Corrosion properties and mechanism of Ti43Zr27Mo5Cu10Be15 amorphous composites in various conditions

The in situ Ti43Zr27Mo5Cu10Be15 amorphous composites were investigated for their corrosion properties in solutions of NaCl, HCl, H2SO4 and NaOH. Electrochemical testing, SEM, EDS and XPS analyses revealed that pitting in NaCl and HCl solutions caused local surfaces damage. Amorphous matrix corrodes slightly in H2SO4 solution. Uniform corrosion occurred in NaOH solution without passive film formed, leading to the worst corrosion resistance. The optimal corrosion performances for NaCl and HCl solution are achieved at 0.5 mol l−1 and 0.75 mol l−1 separately which is related to the shortage of oxygen content in the solutions. While, the best corrosion performances for H2SO4 and NaOH solution are at 0.25 mol l−1. Moreover, the research on the effect of temperature was conducted in 3.5 wt% NaCl solution, it was found that Ti43Zr27Mo5Cu10Be15 amorphous composites exhibited good corrosion resistance at 298 K, while the E corr declined with increasing solution temperature.


Introduction
Amorphous composites are widely recognized for their superior physical, chemical, and mechanical characteristics, including high strength, hardness, and wear resistance, as compared to crystalline alloys [1,2].The long-range organized structure and lack of notable crystalline imperfections, like dislocations and grain boundaries, are responsible for these characteristics.These characteristics make amorphous composites extremely promising for use in the military, aerospace, medicinal, chemical engineering, and machinery industries [3][4][5].However, the weak ductility severely limits their widespread use [6][7][8].To increase the plasticity, a large amount of research has been committed to the in situ amorphous composites, among which the in situ Ti-based amorphous composites had excellent mechanical properties in compressive plasticity and tensile ductility [9][10][11].The dendritic phases prepared by in situ method can improve mechanical properties, but may reduce corrosion properties due to the interface between the matrix and dendrites [3,[12][13][14][15][16].
Even though the corrosion properties of in situ Ti-based amorphous composites are a vital performance indicator for future application, few researchers have focused on them [17,18].Debnath et al [19,20] discovered that introducing secondary phases into the amorphous matrix had a negative influence on corrosion behavior when comparing Ti-based amorphous composites to their crystalline counterparts.This was due to the preferential dissolution between the matrix and the dendrites [19].Xu et al [21] investigated the pitting susceptibility of in situ Ti-based metallic glass matrix composites (TGMCs) with four different compositions in 3.5 wt% NaCl solutions.The distribution of components in the matrix and dendrites was discovered to be connected to the distinct corrosion behavior.The corrosion behavior of Ti 46 Zr 20 V 12 Cu 5 Be 17 in 10% H 2 SO 4 solution was studied by Qiao et al [22].Chemical and electrochemical studies at room temperature show that the bigger sample had better corrosion resistance.In their researches, the solution type is relatively single.Some researchers [5,23] have found the TGMCs exhibited different corrosion behaviors in NaCl, HCl, H 2 SO 4 and NaOH solutions.The selective dissolution and pitting damage occurred in chloride-containing solutions, while in the sulfuric acid solution, a stable passive film formed on the sample surface.However, in the strong alkaline solution, the corrosion resistance is the worst.Although the influence of solution type is examined in their studies, the effect of concentration is overlooked.
In this work, the corrosion characteristics of Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous composites were assessed with respect to solution type and concentration.Since the 3.5 wt% NaCl solution was used to simulate seawater, the impact of temperature was investigated.Simultaneously, the corrosion mechanism analysis under various situations was conducted.

