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.     

Studies of the impedance models and water transport behaviors of cathodically polarized coating

Cathodic protection (CP) is usually combined with organic coatings to protect metallic structures exposed to seawater. However, the application of CP would enhance coating failure, such as cathodic delamination. To date, there has been few works characterizing the impedance models and water transport behaviors of cathodically polarized coating. In the present article, the analyses of impedance models and water uptake processes of chlorinated rubber coating subjected to various levels of cathodic protection were studied during coatings aging process by electrochemical impedance spectroscopy (EIS).Four distinguished electrical equivalent circuits (EEC) were used to fit the EIS plots of coatings without CP, while only two were employed for samples with CP. Since no corrosion was expected to take place at the metal/coating interface for sample which was polarized cathodically. Coating capacitance was used to investigate the sorption characteristic of water in coating since the increase of C_c was associated with water penetration into the coating. Compared with the sample without CP, those coating systems under CP have a smaller water diffusion coefficient and a further water uptake process after the saturation period.     

Comparisons of clear coating degradation in NaCl solution and pure water

Organic coating's degradation behavior is essential to its corrosion protective function and has been widely studied. A main function of anti-corrosive organic coatings is acting as barriers to water uptake and ion diffusion. It is of great fundamental importance to study the influence of different working fluids on the degradation of organic coatings. In this study, a 3.5 wt% NaCl solution and the pure water are adopted as the working fluids based on their distinct properties. The commercially available polyurethane and epoxy based clear coatings are chosen for evaluation. The coating degradation is monitored by electrochemical impedance spectroscopy (EIS) measurement. Equivalent circuit models are employed to interpret the EIS spectra. The time evolution of coating resistance, capacitance, and water volume fraction of the coating is analyzed. Besides the fact that the coating's barrier property is deteriorated by the percolating of both NaCl solution and pure water, we also discover that pure water leads to faster coating degradation, demonstrated by a more substantial decrease in coating resistance, a more prominent increase in coating capacitance, and a greater saturated water volume fraction.     

Evaluation of a high resistance paint coating with EIS measurements: Effect of high AC perturbations

Electrochemical impedance spectroscopy (EIS) was used to evaluate a high resistance paint coating immersed in 10% sodium chloride solution. Although this method was valuable in evaluating this type of coating there were many fluctuations in measured data at low AC perturbations. In this work the role of high AC perturbation to fit EIS data of high resistance coating with equivalent electrical circuit (EEC) was studied. EIS measurements showed that these fluctuations could be reduced by high AC perturbation. The impedance plots showed best fitting at 400 mV AC perturbation at the initial time of immersion and at 100 mV AC perturbation after 90 days of immersion. The coating resistance and coating capacitance were extracted from the Bode and Nyquist plots during the period of 90 days of immersion.     

Simulation of transient electrochemical impedance spectroscopy due to water uptake or oxide growth

Coatings are often employed to prevent, or at least delay, corrosion of a metallic substrate. The topcoat is often also intended to be a barrier to environmental moisture and any entrained, corrosive ions reaching the substrate. The protective ability of a coating system is very well monitored by electrochemical impedance spectroscopy since it can measure changes as the protective coatings deteriorate, as they absorb water and as they permit the substrate to corrode. Infiltration of water and ions into the coating can also be measured by EIS to deduce approximate water uptake. EIS analysis in coating systems is often predicated on the system being stable over the timeframe of the spectral measurement, which if taken from 100 kHz to 0.01 Hz, or even 0.001 Hz, may take as long as an hour, or more. EIS measurement is most often accomplished by the use of an aqueous electrolyte solution as the electrical contact at the top of the coating, with the metallic substrate used as the other electrical contact. The ingress of aqueous solution is likely to result in time-dependent dielectric or chemical changes to the coating/substrate system. This work shows how these changes, during the spectral measurement period, produce a Bode plot that has a slope of magnitude slightly less than 1. This capacitive response to non-steady state water uptake is commonly characterized by a constant phase element (CPE) in an equivalent circuit model for the material, and equations developed here can be used to deduce the diffusion coefficient based on the CPE.     

