Electrochemical corrosion properties of metal alloys used in orthopaedic implants

This study concerns an investigation of the corrosion behavior of 316 stainless steel, CoCrMo and Ti6Al4V alloys in simulated body conditions (ringer lactate) at 37 °C by the use of Tafel plots, mixed potential and electrochemical impedance spectroscopy (EIS). Ti6Al4V alloy has the highest corrosion resistance followed by CoCr alloy. Ti6Al4V–CoCrMO was the best couple for galvanic corrosion with the minimum galvanic potential and current values according to mixed potential theory and Tafel method. It was concluded that Ti6Al4V was the most suitable material for implant applications in the human body.     

Corrosion behavior of TiN and TiN/Ti composite films on Ti6Al4V alloy in Hank’s solution

Surface films of TiN and TiN/Ti were deposited on Ti6Al4V alloy by arc ion plating (AIP). Open-circuit potential, potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) were employed to investigate the corrosion performance of TiN and TiN/Ti films in Hank’s simulated body fluid at 37 °C and pH 7.4. Scanning electron microscopy (SEM) was used to study the surface morphology of the corroded samples after the potentiodynamic polarization tests. The results show that the TiN and the TiN/Ti films can provide effective protection for the Ti6Al4V substrate in Hank’s fluid, and the TiN/Ti composite film showed a corrosion resistance superior to that of the TiN film. The outer TiN layer of the composite film mainly acted as an efficient barrier to corrosion during short-term experiments. In contrast to the bare Ti6Al4V, no pitting was observed on the surface of the TiN and TiN/Ti films deposited on the bare alloy after potentiodynamic polarization.     

Synthesis and evaluation of TaC nanocrystalline coating with excellent wear resistance,corrosion resistance,and biocompatibility

To improve the mechanical properties, corrosion resistance, and biocompatibility of implanted titanium alloys, a TaC nanocrystalline coating was deposited on Ti–6Al–4V alloy using a double-cathode glow discharge method. The microstructure of the newly developed coating was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The coating exhibits a dense and uniform structure, composed of equiaxed TaC grains with an average grain size of 15.2 nm. The mechanical properties of the TaC-coated Ti–6Al–4V alloy were evaluated by a scratch tester, a nanoindentation tester, and a ball-on-disc tribometer. The average hardness of the TaC nanocrystalline coating is about 6 times higher than that of uncoated Ti–6Al–4V alloy and the specific wear rate of the coating is two orders of magnitude lower than that for Ti–6Al–4V at applied normal loads of 4.9 N under dry sliding condition. The electrochemical behavior of the TaC nanocrystalline coating after soaking in Ringer's solution for different periods was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Furthermore, in vitro cytocompatibility of the coating was assessed using MC3T3-E1 mouse osteoblastic cells. The results showed that the TaC coating exhibits better corrosion resistance and biocompatibility as compared to uncoated Ti–6Al–4V alloy.     

电流密度对Ti 6Al 4V微弧氧化膜形貌和性能的影响

采用NaAlO2-Na3PO4-NaF溶液体系,研究了电流密度对Ti 6Al 4V合金微弧氧化膜厚度、生长速率、表面形貌、粗糙度、组成相以及氧化膜耐蚀性、耐磨性等影响.结果表明,(1)在试验的电流密度范围内,氧化膜的厚度随电流密度的增大呈线性增大,但氧化膜的粗糙度却几乎呈指数增大,表面质量变差;(2)在质量分数为3.5%的NaCl溶液中显示了比Ti 6Al 4V钛合金更好的耐蚀性;(3)在干摩擦条件下,氧化膜的摩擦系数高于基体的,氧化膜的磨损机制为脆性断裂.     

A study of the potentials achieved during mechanical abrasion and the repassivation rate of titanium and Ti6Al4V in inorganic buffer solutions and bovine serum

