A comparative study between the results of different electrochemical techniques (EIS and AC/DC/AC): Application to the optimisation of the cataphoretic and curing parameters of a primer for the automotive industry

Companies producing organic coatings for the automotive industry often need to test newly formulated products, which may have used different production or application parameters, to determine their actual behaviour before large-scale production and use of the coating. Proper choice of the test for this characterisation and the method of evaluation are critical. The results must be obtained in relatively short times so it is impracticable for manufacturers to wait for the completion of outdoor exposure tests. To reduce lead times in the development process, the use of an accelerated aging test (such as salt spray resistance) to predict durability of the coating is essential. Yet, the time needed to perform such tests is usually at least 500 h. This article proposes the use of electrochemical impedance spectroscopy (EIS) and an electrochemical test, AC/DC/AC, which consists in a combination of impedance results and cathodic polarisations, in order to optimise in a very short time the cataphoretic process parameters (mean applied voltage) and the curing temperature needed for a primer in order to obtain the maximum anticorrosion performance. Results show very good degree of concordance between the two electrochemical tests although the AC/DC/AC method is much faster. The accelerated electrochemical test lasted 24 h to differentiate anticorrosive coatings performance, while same conclusions were obtained after more than 100 days by electrolyte immersion and evaluated by EIS.     

Application of electrochemical techniques to study the effect on the anticorrosive properties of the addition of ytterbium and erbium triflates as catalysts on a powder epoxy network

New low curing temperature epoxy powder coatings cured with o-tolylbiguanide and catalyzed by the use of erbium(III) and ytterbium(III) trifluoromethanesulfonates have been formulated. Their curing kinetics and anticorrosive properties have been studied and compared with a system commonly used in industry (o-tolylbiguanide/epoxy resin). Three different tests for measuring anticorrosive properties (EIS, AC/DC/AC, and salt fog spray) have been used together with an adherence test, in order to establish the ideal system. Results show that a system using 1 phr of ytterbium triflate presents good anticorrosive properties. The technique AC/DC/AC has shown its ability to evaluate properly and much faster than other techniques the anticorrosive properties of powder coatings with similar results.     

Anticorrosive properties of an epoxy-Meldrum acid cured system catalyzed by erbium III trifluromethanesulfonate

New low curing temperature powder epoxy coatings cured with Meldrum's acid and catalyzed by the use of erbium III trifluoromethanesulfonate have been formulated. Their curing kinetics and anticorrosive properties have been studied and compared with a system commonly used in industry (o-tolylbiguanide/epoxy resin) and with an epoxy powder coating homopolymerized by erbium III trifluoromethanesulfonate. Three different tests of anticorrosive properties (EIS, AC/DC/AC and salt fog spray) have been used together with an adherence test, in order to establish the best system. Results show that systems crosslinked with Meldrum's acid and catalyzed with erbium triflate present very fast curing kinetics and very good anticorrosive properties. The technique AC/DC/AC has shown its ability to evaluate properly and much faster than other techniques the anticorrosive properties of powder coatings.     

Rapid assessment of automotive epoxy primers by electrochemical techniques

An epoxy primer commonly used in the automotive industry was applied cataphoretically (using different times and potentials) over metallic substrates, and was investigated by means of different techniques such as accelerated cathodic disbonding, EIS, and AC/DC/AC tests. The variables considered in this study were the metallic substrate (cold rolled steel and phosphatized cold rolled steel), the thickness (20 and 25 μm), and finally the deposition potential in the cataphoretic cell (140 V and 190 V). The primer performed better when applied over phosphatized steel at the maximum thickness and when the lowest cataphoretic potential was used. Primer quality results obtained by the different techniques used in this study were quite similar and led to almost the same conclusions, although the AC/DC/AC and cathodic disbonding techniques provided results in less time than the EIS technique. Departamento de Tecnología. Area de Ciencia de los Materiales e Ingeniería Metalúrgica. Campus Riu Sec., Castellón 12071 Spain.     

