Effect of surface state on corrosion resistance of TC4 titanium alloy
The surface morphology and roughness of TC4 titanium alloy after shot peening were analyzed by transmission electron microscope, energy dispersive spectrometer, laser confocal microscope and X-ray diffraction; The corrosion resistance of TC4 titanium alloy before and after shot peening in 3.5% NaCl solution was analyzed by potentiodynamic polarization curve, electrochemical impedance spectroscopy and Mott Schottky curve. The results show that the surface roughness has a greater effect on the corrosion resistance than a small amount of shot peening residue. After polishing, TC4 titanium alloy has the lowest corrosion current density, the largest capacitive arc radius, the least passivation film defects and the strongest corrosion resistance. After shot peening, the passivation film on the shot peened surface of cast steel is the most stable and the corrosion resistance is relatively high. Therefore, the smooth surface is helpful to form a uniform passivation film and increase the corrosion resistance of TC4 titanium alloy.
Titanium alloy has been widely used in aerospace, marine, medical and other fields because of its good comprehensive properties . TC4 (Ti6Al4V) is a α+β Biphasic titanium alloy containing 6% (mass fraction, the same below) α Phase stable elements Al and 4% β The phase stable element v  accounts for about 60% of the titanium alloy products used at present. At present, domestic TC4 titanium alloy is being studied and used in steam turbine blades to replace martensitic heat-resistant stainless steel and precipitation hardening stainless steel . It is expected to reduce the corrosion damage of the last stage blade and prolong the service life. However, the surface of titanium alloy also has some disadvantages, such as low hardness, poor wear resistance and high temperature oxidation resistance, which limits its further application. In order to make titanium alloy parts work normally in complex environment, more and more researchers pay attention to improving the surface properties of titanium alloy by surface modification. Among them, shot peening has become an important surface treatment method for TC4 titanium alloy structural parts .
Shot peening treatment causes high-energy projectiles to continuously impact the surface of titanium alloy, resulting in severe plastic deformation, increased fluctuation and changed roughness. At the same time, it also introduces a large number of dislocations and grain boundaries to make the surface grains of titanium alloy nano . Since the diffusion coefficient along the grain boundary is much larger than that in the grain and along the dislocation pipeline, the grain boundary greatly promotes the diffusion of atoms [6,7], which makes the surface passive film form rapidly and is conducive to the improvement of corrosion resistance. However, the corrosion resistance of titanium alloy is also related to the surface state. Excessive surface roughness caused by shot peening and shot debris left on the surface during shot peening will deteriorate the corrosion resistance of the material .
At present, the research on shot peening of titanium alloy focuses more on fatigue properties. Li Kang et al.  studied the effect of wet shot peening technology on the fatigue life of TC4 titanium alloy. Zhang Conghui et al.  improved the fatigue limit of TC4 titanium alloy by ultrasonic shot peening. Huang Yu et al.  studied the effect of cryogenic laser peening on vibration fatigue life of TC6 titanium alloy. Tan et al.  studied the surface integrity and fatigue properties of TC17 alloy blades under the condition of integrated manufacturing process. Soyama et al.  clarified the mechanism of increasing the fatigue strength of Ti6Al4V made of additive through the plane bending fatigue experiment of Ti6Al4V titanium alloy treated by cavitation shot peening, laser shot peening and particle shot peening. There are few reports on improving the corrosion resistance of titanium alloy by shot peening, especially the lack of research on the effect of surface state after shot peening on the corrosion resistance of titanium alloy. In this work, the surface of TC4 titanium alloy was treated by cast steel shot peening, glass shot peening and composite shot peening of cast steel shot and glass shot. Through the electrochemical experiment in 3.5% (mass fraction) NaCl solution, the effect of surface state after shot peening on the corrosion resistance of TC4 titanium alloy was discussed.
