Adhesion and corrosion resistance of epoxy primers used in the automotive industry

One of the most important factors in corrosion prevention by protective coatings is the loss of adhesion of the coating under environmental influence. Thus, adhesion strength is often used when characterizing protective properties of organic coatings on a metal substrate. In this work, the adhesion of different epoxy primers (pigment-free, zinc-rich and chromate-based) was examined on steel. Both the dry and wet adhesion strengths of organic primers were measured directly by a pull-off standardized procedure, as well as indirectly by the NMP test. The corrosion stability of coated samples was investigated by electrochemical impedance spectroscopy. It was shown that under dry test conditions all the samples showed very good adhesion. However, different trends in adhesion for different primers during exposure to the corrosive agent (3% NaCl solution) were observed. The lowest adhesion values were obtained for chromate-based epoxy primer; however, the change in adhesion of this protective system during immersion in 3% NaCl solution for 25 days was the smallest of all investigated samples. Electrochemical impedance measurements in 3% NaCl solution confirmed good protective properties of pigmented epoxy primers on steel, i.e., greater values of pore resistance and charge-transfer resistance, and smaller values of coating capacitance and double-layer capacitance, were obtained for these protective systems.     

Corrosion stability of polyester coatings on steel pretreated with different iron–phosphate coatings

The influence of steel surface pretreatment with different types of iron–phosphate coatings on the corrosion stability and adhesion characteristics of polyester coatings on steel was investigated. The phosphate coating was chemically deposited either from the simple novel plating bath, or with the addition of NaNO₂, as an accelerator in the plating bath. The morphology of phosphate coatings was investigated using atomic force microscopy (AFM). The corrosion stability of polyester coatings on steel pretreated by iron–phosphate coatings was investigated by electrochemical impedance spectroscopy (EIS) in 3% NaCl solution, while “dry” and “wet” adhesion were measured by a direct pull-off standardized procedure. It was shown that greater values of pore resistance, R_p, and smaller values of coating capacitance of polyester coating, C_c, on steel pretreated with iron–phosphate coating were obtained, as compared to polyester coating on steel phosphated with accelerator, and on the bare steel. The surface roughness of phosphate coating deposited on steel from the bath without accelerator is favorable in forming stronger bonds with polyester coating. Namely, the dry and wet adhesion measurements are in accordance with EIS measurements in 3% NaCl solution, i.e. lower adhesion values were obtained for polyester coating on steel phosphated with accelerator and on the bare steel, while the iron–phosphate pretreatment from the novel bath enhanced the adhesion of polyester coating on steel.     

The effect of silane layer drying temperature on epoxy coating adhesion on silane-pretreated aluminum substrate

In this study, 3-glycidoxypropiltrimethoxysilane was used as an adhesion promoter to enhance the adhesion strength of epoxy coating on an aluminum (Al) substrate. Silane layer drying temperature was investigated as a factor that has an influence on the adhesion of polymeric coating on metal substrate and also on its performance in wet and corrosive environments. FTIR tests were carried out to study Al/silane interactions. Drying the silane layer at high temperatures formed a condensed siloxane layer that improved the bonding strength as well as the performance of the protective coating in corrosive environments. The highest dry and wet pull-off strengths were obtained at drying temperatures of 100 and 125°C, respectively.     

Barrier and adhesion properties of anti-corrosion coatings based on surfactant-free latexes from anhydride-containing polymers

We have successfully obtained surfactant-free latexes from anhydride-containing polymers, including poly(styrene-alt-maleic anhydride) (PSMA), maleinized polybutadiene (PBDMA), and poly(octadecene-alt-maleic anhydride) (POMA). Here we report barrier and adhesion properties of the coatings made from these surfactant-free latexes, which were crosslinked with adipic acid dihydrazide (ADH). The barrier property of these coatings was studied with electrochemical impedance spectroscopy in NaCl solutions on pretreated aluminum substrates. PSMA and PBDMA-based coatings demonstrated good barrier property, indicated by the absence of corrosion after 1-month immersion in NaCl solutions, which was comparable to that of a commercially available polyester coating. The polyester coating, however, was found to hydrolytically degrade when immersed in a NaCl solution, which was not observed for our latex-based coatings. The POMA-based coating showed inferior barrier property on un-pretreated aluminum, likely due to inhomogeneous film formation and poor adhesion to the substrate, but the adhesion could be improved by an alkali pretreatment of the substrate. In comparison, PBDMA and PSMA-based coatings showed much better adhesion toward the aluminum substrates. The anti-corrosion behavior appeared to be closely related to both the barrier and adhesion properties of the coatings on the metal substrate.     

