Effect of mechanical stresses on marine organic coating ageing approached by EIS measurements

In this work, the role of a stress-strain state (visco-elastic domain) on the protective properties of two marine epoxy coatings (with and without VOC) applied onto mild steel was studied. Different stress values were applied on coated substrates and bent samples were immersed in 3 wt.% NaCl solution at different temperatures. Non-bent coated samples were also immersed in the same conditions as references. Electrochemical Impedance Spectroscopy was used to evaluate the organic coating degradation on the compressed and the stretched sides periodically.The degradation kinetics showed that the tensile mode was very damaging for one coating while a slight effect was observed on the other coating. In the first case, the water uptake was found to be more important in the tensile mode for higher stress values. A particular attention was focussed on the initial relative permittivity which appeared as a thermo-activated function of the absolute value of the applied stress, for both coatings. Using a thermodynamic approach, the influence of the enthalpic and entropic part of the permittivity was discussed. The diffusion coefficient of the solution into the coating was also measured. The results showed that the diffusion coefficient is strongly modified by the mechanical stress but different behaviours were obtained with both coatings. It is proposed that the entropic contribution plays a major role on the modification of this coefficient.     

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).     

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.     

Effects of phase separation on stress development in polymeric coatings

A cantilever deflection technique was used to monitor stress in situ during drying of cellulose acetate coatings. Porosity was introduced in some coatings using dry-cast phase separation. Stress and weight loss profiles for dense coatings, a coating that contained small (∼1 μm) pores, and a coating that contained small (∼1 μm) pores and macrovoids (∼200 μm) are compared. In-plane tensile stress after drying ranged from 30 MPa (dense coatings) to 5 MPa (macrovoid-containing coating). The stress profiles for dense coatings feature a period of rapidly and then slowly increasing stress due to constrained shrinkage. For a coating that formed small pores, drying and stress development are delayed, stress rises and then drops a small amount due to capillary pressure relief. The stress profiles for the small pore and macrovoid-containing coatings are similar, except for a stress plateau at early stages of drying, which may be caused by macrovoid growth.     

Effect of nano-ZnO particles on the corrosion resistance of polyurethane-based waterborne coatings immersed in sodium chloride solution via EIS technique

Nano-composite coatings were formed by incorporating 3 wt% nano-ZnO in a polyurethane-based waterborne coating. The nano-ZnO based composite coatings were applied on standard phosphated steel panels by cathodic electrodeposition. The electrodeposited nano-composite coatings were then baked for 20 min at 165 °C. To investigate the corrosion resistance of the coatings, the coated panels were immersed in 3.5 wt% NaCl solutions for 2880 h (120 days). The improvement in corrosion performance of the composite coatings was evaluated using electrochemical impedance spectroscopy technique. It was found that the films containing nano-sized ZnO particles show a corrosion resistance of 2 orders of magnitude higher than that of the neat films.     

The investigation of the tribocorrosion properties of DLC coatings deposited on Ti6Al4V alloys by CFUBMS

Various machine components produced from titanium alloys used in various industries are subject to a combination of electrochemical and mechanical effects. The science of surface transformations resulting from the interaction of mechanical loading and chemical reactions that occur between elements of a tribosystem exposed to corrosive environments is described as tribocorrosion. This research focuses on the tribocorrosion behaviour of Ti6Al4V alloys after coated by using closed field unbalance magnetron sputtering (CFUBMS). The structural analyses of the coatings were performed using Raman spectroscopy and scanning electron microscopy (SEM). Tribocorrosion experiments were performed in a pin-on-disc tribotester under electrochemical polarisation in NaCl 1 wt.% solution. This study shows that the Ti-DLC coating is protecting the Ti6Al4V alloy and having good performance in corrosion and tribocorrosion conditions. The OCP values for Ti6Al4V substrate and Ti-DLC protective coatings during tribocorrosion tests were measured as −560 V and −330 V, respectively. These results showed that Ti-DLC protective coating on Ti6Al4V substrates increased the tribocorrosion resistance by acting as a barrier layer.     

