Sol–gel based protective coatings for copper products

Corrosion, fouling, and wearing of metal surfaces are the most common problems faced in industry as well as in environmental use. In the recent study, sol–gel based protective coatings were developed and deposited on different copper substrates. Different chemical pretreatments were used to enhance the adhesion and spreadability of coatings on copper surfaces. Substantial improvement of the corrosion resistance of sol–gel coated copper surfaces was obtained by a salt spray test. In addition, the sol–gel coatings increased hydrophobic and easy-to-clean properties of the copper surface. Variation of the curing temperature of the coatings caused changes in the morphology, adhesion, and topography of the sol–gel coatings. On the basis of the important information obtained in this study, the protective properties of sol–gel coatings can be tailored for copper and copper alloy substrates.     

Corrosion protection behaviour of sol–gel derived N,N-dimethylthiourea doped 3-glycidoxypropyltrimethoxysilane on aluminium

The present work describes the anticorrosion features of inhibitor doped sol–gel coating on Al metal. Sol–gel coatings were prepared by using 3-glycidoxypropyltrimethoxysilane (GPTMS) as parent precursor. In order to improve the corrosion resistance property of coating, N,N-dimethylthiourea was added into the sol–gel matrix. The corrosion inhibitor doped sol–gel coating on metal was characterized by Fourier transform infrared analysis (FTIR) and scanning electron microscope (SEM). Inhibition effect of N,N-dimethylthiourea doped GPTMS coating on Al substrates in 1% NaCl solution was investigated using electrochemical impedance (EIS) and polarization studies. EIS results showed that the corrosion resistance of sol–gel coating significantly improved upon addition of N,N-dimethylthiourea. The study had outlined the nuances of doping an organic inhibitor to enhance the protection ability of sol–gel coating on Al metal.     

Complex anticorrosion coating for ZK30 magnesium alloy

This work aims at developing a new complex anticorrosion protection system for ZK30 magnesium alloy. This protective coating is based on an anodic oxide layer loaded with corrosion inhibitors in its pores, which is then sealed with a sol–gel hybrid polymer. The porous oxide layer is produced by spark anodizing. The sol–gel film shows good adhesion to the oxide layer as it penetrates through the pores of the anodized layer forming an additional transient oxide–sol–gel interlayer.The thickness of this complex protective coating is about 3.7–7.0 μm. A blank oxide–sol–gel coating system or one doped with Ce³⁺ ions proved to be effective corrosion protection for the magnesium alloy preventing corrosion attack after exposure for a relatively long duration in an aggressive NaCl solution.The structure and the thickness of the anodized layer and the sol–gel film were characterized by scanning electron microscopy (SEM). The corrosion behaviour of the ZK30 substrates pre-treated with the complex coating was tested by electrochemical impedance spectroscopy (EIS), scanning vibrating electrode technique (SVET), and scanning ion-selective electrode techniques (SIET).     

Development of new processes to protect zinc against corrosion,suitable for on-site use

Protection against corrosion of metals is well known as an important issue in numerous fields. In all cases, the improvement of durability of these metals has to be connected to the development of environmentally friendly processes. Sol–gel protective coatings have shown excellent chemical stability and enhanced corrosion resistance for zinc substrates. Further, the sol–gel method, used as technique of surface protection, showed the potential for the replacement of toxic pre-treatments. This paper highlights the recent developments and applications of silane based sol–gel coatings on zinc substrates. Then, the challenges for industrial transfer of the developed process are also discussed because this process presents a disadvantage for on-site use, which is the too time-consuming thermal treatment. So, the goal of this study was to determine the convenient experimental conditions to reduce the duration of heat treatment of the hybrid sol–gel layer, compatible with the severe industrial requirements, without reducing the protection against corrosion. To reach this objective, a correlation between the results of chemical analyses and the protection against corrosion efficiency was established.     

Self-healing corrosion protection by nanostructure sol–gel impregnated with propargyl alcohol

Zirconia based nanostructured hybrid sol–gel coating, impregnated with propargyl alcohol (PA) to hinder corrosion of mild steel was studied. Zirconia nano-particles (TPOZ) contained sol–gel was synthesized using 3-glycidoxypropyltrimethoxysilane (GPTMS) as precursor and subsequently optimized for internal cohesion as well as adhesion to the mild steel substrate to assess the consequent effect on suppression of corrosion damage in 0.5 M NaCl solution. Electrochemical Impedance Spectroscopy (EIS) and Scanning Electron Microscopy (SEM) results demonstrated the effect of coating integrity on impeding corrosion. Experiments on mild steel specimens coated with hybrid sol–gel films following mechanical polishing render superior corrosion resistance to mild steel in contrast to those undergoing electrochemical polishing with marked lower surface roughness. Results also proved of beneficial effect of low and exact concentration of PA on sol–gel coat's barrier properties evident from diminished corrosion current and other electrochemical indicators.     

