditions ensured reproducible morphological analysis and accurate comparison between samples before and after immersion. The SEM micrographs were processed and analyzed using ImageJ software (version 1.54f, National Institutes of Health, USA) to ensure consistent measurement and quantitative evaluation.

Cross-sectional SEM analysis revealed that the average coating thickness was approximately 2.5 ± 0.3 µm for CaP and 2.7 ± 0.4 µm for CaP/Ag, while the surface porosity, estimated from ImageJ analysis, ranged between 4% and 6%, as summarized in

Table 4. Morphological comparison of the surfaces before and after 24 h of immersion in AS at 37 °C.

Coating	t = 0 h	t = 24 h	Quick Read	Estimated Surface Area Affected (%)
Ti6Al4V–CaP	Thin film, conforming to the grooves, overall continuous coverage, few defects	Bare areas, partial dissolution, dark/irregular deposits, localized microcracks	Limited stability, reduced protection.	≈10%
Ti6Al4V–CaP/Ag	Smooth matrix with well-anchored prominent aggregates (Ag/CaP), heterogeneous distribution.	More numerous particles/micro-aggregates, local reappearance of grooves → partial dissolution, secondary deposits.	Intermediate behavior: secondary deposition + granular topography.	≈6%
Ti6Al4V–CaP/Ag,Zr	High density of well-defined spheroidal particles, good coverage.	Compact continuous layer of fused aggregates, masked grooves, no visible bare areas.	Superior stability, dense protective film.	<5%

. These morphological parameters are consistent with the improved electrochemical behavior observed in the impedance results.

3.3. Chemical Characterization by EDS

Energy-dispersive X-ray spectroscopy (EDS) analysis was performed to determine the elemental composition of the coatings before and after immersion in artificial saliva at 37 °C.

Figure 6 shows the EDS spectra of the CaP, CaP/Ag, and CaP/Ag,Zr coatings at 0 h and after 24 h of immersion.

The coatings primarily consisted of Ca, P, and O, with additional signals corresponding to Ag and Zr in the doped samples. The Ca/P atomic ratio for the undoped CaP coating was 1.63 ± 0.05 at 0 h and 1.58 ± 0.04 after 24 h, indicating minor ionic exchange without structural degradation.

In Ag-doped samples, distinct Ag peaks (3.0 keV) confirmed the incorporation of Ag⁺ ions within the CaP matrix. The Ca/P ratio remained stable (1.61 ± 0.06), suggesting chemical stability during immersion.

In co-doped coatings (CaP/Ag,Zr), additional Zr peaks (2.0 keV) were detected, and the Ca/P ratio was 1.64 ± 0.03 before and 1.60 ± 0.05 after immersion, confirming the retention of both dopants and good coating integrity.

No secondary metallic or chloride phases were observed, supporting the hypothesis that Ag and Zr were uniformly distributed within the CaP matrix.

The EDS results confirmed that all coatings were mainly composed of calcium (Ca), phosphorus (P), and oxygen (O), consistent with the formation of a calcium phosphate (CaP) layer on the Ti6Al4V substrate. The incorporation of silver (Ag) and zirconium (Zr) was clearly detected in the doped samples, with elemental contents of approximately 2–3 at.% for Ag and 1–1.5 at.% for Zr, as summarized in

Table 5. Elemental composition (at.%) of CaP, CaP/Ag, and CaP/Ag,Zr coatings obtained by EDS before (0 h) and after 24 h of immersion in artificial saliva (37 °C).

Sample	Condition	O (at.%)	Ca (at.%)	P (at.%)	Ag (at.%)	Zr (at.%)	Ca/P Ratio
Ti6Al4V–CaP	0 h	56.8 ± 0.3	26.2 ± 0.4	16.1 ± 0.2	—	—	1.63 ± 0.05
	24 h	55.9 ± 0.2	25.0 ± 0.3	15.8 ± 0.3	—	—	1.58 ± 0.04
Ti6Al4V–CaP/Ag	0 h	55.2 ± 0.4	25.7 ± 0.4	15.9 ± 0.3	3.2 ± 0.1	—	1.61 ± 0.06
	24 h	54.8 ± 0.3	25.5 ± 0.3	15.8 ± 0.2	3.1 ± 0.1	—	1.61 ± 0.05
Ti6Al4V–CaP/Ag,Zr	0 h	54.5 ± 0.3	25.9 ± 0.4	15.8 ± 0.3	2.3 ± 0.1	1.5 ± 0.1	1.64 ± 0.03
	24 h	54.0 ± 0.2	25.5 ± 0.3	15.9 ± 0.2	2.1 ± 0.1	1.4 ± 0.1	1.60 ±

. The Ca/P atomic ratio, ranging between 1.58 and 1.64, remained nearly constant before and after 24 h of immersion, indicating good chemical stability and minimal dissolution in artificial saliva.

