Development of an electrochemical impedance spectroscopy sealant test: II. Conductive sealants

Distinguishing between sealants and providing laboratory prognostic tools is essential for qualifying new sealants. Current test methods do not provide adequate discrimination between sealants. A laboratory test based on electrochemical impedance spectroscopy (EIS) supplemented by adhesion strength measurements and examination of sealed, bimetallic surfaces is an approach for discriminating between sealants. This approach gives an early indication of sealant performance so that suitable materials can be chosen for aircraft and other applications. Sealant specimens (three conductive sealants) underwent hot-salt-water exposure followed by EIS inspection, pull strength measurements, and visual inspection. Several specimens exhibited a temporary loss of conductivity during exposure. The maximum low-frequency impedance was correlated to both corrosion inside the sealant ring and pull strength.     

Structural characterization of bis-[triethoxysilylpropyl]tetrasulfide and bis-[trimethoxysilylpropyl]amine silanes by Fourier-transform infrared spectroscopy and electrochemical impedance spectroscopy

Bis-[triethoxysilylpropyl]tetrasulfide (or bis-sulfur silane) and bis-[trimethoxysilylpropyl] amine (or bis-amino silane) were deposited on 2024-T3 aluminum alloy (AA 2024-T3). The structures of the films were characterized using Fourier-transform infrared spectroscopy (FTIR) and electrochemical impedance spectroscopy (EIS) techniques. The results showed that: (1) The silane structures were affected significantly by the hydrolysis time of the silane solutions. A minimum hydrolysis time is required to obtain a crosslinked silane film. (2) Hydrolysis progressed more readily and faster in the bis-amino silane system than in the bis-sulfur silane system, probably due to the catalytic action of the amine of the bis-amino silane. (3) Both silane systems experienced significant crosslinking upon curing at 100°C, during which denser interfacial layers were formed via crosslinking in the interfacial regions. The interfacial layer contributes to corrosion protection of metals by silanes. (4) A new phase was observed in the fully cured bis-amino silane film after aging in the atmosphere. This new phase is likely to be carbamates and bicarbonates formed via a reaction between the secondary amino groups, carbon dioxide, and moisture absorbed from the atmosphere.     

Inspection of composite and metal adhesive bonds with an electrochemical sensor

An in-situ sensor, based on Electrochemical Impedance Spectroscopy (EIS), has been used to monitor the health of adhesive bonds constructed from various combinations of aluminum, graphite/epoxy, glass/epoxy, glass/polyester, and glass/vinylester composites and exposed to high humidity and temperature conditions. Modeling of the EIS data as an electric circuit demonstrated that several circuit parameters of the impedance spectra were sensitive to bond performance, as determined by wedge tests and lap shear tests. Moisture absorption by the adhesive and composite was calculated from the circuit capacitance, which was also a function of bonded area and bondline thickness (bondline + composite thickness for glass composites). Material differences, including saturation level of moisture, rate of absorption, and bondline stability, were readily seen among the various materials sets. The sensor electrodes are attached to opposite sides of a bond after fabrication, i.e. they are not embedded. Thus, they are suitable for monitoring existing bonded structures. They have the potential to identify bondlines that are in the early stages of degradation, prior to significant loss of bond strength. As an input to a condition-based maintenance system, they would identify weakening bondlines and allow preventative action to be scheduled and performed.     

Electrochemical impedance spectroscopy investigation of the corrosion at metallic substrates covered by organic coatings

The corrosion protection characteristics of an epoxy-polyamide coating were investigated by electrochemical impedance spectroscopy in immersion tests performed in a 3.0% NaCl solution. Carbon steel and galvanized steel substrates were employed. A porous film was produced on the metallic substrates, which allows for electrochemically reactive areas to be developed inside the pores. An effective corrosion-resistant system was produced in the case of the galvanized steel substrates, due to the precipitation of zinc-containing corrosion products that contribute to the sealing of the coating. Conversely, no effective protection was found for carbon steel specimens, since in this case the local accumulation of the corrosion products causes swelling of the coating.     

Electrochemical impedance spectroscopy inspection of composite adhesive joints

Lap shear joints comprised of aluminum-aluminum, aluminum-glass fiber-reinforced polymer (GFRP), aluminum-carbon fiber-reinforced polymer (CFRP), CFRP-CFRP, CFRP-GFRP and GFRP-GFRP, were exposed to 95% relative humidity (RH) at 50°C, 70°C and 90°C, to salt fog and to a 90°C/95%RH freeze cycle. Electrochemical impedance spectroscopy (EIS) spectra were taken across the whole bonded assembly using an EIS corrosion sensor. Periodically, some specimens were pulled to obtain bond strength as a function of exposure. As expected, the higher the temperature, the faster the bond degradation. The low-frequency impedance correlated with bond strength of the humidity-exposed specimens and showed the same Arrhenius dependence, suggesting that moisture absorption by the adhesive was the limiting factor in bond performance and that EIS has the potential to nondestructively track bond health and warn of deterioration.     

