The effect of surface modification on the cathodic disbondment rate of epoxy and alkyd coatings

Surface modification of carbon steel substrates using appropriate functionalised silanes was carried out to investigate their effect on the dry and wet adhesion strength, and the cathodic disbondment rate, of coating binders based on epoxy and alkyd chemistries. Results show that pre-treatment of the steel substrate with 3-glycidoxypropyltrimethoxy silane (3-GPS) enhanced the dry and wet adhesion of an epoxy-based coating. Similarly, pre-treatment with 3-aminopropyltriethoxy silane (3-APS) enhanced the dry and wet adhesion of alkyd-based systems. However, although pre-treatment with 3-GPS reduced the cathodic disbondment rate for epoxy by a factor of 3, no effect on the disbondment rate for alkyd-based binders on substrates pre-treated with 3-APS was found. This strongly suggests that cathodic disbondment of epoxy proceeds by disruption of interfacial bonds (i.e. at the binder/substrate interface) but that disbondment of alkyds proceeds by direct degradation of the binder and that the interface plays little part in the process.     

The effect of organosilane on corrosion resistance of epoxy coating containing cerium nitrate

Structure and corrosion resistance behavior of epoxy coating containing cerium nitrate in different amounts of γ-glycidoxypropyltrimethoxysilane (γ-GPS) were investigated. Corrosion resistance ability of coating systems was characterized by using electrochemical impedance spectroscopy and polarization techniques. Also atomic force microscopy was used to evaluate dispersion of cerium nitrate in the presence of γ-GPS. Furthermore, the effect of γ-GPS on flexibility of coatings was studied by using the cupping test. Increment of 10?wt.% of γ-GPS into the coating formulation led to the superior dispersion of cerium nitrate and also highest corrosion protection performance.     

Cathodic disbondment resistance with reactive ethylene terpolymer blends

Adhesion to metallic substrates can be improved through the addition of polar functional groups, which bond with surface groups on the metal substrate. Additionally, polar interactions have been shown to increase adhesive strength even in wet environments (such as in the case for cathodic protection). A polymer blend is proposed as a coating material to provide adequate protection against the diffusion of moisture and air to the metallic surface along with superior adhesion even in the presence of wet and corrosive environments to resist cathodic disbondment. A reactive ethylene terpolymer (RET) of ethylene/n-butyl acrylate/glycidyl methacrylate (E/nBA/GMA) was compounded with HDPE to develop a potential coating material. The HDPE component offers high chemical and moisture resistance to permeation, while the RET component provides the material with high polarity and reactivity, which enhances adhesion to the substrates to be coated. The introduction of the reactive ethylene terpolymer decreases the magnitude of cathodic disbondment area of polyethylene coatings. After applying a cathodic potential to the coating substrate, the adhesive strength was observed to remain the same for silane-pretreated steel dollies. Without silane pretreatment, post-CD adhesive loss resembles that of the open circuit “wet” condition. EDAX data in conjunction with oxygen and water vapor transmission rates suggest an initial stage of disbondment where interfacial oxide is dissolved resulting in the delamination of coating around the initial defect. This initial disbondment zone acts like a moving crack tip creating larger areas of disbondment where interfacial bonds are degraded by the ingress of moisture and ions along the interface.     

Cathodic disbondment resistance with reactive ethylene terpolymer blends

Adhesion to metallic substrates can be improved through the addition of polar functional groups, which bond with surface groups on the metal substrate. Additionally, polar interactions have been shown to increase adhesive strength even in wet environments (such as in the case for cathodic protection). A polymer blend is proposed as a coating material to provide adequate protection against the diffusion of moisture and air to the metallic surface along with superior adhesion even in the presence of wet and corrosive environments to resist cathodic disbondment. A reactive ethylene terpolymer (RET) of ethylene/n-butyl acrylate/glycidyl methacrylate (E/nBA/GMA) was compounded with HDPE to develop a potential coating material. The HDPE component offers high chemical and moisture resistance to permeation, while the RET component provides the material with high polarity and reactivity, which enhances adhesion to the substrates to be coated. The introduction of the reactive ethylene terpolymer decreases the magnitude of cathodic disbondment area of polyethylene coatings. After applying a cathodic potential to the coating substrate, the adhesive strength was observed to remain the same for silane-pretreated steel dollies. Without silane pretreatment, post-CD adhesive loss resembles that of the open circuit “wet” condition. EDAX data in conjunction with oxygen and water vapor transmission rates suggest an initial stage of disbondment where interfacial oxide is dissolved resulting in the delamination of coating around the initial defect. This initial disbondment zone acts like a moving crack tip creating larger areas of disbondment where interfacial bonds are degraded by the ingress of moisture and ions along the interface.     

