Corrosion behaviour of Ti-6Al-7Nb and Ti-6Al-4V ELI alloys in the simulated body fluid solution by electrochemical impedance spectroscopy

The corrosion behaviour of Ti-6Al-7Nb and Ti-6Al-4V ELI (extra low interstitial) was investigated as a function of immersion hours in simulated body fluid (SBF) condition, utilizing potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques. Polarisation experiments were conducted after 0, 120, 240 and 360 h of immersion in SBF solution. From the polarisation curves, very low current densities were obtained for Ti-6Al-7Nb alloy compared to Ti-6Al-4V ELI, indicating a formation of stable passive layer. Impedance spectra were represented in the form of Bode plots and it was fitted using a non-linear least square (NLLS) fitting procedure, in which it exhibited a two time constant system suggesting the formation of two layers. The surface morphology of the titanium alloys have been characterized by SEM and EDAX measurements.     

Development of HA-CNTs composite coating on AZ31 Magnesium alloy by cathodic electrodeposition. Part 2: Electrochemical and in-vitro behavior

The carbon nano-tubes (CNTs) reinforced hydroxyapatite (HA), with various functionalized CNTs concentration ranging from 0 to 1.5?wt%, were deposited on AZ31 magnesium alloy by direct and pulse cathodic electrodeposition methods. The corrosion resistance of the coatings was tested in simulated body fluid (SBF) using different electrochemical methods such as open circuit potential, polarization and electrochemical impedance spectroscopy. The in-vitro behavior, changes in solution pH as well as the amount of evolved hydrogen of these coatings were also evaluated during five days immersion in SBF. The results indicated that the pulse deposited HA having 1% CNTs coating was the optimum condition which decreased the corrosion current density of AZ31 magnesium alloy from 44.25?µA/cm² to 0.72?µA/cm². Moreover, it stabilized the alkalization behavior of AZ31 alloy and caused a tenfold decrease in the amount of hydrogen generation in SBF. Additionally, the formation of new hydroxyapatite layer on the surface of the pre-exist coatings after five days immersion in SBF was confirmed by SEM characterization.     

预钙化加速类骨磷灰石在镍钛合金表面生长的电化学阻抗谱

通过酸碱处理活化NiTi合金表面,在模拟体液中仿生生长类骨磷灰石层以改善其生物相容性.采用电化学阻抗谱研究了预钙化对加速磷灰石沉积的影响,并基于双层模型建立了电子等效电路.结果表明:随着在模拟体液中浸泡时间的延长,化学处理的NiTi合金表面类骨磷灰石不断生长,并且添加预钙化试样浸泡3 d,即可在合金表面生长出均匀完整的类骨磷灰石层,而未预钙化试样表面沉积物稀少.对应电子等效电路中,预钙化试样电阻值明显大于未预钙化试样的,显示预钙化促进了活化NiTi合金表面类骨磷灰石的生长.     

The effect of surface treatment on CaP deposition of Ti6Al4V open cell foams in SBF solution

The effects of alkali and nitric acid surface treatment and acid etching on the CaP deposition of an open cell Ti6Al4V foam (60% porous and 300–500 μm in pore size) developed for biomedical applications were investigated in a simulated body fluid (SBF) solution for 14-day. The surface roughness of the foam specimens ground flat surfaces was measured in nano-metric scale before and after SBF immersion using an atomic force microscope (AFM). A significant increase in the surface roughness of alkali treated foam specimen after SBF immersion indicated a smaller crystal size CaP deposition, which was also confirmed by the AFM micrographs. The microscopic evaluation clearly showed that alkali treatment and nitric acid treatment induced a continuous, uniform CaP deposition on the cell wall surfaces of the foam (interior of cells). While in untreated foam specimen the cells are filled with CaP precipitates and acid etching did not produce a continuous coating layer on particles interior of the cells. The coating layer thickness was ～3 μm in alkali treated foam specimens after 14-day of SBF immersion, while nitric acid treatment induced relatively thinner coating layer, 0.6 μm.     

Preparation and bioactivity of apatite coating on Ti6Al4V alloy by microwave assisted aqueous chemical method

To improve the bioactivity of titanium and its alloys, dense and uniform apatite coatings were prepared on Ti6Al4V titanium substrates using microwave assisted aqueous chemical method. The influence of the pretreatment to the titanium substrates and the Ca/P molar ratio of the microwave solution on the coating deposition and morphology, as well as the bioactivity of the coated samples were studied. Results showed that during the microwave process, alkali treatment followed by heat treatment to the titanium substrates would promote the rapid deposition of hydroxyapatite to form coating. And the morphologies of the apatite coatings could be adjusted by the Ca/P molar ratio of the microwave solution. After immersion test in simulated body fluid (SBF), the coated titanium alloy exhibits a good bioactivity by inducing the formation of apatite depositions.     

