Laboratory evaluation of corrosion resistance at metallic substrates by an organic coating: delamination effects

The corrosion behaviour of an epoxy-polyamide primer applied on galvanized steel specimens during immersion in 3% NaCl aqueous solution was examined by electrochemical impedance spectroscopy (EIS). The investigation of both intact and defective coatings allowed for the comparison of their electrochemical behaviours in order to assess the anticorrosive characteristics of the system. The impedance response of the intact coating was found to correspond to a porous film presenting localised electrochemically active areas, in which the precipitation of zinc-containing corrosion products contributes to the sealing of the coating. Conversely, scribed defects cannot be spontaneously sealed and no effective protection of the metal can be achieved.     

Electrochemical impedance spectroscopy investigation of the corrosion at metallic substrates covered by organic coatings

The corrosion protection characteristics of an epoxy-polyamide coating were investigated by electrochemical impedance spectroscopy in immersion tests performed in a 3.0% NaCl solution. Carbon steel and galvanized steel substrates were employed. A porous film was produced on the metallic substrates, which allows for electrochemically reactive areas to be developed inside the pores. An effective corrosion-resistant system was produced in the case of the galvanized steel substrates, due to the precipitation of zinc-containing corrosion products that contribute to the sealing of the coating. Conversely, no effective protection was found for carbon steel specimens, since in this case the local accumulation of the corrosion products causes swelling of the coating.     

Effect of the physical and mechanical properties of epoxy resins on the adhesion behavior of epoxy/copper leadframe joints

In this study, the adhesion strength of three epoxy resins, which are used as basic materials for epoxy molding compound (EMC) in microelectronics, to copper leadframe was determined using the peel test. The epoxy resins used were O-cresol Novolac (OCN), dicyclopentadiene (DCPD), and biphenyl sulfide (BIPHS) epoxy resins. It was found that DCPD showed the highest peel strength and OCN had the lowest value. The difference in the peel strength was explained by investigating the physical and mechanical properties, as well as the surface properties of the epoxy resins. These properties included the surface energy, viscosity and gelation time, fracture toughness, and the coefficient of thermal expansion. As a result of the lower viscosity of BIPHS and DCPD than OCN epoxy resin, BIPHS and DCPD have a better peel strength than OCN. The DCPD resin has a better peel strength than BIPHS because of its higher fracture toughness.     

Development of an electrochemical impedance spectroscopy sealant test: I. Nonconductive sealants

Distinguishing between sealants and providing laboratory prognostic tools is essential for qualifying new sealants. Current test methods do not provide adequate discrimination between sealants. A laboratory test based on electrochemical impedance spectroscopy (EIS) supplemented by adhesion strength measurements and examination of sealed, bimetallic surfaces has been developed to give an early indication of sealant performance. Specimens using one of two different nonconductive sealants underwent hot-salt-water exposure followed by EIS measurements, pull strength measurements, and visual inspection. Defective sealant bonded specimens were readily detected by EIS. Absorption of moisture by the sealants was also detected and quantified.     

Structural characterization of bis-[triethoxysilylpropyl]tetrasulfide and bis-[trimethoxysilylpropyl]amine silanes by Fourier-transform infrared spectroscopy and electrochemical impedance spectroscopy

Bis-[triethoxysilylpropyl]tetrasulfide (or bis-sulfur silane) and bis-[trimethoxysilylpropyl] amine (or bis-amino silane) were deposited on 2024-T3 aluminum alloy (AA 2024-T3). The structures of the films were characterized using Fourier-transform infrared spectroscopy (FTIR) and electrochemical impedance spectroscopy (EIS) techniques. The results showed that: (1) The silane structures were affected significantly by the hydrolysis time of the silane solutions. A minimum hydrolysis time is required to obtain a crosslinked silane film. (2) Hydrolysis progressed more readily and faster in the bis-amino silane system than in the bis-sulfur silane system, probably due to the catalytic action of the amine of the bis-amino silane. (3) Both silane systems experienced significant crosslinking upon curing at 100°C, during which denser interfacial layers were formed via crosslinking in the interfacial regions. The interfacial layer contributes to corrosion protection of metals by silanes. (4) A new phase was observed in the fully cured bis-amino silane film after aging in the atmosphere. This new phase is likely to be carbamates and bicarbonates formed via a reaction between the secondary amino groups, carbon dioxide, and moisture absorbed from the atmosphere.     