Table of Contents
The sample material is TC4 titanium alloy (Ti6Al4V). The main chemical components (mass fraction,%): Al 6.24, V 4.01, Fe 0.20, Si 0.12, C 0.03, n 0.02, H 0.0021, O 0.14, and Ti residual. The size of TC4 titanium alloy specimen is 15 mm × 15 mm × 10mm, polished from coarse to fine with SiC sandpaper to 2000# and then polished with 1.5 μ M diamond polishing paste, degrease and decontaminate with absolute ethanol and deionized water, and then blow dry for use. Pneumatic shot peening machine is used for shot peening. The shot peening medium is cast steel shot and glass shot, and the coverage is greater than 100%. Shot peening methods include cast steel shot peening (CSSP), glass shot peening (GSP), and composite shot peening of cast steel shot and glass shot (CSP).
The surface morphology after shot peening was observed and analyzed by field emission scanning electron microscope (FE-SEM, Thermo Fisher apreo), and the composition of the sample surface before and after shot peening was analyzed by its equipped electronic energy spectrometer (EDS).
The surface three-dimensional morphology, surface specific surface area and roughness profile of TC4 titanium alloy before and after shot peening were observed and analyzed by lext-ols5000 laser confocal microscope (CLSM). The arithmetic mean RA, root mean square RQ and average maximum height RZ of each point of the profile were selected to characterize the surface roughness of TC4 Titanium alloy. Each sample shall be tested in 6 areas, and then the average value shall be taken. Before measurement, the sample shall be cleaned with absolute ethanol to remove surface pollution.
The phase composition of TC4 titanium alloy surface after shot peening was analyzed by X-ray diffraction (XRD, Bruker D8 advance), Cu Target and K α Wavelength 0.15418 nm, tube voltage 40 kV, tube current 40 Ma, 2 θ The scanning angle range is 20 ° ~ 80 °, the scanning step is 0.1 °, and each step stays for 0.15 s.
Zahner zennium electrochemical workstation was used for electrochemical test. The electrolyte was 3.5% NaCl solution and the experimental temperature was 23 ℃. The test adopts a three electrode system, saturated calomel electrode (SCE) as the reference electrode, platinum sheet as the auxiliary electrode, and TC4 titanium alloy encapsulated in silica gel as the working electrode.
Before the test, TC4 titanium alloy was immersed in 3.5% NaCl solution for 2 hours, and the electrochemical test was carried out after the open circuit potential was stable. The test frequency range of electrochemical impedance spectroscopy (EIS) is 105 ~ 10-2 Hz and the AC disturbance voltage is 10 MV. The data are fitted and analyzed by zsimpwin software. The potential scanning range of potentiodynamic polarization test is – 0.5 (vs. OCP) ~ + 2 V (vs. SCE), and the scanning speed is 1 mV / s. Mott Schottky test was carried out after 3 hours of potentiostatic polarization at (+ 1.1 and + 1.3 V) of two kinds of passivation film-forming potentials selected in the experiment, scanning from high potential to low potential, that is, the test interval was scanned from film-forming potential to – 1.2 V (vs. SCE), the test frequency was 1 kHz, the step size was 25 MV, and the AC signal was 10 MV.
Results and discussion
Surface morphology analysis
Figure 1 shows the surface morphology of TC4 titanium alloy after polishing. It can be seen that the surface of TC4 titanium alloy is smooth and flat without defects before shot peening. Figures 2a and b show the surface morphology of TC4 titanium alloy cast steel after shot peening. It can be seen that the impact of high-energy cast steel shot produces pits, but the pits are shallow, and the periphery of the pits is tilted due to the impact of the shot. Figures 2C and D, 2e and f show the surface morphology after glass shot peening and composite shot peening respectively. Compared with cast steel shot peening, the pit becomes smaller, but the pit density impacted by the shot obviously increases, the sample surface fluctuates greatly, many warped folds appear, and even peeling to a certain extent. This is because when the glass shot hits the surface of TC4 titanium alloy, many pits squeeze each other, and there are warped folds around the pits; At the same time, certain microcracks are generated, and the microcracks continue to cross and expand, making the warped folds form peeling . In addition, it can also be seen that a large number of black cluster shot peening residues are distributed on the surface of glass shot and composite shot peening TC4 titanium alloy.
