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.     

Voltammetric stability of anodic films on the Ti6Al4V alloy in chloride medium

The biocompatibility and mechanical integrity of Ti and Ti6Al4V alloy can be affected by corrosion processes. This paper presents studies on the stability of anodic oxide films on Ti6Al4V and Ti in chloride medium. The oxides were grown potentiodynamically up to 8.0 V in the phosphate buffer saline (PBS) solution (pH 6.8) at 25 and 37 °C. The morphology of the obtained anodic oxides and the type of corrosion that occurred were analyzed by SEM–EDS. The Ti6Al4V alloy presented less corrosion resistance than pure Ti. Elemental analyses showed that the decrease of the alloy corrosion resistance is due mainly to the corrosion of Al.     

Electrochemical synthesis of polyindole on 304-stainless steel in LiClO4–acetonitrile solution and its corrosion performance

The use of electrochemically synthesized polyindole (PIN) film was investigated for protective coating on 304-stainless steel (SS). Polyindole was deposited via anodic oxidation of the corresponding monomer, indole in acetonitrile (ACN) solution containing LiClO₄. It provided an adherent and stable coating on SS. The corrosion performance of coated and uncoated SS was investigated in 3.5% NaCl solution using electrochemical impedance spectroscopy (EIS), anodic polarization curves and open-circuit potential–time (E_ocp–t) diagrams. These tests demonstrated that PIN coating provided important barrier effect to SS for important immersion times in aggressive medium.     

Methacryloxypropyltrimethoxysilane films on aluminium: Electrochemical characteristics, adhesion and morphology

The electrochemical characteristics, adhesion and morphology of methacryloxypropyltrimethoxysilane (MAPT) films on aluminium were investigated during exposure to 3 wt.% NaCl. The MAPT films were deposited on aluminium surface from 2 to 5 vol.% methacryloxypropyltrimethoxysilane solutions, with the aim to investigate the influence of deposition parameters (silane solution concentration and curing time) on electrochemical characteristics, adhesion and morphology of MAPT films on aluminium.Using electrochemical impedance spectroscopy (EIS), potential–time measurements, adhesion measurements and optical microscopy coupled with image analysis, it was shown that films deposited from 5 vol.% solution exhibited better corrosion stability and adhesion, as well as lower porosity comparing to 2 vol.% solution and improved the corrosion protection of aluminium substrate, while the curing time had no influence on these characteristics.     

Electrochemical study of TEOS,TEOS/MPTS,MPTS/MMA and TEOS/MPTS/MMA films on tin coated steel in 3.5% NaCl solution

This work compares the anticorrosion features of siloxane layers as eco-friendly alternatives for chromium passivation process on industrials tin coated steel and unravels the influence of each component of the film. The films were prepared with synergistic blends of siloxanes as tetraethyl orthosilane (TEOS), 3-methacryloxy-propyl-trimethoxysilane (MPTS) and methyl methacrylate (MMA). To assess the influence of each component, five different films were prepared: TEOS-based, MPTS-based, TEOS/MPTS, MPTS/MMA and TEOS/MPTS/MMA. The corrosion resistance of the coatings was evaluated by means of open circuit potential, anodic polarization curves and electrochemical impedance spectroscopy measurements, and the anticorrosion properties discussed based on electrical equivalent circuit fitting. Coated surfaces were analyzed using atomic force microscopy and the coatings’ thicknesses were evaluated by means of glow discharge optical emission spectroscopy. The results plainly showed the efficiency of the anticorrosion properties of the film in a 3.5 wt.% NaCl solution and have clearly revealed the improvement of the protective properties of the coating when the MPTS was added to the formulations, pointing this component as the main responsible for the coating anticorrosion action. The addition of MMA to the formulation led to formation of coatings with low long term anticorrosion protection, which was ascribed to the low thickness.     

Electrochemical impedance behavior of graphite-dispersed electrically conducting acrylic coating on AZ31 magnesium alloy in 3.5 wt.% NaCl solution

The paper deals with understanding the electrochemical impedance behavior of (a) chromate coating (b) electrically conducting polymer coating and (c) polyurethane coating under different combinations applied over AZ31 magnesium alloy in an effort to develop a conducting organic coating that can offer corrosion protection as well as electrical conductivity. The study indicates that dispersion of graphite in acrylic coating though significantly brings down the electrical resistance; application of such a coating on the magnesium alloy does not offer good resistance against corrosion. In order to gain both the corrosion resistance as well as electrical conductivity an intermediate coating such as polyurethane needs to be provided. The paper discusses the electrochemical impedance behavior of the above coatings.     