Electrochemical investigation of high-performance silane sol–gel films containing clay nanoparticles

Silane sol–gel coatings are widely used as adhesion promoters between inorganic substrates, such as metals, and organic coatings. The aim of these pre-treatments is to enhance the corrosion protection performance of the organic coating improving the adhesion to the substrate and acting as a barrier against water and aggressive ions diffusion. It is a matter of fact that the silane sol–gel pre-treatments do not provide an active protection against corrosion processes except for the partial inhibition of the cathodic reaction. Inorganic pigments can improve the barrier properties of the silane sol–gel film, enhancing the resistance against corrosion. In this study, different amounts of montmorillonite nanoparticles were added to a water based silanes mixture in order to improve the barrier properties of the sol–gel coating. Hot dip galvanized steel was used as substrate. The sol–gel film consists of a combination of three different silanes, GPS, TEOS and MTES. The clay nanoparticles used in this study were mainly neat montmorillonite. The proper concentration of filler inside the sol–gel films was determined comparing the corrosion resistance of silane layers with different nanoparticles contents. Additionally, the effect of CeO₂ and Ce₂O₃ enriched montmorillonite particles. The EIS analysis and the polarization measurements demonstrated that the optimal amount of neat montmorillonite nanoparticles is about 1000 ppm. The same electrochemical techniques highlighted the limited effect of the cerium oxides grafted to the clay nanoparticles on the corrosion resistance of the silane sol–gel film. The TEM analysis proved the presence of a nano-crystalline structure inside the silane sol–gel film due to the formation of crystalline silica domains.     

Effect of organic chelates on the performance of hybrid sol–gel coated AA 2024-T3 aluminium alloys

Sol–gels are organic–inorganic polymers formed by hydrolysis/condensation reactions of alkoxide precursors, primarily silanes, which have found applications as electronic, optical and protective coatings. These coatings possess important characteristics such as chemical stability, physical strength and scratch resistance. Further performance improvement is achieved through the incorporation of zirconium and titanium based nanoparticles, also formed through the sol–gel process. However due to the inherent difference in the reactivity of the precursors, the hydrolysis of each precursor must be carried out separately before being combined for final condensation. Zirconium precursors are commonly chelated using acetic acids, prior to hydrolysis, to lower the hydrolysis rate.In this body of work various ligands such as organic acids, acetyl acetone (AcAc) and 2,2′-bipyridine (Bipy) were used to control the zirconium hydrolysis reaction and form nanoparticles within the silane sol matrix.Nanoparticle modified coatings formed from the silane sol on AA 2024-T3 aluminium were characterised spectroscopically, electrochemically and calorimetrically to evaluate the potential effect of the different chelates on the final film properties while neutral salt spray tests were performed to study their anti-corrosion performance. Results indicate that the acid ligand modified coatings provided the best performance followed by AcAc, while Bipy was the poorest. In all cases the zirconium nanoparticle improved the protective properties of the sol–gel coating.     

Optimization of hybrid sol–gel coatings by combination of layers with complementary properties for corrosion protection of AA2024

The coating system usually employed for corrosion protection of metal substrates consists of different layers which can be constituted of a chemical conversion coating applied on the metal surface followed by a number of organic layers. Hybrid films prepared by the sol–gel method provide a good approach as protective layers on metallic surfaces, although it is necessary to combine the barrier functionality with an active protection mechanism to avoid corrosion when the coating is damaged. Previous works have shown that it is very difficult to reach this result in a mono-layer sol–gel because the amount of inhibitors incorporated tends to increase significantly the porosity of the coating and reduces drastically the barrier properties. This work presents the characterization of a multi-layer sol–gel hybrid inorganic–organic coating system with a structure composed of an intermediate cerium inhibited layer deposited between two un-doped layers on AA2024 alloy. The comparison between the inhibited system and a bi-layer non-inhibited one has allowed to assess the migration of the cerium ions into the hybrid coating towards the substrate corresponding to the improvement of the corrosion properties. The combination of the physical barrier and the active protection enables to obtain an effective protective system.     

