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.     

The mechanism for the cathodic delamination of organic coatings from a metal surface

A detailed mechanism of cathodic delamination from a defect in an organic coating is presented, which takes into account the reactants and how they get to the delaminating front. Oxygen and water pass through the organic coating whereas cations, at least in part, reach the front by lateral diffusion. Five means for minimizing cathodic delamination are suggested: (1) No bare metal, or superficially oxidized metal, should be present at the coating/substrate interface. (2) There should exist at the coating/substrate interface a layer which is a very poor conductor of electrons. (3) The interfacial layer at the coating/substrate interface should be a poor catalyst for the cathodic reaction. (4) The boundary between the substrate and the coating should be rough in order to provide a tortuous path for lateral diffusion. (5) The interfacial region should be resistant to alkaline attack.     

Reduction of cathodic delamination rates of anticorrosive coatings using free radical scavengers

Cathodic delamination is one of the major modes of failure for anticorrosive coatings subjected to a physical damage and immersed in seawater. The cause of cathodic delamination has been reported to be the result of a chemical attack at the coating–steel interface by free radicals and peroxides formed as intermediates in the cathodic reaction during the corrosion process. In this study, antioxidants (i.e., free radical scavengers and peroxide decomposers) have been incorporated into various generic types of coatings to investigate the effect of antioxidants on the rate of cathodic delamination of epoxy coatings on cold rolled steel. The addition of <5 wt% free radical scavengers to epoxy coatings improved the resistance toward cathodic delamination by up to 50% during seawater immersion, while peroxide decomposers had a limited effect. Testing using substrates prepared from stainless steel, copper, aluminum, galvanized steel, and brass also showed a reduction in the rate of cathodic delamination when the coating was modified with a free radical scavenger. The protective mechanism of free radical scavengers investigated for the primers are similar to that of antioxidants used for protection against photochemical degradation by UV-radiation of top coatings. Both substrate corrosion and degradation of a coating exposed to UV-radiation lead to the formation of free radicals as reactive intermediates.     

Determination of the oxygen reduction products on ASTM A516 steel during cathodic protection

The electrochemistry of steel in aerobic and anaerobic aqueous alkaline solutions was studied with or without forced convection to investigate the cathodic processes occurring on steel exposed by defects in polymer coated steel pipe. The results are relevant to the mechanistic understanding of the effect of cathodic protection on the disbonding of fusion bonded epoxy (FBE) coatings on steel. Moderate (pH9.8) and strongly (pH14) alkaline aqueous solutions were used to simulate the water layers at the cathodically polarized steel surface on the soil-side of buried pipe. A rotating gold ring and steel disc electrode (RRDE) in alkaline aqueous electrolyte equilibrated with 1atm oxygen over solution was used to measure the rotation rate dependent current for the electroreduction of oxygen, O2, on an ASTM A516 steel disc and the resulting peroxide generation, which was determined by monitoring the oxidation current on the gold ring. An appreciable fraction of the oxygen reduction current on the steel disk gave rise to peroxide generation over a wide range of potentials, from –0.2 to –0.9V vs SCE in 1M KOH. The observation of peroxide generation is noteworthy, because oxidizing agents, such as peroxide and its decomposition products, superoxide and hydroxy radical, can degrade the polymers used for coating pipelines. As result, oxidative degradation of polymer or interfacial compounds may be a cause of the accelerated disbonding observed for protective coatings on steel pipelines under cathodic protection.     

A study of the bond degradation of rebar due to cathodic protection current

Bond degradation of rebar embedded in concrete due to impressed cathodic protection current was studied and is reported in this paper. Different mix designs of concrete are found to have influence on the degradation percentage of bond strength. More specifically, bond strength degradation percentage with higher water-to-cement (w/c) ratio is found to be larger than that with lower w/c ratio. The microstructure scanning electron microscope (SEM) photos of the concrete near the rebar-concrete interface showed that a loose structure with larger microvoids existed in the interface zone. Further, microhardness tests on the concrete near the interface and chemical titration to determine contents of potassium and sodium ions were performed to ensure that the main cause of bond degradation is the softening effect of concrete. A unified parameter, which combined the effects of cathodic current density and polarization time, is used to build up the relationship between experimental data. This concept allows engineers to quickly obtain design information from the experimental data of accelerated tests.     