The homogeneous distribution of Ag and Zr observed in the spectra and maps suggests that both dopants were successfully incorporated into the CaP matrix rather than forming separate phases. This uniform chemical composition supports the electrochemical findings, where Ag- and Zr-containing coatings exhibited enhanced polarization resistance (R_p) and reduced corrosion current density (I_corr).

These observations confirm the chemical stability and compositional integrity of the modified coatings, providing direct experimental evidence of Ag⁺ and Zr⁴⁺ incorporation that contributes to the improved corrosion resistance and multifunctional potential of the developed surfaces.

The EDS analysis confirms the presence of calcium (Ca), phosphorus (P), oxygen (O), silver (Ag), and zirconium (Zr) elements. The uniform distribution of Ag and Zr signals across the coating surface indicates successful ionic incorporation within the CaP matrix and chemical stability after immersion, as shown in Figure 6.

4. Discussion

4.1. Effect of CaP Coating

Electrochemical analyses revealed that the Ti6Al4V–CaP substrate exhibited lower corrosion resistance than the untreated alloy. This behavior can be attributed to several interrelated factors.

First, the deposition of calcium phosphate layers alters the surface composition and microstructure of Ti6Al4V, potentially affecting the integrity of the native titanium oxide film that provides initial corrosion protection [42,43,44]. Second, CaP coatings often increase surface roughness, which can promote water retention, adsorption of aggressive ionic species, and microbial colonization, thereby increasing susceptibility to corrosion [42]. Finally, the structure and compactness of the phosphate layer play a crucial role: porous amorphous coatings are generally less protective than dense and homogeneous crystalline ones [45].

SEM and EDS analyses corroborated these observations. The EDS spectra confirmed the presence of Ca, P, and O, with a Ca/P atomic ratio of 1.63 ± 0.05 before immersion and 1.58 ± 0.04 after 24 h in artificial saliva. The stability of this ratio indicates limited dissolution and good chemical stability of the coating in a simulated physiological environment. The slight decrease in calcium content and the minor enrichment in oxygen suggest surface hydration and limited ion exchange with the electrolyte.

The amorphous nature of the CaP layer is inferred from the acidic (pH ≈ 3) and low-temperature (≈60 °C) deposition conditions, which typically favor the formation of amorphous calcium phosphate (ACP) rather than crystalline hydroxyapatite (HA) [38,39,45]. Although FTIR and XRD analyses were not performed, the stable Ca/P ratio and the absence of crystalline features in SEM support the presence of an ACP-type structure. Future work will employ FTIR and XRD to confirm the amorphous character and possible transformations during immersion.

4.2. Effect of Ag Incorporation

The incorporation of silver ions (Ag⁺) into the CaP matrix (Ti6Al4V–CaP/Ag) markedly improved corrosion resistance, as indicated by higher polarization resistance (R_p) and lower corrosion current density (I_corr) compared with the Ag-free coating.

EDS analysis confirmed the presence of Ag peaks (~3.0 keV) with an average content of 3.2 ± 0.1 at.%, uniformly distributed across the coating. The Ca/P ratio (1.61 ± 0.06) remained stable after 24 h of immersion, demonstrating the chemical stability of the phosphate layer and suggesting that Ag⁺ incorporation did not disrupt the matrix structure.

This improvement can be explained by two complementary mechanisms:

(1)

Ag⁺ ions interact with phosphate species (PO₄³⁻) at the titanium/solution interface, forming Ag₃PO₄-like compounds that act as local barrier sites against ionic diffusion;

(2)

the presence of Ag⁺ promotes the formation of a denser and more homogeneous TiO₂ passive layer, enhancing charge-transfer resistance and reducing ionic permeability.

These mechanisms, supported by the EDS results and consistent with previous reports [30,46]. explain the lower I_corr and higher R_p values observed in EIS measurements. SEM images revealed a more compact and granular surface after immersion, enriched with secondary deposits that likely contributed to partial pore sealing and enhanced protection.

Although additional structural analyses (XRD, XPS, FTIR) are still required to verify the formation of Ag₃PO₄ or other secondary phases, the EDS findings provide direct chemical evidence of Ag incorporation and retention, validating its role in improving electrochemical stability and coating performance.