Fig.1 Surface morphology of TC4 Ti-alloy before shot peening
Fig.2 SEM surface images of TC4 Ti-alloy with CSSP (a, b), GSP (c, d) and CSP (e, f) treated
Figure 3 shows the three-dimensional surface morphology of TC4 titanium alloy before and after shot peening. Local bulges (red area) and pits (blue area) appear in the figure, which can more intuitively show the high and low fluctuation morphological characteristics formed by the impact of the projectile on the surface of TC4 titanium alloy sample after shot peening. The maximum height difference of TC4 titanium alloy after cast steel shot peening, glass shot peening and composite shot peening is 3.377, 11.081, 14.224 and 17.348 respectively μ m. It shows that the surface fluctuation caused by cast steel shot, glass shot and composite shot peening increases gradually.
Fig.3 Three-dimensional surface topographies of TC4 Ti-alloy with untreated (a) and CSSP (b), GSP (c) and CSP (d) treated
Table.1 shows the surface roughness parameters and specific surface area of TC4 titanium alloy. It can be seen that the roughness Ra of TC4 titanium alloy after polishing is the smallest, which is 0.059 μ m； The roughness Ra of cast steel shot peening, glass shot peening and composite shot peening are 0.550, 0.602 and 0.676 respectively μ m. The surface roughness parameters increase in turn, which is consistent with the conclusion in Figure 3. With the increase of surface roughness, the specific surface area is also increasing, indicating that the larger the effective real area of TC4 titanium alloy in contact with corrosive medium, the higher the corrosion rate [15,16]. In addition, excessive roughness will also cause surface damage, stress concentration and microcrack initiation, which is not conducive to the improvement of surface properties . Surface roughness also plays an important role in the formation of material passivation layer. Generally speaking, smooth surface has better corrosion resistance than rough surface .
Table.1 Surface roughnesses and specific surface areas of TC4 Ti-alloy with untreated and CSSP, GSP and CSP treated
|Sample||Surface roughness / μm||Specific surface area|
Surface composition analysis
Figure 4 shows the EDS results of TC4 titanium alloy and after cast steel shot peening, glass shot peening and composite shot peening. It can be seen that the Fe content of TC4 titanium alloy surface increases after cast steel shot peening, and the Fe content decreases slightly after composite shot peening, but it is still higher than that of TC4 titanium alloy and glass shot peening, indicating that glass shot peening on the basis of cast steel shot can reduce the content of shot peening residue of cast steel shot. In addition, after glass shot peening and composite shot peening, the surface O and Si of TC4 titanium alloy increased significantly, and the contents of Ti and V decreased. Combined with the roughness analysis of TC4 titanium alloy surface, it shows that in the process of glass shot peening, the pits produced by the impact of glass shot are more evenly distributed, the debris of glass shot is embedded into the sample surface, O and Si are introduced, the coverage of shot peening residue is larger, and the contents of Ti and V detected by energy spectrometer are reduced. Therefore, glass shot peening based on cast steel shot peening can reduce the content of cast steel shot peening residue, but it also introduces new glass shot peening residue.
Fig.4 Surface EDS analysis results of the surfaces of TC4 Ti-alloy with untreated (a), CSSP (b), GSP (c) and CSP (d) treated
XRD spectrum analysis
Figure 5 shows the XRD spectrum of TC4 titanium alloy surface before and after shot peening. It can be observed that the close packed hexagonal close packed (HCP) crystal structure of Ti and the diffraction peaks corresponding to the body centered cubic (BCC) crystal structure. The sharp and strong peaks confirm the high crystallinity of the alloy. There was no diffraction peak of other phases in XRD spectrum. It can be seen from EDS analysis that the content of other elements including shot peening residual elements is relatively small. Compared with the untreated TC4 titanium alloy, the X-ray Bragg diffraction peak of TC4 titanium alloy did not change significantly after shot peening. Using Scherrer Wilson Equation, the average grain sizes of TC4 titanium alloy, cast steel shot peening, glass shot peening and composite shot peening can be approximately calculated, which are 154166180 and 152 nm respectively. The results show that compared with the untreated TC4 titanium alloy, the average grain size of TC4 titanium alloy does not change significantly after shot peening, and the effect of shot peening process on the surface layer of TC4 titanium alloy is limited. Therefore, the corrosion resistance of TC4 titanium alloy after shot peening is mainly determined by the surface state.