An optimized strategy toward multilayer ablation coating for SiC-coated carbon/carbon composites based on experiment and simulation

Carbon/carbon (C/C) composites are widely used as thermal protection systems for atmospheric re-entry, where they are subjected to strong oxidation and mechanical denudation. Sublayer thickness of multilayer coating has considerable influence on its stress, which further governs their service life in critical environments. In this study, a multilayer coating with different sublayer thicknesses was fabricated on SiC-coated C/C composites using plasma spraying. Prior to the fabrication, finite element analysis (FEA) was firstly established to investigate the relationship between sublayer thickness and thermal stress. Thereafter, the coatings with typical sublayer thickness were verified through practical experiments. Raman spectra and ablated appearances showed well coincidence with the FEA results, pointing out close relationship among sublayer thickness, residual stress and ablation behavior. After testing for 90 s, the sample with optimized thickness owned the least stress (294 MPa) and lowest ablation rates (?0.467 µm/s and ?0.343 mg/s) as compared to other coated ones.     

A hot-pressing reaction technique for SiC coating of carbon/carbon composites

A hot-pressing reactive sintering (HPRS) technique was explored to prepare SiC coating for protecting carbon/carbon (C/C) composites against oxidation. The microstructures of the coatings were analyzed by X-ray diffraction and scanning electron microscopy. The results show that, SiC coating obtained by HPRS has a dense and crack-free structure, and the coated C/C lost mass by only 1.84 wt.% after thermal cycles between 1773 K and room temperature for 15 times. The flexural strength of the HPRS-SiC coated C/C is up to 140 MPa, higher than those of the bare C/C and the C/C with a SiC coating by pressure-less reactive sintering. The fracture mode of the C/C composites changes from a pseudo-plastic behavior to a brittle one after being coated with a HPRS-SiC coating.     

UV-cured clay/based nanocomposite topcoats for wood furniture. Part II: Dynamic viscoelastic behavior and effect of relative humidity on the mechanical properties

Topcoat constituting multi-layer coatings for wood furniture used in high humidity environments, like bathrooms, must have not only good barrier properties, but also good mechanical properties. Three different types of commercial organoclays, namely Cloisite 10A (C10A), Cloisite 15A (C15A) and Cloisite 30B (C30B), were chosen in this study as reinforcing agents. These nanoparticles were dispersed (1 and 3 wt% into the formulation) into a commercial epoxy acrylate oligomer by means of a three roll mill. Samples obtained from free standing UV-cured coatings were used for mechanical assessments. Mechanical tests were performed in both dynamic and static mode in order to investigate the viscoelastic behavior and tensile properties of coatings. Results from dynamic mechanical analysis have shown that all nanocomposite coatings have higher (72–75 °C) glass transition temperature compared to that observed (71 °C) in unreinforced coatings. The restriction of polymer chains mobility, due to the presence of layered silicate nanoparticles, has been used to explain the increase of glass transition temperature related to the decrease of the free volume. The storage modulus for nanocomposites containing 3 wt% of C10A, C15A and C30B was found to be slightly higher than that observed in pure coatings. The analysis of tensile stress–strain curves has revealed that tensile properties are affected by relative humidity (RH) due to the plasticization effect of humidity. In fact, results have shown that regardless of the organoclay type, the increase of RH decreases both Young's modulus and tensile strength while increasing maximum strain. We believe that low interfaces between photocrosslinked polymer chains and organoclays explain the lack of any effect of organoclays on both storage and Young's moduli. Among samples from each type of UV-cured coating tested at 0, 20 and 80% of RH, regardless of the organoclay type and content, only samples tested (tensile tests in static mode) at RH = 80% were broken. SEM images obtained from the fractured surface of these samples have shown that unreinforced UV-cured coatings and nanocomposite coatings are respectively characterized by smooth and rough fracture surface.     

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.     

Investigation of the corrosion protection of SiOx-like oxide films deposited by plasma-enhanced chemical vapor deposition onto carbon steel

The paper reports on the corrosion behavior of carbon steel coated with thin SiO_x-like oxide films. The SiO_x-like coatings were deposited by plasma-enhanced chemical vapor deposition (PECVD) and their thickness was varied between 20 and 200 nm. The coated carbon steel interfaces were investigated for their corrosion protection efficiency when immersed in an aqueous saline solution of 3% NaCl. FTIR measurements and electrochemical impedance spectroscopy (EIS) experiments revealed that thin SiO_x-like coating layers (20 nm thick) do not prevent the carbon steel from corrosion, while thicker silica layers (d ≥ 100 nm) protect efficiently carbon steel interfaces in highly saline media with a protection efficiency of about 96% for a 200 nm thick coating.     

Mechanical properties of copper-coated carbon foams

The mechanical properties of copper-coated carbon foam were investigated. Reticulated vitreous carbon cell type foams, with 97% porosity and 10 ppi pore size, were electroplated with copper for different periods of time to obtain coatings with different thicknesses and foams with different porosities. Compression tests were performed to determine the Young’s modulus and the plateau stress. The copper electroplating technique improved these two properties, with the modulus increasing from 4.5 to 8.6 MPa for the sample electroplated for 40 min and the plateau stress increasing from 54 to 171 kPa for the foam coated for 80 min. The relationships between the measured properties and the copper weight ratio were determined.     