The influence of zinc surface pretreatment on the adhesion of epoxy coating electrodeposited on hot-dip galvanized steel

The adhesion and electrochemical properties of epoxy coatings electrodeposited on hot-dip galvanized steel with and without passive films were investigated during exposure to 3% NaCl. The passive films were formed in hot air, in boiling water and by chromating. Adhesion was measured both by a standardized pull-off method and by swelling in N-methyl pyrrolidone. Pretreatment of hot-dip galvanized steel with passive film formed in hot air increases both dry and wet adhesion strength of the epoxy coating compared to pretreatment with passive film formed in boiling water and chromate coating. The overall increase of wet adhesion for this sample was maintained throughout the whole investigated time period. It was shown that the change in adhesion of epoxy coating on a chromate coating is smallest of all investigated samples, although the initial value of adhesion on this surface had the lowest value. The corrosion stability of coated Zn samples pretreated by different methods, was investigated by electrochemical impedance spectroscopy and in the initial time of exposure to NaCl the highest values of pore resistance were also obtained for the epoxy coating on Zn pretreated in hot air, whereas the epoxy coating on a HDG steel with a chromate coating showed the smallest change in electrochemical properties (pore resistance, coating capacitance, charge-transfer resistance) during prolonged exposure time.     

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.     

Electrochemical effects of silane pretreatments containing cerium nitrate on cathodic disbonding properties of epoxy coated steel

In the recent years, silane materials, because of their environmental friendly nature and ease of application have been attended as an alternative for chromate conversion coatings. Different materials were searched for improvement of the efficiency of silane formulation. In this research, pretreatment of carbon steel substrates was carried out using γ-glycidoxypropyl-trimethoxysilane (γ-GPS) as functionalized silane. Cerium nitrate as a corrosion inhibitor material was introduced into the silane material and epoxy resin was applied on the pretreated steel substrates. Effects of the pretreatment on electrochemical properties, cathodic disbondment, dry and wet adhesion strength, and surface morphology of resultant epoxy coating were investigated. Results showed that pretreatment of steel substrate with γ-glycidoxypropyl-trimethoxysilane (γ-GPS) doped with cerium nitrate leads to improvement of cathodic disbondment and also dry and wet adhesion of epoxy coating. Furthermore, this type of pretreatment reduced the disruption of interfacial bonds at the binder/substrate interface. Addition of 2?wt% cerium nitrate into the silane formulation led to the maximum efficiency of resultant coating.     

High performance corrosion protective DGEBA/polypyrrole composite coatings

Conducting polymers (CP) have been envisaged as nontoxic substitutes for chromates in corrosion protective coatings owing to their environmental stability, excellent corrosion resistant performances and good adhesion to metal substrate. Recent advancements in the field of corrosion protective coatings have therefore headed towards the utilization of conducting polymers as “smart corrosion inhibitors” in the formulation of eco-friendly anti-corrosive coatings. With the aim to develop a facile method for the synthesis of anti corrosive coating material having superior corrosion protective properties, the present work reports the corrosion studies of polypyrrole/DGEBA composite coatings on mild steel (MS). The coatings were characterized by physicochemical as well as physicomechanical studies while the corrosion protective performance was evaluated by chemical resistance in different corrosive media, corrosion rate and open circuit potential (OCP) measurements. The morphologies of corroded, coated, uncorroded and uncoated samples were investigated by SEM studies. The composite coatings were found to show far superior anti corrosive performance than PANI/epoxy coatings in acid, alkaline as well as saline environments.     

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.     