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.     

The effect of interfacial chemistry on coating adhesion and performance: A mechanistic study using aminobutylphosphonic acid

Failure of adhesion of organic coating (paints) to steel and other metallic substrates is one of the key mechanisms for local coating failure and for consequent restriction of coating lifetime. This study thus focuses on modification to the chemistry of the metal interface in order to promote enhanced (dry and wet) coating adhesion. The work uses an appropriate bi-functional amino alkyl phosphonate to provide the desired interfacial properties. The study uses X-ray photoelectron spectroscopy and water contact angle measurement to examine changes in the interfacial surface chemistry and electrochemical impedance spectroscopy and cathodic disbonding to determine improvements to the anti-corrosion performance. Choosing 4-amino-butyl-phosphonic acid as the surface modifier a significant influence on its efficiency as an adhesion promoter was observed as a function of pH. Thus, when the amino group was protonated at a pH of 5.3, the molecule demonstrated attachment to carbon steel at both ends with no significant improvement in performance. However at pH 8 the molecule demonstrated greatly improved surface packing density with the amino group outwards from the surface in the preferred orientation. In this condition, an epoxy coating demonstrated substantial resistance to interfacial hydrolysis with overall improved adhesion and reduced cathodic disbondment rate.     

Tailoring of the practical adhesion between polyethylene and galvanised steel

The practical adhesion of maleic anhydride grafted polyethylene (MAH-PE) to galvanised steel was studied using 3-point flexure tests, before and after hydrothermal ageing. Before bonding, the electro-galvanised steel was treated with γ-aminopropyltriethoxysilane (γ-APS). The influence of the silane coating thickness and deposition pH on the practical adhesion of MAH-PE to steel was investigated. FT-IR spectroscopy and microscopy enabled to gain understanding of the interphase formation between the silane and the metal substrate. It was found that, at the natural pH of the γ-APS, Zn ions dissolved in the silane coating with subsequent formation of crystals. This interphase could be held responsible for the better durability of the bonds than for silane coatings applied at quasi-neutral pH, for which dissolution of Zn ions was not observed.     

Investigation of Processes Occurring at the Metal/Polymer Coating/Electrolyte Interface

The methodical approach and the cell to study electrochemical processes occurring during cathodic disbondment of a polymer coating are worked out. They permit one to investigate the role of each process separately when supervising the metal substrate potential, electrolyte and polymer coating composition at a metal/polymer/electrolyte interface. The cathodic disbondment of ethylene-vinyl acetate copolymer, polyisoprene and poly(vinyl chloride) coatings are studied. It is found that the cathodic disbondment rate for ethylene-vinyl acetate copolymer coatings depends on double layer parameters at the interface. These parameters are determined by specific volume charge of hydrated cations of the electrolyte, potential of the substrate, the presence of oxygen, surface active substances, etc. Based on the data of IR spectroscopy in internal reflection applied to disbonded films, it is established that during the cathodic disbondment an electron transfer to polymer functional groups, as well as an attacking of the adhesion bonds by active intermediates of oxygen reduction, occurs resulting in an electrochemical degradation of the polymer and an adhesion loss. It is shown that the electrochemical transformations at the steel/poly(vinyl chloride) interface can lead to the appearance of new adhesion bonds, increasing adhesion strength and decelerating the cathodic disbondment.     

