Novel approach to correlate degree of surface deterioration to coating impedance for laboratory test panels coated with two types of primers

Abstract

Relationships between deteriorated coating surface conditions of laboratory test panels determined by image processing software and coating impedance data collected with electrochemical impedance spectroscopy (EIS) were investigated. A unique relationship was found between the log?|Z|_0·1 values of experimental EIS data and the corresponding deteriorated area ratios determined by the image processing method. A well defined one to one relationship was also found between log?|Z|_0·1 values and log?(R_po) values extracted through EIS data fitting analysis. This study demonstrates that estimation of log?(R_po) at a certain deterioration stage is feasible via field image processing alone without running in situ EIS measurements and going through time consuming EIS fitting analysis. A field coating maintenance program established by this methodology can be an effective tool for bridge maintenance engineers who need to make timely decisions on coating repair/rehabilitation work based on the quantitative ‘visual’ image scan results and ‘invisible’ coating pore resistance.     

Determination of the long term corrosion behavior of coated steel with A.C. impedance measurements

A previous study of the a.c. impedance of thin polymer films on aluminum alloys, steel, and phosphated steel, determined over a wide frequency range, has resulted in a general model for the electrochemical impedance across the coated surfaces. According to this model, penetration of the coating by the electrolyte results in an impedance behavior which is typical of a dielectric film short-circuited by conducting electrolytic paths perpendicular and parallel to the polymer/metal interface. Comparison of the time dependence of the impedance of coated steel substrates with the impedance across a free film has demonstrated that the corrosion of the substrate enhances the development of parallel paths of ionic conduction in the coating. A.c. impedance measurements carried out on polybutadiene-coated steels pretreated by different procedures determine quantitatively the resistance R_p of the conducting paths. The time dependence of R_p for the coated substrates has been determined for exposure periods of up to three weeks. Visual observation of the corroded and delaminated areas according to ASTM D610 has been carried out simultaneously, and correlations have been established with R_p. The results are discussed in terms of the possibility to predict the corrosion behavior of a coated metal, based on the measurement of R_p.     

Corrosion electrochemical characteristics of red iron oxide pigmented epoxy coatings on aluminum alloys

Red iron oxide pigmented epoxy coatings were prepared on aluminum alloys and were characterized by electrochemical impedance spectroscopy (EIS) immersed in NaCl solution. The evolutions of impedance models of coated metals were obtained by the fitting analysis of experimental data using suitable equivalent electrical circuits (EEC). The results indicated that the composite electrode system could be fitted by three impedance models. At the initial immersion stage, coatings acted as a barrier layer and only performed a simple circuit consisting of a coating resistance (R_c) parallel to a coating capacitance (C_c). After a certain time of exposure, water and (or) oxygen arrived at the metal surface through the coating, leading to the formation of electrochemical corrosion sites at metal interface and thereby the appearance of double-layer capacitance (C_dl) and charge transfer resistance (R_ct). After intensive attacking of metal substrates, the mass transfer of corrosion products was in difficulty. In this case, the diffusion elements were added to the EEC. It was found that due to the presence of inert pigment particles, the mass transfer behaviors were interestingly different from those of the varnish polymer coatings. Electrical parameters were also obtained from the EIS data. lnC_c-time curve showed a Case II water sorption kinetics, typical non-Fickian diffusion for water uptake.     

Analysis of impedance spectra on corroding metals

The dispersion occurring during impedance measurements on corroding metal surfaces can be mathematically represented by the introduction of a frequency-dependent impedance Z₂(ω). In order to determine this impedance physically, an analysis of the term Z₂(ω) = K₂⁻¹(jω)^−α2 is carried out. Supplementary to the conventional transmission line model for solid electrodes, a new model is presented. The occurrence of frequency-dependent elements is explained here as a superposition of many partial reactions in the form of R_p — C_p elements in series circuit. The frequency of the individual relaxation times is determined by means of the empirical distribution expression by Cole and Cole. It is shown that the results of impedance measurements on zinc-plated electrodes with a defective polymer layer after an intensive chemical loading can be represented by this model. The causes of dispersion are discussed in a general way.     

