Corrosion-Resistant Plasma Electrolytic Oxidation Composite Coatings on Ti6Al4V for Harsh Acidic Environments

Abstract

Titanium alloys are widely employed in structural and electrochemical applications owing to their excellent mechanical properties and inherent corrosion resistance. However, their stability in harsh acidic environments, such as those encountered in energy storage systems, remains a critical issue. In this study, composite ceramic coatings were synthesized on a Ti6Al4V alloy using plasma electrolytic oxidation (PEO) in silicate-, phosphate-, and sulfate-based electrolytes, with and without the addition of α-alumina nanoparticles. The resulting coatings were comprehensively characterized to assess their surface morphology, chemical and phase compositions, and corrosion performance. Thus, the corrosion current density decreased from 9.7 × 10⁴ for bare Ti6Al4V to 143 nA/cm² for the coating fabricated in phosphate electrolyte with alumina nanoparticles, while the corrosion potential shifted anodically from –0.68 to +0.49 V vs. silver chloride electrode in 5 M H₂SO₄. Among the tested electrolytes, coatings produced in the phosphate-based electrolyte with Al₂O₃ showed the highest polarization resistance (113 kΩ·cm²), outperforming those fabricated in silicate- (71.6 kΩ·cm²) and sulfate-based (89.0 kΩ·cm²) systems. The composite coatings exhibited a multiphase structure with reduced surface porosity and the incorporation of crystalline oxide phases. Notably, titania–alumina nanoparticle composites demonstrated significantly enhanced corrosion resistance. These findings confirm that PEO-derived composite coatings provide an effective surface engineering strategy for enhancing the stability of the Ti6Al4V alloy in aggressive acidic environments relevant to advanced electrochemical systems.

1. Introduction

Titanium alloys, particularly Ti6Al4V, have emerged as critical structural materials in various industrial applications owing to their exceptional combination of high strength-to-weight ratio, excellent corrosion resistance, and superior mechanical properties [1,2,3]. Alloys are extensively used in the aerospace, offshore oil and gas, and energy storage industries, where components must withstand harsh environmental conditions while maintaining their structural integrity [4,5,6]. However, despite its inherent corrosion resistance, Ti6Al4V faces limitations when exposed to aggressive acidic environments commonly encountered in advanced electrochemical systems and related manufacturing processes.

Plasma electrolytic oxidation (PEO) has emerged as an effective surface modification strategy for enhancing the corrosion resistance and functional surface characteristics of Ti6Al4V. By applying high-voltage discharges in an electrolyte bath, a hard, adherent, and porous ceramic-like oxide layer is formed, which is typically composed of crystalline rutile, anatase, and mixed oxides [7,8,9,10]. PEO coatings exhibit significantly enhanced corrosion resistance compared to uncoated titanium substrates; however, their performance is strongly influenced by the coating density, thickness, and phase composition [11,12]. Despite these promising results, only a limited number of studies have focused on evaluating the behavior of Ti alloy coatings under harsh acidic conditions. For example, Seo et al. showed the incorporation of costly elements such as Ru, Nb, and Ta into Ti grade 13 alloy. Their findings indicated that Nb and Ru primarily contribute to cathodic modification, and Ta significantly enhances the stability of the passive TiO₂ film, which is crucial for maintaining high corrosion resistance in aggressive reducing environments [13].

Titanium components play a vital role in energy storage systems, such as redox-flow batteries (RFBs) and lithium-ion (Li-ion) batteries, owing to their unique combination of chemical, electrochemical, and physical properties that meet stringent operational demands. In RFBs, titanium’s outstanding corrosion resistance in strongly acidic environments, including concentrated sulfuric acid, makes it an excellent choice for electrodes, current collectors, and flow-field components [14]. This chemical stability supports an extended service life and reduces maintenance requirements, even when exposed to aggressive redox couples such as Ti/Mn or Ti/Fe in strongly acidic electrolytes. In Li-ion batteries, titanium-based materials exhibit high stability as anode materials against electrolyte decomposition, contributing to improved safety and extended cycling performance [15,16]. Additionally, titanium oxide has been explored in the design of lithium/sulfur batteries as a binder for polysulfide anions, helping to mitigate functionality as an anode and improve its corrosion properties [17].