Surface evaluation and electrochemical behaviour of doped silane pre-treatments on galvanised steel substrates

The present work aims at evaluating the surface morphology and the corrosion resistance of galvanised steel substrates pre-treated with bis-[triethoxysilylpropyl] tetrasulfide silane (BTESPT) solutions doped with cerium nitrate or zirconium nitrate. The silane pre-treatment leads to the formation of a silane coating in the metallic surface. The surface morphology of this coating was studied by atomic force microscopy (AFM) and scanning electron microscopy (SEM).     

Disbondment Mechanisms for a Heat Shrink Polyethylene Coating on a Cathodically Protected Hot Pipeline

Interfacial adhesive failure of a pipeline coating was found to be related to the operating temperature of the pipeline, the presence of moisture at the coating/steel interface and cathodic polarisation of the steel. The application of cathodic protection was found to be more detrimental to a pipeline coating than was the immersion of the coating specimens in alkaline environments without polarisation. It is suggested that in the system examined cathodic disbondment is initiated at a coating holiday by the electrochemical reduction of Fe₃O₄ in the interfacial oxide film and that propagation of the disbondment is associated with electrocapillary action which reduces the surface tension between the steel and the crevice solution. This process increases the thermodynamic disbonding force between the adhesive and the steel in the aqueous environment.     

Corrosion and mechanical behavior of fusion bonded epoxy (FBE) in aqueous media

In recent years there have been many reported cases of corrosion failure in cement concrete pipelines. In the majority of cases, the failures have been attributed to rebar corrosion which is caused by the permeability of chloride from low resistivity soil and subsequent attack on a passive layer on an iron bar in the structure. As a possible alternative to cementitious materials, some organic coatings based on olefin, vinyl or epoxy-based polymers have been considered. However, due to a paucity of data on the behavior of these coatings in aqueous media— particularly product water—the possibility of their application in water transmission systems in the Kingdom has not been fully exploited. This paper deals with the studies carried out on the corrosion and mechanical behavior of fusion bonded epoxy (FBE) coating on steel in aqueous media which include product water, distilled water and saline water. The mechanical testings on coating include adhesion, bending and cathodic disbondment testings. The corrosion studies include immersion testing under static and dynamic conditions, autoclave tests and accelerated (salt-fog) tests. The analysis of results indicates chemical inertness of FBE coating in either of the aforementioned water used during testing, good adhesion and no damage to the coating during bending. Cathodic disbondment tests indicate that FBE coating sustains under cathodic protection (CP) conditions. In general, the results of mechanical and corrosion tests indicate that FBE is a promising material for internal coating on steel in water transmission systems.     

Cathodic disbondment of epoxy coating with zinc aluminum polyphosphate as a modified zinc phosphate anticorrosion pigment

As an approach to improve the resistance of protective coatings to the disbondment, modification of the formulation through incorporation of zinc aluminum polyphosphate anticorrosion pigment representing third generation phosphates was examined in this paper. The data obtained from cathodic disbonding test, electrochemical impedance spectroscopy and pull-off indicated that introduction of zinc aluminum polyphosphate within epoxy coating could provide improved resistance to cathodic disbonding as well as superior adhesion strength. The superiority in the presence of the modified pigment was connected to deposition of a layer at the disbonding front and locally controlled pH as well. The precipitation restricting active zone available for electrochemical reaction was confirmed by SEM.     

Electrochemical impedance spectroscopy as a tool to measure cathodic disbondment on coated steel surfaces: Capabilities and limitations

The disbondment of protective organic coatings under excessive cathodic protection potentials is a widely reported coating failure mechanism. Traditional methods of evaluating cathodic disbondment are based on ex situ visual inspection of coated metal surfaces after being exposed to standard cathodic disbondment testing conditions for a long period of time. Although electrochemical impedance spectroscopy (EIS) has been employed as an effective means of evaluating various anti-corrosion properties of organic coatings; its application for assessing the cathodic disbondment resistance of coatings has not been sufficiently exploited. This paper reports an experimental study aimed at developing EIS into a tool for in situ measurement and monitoring of cathodic disbondment of coatings. A clear correlation between EIS parameters and the disbonded coating areas has been confirmed upon short term exposure of epoxy-coated steel electrodes to cathodic disbondment conditions; however the degree of this correlation was found to decrease with the extension of exposure duration. This observation suggests that EIS loses its sensitivity with the propagation of coating disbondment, and that in order to achieve quantitative determination of the coating cathodic disbondment localized EIS measurements are required to measure the parameters related to local disbonded areas.     