Evaluating water transport through high solid polyurethane coating using the EIS method

Buried pipelines are commonly coated with high solid polyurethane to prevent corrosion. The diffusion of water through the coating plays an essential role in the performance and endurance of the coating. This study focuses on the evaluation of water diffusion through polyurethane coatings. The effects of thickness and ambient temperature on water diffusion through polyurethane, which is coated on mild steel and immersed in 3.5% (weight fraction) NaCl solution, are evaluated using electrochemical impedance spectroscopy (EIS). Experimental observations clearly indicate that the water diffusion coefficient strongly depends on service temperature and the thickness of coating. Higher temperature (lower than T _g) increases water uptake of the polymer but no measurable changes were observed for the temperature change studied. EIS results show that the mechanism of corrosion does not change in the range of thicknesses and temperature investigated in this study.     

Electrochemical synthesis of poly(5-nitroindole) on 316L-stainless steel in LiClO4-acetonitrile solution and its corrosion performance

The preparation of poly(5-nitroindole) (P5NI) coating was achieved on 316L-stainless steel (SS). Poly(5-nitroindole) was deposited via anodic oxidation of the corresponding monomer in acetonitrile (ACN) solution containing LiClO₄. The influence of P5NI coating against SS corrosion was studied in 3.5% NaCl solution by electrochemical impedance spectroscopy (EIS), anodic polarization curves and the open circuit potential–time (E_ocp–t) diagrams. The results obtained suggest that P5NI coating forms a sacrificial layer but the efficiency against corrosion is limited with increasing time.     

EIS and in situ AFM study of barrier property and stability of waterborne and solventborne clear coats

Various processes can occur when paints are in contact with moisture, such as ingress of water and aggressive ions into the coating. As a consequence, the microstructure and properties of the paints can be affected. The present study combines electrochemical impedance spectroscopy (EIS) and in situ atomic force microscopy (AFM) to investigate the barrier property of waterborne and solventborne coatings on mild steel, paying particular attention to the occurrences in the first 24 h after contact between the coating surface and electrolyte. The sequential in situ AFM images revealed that changes on the order of hundreds of nanometres at the coating surface have occurred shortly after the exposure to the electrolytes. EIS observations for the clear waterborne alkyd coating revealed a rise in the |Z|_0.015Hz and a decrease in the coating capacitance after a few hours of exposure. Evidences that water uptake caused swelling of the coating and promoted the closure/blockage of pores were given by means of in situ AFM. The solventborne alkyd emulsion has demonstrated lower reactivity to the presence of the electrolyte and a correlation between the coating resistance and defects/pores evolution is suggested.     

EIS investigation of the deposition of trivalent chromium coatings on Al 6063 alloy

The influence of five variables (i.e. deposition temperature, time, bath pH, and concentrations of Cr³⁺ compound (KCr(SO₄)₂) and H₃PO₄) on the preparation of Cr³⁺ coating on Al 6063 alloy was investigated using AC impedance spectroscopy (EIS) in 3.5 wt% NaCl. The optimal conditions were determined by AC impedance spectroscopy. The results indicated that the formation and quality of the coating were very sensitive to the deposition bath pH. A mechanism was proposed to explain the results. A simple model was derived and experimentally tested in terms of an equivalent circuit. Good agreement was found between the model predictions and the experimental results. The morphologies of the coated and uncoated electrodes were examined by scanning electron microscopy (SEM), and the results also support the proposed surface model.     