Modification of epoxy resins for improvement of adhesion: a critical review

Modification of epoxy resins for improvement of adhesion has been the subject of intense research throughout the world. Unlike for thermoplastics, physical blending is not successful for improvement of bond strength and impact strength of epoxy resins. The bond strength of an epoxy resin can be improved only by chemical modification with a suitable flexible modifier. Such chemical modification may either plasticize the epoxy matrix or lead to a two-phase microstructure. Both methods of chemical modifications are discussed critically in the present review.     

Electrochemical impedance of mercury electrodes with hematite particles adhered

The effect of hematite particles adhesion on the electrochemical impedance of mercury electrodes was studied at different electrode potentials. The impedance decreases as the number of attached particles increases; this impedance decrease is related to strong adhesion of particles. The impedance diagrams show, in the low frequency range, the presence of a constant phase element (CPE), with an exponent of ca. 0.5. The experimental results are analyzed in terms of an equivalent circuit including the CPE. The magnitude of this CPE is directly related to the coverage of the electrode. A qualitative interpretation for this behavior, when an AC signal is applied, is proposed in terms of a pore model for the metal/hematite particles interphase.     

Effect of skin pass rolling on the primer adhesion and corrosion resistance of hot-dip galvanized (HDG) steel

By using three different skin pass reductions, 0%, 0.75%, and 1.5%, the influence of skin pass rolling on the primer adhesion and corrosion resistance of primed hot-dip galvanized (HDG) steel has been studied. The corrosion resistance of primed panels was determined by a cyclic prohesion test, and the primer adhesion was examined with a combined cross-cut and impact test. Surface roughness was determined for untreated and pretreated skin passed panels and the samples were also studied using an optical microscope and a scanning electron microscope. Electron spectroscopy for chemical analysis (ESCA) and glow discharge optical emission spectroscopy (GD-OES) were used to characterize the chemical surface composition of the panels. According to the prohesion test results, the roughest samples showed the best corrosion resistance and also slightly improved adhesion test results. The ESCA and GD-OES results showed that the outermost surface was enriched by aluminium in the zinc coating. During skin pass rolling, the aluminium oxide-rich surface is broken and zinc is revealed to the surface. An increase in the skin pass reduction resulted in an almost linear increase in the surface roughness. Mechanical removal of the surface aluminium also affected the amount of aluminium dissolved during the chemical pretreatment. The best results obtained for the roughest samples are mainly due to the most homogeneous skin pass pattern obtained with the highest skin pass reduction.     

Adhesion and corrosion resistance properties of coatings obtained from modified low-molecular-weight polystyrenes

In this study, as a continuation of our previous studies, chemical modification of low-molecular-weight polystyrenes (PSs) was carried out with various functional group modifiers: epichlorohydrin (ECH), maleic anhydride (MA) and acetic anhydride (AA), in a single stage using a cationic catalyst. It was determined that the amounts of the functional groups bound to the structure of the polymer depended on the molecular weight of the polymer used, and more functional groups were bound to the lower-molecular-weight PSs. It was found that the coating properties (adhesion properties and resistance to aggressive conditions) of the functional group containing PS to the metal surface depended on the structure and the amount of the functional groups bound to the aromatic ring of the polymer. In addition, it was observed that the PS modified with MA and ECH having carboxyl- and epoxy-groups in their aromatic rings had higher adhesion, as well as higher corrosion resistance properties. Various functional groups bound to the aromatic ring of the polystyrene and their amounts were determined by spectral and chemical analysis methods.     

Surface reactivity of polyimide and its effect on adhesion to epoxy

Polyimides are commonly used as organic passivation layers for microelectronic devices due to their unique combination of properties such as low dielectric constant, high thermal stability, excellent mechanical properties and superior solvent resistance. Unfortunately, polyimides are well known to be difficult to bond to other materials, especially to epoxy resins. Many surface treatments have been developed to increase epoxy–polyimide adhesion. These treatments include exposure to ion beams, plasmas and chemical solutions. The goal of our research was to relate surface reactivity of epoxy and polyimide resins to the strength of epoxy–polyimide interfaces. The surface reactivity of four polyimides was studied and quantified using contact angle measurements, flow microcalorimetry (FMC), Fourier transform infrared (FT-IR) spectroscopy (using an attenuated total reflection (ATR) accessory) and X-ray photoelectron spectroscopy (XPS). Several ways of analyzing contact angles were tried and only a weak correlation between the polar component or the acid–base components of the surface free energy with the critical interfacial strain energy release rate (i.e., the interfacial fracture strength) was observed. FMC results suggest that the strength of epoxy–polyimide interfaces is related to the molecular interactions between the curing agent and polyimide. The molecular interactions between the curing agent and polyimide surfaces were found to be either greater than epoxy and polyimide interactions or more irreversible. Therefore, the curing agent (2,4-EMI) is thought to play a critical role in controlling adhesion strength.     

