Electrochemical Behavior of Passive Films Formed on the Surface of Coarse-,Fine-and Ultra-fine-Grained AA1050 Based on a Modified PDM

Electrochemical impedance spectroscopy(EIS) and Mott-Schottky analysis were carried out to evaluate the electrochemical behavior of the passive films formed on the surface of coarse-grained(CG),fine-grained(FG) and ultrafine-grained(UFG) 1050 Al alloy(AA1050) samples in alkaline media(pH value of 8.0) based on a modification of point defect model(PDM).The EIS results revealed that the polarization resistance increased from about 22.71-120.33 kΩ cm~2for UFG sample when compared to CG sample(annealed sample).The semiconductor properties of the passive films formed on CG,FG and UFG AA1050 samples in the test solution were investigated by employing Mott-Schottky analysis in conjunction with PDM.The results indicated that donor densities were in the range of 2.19 × 10~(21)-0.61 × 10~(21) cm~(-3)and decreased with grain refinement.Finally,all electrochemical tests showed that the electrochemical behavior of AA1050 alloy was improved by decreasing the grain size,mainly due to the formation of thicker and less defective oxide films.     

Corrosion Behavior of Pure Copper Surrounded by Hank’s Physiological Electrolyte at 310 K (37 °C) as a Potential Biomaterial for Contraception: An Analogy Drawn Between Micro- and Nano-grained Copper

This work aims to evaluate the corrosion behavior of pure copper from the microstructural viewpoint for a biomedical application, namely intrauterine devices. For this purpose, Tafel polarization and electrochemical impedance spectroscopy (EIS) techniques were used to evaluate the corrosion behavior of annealed pure copper (with the average grain size of 45 ± 1 µm) and nano-grained microstructure in physiological electrolyte of Hank at 310 K (37 °C). Pure copper in nanoscale grain size, typically an average of 90 ± 5 nm, was successfully made by eight-cycle accumulative roll bonding process at room temperature. On the basis of Tafel polarization results, it was revealed that nano-grained sample had lower corrosion current density and more noble corrosion potential for prolonged exposure in Hank’s physiological solution at 310 K (37 °C). In addition, the EIS results showed that the nano-grained sample had more corrosion resistance compared to the coarse-grained one for long-time immersion.     

The Passive Film Characteristics of Cold Deformed Pure Copper

In the present study, the effect of cold deformation on the electrochemical and passive behaviors of pure copper in 0.01 M NaOH solution was investigated. The dislocation density in cold deformation was calculated using a recently developed JAVA-based software, materials analysis using diffraction, based on Rietveld’s whole x-ray pattern fitting methodology. At the thickness reduction of 70%, the microhardness measured as 125.30 HV, which is 1.56 times than that in the annealed pure copper (80.25 HV). Potentiodynamic polarization plots and electrochemical impedance spectroscopy measurements showed that increasing the cold deformation offers better conditions for forming the passive films. In the Mott-Schottky analysis, no evidence for n-type behavior was obtained which indicates that the oxygen vacancies and the copper interstitials did not have any significant population density in the passive films. Also, this analysis revealed that with increasing cold deformation, the acceptor density of the passive films decreased.     

Dependence of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> coating thickness and annealing conditions on microstructural and electrochemical properties of LiCoO<Subscript>2</Subscript> film

We studied the dependence of Al₂O₃ coating thickness and annealing conditions on the surface morphology and electrochemical properties of Al₂O₃ coated LiCoO₂ films. The optimum coating thickness allowing for the highest capacity retention was about 24 nm. A sample consisting of Al₂O₃ coated on annealed LiCoO₂ film with additional annealing at 400 °C had a uniform coating layer between the coating materials and cathode films. This sample showed the best capacity retention of ∼91 % with a charge-cut off of 4.5 V after 30 cycles, while the bare cathode film showed a capacity retention of ∼32 % under the same conditions. The formation of second phases such as Co-Al-O was observed in the coating films by X-ray photoelectron spectroscopy (XPS). The Co-Al-O containing samples showed a higher initial capacity because of their smaller grain size, but less capacity retention than the Al₂O₃ containing samples.     