Corrosion behaviour of electrogalvanized steel in sodium chloride and ammonium sulphate solutions; a study by E.I.S.

Zinc and zinc-nickel (13% Ni) electrodeposits were passivated by dipping in chromate baths and characterized by scanning electron microscopy. The corrosion behaviour was studied using a.c. electrochemical techniques; electrochemical impedance spectroscopy (EIS) measurements were performed at open circuit and under galvanostatic control during the 24 h immersion time. In sodium chloride solution the zinc-nickel electrodeposits show a better corrosion resistance compared to the pure zinc coatings. During the immersion time, a surface nickel enrichment was observed which, together with the zinc corrosion products, acts as a barrier layer reducing the total corrosion rate. In the same solution the passivation treatment improves the corrosion resistance of the electrodeposits; nevertheless, on zinc substrates, the protection exerted by the chromate film is not, always effective during the immersion time. On the contrary the chromate coating on zinc-nickel substrates induces a remarkable and durable improvement of the corrosion resistance reducing the zinc dissolution almost completely. In the ammonium sulphate solution, the corrosion mechanism is significantly influenced by hydrogen reduction on the zinc-nickel surfaces, and by the production of a local surface acidity which is aggressive for the chromate coatings.     

In vitro corrosion and mineralization of novel Ti–Si–C alloy

The in vitro electrochemical behaviour of a new titanium based α-alloy (Ti–0.5 wt% Si–0.65 wt% C), fabricated via casting and rapid cooling route, was determined using linear, Tafel, potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS), complemented with ex situ SEM-EDS analysis to evaluate the corrosion mechanism. The experimental results revealed that silicon and carbon, in addition to titanium, resulted in the enhancement of mechanical properties. The polarization tests confirmed that Ti–Si–C alloy possessed excellent corrosion resistance (a low corrosion current density of 0.033 μA cm⁻²), comparable to cp Ti and better than Ti6Al4V in phosphate buffered saline (PBS). The mechanism of corrosion was identified as selective dissolution of titanium solid solution matrix. EIS studies indicated the formation of a stable, passive oxide film on the alloy. Further, in vitro bioactivity was evaluated using mineralization tests i.e. by immersing the pre-treated alloy in a concentrated simulated body fluid (10× SBF). Chemical and microstructural characterization of the mineral layer, formed during immersion, revealed the deposition of fine, porous micron-sized globules of a phase rich in calcium-phosphate (Ca-P). In summary, the bulk properties and excellent in vitro electrochemical and mineralization behaviour of the as-cast Ti–Si–C alloy reveal a high potential for its application as load bearing metallic implants.     

Formation of nanoporous oxide layer over a binary β-phase titanium in simulated body fluid

In the present investigation, the surface of Ti-15Mo (??-Ti) alloy was oxidized using hydrogen peroxide (H₂O₂) and the layer was densified by thermal treatment. Morphological characterization of treated surface by Field Emission Scanning Electron Microscope (FE-SEM) revealed the formation of nanoporous layer. Electrochemical studies of H₂O₂ treated specimen exhibited higher corrosion resistance in simulated body fluid (SBF) solution compared to untreated ??-Ti alloy. Further, the formation of nanoporous layer and their electrical components were evinced from impedance studies by fitting it to an circuit model.     

Corrosion behavior of electrochemically assembled nanoporous titania for biomedical applications

Corrosion resistance of nanoporous titania was investigated in Hank’s solution using potentiodynamic polarization and electrochemical impedance spectroscopic techniques. The phase structure, surface morphology and elemental composition of the untreated, anodized heat treated and anodized heat treated titanium specimens immersed in Hank’s solution for seven days were characterized using X-ray diffraction, atomic force microscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy techniques, respectively. The X-ray diffraction technique revealed that the anodized heat treated titanium exhibited anatase structure. The atomic force microscopic and scanning electron microscopic results showed that the titanium surface has transformed from a smooth to nanoporous surface depending on the anodization conditions. The energy dispersive X-ray spectroscopy results confirmed the formation of hydroxyapatite over the anodized titanium after immersion for seven days in Hank’s solution. The electrochemical results revealed that the anodized heat treated titanium after seven day immersion in Hank’s solution showed nobler shift in corrosion potential compared to untreated and anodized titanium. Hence, the results suggested that the nanoporous titania layer developed on titanium is a promising material for application as orthopaedic implants.     