The electrolyte selection in the PEO process plays a critical role in determining the composition and protective performance of the resulting oxide coatings. Fattah-Alhosseini et al. evaluated the effect of different electrolyte systems, such as sodium aluminate, sodium phosphate, and sodium metasilicate, on the formation of TiO₂ coatings on commercial Ti grade 2 alloy [9]. Their findings indicate that the most uniform and compact coating was obtained using an aluminate-phosphate-based electrolyte. Later, Wu et al. reported that PEO coatings formed on the Ti6Al4V alloy in silicate-based electrolytes exhibited superior corrosion resistance in Ringer’s solution compared to those formed in phosphate or aluminate electrolytes [18].

Recently, composite PEO coatings have emerged as an effective strategy to improve the surface properties of titanium [8,19,20,21], magnesium [22,23], zirconium [24,25], and aluminum alloys [26,27]. Yang et al. reported a notable enhancement in the anticorrosive behaviour of TiO₂ coatings through the incorporation of ZrO₂ nanoparticles [28]. In a related study, the inclusion of Al₂O₃ microparticles into the TiO₂ coating resulted in a TiO₂-Al₂O₃ composite with enhanced wear resistance attributed to the pore-filling effect of the ceramic particles [29]. Moreover, previous studies have established that an optimal Al₂O₃ particle concentration in the electrolyte is approximately 5 g/L [30,31,32,33].

The current study investigates the formation of composite coatings on Ti6Al4V alloy by PEO process in silicate-, phosphate-, and sulfate-based electrolytes, each supplemented with α-alumina nanoparticles. The corrosion properties of the resulting composite PEO coatings were systematically investigated in 5 M H₂SO₄ to assess their effectiveness in protecting the substrate under harsh acidic environments.

2. Materials and Methods

2.1. Coating Fabrication

Rectangular samples of the Ti-6Al-4V alloy (TVA, Al 6 wt. %, V 4 wt.%, bal. Ti, Testbourne Ltd., Basingstoke, UK) with dimensions of 20 × 40 × 1 mm³ were used in this study. Prior to PEO surface treatment, the samples were polished using #600 and #1200 grit abrasive paper, followed by rinsing in distilled water, acetone, and ethanol, and finally, air-dried.

The PEO treatment was performed using an MP2-AS 35 (Magpuls, Sinzheim, Germany) power supply with an anodic unipolar square current of a 50% duty cycle. The applied voltage was set to 300 V, the current frequency was 50 Hz, and the treatment duration was 10 min. The PEO process was conducted in a 1 L double-jacketed beaker cooled with a WBL-700 (MRC, Swindon, UK) system set to 20 °C with magnetic stirring at 400 RPM. The electrical parameters were monitored using a Fluke Scope-Meter 199C (2.5 GS/s, Everett, WA, USA). The effect of three types of electrolyte systems on coating formation was examined: (1) sodium metasilicate and potassium hydroxide (Na₂SiO₃·5H₂O 15 g/L, (≥95.0%, Sigma Aldrich, St. Louis, MO, USA), and KOH 2 g/L (>85.0%, Daejung, Siheung-si, Republic of Korea), (2) sodium sulfate (Na₂SO₄ 10 g/L, (>99.0%, Carlo Erba GmbH, Emmendingen, Germany), and (3) sodium phosphate (Na₃PO₄ 10 g/L, (>96.0%, Sigma Aldrich, St. Louis, MO, USA). Composite coatings were synthesized by incorporating α-Al₂O₃ nanoparticles (200 nm, 5 g/L, 99.8%, Io-Li-Tec nanomaterials, Heilbronn, Germany). The nanoparticles were dispersed using bath ultrasonication, ACP-120H (MRC), at 100 W for 30 min.

The sample naming system follows the format [type of electrolyte, additive absence/presence], where SM denotes sodium metasilicate, SS denotes sodium sulfate, and SP denotes sodium phosphate. The symbols “o” and “a” indicate the absence and presence of an alumina additive, respectively. For example, [SP, a] refers to a sample treated in a sodium phosphate-based electrolyte with alumina addition.

2.2. Coating Characterization

Surface morphology examinations were analyzed using a scanning electron microscope (SEM), Jeol 6510 LV (Jeol Ltd., Tokyo, Japan). The elemental composition was determined using an energy-dispersive spectroscopy (EDS) system, Thermo Scientific Fisher EDX 7 (Thermo Fisher Scientific Inc., Oxford, UK). Phase identification was conducted by X-ray diffraction (XRD, X’Pert Pro diffractometer, PANalytical B.V., Almelo, The Netherlands) with CuKα radiation (λ = 1.542 Å) over a 2θ range of 20–60°, with a step size of 0.02° and a rate of 1 s/step operated at 40 kV and 40 mA in the grazing incident mode of 1°. The porosity and pore sizes were estimated with ImageJ 2, version 2.17.0 software on the SEM images. The thickness of the PEO coatings was determined from cross-sections using optical microscopy RH-2000 (Hirox Ltd., Tokyo, Japan).

