Effect of mechanical stress on kinetics of degradation of marine coatings

Water, UV and temperature are well-known factors for organic coating degradation. Mechanical stress can also affect long-term lifetime in marine environments and probably is conducive to synergistic effects with other ageing parameters. The present work proposes a method to estimate the role of a stress–strain state on the protective properties of two marine epoxy coatings onto mild steel (with and without VOC). Preliminary mechanical measurements on free films by Dynamic Mechanical Analysis (DMA) indicated that the two coatings have a visco-elastic behaviour for a stress level lower than 3.3 MPa. Consequently, a stress equal to 3 MPa was applied on coated substrates using bent samples which were immersed in 3% NaCl solution and in natural seawater (“Les Minimes” yachting harbour in La Rochelle). This test is innovative because a visco-elastic deformation implies that the chain motion remain unchanged after a time of recovery (total restoration of strain in unloading stage) so coatings are not disturbed without applied stress. Non-bent coated samples were also immersed in the same environments as references. The coating degradation was followed by Electrochemical Impedance Spectroscopy (EIS) on both sides of the sample (compressed and stretched sides). The results allowed evaluation of the influence of mechanical state on the coatings degradation in visco-elastic regime and also demonstrate that the mechanical effect depends on the direction of the stress loading.     

Evaluating water transport through high solid polyurethane coating using the EIS method

Buried pipelines are commonly coated with high solid polyurethane to prevent corrosion. The diffusion of water through the coating plays an essential role in the performance and endurance of the coating. This study focuses on the evaluation of water diffusion through polyurethane coatings. The effects of thickness and ambient temperature on water diffusion through polyurethane, which is coated on mild steel and immersed in 3.5% (weight fraction) NaCl solution, are evaluated using electrochemical impedance spectroscopy (EIS). Experimental observations clearly indicate that the water diffusion coefficient strongly depends on service temperature and the thickness of coating. Higher temperature (lower than T _g) increases water uptake of the polymer but no measurable changes were observed for the temperature change studied. EIS results show that the mechanism of corrosion does not change in the range of thicknesses and temperature investigated in this study.     

Ageing of marine coating in natural and artificial seawater under mechanical stresses

In order to study the effect of a visco-elastic stress (tension and compression mode) onto the performances of a thick marine organic coating, free films and coated panels were immersed in natural seawater and in NaCl 3 wt.% solution at room temperature (20 °C), fixed temperature (45 °C) or under cyclic temperatures. Free films were analysed using uniaxial elongation and Dynamic Mechanical Analysis (DMA) and the degradation of coated samples was investigated using Electrochemical Impedance Spectroscopy (EIS).     

Preparation and characterization of hydroxyapatite coating by magnetron sputtering on Mg–Zn–Ag alloys for orthopaedic trauma implants

The aim of the present paper was to evaluate the effect of hydroxyapatite coatings on the two types of Mg–Zn–Ag alloys as a possible solution to control magnesium alloy degradation. The coatings were prepared by the radio frequency magnetron sputtering method at a deposition temperature of 300 °C. To perform this evaluation, the coated alloys were immersed in a simulated body fluid solution at body temperature (37 ± 0.5 °C) to determine the corrosion resistance through electrochemical and immersion tests. Moreover, the investigation also consisted of the evaluation of microchemical, mechanical, and morphological properties. The deposition temperature of 300 °C was enough to obtain a crystalline hydroxyapatite structure with a Ca/P ratio close to the stochiometric one. The adhesion of coatings was not influenced by the nature of Mg–Zn–Ag alloys, so similar values for both coated alloys were found. The results showed that the coating was homogonous deposited on the Mg–Zn–Ag alloys and the corrosion resistance of uncoated magnesium alloys was improved.     