Fig.5 XRD patterns of TC4 Ti-alloy before and after treatment by CSSP, GSP and CSP
Potentiodynamic polarization test
Figure 6 shows the potentiodynamic polarization curves of four samples in 3.5% NaCl solution. It can be seen that there is no obvious activation passivation transition on the polarization curves of the four TC4 titanium alloys. In a wide potential range, the sample maintains a low anode current density, indicating that TC4 titanium alloy can be passivated spontaneously before and after shot peening , and the polarization curves of TC4 titanium alloy after shot peening in 3.5% NaCl solution are basically the same. The relevant electrochemical parameters fitted according to the Tafel curve extrapolation method are shown in Table 2.
Fig.6 Dynamic potential polarization curves of TC4 Ti-alloy without and with CSSP, GSP and CSP treatment in 3.5%NaCl solution
Table.2 Fitting parameters of dynamic potential polarization curves of TC4 Ti-alloy without and with CSSP, GSP and CSP treatment in 3.5%NaCl solution
|Sample||Ecorr / V vs.SCE||Icorr / A·cm-2||Ipass / A·cm-2|
Table 2 shows the corrosion potential Ecorr, corrosion current density icorr and passivation current density IPASS of TC4 titanium alloy. It can be seen that shot peening makes the corrosion potential of TC4 titanium alloy move forward, but the corrosion current density increases. Ecorr can only reflect the tendency of corrosion and can not explain the speed of corrosion rate . The corrosion rate of electrode materials is related to the corrosion current density. The greater the corrosion current density, the faster the corrosion rate . Therefore, after shot peening, icorr of TC4 titanium alloy increases and corrosion resistance decreases, and the corrosion resistance of cast steel shot peening, glass shot peening and composite shot peening decreases in turn. From the perspective of IPASS, IPASS increases after shot peening of TC4 titanium alloy, indicating that the passivation film on smooth surface is more stable . After shot peening, the IPASS of TC4 titanium alloy after cast steel shot peening is low, indicating that the passivation film formed on the surface of TC4 titanium alloy after cast steel shot peening is relatively stable, and the residue of cast steel shot peening has no impact on the corrosion resistance.
Electrochemical impedance test
Fig. 7 is the Nyquist diagram and Bode diagram obtained by electrochemical impedance test of TC4 titanium alloy in 3.5% NaCl solution. The radius of capacitive reactance arc in Nyquist diagram is directly proportional to corrosion resistance, that is, the larger the capacitive reactance arc, the better the corrosion resistance. The high frequency region in Bode diagram reflects the corrosion characteristics of the interface between TC4 titanium alloy and corrosion solution, and the medium and low frequency regions reflect the characteristics of TC4 titanium alloy . The amplitude of impedance in low frequency region can directly explain the corrosion resistance of TC4 titanium alloy. The greater the impedance amplitude, the better the corrosion resistance . It can be seen from the figure that the impedance modulus | Z | and phase angle of four TC4 titanium alloys θ The variation trend with frequency is roughly the same. The lg|z|lg f curve shows a straight line with a slope close to – 1 in the middle and low frequency region, and the phase LG f curve shows a platform with a wide phase angle in the middle and low frequency region, indicating that TC4 titanium alloy has strong corrosion resistance . The radius of capacitive resistance arc of untreated TC4 titanium alloy is larger than that after shot peening, indicating that the corrosion resistance of TC4 titanium alloy is better than that after shot peening. The capacitive resistance arc of cast steel shot peening in TC4 titanium alloy after shot peening is larger, which further proves that the corrosion resistance of cast steel shot peening is better, and the impedance amplitude in the low-frequency region in Bode diagram can also explain this.
Fig.7 Nyquist (a) and Bode (b) plots of TC4 Ti-alloy without and with CSSP, GSP and CSP treatment in 3.5%NaCl solution
Fig. 8 is the equivalent circuit diagram of TC4 titanium alloy in 3.5% NaCl solution, and the fitting results are shown in Table 3. Where RS represents the solution resistance between the reference electrode and the sample surface; Q and N are two parameters of constant phase angle element, q is constant phase angle element (CPE), n is CPE constant, – 1 < n < 1. Using constant phase angle element Q instead of pure capacitance is mainly because the system is in a non ideal state and there is dispersion effect on the sample surface. The n value is related to the surface roughness of the sample. The greater the N, the denser the passivation film on the sample surface, which can more effectively block the corrosive medium and reduce the corrosion rate of the sample . It is generally believed that the passivation film of titanium exists in layers , which is divided into an outer layer with loose structure and a dense inner barrier layer . Therefore, Q1, R1, Q2 and R2 represent the capacitance and resistance of the outer porous layer and the inner dense layer of the passive film, respectively.