Advanced chrome-free organic–inorganic hybrid pretreatments for aerospace aluminum alloy 2024-T3—application of novel bis-ureasil sol–gel precursors

Effective pretreatment of aluminum alloys is very critical to success of protective coating systems for aerospace applications. While chromate-based pretreatments have been very successful for corrosion protection, they have been a target for replacement due to the increasingly stricter regulatory requirements arising from toxicity and carcinogenic nature of Cr(VI) used in such pretreatments. Among many approaches to develop alternative systems, the organic–inorganic hybrid (OIH) coatings based on sol–gel technology has advanced rapidly. We have successfully developed OIH coating systems by using suitably tailored organosilane precursors and sol–gel processing conditions. A series of novel bis-ureasil precursors have been developed and employed as organic precursor of OIH systems. Statistical design of experimental methodology (DoE) has been used to study and optimize compositional and process parameters using multifactor analysis of variance (ANOVA) analysis method. The corrosion resistance study (Potentiodynamic polarization, salt-spray corrosion test) shows that by proper choice of sol–gel precursors, cross-linkers, and reaction conditions, very dense, adherent and protective hybrid coatings, comparable in performance to chromate-based ones, can be obtained for aerospace aluminum alloy 2024-T3. This paper was awarded the Southern Society A.L. Hendry Award for best student paper. The paper was presented at FutureCoat! 2008, sponsored by Federation of Societies for Coatings Technology, held October 14–16, 2008, in Chicago, IL.     

Enhancement of corrosion protection of mild steel by chitosan/ZnO nanoparticle composite membranes

The development of active corrosion protection systems for metallic substrates is an issue of prime importance for many industrial applications. Nanostructured chitosan/ZnO nanoparticle films were coated on mild steel by sol–gel process, dip coating technique. Sol–gel protective coatings have shown excellent chemical stability, oxidation control and enhanced corrosion resistance for metal substrates. Further, the sol–gel method is an environmentally friendly technique of surface protection which has traditionally been used for increasing corrosion resistance of metals. Films so formed were characterized by UV–vis absorption spectroscopy (UV–vis), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray fluorescence spectrometry (EDX). Corrosion protection behavior of these coated mild steel substrates in 0.1 N HCl solutions was evaluated by potentiodynamic polarisation studies (Tafel), linear polarisation studies (LPR), electrochemical impedance spectroscopy studies (EIS).     

Hydrogen permeation through sol–gel-coated iron during galvanostatic charging

One-layer sol–gel silica–zirconia and two-layer silica–zirconia and zirconia coatings were deposited on one side of iron membranes by spin-coating, densified in air and annealed up to 800 °C in vacuum. Hydrogen permeation through the membranes, coated and uncoated, polarised cathodically under galvanostatic control in 0.1 M NaOH solution was studied using the electrochemical permeation technique. During the initial period, the effect of the sol–gel coatings was insignificant. However, the coatings quite efficiently prevented the iron surface become more active to hydrogen entry during a long-lasting cathodic polarisation. In addition, the electrochemical-corrosion behaviour of the coated iron and the effect of the sol–gel coatings on the effective diffusivity of hydrogen in the coated membranes were studied. On the basis of the polarisation curves and the hydrogen permeation data it was proved that the sol–gel coatings blocked the iron surface for the hydrogen evolution reaction and, consequently, for the hydrogen entry into iron. The effective coating coverage was determined by comparison of the hydrogen fluxes permeating the coated and uncoated membranes. Finally the real concentration of hydrogen beneath the uncoated iron sites and the amount of hydrogen stored in a membrane were evaluated.     

The Influence of Low- and High-Molecular Hydroxyl-Containing Additives on the Stability of Sol–Gel Tetraethoxysilane-Based Systems and on the Structure of Hybrid Organic–Inorganic Coatings

Organic–inorganic composites are synthesized by introducing a number of low- and high-molecular water-soluble polyhydroxyl compounds (with different molecular weights and different molecular structures) into sols based on tetraethoxysilane. The coatings on nickel substrates are prepared by adding chromium dioxide as a finely disperse filler to the soles. The rheological properties of the sol–gel systems synthesized are investigated, and the surface condition and the microstructure of the coatings formed are examined. The xerogels and the coatings are studied by differential thermal and X-ray powder diffraction analyses.     