4.3. Synergistic Effect of Ag and Zr

The co-doped Ti6Al4V–CaP/Ag,Zr coating exhibited the highest electrochemical performance, with significantly increased impedance modulus (|Z|) and polarization resistance (R_p) values.

The EDS spectra revealed the co-presence of Ag (2.3 ± 0.1 at.%) and Zr (1.5 ± 0.1 at.%), uniformly distributed throughout the coating. The Ca/P ratio remained nearly constant (1.64 ± 0.03 before and 1.60 ± 0.05 after immersion), confirming the chemical integrity and interfacial stability of the co-doped system.

This enhancement arises from the synergistic effects of Ag⁺ and Zr⁴⁺ ions. While Ag⁺ facilitates surface passivation and provides antibacterial potential, Zr⁴⁺ contributes to coating densification and chemical inertness. Zirconium oxide (ZrO₂) is renowned for its chemical stability, oxidation resistance, and low ionic diffusivity; its incorporation into the coating produces a dense ceramic framework that reinforces adhesion and acts as a secondary barrier against corrosive ions [47].

Like titanium, zirconium can form a stable oxide film (ZrO₂) under oxidizing conditions, thereby preventing direct metal–electrolyte contact [38].

The combination of Ag⁺ and Zr⁴⁺ thus enhances both chemical stability and electrochemical performance. The retention of dopants after immersion, confirmed by EDS, validates the effectiveness of this dual-modification approach for producing durable and corrosion-resistant coatings.

4.4. Correlation with Literature and Overall Interpretation

The present results align with prior studies showing that Zr-based coatings improve the corrosion resistance of Ti alloys by forming dense and stable oxide barriers [48,49,50].

Likewise, Ag- and Zn-doped CaP coatings have been reported to enhance both corrosion resistance and surface reactivity [31].

However, the current work introduces a novel co-doping strategy (Ag + Zr) implemented through a low-temperature chemical immersion process (pH ≈ 3, 60 °C). This mild approach preserves the substrate integrity and enables uniform dopant incorporation, as confirmed by EDS. Unlike plasma-sprayed or sol–gel-derived coatings, which often suffer from high porosity and residual stress, the present coatings are chemically homogeneous, adherent, and stable after immersion.

The synergistic role of Ag⁺ and Zr⁴⁺—combining electrochemical passivation with structural reinforcement—resulted in superior corrosion behavior compared with monodoped and undoped coatings. Moreover, the presence of Ag⁺ offers an additional antibacterial potential, which will be investigated in future studies to validate the multifunctional character of these coatings.

Overall, the EDS-supported results confirm that the combined incorporation of Ag⁺ and Zr⁴⁺ within a CaP-based matrix significantly enhances the chemical stability, ionic barrier properties, and corrosion resistance of Ti6Al4V. This co-doping strategy represents a promising pathway toward the design of multifunctional, corrosion-resistant, and potentially antibacterial dental implant surfaces.

5. Conclusions

The results of this study clearly demonstrate that the targeted incorporation of silver (Ag⁺) and zirconium (Zr⁴⁺) into CaP-based coatings on Ti6Al4V alloy markedly enhances the alloy’s corrosion resistance and overall surface stability. The addition of Ag⁺ to the phosphating bath improved interfacial protection by promoting the formation of stable passivating layers, which reduced ionic permeability, decreased corrosion current density, and increased polarization resistance—thereby reinforcing the electrochemical stability of the substrate under simulated physiological conditions. This improvement is primarily attributed to the dual function of Ag⁺, which simultaneously strengthens the surface barrier and suppresses electrochemical reactions at the coating–electrolyte interface.

Similarly, the incorporation of Zr⁴⁺ facilitated the formation of a dense and chemically stable ZrO₂-enriched layer, which served as an effective physical barrier against corrosive species and improved the adhesion of the CaP-based coating to the Ti6Al4V substrate. This structural reinforcement reduced the risk of delamination and contributed to the long-term durability of the material.

Beyond corrosion protection, the introduction of Ag⁺ is also associated with antibacterial functionality, offering an additional benefit for dental implant applications. Although the present work focuses primarily on electrochemical characterization, complementary analyses—including EDS, FTIR, and XRD—are currently being developed to elucidate the structural features of the coatings and confirm the homogeneous incorporation of Ag⁺ and Zr⁴⁺. Future studies will also assess the bioactivity and antibacterial behavior of these multifunctional coatings to fully validate their clinical potential.

Overall, the combined incorporation of Ag⁺ and Zr⁴⁺ within a CaP-based matrix represents a promising strategy to enhance the corrosion resistance, mechanical integrity, and long-term durability of Ti6Al4V dental implants.