Characterisation of anodic films formed on titanium and its alloys

The properties of the anodic oxide films formed on titanium and its implant alloys Ti‐5Al‐4V and Ti‐6Al‐4Fe are investigated in this paper. Anodic oxide films were prepared by electrochemical treatment in 3M sulphuric acid solution at 60 V for 1 min, followed by a thermal treatment consisting of heating at 500°C for 24 h and then cooling in water. Electrochemical impedance spectroscopy (EIS) measurements were carried out at open circuit potential. Nyquist and Bode spectra exhibit a two‐time constant system, attesting an oxide with two layers: a barrier inner oxide and a porous outer one. All spectra change in time, indicating a thickening of the films. Open circuit potentials were monitored (long‐term, 25,000 exposure hours) in Ringer 2 solution of different pH values (6.98, 4.35, 2.5) for titanium, Ti5Al‐4V and Ti‐6Al‐4Fe alloy but without treatment with the purpose to compare the biomaterial behaviour. It was pointed out some variations of the open circuit potentials around electropositive values, suggesting slow dissolution, re‐passivation or adsorption processes.     

On the stability of anodic oxide films formed on molybdenum in various aqueous solutions

The dissolution behaviour of the anodically formed oxide films on molybdenum was studied in normal solutions of H₂SO₄, H₃PO₄, HCl, Na₂SO₄ and NaOH using impedance and potential measurements. The effect of the different parameters, e.g. the formation voltage and the pH of the test solution, on the stability of the oxide film was also studied.

The results reveal that the oxide film is highly stable in all solutions, regardless of the solution pH or the formation voltage. The stability of the oxide film was attributed to its compact and non-defective structure. The potential of the anodized electrode did not vary with time and was found to be independent of the formation voltage or the pH of the solution, which reflects its high insulating properties. Furthermore, the electrode impedance is purely capacitive in all measurements.     

Surface characteristics and bioactivity of oxide films formed by anodic spark oxidation on titanium in different electrolytes

Porous oxide films were fabricated on commercially pure titanium (CP-Ti) using an anodic spark oxidation technique with different electrolytes, 1 M H₂SO₄, 1 M H₃PO₄, and 1 M CH₃COOH. The micro-morphology, surface roughness, and crystalline structure were evaluated by scanning electron microscopy, profilometry, and X-ray diffraction, respectively. The chemical composition and binding state of the specimen groups were evaluated by X-ray photoelectron spectroscopy (XPS). TiO₂ films were observed on the specimens anodized in the acetic acid and sulfuric acid electrolytes. However, a TiP₂O₇ film was mainly observed on the specimen anodized in the phosphoric acid electrolyte. The dominant Ti⁴⁺ peaks for all sample groups and the additional Ti³⁺ peaks for the groups anodized in the acetic and phosphoric acid electrolytes were detected by high-resolution XPS. The effects of the surface characteristics of the specimens on the bioactivity were examined using an immersion test in a minimum essential medium (MEM) solution. There was a higher level of calcium formed on the anodized specimens than on the as-received titanium while there was no significant difference in the calcium content between the anodized specimen groups. Fourier transform infrared spectroscopy showed a different content of chemical function groups in the deposits formed in MEM according to the type of electrolyte used. These results were attributed to the different surface chemical states of the oxide films.     

Gel-like layer development during formation of thin anodic films on titanium in phosphoric acid solutions

Anodising of commercial pure titanium in phosphoric acid solutions at different concentrations (0.5-4 M) have been investigated using galvanostatic and potentiodynamic polarisation techniques. Under galvanostatic conditions at low current densities (0.1-0.6 A m⁻²), the chrono-potentiometric curves always show a linear section in the early stage of the process. The slope of the linear section, dE/dt, decreases initially and then increases with increasing H₃PO₄ concentration, with the minimum slope at a concentration, ∼2 M. Under potentiodynamic conditions at this concentration the samples exhibit a different anodic behaviour. The development of gel-like layer during formation of thin anodic films on titanium in H₃PO₄ solutions is proposed to account for experimental observations.     

Luminescence from band-gap photo-excitation of titanium anodic oxide films

Luminescence light from the anodic thin oxide films formed on titanium in neutral phosphate solution was measured by photo-excitation of ultra-violet light irradiation with 3.82 eV energy. The luminescence light is observed with a peak of about 3 eV energy that corresponds to the band-gap energy of the n-type semiconductive TiO₂ oxide. It can therefore be presumed that the luminescence is induced by de-excitation or recombination between electrons in the conduction band and holes in the valence band. The peak wavelength of the luminescence light changes with the formation potential of the oxide film from 409 nm at 1 V to 425 nm at 6 V. The shift of the peak may reflect an amorphous-crystalline transition with increase of potential.     

