Biofunctional calcium titanate coating on titanium by simple chemical treatment process using calcium-hydroxide slurry—Effects of the heating temperatures

The present study investigates the effect of the heating temperature on the characteristics of the surface layer in a simple treatment process using calcium-hydroxide slurry. Furthermore, biofunctions of the treated titanium surfaces, such as bioactivity in a simulated body fluid and corrosion resistance under a bio-environment, are also investigated. Our treatment process using calcium-hydroxide slurry is as follows: a titanium substrate is buried in calcium hydroxide slurry, and the slurry is then heated in air. The calcium hydroxide slurry was prepared by mixing of calcium hydroxide powder and water. When treated at heating temperature beyond 873 K, a crystallized calcium titanate coating is formed on a titanium substrate and further, the thickness increases with the increase of heating temperature. On the crystallized calcium titanate coating, calcium phosphate is precipitated in a simulated body fluid; therefore, the treated titanium surface has bioactivity. Furthermore, anodic current density of the treated surface is 10–10³ times lower than that of the non-treated surface.     

Surface characteristics of titanium anodized in the four different types of electrolyte

This study evaluated the surface characteristics of titanium modified by anodic spark oxidation and a subsequent hydrothermal treatment. The electrolytic compositions of the experimental groups are as follows: GA: 0.015 M dl-α-glycerophosphate disodium salt hydrate (dl-α-GP) and 0.2 M calcium acetate (CA), GB: anodized in 0.015 M β-GP (glycerophosphate disodium salt) and 0.2 M CA, GC: anodized in 0.015 M GP (glycerophosphate disodium salt) and 0.2 M CA, and GD: anodized in 0.015 M GP-Ca (glycerophosphate calcium salt) and 0.2 M CA. Anodic spark oxidation was carried out at 30 mA/cm² to 290 V. In addition, the anodized samples were treated hydrothermally at 300 °C for 2 h in an autoclave system. Regardless of the electrolytic composition, the anodic oxide films on the titanium surface contained pores ∼5 μm in size and the diameter was larger at the protrusion parts than that at the lower parts. The phase of the anodic oxide layer consisted mainly of anatase with a small amount of rutile. HA crystals precipitated on the porous titanium oxide layer after a hydrothermal treatment. Moreover, the morphology of the HA crystals was a dense fine needle shape, which changed according to the electrolytic composition. The mean surface roughness (R_a) was highest in group GB at 0.437 μm. The R_a values of the hydrothermally treated group was approximately 0.14-0.2 μm higher than the anodized groups. Anodic spark oxidation and the hydrothermal treatment resulted in increasing corrosion potential and decreasing corrosion current density, which means an improvement in the corrosion resistance. The surface activity of the specimens in Hanks’ solution was GD > GA > GB > GC.     

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.     

Electrochemical and corrosion behavior of Ti-xAl-yFe alloys prepared by direct metal deposition method

The electrochemical and corrosion behavior of Ti-based alloys was investigated. The direct metal deposition technique was used to fabricate 21 alloys with different ratio of metals (0 ≤ Al ≤ 27 wt.%, 0 ≤ Fe ≤ 25 wt.%). Corrosion resistance of each alloy was evaluated both qualitatively and quantitatively by voltammetric measurements in the simulated human body fluid conditions (Hank's solution). The corrosion rates of the materials were compared in Hank's solution using Tafel extrapolation method. Among the Ti-xAl-yFe alloys the Ti-7Al-4Fe alloy exhibited the slowest corrosion rate of 7.7 × 10⁻⁴ mm/year and the least value of passive current density (6.3 × 10⁻³ A/m²). The alloy is resistant to pitting corrosion as well.     