Titanium alloys in orthopaedic implants are susceptible to mechanical disruption of the passive film (fretting corrosion). To study this effect, open-circuit potential (ocp) measurements before, during and after mechanical disruption of the passive film in a tribo-electrochemical cell on commercial pure titanium and Ti6Al4V alloy in inorganic buffer solutions in the pH range from 2.0 to 12.0 and calf bovine serum at pH 4.0 and 7.0 are reported. Additionally, the effect of pH, electrolyte and sample composition on the repassivation rate has been investigated. The potentials achieved during the abrasion of Ti6Al4V are the same as those characterizing pure titanium, which indicates that the corrosion current of both materials in the active state is due to the oxidation of titanium. However, commercial pure titanium displays a tendency to repassivate faster than Ti6Al4V in inorganic buffer solutions thanks to the lower critical current density and the higher catalytic activity towards the hydrogen evolution reaction observed on the pure metal in comparison with the alloy.Proteinaceous solutions like bovine serum, significantly slow down the anodic dissolution and the cathodic reactions both on titanium and the alloy. However, the repassivation rate of the Ti6Al4V is not affected by serum, while that of cp titanium significantly decreases both at pH 4.0 and 7.0.     

In vitro cytotoxicity,corrosion and antibacterial efficiencies of Zn doped hydroxyapatite coated Ti based implant materials

Ti6Al4V alloy is successfully used as implant material in dental and orthopedic surgeries for years due to its much better compatibility, lower density, corrosion resistance, etc. compared to the other metals. Meantime, modification of the surface of these alloys is needed to enhance material-tissue interaction and osteointegration between the implant and the bone. In this study, Ti6Al4V alloy surfaces were modified by application of RF magnetron sputtering technique and coated with zinc (Zn) doped hydroxyapatite (HAp). The obtained coating was very stable with highly crystalline structure, demonstrated enhanced corrosion resistance, osteointegration and antimicrobial effectiveness against Escherichia coli (E. coli) bacteria.     

Tantalum carbide coating on Ti‐6Al‐4V by electron beam physical vapor deposition method: Study of corrosion and biocompatibility behavior

This study aimed to improve the corrosion resistance and biocompatibility of titanium alloy (Ti‐6Al‐4V) by tantalum carbide (TaC) deposition through electron beam physical vapor deposition (EB‐PVD) method. The physical and chemical characteristics of the coated surface are comprehensively evaluated. The corrosion resistance and ion release are assessed. Cytocompatibility assay and cell morphology observation are performed to assess toxicity and cell interaction, respectively. The TaC‐coated Ti‐6Al‐4V exhibits more resistance to corrosion and ion release. It provides a surface, which is appropriate for cell adhesion, an expansion as well as better biocompatible performance. So, it could improve osseointegration Ti‐alloy implants in clinical applications.     

Controlled surface modification of Ti6Al4V using biomimetic mineralization via thermo-chemical route improves bioactivity

Ti alloy (Ti6Al4V) sheets were bio-activated by a two-step thermo-chemical treatment followed by biomimetic mineralization. The samples were then characterized by standard techniques and evaluated of their mechanical properties, electrochemical corrosion potential and biological performance. The intermediate layer corresponding to thermo-chemical treatment displayed anatase TiO₂ peaks and the final bio-mineralization resulted in a globular hydroxyapatite (HAP) layer. Thermo-chemical treatment yielded a two-fold increase (98.79% increment) in microhardness value, whereas, the biomimetically activated samples showed a very small decrease in the same owing to their ceramic behavior. The surface hydrophobicity of the bio-activated surface was found reduced significantly, might assist to facilitate improved cell adhesion. Electrochemical corrosion measurements exhibited an increase in corrosion potential and decrease in current density of the samples, suggested increased corrosion resistant. The surface coating on the Ti6Al4V sheet also demonstrated enhanced cytocompatibility as no toxic effect of the samples could be perceived to human keratinocyte cell line (HaCaT). Similarly, the samples showed higher hemocompatibility and enhanced bactericidal activity. Our study concluded that the surface coating of Ti6Al4V sheets significantly improved corrosion resistance and bioactivity of the substrates, which can be applied for various biomedical applications.     

Electrochemical and structural evaluation of functionally graded bioglass-apatite composites electrophoretically deposited onto Ti6Al4V alloy