2.3. Corrosion Resistance Test

Corrosion resistance was examined using an Ivium XRe potentiostat (Ivium Technologies B.V., Eindhoven, The Netherlands) in a harsh acidic environment, employing an aqueous solution of 5 M H₂SO₄ (96%, Carlo Erba GmbH, Emmendingen, Germany) as the electrolyte in a three-electrode cell configuration. A Pt electrode served as the counter electrode (CE), and a Reference Silver Chloride Electrode 3.0 M KCl (Metrohm Autolab B.V., Utrecht, The Netherlands) served as the reference electrode (RE). Prior to the electrochemical measurements, all samples were immersed in a sulfuric acid solution until a steady state was established for the working electrode (WE), which was achieved after 30–60 min of immersion. Potentiodynamic polarization (PDP) tests were conducted at a scanning rate of 1 mV/s, with a potential range of –250 to +250 mV vs. the RE. Each specimen was tested at six different locations to ensure uniform corrosion performance of the coating.

Two sets of samples were investigated: the first series was produced using various electrolytes without any particle additives, while the second involved incorporated α-Al₂O₃ nanoparticles as an additive in the PEO electrolyte.

3. Results and Discussion

3.1. Synthesis of Composite Coatings

The PEO process involves the oxidation of the substrate under an applied field within an electrolyte medium. The key characteristics of the process include the measured voltage and current, particularly the sparking voltage. Figure 1 presents the voltage-time and current-time responses recorded during the PEO treatment of the Ti6Al4V alloy in different electrolytes, both with and without the presence of α-Al₂O₃ nanoparticles.

The growth of composite ceramic coatings in electrolytes without the addition of α-alumina nanoparticles demonstrated typical applied current-voltage behaviour during the PEO process. Initially, a rapid voltage increase was observed, corresponding to the formation of the barrier layer, which was very similar to the growth of the barrier layer in anodizing processes [34,35,36,37,38,39]. This was followed by the sparking voltage, corresponding to the appearance of microdischarges, and subsequently a steady-state region with a more gradual voltage increase. The sparking voltage increased in the order of [SS, o] < [SM, o] < [SP, o], which reflects the increasing dielectric strength of the produced coatings. Notably, only the [SM, o] and [SP, a] samples demonstrated a further voltage increase, corresponding to the formation of denser and thicker composite PEO coatings. In contrast, the presence of sulphate anions likely leads to the formation of dissoluble compounds of titania through an intermediate form, e.g., TiO²⁺, such as titanyl sulfate or its derivative, thereby suppressing coating growth according to [40,41,42,43]. Thus, the [SS, a] sample demonstrated even higher current and lower voltage, suggesting ineffective coating formation or failure of the PEO process. The incorporation of α-alumina nanoparticles did not significantly change the overall current-voltage characteristics of the PEO process. However, a slight increase in steady state current was observed for the [SM, a] sample, which may be attributed to the formation of more stable silica-containing compounds during repetitive sparking provided by the sodium metasilicate electrolyte

3.2. Phase Analysis of Composite Coatings

XRD revealed that the composite coatings primarily consisted of titanium (ICDD #00-001-1198), anatase (ICDD #01-083-2243), rutile (ICDD #01-078-1510), and corundum (ICDD #00-001-1296), as previously reported [8,10,43], as presented in Figure 2.

Samples [SM,o] and [SP,o] exhibited weak diffraction peaks corresponding to anatase and rutile phases, indicating low crystallinity along with a high portion of amorphous content within the PEO coatings. Additionally, strong titanium substrate peaks were observed in the samples [SM, o], [SM, a], [SS, o], and [SS, a], indicating a limited coating thickness, as inferred from the grazing incident XRD mode. The dominance of anatase further suggests the predominantly amorphous nature of the PEO coatings formed under these conditions. Notably, diffraction peaks corresponding to the α-alumina phase were detected in all [X, a] sample series, confirming the successful incorporation of α-Al₂O₃ nanoparticles into the composite coating.

3.3. Surface Morphology and Chemical Composition Studies of Composite Coatings

Figure 3 demonstrates the surface morphology of the composite PEO coatings. The surfaces of [SM, o] and [SP, o] exhibited typical porosity for PEO coatings. In contrast, the [SS, o] sample mostly demonstrated nanoporosity, resembling that produced by conventional anodic oxidation, along with several pores attributed to electrical breakdown during sparking. Notably, all samples treated with α-Al₂O₃ nanoparticles demonstrated a reduction in porosity (

Table 1. Surface porosity, pore size, and thickness of the produced coatings.