AFM Scratching for Adhesion Studies of Thin Polymer Coatings on Steel

Abstract

Atomic force miscroscopy (AFM) scratching at constant applied forces was used to quantify the adhesion of polymer coatings to cold rolled steel (CRS) and to study the effectiveness of a pretreatment for improving the adhesion. The pretreatment was a phosphate-free zirconia-based coating. Thin layers of commercially available epoxy, acrylic and polyester-based polymer coatings, were applied to polished or pretreated cold rolled steel substrates and the surface was scratched at the edge of the polymer coatings with the AFM tip at increasing values of normal loads until the coating was removed. Adhesion strengths were determined from the minimum tip-sample interaction force and number of cycles (scans) at a particular applied force. The pretreatment significantly improved adhesion of the epoxy and acrylic-based coatings on CRS. Adhesion of the acrylic-based coating was found to be better than the epoxy coatings on the bare as well as pretreated steel. Adhesive strength of the polyester-based coating was inconclusive because it was very easily removed on application of small forces using the AFM tip. The AFM scratching technique was found to provide a quick, easy and effective way to make quantifiable comparisons in relative adhesive strengths of polymer coatings and the effect of pretreatments.     

Corrosion resistance of steel treated with different silane/paint systems

This article reports on a comparative study on the corrosion resistance of low-carbon steel substrates pretreated with different silane solutions and painted. The pure silanes used to pretreat the steel panels were 3-aminopropyltriethoxysilane (γ-APS), 3-glycidoxypropyltrimethoxysilane (γ-GPS), and bis(3-triethoxysilylpropyl)amine. The study also considered other silane solutions with ureido, amino, and epoxy organofunctional groups, and two bis-functional silanes: bis(γ-trimethoxysilylpropyl)amine (BAS) and 1,2-bis(triethoxysilyl)ethane (BTSE). A conventional phosphate-type pretreatment was also applied for reference purposes. The pretreated panels were then finished with an alkyd/polyester aminoplast base paint. As a branch test, an acrylic/urethane paint was also applied. Different tests were conducted to evaluate the anticorrosive ability of the different silane/paint systems: outdoor exposure in an atmosphere of moderate aggressivity; accelerated corrosion test (salt fog test); and electrochemical impedance spectroscopy (EIS). The results show that the steel pretreated with certain silanes, especially γ-APS, yields similar results to steel subjected to conventional phosphate pretreatment.     

Characterization of high performance composite coating for the northern pipeline application

High performance composite coating (HPCC) provides a potential, excellent coating alternative for integrity maintenance of pipelines in the northern area. In this work, the physical, chemical and mechanical properties of HPCC were investigated to determine the microstructure, water permeability, cathodic disbondment resistance, electrochemical impedance, adhesion and impact resistance of the coating. It is shown that the addition of polyethylene layer significantly improves the compactness of the coating and enhances its resistance to water and chemical penetration, resulting in a small water vapor transmission rate and permeance. There is a quite small cathodic disbondment of HPCC under the standard test. The impedance characteristic measured on HPCC-coated steel shows a capillary behavior, indicating an effective protection over the underlying steel from corrosion. The adhesion of HPCC to the substrate ranks top one according to both ASTM and CSA standards. The impact energy of HPCC is 9.7 J at 22 °C, and about 10.2 J around 0 °C.     

