Silane pre-treatments on copper and aluminium

A large part of aluminium products are coated with an organic layer in order to improve the corrosion resistance. Copper surfaces are also sometimes protected with an organic coating to improve the durability or the aesthetic properties. Examples of industrial applications are household appliances and heat exchanger components. For these applications it is not rare to have the industrial need to treat at the same time components made of aluminium and copper.In order to extend the service life of the organic coated copper a specific surface pre-treatment is often required. Nevertheless, probably because of the limited market of this application, no specific pre-treatments for copper are industrially developed, with the exception of cleaning procedures, but simply extensions of existing pre-treatments optimised for other metals (aluminium, zinc) are used.The application of silane pre-treatments as adhesion promoters for organic coated metals is remarkably increasing in the last decade, because silanes offer very good performance together with high environmental compatibility.The idea is therefore to try to develop a specific silane based pre-treatment for copper. The starting point is the existing silane products for aluminium, optimising the composition and the application conditions (concentration, temperature, pH of the bath, etc.) in order to develop a high performance copper alloy pre-treatment increasing the protective properties and the adhesion of a successively applied organic coating. Moreover these pre-treatments could be used for aluminium alloys too and therefore could be suggested for multi-metals components.The deposits were analysed using FTIR spectroscopy and optical and electron microscopic observations. A careful electrochemical characterisation, mainly by electrochemical impedance spectroscopy measurements (EIS) was carried out to highlight the presence of silane and to evaluate the performance of the different deposits.In order to study an environmentally friendly protective system, an epoxy-polyester powder coating was deposited on the silane treated metal surfaces.The influence of the pre-treatments on the adhesion of an organic coating was evaluated by adhesion tests in dry and wet conditions and the delamination of the coating, starting from a defect under specific environmental conditions, was measured.Some preliminary results appear very interesting and the deposition conditions were optimised, in particular the thermal curing (200 °C) of the silane layer is necessary to have a good coverage in the case of a copper substrate. The good adhesion also in wet conditions of the silane pre-treatments was also proven.     

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

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

A comparative study on the corrosion resistance of AA2024-T3 substrates pre-treated with different silane solutions: Composition of the films formed

This work reports a comparative study on the corrosion resistance of AA2024-T3 pre-treated with three different silane solutions. The silanes used for the pre-treatments of the AA2024-T3 panels were: 1,2-bis(triethoxysilyl)ethane (BTSE), bis-[triethoxysilylpropyl]tetrasulfide (BTESPT) and γ-mercaptopropyltrimethoxysilane (γ-MPS). The analytical characterisation of the silane films was performed by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The corrosion performance of the pre-treated substrates was evaluated by electrochemical impedance spectroscopy (EIS). The results show that the pre-treatments based on silanes provide good corrosion protection of unpainted AA2024-T3. Painted substrates, previously pre-treated with the silane solutions also revealed improved corrosion resistance and good adhesion properties. Fatigue tests show that the silane pre-treatments do not affect the fatigue behaviour of the AA2024-T3. The work also discusses the formation of the protective silane films.     

Comparative EIS study of pretreatment performance in coated metals

Various coated metal samples with different pretreatments were investigated by electrochemical impedance spectroscopy (EIS). Variables were the substrate (cold-rolled steel and hot-dipped galvanized steel), phosphate system (iron and zinc phosphate), post rinse (chromate and silane/zirconium rinse) and paint systems. The corrosion performance was determined on the basis of coating degradation, water uptake and interface delamination of the tested samples. The zinc phosphate performed better than iron phosphate on CRS. The silane/Zr rinse did not perform well in the CRS/iron-phosphate system. However, it showed a better performance than the chromate when used as a post rinse of zinc phosphate. Salt spray test (SST) and adhesion test results of the same samples are also reported in this paper and compared to the EIS data. The correlation among three test methods was poor.     

A comparative study between Cr(VI)-containing and Cr-free films for coil coating systems

In this work, the protection conferred by Cr(VI)-containing and Cr-free pre-treatments and primers used in hot dip galvanized steel (HDG) coated systems were studied. The EIS results showed a differentiated behaviour for the specimens with Cr(VI) compared to the chromium-free ones. The samples with Cr(VI), both in the pre-treatment and in the primer, presented a better corrosion performance when compared to the Cr-free ones.

Moreover, it can be said that the pre-treated and primed samples without Cr(VI) presented lower resistive properties and higher delamination fractions throughout the immersion time. The amount of Cr in the primer also influenced the coating resistance for shorter periods of immersion, but for long periods the passivating effect of Cr seems to be determinant.

When topcoat was applied, i.e. for a complete system, the Cr-free specimens performed better than the Cr(VI)-containing ones, perhaps due to a better adhesion of the topcoat to the primer in the former case. This leads to the conclusion that there is a risk in assessing the behaviour of a paint scheme on the basis of the individual behaviour of each layer.     

