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.     

Effect of curing time on the physicomechanical characteristics of the hardened cement pastes containing limestone

The effect of curing time on the physico-mechanical properties of the hardened Portland cement pastes containing limestone was studied. Five cement-limestone blends were prepared using 0%, 5%, 10%, 15%, and 20% of limestone as a partial substituent of Portland cement. The cement pastes were prepared using the standard water of consistency of 0.255, 0.255, 0.258, 0.261, and 0.263, respectively. The fresh pastes, thus produced, were moulded into 2×2×2-cm cubes. The pastes were first cured within the moulds at 100% relative humidity for 24 h, then the specimens were demoulded and cured under tap water for 3, 7, 14, and 28 days. At each hydration age, the hardened pastes were tested for bulk density, compressive strength, differential scanning calorimetery (DSC), and X-ray diffraction analysis (XRD). The results obtained were related as much as possible to the mechanical properties of the hardened cement pastes. The inclusion of limestone results in a notable improvement of the mechanical properties of the cement pastes containing limestone.     

Effect of Yb2SiO5 addition on the physical and mechanical properties of sintered mullite ceramic as an environmental barrier coating material

In this study, ytterbium monosilicate (Yb₂SiO₅)-added sintered mullite ceramics are prepared as candidate materials for environmental barrier coatings (EBCs). The effect of adding Yb₂SiO₅ on the physical and mechanical properties of the sintered mullite ceramics is investigated. The Yb₂O₃–SiO₂–Al₂O₃ ternary phase diagram indicates that adding Yb₂SiO₅ to the mullite goes beyond simply mixing; instead, liquid sintering occurs. Therefore, when we add Yb₂SiO₅ to the mullite, the sintered body possesses a denser microstructure and faster densification rate than does pure mullite. The density rapidly increases with the addition of 6 wt% Yb₂SiO₅ in the mullite, and almost full densifications are achieved with the addition of 12 wt% and 18 wt% Yb₂SiO₅. In this study, mullite ceramic that contains 12 wt% Yb₂SiO₅ exhibits the smallest plastic deformation and the highest elastic modulus among ceramics containing 6, 12, and 18 wt% Yb₂SiO₅, according to Hertzian indentation results. The results suggest that 12 wt% Yb₂SiO₅-doped mullite may be expected to act as a potential EBC material based on its excellent elastic properties, dense microstructure, and appropriate coefficient of thermal expansion.     

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).     

Resistance of metallic substrates protected by an organic coating containing aluminum powder

The corrosion behavior of an epoxy primer containing aluminum powder (10 vol.%) applied on carbon steel and on galvanized steel was examined by electrochemical impedance spectroscopy (EIS). The data show that this coating is more protective when applied onto carbon steel substrates, and that on galvanized steel thicker coatings allow to achieve similar protection levels as those obtained for carbon steel. These effects are probably due to aluminum pigments providing a cathodic protection of the substrate, and to the resulting products precipitating inside the pores of the polymeric coating. Three stages can be distinguished during exposure of the coated specimens. Upon immersion of the coated samples in the test solution, a pre-saturated stage is observed. After a certain period of immersion, which strongly depends on the thickness of the applied coating, a saturation stage is reached in which an effective protection of the metallic substrate against corrosion is achieved. Finally, at sufficiently long exposure times, swelling through the coating eventually leads to the detachment of the coating.     

The effect of micro and nano-sized particles on mechanical and adhesion properties of a clear polyester powder coating

In this study, the effect of various micro and nano-sized particles on the mechanical and adhesion properties of a commercial non-pigmented polyester-TGIC powder coating was studied. Different coating formulations containing various wt.% of aluminum hydroxide (alumina) particles, TiO₂ and two different types of fumed silica nanoparticles were prepared via a two-stage process. Tensile strength measurements, DMTA analyses and vertical pull off adhesion test were conducted to evaluate mechanical and adhesion properties of the powder coating samples.     