Hard coatings on elastomers for reduced permeability and increased wear resistance

Abstract

In this study, three different elastomers, namely hydrogenated nitrile butadiene rubber, fluoroelastomer and silicone, have been subjected to two different hard metallised coatings by ion implantation process. The three different elastomers are commonly used in various seal applications, where reduced wear and gas permeability are essential in maintaining seal performance and functionality. Samples of these rubbers have been coated with chromium coating in one set of tests. In the second set of tests, samples of elastomers have been coated with tungsten carbide coating being deposited on all the three different elastomers. Wear, gas permeability and mechanical behaviour of the coated samples were compared with each other and with the control uncoated elastomers. All the coated samples showed good reduction in gas permeability. With the use of metallised coatings, there has been improved resistance to wear in all the coated samples. Adhesion strength and effect of coating on the elastomer have been investigated by mechanical testing. Mechanical tests revealed good adhesion of metal coatings on all the rubber samples, and there was no detrimental effect on the mechanical properties after coating.     

Damage accumulation in plasma-sprayed zirconia under cyclic loading

The stiffness and hysteretic response due to mechanical and thermo-mechanical cycling have been studied in plasma-sprayed yttria-stabilized zirconia (YSZ). Mechanical cycling of free-standing cantilevers of YSZ shows that the progressive decrease in stiffness is accompanied by monotonic increase in hysteretic energy dissipation per cycle and a permanent ratcheting displacement of ∼20 nm/cycle. Below a critical stress, it varies from coating to coating, ratcheting accompanied by a slow decrease in stiffness does not lead to failure even after ∼1000 cycles. In contrast, at higher stresses, the rates of ratcheting and decrease in stiffness increase rapidly, leading to the nucleation of macrocracks that lead to fracture failure. Prior thermal cycling of coatings on Inconel substrate up to 700°C, which induced an estimated cyclic thermal stress of ∼35 MPa, led to a pronounced reduction in stiffness and mechanical cycling life. During bending, damage accumulates in the tension side of the cantilever and the volume going through the compression cycle remains relatively undamaged.     

Preparation and characterization of a greenish yellow lackluster coating with low infrared emissivity based on Prussian blue modified aluminum

Greenish yellow lackluster coatings with low infrared emissivity were prepared by Prussian blue (PB) surface modified Al powders and polyurethanes. The morphology and component of PB/Al powder were characterized by scanning electron microscopy and X-ray diffractometer. The infrared emissivity, surface gloss and visible light color of PB/Al composite coating were investigated by an infrared emissometer, a glossmeter and a colorimeter, respectively. Mechanical properties of PB/Al composite coatings were studied by using adhesion test and impact strength test. The results indicate that PB/Al powder decreases not only the gloss of the coating, but also its emissivity within the wavelength range of 8–14 μm. The composite coatings have good adherence and impact strength at PB/Al content below 50 wt.%, and then the mechanical properties decrease in the PB/Al content range from 50 wt.% to 60 wt.%. By comparing PB/Al composite coating and Al powder tinting coating with the same color and surface gloss, PB/Al composite coating exhibits significant lower infrared emissivity, which is attributed to closer inter-powder distances of metallic fillers and higher electrical conductivity in the coating.     

Mechanism of corrosion protection of aluminum alloy substrate by hybrid polymer nanocomposite coatings

The corrosion protection of polymer clay nanocomposite, PCN coatings consisting of polyurea, siloxanes, epoxy ester and montmorillonite clay was determined. Corrosion resistance of the coating, was assessed by monitoring the polarization resistance and impedance of coated aluminum alloy, Al 2024-T3, coupons immersed in 3.5 wt.% of sodium chloride, NaCl, solution. Direct current polarization and electrochemical impedance spectroscopic techniques were used to measure polarization resistance and impedance of the samples, respectively. Diffusion of saturated salt solution into free-standing PCN films was measured gravimetrically and diffusivity of the nanocomposites was determined. The presence of clay decreases diffusivity and increases corrosion resistance of the non-scribed coatings containing up to 10 wt.% of clay. A correlation between polarization resistance and diffusivity was made. It was shown that for non-scribed coatings, polarization resistance increases with decreasing diffusivity. A relationship between coating's diffusivity and weight fraction of clay was established. Increasing clay concentration also resulted in decreasing diffusivity. The scribed nanocomposite coatings show slightly decreasing polarization resistance with increasing weight fraction, however, the polarization resistance of scribed coatings containing low clay weight fraction in the range between 0.5 and 2.0 wt.% was higher than that for the matrix. A barrier mechanism of corrosion prevention of the coated substrate is proposed for non-scribed coatings. The viscoelastic property of the nanocomposites was determined by using dynamic mechanical spectrometer. A correlation between polarization resistance of the coatings and the rubbery plateau modulus on the one hand and polarization resistance and tan δ peak area for α-transition of the nanocomposites is made. Decreasing tan δ peak area for α-transition and increasing rubbery plateau modulus resulted in increasing coatings polarization resistance.     

