Influence of Zn²⁺ on Corrosion Behavior of Ti in H₂SO₄ Solution with F⁻

Abstract

The influence of Zn²⁺ on the corrosion behavior of Ti in 1.68 mol L⁻¹ H₂SO₄ with 0.058 mol L⁻¹ F⁻ solutions was investigated by mass loss measurements, electrochemical tests, and surface observation and analysis. Different concentrations of Zn²⁺ (0.0002 mol L⁻¹, 0.002 mol L⁻¹, 0.02 mol L⁻¹, 0.2 mol L⁻¹) were added to the solutions to investigate the effect of Zn²⁺ concentration. The mass loss results showed that the presence of Zn²⁺ in the corrosive solution could accelerate the corrosion of Ti. From the electrochemical tests results, the corrosion resistance is decreased while increasing the Zn²⁺ concentration. An adsorption-desorption model of TiF_x^3−x on Ti was proposed: Zn²⁺ could inhibit the adsorption process of TiF_x^3−x and reduce the surface coverage of TiF_x^3−x, which could provide more active site for Ti to dissolve, resulted in the severe corrosion of Ti.

1. Introduction

Titanium has garnered significant attention for its exceptional strength-to-weight ratio, corrosion resistance, and fatigue limit, ensuring durability in various applications.^1–3 Titanium is a widely used lining material in chimneys, which is exposed to a severely corrosive environment containing acid condensates (H₂SO₄ solution, fluoride ions, zinc ions, etc.).^4–6 In general, the anti-corrosion ability of titanium is owing to the spontaneous formation of a passive film (TiO₂) upon exposure to an aqueous solution.^2,7–10 This TiO₂ layer acts as a barrier against rapid dissolution, providing anti-corrosion ability in acidic media.^11,12 However, the stability and electrochemical properties of this TiO₂ film are contingent upon its structure and thickness.^13,14 Nevertheless, presence of F⁻ and Zn²⁺ in the acid solution makes the corrosive environment more aggressive bringing additional complexities to the corrosion of titanium.

It has been reported that the presence of a very low concentration of F⁻ in H₂SO₄ solution has a detrimental effect on the passive film.^3,15–19 Electrochemical test results have shown that higher fluoride (F⁻) concentrations are associated with lower impedance at low frequencies, suggesting that the dissolution of the TiO₂ film diminishes its protective capacity. The chemical equation for the reaction is as follows:¹⁷

  

\begin{equation} \text{TiO}_{2} + 4\text{HF} \to \text{TiF}_{4} + 2\text{H$_{2}$O} \end{equation} (1)

Also, the surface morphology of Ti exposed to a similar environment showed the formation of micro-pits on the surface, along with a significant decrease in Ti and O peak intensities (based on XPS analysis), indicating film deterioration.²⁰

The role of Zn²⁺ on the corrosion behavior of various materials like Mg, steel, etc. in different environments has been reported.^21–27 Zn²⁺ acts as a corrosion accelerator on Mg alloy in a NaCl environment.²² On the other hand, Zn²⁺ inhibits the corrosion of mild steel in an aqueous environment.^25,27 Based on the literature, it can be stated that Zn²⁺ acts as a corrosion inhibition or accelerator depending on the nature of the corrosive environment.^21–27 However, the studies about the effect of Zn²⁺ on the corrosion behavior of Ti in acid solution with F⁻ are scanty. This knowledge gap highlights the significance of investigating the effect of Zn²⁺ on titanium corrosion.

This study aims to investigate the effect of Zn²⁺ on the corrosion behavior of Ti in H₂SO₄ solution in the presence of F⁻. Mass loss measurements and electrochemical tests were conducted to investigate the effects of Zn²⁺ and its concentration on titanium corrosion. These methods helped clarify the electrochemical properties and corrosion behavior affected by Zn²⁺. This work contributes to the basic understanding of the corrosion of Ti in complex environments and is beneficial for identifying the practical failures in industries.

