Nanostructured coatings approach for corrosion protection

Nanostructured surface treatment coatings based on the Self-assembled Nanophase Particle (SNAP) approach were investigated as potential replacement for chromate-based surface treatments on aircraft aluminum alloys. In the traditional sol–gel method, hydrolysis-condensation processes are followed by condensation polymerization upon film application. This process sequence provides a low temperature route to the preparation if thin coatings which are readily applied to most metallic substrates. The recent discovery of a method of forming functionalized silica nanoparticles in situ in an aqueous sol–gel process, and then cross-linking the nanoparticles to form a thin film, is an excellent example of a nanoscience approach to coatings. This Self-assembled Nanophase Particle (SNAP) process can be used to form thin, dense protective organic surface treatment coatings on Al aerospace alloys. The ability to design coating components from the molecular level upward offers tremendous potential for creating multifunctional coatings.

The important components of Al alloy corrosion inhibition by chromate are storage and release of Cr^VI species, inhibition of cathodic reactions (primarily oxygen reduction), and inhibition of attack at active sites in the alloy. Unlike chromate-based treatments, current SNAP coatings provide barrier-type corrosion resistance but do not have the ability to leach corrosion inhibitors upon coating damage and minimize corrosion of the unprotected area. In this study, organic inhibitors were tested for corrosion protection of aluminum alloys in combination with the (SNAP). Scanning Vibrating Electrode Technique, anodic polarization, electrochemical impedance spectroscopy, and salt spray test were used to study this new approach for chromate replacement.     

Hybrid organosiloxane material/metal oxide composite as efficient anticorrosive coatings for mild steel in a saline medium

Hybrid sol–gel materials have been found very promising anticorrosive coatings for metal substrates. In this article, the synthesis of novel hybrid organic‐inorganic sol–gel polymer; starting from tetraethyl orthosilicate, (3‐aminopropyl) trimethoxysilane, dimethoxy‐methyl‐octadecylsilane and polydimethylsiloxane, silanol terminated precursors, is reported. The hybrid polymer has been further loaded individually as well with five different metal oxides, then deposited on mild steel panels. All cured coating formulations have been characterized using thermogravimetric analysis, contact angle measurements, electrochemical impedance spectroscopy, vicker‐microhardness, surface roughness, and critical load analyses. Results have revealed that the parent coating exhibits excellent thermal stability and hydrophobic nature with minor observed changes on the two properties for the metal oxide‐loaded coatings. Electrochemical impedance and visual inspection results indicated excellent corrosion protection performance for all metal oxide composite coatings (except magnesium oxide) on steel when immersed in 3.5% NaCl solution for a prolonged time. Furthermore, the coating containing molybdenum oxide exhibited a maximum hardness, homogeneity, and adherence to the steel surface. The developed coating formulations in this study can be considered as a promising alternative to the currently‐used toxic chromate and phosphate coatings. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 134, 46718     

Improved mechanical and anticorrosion properties of metal oxide-loaded hybrid sol–gel coatings for mild steel in a saline medium

Abstract

A hybrid organic–inorganic sol–gel coating was successfully prepared and subsequently functionalized individually with five different metal oxide additives. The effect of the incorporated oxides on the corrosion protection performance and scratch-resistance properties of the hybrid base coating on mild steel substrates was investigated using electrochemical techniques, namely electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) as well as mechanical testing. The steel-coated specimens were immersed in 3.5?wt.% NaCl corrosive medium for two weeks and the results reveal an excellent corrosion protection performance by all coating formulations with a significant high corrosion-resistance property for the sample loaded with molybdenum oxide. Scanning electron microscopy (SEM) images proved the absence of corrosion signs, defects, micro cracks, or delamination on the surface of the coated samples. Compared with the pure hybrid coating, all the metal oxide-embedded coatings (except for the sample loaded with yttrium(III) oxide) show comparable aqueous contact angle values as well as enhanced hardness and adherence properties. No noticeable dependence was observed for the surface roughness parameters as a function of the type of incorporated metal oxide within the sol–gel matrix. Overall, the results of this study demonstrate that metal oxides can be advantageous to the desired properties of hybrid sol–gel coatings applied to steel surfaces.     