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.     

Silane and epoxy coatings: A bilayer system to protect AA2024 alloy

This work has proved that a good combination of a simple and fast metal pre-treatment, followed by the deposition of a thin layer of an organic–inorganic silane coating and further layer of epoxy coatings, are able to protect the aluminium alloy AA2024-T3 against corrosion in high concentrations of NaCl solution. The alloy AA2024 is one of the most employed aluminium alloy in structural applications due to its good mechanical properties. However, AA2024 alloy series commonly presents galvanic corrosion due to the rich content of copper element. The influence of different surface pre-treatments, the presence of a silane layer as pre-coating treatment and the influence of phosphonic acids combined with the silane layer on the corrosion protection and adhesion to the aluminium alloy have been examined using accelerated corrosion tests. High roughness and the presence of a pre-coating film between the metal surface and the organic coating were essential for a good protection and resistance to blistering appearance in the surface of AA2024-T3.     

Effect of functional groups (methyl,phenyl) on organic–inorganic hybrid sol–gel silica coatings on surface modified SS 316

Organic–inorganic hybrid sol–gel based silica coatings derived from hydrolysis and condensation of organically modified silane precursors like phenyltrimethoxysilane and methyltriethoxysilane along with tetraethoxysilane were deposited on different surface pre-treated (as-cleaned, plasma-treated, shot-blasted) SS 316 grade stainless steel substrate, using dip coating technique. The coatings were heat treated at 150 °C for 2 h in air. The pre-treated surfaces were characterized using X-ray Photoelectron Spectroscopy and Scanning Electron Microscopy. The water content of the sols was determined by Karl Fischer titration to evaluate the degree of completion of hydrolysis and condensation reactions. Cured coatings were characterized to evaluate thickness, water contact angle, pencil scratch hardness, gloss, and shrinkage in coating thickness. Impact test was carried out on pigmented coatings derived from sols synthesized using the two silane precursors. The corrosion resistance and water durability tests were carried out to compare the coatings derived from using different precursors and different surface pre-treatments. The corrosion tests were carried out for 1 h and 24 h exposure to a 3.5% NaCl solution by electrochemical polarization measurements. It was found that coatings from methyl substituted organically modified alkoxysilane exhibited better hydrophobicity, scratch hardness, impact resistance and barrier properties with respect to corrosion, when compared to those derived from phenyl substituted trialkoxysilane. The difference in performance of coatings was explained on the basis of difference in hydrolysis and condensation rates between the two organically modified silane precursors used for the sol synthesis.     

Investigation of the corrosion behaviour of a bilayer cerium-silane pre-treatment on Al 2024-T3 in 0.1 M NaCl

In the last few years great efforts have been made in order to find environmentally friendly substitutes for Cr⁶⁺ pre-treatments applied to aluminium alloys used in the aircraft industry. In this work we have investigated the electrochemical response of a bilayer pre-treatment consisting of a Ce conversion bottom layer and a non-functional silane (bis-1,2-(triethoxysilyl) ethane (BTSE)) top layer applied on Al 2024-T3, and compared its behaviour with monolayer coated samples. The investigation was carried out in 0.1 M NaCl solution, and the electrochemical techniques employed were anodic polarization curves and electrochemical impedance spectroscopy (EIS). EIS experiments performed with bilayer coated samples have shown a continuous increase of the impedance response during the whole test period, which was interpreted on the basis of a pore blocking mechanism supported by scanning electron microscopy (SEM) images and equivalent circuit fitting. Moreover, the impedance of the bilayer coated samples was approximately one order of magnitude higher than that presented by monolayer coated ones. On the other hand, mechanical tests have evidenced the good adhesion of the silane layer to the Ce conversion layer, which can be likely attributed to a better linking between the silane molecules and the cerium bottom layer.     

Polyelectrolytes to promote adhesive bonds of laser-structured aluminium

Aluminium is one of the most popular construction materials in machine and equipment manufacture as well as vehicle and aircraft construction. Particularly, in automotive and aircraft industries, the adhesive bonding of aluminium requires the pre-treatment of the adhesive surfaces. In this study laser pre-treatments were used to laterally control the surface roughness and clean the substrate surfaces by forming fresh aluminium oxide layers. In order to keep the adhesive properties stable over time, the laser pre-treated aluminium surfaces were subsequently coated with weak polyelectrolytes. The applied polyelectrolytes lower the driving forces for the adsorption of unwanted surface contaminations and provide reactive amino groups for the subsequent coupling of reactive adhesives. The surface topographies of the laser-treated aluminium surfaces were investigated in relation to the applied laser parameters (such as pulse frequency, and laser power) by means of scanning electron microscopy (SEM) and light-microscopic techniques (confocal microscopy). The adsorption of the polyelectrolytes was studied by X-ray photoelectron spectroscopy (XPS). Inverse water contact angle measurements using captive air bubbles were carried out to study the wettability (hydrophilicity/hydrophobicity) of the modified aluminium surfaces. Single lap joint tests carried out on joined AlMg3 sheets showed that the shear strengths can be significantly increased by pre-treatment with laser and coating of the alloy surfaces with weak polyelectrolytes. Furthermore, the application of polyelectrolytes improved the stability against corrosion. The article shows the increase of tensile shear strengths at adhesively bonded single lap shear samples after laser pre-treatment and also an increase in long-term stability due to of the combination of laser pre-treatment and coating with polyelectrolytes. Adhesive bonds of laser treated samples with and without polyelectrolyte coating have a higher stability against corrosion compared to untreated samples.     

