Investigation of corrosion protection properties of an epoxy nanocomposite loaded with polysiloxane surface modified nanosilica particles on the steel substrate

In this study, it has been aimed to investigate the corrosion protection properties of an epoxy/polyamide coating loaded with different concentrations (ranged from 3 to 6% (w/w)) of the polysiloxane surface modified silica nanoparticles (nano-SiO₂). The nanocomposites were applied on the steel substrates. Field emission scanning electron microscope (FE-SEM) and UV–vis techniques were utilized in order to investigate the nanoparticles dispersion in the coating matrix. The effects of addition of nanoparticles on the corrosion resistance of the coating were studied by an electrochemical impedance spectroscopy (EIS) and salt spray test. The coating surface degradation was studied by optical microscope and Fourier transform infrared radiation (FT-IR) spectroscopy. Results obtained from UV–vis and FE-SEM analyses revealed proper and uniform distribution of surface modified nanoparticles in the epoxy coating matrix. It was shown that the coating corrosion protection properties were significantly enhanced in the presence of 5 wt% silica nanoparticles. Less degradation occurred on the surface of the coatings loaded with 5 wt% nanoparticles.     

Tailored AA1050 alloy surfaces by electrograining

The effects of anodic and cathodic polarisation during ac electrograining of aluminium AA1050 alloy in nitric acid on the resultant surface appearance have been examined. A strong dependency of the surface convolution on the amplitude of the anodic and cathodic polarisation is revealed. Cathodic polarisation promotes pit initiation and suppresses pit growth; consequently, a high population of fine hemispherical pits is observed after electrograining with cathodic bias. The passive region of anodic polarisation below the pitting potential is associated with dissolution of segregates and second phase material from the filmed macroscopic alloy surface, which influences the pit initiation stage. Anodic polarisation above the pitting potential encourages pit growth. Thus, by controlling the extents of anodic or cathodic polarisation and by consideration of the local interfacial solution conditions, the means for developing tailored and optimised pit morphologies is revealed.     

Interactions of BTESPT silane and maleic anhydride grafted polypropylene with epoxy and application to improve adhesive durability between epoxy and aluminium sheet

For improvement of adhesive strength and durability of adhesion between epoxy and aluminium sheet bis-(triethoxysilylpropyl)tetrasulfide (BTESPT) silane and maleic anhydride grafted polypropylene (PP-g-MAH) are chosen for surface pretreatment of the aluminium sheet respectively. Fourier transform infrared (FTIR) spectroscopy was used for characterization of the structure and the interactions in the systems. It is shown that BTESPT silane and the anhydride on PP-g-MAH take part in the curing reactions of the epoxy/polyamide system. The adhesive shear strength of the samples, prepared under different curing temperatures, and after immersion in boiling water and 3.5% NaCl water solution respectively, was tested. The features of the shear fractured surfaces were examined by scanning electron microscope (SEM). For the aluminium sheet pretreated by BTESPT silane, the maximum adhesive shear strength is 22.2 MPa, which is higher than that of 17.5 MPa for aluminium sheet without pretreatment by the silane. After immersion in boiling water for 80 h and in NaCl water solution at 50 °C for 180 h the adhesive shear strengths are 13.39 MPa and 18.4 MPa respectively, which are higher than these (below 6 MPa) for aluminium sheet without pretreatment by the silane. As for the aluminium sheet pretreated by PP-g-MAH, the maximum adhesive shear strength is 13.17 MPa. After immersion in boiling water for 80 h and in NaCl water solution at 50 °C for 180 h the adhesive shear strengths decline to 10.67 MPa and 8.1 MPa respectively.     

A comprehensive study of the green hexafluorozirconic acid-based conversion coating

The effect of four practical parameters (deposition temperature, time, bath pH and concentration of Zr) on electrochemical and morphological properties of zirconium based conversion coating (ZrCC) on cold rolled steel (CRS) substrates was investigated. DC polarization and electrochemical impedance spectroscopy (EIS) measurements were used for electrochemical studies. Micro structural characterization was carried out by FE-SEM, AFM and EDS. The optimal conditions were determined as follows: solution temperature 20–30 °C, immersion time 60–120 s, pH = 4 and acid concentration of 4% vol. In such condition, corrosion resistance values were maximum and uniformity was improved. Microscopical observations revealed that ZrCC comprises a two-layered structure. The film formation mechanism of ZrCC was discussed in detail. After achieving the optimum conditions, epoxy nanocomposites were applied on ZrCC treated CRS samples and corrosion performance of fully painted system was investigated. Finally the adhesion strength of subsequently applied organic coating on ZrCC treated substrates was measured by pull-off technique which exhibited considerable high values.     