Development of an environmentally friendly protective coating for the depleted uranium-0.75 wt.% titanium alloy: Part II: Coating formation and evaluation

Molybdenum oxide based coatings have been formed on the surface of the depleted uranium-0.75 wt.% titanium alloy. Surface activation prior to coating formation has been examined using fluorides and concentrated nitric acid. The electrochemical characteristics of the coating formation processes were studied using open circuit potential measurements. Residual fluoride from the activation process has been found to interfere with coating formation and surface activation by nitric acid yields a relatively thinner but more robust coating.The corrosion protection characteristics of the coatings were evaluated by potentiodynamic polarization testing in quiescent 0.05 M sodium chloride. The coatings have been studied using scanning electron microscopy, energy dispersive spectroscopy and optical microscopy.     

The self-assembled nanophase particle (SNAP) process: a nanoscience approach to coatings

In the corrosion protection of aluminum-skinned aircraft, surface pretreatment and cleaning are critical steps in protecting aerospace alloys from corrosion. Our recent discovery of a revolutionary new method of forming functionalized silica nanoparticles in situ in an aqueous-based sol–gel process, and then crosslinking the nanoparticles to form a thin film, is an excellent example of a nanoscience approach to coatings. This coating method is called the self-assembled nanophase particle (SNAP) process.

The SNAP coating process consists of three stages: (1) sol–gel processing; (2) SNAP solution mixing; (3) SNAP coating application and cure. Here, we report on key parameters in the ‘sol–gel processing’ and the ‘coating application and cure’ stages in the GPTMS/TMOS system. The SNAP process is discussed from the formation of the nanosized macromolecules to the coating application and curing process.

The ‘sol–gel processing’ stage involves hydrolysis and condensation reactions and is controlled by the solution pH and water content. Here, the molar ratio of water to hydrolysable silane is a key factor. SNAP solutions have been investigated by NMR, IR, light scattering, and GPC to identify molecular condensation structures formed as a function of aging time in the solution. In moderate pH and high water content solutions, hydrolysis occurs rapidly and condensation kinetic conditions are optimized to generate nanophase siloxane macromolecules.

In the ‘SNAP solution mixing’ stage, crosslinking agents and additives are added to the solution, which is then applied to a substrate by dip-coating to form the SNAP coating. The chemical structure and morphology of the films have been characterized using X-ray diffraction (XRD), time-of-flight secondary ion mass spectrometry (TOF-SIMS) and atomic force microscopy (AFM). SNAP films are amorphous but exhibit nanostructured assembly of siloxane oligomers at a separation of about 1.8 nm as well as molecular level ordering of O–Si–O species. The surface analytical data indicate that the films retain the basic chemical arrangement of the siloxane macromolecules/oligomers and crosslinking process creates a network of siloxane oligomers tethered together. Results of these analyses are then used to construct a model of the SNAP coating. Results of these analyses are discussed in detail.     

Real time mapping of corrosion activity under coatings

Current accelerated testing of aircraft coating systems for corrosion protection relies heavily on salt spray methods. Electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and electrochemical noise methods (ENM) provide insight into the global properties of a coating system, and both techniques are being used on a limited basis. However, there is a need to investigate corrosion events with greater spatial resolution under coatings at the metal/coating interface. Such corrosion activity may be related to coating defects and variations in the surface chemistry of the underlying metal.

The scanning vibrating electrode technique (SVET) has been developed to allow high spatial resolution investigation of localized corrosion activity that may be associated with coating defects or galvanic coupled regions of the metal surface. The SVET offers high resolution in current measurements of the order of 0.5 μA/cm² and is able to detect in-situ initiation and progress of corrosion activity under a protective coating. Using the SVET, minute variations in d.c. current associated with localized corrosion activity are detected and used to map both anodic and cathodic corrosion activities in a localized area. The difference in initial corrosion activity under various coatings can be correlated to the performance life of the coatings. The application of SVET to aircraft coatings and corrosion is reported to demonstrate the utility of this important new electrochemical tool.

