Improving the adhesion between epoxy coatings and aluminium substrates

Two different methods were followed to improve the adhesion and durability of the adhesion of a commonly used epoxy coating on an aluminium substrate. The first method was by application of a thin polymeric layer, having a thickness of around 10 nm, on the aluminium substrate prior to application of the epoxy coating. The functional groups in the polymers were chosen so as to be capable of chemisorption to the oxide surface and should also to be capable of being involved in the curing reaction of the epoxy resin. These polymers were poly(acrylic acid) (PAA), poly(ethylene-alt-maleic anhydride) (PEMah) and poly(vinyl phosphonic acid) (PvPA). An investigation of the interphasial region between the epoxy coating and the aluminium substrate in the final cured system showed that the polymeric layers were indeed involved in the curing reaction with the epoxy.

For the poly(ethylene-alt-maleic anhydride)-based system, this resulted in the formation of a cured, mixed poly(ethylene-alt-maleic anhydride)/epoxy interphasial region between coating and substrate while for the two other polymers, a weakly cured interphasial region was formed. The second method of adhesion and durability improvement was by hydration of the aluminium substrates, performed by immersion in boiling water. This procedure results in the formation of a porous pseudoboehmite oxyhydroxide layer. The epoxy coating was found to be capable of fully penetrating into the layer. The adhesion of the epoxy coatings was tested initially and after exposure to 40 °C water and 40 °C 5% acetic acid. The poly(ethylene-alt-maleic anhydride)-based system resulted in a very good initial adhesion and durability in presence of water for the epoxy coating, while the systems based on the other two polymers did not. The pseudoboehmite-based system also resulted in very good initial adhesion and durability in the presence of water. None of the improved systems were, however, found to be able to withstand 40 °C 5% acetic acid and showed severe corrosion underneath the epoxy coating.     

Modes and mechanisms for the degradation of fusion-bonded epoxy-coated steel in a marine concrete environment

Fusion-bonded epoxy is used extensively to protect steel reinforcing bars from corrosion. This coating has proven to be a cost-effective material for increasing the service life of reinforced concrete structures. However, field observations have reported premature corrosion of fusion-bonded epoxy-coated reinforcing bars used in marine concrete environments, leading to severe cracking and spalling of the reinforced concretes. This study was initiated to provide a better understanding of the modes and mechanisms of the corrosion of fusion-bonded epoxy-coated steel exposed in a marine concrete environment. Grit-blasted steel panels, were coated with two commercial fusion-bonded powder epoxy coatings at two thicknesses. Half of the coated panels were scribed, the other half were non-scribed. The panels were immersed in a saturated calcium hydroxide aqueous solution containing 0.6 mole/liter sodium chloride maintained at either 35°C or 50°C. Degradation was characterized and followed by infrared thermography, wet adhesion, and microscopic and analytical chemical techniques. Unscribed panels exhibited only water-induced adhesion loss, most of which was recovered after drying. However, in addition to the water-induced adhesion loss, scribed panels exhibited two modes of failure: anodic blistering near the scribe mark and cathodic delamination around the anodic blisters. Anodic blistering was attributed to localized crevice corrosion under the coating followed by blistering via an osmotic pressure mechanism. Cathodic delamination was probably induced by the alkaline cathodic reaction products, and water-induced adhesion loss was due to the presence of a water layer at the coating/steel interface. Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology, November 12–14, in Philadelphia, PA.     

Improvement of the resistance to water of an adhesive joint between polymers and aluminium by using thin adhesion layers

The influence of a thin adhesive layer (AL) of a polymer on the wet adhesion of an epoxy coating on an aluminium substrate has been studied by the peel and tape test method. It is shown that thin layers considerably improve the stability of adhesive joints in the presence of water. The mechanism of this improvement and the mechanical properties required for such thin layers are discussed. Some experimental evidence for improved wet adhesion of polymer binders to an aluminium support is presented and the mechanism explained. These results on interfacial processes contribute to a better understanding of the mechanism of adhesion.     

Surface chemistry of galvanized steel sheets relevant to adhesion performance

A review is presented on the recent development of surface treatment technologies for hot-dip galvanized steels relevant to adhesion of organic coatings. Applications of surface analytical techniques have elucidated that the surface layers of the nanometer scale dramatically govern the adhesion performance of painting or adhesive bonding. Surface enrichment of aluminium in the zinc layer deteriorates paint adhesion due to the reduction in phosphatability on the galvanized steel sheets and decreases the adhesive strength of the epoxy/dicyandiamide-bonded sheets due to the loss of acid-base interaction at the adhesive-substrate interface. In addition, the co-segregation of Al and Pb into the surface layer is responsible for the intergranular corrosion of zinc and facilitates the formation of a weak boundary layer, resulting in poor bond durability in a wet atmosphere. Improved adhesion performance has been established by developing new technologies that reduce the surface enrichment of minor elements or impurities in the zinc layer on the galvanizing line or that adopt a surface conditioning process prior to pretreatment in subsequent coil coating lines.     