The effect of different exposure conditions on the biofilm/copper interface

The effects of the different exposure conditions on the electrochemical behavior of copper were evaluated in a growth medium containing Shewanella oneidensis MR-1. Impedance spectra were recorded at the corrosion potential (E_corr) in three different cells for one week of exposure followed by cyclic voltammetry. A second time constant was observed in the impedance spectra of copper that was partially immersed in the test cell, where the electrode was in contact with an air/liquid interface (cell B). These spectra resembled those usually observed for metals covered with a polymer coating. Complete immersion of copper in the electrolyte (no air/liquid interface) or deaeration of cell B resulted in one-time-constant spectra that are typical of those found for passive metals. Excellent corrosion protection was provided by MR-1 regardless of exposure condition. E_corr increased with time for the partially immersed Cu electrode exposed to the aerated solution in cell B, while it decreased for the other two exposure conditions. Cathodic polarization curves recorded after exposure for 7 days showed two reduction peaks for copper tested in cell B, while no reduction peaks were observed for the other cases. Similar results were obtained using cyclic voltammetry.     

Analysis of electrochemical impedance of polyaniline films prepared by galvanostatic,potentiostatic and potentiodynamic methods

Polyaniline (PANI) films have been deposited on stainless steel (SS) substrate by galvanostatic, potentiostatic and potentiodynamic methods. Electrochemical impedance spectra of the electrodes have been analyzed using a transmission line model consisting of two rails of finite resistances. The impedance spectra of the galvanostatically prepared electrodes are similar in shape to those of potentiostatically grown electrodes. The resistance of aqueous pore (R_aq) due to ionic doping–dedoping process is greater than the resistance (R_p) of electronic charge-transfer on polymer chain. These parameters exhibit an increase in magnitude with thickness of PANI, as expected from the transmission line theory. The shape of impedance spectra of potentiodynamically grown PANI, however, is different and its R_aq ≤ R_p. The impedance parameters of potentiodynamically prepared PANI/SS electrodes have been found to depend on the sweep rate used for the deposition of PANI. Several possible explanations have been discussed to understand the influence of sweep rate during deposition on impedance parameters of the electrodes. In contrast to the behavior of PANI deposited by galvanostatic and potentiostatic methods, R_p decreases with an increase in thickness of PANI deposited by the potentiodynamic method. The effect of concentration of aniline used for the deposition of PANI on impedance parameters has also been studied. The results of capacitance of PANI obtained from the ac impedance studies have been found to be comparable with those obtained from the cyclic voltammetric studies.     

Evaluation of localized corrosion phenomena with electrochemical impedance spectroscopy (EIS) and electrochemical noise analysis (ANA)

The use of electrochemical impedance spectroscopy (EIS) and electrochemical noise analysis (ENA) for non-destructive evaluation of corrosion processes is illustrated for three model systems. EIS can be used to detect and monitor localized corrosion of Al alloys and determine pit growth laws which can be used for lifetime prediction purposes. Electrochemical potential and current noise data can be analyzed in the time and the frequency domain. A comparison of noise data obtained for Pt and an Al 2009/SiC metal matrix composite (MMC) exposed to 0.5 N NaCl has shown that the use of potential noise data alone can lead to erroneous conclusions concerning corrosion kinetics and mechanisms. The electrochemical noise data have been evaluated using power spectral density (PSD) plots in an attempt to obtain mechanistic information. The system Fe/NaCl has been used to determine the relationship between the polarization resistance R_p obtained from EIS data and the noise resistance R_n determined by statistical analysis of potential and current noise data. Potential and current noise can be recorded simultaneously allowing construction of noise spectra from which the spectral noise resistance R can be obtained as the limit for zero frequency. Good agreement between R_P, R_n and R has been observed for iron exposed to NaCl solutions of different corrosivity. For polymer coated steel exposed to 0.5 N NaCl for five months analysis of EIS data allows to draw conclusions concerning the degree of disbonding of the coating and the decrease of the coating resistivity with exposure time. R_n and R obtained from electrochemical noise data for an alkyd coating on cold rolled steel agree with each other and show the same time dependence as R_p and the pore resistance R_po determined from EIS data, but are significantly lower than R_p and R_po. The relationships of derived noise parameters such as R_n and R to coating properties and to the remaining lifetime of a polymer coating are not clear at present.     