Anticorrosive properties of an epoxy-Meldrum acid cured system catalyzed by erbium III trifluromethanesulfonate

New low curing temperature powder epoxy coatings cured with Meldrum's acid and catalyzed by the use of erbium III trifluoromethanesulfonate have been formulated. Their curing kinetics and anticorrosive properties have been studied and compared with a system commonly used in industry (o-tolylbiguanide/epoxy resin) and with an epoxy powder coating homopolymerized by erbium III trifluoromethanesulfonate. Three different tests of anticorrosive properties (EIS, AC/DC/AC and salt fog spray) have been used together with an adherence test, in order to establish the best system. Results show that systems crosslinked with Meldrum's acid and catalyzed with erbium triflate present very fast curing kinetics and very good anticorrosive properties. The technique AC/DC/AC has shown its ability to evaluate properly and much faster than other techniques the anticorrosive properties of powder coatings.     

Electrodeposited PbO2 thin film as positive electrode in PbO2/AC hybrid capacitor

Lead dioxide (PbO₂) thin films were prepared on Ti/SnO₂ substrates by means of electrodeposition method. Galvanostatic technique was applied in PbO₂ film formation process, and the effect of deposition current on morphology and crystalline form of the PbO₂ thin films was studied by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The energy storage capacity of the prepared PbO₂ electrode was investigated by means of cyclic voltammetry (CV) and charge/discharge cycles, and a rough surface structure PbO₂ film was selected as positive electrode in the construction of PbO₂/AC hybrid capacitor in a 1.28 g cm⁻³ H₂SO₄ solution. The electrochemical performance was determined by charge/discharge tests and electrochemical impedance spectroscopy (EIS). The results showed that the PbO₂/AC hybrid capacitor exhibited high capacitance, good cycling stability and long cycle life. In the voltage range of 1.8-0.8 V during discharge process, considering the weight of all components of the hybrid capacitor, including the two electrodes, current collectors, H₂SO₄ electrolyte and separator, the specific energy and power of the device were 11.7 Wh kg⁻¹ and 22 W kg⁻¹ at 0.75 mA cm⁻², and 7.8 Wh kg⁻¹ and 258 W kg⁻¹ at 10 mA cm⁻² discharge currents, respectively. The capacity retains 83% of its initial value after 3000 deep cycles at the 4 C rate of charge/discharge.     

Elucidation of corrosion failure mechanisms of coated phosphated steel substrates

Coatings are commonly believed to protect metal surfaces from corrosion based on some combination of their barrier properties and electrochemical properties. Various physical and electrochemical tests were performed on seven different coatings (latex, alkyd, 2-PK epoxy, and electrocoat) to determine which properties were the main determinants of corrosion resistance in continuous and cyclic corrosion tests. Physical property tests and AC electrochemical tests were all related to barrier behavior, while DC electrochemical tests were related to electrochemical behavior. DC electrochemical properties are commonly associated with inhibitor chemistry, but can be broadly related to all components of the paint. These test results were compared with corrosion test results from both continuous (B-117) and cyclic (GM9540) accelerated tests. The best correlation was seen with a model emphasizing barrier behavior with a secondary component relating to electrochemical protection. Presented at 2007 FutureCoat! Conference, sponsored by Federation of Societies for Coatings Technology, October 3–5, 2007, in Toronto, Ont., Canada.     

The combined role of inhibitive pigment and organo-modified silica particles on powder coatings: Mechanical and electrochemical investigation

The addition of organo-modified silica particles (OSP) to powder coating containing zinc molybdenum phosphate pigment (ZMP) has been investigated. The OSP were directly incorporated at different concentrations (1, 2.5, 3.5 and 4.5 wt%) in a polyester powder monolayer coating with 10 wt% ZMP. The adhesion and anticorrosion properties were evaluated by means of adhesion studies and different electrochemical techniques: electrochemical impedance spectroscopy (EIS) and an accelerated cyclic electrochemical technique (ACET). The optimum quantity of OSP that gave maximum anticorrosive performance of the coating system was determined (1.0%), however two opposite effects of addition on the coatings led to a reduction of anticorrosive properties for higher contents. Finally, the electrochemical behaviour of the optimized combined system was compared to the effects achieved when both additives were separately added to powder coatings by means of an additional electrochemical test.     