The mode of action of chromate inhibitor in epoxy primer on galvanized steel

Investigations of the mechanism of the protective action of strontium chromate pigment in an epoxy primer were carried out with electrochemical impedance spectroscopy (EIS), atomic absorption spectroscopy (AAS), scanning reference electrode techniques (SRET) and water uptake measurements. Epoxy primers applied to galvanized steel were studied in a corrosion environment which models the atmospheric precipitation of European countries. Corrosion and electrochemical properties of samples of bare galvanized steel and coated galvanized steel were investigated. It was established that the protective function of the chromate in the primer is primarily due to a cathodic/mixed inhibition of the surface of galvanized steel in defect areas of the polymer coating. It is suggested that the process of leaching of chromate ions from epoxy primer into the environment takes place because the decrease in pH at anodic defect sites causes the destruction of the primer film and accelerates the dissolution of the chromate pigment.     

Electrochemical study of modified bis-[triethoxysilylpropyl] tetrasulfide silane films applied on the AZ31 Mg alloy

This work investigates the protective behaviour of bis-[triethoxysilylpropyl] tetrasulfide silane pre-treatments on the AZ31 Mg alloy. The silane solution was modified by the addition of cerium nitrate or lanthanum nitrate in order to introduce corrosion inhibition properties in the silane film.The corrosion behaviour of the pre-treated AZ31 magnesium alloy was studied during immersion in 0.005 M NaCl solution, using electrochemical impedance spectroscopy and the scanning vibrating electrode technique (SVET). The electrochemical experiments showed that the presence of cerium ions or lanthanum ions improve the protective behaviour of the silane film. The SVET experiments evidenced that the presence cerium in the silane film led to an important reduction of the corrosion activity.The results demonstrate that either cerium ions or lanthanum ions can be used as additives to the silane solutions to improve the performance of the pre-treatments for the AZ31 magnesium alloy.     

Coil-coated steel: corrosion resistance and adhesion as a function of the composition of the intermediate galvanic layer

The corrosion behaviour of coil-coated steel (CCS) sheets with different intermediate zinc-based coatings was examined by electrochemical impedance spectroscopy (EIS). Corrosion tests were performed in a 0.5 M NaCl aqueous solution in contact with the air, at ambient temperature. The results of pull-off adhesion test are also presented. The impedance data show that the improvement in the protective behaviour of the composite system is connected with the higher resistance values for the samples in which aluminium was added to the galvanized layer, and the best results are shown by CCS with aluzinc (Zn-55Al). These effects have been related to differences in the adhesion of the organic coating to the underlying metal substrate, as determined from both non-exposed samples and samples exposed to the electrolyte solution.     

Electrochemical impedance spectroscopic investigation of the role of alkaline pre-treatment in corrosion resistance of a silane coating on magnesium alloy, ZE41

The protective performance of the coatings of bis-1,2-(triethoxysilyl) ethane (BTSE) on ZE41 magnesium alloy with different surface pre-treatments were evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 0.1 M sodium chloride solution. Electrical equivalent circuits were developed based upon hypothetical corrosion mechanisms and simulated to correspond to the experimental data. The morphology and cross section of the alloy subjected to different pre-treatments and coatings were characterized using scanning electron microscope. A specific alkaline pre-treatment of the substrate prior to the coating has been found to improve the corrosion resistance of the alloy.     

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.     

Surface treatment of carbon steel substrates to prevent cathodic delamination

Steel structures immersed in seawater are protected from corrosion using cathodic protection systems (sacrificial anodes or impressed current), by paint coatings or by duplex systems in which cathodic protection hinders corrosion at coating's failure sites. The main problem related to those duplex systems is how to avoid cathodic delamination due to the alkalinity generated at cathodic sites.The present paper analyses the effectiveness of red mud suspensions as pre-treatment to avoid cathodic delamination. The study was conducted using mainly electrochemical impedance spectroscopy (EIS). The evolution of the electrode potential with immersion time was also recorded because it concerns the practical aspect of current density necessary to reach the protection potential.The study shows that red mud pre-treatments are effective in avoiding coating's delamination.     