Humidity sensitive properties and sensing mechanism of π-conjugated polymer p-diethynylbenzene-co-propargyl alcohol

A novel thin film resistive humidity sensor based on soluble FeCl₃-doped copolymer of p-diethynylbenzene with propargyl alcohol (p-diethynylbenzene-co-propargyl alcohol, DEB-co-OHP) was prepared. Its humidity sensitive properties were investigated by electrochemical methods (complex impedance spectra, chronoamperometric and voltammetric measurements), and sensing mechanism presented. The impedance of the sensor changed from 10⁷ to 10⁴ Ω in the range of 33–95% relative humidity (RH), showing high sensitivity. Comparison of steady-state current–RH plots of film with and without DEB-co-OHP indicated that conjugated polymer plays a dominant role in the realization of conductivity change with humidity.     

Electrochemical Impedance Spectroscopy of protective coatings

Electrochemical impedance spectroscopy (EIS) carried out over a wide frequency range (10⁵ to 10^?2 Hz) allows us to obtain mechanistic information concerning corrosion protection by coatings. Examples are given for polymer coatings on an Al alloy as a function of surface petreatment and for anodized Al alloys. For polymer coatings on metals, information concerning the coating's properties and its changes with exposure time can be obtained from the high and medium frequency regions of the impedance spectrum, while the corrosion reaction at the metal/coating interface can be evaluated from the low frequency part. The pore resistance of the coating and its changes with exposure time have been used to rank different pretreatment procedures for a given metal/coating combination. For Al alloys, pronounced differences in corrosion resistance between a conversion coating and an anodized layer under a polyurethane coating have been observed. The use of a segmented electrode allows measurements of the impedance across as well as under a coating. From these data, information concerning delamination can be obtained. An example is given for an epoxy coating on steel. The use of EIS as a quality control and corrosion test for anodized Al alloys is discussed. The effects of the anodization procedure (sulfuric acid and chromic acid), sealing procedure (hot water and dichromate) and alloy chemistry (Al 2024, 6061 and 7075) have been studied during exposure to aerated 0.5 N NaCl. All these parameters play an important role in the corrosion resistance of the alloys. The sealing process can be evaluated from the high frequency part of the spectrum. Pronounced differences in the spectra for the two sealing procedures are observed. The corrosion behavior is reflected in the low frequency part, which is essentially dominated by barrier layer properties.     

Impedance of NiSi electrode in sulfuric-acid solution in the active-passive transition range

The behavior of NiSi electrode in 0.5 M H₂SO₄ in a potential range of active-passive transition (from ?0.02 to 0.40 V (N.H.E.)) is studied with the use of impedance spectroscopy. Nyquist plots with an arc in the second quadrant (Z′ < 0 and Z″ > 0) in the lowest frequency range were constructed for the beginning of the active-passive transition. The shape of impedance plots is explained by the effect of the solid-phase diffusion of selectively dissolved nickel on the character of passivation of nickel silicide.     

Cracking investigation of W and W(C) films deposited by physical vapor deposition on steel substrates

This paper presents the investigation of the cracking of coatings deposited on steel substrates. The coating on substrate systems consisted on pure tungsten films (W) and films of solid solutions of carbon in tungsten [W(C)], which were deposited by direct current reactive magnetron sputtering on stainless steel substrates. The systems were strained uniaxially with a microtensile device adapted to a scanning electron microscope. The mechanical response was analyzed from the experimental results: the straining of the samples showed an evolution of the density of cracks in the coating, which was described trough an empirical equation based on the Weibull distribution function. The density of cracks, which corresponds to the crack saturation of the coating, appeared to vary inversely with coating thickness. Critical parameters relative to their mechanical stability were also determined from the experimental results: the strain energy release rate for crack extension through the film, G^f_c, and the fracture toughness, K^f_Ic, of the coatings. These values are included between 0.2 and 14 J m⁻², and between 0.1 and 2.5 MPa m^−1/2. The fracture resistance of W and W(C) coatings was found to be correlated to their thickness and microstructure.     

Correlation between electrochemical impedance and noise measurements of waterborne coatings

A modified equivalent electric circuit is proposed to establish correspondence between impedance and noise measurements in waterborne coatings in saline media (NaCl). The polarization resistance (R_p) given by the impedance modulus when frequency approaches to zero can not directly be compared with noise measurements since this latter accounts only for the pore resistance of the paint of low barrier protective coatings while R_p parameter represents the sum of all the resistances at the system. The results have shown that the pore resistance data can be attributed to noise resistance when (i) the coating exhibit porosity presenting diffusion mechanism as rate-determining step, and (ii) when the sampling frequency of recorded data gives a Nyquist frequency of 1 Hz.     