Influence of rare earth Y on the corrosion behavior of as-cast AZ91 alloy

The influence of different contents of rare earth Y on the corrosion resistance of AZ91 alloy was investigated by the salt spray test and electrochemical measurements. It was found that the proper amount of Y was effective on improving the corrosion resistance of AZ91 alloy. The optimal modification effect was obtained when the Y content in the alloys was 0.3 wt.%. However, with the increase of rare earth Y, the corrosion rate became bigger slightly, and further addition of Y content over 0.3 wt.% resulted in the increment of the corrosion rate. It is suggested that the excessive rare earth Y can reduce the corrosion resistance of AZ91 alloy.     

Electrochemical study of effect of the concentration of azole derivatives on corrosion behavior of stainless steel in H2SO4

In order to improve corrosion resistance of the stainless steel structures exposed to acidic media, a variety of corrosion inhibitors particularly organic ones have been examined. In this work, the corrosion inhibition performance of two azole derivatives namely benzotriazole and benzothiazole on stainless steel in 1 M sulfuric acid was studied through taking advantage of electrochemical techniques as well as SEM surface analysis. Revealing effectiveness of the two inhibitors, the AC impedance spectra indicated no change in corrosion mechanism. The noise resistance and average current density as parameters extracted from electrochemical noise measurements revealed the direct proportion of inhibition function to the inhibitor concentration. In accordance with the polarization curves, benzotriazole and benzothiazole appeared to act as mixed type inhibitors. The adsorption of the two corrosion inhibitors was shown to obey Langmuir isotherm. Moreover, it was deduced from the isotherm that the type of adsorption can be physical and chemical in nature. The corrosion damage mitigation was also confirmed through SEM in the presence of benzothiazole.     

Electrochemical behaviour of passive films on molybdenum-containing austenitic stainless steels in aqueous solutions

Passivation and its breakdown reactions have been studied on Mo-containing stainless steel specimens using different electrochemical techniques. Mo-containing stainless steel specimens were polarized in both naturally aerated NaCl and Na₂SO₄ solutions of different concentrations at 25 ± 0.2 °C between −1000 and 1500 mV versus saturated calomel electrode (SCE). The results of potentiodynamic polarization showed that i_corr and i_c increases with increasing either Cl⁻ or SO₄²⁻ concentration indicating the decrease in passivity of the formed film. EIS measurements under open circuit conditions confirmed that the passivity of the film decrease with increase in either Cl⁻ or SO₄²⁻ concentration.     

Electrochemical corrosion study of Sn–3Ag–3Cu solder alloy in NaCl solution

The corrosion behaviour of Sn–3Ag–3Cu (at%) alloy was investigated in 0.1 M NaCl solution by potentiodynamic polarization and impedance spectroscopy measurements and compared with that of the Sn–3Ag–0.5Cu (at%) solder employed in the packaging of some microelectronic components and devices. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) were used to characterize the samples prior to and after the electrochemical tests. Results showed that in NaCl solution the corrosion resistance of the Sn–3Ag–3Cu alloy was better than that of the Sn–3Ag–0.5Cu solder. The presence of tin oxychlorides or oxyhydroxychlorides was detected at the surface of both alloys investigated after the electrochemical tests. The better corrosion behaviour of the Sn–3Ag–3Cu alloy compared to the Sn–3Ag–0.5Cu solder can be ascribed to a more adherent and compact corrosion products layer.     

Fabrication of a Glass‐Ceramic‐to‐Metal Seal Between Ti–6Al–4V and a Strontium Boroaluminate Glass

Glass‐ceramics are widely utilized in the electronics industry to provide electrical insulation and to form leak‐tight joints with a range of metals. The coefficient of thermal expansion (CTE) of the glass‐ceramic can be controlled by the extent of crystallization to reduce detrimental tensile stresses in the joint. In recent years, there has been interest in using titanium alloys, in place of stainless steels, due to their lower density and superior specific strength. In this study, the heat treatment of a strontium boroaluminate glass has been tailored to create glass‐ceramics with mean CTEs ranging from 5.7 ± 0.1 × 10^?6/K to 9.7 ± 0.1 × 10^?6/K over the temperature range 303–693 K. The resultant glass‐ceramic consists of three crystalline phases and residual glass. A glass‐ceramic with a mean CTE of 6.9 ± 0.1 × 10^?6/K was subsequently fabricated to form a compression seal with a Ti–6Al–4V housing and a preoxidized Kovar pin. Single pin assemblies were shown to be reproducible in terms of microstructure and all passed a standard helium leak test, indicating that a successful seal had been produced.     

Development of an environmentally friendly protective coating for the depleted uranium-0.75 wt.% titanium alloy: Part V. Electrochemical impedance spectroscopy of the coating

Molybdenum oxide based conversion coatings have been formed on the surface of the depleted uranium-0.75 wt.% titanium alloy. Electrochemical impedance spectroscopy (EIS) measurements have been performed on the as-made and aged coatings and compared with the untreated depleted uranium (DU) alloy. The Nyquist and Bode plots of the as-made coating were similar to the untreated samples and contained capacitive and inductive loops. The aged coating exhibits significantly different behavior from the as-made coating and has been modeled with a four element equivalent circuit that contains a constant phase element (CPE).     