Failure process of acrylic polyurethane coating under alternate wetting and drying condition

The failure process of an acrylic polyurethane coating in simulated sea water under a wet‐dry cyclic condition and immersion condition was studied with the methods of electrochemical impedance spectroscopy (EIS), Fourier transform infrared spectroscopy (FT‐IR), and scanning electron microscope (SEM). The results show that the failure rate of acrylic polyurethane coating under the condition of wet‐dry alternation is obviously greater than that under complete immersion. Under wet‐dry condition, both the porosity and the water absorptivity of the coating are also greater than those under complete immersion condition. Owing to the physical effect of wet‐dry alternation, the fillers in the coating surface layer may fall off and result in micro‐pores, which could multiply the defects in the coating and accelerate the coating degradation. FT‐IR analysis shows that the isocyanate group in the coating not only participates in the curing of coating, but also hydrolyzes with water molecule. The C‐O bonds fracture partly due to hydrolysis of the main molecule chains, which is one reason of the coating failure. The failure process for acrylic polyurethane coating under a wet‐dry cyclic condition is discussed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41989.     

In situ measurement of water at the organic coating/substrate interface

In situ and quantitative information on the water layer at the organic coating/substrate interface is crucial for understanding and preventing the failure of organic coating systems. A technique, based on a two-layer model derived rigorously from internal reflection theory, has been developed for measuring in situ the thickness and amount of the water layer at the organic coating/substrate interface. The technique gives new insight into the processes by which water degrades the coating/substrate bonds. In this technique, a transparent or an opaque organic coating of sufficient thickness is applied to an internal reflection element (IRE) with or without a thin metallic film, which is used as the substrate. A water chamber is attached to the organic-coated specimen. After adding water to the chamber, Fourier transform infrared-multiple internal reflection (FTIR-MIR) spectra are taken automatically at specified time intervals without disturbing the specimens or the instrument. Water uptake in the coating and FTIR-MIR spectra of water on the coating-free substrate are also used for the analysis. Examples of clear and pigmented coatings on untreated and treated substrate surfaces are given to demonstrate the technique. Results of water accumulation at the coating/iron interface with and without applied electrical potentials are given. In addition to measuring water at the coating/substrate interface, the technique provides a means for studying the transport of water through a coating adhered to a substrate. Information on water at the interface and its transport properties through coatings applied to a substrate is valuable for interpreting corrosion, blistering and delamination of organic coating systems, and for developing models for use in predicting the serivce lives of protective coatings.     

Crack-bridging ability of organic coatings for concrete: influence of the method of concrete cracking, thickness and nature of the coating

Some concern about the determination of a standardised test for crack-bridging ability (CBA) measurements exists. The present work focuses on the analysis of the results of experimental tests performed to simulate the mechanical behaviour of a coating when a crack in the beneath concrete opens and grows. The influence of the method of concrete cracking, thickness and nature of the coating on the CBA are studied. Experimental results suggest that: (i) the same coating has higher CBA when the crack in the beneath concrete is produced through slight bending rather than through tensile loading; (ii) the CBA clearly depends on the nature (then mechanical properties) of the applied coating; (iii) power law relationships between CBA and thickness of the coating exist.     

Improved hydrothermal aging performance of glass fiber-reinforced polymer composites via silica nanoparticle coating

The long-term exposure to a hot and humid environment severely damages the bonding integrity of fiber-reinforced polymer composites and thus significantly degrades their mechanical performances. In this work, we aim to develop an improvement procedure for effectively enhancing the bonding strength in glass fiber-reinforced polymers (GFRPs). Glass fibers were coated with a thin layer of silica nanoparticles of different concentrations by the use of the evaporative deposition method. Micromorphological comparisons in terms of scanning electron microscope imaging demonstrate significant improvements on the surface roughness of glass fibers. With the coated glass fibers, GFRP composite laminates were designed, molded through the vacuum-assisted resin infusion technique, and experimentally tested for quantitatively studying their hydrothermal aging performance. The water absorption tests conducted for three exposure temperatures suggest that both the water diffusion rate and the equilibrium water content can be effectively reduced due to the introduction of the silica coating. With increased exposure temperatures, however, the desired reductions become much less significant. A so-called water-channel diffusion mechanism along fiber/resin interfaces was proposed to explain the coupling effects of silica coating and exposure temperature. Reductions of water diffusion rate and equilibrium water content were expected to slow down the hydrothermal aging performance of GFRPs. For this purpose, both uniaxial tensile test and three-point bending test were subsequently performed on GFRP specimens that have been subjected to different coating concentrations, exposure temperatures, and exposure durations. When compared with untreated GFRP specimens, both experiments demonstrate that the residual strength and stiffness can be effectively promoted through coating a thin layer of silica nanoparticles on glass fiber surfaces. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48652.     