Mechanisms for the Improvement in Interfacial Adhesion Between UHMWPE Reinforcement and Nano-epoxy Resins with Reactive Graphitic Nanofibers

Previously developed nano-epoxy matrices with reduced viscosity showed both substantially enhanced mechanical properties and interfacial adhesion with ultra-high molecular weight polyethylene (UHMWPE) fibers vis-à-vis pure epoxy. In this work, mechanisms for the improvement in the interfacial adhesion were investigated. Atomic Force Microscopy and Energy Dispersive X-ray with Scanning Electron Microscopy analyses indicated that improved performance of the UHMWPE fiber composites with the nano-epoxy containing reactive graphitic nanofibers (r-GNFs) is attributed to mechanical interlocking and a diffusion mechanism. The nano-epoxy with the 'liquid nano-reinforcement' resulting in reduced viscosity provided better wettability, diffusion capability and reinforcing effect, which produced an effective improvement in the adhesion properties.     

Inspection of composite and metal adhesive bonds with an electrochemical sensor

An in-situ sensor, based on Electrochemical Impedance Spectroscopy (EIS), has been used to monitor the health of adhesive bonds constructed from various combinations of aluminum, graphite/epoxy, glass/epoxy, glass/polyester, and glass/vinylester composites and exposed to high humidity and temperature conditions. Modeling of the EIS data as an electric circuit demonstrated that several circuit parameters of the impedance spectra were sensitive to bond performance, as determined by wedge tests and lap shear tests. Moisture absorption by the adhesive and composite was calculated from the circuit capacitance, which was also a function of bonded area and bondline thickness (bondline + composite thickness for glass composites). Material differences, including saturation level of moisture, rate of absorption, and bondline stability, were readily seen among the various materials sets. The sensor electrodes are attached to opposite sides of a bond after fabrication, i.e. they are not embedded. Thus, they are suitable for monitoring existing bonded structures. They have the potential to identify bondlines that are in the early stages of degradation, prior to significant loss of bond strength. As an input to a condition-based maintenance system, they would identify weakening bondlines and allow preventative action to be scheduled and performed.     

The critical humidity effect in the adhesion of epoxy to glass: role of hydrogen bonding

Adhesion between an epoxy [diglycidyl ether of bisphenol F (DGEBF) cured with diethylene triamine] and glass was lowered abruptly when the epoxy was equilibrated with air whose relative humidity (RH) exceeded a critical value of approximately 70% RH. The critical humidity marking the onset of adhesion loss was also associated with a sudden increase of water uptake by the epoxy. In earlier work, it was shown that this 'transition' was not due to capillary condensation, osmotic cell formation or a decrease in the T _g of the material. Instead, it was speculated that the critical humidity effect was due to the trapping of water by hydroxyl groups which become available as inter-chain hydrogen-bonded structures are broken. To verify the above hypothesis, two model compounds were synthesized. One closely mimicked the cured DGEBF resin and the other had all of its hydroxyl groups replaced by hydrogen. Comparison of the water sorption isotherms of these two model compounds clearly suggested that hydroxyl groups played a key role in the critical humidity effect. Using molecular simulation software, hydrogen bonding between the various polar sites of the hydroxylated model compound was also studied. In the dry state or at low water concentrations, simulations predicted the formation of hydrogen bonds between polar sites. These hydrogen bonds always involved one or more hydroxyl groups. At higher water concentrations, molecular simulations showed that water tended to displace the hydrogen bond network of the epoxy, and in the process, water-mediated 'bridges' between polar groups were formed. The large decrease in entropy associated with the formation of such macrocyclic conformers is thought to be offset by the decreased enthalpy of condensation of water made possible by multiple hydrogen bonding. This suggests that the critical humidity effect might be an 'order-disorder' transition associated with the formation of ring structures closed by hydrogen-bonded water linkages between polar groups. The first-order energetics of this type of transition is consistent with the abrupt nature of the critical humidity effect.     

