Resistance of metallic substrates protected by an organic coating containing aluminum powder

The corrosion behavior of an epoxy primer containing aluminum powder (10 vol.%) applied on carbon steel and on galvanized steel was examined by electrochemical impedance spectroscopy (EIS). The data show that this coating is more protective when applied onto carbon steel substrates, and that on galvanized steel thicker coatings allow to achieve similar protection levels as those obtained for carbon steel. These effects are probably due to aluminum pigments providing a cathodic protection of the substrate, and to the resulting products precipitating inside the pores of the polymeric coating. Three stages can be distinguished during exposure of the coated specimens. Upon immersion of the coated samples in the test solution, a pre-saturated stage is observed. After a certain period of immersion, which strongly depends on the thickness of the applied coating, a saturation stage is reached in which an effective protection of the metallic substrate against corrosion is achieved. Finally, at sufficiently long exposure times, swelling through the coating eventually leads to the detachment of the coating.     

Characterization of active pigments in damage of organic coatings on steel by means of electrochemical impedance spectroscopy

Active anticorrosive pigments are solid additives for primers which can give further protection for areas with coating damage in addition to their barrier effect. These pigments are expected to prevent corrosion of metal substrate in coating damage by build-up of permanently passive conditions at the metal surface (electrochemical protection) and/or by build-up of solid compounds which plug the coating damage (chemical protection). Electrochemical Impedance Spectroscopy (EIS) was applied to characterize the corrosion protection behaviour of alkyd primers containing different pigments. Impedance spectra were recorded in the frequency range 50 mHz f 50 kHz at the open-circuit potential as a function of the type of pigment and the exposure time in different corrosive media. In general, two different parts can be distinguished in the impedance diagrams. The higher frequency part is related to the insulating properties of the primer and the lower frequency part can be attributed to electrochemical processes taking place within the coating defects. The parameters derived from EIS results show that the low frequency data can be used for characterization of the protective properties of anticorrosive pigments in the presence of defects in organic coatings.     

Electrochemical impedance studies of corrosion protected surfaces covered by epoxy polyamide coating systems

Electrochemical impedance spectroscopy (EIS) was used to study the corrosion behavior of different types of commercial quality epoxy polyamide dry coatings on mild steel, with thickness between 150 and 250 μm, which were previously weather accelerated in a wet chamber. The following data were established on the basis of impedance curves and corresponding equivalent circuits: the coating capacity that relates to the coating dimension, the pore resistance that represents conductive paths through the pores, and Warburg coefficients, which are the measure of ion diffusion through the coating. EIS data were compared with a criterion in the European standard, and samples were classified on the basis of their quality, also receiving a final ranking by summing-up all the individual rankings. Suggestions have been made, namely that the use of anticorrosive active pigments is obsolete, when the thickness of the coatings exceeds 200 μm.     

Corrosion protection coating containing polyaniline glass flake composite for steel

Corrosion protection of steel by glass flake (GF) containing coatings is widely used in marine atmosphere. Even though, the coatings containing glass flake are highly corrosion resistant, their performance is decreased due to the presence of pinholes and coating defects. It is well established that polyaniline containing coating is able to protect the pinhole defects in the coatings due to passivating ability of polyaniline. Hence a study has been made on the corrosion protection ability of steel using polyaniline-glass flake composite containing coating with 10% loading of glass flake in epoxy binder. The polyaniline glass flake composite (PGFC) was synthesized by chemical oxidation of aniline by ammonium persulphate in presence of glass flake. The corrosion protection ability of GF and PGFC containing coating on steel was found out by salt spray test and EIS test in 3% NaCl. In both the tests, the resistance value of the PGFC containing coating has remained at 10⁸-10⁹ Ω cm² where as for the GF containing coating, the resistance values decreased to 10⁵ Ω cm². The enhanced corrosion protection ability of the PGFC containing coating is due to the passivation ability of the polyaniline present in the coating.     