Resistance of metallic substrates protected by an organic coating containing glass flakes

The resistance against corrosion of an epoxy-polyamine-based coating immersed in a 3 wt.% sodium chloride solution was investigated by electrochemical impedance spectroscopy (EIS). The organic coating contained glass flakes as pigment in order to enhance its barrier characteristics. The data show that this coating is more strongly adhered and exhibits higher protection characteristics when applied onto carbon steel substrates than on galvanized steel. Though the capacitance of the coating (C_C) does not show any appreciable variation with immersion time, the resistance (R_PO) of the film is observed to increase with time upon immersion. The analysis of the data sustains that the organic film behaves as a porous, non-barrier coating. Two time constants are observed even at earlier exposures, and the improved corrosion resistance developed after the coating system was exposed to the test electrolyte is considered to originate from the precipitation of corrosion products within the pores in the film.     

Effect of two-step sintering on the hydrothermal ageing resistance of tetragonal zirconia polycrystals

The effects of two-step sintering (TSS) on the mechanical properties and hydrothermal ageing resistance of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) were investigated. In TSS, the first step involved heating the samples up to 1400 °C at a heating rate of 10 °C/min and holding the samples at this temperature for 1 min. The second step involved sintering by cooling the samples down to 1200 °C and holding the samples at this temperature for various holding times (t) ranging from 0 to 30 h before cooling to room temperature. Moreover, TSS promoted densification with increasing holding time without sacrificing the mechanical properties of the sintered body and causing abnormal grain growth. The average grain size was found not to be affected by the long holding times, and the final microstructure composed of a uniformly distributed tetragonal grain having sizes ranging from 0.24 to 0.26 µm. The beneficial effect of TSS in suppressing the hydrothermal ageing of Y-TZP has been revealed in the present work. In particular, samples sintered at t=20 and 30 h exhibited excellent resistance to low-temperature degradation when exposed to superheated steam at 180 °C, attributed mainly to the enhance densification of the sintered bodies.     

Ingress of water into organic coatings: Real-time monitoring of the capacitance and increase in mass

Water ingress into a cataphoretic coating employed in the car industry was studied using real-time mass and capacitance measurements. The results showed the presence of three well-differentiated time constants associated with the process. The corresponding time scales were 2–3 min, 1 and 10 h. The first time constant (fastest process) was assumed to be associated with the filling of surface pores by water. This process involved about 50% of the total water uptake. The second time constant was assumed to be due to the water occupancy of pores in the bulk of the coating. Finally, the slowest process was assumed to be due to water diffusion through the bulk coating. This process presented capacitive effects only if fillers were present in the coating.     

Effect of high temperature or fire on heavy weight concrete properties

Temperature plays an important role in the use of concrete for shielding nuclear reactors. In the present work, the effect of different durations (1, 2 and 3 h) of high temperatures (250, 500, 750 and 950 °C) on the physical, mechanical and radiation properties of heavy concrete was studied. The effect of fire fitting systems on concrete properties was investigated. Results showed that ilmenite concrete had the highest density, modulus of elasticity and lowest absorption percent, and it had also higher values of compressive, tensile, bending and bonding strengths than gravel or baryte concrete. Ilmenite concrete showed the highest attenuation of transmitted gamma rays. Firing (heating) exposure time was inversely proportional to mechanical properties of all types of concrete. Ilmenite concrete was more resistant to elevated temperature. Foam or air proved to be better than water as a cooling system in concrete structure exposed to high temperature because water leads to a big damage in concrete properties.     

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.     

Effect of thermo-oxidative aging on surface characteristics of benzoxazine and epoxy copolymer

The copolymer of bisphenol A-based benzoxazine and cycloaliphatic epoxy resin possesses greater processability and glass-transition temperature compared to that of benzoxazine homopolymer, making it suitable for high-temperature aerospace applications. The aim of this work is to investigate the behavior of the copolymers under thermos-oxidative aging conditions using the surface characterization techniques. The benzoxazine homopolymer and copolymer samples were thermally aged in an air-circulating oven under controlled temperatures of 180 and 200°C for various periods of time up to 24 weeks. The thermo-oxidative-induced weight change and surface cracking were monitored for samples during the aging period. As an alternative method to characterize thermal aging, static contact angle, and contact angle hysteresis of samples were measured over time. Moreover, droplet impact tests using deionized water were performed on stationary and moving samples to investigate the effects of aging time and temperature on impact dynamics of droplets. In addition, FTIR analysis was performed to evaluate the chemical changes on the surface of samples. The results indicated the extent of degradation of aged samples increased with increasing the aging time and temperature. In addition, the copolymer of cycloaliphatic epoxy and benzoxazine showed a greater amount of thermos-oxidative degradation than the one for benzoxazine homopolymer. More importantly, the results obtained from the surface analysis measurements (e.g., contact angle) followed a similar trend to the one obtained from the traditional aging experiments (e.g., weight change). This showed the former nondestructive and simple method can be used to monitor thermal aging of materials in service.     