In the current study, the SVET was used to discriminate the corrosion protection performance of selected sol–gel based coating systems. Sol–gel based surface treatments are being developed as part of an environmentally compliant coating system alternative to the currently used chromate-based systems. The SVET results are compared with data obtained from chromium inhibition coating systems. The SVET analyses are compared with electrochemical impedance measurements. The comparison of such data will provide the basis to adopt SVET measurements as an early performance discriminator for newly developed coating systems.     

Nanostructured silicon sol-gel surface treatments for Al 2024-T3 protection

Current coatings for aircraft corrosion protection are based on chromate surface treatments, primers, and topcoats. One approach to developing a chromate-free surface treatment is through the use of sol-gel materials that interact strongly with both the substrate and the subsequent polymer layers. Results are reported here on a new sol-gel technique using a pre-formed, self-assembled, nano-phase particle (SNAP) sol-gel system. SNAP films have been investigated by infrared spectroscopy, atomic force microscopy, and electrochemical methods. Potentiodynamic polarization and electrochemical impedance spectroscopy demonstrated that the SNAP system generates high quality, defect-free, durable films. Formulation parameters, including crosslink density and coupling agent application, were optimized based on experimental results. Presented at the 78th Annual Meeting of the Federation of Societies for Coatings Technology, on October 18–20, 2000, in Chicago, IL Materials and Manufacturing Directorate, Nonmetallic Materials Division, Coatings Research Group, Wright-Patterson AFB, OH 45433-7750.     

A new acceleration factor for the testing of corrosion protective coatings: flow-induced coating degradation

For corrosion protective coatings that are designed to give lifetimes of protection that may extend to 50 years, valid accelerated test methods are necessary to develop improved systems and validate performance. Fluid flow over metals has long been believed to influence the corrosion process. Studies have been focused on the effects of flow rate on the corrosion of bare metals. The influence of fluid flow on the degradation of metal-protective coatings has received less attention. This paper describes a preliminary study on the influence of laminar flow on organic coatings. A Hele-Shaw cell and its associated fluid control apparatuses are incorporated into the electrochemical cell setup. The barrier properties of the coating as a function of immersion time and flow rate have been monitored by electrochemical impedance spectroscopy. We observe that the barrier properties of the coating measured electrochemically decrease exponentially with the increasing flow rate. We propose that the flowing electrolyte solution could be used in acceleration tests for the lifetime prediction of organic coatings as the acceleration of failure we have observed does not appear to change the mechanism of failure. Further analysis is proposed to validate immersion flow rate as a universal accelerating parameter for coating failure.     

Fabrication of ZnO–GO hybrid for enhancement of chemically bonded phosphate ceramic coatings corrosion protection performance on AISI304L stainless steel

In this paper, a solvothermal method is used to prepare nano-sized zinc oxide-graphene oxide (ZnO–GO) hybrid, and the ZnO–GO hybrid is characterized by X-ray diffraction analysis, Raman, Fourier transform infrared spectroscopy, and scanning electron microscopy. In addition, chemically bonded phosphate ceramics coatings with different content of ZnO–GO hybrid are prepared on the stainless steel through the sol-gel method. The corrosion performance of the coatings is evaluated by electrochemical properties and the analysis of the surface and cross morphology of the coating. Results indicate ZnO–GO hybrid significantly enhances the compactness and corrosive behavior of the coating because the overlapping structure of GO flake improves the barrier performance of the coating. Besides, ZnO nanoparticles on the surface of GO can react with aluminum dihydrogen phosphate binder, in that case the adhesion between GO and the coating is improved.     

High performance polyaniline containing coating system for wet surfaces

Application of paint coatings on wet surfaces is rather difficult due to poor adhesion of coatings. For painting of wet surfaces, moisture curable coating systems based on epoxy resin and ketimine are found to be useful. Hence a study has been made on the corrosion protection ability of coating on wet surfaces using epoxy resin, ketimine and polyaniline. Paints with 20–30% PVC were prepared and applied over the wet steel surface and the corrosion protection performance of the coating was found out by salt spray and electrochemical impedance spectroscopic techniques. Coating with 20% PVC is found to offer very high protection since the impedance values are remained at greater than 10⁹ Ω cm² after immersion and salt spray tests.     