Experimental
The amorphous composites with a nominal composition of Ti 43 Zr 27 Mo 5 Cu 10 Be 15 (at %) were arc melted to master ingots in a high purity argon gas.The raw materials of Ti, Zr, Mo, Cu and Be were bought from Bei Jing Ryubon New Material Technology Co., Ltd, and the purity exceeds 99.99% (at %).The composition homogeneity was achieved by a minimum of four remelting processes.Following melting, copper mold casting was used to create the cylinder-shaped rod samples with f 6 mm (diameter) ×80 mm (length).
All samples were cut from as-cast rods with dimensions of f 5 mm (diameter) ×4 mm (thickness).Samples were ground up to 800-grit SiC sandpaper, polished with micrometer-sized diamond powder, ultrasonically cleaned with ethanol, and dried in the air.The x-ray diffraction (XRD, D/max-2500, Cu Kα) was employed to get the phase structure information with the scanning rate of 8°/min.A metallurgical microscope (Zeiss Axio Scope A1) was used to analyze the microstructure after samples were etched by mixed solution of HNO 3 , HF and H 2 O in a ratio of 4:2:94.The samples used for electrochemical testing were linked to a copper wire and encased in epoxy resin, just leaving working face exposed.Electrochemical tests were conducted by electrochemical workstation with a three-electrode cell (CHI660).Saturated calomel electrode (SCE) serves as the reference electrode, while Pt electrode serves as the counter electrode.The sweep rate of potentiodynamic polarization is 5 mV s −1 , while the frequency range of electrochemical impedance spectroscopy (EIS) is 10 −2 to 10 4 Hz.Different conditions were used for the electrochemical measurements, such as variations in temperature, concentration, and solution.When the electrochemical tests finished, the corroded samples were removed and cleaned immediately.Scan electron microscopy (SEM, Zeiss SIGMA 500) equipped with energy-dispersive x-ray spectrum (EDS) was used to characterize the corrosion morphology and elements distribution on the corroded surface.X-photoelectron spectroscopy (XPS, PHI-5400) with photon energy ranges from 1000 to 1500 eV was performed on given samples which testing in 0.5 mol l −1 HCl solution.

Characterization of microstructure
The XRD pattern of as-cast Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous composites is displayed in figure 1(a).The XRD pattern shows a wide diffuse diffraction peak superposed by four distinct crystalline peaks.The wide diffuse peak around 2θ = 38°was identified as amorphous matrix, which was reported by Yang [5].Confirmed by High Score Plus 3.0e software, the four crystalline peaks are the (110), ( 200) and (211) crystal planes of bcc β-Ti solid solution (JCPDS, 01-089-3726#).This further proves that the Ti 43 Zr 27 Mo 5 Cu 10 Be 15 is indeed a composite of amorphous and crystal dendritic phases.The image shows that the continuous glass matrix has many uniformly dispersed flower-like dendrites.Based on the calculation, the volume fraction of dendrites is about 54%, and the spanning length for each dendrite is approximately 2.8 μm [24].This is higher than the dendritic volume percentage (41%) and average dendritic size (1.0 μm) of the Ti 42 Zr 22 V 14 Cu 5 Be 17 produced by Zhang [25].

Corrosion properties in different solutions of various concentrations
This section investigated the corrosion properties of Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous in different types and concentrations of corrosive media.Because high temperatures are uncommon in the strong acid and alkali conditions in which amorphous materials are used, along with the extreme volatility of strong acids and alkali, higher the temperature, greater the volatility, causing injury to researchers and equipment.Therefore, only the corrosion properties in NaCl, HCl, H 2 SO 4 , and NaOH solutions at 298K were studied.