The influence of Ce-based coatings as pretreatments on corrosion stability of top powder polyester coating on AA6060

Cerium-based conversion coatings (CeCCs) are one of the most prospective alternatives to the widely used chromate conversion coatings (CCCs) due to their anticorrosion efficiency, environmentally friendly nature and low cost. In this work, the CeCCs on AA6060 were prepared by immersion into aqueous cerium salt solutions at room temperature, and subsequently post-treated in heated phosphate solution. The effect of counter ion (nitrate and chloride) on the coating properties was studied testing CeCCs as sole or conversion layers for the top polyester coating. Since the 60 μm thick polyester coating was applied, an artificial defect of 0.8 mm hole was introduced to faster assess the differences between pretreatments. The system with CCC pretreatment was used as reference. Corrosion properties were investigated in 0.5 M NaCl solution by electrochemical impedance spectroscopy while the adhesion strength was measured by NMPR (N-methyl-2-pyrrolidone) and pull-off tests. As shown, the post-treated chloride-based CeCC offered better protection than crack-free thin nitrate-based CeCC, when used as sole coatings. On the other hand, it was brought to evidence that in combination with top powder polyester coating, the CeCC deposited from nitrate solution exhibited better protection compared to protective system pretreated with chloride-based one. Excellent polyester coating adhesion was found independently on aluminium surface pretreatment.     

Enhancement of Corrosion Performance of Epoxy Coatings by Chemical Modification With GPTMS Silane Monomer

In this paper, commercial epoxy resin was chemically modified by different amounts of 3-glycidoxypropyltrimethoxysilane (GPTMS) monomer using an organotin compound as catalyst, aiming to improve the anti-corrosion performance of epoxy coatings on 2024-T3 aluminum alloy substrate. Electrochemical impedance spectroscopy (EIS) was used to evaluate the barrier properties against water permeation and protectiveness of silane-modified epoxy coatings. The results showed that all the modified coatings presented higher barrier performance and better corrosion performance than pure epoxy coating, which were characterized by higher charge transfer resistance (R _ct) and lower double-layer capacitance (C _dl) at the electrolyte/metal interface. The improvements in corrosion performance and wet adhesion of modified epoxy coatings were also observed by the Machu test and boiling water test, respectively. Interestingly, it was found that the glass-transition temperature (T _g) of silane-modified epoxy coatings decreased only slightly during immersion in 3.5 wt% NaCl solution, in contrast with pure epoxy coating, which was observed to decrease significantly after water permeation. The corrosion performance of epoxy coatings was, thus, improved when the amount of chemically grafted silane monomer increased in the content range investigated in the present work.     

Water uptake of epoxy coatings modified with γ-APS silane monomer

Epoxy resin was modified by a silane monomer, γ-aminopropyltrimethoxy silane (γ-APS), and was used as the protective coatings for LY12 aluminum alloys. The aim of the modification is to reduce the water uptake of polymeric coatings. The water absorption of coatings was measured by coating capacitance method in 3.5 wt.% NaCl aqueous solution. The result indicates that water uptake of epoxy coatings modified with 1.0 wt.% γ-APS decreases compared with pure epoxy coatings, whereas larger amounts of sliane result in the deterioration in performance against water permeation, due to the excessive consumption of epoxide group in epoxy resin by amino-group in silane agent, thus reduce the cross-linking of epxoy coating as a result of presence of excessive curing agent (polyamide). T_g of silane-modified coatings increases slightly after immersion, extremely contrasting with that of pure epoxy coating, which was observed to decrease significantly after water permeation. The formation of Si–O–Si structure resulting from the hydrolysis and condensation of silane components during the immersion in aqueous media may be a reasonable explanation for the abnormal change in T_g of silane-modified coatings. In addition, all silane-modified coatings display better protection performance, which is characterized by higher charge transfer resistances (R_ct) and lower double layer capacitance (C_dl) at substrate/electrolyte interface.     

Characterization and corrosion performance of poly(pyrrole-siloxane) films on commercial Al alloys

An alternative approach was developed for the surface treatment of as-received commercial aluminium alloys 6082-T6, 5083-H111 and 2024-T3 by using a pyrrole-based silane. One immersion step is sufficient for poly(pyrrole-siloxane) film deposition, with good adhesion and desirable morphological features. The combination in a single coating of polysiloxane linkages, acting as a barrier for aggressive species penetration towards the metal surface, and polypyrrole units manifesting active anodic protection, together with high degree of compactness, explains the significantly improved corrosion protection of the composite film in comparison to simple coatings of polymethylsiloxane and polypyrrole.     