Tantalum carbide coating on Ti‐6Al‐4V by electron beam physical vapor deposition method: Study of corrosion and biocompatibility behavior

This study aimed to improve the corrosion resistance and biocompatibility of titanium alloy (Ti‐6Al‐4V) by tantalum carbide (TaC) deposition through electron beam physical vapor deposition (EB‐PVD) method. The physical and chemical characteristics of the coated surface are comprehensively evaluated. The corrosion resistance and ion release are assessed. Cytocompatibility assay and cell morphology observation are performed to assess toxicity and cell interaction, respectively. The TaC‐coated Ti‐6Al‐4V exhibits more resistance to corrosion and ion release. It provides a surface, which is appropriate for cell adhesion, an expansion as well as better biocompatible performance. So, it could improve osseointegration Ti‐alloy implants in clinical applications.     

Electrochemical and structural evaluation of functionally graded bioglass-apatite composites electrophoretically deposited onto Ti6Al4V alloy

Titanium and its alloys are widely used as materials for implants, owing to their corrosion resistance, mechanical properties and excellent biocompatibility. However, clinical experience has shown that they are susceptible to localised corrosion in the human body causing the release of metal ions into the tissues surrounding the implants. Several incidences of clinical failures of such devices have demanded the application of biocompatible and corrosion resistant coatings and surface modification of the alloys. Coating metallic implants with bioactive materials is necessary to establish good interfacial bonds between the metal substrate and the bone. Hence, this work aimed at developing a bioglass-apatite (BG-HAP) graded coating on Ti6Al4V titanium alloy through electrophoretic deposition (EPD) technique. The coatings were characterized for their properties such as structural, electrochemical and mechanical stability. The electrochemical corrosion parameters such as corrosion potential (E_corr) (open circuit potential) and corrosion current density (I_corr) evaluated in simulated body fluid (SBF) have shown significant shifts towards noble direction for the graded bioglass-apatite coated specimens in comparison with uncoated Ti6Al4V alloy. Electrochemical impedance spectroscopic investigations revealed higher polarisation resistance and lower capacitance values for the coated specimens, evidencing the stable nature of the formed coatings. The results obtained in the present work demonstrate the suitability of the electrophoretic technique for the preparation of graded coating on Ti6Al4V substrates.     

Electrochemical behavior of (Cr,W, Al,Ti, Si)N multilayer coating on nitrided AISI 316L steel in natural seawater

AISI 316L steel is often used in materials applied toward nuclear power but are subjected to pitting corrosion in a marine environment. In this study, (Cr, W, Al, Ti, Si)N multilayer coatings were deposited using multi-arc ion plating on the surface of non-nitrided and nitrided AISI 316L steel. The microstructure and corrosion resistance of four different systems were investigated, namely, (i) untreated AISI 316L steel, (ii) plasma nitrided (PN), (iii) coated on an untreated matrix (coating) only, and (iv) coated on nitrided (hybrid) specimens. The phase structures, morphologies, and compositions of the different specimens were characterized using X-ray diffraction, transmission electron microscope, Atomic Force Microscope, scanning electron microscope, X-ray photoelectron spectroscopy, and energy dispersive x-ray spectroscopy. The results show that a thin CrWAlTiSiN multilayer coating, approximately 2.3 μm in thickness, is deposited on the surface of an ~12 μm nitrided layer. Potentio-dynamic polarization and electrochemical impedance spectroscopy were used to evaluate the assessment of the electrochemical behavior in the natural seawater of China's Yellow Sea. The hybrid specimens exhibited excellent corrosion resistance compared to both the nitrided and coated specimens.     

Electrochemical behavior and anticorrosion properties of modified polyaniline dispersed in polyvinylacetate coating on carbon steel

Conducting polyaniline (Pani) was prepared in the presence of methane sulfonic acid (MeSA) as dopant by chemical oxidative polymerization. The Pani-MeSA polymer was characterized by FT-IR, UV-vis, X-ray diffraction (XRD) and impedance spectroscopy. The polymer was dispersed in polyvinylacetate and coated on carbon steel samples by a dipping method. The electrochemical behavior and anticorrosion properties of the coating on carbon steel in 3% NaCl were investigated using open-circuit potential (OCP) versus time of exposure, and electrochemical techniques including electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and cyclic voltammetry (CV). During initial exposure, the OCP dropped about 0.35 V and the interfacial resistance increased several times, indicating a certain reduction of the polymer and oxidation of the steel surface. Later the OCP shifted to the noble direction and remained at a stable value during the exposure up to 60 days. The EIS monitoring also revealed the initial change and later stabilization of the coating. The stable high OCP and low coating impedance suggest that the conducting polymer maintains its oxidative state and provides corrosion protection for carbon steel throughout the investigated period. The polarization curves and CV show that the conducting polymer coating induces a passive-like behavior and greatly reduces the corrosion of carbon steel.     