In vitro assessment of degradation and bioactivity of robocast bioactive glass scaffolds in simulated body fluid

In this study porous three-dimensional scaffolds of borate (13-93B3) bioactive glass were prepared by robocasting and in vitro degradation and bioactivity was evaluated. Grid like scaffolds with interconnected pores was assembled using robotic deposition technique which is a direct ink writing method. After binder burnout, the constructs were sintered for 1 h at 560 °C to produce scaffolds (porosity≈60%) consisting of dense glass struts (300±20 μm in diameter) and interconnected pores of width 580±20 μm. Hydroxyapatite formation on borate bioactive glass scaffolds was investigated in simulated body fluid (SBF) using three different scaffold/SBF (S/S) ratios (1, 2 and 10 mg/ml) at 37 °C. When immersed in SBF, degradation rate of the scaffolds and conversion to a calcium phosphate material showed a strong dependence to the S/S ratio. At high solid concentration (10 mg/ml) surface of the glass scaffolds converted to the calcium rich amorphous calcium phosphate after 30 days. At lower solid concentrations (2 and 1 mg/ml) an amorphous calcium phosphate layer formation was observed followed by the conversion to hydroxyapatite.     

Electrochemical study of the in vitro degradation of plasma-sprayed hydroxyapatite/bioactive glass composite coatings after heat treatment

Hydroxyapatite (HA)-coated implants plasma-sprayed on metallic substrates have been widely used in load-bearing applications. In the present work, the in vitro behaviors of bioactive glass-containing HA coatings with post-deposition heat treatment in a simulated body fluid has been performed by means of electrochemical techniques. Annealing of the coatings in air at 650 °C led to recrystallization of amorphous calcium phosphate and effectively increased the conversion of non-apatite phases into apatite. The heat treatment also resulted in a reduction of layer defects associated with plasma-sprayed coatings without adversely affecting bond strength of the coatings. Moreover, the heat-treated coatings significantly increased the corrosion potential and polarization resistance value by approximately two times as compared to untreated samples. Improved corrosion resistance could be attributed to reduction of layer defects and enhancement of coating crystallinity.     

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.     

In vitro behavior of bioactive phosphate glass–ceramics from the system P2O5–Na2O–CaO containing titania

Soda lime phosphate bioglass–ceramics with incorporation of small additions of TiO₂ were prepared in the metaphosphate and pyrophosphate region, using an appropriate two-step heat treatment of controlled crystallization defined by differential thermal analysis results. Identification and quantification of crystalline phases precipitated from the soda lime phosphate glasses were performed using X-ray diffraction analysis. Calcium pyrophosphate (β-Ca₂P₂O₇), sodium metaphosphate (NaPO₃), calcium metaphosphate (β-Ca(PO₃)₂), sodium pyrophosphate (Na₄P₂O₇), sodium calcium phosphate (Na₄Ca(PO₃)₆) and sodium titanium phosphate (Na₅Ti(PO₄)₃) phases were detected in the prepared glass–ceramics. The degradation of the prepared glass–ceramics was carried out for different periods of time in simulated body fluid at 37 °C using granules in the range 0.300–0.600 mm. The released ions were estimated by atomic absorption spectroscopy and the surface textures were measured by scanning electron microscopy. Investigation of in vitro bioactivity of the prepared glass–ceramics was done by the measurement of the infrared reflection spectra for the samples after immersion in the simulated body fluid for different periods at 37 °C. The result showed that no apatite layer was formed on the surface of the samples and the dominant phase remained on the surface was β-Ca₂P₂O₇, which is known for its bioactivity.     

Optimal synthesis of mesostructured hollow titania nanotubes templated on CaCO3 nanoparticles

Approximate 10 µm length of mesostructured hollow titania nanotubes with intact configuration was successfully prepared by using needle-like calcium carbonate and octadecylamine as double templates at room temperature in nonaqueous system. During the whole preparation, two parameters i.e. tetrabutoxytitanium/calcium carbonate molar ratio and annealing temperature, were optimized to obtain titania nanotubes with well-defined tubular morphology and mesoporous structure in tube walls. The as-prepared samples were characterized by scanning electron microscopy, transmission electron microscopy, broad-angle X-ray diffraction, pore size distribution and Brunauer-Emmett-Teller. The results showed that under optimal experimental conditions i.e. tetrabutoxytitanium/calcium carbonate molar ratio (50 wt.%) and annealing temperature (773 K), the tube materials exhibited uniform tubular structure with a length of 8-15 µm and an inner diameter of ∼ 400 nm, a wall thickness of ∼ 40 nm, a surface area of 112.2 m²/g and a pore volume of 0.18 cm³/g. The optimized titania nanotubes were utilized as a carrier for the immobilized of ibuprofen via a simple adsorption method. It was found that the loaded drug presented good sustained release property in three release media, i.e. simulated body fluid, normal saline and pure water.     