A comparative study of three adhesion tests (EN 582, similar to ASTM C633, LASAT (LASer Adhesion Test), and bulge and blister test) performed on plasma sprayed copper deposited on aluminium 2017 substrates

The aim of this study was to compare three adhesion tests carried out on plasma-sprayed copper coatings on aluminium substrates. The first test, the bond pull test, designated EN 582 or ASTM C633, involves a uniaxial static stress and is commonly used in the coating industry. The second test, the LASAT (LASer Adhesion Test), is a recently developed technique based on spallation phenomenon due to laser induced shock waves. In this test, the coating delamination results from spallation at the coating/substrate interface due to uniaxial tensile stress. The last test, the bulge and blister test, involves a quasi-static measurement of the crack propagation energy at the coating/substrate interface. These three techniques have been used to evaluate the influences of different process parameters involved in the coating adhesion such as aluminium surface roughness, substrate pre-heating and plasma spray conditions.     

Relationship Between Interfacial Water Layer Adhesion Loss of Silicon/Glass Fiber-Epoxy Systems: A Quantitative Study

Water at the polymer/substrate interface is often the major cause of adhesion loss in coatings, adhesives, and fiber-reinforced polymer composites. This study critically assesses the relationship between the interfacial water layer and the adhesion loss in epoxy/siliceous substrate systems. Both untreated and silane-treated Si substrates and untreated and silane-treated E-glass fibers were used. Thickness of the interfacial water layer was measured on epoxy/Si systems by Fourier transform infrared-multiple total internal reflection (FTIR-MTIR) spectroscopy. Adhesion loss of epoxy/Si systems and epoxy/E-glass fiber composites was measured by peel adhesion and short-beam shear tests, respectively. Little water accumulation at the epoxy/Si substrate interface was observed for silane-treated Si substrates, but about 10 monolayers of water accumulated at the interface between the epoxy and the untreated Si substrate following 100 h of exposure at 24 °C. More than 70% of the initial epoxy/untreated Si system peel strength was lost within 75 h of exposure, compared with 20% loss after 600 h for the silane-treated Si samples. Shear strength loss in composites made with untreated E-glass fiber was nearly twice that of composites fabricated with silane-treated fiber after 6 months of immersion in 60 °C water. Further, the silane-treated composites remained transparent, but the untreated fiber composites became opaque after water exposure. Evidence from FTIR-MTIR spectroscopy, adhesion loss, and visual observation strongly indicated that a water layer at the polymer/substrate interface is mostly responsible for the adhesion loss of epoxy/untreated siliceous substrate systems and epoxy/untreated glass fiber composites and that FTIR-MTIR is a viable technique to reliably and conveniently assess the adhesion loss attributable to water sorption at the interface.     

Electrochemical impedance spectroscopic investigation of adhesion formation mechanism at an epoxy/aluminum interface

A theoretical method for characterizing the structure of a coating/metal interface by electrochemical impedance spectroscopy using water molecules as the probe was established. The properties of coating/metal interfaces for a series of epoxy resins with different water affinities were studied using this method. It was found that as the water affinity of the coating decreased, it became much more difficult for water molecules to reach the coating/metal interface. This suggests that, during the adhesion formation, a more hydrophobic layer is formed along the epoxy/metal interface.     

Hydrophobic interfacing layers for improvement of corrosion protection by polymeric coatings

Water sorption of coating materials is the main cause of coating deterioration, adhesion loss and substrate corrosion. By introducing alkanethiol self-assembled monolayers (SAMs), a hydrophobic interfacing layer between coating and substrate metal can be constructed. The effect of the hydrophobic SAMs interfacing layer on the corrosion protection of epoxy coatings was evaluated using electrochemical techniques including Tafel polarization, electrochemical impedance spectroscopy and impedance–time transition measurement. It was found that the SAMs interfacing layer improved the corrosion protection of the coating significantly. The improvement was attributed to the strong interaction between SAMs and the metal substrate, the compact structure and low water affinity of the SAMs interfacing layer, which prevent water absorbed by the coating from reaching the coating–metal interface and spreading along the interface.     