The role of Ce ions in electrochemical corrosion of 304 SS in Ce(NO<Subscript>3</Subscript>)<Subscript>3</Subscript>.6H<Subscript>2</Subscript>O

Effects of temperature and potential on the electrochemical corrosion behavior of alloy AISI 304 (UNS S30400) Stainless steel were investigated in 3 wt.% cerium nitrate (Ce[NO₃]₃.6H₂O) solution. With an increase in electrolyte temperature from ambient temperature to 90°C, the corrosion potential of the alloy shifted towards the noble direction, and the resistance to polarization increased due to the formation of Ce-oxide on the electrode surface. The oxide films formed at the open circuit potential (OCP) and a passive potential of 0.4 V_SCE were examined by x-ray photoelectron spectroscopy (XPS). The oxide film formed at 50°C and a passive potentialof 0.4 V_SCE consists of mixed oxides of Ce and Cr, whereas that at OCP consists of only Cr oxide. The formation of Cr oxides on the electrode surface was primarily due to the nitrate (NO₃ ⁻) ions in Ce(NO₃)₃.6H₂O electrolyte.     

Microstructural and electrochemical properties of Ti-doped Al<Subscript>2</Subscript>O<Subscript>3</Subscript> coated LiCoO<Subscript>2</Subscript> films

We studied the microstructural and electrochemical properties of Ti-doped Al₂O₃ (Ti-Al₂O₃) coated LiCoO₂ thin films depending on the Ti composition. The 1.27 at.% Ti-Al₂O₃ coated films had an amorphous structure with better conductivity than that of pure Al₂O₃ films. The Ti-Al₂O₃ coating layer effectively suppressed the dissolution of Co and the formation of lower Li conductivity SEI films at the interface between the LiCoO₂ film and electrolyte. The Ti-Al₂O₃ coating improved the cycling performance and capacity retention at high voltage (4.5 V) of the LiCoO₂ films. The Ti-Al₂O₃ coated LiCoO₂ films showed better electrochemical properties than did the pure Al₂O₃ coated LiCoO₂ films. These results were closely related to the enhanced Li-conductivity and interfacial quality of the Ti-Al₂O₃ film.     

Electrochemical properties of spinel compound LiNi_（0.5）Mn_（1.2）Ti_（0.3）O_4 as cathode materials for lithium ion batteries

The electrochemical properties of spinel compound LiNi0.5Mn1.2Ti0.3O4 were investigated in this study.The chemicals LiAc·2H2O,Mn(Ac)2·2H2O,Ni(Ac)2·4H2O,and Ti(OCH3)4 were used to synthesize LiNi0.5Mn1.2Ti0.3O4 by a simple sol-gel method.The discharge capacity of the sample reached 134 mAh/g at a current rate of 0.1C.The first and fifth cycle voltammogram almost overlapped,which showed that the prepared sample LiNi0.5Mn1.2Ti0.3O4 had excellent good cycle performance.There were two oxidation peaks at 4.21 V and 4.86 V,and two reduction peaks at 4.55 V and 3.88 V in the cycle voltammogram,respectively.By electrochemical impedance spectroscopy and its fitted result,the lithium ion diffusion coefficient was measured to be approximately 7.76 × 10?11 cm2/s.     

Evaluation of in vitro and in vivo tests for surface-modified titanium by H<Subscript>2</Subscript>SO<Subscript>4</Subscript> and H<Subscript>2</Subscript>O<Subscript>2</Subscript> treatment

