Virgil Constantin

Thermodynamic assessment of the probability of interaction of metal oxides of group VI-B with tungstate melts, the results of the study of the acid-base properties of tungstate melt by potentiometric method. Analysis of the presented experimental data on the study of the electrochemical behavior of chromium, molybdenum and tungsten under equilibrium and non-equilibrium conditions allows us to conclude that it is possible to implement multi-electron reversible equilibria and electroreduction processes involving oxide forms of chromium, molybdenum and tungsten (VI) in tungstate melts. The mechanism and final product of electroreduction of oxide forms of metal (VI) depend on the acid-base properties of the medium. By setting the latter, it is possible to control the electrode process.

Nd-Fe-B nanostructured thin layers have been growth on the glass substrate by the "flash" evaporation (FE) method. The microscopic and surface structure analysis of the Nd-Fe-B thin films investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) demonstrate these thin films to be nanostructured.The temperature dependence of the specific magnetization of the films in the 80 ≤ T ≤ 800 K range, measured by the ponderomotive method showed that the magnetization of the Nd-Fe-B nanostructured thin films before corrosion did not exceed 85 A×m2×kg-1 at 77 K. In such Nd-Fe-B nano thin films the long-range structural order is destroyed. After corrosive action on the thin films, their specific magnetization value increased to 140-155 A×m2×kg-1, most probably because of the formation of Fe3O4 (iron oxide) and the presence of free iron ions on the surface of the nanometric Nd-Fe-B layer upon heating the samples to 1050 K. XPS spectra confirmed the presence of Fe3O4, FeOOH and Nd(OH)3.

Sintered NdFeB magnets possess excellent magnetic properties, but behave passively in alkaline and saline media and are susceptible to corrosion in acidic environments. The corrosion behavior of NdFeB magnets in industrial environments: potassium hydroxide (KOH), perchloric acid (HClO4) and sodium chloride (NaCl), was investigated by immersion and electrochemical tests. The immersion test concluded that the most corrosive electrolyte of the three studied was HClO4. Electrochemical tests showed that the NdFeB sample without magnetic order had better corrosion resistance. Metallographic microscopy and scanning electron microscopy support the obtained corrosion data. X-ray photoelectron spectroscopy indicates that the main elements on the surface of the analyzed sample (Nd, Fe, B) are in the oxidized state after corrosion.

We investigate the corrosion behavior on the microstructure of CoCrFeMnNi high-entropy thin film potentiodynamic alloys electrodeposited on aluminum support. Using electrolytes based on dimethyformamide (DMF), (HCON(CH₃)₂), dimethylsulfoxide (DMSO, (CH₃)₂SO), an acetonitrile (AN, CH₃CN) organic system (4:1 vol. ratio), LiClO₄ in different concentrations and CoCl₂, CrCl₃ × 6H₂O, FeCl₂ × 4H₂O, MnCl₂ × 4H₂O and NiCl₂ × 6H₂O in different conditions, the HEA components were co-deposited on an aluminum foil substrate. Using the CALPHAD method and the MatCalc PC software, the structure of the HEA system under study was investigated by calculating the characteristic kinetic and thermodynamic criteria. The influence of each metal from HEA inside the solid solution zone and over the formation of the solid solution phase were obtained from the ratio between the derived parameters. The electrochemical measurements, which demonstrate that on an aluminum support the HEA thin films have lower corrosion resistance, were performed at ambient temperature in an aerated artificial solution. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to investigate the microstructure of HEA thin films before and after corrosion in artificial seawater. This complex study demonstrates that the electrodeposition of HEA thin films on an aluminum support is more difficult and that the resistance to corrosion is much lower compared to that in the previous work on HEA deposits on a copper support.

The corrosion of OL 52 and OL52.4 steels exposed to seawaters (Black, Aegean and Mediterranean Sea) has been investigated by weight loss method and the corresponding corrosion rates in the three sea waters are calculated. Before and after immersion in the corrosive medium micrographic images are obtained. XRD and ponderomotive methods have been used to determine the influence of the seawaters corrosion processes on the structure and magnetization of the studied steels. Obtained results show that both OL52 and OL52.4 steels have good corrosion resistance in the studied seawaters.

