A study on the electrochemical inhomogeneity of organic coatings

The inhomogeneity of three organic coatings, phenolic resin, alkyd resin and poly(urethane) varnish, was studied using a wire beam electrode and a high resistance measurement technique by measuring the distribution of d.c. resistances on different surface areas of the coatings in a 3% NaCl solution. Two types of areas which showed a significant difference in their d.c. resistance were found to be a basic pattern of this inhomogeneity. Experimental findings in this work nicely proved the reality of ‘D' and ‘1' areas in organic coatings. The results are in good agreement with those of Mayne et al.. Results about the influence of the coating's thickness and layers on its inhomogeneity are also reported in this paper.     

Ingress of water into organic coatings: Real-time monitoring of the capacitance and increase in mass

Water ingress into a cataphoretic coating employed in the car industry was studied using real-time mass and capacitance measurements. The results showed the presence of three well-differentiated time constants associated with the process. The corresponding time scales were 2–3 min, 1 and 10 h. The first time constant (fastest process) was assumed to be associated with the filling of surface pores by water. This process involved about 50% of the total water uptake. The second time constant was assumed to be due to the water occupancy of pores in the bulk of the coating. Finally, the slowest process was assumed to be due to water diffusion through the bulk coating. This process presented capacitive effects only if fillers were present in the coating.     

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

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

Phenol electropolymerization: a straight route from monomers to polymer coatings

Recent results on phenol electropolymerization aimed at the simultaneous production and deposition of protective coatings are described. Discussion is especially focused on the electrochemical growth mechanism of the non-conducting polymer films (previously unpublished chronoamperometric and chronopotentiometric results are provided and compared with a simple model), incorporation of corrosion inhibitors, the selected inhibitor being 2-benzothiazolylthiosuccinic acid, coating of phosphated mild steel and phosphated galvanized steel via the cathodic deposition of minor Zn amounts in the phosphate layer pores and protective performance of the coated samples, studied by a.c. impedance. It is shown that significant advances have been achieved since 1987, when a comprehensive review was published, in both the fundamental understanding of the process and the protective capabilities of the coatings.     

Evidencing antagonist effects of water uptake and leaching processes in marine organic coatings by gravimetry and EIS

Two distinct marine epoxy organic coatings were aged in a sodium chloride (30 g L⁻¹) solution at different temperatures. The water absorption was evaluated using gravimetric tests onto free films and EIS experiments were realized onto attached films. A strong leaching process was evidenced by gravimetry and the FT-IR analysis of the solution after ageing revealed the presence of extracted organic components. However, EIS results were not sensitive to this phenomenon and the water uptake values, obtained with a modified Brasher–Kingsbury equation, were found to be higher than gravimetric values. The comparison of the water volume fraction obtained by EIS with the apparent water mass fraction allowed following the species desorption with immersion time. It was proposed that this leaching process was related to a loss of the polymer density and the results showed that this phenomenon was thermally activated.     

Challenges of chromate inhibitor pigments replacement in organic coatings

General considerations concerning pigment grade corrosion inhibitors are presented and new characteristic functional parameters proposed. The chemistry, contemporarily practiced for corrosion inhibitor pigment synthesis, as well as known mechanistic considerations relevant to the corrosion inhibitor species available in pigment grades, are reviewed. Inherent limits of chemistry are outlined regarding the feasibility of developing equally effective and versatile non-toxic alternatives for chromates.

Fundamental aspects of corrosion inhibitor pigment behavior are discussed in the context of organic coatings degradation mechanisms. Disclosed experimental data demonstrate that correlation exists between solubility of corrosion inhibitor pigments, leaching from, and osmotic blistering of organic coatings. A mathematical expression is proposed and empirically proven to adequately describe the leaching rate of corrosion inhibitor pigments from organic coatings.

The barrier function of some high performance organic coatings, such as aircraft or coil, is negligible and requires the contribution of an effective corrosion inhibitor pigment, which, typically, is strontium chromate. In this context, the feasibility of replacing chromate inhibitor pigments is assessed and the “gap” observable between the inhibitor performance of chromates and traditional non-chromate pigments is noted.

Experimental data are displayed regarding the corrosion inhibitor performance of a novel, organic–inorganic, hybrid type corrosion inhibitor pigment in typical aircraft primer application on aluminum and plated steel. The presented data demonstrate the feasibility of chromate replacement in this specific case.     

Assessment of protective properties of organic coatings by thermal cycling

Researchers’ efforts are focused on understanding how coatings can be tested in order to determine their real capabilities and selected for different purposes. Outdoor exposures are certainly reliable and offer a good representation of the actual service life. However, such tests cannot be considered quick.