Titanium is widely used as an implant material for artificial teeth. Furthermore, various studies have examined surface treatment with respect to the formation of a fine passive film on the surface of commercial titanium and its alloys and to improve the bioactivity with bone. However, there is insufficient data about the biocompatibility of implant materials in the body. The purpose of this study was to examine whether surface modification affects the precipitation of apatite on titanium metal. Specimens were chemically washed for 2 min in a 1∶1∶1.5 (vol.%) mixture of 48 %HF, 60%HNO₃ and distilled water. The specimens were then chemically treated with a solution containing 97%H₂SO₄ and 30%H₂O₂ at the ratio of 1∶1 (vol.%) at 40°C for 1h, and subsequently heat-treated at 400°C for 1h. All the specimens were immersed in HBSS with pH 7.4 at 36.5°C for 15d, and the surface was examined with TF-XRD, SEM, EDX and XPS. In addition, specimens of commercial pure Ti, with and without surface treatment, were implanted in the abdominal connective tissue of mice for 28 d. Conventional aluminum and stainless steel 316L were also implanted for comparison. An amorphous titania gel layer was formed on the titanium surface after the titanium specimen was treated with a solution of H₂SO₄ and H₂O₂. The average roughness was 2.175 μm after chemical surface treatment. The amorphous titania was subsequently transformed into anatase by heat treatment at 400°C for 1h. The average thickness of the fibrous capsule surrounding the specimens implanted in the connective tissue was 47.1μm in the chemically treated Ti, and 52.2, 168.7 and 101.9μm, respectively, in the untreated commercial pure Ti, aluminum and stainless steel 316L.     

Corrosion behaviour of polycrystalline Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> thin films and its size effects

This study examines the effect of film thickness ranging from 230 to 404 nm on the corrosion resistance of Nb₂O₅ thin films grown by chemical solution deposition. The films were characterized to obtain the relationships between the deposition parameters and the most relevant physical properties (structural, surface morphology and corrosion resistance). From X-ray diffraction and XPS analyses we can conclude that the films were stoichiometric Nb₂O₅ and crystalline. The internal strain and morphology of the film changes as the number of layers increases indicating a thickness dependent grain size. The surface roughness, corrosion resistance were also affected by the film thickness. Electrochemical impedance spectroscopy (EIS) shows that the thicker film have higher passive and charge transfer resistance than the control samples. These results coating layer of Nb₂O₅ improves the corrosion resistance on an API 5L X80 steel alloy due to the formation of a film on the surface.     

Facile Electrochemical Method to Improve Surface Features of Pure Copper in Dilute Basic Solutions

Electrochemical properties of coarse and nano-grained pure copper can be modified and improved effectively through applying cyclic potentiodynamic passivation (CPP) treatment. It is found that the success of this method depends up to a large extent on grain size. Eight passes of accumulative roll bonding processing are successfully used at room temperature to produce nano-grained pure copper. Transmission electron microscopy image and selected area diffraction pattern both attest to the occurrence of intense grain refinement under the influence of aforementioned process, in which an average grain size <100 nm is attainable. Using several electrochemical characterization methods reveals that CPP treatment fully exploits potentials of nano-grained samples to form a dense and thick protective passive film. It is speculated that high-quality passive layers relate to the presence of high-density structural defects on the surface of nano-grained samples.     

Investigation of Corrosion Protection Performance of Multiphase PEO (Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript>, MgO,MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript>) Coatings on Mg Alloy Formed in Aluminate-Silicate- based Mixture Electrolyte

Plasma electrolytic oxidation (PEO) coatings in the aluminate-silicate-based mixture electrolyte solution with different duty cycles were successfully applied on Mg alloy. The corrosion behavior of the samples was evaluated by water contact angle test, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and immersion tests. Hydrophobic PEO coating could be obtained by adjusting the duty cycle of the applied electric signal. This coating considerably diminished the Mg dissolution and could enhance the impedance values of Mg alloy in 3.5 wt % NaCl solution. However, the surface of other PEO coated samples showed more hydrophilic properties compared to that of the uncoated sample. Dense structure of the modified PEO multiphase (including Mg₂SiO₄, MgO and MgAl₂O₄ phases) coating and also its appropriate thickness provided an effective barrier to remarkably delay corrosive solution penetration into the PEO coating. This phenomenon led to major decrease in anodic current density of alloy in chloride solution.     