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.     

The effect of water on the adhesion of organic coatings on aluminium

Water normally decreases the strength of adhesive joints. In the case of epoxy coatings on aluminium, however, after an initial decrease the adhesive strength increased with the time of exposure to water. It is suggested that this increase is caused by the hydration of aluminium oxide adjacent to the adhesive joint, thus enabling additional hydrogen bonding between the organic coating and its support.

Results obtained by measuring adhesion with peel and tape tests on aluminium foil and an alloy with different surface pretreatments and different curing conditions have been compared. It is shown that the tape test is useful for the semi-quantitative determination of the stability towards water of an adhesive joint.     

Effects of different surface cleaning procedures on the superficial morphology and the adhesive strength of epoxy coating on aluminium alloy 1050

This study reports the effects of different cleaning procedures on the surface characteristics of the aluminium alloy 1050 substrates and on the adhesive strength of the epoxy coating to this alloy's surface. The cleaning procedures used in this study were (1) degreasing by acetone, (2) alkaline etching by 5 w/w% NaOH solution and (3) alkaline etching by 5 w/w% NaOH solution followed by acid cleaning by 50 v/v% HNO₃ solution. The surface morphology, chemical composition and topography of the cleaned substrates were investigated by field-emission scanning electron microscope (FE-SEM), energy dispersive spectroscopy (EDS) and atomic force microscope (AFM), respectively. The effectiveness of the cleaning procedures was also studied by polarization test and open circuit potential (OCP) measurements. The surface free energy and work of adhesion were obtained on the cleaned samples using contact angle measuring device. Pull-off test was conducted to evaluate the adhesion strength of the epoxy coating on the aluminium substrates. Results revealed that the surface cleaning of aluminium alloy by alkaline etching followed by acid cleaning method was the most efficient procedure for removing the oxide layer from the surface of aluminium compared to other cleaning procedures. The surface roughness, surface free energy, electrochemical activity and adhesion strength of the epoxy coating to the aluminium surface were significantly increased using this surface cleaning procedure.     

Effect of Cu strike coating on adhesion between Cu–Sn coated steel and rubber

In order to improve the adhesion between steel and rubber, a novel coating deposition technique has been developed, where steel substrate with orchestrated surface roughness was coated with double-layer coatings consisting of a thin Cu strike layer followed by a Cu–Sn layer with varying Sn compositions by immersion route. Coating surface characteristics studied using scanning electron microscope coupled with energy dispersion spectroscopy analyzer, electron probe micro analyzer, and inductively coupled plasma optical emission spectroscopy showed improvement in surface coverage with coating after employing the strike layer coating attributed to the better coating penetration in the deep roughness troughs. Peel test of the coated samples vulcanized with styrene butadiene rubber (SBR) was carried out which showed improvement in adhesion strength of the double-layer-coated samples inferring more uniform Cu-sulfide layer formation at interface due to more uniform coating coverage in these samples. Highest peel strength with uniform cohesive fracture within rubber was observed for optimum 2–3?wt% Sn content in the coatings. This result was further supported by pull-out test conducted on coated wire samples vulcanized with SBR.     

Adhesion and adhesion degradation of a pressure sensitive tape on carbon steel

Corrosion protection of carbon steels by organic coatings is dependent on a good adhesion between coating and substrate. In this work the blister test method was used to study the adhesion of a pressure sensitive tape applied on carbon steel. Deionized water was used to pressurize a blister formed by the tape covering a through-hole in the steel substrate. Values of adhesion strength as measured by the strain energy release rate were determined for two different blister pressurization rates or water injection rates. The adhesion strength was higher for the larger injection rate, which is expected for pressure sensitive tape. These values were probably overestimates of the true adhesion strength owing to plastic yielding of the membrane. Resistance to adhesion degradation near coating defects or macroscopic scribes was characterized using a height regulating scanning Kelvin probe on samples exposed to sodium chloride solution or a salt spray environment. Although the tape exhibited good barrier properties to water and ions, cathodic delamination was observed near the scribe after salt spray exposure. Wet/dry cycles were more aggressive than continuous salt spray exposure, since during continuous exposure corrosion products are washed away. It was observed that the oxide layer grows to a larger extent during wet/dry cycles; therefore, acceleration of the delamination process is probably associated with a “wedge effect” of the oxide on the interface.     

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.     