The effect of solution type on corrosion properties
Polarization curves measured at 298K for NaCl, HCl, H 2 SO 4 , and NaOH solutions are displayed in figure 2. The kinetic parameters that were determined using Tafel extrapolation method are listed in table 1. Cut two linear parts on the cathode and anode of the polarization curve (which must be within the Tafel zone), and determine the Tafel slope corresponding to these two linear parts.The horizontal and vertical coordinates corresponding to the intersection of the two slopes are the corrosion current density (i corr ) and corrosion potential (E corr ).
The polarization curves in NaCl solution are shown in figure 2(a).In the anodic polarization region, the i corr increases slowly with the E corr increase, indicating that sample surface is damaged and the oxygen interacts with the corroded surface to form oxides (which will be identified in later), hindering the further dissolution of metal ions [26].A brief passivation phenomenon occurs when the solution concentration is 0.25 mol l −1 , that is, in the −0.4 ∼ −0.2 V E corr range, the i corr remains almost unchanged.At this point, a passive flim forms on the corroded surface, which can impede further dissolving of the surface metals [26].When the potential up to −0.2 V, the i corr increases rapidly, which is related to the Cl − ions enter the passive film and induce pitting corrosion on the sample surface [27,28].Compared to NaCl solution, Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous have lower E corr and higher i corr in HCl solution, as shown in figure 2(b), resulting in a poorer corrosion resistance.The passivation phenomenon in HCl solution is obvious.It is ascribed to the presence of H + ions, the sample surface was corroded, and the metal ions on the corroded surface will react with oxygen to form a passivation film [5].The corrosion behaviors of Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous in the NaCl and HCl solutions are quite similar to the results reported by Tian et al [29].The polarization curves of amorphous composites in H 2 SO 4 solution are shown in figure 2(c), the relatively large plateau on anodic polarization curves clearly indicates a passivation film formed on the sample surface, similar results have been already reported in Zr-based amorphous [30,31].The polarization curves of amorphous composites in NaOH solution are shown in figure 2(d).The passivation region in the polarization curve is not obvious, maybe the passivation film is relatively loose.
One of the most used methods for examining the anti-corrosion properties is electrochemical impedance spectroscopy (EIS) [32].In general, the impedance arc diameter is a reliable predictor for corrosion resistance.Figure 3 displays the Nyquist plots of amorphous composites in NaCl, HCl, H 2 SO 4 and NaOH solutions at various concentrations.To get EIS findings, an appropriate fitting analysis was performed using Z-view software, as shown in table 2. The insets in figure 3 show the equivalent circuits.The solution resistance, chargetransfer resistance, constant phase element capacitance, and inductive reactance are denoted as Rs, Rt, CPE (or Qp), and L in this instance.Usually, the Rt value is a good measure of corrosion property.It is obvious that the  Comparing these four type solutions, we discovered that passivation occurred in NaCl, HCl, and H 2 SO 4 solutions, but not in NaOH solution.In general, larger i corr typically signifies a higher corrosion rate, whereas lower E corr suggests the materials have lesser chemical stability and a greater corrosion inclination [5].Therefore, Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous have the lowest corrosion potential and the worst corrosion resistance in NaOH solution at the same concentration, as shown in table 1.Similar results have been found in Zr-based MGMCs [29].

The effect of solution concentration on corrosion properties
In NaCl solution, polarization curve analysis shows that as the concentration increases, the E corr and i corr gradually decrease.However, EIS analysis shows that the diameter of the capacitance arc is maximum at 0.5 mol l −1 and minimum at 0.25 mol l −1 .In HCl solution, as the concentration increases, the E corr gradually increases, but the i corr decreases firstly and then increases.EIS analysis shows that the diameter of the capacitive arc is the largest at 0.75 mol l −1 .For H 2 SO 4 and NaOH solutions, they have the similar trend.As the concentration increases, the i corr does not change much, but the E corr continues to decrease.This trend indicates that the corrosion resistance is optimal when the concentration of these two solutions is 0.25 mol l −1 .This conclusion has also been confirmed in the EIS analysis (as shown in figure 3 and table 2).

