Electrochemical Corrosion Behavior and the Related Mechanism of Ti₃SiC₂/Cu Composites in a Strong Acid Environment

Abstract

The electrochemical corrosion behaviors of Ti₃SiC₂/Cu composites in harsh media including dilute HNO₃ and concentrated H₂SO₄ were studied in detail and the related corrosion mechanisms were explored. Under open-circuit potential, the corrosion resistance of Ti₃SiC₂/Cu in dilute HNO₃ was worse than that in concentrated H₂SO₄. In dilute HNO₃, Ti₃SiC₂/Cu exhibited a typical passivation character with a narrow passivation interval. During the corrosion process, the dissolution of Cu-Si compounds resulted in the destruction of the passivation layer on the surface. Additionally, with the increasing of the potentials, the oxidation of Cu and Si atoms led to the generation of the oxide film again on the surface. In concentrated H₂SO₄, the Ti₃SiC₂/Cu composite was covered by a double-layered passivation layer, which was composed of an internal layer of TiO₂ and an external layer of Cu₂O and SiO₂. This was because Cu diffused into the surface and was oxidized into Cu₂O, which formed a denser oxidized film with SiO₂. In addition, it was found that Ti₃SiC₂/Cu has better corrosion resistance in concentrated H₂SO₄.

1. Introduction

Since the preparation of the MAX phase, research on its properties and applications, such as electrical conductivity, thermal conductivity, machinability, bending resistance, superconductivity, and optical property, has continuously emerged [1,2,3]. As an important character of ceramics, the corrosion resistances, including frictional corrosion, hydrothermal corrosion, resistance to strong acid and alkali corrosion, electrochemical corrosion, etc., have been substantially studied [4,5]. It is well known that electrochemical corrosion generally occurs in some circumstances, such as in the atmosphere, sea, acid or alkali solutions, and wastewater. Currently, metals or alloys are used as the structural parts. If corrosion happened, especially under the influence of mechanical action, the property of materials was impaired, and even some catastrophes happened [6,7]. The corrosive behaviors of Ti₃SiC₂ in various acid, alkali and salt solutions have been studied, and it was found that the formation of oxides in its service environment was beneficial for the improvement of its corrosion resistance. However, the effect of the passivation film on the electrochemical corrosion of Ti₃SiC₂ has been less studied [6,8]. Up until now, the corrosive behavior of Ti₃SiC₂ in HCl and H₂SO₄ has been thoroughly investigated. In both acids, under the open potential and high anode potential, Ti atoms were leached out and Si atoms were oxidized in situ, forming a passivation film [9]. Barsoum et al. [10] studied the electrochemical behaviors of some common MAX phases in strong acids and alkali solutions (such as HCl, H₂SO₄, and NaOH) and they found that the corrosion resistance of Ti₃SiC₂ was better than that of pure Ti, which resulted from the formation of the thin passivation film of SiO₂. Ming Zhu et al. [9] investigated the electrochemical behaviors of Ti₃SiC₂ in the 3.5% NaCl solution, and they discovered that it exhibited excellent corrosion resistance. However, the passivation behavior of Ti₃SiC₂ was weaker than Ti, because the special layered structure of Ti₃SiC₂ provided a path for the spread of Si, which resulted in the low passivation efficiency of atoms diffusing to the obstacle layer.

Previously, we have investigated the static corrosion of Ti₃SiC₂/Cu composites in a strong acid environment and the electrochemical corrosion in a 3.5% NaCl environment, explaining the corrosion mechanisms involved [11,12]. However, in industrial production environments, most of the structural components are exposed to acidic conditions [13]. In this study, the electrochemical corrosion resistance of Ti₃SiC₂/Cu composites in the harsh media of dilute HNO₃ and concentrated H₂SO₄ has been thoroughly investigated in the context of strong-acid and electrochemical corrosion environments, and the related corrosion mechanisms have been discussed.