交流阻抗谱研究钙磷陶瓷电沉积层的结构

用交流阻抗技术研究了不同电沉积时间的钙磷陶瓷沉积层。结果表明，沉积时间不同的陶瓷沉积层的交流阻抗谱存在显著的差别。采用双层结构物理模型，对不同沉积时间陶瓷沉积层的交流阻抗谱进行计算机拟合，得到的等效电路各元件拟合值显示：电极表面钙磷陶瓷为内密外疏型梯度沉积层。在陶瓷电沉积过程中，钙磷晶粒同时沉积于距电极表面不同距离的部位，沉积层厚度增加的同时内层密度也在逐渐增加，整个 只层的梯度结构处于动态变化之中。     

聚天门冬氨酸酯聚脲涂层湿热失效行为的电化学

采用电化学阻抗谱研究了聚天门冬氨酸酯聚脲涂层在湿热环境作用下的失效行为,采用SEM研究了聚天门冬氨酸酯聚脲涂层失效前后表面形貌的变化。电化学研究结果表明:涂层湿热失效过程分为3个主要阶段:前期电解质在涂层中的渗透、中期基体金属腐蚀发生起始的过渡、后期基体金属的腐蚀扩展并导致涂层的失效。湿热试验中,涂层孔隙率和涂层吸水率均呈3个阶段变化,即快速增长阶段、缓慢增长阶段和稳定阶段,很好地验证了PAEP涂层湿热失效的3个过程。表面形貌研究发现,湿热试验4572 h后,涂层表面粗糙度增加,出现了裂缝等缺陷,表明涂层已经失去防护作用。     

EIS characterisation of new zinc-rich powder coatings

A zinc-rich powder coating, applied onto steel substrate, was studied using electrochemical impedance spectroscopy (EIS). Before immersion when coating is dry, EIS spectra revealed that the percolation threshold was reached. Then, an equivalent circuit including a transmission line was applied to model electrochemical response. In this model, which considers isolated and semi-isolated zinc particles within the binder, it was found that both phases were equally distributed in the coating.     

Electrochemical impedance spectroscopy response of water uptake in organic coatings by finite element methods

The corrosion of metals is a long standing topic of research which is of immense economic importance. One of the principal means of preventing surface corrosion of metals is thin polymer coatings, i.e. paint. These coatings often fail because of the passage of water or other active ionic species into and through the polymer coating. Since both the capacitance and the resistance of a polymer coating change as it absorbs water, electrochemical impedance spectroscopy (EIS) gives a general idea of how much water a polymer has been absorbed. Theories, such as the Brasher-Kingsbury approximation, are effective medium theories based on the assumption that the absorbed water is randomly distributed in spherical inclusions. The water in a coating may be distributed as spherical inclusions, as discrete channels, or as some combination that transports water from the coating surface until the water reaches the metal substrate and corrosion can begin.The resistance and the capacitance of a coating depend on both the amount of water (volume fraction) and the shape of the water inclusions. EIS gives only a general idea of how much water has been absorbed by a coating but does not provide the distribution or shape of the water inclusions. EIS circuit response is often modeled with the equivalent circuit elements describing the material properties for water inclusions that are implicitly assumed to be randomly distributed spherical inclusions. Numerical calculations using the finite element analysis (FEA) are reported here to solve Maxwell's equations for various shapes and sizes of water inclusions within the polymer. Calculations here have been based on the electrical properties of a polyvinyl fluoride film, as an exemplar, with water inclusions of different shapes and concentrations (water volume fraction). The Brasher-Kingsbury approximation gives the correct outcome only for a random distribution of spherical inclusions, as expected. Other shapes and distributions can vary from the Brasher-Kingsbury prediction of water volume fraction by more than 50% of the actual gravimetric water volume fraction. Results are presented here for spherical and cylindrical randomly distributed water inclusions. Understanding the sensitivity to different distributions and numbers of inclusions is an objective planned for future research.     