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.     

The influence of aluminium surface pretreatment on the corrosion stability and adhesion of powder polyester coating

One of the most important factors in corrosion prevention by protective coatings is the coating adhesion loss under environmental influence. Thus, adhesion strength is often used when characterizing protective properties of organic coatings on a metal substrate. In order to improve the adhesion of organic coating the metal substrate is often pretreated in some way. In this work, the adhesion of polyester coatings on differently pretreated aluminium surface (by anodizing, with and without sealing, by phosphating and by silane film deposition) was examined. The dry and wet adhesion of polyester coatings were measured by a direct pull-off standardized procedure, as well as indirectly by NMP test. It was shown that under dry test conditions all polyester coatings showed very good adhesion, but that aluminium surface pretreated by silane film showed superior adhesion. The overall increase of wet adhesion for polyester coating on aluminium pretreated by silane film was maintained throughout the whole investigated time period. The different trends in the change of adhesion of polyester coatings were observed for different aluminium pretreatments during exposure to the corrosive agent (3% NaCl solution). The highest adhesion reduction was obtained for polyester coating on aluminium pretreated with phosphate coating. The corrosion stability of polyester coated aluminium was investigated by electrochemical impedance spectroscopy in 3% NaCl solution. The results confirmed good protective properties of polyester coating on aluminium pretreated with silane film, i.e. greater values of pore resistance and smaller values of coating capacitance were obtained in respect to other protective systems, whereas charge-transfer resistance and double-layer capacitance were not measurable during 2 months of exposure to a corrosive agent.     

Corrosion protection of steel by polyaniline blended coating

Phosphate doped polyaniline was synthesized from aqueous phosphoric acid containing aniline by chemical oxidation method using ammonium persulphate as oxidant. The polymer was characterized by UV–vis and FT-IR spectroscopic techniques. Using this polymer, a paint with 1% polyaniline was prepared with epoxy binder. The corrosion resistant property of the polymer containing coating on steel was found out by open circuit potential measurements and electrochemical impedance spectroscopic method in 0.1 N HCl, 0.1 N H₃PO₄ and 3% NaCl for a duration of 50 days. The coating was able to protect the steel more in 0.1 N H₃PO₄ and 3% NaCl media than in 0.1 N HCl.     

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.     

EIS study of environmentally friendly coil coating performances

The corrosion protection of metal structures by the application of organic coatings is obtained using complex paint cycles. The metal pretreatment as well as primer, intermediate and top coat are the usual layers composing the protection system. Although chromate pretreatment processes and chromate containing primers are widely used, they require highly toxic chromic acid solutions, with consequent effluent disposal and ecological problems. This fact pushed many research laboratories to develop new chromium-free pretreatments and primer inhibitors. Electrochemical impedance spectroscopy (EIS) proved to be a very useful technique to study the protection properties of the organic coating because of the possibility of evaluating the corrosion process on the metal substrate and to measure the electric and dielectric characteristics of the coating. In this study, new chromium-free pretreatments and new primers containing ecological inhibitors were applied to steel substrates coated using an Al-Zn alloy. The results were compared with those obtained using traditional protection systems. The characterization of the complete system (pretreatment and primer) in sodium sulfate solution clearly showed the different water uptake behavior of the two primers, which highlights the better barrier properties of the one containing phosphate. However the corrosion protection of the primer containing chromates is also due to the inhibitive action of the chromates. Hence such a type of primer better withstands the presence of defects in the coating. Moreover the presence of environmentally friendly pretreatments do not worsen the corrosion protection which appears comparable to that observed when chromatation is employed.     

Evaluation of thick organic coatings degradation in seawater using cathodic protection and thermally accelerated tests

Organic coatings are often associated with cathodic protection to fight against the corrosion of metallic structures when immersed in seawater. However, cathodic protection leads to the generation of a strong alkalinity at the metal/coating interface, which causes the degradation of the coating. It is then necessary to develop a reliable method to evaluate the compatibility between organic coatings and the application of cathodic protection.