Fabrication of corrosion resistant, bioactive and antibacterial silver substituted hydroxyapatite/titania composite coating on Cp Ti

The present work is aimed at developing a bioactive, corrosion resistant and anti bacterial nanostructured silver substituted hydroxyapatite/titania (AgHA/TiO₂) composite coating in a single step on commercially pure titanium (Cp Ti) by plasma electrolytic processing (PEP) technique. For this purpose 2.5 wt% silver substituted hydroxyapatite (AgHA) nanoparticles were prepared by microwave processing technique and were characterized by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy and transmission electron microscopy (TEM) methods. The as-synthesized AgHA particles with particle length ranging from 60 to 70 nm and width ranging from 15 to 20 nm were used for the subsequent development of coating on Cp Ti. The PEP treated Cp Ti showed both titania and AgHA in its coating and exhibited an improved corrosion resistance in 7.4 pH simulated body fluid (SBF) and 4.5 pH osteoclast bioresorbable conditions compared to untreated Cp Ti. The in vitro bioactivity test conducted under Kokubo SBF conditions indicated an enhanced apatite forming ability of PEP treated Cp Ti surface compared to that of the untreated Cp Ti. The Kirby-Bauer disc diffusion method or antibiotic sensitivity test conducted with the test organisms of Escherichia coli (E. coli) for 24 h showed a significant zone of inhibition for PEP treated Cp Ti compared to untreated Cp Ti.     

Evaluation of an environmentally friendly anticorrosive pigment for alkyd primer

An alkyd formulation containing zinc phosphate (10 wt.%) was prepared and subsequently modified replacing the latter anticorrosive additive by a very low concentration of conducting polymer. Specifically, three modified paints, which contain polyaniline emeraldine base (undoped form), polyaniline emeraldine salt (doped form) and an eco-friendly polythiophene derivative (partially oxidized), were formulated. The properties and corrosion resistance of the four alkyd coatings have been characterized. Among the three modified paints, the one containing polythiophene shows the best adherence and the highest corrosion resistance. This has been attributed to the fact that the miscibility of the polythiophene derivative with the alkyd formulation is better than that of polyaniline. Furthermore, accelerated corrosion assays and electrochemical impedance spectroscopy measurements revealed that the corrosion resistance of the paint with polythiophene is several orders of magnitude higher than that with zinc phosphate. The polythiophene derivative has been found to induce the formation of a passivating and well-adhered layer between the coating and the surface, preventing the access of chloride anions and oxygen to the substrate.     

Synthesis and electrochemical evaluation of polypyrrole coatings electrodeposited onto AA-2024 alloy

Polypyrrole coating was successfully deposited on anodized 2024 unclad aluminum alloy showing that the presence of the anodic film is the key factor to ensure better adhesion to the polymer coating. Corrosion resistance of the conductive polymer layers grown for three and five cycles were evaluated in a 3 wt.% NaCl solution using polarization curves and impedance spectroscopy. It was found that the thermally treated polymer coating with higher thickness (five cycles) exhibited the best corrosion performance. This best coating shifts the corrosion potential towards nobler values, about 650 mV, and the exchange current density decreases two orders of magnitude regarding the anodic layer. The partial reduction of the structure of the polymer promoted by the thermal treatment plays a key role in the corrosion resistance of the coating allowing to the thermally treated polymer layer to act as a physical barrier against corrosion.     