2. Experimental

2.1. Sample Preparation

Ti₃SiC₂/Cu composites were synthesized by the spark plasma sintering (SPS) process. The original Ti₃SiC₂/Cu powder mixtures were composed of 15 vol.% Cu powder (average grain size: 74 μm, purity ≥ 99.9%, China Shanghai McLean Biochemical Co., Ltd., Shanghai, China) and 85 vol.% Ti₃SiC₂ powder (average grain size: 38 μm, purity ≥ 98%, China Jilin 11 Technology Co., Ltd., Jilin, China). The powder mixtures were uniformly mixed, then sintered by an SPS furnace (SPS, Model Labox-350, SINTER LAND INC., Niigata, Japan) and finally cooled with the furnace. The sintering temperature was 1000 °C. The sintering pressure was 35 MPa. The swelling duration was 20 min. This Ti₃SiC₂/Cu preparation scheme has been reported previously, and it is confirmed that it has high mechanical properties such as hardness and compressive strength. Not only that, its excellent tribological properties have been widely studied [14,15]. The diameter of the as-prepared composite was 25 mm and its thickness was 10 mm. The relative density of the composites was 95%~96%. The as-synthesized composites were processed into pieces (10 mm × 10 mm × 5 mm) by wire cutting. These pieces were polished by diamond grinding discs to 3000 mesh. Then, they were ultrasonically washed in acetone and ethanol in turn, followed by drying in air at room temperature. Finally, the pieces were linked by Cu wires and sealed with epoxy to prepare a work electrode.

2.2. Electrochemical Measurements

Electrochemical measurements were conducted on a DH7003 potentiostat/galvanostat controlled by DHMultiElec software (V6.21.9.23h) linked by a general three-electrode electrochemical cell full of dilute HNO₃ (11.6 wt%) and concentrated H₂SO₄(98 wt%) solutions, at room temperature. The counter electrode was the Pt electrode and the reference electrode was the Ag/AgCl electrode (0.197 V vs. standard H electrode, SHE). These three electrodes were correctly positioned in the fitted locations of the cell. Potentiodynamic polarization experiments were performed from −0.3 to 3 V (vs. open-circuit potential (OCP)). The scan rate was 0.333 mV/s. It has been stated in the relevant literature that the potential scan rate plays an important role in minimizing the effects of aberrations in Tafel slope and corrosion current density analysis, and that the 0.333 mV/s used has no deleterious effect on the Tafel extrapolation method for determining the corrosion current density of test samples [16,17,18]. The electrochemical impendence spectroscopy (EIS) data gained by immersion in dilute HNO₃ (11.6 wt%) and concentrated H₂SO₄ (98 wt%) solutions for 30 min at the open potential, or by potentiostatic polarization at different potentials for 12 h, were acquired by changing the frequency from 100 kHz to 10 mHz, with the current amplitude of 5 mV.

2.3. Characterization

The surface morphology and composition of the composites before and after immersion in harsh media were characterized by scanning electron microscope (SEM, JIB-4700F, JEOL, Akishima City, Tokyo, Japan) affiliated by an energy-dispersive spectrometer (EDS). The chemical valence states of the passivation layers generated during potentiostatic polarization were examined by X-ray photoelectron spectroscopy (XPS, PHI 5000 VersoProbe III, Ulvac-PHI, Akishima City, Tokyo, Japan). The X-ray beam spot was <10 μm. The accelerating voltage was 5 kV. Its energy resolution was <0.5 eV and its vacuum pressure was 5 × 10⁻⁸ Pa. The C 1s with a binding energy of 284.8 eV was used as a reference for all chemical elements (including Ti, Si, C, O, and Cu).