Evaluation of cure temperature effects in cataphoretic automotive primers by electrochemical techniques

Companies producing organic coatings for the automotive industry frequently need to test newly formulated products or materials, habitually according to different production parameters, in order to determine their actual behaviour before large-scale production of the coating is implemented. Proper choice of the test for this characterisation and the method of evaluation are critical. On the one hand, the choice is important because it is often necessary to distinguish coatings with very similar performance. On the other hand, results must be obtained in relatively short times, for it is impracticable for the manufacturers to wait for the completion of outdoor exposure tests. To reduce lead times in the development process, the use of accelerated aging tests to predict durability of the coating is essential (the time needed to perform them, however, is usually at least 500 h). This article proposes the use of EIS and an electrochemical test, AC/DC/AC, which consists in a combination of impedance results and cathodic polarisations, in order to find out the performance of coatings with precision and in a very short time. These techniques have been used to determine the optimum range of curing temperatures of a primer in order to obtain the maximum anticorrosion properties. Results show very good degree of concordance between the two electrochemical tests, although the AC/DC/AC method is much faster.     

Compatibility between cataphoretic electro-coating and silane surface layer for the corrosion protection of galvanized steel

The compatibility between a cataphoretic electro-coating and a silane layer applied on galvanized steel was evaluated by performing electrochemical impedance measurements on coated and uncoated samples. During electro-deposition, the water electrolysis induces hydrogen production. This process can induce degradation or destruction of the silane layer. This process was simulated by reproducing the application conditions of electro-coating in an aqueous solution of same pH (6) and conductivity (1600 μS) than the electro-coating bath, but without any pigments and binder. A current of 2 mA/cm² was applied between the sample and the counter-electrode during 10 and 20 s. These conditions are representative of the mean real application conditions just before the coating formation. The loss of the barrier effect offered by the silane layer was evaluated by EIS before and after simulation. This simulation shows whether it is possible to conveniently design the properties of the silane layer to maintain its protection and adhesion promotion properties after polarization. The barrier properties and the water uptake of the electro-coated samples were evaluated by EIS as a function of immersion time in a sodium chloride solution (0.1 M). The coated silane pre-treated samples show a good behaviour compared to the samples coated without pre-treatment. By properly managing the deposition conditions of sol–gel films it is possible to obtain cataphoretic coating with improved corrosion resistance. Silane sol–gel films of different thicknesses and curing temperature were produced. It was demonstrated that a 120 nm thick silane sol gel film cured at 180 °C ensures a very good compatibility with the electro-coat. In fact, this system shows a very high corrosion resistance even after 50 days of immersion in a sodium chloride solution. Also the resistance in the salt spray chamber of the electro-coated thin silane layer cured at 180 °C is remarkable. The results confirm that, if conveniently designed, silane sol–gel film properties, the silane layer is a good adhesion promoter of the cataphoretic coating on galvanized steel and this property is maintained for long exposure times.     

Study of the anticorrosive behaviour of epoxy binders containing non-toxic inorganic corrosion inhibitor pigments

The anticorrosive performance of epoxy coatings pigmented with non-toxic corrosion inhibitors pigments was investigated in this work. The coatings used contained the following pigments: zinc phosphate (ZP), zinc phosphomolybdate (ZPM) and zinc calcium phosphomolybdate (ZCPM). For comparative studies epoxy coatings with the following compositions were made up: one only with filler (CRG); one without pigments, varnish (VR) and other with zinc chromate (ZC) pigment. The corrosion inhibitor performance of the coatings was evaluated by immersion tests in 0.01 mol L⁻¹ NaCl aqueous solutions and accelerated tests in a salt spray chamber. The corrosion inhibitor performance of the samples was monitored using open-circuit potential (E_oc) measurements and electrochemical impedance spectroscopy (EIS) technique. Complementary tests were carried out using water vapour permeability of free-standing films and thermogravimetric (TG) analysis. The permeability test showed that the addition of the studied pigments did not modify the barrier properties of the free-films in comparison that pigmented with chromate. Thermal analysis indicated that the addition of the pigments improved the thermal stability of the coatings and it suggested a resin/pigment interaction. The total immersion tests and salt spray tests demonstrated that the barrier properties of the coatings pigmented with the inhibitors were not degrading as much as that pigmented with ZC. Therefore, all the three pigments could replace ZC as an anticorrosive pigment in similar conditions to those described here. The best corrosion inhibitor performance in the total immersion test was presented by the ZPM and ZCPM coatings while in the salt spray test the corrosion inhibitor performance of all the three pigmented coatings was similar, suggesting that only in the less aggressive test is possible to detect any difference between the coatings with the non-toxic pigments.     

