Influence of passive potential on the electronic property of the passive film formed on Ti in 0.1 M HCl solution during ultrasonic cavitation

Cavitation erosion-corrosion properties of as-cast TC4 and LPBF TC4 in 0.6 mol/L NaCl solution: A comparison investigation

In this work study, a comparative analysis was undertaken to investigate investigation into the cavitation erosion (CE) and corrosion behavior of laser powder bed fusion (LPBF) TC4 and as-cast TC4 in 0.6 mol/L NaCl solution. Relevant results indicated that LPBF TC4 revealed a rectangular checkerboard-like pattern with a more refined grain size compared to as-cast TC4. Meanwhile, LPBF TC4 surpassed its as-cast counterpart in CE resistance, demonstrating approximately 2.25 times lower cumulative mass loss after 8 h CE. The corrosion potential under alternating CE and quiescence conditions demonstrated that both LPBF TC4 and as-cast TC4 underwent a rapid potential decrease at the initial stages of CE, while a consistent negative shift in corrosion potential was observed with the continuously increasing CE time, indicative of a gradual decline in repassivation ability. The initial surge in corrosion potential during the early CE stages was primarily attributed to accelerated oxygen transfer. As CE progressed, the significant reduction in corrosion potential for both LPBF TC4 and as-cast TC4 was attributed to the breakdown of the passive film. The refined and uniform microstructure in LPBF TC4 effectively suppresses both crack formation and propagation, underscoring the potential of LPBF technology in enhancing the CE resistance of titanium alloys. This work can provide important insights into developing high-quality, reliable, and sustainable CE-resistant materials via LPBF technology.

Effect of low dissolved oxygen concentration on the defects and composition of regenerated passive film of Ti-6Al-4V alloy under continuous wear

Tribocorrosion is one of the most common forms of failure of biomedical titanium alloys. As the passive film of titanium alloys is highly dependent on oxygen conditions, the passivation behavior and the microstructure of the passive film of Ti-6Al-4V under tribocorrosion in 1 M HCl with a low dissolved oxygen concentration (DOC) were studied by means of electron probe microanalysis (EPMA), Ar-ion etched X-ray photoelectron spectroscopy (XPS), focused ion beam (FIB) milling and high resolution transmission electron microscopy (HRTEM). The results showed that the protective ability of the regenerated passive film decreased sharply under low DOC. Al and V ions dissolved in excess, and a large number of oxygen atoms entered the matrix, leading to internal oxidation. Structural characterization indicated that Ti atoms occupied more metal lattice points in the regenerated passive film and that the high dislocation density in the deformed layer caused by wear facilitated the diffusion of Al and V. Finally, the first-principles calculation showed that Al had the minimum vacancy formation energy.

Effect of Cavitation Intensity on the Cavitation Erosion Behavior of 316L Stainless Steel in 3.5 wt.% NaCl Solution

In this study, the cavitation erosion behavior of 316L stainless steel under different cavitation intensities in 3.5 wt.% NaCl solution was investigated with scanning electron microscopy and various electrochemical tests. Results indicated that cavitation intensity corresponding to CE amplitude of 5 μm was lower than that of the mechanical bearing capacity of passive films. When subjected to cavitation erosion (CE), Open circuit potential (OCP) shifted to the noble direction instantaneously, which was mainly attributed to enhanced oxygen transfer of the cathode due to stirring effects of CE. By contrast, high cavitation intensities corresponding to CE amplitudes of 25 μm and 55 μm exceeded the mechanical bearing capacity of passive films, causing significantly reduced OCP associated with metal dissolution of the anode from mechanical damage. Potentiostatic polarization and Mott–Schottky tests showed that 316L SS subjected to low cavitation intensities displayed good repassivation properties. However, repassivation performance was markedly weakened when high cavitation intensity was applied, resulting in weaker protection of the passive films with high carrier density.

