Browsing by Author "Girginov C."
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Item Addition of transition elements to the phosphate sealing of cerium oxide protective primer deposited on AA2024-T3 alloy(2016-01-01) Girginov C.; Avramova I.; Kozhukharov S.A concise investigation was undertaken to determine the impact of transitional elements on the phosphate sealing of cerium oxide primer layers deposited on AA2024-T3 aircraft alloy. The cerium oxide primer layers were deposited electrochemically on preliminary treated alloy specimens under potentiostatic conditions. The subsequent phosphate sealing layer deposition was performed in equimolar solutions of NaMoO4, NaVO3 or ZrO(NO3)2 in (NH4)2HPO4 medium for 30 min at 90°C. The already coated and sealed specimens were submitted to comparative measurements via electrochemical impedance spectroscopy and linear voltammetry after 168 h of exposure to 3.5% NaCl model corrosive medium. The obtained bi-layer coatings were characterized by systematic analyses using scanning electron microscopy, energy dispersion X-ray spectroscopy and X-ray photoelectron spectroscopy. The latter analysis evinced that cerium present in a cerium phosphate coating was predominantly in Ce(III) state, while doping by Mo, V and Zr caused oxidation to Ce(IV) species. The results obtained reveal successful inclusion of these elements inside the phosphate sealing layer.Item CHANGE OF THE ANTIMONY ANODIZING KINETICS BY COATING THE ELECTRODE WITH A THIN FILM(2023-01-01) Lilov E.; Lilova V.; Nedev S.; Kozhukharov S.; Adam A.M.; Girginov C.The present research work is dedicated on the processes kinetics description of the antimony anodization in potassium phosphate (K3PO4) aqueous solutions. The data have shown that the slope of the kinetic curves is dependent on the phosphate solution concentration. Besides, their mathematic approximation has revealed occurrence of a critical phosphate solution concentration of 0.11 mol dm-3. At this concentration, the slope of the kinetic curve tends to zero. Above this critical concentration, induction periods occur during the anodization. Additional experiments were performed with thin Sb2O3 layers preliminary deposited in vacuum. This approach has enabled to define the impact of such layers on the kinetic behavior during the anodization process. The results have shown that various stages of the process could be skipped. The final data analysis has confirmed the suggestion that the induction periods and voltage variations during the anodization process are caused by the formation of a thin Sb2O3 film, whose dielectric properties change with the film thickness increment.Item Characterization of anodized Al 1050 with electrochemically deposited Cu, Ni and Cu/Ni and their behavior in a model corrosive medium(2021-01-01) Girginov C.; Kozhukharov S.; Tsanev A.; Dishliev A.The specific benefits of the modified films formed on preliminary anodized aluminum, including the versatility of their potential applications impose the need for evaluation of the exploitation reliability of these films. In this aspect, the dura-bility of Cu and Ni modified anodized aluminum oxide (AAO) films on the low-doped AA1050 alloy was assessed through extended exposure to a 3.5% NaCl model corrosive medium. The electrochemical measurements by means of electrochemical impedance spectroscopy (EIS) and potentiodynamic scanning (PDS) after 24 and 720 hours of exposure have revealed that the obtained films do not change their obvious barrier properties. In addition, supplemental analyses of the coatings were performed, in order to elucidate the impact of the AC-deposition of Cu and Ni inside the pores. The scanning electron microscopy (SEM) images have shown that the surface topology is not affected and resembles the typical surface of an etched metal. The subsequent energy dispersive X-ray spectroscopy (EDX) tests have revealed a predominance of Cu in the combined AAO-Cu/Ni layers, whereas additional X-ray photoelectron (XPS) analyses showed that both metals form oxides with different oxidation states due to alterations in the deposition conditions, promoted by the application of AC-polarization of the samples.Item Characterization of anodized Al 1050 with electrochemically deposited Cu, Ni and Cu/Ni and their behavior in a model corrosive medium(2021-01-01) Girginov C.; Kozhukharov S.; Tsanev A.; Dishliev A.The