The use of electrochemical noise methods (ENM) to study thick, high impedance coatings

Thick, high impedance organic coatings are those class of coatings used to provide corrosion protection to naval vessels, pipelines, gasoline storage tanks, and other large structures such as bridges and plant structures. These coatings, especially the newest generations now being used in practice, can provide exceptional protection and lifetime of performance such that properly and accurately assessing and differentiating among competing coatings is a very difficult task. The standard protocol of salt fog testing (ASTM B117), immersion testing, and outdoor exposure in a corrosive environment with subjective evaluation of a coating's performance durings and after testing, does not adequatcly rank and predict coating lifetimes for new coating systems, especially for the environmentally compliant coating systems such as powder coatings (especially the thick, fusion bonded epoxy (FBE) coatings used for pipelines), two component epoxy and urethane coatings and waterborne coatings. New, objective test methods are desperately needed by users and manufacturers of coatings. A relatively new electrochemical test procedure, electrochemical noise methods (ENM), as developed by Skerry and Eden, has been shown in our laboratory to be very successful in the ranking and prediction of relative coating performance. We have used the method successfully on naval ship coatings, several pipeline coatings and other related systems, and Skerry has used them successfully on industrial maintenance coatings. We have used these methods in conjunction with electrochemical impedance spectroscopy, d.c. resistance measurements and cyclic salt fog testing of the Prohesion^TM type. In our studies of pipeline coatings, we needed to investigate thermal effects because of their extended range of use temperature. In these studies, we have discovered that electrochemical methods can be used for an in situ measurement of the T_g of coatings in electrolyte immersion. Further, the ‘plasticizing’ effect of aqueous electrolyte absorption as well as its relative irreversibility has been shown. For all coatings studied, ENM provided useful, objective, numerical data which rapidly ranks coatings and provides useful information on the relative lifetime prediction of coatings which may provide up to 30 years of service.     

Study of degradation of organic coatings in seawater by using EIS and AFM methods

In this article, electrochemical behaviors and their topography observation for four organic coatings used in seawater, by using both electrochemical impedance spectroscopy (EIS) and atomic force microscopy (AFM) methods to study environment behaviors of different coatings as well as the effects of their film formation, pigments, and fillers on anticorrosion behaviors, were measured. The results show that polyurethane, epoxy, and chlorinated rubber coatings all present one capacitive loop in their tested EIS which contains phenomenally only one time constant, whereas alkyd coating presents two capacitive semicircle arcs. With two capacitive loops, the capacitive semicircle in the high frequency range represents barrier layer property, but the semicircle in the low frequency range represents corrosion reaction of metals under the film. Polyurethane coating used in seawater has well anticorrosion property in seawater immersion test. The appearance features of different layers are visible different between different layers of tested coatings at their surface topography. The property of polyurethane paint film coated on metal is better than other layers, and film of alkyd coating has many pits at its surface by observing the layer's images. AFM photos imaged have also been used to further detail surface topography for four organic coatings, and to approve effects of topography of these coatings on its electrochemical behaviors, from two views of both height and phase modes. It is beneficial to explain deeply the environment behaviors and degradation mechanism of organic coatings. To further study failure of these organic coatings and dynamic processes of corrosion of metal under the film, two equivalent circuit models, according to these tested EIS, have been suggested to explain the corrosive kinetics of these four coatings. To polyurethane, epoxy, and chlorinated rubber coatings used in seawater which have good protection effects for substrate metal, the diffusion process for water, from their layer's surface to interface of film/metal, is mainly controlled factor for degradation. However, the electrochemical reaction process has may become a control procedure for corrosion of alkyd coated metal. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Hybrid sol–gel coatings for corrosion protection of galvanized steel in simulated concrete pore solution