Effect of pH on the Corrosion and Repassivation Behavior of TA2 in Simulated Seawater

The effect of pH on the corrosion and repassivation behavior of TA2 in simulated seawater was studied using electrochemical tests, immersion experiments, and surface morphology topology analysis. The results show that E_corr and R_f increased while i_pass and weight loss rate decreased as the pH of simulated seawater increased. The TA2 passive film was determined to be mainly composed of a large amount of TiO₂ and a small amount of TiO. The repassivation function of TA2 can be expressed as E = −0.1375 + 0.0532ln(t − 1.241) for a simulated seawater pH of 8.2. The parameter b, which represents the slope of the potential–time curve during the friction electrode test, was used to evaluate the repassivation behavior of TA2. The increase in pH value was observed to promote the repassivation speed of the passive film, which is beneficial to the corrosion resistance of TA2.

The influence of semiconducting properties of passive films on the cavitation erosion resistance of a NbN nanoceramic coating

To alleviate the cavitation damage of metallic engineering components in hydrodynamic systems operating in marine environments, a NbN nanoceramic coating was synthesized on to a Ti-6Al-4V substrate via a double cathode glow discharge technique. The microstructure of the coating consisted of a ~13 μm thick deposition layer of a hexagonal δ'-NbN phase and a diffusion layer ~2 μm in thickness composed of face-centered cubic (fcc) B1-NaCl-structured (Ti,Nb)N. The NbN coating not only exhibited higher values of H/E and H²/E than those measured from NbN coatings deposited by other techniques, but also possessed good adhesion to the substrate. The cavitation erosion resistance of the NbN coating in a 3.5 wt% NaCl solution was investigated using an ultrasonic cavitation-induced apparatus combined with a range of electrochemical test methods. Potentiodynamic polarization measurements demonstrated that the NbN coated specimens demonstrated both a higher corrosion potential (E_corr) and lower corrosion current density (i_corr) than the uncoated substrate. Mott-Schottky analysis, combined with the point defect model (PDM), revealed that, for a given cavitation time, the donor density (N_D) of the passive film on the NbN coating was reduced by 1 ~ 2 orders of magnitude relative to the uncoated Ti-6Al-4V, and the diffusivity of the point defects (D₀) in the passive film grown on the NbN coating was nearly one order of magnitude lower than that on the uncoated substrate. In order to better understand the experimental observations obtained from Mott-Schottky analysis and double-charge layer capacitance measurements, first-principles density-functional theory was employed to calculate the energy of vacancy formation and the adsorption energy for chloride ions for the passive films present on both the NbN coating and bare Ti-6Al-4V.

In-situ monitoring of the electrochemical behavior of cellular structured biomedical Ti-6Al-4V alloy fabricated by electron beam melting in simulated physiological fluid

Ti-6Al-4V alloys with cellular structure fabricated by additive manufacturing are currently of significant interest because their modulus is comparable to bone and the cellular structure allows the cells to penetrate and exchange nutrients, promoting osseointegration. We describe here a unique simulation device that replaces the traditional steady electrochemistry approach, enabling in-situ study of variation of ion concentration and surface potential with pore depth for cellular structured Ti-6Al-4V alloys fabricated by electron beam melting (EBM) in phosphate buffered saline (PBS). This approach addresses the scientific gap on the electrochemical behavior of cellular structured titanium alloys. The study indicated that concentration of H⁺ and Cl^- increased with the increase of pore depth, while the surface potential decreased. The exposed surface of inner cellular structure was not corroded but passivated after immersing in PBS at 37 °C for 14 days, which was independent of pore depth. Furthermore, X-ray photoelectron spectroscopy (XPS) and Mott-Schottky (M-S) studies suggested that a thinner passive film containing a greater donor density was formed on the surface of cellular structured Ti-6Al-4V alloy at the deepest pore depth. This is attributed to insufficient oxygen supply and Cl^-adsorption on the surface inside the pores. STATEMENT OF SIGNIFICANCE: Porous titanium alloys are promising implants in biomedical applications. However, it is a challenge to accurately characterize the corrosion behavior of porous titanium alloys with complex pore structure using traditional electrochemical methods. In this study, we have adopted a special device to simulate the environment within the pore structure. The variation in ion concentration and surface potential of Ti-6Al-4V fabricated by EBM with pore depth was in-situ monitored. After immersing in PBS for 14 days, Ti-6Al-4V exhibited good corrosion properties and the samples with less than 60 mm pore depth were not corroded but passivated. Also, we analyzed the difference in corrosion property at different pore depth. This type of in-situ corrosion performance monitoring in EBM-produced Ti-6Al-4V has not been previously studied.