Fig.8 Equivalent circuit model of TC4 Ti-alloy without and with CSSP, GSP and CSP treatment in 3.5% NaCl solution
Table.3 Fitting results of EIS of TC4 Ti-alloy without and with CSSP, GSP and CSP treatment in 3.5%NaCl solution
|Sample||Rs / Ω·cm2||Q1 / Ω-1·cm-2·sn||n1||R1 / Ω·cm2||Q2 / Ω-1·cm-2·sn||n2||R2 / Ω·cm2||χ2|
It is 10-5 order of magnitude and within the error range, indicating that the equivalent circuit model used meets the requirements. The outer resistance R1 of the passivation film of TC4 titanium alloy is much less than R2, indicating that the corrosion resistance of TC4 titanium alloy mainly depends on the dense layer inside the passivation film, while the external loose porous layer has little contribution to the corrosion resistance . In the table, the R2 of TC4 titanium alloy after polishing is the largest, indicating that it has the best corrosion resistance; Secondly, TC4 titanium alloy was treated by cast steel shot peening, glass shot peening and composite shot peening, and the corrosion resistance decreased in order. In addition, after shot peening, the dispersion coefficient N2 of the dense layer on the surface of TC4 titanium alloy is relatively large, indicating that the passive film on the surface of TC4 titanium alloy after shot peening is more stable and has better self-healing ability, which is consistent with the conclusion of polarization curve.
Mott Schottky test
The passive film formed on the material surface is usually semiconductor, and its electronic properties can be evaluated by Mott Schottky analysis . According to Mott Schottky theory, the charge distribution at the metal electrolyte interface can be described by the Mott Schottky equation as a function of the space charge capacitance CSC of the passivation film and the electrode potential E .
For n-type semiconductors:
For p-type semiconductors:
Where CSC is space charge capacitance, ε R is the relative dielectric constant of the passivation film (60 ), ε 0 is the vacuum dielectric constant (8.854 × 10-14 f / cm), e is the electric charge (1.602 × 10-19 C), Nd and Na are the donor and acceptor carrier concentrations respectively, a is the real contact area between the sample and the liquid (the measured specific surface areas of TC4 titanium alloy after shot peening are 1.076, 1.105 and 1.119 respectively, multiplied by the surface area of the sample before shot peening, the real areas of cast steel shot peening, glass shot peening and composite shot peening TC4 titanium alloy are 2.42, 2.49 and 2.52 cm2 respectively), EFB is the flat band potential and K is the Boltzmann constant (1.38 × 10-23 J / k), t is the thermodynamic temperature. Kt / E at room temperature is about 25 MV, which is usually negligible.
In the passivation zone, the main electrochemical reaction on the surface of TC4 titanium alloy is the formation of passivation film. As long as a certain passivation potential is applied, a passivation film with high resistance can be formed on the surface, and the passivation film will continue to grow with the increase of potential . Therefore, select + 1.1 and + 1.3 V potentials in the passivation section of potentiodynamic polarization curve, conduct potentiostatic polarization film formation, and test Mott Schottky curve. Figure 9 shows Mott Schottky curves of four TC4 titanium alloys tested at + 1.1 and + 1.3 V constant potentials in 3.5% NaCl solution. It can be seen from the figure that the Mott Schottky curves of four TC4 titanium alloys have roughly the same change trend, and there are two linear regions with different slopes. The linear region of the high potential region reflects the dielectric behavior of the test material, while the linear region of the low potential region represents the semiconductor characteristics of the test material [29,30]. In the figure, the slope of the low potential region of the Mott Schottky curve of TC4 titanium alloy is positive, and the slope increases with the increase of film-forming potential, indicating that the passivation films formed on the surfaces of the four samples are n-type semiconductors, that is, the main carriers are interstitial Ti ions and oxygen vacancies, and the carriers in the passivation films decrease with the increase of film-forming potential, which is consistent with the conclusion obtained by Fattah alhosseini . The low potential linear region is selected to analyze the semiconductor characteristics of the sample. The carrier density nd and the flat band potential EFB are calculated according to the Mott Schottky equation. Nd can pass through the slope 2 / (E) of the 1 / c2-e curve ε r ε 0ka2), the flat band potential EFB can be obtained from 1 / C2 = 0.