specific benefits of the modified films formed on preliminary anodized aluminum, including the versatility of their potential applications impose the need for evaluation of the exploitation reliability of these films. In this aspect, the dura-bility of Cu and Ni modified anodized aluminum oxide (AAO) films on the low-doped AA1050 alloy was assessed through extended exposure to a 3.5% NaCl model corrosive medium. The electrochemical measurements by means of electrochemical impedance spectroscopy (EIS) and potentiodynamic scanning (PDS) after 24 and 720 hours of exposure have revealed that the obtained films do not change their obvious barrier properties. In addition, supplemental analyses of the coatings were performed, in order to elucidate the impact of the AC-deposition of Cu and Ni inside the pores. The scanning electron microscopy (SEM) images have shown that the surface topology is not affected and resembles the typical surface of an etched metal. The subsequent energy dispersive X-ray spectroscopy (EDX) tests have revealed a predominance of Cu in the combined AAO-Cu/Ni layers, whereas additional X-ray photoelectron (XPS) analyses showed that both metals form oxides with different oxidation states due to alterations in the deposition conditions, promoted by the application of AC-polarization of the samples.Item Efficiency of the Galvanostatic Formation of Anodic Antimony Oxide in Oxalic Acid Solutions(2022-03-12) Girginov C.; Lilov E.; Kozhukharov S.; Lilova V.The formation of thick anodic oxide films on antimony in diluted solutions of oxalic acid (CO(OH)2) was studied under galvanostatic and isothermal conditions. The film formation was always accompanied by a dissolution process which strongly depended on the growth conditions. The formation efficiency, as determined by the dissolved metal amount, was affected by the CO(OH)2 concentration, the current density and the anodization time. The dissolved antimony amount increased with higher CO(OH)2 concentrations and anodization time, and with lower current densities. The analysis of the total current density suggests the occurrence of a formed film and a dissolving component. According to the calculations, the anodic oxides composition is close to Sb2O3. The growth of anodic Sb2O3 took place at high electric fields within the oxide film. The thicknesses of the formed films were calculated by taking into account their dissolution. The film formation efficiency was determined at various current densities.Item Electrochemical assessment of the barrier ability of galvanic Zn coating primers electrodeposited on low-carbon steel(2024-01-01) Girginov C.; Kozhukharov S.; Boshkova N.Low-carbon steel is highly susceptible to corrosion, necessitating protective coatings capable of efficiently shielding its surface from corrosive environments. This brief research focuses on evaluating the protective capacity of electrochemically deposited Zn coatings, both with and without the addition of benzalacetone. Two independent electrochemical methods, Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Scanning (PDS), were employed after exposing the samples to a 0.01 M NaCl model corrosive medium (MCM) for 24 hours. The results demonstrate the discernible barrier performance of the Zn protective films compared to the bare steel substrates.Item Enhancement of the cerium oxide primer layers deposited on AA2024-T3 aircraft alloy by preliminary anodization(2018-01-01) Kozhukharov S.; Girginov C.The possibility for combination between anodized aluminum oxide (AAO) and cerium oxide primer layer (CeOPL) for elaboration of efficient protective coatings for AA2024-T3 aircraft alloy is proposed in the present research. The combined AAO/CeOPL coating characterizations include electrochemical impedance spectroscopy (EIS) combined with linear voltammetry (LVA), for extended times (until 2520 hours) to a model corrosive medium (3.5 % NaCl). Topographical and cross-sectional (SEM and EDX) observations were performed in order to determine the AAO/CeOPL film thickness and composition. The AAO/CeOPL layer durability tests were confirmed by standard neutral salt spray (NSS). The data analysis from all the used measurement methods has undoubtedly shown that the presence of AAO film significantly improves the cerium oxide primer layer (CeOPL) protective properties and performance.Item Evaluation of the electrochemical