Non-substrate EIS monitoring of organic coatings with embedded electrodes

Electrochemical impedance spectroscopy provides a quantitative evaluation of the protection afforded by coatings on metals. Two constraints are that the coating is under immersion and that the substrate acts as the working electrode with the counter and reference electrodes located in the electrolyte. The use of embedded electrodes placed between a topcoat and primer can relax these constraints and make EIS monitoring more applicable to coatings in the field. A two-electrode, non-substrate configuration involves two embedded electrodes on a coated panel acting as the working and counter/reference electrodes. This configuration has been used to characterize the interlayer between a topcoat and primer under the assumption that the current passed through the interlayer. Simulated results have been presented where current passage for a non-substrate configuration was through the metal substrate. The results associated with a urethane topcoat/epoxy primer system and an alkyd topcoat/alkyd primer system are presented to demonstrate the feasibility of monitoring the substrate where the substrate is not an electrode. The degradations of the coatings were induced using the ac–dc–ac accelerated test where the immersed coatings were subjected to cycles that involved a dc cathodic potential condition that promoted the cathodic reactions at the metal/coating interface.     

Degradation of adhesive composites

Composite materials consisting of natural leather and polyurethane elastomers, and bonded together by polyurethan adhesives, were submitted to accelerated ageing at various testing conditions. The extent of the degradation of these composites was evaluated through the determination of mechanical strength reduction and through the insight into the structural and chemical transformations of the adhesive polymeric materials. Founded data correlations indicate to the mechanism and to the relevant factors that influence of ageing processes.     

Probing photodegradation beneath the surface: a depth profiling study of UV-degraded polymeric coatings with microchemical imaging and nanoindentation

Photodegradation of polymer coatings generally involves photooxidation, resulting in the formation of oxidized products, chain scission, and crosslinking. On severe exposure to ultraviolet (UV) light in the presence of air, chemical degradation transforms into substantial changes in the physical and mechanical properties, leading to failures of the coatings. Systematic research by NIST on service life prediction of polymeric coatings indicates that the degradation of polymer coatings starts from the sub-micrometer degradation-susceptible regions at the surface and then grows in width and depth. Additionally, due to the oxygen diffusion effect and the attenuation of the UV light passing through the polymer, the degradation can be spatially heterogeneous. In this study, the changes with depth of the mechanical and chemical properties of a UV-exposed epoxy/polyurethane system were measured by nanoindentation and Fourier transform infrared spectroscopy (FTIR) microscopy using cross-sectioned specimens. Multilayers of epoxy/polyurethane samples were prepared by a draw-down technique. After curing, samples were exposed to the outdoors in Gaithersburg, MD, for four months. Cross-sectioned slices of the exposed and unexposed samples, approximately 500 nm thick as-prepared by microtoming, were used for micro-FTIR imaging. Samples for nanoindentation were prepared by embedding the epoxy/polyurethane multilayers (both exposed and unexposed) in a molding compound, followed by microtoming and polishing the embedded films in the thickness direction. Micro-FTIR images clearly show that, for the outdoor exposed samples, substantial amounts of oxidation products are distributed in the 60 μm deep region from the surface to the epoxy bulk, decreasing in the center of epoxy region and increasing again toward the epoxy/urethane interface. Nanoindentation results also show that the modulus significantly increases in the first 60 μm region after UV degradation, and then decreases gradually with depth until a value slightly higher than the modulus of the undegraded epoxy is reached. The modulus rises again in the region near the epoxy/urethane interface. These similarities in the depth profiles of the properties indicate the linkage between the chemical degradation and the mechanical degradation. The study clearly shows that the spatial distribution of chemical species and mechanical properties is heterogeneous in the thickness direction for polymer coatings after UV degradation. It also demonstrates that cross-sectional analysis using nanoindentation and micro-FTIR imaging techniques is a useful method to characterize the mechanical and chemical depth profiles of polymer coating degradation.

Improvement of the Durability of Zinc-Coated Steel/Epoxy Bonded Joints

The durability properties of bonded lap shear joints made from an epoxy/dicyandiamide adhesive and hot-dipped galvanized (G2F) or electroplated-phosphated (EZ2) steel have been investigated. The degradation mechanisms have been studied after three accelerated ageing tests: the “cataplasme humide” (“C.H.T.”), immersion (“I.T.”), and salt spray (“S.S.T.”) tests. X-ray photoelectron spectroscopy (XPS) analysis of fracture surfaces after ageing have shown that anodic dissolution of the zinc-coating is responsible for debonding in all cases and that intergranular corrosion phenomena account for poorer performances of the hot-dipped galvanized substrate during “C.H.T.” and “I.T.” Silane coupling agents were successfully used as primers on both substrates to increase the hydrolytic stability of the metal/adhesive interface. XPS results indicate that both the interfacial dissolution of the phosphate coating of EZ2 and intergranular corrosion of G2F are delayed for silane-primed specimens. The observed improvements do not appear to depend on the nature of the silane coupling agents. Alkylsilanes have been found to perform as well as silanes having a group capable of reacting with the epoxy/dicyandiamide system.