Epoxysilane as adhesion promoter in duplex system

Coatings are one of the most used protection methods for metals. Metallic coatings, such as zinc and its alloys, are used to protect steel in mild corrosive environments. In aggressive environments, on the other hand, organic coatings must be employed in the so-called duplex systems. However, the galvanized steel/organic coating adhesion is a problem and many attempts had been done to solve it with the incorporation of a chromate-based or phosphate-based interlayer. Nowadays, the use of these compounds is questioned due to their environmental impact and new adhesion promoters, like silanes, are being investigated. The aim of this paper was to study the adhesion and the anticorrosive behavior of a duplex system with a layer of glycidoxypropyltrimethoxysilane (γ-GPS) between the zinc and the coating. Polarization tests and corrosion potential measurements were done on the γ-GPS/galvanized steel to select the better anticorrosive pretreatment conditions for the application of an organic traditional paint. Dried and wet adhesion of the coating to the pretreated substrate was studied by the standard tape test. Salt spray test and electrochemical noise technique were employed to study the corrosion behavior of the duplex systems. Results showed that the films of γ-GPS formed on galvanized steel diminished the corrosion current of the metal, but they do not protect the substrate by a barrier effect. The incorporation of the pretreatment in the duplex system increased the adhesion of the paint, especially when the pretreated substrate was cured 1?h at 200?°C.     

Protective properties of epoxy-based organic coatings on mild steel

The protective properties of epoxy-based organic coatings on mild steel substrates have been studied using the electrochemical impedance spectroscopy (EIS) technique. The effects of coating composition, film thickness and curing temperature on the lifetime of the epoxy coating were investigated. The results obtained indicate that the film composition dramatically affects the rate of corrosion of the metallic substrate. Coatings with low and high content of curing agent exhibit early failure when exposed to an air-saturated 0.5 M aqueous solution of NaCl. Extended lifetimes were observed for samples with a content of the curing agent equal to 8.5% by weight. An increase in lifetime was also observed on increasing the film thickness and the curing temperature. The failure mechanism seems to occur in two steps: the first step is related to water uptake in the film while the second step is related to Cl⁻ ion diffusion through the coating. The results have been interpreted in terms of a model in which the dielectric properties (capacitance and resistance) of the coating depend strongly on the coating composition and are affected by both the water and salt uptake into the film.     

Characterization of ceramic sol-gel coatings as an alternative chemical conversion treatment on commercial carbon steel

Sol-gel yttria-stabilized zirconia (YSZ) thin films were prepared on commercial carbon steel sheets by dip-coating technique followed by a low temperature heat treatment (473, 573, and 673 K for 1 h) in order to improve both corrosion properties and adhesion. For comparison, zirconia (ZrO₂) coatings have been also analyzed. Electrochemical techniques, Fourier Transform Infrared (FT-IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the anticorrosion behavior of the coatings in a 3.5 wt% NaCl solution. The adhesion with a polyester organic coating was evaluated by the pull-off technique. The typical thickness of the deposited layers ranged from 1 to 1.3 μm depending on process parameters. The obtained results indicated that sol-gel ZrO₂ and YSZ coatings without an organic coating can act as protective barriers against wet corrosion during the first hours, but they fail when the time exposure is longer than 1 day. However, when synthesized films were used as a pre-treatment and an organic coating was added (top-coated), the anticorrosive and adhesion properties were strongly affected by the temperature of the treatment, and an increase in both properties was observed at higher temperatures. The structural and morphological characteristics of the coating provide an explanation of the role of each film in the electrochemical behavior in this aggressive medium. Comparing both systems, YSZ displayed greater protective and adhesion values than exhibited for ZrO₂ which can be correlated with the stabilization of the cubic phase.     