Adhesion between rubber compounds and ternary-alloy-coated steel cords,Part I: effect of cobalt plating amount in ternary-alloy-coated steel cords

The adhesion between a rubber compound and ternary-alloy-coated steel cords with different cobalt plating amounts (0, 2 and 4 wt%) was investigated to understand the role of cobalt in stabilizing adhesion to the rubber compound. The adhesion property of ternary alloy coated steel cords to the rubber compound did show significant enhancement after cure for the ternary alloy coated steel cord with 2 wt% cobalt plating. Further increase of cobalt plating in ternary-alloy-coated steel cord was responsible for the poor adhesion to the rubber compound. An improvement in adhesion durability after aging in various hostile environments was shown for the ternary-alloy-coated steel cord with 2 wt% cobalt plating. The interphase between the ternary-alloy-coated steel cords and the rubber compound studied using AES showed a stable adhesion interphase by optimum cobalt plating, resulting in enhancement of adhesion retention.     

The Effect of Oily Finish Components on the Adhesion Between Aramid Fibers and Rubber

The use of aramid fibers as a reinforcing material in both tires and mechanical rubber goods, such as hoses, belts, etc., is growing. In these dynamic applications, the adhesion between fiber and rubber is critical. This can be optimized by activating the aramid with an epoxy formulation, followed by RFL (Resorcinol Formaldehyde Latex) treatment. In the past, various combinations of analytical techniques have been used to study the relationship between the fiber surface treatment, the resulting microscopic interphase structure and the macroscopic rubber properties. The fundamental knowledge acquired from these past studies has been exploited here to investigate the effect of oily finish components on the aramid–rubber adhesion. For this purpose, aramid yarn has been treated with various combinations of an adhesion improving (epoxy–amine) component and a processability improving (oily) component. Contrary to general belief, the oily components do not directly reduce the SPAF (Strap Peel Adhesion Force) to rubber, rather show some positive effect. Furthermore, there is a relative broad 'safe' oil range, i.e., fluctuations in the amount of oil will not directly lead to adhesion problems. This is in line with earlier observations, but this study using appropriate analytical techniques provides quantitative confirmation and additional understanding of the fundamental principles behind these effects.     

Enhanced adhesion of silica for epoxy molding compounds (EMCs) by plasma polymer coatings

Silica for epoxy molding compounds (EMCs) was coated via plasma polymerization using an RF plasma (13.56 MHz) as a function of the plasma power, gas pressure, and treatment time. The monomers utilized for the plasma polymer coatings were 1,3-diaminopropane, allylamine, pyrrole, 1,2-epoxy-5-hexene, allyl mercaptan, and allyl alcohol. The EMC samples were prepared from biphenyl epoxy resin, phenol novolac, triphenyl phosphine, and plasma polymer-coated silica, and the loading of silica was controlled to 60 wt%. The EMC samples were cured at 175°C for 4 h and subjected to T_g, CTE, and water absorption measurements. The adhesion of silica to epoxy resin was evaluated by measuring the flexural strength of EMC samples and the fracture surfaces were analyzed by SEM. Plasma polymer coatings were also characterized by FT-IR and coating thickness measurements. The plasma polymer coating of silica with 1,3-diaminopropane and allylamine enhanced the flexural strength of EMC samples (167 and 165 MPa), compared with the control sample (140 MPa), and exhibited a higher T_g, a lower CTE, and lower water absorption. The enhanced properties with 1,3-diaminopropane and allylamine plasma polymer coatings can be attributed to the amine functional groups in the plasma polymer coatings.     

Adhesion Between Rubber Compounds Containing Various Adhesion Promoters and Brass-Plated Steel Cords. Part I. Effect of Sulfur Loading in Rubber Compounds

The adhesion property between rubber compounds containing different types of adhesion promoters (resinous adhesion promoter (containing both methylene donor and methylene acceptor), cobalt salt and zinc borate) and different loading amounts of sulfur and brass-plated steel cords was investigated to understand the effect of sulfur loading in the rubber compounds on their adhesion characteristics to the brass-plated steel cords. The adhesion property of the rubber compounds to brass-plated steel cords was largely dependent on both the type of adhesion promoter and the loading amount of sulfur in the rubber compounds. The pull-out force of adhesion samples increased significantly with increasing loading amount of sulfur in the rubber compounds containing resinous adhesion promoter, whereas it decreased slightly with increasing loading amount of sulfur in rubber compounds containing cobalt salt or zinc borate. In humidity aging, the best adhesion retention was observed in the rubber compound containing zinc borate and low loading of sulfur. Regardless of the type of adhesion promoter, adhesion retention after thermal aging treatments improved with increasing loading amount of sulfur in the rubber compounds. The adhesion property was interpreted in terms of the interphases formed between the rubber compounds and the brass-plated steel cords as studied using Auger electron spectroscopy.     