3. Results and Discussion

3.1. Potentiodynamic Polarization

Potentiodynamic polarization curves of Ti₃SiC₂/Cu composites in dilute HNO₃ and concentrated H₂SO₄ solutions are shown in Figure 1. A passivation behavior was observed for the composites under anodic polarization conditions, but the passivation gap of the composites in the dilute HNO₃ was very narrow, which resulted from the weak stability of the as-formed oxide film of the composites. In dilute HNO₃ (11.6%), the anodic curve of the Ti₃SiC₂/Cu composites exhibited a tipping point at 0.2 V, and subsequently underwent a passivation region, with a passivation current density of 5.01 × 10⁻⁵ A/cm². When the voltage was higher than 0.55 V, the anode current density of Ti₃SiC₂/Cu obviously showed a quick upward tendency, which resulted from the destruction of the passivation layer, resulting in the accelerating of corrosion. When the voltage was in a range of 1.4 V–1.8 V, the anode current density of Ti₃SiC₂/Cu decreased. The reason was that Ti atoms were oxidized to TiO₂, forming an oxide film. When the potential was greater than 1.8 V, the anodic current density showed a step-up trend, which may be due to hydrogen precipitation corrosion on the material surface. In the concentrated H₂SO₄ solution, the anodic curve of the Ti₃SiC₂/Cu composites exhibited a tipping point at 0.1 V, and it subsequently underwent a relatively wide passivation region, with a passivation current density of 6.87 × 10⁻⁶ A/cm². This was because a double-layered passivation layer made up of TiO₂ and SiO₂ was formed on the surface of the composites. While the voltage was in the range of 1 V–2 V, the current density of Ti₃SiC₂/Cu slightly decreased and remained around 8.09 × 10⁻⁶ A/cm², which resulted from the transformation of Ti from oxides (II) to TiO₂.

As seen in

Table 1. The electrochemical parameters of Ti₃SiC₂ and Ti₃SiC₂/Cu in dilute HNO₃ and concentrated H₂SO₄.

	Ecorr (V)	icorr (A/cm2)	ip (A/cm2)	Ep (V)
HNO3 (11.6%)	0.06	7.05 × 10−4	5.01 × 10−5	0.55
H2SO4	0	3.01 × 10−6	6.87 × 10−6	0.6

, the open potentials of Ti₃SiC₂/Cu composites in two strong acids were similar. However, the self-corrosion current density of the composites in dilute HNO₃ and concentrated H₂SO₄ were 7.05 × 10^-4 A/cm² and 3.01 × 10^-6 A/cm², respectively. Therefore, at the open potential, the corrosion resistance of the composites in concentrated H₂SO₄ was superior than that in dilute HNO₃. When the potential was higher than 1 V, the Ti₃SiC₂/Cu composites exhibited a secondary passivation phenomenon in two strong acids.

3.2. EIS

For further investigating the corrosion process of the Ti₃SiC₂/Cu composites, the EIS data of the composites in two strong acids were acquired at the open potential and different polarization potentials (see Figure 2). As shown in Figure 2a,c, at high frequencies, the value of log|Z| tended to be stable and the phase angle was near zero, which indicated that the impedance was dominated by the solution resistance (R_s). As shown in Figure 2a, when the polarization potential increased, the phase curve of the composites moved to left first, and subsequently moved to right. It was speculated that the passivation layer first became thin, and then became thick [19]. At the passivation interval, the oxide film of the composites was relatively thin. There was a platform at the high frequency of the Bode diagram, which related to the solution resistance (R_s). Under the medium and low frequencies, the slope of log|Z|~logf tended to be −1 and the phase angle approached −90°. At high frequencies, a platform was seen in the Bode diagram, which was related to the solution resistance (R_s). As the voltage increased, the capacitive arc radius of the Ti₃SiC₂/Cu composites first rose, then declined (see Figure 2b). At 0.55 V, the corrosion resistance of the composites was the best. At the open potential (OCP) and 0.7 V, the capacitive arc radius was similar, which indicated that the composites showed a similar corrosion resistance behavior at OCP and 0.7 V.