Titanium and its alloys are widely used as materials for implants, owing to their corrosion resistance, mechanical properties and excellent biocompatibility. However, clinical experience has shown that they are susceptible to localised corrosion in the human body causing the release of metal ions into the tissues surrounding the implants. Several incidences of clinical failures of such devices have demanded the application of biocompatible and corrosion resistant coatings and surface modification of the alloys. Coating metallic implants with bioactive materials is necessary to establish good interfacial bonds between the metal substrate and the bone. Hence, this work aimed at developing a bioglass-apatite (BG-HAP) graded coating on Ti6Al4V titanium alloy through electrophoretic deposition (EPD) technique. The coatings were characterized for their properties such as structural, electrochemical and mechanical stability. The electrochemical corrosion parameters such as corrosion potential (E_corr) (open circuit potential) and corrosion current density (I_corr) evaluated in simulated body fluid (SBF) have shown significant shifts towards noble direction for the graded bioglass-apatite coated specimens in comparison with uncoated Ti6Al4V alloy. Electrochemical impedance spectroscopic investigations revealed higher polarisation resistance and lower capacitance values for the coated specimens, evidencing the stable nature of the formed coatings. The results obtained in the present work demonstrate the suitability of the electrophoretic technique for the preparation of graded coating on Ti6Al4V substrates.     

The investigation of the tribocorrosion properties of DLC coatings deposited on Ti6Al4V alloys by CFUBMS

Various machine components produced from titanium alloys used in various industries are subject to a combination of electrochemical and mechanical effects. The science of surface transformations resulting from the interaction of mechanical loading and chemical reactions that occur between elements of a tribosystem exposed to corrosive environments is described as tribocorrosion. This research focuses on the tribocorrosion behaviour of Ti6Al4V alloys after coated by using closed field unbalance magnetron sputtering (CFUBMS). The structural analyses of the coatings were performed using Raman spectroscopy and scanning electron microscopy (SEM). Tribocorrosion experiments were performed in a pin-on-disc tribotester under electrochemical polarisation in NaCl 1 wt.% solution. This study shows that the Ti-DLC coating is protecting the Ti6Al4V alloy and having good performance in corrosion and tribocorrosion conditions. The OCP values for Ti6Al4V substrate and Ti-DLC protective coatings during tribocorrosion tests were measured as −560 V and −330 V, respectively. These results showed that Ti-DLC protective coating on Ti6Al4V substrates increased the tribocorrosion resistance by acting as a barrier layer.     

Understanding the protective role of a gradient titanium oxide ceramic layer on Ti6Al4V against corrosion via analyses of Mott-Schottky curve and electron work function (EWF)

Thermal oxidation (TO) process was employed to generate a gradient titanium oxide ceramic layer for improving corrosion performance and service safety of Ti6Al4V alloy. The semiconductor characteristic of the TO layer was evaluated in CO₂-saturated simulated oilfield brine. The generated TO layer with a thickness of about 20 μm was dense and continuous without cracks or spalling characteristics. The TO layer mainly comprised of an oxide ceramic layer (rutile TiO₂ ceramic phase, minor anatase one, and Al₂O₃) and an oxygen diffusion layer. The conducted electrochemical analysis suggested that the corrosion resistance of Ti6Al4V alloy was improved using TO surface strengthening process. It was demonstrated that the TO layer with semiconductor characteristics showed a transition from n-type (donor) to p-type (acceptor) with the increasing applied electric potential. The electron work function of the TO layer was higher than that of Ti6Al4V alloy with a naturally formed passive film. The improvement in corrosion properties was attributed to the excellent chemical stability and semiconductor properties of the metal oxide ceramic phases (TiO₂, Al₂O₃) in the TO layer.     

Pitting corrosion of Al and Al–Cu alloys by ClO4 ions in neutral sulphate solutions

The influence of various concentrations of NaClO₄, as a pitting corrosion agent, on the corrosion behaviour of pure Al, and two Al–Cu alloys, namely (Al + 2.5 wt% Cu) and (Al + 7 wt% Cu) alloys in 1.0 M Na₂SO₄ solution was investigated by potentiodynamic polarization and potentiostatic techniques at 25 °C. Measurements were conducted under the influence of various experimental conditions, complemented by ex situ energy dispersive X-ray (EDX) and scanning electron microscopy (SEM) examinations of the electrode surface. In free perchlorate sulphate solutions, for the three Al samples, the anodic polarization exhibits an active/passive transition. The active dissolution region involves an anodic peak (peak A) which is assigned to the formation of Al₂O₃ passive film on the electrode surface. The passive region extends up to 1500 mV with almost constant current density (j_pass) without exhibiting a critical breakdown potential or showing any evidence of pitting attack. For the three Al samples, addition of ClO₄⁻ ions to the sulphate solution stimulates their active anodic dissolution and tends to induce pitting corrosion within the oxide passive region. Pitting corrosion was confirmed by SEM examination of the electrode surface. The pitting potential decreases with increasing ClO₄⁻ ion concentration indicating a decrease in pitting corrosion resistance. The susceptibility of the three Al samples towards pitting corrosion decreases in the order: Al > (Al + 2.5 wt% Cu) alloy > (Al + 7 wt% Cu) alloy. Potentiostatic measurements showed that the rate of pitting initiation increases with increasing ClO₄⁻ ion concentration and applied step anodic potential, while it decreases with increasing %Cu in the Al samples. The inhibitive effect of SO₄²⁻ ions was also discussed.     