Electrochemical behavior of magnesium alloys AZ91D,AZCe2, and AZLa1 in chloride and sulfate solutions

The influence of Cl⁻ and SO₄²⁻ on the electrochemical behavior of AZ91D, AZCe2, and AZLa1 was studied. For all alloys, there was a current plateau in the anodic polarization curves in Na₂SO₄ solutions. In 0.5% NaCl solution, there was a small current plateau, whereas there was none in the 3.5% and 5% NaCl solutions. This indicated that SO₄²⁻ is less aggressive than Cl⁻. The range of the current plateau decreased with increasing SO₄²⁻ concentration. For all alloys, the high frequency capacitive loop in the Nyquist plots decreased with increasing concentration consistent with the decrease in corrosion resistance with increasing Cl⁻ and SO₄²⁻ concentration.     

Electrochemical behavior of K<Subscript>4</Subscript>Fe(CN)<Subscript>6</Subscript> in [bmim]PF<Subscript>6</Subscript>/TX-100/H<Subscript>2</Subscript>O based microemulsions

The electrochemical behavior of potassium ferrocyanide [K₄Fe(CN)₆] at Pt/ionic liquid (IL) microemulsion interfaces was investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). H₂O/TX-100/bmimPF₆ was used to prepare three IL microemulsions: water in 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF₆) (W/IL), bicontinuous (WIL) and bmimPF₆ in water (IL/W). The results show that the IL microemulsion systems have relatively narrower potential windows compared with the pure IL system. The redox potential gap is about 100 mV in the pure water and the three IL microemulsions. The redox potentials of K₄Fe(CN)₆/K₃Fe(CN)₆ and the redox peak currents decrease in the order pure water, IL/W, WIL, W/IL. Furthermore, the peak currents increase linearly with the square root of the scan rate, while the diffusion coefficient increased in the order W/IL, WIL, IL/W. The Nyquist plots obtained in the WIL and IL/W systems show capacitive resistance arcs at high frequencies and 45° straight lines at low frequencies, implying that the electrochemical reactions are controlled by charge transfer and diffusion steps. For the W/IL system there is only a 45° straight line in the Nyquist plot, indicating that diffusion is the controlling step at all frequencies.     

Corrosion resistance of pulsed laser-treated Ti-6Al-4V implant in simulated biofluids

A commercial Ti-6Al-4V alloy was treated with a pulsed-wave Nd:YAG laser under various process conditions to obtain surface oxide layer for corrosion resistance. The corrosion behaviors of bare and laser-treated Ti-6Al-4V alloy exposed to three different simulated biofluids (SBFs), namely, (1) NaCl solution, (2) Hank's solution, and (3) Cigada solution, were studied by using the electrochemical techniques like open circuit potential (OCP), Tafel analysis, and electrochemical impedance spectroscopy (EIS). The Tafel analysis showed that the laser-treated Ti-6Al-4V specimens were more corrosion resistant than the bare specimens in any of the above SBFs. The various electrical equivalent circuit models were applied to fit the EIS results to further understand corrosion mechanisms due to different surface layers formed on the alloy surface before and after the laser treatment. Optical and AFM imaging techniques were used to evaluate the topographic and morphologic features of the alloy exposed to such SBFs. The corrosion behavior of the laser-treated surfaces was explained by the melting and solid-state oxidation processes, the morphology of the surface oxide, and the underlying alloy microstructure. It is realized during the present investigation that better corrosion resistance and surface stability can be achieved by oxide growth in solid-state, under a pulsed laser condition.     

Electrochemical study of protective behavior of organic coating pigmented with zinc aluminum polyphosphate as a modified zinc phosphate at different pigment volume concentrations

Electrochemical impedance spectroscopy was employed to evaluate protective performance of the solvent-borne epoxy coatings pigmented with zinc aluminum polyphosphate as a representative of phosphate-based anticorrosion compounds at different Lambda values. Furthermore, the effective ratio of the pigment volume concentration (PVC) to the critical pigment volume concentration (CPVC) was determined. To compare the function of zinc aluminum polyphosphate and zinc phosphate incorporated into coatings, electrochemical noise method as well as electrochemical impedance spectroscopy was taken into consideration. The trend and magnitude of charge transfer, coating and noise resistances plus the amplitude of the current noise fluctuation indicated superiority of the modified pigment. In order to provide an insight into the mechanism by which anticorrosion pigments improve protective behavior of coating, performance of bare metals exposed to pigment extracts was assessed through taking advantage of electrochemical impedance spectroscopy and electrochemical noise method as well.