Parametric study on the electrochemical impedance spectroscopy of organic-coated steels in hydrochloric acid solutions

Electrochemical impedance technique was used to study the corrosion resistance of polymer-coated steels in hydrochloric acid solutions. The mechanism of the degradation process of the coating can be described using a Randles-type equivalent circuit model with the inclusion of Warburg diffusion impedance. The impedance parameters: coating resistance R_po, coating capacitance C_c, charge transfer resistance R_ct and double layer capacitance C_dl of the metal/coating interface, and the Warburg coefficient σ were calculated using a nonlinear least-square minimization scheme. The time dependency of impedance parameters was compared to the method of breakpoint frequency. It was found that the delamination of the metal/coating interface is more associated with the charge transfer resistance than the coating resistance. Open circuit potential measurement is indicative of the corrosion status of the coating. Department of Chemical Engineering, Neili, Taoyuan, Taiwan 32026 Fax: 886.3.4559373 email:cekenyin@saturn.yzu.edu.tw     

Instantaneous electrochemical impedance spectroscopy of electrode reactions

Continuous STFT transforms of a sinusoidal perturbation signal and current have been found for the first order electrode reaction. Electrode impedance has been determined in the joint time-frequency domain. Measurements of the Cd(II) reduction reaction have been performed on a dropping mercury electrode as a function of time. A possibility of instantaneous impedance spectra generation has been presented. Time characteristics of charge transfer resistance, Warburg coefficient and double layer capacitance has been described.     

Effects of solution temperature on the kinetic nature of passive film on Ni

Effects of solution temperature on the kinetic nature of passive film on Ni were investigated using polarization test, Mott–Schottky analysis and electrochemical impedance spectroscopy to reveal why the corrosion resistance of Ni is degraded with an increase in solution temperature. The increase in the corrosion rate of Ni with solution temperature was confirmed by the increase in the passive current density and also in the steady-state current density. Mott–Schottky analysis revealed that the passive film formed on Ni exhibits a p-type semiconducting characteristics irrespective of the solution temperature, and the concentration of cation vacancy in the passive film increases with temperature. By optimizing the reduced PDM (point defect model) on the experimental impedance data, base rate constants and transfer coefficients for the charge transfer reactions occurring at the metal/film and film/solution interfaces were extracted, and the Warburg coefficient for the cation vacancy transport was also determined. According to the calculated kinetic parameters (rate constants for the interfacial reactions, diffusivity of cation vacancy, etc.), the mechanism for the degradation of corrosion resistance of Ni with solution temperature was explained.     

Kinetics and mechanism of the magnesium electrode reaction in molten magnesium chloride

Using electrochemical impedance spectroscopy (EIS) and relaxation method with galvanostatic perturbation (RM) the kinetics and mechanism of the magnesium electrode reaction in pure molten M9Cl₂ have been determined at several temperatures. A three-step electrode process has been found, the high frequency process being pure charge transfer with the low frequency process showing mixed charge transfer-diffusion character. The low frequency step has also been treated as a preceding chemical reaction followed by charge transfer. On the basis of the corresponding exchange current densities and Warburg diffusion impedance, a mechanism of the overall electrode reaction in this melt is proposed.     

陕北盐渍土壤对X80管线钢电化学腐蚀行为的影响

采用电化学测试、SEM及EDS微观分析等方法,研究了X80管线钢在陕北水饱和盐渍土壤中的电化学腐蚀行为。结果表明,在水饱和盐渍土壤中,随腐蚀时间延长,X80钢腐蚀趋势和腐蚀速率均明显增大,局部腐蚀面积和深度不断增加,钢基体表面由以全面腐蚀为主转为以局部腐蚀为主,腐蚀机理为氧浓差腐蚀电池和局部腐蚀自催化效应,腐蚀速度主要受氧扩散过程控制;EIS图谱具有双容抗弧与Warburg阻抗特征,电荷传递和扩散传质的阻力随时间越来越大,而结合层电阻明显减小,这与钢表面生成腐蚀产物膜的完整性和致密性有关,腐蚀产物主要为铁的氧化物、硫化物和土壤中盐类的混合物。     

Theory of quasi-reversible charge transfer admittance on finite self-affine fractal electrode

