Influence of fluoride concentration and pH on corrosion behavior of titanium in artificial saliva

Titanium (Ti) exhibits excellent corrosion resistance in most aqueous media due to the formation of a stable oxide film and is chosen for surgical and odontological implants for this resistance and its biocompatibility. Treatment with fluorides (F) is the main method to prevent plaque formation and dental caries. Toothpastes, mouthwashes and prophylactic gels can contain from 200 to 20,000 ppm F and have neutral to acidic character, which can affect the corrosion behavior of titanium devices present in the oral cavity. In this work the behavior of Ti has been evaluated in artificial saliva of pH 2, 5 and 7 and different fluoride concentrations (0, 1,000, 5,000 and 10,000 ppm F), through open-circuit potential measurements, potentiodynamic polarization and electrochemical impedance spectroscopy. Limits of pH value and fluoride concentration at which Ti corrosion behavior changed could be established. Active behavior was observed for pH 2 and 1,000–10,000 ppm F, and for pH 5 and 5,000 and 10,000 ppm F. The other conditions led to passive behavior. Decrease in corrosion resistance and less tendency for passivation were observed as fluoride concentration increased and pH decreased.     

Electrochemical behavior of palladium-gold alloys

The behavior of Pd-Au alloys, prepared by electrochemical codeposition, has been studied in acidic solutions (1 M H₂SO₄) using mainly the cyclic voltammetry technique. Morphology of the alloy surface and bulk compositions were examined by SEM/EDAX method. Various types of voltametric behavior during potential cycling in the oxygen region are presented. The influence of hydrogen absorption on electrochemical properties of surface oxides is demonstrated. The problem of the nature of oxygen electrochemisorbed on Pd-Au alloys is discussed.     

Electrochemical behavior of non-precious dental alloys in bleaching agents

Corrosion behavior of dental alloys in artificial saliva containing different concentrations of hydrogen peroxide, carbamide peroxide and fluoride ions was studied using electrochemical technique. The alloys have been arranged according to its corrosion resistance in the hydrogen peroxide and carbamide peroxide as follows: wiron99 > wirolloy > wironit. Alloys suffer less corrosion rates in presence of carbamide peroxide than hydrogen peroxide. The effect of CH₆N₂O₃ on the corrosion is less than the effect of H₂O₂ and this is due to the presence of centers rich by electrons such as nitrogen and oxygen in CH₆N₂O₃. Nitrogen and oxygen adsorbed on the alloy surface forming protective layer isolate it relatively from the corrosive medium. The order of increasing resistivity of the alloys in fluoride solution is as follows: wironit > wiron99 > wirolloy. SEM for the three alloys in artificial saliva containing 0.015 M F⁻ shows that wironit has less corrosion compared to the two other alloys and this agrees with the results obtained from potentiodynamic polarization curves and Nyquist impedance diagrams.     

Electrochemical behavior of magnesium alloys as biodegradable materials in Hank's solution

The electrochemical behavior of extruded AZ31E and AZ91E alloys was investigated in Hank's solution at 37 °C. The behavior of the two alloys was studied with immersion time using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and weight loss tests. It was found that the corrosion resistance of AZ31E alloy is higher than that of AZ91E. Also, the effect of adding different concentrations of a commercial drug called glucosamine sulphate (as inhibitor) to Hank's solution was studied for AZ31E alloy. The corrosion was effectively inhibited by the addition of 0.01 mM glucosamine sulphate that reacts with AZ31E alloy and forms a protective film on its surface. The results were confirmed by surface examination via scanning electron microscope.     

Electrochemical corrosion behavior of a Ti-35Nb alloy for medical prostheses

Since the 1980s, the titanium alloys show attractive properties for biomedical applications where the most important factors are, firstly, biocompatibility, corrosion and mechanical resistances, low modulus of elasticity, very good strength to weight ratio, reasonable formability and osseointegration. The aim of this study was to investigate the effects of two different heat treatments; furnace cooling and water quenching, on the general electrochemical corrosion resistance of Ti-35 wt%Nb alloy samples immersed in a 0.9% NaCl (0.15 mol L⁻¹) solution at 25 °C and neutral pH range. The samples were obtained using a non-consumable tungsten electrode furnace with a water-cooled copper hearth under argon atmosphere. The microstructural pattern was examined by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). In order to evaluate the electrochemical corrosion behavior of such Ti-Nb alloy samples, corrosion tests were performed by using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves. Analyses of an equivalent circuit have also been used to provide quantitative support for the discussions and understanding of the corrosion behavior. It was found that water quenching provides a microstructural pattern consisting of an alpha-martensite acicular phase which decreases the material electrochemical performance due to the stress-induced martensitic transformation.     