Methacryloxypropyltrimethoxysilane films on aluminium: Electrochemical characteristics, adhesion and morphology

The electrochemical characteristics, adhesion and morphology of methacryloxypropyltrimethoxysilane (MAPT) films on aluminium were investigated during exposure to 3 wt.% NaCl. The MAPT films were deposited on aluminium surface from 2 to 5 vol.% methacryloxypropyltrimethoxysilane solutions, with the aim to investigate the influence of deposition parameters (silane solution concentration and curing time) on electrochemical characteristics, adhesion and morphology of MAPT films on aluminium.Using electrochemical impedance spectroscopy (EIS), potential–time measurements, adhesion measurements and optical microscopy coupled with image analysis, it was shown that films deposited from 5 vol.% solution exhibited better corrosion stability and adhesion, as well as lower porosity comparing to 2 vol.% solution and improved the corrosion protection of aluminium substrate, while the curing time had no influence on these characteristics.     

Electrochemical assessment of the self-healing properties of Ce-doped silane solutions for the pre-treatment of galvanised steel substrates

The present work aims at assessing the electrochemical behaviour of galvanised steel (GS) substrates pre-treated with bis-[triethoxysilylpropyl] tetrasulfide silane (BTESPT) doped with cerium nitrate. Furthermore, the work aims at evaluating the self-healing properties of the dopant and discussing the possible mechanisms involved in this process. The study was performed by electrochemical impedance spectroscopy (EIS) and by the scanning vibrating electrode technique (SVET), during immersion in NaCl solutions. X-ray photoelectron spectroscopy (XPS) was also used to complement the electrochemical results. The results show that the protective behaviour of the pre-treatments based on Ce-doped silane solutions is dependent on the concentration of the dopant. The results also show that the dopant improves the anti-corrosion performance of the silane coatings formed on galvanised steel substrates.     

Application of EIS and salt spray tests for investigation of the anticorrosion properties of polyurethane-based nanocomposites containing Cr2O3 nanoparticles modified with 3-amino propyl trimethoxy silane

The Cr₂O₃ nanoparticles were modified with 3-amino propyl trimethoxy silane in order to obtain proper dispersion and increment compatibility with the polyurethane coating matrix. The nanocomposites prepared were applied on the St-37 steel substrates. The existence of 3-amino propyl trimethoxy silane on the surface of the nanoparticles was investigated by Fourier transform infrared (FTIR) spectroscopy and thermal gravimetric analysis (TGA). Dispersion of the surface modified particles in the polyurethane coating matrix was studied by a field emission-scanning electron microscope (FE-SEM). The electrochemical impedance spectroscopy (EIS) and salt spray tests were employed in order to evaluate the corrosion resistance of the polyurethane coatings. Polarization test was done in order to investigate the corrosion inhibition properties of the Cr₂O₃ nanoparticle on the steel surface in 3.5 wt.% NaCl solution. The adhesion strengths of the coatings were evaluated by pull-off adhesion tester before and after 120 days immersion in 3.5 wt.% NaCl solution. FT-IR and TGA analyses revealed that surface modification of the nanoparticles with 0.43 silane/5 g pigment resulted in the greatest amount of silane grafting on the surface of particles. Results obtained from FE-SEM analysis showed that the surface modified nanoparticles dispersed in the coating matrix properly. Results obtained from EIS and salt spray analyses revealed that the surface modified particles enhanced the corrosion protection performance of the polyurethane coating considerably. The improvement was more pronounced for the coating reinforced with 0.43 g silane/5 g pigment. Moreover, the adhesion loss decreased in the presence of surface modified nanoparticles with 0.43 silane/5 g pigment.     

Evaluation of the corrosion behavior of galvannealed steel in chloride aqueous solution and in tropical marine environment

An investigation was made into the corrosion behavior of commercial galvannealed steel in 10⁻² mol dm⁻³ NaCl aqueous solutions and in a tropical marine environment, using scanning electron microscopy (SEM), galvanostatic electrochemical stripping (GES), potentiodynamic linear polarization (PLP), and electrochemical impedance spectroscopy (EIS) techniques and open circuit measurements (E _oc). For purposes of comparison, a commercial galvanized steel was also subjected to similar corrosion tests. GES and SEM techniques allowed for the identification of ζ, δ and Γ intermetallic phases and revealed cracks in the galvannealed steel. The PLP, EIS and E _oc results indicated that the galvannealed coating was more corrosion resistant than galvanized coating in an aqueous medium, but that their corrosion behaviors were similar in the marine environment. The corrosion behavior of the galvannealed steel was affected by the evolution of the cracking process in the Zn–Fe layer due to the dissolution of zinc-rich phases, while the galvanized steel displayed generalized corrosion in the aqueous medium and localized corrosion in the marine environment.