The aim of this experimental research was to study the electrochemical behavior of organic–inorganic hybrid (OIH) coatings for corrosion protection of hot-dip galvanized steel (HDGS) in the first instants of immersion in simulated concrete pore solution (SCPS) (pH > 12.5). The electrochemical performance of the OIH coatings was assessed by electrochemical impedance spectroscopy, potentiodynamic polarization curves, macrocell current density, and polarization resistance. The OIH coatings were prepared via the sol–gel method and were deposited on HDGS surfaces by dip-coating using one or three dip steps. The electrochemical results obtained for HDGS samples coated with OIH matrices in SCPS showed higher corrosion resistance than bare HDGS; as the molecular weight (MW) of Jeffamine^® increased the barrier protection of the coating decreased. The lowest protection efficiency was found for HDGS samples synthesized with oligopolymers with an MW of 2000. Coatings produced with an oligopolymer of 230 MW conferred the highest protection. The surface morphology of the OIH coatings deposited on HDGS surfaces was studied by atomic force microscopy. The results show that the roughness of the OIH films depends on the MW of Jeffamine^® and on the number of dip-coating steps used. Thermogravimetry results show that the Jeffamine^® MW affected the thermal properties of the prepared OIH samples. The prepared OIH materials are thermally stable within the range of 20–80°C.     

Modeling of nano-sized macromolecules in silane-based self-assembled nano-phase particle coatings

Molecular simulation approaches have been used to enhance the understanding of complex chemical interactions in coatings related processes. The Self-assembled NAno-phase Particle (SNAP) coating process relies on aqueous solution processes, similar to those used in conventional sol–gel synthesis, to form siloxane nano-sized structures, which are subsequently cross-linked upon film application. This process has been shown to produce a dense, protective thin film on metal substrates. The SNAP process involves design and selection of the coating constituents, based on the desired functionalities for network formation and cross-linking chemistry. In order to facilitate the design of coating components at the molecular level, it is imperative to gain a fundamental understanding of these complex phenomena.

Molecular simulations on several oligomers with different side chains have been performed to study components of the of Si–O networks during the SNAP particle formation process. Several ring structures of tetramethyl orthosilicate (TMOS) and 3-glycidoxypropyltrimethoxysilane (GPTMS) have been considered. Geometry optimization of the cyclic Si–O structure formation has been performed, and ring strain parameters have been calculated.     

Self-assembled nano-phase particles for enhanced oxygen barrier coatings on polymeric materials

Aqueous-based inorganic–organic hybrid coating materials comprising self-assembled nanophase particles (SNAP) were investigated for their potential to confer high gas barrier performance to flexible polymeric substrates, particularly to improve suitability of the substrates as encapsulation materials for organic photovoltaic (OPV) devices. Potential benefits of this approach include versatility in barrier coating formulation to achieve specific barrier properties, and application of coatings in a reel-to-reel process under ambient conditions. The present study focused on enhancement of the oxygen barrier performance of polypropylene (PP) substrates by applying SNAP-based coatings with and without the addition of an oxygen scavenging additive. SNAP particles were characterised using ²⁹Si NMR and dynamic light scattering, and coatings were analysed using atomic force microscopy and X-ray photoelectron spectroscopy. SNAP particle preparation and coating formulation was optimised with respect to oxygen transmission rate (OTR) of the coating on PP, and mechanical properties of the coating solution. In the absence of oxygen scavenger, the lowest OTR attained for the SNAP-based coatings was 0.87 cm³ mil m⁻² day⁻¹ atm⁻¹. The OTR was further reduced to 0.22 cm³ mil m⁻² day⁻¹ atm⁻¹ on addition of 9,10-anthraquinone-2,6-dissulfonic acid (AQDS) into the coating as an oxygen scavenger. These results represent a decrease in OTR by 4 orders of magnitude compared with uncoated PP, and the oxygen barrier obtained by addition of AQDS surpasses the performance of many plastic materials considered to be high oxygen barriers in the food and pharmaceutical industries.     

