V.B. Mišković-Stanković

Electrolyte penetration through epoxy coatings electrodeposited on steel

Abstract A.C. impedance measurements carried out on epoxy-resin elcctrocoated steels were used to determine the pore resistance of the organic film as a function of time of exposure to 3% sodium chloride solution. The thermogravimctric analysis of coatings after various exposure times enabled determination of the quantity of electrolyte inside the pores, while gravimetric liquid sorption experiments were used to evaluate water uptake inside the coating. The results were interpreted in terms of a model in which water rapidly penetrates macrocapillaries (macropores) present as defects in the external portion of the coating and extra fine capillaries formed by the polymer net, followed by water and ions penetration through the macropores. The number, dimensions and shape of the pores through the film, estimated by optical microscopy, additionally confirmed the proposed model of electrolyte penetration. It was shown that conduction through the coating depends only on conduction through the macropores, although the quantity of electrolyte inside the macropores is only one tenth of that inside the polymer net.

The sorption characteristics of epoxy coatings electrodeposited on steel during exposure to different corrosive agents

Organic protective coatings on steel were formed by cathodic electrodeposition of an epoxy resin modified by an amine and isocyanate using a constant voltage method (resin concentration 10%wt, temperature 26 °C, applied voltage 250 V). The corrosion behaviour of these coatings was investigated during exposure to different corrosive agents (H2O, 3% NaCl, 3% Na2SO4, 3% sodium salt of 2-naphthol-3,6 disulphonic acid) using a.c. impedance measurements, gravimetric liquid sorption experiments and differential scanning calorimetry (DSC), with the aim of explaining the mechanism of coating degradation. The time required to saturate the coating with pure water, obtained from the sorption data, coincides with the initial increase in coating capacitance and initial decrease in pore resistance, obtained from a.c. impedance measurements, denoting the entry of electrolyte into the coating. The first step of electrolyte penetration through the coating is related to water uptake, when molecules of pure water diffuse in the micropores of polymer net according to Ficks law and it is independent of type and dimensions of ions in the electrolyte. Similar values of enthalpy of vaporization of different volatile electrolytes and the quantity of water inside the coating, obtained from DSC measurements were in agreement with the proposed mechanism. From the calculated diffusion coefficient for coatings where external layers were mechanically removed, it can be concluded that the micropores of the polymer net are homogeneously distributed throughout the coating.

Corrosion protection of aluminium by a cataphoretic epoxy coating

Abstract The corrosion behavior and thermal stability of epoxy coating electrodeposited on aluminium have been investigated during exposure to 3% NaCl. Electrochemical impedance spectroscopy (EIS), gravimetric liquid sorption experiments, thermogravimetric analysis (TGA) and polarization measurements were used. From the results obtained from EIS (pore resistance, coating capacitance, relative permittivity of coating, charge-transfer resistance, double-layer capacitance), gravimetric liquid sorption experiments (diffusion coefficient, energy of activation of water diffusion) and TGA (water content inside the coating and thermal stability), it can be concluded that the electrochemical, transport and thermal properties of an epoxy coating on aluminium are significantly improved with respect to the same epoxy coating on other substrates (steel, phosphatized steel, steel modified by Zn–Ni alloy). The better protective properties can be explained by less porous structure of the coating on aluminium, caused by lower rate of H 2 evolution, while the prolonged corrosion protection is due to the passive, mainly barrier film of Al 2 O 3 on its surface.

Electrochemical deposition and characterization of ZnCo alloys and corrosion protection by electrodeposited epoxy coating on ZnCo 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.

Corrosion Stability and Bioactivity in Simulated Body Fluid of Silver/Hydroxyapatite and Silver/Hydroxyapatite/Lignin Coatings on Titanium Obtained by Electrophoretic Deposition

Hydroxyapatite is the most suitable biocompatible material for bone implant coatings. However, its brittleness is a major obstacle, and that is why, recently, research focused on creating composites with various biopolymers. In this study, hydroxyapatite coatings were modified with lignin in order to attain corrosion stability and surface porosity that enables osteogenesis. Incorporating silver, well known for its antimicrobial properties, seemed the best strategy for avoiding possible infections. The silver/hydroxyapatite (Ag/HAP) and silver/hydroxyapatite/lignin (Ag/HAP/Lig) coatings were cathaphoretically deposited on titanium from ethanol suspensions, sintered at 900 °C in Ar, and characterized by X-ray diffraction, scanning electron microscopy, field emission scanning electron microscopy, attenuated total reflection Fourier transform infrared, and X-ray photoelectron spectroscopy. The corrosion stability of electrodeposited coatings was evaluated in vitro in Kokubos simulated body fluid (SBF) at 37 °C using electrochemical impedance spectroscopy. Bioactivity was estimated by immersion in SBF to evaluate the formation of hydroxyapatite on the coating surface. A microcrystalline structure of newly formed plate-shaped carbonate-hydroxyapatite was detected after only 7 days, indicating enhanced bioactive behavior. Both coatings had good corrosion stability during a prolonged immersion time. Among the two, the Ag/HAP/Lig coating had a homogeneous surface, less roughness, and low values of contact angle.