Repassivation behavior of titanium in artificial saliva investigated with a photon rupture method

Repassivation kinetics of titanium were investigated in artificial saliva by the photon rupture method, PRM, combined with anodizing. PRM uses focused pulsed Nd–YAG laser beam irradiation to remove oxide film and metal. The effects of the applied potential and F⁻ ions on the repassivation kinetics of titanium were examined electrochemically. All conditions in this study use specimens that were repassivated within about 100 ms after the laser beam irradiation. The current transients after the laser beam irradiation showed a rapid increase, followed by a decrease with a slope of the log i vs. log t plots of about −1.5. The measured slope is steeper than what would be expected with the high field oxide film formation theory. This result suggests that the oxidation of titanium takes place through a combination of electrochemical and chemical reactions. More NaF results in a higher peak current density and amount of charge after laser beam irradiation. The influence of F⁻ ions on the repassivation kinetics may be explained by localized pH changes caused by dissolution of titanium immediately after the laser beam irradiation.     

Electrochemical properties of aluminium alloys containing indium,gallium and thallium

The electrochemical behaviour of aluminium alloyed with small amounts (up to 0.2%) of indium, gallium and thallium has been investigated. It has been found that these additions result in:

1. A considerable shift of the rest potential in the negative direction (to 1.4–1.7 V versus SCE),
2. A significant increase of the passivating current density (up to 0.1–1 Acm^?2) and
3. A considerable decrease in the negative difference effect (increase of the faradaic efficiency to

99.5%) compared to the behaviour of pure aluminium. Except for the gallium alloy, the rate of corrosion of the alloys in neutral salt solutions is also decreased compared to that of pure aluminium. A ternary alloy, Al-0.01 In-0.01 Ga, exhibited a more negative rest potential than the Al-In alloy and a corrosion stability superior to that of the Al-Ga alloy. The negative difference effect was found to depend on the cation of the neutral salt in solution. The lowest effect was obtained in ammonium chloride solutions.     

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.     

Electrochemical behavior of pyrite in sulfuric acid solutions containing silver ions

A systematic electrochemical study of pyrite in H₂SO₄ solutions containing dissolved silver was undertaken to gain more information about the transfer of silver ions to pyrite and their role in enhancing the direct oxidation of pyrite. The results of cyclic voltammetry experiments provide additional evidence of the formation of metallic silver on the FeS₂ surface under open-circuit conditions. A pyrite electrode held at the open-circuit potential for 2 h in the presence of 10^–3 m Ag⁺ exhibits a large and sharp anodic peak at about 0.7V. The current associated with this peak is the result of the dissolution of metallic silver deposited during the initial conditioning period. There is no evidence of silver deposition without preconditioning until the potential drops below about 0.6V for Ag⁺ concentrations ranging from 10^–4 to 10^–2 m. However, subsequent silver deposition appears to be very sensitive to the dissolved silver concentration in this range. There is also evidence that the state of the pyrite surface has a pronounced influence on its interaction with silver ions. Agitation has also been found to have a significant effect on the electrochemistry of the Ag–FeS₂ system.     

Electrochemical behaviour of titanium in fluoride-containing saliva

The effect of fluoride on the electrochemical behaviour of titanium was studied. Open circuit potentials, breakdown potentials (E _b) and potentiostatic transient currents were measured in synthetic salivas of different compositions. Optical and scanning electron microscopic observations were also made. Results show that the growth rate of Ti oxide layer is affected by fluoride anions and tensile stresses are developed. The OCP/time relationship of Ti immersed in salivas A and B obeys a logarithmic law which depends on the saliva composition. The E _b value is influenced by the thickness of the oxide layer, by the composition of the saliva (including fluoride concentration), and by the technique utilised for its evaluation. Thus, results reported in the literature, which seem to be contradictory, could be explained taking into account the experimental conditions assayed. A careful control of the titanium-containing dental materials should be made after long treatments with fluoride-containing prophylactic products or when fluoride-releasing restorative materials are present in the vicinity.     