Laboratory evaluation of corrosion resistance at metallic substrates by an organic coating: delamination effects

The corrosion behaviour of an epoxy-polyamide primer applied on galvanized steel specimens during immersion in 3% NaCl aqueous solution was examined by electrochemical impedance spectroscopy (EIS). The investigation of both intact and defective coatings allowed for the comparison of their electrochemical behaviours in order to assess the anticorrosive characteristics of the system. The impedance response of the intact coating was found to correspond to a porous film presenting localised electrochemically active areas, in which the precipitation of zinc-containing corrosion products contributes to the sealing of the coating. Conversely, scribed defects cannot be spontaneously sealed and no effective protection of the metal can be achieved.     

Novel capacitance sensor design for measuring coating debonding: the effects of thermal-hygroscopic cycling

The effects of combined thermal and hygroscopic cycling on the adhesion performance of an epoxy coating were measured using a novel electrode sensor. The sensor is uniquely designed, consisting of a series of independent interdigitated electrode traces which are arranged parallel to the sensor edges. Coupled with single-frequency capacitance measurements, the sensor detects changes in capacitance in the adhered coating–sensor interfacial region as a function of the distance from the edge of the sensor, x. Recently, this sensor was utilized by O'Brien and co-workers to measure interfacial diffusion and the concentration profile of fluid in an adhesive joint (Int. J. Adhesion Adhesives 23, 335–338 (2003)). In the present work, large capacitance changes due to debonding and displacement of the coating by fluids at the sensor surface were used to monitor coating delamination. The apparent debond growth rate and number of cycles until failure were determined as a function of coating thickness, fluid environment and sensor surface chemistry. The results show that the coating becomes more durable as the thickness is reduced; and also that thermal and hygroscopic cycling of coatings produces different results than conventional continuous adhesion tests. This study suggests that this novel sensor or a similar design is applicable for the study of adhesion loss and interfacial diffusion processes, and could be extended to other coatings or adhesives in a variety of environments. General trends about coating durability are also discussed.     

Electrical characterisation of Pb2Bi3SmTi5O18 ceramic using impedance spectroscopy

Abstract

Polycrystalline sample of Pb₂Bi₃SmTi₅O₁₈, a member of tungsten–bronze (TB) family, was prepared using a high temperature solid state reaction technique. X-ray diffraction (XRD) analysis indicated the formation of a single phase orthorhombic structure. AC impedance plots were used as a tool to analyse the electrical behaviour of the sample as a function of frequency at different temperatures. The ac impedance studies revealed the presence of grain boundary effect, from 350°C onward. Complex impedance analysis indicated non-Debye type dielectric relaxation. The Nyquist plot showed the negative temperature coefficient of resistance (NTCR) character of Pb₂Bi₃SmTi₅O₁₈. AC conductivity data were used to evaluate the density of states at Fermi level and activation energy of the compound.     

电化学阻抗谱在电沉积研究中的应用(一)

介绍了电化学阻抗谱在各种金属及合金的电沉积研究中的应用。文章分3期连载。第一部分介绍了电化学阻抗谱的基础知识,包括复数、复阻抗的概念,以及在各种常见条件下电解池的等效电路图。     

Determination of the adhesion properties of an alkyd pigmented coating by electrochemical impedance spectroscopy

One of the most modern methods of characterizing the electrical properties of coatings is by electrochemical impedance spectroscopy (EIS). EIS can provide useful information about the transport of water and corrosive species through a coating. In this study, impedance measurements were performed in different frequency ranges at open circuit potential for an alkyd coating with TiO₂ as a mineral pigment in 3% NaCl. The most probable impedance equivalent circuit method (MPI) was considered for data analysis. The interpretation of the impedance spectra permitted the determination of water permeation, the formation of blisters, swelling of the coating, and the loss of adhesion.     

Evaluation of sub-micrometer parylene C films as an insulation layer using electrochemical impedance spectroscopy

We investigated the insulation performance of sub-micrometer parylene C films over time using electrochemical impedance spectroscopy (EIS). For this, interdigitated electrodes were fabricated and completely encapsulated with parylene C in thicknesses of 50, 100, 200, and 500 nm. The EIS was measured in phosphate buffered saline (PBS) solution under an accelerated aging condition at 90 °C over 45 days. To analyze the EIS data, the equivalent circuit models of coating at different stages of coating degradation were used and the lumped circuit parameters of the best fitted equivalent circuit model were extracted by curve fitting. The analysis of impedance using the equivalent circuit model and the FTIR measurements suggest that sub-micrometer parylene C coatings exhibited delamination resulting from water diffusion from the top surface as soon as being immersed in PBS solution, although the degree of delamination varied depending on the film thickness. The penetration of water through sub-micrometers thick parylene C films can occur as quickly as the film is in contact with solution, unlike for thicker coatings in several micrometers where water diffusion would be saturated before water reaches the bottom surface of the coating.     