Fig.9 Mott-schottky curves of TC4 Ti-alloy without (a) and with CSSP (b), GSP (c) and CSP (d) in 3.5%NaCl solution
Fig.10 Shows nd and EFB of passive film formed by potentiostatic polarization in 3.5% NaCl solution after untreated TC4 titanium alloy, cast steel shot peening, glass shot peening and composite shot peening. It can be seen that the nd of TC4 titanium alloy is the smallest, and the carrier density of passive film on the surface of cast steel shot peened is the smallest. The smaller the carrier density, the fewer defects in the passive film. The reduction of donor density will inhibit the electron transfer and then inhibit the electrochemical reaction, so as to enhance the protection ability of the passive film . With the increase of film-forming potential, EFB moves forward, which also shows that the protective effect of passive film on TC4 titanium alloy is improved . Therefore, the passivation film formed by TC4 titanium alloy after polishing has the least defects, so it maintains a low IPASS.
Fig.10 Nd (a) and Efb (b) of passive films formed on TC4 Ti-alloy without and with CSSP, GSP and CSP treatment in 3.5%NaCl solution
According to the above analysis, the passivation film generated by polishing TC4 titanium alloy has strong stability and the highest corrosion resistance, indicating that the smooth surface is helpful to form a uniform passivation film layer and increase the corrosion resistance of the matrix. After shot peening, the corrosion resistance of passive film of TC4 titanium alloy is the best. Combined with the surface morphology, EDS and XRD analysis, compared with a small amount of shot peening residue, the surface roughness after shot peening has a greater impact on the corrosion resistance of TC4 titanium alloy. Because the surface roughness of TC4 titanium alloy is large after glass shot peening and composite shot peening, the corrosion resistance is reduced compared with cast steel shot peening.
- (1) The surface of TC4 titanium alloy polished before shot peening is smooth and flat without defects. There are pits on the surface of TC4 titanium alloy treated by shot peening of cast steel, but the pits are shallow, and the periphery of the pits is warped due to the impact of shot. Compared with cast steel shot peening, the pits on the surface of TC4 titanium alloy become smaller after glass shot peening and composite shot peening, but the pit density increases obviously, the surface fluctuation is relatively large, there are many warped folds and even peeling to a certain extent. Moreover, there is a small amount of shot peening residue on the surface of TC4 titanium alloy after shot peening. The shot peening residue of cast steel shot is mainly Fe, the shot peening residue of glass shot is mainly Si, and the surface residue after composite shot peening contains both Si and Fe.
- (2) After polishing, the roughness Ra of TC4 titanium alloy is the smallest, which is 0.059 μ m； After shot peening, plastic deformation occurs on the surface and the roughness changes. The roughness Ra of cast steel shot peening, glass shot peening and composite shot peening are 0.550, 0.602 and 0.676 respectively μ m. The surface roughness parameters increase in turn. The effect of shot peening process on the surface of TC4 titanium alloy is limited. The corrosion resistance of TC4 titanium alloy after shot peening is mainly determined by the surface state.
- (3) In 3.5% NaCl solution, the icorr of polished TC4 titanium alloy is the smallest, the resistance R2 of dense layer is the largest, and the defects of passive film formed are less, so it can maintain low IPASS and has the strongest corrosion resistance. Among TC4 titanium alloy shot peened, the passivation film of cast steel shot peened is the most stable and has relatively high corrosion resistance. Compared with a small amount of shot peening residue, the surface roughness has a great influence on the corrosion resistance. Therefore, the smooth surface is helpful to form a uniform passivation film and increase the corrosion resistance of TC4 titanium alloy.
Authors: Liu xing, ran dou, Meng Huimin, Li quande, Gong xiufang, long bin
Source: China Titanium Flange Manufacturer: www.titaniuminfogroup.com
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