Corrosion behaviour of epoxy coatings electrodeposited on galvanized steel and steel modified by Zn–Ni alloys

The electrochemical and transport properties and thermal stability of epoxy coatings electrodeposited on hot-dip galvanized steel and steel modified by Zn–Ni alloys were investigated during exposure to 3% NaCl solution. Zn–Ni alloys were electrodeposited on steel by direct and pulse current. From the time dependence of pore resistance, coating capacitance and relative permittivity of epoxy coating, diffusion coefficient of water through epoxy coating, D(H₂O) and thermal stability, it was shown that Zn–Ni sublayers significantly improve the corrosion stability of the protective system based on epoxy coating. Almost unchanged values of pore resistance were obtained over the long period of investigated time for epoxy coatings on steel modified by Zn–Ni alloys, indicating the great stability of these protective systems, due to the existence of the inner oxide phase layer and the outer layer consisting of basic salts.     

Effect of vanadium additive and phosphating time on anticorrosion,morphology and surface properties of ambient temperature zinc phosphate conversion coatings on mild steel

An attempt has been made to investigate the effect of phosphating time and vanadium additive on the anticorrosion and surface properties of ambient temperature zinc phosphate coatings. Zinc phosphate coatings with different phosphating times and vanadium concentrations were applied to low carbon steel samples. A potentiostatic polarization test in 3.5 wt% NaCl solution was carried out to investigate the electrochemical properties of coated samples. Field emission scanning electron microscopy, energy-dispersive spectroscopy, and atomic force microscopy were utilized to evaluate the microstructure, chemistry and roughness of coatings. Surface properties such as wettability, surface tension, and work of adhesion were measured. Results indicate that the sample which was immersed for 30 min in the phosphating bath exhibits the lowest corrosion current density, one tenth of bare steel, due to formation of a compact coating while having a low number of microcracks. Addition of 500 ppm vanadium to the coating in a secondary bath decreases the corrosion rate of zinc phosphate coating remarkably, by almost 80%. Microstructural results reveal that vanadium-rich precipitates are formed and enhance the coating coverage on the steel substrate. Vanadium addition increases the surface roughness, surface free energy, and work of adhesion of the phosphate coating.     

表面粗糙度对涂装铝合金应力腐蚀敏感性的影响

在粗糙度不同的A7N01S-T5铝合金表面上进行涂装处理后,再通过慢应变拉伸试验来获取该铝合金在3.5%Na Cl溶液中的应力-应变曲线,解析这些曲线后得到了表征铝合金应力腐蚀开裂(SCC)敏感性的特性指数,并由此得到了粗糙度对涂装A7N01S-T5铝合金SCC敏感性影响的作用规律。     

Polypyrrole films modified with graphite layer on mild steel

Electrochemically synthesized polypyrrole coating was modified with very thin graphite layer and top coated with another polypyrrole film. The corrosion behaviour of this coating has been investigated in aqueous sodium chloride solution. The synthesis of polypyrrole coatings was carried out by cyclic voltammetry technique, from aqueous oxalic acid solution. Electrochemical impedance spectroscopy and potentiodynamic measurements were used for corrosion tests. The cyclic voltammograms obtained in oxalic acid solution and the polarisation curves obtained in sodium chloride solution showed that the stability of coating was improved significantly by graphite layer. The impedance spectra also showed that the corrosion process was controlled by the diffusion rate along the coating, even after 96 h immersion period. The Warburg coefficient values were calculated and used to evaluate the barrier property of coating with time. It was shown that the water up taking process was slowed down by the hydrophobic nature of the graphite layer sandwiched between the two polypyrrole films.     

Relaxation of the axial strain induced stresses in double–coated optical fibers

The axial strain induced stresses in double‐coated optical fibers are analyzed by the viscoelastic theory. A closed form solution of the axial strain induced viscoelastic stresses is obtained. The viscoelastic stresses are a function of the radii, Young's moduli, relaxation times and Poisson's ratios of the polymeric coatings. If the applied axial strain linearly increases, the induced stresses increase with the time. On the other hand, if the axial strain is fixed, besides the axial stress in the glass fiber, the stresses exponentially decrease with the time. The relaxation of stresses is strongly dependent on the relaxation times of the polymeric coatings. If the relaxation time of the polymeric coating is very long, the viscous behavior of the polymeric coatings will not appear, and the axial strain induced stresses solved by the viscoelastic theory are the same as those solved by the elastic theory. On the other hand, if the relaxation time of the polymeric coating is very short, the relaxation of stresses is very apparent. A compressive radial stress at the interface of the glass fiber and primary coating will result in an increase of the transmission losses, and a tensile interfacial radial stress will possibly produce debonding at the interface of the glass fiber and primary coating. To minimize this interfacial radial stress, the radius, Young's modulus and Poisson's ratio of the polymeric coatings should be appropriately selected, and the relaxation time of the primary coating should be shortened. Finally, the stresses in single‐coated and double‐coated optical fibers are discussed.     