Determination of water uptake and diffusion of Cl ion in epoxy primer on aluminum alloys in NaCl solution by electrochemical impedance spectroscopy

The electrochemical impedance spectroscopy (EIS) of epoxy-coated aluminum alloy LY12 has been investigated during exposure to 3.5% NaCl solution. Using the continuous simulation of EIS by expanded general electrical model, the time-dependent impedance model of the alloy/coating/solution system was deduced. The results shown that the composite electrode displayed a barrier behavior before water and oxygen penetrated to alloy base. After water and oxygen reached the base, the impedance associated with corrosion of alloy base changed with the immersion time as following: (i) active corrosion period at the beginning (double-layer capacitance, C_dl, in parallel to the charge transfer resistance of electrochemical corrosion R_ct), (ii) impeding of the diffusion of corrosion production at the intermediate period as a result of the presence of coated film (a constant phase element Z_diff was additionally in series with R_ct), and (iii) appearance of the characteristic impedance related to Cl⁻ ion-participating reaction with alloy base at the later stage. From the linear part of ln C_c–t^0.5 curve in the early immersion stage, the apparent diffusion coefficient of water was obtained. The diffusion coefficient of water and Cl⁻ ion through the coating was also calculated by the required time for diffusion of permeation species through the coating to the metal interface obtained from the simulation of EIS data by which the occurrence of characteristic impedance element(s) corresponding to special species arrival can be determined.     

Development of a new experimental method to determine critical pigment-volume-concentrations using impedance spectroscopy: Part III: Water-based coatings with components typical for commercial organic anticorrosion coatings

Electrochemical impedance spectroscopy (EIS), which has been previously demonstrated to be a suitable method to determine the CPVC of organic coatings [R.E. Lobnig, W. Villalba, K. Goll, J. Vogelsang, I. Winkels, R. Schmidt, P. Zanger, J. Soetemann, Development of a new experimental method to determine critical pigment-volume-concentrations using impedance spectroscopy, Prog. Org. Coat. 55 (2006) 363–374], was applied to three series of coatings with components typical for water-based commercial organic anticorrosion coatings and to one with components typical for a wall paint. The new method led to CPVC-values for all four coating series, but up to 5–10% higher than those determined from cross sections with scanning electron microscopy (SEM), for one coating series to the exact reference value. The CPVC could be identified best from EIS data by visual comparison of the shape of Bode plots or from pore resistance (R_p)-time plots. The constant phase element (CPE), that characterizes the coating, was less useful for the determination of the CPVC of most water-based coatings.     

Microstructure and electrochemical characterization of trivalent chromium based conversion coating on zinc

A trivalent chromium based conversion coating (CCC), based on chromium nitrate solution with Co(II) ions, was developed on Zn substrate. The corrosion resistance of the trivalent CCC, measured in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution using anodic polarization and electrochemical impedance spectroscopy (EIS), was very sensitive to both immersion time and bath pH. Micro-cracks were found on the surface of the CCC. Besides, the density of micro-crack and the coating thickness also depended on immersion time and bath pH. With increasing the coating thickness its pitting potential increased and passive current density decreased. The trivalent CCC formed on Zn for 40 s in pH 1.7 bath showed the best corrosion resistance, and the pitting potential increased significantly from −355 mV_SCE for Zn to 975 mV_SCE for the trivalent CCC on Zn. To explain the corrosion behavior of the trivalent CCC using EIS analysis, a modified equivalent circuit, which considered the micro-cracks in the coating and chromium corrosion product (CCP) deposited in the micro-cracks, was designed and the variation of each electrical parameter was examined. Especially, its corrosion behavior was well described by the variation of the resistance of CCP (R_ccp).     

Corrosion protection assessment of sacrificial coating systems as a function of exposure time in a marine environment

The present investigation assessed the corrosion protection performance of 17 different Zn and Al sacrificial coating system configurations during marine atmospheric exposure at Kure Beach, NC. The coating systems incorporated several conversion coating layers, primers and organic topcoats. Visual observations and electrochemical measurements (including electrochemical impedance spectroscopy, EIS) were made on six different occasions throughout the 20-month exposure time. Milled scribes on each of the coating specimens allowed for defect protection as well as barrier protection to be investigated. A novel corrosion analysis technique utilizing a specialized conducting agar (SCAR) cell enabled impedance measurements to be made on both intact and defect areas. Visual observations, E_oc's, and EIS as a function of atmospheric exposure time provided complementary results. Impedance results were found to be useful in determining a coating's barrier protection and scribe damage analysis accurately represented defect protection.     