On the other hand, a quick test, even if reliable, very often disagrees with the actual degradation mechanisms occurring under natural conditions. In fact, in order to determine an acceleration of the natural weathering, it is necessary to increase the effect of natural parameters affecting the protection properties of a coating.

The usual modern way to operate is to take advantage of ageing tests where temperature plays a big role in the ageing of the material, permitting to gather faster information for coating corrosion resistance evaluation.

Following the recent new experience realised by the Bierwagen group, we carried out different thermal cycling tests consisting in daily series of electrochemical measurements on coated samples, carried out using electrochemical impedance spectroscopy (EIS).

The cumulative effects of such a thermal cycling on the film, based on a large variety of theoretical explanations, should permit the ranking of a variety of materials, by constituents, characteristics and application purposes, in a short time while remaining objective and reliable.

The used ageing procedure and data evaluation allowed to quickly and precisely obtain information concerning both barrier properties and adhesion properties of the studied materials.     

Recent studies of particle packing in organic coatings

Organic coatings often contain sufficient pigmentation that the pigment volume concentration (PVC) and the critical PVC (CPVC) must be carefully considered. Past work has shown that the CPVC can be identified with the random densest packing of pigment particles after considerations of the adsorbed polymer layer on the particles is taken into account, and that this packing can be predicted by empirical particle packing algorithms developed for mixtures of particle sizes. Recent studies have shown that local fluctuations of pigment and polymer concentrations in a film can lead to local film volumes that can exceed the CPVC even though the average PVC for the whole film (the global PVC) is below the CPVC. These effects are consistent with observations of void formation in films below the CPVC, and can be analyzed by local film statistics. We have also recently performed simulations of particle packing by the compression of multiple hard spheres that allows further insight into local concentration fluctuations in PVC within a film.     

Improvement of corrosion protective performance of organic coating on low carbon steel by PEO pretreatment

Employing pretreatment is a pressing need for preparing anticorrosive coatings on carbon steels. However, conventional pretreatments are usually based on Cr, P and some other toxic elements, which are harmful to human body. For this reason, green and environmental techniques attract more and more attention. In this paper, plasma electrolytic oxidation (PEO) process was used as a pretreatment to fabricate an underlayer for the organic coating on low carbon steel. The anticorrosive performance of the organic coated samples with and without PEO pretreatment was studied by potentiodynamic polarization, ac/dc/ac electrochemical impedance spectroscopy, salt spray and immersion tests, respectively. Results show that the PEO process produces an oxide layer with porous and rough surface structure on the low carbon steel substrate. The porous and rough PEO layer is beneficial for enhancing the adhesion strength and thickness of the organic topcoatings. The organic coated sample with PEO pretreatment exhibits improved corrosion resistance and longer service life in corrosive environment compared to that without PEO pretreatment.     

A brief history of the Athens conference on organic coatings, science and technology

On 5–9 July 1999 the 25th conference on organic coatings took place in Vouliagmeni near Athens in Greece. To celebrate this anniversary and to honor the memory of the founder and director of the conference, Professor Angelos V. Patsis, a brief review of the history of the conference is given. Typical trends in coatings technology and applications reflected in the programs throughout the years are analyzed.     