Hydrothermal nucleation of hydroxyapatite on titanium surface

Titanium plates were submitted to nucleation and growth of hydroxyapatite (HAp) under hydrothermal conditions. A group of these plates were submitted to nucleation without any previous treatment and another group was treated with NaOH 1 M at 130°C inside an autoclave followed by heat-treatment at 600°C. The nucleation were performed by soaking all these titanium pieces, in a simulated body fluid (SBF) solution, containing calcium, phosphate and other ions, in order to promote the nucleation and growth of hydroxyapatite under hydrothermal conditions on the titanium surface. The results show that hydrothermal nucleation and growth of HAp occurs even on the non-treated titanium surface at 150°C. However, a better uniformity of the hydroxyapatite layer was observed on the pre-treated titanium surface, at 80°C, with the renewal of the SBF solution.     

Modification of titanium oxide layer by calcium and phosphorus

The aim of the study was to investigate the possibility of calcium and phosphorus ion implantation into an oxide film applied onto titanium during anodic passivation. The corrosion resistance of modified titanium in Tyrode's physiological solution has been identified. Anodic oxidation was carried out in two solutions. The first contained 20 g dm⁻³ NaH₂PO₂ in 4.3 M H₃PO₄ (K1), whereas the other, 20 g dm⁻³ Ca(H₂PO₂)₂ in 4.3 M H₃PO₄ (K2). Voltage of 100 and 150 V was applied. It has been found out that it is possible to incorporate Ca and P into the emerging passive layer. The application of the voltage of 150 V makes it very porous. It has been also demonstrated that titanium so modified presents higher resistance to corrosion in the investigated environment than titanium not modified in Tyrode's solution.     

Low-Temperature Preparation of Anatase and Rutile Layers on Titanium Substrates and Their Ability To Induce in Vitro Apatite Deposition

Surface modification of titanium and its alloys to induce apatite deposition within a short period is of practical importance in clinical applications. In this study, titanium substrates were treated with hydrogen peroxide solutions at low temperatures to yield titania layers consisting of anatase and rutile. Those titania layers, regardless of the fraction of anatase and rutile, were bioactive to induce deposition of apatite in Kokubo's simulated body fluid within 24 h. The bioactivity was attributed to both the epitaxial effect and the abundant Ti–OH group of the titania layer.     

Optimization of oxidative desulfurization of thiophene using Cu/titanium silicate-1 by box-behnken design

The oxidative desulfurization of thiophene in a synthetic mixture of thiophene and iso-octane was investigated with copper loaded titanium silicate-1 (TS-1) catalyst in presence of hydrogen peroxide as oxidising agent and the conversion was enhanced by 22% at 240 min on addition of 1.05 wt.% copper in TS-1. The optimal design of experiments using box-behnken method was employed to evaluate the effects of individual process variables such as, reaction temperature, amount of catalyst and moles of hydrogen peroxide per mole of thiophene and their optimum values were found to be 70 °C, 0.45 g (22.5 mol/L of iso-octane) and 19.9 mol (in 20 ml of iso-octane), respectively, to achieve a conversion of 93%. The influence of mass transfer effects on the desulfurization reaction was minimized by selecting proper degree of agitation and catalyst size. An empirical kinetic model was used to interpret the rate data. The apparent activation energy was found to be 28.67 kJ/mol.     

Bioactive and electrochemical characterization of TiO2 nanotubes on titanium via anodic oxidation

The selection of bioactive and electrochemically stable materials for implants having effective corrosion resistance during long-term use in the body is essential. In this study, the bioactive and electrochemical properties of titanium implant materials with a nanotube surface treatment and various types of post-treatments were examined. Two types of amorphous TiO₂ nanotubes were grown homogeneously on the surface: one with a larger diameter (approximately 85 nm) and one with a smaller diameter (approximately 50 nm). Amorphous TiO₂ nanotubes were partially crystallized to anatase and rutile by heat treatment at 500 °C for 2 h. The corrosion potential (E_corr) of the heat-treated sample (HT) had a novel value of 0.102 V due to the stable TiO₂ crystal phase compared to the −0.106 V observed in the anodic oxidation sample (AN). The corrosion current density (I_corr) ranged from 0.20 to 0.64 μA/cm² according to the post-treatment conditions. However, at 0.6 V, where a passive layer had formed, the corrosion resistance of the HT was approximately ten times that of the AN and untreated (UT) samples. After evaluating the hydroxyapatite (HA)-forming ability by immersion in a simulated body fluid (SBF) solution, the CP process induced the adsorption of Ca and P onto HT. A comparison of the time-dependent amount of Ca and P adsorption showed that Ca adsorption plays a role in determining the rate at which hydroxyapatite (HA) is formed. For the induction of HA formation, a level of Ca adsorption above a critical level is required.     

