Electrochemical corrosion behavior of composite coatings of sealed MAO film on magnesium alloy AZ91D

Protective composite coatings were prepared on magnesium alloy AZ91D by micro-arc oxidation (MAO) treatment plus a top coating with sealing agent using multi-immersion technique under low-pressure conditions. The corrosion resistance of AZ91D alloy with composite coatings was superior evidently to that with merely MAO film. SEM observations revealed that the sealing agent was integrated with MAO film by physically interlocking; therewith covered uniformly the surface as well as penetrated into pores and micro-cracks of MAO film. The anti-corrosion properties in 3.5% NaCl solution of the composite coatings were evaluated by using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. Based on the results of chronopotentiometric (E ∼ t) and EIS measurements for long time immersion in 3.5% NaCl solution, appropriate equivalent circuits for the composite coatings system were proposed. It follows that due to the blocking effect of the sealing agent in pores and cracks in MAO film, the composite coatings can suppress the corrosion process by holding back the transfer or diffusion of electrolyte and corrosion products between the composite coatings and solution during immersion.     

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.     

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.     

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.     

Electrochemical impedance spectroscopy; a tool for organic coatings optimizations

Electrochemical impedance spectroscopy (EIS) was applied to the optimization of automotive electrodeposited coatings, container interior coatings and industrial maintenance coatings. The electrochemical impedance data were used to predict corrosion protection, film porosity, solution absorption into the coatings and film delamination properties. Variables such as resin contents, crosslink densities, cure temperatures, and solvent types and contents were evaluated for these various types of coatings. In general the electrochemical impedance data correlated well with conventional exposure tests results such as salt fog, cyclic scab corrosion and delamination tests. The impedance spectra permits a rather rapid (15–75 min per sample) assessment of the film's characteristics even when no visually observable changes have occurred. Electrochemical impedance spectroscopy provides a technique to optimize coatings while reducing the time of coating evaluations and gives insight into the chemical and physical properties of the coatings.     

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).     

Trimethylsilane-based pretreatments in a Mg-rich primer corrosion prevention system

A trimethylsilane-based coating was investigated as a pretreatment for Al-2024 T3 in a novel Mg-rich primer corrosion prevention system. SiC-based thin films were deposited onto Al substrates by plasma-enhanced chemical vapor deposition (PECVD). A screening study of the pressure (P) dependence of films deposited at 350 °C showed an increase in growth rate from 0.6 to 1.9 Torr. A second screening study where P was fixed at 1.9 Torr and temperature (T) was varied from 125 to 550 °C showed decreasing growth rates with increasing temperature with an apparent transition around 300 °C. Electrochemical impedance spectroscopy (EIS) of the SiC-based films on Al-2024 after exposure to a corrosive environment (i.e., dilute Harrison solution) indicated that samples coated using SiC-based films exhibit higher low frequency impedance (i.e., 100–1000× higher) than bare Al-2024 with open circuit potential remaining 0.1 V higher for the former suggesting the SiC-based films slow the corrosion process. A Mg-rich primer was coated onto the SiC on Al-2024 with the galvanic function of the system determined by EIS. As compared to SiC on Al-2024, a similar behavior for the low frequency impedance was observed for the Mg-rich primer-coated samples with some films exhibiting 1E + 8 Ω at 0.1 Hz indicating a strong barrier property. Initial gas jet erosion using acrylic media indicates the Mg-rich primer coatings are removed in preference to the Si–C films—the first step toward demonstrating a permanent pretreatment. When successfully developed and optimized, the value of such a hard, protective coating is the reduction of a three-component coatings system (i.e., pretreatment, primer, and topcoat) to a two-component system (i.e., primer and topcoat).     

Improved electrochemical performance of trivalent-chrome coating on Al 6063 alloy via urea and thiourea addition

This project aims at improving the electrochemical performance of trivalent-chrome coating through urea and thiourea addition. The electrochemical behaviors of coatings formed with different concentrations of urea and thiourea were investigated in 3.5 wt.% NaCl solution at 25 °C, using potentiodynamic polarization curves and EIS. The corrosion resistance of coatings is improved greatly by adding a small amount of inhibitors, whereas the excessive addition deteriorates the corrosion resistance. Thiourea addition presents better effect than urea. To explain the EIS results of the coatings, a simple equivalent circuit was designed. The EIS parameters were obtained by fitting the EIS plots. The results of the polarization curves and EIS show that the inhibitor-containing coatings present better corrosion resistance than the coating without inhibitor. The morphology and composition and valence state of the conversion coatings were examined by SEM and EDS and XPS, respectively. The results indicated that the trivalent chromium coating was developed on Al 6063 alloy, urea and thiourea inhibitors were also deposited on the substrates, respectively. A noticeable chemical shift was also observed.     

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.     