Sample Name	Porosity [%]	Pore Size [µm]	Thickness [µm]
[SM, o]	10.1 ± 3.3	1.02 ± 0.35	4.9 ± 0.7
[SM, a]	4.3 ± 0.6	0.20 ± 0.07	2.3 ± 0.3
[SS, o]	2.9 ± 0.7	0.17 ± 0.04	2.4 ± 0.8
[SS, a]	1.0 ± 0.4	0.31 ± 0.08	3.4 ± 0.9
[SP, o]	5.7 ± 0.7	0.74 ± 0.20	3.6 ± 1.1
[SP, a]	2.5 ± 0.6	0.61 ± 0.14	3.5 ± 0.4

), despite evidence of sparking activity during the PEO process. Nanoparticles were also observed on the surfaces of the α-Al₂O₃-treated samples, confirming the incorporation of alumina nanoparticles into the ceramic coating. The relatively small coating thicknesses were confirmed by optical microscopy measurements. Overall, the PEO composite coatings showed comparable thicknesses, with the exception of the [SM, o] sample, where a higher thickness was observed due to the active incorporation of silica from the decomposition of sodium metasilicate.

The chemical compositions of the composite PEO coatings are summarized in

Table 2. Elemental composition of composite PEO coatings formed on Ti6Al4V alloy in various electrolytes, with and without α-alumina nanoparticle addition, as determined by energy-dispersive X-ray spectroscopy.

Sample Name	Element Content [at.%]
	Ti	Al	V	Si	P	S	Na	K
[SM, o]	45.6 ± 7.2	4.6 ± 0.8	1.9 ± 0.3	46.8 ± 8.2	n.d.	n.d.	0.6 ± 0.1	n.d.
[SM, a]	74.4 ± 0.6	10.9 ± 0.2	3.2 ± 0.2	9.8 ± 0.5	n.d.	n.d.	1.3 ± 0.2	n.d.
[SS, o]	85.6 ± 0.6	8.2 ± 0.5	3.4 ± 0.2	n.d.	n.d.	2.3 ± 0.3	0.3 ± 0.2	n.d.
[SS, a]	76.0 ± 1.1	18.7 ± 1.2	3.2 ± 0.2	n.d.	n.d.	1.6 ± 0.1	0.4 ± 0.2	n.d.
[SP, o]	70.5 ± 1.2	7.4 ± 0.3	3.2 ± 0.2	n.d.	15.9 ± 0.8	n.d.	0.6 ± 0.2	n.d.
[SP, a]	63.6 ± 2.4	20.0 ± 1.5	2.9 ± 0.2	n.d.	10.4 ± 0.6	n.d.	0.6 ± 0.2	n.d.

n.d.—non-detectable.

. The [SM, y] series exhibited significant silicon content, consistent with the use of silicate salts, along with detectable levels of sodium incorporated into the coating. Samples from the [SP, y] and [SS, y] series exhibited phosphorus and sulfur according to their presence in the electrolyte. The addition of α-Al₂O₃ nanoparticles resulted in an increased aluminum content in all [X, a] samples, confirming their successful incorporation into the coating. However, the relatively high titanium content detected in most coatings suggests limited coating thickness owing to X-ray penetration into the substrate. Only the [SM, o] sample showed a reduced Ti signal, indicative of a thicker oxide layer.

3.4. Corrosion of Composite Coatings in the Harsh Acidic Environment

PDP analysis was conducted using 5 M H₂SO₄ to assess the corrosion behavior of the synthesized coatings, as demonstrated in Figure 4. All the samples exhibited significantly enhanced corrosion resistance compared to the original Ti-6Al-4V alloy. This improvement was evidenced by reduced corrosion current densities and more noble corrosion potentials, indicating the effective barrier properties of the oxide coatings under highly aggressive acidic conditions.

The corrosion potentials (E_corr) vs. RE, current densities (i_corr), and anodic and cathodic Tafel slopes (β_a and β_c) were determined by linear fitting of the respective branches of the Tafel plots. The polarization resistance (R_p,_s-g) was calculated using the Stern–Geary Equation (1), based on the extracted electrochemical parameters.

$${\mathrm{R}}_{\mathrm{p},\mathrm{s}-\mathrm{g}}={\displaystyle \frac{{\mathsf{\beta }}_{\mathrm{a}}\times {\mathsf{\beta }}_{\mathrm{c}}}{2.3\times {\mathrm{i}}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}({\mathsf{\beta }}_{\mathrm{a}}+{\mathsf{\beta }}_{\mathrm{c}})}}$$

(1)

where β_a and β_c are the anodic and cathodic Tafel slopes, respectively, E_corr vs. RE, and i_corr are the corrosion potential and current density from the Tafel plot fitting.