2. Experimental

A Ti sheet (thickness 0.3 mm) was cut into 10 mm × 10 mm specimens. The Ti sheet was purchased from Nilaco Corporation, Japan. The specimens used for electrochemical tests were connected with copper wire as the working electrode. All the specimens were molded with resin (Struers Ltd., EpoFix Resin). The exposed surfaces of specimens were ground from #240 to #4000 SiC paper and finally polished with colloidal silica. Further, the specimens were washed with ethanol several times and finally cleaned with highly purified water using an ultrasonic bath (SIBATA ultrasonic cleaner, SU-2T) for 15 min. Before the experiments, the specimens were stored in a desiccator for 24 h to naturally form the oxide film.

S is defined as the symbol of solution. The base solution S_mixed contains 0.058 mol L⁻¹ of NaF and 1.68 mol L⁻¹ of H₂SO₄, without Zn²⁺. Based on this, a series of solutions were prepared by adding different concentrations of Zn²⁺: S_{mixed + 0.0002 Zn} (0.0002 mol L⁻¹ Zn²⁺), S_{mixed + 0.002 Zn} (0.002 mol L⁻¹ Zn²⁺), S_{mixed + 0.02 Zn} (0.02 mol L⁻¹ Zn²⁺), and S_{mixed + 0.2 Zn} (0.2 mol L⁻¹ Zn²⁺).

The special-grade chemicals used were commercially available from Kanto Chemical Co., Ltd. The solutions used in this study were not deaerated.

Specimens were immersed in the test solutions for 7 d at 298 K. The mass of specimens was measured before and after immersion by a micro balance (Mettler Toledo, MX5 Micro Analytical Balance). The corrosion rate (C_R: mg (cm² year)⁻¹) was calculated by the following Eq. 2:

  

\begin{equation} C_{\text{R}} = \frac{M_{\text{b}} - M_{\text{a}}}{t \times \text{A}} \times 365 \end{equation} (2)

where, M_b is the mass of the Ti specimen before the immersion test (mg), M_a is the mass of the Ti specimen after the immersion test (mg), t is the immersion time (7 d), and A is the surface area of the Ti specimen (cm²). Reproducible data were obtained from the mass loss measurements.

After the mass loss measurements, the surface roughness of specimens was observed by a confocal laser scanning microscope (LSCM, Lasertec Co., 1LM21D) using a 20x magnification. The surface morphology of the specimens was observed by a scanning electron microscope (SEM, JEOL Ltd., JSL6510-LA). The surface composition of specimens was analyzed by energy dispersive spectrometry (EDS).

The electrochemical tests were carried out in a three-electrode cell system using a potentiostat (HZ-7000, HOKUDO DENKO). A platinum plate was used as the counter electrode. A standard Ag/AgCl electrode immersed in saturated KCl solution (SSE) was used as the reference electrode. The specimen was used as the working electrode. Open-circuit potential (OCP) was measured for 1500 s to attain a stable state. After the OCP measurements, the electrochemical impedance spectroscopy (EIS) tests were carried out at the OCP (after the 1500 s immersion) in the frequency range from 10 kHz to 10 mHz, and the modulation amplitude was 10 mV. All the tests were carried out at least three times in each condition to check the reproducibility of the data.

3. Results and Discussion

3.1 Mass loss results

Figure 1 illustrates the corrosion rate (C_R) of Ti, calculated using Eq. 2, after immersion in solutions for 7 d with different Zn²⁺ concentration (C_Zn²⁺). In the S_mixed, the corrosion rate (C_R) of Ti is 1923 mg (cm² year)⁻¹. Adding 0.0002 mol L⁻¹ Zn²⁺ to S_mixed (S_{mixed + 0.0002 Zn}) increases the C_R to 1972 mg (cm² year)⁻¹. With an increase in Zn²⁺ concentration to 0.002 mol L⁻¹ (S_{mixed + 0.002 Zn}), the C_R increases further to 2013 mg (cm² year)⁻¹. The C_R continues to increase to 2308 mg (cm² year)⁻¹ in S_{mixed + 0.02 Zn} and 2702 mg (cm² year)⁻¹ in S_{mixed + 0.2 Zn}.

Figure 1.

Corrosion rate of Ti, C_R, after the immersion test in solutions with different concentrations of Zn²⁺, $C_{\text{Zn}^{2 + }}$.

These results demonstrate that higher concentrations of Zn²⁺ in S_mixed significantly accelerate the corrosion of Ti.