Scratch resistance and electrochemical corrosion behavior of hydroxyapatite coatings on Ti6Al4V in simulated physiological media

Using an electrochemical process, needle-like hydroxyapatite crystals with Ca/P ratio of 1.67 were synthesized on Ti₆Al₄V without the formation of any precursor. In vitro dissolution/precipitation process was investigated by immersion of the coated substrate into Hank??s solution up to 14?days. Physical and chemical characterizations were performed by scanning electron microscope coupled with energy dispersive X-ray spectroscopy and by X-ray diffraction. In particular, through a sequence of reactions including dissolution, precipitation, and ions exchange during immersion tests, a precipitated bone-like apatite coating homogenous and less porous was formed. Further, the corrosion behavior of the untreated and HA-coated specimens in simulated body fluid was evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy. The results showed that the corrosion rates of the samples with HA layer before and after immersion tests were 72 and 80?% lower than that of the bare titanium alloy. At last, the adhesion of the HA layer was determined through the use of scratch tests. A particular tribological behavior and a strong link to the substrate were revealed.     

Effect of carbonate content on the in vitro bioactivity of carbonated hydroxyapatite

The effect of carbonate content on the apatite-forming ability of carbonated hydroxyapatite (CHA) in simulated body fluid (SBF) has been investigated. Five different nanocrystalline B-type CHA with carbonate content ranged from 2.01 to 5.25 wt% were prepared, sintered, and assessed for their in vitro bioactivity in SBF solution for 7-weeks under static conditions at 36.5 °C. The formation of the apatite layer and the surface morphology of CHA were examined by using a scanning electron microscope (FESEM) at week 1, 3, and 7 of SBF immersion, respectively. The Ca/P molar ratio of the CHA was determined by X-ray fluorescence (XRF). In addition, the sample weight changes and the pH of the SBF solution were measured. The results show that the formation of apatite layer depends on the carbonate content of CHA. Increasing the carbonate content caused significant increases in the surface area of CHA and the rate of apatite formation. Weight loss was observed for all CHA samples during the first week of SBF immersion, and thereafter followed by weight regain weekly until week 7. The changes in the pH of SBF and the Ca/P molar ratio were proportional to the carbonate content of CHA. This study thus highlights the importance of determining carbonate content aspect that govern the bioactivity of CHA.     

The Influence of Ni Content on the Stability of Copper—Nickel Alloys in Alkaline Sulphate Solutions

The electrochemical behaviour of copper–nickel alloys with different Ni content (5–65%) in sulphate solutions of pH 12 was investigated. The effects of temperature, immersion time, and concentration of sulphate ions were also studied. Different electrochemical methods such as open-circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy (EIS) were used. Potentiodynamic measurements reveal that the increase in nickel content increases the corrosion rate of the alloy in sulphate solution linearly. Nevertheless, an increase in the nickel content along with increase in immersion time improves the stability of the Cu–Ni alloys due to the formation of a stable passive film. An equivalent circuit model for the electrode/electrolyte interface under different conditions was proposed. The experimental impedance data were fitted to theoretical data according to the proposed model. The relevance of the model to the corrosion/passivation phenomena occurring at the electrode/solution interface was discussed.     

The use of polyindole for mild steel protection

Synthesis of polyindole was achieved on mild steel electrode previously coated with a very thin polypyrrole layer (PPy). Cyclic voltammetry technique was used for both syntheses; oxalic acid solution was used for synthesis of primer PPy coating and polyindole film (PI) was obtained from LiClO₄ containing acetonitrile medium. The corrosion performance of this PPy/PI coating was investigated properly in 3.5% NaCl solution by using anodic polarization and open circuit potential (E_ocp)–time curves and electrochemical impedance spectroscopy (EIS). This coating exhibited excellent barrier efficiency for a long time (about 190 h) and it was also able to provide a certain anodic protection. After 240 h of immersion time in corrosive test solution, the protection efficiency value was determined to be 98.9%.     

仿生法制备钛合金/羟基磷灰石复合涂层的研究

本研究了仿生溶液中钛合金表面沉积羟基磷灰石的技术路线。钛合金经抛磨、清洗．进行酸碱活化处理后，置于模拟体液(SBF)中，最终在钛合金基体上生成羟基磷灰石涂层。     

Electrochemical behaviour of cadmium in NaOH solution

The electrochemical behaviour of cadmium in 1 M NaOH solution was studied. Different electrochemical methods such as potentiodynamic measurements, polarization techniques and electrochemical impedance spectroscopy (EIS) were used. During the course of polarization, several characteristic features were observed, including prepassivation and a relatively wide passive range extending over 1.4 V. Equivalent circuit models for the electrode–electrolyte interface under different applied potentials were proposed. The impedance data measured at steady state were fitted to calculated data according to proposed equivalent circuits. The relevance of the proposed equivalent circuits to the different phenomena occurring at the electrode–solution interface was discussed. The passive film formed on Cd is composed of two layers: a barrier layer in contact with metal and a deposit on the barrier layer. The barrier layer appears to form directly from the metal through a solid state reaction, rather than by a dissolution–precipitation mechanism. For an applied potential >+0.7 V the relative passive film thickness and resistance decrease to lower values indicating transpassive dissolution at this potential.     