Crystallization behavior of poly(vinylidene fluoride) composites containing zinc phenylphosphonate

Poly(vinylidene fluoride) (PVDF) composites with different ratios of zinc phenylphosphonate (ZPP) were prepared by mixing in a kneader. The crystallization behavior of the composites from the melt was investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), and wide‐angle X‐ray diffraction (WAXD). The POM observations directly showed that nucleation of the PVDF crystal was promoted by adding ZPP. The data obtained from DSC measurements also indicated that the crystallization temperature of the composites became higher than that of pure PVDF. These results suggested that ZPP acted as a good nucleating agent for PVDF. Nonisothermal crystallization behavior was analyzed by Avrami equation adopted by Jeziorny. The results of the analysis were found to be consistent with the POM observations for the composites. WAXD analysis suggested that the PVDF crystal structure was influenced by ZPP addition. These results proved that as a nucleating agent, ZPP increased the crystallization rate and temperature of PVDF. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Electrochemical behavior of Mg and some Mg alloys in aqueous solutions of different pH

The electrochemical behavior of Mg, Mg-Al-Zn and Mg-Al-Zn-Mn alloys were investigated in aqueous acidic, neutral and basic solutions. Conventional electrochemical techniques such as open-circuit potential measurements, polarization methods and electrochemical impedance spectroscopy (EIS) were used. The results have shown that the rate of corrosion in acidic solution is relatively high compared to that in neutral or basic solutions. The presence of Al, Zn and Mn as alloying elements decreases the rate of corrosion of the alloy. The activation energy of the corrosion process occurring at the surface of Mg or Mg alloys in aqueous solutions is less than 40 kJ mol⁻¹. This value indicates a one electron transfer electrode as a rate controlling process. The impedance data were fitted to equivalent circuit models that explain the different electrochemical processes occurring at the electrode/electrolyte interface.     

Electrochemical behaviour of heat-treated AlZnMg alloys in chloride solutions containing sulphate

The electrochemical corrosion and passivation of Al5Zn1.7Mg0.23Cu0.053Nb alloys, submitted to different heat treatments (cold-rolled, annealed, quenched and aged, and quenched in two steps and aged), in sulphate-containing chloride solutions, has been studied by means of cyclic polarization, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and X-ray photoelectron spectroscopy (XPS). The cyclic polarization curves showed that sulphate addition to the chloride solution produced a poor reproducible shift of the breakdown potential to more positive potentials. The repassivation potentials, much more reproducible, and practically separating the passive from the pitting potential region, were slightly displaced in the negative direction with that addition. When the alloys were potentiodynamically polarized in the passive potential region, sulphate was incorporated in the oxide film, thus precluding chloride ingress. In addition, Zn depletion was favoured, whereas Mg losses were avoided. Different equivalent circuits corresponding to different alloys and potentials in the passive and pitting regions were employed to account for the electrochemical processes taking place in each condition. This work shows that sulphate makes these alloys more sensitive to corrosion, increasing the fracture properties of the surface layer and favouring the pitting attack over greater areas than chloride alone.     

Electrochemical behavior of centrifuged cast and heat treated Ti-Cu alloys for medical applications

The aim of this study was to evaluate the general electrochemical corrosion resistance of Ti-5, 7.1 and 15 wt.% Cu alloys with a view to medical applications. A centrifuged casting set-up and a solution heat treatment at 900 °C for 2 h were used to prepare the samples. Electrochemical impedance spectroscopy (EIS) and potentiodynamic anodic polarization techniques were used to analyze the corrosion resistance in a 0.15 M NaCl solution at 25 °C. An equivalent circuit analyses was also conducted. It was found that the corrosion rate increased with increasing Cu content. The results have shown that the addition of Cu has not stabilized the β phase. Martensite and Ti₂Cu intermetallic particles provided by casting and heat treatment processes, respectively, have important roles on the resulting impedance parameters and passive current densities of the Ti-Cu alloys.     