Inorganic/organic hybrid coatings for aircraft aluminum alloy substrates

A series of water-based stable sol–gel systems have been developed. Various functional groups including amino, epoxy, vinyl, and allyl groups can be incorporated into the sol–gel network to interact with organic polymer resins. The solid content of these sol–gel-based coating formulations varies from 2.5 to 45%. The sol–gel coating of alumina–silica networks derived from low solid content solutions (2.5%) has been developed and evaluated to replace the current conversion coating pretreatment process. Sol–gel coatings derived from the high solid content solutions (17–45%) have shown excellent mechanic strength, good adhesion, and provide corrosion protection of the aluminum substrate when cured at elevated temperatures. Sol–gel/epoxy resin hybrid coatings have been formulated and studied. The hybrid coating showed enhanced mechanical strength such as hardness and abrasion resistance. When cured at elevated temperatures (80°C), all of the hybrid coatings studied passed wet adhesion testing. Some of the hybrid coatings pass wet adhesion testing when cured at room temperature. However, water-sensitivity remains for most of the room temperature cured hybrid coatings.     

Electropolymerized bilayer coatings of polyaniline and poly(N-methylaniline) on mild steel and their corrosion protection performance

Polyaniline (PANI) and poly(N-methylaniline) (PNMA) have been electrodeposited on mild steel from oxalic acid bath using cyclic voltammetric technique. Pretreatments like passivation and primer polymer coatings were required for effective coating. Differently stacked composite polymer layers on the metal surface by layer-by-layer approach have also been obtained and their properties have been compared with their corresponding copolymer coatings. FTIR study confirms the formation of electroactive polymer compounds on mild steel. Evaluation of these coatings in 3.5% NaCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy reveals significant corrosion resistant behavior. Relatively higher corrosion protection is exhibited by copolymer coatings and composite-bilayer coatings than the corresponding homopolymer coatings. The composite metal–PANI–PNMA layer shows higher stability and better protection than the metal–PNMA–PANI layer.     

Study on cerium-doped nano-TiO2 coatings for corrosion protection of 316?L stainless steel

Many methods have been reported on improving the photogenerated cathodic protection of nano-TiO₂ coatings for metals. In this work, nano-TiO₂ coatings doped with cerium nitrate have been developed by sol–gel method for corrosion protection of 316 L stainless steel. Surface morphology, structure, and properties of the prepared coatings were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. The corrosion protection performance of the prepared coatings was evaluated in 3 wt% NaCl solution by using electrochemical techniques in the presence and absence of simulated sunlight illumination. The results indicated that the 1.2% Ce-TiO₂ coating with three layers exhibited an excellent photogenerated cathodic protection under illumination attributed to the higher separation efficiency of electron–hole pairs and higher photoelectric conversion efficiency. The results also showed that after doping with an appropriate concentration of cerium nitrate, the anti-corrosion performance of the TiO₂ coating was improved even without irradiation due to the self-healing property of cerium ions.     

The corrosion resistance of 316L stainless steel coated with a silane hybrid nanocomposite coating

The present work aims at evaluating the corrosion resistance of 316L stainless steel pre-treated with an organic–inorganic silane hybrid coating. The latter was prepared via a sol–gel process using 3-glycidoxypropyl-trimethoxysilane as a precursor and bisphenol A as a cross-linking agent. The corrosion resistance of the pre-treated substrates was evaluated by neutral salt spray tests, linear sweep voltammetry and electrochemical impedance spectroscopy techniques during immersion in a 3.5% NaCl solution. In addition, the effect of the drying method as an effective parameter on the microscopic features of the hybrid coatings was studied using Fourier transform infrared spectroscopy and scanning electron microscopy. Results show that the silane hybrid coatings provide a good coverage and an additional corrosion protection of the 316L substrate.     

不同表面处理条件下复合涂层体系失效过程的EIS特征

研究了不同表面处理条件下环氧富锌/环氧云母氧化铁/氯化橡胶涂层体系的电化学阻抗谱特征。利用Bode图、涂层吸水率、涂层电阻及特征频率的变化评价了表面处理对涂层防护性能的影响。结果表明,基材表面状态不同的复合涂层体系吸水率相对稳定阶段所持续的时间长短顺序为：手工打磨>表面锈蚀>表面未处理,与涂层的防护寿命长短、涂层/基材间的黏附力大小顺序一致。此外,不论基材表面处理程度如何,当涂层体系的特征频率增加到1400 Hz左右时,涂层电阻均发生较快降低,吸水率发生较大增长,涂层失去防护作用。