In-pile electrochemical tests of stainless steel under PWR conditions: Interpretation of electrochemical impedance spectroscopy data

In-pile electrochemical measurements were performed in order to investigate the effect of radiation on the electrochemical corrosion behaviour of AISI 304 and AISI 316 in PWR primary water (400 ppmB and 2 ppmLi) at 300 °C. All measurements were performed on both an in-flux and an out-of-flux three-electrode electrochemical cell, each containing a high temperature reference electrode. In this paper we focus on the EIS measurements that have been carried out. The quality of these EIS data varied from good to rather poor as the electrical feed through degraded in time due to the harsh test conditions, i.e., radiation, high temperature and high pressure solution with boric acid and lithium hydroxide. The Nyquist diagrams do change in time, but still reflected the phenomena that took place at the metal-metal oxide-solution interface. The electrical feed through used to electrically isolate the working and reference electrodes from the test rig had a life time of minimum 1 month. The degradation of the connectors resulted in a decrease of the isolation resistance of the electrodes. This could be modelled with an equivalent circuit where the isolation resistance was placed in parallel with the actually equivalent circuit for the working electrode impedance. Measured and simulated Nyquist plots were qualitatively in good agreement and show a difference in the oxide layer structure of the in-flux and out-of-flux electrodes.     

The corrosion protection mechanism of rust converters: An electrochemical impedance spectroscopy study

Oxide converters represent an interesting alternative for the protection of steel surfaces that have some degree of rust. Although the mechanism of these converters is not clear, it is assumed that they react with iron oxides and generate new compounds that may have a passivation effect on the steel surface. This last point is not well established, and several authors have even spoken of an accelerating effect of these compounds.We present here a study of the electrochemical behaviour of iron oxides immersed in the rust converter. The modulus of the impedance increases significantly after a certain time of immersion, suggesting that the electronic conductivity and, consequently, the rate of the cathodic reaction tend to decrease.     

Hydrodynamic effect on the behaviour of a corrosion inhibitor film: Characterization by electrochemical impedance spectroscopy

Influence of hydrodynamic conditions on the behaviour of an inhibitor film used in the high speed mechanical field is studied. Experimentations are realized with a Rotating Disk Electrode (RDE). Electrochemical Impedance Spectroscopy (EIS) is carried out at the corrosion potential and also at anodic and cathodic over voltages under various rotation rates and after different immersion times. EIS technique is used to determine the beneficial effect of hydrodynamic conditions on the film formation. This technique characterizes also inhibitive layer modifications for high electrode rotation speeds and after long immersion times. Different analyses, XPS and AFM, were carried out and have confirmed film structure evolution with flow conditions.     

Structure of waterborne coatings by electrochemical impedance spectroscopy and a thermostimulated current method: influence of fillers

Electrochemical impedance spectroscopy (EIS) was coupled with a thermostimulated current (TSC) method to investigate the effect of fillers on the barrier properties of an epoxy-polyamido amine waterborne coating on a 2024 aluminium alloy. Four systems were studied: a clear coat and three pigmented coatings (with and without chromates). The results obtained by TSC highlight the specific action of chromates which decrease the molecular mobility (hardening) of the coating by comparison with neutral fillers. This has been explained by the high polarity of chromates that enhances interactions with the binder. Impedance measurements carried out for free-standing films and for coated aluminium alloy have corroborated the role of chromates on the barrier properties of the coating which remain high as a function of exposure time in a 0.5 M NaCl solution. TSC was also used to measure the glass transition temperature (T_g) of the hydrated systems. T_g was significantly decreased when the coatings were hydrated due to a strong plasticization effect induced by water uptake. For the different systems, we conclude that the weak barrier properties of the films (attached films) or the decrease of the barrier properties with time (free-standing films) were linked to the variation of the glass transition temperature by water uptake.     