Materials used in the marine industry are exposed to extreme conditions, so it is necessary to meet remarkable characteristics, such as mechanical resistance, low density, and good corrosion resistance. The challenging environment requires continuous performance improvements, so this work is focused on developing new materials with superior properties, using the electrochemical deposition technique, which are convenient for marine engineering. High-entropy alloys have been attracting tremendous interest in many applications, due to their simple crystal structures and advantageous physical-chemical properties, such as high strength, anti-corrosion, erosion, and electro-magnetic capabilities. To identify the most appropriate compositions, MatCalc software was used to predict the structure and characteristics of the required materials, and thermodynamic and kinetic criteria calculations were performed. The modelling processes generated a series of optimal compositions in the AlCrCuFeNi alloy system, that are suitable to be used in anticorrosive and tribological applications. The composition and morphology of the obtained high entropy alloy thin films revealed a uniform structure, with a small grain profile. The corrosion resistance was investigated in artificial seawater to observe the behavior of the newly developed materials in demanding conditions, and the results showed improved results compared to the copper foil substrate.

In this paper, we investigate what effects heat treatment can have on potentiodynamically electrodeposited high-entropy thin film (HEA) CoCrFeMnNi alloys. We focused our study on the corrosion resistance in synthetic seawater, corroborated with the structure and microstructure of these thin films. Thin films of HEA alloys were deposited on a copper foil substrate, using an electrolyte based on the organic system dimethyl-sulfoxide (DMSO-(CH₃)₂SO)-acetonitrile (AN-CH₃CN) (in a volume ratio of 4:1), which contains LiClO₄ as electrolyte support and chloride salts of CoCl₂, CrCl₃ × 6H₂O, FeCl₂ × 4H₂O, MnCl₂ × 4H₂O and NiCl₂ × 6H₂O. Using MatCalc PC software, based on the CALPHAD method, the structure and characteristics of the HEA system were investigated, and thermodynamic and kinetic criteria were calculated. The modeling process generated in the body-centered-cubic (BCC) or face-centered-cubic (FCC) structures a series of optimal compositions that are appropriate to be used in anticorrosive and tribological applications in a marine environment. Electrochemical measurements were carried out in an aerated artificial seawater solution at ambient temperature. In the experimental media, HEA thin films proved to have good corrosion resistance and were even better than the copper substrate. Corrosion resistance was improved after heat treatment, as shown by polarization and EIS tests. The structure and microstructure of HEA thin films before and after corrosion in artificial seawater were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The XRD data showed no significant changes in the structure of HEA heat-treated thin films after the corrosion in saline media. The data obtained by polarization and ESI are supported by results from SEM-EDS. This complex study reveals that, for HEA thin films, heat treatment leads to an increase in corrosion resistance. So, this finding suggests that thermal annealing is an appropriate method for improving the corrosion performance of HEA thin films.

We investigated the phase structure, magnetic properties, corrosion resistance, microstructure of 1.4541 (V2A) stainless steel through X-ray diffraction (XRD), magnetic measurement device, gravimetric/electrochemical methods, metallographic microscopy. Corrosion resistance was evaluated in natural seawaters of Mediterranean, Black and Aegean seas. The results show that crystal structure and specific magnetic characteristics are resistant to corrosion action of all seawaters at a temperature of 295 K. The greatest corrosive effect on studied alloy has the Mediterranean Seawater, corrosion penetration is about 0.1290 mm/year. Microscopy confirmed corrosion data.

The corrosion behaviour of SR355JR and S355J2 carbon steels exposed to different seawaters (Black, Aegean and Mediterranean Sea) has been investigated by polarization measurements. The corresponding corrosion parameters were calculated. Micrographic images before and after immersion in the used seawaters corrosive medium were obtained. X-ray diffraction and ponderomotive methods respectively were used to determine the influence of the corrosion process on the crystal structure and specific magnetization of the studied steels. Resumen. En este estudio se investigó el comportamiento a la corrosión de los aceros al carbono SR355JR y S355J2 expuestos a diferentes aguas marinas (Mar Negro, Mar Egeo y Mar Mediterráneo) mediante mediciones de polarización. Se calcularon los parámetros de corrosión correspondientes. Se obtuvieron imágenes micrográficas antes y después de la inmersión en el medio corrosivo de las aguas marinas utilizadas. Para determinar la influencia del proceso de corrosión en la estructura cristalina y la magnetización específica de los aceros estudiados se utilizaron los métodos de difracción de rayos X y ponderomotivos respectivamente.