On one hand, cathodic disbonding tests (ASTM G-8 and G-80) can be driven with an artificial defect but this defect is mainly responsible for the electrochemical response. In addition, calcareous deposit rapidly forms onto the defect zone when cathodic protection is applied which can make difficult the evaluation of coating delamination. On the other hand, immersion of defect-free specimens requires very long testing periods (several months or even years) in order to detect the coating degradation.

In this work, an attempt to accelerate the coatings degradation by imposing a high temperature and thermal cycles were made in order to decrease the test-time duration. The influence of the applied cathodic potential was also investigated. The coating degradation was evaluated by EIS, considering the defect-free zone of coatings. It was shown that the coating degradation is faster in the presence of a defect and for high temperature (45 °C). Moreover, thermal cycles allow to greatly accelerate the degradation of defect-free coatings and then to compare the compatibility of both coatings with cathodic protection.     

Techniques for the measurement of natural product incorporation into an antifouling coating

The control of biofouling can be achieved by a variety of methods but for an open system, such as a ship's hull, a protective paint coating is the most adopted method. The incorporation of a natural product extract directly into a coating has received little previous attention. This study has investigated a combination of the antifouling compound (a natural product extract) and the delivery system (control depletion polymer) investigated together. It was necessary to investigate the natural product incorporation into a coating and finally assess the antifouling system including the primer layers in the natural marine environment. Natural products must first be practical as antifoulants to be developed further into a functional system by their incorporation into surfaces or coatings. To demonstrate this, the natural product under investigation was homogenised into a blank proprietary antifouling paint system binder, applied to primed and un-primed ship grade steel and immersed in marine environments. Electrochemical techniques were used to investigate the effects of natural product incorporation into a coating. In addition, optical and scanning electron microscopes were used to assess the physical characteristics of the coating system. The most rigorous test for an antifouling system is a field trial. Field trials were completed at a raft exposure facility, in estuarine dock conditions at the Empress dock, National Oceanography Centre, Southampton, UK.     

Underwater EIS measurements

The “on site” EIS evaluation of coatings on steel constructions immersed in water or splashed by water is much more difficult than in atmospheric conditions. There are many technical problems with insulation of electrical connections, the mounting and leak tightness of measurement cell and the exclusion of measurement of electric capacitance between the counter electrode and surrounding constructions. These problems were successfully solved by the team of scientists from Road and Bridge Institute, Atlas Sollich Electronic Systems Ltd. and Trento University. The construction of the measurement cell, the investigation method as well as some of the results from “on site” coating investigation were presented.     

Electrochemical and anticorrosion behavior of functionalized graphite nanoplatelets epoxy coating

Functionalized graphite nanoplatelets (FGNP) were used as efficient and compatible nano-particles to produce homogenous epoxy nano-coating with impressive anticorrosion behavior for carbon steel. The characterizations of the nano-particle and nano-coating were carried out by SEM, FT-IR, XRD, TEM and pull-off test. Fine distribution of nano-particles in the cured nano-coating with particle sizes of 20–40 nm was obtained. Electrochemical experiments, salt spray and X-ray fluorescence showed that the nano-coatings protect the metal substrate by formation of passive layer and physical barrier characteristics. Three samples (0.25%, 0.5% and 1%) of FGNP-epoxy coatings were prepared that 0.5% showed better anticorrosion properties.     