performance of Ag containing aao layers after extended exposure to a model corrosive medium(2020-01-01) Kozhukharov S.; Girginov C.; Kiradzhiyska D.; Tsanev A.; Avdeev G.The coating procedure appears to be an indispensable finishing stage in the production of Al based industrial products, engineering facilities and equipment. For this reason, there is an ever-increasing interest towards the elaboration of reliable corrosion protective layers with apparent coverage, adhesion, and barrier properties. In this sense, both the formation of anodized aluminum oxide (AAO) layer and its further modification with silver enable the elaboration of advanced (Al-O-Ag) films with extended beneficial characteristics. The present research activities are aimed at the determination of the corrosion protective properties of electrochemically synthesized Al-O-Ag layers on the technically pure AA1050 alloy. The structures and compositions of the obtained Al-O-Ag layers were characterized by X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). The research activities were accomplished by means of two independent electrochemical characterization methods: electrochemical impedance spectroscopy (EIS) and potentiodynamic scan-ning (PDS). The electrochemical measurements were performed after 24, 168 and 672 hours of exposure to 3.5 % NaCl solution used as a model corrosive medium (MCM), in order to determine the barrier properties and durability of the elaborated Al-O-Ag layers. The analysis of the obtained results has undoubtedly shown that the proposed electrochemical Al-O-Ag layer formation can successfully be used for the creation of self-standing layers with apparent corrosion protective properties. Besides, Al-O-Ag system can be used as a basis for development of efficient protective layers suitable for application in biologically contami-nated media.Item Impact of the final thermal sealing of combined zinc/cerium oxide protective coating primers formed on low carbon steel(2022-01-01) Kozhukharov S.; Girginov C.; Boshkova N.; Tzanev A.The final sealing possesses a proven beneficial effect on the protective properties of anodic oxide films on aluminum. In this sense, the present research is devoted to the evaluation of the impact of this procedure on the barrier ability of combined Zn/Ce oxide layers deposited on low carbon steel samples. For this purpose, four samples were submitted to galvanic zinc deposition, followed by spontaneous formation of cerium oxide primer layer (CeOPL). Afterwards, two of the samples underwent thermal sealing in boiling water in order to enhance their barrier ability. Its evaluation was performed by two electrochemical methods: electrochemical impedance spectroscopy (EIS) and potentiodynamic scanning (PDS) after 24 hours of exposure to a diluted model corrosive medium (MCM). Other instrumental methods were used in order to describe the effect of this final procedure on the color characteristics and hydrophobicity of the films. The results were collected from multiple tests, followed by statistical data treatment. In addition, the surfaces of the obtained films were submitted to direct observation by scanning electron microscopy (SEM), coupled with energy dispersion X-ray (EDX). Their composition was determined by means of X-ray Photoelectron Spectroscopy (XPS). The acquired data have revealed a detrimental effect of the final sealing in boiling water. It was expressed by the loss of the barrier properties of the Zn/CeOPL films, combined with additional decolorization and hydrophilization. Finally, the mechanism of this detrimental effect was determined by further SEM, EDX and XPS analyses.Item INFLUENCE OF THE FINAL THERMAL SEALING ON THE PROPERTIES OF COMBINED ANODIC ALUMINA/CERIUM CONVERSION COATINGS ON AA2024-T3 AIRCRAFT ALLOY(2023-01-01) Girginov C.; Kozhukharov S.; Tsanev A.; Georgieva M.; Petrova M.; Lilov E.; Petkov P.The current research presents results following corrosion tests, performed on combined anodic alumina/cerium conversion coatings (Al2O3/CeCC), thermally sealed for 15 min at 100°C either in boiling water or in hot-air medium. The coatings were formed on AA2024-T3 aircraft alloy substrates, at the optimum conditions of anodization and cerium conversion coating deposition, described in previous works. Prior to the corrosion tests, both the color characteristics and wetting ability of the combined films were