Response of an electrode under finite charge transfer rate is strongly influenced by morphological and electrochemical heterogeneity of the surface. Here, we developed a theoretical model for quasi-reversible charge transfer admittance of a random finite fractal electrode. These random heterogeneities are characterized by their roughness power spectrum. The power spectrum of the finite fractal roughness is approximated in terms of a power-law function for the intermediate wave numbers (spatial frequency components in roughness). The mathematical expression for the admittance response of a finite fractal electrode depends on diffusion length , charge transfer resistance (R_CT) and its fractal morphological characteristics. Fractals are characterized by the fractal dimension (D_H) and the surface roughness amplitude (μ), but finite fractals includes two lateral cutoff lengths. These lengths are the smallest length of roughness (?) and the lateral correlation length (L). The impedance of the electrode shows three frequency regimes, viz., (i) low frequency region, which has a classical Warburg impedance, (ii) anomalous Warburg behavior for intermediate frequency and their phase angle, ?(ω) > 45°, and (iii) high frequency impedance is controlled by charge transfer kinetics, and its phase angle follows the constraint: 0 ≤ ?(ω) < 45°. Finally, we can say that finite fractal model accounts for fractal as well as nonfractal roughnesses of the electrode, and it strongly influences the admittance response.     

Analysis of a Fuel Cell System by a Step Response

A simple method for analysing a fuel cell system using the voltage response to a step change in load resistance is presented in this work. With the modified Randles model, where two resistors and two capacitors are implemented for the Warburg impedance, the response curve is fitted with three exponential curves. Six independent equations corresponding to six parameters of the model can be obtained from the fitted values, and they are solved under two specific assumptions for the initial state. The parameters are the charge double layer capacitance, the charge transfer resistance, a resistor–capacitor pair related to gas diffusion and another pair related to water diffusion. A down curve of the voltage for the step down change in load and an up curve for the step up change are fitted and parameters for both cases are evaluated separately. Analysis is performed by comparing the parameters at different current levels. The impedance is also simulated using the estimated parameters in order to show which parameter is the rate‐determining factor of the fuel cell reaction with increasing current levels.     

Impedance study of electropolymerized films of polyNi(II)-macrocycles

Glassy carbon electrodes modified by electropolymerization of two water-soluble compounds, Ni(II)tetrasulphophthalocyanine (NiTSPc) and Ni(II)tetrasulphophenylporphyrin (NiTSPP), and of two water-insoluble compounds, Ni(II)tetraaminophthalocyanine (NiTAPc) and Ni(II)tetraaminophenylporphyrin (NiTAPP), have been studied by impedance measurements. While in the tetrasulpho films the Ni(II)/Ni(III) process was operative, the tetraamino films required activation by repetitive potential cycling before the peaks of this process appeared in the CV, and even then the charge was a fraction of that shown by the tetrasulpho films. The Nyquist plots of all these films shown in the potential range of the Ni(II)/Ni(III) process the semicircle typical of a charge transfer, the charge-transfer resistance decreasing very much with increasing potential. The uncompensated resistance of the GC electrode was barely affected by the films, showing that they were very porous. The impedance at low frequencies of the thick polyNiTSPP film, the thin polyNiTSPc film, and also the activated polyNiTAPc film fitted the Warburg model, the diffusing species being probably the hydroxyl ions involved in the process.     

The application of the measurement of impedance to the corrosion of dental amalgams

A preliminary experimental investigation of the electrochemical characteristics of a high and a low copper dental amalgam in contact with saline solution has been carried out. The impedance and the corrosion current (in the absence of dissolved oxygen) have been measured at each potential in the near steady state for fresh amalgam surfaces. Analysis of the electrochemical data produces charge transfer resistance, double layer capacity, ohmic resistance and Warburg coefficient curves, which are briefly discussed. The aim of the work is, ultimately, to determine thein vivo corrosion rates and heavy metal ion release from amalgams.Nomenclature a anodic Tafel slope - C _dl differential capacity - D _A diffusion coefficient of A - E potential with respect to SCE electrode - E ₀ standard potential with respect to SCE - i current density - j square root of-1 - R ohmic loss resistance - R _cl charge transfer resistance - Z() impedance - frequency of a.c. potential - diffusion layer thickness - Warburg impedance parameter     