In concentrated H₂SO₄, the Bode plots tend to increase at higher frequencies when the potentials are 0.7 V and 1.7 V (see Figure 2c). As seen in Figure 2d, with the increase in the potential, the capacitive arc radius of the Ti₃SiC₂/Cu composites in the concentrated H₂SO₄ increased and the corrosion resistance of the composites was the best at 1.75 V. The capacitive arc radius of the Ti₃SiC₂/Cu composites in dilute HNO₃ was much smaller than that in concentrated H₂SO₄, which suggested that the corrosion resistance of the composite in concentrated H₂SO₄ was better. The impedance characteristics and corrosive behaviors of the composites in different situations were represented by the appropriate equivalent circuit. Based on the point-defect model (PDM), a passivation layer had a double-layered structure. In accordance with the PDM, the passivation process can be represented by the two chemical reactions below, which interpret the growth of the film and the transformation of the matrix, respectively [20].

$$\mathrm{M}+{\displaystyle \frac{x}{2}}{\mathrm{H}}_2\mathrm{O}=[{\mathrm{M}}_{\mathrm{M}}+{\displaystyle \frac{x}{2}}{\mathrm{O}}_{\mathrm{O}}]+x{\mathrm{\delta }}^{-}$$

(1)

M → M^δ+ (aq) + V_m + δe⁻

(2)

Two time-constants were determined by combining PDM and the feature of impedance pattern, then a suitable equivalent circuit was applied to fit the experimental data [12,16,21]. As seen in Figure 3, the EIS data of the Ti₃SiC₂/Cu composites in dilute HNO₃ at OCP, 0.55 V and 1.75 V, and in concentrated H₂SO₄ at OCP and 1.7 V were fitted by the equivalent circuit of R_s(Q₁R_t)(Q₂R_b). Meantime, the EIS data of the Ti₃SiC₂/Cu composites in dilute HNO₃ at 0.8 V and in concentrated H₂SO₄ at 0.7 V were fitted by the equivalent circuit of R_s((Q₁R_t(Q₂R_b))). In dilute HNO₃, when the potential was 0.55 V, the generation of a single-layer oxide film benefitted the corrosion resistance of the composites. When the potential was 0.8 V, a porous double-layer oxide film was formed, and its inner oxide film showed relatively poor corrosion resistance. At 1.75 V, the corrosive current density of the composites substantially decreased, because at low overpotentials the oxide film of the Ti-containing materials transformed from the noncrystalline state to the crystalline state, forming a densified oxide layer on the surface of the composites. In concentrated H₂SO₄, when the potential was 0.7 V, a double-layer oxide film was formed on the surface of the composites, which was favorable for its corrosion resistance. When the potential was 1.7 V, a single-layer oxide film was formed, and its corrosion resistance was further enhanced.

Polarization resistance (R_p) can be thought of as the association of R_b and R_t. According to the fitted results (see

Table 2. The equivalent circuit fitting parameters of Ti₃SiC₂/Cu in dilute HNO₃ and concentrated H₂SO₄ under different conditions.

		Rs (Ω·cm2)	Q1 (10−4F/cm2)	Rt (Ω·cm2)	Q2 (10−4 F/cm2)	Rb (Ω·cm2)
HNO3 (11.6%)	OCP	0.7151	120.38	9.785	118,600	4.79
	0.55 V	0.84	609.96	14.66	816.66	34.75
	0.8 V	0.7277	127.52	9.778	17,630	8.679
	1.75 V	1.023	143.88	7.129	2875.1	17.46
H2SO4	OCP	5.67	8.91	430.9	24.192	1843
	0.7 V	5.251	6.81	300.1	10.417	6731
	1.7 V	4.744	1.52	77.81	13	15,275

), the corrosion resistance of the Ti₃SiC₂/Cu composites in concentrated H₂SO₄ was superior. At 0.55 V, the R_b value of the Ti₃SiC₂/Cu composites in dilute HNO₃ was relatively high, which was due to the generation of a single-layer oxide film on its surface. As the potential increased, for example, at 0.7 V, the oxide film of the composites converted from the single-layer film to a double-layer film, and the R_p value decreased [19]. When the potential was 1.75 V, the R_p value increased, because a dense single-layer oxide film composed of TiO₂ was formed on the surface of the composites. In concentrated H₂SO₄, the polarization resistance of the Ti₃SiC₂/Cu composites increased with the increase in the potential. With the increase in the potential, the R_b value increased, because the doping ion made it thinner, with much larger surface area, causing the increase in the resistance of the positive current flow. As the potential increased, the R_t value decreased, because at the specific potentials the doping ions formed an energy barrier, which was related to the structural adjustment. On the other hand, it promoted the performance of the reverse current, decreasing the charge-transfer resistance [22,23,24].