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

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

Preparation of PANI/epoxy/Zn nanocomposite using Zn nanoparticles and epoxy resin as additives and investigation of its corrosion protection behavior on iron

Conducting polyaniline, zinc and epoxy resin solely have anticorrosive properties by different mechanisms on metallic substrates. In this work the triple hybrid of PANI/epoxy/Zn nanocomposite was prepared as a thin layer coating (70 ± 5 μm) on iron coupons and its anticorrosion performance was investigated in HCl (0.1 M) as corrosive solution. Epoxy resin and zinc nanoparticles were applied as additives in the PANI matrix to improve the mechanical properties of PANI coating and investigate their synergetic effects on the anticorrosion performance of PANI coating. At first PANI/Zn nanocomposite coatings with different Zn contents were prepared and the zinc content optimized so that the coating achieve the best anticorrosion performance. Accordingly the iron coupons coated by PANI/Zn coating having 4 wt% Zn content showed more noble open circuit potential and lower corrosion current values. Then epoxy resin was applied as additive to the optimized formulation of PANI/Zn coating in different weight percents (0–20 wt%) and the anticorrosion performance of the related PANI/epoxy/Zn triple hybrid nanocomposite coatings was evaluated. Results showed that the addition of epoxy resin causes to the decreasing of corrosion current of iron samples coated by PANI/epoxy/Zn nanocomposite. An optimum range of 3–7 wt% was obtained for the epoxy content in the composition of PANI/epoxy/Zn nanocomposite in which the coating exhibits the best anticorrosion performance. Iron metal coupon was elementally analyzed and the PANI/Zn and PANI/epoxy/Zn nanocomposites were characterized using Fourier Transform Infrared spectroscopy, X-ray diffraction patterns and Scanning Electron Microscopy techniques.     

Microstructure and thermo-physical properties of yttria stabilized zirconia coatings with CMAS deposits

Yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) are used to protect hot-components in aero-engines from hot gases. In this paper, the microstructure and thermo-physical and mechanical properties of plasma sprayed YSZ coatings under the condition of calcium-magnesium-alumina-silicate (CMAS) deposits were investigated. Si and Ca in the CMAS rapidly penetrated the coating at 1250 °C and accelerated sintering of the coating. At the interface between the CMAS and YSZ coating, the YSZ coating was partially dissolved in the CMAS, inducing the phase transformation from tetragonal phase to monoclinic phase. Also, the porosity of the coating was reduced from ∼25% to 5%. As a result, the thermal diffusivity at 1200 °C increased from 0.3 mm²/s to 0.7 mm²/s, suggesting a significant degradation in the thermal barrier effect. Also, the coating showed a ∼40% increase in the microhardness. The degradation mechanism of TBC induced by CMAS was discussed.     

Sintering resistance of suspension plasma sprayed 7YSZ TBC under isothermal and cyclic oxidation

This study examines sintering resistance of a thermal barrier coating (TBC), composed of a 7YSZ suspension plasma sprayed (SPS) top coat (TC), an air plasma sprayed (APS) NiCoCrAl bond coat (BC), and an INCONEL 625 substrate, under isothermal and cyclic conditions with a peak temperature of 1080 °C for 400, 800, and 1300 h/cycles. Microstructure, phase composition and microstrain were examined using SEM and XRD. Mechanical properties of fracture toughness, hardness and elastic modulus were obtained using nano-indentation. Samples under cyclic conditions presented faster sintering rate than under isothermal condition due to larger compressive strain and frequent heating and cooling cycles. Faster degradation of mechanical properties due to sintering leads to shorter lifetime of SPS coating under cyclic conditions. Moreover, vertical cracks within SPS coatings reduces compressive stress leading to a greater lifetime as compared to APS coatings exposed to similar conditions.