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.     

Effects of electrolytes on the electrochemical performance of Si/graphite/disordered carbon composite anode for lithium-ion batteries

The influences of LiBF₄, LiClO₄, lithium bis(oxalato) borate (LiBOB), LiPF₆ with VC and without VC, and the mixed electrolytes composed of different ratios of LiBOB and LiPF₆ or LiClO₄ on the electrochemical properties of Si/graphite/disordered carbon (Si/G/DC) composite electrode were systematically investigated by constant current charge-discharge and electrochemical impedance spectra (EIS) techniques. Scanning electron microscopy (SEM) was used to observe the change of electrodes in morphology after given cycle numbers. X-ray photoelectron spectroscopy (XPS) was employed to understand the influences of different mixed electrolytes on the composition of SEI layers. The results showed that Si/G/DC composite electrode in the mixed electrolytes presented better electrochemical performance than in single electrolyte. The compactness and compositions of SEI layers intensively influenced the cycle performance of Si/G/DC composite materials. LiBOB and additive VC had a good synergistic effect on the formation of the dense SEI layers. In particular, Si/G/DC in 0.5 M LiBOB + 0.38 M LiPF₆ electrolytes containing VC exhibited a high reversible capacity and excellent cycle performance.     

Electrochemical performance of nano-SiO2 modified LiCoO2 thin films fabricated by electrostatic spray deposition (ESD)

With a mixture of a SiO₂ sol and a solution of lithium and cobalt acetates as the precursor, nano-SiO₂ modified LiCoO₂ films were fabricated by the electrostatic spray deposition (ESD) technique. The SiO₂ content of these films was 0, 5, 10, 15 and 20 wt%, respectively. Their structure and electrochemical properties were characterized by means of X-ray diffraction, scanning electron microscopy, galvanostatic cell cycling, AC impedance spectroscopy and cyclic voltammetry. Li₂CoSiO₄ was found formed in the SiO₂-containing films. The film with 15 wt% SiO₂ shows the best cycling stability with the capacity of 130 mAh/g in the voltage range between 2.7 and 4.3 V at the current density of 0.1 mA/cm². Due to its resulted small cell impedance, it has excellent rate capability. A LiCoO₂ (shell)/SiO₂ (core) structure model is proposed to explain the improved properties of these films.     

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.     

Li-intercalation electrochemical/electrochromic properties of vanadium pentoxide films by sol electrophoretic deposition

Thin films of orthorhombic V₂O₅ have been prepared by sol electrophoretic deposition (EPD) followed by post-treatment at 500 °C. Their electrochemical and optical performances have been investigated for possible applications in electrochemical/electrochromic devices. Li⁺-intercalation properties of the films have been explored in two voltage ranges: 0.4 to −1.1 V and 0.4 to −1.6 V versus Ag/Ag⁺, respectively. High capacities of over 300 mAh/g are acquired in the wider voltage range at a current density of 50 μA/cm² and moderate capacities of 140 and 110 mAh/g are obtained in the narrower voltage range at a current density of 25 and 50 μA/cm², respectively. Electrochemical measurements have shown that the films demonstrate good cyclability in both voltage ranges. X-ray diffraction, scanning electron microscopy and optical spectra have been used to examine the changes in crystallinity, microstructure, morphology and transmittance of the films during cycling. Films cycled to a deeper voltage of −1.6 V versus Ag/Ag⁺ deliver higher capacity with appreciable morphological change, while films cycled in the narrower voltage range show moderate capacity and maintain the morphology, optical responses and crystalline structure. Voltage range can be optimized in between to acquire both high capacity and stability in structure, electrochemical and optical properties. High Li⁺-intercalation capacity and good cyclic stability are attributed to the porous structure of V₂O₅ films prepared by EPD.     