Characterization and corrosion behavior of plasma sprayed calcium silicate reinforced hydroxyapatite composite coatings for medical implant applications

Titanium and its alloys are widely used for medical implant applications, but their corrosion in the physiological environment leads to the discharge of metal ions, which can trigger severe health issues. In the present study, calcium silicate reinforced hydroxyapatite (HA-CS) coatings were deposited on the Ti6Al4V substrate by using atmospheric plasma spray (APS) process with an aim to improve the corrosion resistance and bioactivity. The coatings were prepared by varying the weight percentage (wt %) of calcium silicate (CS) reinforcement in hydroxyapatite (HA) as Ha/x CS (x = 0, 10, 20 wt %). The SEM analysis of the pure HA coating revealed the presence of surface microcracks, whereas HA-CS coatings displayed the crack-free surface morphology. The corrosion investigation revealed that with the progressive increment of CS content in HA coating, the corrosion resistance of HA-CS coatings improved. In addition, surface roughness, porosity, microhardness and crystallinity increased with the increase of CS content in HA. The findings of this study indicate that the development of plasma sprayed HA-CS coatings is a promising approach to improve the performance of Ti6Al4V alloy for medical implant applications.     

压铸镁合金阳极氧化膜的研究

研究了压铸镁合金AZ91的阳极氧化膜的工艺及其耐蚀性，探讨了镁合金表面阳极氧化膜的组织、相、成分及其耐蚀性。研究结果显示，压铸镁合金AZ91阳极氧化膜表面系氧化物的聚集，阳极氧化膜在3．5％NaCl中的极化曲线与AZ91压铸镁合金的极化曲线对比，阳极氧化膜的极化曲线有明显的钝化区，但在极化区只呈锯齿状变化，耐蚀性较好。     

Electrochemical corrosion behavior of magnesium and titanium alloys in simulated body fluid

The electrochemical behavior of AZ91D and Ti–6Al–4V alloys was investigated in simulated body fluid (SBF) at 37 °C. The aim of the present study was to evaluate their corrosion performance through the analysis of corrosion resistance variation with time, using electrochemical impedance spectroscopy (EIS) tests and corrosion current density using potentiodynamic polarization measurements. Very low current density was obtained for Ti–6Al–4V alloy compared to that of AZ91D alloy, indicating a typical passive behavior for Ti alloy. EIS results exhibited high corrosion resistance indicating a highly stable film on titanium alloy compared to magnesium alloy in SBF.     

Corrosion,wear and wear–corrosion behavior of graphite-like a-C:H films deposited on bare and nitrided titanium alloy

This work presents a comparative wear, corrosion and wear–corrosion (the last one in a simulated physiological solution) study of graphite-like a-C:H (GLCH) films deposited on bare and nitrided Ti6Al4V alloy. Films, deposited by r.f. PACVD, presented low porosity and promoted high corrosion resistance. The friction coefficient of the films was very low with appreciable wear resistance at room conditions. However, due to the simultaneous action of both load and the corrosive environment in wear–corrosion tests a marked reduction in the coating lifetime was observed. Unexpectedly, films deposited on the nitrided alloy presented a lifetime at least ten times shorter than that of films on bare alloy. We explain such a result in terms of film/substrate interaction. The weak GLCH/nitrided alloy interaction facilitates fluid penetration between the film and the substrate which leads to a fast film delamination. Such an interpretation is supported by force curve measurements, which show that the interaction between GLCH and nitrided alloy is four times weaker than that between GLCH and bare alloy.     