3.2 Surface observation and analysis

The micro structure of the specimens is observed using SEM. Figure 2a to Fig. 2e show the SEM images of Ti surfaces after immersed in different solutions for 7 d. From Fig. 2a, after immersion in the S_mixed, the surface of Ti has dimples. When Zn²⁺ is added to the S_mixed (S_{mixed + 0.0002 Zn}), the surface shows different structure. With increasing the Zn²⁺ concentration (from 0.002 mol L⁻¹ to 0.2 mol L⁻¹ Zn²⁺) shown in Fig. 2c to Fig. 2e, the dimple size increased, indicating a more severe corrosion.

Figure 2.

SEM images of Ti after 7 d immersion test in different solutions; (a) S_mixed, (b) S_{mixed + 0.0002 Zn}, (c) S_{mixed + 0.002 Zn}, (d) S_{mixed + 0.02 Zn}, and (e) S_{mixed + 0.2 Zn}.

The roughness of Ti specimens exposed to S_mixed and S_{mixed + 0.2 Zn} is studied using. The LSCM results are shown in Fig. 3a S_mixed and Fig. 3b S_{mixed + 0.2 Zn}. From the 3D images, the presence of Zn²⁺ (in S_mixed) increases the heterogeneity on the surface. This is confirmed by an increase in the average roughness (R_a) value from 48.4 µm to 64.2 µm, indicating severe corrosion in S_{mixed + 0.2 Zn}.

Figure 3.

LSCM 3D images of Ti after immersion test in different solutions: (a) S_mixed and (b) S_{mixed + 0.2 Zn}.

The chemical composition (from EDS) of the specimens exposed to different solutions is shown in Table 1. The top surface shows a high Ti concentration. No O is detected in all the tests solutions. Zn is detected on the specimen surface suggesting the presence of Zn-related substances on the Ti surface.

Table 1. EDS surface analysis results of Ti after immersed in different solutions (ND: Not Detected).

Solution	Ti (Mass%)	O (Mass%)	S (Mass%)	Zn (Mass%)	F (Mass%)
Smixed	99.87	ND	0.08	—	0.04
Smixed + 0.0002 Zn	99.51	ND	0.10	0.16	0.22
Smixed + 0.002 Zn	99.88	ND	0.01	ND	0.11
Smixed + 0.02 Zn	99.95	ND	0.03	0.02	ND
Smixed + 0.2 Zn	99.93	ND	0.02	0.05	ND

3.3 Electrochemical tests

3.3.1 Open circuit potential

The open circuit potential, OCP, is monitored for 1500 s, and the results are shown in Fig. 4. The OCP values are shown in Table 2. The OCP in all solutions displayed an initial drop indicating the dissolution of the naturally formed oxide film. Further, the OCP gradually increases and then reaches a steady state. As shown in Table 2, OCP in S_mixed stabilizes at −0.664 V _{vs SSE} at 1500 s. With the addition of Zn²⁺, the OCP trend remains unchanged. The OCP in the S_{mixed + 0.0002 Zn} has slightly shifted (30 mV) and reaches a steady state at −0.694 V _{vs SSE}. With increasing the Zn²⁺ concentration (0.0002 mol L⁻¹, 0.002 mol L⁻¹, and 0.2 mol L⁻¹), the OCP is more pronounced (−0.720 V _{vs SSE}, −0.744 V _{vs SSE}, and −0.759 V _{vs SSE}, respectively). The result indicates the stability of the TiO₂ film decreases with increasing the concentration of Zn²⁺.

Figure 4.

Open circuit potential of titanium immersed in different solutions for 1500 s.

Table 2. Open circuit potential, OCP, at 1500 s in the different solutions.

Solution	Smixed	Smixed + 0.0002 Zn	Smixed + 0.002 Zn	Smixed + 0.02 Zn	Smixed + 0.2 Zn
OCP (V vs SSE)	−0.664 ± 0.039	−0.694 ± 0.058	−0.720 ± 0.016	−0.744 ± 0.016	−0.759 ± 0.019

These observations provide valuable insights into the electrochemical behavior of the studied systems with the influence of zinc ions on Ti corrosion.