Corrosion protection performance of nanocomposite coatings under static,UV, and dynamic conditions

Silicone-modified epoxy polymeric matrix was successfully fabricated and reinforced with 1–2 wt% SiO₂, TiO₂, and TiSiO₄ nanoparticles. Fourier-transform infrared spectroscopy, contact angle measurements, differential scanning calorimetry, and field-emission scanning electron microscopy together with energy-dispersive X-ray spectroscopy were employed to investigate different characteristics of the prepared coatings. To simulate operating conditions, all samples were characterized via electrochemical impedance spectroscopy (EIS) after being subjected to different conditions. Corrosion under static conditions, in which the samples were exposed to a static electrolyte without further change in other parameters, was investigated. Furthermore, to study the effects of ultraviolet (UV) radiation in accelerating the degradation of the coatings, samples were characterized after being subjected to UV while immersed statically in the electrolyte. Additionally, the corrosion protection performance was investigated after subjecting the coated substrates to dynamic conditions involving continuous movement of the sample in the electrolyte, simulating continuous wear of the coated surfaces. Compared with the static condition, the EIS results revealed the vital role of the silicone resin and nanoparticles in improving the stability of the coating film against corrosion degradation in the presence of UV radiation, while poor performance in dynamic condition was recorded for all the coating systems.     

In-vitro corrosion inhibition mechanism of fluorine-doped hydroxyapatite and brushite coated Mg–Ca alloys for biomedical applications

Magnesium alloys have received great attention as a new kind of biodegradable metallic biomaterials. However, they suffer from poor corrosion resistance. In this study, Mg–Ca alloy was coated with nano-fluorine-doped hydroxyapatite (FHA), and brushite (DCPD); via electrochemical deposition (ED). Coatings were characterized by X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The results revealed that nano-fluorine-doped hydroxyapatite coating produced more dense and uniform coating layer, compared to the brushite coating. The compression tests of the ED-coated Mg alloy samples immersed in simulated body fluid for different time periods showed higher yield strength (YS) and ultimate tensile strength (UTS), compared to those of the uncoated samples. The degradation behavior and corrosion properties of the ED-coated Mg alloy samples were examined via electrochemical measurements and immersion tests. The results showed that FHA coating could effectively induce the precipitation of more Ca²⁺ and PO₄³⁻ ions than DCPD coating, because the nanophase can provide higher specific surface area. It was also found that FHA and DCPD coatings can significantly decline the initial degradation rate of the alloy. A corrosion mechanism of the ED-coated alloy is proposed and discussed in this paper.     

Experimental and numerical life evaluation of TBCs with different BC and diffusion coating under cyclic thermal loading

This paper experimentally and numerically investigates the thermally grown oxide (TGO), lifetime, and stress values in thermal barrier coatings with different bond coat (BC), without top coat (TC), and diffusion coating under cyclic thermal loading. Scanning electron microscope (SEM) analysis shows that the atmospheric plasma spraying (APS) and high-velocity oxygen fuel (HVOF) fabricated sample has the highest and lowest TGO thickness and growth rate, respectively. The new coating with two BCs has a maximum lifetime of 102 cycles. After that, the lifetime of the coatings with HVOF-BC, diffusion coating, and APS-BC reach 84, 56, and 44 cycles, respectively. The diffusion coating does not have much effect on the TGO thickness; however, it delays the Al interdiffusion to the substrate. In the sample without the TC layer, oxygen contact with the BC layer has increased, leading to a rise in the BC oxidation rate. The numerical analysis of the stress values based on SEM images shows that the more intense TGO layer growth in APS coating caused an increase in TC layer stress values. Furthermore, the results show that the new coating with two-layer BC has the lowest stress values. The TC absence causes the loss of compressive stresses caused by TC on TGO and increases the tensile stress values in this layer.     