3.3. Surface Morphology and Composition of Polarization Surface

To reveal the passivation mechanism of the Ti₃SiC₂/Cu composites, XPS (see Figure 4 and Figure 5) was applied to investigate the chemical composition of the passivation surface under various voltages. As shown in Figure 4 and Figure 5, the chemical composition of the composites in two strong acids were obviously different. In HNO₃ (11.6%), under all potentials, TiO₂ positioned at 464 eV and 459 eV (see Figure 4) was detected for Ti 2p. However, in concentrated H₂SO₄, the Ti 2p was examined in the shape of TiO₂ or TiO_2−X (positioned at 455 eV and 459 eV). The Si 2p was measured in the form of SiO₂ in dilute HNO₃ (11.6%) (see Figure 4), while it was present as SiO₂ or SiO_2-X (positioned at 103 eV and 101 eV) in concentrated H₂SO₄ (see Figure 5). The Cu 2p was detected in the shape of CuO or Cu₂O (positioned at 933 eV and 931 eV) in dilute HNO₃ (11.6%), while it was present as Cu₂O in concentrated H₂SO₄. After treatment in concentrated H₂SO₄, S2p was seen on the surface of the composites in the shape of SO(CH₃O)₂ or (CH₃)₂SO₂ (positioned at 168 eV and 169 eV).

The chemical composition of the composites after immersion in two strong acids were compared. In dilute HNO₃ (11.6%), with the increase in the potential, Ti 2p was examined only as TiO₂, while those of Si and Cu varied with the potential. When the potentials were 0.55 V and 0.8 V, the Si and Cu were present in the shapes of SiO₂ and CuO, respectively. As the voltage increased, for example, 1.75 V, SiO₂ and CuO were transformed into SiO_2−X and CuO_2−X. In comparison, in concentrated H₂SO₄, with the increase in the potential, the Ti was always examined in the shapes of TiO₂ and TiO_2−X. Additionally, the Si and Cu were present as SiO₂ and Cu₂O, respectively. Interestingly, the S existed in different oxidation states with the change in the potential. Under low potentials (for example, 0.7 V), S 2p was present as SO(CH₃O)₂. When the potential increased, SO(CH₃O)₂ was transformed to (CH₃)₂SO₂.

3.4. Corrosion Mechanism

To clearly explore the electrochemical corrosion mechanism in harsh media, the corrosion morphology of the Ti₃SiC₂/Cu composites (see Figure 6 and Figure 7) was analyzed, accompanied with a proposed-mechanism diagram (see Figure 8). In dilute HNO₃, the Ti-Si bonds and Cu-Si bonds among the composites were relatively weak. During the corrosion process, Cu atoms and Si atoms were first dissolved. Then, the Si atoms diffused to the outside and were oxidized. On the other hand, the Ti atoms were oxidized in situ. Therefore, a double-layer oxide film consisting of an internal layer of TiO₂ and an external layer of SiO₂ formed. When the potential (from OCP to 0.2 V) was relatively low, the main reaction was reaction (1). As the voltage was >0.2 V, the dominant reaction was reaction (2) and a single-layer oxide film was generated. The character of the layered structure of the composites and the weak interaction of Cu-Si and Ti-C was beneficial for the outside diffusing of Cu and Si atoms, accelerating the reaction (2). Thus, a large amount of Cu and Si atoms was lost (see Figure 8a). At 0.55 V, the passivation gap was not wide, because the dissolution of Cu-Si compounds was favorable for the formation of holes or defects, resulting in the destruction of the passivation layer (see Figure 6a). When the potential was 0.8 V, the current density decreased again, and a passivation interval appeared, too. It was guessed that Cu atoms were oxidized around the holes or defects, repairing the passivation layer and forming a double-layer oxide film comprising an internal layer of TiO₂ and an external layer of Cu₂O and SiO₂. However, the CuO formed was dissolved by the highly corrosive HNO₃. According to Figure 1, when the potential was 1.4 V, the current density began to decrease, because the outer layer of SiO₂ was dissolved and the inner layer of TiO₂ was exposed.