Additional tests were carried out in view of the possible application of organosilane reagents as additives in corrosion-protective oils. Good durability properties have been obtained by priming the metal coupons with a standard oil/silane mixture prior to bonding.

When corrosion was the controlling degradation mechanism as is the case during the salt spray test, silane treated specimens did not generally perform better than control specimens.     

Study the effect of dispersion of filler in polymer composite for radiation shielding

Good filler dispersion in a polymer resin is crucial for achieving performance. Main focus of this study is the effect of dispersion of various fillers in epoxy resin to produce radiation hardened plastic packaging which can be used as a shielding against a high frequency radiation attack. High‐frequency rays have tendency to intrude in the resin and polymer especially are quite vulnerable to be penetrated by high‐frequency rays which may create an early degradation of product. Hence, stabilizers are also used which can absorb the high‐frequency radiation and save the material from early degradation. However, particular fillers are also required that can uniformly disperse in the resin and create a film which can provide extra protection against high‐frequency radiation attack. Three types of compositions are prepared using epoxy/graphite, epoxy/lead, and epoxy/boron nitride nanopowder. Scanning Electron Microscope (SEM) images are used to study dispersion in resin. Small batches using gravity casting method at laboratory using compatibilizers are prepared to carry out experiments. Results of linear absorption coefficient carried out using Am–Be neutron source are also discussed in the paper. POLYM. COMPOS., 35:1263–1266, 2014. © 2013 Society of Plastics Engineers     

Hygrothermal ageing of adhesive joints with nanoreinforced adhesives and different surface treatments of carbon fibre/epoxy substrates

The effect of water absorption on the strength of single lap adhesive joints subjected to accelerated hygrothermal ageing (55 °C, 95% relative humidity, 800 h) was analysed. Two different variables were studied: the surface treatment of the carbon fibre/epoxy laminates (peel ply, grit blasting and atmospheric pressure plasma) and the addition of carbon nanofillers (0.5 wt% nanofibres and 0.25 wt% nanotubes) to the epoxy adhesive. The joint strength and the failure mode of the joints were investigated. Furthermore, the amount of water absorbed by the adhesive was determined.Adhesive joints with peel ply-treated laminates exhibit an increase in their strength, which is attributed to a relaxation of stresses in the adhesive/laminate interface; with grit blasting, this property remains almost constant. Plasma treatment provides the worst ageing behaviour because this treatment results in a surface with a higher surface free energy, which is more susceptible to environmental attack. The nanoreinforcement of the adhesive has a beneficial effect: it decreases the amount of absorbed water.     

Effects of interfaces on the thermal degradation of polymer–metal composites

Previous studies by Black and Blomquist on the degradative failure of polymer–metal adhesive bonds have shown that composite failure depends largely on the type of metal substrate employed. In the work reported herein, metal powders of high surface area have been employed to maximize the metal–polymer interface. The composite systems studied consisted for aluminum and iron with polycondensates of bisphenol A–diglycidyl ether, phenol–formaldehyde and poly-2,2′-(m-phenylene)-5,5′-bibenimidazole. The composites were prepared in the absence of air and thermally degraded in a time-of-flight mass spectrometer while the degradation products were continuously monitored from mass 1 to 200. In the polymer and polymer–metal systems investigated, iron accelerated the decomposition of all polymers studied. This was determined by plotting m/e against degradation temperature for the more common mass peaks such as hydrogen and carbon monoxide for the carbon–hydrogen–oxygen-containing polymer and hydrogen cyanide and ammonia for the carbon–hydrogen–nitrogen-containing polymer. This technique offers promise in determining the nature of the interface as well as the effect of the interface on polymer degradation.     

Evaluation of thick organic coatings degradation in seawater using cathodic protection and thermally accelerated tests

Organic coatings are often associated with cathodic protection to fight against the corrosion of metallic structures when immersed in seawater. However, cathodic protection leads to the generation of a strong alkalinity at the metal/coating interface, which causes the degradation of the coating. It is then necessary to develop a reliable method to evaluate the compatibility between organic coatings and the application of cathodic protection.