Photoelectrochemical properties of oxide films formed by anode plasma electrolytic oxidation on titanium in water solutions

It is shown that films of titanium dioxide obtained by anodic plasma electrolytic processing of commercial titanium in an aqueous solution of ammonium chloride are photosensitive. Incident photon to charge carrier efficiency (IPCE) in the near ultraviolet (366 nm) reached 8.5%. Magnitude of the photocurrent generated by the films obtained under optimum processing conditions decreases no more than 20% at the transition from the ultraviolet to the mixed light.     

Characterization of oxide films formed on magnesium alloys using bipolar pulse microarc oxidation in phosphate solutions

The surface morphology and chemical composition of the oxide films formed on pure magnesium and AZ91D alloy in aqueous electrolytes which contained sodium hexafluorinealuminate(Na3 AlF6 ), potassium hydroxide (KOH), sodium hexametahposphate ((NaPO3)6 ), and triethanolamine were investigated by X-ray diffraction (XRD), scanning electron microscope(SEM) and energy dispersive spectroscopy(EDX). The results show that the input of the negative pulse has great influences on the quantity and the appearance of the microdischarges. Three types of pores can be distinguished on the surface of the oxide film and their size ranges are 0.5- 1μm, 1 - 2μm and 4- 7μm, respectively. A few microcracks are seen around the large pores. There exists a remarkable fluorideenriched zone of about 4 - 6μm for pure magnesium and 3 - 5μm for AZ91D alloy at the coating/substrate interface.     

The anodic behaviour of copper in static and flowing hydrochloric acid solutions

The dissolution of copper in 0·36 to 3·6 HCl was studied both in static and in flowing solutions using flow rates between 0·098 and 1·14 ms⁻¹, all in the region of laminar flow.Steady state anode potential current curves, potentiodynamic sweep and potentiostatic pulse techniques and impedance measurements, in the range of 0·02–7 kHz, were used. In both static and flowing solutions dissolution of copper occurs to a monovalent state, as chloro-complexes CuCl⁻₂ and CuCl²⁻ with exchange currents for the two reactions of 1·9 × 10⁻⁵ A mm⁻² and 8 × 10⁻⁷ Amm⁻² respectively in 1·44 MHCl. The cuprous chloride layer first forms a monolayer and subsequently grows to considerable thicknesses. The double layer capacity is constant at 0·17 ± 0·03 μFmm⁻² at potentials below multilayer formation and this is interpreted as implying that there is no specific adsorption of chloride ions prior to formation of the cuprous chloride layer. As the flow rate increases the film becomes thinner so delaying the formation of the film of critical thickness required for passivation.     

Influence of the nature of alloying elements on the adherence of oxide films formed on titanium alloys

In order to study the influence of aluminum, chromium, and silicon on the adherence of oxide layers to the metallic substrate for binary alloys of titanium, adhesion measurements were carried out and the results were correlated with the oxidation behavior of these materials. Ti-Al, Ti-Cr, and Ti-Si alloys were prepared and oxidized at temperatures of 550–700°C in air and in oxygen, for oxidation times from a few hundred to several thousand hours. The reaction kinetics were followed using continuous thermogravity or daily weighing of samples. For Ti-Al and Ti-Si alloys, a slight decrease in the adherence of oxide layers to the substrate was observed for the lowest aluminum contents (1.65 and 3%) and silicon content (0.25%) as compared with the behavior of unalloyed titanium; at higher levels, the adherence increased with the alloying element content. For Ti-Cr alloys, the addition of chromium increased the adhesion of oxide layers to the substrate for all cases; however, the change in adhesion with chromium content was not monotonic.     

Optical properties of anodic aluminum oxide films on Al1050 alloys

In this study the reflectance and nano-structure of anodized aluminum oxide (AAO) films that formed on Al1050 alloys in a 15% w/w sulphuric acid solution were examined. It was found that AAO films that formed under a high bath temperature and/or low anodic current density had a high pore density (count dm^− 2). The reflectance of AAO films formed at different anodizing times is a function of their thickness. The surface reflectance spectra showed interference when the AAO films exceeded a critical thickness (~ 100 nm). The reflectance curves oscillated and were affected by the pore density. We calculated the refractive index (n) and extinction coefficient (k) of the films based on the measured film thickness and reflectance. We can see from the experimental results that reflectance decreased with increasing thickness, while increasing pore density led to an increased refractive index but decreased extinction coefficient.     