Preparation of spherical spinel LiCr<Subscript>0.04</Subscript>Mn<Subscript>1.96</Subscript>O<Subscript>4</Subscript> cathode materials based on the slurry spray drying method

Regular spherical chromium doped spinel lithium manganese oxides (LiCr_0.04Mn_1.96O₄) with an average particle size of about 20 μm were prepared by the slurry spray drying process. The materials were compared with non-spherical LiCr_0.04Mn_1.96O₄ materials prepared by the common drying process, and all materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), laser particle analyzer and Brunauer-Emmett-Teller (BET) specific surface area test. Electrochemical performances of these cathode materials were studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Li/LiCr_0.04Mn_1.96O₄ battery test. The results show that the spherical active material is single spinel structure, compact, and with narrow particle size distribution and low BET specific surface area. Compared with the non-spherical material, the spherical material prepared by the spray drying process shows a lower electrochemical impedance, a fewer electrochemical polarization and a better charge/discharge rate capability and capacity retention at elevated temperatures.     

Effect of heat treatment of titanium substrates on the properties of IrO<Subscript>2</Subscript>-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> coated anodes

The Ti substrates of IrO 2 -Ta 2 O 5 coated anodes were treated by solid-solution and aging, stress relieving annealing, and recrystallization annealing, and the coatings were prepared by thermal decomposition of a mixture of H 2 IrCl 6·6H 2 O dissolved in hydrochloric acid and TaCl 5 dissolved in alcohol. Scanning electron microscopy (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and accelerated life test (ALT) were employed to study the microstructure and electrochemical properties of the anodes. Compared with the anode without heat treatment, the anodes with heat treatment are of higher electrochemical activity and longer accelerated life; especially, the anode with recrystallization annealing treatment has the best electrochemical properties and the longest accelerated life.     

Grain refinement and mechanism of CAl_（0.5）W_（0.5） compound in Mg-Al alloys

The effect of CAl0.5W0.5(CAW) compound on the grain refinement of Mg-Al based alloys was investigated.The results show that CAW compound is an effective and active grain refiner.The grain size of binary Mg-Al alloys is more than 500 μm,and it is changed to about 110 μm with a 1 wt.% CAW addition.The hardness increased with the decease of grain size monotonously.The mechanical properties are improved by the addition.The fine grain size is mainly ascribed to the dispersed Al2CO particles,which are very potent nucleating substrates for Mg-Al alloys.The nucleation cores formed by chemical reaction directly are well-distributed in the matrix.     

Effect of pH on the Electrochemical Behavior of Tantalum in Borate Buffer Solutions

In this research, various electrochemical methods were used to investigate the electrochemical behavior of tantalum in borate buffer solutions of various pH values, ranging from 9.0 to 6.5. Potentiodynamic polarization curves revealed that tantalum showed excellent passive behavior in borate buffer solutions. The potentiodynamic polarization and electrochemical impedance spectroscopy results showed that the passive film formed on tantalum offered its best protective behavior when the pH is 8.0, with the passivity undergoing a drastic change as the pH moved toward higher values. The semiconductive behavior of the passive films formed on tantalum was investigated by employing Mott-Schottky analysis in conjunction with a point defect model. The results indicated that the passive film exhibited n-type semiconductive behavior and that donor densities were in the range of 1.958-7.242 × 10²⁰ cm^?3. Moreover, this analysis showed that the donor density and flat band potential were quite sensitive to the pH.     

The effect of natural ageing of quickly quenched amorphous Fe<Subscript>75</Subscript>Si<Subscript>12</Subscript>B<Subscript>10</Subscript>Nb<Subscript>1</Subscript>Cu<Subscript>2</Subscript> alloys on their corrosion behavior sin acidic sulfate environments

The effect of natural ageing on the electrochemical and corrosion behavior of an amorphous Fe₇₅Si₁₂B₁₀Nb₁Cu alloy in acidic sulfate environments is studied. In its shape, the polarization curve is similar to that of the alloy, which was heat treated at 400°C. The ageing made the alloy brittle and unable to be soldered with acidic fluxes.     

Electrochemical corrosion behavior of bulk ultra-fine grained Fe-Ni-Cr alloy

The corrosion behavior of bulk ultra-fine grained (UFG) Fe-Ni-Cr alloy prepared by equal-channel angular pressing technique was investigated in 0.25 mol/L Na₂SO₄+0.05 mol/L H₂SO₄ solution by electrochemical measurements. As compared to the coarse grained (CG) counterpart, the UFG alloy exhibits an acceleration of the active dissolution and a shrunk passive region with a higher passive current. The Mott-Schottky analysis in conjunction with the point defect model indicates that the donor diffusion coefficient in the passive films of the UFG sample increases greatly to one magnitude order higher and the donor density is slightly lower than that of the CG sample.     