Electrochemical and thermal behavior of copper coated type MAG-20 natural graphite

Natural graphite powders, type MAG-20, were modified by electroless deposition of copper onto the graphite surface. The scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) of the modified graphite showed that almost all the copper covered the edge plane of graphite particles and the copper content increased from 6 to 17 at.% depending upon the plating time. Thermal gravimetric analysis (TGA) confirmed the copper composition obtained using EDS. TGA carried out under oxygen atmosphere showed that the combustion temperature decreased by 200 °C for a copper content from 0 to 16 at.%. Cycling the modified graphite at a C/10 rate showed that the initial large irreversible capacity typically present in untreated natural graphite was significantly reduced when the modified graphite contained 6-17 at.% copper coatings. A differential scanning calorimetry (DSC) comparison between fully lithiated copper coated MAG-20 with fully lithiated MCMB showed that the major exothermic peak for the MAG-20 modified graphite is lower in magnitude and is shifted to higher temperatures. However, both MAG-20 and MCMB types of lithiated graphite gave the same amount of total heat generation up to 350 °C.     

Passive behavior of magnesium alloys (Mg-Zr) containing rare-earth elements in alkaline media

The passive behavior of magnesium alloys ZK31, EZ33 and WE54 was studied in alkaline media (NaOH - pH 13) in the presence and absence of chloride ions. The electrochemical properties were investigated by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and capacitance measurements.X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed for the study of the chemical composition and surface morphology of the surface films, respectively.The electrochemical impedance results revealed that the film formed on the surface of the three alloys is characterized by an increasing resistance, which stabilized with time. In the absence of chloride the film resistance was identical for all the three alloys. However, in the presence of chloride, the resistance of the film formed on the EZ33 alloy dropped nearly one order of magnitude comparatively to the other alloys. Generally, in the presence of chloride there was a decrease of the conductive character of the film.The films are homogeneous and, according to the XPS results, the outer layer seemed mainly composed of Mg(OH)₂ and the internal layer composed of MgO, independently of the presence of chloride. The AFM study revealed that the presence of chloride affected film morphology, namely nano-crystallites dimensions and aggregates size that increased.     

Spectroscopic investigation of synergetic bioactivity behavior of some ternary borate glasses containing fluoride anions

Ternary borate glasses containing LiF, ZnF₂, NaF or CaF₂ were prepared by full replacement of silica by borate in patented Hench′s bioglass. Prepared samples were examined for their corrosion behavior with the expected final formation of fluoroapatite after immersion in SBF (simulated body fluid). Characterization of the glasses was carried by FTIR (Fourier transform infrared) absorption spectra before and after immersion. DAT (deconvolution analysis technique) was used to identify the formation of fluoroxyapatite from FTIR data after immersion in SBF. X-ray diffraction analyses were done for all samples to identify the crystalline phases that were formed after immersion in SBF and also to determine the degree of crystallinity for each sample. Also, scanning electron microscopic (SEM) investigations were carried out to examine the morphological changes of the surfaces upon immersion and the effects of different individual fluoride additives. The solubility testing for glassy samples was performed and the changes in the pH of the leaching solution were measured and evaluated.     

Electrochemical dissolution of hard metal alloys

Different components of hard metals such as TiC, TiN, pure Ti and the binder metals Fe, Co and Ni were investigated in neutral sodium nitrate solutions by cyclovoltammograms at current densities up to 35 A/cm². All combinations of TiN and TiC dissolve anodically. The surface of pure Ti is blocked by the anodic surface oxide which is not removed by cathodic current densities up to 30 A/cm². Cathodically all samples were conductive and showed a strong hydrogen evolution. The binder metals Fe, Co and Ni should be free of oxide films at cathodic potentials. Anodically, Co dissolves in an active state while Fe and Ni show a potential shift to the passive region. Our surface model developed for machining of Fe seems to be applicable to all other materials except pure Ti.     

Electrochemical preparation of amorphous Fe-P alloys

Optimum conditions have been established for the electrodeposition of amorphous fe-P alloys from sulphate electrolytes, containing complex-forming additives (glycine and oxalic acid) and sodium hypophosphite. It was shown that the increase of pH, current density and glycine content in the plating solution leads to a decrease of the amount of phosphorus in the Fe-P alloy. The cathodic current yield decreases with the increase of glycine concentration in the electrolyte. Small amounts of Cu²⁺ and Mn²⁺ act as brighteners. Sodium hypophosphite exerts a depolarizing effect on the alloy formation process. This effect is more pronounced at low hypophosphite concentrations and low cathodic current densities (CCD). The measurement of the cathodic potential during the deposition of the investigated alloy provides no evidence for a concentration change of phosphorus in the electrodeposited layers. No qualitative alterations of the surface morphology of the amorphous coatings studied have been established as the composition of the alloy and the plating conditions are changed.