Characterization of sol–gel coated 316L stainless steel for biomedical applications

Silica based organic–inorganic hybrid coatings were deposited on 316L stainless steel by sol–gel technique. The hybrid sols were prepared by hydrolysis and condensation of 3-methacryloxypropyltrimethoxysilane (TMSM) and tetraethylorthosilicate (TEOS) at different molar ratios. Electrochemical experiments were performed to evaluate the corrosion resistance properties of the coatings. Structural characterization of the coatings was performed using scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. Contact angle measurement and cell morphology assay were performed to investigate the hydrophilicity and in vitro cytotoxicity of the coatings, respectively. The results indicate formation of a crack-free and highly adherent film acting as a protective barrier against the physiological medium. Corrosion resistance of hybrid coatings was influenced by the molar ratios of TMSM:TEOS. The best corrosion protection was obtained at TMSM:TEOS molar ratio of 1:1. Sol–gel coatings enhanced the hydrophilicity of 316L steel surfaces. Also, these coatings showed non-toxicity for L929 cells.     

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.     

Corrosion resistance of compositionally modulated multilayered Zn–Ni alloys deposited from a single bath

By consecutive deposition at two different current densities from a single sulfate–chloride bath, compositionally modulated multilayered (CMM) coatings of Zn–Ni alloys, with different number, thickness and sequence of the sublayers were obtained. The corrosion resistance of the coatings was studied by potentiodynamic dissolution and by corrosion potential measurement. In the current–potential (stripping) curves two well-defined peaks were observed. With increase in the number of sublayers, regardless of their individual thickness, the correlation between the amount of Zn, dissolved at more negative potentials and the whole amount of the metal in the CMM coatings, decreases. The corrosion potentials of CMM coatings are most positive (–0.940 V vs SSE) when they end with a Zn–Ni18% oversublayer. As a result of the alternation of Zn–Ni alloy sublayers with different Ni content the obtained CMM coatings have increased corrosion resistance in comparison with the monolayer coatings of the composing alloys.     

Passivation of metal alloys using sol–gel-derived materials — a review

The sol–gel method is being investigated as an environmentally compliant alternative for chromate-based conversion coatings currently in use. An overview of recent advances in the use of sol–gel derived coatings for improved corrosion resistance of aluminum and steel metal surfaces is given.     

Observations on chromate conversion coatings from a sol–gel perspective

Chromate conversion coatings (CCCs) have been applied to aluminum substrates for many years to improve corrosion resistance and paint adhesion. In recent years the so-called sol–gel chemistry based coating systems have been examined as possible replacements for CCCs on aluminum. A mechanism is proposed for the formation of CCC on aluminum that is consistent with sol–gel chemistry principles. The conclusions illustrate important considerations for developing non-CCCs for aluminum.     

Siliconcarbide Embedded Hybrid Nanocomposites as Abrasion Resistant Coating

Siliconcarbide (SiC) ceramic powder incorporated inorganic–organic nanocomposites were prepared by sol–gel spin coating method. Coating properties and abrasion resistance of the hybrid polymers were examined by scanning electron microscopy (SEM), EDX analysis, FT-IR spectroscopy, thermogravimetric differential thermal analysis (TG-DTA). Mechanical tests like surface static corrosion analysis, abrasion test, scratch-tape test, adhesion test, chemical and solvent resistance, surface hardness were performed in order to evaluate the coatings for possible industrial applications. Prepared polymers were applied onto the aluminium substrates. Results revealed that industrially available SiC ceramic powder incorporated inorganic–organic nanocomposites can be prepared with an easy and controlled way using sol–gel method and applied onto the aluminium surfaces by spin coating without using primer solution and shows excellent abrasion resistance.     

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.     

Critical aspects in the electrochemical study of unstable coated metallic substrates

Electrochemical methods are largely employed for the study of coated metallic substrates. Electrochemical techniques are very useful for investigation of coating performances also in the field of corrosion protection of aluminum alloys where replacement of chromate based coatings is an important issue. Application of sol–gel technique has recently made available thin inorganic protective coatings that can be considered as an alternative to chromate systems. Therefore, it is important to evaluate the electrochemical behaviour of defect free inorganic films which can be considered an inert material. As a consequence, investigation of this inert material might generate critical aspects related to rapid fluctuations of its open circuit potential and current density during open circuit potential and potentiodynamic polarization measurements. Moreover, the unstable nature of the sol–gel very thin films is a critical issue also in electrochemical impedance measurements carried out under potential control because variation of the open circuit potential of the system during data acquisition might lead to sample damage. Impedance measurements under current control is an alternative method for data acquisition. This technique is considered in the present paper for the investigation of AA2024 substrate coated with a thin sol–gel layer and for chromate conversion coated alloy.