The results of six independent tests for each sample, with standard deviations as described in the Methods section, are summarized in

Table 3. Electrochemical corrosion parameters of composite PEO coatings.

Sample Name	Ecorr,tafel [V vs. RE]	Icorr,tafel [nA/cm2]	ba [V/dec]	bc [V/dec]	Rp,s−g [kΩ·cm2]
Ti6Al4V	−0.680 ± 0.006	(9.7 ± 0.8)·104	0.341 ± 0.022	0.162 ± 0.005	0.50 ± 0.04
[SM, o]	0.343 ± 0.095	611 ± 464	0.164 ± 0.049	0.081 ± 0.107	25.95 ± 7.90
[SM, a]	0.433 ± 0.040	205 ± 62	0.162 ± 0.053	0.042 ± 0.005	71.55 ± 13.03
[SS, o]	0.235 ± 0.059	392 ± 60	0.174 ± 0.003	0.101 ± 0.005	71.99 ± 10.36
[SS, a]	0.333 ± 0.036	302 ± 33	0.172 ± 0.026	0.096 ± 0.006	89.01 ± 9.53
[SP, o]	0.159 ± 0.056	253 ± 121	0.159 ± 0.034	0.043 ± 0.013	70.86 ± 41.15
[SP, a]	0.492 ± 0.003	143 ± 22	0.218 ± 0.022	0.044 ± 0.003	113.00 ± 11.76

.

All the PEO-coated samples demonstrated a beneficial anodic shift in the corrosion potential, attributed to the transformation of the surface from metallic to oxide-coated state. The influence of the different anionic species was predominantly observed in the cathodic regions of the Tafel plots, whereas the anodic branches were generally indicative of passivation behavior. Notably, the [SM, o] sample exhibited poor corrosion behavior, likely due to the formation of a multi-phase system consisting of titanium oxide, silicates, and aluminum oxides. Such a multicomponent nature may facilitate charge transport along the grain boundaries, accelerating the corrosion processes. Samples synthesized without alumina nanoparticle additives demonstrated less pronounced anodic shifts and higher corrosion current densities, which can be attributed to the reduced coating uniformity. In contrast, the composite PEO coatings with incorporated alumina nanoparticles exhibited a more significant anodic shift and consistently higher average polarization resistances, indicating increased corrosion protection in a harsh acidic environment. This enhancement is primarily associated with the incorporation of highly crystalline and inert alumina nanoparticles, which are likely embedded both within grain boundaries and through the bulk matrix. Moreover, the XRD analysis confirmed a reduction in the amorphous content of these composite coatings, further supporting their superior corrosion protection. Overall, the addition of alumina nanoparticles during the PEO treatment represents an effective strategy for significantly enhancing the corrosion resistance of Ti6Al4V alloy under harsh acidic environments.

4. Conclusions

Composite ceramic coatings were successfully synthesized on a Ti6Al4V alloy via PEO in silicate-, phosphate-, and sulfate-based electrolytes, with and without the incorporation of α-Al₂O₃ nanoparticles. The addition of alumina nanoparticles into the electrolyte led to their successful integration into the oxide matrix, as confirmed by SEM imaging, X-ray diffraction, and EDS analysis, which revealed the presence of corundum alongside anatase and rutile titanium oxide phases. Despite the mainly observed decrease in coating thickness associated with nanoparticle incorporation, the corrosion resistance of the composite coatings improved markedly in 5 M H₂SO₄. The addition of α-Al₂O₃ resulted in a significant anodic shift in the corrosion potential and a pronounced reduction in the corrosion current density. For instance, the corrosion potential shifted from –0.680 for bare Ti6Al4V to +0.492 V vs. RE for the [SP, a] sample, while the corrosion current density dropped from 9.7 × 10⁴ to 143 nA/cm². Among the composite coatings, the corrosion resistance increased in the following order: [SM, a] < [SS, a] < [SP, a]. Overall, the incorporation of α-Al₂O₃ nanoparticles into the PEO electrolyte facilitated the formation of robust composite coatings with enhanced microstructural quality. These coatings provided significantly enhanced protection in highly acidic environments, making them promising candidates for electrochemical and energy storage system applications.