Anticorrosive properties of polypyrrole films modified with zinc onto SAE 4140 steel

Polypyrrole (PPy) films modified with zinc were electrosynthesized onto SAE 4140 steel in presence of bis(2-ethylhexyl) sulfosuccinate (AOT). The Zn and PPy electrodeposition was realized by using cyclic voltammetry at different temperatures. The corrosion protection properties of the films were examined in chloride solution by open circuit measurements, linear polarization and electrochemical impedance spectroscopy (EIS). The obtained results indicate that the presence of Zn in the polymer matrix improves the anticorrosive performance of PPy films. The best anticorrosion efficiency was obtained for the coatings modified at 20 °C which provided anodic protection to the steel substrate for a long period of immersion in chloride solution. Cathodic protection was observed when the electrodeposition temperature was increased. Adherence and anticorrosive properties declined sharply for the coatings electrosynthesized at 5 °C.     

Corrosion behavior of NiTi alloy in fetal bovine serum

The corrosion behavior of NiTi alloy in fetal bovine serum (FBS) at 37 °C is investigated using open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry (ICP-MS). The presence of FBS moves the OCP to the negative direction and makes the oxide film thinner and more porous than that in phosphate buffer saline (PBS, pH = 7.4). The impedance of the oxide film formed in FBS is smaller than that in PBS, but the total interface impedance is bigger in FBS because of the formation of a surface bio-film. Pits form on the NiTi alloy after immersion in FBS for 200 h but they are not observed on the sample immersed in PBS. XPS shows that the composition of the oxide film formed in FBS is similar to that formed in PBS and it is composed of mainly Ti oxides with a small amount of Ti hydroxide. Hydrated Ti is observed on the outermost surface of the NiTi alloy. The thickness of the oxide film on the NiTi alloy immersed in PBS is 17 ± 3.0 nm and that on the sample immersed in FBS is 10 ± 3.2 nm. The results are consistent with those obtained by EIS. The presence of FBS can accelerate leaching of Ni ions and the mechanism is investigated and discussed.     

Correlation of morphology and barrier properties of thin microwave plasma polymer films on metal substrate

The barrier properties of thin model organosilicon plasma polymers layers on iron are characterised by means of electrochemical impedance spectroscopy (EIS). Tailored thin plasma polymers of controlled morphology and chemical composition were deposited from a microwave discharge. By the analysis of the obtained impedance diagrams, the evolution of the water uptake ?, coating resistance and polymer capacitance with immersion time were monitored and the diffusion coefficients of the water through the films were calculated. The impedance data correlated well with the chemical structure and morphology of the plasma polymer films with a thickness of less than 100 nm. The composition of the films were determined by means of infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The morphology of the plasma polymer surface and the interface between the plasma polymer and the metal were characterised using atomic force microscopy (AFM). It could be shown that, at higher pressure, the film roughness increases which is probably due to the adsorption of plasma polymer nanoparticles formed in the plasma bulk and the faster film growth. This leads to voids with a size of a few tens of nanometers at the polymer/metal interface. The film roughness increases from the interface to the outer surface of the film. By lowering the pressure and thereby slowing the deposition rate, the plasma polymers perfectly imitate the substrate topography and lead to an excellent blocking of the metal surface. Moreover, the ratio of siloxane bonds to methyl-silyl groups increases which implies that the crosslink density is higher at lower deposition rate. The EIS data consistently showed higher coating resistance as well as lower interfacial capacitance values and a better stability over time for the film deposited at slower pressure. The diffusion coefficient of water in thin and ultra-thin plasma polymer films could be quantified for the smooth films. The measurements show that the quantitative evaluation of the electrochemical impedance data requires a detailed understanding of the film morphology and chemical composition. In addition, the measured diffusion coefficient of about 1.5×10⁻¹⁴ cm² s⁻¹ shows that plasma polymers can act as corrosion resistant barrier layers at polymer/metal interfaces.     

Zinc deposited polymer coatings for copper protection

Polypyrrole (PPy) and polyaniline (PAni) coatings were electrosynthesized on copper, by using cyclic voltammetry technique. Then, these coatings were modified with the deposition of zinc particles from aqueous zinc sulphate solution. The electrodeposition of zinc was achieved at a constant potential value of −1.20 V, in the amount of ∼0.75 mg/cm². The corrosion performance of zinc modified polymer coatings were investigated in 3.5% NaCl solution; by using the electrochemical impedance spectroscopy (EIS), and anodic polarization curves. The zinc particles improved the barrier property of polymer films, thanks to formation of voluminous zinc corrosion products within the pores of polymer coating. Also, the zinc particles provided cathodic protection to the substrate, where the polymer film played the role of conductance between zinc particles and copper.     