Electrochemical properties of Al and Al alloys relevant to corrosion protection in seawater environments

A series of electrochemical experiments on Al alloys were undertaken to determine their optimum protection potentials in seawater. With 1050 and 5456 alloys, passive films form during anodic polarization but are destroyed by the Cl in seawater, only to regrow as a result of the self-healing capacity of aluminum. The current density of 5456 Al alloy proved to be lower than that of 1050 as a whole. Any shift to more anodic or cathodic conditions in the potential range of-1.5∼-0.68 V resulted in a sudden increase in current density. Current densities in the high-strength 7075 Al alloy showed the greatest values. In contrast, the current densities of 5456 alloy, known to have excellent corrosion resistance in seawater, were the lowest in the range of -0.70∼-1.3 V, and we concluded that this potential range offered optimal protection.     

In vitro biocompatibility of a nanocrystalline β-Ta2O5 coating for orthopaedic implants

To improve the durability and bioactivity of Ti–6Al–4V alloy used for medical implants, the β-Ta₂O₅ nano-crystalline coatings were introduced using double cathode glow discharge technique. The coating microstructure was characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The coating exhibits an assembly of near-equiaxed grains, locally aligned normal to the coating surface. The β-Ta₂O₅ coating exhibits strong adhesion to substrate and a strong resistance to deformation and cracking under applied loads. Cells culture tests showed that the coating is more beneficial to the adhesion and proliferation of NIH-3T3 cells as compared to the uncoated alloy. In-vitro bioactivity was evaluated by immersion of the coating in simulated body fluids (SBF) for different periods up to 14 days at 37 °C. The results indicated that bioactivity of Ti–6Al–4V was dramatically improved after the deposition of β-Ta₂O₅, since the coating has a higher apatite forming ability than the Ti–6Al–4V substrate. Finally, the electrochemical behavior of the β-Ta₂O₅ coating after soaking in SBF at 37 °C for 0, 3, 7, and 14 days was studied through potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). EIS measurements also confirm that the presence of a hydroxyapatite layer on the coating becomes thicker and denser during soaking in SBF. Moreover, the coating exhibits better corrosion resistance than the bare alloy. Hence, the β-Ta₂O₅ coating is a promising candidate coating for protection of orthopedic implants with enhanced bioactivity and corrosion resistance.     

Novel structured spark plasma sintered thermal barrier coatings with high strain tolerance and oxidation resistance

Titanium alloys play an important role in lightweight aircraft engines owing to their low densities and high specific strengths. However, an increase in the thrust-to-weight ratio causes the engine operating temperature to be much higher than the service temperature, which deteriorates the oxidation resistance and mechanical properties. In this study, yttria-partially stabilised zirconia (8YSZ)/NiCrAlY thermal barrier coatings (TBCs) with a bimodal structure were prepared on Ti–6Al–4V by using spark plasma sintering (SPS) to improve the service temperature. The distinctive bimodal structure possessed dense particle contacts and a uniform distribution of porous nanoparticles, resulting in higher strain tolerance, sintering resistance, and lower thermal conductivity. Therefore, the bimodal structure prepared by lowering the SPS preparation temperature increased the high-temperature service time of TBCs on titanium alloy. The ceramic top coating (TC) and bond coating (BC) were well connected after isothermal oxidation at 800 °C for 100 h. The TBCs only shed 6% of their surface area at high temperature and large-angle bending. In addition, the bimodal-structured TBCs effectively improved the oxidation resistance of the Ti–6Al–4V substrate. The Ti–6Al–4V substrate with bimodal-structured TBCs only gained 0.51 times the mass gained by the bare Ti–6Al–4V after 100 h of isothermal oxidation.     

Galvanic corrosion of titanium-based dental implant materials

This paper describes an investigation of the corrosion behavior of Ti-based dental materials with Au, CrNi and CoCr in Ringer solution by the use of Tafel plots, Evans diagrams and EIS Nyguist diagrams. The galvanic potentials and currents obtained for various implant couples are as follows: For, Ti6Al4V/CrNi couple −0.030 V (Ag/AgCl (3 M NaCl)) and 7.94 μA cm⁻²; for Ti6Al4V/CoCr couple −0.020 V (Ag/AgCl (3 M NaCl)) and 7.08 μA cm⁻²; for Ti6Al4V/Au couple −0.020 V (Ag/AgCl (3 M NaCl)) and 5.62 μA cm⁻². The Ti6Al4V/Au couple was found to be the most suitable one against galvanic corrosion according to both the Tafel method and mixed potential theory. The corrosion behaviors of Ti6Al4V/CoCr and Ti6Al4V/CrNi couples were found to be similar.