Ionic liquid enhanced electrochemical characterization of organic coatings

Our laboratory recently began work on the use of room temperature ionic liquids ((RTIL's) to enhance our capabilities for the electrochemical characterization of organic coatings [A.M. Simões, D. Tallman, G.P. Bierwagen, The use of ionic liquids for the electrochemical characterization of water transport in organic coatings, Electrochem. Solid-State Lett. 8 (2005) 60]. The RTIL's are electrically conductive liquids consisting of large molecules that can be used to investigate the electrochemical properties of coatings in a non-aqueous medium. The enhancement of coating characterization comes from the fact that RTIL's have sufficient conductivity to be an immersion medium for electrochemical measurements, but they do not directly penetrate and effect organic coatings as do aqueous electrolyte solutions. This allows the separate examination of the effects of water on coatings in immersion or cyclic exposure. Indeed, our initial studies showed that a hydrophilic RTIL could be used to electrochemically characterize the drying of a coating after immersion, a process which heretofore had not been followed electrochemically. Thus, electrochemical measurements of coatings based on aqueous electrolyte immersion can be enhanced by the use of RTIL's and the effects of water on the coatings under study isolated and analyzed separately, especially the diffusion of water out of coatings during drying processes. Recent papers from our group have introduced the methodology whereby RTIL's in conjunction with capacitance monitoring via electrochemical impedance spectroscopy (EIS) can be used to determine the diffusion coefficient of water out of a non-pigmented, additive free coating [A.M. Simões, D. Tallman, G.P. Bierwagen, The use of ionic liquids for the electrochemical characterization of water transport in organic coatings, Electrochem. Solid-State Lett. 8 (2005) 60; K. Allahar, B. Hinderliter, A. Simoes, D. Tallman, G. Bierwagen, S. Croll, Simulation of wet–dry cycling of organic coatings using ionic liquids, J. Electrochem. Soc. 154 (2007) 177–185; B. Hinderliter, K. Allahar, O. Stafford, S. Croll, Using Ionic Liquids to Measure Coating Properties via Electrochemical Impedance Spectroscopy, Presented the 2006 International Coatings Exposition, Federation of Societies for Coatings Technology, New Orleans, LA, 2006 Oct.; B.R. Hinderliter, K.N. Allahar, G.P. Bierwagen, D.E. Tallman, S.G. Croll, Thermal cycling of epoxy coatings using room temperature ionic liquids, J. Electrochem. Soc. 155 (3) (2008) 1]. The technique has been extended to several types of coatings as well as the study of the cyclic wetting and drying of coatings [K. Allahar, B. Hinderliter, A. Simoes, D. Tallman, G. Bierwagen, S. Croll, Simulation of wet–dry cycling of organic coatings using ionic liquids, J. Electrochem. Soc. 154 (2007) 177–185]. This latter set of processes is one of the key set of events in exterior exposure that causes the failure of exterior protective coatings. Recently, RTIL's have been used to simulate the alternate wetting and drying of a Zn-rich epoxy coating system. EIS experiments were conducted on the Zn-rich epoxy under constant immersion in 0.05 M NaCl and RTIL. The experimental results were analyzed to determine the dielectric response and changes due to Zn oxidation within the Zn-rich system.     

Anticorrosion organic coating with layered double hydroxide loaded with corrosion inhibitor of tungstate

Mg–Al layered double hydroxides, loaded with tungstate anions, were synthesized via the co-precipitation method. The obtained compounds were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS), inductively coupled plasma (ICP), and transmission electron microscopy (TEM), respectively. The ICP results demonstrated the partial anion-exchange process, involving the release of tungstate anions from layered double hydroxide by the chloride anions. Polarization measurements showed that the filtrate as electrolyte possessed a low corrosion current density value of 3.042 μA/cm². The electrochemical impedance spectroscopy (EIS) exhibited that the coating could effectively protect the alloy from corrosion. The function of the layered double hydroxide loaded with tungstate doped in organic coating was also discussed.     

Layered double hydroxides as containers of inhibitors in organic coatings for corrosion protection of carbon steel

The present work focuses on the use of layered double hydroxides (LDH) as containers for corrosion inhibitors in an epoxy coating. 2-Benzothiazolylthio-succinic acid (BTSA), used as corrosion inhibitor, was intercalated by co-precipitation in magnesium–aluminum layered double hydroxides. The obtained LDH–BTSA was characterized by infrared spectroscopy, X-ray diffraction and scanning electron microscopy. BTSA release from LDH–BTSA in NaCl solutions was investigated by UV–vis spectroscopy. The inhibitive action of LDH–BTSA on carbon steel corrosion was characterized by electrochemical methods and the protective properties of an epoxy coating containing LDH–BTSA were evaluated by electrochemical impedance spectroscopy. It was shown that the BTSA was intercalated in the layered double hydroxide and its loading was about 33%. The BTSA release was dependent on the NaCl concentration in the electrolyte. The polarization curves obtained on the carbon steel sample showed that the LDH–BTSA is an anodic inhibitor. Its efficiency was about 90% at a concentration of 3 g/l. The impedance results showed that the incorporation of LDH–BTSA (3%) in the epoxy matrix improved the corrosion protection of the carbon steel.     

Effects of electrolytic composition on the electric double-layer capacitance at smooth-surface carbon electrodes in organic media

As a fundamental research on the optimization of electrolyte composition in practical electrochemical capacitor device, double-layer capacitance at Glassy Carbon (GC) and Boron-doped Diamond (BDD), as typical smooth-surface carbon electrodes, has been studied as a function of the electrolyte composition in organic media. Specific capacitance (differential capacitance: F cm⁻²) determined by an AC impedance method, in which no contribution of mass-transport effects is included, corresponded well to integrated capacitance evaluated by conventional cyclic voltammetry. The specific capacitance at the GC electrode varied with polarized potential and showed clear PZC (potential of zero charge), while the potential dependence of the capacitance at BDD was very small. The effects of the solvent and the electrolytic salt on the capacitance behavior were common for both electrodes. That is, the sizes of the solvent molecule and the electrolytic ion (cation) strongly affected the capacitance at these smooth-surface carbon electrodes.     