Structure and corrosion resistance of ZrO2 ceramic coatings on AZ91D Mg alloys by plasma electrolytic oxidation

The aim of this work is to study the structure and the corrosion resistance of the plasma electrolytic oxidation ZrO₂ ceramic coatings on Mg alloys. The ceramic coatings were prepared on AZ91D Mg alloy in Na₅P₃O₁₀ and K₂ZrF₆ solution by pulsed single-polar plasma electrolytic oxidation (PEO). The phase composition, morphology and element distribution in the coating were investigated by X-ray diffractometry, scanning electron microscopy and energy distribution spectroscopy, respectively. The results show that the coating thickness and surface roughness were increased with the increase of the reaction time. The ceramic coatings were of double-layer structure with the loose and porous outer layer and the compact inner layer. And the coating was composed of P, Zr, Mg and K, of which P and Zr were the main elements in the coating. P in the coating existed in the form of amorphous state, while Zr crystallized in the form of t-ZrO₂ and a little c-ZrO₂ in the coating. Electrochemical impedance spectra (EIS) and the polarizing curve tests of the coatings were measured through CHI604 electrochemical analyzer in 3.5% NaCl solution to evaluate the corrosion resistance. The polarization resistance obtained from the equivalent circuit of the EIS was consistent with the results of the polarizing curves tests.     

The application of the transform of Kramers- Kronig for computing the polarization resistance

Some general considerations on the use of the Kramers-Kronig transform for computing the real and imaginary parts of the electrode impedance Z(ω) are given and the relationship between R_p and Zr(0) is established by considering a simple electric network. Experimental applications concern the behaviour of iron in 5 wt% HCl solutions, containing a specific inhibitor for this environment with concentration values ranging from 0.1 to 3.0 g/kg, at temperatures of 65, 75, 80 and 90°C. Electrode impedance determinations were performed over the [0.08, 2 × 10⁴] Hz frequency interval under current control. The values of R_s were usually determined by processing experimental data belonging to the [0.08, 2 × 10⁴] Hz frequency interval. The values of R_p, based on the use of the first Kramers-Kronig relation between Z_r(ω) and Z_i(ω), were computed using the D01GAF subroutine of the NAG library. The accuracy of this procedure was verified by comparing the experimental and computed values of Z_r(ω). Comparison of the two sets of values of Z_r(ω) confirmed that the use of the Kramers-Kronig transform was pertinent to analyse experimental data because the reproduction of all examined experimental data was rather faithful over the [0.08, 2 × 10⁴] Hz frequency interval. At last, the first Kramers-Kronig relation provide a valid mathematical tool to compute the correct magnitude order of R_p as a function of inhibitor concentration.     

Interfacial adhesion characterization of plasma coatings by V(z) inversion technique and comparison to interfacial indentation

This paper presents the development of V(z) inversion technique and its application to quantitative determination of interface adhesion by measuring interface tangential stiffness parameter K_T. The measurement is performed in two steps on an air plasma sprayed bronze-aluminum alloy coating (on a steel substrate) having different adhesion levels: the determination of the coating elastic constants and the determination of the interface tangential stiffness K_T are performed by optimizing the inverted angular-frequency reflectance function R(θ,f). The results obtained by the V(z) inversion are then compared to mechanical testing by interfacial indentation on the same sample. The same behavior (quality of interface adhesion) can be seen by both methods: the coating adhesion decreases as the interfacial stiffness parameter K_T decreases.     

BTA@SPANI-POSS环氧涂层的制备及防腐性能

为提高环氧涂层在腐蚀环境下的防腐性和持久性,合成一种负载有缓蚀剂苯并三唑(BTA)的苯并三唑@磺化聚苯胺功能化倍半硅氧烷(BTA@SPANI-POSS),随后将BTA@SPANI-POSS与环氧树脂共混得到BTA@SPANI-POSS环氧涂料,最后在Q235碳钢上制备数种复合环氧涂层。通过红外光谱、紫外可见光谱、扫描电子显微镜对BTA@SPANI-POSS的结构、缓蚀性能、表面形貌进行表征,利用接触角测量仪、电化学工作站研究所制备涂层的疏水性能和防腐性能。研究表明,随着SPANI-POSS的添加,涂层沾湿性能降低。电化学阻抗谱(EIS)和塔菲尔极化曲线测试结果表明,与SPANI-POSS环氧涂层相比,负载有BTA的BTA@SPANI-POSS环氧涂层对金属基底具有更高和更持久的保护能力,其中试样EB1.5%的腐蚀电流密度icorr为16.67?A·cm^-2,其极化电阻Rp为2.467 M?·cm²,具有较低的腐蚀动态速率。在3.5 wt.%NaCl溶液中浸泡15 d后环氧涂层仍具有良好的防腐蚀效果,其阻抗值Z0.01Hz仍保留有第1 d时的2...     