Corrosion and time dependent passivation of Al 5052 in the presence of H<Subscript>2</Subscript>O<Subscript>2</Subscript>

Corrosion and time–dependent oxide film growth on AA5052 Aluminum alloy in 0.25M Na₂SO₄ solution containing H₂O₂ was studied using electrochemical impedance spectroscopy, potentiodynamic polarization, chronoamperometric and open circuit potential monitoring. It was found that sequential addition of H₂O₂ provokes passivation of AA5052 which ultimately thickens the oxide film and brings slower corrosion rates for AA5052. H2O2 facilitates kinetics of oxide film growth on AA 5052 at 25° and 60 °C which is indicative of formation of a thick barrier film that leads to an increment in the charge transfer resistance. Pitting incubation time increases by introduction of H₂O₂ accompanied by lower pitting and smoother surface morphologies. At short exposure (up to 8 h) to H₂O₂–containing solution, the inductive response at low frequencies predominantly determined the corrosion mechanism of AA5052. On the other hand, at prolonged exposure times (more than 24 h) to 0.25M Na₂SO₄+1vol% H₂O₂ solution, thicker oxide layers resulted in the mixed inductive–Warburg elements in the spectra.     

Dielectric properties of Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> thin films deposited onto Ti and TiO<Subscript>2</Subscript> layer

The present study describes the dielectric properties of RF sputtered Ta₂O₅ thin films as a function of the buffer layer and annealing condition. The buffer layers were Ti or TiO₂. And the thin film was annealed in various conditions. The X-ray pattern results showed that the phase of the RF sputtered Ta₂O₅ thin films was amorphous and this state was kept stable to RTA (rapid thermal annealing) even at 700°C. Measurements of the electrical and dielectric properties of the reactive sputtered Ta₂O₅ fabricated in two simple metal insulator semiconductor (MIS) structures, (Cu/Ta₂O₅/Ti/Si/Cu and Cu/Ta₂O₅/TiO₂/Si/Cu) indicated that the amorphous Ta₂O₅ grown on Ti possesses a high dielectric constant (30–70) and high leakage current (10⁻¹–10⁻⁴ A/cm²), whereas a relatively low dielectric constant (−10) and low leakage current (−10⁻¹⁰ A/cm²) were observed in the amorphous Ta₂O₅ deposited on the TiO₂ buffer layer. In addition, the leakage current mechanisms of the two amorphous Ta₂O₅ thin films were investigated by plotting the relation of current density (J) vs. applied electric field (E). The Ta₂O₅/Ti film exhibited three dominant conduction mechanism regimes contributed by the Ohmic emission at low electrical field, by the Schottky emission at intermediate field and by the Poole-Frenkel emission at high field. In the case of Ta₂O₅/TiO₂ film, the two conduction mechanisms, the Ohmic and Schottky emissions, governed the leakage current density behavior. The conduction mechanisms at various electric fields applied were related to the diffusion of Ta, Ti and O, followed by the creation of vacancies, in the rapid thermal treated capacitors.     

Dielectric properties of ZrTiO<Subscript>4</Subscript> thin films synthesized by sol-gel method

ZrTiO₄ thin films were successfully prepared on Pt/Ti/SiO₂/Si(100) substrates by a sol-gel process and gel films were heat-treated at various temperatures. The surface morphology, crystal structure, and dielectric properties of the thin films were investigated. It was possible to obtain ZrTiO₄ phase at temperatures above 650 °C for 2 h, which is much lower than the bulk sintering temperature. The microstructure of well-crystallized ZrTiO₄ thin films was a fine-grained microstructure less than 70 nm in grain size and the surface morphology was smooth with 22.4 rms roughness. The dielectric constant and loss of ZrTiO₄ thin films were 38 and 0.006, respectively, for thin films with 450 nm thickness heat-treated at 900 °C for 2 h.