Corrosion behaviour in physiological fluids of surface films formed on titanium alloys

The properties of the surface films formed on Ti and its Ti–5Al–4V and Ti–6Al–3.5Fe alloys by chemical (immersion in 10 M NaOH) and thermal treatment (500 °C) were studied in this paper. Their corrosion behaviour in Ringer solution was investigated. After treatments, the sample microhardness was measured. Also, the sample surfaces were observed by an optical microscope before and after immersion in Ringer solutions. The electrochemical behaviour of the surface films obtained by the chemical and thermal treatments was studied using the methods of the electrochemical impedance spectroscopy (EIS) and monitoring of open circuit potentials at different immersion periods (up to 60 days) in Ringer solution. Microhardness increased with the increasing loads for the studied materials, showing the existence of a compact protective layer. Microscopic observations exhibited more compact coatings after 60 days of exposure in Ringer solution, due to the formation of an apatite layer. EIS spectra revealed surface films with two layers: an inner, barrier protective layer and an outer, porous apatite layer. The impedance increases in time denoting that the films have grown by apatite nucleation. Also, EIS spectra showed that the complex treatment by chemical + heat method is the most efficiently. For the samples treated by the two processes (chemical + heat) the values of the open circuit potentials are nobler than of the chemical treated samples, denoting better protective, bioactive films.     

Stability of protective oxide films formed on a porous titanium

Porous titanium has mechanical advantages over its solid counterpart, especially for use in surgical implants, however, its corrosion resistance is not as good. The relative stability of various oxide films on both porous and solid titanium was thus investigates. The point of initial failure of the oxide films on porous titanium was found to be within the depths of the pores. It is also postulated that galvanic coupling to the outer surface, via a mechanism akin to crevice corrosion, may promote the rapid dissolution in the pores. Preliminary, results suggest that thermal oxides offer better corrosion protection to porous titanium than anodic oxides.     

Inhibition of Corrosion resistant alloys in hot hydrochloric acid solutions

Weight loss measurements were performed in 20% and 28% hydrochloric acid at 90 °C on carbon steel, 22Cr5Ni duplex stainless steel, and two superaustenitic steels – 27Cr31Ni3Mo and 19Cr25Ni4Mo –, in presence of organic substances used singly or mixed. The organic substances examined were quaternary ammonium salts (1–4-pyridyl)-pyridinium chloride hydrochloride, dodecylpy-ridiniumchloride, benzyldimethylstearylammonium chloride, and (dodecyltrimethylammonium bromide), alkynols (1-octyn-3-ol, propargyl alcohol) and trans-cinnamaldehyde. The synergistic effect of potassium iodide on the inhibitive efficiency of the organic substances was studied. The variations of corrosion rate during the test time (normally six hours) were recorded by means of polarization resistance measurements. Polarization curves were also recorded. The results showed that the corrosion rates of the four steels examined can be reduced to less than 1 mg/cm² · h using ternary inhibitor mixtures containing quaternary ammonium salts, trans-cinnamaldehyde and potassium iodide (0.2% of each component). Electrochemical tests gave useful indications on the action mechanism of the inhibitors studied.     

Electronic structure of anodic oxide films formed on cobalt by cyclic voltammetry

The chemical composition and degrees of oxidation of thin oxides films formed on cobalt by cyclic voltammetry have been investigated by Auger electron spectroscopy and XPS analysis. In addition the electronic properties of the films have been examined by capacitance measurements using the Mott-Schottky method and photoelectrochemistry. The analytical results show that the thickness of the cobalt oxide films increases with the number of cycles and varies from a few tens to a few hundreds of angströms. When observed by transmission electron microscopy and diffraction, the films appear compact and well crystallised (spinel structure). Capacitance measurements show that both very thin and relatively thick films exhibit p-type semiconductivity. The band structure model proposed and the interpretation of the oxidation processes in terms of lattice ionic defects, can explain the film growth mechanism. The study shows how the electric fields created by the development of space charges influence both ionic transport and electronic transfer of charges.     