Effect of the microstructure of copper oxide on the adhesion behavior of epoxy/copper leadframe joints

The thermal oxidation of copper leadframe was carried out at 175°C and the adhesion behavior of the epoxy/copper leadframe joint was analyzed by investigating the microstructure changes of copper oxide with the thermal oxidation time of copper. The peel strength increased sharply at an early stage of oxidation (~20 min) followed by a slight increase. After further oxidation (120 min), the peel strength showed a slight decrease. The contact angles of water and diiodomethane decreased sharply at an early stage of oxidation with negligible change afterwards. As the oxidation time increased, X-ray photoelectron spectroscopy (XPS) results revealed that the chemical composition of copper oxide had changed (Cu/Cu₂O → Cu₂O → CuO); this change improved the wettability of the copper surface, which affected the peel strength. Increase of the surface roughness of copper oxide, investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM), causes the epoxy resin and copper oxide to undergo mechanical interlocking, which increases the peel strength. Failure analysis by SEM and XPS indicated that failure was largely in the copper oxide, and the amount of copper oxide on the peeled epoxy increased as the oxidation time increased, due to the weak mechanical strength of the oxide layer. However, a small portion of the epoxy resin was also fractured during the failure process, regardless of the oxidation time. Consequently, fracture proceeded mainly in the copper oxide close to the epoxy resin/copper oxide interface.     

Adhesion between rubber compounds and ternary-alloy-coated steel cords. Part II: Effects of sulfur and cobalt salt in rubber compounds

The adhesion properties between rubber compounds with different amounts of cobalt salt and sulfur and ternary-alloy-coated steel cord with 2 wt% cobalt plating amount were investigated to understand the effects of cobalt salt and sulfur in rubber compounds on their adhesion characteristics to the ternary-alloy-coated steel cord. The adhesion properties of the rubber compounds to ternary-alloy-coated steel cord were largely dependent on the amounts of both cobalt salt and sulfur in the rubber compounds. The pull-out force of adhesion sample increased significantly with increasing concentration of cobalt salt in the rubber compound with constant sulfur loading, while it decreased slightly with increasing sulfur into the rubber compound with constant cobalt salt loading. Adhesion retention after various hostile aging treatments improved in the rubber compounds incorporating both cobalt salt and high loading of sulfur. The interphases between the rubber compounds and the ternary-alloy-coated steel cord studied using AES showed stable adhesion patterns by incorporating cobalt salt, resulting in enhancement of the adhesion retention.     

Evaluation of infrared spectroscopic techniques to determine the Drago constants of a cycloaliphatic epoxy

The task of understanding adhesion is not complete without considering acid–base interactions. Such site-specific interactions play a major role in promoting adhesion. However, these interactions are often difficult to characterize or quantify. In this work, a cycloaliphatic epoxy resin is studied by Fourier Transform Infrared (FT-IR) spectroscopy to identify and quantify possible molecular interactions. As controls, simple molecules such as acetone and ethyl acetate were also studied. Moreover, interactions of molecules with structural similarities to the epoxy resin were studied to provide additional insight. Two infrared spectroscopic techniques, carbonyl peak shifts and hydroxyl peak shifts, were employed to quantify the acid–base interactions of these organic molecules. The Drago constants from carbonyl peak shifts were determined from predicted heats of complexation and also directly from hydroxyl peak shifts. The constants obtained for the control molecules were compared with published data. The Drago constants of the control molecules determined by hydroxyl peak shifts agreed well with literature values, in contrast, to those derived from the carbonyl peak shifts. This lack of correlation may be attributed to the influence of solvent effects and concentration dependence on carbonyl shifts. Using the hydroxyl peak shift approach, the Drago constants for the cycloaliphatic epoxy group of cycloaliphatic epoxy resin were found to be E _B = 2.69 (kJ/mol)^1/2 and C _B = 4.04 (kJ/mol)^1/2 and E _B = 3.45 (kJ/mol)^1/2 and C _B = 2.11 (kJ/mol)^1/2 for the ester group of the epoxy resin. The average Drago constants for the resin were found to be E _B = 3.28 ± 0.14 (kJ/mol)^1/2 and C _B = 2.00 ± 0.09 (kJ/mol)^1/2.