The influence of organosilane condition on films formed on aluminium characterised using RAIR and XPS

Aqueous solutions prepared from γ-glycidoxypropyltrimethoxysilane (γ-GPS) and 1,2-bis(trimethoxysilyl)ethane (BTSE) silane were used to treat Al2024-T3-clad aluminium alloy. Both fresh and naturally-aged organosilane chemicals were used to prepare the aqueous solutions. X-ray photoelectron spectroscopy and reflection absorption infrared spectroscopy were used to characterise the films produced on the aluminium substrates. The organosilane films were also rinsed in water and chloroform and reanalysed to further assess their chemical composition. Aqueous solutions prepared using fresh silane produced films that had a higher concentration of silanol groups. Silanol groups could be removed from the films by rinsing with water; however, after heating in a vacuum oven the formation of siloxane groups produced films that were insoluble in both water and chloroform. Films prepared from solutions that used naturally aged organosilane, which had been stored in a laboratory environment for five years, were thicker and contained high concentrations of methoxy groups that produced a hydrophobic layer that was soluble in chloroform. After vacuum oven drying, the films prepared using naturally-aged γ-GPS appeared to cross-link, making the films insoluble in both water and chloroform. In contrast, the film prepared using naturally aged BTSE remained soluble in chloroform after oven drying.     

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.     

Corrosion resistance performance of cerium doped silica sol–gel coatings on 304L stainless steel

The aim of this work is the synthesis and investigation of silane based organic–inorganic hybrid coatings, which can be used to improve the corrosion performance of steel structures subjected to a marine environment. The silane based sol–gel coatings were prepared by dip coating 304L stainless steel in a solution of organically modified silica sol made through hydrolysis and condensation of 3-glycidoxypropyl-trimethoxysilane (GPTMS) as precursor and bisphenol A (BPA) as a cross-linking agent in an acid catalyzed condition. The influence of the addition of cerium and the use of bisphenol A as a cross-linking agent on the microscopic features and morphology as well as on the corrosion resistance of the coatings were examined using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), neutral salt spray tests, potentiodynamic polarization and electrochemical impedance techniques. Results show that cerium modified nano-hybrid coatings exhibit a superior corrosion inhibition performance to that displayed by silica hybrid coatings. Additionally, data showed that the bisphenol A as a cross-linking agent has a significant effect on the morphology and corrosion resistance of the cerium doped silica coating. Omitting the use of bisphenol A causes the creation of defects/cracks in the coating, thereby promoting diffusion of the aggressive electrolyte toward the substrate and decreasing the corrosion resistance of the coating.     

Novel Self-Assembling Silane for Abhesive and Adhesive Applications

The potential benefits from using a novel vinyl-terminated long alkyl-chain self-assembling silane (VTS) have been investigated. A relatively high water contact angle of 83° was found for water on the VTS-pretreated soda-lime glass surface. This revealed that the degree of hydrophobicity of this surface was far more pronounced than for the ultrasonically-cleaned glass surface or the γ-glycidoxypropyltrimethoxy silane (GPS)–pretreated glass surface, which gave contact angles of 14° and 42°, respectively. Further, good agreement between the measured and the theoretical contact angles for a fully adsorbed vinyl-terminated self-assembled monolayer surface implied that the VTS molecules were adsorbed with the vinyl-terminal groups orientated away from the glass surface. Indeed, X-ray photoelectron spectroscopy (XPS) analysis confirmed that the VTS molecules were adsorbed as a monolayer with the vinyl-terminal groups orientated away from the glass surface. Double-cantilever beam joints were prepared using these various pretreated-glass substrates, using an epoxy adhesive, and they were tested employing a fracture-mechanics approach. The adhesive fracture energy, G _C , of the VTS-pretreated glass/epoxy joints was found to be far lower in value than for the ultrasonically cleaned joints or the GPS-pretreated joints. The potential applications of this novel long alkyl-chain silane are twofold: (a) the relatively very poor adhesion of the VTS pretreatment enables this silane to be used as an excellent abhesive layer, i.e., as a release agent, and (b) the presence of reactive vinyl-terminal groups might allow this silane to be activated to form functional groups that may then react with an adhesive, such as an epoxy resin, and so act as an excellent adhesion promoter to increase the durability of the adhesive joint.