Semiconductive properties of titanium anodic oxide films in McIlvaine buffer solution

The semiconductive properties of anodic oxide films grown on titanium surface at different formation potentials: 1.0, 2.0, 3.0 and 4.0 V were investigated by means of electrochemical impedance spectroscopy and cyclic voltammetry in McIlvaine buffers at pH 2, 4 and 5. The passive films show a Mott-Schottky behavior typical for an n-type semiconductor at the studied potential range with a high concentration of donors. On increasing the film formation potential, the flat band potential and the donor density decrease. Further, lower flat band potentials were obtained for higher pH buffer. These experimental results were related to the film thickness and composition. The influence of film thickness on the oxidation reactions taking place at the titanium electrodes covered by oxide film was evaluated.     

3种溶液体系下铝合金阳极氧化膜的性能

通过表面形貌观察、升压曲线测试、电化学阻抗谱(EIS)测试和疲劳性能测试等研究LY12铝合金铬酸阳极氧化(CAA)、硼酸硫酸阳极氧化(BSAA)和己二酸硫酸阳极氧化(ASAA)膜的性能。结果表明:CAA膜和ASAA膜缺陷较少且缺陷边缘光滑,BSAA膜缺陷数目较多且边缘较为粗糙。3种氧化膜的耐蚀性良好,在5%NaCl(质量分数)溶液中浸泡80 d后,均未出现明显的破坏。对比氧化膜的壁垒层阻抗(Rb)发现,BSAA的Rb值下降最大,约为94%,CAA和ASAA优于前者,其阻抗分别下降了75%和78%。3种阳极氧化技术对基体疲劳性能的影响结果表明,CAA处理降低基体的疲劳寿命值最小,约为14%;ASAA次之,约为20%;BSAA对基本疲劳性能的影响最大,约25%。     

阳极氧化工艺对6061铝合金草酸氧化膜耐磨耐蚀性的影响

为改善6061铝合金的表面耐磨性能和耐蚀性能,利用阳极氧化技术在草酸电解液中制备了阳极氧化膜。采用显微硬度仪测试了膜层的显微硬度,利用摩擦磨损试验机(CSEM)研究阳极氧化膜的摩擦性能,利用电化学阻抗谱(EIS)分析氧化膜的抗腐蚀性能。研究了阳极氧化电流密度、电解液温度以及阳极氧化时间对氧化膜的硬度、耐磨性及耐蚀性的影响规律,给出综合性能较好的阳极氧化工艺参数:电解液为40 g·L-1的草酸,阳极氧化温度为10℃,阳极氧化电流密度为1.0 A·dm-2,阳极氧化时间为120 min。     

Photoacoustic study on cathodic reduction of anodic oxide films formed on copper in borate solution

An in-situ photoacoustic (PAS) technique, using a piezoelectric detector with high sensitivity was applied to the study on duplex oxide films anodically formed on copper in pH 8.4 borate solution. The PAS signals from the copper electrode were produced by an irradiation of light beam with a wavelength of 514.5 nm. The PAS amplitude during cathodic reduction of the outer oxide layer to Cu₂O changed in the opposite direction, depending on the anodic potential of film formation and oxidation time. Assuming that the change in PAS amplitude is proportional to both optical absorption coefficient and film thickness, it was deduced from comparison of the estimated absorption coefficients for Cu (OH)₂, CuO and CuO_0.67 films that dehydration of the outer layer having an average composition of CuO_x (OH)_2?2x proceeded with increasing anodic potential of film formation and oxidation time during growth of the duplex oxide film. Moreover, it was found that the change in PAS amplitude during cathodic reduction of the total Cu₂O film involving the inner layer to metallic copper was proportional to the electric charge required for cathodic reduction, i.e., the film thickness, irrespective of anodic potential of film formation and oxidation time, which proved the validity of the above assumption.     

Characterisation of unsealed anodic oxide layers on aluminium

The inhibiting effect of unsealed anodic oxide layers on filiform corrosion (FFC) was studied. This was done by means of impedance measurements, polarisation measurements and accelerated corrosion tests (Lockheed test). The electrochemical measurements indicate that both the porous layer and the barrier layer contribute to an enhanced corrosion resistance. In the Lockheed test, the specimens with a porous layer, did not show FFC. The specimens with a barrier layer revealed first a decrease of the number of filaments but after a certain point it increased. In contrast to this, the corroded area of the filaments showed a gradual decrease with increasing barrier layer thickness.