Indirect estimation of fiber/polymer bond strength and interfacial friction from maximum load values recorded in the microbond and pull-out tests. Part I: local bond strength

The techniques aimed at adhesion strength measurement between reinforcing fibers and polymer matrices (the pull-out and microbond tests) involve the measurement of the force, F _max, required to pull out a fiber whose end is embedded in the matrix. Then, this maximum force value is used to calculate such interfacial parameters as the apparent bond strength, τ_app, and the local interfacial shear strength (IFSS), τ_d. However, it has been demonstrated that the F _max value is influenced by interfacial friction in already debonded regions, and, therefore, these parameters are not purely 'adhesional' but depend, in an intricate way, on interfacial adhesion and friction. In the last few years, several techniques for separate determination of adhesion and friction in micromechanical tests have been developed, but their experimental realization is rather complicated, because they require an accurate value of the external load at the moment of crack initiation. We have developed a new technique which uses the relationship between the maximum force and the embedded length ('scale factor') to separately measure fiber-matrix interfacial adhesion and friction. Using the equation for the current crack length as a function of the applied load, based on a stress criterion of interfacial debonding, we modeled the pull-out and microbond experiments and obtained the maximum force value versus the embedded length. By varying τ_d and interfacial friction, τ_f, to fit experimental plots, both interfacial parameters were estimated. The micromechanical tests were modeled for three types of specimen geometries (cylindrical specimens, spherical droplets, and matrix hemispheres in the pull-out test) with different levels of residual thermal stresses and interfacial friction. The effect of all these factors on the experimental results is discussed, and the importance of specimen geometry is demonstrated. One of the most interesting results is that the 'ultimate' IFSS (the limiting τ_app as the embedded length tends to zero) is not always equal to the 'local' bond strength.     

Acceleration and quantitative evaluation of degradation for corrosion protective coatings on buried pipeline: Part I. Development of electrochemical test methods

The electrochemical degradation of polyethylene coated onto SS400 was examined in synthetic groundwater. Electrochemical techniques (electrochemical impedance spectroscopy, potentiodynamic and potentiostatic polarization tests) and surface analysis (scanning electron microscopy) were used to accelerate and evaluate the coating degradation. The pulsed potentiostatic polarization test accelerating both the cathodic reduction and anodic oxidation reactions was applied to reproduce the coating degradation mechanisms of cathodic disbondment and oxide lifting. The applied potentials were determined to be ±300 mV_SCE versus open-circuit potential from the analysis of the anodic and cathodic polarization data. Results from the EIS confirmed that coating degradation is accelerated effectively by the pulsed potentiostatic polarization testing.     

Testing of PEM fuel cell performance by electrochemical impedance spectroscopy: Optimum condition for low relative humidification cathode

Electrochemical impedance spectroscopy (EIS) was used to investigate the influence of several parameters on the performance of PEMFC. The applied frequency was in the range of 50 mHz–10 kHz. The experiment was designed by using a 2 ^k factorial design to identify the effects of various parameters including cell voltage, flow rates of gaseous fuels and cell temperature at the saturated humidification in anode and 60% relative humidity cathode. The results indicated that the cell temperature, cell voltage and interactions of cell voltage, flow rate of H₂ and O₂ had a significant effect on the cell performance. In addition, the flow rate of O₂ had a strong effect on the ohmic resistance and the charge transfer resistance in the system. Models describing the relationship between previous parameters and ohmic resistance, charge transfer resistance and capacitance were also developed.     

Electrochemical test methods for evaluating organic coatings on metals: an update. Part I. Introduction and generalities regarding electrochemical testing of organic coatings

The status of evaluating organic coated metals utilizing electrochemical means was reviewed for the period of 1988–1994. The general improvements in the overall technology are presented in three sections. Part I covers the test cell configurations, changes in testing approaches and a brief survey of measurement equipment. Part II presents the test methods involving a single test parameter such as the panel potential relative to a reference electrode, electrochemical voltage and/or current noise, as well as the dc resistance of the coating on the metal substrate. Multiple test parameter measurements such as potentiodynamic curves and electrochemical impedance spectroscopy are covered in Part III. Although the majority of data were taken from the literature, some supplementary data are included from NSWCCD studies.     

Electrochemical composite formation of thiophene and N-methylpyrrole polymers on carbon fiber microelectrodes: Morphology,characterization by surface spectroscopy,and electrochemical impedance spectroscopy

Electrochemical composite thin film formation (∼0.6–0.7 μm) of thiophene and N-methylpyrrole on carbon fiber microelectrodes (diameter ∼7 μm) was carried out by cyclic voltammetry in order to understand and improve the surface properties and capacitance behaviour of carbon fibers. Carbon fiber microelectrodes were coated with polythiophene and N-methylpyrrole was electrografted onto the thiophene electrode. The electrocoated carbon fiber surface mophology was characterized by scanning electron microscopy and atomic force microscopy and by FTIR-reflectance spectroscopy for their composition. The effect of monomer concentration and scan number on electropolymerization has also been investigated. The impedance behaviour of composite electrodes was characterized by electrochemical impedance spectroscopy. The composite of polythiophene and poly-N-methylpyrrole exhibits better charge storage properties than polythiophene coated carbon fiber microelectrodes.