An elevated temperature infrared emissivity ceramic coating formed on 2024 aluminium alloy by microarc oxidation

An infrared emissivity coating material containing γ-Al₂O₃ was prepared on 2024 aluminium alloy surface by the microarc oxidation (MAO) method. The microstructure of the coatings was analysed by SEM, XRD and EDS techniques. The infrared emissivity properties tested at 500 °C were investigated by an infrared radiometer based on a Fourier transform infrared spectrometer. The results show that the infrared emissivity values of coated Al samples depend on the phase composition and surface roughness of the coatings. Corresponding to increasing coatings thickness, the gradually increasing γ-Al₂O₃ content and some oxide compounds containing Si and P contribute to the higher infrared emissivity value (about 0.85) in the wavelength range of 8–20 μm. The increasing surface roughness leads to an obvious increase in emissivity from 0.2 to 0.4 at wavelength 3–5 μm.     

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.     

Influence of adhesive bond line thickness on joint strength

While the geometry of aerospace assemblies is carefully controlled, for many industrial applications such as marine structures bond line thickness can vary significantly. In this study epoxy adhesive joints of different thicknesses between aluminium substrates have been characterized using physico-chemical analyses (differential scanning calorimetry, DSC; dynamic mechanical analysis, DMA; spectroscopy), nano-indentation and mechanical testing. Thermal analyses indicated no influence of thickness on structure. Nano-indentation revealed no evidence of an interphase at the metal/epoxy interface, nor any change in modulus for different thicknesses, though Raman spectroscopy suggested there may be slight variations in composition close to the substrates. However, mechanical testing using the modified Arcan fixture indicated a significant drop in strength and failure strain under pure tension and a smaller reduction for tension/shear and pure shear loads as thickness increased. Examination of sections through joints did not indicate any physical reason for this, but numerical analysis of the stress state revealed larger stress concentration factors for tensile loading in thick joints, which may explain the thickness effect. It is recommended that joint thickness should be kept below 0.8 mm to avoid obtaining artificially low values with the Arcan test.     

The protective properties of epoxy coating electrodeposited on Zn–Mn alloy substrate

The epoxy coating was cataphoretically deposited on steel and steel modified by electroplated Zn–Mn alloy of different chemical contents. The samples were immersed in 0.5 mol dm⁻³ NaCl solution for 60 days. The electrochemical impedance spectroscopy (EIS) analysis showed that the values of pore resistance for epoxy coating on steel and Zn–Mn alloy with 16 at.% Mn were two orders of magnitude higher, while the capacitance values were two orders of magnitude lower than those for the epoxy coating on Zn–Mn alloy substrates with 5 and 8 at.% Mn. It was assumed that the main reason for such a difference was metallic substrate dissolution during cataphoretic deposition, due to high pH (12.9). This assumption was supported by energy dispersive X-ray spectrometry (EDS) measurements showing that the amount of released Zn in epoxy coatings decreased as Mn percent in the Zn–Mn alloys increased. In addition, Zn–Mn alloy coatings on steel, as well as bare steel, were immersed in 0.1 mol dm⁻³ NaOH solution, pH 12.9, simulating conditions during cataphoretic deposition, and polarization resistance measurement in this solution indicated that Mn inclusions in Zn–Mn alloy substrate prevent Zn dissolution in alkaline medium.     

Electrochemical deposition and characterization of ZnCo alloys and corrosion protection by electrodeposited epoxy coating on ZnCo alloy

ZnCo alloys electrochemically deposited on steel under various deposition conditions were investigated. The influence of deposition current density, temperature and composition of deposition solution on the phase structure and corrosion properties of ZnCo alloys were studied. It was found that ZnCo alloy obtained from chloride solution at 5 A dm⁻² showed the best corrosion properties, so this alloy was chosen for further examination. Epoxy coating was electrodeposited on steel and steel modified by ZnCo alloy using constant voltage method. The effect of ZnCo alloy on the corrosion behavior of the protective system based on epoxy coating is interpreted in terms of electrochemical and transport properties, as well as of thermal stability.     

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

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

Influence of the curing temperature of a cataphoretic coating on the development of filiform corrosion of aluminium

Aluminium alloys are known to be particularly sensitive to filiform corrosion. The initiation of this particular type of corrosion is related to different parameters such as the presence of defects, the permeability of the coating to water and oxygen, the adherence of the paint system and the presence of salts.

In this work, the filiform corrosion resistance of Al 6016 substrates coated with a cataphoretic paint was studied. The curing of the coating was performed at different temperatures (185, 175, 165, 155 and 135 °C) in order to modify its mechanical properties and its permeability to oxygen.

The paint properties were studied by different techniques allowing the estimation of the degradation of the metal–primer system and giving some information about intrinsic paint properties.

The corrosion protection of the coating was evaluated by a normalized filiform corrosion test and by electrochemical impedance spectroscopy on scratched samples. The glass transition value and the internal stresses of the cataphoretic coatings obtained for different curing temperatures were determined by a stressmeter equipment.

This study enabled us to underline the influence of the curing temperature on the intrinsic properties of the coatings such as the glass transition temperature, the internal stresses, the adherence, the permeability and the corrosion protection properties.     

A 3D EFFECT IN THE WEDGE ADHESION TEST: APPLICATION OF SPECKLE INTERFEROMETRY

The wedge test is of considerable use for evaluating adhesion between two bonded rigid substrates. In its (usual) static form, release of elastic strain energy is equated to effective adhesion energy during crack growth. However, the test is usually treated as two-dimensional. In fact, it is really three-dimensional due to anticlastic bending effects of the bent beam(s) during crack propagation.