3.3.2 Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) measurements were conducted to elucidate the corrosion resistance of titanium (Ti) in diverse solutions. The Bode diagrams presented in Fig. 5a impedance and Fig. 5b phase shift. Lower impedance magnitudes at low frequencies correlate with poor corrosion resistance of the specimens. Figure 5a shows that the corrosion resistance of Ti decreases in the presence of Zn²⁺ and is strongly correlated with Zn²⁺ concentration.

Figure 5.

EIS results of titanium immersed in different solutions.

From Fig. 5b, in S_mixed, the phase angle reaches 50° as the frequency decreases from medium to low, indicating the presence of non-uniform film under these conditions. With the addition of Zn²⁺ to the solutions, the phase angle decreased: the higher the Zn²⁺ concentration, the lower the phase angle. In the presence of Zn²⁺, this phase angle is smaller compared to that in the Zn²⁺ free solution (S_mixed), suggests the Zn²⁺ may affect the deposition/dissolution process.

To obtain deeper insights into effects of Zn²⁺ on the structural and electrochemical properties of the TiO₂ film in the aggressive environment, two-time constant equivalent circuit model is employed. It was designed to characterize TiO₂ film with defects which formed on the Ti in the sulfuric acid solution with F⁻.^3,19,28 Nonideal behavior of capacitance is shown in Fig. 5, a constant phase element (CPE) is used to improve the quality of fitted results. The CPE is calculated by the following equation:²⁹

  

\begin{equation} Z_{\text{CPE}} = [Q(j\omega )^{\text{n}}]^{ - 1} \end{equation} (3)

where the imaginary unit is j, ω represents the angular frequency, and the factor, n, is defined as a CPE power, which is an adjustable parameter that lies between −1 (for an ideal inductor) and 1 (for an ideal capacitor). Q is the constant phase element (CPE) parameter. The equivalent circuit is shown in Fig. 6. The equivalent circuit consists of several components: Q_f is utilized to describe the CPE parameter of the film, while Q_dl is employed to characterize the CPE parameter of the double layer at the solution/metal interface. R_s represents the solution resistance, R_f denotes the resistance of the defective/porous film, and R_ct represents the charge transfer resistance at the metal/solution interface.

Figure 6.

Used equivalent circuits for fitting EIS results of titanium immersed in solutions.

The results obtained from fitting the equivalent circuits are summarized in Table 3. With the addition of Zn²⁺, both R_f and R_ct decreased, indicating a reduction in corrosion resistance. The corrosion resistance (R_c) was be calculated by Eq. 4 and shown in Table 3.²⁴

  

\begin{equation} R_{\text{c}} = R_{\text{f}} + R_{\text{ct}} \end{equation} (4)

Table 3. Fitted results of electrochemical impedance parameters after immersed in different solutions.

Solution	Qf ($\unicode{x00B5} \text{S}^{\text{n}_{\text{f}}}$/Ω cm2)	nf	Rf (Ω cm2)	Qdl ($\unicode{x00B5} \text{S}^{\text{n}_{\text{dl}}}$/Ω cm2)	ndl	Rct (Ω cm2)	Rc (Ω cm2)
Smixed	351	0.89	11.53	2.05 × 104	0.97	31.26	42.79
Smixed + 0.0002Zn	744	0.89	7.09	4.31 × 104	0.73	21.53	28.62
Smixed + 0.002Zn	189	0.85	9.20	4.99 × 104	0.55	19.01	28.21
Smixed + 0.02Zn	344	0.92	8.12	6.20 × 104	0.66	14.25	22.37
Smixed + 0.2Zn	597	0.84	7.74	6.28 × 104	0.65	8.39	16.13

R_c as the function of Zn²⁺ concentration is shown in Fig. 7. R_c obtained in Zn²⁺ free solution (S_mixed) is shown as the control data. The results show that the R_c decreases with an increase in Zn²⁺ concentration. The correlation coefficient and linear regression line are also shown in Fig. 7. A correlation coefficient of 0.95 indicates a strong relationship between the corrosion resistance of Ti and the concentration of Zn²⁺ in the solution. These results are consistent with the mass loss measurements results.

Figure 7.

Corrosion resistance, R_c, in solutions with different concentrations of Zn²⁺, $C_{\text{Zn}^{2 + }}$.