Surface characterization of corroded samples
In order to clarify the corrosion mechanisms under different corrosive media, the analyses are conducted on the corroded samples' surface.
Figure 4 shows XRD patterns for Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous at 298K after electrochemical test.The different oxide compounds of ZrO 2 (JCPDS, 00-033-1483#), TiO 2 (JCPDS, 01-088-1174 #) and (Zr Ti) O 4 (JCPDS, 01-080-1783#) were identified and marked according to the diffraction peaks.These oxide compounds were thought to give the matrix a high level of corrosion resistance [19].The diffraction peaks of corrosion products not detected in NaCl solution may be due to the quantity of corrosion products is lower than the XRD detection limit.Figure 5 shows the typical corroded surface micrographs of samples after electrochemical testing.NaCl and HCl solutions, which include chlorides, have generated corrosion pits on the surfaces, As demonstrated in figures 5(a) and (b).The pitting corrosion in HCl solution is more severe because of the presence of H + ions, and the dendritic clusters are clearly seen stacked in the corrosion pits.Figure 5(c) shows the corroded surface in 1 mol l −1 H 2 SO 4 solution.Parts of the dendrites are visible, suggesting that only a small amount of corrosion took place at the amorphous matrix.In the NaOH solution, the corroded surface is extremely smooth, as seen in figure 5(d).Only a little protuberance is visible, but crystalline dendrites are difficult to observe, indicating that the sample surface is corroding uniformly.Perhaps the passive film generated on the corroded surface is stable.