Effect of Ba concentration on phase stability and mechanical and thermal properties of La2Mo2O9

Ba-substituted La₂Mo₂O₉ ((La_1−xBa_x)₂Mo₂O_9−δ, x = 0–0.12) was prepared and the thermal and mechanical properties were evaluated. The thermal expansion coefficients (TECs) were determined from high-temperature X-ray diffraction (XRD) analysis. Phase transition in La₂Mo₂O₉ was suppressed via substitution of Ba for La, as demonstrated by differential scanning calorimetry (DSC) analysis. The mechanical properties, such as the bulk modulus, shear modulus, Young’s modulus, compressibility, and Debye temperature were evaluated from the measured sound velocities. The thermal conductivity was evaluated from the thermal diffusivity, heat capacity, and density in the temperature range from room temperature to 1073 K. The thermal conductivity decreased with increasing Ba content. Theoretical calculations based on the Klemens–Callaway model were performed to analyze the thermal conductivity, and the results suggest that the reduction of the thermal conductivity was mainly attributed to oxygen defects in the anion sublattice of La₂Mo₂O₉.     

Effects of curing program on mechanical behavior and water absorption of DGEBA/TETa epoxy network

Water uptake in organic coatings remains an interesting challenge for fundamental and applied researches because chemical, physical, and mechanical properties are concerned. The polymer network, which is affected by the curing program, is a key factor for water absorption. In this work, an epoxy network based on diglycidyl ether of bisphenol A and a hardener triethylentetramine was cured at different temperatures: below T_g (protocol 1) and above T_g (protocol 2). DMA, Differential Scanning Calorimetry (DSC), and FT‐IR measurements showed that both protocols allow to obtain totally cured networks. However, DSC and DMA results revealed that both cured networks present different levels of homogeneity, depending on the different curing conditions, which affect the free volume and the activation volume associated with visco‐elastic properties. The mechanical properties of free films and water sorption behaviors were investigated as function of cured conditions. It was found that protocol 1‐cured networks present higher mechanical properties and was less affected by water ingress than protocol 2‐cured systems, leading to better barrier properties. These results highlight the influence of the curing program onto the heterogeneous distribution of the epoxy network. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effect of the passivating coating type, particle size, and storage time on oxidation and nitridation of aluminum powders

Processes of nonisothermal oxidation, nitridation, and ageing of aluminum powders with different particle sizes (nano-sized powder, ASD-1 powder, and PAP-2 powder) are considered. Application of non-oxide coatings onto particles of aluminum nanopowders reduces their thermal stability. Owing to scale-shaped particles, the PAP-2 powder after long-time storage preserves high activity of oxidation and nitridation, which is commensurable with that of the aluminum nanopowder. The activity of the coarse ASD-1 powder consisting of spherical particles in terms of oxidation and nitridation is low and only slightly changes during ageing. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 2, pp. 61–69, March–April, 2006.     

Effect of Alkalization on Mechanical Properties of Water Hyacinth Fibers-Unsaturated Polyester Composites

Characteristic of local water hyacinth (WH) fibers and composites that consist of mixing WH fibers and unsaturated polyester (UPR) were studied. Composites mixed with the WH fibers treated in different alkali concentration for 1 h soaking time were tested by tensile and flexure machine and their fracture surface was observed by using scanning electron microscope (SEM). The results show that 7% NaOH, 1 h, treated WH fibers provided better mechanical properties on UPR matrix composites in comparison with other alkali concentrations. From SEM observation, some untreated WH fibers pulled out from their matrix were observed clearly in fracture surface of composites. The high alkali concentration created damage of cellulosic structure, thus decreasing of mechanical properties of the composites.