evaluated. The corrosion protective properties of the sealed films were assessed by means of Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization (PDP). The measurements were performed after 672 hours of exposure to a 5 % NaCl model corrosive medium (MCM). Additional long term (up to 1344 hours of exposure) durability tests were then performed under the same conditions for the samples and superior performance was established. The electrochemical measurements were conducted regularly, once a week. Low and high-resolution Scanning Electron Microscopy (SEM) studies were performed on selected samples. The experimental results have shown that the sealing procedure in an aqueous environment enhances the corrosion protective ability and durability of the coating, probably due to formation of a hydrate layers, that suppresses the access of corrosive species to the surface of the substrate. This inference was additionally confirmed by the subsequent chemical analysis by means of X-Ray Photoelectron Spectroscopy (XPS). The results acquired have also shown that the incorporated Ce is predominantly in the form of Ce(IV)-oxides/hydroxides and after the thermal treatment almost entirely consists of cerium.Item MODEL FOR PHOTODEGRADATION WITH A MODIFIED RATE CONSTANT. PART 1: USE OF ANODICALLY-PREPARED THIN ZnO FILMS FOR METHYL ORANGE PHOTODEGRADATION(2022-01-01) Lilov E.; Lilova V.; Nedev S.; Kozhukharov S.; Girginov C.; Yancheva D.Four sets of samples of anodically prepared ZnO thin films were tested as potential photocatalysts for degradation of methyl orange (MO). The results show that the best properties belong to the samples prepared by anodization in oxalic acid and subsequently annealed 3 hours at 341°C. The dependences of the rate of the reaction on the time of use of the photocatalyst, the temperature of the decomposition reaction, the pH of the solution and the illumination were investigated too. The activation energies of the reactions were derived from the dependence of the rate of the reaction on the temperature. The properties of the acquired ZnO anodic films, as well as their compositions and structures were investigated by various analytical techniques, as follows: Attenuated Total Reflection Fourier Transformation Infrared Spectroscopy (ATR-FTIR), X-ray Diffractometry (XRD), Atomic Force Microscopy (AFM), Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES), Scanning Electron Microscopy (SEM) and UV-VIS Spectrophotometry.Item MODEL FOR PHOTODEGRADATION WITH A MODIFIED RATE CONSTANT. PART 2: МODEL DESCRIPTION(2022-01-01) Lilov E.; Lilova V.; Nedev S.; Kozhukharov S.; Girginov C.The dependence of the photodecomposition rate on the initial concentration of methyl orange (MO) as model pollutant was investigated. On the basis of relatively simple calculations, it was concluded that the role of the thermally exited charge carriers could not be neglected in case of high pollutant concentrations. It was found that the correlation between the reaction rate and the methyl orange concentration possesses a complicated N-shaped form. A model was proposed, on the basis of two reactions obeying the Langmuir-Hinshelwood mechanism. It describes the dependence of the reaction rate on the model pollutant concentration, on the basis of an equation, which comprises three parameters. Here, two of them were estimated by two, different ways and show excellent match, confirming that the model describes the dependence correctly.Item Optical and color characteristics of porous alumina with electrochemically incorporated silver(2018-01-01) Kiradzhiyska D.; Mantcheva R.; Girginov C.; Kozhukharov S.Following the ever increasing interest towards the combination between aluminum anodization and incorporation of relatively affordable metals, a brief study was performed on the impact of the duration of AC-electrochemical incorporation of silver on the resulting optical properties and color characteristics. The experiments performed on technically pure aluminum included: anodization in galvanostatic regime; AC-electrochemical silver incorporation, characterization of the optical properties by light reflectance and refractive colorimetric measurements. This systematic study enabled to determine some correlation between the coloring process duration and the resulting optical and color features.Item REPRODUCIBILITY OF THE SURFACE CHARACTERISTICS OF ANODIZED