The magnitude of Q_dl is related to the capacitive characteristics of the system: the larger Q_dl, the more defect area of the passive film. The addition of Zn²⁺ increases the Q_dl (at least 2 times), indicating a larger total defect area within the passive film, which suggests a more porous structure and reduced corrosion inhibition ability.

These results suggest that the presence of Zn²⁺ may expand the area of defects in the film, ultimately influencing the corrosion behavior of titanium in complex aqueous environments.

3.4 Kinetic parameter calculation

As the TiO₂ film dissolves shown in Fig. 8a, the underlying Ti substrate is exposed to the solution, resulting in the dissolution of Ti. A two-step metal dissolution process, previously proposed and used to study the anodic behavior of metals,^16,30,31 is extended here to investigate the influence of Zn²⁺ on accelerating titanium dissolution. This model considers TiF_x^3−x as an intermediate species, which first adsorbs onto the titanium surface (as described in Eq. 6) and subsequently desorbs into the solution (Eq. 7). This two-step process is schematically shown in Fig. 8b and Fig. 8c, illustrating the adsorption and desorption processes.

Figure 8.

The adsorption model of TiF_x^3−x; (a) dissolution of TiO₂ film, and (b) in S_mixed and (c) in S_{mixed + Zn}.

By applying this model, the adsorption and desorption rate constants of TiF_x^3−x, along with the titanium surface coverage by TiF_x^3−x, are quantitatively analyzed under conditions with and without Zn²⁺.

The dissolution of Ti could be explained by the two steps process by the following equations:¹⁶

  

\begin{equation} \text{Ti} + x\text{F}^{ - } \xrightarrow{k_{\text{ads}}}\text{TiF}_{x}{}^{3 - x}\,_{\text{adsorbed}} + 3\text{e}^{ - } \end{equation} (5)

  

\begin{equation} \text{TiF}_{x}{}^{3 - x}\,_{\text{adsorbed}} \xrightarrow{k_{\text{des}}}\text{TiF}_{x}{}^{3 - x}\,_{\text{aqueous}} \end{equation} (6)

The TiF_x^3−x _aqueous will dissolve into the solution as shown in Fig. 8b. The rate constant of TiF_x^3−x _adsorbed and TiF_x^3−x _aqueous formation is denoted as k_ads and k_des, respectively. Since Eq. 5 describes an electrochemical reaction, k_ads can be expressed using Tafel’s law,^16,32 as shown in Eq. 7:

  

\begin{equation} k_{\text{ads}} = k_{\text{ads}}^{0}\exp (\text{b}E) \end{equation} (7)

The standard rate constant is denoted as $k_{\text{ads}}^{0}$. The Tafel constant is represented as b. The E is overpotential. The TiF_x^3−x _adsorbed desorption (Eq. 6) is regarded as the potential independently process.

To better explain the monolayer adsorption process, the Faradaic current (i_F) is analyzed in conjunction with the Langmuir isotherm, to study the coverage of the intermediate on the electrode surface. Given that the dissolution of Ti represents the primary electrochemical process, at steady state, the faradaic current (i_F) is equivalent to the dissolution current (i_d). At the steady state, i_F = i_d. In other words, the faradaic current is entirely attributed to the dissolution reaction of Ti. Based on this understanding, the influence of Zn²⁺ on accelerating the Ti corrosion will be further analyzed.

Assuming that the TiF_x^3−x _adsorbed follows the Langmuir isotherm, the Faradaic current (i_F) corresponding to Eq. 5:

  

\begin{equation} i_{\text{F}} = n\text{F}k_{\text{ads}}C_{\text{F}^{ - }}^{\alpha }\Gamma_{0}( 1 - \theta ) \end{equation} (8)

where n is the electron number involved in the Faradaic reactions. F is the Faraday constant. In this study, the reactant concentration, $C_{\text{F}^{ - }}$ is 0.058 mol L⁻¹ in the solutions without and with Zn²⁺. α is the reaction order in the Eq. 8. Γ₀ represents the total number of adsorption sites available for TiF_x^3−x adsorbed on the titanium surface, corresponding to a mono-layer of adsorbed species on metals, with a typical value of 8.8 × 10⁻⁸ mol cm⁻² (1 cm ×1 cm Ti plate). The fractional surface coverage by adsorbed TiF_x^3−x is represented as θ, and the active cite numbers covered by adsorbed TiF_x^3−x are θΓ₀. The site numbers for Ti to dissolve are Γ₀(1 − θ). The $k_{\text{ads}}C_{\text{F}^{ - }}^{\alpha }\Gamma_{0}$ could be written as K_ads.