Polypyrrole films modified with graphite layer on mild steel

Electrochemically synthesized polypyrrole coating was modified with very thin graphite layer and top coated with another polypyrrole film. The corrosion behaviour of this coating has been investigated in aqueous sodium chloride solution. The synthesis of polypyrrole coatings was carried out by cyclic voltammetry technique, from aqueous oxalic acid solution. Electrochemical impedance spectroscopy and potentiodynamic measurements were used for corrosion tests. The cyclic voltammograms obtained in oxalic acid solution and the polarisation curves obtained in sodium chloride solution showed that the stability of coating was improved significantly by graphite layer. The impedance spectra also showed that the corrosion process was controlled by the diffusion rate along the coating, even after 96 h immersion period. The Warburg coefficient values were calculated and used to evaluate the barrier property of coating with time. It was shown that the water up taking process was slowed down by the hydrophobic nature of the graphite layer sandwiched between the two polypyrrole films.     

Commercial polymer films as calibration standards for EIS measurements

The current protocol regarding calibration and troubleshooting of potentiostats used to conduct electrochemical impedance spectroscopy (EIS) measurements on coated metals involves dummy cells made of simple electrical components. This concept has two major limitations: first, the instrument is used at an impedance range it has not been calibrated for—the instrument is calibrated for impedances in the order of ohms, but corrosion protective coatings usually have impedance values in the range of GΩ cm² or higher; second, commercial coating systems do rarely behave like ideal equivalent circuits of electronic components. In view of these limitations, we propose the use of thin polymer films such as PVF or poly ethylene terephthalate (PET) as new calibration standards for EIS measurements. These films are well defined in thickness and composition and give a highly reproducible response when subjected to EIS measurements. In addition, they closely mimic commercial coating systems with regard to target impedance range and electrical characteristics.     

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.     

Synthesis and characterization of poly(aniline) and poly(o-anisidine) films in sulphamic acid solution and their anticorrosion properties

Poly(o-anisidine) (POA) and polyaniline (PANI) coatings were synthesized on platinum (Pt) surface and stainless steel (SS) in monomer containing 0.50 M sulphamic acid (SA) solution by means of cyclic voltammetry (CV) technique. Meanwhile, poly(o-anisidine) film was also deposited with a different scan rate on SS electrode. The behaviour of PANI and POA films obtained on stainless steel examined by CV was different from the one obtained for PANI and POA on Pt electrode. The corrosion performances of PANI and POA coatings in 3.5% NaCl solution were investigated with anodic polarization technique and electrochemical impedance spectroscopy (EIS). EIS measurements verified the effect of monomers and that of scan rate on corrosion inhibition of coatings on SS electrode. The results showed that POA film synthesized at low scan rate exhibited an effective anticorrosive property on SS electrode. POA synthesized at low scan rate and PANI coatings provided a remarkable anodic protection to SS substrate for longer exposure time than the one observed for POA coating produced at high scan rate as well as that of bare SS electrode.     

Effective corrosion protection of AA6061 aluminum alloy by sputtered Al-Ce coatings

Al-Ce coatings were deposited on silicon and AA6061 aluminum alloy substrates by DC magnetron sputtering using aluminum in combination with pure cerium targets. The materials were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and electrochemical impedance spectroscopy (EIS) in order to consider their application as high corrosion resistance coatings. The corrosion behavior of the films was studied using a NaCl aqueous solution (3.5 wt%). As for the characterization results, an apparent amorphous phase of aluminum oxide with small cerium compounds embedded in the matrix was detected by the X-ray diffraction patterns and HRTEM on the deposited films at 200 W and 4 Pa. At these conditions, AFM and SEM images evidenced crack-free coatings with low-roughness nanometric structures and columnar growth. EIS and Tafel results converged to indicate an inhibition of the corrosion reactions. The film displayed good stability in the aggressive medium and after 1 day of exposure underwent very little degradation. The variations in the impedance and Tafel characteristics were found to occur as a function of cerium content, which provokes important changes in the film protective properties.     

Corrosion‐protection aspects of electrochemically synthesized poly(o‐anisidine) coatings on mild steel: An electrochemical impedance spectroscopy study

The corrosion‐protection aspects of poly(o‐anisidine) (POA) coatings on mild steel in aqueous 3% NaCl solutions were investigated with electrochemical impedance spectroscopy, a potentiodynamic polarization technique, and open circuit potential measurements. The POA coatings were electrochemically synthesized on mild steel with cyclic voltammetry from an aqueous salicylate medium. The corrosion behavior of the POA coatings was investigated through immersion tests performed in aqueous 3% NaCl solutions, and the recorded electrochemical impedance spectra were fitted with an equivalent circuit to obtain the characteristic impedance parameters. The use of a single equivalent circuit was inadequate to explain the various physical and electrochemical processes occurring at different exposure times. It was suggested that some characteristic element(s) should be incorporated into the equivalent circuit at different stages of the immersion to elucidate the various processes occurring at different exposure times. The evolution of the impedance parameters with the immersion time was studied, and the results showed that POA acted as a protective coating on the mild steel against corrosion in a 3% NaCl solution. From these data, the water uptake and delamination area were determined to further support the corrosion‐protection performance of the POA coating. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

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.