铜在3.5％NaCl体系中浸泡过程的阻抗谱研究

采用阻抗谱研究铜在3.5％NaCl体系中的浸泡过程中的腐蚀行为,研究表明随着浸泡时间的延长,铜电极表面逐渐产生致密的具有保护作用的CuCls,从而电荷转移电阻和金属表面膜电阻均增大,铜的腐蚀速度逐渐减小;同时,CuCl2^-由电极表面向本体溶液扩散逐渐趋向于半无限扩散,表现为膜电阻的弥散指数逐渐减小趋向于0.5,阻抗谱上出现Warburg阻抗。     

Kinetic investigation of sulfurized polyacrylonitrile cathode material by electrochemical impedance spectroscopy

To understand more about the sulfur composite prepared by sulfurized polyacrylonitrile at 300 °C, electrochemical impedance spectroscopy (EIS) is employed to investigate the electrochemical properties of the sulfur composite cathode during discharge process. The impact of discharge depth on the performance of sulfur composite materials is investigated. The electrolyte solution resistance, the charge transfer resistance and the interface impedance are evaluated from the EIS analysis. The charge transfer resistance and the interface impedance increase during delithiation and decrease during lithiation, while the concentration of Li⁺ in the composite decreases and increases, respectively. Meanwhile the electrolyte solution resistance is likely to keep stable. The interface resistance and charge transfer resistance of the sulfur composite cathode decrease rapidly after initial lithiation, while Li⁺ diffusion coefficient and exchange current density increase rapidly. After the initial lithiation, they are likely to keep stable. This study reveals more characteristics of the sulfur composite, which is considered to be a promising candidate for large capacity cathode material.     

The estimation of the charge transfer resistance by graphical analysis of inclined semicircular complex impedance diagrams

Recently a graphical method for the determination of the charge transfer resistance, obtained from impedance measurements, was developed. The original method was limited to perfect, semicircular Nyquist plots. Extension to inclined or depressed semicircular arcs is made possible by a few simple mathematical and graphical manipulations.     

Sol-gel–mediated synthesis of TiO2 nanocrystals: Structural,optical, and electrochemical properties

Nanoparticles of titanium dioxide were prepared using the sol-gel method without any impurity. Rietveld refinement of XRD data confirmed the anatase phase of synthesized nanoparticles with space group I4_1/amd (141). XRD pattern revealed the crystalline nature of synthesized nanopowder. The average crystallite size of synthesized nanoparticles was calculated 7.5 nm. The electrochemical performance of synthesized TiO₂ nanopowder was investigated as working electrode. The electrochemical reaction was found diffusion-controlled as observed from cyclic voltammetry (CV) studies at different scan rates. The diffusion-controlled charge storage mechanism also confirmed by charge transfer resistance and Warburg impedance, as calculated from the EIS analysis. SEM micrograph showed the plate-like structure grown in cluster cloud of particles of synthesized TiO₂ nanocrystals. Absorbance and optical bandgap were obtained using UV-Vis spectra. De-convoluted PL spectra provided the emission pattern from the ultra-violet region to green region due to the presence of interstitial oxygen vacancies. The tune bandgap with EIS measurements of synthesized TiO₂ nanoparticles offers its potential application in energy storage devices and photovoltaic applications.     

EIS measurements in the diagnosis of the environment within a PEMFC stack

The diagnosis of flooding or drying is an important factor in improving the reliability and durability of PEMFCs. Indeed, flooding and drying lead to a reduction in the output voltage delivered by the fuel cell. Among the various diagnostic techniques, electrochemical impedance spectroscopy (EIS) is a powerful tool. In this study, EIS measurements were carried out on a 500 W stack comprised of 16 elementary cells. Electrochemical impedance spectra were recorded either on each cell or on the stack. Use of an electrical equivalent circuit model revealed that flooding mainly affects the transfer resistance and the cathodic Warburg impedance. A diagnostic tool based on these two components is proposed.     