Investigation on ultrasonic cavitation erosion of TiMo and TiNb alloys in sulfuric acid solution

Two kinds of Ti-alloys, i.e., TiMo and TiNb alloys are manufactured in this paper, and their ultrasonic cavitation erosion behaviors in 0.1 M H₂SO₄ solution are evaluated by the mean depth erosion (MDE), SEM and white light photograph. The results show that MDE of TiMo and TiNb alloys obviously increase with increasing the cavitation erosion time, however, they evidently decrease with the increment of Mo or Nb content at each fixed cavitation erosion time, and even some large blank areas (uneroded areas) still exist on the sample surface after ultrasonic cavitation erosion for 2 h in the case of Ti10Mo and Ti20Nb samples, implying the enhanced anti-cavitation erosion property of Ti-alloy by adding Mo or Nb element. The MDE of Ti10Mo or Ti20Nb sample is lower than that of TC4 sample in the case of each cavitation erosion time, indicating the better cavitation erosion resistance of of Ti10Mo or Ti20Nb sample. The influences of Mo and Nb on the passivity of TiMo and TiNb alloys during the ultrasonic cavitation erosion are detected by potentiodynamic curves. The results display that Ti, TC4, TixMo (x = 1, 5, 10) and TixNb (x = 5, 10, 20) samples are all almost in the passive state within the potential region from 0V_SCE to 1.5V_SCE during ultrasonic cavitation erosion, and the passive current density evidently decreases with increasing Mo or Nb content, indicating the enhanced passive characteristic by adding Mo or Nb alloys during the ultrasonic cavitation erosion.

Corrosion Behavior of the Ti–6Al–4V Alloy in Sulfate-Reducing Bacteria Solution

The corrosion behavior of the Ti–6Al–4V alloy was investigated in a sulfate-reducing bacteria (SRB) solution. The results showed that sulfate-reducing bacteria has good affinity with the surface of the Ti–6Al–4V alloy after 5 days. A potentiodynamic polarization test demonstrated that the corrosion resistance of the Ti–6Al–4V alloy was initially improved but deteriorated quickly in the subsequent period. The corrosion mechanism of the Ti–6Al–4V alloy was revealed by analyzing its microstructure with the aid of scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray fluorescence. The pitting corrosion was deemed to be a typical cause of the corrosion behavior of the alloy in the SRB solution. The underlying mechanism of the pitting corrosion was proposed for the alloy.

Enhancing the cavitation erosion resistance of D8m-Ta5Si3 nanocrystalline coatings through Al alloying

To investigate the effects of Al alloying on the erosion-corrosion resistance of β-Ta₅Si₃, both a β-Ta₅Si₃ coating and an Al-alloyed β-Ta₅(Si_0.83Al_0.17)₃ coating were synthesized on a 316 substrate by the double cathode glow discharge technique. The phase constitution, composition and microstructure of the two coatings were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The two coatings were composed of nearly rounded D8_m-β-Ta₅Si₃ grains with an average size of ∼4 nm, and after the addition of Al, the preferred growth orientation for the β-Ta₅Si₃ coating changed from (4 0 0) to (0 0 2). The hardness, elastic modulus and contact damage resistance of the coatings were measured using a nanoindentation tester. The results showed that Al alloying improved the contact damage resistance of β-Ta₅Si₃ with only a slight decrease in hardness. The erosion-corrosion behavior of the two coatings was performed in a 3.5 wt% NaCl solution containing a 12 wt% concentration of silica sand under two phase slurry flow condition and in a 3.5 wt% NaCl solution under ultrasonic cavitation erosion conditions. This revealed that the Al alloyed β-Ta₅Si₃ has a higher resistance to both erosion-corrosion and ultrasonic cavitation erosion as compared to the binary β-Ta₅Si₃ coating.