Diesel fuel desulfurization with hydrogen peroxide promoted by formic acid and catalyzed by activated carbon

Desulfurization of diesel fuels with hydrogen peroxide was studied using activated carbons as the catalysts. Adsorption and catalytic properties of activated carbons for dibenzothiophene (DBT) were investigated. The higher the adsorption capacity of the carbons is, the better the catalytic performance in the oxidation of DBT is. The effect of aqueous pH on the catalytic activities of the activated carbons was also investigated. Oxidation of DBT is enhanced when the aqueous pH is less than 2, and addition of formic acid can promote the oxidation. The effect of carbon surface chemistry on DBT adsorption and catalytic activity was also investigated. Adsorption of DBT shows a strong dependence on carboxylic group content. The oxidative removal of DBT increases as the surface carbonyl group content increases. Oxidative desulfurization of a commercial diesel fuel (sulfur content, 800 wt. ppm) with hydrogen peroxide was investigated in the presence of activated carbon and formic acid. Much lower residual sulfur content (142 wt. ppm) was found in the oxidized oil after the oxidation by using the hydrogen peroxide-activated carbon-formic acid system, compared with a hydrogen peroxide-formic acid system. The resulting oil contained 16 wt. ppm of sulfur after activated carbon adsorption without any negative effects in the fuel quality, and 98% of sulfur could be removed from the diesel oil with 96.5% of oil recovery. Activated carbon has high catalytic activity and can be repeatedly used following simple water washing, with little change in catalytic performance after three regeneration cycles.     

Screening study of spray solution parameters for depositing cerium-based conversion coatings on Al alloy 2024-T3

Cerium-based conversion coatings were deposited on aluminum alloy 2024-T3 by a spray process using a solution containing cerium chloride, hydrogen peroxide, and gelatin. As deposited coatings were composed of hydrated cerium oxide and were post-treated in a phosphate solution to improve corrosion performance. Coating solution parameters, including the pH (1–2.5), cerium chloride concentration (0.025–0.125 M), and hydrogen peroxide content (0–1.2 M), were varied to investigate the effect(s) of solution parameters on the corrosion performance of the post-treated coatings. Results indicated that thickness of coatings deposited from solutions with different pH values were similar, while coating thickness increased with increasing concentration of cerium chloride and hydrogen peroxide in the solutions. Electrochemical impedance spectroscopy and observations of the surface appearances of the coatings indicated that coatings deposited from solutions with a pH 2, a cerium concentration of 0.1 M, and a hydrogen peroxide concentration of 0.8 M exhibited the best corrosion resistance.     

Electrochemical behavior of organic and inorganic complexes of Zn(II) as corrosion inhibitors for mild steel: Solution phase study

This work intends to study inhibitive performance of organic and inorganic complexes of Zn(II) using electrochemical techniques along with surface analysis. In this regard, inorganic zinc aluminum polyphosphate pigment as modified zinc phosphate and zinc acetylacetonate and benzimidazole mixture representing organic replacement of zinc phosphate were employed. Through taking advantage of electrochemical impedance spectroscopy and DC polarization, two mentioned approaches were indicated to be efficient. Charge transfer resistance and corrosion current density values exhibited superiority of zinc aluminum polyphosphate and mixture of zinc acetylacetonate and benzimidazole compared to zinc phosphate and also zinc acetylacetonate and benzimidazole as individual inhibitors. Corrosion inhibition efficiencies calculated based on charge transfer resistance in consistent with those calculated from corrosion current density showed the following sequence; zinc aluminum polyphosphate > mixture of zinc acetylacetonate and benzimidazole > zinc acetylacetonate > zinc phosphate > benzimidazole. Showing film formation, surface analysis SEM/EDX confirmed the results obtained by electrochemical methods.