DC and AC voltammetry of a free-base porphyrin adsorbed onto basal-plane graphite under acidic conditions: An example of a close to ideal reversible two-electron surface-confined redox process at sub-monolayer coverages

The free-base porphyrin, 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphine (H₂TMPyP), adsorbs onto a basal-plane graphite electrode. Under DC cyclic voltammetric conditions, the fully protonated dication, [H₄TMPyP(0)]²⁺, undergoes an apparently close to ideal surface-confined two-electron reduction to the neutral [H₄TMPyP(-II)] species when the supporting electrolyte consists of aqueous 1 M HCl and 1 M NaCl and coverages are sub-monolayer. The reversible potential calculated from the average of the oxidation and reduction peak potentials is 0.138 ± 0.002 V (vs Ag/AgCl, 3 M NaCl) whilst their separation ΔE_p, approaches 0 mV at slow scan rates, as expected theoretically for an ideal surface-confined electron transfer process. Comparisons of simulated and experimental data imply that the increase in ΔE_p observed at scan rates above 10 V s⁻¹ is consistent with uncompensated Ohmic IR_u drop effects, and not limitations imposed by electron transfer kinetics. Analysis of fundamental and higher harmonic components derived from large-amplitude sine-wave AC voltammetry is consistent with a very fast electron transfer rate constant, k⁰, in excess of 10⁶ s⁻¹ for the overall two-electron process. However, careful comparison with AC theory highlights minor levels of non-ideality not attributable to purely capacitative background or uncompensated resistance effects. These are particularly evident when greater than monolayer surface coverages are employed. It is likely that subtle contributions from heterogeneity in the adsorbed layer and complexities in the reaction mechanism are present in this close to ideal surface-confined process, but they are more readily detected under conditions of large-amplitude Fourier transformed AC cyclic voltammetry than with the conventionally used DC cyclic format.     

Characterization of structure and corrosion resistivity of polyurethane/organoclay nanocomposite coatings prepared through an ultrasonication assisted process

In this study, a series of castor oil based polyurethane/organically modified montmorillonite (OMMT) clay nanocomposite coatings have been successfully prepared by effective dispersing of OMMT nano-layers in polyurethane matrix through an ultrasonication assisted process. Effectiveness of ultrasonication process in de-agglomeration of clay stacks in castor oil dispersions was evaluated by optical microscopy and sedimentation test. Structure of nanocomposite coatings was investigated by wide angle X-ray diffraction (WAXD) and Fourier-transform infrared spectroscopy (FT-IR). The anticorrosive properties of nanocomposite coatings were characterized by electrochemical impedance spectroscopy (EIS), Tafel polarization study, water absorption and pull-off adhesion tests. The experimental results showed that PU/OMMT nanocomposite coatings were superior to the neat PU in corrosion protection effects. Also, it was observed that the corrosion protection of polyurethane organoclay nanocomposite coatings is improved as the clay loading is increased up to 3 wt.%.     

Electrochemical impedance spectroscopy of poly(N-methyl pyrrole) on carbon fiber microelectrodes and morphology

This work reports the supercapacitive properties of electrochemically grown homopolymer films on carbon fiber microelectrode (CFME) via poly(N-methyl pyrrole) (P(NMPy)) which is characterized by cyclic voltammetry (CV), Fourier transform infrared reflectance (attenuated total reflection) spectroscopy (FTIR-ATR), scanning electron microscopy (SEM), atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS). Microporosity of P(NMPy) electrocoated carbon fiber microelectrodes facilitated improved capacitance and redox behaviours by applying different DC potentials in electrochemical impedance spectroscopic measurements. The capacitance values of P(NMPy)/CFME is obtained (0.059 F) which is in the range of manufactured values.     

Li-ion diffusion kinetics in LiMn2O4 thin films prepared by pulsed laser deposition

LiMn₂O₄ thin films were deposited on Au substrates by pulsed laser deposition (PLD). The Li-ion chemical diffusion coefficients of the films, , were measured by cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT), potentiostatic intermittent titration technique (PITT), and electrochemical impedance spectroscopy (EIS). It was found that the values by CV and PITT were in the order of 10⁻¹³ cm² s⁻¹, and those by EIS and GITT were in the range of 10⁻¹³ to 10⁻¹¹ and 10⁻¹⁴ to 10⁻¹¹ cm² s⁻¹, respectively. These data were compared with the previously reported values.