On one hand, cathodic disbonding tests (ASTM G-8 and G-80) can be driven with an artificial defect but this defect is mainly responsible for the electrochemical response. In addition, calcareous deposit rapidly forms onto the defect zone when cathodic protection is applied which can make difficult the evaluation of coating delamination. On the other hand, immersion of defect-free specimens requires very long testing periods (several months or even years) in order to detect the coating degradation.

In this work, an attempt to accelerate the coatings degradation by imposing a high temperature and thermal cycles were made in order to decrease the test-time duration. The influence of the applied cathodic potential was also investigated. The coating degradation was evaluated by EIS, considering the defect-free zone of coatings. It was shown that the coating degradation is faster in the presence of a defect and for high temperature (45 °C). Moreover, thermal cycles allow to greatly accelerate the degradation of defect-free coatings and then to compare the compatibility of both coatings with cathodic protection.     

Thermal degradation kinetics of insulating/conducting epoxy/Zn composites under nonisothermal conditions

This article deals with the nonisothermal degradation kinetics of insulating and conducting epoxy/Zn composites. A comparison of thermal degradation data obtained from epoxy/Zn composites revealed that the addition of zinc content in epoxy significantly increases its degradation rate. However, the zinc content activates the degradation until its melting point (419.5°C) and then it starts stabilizing the matrix due to its higher specific heat in molten state. Kinetics of the phenomena fairly explains this behavior in terms of the comparison of general kinetic equations for epoxy/Zn composites. It is to notice that both the values of effective activation energy and reaction model (Šestâk Berggren/SB‐m, n) for insulator/conductor composite pair have been found almost the same emphasizing upon negligible polymer–metal interactions in both cases. These mechanistic clues derived from comparative kinetic study have been found in good agreement with the results obtained through morphological analysis of samples by scanning electron microscopy and X‐ray diffraction techniques. POLYM. COMPOS., 34:2049–2060, 2013. © 2013 Society of Plastics Engineers     

Improvement of the Durability of Zinc-Coated Steel/Epoxy Bonded Joints

The durability properties of bonded lap shear joints made from an epoxy/dicyandiamide adhesive and hot-dipped galvanized (G2F) or electroplated-phosphated (EZ2) steel have been investigated. The degradation mechanisms have been studied after three accelerated ageing tests: the “cataplasme humide” (“C.H.T.”), immersion (“I.T.”), and salt spray (“S.S.T.”) tests. X-ray photoelectron spectroscopy (XPS) analysis of fracture surfaces after ageing have shown that anodic dissolution of the zinc-coating is responsible for debonding in all cases and that intergranular corrosion phenomena account for poorer performances of the hot-dipped galvanized substrate during “C.H.T.” and “I.T.” Silane coupling agents were successfully used as primers on both substrates to increase the hydrolytic stability of the metal/adhesive interface. XPS results indicate that both the interfacial dissolution of the phosphate coating of EZ2 and intergranular corrosion of G2F are delayed for silane-primed specimens. The observed improvements do not appear to depend on the nature of the silane coupling agents. Alkylsilanes have been found to perform as well as silanes having a group capable of reacting with the epoxy/dicyandiamide system.

Additional tests were carried out in view of the possible application of organosilane reagents as additives in corrosion-protective oils. Good durability properties have been obtained by priming the metal coupons with a standard oil/silane mixture prior to bonding.

When corrosion was the controlling degradation mechanism as is the case during the salt spray test, silane treated specimens did not generally perform better than control specimens.     

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.     

Durability of Aluminium Adhesive Joints Bonded with a Homopolymerised Epoxy Resin

The durability of epoxy-aluminium joints that use a homopolymerised epoxy resin was studied, and the effects of relative humidity, temperature, and salt concentration were analysed. The adhesive properties were measured by lap-shear tests, and the water uptake of the epoxy resin was determined by gravimetric measurements. Ageing and degradation effects on the epoxy resin and on the aluminium substrates were also analysed.

The homopolymerised epoxy resin absorbs little water (1.5 wt%) because of its nonpolar network structure. The water uptake is enhanced by increasing relative humidity and temperature; however, the joint strength remains constant because of epoxy plasticization. A saline environment is damaging to the adhesive joints, because of metal corrosion, but was not significantly harmful to the epoxy resin, because of a lower diffusion coefficient of salt water. The T_g decrease of the epoxy adhesive due to water absorption depends only on the amount of absorbed water and is independent of the hydrothermal ageing conditions.     