Corrosion mechanism
Due to the presence of pitting corrosion in both NaCl and HCl solutions, the corrosion mechanism will be clarified together.Figure 6(a) shows the pitting morphology of Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous composites in 0.5 mol l −1 HCl solution.There are many corrosion pits with different sizes distributed on the corroded surface.When a corrosion pit is enlarged (as illustrated in figure 6(b)), the dendrites emerge entirely and display a threedimensional structure, while the amorphous matrix goes away.The EDS examination of the corroded area, as shown in figure 6(c), demonstrates that the corroded surface contains more Ti and Zr elements, which are abundant in the dendrite phases and matrix separately.Cu is abundant in matrix, while Mo is widely distributed across the dendrites.In addition, a large amount of O elements was detected on the corroded surface, indicating the formation of oxidation products, which is consistent with the detection results of XRD.
To clarify the type of oxides, XPS analysis was carried out on the corrosion surface of the sample in 0.5 mol l −1 HCl solution, as seen in figure 7. Ti 2p, Zr 3d, Cu 2p, O 1s, C 1s, and Mo 3d all show broad binding energy area peaks, but the Mo element's binding energy peak is relatively low.A polluting hydrocarbon layer on the sample surface was the source of the C 1s spectra, which peaked at 286.7 eV.The O 1s spectrum, as shown in figure 7(b), is composed of two peaks: the oxygen from metal oxide at 529.9 eV and the higher binding energy peak at 531.8 eV, which is attributed to bound water and OH oxygen.In the O 1s spectra, the O 2− peak was stronger than the OH − + H 2 O peak, indicating that the metal oxide makes up most of the passive film.As seen in figure 7(c), Ti 2p1/2 (463.2 eV) and Ti 2p3/2 (457.6 eV) are the two prominent peaks in the Ti 2p spectrum.These two peaks can be identified as Ti 4+ (456.9 eV) and Ti 3+ (471.0 eV) with analytical deconvolution fitting.Even with five metallic components, corrosion in NaCl solution may be thought as a simple corrosion cell, the corrosion model is shown in figure 8(a), with anodic dissolution of metal atoms followed by cathodic oxygen reduction.Ti, Zr, Mo, and Cu have standard equilibrium electrode potential (SEP) of −1.210 V, −1.529 V, −0.220 V, and 0.337 V, respectively [22,29].At the applied potential, Zr are more prone to lose electrons and becoming Zr 4+ .The cathode undergoes a reduction reaction in which O 2 molecules adsorbed on the corroded surface accept electrons and react with H 2 O to produce OH − ions.When Zr 4+ combine with OH − , unstable precipitate phases like Zr(OH) 4 are formed [29].Subsequently, ZrO 2 will be generated under the action of dissolved oxygen.The formation of titanium oxide (TiO 2 and Ti 2 O 3 ) is similar to that of Zr, but due to the higher SPE of Ti, titanium oxide is more stable.Several studies have demonstrated that protective passive films containing ZrO 2 , TiO 2 and Ti 2 O 3 are generated during anodization are extremely stable [33][34][35][36].According to the EDS analysis in figure 6, the Zr and Cu elements are abundant in matrix, while Ti and Mo elements are rich in dendrites.The distribution differences of elements cause selective dissolution on the matrix.Subsequently, the Cl − ions penetrate the passive films easily due to their tiny size and cause pitting corrosion at the selective dissolution area, such as dendrites-matrix interface.It is worth noting that the Cu in glassy matrix is rapidly attacked by the chloride-ions, resulting in a low corrosion resistance of the amorphous matrix [37][38][39].Therefore, the selective dissolution at the interfaces between glassy matrix and dendrites, and then spread rapidly throughout the amorphous matrix [29].This phenomenon has been confirmed in other references [21,23].
As mentioned above, oxygen element plays a crucial role in the corrosion process.But The oxygen content in the NaCl solution would decrease with the increase of NaCl concentrations [21].The lack of oxygen supply during the cathodic reduction reaction causes an increase in the degree of cathodic polarization.Two effects are evident: the first is a reduction in corrosion potential, and the second is a drop in corrosion current densities [21].The decrease in corrosion current densities caused by this lack of oxygen supply counteracted the rise in densities caused by the higher concentration of chloride.In HCl solution, galvanic corrosion occurs due to the presence of H + ions.Zr, as an anode, is more prone to losing electrons and becoming Zr 4+ .At the cathode, H + gains electrons and generates H 2 , the corrosion model is shown in figure 8(b).A passivation coating will be formed simultaneously by the reaction of metal ions on the damaged surface with oxygen.The pitting mechanism is similar to that of NaCl solution.However, when the H + concentration rises, the passivation film thickens, the E corr rises, and the i corr falls.But as the concentration increases to 1 mol l −1 , the shortage of O element causes the passivation film begins to be destroyed by H + and Cl − ions, resulting in an increase in i corr .
Based on the analyses of electrochemistry, corrosion morphology and XRD, as shown in figures 12(c), 3(c), 4(c) and 5(c), it was found that a slight corrosion attack takes place on the sample surface in H 2 SO 4 solution.The considerable stability passive films formed on the corroded surface.With the concentration increases, the shortage of O element causes the passivation film begins to be destroyed by H + , resulting in a decrease in E corr .
The SEM morphologies and EDS analysis of Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous composites in 0.5 mol l −1 NaOH solution are displayed in figure 9.The distribution of Ti, Zr, Mo, and Cu elements is basically consistent with the analysis in figure 6, but the O element only corresponding to the protuberance.It indicates that the amorphous matrix and dendrites are corroded simultaneously, and no passive film is formed on the corroded surface, only oxide particles are generated.Different from the findings of other researchers [23,29], the corrosion of Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous in NaOH solution is more severe, which is consistent with the electrochemical results.The Be element with SEP of −1.847 V is active in all aqueous solutions, especially in alkaline environments [29].The decreases in E corr found in the NaOH solution as the concentration increases may be due to the higher dissolution ability of Be oxide in NaOH solution.Figure 10 displays the polarization curves at temperature ranging from 298 K to 373 K.The polarization profiles of the samples at 298 K and 323 K exhibit a broad passivation zone within the range of −0.50 ∼ −0.15 V.The passivation area becomes narrower as the temperature increases to 348 K and 373 K. Table 3 lists the corrosion potential and corrosion current density.It is notable that the E corr clearly decreases as the temperature rises.Figure 11 displays the corroded surface micrographs after electrochemical testing in 3.5 wt% NaCl solutions.The corroded surface is flat and smooth at 298 K.When the temperature rises to 323 K, various sizes of corrosion pits form on the corroded surface, with visible dendritic crystals inside.As the corrosion temperature continues to increase, the corrosion pits enlarge, as seen in figures 11(c) and (d), the amorphous matrix and dendrites inside the pits corrode.
In NaCl solution, Cl − competes with OH − for sample surface adsorption.Once Cl − reacts with the metal elements, the selective dissolution is accelerated, thus the formation of the passive film is destroyed [40].There is more Cl − in the activated state at a higher temperature in the corrosion solution.Pitting corrosion is more likely to occur on the damaged surface, resulting in more severe corrosion.Moreover, the research on Fe-based amorphous by Lu [40] has found the protective effect of passive film was weakened due to the increase of temperature decreased the passivation index and the polarization resistance, so the induction time for pits growth is shortened.Above all, Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous composites have better corrosion resistance at low temperature.