AA2024-T3 AIRCRAFT ALLOY WITH DEPOSITED Ce-CONVERSION COATING(2022-01-01) Kozhukharov S.; Dishliev A.; Girginov C.The establishment of any technology requires data verification regarding the repeatability and reproducibility of the obtained products. It includes the capability to repeat the properties of the respective products, in order to ensure quality reproduction. Namely, the ability of reproducing the characteristics of the final products determines the reliability of the used technological regime. In the present study a large amount of data was collected and analyzed. The data contain information for the color characteristics and the hydrophobicity of 28 samples of the AA2024-T3 aircraft alloy, coated with combined oxide layers. The coating primers were formed by means of anodization and subsequent sealing with cerium oxide layers. The combination of these methods results in the formation of hybrid layers, composed of Anodic Aluminum Oxide (AAO) and Cerium Conversion Coating (CeCC). Finally, the combined AAO-CeCC protective coatings were submitted to a total of 56 measurements and the obtained results underwent statistical data treatment. As a main result, the acquired statistical data have revealed a high level of repeatability, and can be used as a standard for the further production of similar coating systemsItem Silver doped TiO2 photocatalyst for methyl orange degradation(2012-01-01) Girginov C.; Stefchev P.; Vitanov P.; Dikov H.The photocatalytic efficiency of nanocrystalline TiO2 is strongly influenced by light sources and recombination of photoinduced electrons and holes. It is known that TiO2 doped with Ag has self-sterilizing and self-cleaning effects. A study has been carried out on the photocatalytic behavior of silver modified titanium dioxide for decomposition of methyl orange after UV black lamps and solar irradiation treatment. The base nanoparticles were standard photocatalyst TiO2 (P25). AgNO3 aqueous solution with the presence of TiO2 were irradiated with UV black lamps and reduced Ag doped the surface of the TiO2 nanoparticles. The silver modified titanium dioxide particles were studied by XRD and TEM. Silver ions (Ag+) were reduced to Ag0 as shown by the XRD. Silver particles of about 10 nm in size were detected on the surface of the TiO2 and partially in the aqueous solution. The presence of Ag0 on the surface of TiO2 nanoparticles, after UV black lamps and solar irradiation, highly increased the photocatyalytic activity in comparison to the photocatalyst P25. This is explained with the reduced recombination of electrons and holes in the presence of Ag0. © 2012 Kavala Institute of Technology.Item Solution conductivity as a factor determining the photocatalytic reaction rate(2024-06-01) Lilov E.; Nedev S.; Lilova V.; Kozhukharov S.; Girginov C.A rather simple model was developed, which predicted a hyperbolic relation between the photocatalytic reaction rate and solution conductivity. Experimental validation was performed using methyl orange as a model pollutant and titanium dioxide as a photocatalyst. The experiments, conducted with three different salts confirm the hyperbolic correlation between the photocatalytic degradation rate and solution conductivity. The results have evinced this correlation, albeit the model ignores crucial factors, such as the role of one of the two types of charges generated during light absorption and the chemical decomposition pathway. In addition, the experimentally derived hyperbolic function displays a positive shift along the ordinate.Item Synthesis of new complex [Bi6O6(OH)2](ClC6H4SO3)4and investigation of its thermal decomposition(2023-05-01) Kaloyanov N.; Zahariev A.; Parvanova V.; Avdeev G.; Girginov C.A new [Bi6O6(OH)2](ClC6H4SO3)4complex is synthesized through a reaction of Bi2O3 with aqueous 4-chlorobenzenesulfonic acid at elevated temperature. The product composition is elucidated by elemental analysis, ICP-OES and FTIR techniques. By means of XRD analysis, the compound s indexation, monoclinic P2(3) space group and unit cell parameters: A=31.25(3) A, b=19.78(1) A, c=8.795(8) A, γ = 94.33(4)o as well as a cell volume = 5424.21 A3 are determined. Based on DTA-TG technique, three specific temperatures aimed at studying product s thermal decomposition are determined. The compositions of solid residues after isothermal heating are proved following the XRD and FTIR data.