Preparation and characterization of a double-layer coating on magnesium alloy AZ91D

A double-layer coating was prepared on AZ91D alloy by plasma electrolytic oxidation (PEO) plus electroless plating (EP). The plasma eletrolytic oxidation film was prepared in a silicate bath as an inner layer of the coating. Electroless plated Ni-P layer grew from the pores of the PEO film in a nickelous acetate bath and formed as the outer layer of the coating. The microstructure and crystallographic structure was observed with FESEM and XRD. The corrosion resistance of the double-layer coating was evaluated by means of chronopotentiometric (E-t), potentiodynamic polarization (E-i), neutral salt spray test and electrochemical impedance spectroscopy (EIS) test. Compared with the data of as-cast AZ91D magnesium, the open circuit potential of the double-layer coated AZ91D alloy increased by 1.1815 V, while the self-corrosion current density decreased by two orders of magnitude. E-i, EIS result showed that the corrosion resistance of magnesium alloy AZ91D was improved by the double-layer coating. The salt spray test and polarization test results show that the pitting corrosion resistance of AZ91D alloy was improved greatly. An equivalent circuit was proposed to fit the impedance diagrams of AZ91D alloy with the coating.     

Mica/polypyrrole (doped) composite containing coatings for the corrosion protection of cold rolled steel

Micas/polypyrroles (PPys) doped with molybdate, p-toluene sulfonate, dodecyl benzene sulfonate, and 2-naphthalene sulfonate composite pigments were synthesized by chemical oxidative polymerization and characterized in coatings for corrosion protection on cold rolled steel substrate by various electrochemical techniques. Synthesized composite pigments were characterized for morphology by scanning electron microscopy, which indicated physical formation of PPy on the surface of mica. Chemical composition of the composite pigments was analyzed by X-ray photoelectron spectroscopy which chemically confirmed doped PPy formation on the mica surface. Coatings were formulated at 20% pigment volume concentration (composite pigments or as-received mica pigment) and were applied on cold rolled steel substrate. Coatings were exposed to salt spray test conditions (ASTM B117) for 30 days and were periodically assessed for corrosion with electrochemical impedance spectroscopy (EIS), open circuit potential (OCP), and potentiodynamic polarization. EIS and circuit modeling results demonstrated higher coating resistance (R _c) for mica/PPy (doped) composite coatings as compared to as-received mica pigment containing coating after 30 days of salt spray exposure. Lower current density and more positive corrosion potential values were observed for mica/PPy (doped) composite coatings as compared to mica pigment-based coating in potentiodynamic polarization measurements, indicating improved corrosion protection for cold rolled steel substrate. OCP measurements revealed more positive values for mica/PPy (doped) composite coatings as compared to mica pigment-based coating suggesting superior corrosion protection for mica/PPy (doped) composites.     

The corrosion protection of mild steel by single layered polypyrrole and multilayered polypyrrole/poly(5-amino-1-naphthol) coatings

Electrochemical synthesis of polypyrrole (PPy) and top coat of poly(5-amino-1-naphthol) (PANAP) on PPy films from oxalic acid solution was achieved on mild steel (MS) by cyclic voltammetry technique. The morphology and the structure of the films were investigated by scanning electron microscopy (SEM). The corrosion performance of this multilayer coating and single PPy coating were investigated in 3.5% NaCl solution by using open circuit potential (E_ocp)–time curves, polarization curves and electrochemical impedance spectroscopy (EIS). It was found that the multilayer PPy/PANAP coating could provide much better protection than single PPy coating for corrosion of MS. It was observed that corrosion performance of coatings was increasing with immersion period. This was explained by auto-undoping properties of PPy coatings during immersion in corrosive solution. The improved corrosion performance in the presence of PANAP top coat on PPy was explained by increase in barrier effect of bilayer films.