Stainless steel as new substrate for coil coating

Increasing needs of very high resistance to cosmetic corrosion, of more extended service life and reduced maintenance costs for infrastructures, civil and industrial buildings open new fields of application for coil coated stainless steel. This paper describes the adhesion and corrosion properties of new coil coated stainless steel materials produced in industrial coil coating lines. The use of an electrochemical test (electrochemical impedance spectroscopy) can give detailed information on the reactivity of the system and allow the performance of different substrates (AISI 409, 430, 316 and 304) coated with different polymers (polyvinylidene and polyester) to be compared. The results obtained show the interesting properties of this new class of coil coated products. The materials were tested for a long time (about 200 days) in an aggressive environment (3.5% sodium chloride solution) also in the presence of macrodefects. In particular, VIVINOX 430, 304 and 316 revealed no reactivity, corrosion or disbonding, thus supporting the expectancy of very long trouble free exposure also in very aggressive natural environments. (VIVINOX is the brand name of the AST (Acciali Speciali Terni) line of coil coated stainless steel.)     

Environmental studies of smart/self-healing coating system for steel

Smart/self-healing micro-capsulated inhibitor incorporated in epoxy primer before painting on a steel surface was evaluated for its corrosion protection effectiveness on exposure to ASTM D 5894 electrolyte in laboratory and natural tropical sea-shore environment. The “healant” inhibitor was industrial custom-made and non-chromate organic-based microcapsules which were mixed into the primer before applying a polyurethane topcoat layer on steel surface. The results indicate that the active components in ruptured embedded inhibitor microcapsules were released into an inflicted scribe primer and topcoat film on steel surface on exposure to inhibit development of an electrochemical cell. Undamaged surface film of the test and control specimens exposed in the environments demonstrated excellent corrosion-inhibition performance as reflected by both visual inspection and electrochemical impedance spectroscopy experimental data. The results obtained on the performance of self-healing inhibitors should provide an understanding of the fundamental material-property relationships of smart inhibitor coatings. And, thus, should facilitate the development of optimized paint compositions in order to extend the useful service life of steel-infrastructure applications.     

Evaluation of different coating systems by electrochemical methods

Corrosion protection of organic coated steel is determined by free corrosion potential measurements and impedance spectroscopy. The results are classified by means of very basic considerations and evaluation figures are developed. These electrochemical data are correlated with the extent of the corrosion for each coating system. The evaluation figures are used for ranking coating systems and this ranking is correlated with the ranking with reference to the results of standardized and exposure tests.     

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.     

Hot wet aging of glass syntactic foam coatings monitored by impedance spectroscopy

This paper examined both short and long term behaviors of glass-epoxy syntactic foam coatings containing various types of microspheres while exposed to hot deionized water. Electrochemical impedance spectroscopy (EIS) was used to monitor in situ the water uptake of the coating in complement to gravimetry but also to investigate the degradation phenomenon occurring after the matrix had reached saturation (monitoring over 420 days). Diffusion properties were discussed from capacitance measurements in comparison to classical gravimetric data. In addition, attention was paid to the increase of ionic conductivity suggesting the creation of water paths through the material and/or the occurrence of ionic extraction from the microsphere glass due to water leaching. Possible degradation mechanisms were discussed from the modeling of the results obtained by in situ monitoring. The interest of EIS for the investigation of composite coating aging was highlighted.     

Adhesion characterization of protective organic coatings by electrochemical impedance spectroscopy

Adhesion is considered in many situations to be a very important property of organic coatings for corrosion protection and much scientific work is devoted both to the study of the mechanism involved in polymer-metal adhesion and to the ways of measuring this property. The large number of experimental methods in existence to obtain information on coating adhesion is an indication of both the scientific and the technological interests in this material science and engineering area, but it is also a consequence of the difficulty in measuring adhesion in a general sense. As a partial alternative to the traditional adhesion measurement approaches for organic coatings, the evaluation of adhesion by electrochemical techniques such as electrochemical impedance spectroscopy (EIS) is discussed for different examples. The influences on adhesion of different pretreatments or organic coatings are discussed, considering aluminium, galvanized steel, and stainless steel substrates, and we have shown that the information obtained by using an electrochemical approach can be used for adhesion evaluation, with particular attention to the monitoring of adhesion in an aqueous environment, which is the most detrimental for protective organic coatings.