EIS and ENM measurements for three different organic coatings on aluminum

Electrochemical impedance spectroscopy (EIS) and Electrochemical noise measurements (ENM) were used to evaluate protective properties of three different organic coating systems. The coatings under investigation were two-component aerospace coatings, applied on aluminum substrate. They were immersed in a 0.5 mol l⁻¹ sodium chloride (NaCl) solution, within a controlled flow cell and were tested for 1 year.

The impedance modulus in the low frequency domain and the noise resistance were calculated and compared. From EIS data, coating capacitances and coating resistances of coating performance were estimated. The electrochemical results are in good agreement with final visual observations. The results of this study yield a performance ranking of the three different coatings.     

The properties of hydroxyapatite ceramic coatings produced by plasma electrolytic oxidation

Calcium phosphate coatings produced on the surface of Ti6Al4V by plasma electrolytic oxidation (PEO) using different concentrations of hydroxyapatite (HA) in a 0.12 M Na₃PO₄ (NAP) electrolyte solution was investigated. It was found that the amount of calcium phosphate particles infiltrated into the coating layer as well as the thickness and the surface roughness of the coating increased with increasing HA concentration. The porosity of the ceramic coatings indicated an inverse relationship with the concentration of HA particles dispersed in the NAP solution. The result also demonstrates that higher scratch adhesive strength was achieved using 1.5 g/L HA solution, producing a critical load of 2099 mN, while 0 g/L HA only produced a critical load of 1247 mN. The adhesion becomes independent of thickness when the concentration of HA exceeds 1.5 g/L. The failure of the coating was characterized by large periodic hemispherical chipping, while intermittent delamination was noticed with the coating embedded with HA particles. This study demonstrate the viability of using PEO to produce a thin layer of HA ceramic coating on Ti6Al4V suitable for biomedical applications.     

Metal-ceramic diffusion barrier nanocomposite coatings on nickel based superalloys for corrosion and high temperature oxidation resistance

Ceramic diffusion barrier coatings are inevitable in high temperature aerospace and nuclear applications to protect superalloys from oxidation and hot corrosion. Compositionally graded coating (CGC) of Ni-YSZ with five layers was deposited on an Inconel-690 substrate through electron beam physical vapor deposition (EBPVD) method. After heat treatment, the phase formation and crystallite determination in each layer of the CGC were studied by X-ray diffraction. Pulsed radio frequency glow discharge optical emission spectroscopy (RF-GDOES) showed outward diffusion of Ni towards the surface of the CGC. High resolution transmission electron microscopy (HRTEM) studies of the cross-sectional region of the heat-treated coating at 1273 K revealed no secondary phases within the coating as well as at substrate-coating interface. The corrosion behavior of Ni-YSZ coating under 3 M HNO₃ medium showed that the heat-treated CGC of Ni-YSZ exhibited better corrosion resistance due to the formation of NiO than as-deposited Ni-YSZ coating.     

Heat exchangers corrosion protection by using organic coatings

Corrosion phenomena in aluminium heat exchangers represent a problem in terms of durability and efficiency of thermal exchange. This work evaluates the barrier properties of two coatings that represent the state of the art for the protection in the heat, ventilating, air conditioning/refrigerating field (HVAC/R): electrophoretic coating (E-coating) using epoxy resin and spraying of polyurethane coating with addition of metallic pigments. The ability of the coatings to cover the surface of heat exchanger has been evaluated by means of optical microscopy in order to highlight critical zones of the system for the application of the coatings. The electrochemical behaviour of coated heat exchangers has been studied by means of electrochemical impedance spectroscopy in 3.5% NaCl solution. The local electrochemical behaviour of coating defects has been investigated using the electrochemical micro-cell, which enables to perform potentiodynamic polarization measurements on single defects. E-coatings evidenced difficulties to provide uniform thickness of the coating at the extremities of the fins. Spraying of the polyurethane coating containing pigments require particular care in order to fully cover zones of the heat exchanger with difficult accessibility. The electrochemical behaviour of coated heat exchangers is affected by the existence of defects in the coatings.     

Studies on the evaluation of the performance of organic coatings used for the prevention of corrosion of steel rebars in concrete structures

Corrosion of steel rebars in reinforced concrete constructions, particularly those located in marine environments and industrially polluted areas is one of the major problems baffling the construction industry all over the world including India. A suitable protective coating to rebar is found to improve the durability of such structures under aggressive exposures. The performance of a few polymeric coatings based on different resins such as acrylic polyol-aromatic isocyanate, polyester polyol-aromatic isocyanate, acrylic resin and epoxy–silicone–polyamide containing ordinary Portland cement or flyash as extenders and titanium dioxide and zinc phosphate as main pigments on rebar in concrete has been evaluated using mechanical strength tests and accelerated corrosion tests. It is observed that, of the 16 coating formulations, four have been found to perform well as effective and durable coatings.