Correspondingly, the dissolution current of Eq. 6 is shown as 9:

  

\begin{equation} i_{\text{d}} = n\text{F}k_{\text{des}}\Gamma_{0}\theta \end{equation} (9)

The k_desΓ₀ could be written as K_des.

Due to the mass is balanced in the Eqs. 5 and 6, the Eq. 10 could be concluded based on Eqs. 8 and 9:

  

\begin{equation} (1 - \theta)K_{\text{ads}} = \theta K_{\text{des}} \end{equation} (10)

At the steady state, the θ_ss (surface coverage at the steady state) could be attributed to get Eq. 11:

  

\begin{equation} \theta_{\text{ss}} = \frac{K_{\text{ads}}}{K_{\text{ads}} + K_{\text{des}}} \end{equation} (11)

The Faradaic current, i_F, depends on the titanium surface's fractional coverage by the intermediate species, θ, and the potential, E. As a result, Eq. 12 could be given:¹⁶

  

\begin{equation} \Delta i_{\text{F}} = \left(\frac{\partial i_{\text{F}}}{\partial E} \right)_{\theta } {}\cdot \Delta E + \left(\frac{\partial i_{\text{F}}}{\partial \theta } \right)_{E} {}\cdot \Delta \theta \end{equation} (12)

Based on the equivalent circuit used in this study, the Z_F is:

  

\begin{equation} Z_{\text{F}} = R_{\text{f}} + \frac{R_{\text{ct}}}{1 + R_{\text{ct}}j\omega Q_{\text{dl}}} = \frac{\Delta E}{\Delta i_{\text{F}}} \end{equation} (13)

Since R_f can be used to describe the resistance of defective/porous film, it may indirectly influence R_ct and i_F by affecting interfacial conditions (e.g., surface coverage).¹⁶ From the equations above, the equivalent circuit parameters with the kinetic parameters could be obtained as Eqs. 14 and 15:

  

\begin{equation} \left(\frac{1}{R_{\text{ct}}} + \frac{1}{R_{\text{f}}} \right) \cdot \frac{1}{Q_{\text{dl}}} = \frac{K_{\text{ads}}}{\theta_{\text{SS}}\Gamma_{0}} \end{equation} (14)

  

\begin{equation} K_{\text{des}} = \frac{\Gamma_{0}}{R_{\text{ct}}Q_{\text{dl}}} \end{equation} (15)

From the above equations, the kinetic parameter values of K_ads, K_des and θ_ss could be calculated.

The analysis of kinetic parameter values provides insight into the accelerating corrosion mechanism of Ti in the solutions based on the equivalent circuit parameters presented in Table 4. In the solutions, K_ads is larger than K_des, indicating that the charge transfer process predominates. However, with the addition of Zn²⁺ (ranging from 0.0002 mol L⁻¹ to 0.02 mol L⁻¹), K_ads reduces with increasing Zn²⁺ concentration, while K_des is significantly increased in the S_{mixed + 0.2 Zn}. Besides, the addition of Zn²⁺ leads to a significant decrease in surface coverage at the steady state, θ_ss, from 73.051 % in S_mixed to 52.016 % in S_{mixed + 0.2 Zn}. It could be concluded that during the adsorption and desorption process, the addition of Zn²⁺ results in a decrease in the adsorption rate constant, an increase in the desorption rate constant, and a reduction in the surface area covered by the TiF_x^3−x _adsorbed.

Table 4. Calculated kinetic parameters in different solutions.