The Formation of Epoxy/Metal Interphases: Mechanisms and Their Role in Practical Adhesion

When epoxy/diamine systems are applied onto metallic substrates and cured, an interphase, having chemical, physical, and mechanical properties quite different from bulk polymer is created between the substrate and the polymer. The aim of this work is to understand the interphase formation mechanisms and their role in practical adhesion. Mechanisms were deduced from comparison of behaviors when either epoxy and diamine monomers or epoxydiamine monomer mixtures were applied onto aluminum, titanium, and gold-coated surfaces. Using various analytical techniques (DSC, FTIR, FTIR-RAS, ICP, and POM) we will show both a chemical sorption of the diamine monomers and a partial dissolution of the surface oxide and/or hydroxide metallic layer. Then, metallic ions diffuse through the liquid monomer layer and react with amine groups to form an organo-metallic complex by coordination bonding. When the complex concentration is higher than its solubility limit, these complexes may partially precipitate to form needle-sharp crystals. The liquid part of the organo-metallic complex forms, with the epoxy prepolymer, a new amorphous network having a lower glass transition temperature. This new biphase material can also contain complex crystals which act as short fibers, randomly dispersed in the polymer matrix or oriented in the vicinity of the polymer/metal interface, inducing an increase of the Young's modulus and a decrease of the elongation at break. By using a three-point flexure test, we have determined the effect of the interphase formation on the practical adhesion before and after hydrothermal aging. Results obtained point out that the epoxy/metal interphase significantly affects the initial practical adhesion. However, formation of organo-metallic complexes greatly improve practical adhesion after aging. The created complexes act as corrosion inhibitors.     

The Formation of Epoxy/Metal Interphases: Mechanisms and Their Role in Practical Adhesion

When epoxy/diamine systems are applied onto metallic substrates and cured, an interphase, having chemical, physical, and mechanical properties quite different from bulk polymer is created between the substrate and the polymer. The aim of this work is to understand the interphase formation mechanisms and their role in practical adhesion. Mechanisms were deduced from comparison of behaviors when either epoxy and diamine monomers or epoxydiamine monomer mixtures were applied onto aluminum, titanium, and gold-coated surfaces. Using various analytical techniques (DSC, FTIR, FTIR-RAS, ICP, and POM) we will show both a chemical sorption of the diamine monomers and a partial dissolution of the surface oxide and/or hydroxide metallic layer. Then, metallic ions diffuse through the liquid monomer layer and react with amine groups to form an organo-metallic complex by coordination bonding. When the complex concentration is higher than its solubility limit, these complexes may partially precipitate to form needle-sharp crystals. The liquid part of the organo-metallic complex forms, with the epoxy prepolymer, a new amorphous network having a lower glass transition temperature. This new biphase material can also contain complex crystals which act as short fibers, randomly dispersed in the polymer matrix or oriented in the vicinity of the polymer/metal interface, inducing an increase of the Young's modulus and a decrease of the elongation at break. By using a three-point flexure test, we have determined the effect of the interphase formation on the practical adhesion before and after hydrothermal aging. Results obtained point out that the epoxy/metal interphase significantly affects the initial practical adhesion. However, formation of organo-metallic complexes greatly improve practical adhesion after aging. The created complexes act as corrosion inhibitors.     

The interplay of physical aging and degradation during weathering for two crosslinked coatings

Polymer molecular relaxation, or ‘physical aging’, is a very important influence on permeability and mechanical properties of any polymer below its glass transition. ‘Physical aging’ occurs as even an unstressed polymer gradually relaxes towards its equilibrium conformation. This and the shorter term response to stress happen over periods much longer than the typical cycle of an accelerated weathering test, thus important properties of a polymeric coating may be affected by the difference in frequency between natural and artificial exposures, in addition to other factors. Further, ‘physical aging’ is affected by chemical changes to the polymer network caused by the degradation during a weathering exposure. In this investigation, purely physical aging was compared with the effect of concurrent chemical degradation by measuring ‘enthalpy recovery’ and mechanical stress relaxation at a variety of temperatures and at various stages during accelerated weathering exposure. The effect of physical aging was quite apparent in both an epoxy-polyamide coating and a polyester-urethane coating. Changes in physical aging behaviour during degradation were different for the two coatings, which points to further reasons for discrepancy between accelerated weathering and natural exposure.