On the passive and semiconducting behavior of severely deformed pure titanium in Ringer's physiological solution at 37°C: A trial of the point defect model

The effects of sever plastic deformation through multi-pass accumulative roll bonding on the passive and semiconducting behavior of pure titanium is evaluated in Ringer's physiological solution at 37°C in the present paper. Produced results by polarization plots and electrochemical impedance spectroscopy measurements revealed a significant advance in the passive response of the nano-grained sample compared to that of the annealed pure titanium. Also, Mott-Schottky test results of the nano-grained pure titanium represented a lower donor density and reduced flat-band potential in the formed passive film in comparison with the annealed sample. Moreover, based on the Mott-Schottky analysis in conjunction with the point defect model, it was suggested that with increase in formation potential, the calculated donor density of both annealed and nano-grained samples decreases exponentially and the thickness of the passive film linearly increases. These observations were consistent with the point defect model predictions, considering that the point defects within the passive film are metal interstitials, oxygen vacancies, or both. From the viewpoint of passive and semiconducting behavior, nano-grained pure titanium appeared to be more suitable for implant applications in simulate human body environment compared to annealed pure titanium.

Effect of micro-particles on cavitation erosion of Ti6Al4V alloy in sulfuric acid solution

The influences of micro-particles on ultrasonic cavitation erosion of Ti6Al4V alloy in 0.1M H₂SO₄ solution were investigated using mass loss weight, scanning electron microscopy (SEM) and white light interferometer. Mass loss results revealed that the cavitation erosion damage obviously decreased with increasing particle size and mass concentration. Open circuit potential recorded during cavitation erosion shifted to positive direction with the decreased mass loss. Meanwhile, the mass loss sharply decreased with applying a positive potential during the entire ultrasonic cavitation erosion, and the relationship between the open circuit potential and the cavitation erosion resistance was discussed.

Local Fine Structural Insight into Mechanism of Electrochemical Passivation of Titanium

Electrochemically formed passive film on titanium in 1.0 M H2SO4 solution and its thickness, composition, chemical state, and local fine structure are examined by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure. AES analysis reveals that the thickness and composition of oxide film are proportional to the reciprocal of current density in potentiodynamic polarization. XPS depth profiles of the chemical states of titanium exhibit the coexistence of various valences cations in the surface. Quantitative X-ray absorption near edge structure analysis of the local electronic structure of the topmost surface (∼5.0 nm) shows that the ratio of [TiO2]/[Ti2O3] is consistent with that of passivation/dissolution of electrochemical activity. Theoretical calculation and analysis of extended X-ray absorption fine structure spectra at Ti K-edge indicate that both the structures of passivation and dissolution are distorted caused by the appearance of two different sites of Ti-O and Ti-Ti. And the bound water in the topmost surface plays a vital role in structural disorder confirmed by XPS. Overall, the increase of average Ti-O coordination causes the electrochemical passivation, and the dissolution is due to the decrease of average Ti-Ti coordination. The structural variations of passivation in coordination number and interatomic distance are in good agreement with the prediction of point defect model.

The role of passive potential in ultrasonic cavitation erosion of titanium in 1 M HCl solution

The influence of the applied passive potential on the ultrasonic cavitation erosion of Ti specimen in 1 M HCl solution was investigated by mass loss experiment, scanning electron microscopy (SEM), electrochemical impedance spectra (EIS) and Mott-Schottky plot. The results showed that Ti was in the passive state within the potential region from -0.3 VSCE to 1.5 VSCE under ultrasonic cavitation erosion. The applied passive potential can obviously decrease the mass loss of Ti caused by ultrasonic cavitation erosion in 1 M HCl solution. The resistance of the passive film increased, the flat band potential moved to positive direction, and the donor density of the passive film decreased with increasing the passive potential. Finally, a physical model was provided to explain the experimental results based on energy band and semi-conductive theories.