Solution	Kads (mol/(Ω $\unicode{x00B5} \text{S}^{\text{n}_{\text{dl}}}$))	Kdes (mol/(Ω $\unicode{x00B5} \text{S}^{\text{n}_{\text{dl}}}$))	θss (%)
Smixed	3.72 × 10−7	1.37 × 10−7	73.051
Smixed + 0.0002Zn	2.88 × 10−7	9.48 × 10−8	75.241
Smixed + 0.002Zn	1.92 × 10−7	9.28 × 10−8	67.388
Smixed + 0.02Zn	1.75 × 10−7	9.96 × 10−8	63.700
Smixed + 0.2Zn	1.81 × 10−7	1.67 × 10−7	52.016

3.5 The adsorption model

Based on the above results, the K_ads and θ_ss of TiF_x^3−x _adsorbed decrease with the addition of zinc ions (Zn²⁺) to the solutions. Additionally, the EIS results show a decline in the corrosion resistance of Ti containing Zn²⁺, and the mass loss experiments indicate an increase in the mass loss of Ti in the presence of Zn²⁺. This suggests that the Zn²⁺ could accelerate the dissolution of Ti in the solutions. From the EDS results, the possible effect of ZnSO₄ on reducing the active sites is also considered. However, both immersion test results and impedance results indicate that Zn²⁺ significantly increases the corrosion of Ti. This implies that the formation of ZnSO₄ may not reduce the number of active sites.

Based on the above, an adsorption model of TiF_x^3−x is shown in Fig. 8. The TiO₂ film is dissolved initially in the solutions as shown in Fig. 8a. With the dissolution of Ti by F⁻, the TiF_x^3−x is adsorbed on the Ti as shown in Fig. 8b.

The presence of Zn²⁺ reduces the adsorption sites for TiF_x^3−x _adsorbed and may also alter the chemical environment at the interface between Ti and solution. The possible reason is the Zn²⁺ approaching to Ti surface and hindering the reaction between F⁻ and Ti (Fig. 8c). Besides, Zn²⁺ may modify the surface charge distribution as shown in Fig. 8c, which may change the affinity of TiF_x^3−x _adsorbed, weakening its adsorption strength. This leads to decreased K_ads and θ_ss of TiF_x^3−x _adsorbed.

As the coverage of TiF_x^3−x _adsorbed decreases, the adsorption sites on the Ti surface become more exposed (as shown in Fig. 8c), providing more reactive sites for Ti to dissolve. The reduction in TiF_x^3−x _adsorbed (lower θ_ss) means a larger exposed metal surface, which leads to an increased dissolution of Ti.

The K_des is typically asymmetric compared to the K_ads. The K_des value does not significantly change with the addition of Zn²⁺. If TiF_x^3−x _adsorbed forms a stable adsorption layer on the Ti surface, the desorption of TiF_x^3−x _adsorbed would require overcoming a significant energy barrier. Therefore, the addition of Zn²⁺ may not significantly alter this process.

Although the effects of Zn²⁺ on the adsorption and desorption rate constants have been thoroughly analyzed and the potential reasons for accelerating Ti corrosion have been discussed, the specific mechanism by which Zn²⁺ accelerates Ti corrosion remains unclear.

4. Conclusions

In conclusion, the effect of Zn²⁺ on Ti corrosion in H₂SO₄ solution with F⁻ was investigated by mass loss measurement, electrochemical tests, and surface observation and analysis.

1. 1)    The results from mass loss measurement show that Zn²⁺ accelerates the Ti corrosion even at a low concentration (greater than 0.0002 mol L⁻¹) in S_mixed.
2. 2)    Based on EIS results, Zn²⁺ could reduce the corrosion resistance of Ti in the solutions and correlate with the concentrations of Zn²⁺.
3. 3)    Based on the kinetic parameters calculated by the TiF_x^3−x adsorption-desorption model, Zn²⁺ could inhibit the TiF_x^3−x adsorption process but not significantly affect the desorption process; a reduction in the surface area covered by the TiF_x^3−x _adsorbed in the Zn²⁺ solutions can be attributed to an increase in the active site for Ti dissolution.

Acknowledgment

A part of this work was conducted at Laboratory of XPS analysis, Joint-use facilities, Hokkaido university, supported by “Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM)” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT). Proposal Number JPMXP12 23HK0028.

CRediT Authorship Contribution Statement

Xinxin Liu: Conceptualization (Supporting), Data curation (Lead), Writing – original draft (Lead)

Yoganandan Govindaraj: Supervision (Supporting), Writing – review & editing (Lead)

Masatoshi Sakairi: Conceptualization (Equal), Supervision (Lead), Writing – review & editing (Lead)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Footnotes

M. Sakairi: ECSJ Active Member

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