Conclusions
This work provides a comprehensive analysis of the corrosion properties of Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous composites in various corrosive conditions.The following conclusions are drawn:  (1) Different types of solutions result in different corrosion behaviors.Pitting corrosion appears on the surface of samples in NaCl and HCl solutions.In H 2 SO 4 solution, a slight corrosion attack takes place on the sample due to the stability of passive films.The worst corrosion resistance is achieved in the NaOH solutions due to uniform corrosion occurs without the formation of a passivation film.
(2) The optimal corrosion performances for NaCl and HCl solution are achieved at 0.5 mol l −1 and 0.75 mol l −1 separately with the concentration increases, which is related to the shortage of oxygen content in the solutions.While, the corrosion performances for H 2 SO 4 and NaOH solution decrease as the concentration increases, and the best concentration is 0.25 mol l −1 .
(3) Temperature has a significant impact on corrosion performance.The corrosion potential of Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous composites in 3.5 wt% NaCl solution peaks at 298 K.As the temperature increases, the activity of Cl − ions increases, and the protective effect of passive film is weakened.The corroded surface morphology develops from uniform corrosion to pitting and large-scale corrosion, and the corrosion potential drops accordingly.
(4) Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous composites can be safely used in chloride free environments, including acidic environments.For environments containing chloride ions, attention should be paid to the concentration of chloride ions in the solution.For alkaline environments, caution is needed.

Figure 6 .
Figure 6.SEM and EDS analysis of corroded surface on Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous composites in 0.5 mol l −1 HCl solution: (a) corrosion morphology; (b) corrosion pit; (c) EDS analysis for the corrosion region.

3. 3 .
Corrosion properties in 3.5 wt% NaCl solutions of various temperature The concentration and ion content of 3.5 wt% NaCl solution are similar to those in seawater, so it can simulate the chemical environment of seawater well and help to study the corrosion behavior of materials in seawater.Therefore, this study utilized this solution to investigate the corrosion performance of Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous at different temperatures.

Figure 8 .
Figure 8. Schematic illustrations of the pitting corrosion process of Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous composites in the chloride-ions solutions: (a) NaCl; (b) HCl.

Figure 9 .
Figure 9. SEM and EDS analysis of corroded surface on Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous composites in 0.5 mol l −1 NaOH solution: (a) corrosion morphology; (b) Enlarged morphology; (c) EDS analysis for the corrosion region.

Table 1 .
Corrosion parameters of Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous composites after polarization in various solutions with different concentration.
EIS curves of samples tested in NaCl and HCl solutions are mostly consisted of a capacitive arc, suggesting the formation of passive film on the sample surface.Nyquist plots in H 2 SO 4 solution consists of capacitive arc in low frequency and inductive arc in high frequency.The generation of inductive arc is caused by the adsorption and desorption of corrosion products.Nyquist plots in NaOH solution consists of a single capacitive arc, which has shifted due to the dispersion effect.

Table 2 .
Equivalent circuit parameters of EIS analysis for Ti 43 Zr 27 Mo 5 Cu 10 Be 15 amorphous composites in various solutions with different concentration.