A study on the anticorrosion performance of the epoxy-polyamide nanocomposites containing ZnO nanoparticles

Epoxy nanocomposites were prepared using different loadings (2, 3.5, 5 and 6.5 wt%) of ZnO nanoparticles. Nanocomposites were applied on steel substrates. Samples were immersed in 3.5 wt% NaCl solution for 1344 h. Corrosion resistance of the coatings was studied by an electrochemical impedance spectroscopy (EIS). The effects of addition of nanoparticles on the mechanical properties of the epoxy coating were studied by a dynamic mechanical thermal analysis (DMTA). Curing behavior of the coatings containing nanoparticles was studied by a differential scanning calorimeter (DSC). Atomic force microscope (AFM) was utilized to investigate the surface topography and surface morphology of the coatings. Coating resistance against hydrolytic degradation was studied by FTIR (Fourier Transform Infrared).Results showed that addition of low loadings of nanoparticles can increase T_g of the composite. Decrease in T_g and cross-linking density of the coating were observed at high loadings of nanoparticles. It was found that nanoparticles can influence the curing behavior of the epoxy coating. Nanoparticles improved the corrosion resistance of the epoxy coating. Increase in coating resistance against hydrolytic degradation was obtained using nanoparticles.     

Wet chemical deposition of BN,SiC and Si3N4 interphases on SiC fibers

Fiber coatings based on BN, BN/SiC and BN/Si₃N₄ were deposited on Hi Nicalon type S SiC fibers. The coating parameters were optimized using a design of experiments study. With optimized parameter sets, the coatings exhibited a high degree of coverage on the fibers and almost no fiber bridging could be observed. The coated fiber bundles are flexible and can be processed further by techniques such as filament winding. In comparison to a non-processed reference sample, the maximum tensile load of the fiber bundles with BN, BN/SiC and BN/Si₃N₄ coatings was reduced by only 5 %, 13 % and 10 %, respectively. The coated fiber bundles retained their tensile strength after thermal annealing up to 1650 °C in a nitrogen atmosphere for 0.5 h. SiC_f/SiC samples with BN/SiC fiber coatings exhibited higher values of bending strength and strain-to-failure as a reference sample without fiber coating indicating the functionality of the fiber coatings.     

陶瓷涂层三明治板的抗热震性

采用解析法研究了第3类边界条件下双面陶瓷涂层三明治板的瞬态温度场及瞬态热应力场.对不同Biot模数的热冲击过程中,Al2O3涂层/硬质合金(WC-8%Co,质量分数)基体/Al2O3涂层三明治板的瞬态热应力进行了数值计算.分析了涂层/基体厚度比、涂层与基体热-物理性能匹配对陶瓷涂层三明治板表面热应力峰值的影响.结果表明:陶瓷涂层三明治板的基体的热导率、线膨胀系数和弹性模量应高于涂层,这样可以降低其表面热应力,获得高抗热震性陶瓷涂层三明治板.此外,涂层厚度应尽可能小,以利于改善涂层的抗热震性.     

Al diffusion coating on Mg alloy by a surface nanocrystallization enhanced CVD process

Surface nanocrystallization by mechanical attrition was used to enhance the chemical vapor deposition process. An aluminum (Al) diffusion coating was produced on AZ91 Mg alloy surface. This process was conducted at a relatively low temperature (400°C) for a short time of 120 min. The results indicated that a continuous and dense Mg₁₇Al₁₂ intermetallic coating with a thickness of ～8 μm formed on the Mg alloy substrate. Almost no corrosion was observed after the coated samples were immersed in 3 wt % NaCl solution for 6 h, reflecting a relatively good corrosion resistance. The formation mechanism of the Al diffusion coating is discussed based on the experimental results.     