Evaluation of thin defect-free epoxy coatings using electrochemical impedance spectroscopy

The water sorption of thin defect free epoxy films and coatings on aluminium electrodes was studied using the gravimetric method and electrochemical impedance spectroscopy. The results show that the double layer capacitance of the wetted surface of the aluminium electrode under the epoxy film is considerably smaller than that of a bare aluminium electrode, except in the early period of immersion. The resistivity of the solution absorbed in the epoxy coating is much higher than that of the bulk solution. A method for approximately determining the equilibrium water sorption of the epoxy coatings on the metal surface from the double layer capacitance is reported.     

Hydrothermal ageing of X65 steel specimens coated with 100% solids epoxy

In some regions of Australia, epoxy-based coatings are commonly used to isolate the outer steel surface of a buried gas pipeline from the aggressive soil environment, but the rate at which the ability of the coating to exclude the environment deteriorates is unknown. The aim of this project was to investigate the accelerated ageing of a pipeline organic coating using a hydrothermal test as a proxy of pipeline's long-term exposure in the field. API-5L-X65 pipeline steel specimens were coated with a commercial 100% solids epoxy coating and immersed into an 80 °C distilled water bath with traces of carbonate–bicarbonate salts whilst electrically connected to an aluminium sacrificial anode. A limited number of specimens were taken out at different predefined ageing times, providing data to evaluate the degradation rate of the coating. Samples aged for 28 weeks were found to have a lower Barcol hardness, greater electrical permeability, and lower dry adhesion strength than non-aged samples.     

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.     

Hygrothermal Aging of Silane-Laced Epoxy Coatings

The impact of silane on the hygrothermal stability of epoxy coatings was investigated by specular neutron reflectivity (NR). By comparing the hygrothermal degradation behavior of neat novolac epoxy coating and corresponding bis[3-(triethoxysilyl)propyl]tetrasulfide-laced epoxy coating, the role of silane was elucidated. Accelerated aging was achieved by exposing the samples to 80°C liquid water. For the pure epoxy coating, degradation occurs at the coating–substrate interface, which makes the coating vulnerable to adhesion failure. For epoxy–silane coating the addition of silane imparts resistance to the interfacial degradation observed in the neat epoxy coating.     

Improved hydrothermal aging performance of glass fiber-reinforced polymer composites via silica nanoparticle coating

The long-term exposure to a hot and humid environment severely damages the bonding integrity of fiber-reinforced polymer composites and thus significantly degrades their mechanical performances. In this work, we aim to develop an improvement procedure for effectively enhancing the bonding strength in glass fiber-reinforced polymers (GFRPs). Glass fibers were coated with a thin layer of silica nanoparticles of different concentrations by the use of the evaporative deposition method. Micromorphological comparisons in terms of scanning electron microscope imaging demonstrate significant improvements on the surface roughness of glass fibers. With the coated glass fibers, GFRP composite laminates were designed, molded through the vacuum-assisted resin infusion technique, and experimentally tested for quantitatively studying their hydrothermal aging performance. The water absorption tests conducted for three exposure temperatures suggest that both the water diffusion rate and the equilibrium water content can be effectively reduced due to the introduction of the silica coating. With increased exposure temperatures, however, the desired reductions become much less significant. A so-called water-channel diffusion mechanism along fiber/resin interfaces was proposed to explain the coupling effects of silica coating and exposure temperature. Reductions of water diffusion rate and equilibrium water content were expected to slow down the hydrothermal aging performance of GFRPs. For this purpose, both uniaxial tensile test and three-point bending test were subsequently performed on GFRP specimens that have been subjected to different coating concentrations, exposure temperatures, and exposure durations. When compared with untreated GFRP specimens, both experiments demonstrate that the residual strength and stiffness can be effectively promoted through coating a thin layer of silica nanoparticles on glass fiber surfaces. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48652.     

Preparation of electrolytic ceramic films on stainless steel conversion coatings

An electrolytic method for the synthesis of an alumina barrier on stainless steel with strong interfacial bonding is described for a Fe-17%Cr alloy. The deposit was laid down electrochemically after a specific conversion treatment by chemical oxidation of the substrate in acid solution. The conversion coating was very porous and had excellent adhesion at the substrate interface. Alumina was obtained by thermal dehydration of aluminium hydroxide deposited from an aqueous solution of an aluminium salt, according to a two-step mechanism: generation of hydroxyl ions at the cathodic substrate by reduction of H₂O or dissolved oxygen and a precipitation reaction forming aluminium hydroxide. Thermal treatment induced interfacial reactions between aluminium oxide and conversion coating compounds which led to spinel formation beneath the superficial alumina layer. The coating presented chemical composition gradients suitable for strong adhesion. Thermal oxidation resistance was studied in air at 1000° C.