Microstructure and corrosion resistance of ceramic coating on carbon steel prepared by plasma electrolytic oxidation

Ceramic coating was prepared on Q235 carbon steel by plasma electrolytic oxidation (PEO). The microstructure of the coating including phase composition, surface and cross-section morphology were studied by X-ray diffraction (XRD), Fourier transform infrared spectroscope (FTIR) and scanning electron microscopy (SEM). The corrosion behavior of the coating was evaluated in 3.5% NaCl solution through electrochemical impedance spectra (EIS), potentiodynamic polarization and open-circuit potential (OCP) techniques. The bonding strength between Q235 carbon steel substrate and the ceramic coating was also tested. The results indicated that PEO coating is a composite coating composed of FeAl₂O₄ and Fe₃O_4. The coating surface is porous and the thickness is about 100 μm. The bonding strength of the coating is about 19 MPa. The corrosion tests showed that the corrosion resistance of Q235 carbon steel could be greatly improved with FeAl₂O₄-Fe₃O₄ composite coating on its surface.     

Comparison of the degradation behaviour of fusion-bonded epoxy powder coating systems under flowing and static immersion

The degradation of three different fusion-bonded epoxy (FBE) powder coating systems under flowing and static immersion condition has been monitored using electrochemical impedance spectroscopy (EIS) when exposed to 3% NaCl aqueous solution at 60 °C. The aim of this project was to determine the impact of flowing condition on the degradation of the protective properties of polymer coatings during exposure to corrosive medium. Using a rotating cylinder apparatus, the immersion tests under the flowing condition were performed. The relative permittivity of coating, , where the coating capacitance C_c was calculated from the high frequency data of impedance spectrum, was selected as the index to monitor property variation with immersion time. Experimental results showed that the flowing condition aggravated the deterioration of coatings. The results were interpreted in terms of a model in which flowing condition changes coating/solution interface state and then accelerates the ions to diffuse through the coating. The electrochemical results were in agreement with the final visual observation. The present investigation suggests that flowing test provides an effective accelerating way to evaluate the degradation of coating system.     

Study of ac impedance spectroscopy of Al doped MnFe2−2xAl2xO4

We have reported electrical properties of Al doped MnFe₂O₄ ferrite using ac impedance spectroscopy as a function of frequency (42 Hz to 5 MHz) at different temperatures (300–473 K). XRD analysis shows that all the compositions are single phase cubic spinel in structure. The complex impedance analysis has been used to separate the grain and grain boundary resistance of MnFe_2−2xAl_2xO₄. From the analysis of impedance spectra it is found that the real (Z′), and imaginary (Z″) part of the impedance decrease with increasing frequency and both are found to decrease with Al doping up to 20%, and thereafter, these increase with further increasing the Al concentration. Experimental results have been fitted with two parallel RC equivalent circuits in series.     

Electrochemical synthesis and corrosion performance of poly o-anisidine on 304 stainless steel

The corrosion inhibition of the synthesis poly o-anisidine on 304 SS has been studied by using a different electrochemical technique such as potential-time measurement, EIS and polarization methods in 0.5 M HCl, and 3.5% NaCl solutions. The poly o-anisidine layer has been obtained on SS electrochemically from 0.5M H₂SO₄ acid solution by using cyclic voltammetric (CV), potentiostatic and galvanostatic techniques. The polymer layer provides around 93% protection in acid solution by potentiostatic coating. Nevertheless, the poly o-anisidine coating is not protected in 3.5% NaCl media by cyclic voltametry and galvanostatic coating. The examination of polymer surface by SEM confirms the results obtained. For characterization of polymer layer the UV-vis spectra and Fourier transform infrared (FTIR) spectra of the polymer films were measured. This work was aimed to study the effect of electrodeposition method of poly o-anisidine on corrosion protection of 304 SS.