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

Abstract 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 (H 2 O) 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.

Corrosion behaviour of epoxy coatings electrodeposited on steel electrochemically modified by Zn–Ni alloy

The corrosion behaviour and thermal stability of epoxy coatings electrodeposited on steel, phosphatised steel and steel previously modified by electrodeposited Zn–Ni alloy was investigated during exposure to 3% NaCl. The electrochemical impedance spectroscopy (EIS), gravimetric liquid sorption experiments, thermogravimetric analysis (TGA) and anodic linear sweep voltammetry (ALSV) were used. Epoxy topcoat was formed by cathodic electrodeposition of an epoxy resin using constant voltage method. From the results obtained from EIS (pore resistance, coating capacitance), gravimetric liquid sorption experiment (diffusion coefficient) and TGA (water content inside the coating and thermal resistance), it can be concluded that electrochemical, transport and thermal properties of epoxy coatings are strongly affected by surface modification of the substrate. The better protective properties of epoxy coating on Zn–Ni sublayer can be explained by its less porous structure, caused by lower rate of H2 evolution on Zn–Ni alloy, while the increased corrosion protection is due to the passive film of ZnO on the alloy surface. The mechanism of electrolyte penetration through an organic coating was also given.

The sorption characteristics and thermal stability of epoxy coatings electrodeposited on steel and steel electrochemically modified by Fe-P alloys

Abstract The corrosion behaviour and thermal stability of epoxy coatings electrodeposited on steel and steel electrochemically modified by Fe–P alloys of different phase structures have been investigated during exposure to 3% NaCl. Electrochemical impedance spectroscopy (EIS), gravimetric liquid sorption experiments and thermogravimetric analysis (TGA) were used. Epoxy coatings were formed by cathodic electrodeposition of epoxy resin on steel and steel modified by Fe–P alloys at various current densities, using a constant voltage method. From the results obtained from EIS (pore resistance, coating capacitance), gravimetric liquid sorption experiments (diffusion coefficient) and TGA (water content inside the coating and thermal resistance), it can be concluded that the electrochemical, transport and thermal properties of epoxy coatings are strongly affected by surface modification of the substrate. The reduced electrochemical and transport properties of epoxy coatings on Fe–P sublayers can be explained by the catalytic activity of Fe–P alloys for the H 2 evolution reaction, while the improved thermal stability of these coatings is probably connected with an increasing number of hydrogen bonds inside the polymer net. The mechanism of electrolyte penetration through an organic coating is also given.

Electrodeposition and characterization of Zn-Ni alloys as sublayers for epoxy coating deposition

The chemical composition and phase structure of Zn–Ni alloys obtained by electrodeposition under various conditions were investigated. The influence of the deposition solution and deposition current density on the composition, phase structure, current efficiency and corrosion properties of Zn–Ni alloys were examined. It was shown that the chemical composition and phase structure affect the anticorrosive properties of Zn–Ni alloys. A Zn–Ni alloy electrodeposited from a chloride solution at 20 mA cm−2 exhibited the best corrosion properties, so this alloy was chosen for further examination. Epoxy coatings were formed by cathodic electrodeposition of an epoxy resin on steel and steel modified with a Zn–Ni alloy. From the time dependence of the pore resistance, coating capacitance and relative permittivity of the epoxy coating, the diffusion coefficient of water through the epoxy coating, D(H2O), and its thermal stability, it was shown that the Zn–Ni sublayer significantly affects the electrochemical and transport properties, as well as the thermal stability of epoxy coatings. On the basis of the experimental results it can be concluded that modification of a steel surface by a Zn–Ni alloy improves the corrosion protection of epoxy coatings.

A comprehensive approach to in vitro functional evaluation of Ag/alginate nanocomposite hydrogels.

In this work, we present a comprehensive approach to evaluation of alginate microbeads with included silver nanoparticles (AgNPs) at the concentration range of 0.3-5mM for potential biomedical use by combining cytotoxicity, antibacterial activity, and silver release studies. The microbeads were investigated regarding drying and rehydration showing retention of ∼ 80-85% of the initial nanoparticles as determined by UV-vis and SEM analyses. Both wet and dry microbeads were shown to release AgNPs and/or ions inducing similar growth delays of Staphylococcus aureus and Escherichia coli at the total released silver concentrations of ∼ 10 μg/ml. On the other hand, these concentrations were highly toxic for bovine chondrocytes in conventional monolayer cultures while nontoxic when cultured in alginate microbeads under biomimetic conditions in 3D perfusion bioreactors. The applied approach outlined directions for further optimization studies demonstrating Ag/alginate microbeads as potentially attractive components of soft tissue implants as well as antimicrobial wound dressings.