We studied a composite material/epoxy/aluminium alloy system and observed a curved crack front during propagation. This leads to doubt as to the value of crack length to be inserted in the adhesion energy formula. In addition, by using the highly sensitive technique of speckle interferometry, it was possible to study anticlastic bending effects in a quantitative manner. Far from the crack front, agreement between theory and experimental is good, yet work remains to be done to understand the zone near the fracture zone.     

A 3D EFFECT IN THE WEDGE ADHESION TEST: APPLICATION OF SPECKLE INTERFEROMETRY

The wedge test is of considerable use for evaluating adhesion between two bonded rigid substrates. In its (usual) static form, release of elastic strain energy is equated to effective adhesion energy during crack growth. However, the test is usually treated as two-dimensional. In fact, it is really three-dimensional due to anticlastic bending effects of the bent beam(s) during crack propagation.

We studied a composite material/epoxy/aluminium alloy system and observed a curved crack front during propagation. This leads to doubt as to the value of crack length to be inserted in the adhesion energy formula. In addition, by using the highly sensitive technique of speckle interferometry, it was possible to study anticlastic bending effects in a quantitative manner. Far from the crack front, agreement between theory and experimental is good, yet work remains to be done to understand the zone near the fracture zone.     

Joint Strength Optimization of Adhesively Bonded Patches

Aircraft face damage from impact with objects or birds or due to ageing that leads to fatigue cracks. The conventional methods of repairing aircraft metallic structures generally include the use of a plate joined by screws or rivets. Although these methods are efficient in the short term, they introduce stress concentrations leading to the initiation of new cracks that are difficult or impossible to detect by non-destructive methods. For these reasons, it is necessary to develop new methods to improve the behaviour of the structure (especially for long term) and its manufacture cost. One of the solutions that have been studied by the aeronautical industry is the use of patches bonded with structural adhesives. However, adhesively bonded patches have problems of stress concentration at the edges where crack initiation is prone to occur. This problem can be reduced by the use of a taper and a spew fillet at the end of the patch and by the use of a mixed adhesive technique where a ductile adhesive is placed at the edges of the patch. Double strap specimens from 3 mm thick 6063-T6 aluminium alloy sheet were analysed. Aluminium and straps (or patches) with an internal taper, an adhesive spew fillet, and dual adhesives were experimentally tested. The results obtained were explained by a finite element analysis. A taper angle is beneficial only for the brittle adhesive. The use of two adhesives is advantageous for the taperless configuration.     

Changes in epoxy-coated aluminium due to exposure to water

An investigation was performed of the changes that occur in a typical epoxy-coated aluminium system due to exposure to water. The adhesion of the epoxy coating upon exposure to water was evaluated for different exposure temperatures and periods. The adhesion test results showed an initial loss of adhesion of the coating but after this the adhesion improved again and even significantly exceeded adhesion prior to exposure. The amount of adhesion improvement and the speed with which adhesion improvement occurred was found to be larger for higher exposure temperatures. The changes that occur in the epoxy-coated aluminium system due to exposure to water were investigated using a number of different analytical techniques. Based on this, a model was proposed for the processes that take place during exposure and which can explain the adhesion test results. First, the adhesion of the epoxy coating is lost upon exposure due to the accumulation of a significant amount of water at the interface. The water at the interface causes formation and growth of a thin oxyhydroxide layer underneath the epoxy coating. After some time, this oxyhydroxide layer re-establishes contact with the epoxy coating and forms a new, water-stable bond, hence explaining the improvement of the adhesion after its initial loss.

The temperature-dependence observed in the adhesion test results is explained by the fact that at a higher exposure temperature, more water accumulates at the interface, the oxyhydroxide layer grows faster and also attains a larger limiting thickness.     

CH680各色环氧背面漆研究

CH680环氧背面漆是底面漆合一的一个品种,通过对环氧树脂、氨基树脂和催化剂的选择,使产品达到良好的综合性能。本文主要介绍CH680各色环氧背面漆的研制及其主要性能。     

氮化铝/氧化石墨烯对环氧树脂胶黏剂导热性能的影响

使用硅烷偶联剂KH-560对氮化铝进行了表面改性,并以其为导热填料,环氧树脂为基体,制备了氮化铝/环氧树脂导热胶黏剂。采用FTIR、SEM、TG、热常数分析仪对导热胶黏剂进行了表征。结果表明:改性后硅烷偶联剂分子成功接枝在氮化铝表面。改性后,氮化铝与环氧树脂的界面粘结力增强,热稳定性和导热性均得到明显改善。当氮化铝质量为导热胶黏剂质量的70%时,改性氮化铝/环氧树脂热胶黏剂的导热系数为2.24W/(m·K),而未改性氮化铝/环氧树脂的导热系数仅为1.73W/(m·K)。为进一步提高其导热性能,制备了改性氮化铝/氧化石墨烯/环氧树脂导热胶黏剂,当改性氮化铝和氧化石墨烯的质量分数分别为50%和3%时,导热胶黏剂导热系数为3.05 W/(m·K)。