In Vitro Corrosion and Cytocompatibility of ZK60 Magnesium Alloy Coated with Hydroxyapatite by a Simple Chemical Conversion Process for Orthopedic Applications

Magnesium and its alloys—a new class of degradable metallic biomaterials—are being increasingly investigated as a promising alternative for medical implant and device applications due to their advantageous mechanical and biological properties. However, the high corrosion rate in physiological environments prevents the clinical application of Mg-based materials. Therefore, the objective of this study was to develop a hydroxyapatite (HA) coating on ZK60 magnesium alloy substrates to mediate the rapid degradation of Mg while improving its cytocompatibility for orthopedic applications. A simple chemical conversion process was applied to prepare HA coating on ZK60 magnesium alloy. Surface morphology, elemental compositions, and crystal structures were characterized using scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction, respectively. The corrosion properties of samples were investigated by immersion test and electrochemical test. Murine fibroblast L-929 cells were harvested and cultured with coated and non-coated ZK60 samples to determine cytocompatibility. The degradation results suggested that the HA coatings decreased the degradation of ZK60 alloy. No significant deterioration in compression strength was observed for all the uncoated and coated samples after 2 and 4 weeks’ immersion in simulated body fluid (SBF). Cytotoxicity test indicated that the coatings, especially HA coating, improved cytocompatibility of ZK60 alloy for L929 cells.     

Application of impedance imaging to evaluation of organic coating degradation at a local scale

A new approach for evaluating the local electrical properties of organic coatings has been proposed. It was performed with contact mode atomic force microscopy (AFM). A current response signal was measured when a single-frequency voltage perturbation was applied between AFM tip and coated metal. Suitability of the proposed technique has been presented on the representative acrylic coating degraded by two different factors—UV radiation and electrolyte exposure. It was possible to disclose the early stages of the coating deterioration, to spatially localize defective regions, and to observe distinctly different modes of the coating degradation as a result of exposure to various degradation factors.     

Corrosion and wear behavior of alumina‐polyester nanocomposite coatings

The corrosion and wear behavior of powder coatings fabricated by the electrostatic method was investigated in this study. Pure polyester coating and fabricated nanocomposite powder coating with 10 and 20 mass.% alumina nanoparticles were coated with electrostatic spraying method on the surfaces of carbon steel substrate. Coatings were cured in two regimes by oven and microwave for the appropriate time. The effects of alumina nanoparticles on the corrosion resistance of coated samples were studied by immersion and electrochemical impedance spectroscopy (EIS) tests. Also, pin‐on‐disk test was applied to evaluate the wear properties and coefficient of friction (COF) of the coatings. The results of the corrosion test reveal that the samples with 10 mass.% alumina show the best corrosion resistance and cause a reduction in corrosion rates which is about 36 times to that of the pure sample. The wear rate of nanocomposite coatings is 10 times lower than that of pure ones and also the coefficient of friction of nanocomposite samples is almost half of the pure samples. Furthermore, the nanocomposite coatings cured in the microwave show better protection properties and wear resistance than that of ones cured in an oven. POLYM. ENG. SCI., 57:846–856, 2017. © 2016 Society of Plastics Engineers     

Evaluation of sub-micrometer parylene C films as an insulation layer using electrochemical impedance spectroscopy

We investigated the insulation performance of sub-micrometer parylene C films over time using electrochemical impedance spectroscopy (EIS). For this, interdigitated electrodes were fabricated and completely encapsulated with parylene C in thicknesses of 50, 100, 200, and 500 nm. The EIS was measured in phosphate buffered saline (PBS) solution under an accelerated aging condition at 90 °C over 45 days. To analyze the EIS data, the equivalent circuit models of coating at different stages of coating degradation were used and the lumped circuit parameters of the best fitted equivalent circuit model were extracted by curve fitting. The analysis of impedance using the equivalent circuit model and the FTIR measurements suggest that sub-micrometer parylene C coatings exhibited delamination resulting from water diffusion from the top surface as soon as being immersed in PBS solution, although the degree of delamination varied depending on the film thickness. The penetration of water through sub-micrometers thick parylene C films can occur as quickly as the film is in contact with solution, unlike for thicker coatings in several micrometers where water diffusion would be saturated before water reaches the bottom surface of the coating.