Corrosion‐protection aspects of electrochemically synthesized poly(o‐anisidine) coatings on mild steel: An electrochemical impedance spectroscopy study

The corrosion‐protection aspects of poly(o‐anisidine) (POA) coatings on mild steel in aqueous 3% NaCl solutions were investigated with electrochemical impedance spectroscopy, a potentiodynamic polarization technique, and open circuit potential measurements. The POA coatings were electrochemically synthesized on mild steel with cyclic voltammetry from an aqueous salicylate medium. The corrosion behavior of the POA coatings was investigated through immersion tests performed in aqueous 3% NaCl solutions, and the recorded electrochemical impedance spectra were fitted with an equivalent circuit to obtain the characteristic impedance parameters. The use of a single equivalent circuit was inadequate to explain the various physical and electrochemical processes occurring at different exposure times. It was suggested that some characteristic element(s) should be incorporated into the equivalent circuit at different stages of the immersion to elucidate the various processes occurring at different exposure times. The evolution of the impedance parameters with the immersion time was studied, and the results showed that POA acted as a protective coating on the mild steel against corrosion in a 3% NaCl solution. From these data, the water uptake and delamination area were determined to further support the corrosion‐protection performance of the POA coating. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Bioactive Calcium Phosphate Coating on Sodium Hydroxide-Pretreated Titanium Substrate by Electrodeposition

This study examined the characteristics of calcium phosphate coatings deposited on a NaOH-pretreated titanium substrate by electrodeposition in a modified simulated body fluid (SBF) immediately after electrodeposition as well as after the coatings had been immersed in the SBF for 5 d in order to determine the effects of a NaOH pretreatment on the bioactive coating prior to electrodeposition. The results showed that a dense and uniform coating that consisted of brushite and hydroxyapatite had formed on the NaOH-pretreated titanium substrate by electrodeposition. This coating had transformed to a bonelike apatite during immersion in the SBF. This was attributed to the increased surface area of the modified titanium formed after the NaOH treatment as well as the Na⁺ ions released from that surface. Therefore, a NaOH pretreatment is recommended as an effective method for preparing a bioactive calcium phosphate coating by electrodeposition.     

An electrochemical study of the behaviour of ear piercing studs immersed in a culture medium

Two commercial studs, of gold-coated stainless steel and copper–zinc alloy, respectively, and a laboratory-made titanium stud were chosen for corrosion studies. Corrosion behaviour in a culture medium (CM) was studied using electrochemical impedance spectroscopy and polarization measurements, as a function of immersion time. The elements that leached out into the CM electrolyte were analysed by instrumental neutron activation analysis. Scanning electron microscopy and energy dispersive X-ray techniques were utilized in the analysis of the stud surfaces before and after their immersion in a CM solution. The cytotoxicity of the tested studs was also determined. The titanium stud showed the best combination of properties, high corrosion resistance and low cytotoxicity, while the gold-coated stainless steel stud occupied an intermediate position.     

碳/碳复合材料表面软复合磷酸钙层

为改进碳／碳复合材料的生物活性，发展了表面软复合磷酸钙层的制备工艺。首先在碳／碳复合材料表面通过离子束辅助沉积技术形成的钛镀层，然后经浓碱液处理后呈多孔网状，该网状结构可在模拟体液（SBF）中诱导沉积出生理磷灰石层，从而在碳／碳复合材料表面形成软复合磷酸钙层。所获得的软复合磷酸钙层厚度约为8μm ，结晶结构为羟基磷灰石，但其Ca,P摩尔比n(Ca)/n(P)低于1．67。     

Influence of a hard transition layer on the microstructure and properties of diamond-like carbon/hydroxyapatite composite coating prepared by magnetron sputtering

The nanostructured diamond-like carbon/hydroxyapatite composite coating (DLC/HA) was deposited using magnetron sputtering technique with a densely packed columnar cross-sectional structure and a uniform granular surface morphology. After heat treatment, the amorphous structure of the coating was transformed into a crystal structure. Nanohardness and scratch tests results demonstrated the DLC transition layer significantly enhanced the nanohardness of Ti6Al4V substrates from 4.8 GPa to 10.4 GPa, and increased critical load from 16.6 N (pure HA layer) to 26.5 N (DLC layer) without obvious brittle fracture, flaking and delamination. Electrochemical and immersion tests results demonstrated that DLC/HA composite coatings with a dense gradient transition interlayer had better corrosion resistance and could prevent harmful metal ions being released into the SBF solution more effectively than single HA coatings. Furthermore, active Ca²⁺ ions can be rapidly released from the coating surface during initial immersion in the SBF solution, and facilitated the formation of bone-like apatite.