Corrosion protection of Mg alloys by cathodic electrodeposition coating pretreated with silane

The corrosion resistance characteristics of three coatings on magnesium alloy AZ31—conventional paint with phosphate film, cathodic electrodeposition coating (E-coating), and E-coating pretreated with silane (Mg/silane/E-coating)—have been studied by means of electrochemical impedance spectroscopy (EIS) in a 3.5 wt% NaCl neutral aqueous solution and salt spray test using ASTM B117. Silane film was obtained by dipping AZ31 specimens in diluted hydroalcoholic silanic solutions and successively curing. It was found that the corrosion resistance of the Mg alloy with E-coating was superior to conventional paint and could be further enhanced with silane pretreatment as an interfacial film. The results of water volume fraction (Φ_saturation) and diffusion coefficient (D) also indicated that the Mg/silane/E-coating possessed excellent compactness and corrosion resistance. A model of the corrosion mechanism for Mg/silane/E-coating has been presented through EIS analysis.     

Evaluation of the effect of vinyltrimethoxysilane on corrosion resistance and adhesion strength of epoxy coated AA1050

The quality of surface treatment of aluminum alloy (AA1050) with vinyltrimethoxysilane (VTMS) was examined. Calculated surface energy with Van Oss, Chaudury and Good (VOCG) method showed higher surface energy of treated specimens. Chemical interaction between silane layer and aluminium surface was examined via FTIR-ATR. In addition, practical adhesion was measured in dry, wet and recovered states via pull-off method and compared to adhesion strength of desmutted and chromated specimens. Results showed good adhesion performance in dry state for silane treated specimens. Corrosion protective performance of epoxy coating in the presence of silane layer on aluminum was studied with Electrochemical Impedance Spectroscopy (EIS) and compared with coated desmutted and chromated specimens. Water sensitivity of silane layer resulted in poor protective performance of specimens treated at pH > isoelectric point (IEP).     

Service life prediction of organic coatings: electrochemical impedance spectroscopy vs actual service life

Electrochemical impedance spectroscopy (EIS) is used to derive an expression for predicting the service life of organic coating in a C4-type environment (industrial and costal areas with moderate salinity) as defined in ISO 12944 standard for paints and varnishes—corrosion protection of steel structures by protective paint systems. Three coating systems with a record of 2, 5, and 10 years of durability were selected for the study. The selection was also based on proven composition and dry film thickness (DFT) of the coatings as per ISO 12944. Electrochemical impedance measurements of the paint-coated panels were carried out by exposing the coated mild steel panels without scribe in different corrosive environments such as immersion in NaCl solution, neutral salt spray, etc. Neutral salt spray exposure was found to be the most severe corrosive environment among all the three coating systems. In most of the cases, EIS gave early indication of coating failure when compared to visual defects such as blistering and over-film corrosion.     

Evaluation of the effects of surface treatments on the cathodic delamination and anticorrosion performance of an epoxy-nanocomposite on steel substrate

The St-37 type steel substrates were pretreated with Cr(VI) and Cr(III) conversion coatings where the latter was then post-treated with Co(II) and Ni(II) chemical treatments. The epoxy coatings containing 3.5 wt% nano-sized ZnO particles were applied over the chemically treated steel samples. The corrosion resistance of the samples was studied by a DC polarization technique. A scanning electron microscope (SEM) was utilized to investigate the morphology of the pretreated and post-treated samples. Electrochemical impedance spectroscopy (EIS) was utilized to investigate the corrosion resistance of the epoxy nanocomposites for different immersion times in 3.5 wt% NaCl solution. The adhesion strengths of the coatings were measured before and after 120 days of immersion in the corrosive electrolyte using a pull-off test. The cathodic delamination (CD) of the painted samples was also investigated. Results showed that conversion coatings can significantly increase the corrosion resistance and adhesion strength of the epoxy coating on the steel, and also reduce the rate of CD in comparison with an untreated sample. The adhesion strength and corrosion resistance of the epoxy coating on the Cr(III) pretreated samples were significantly greater than on the Cr(VI) sample. The increase in adhesion strength and corrosion resistance was more pronounced on the samples that were post-treated with Co(II) and Ni(II) chemical treatments. The cathodic disbonded areas of the Cr(III)–Co(II) and Cr(III)–Ni(II) post-treated samples were significantly lower than the Cr(III) and Cr(VI) pretreated samples. Results showed that Cr(III)-based conversion coatings can improve the anticorrosion performance and reduce CD compared with those with Cr(VI).