In concentrated H₂SO₄, Ti-Si and Cu-Si bonds were first broken, then Cu and Si atoms diffused into the outside and were oxidized into Cu₂O and SiO₂, respectively. Moreover, Ti atoms were oxidized in situ. A double-layer oxide film consisting of an internal layer of TiO₂ and an external layer of Cu₂O and SiO₂ was formed (see Figure 8b). When the potential (from OCP to 0.1 V) was relatively low, the main reaction was reaction (1). As the voltage was >0.1 V, the reaction (2) became the dominant reaction and a passivation layer was formed. With the increase in the potential, Cu-Si was seriously destroyed, resulting in the failure of the passivation layer. As shown in Figure 7 and

Table 3. Atomic ratios of Ti₃SiC₂/Cu in dilute HNO₃ and concentrated H₂SO₄.

Samples		Atomic Percentages
HNO3 (11.6%)	0.55 V	37.3%Ti, 11.17%Si, 31.28%C, 0.66%Cu, 17.75%O
	0.8 V	22.48%Ti, 11.01%Si, 18.82%C, 4.25%Cu, 41.54%O
	1.75 V	38.2%Ti, 13.22%Si, 27.17%C, 0.34%Cu, 19.06%O
H2SO4	0.7 V	34.59%Ti, 17.72%Si, 29.82%C, 2.91%Cu, 17.72%O, 0.13%S
	1.7 V	34.25%Ti, 13.94%Si, 31.26%C, 5.45%Cu, 13.94%O, 0.06%S

, at 0.7 V, a few corrosion pits were detected on the surface of the Ti₃SiC₂/Cu composites and the atomic percentage of Cu decreased, which resulted from the dissolution of the Cu-Si compounds. In the passivation range, the surface of the composites was covered by a double-layer oxide film comprised of TiO₂, SiO₂ and Cu₂O. Compared to the electrochemical corrosion morphology of other metal matrix composites in sulfuric acid, the corrosion samples in this experiment are more complete [25,26]. With the increase in the potential, the dissolution of Cu atoms led to the destruction of the passivation layer. As the voltage was 1.7 V, there was a large amount of Cu₂O around the corrosion pits. A peak around 1 V appeared, and the generation of Cu₂O on the surface repaired the oxide film. Additionally, TiO_2−x was transformed into TiO₂. Therefore, the corrosion resistance of the composites was improved. It was well known that it was possible for the oxide film of the Ti-containing material to transform from amorphous to crystalline under low potentials, causing the appearance of the second peak (see Figure 1). As the voltage was >1.4 V, a secondary passivation occurred for the Ti₃SiC₂/Cu composites. At this moment, TiO_2−x began to transformed into TiO₂, forming a dense single-layer oxide film.

4. Conclusions

The electrochemical corrosion behaviors of the Ti₃SiC₂/Cu composites in dilute HNO₃ and concentrated H₂SO₄ were studied. The results show that the Ti₃SiC₂/Cu composites have better electrochemical corrosion resistance in concentrated H₂SO₄. In dilute HNO₃ (11.6%) solution, when the potential reaches 0.55 V the formation of a single layer of oxide film occurs; when the potential continues to rise, the Ti₃SiC₂/Cu surface tends to stabilize, the corrosion current density appears to be substantially decreased, and the material on the surface of the oxide film gradually becomes dense. In the concentrated H₂SO₄ solution, when the potential reaches 0.7 V, the Ti₃SiC₂/Cu composite surface formed a double-layer oxide film, thus improving the corrosion resistance. With the increase in potential, the oxide film is more stable and the corrosion resistance is stronger.