Influence of nitrate ions on silver electrocrystallization

The influence of nitrate anions on the reduction of silver ions on a silver cathode from a silver nitrate solution was studied with three different experimental methods: potentiokinetic curves, chronopotentiometry and impedance measurements. It was concluded that charge transfer of the Ag⁺ ions is certainly not the rate determining step of the reaction. It was clearly shown with the three methods that an increase in the nitrate ion concentration activates the reduction of the Ag⁺ ions; the overvoltage of the reaction decreases markedly. With the aid of an equivalent circuit it is possible to quantify the activation of the electrode, in particular through the determination of the charge transfer resistance and the resistance and the capacity of adsorbed species.     

Steady-state-current impedance spectroscopy of plasticized PVC membranes containing neutral ion carriers

Impedance spectroscopy measurements on plasticized PVC ion-selective membranes containing neutral carriers provided evidence for both bulk and interfacial features. Under steady-state currents, the systems showed concentration polarization of neutral carriers and ionic species. The shortage of neutral carriers at one of the interfaces was correlated with the presence of a low-frequency kinetic semi-circle and a Warburg impedance. The changes in ionic concentration profiles were associated with an increase in bulk membrane resistance and Warburg impedance.     

Influence of CaF2 and AlF3 on the kinetics and mechanism of the Al electrode reaction in cryolite melts with various alumina contents

Electrochemical techniques were used to study the kinetics and mechanism of the aluminium electrode reaction in two cryolite-based melts containing cryolite with either 11 wt % AlF₃ or 5 wt % CaF₂ additions and variable alumina contents at 1000 °C. A three step electrode process was observed in both melts, comprising a preceding chemical reaction followed by two charge transfer steps. The exchange current density of the cathodic reaction was found to be dependent on the concentration of aluminium fluoride. By a combination of electrochemical impedance spectroscopy (EIS) and galvanostatic relaxation methods (GRM), the exchange current density of the first (slower) charge transfer step, the Warburg diffusion impedance, the double layer capacitance of the aluminium electrode and the rate of the preceding chemical step, were evaluated in the range of 2–8 wt % alumina. The role of the two additives, AlF₃ and CaF₂, was evaluated.     

Impedance of a rotating disc electrode with a reversible reaction

This paper presents a model of electrode impedance for the case of a fast reversible reaction. The various contributions of the impedance were analysed with particular emphasis on the charge transfer resistance: this resistance was shown to be also dependent on mass transfer phenomena. For the case of significant mass transfer control,the diameter of the high-frequency loop increases with the absolute value of the overpotential. The various physicochemical parameters involved in the expression for impedance were determined through previous measurements. The impedance model was validated by experimental measurements carried out with the hexacyanoferrate (II)–(III) couple ona Pt RDE.     

Effect of state of charge on impedance spectrum of sealed cells Part I: Ni-Cd cells

Alternating current impedance spectroscopy (ACIS) was performed on commercial sealed Ni-Cd cells. A method previously developed in the literature was modified to determine the state of charge of sealed Ni-Cd cells by obtaining the impedance spectrum in a wide frequency range. The impedance parameters were sensitive to state of charge at low frequencies. A modified Randles' circuit was used to fit the impedance data. Appropriate modifications were made to account for an additional high frequency arc or a low frequency finite diffusion element. The effect of the state of charge on the equivalent circuit parameters was determined.List of symbols R ohmic resistance of battery (ohms) - C _dl double layer capacitance (F) - Q ₁ constant phase element representing double layer capacitance - Q ₂ constant phase element representing Warburg diffusion - O finite diffusion element - R _t charge transfer resistance () - R _s,R _p equivalent series and parallel resistance () - C _s,C _p equivalent series and parallel capacitance (F)     

Electrochemical impedance studies of corrosion protected surfaces covered by epoxy polyamide coating systems

Electrochemical impedance spectroscopy (EIS) was used to study the corrosion behavior of different types of commercial quality epoxy polyamide dry coatings on mild steel, with thickness between 150 and 250 μm, which were previously weather accelerated in a wet chamber. The following data were established on the basis of impedance curves and corresponding equivalent circuits: the coating capacity that relates to the coating dimension, the pore resistance that represents conductive paths through the pores, and Warburg coefficients, which are the measure of ion diffusion through the coating. EIS data were compared with a criterion in the European standard, and samples were classified on the basis of their quality, also receiving a final ranking by summing-up all the individual rankings. Suggestions have been made, namely that the use of anticorrosive active pigments is obsolete, when the thickness of the coatings exceeds 200 μm.