جستجوی مقالات مرتبط با کلیدواژه « اکسیداسیون الکترولیتی پلاسمایی » در نشریات گروه « مواد و متالورژی »

Plasma electrolytic oxidation is a new and upgraded method of anodizing process to improve the corrosion resistance of aluminum alloys by creating a ceramic coating on their surface.

One of the parameters affecting of PEO process as well as the performance of the prepared coatings is the composition of the substrate. In this study, the effect of increasing the percentage silicon of substrate on the plasma electrolytic oxidation process with bipolar pulse current in a silicate-based electrolytic bath was investigated. Scanning electron microscopy was used to evaluate the morphology and structure of the coating and X-ray diffraction test was used for phase detection. Coating corrosion behavior was evaluated by electrochemical tests after 1 hour immersion in 3.5% NaCl solution with the adjustment of pH 4.

coatings had a pancake structure with craters with irregular micro-cracks and micro-porosity. Investigations showed that with an increase in the percentage of silicon in the substrate, the thickness and porosity of the coatings decreased, and the volcanic morphology prevailed over the pancake morphology in the growth of the coating. Analysis showed the coatings mainly contain a mixture of γ- Al2O3, η- Al2O3, δ-Al2O3, SiO2, a small amount of mullite and some amorphous phases. Tofel polarization test revealed, in addition to reducing the corrosion current density up to 9% of the substrate silicon, we have seen an increase in the polarization resistance with the increase in the silicon percentage of the substrate after coating. The electrochemical spectroscopy test revealed that with the increase in the silicon percentage of the substrate, the coating forms a physical barrier against charge transfer substrate and the resistance of the outer layer decreases, but the resistance of the inner layer increases.

The purpose of this research is to investigate the electrochemical and antibacterial behavior of calcium-phosphate coating formed by the plasma electrolytic oxidation (PEO) method using an electrolyte containing Na3PO4.12H2O, KOH, and Ca3(PO4)2 salts on pure titanium. For this purpose, the surface structure of the coating was evaluated by scanning electron microscopic images and its chemical composition by Raman spectroscopic analysis. The electrochemical behavior of the coating was studied in simulating body fluid (SBF) using electrochemical impedance spectroscopy and potentiodynamic polarization tests. The antibacterial behavior of PEO coatings was evaluated under two conditions without irradiation and under ultraviolet radiation and against gram-positive Staphylococcus aureus bacteria. The results showed that the formed PEO coating had a porous structure and included the anatase crystalline phase. After 7 days of exposure of the samples to SBF solution, the values of polarization resistance (Rp) of pure titanium substrate and PEO coating were 0.097 and 0.263 MΩ.cm2 , respectively. Therefore, by modifying the surface of pure titanium with the help of the PEO coating process, its corrosion resistance increased by about 2.7 times. After exposing the PEO coatings to ultraviolet radiation, their antibacterial behavior was improved due to the production of reactive oxygen species (ROS) by the coatings and as a result damage to Deoxyribonucleic acid (DNA), proteins, and other intracellular components and finally the death of bacteria.

In this study, a hierarchical coating of titanium dioxide/tungsten trioxide (TiO2/WO3) was created using the synergistic methods of plasma/hydrothermal electrolytic oxidation on the surface of pure titanium. For this purpose, TiO2 coating was first created by plasma electrolytic oxidation (PEO) process in phosphate-base electrolyte on titanium substrate. , then WO3 particles were created by hydrothermal process on TiO2 coating. The effect of the duration of the hydrothermal process (12, 18 and 24 hours) on the photocatalytic behavior of the coatings was investigated. Phase studies and microstructural characteristics were performed using X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM). Photocatalytic degradation of methylene blue under visible light in the presence TiO2-WO3 coatings and TiO2 coating were compared. The pure TiO2 PEO coating showed about 47% degradation of methylene blue, while the optimal TiO2-WO3 composite coating, created by the combination of the plasma electrolytic oxidation /hydrothermal methods, resulted in 83% MB degradation. The highest photocatalytic activity of the TiO2-WO3 coating after hydrothermal for 18 h can be attributed to the presence of the visible-light-driven WO3 phase and its appropriate rod microstructure. After 12 h hydrothermal, the morphology of WO3 phase consisted of the nanoparticles and nanorods, while increasing the processing time up to 18 h led to the morphology development including the increase of nanoparticle size increment and growth of the nanorods.

The aim of this research is to investigate the effect of 7 variables of the plasma electrolytic oxidation (PEO) coating process, including the concentration of calcium acetate and potassium hydroxide salts of the electrolyte, frequency, current density, duty cycle, treatment time, and the distance between the anode and cathode on the corrosion current density of the coatings and studying the correlation between the corrosion resistance with the surface roughness and wettability of the coatings on pure titanium using the Taguchi design of experiments method. The surface roughness of the coatings was evaluated with the help of a surface profilometer and their wettability was assessed by measuring the contact angle of their surface with a drop of water. The corrosion resistance of the coatings was investigated using the potentiodynamic polarization test after 72 hours exposure of the samples to Ringer's physiological solution. The surface structure and chemical composition of the optimal coating were studied by a scanning electron microscope and X-ray diffractometer, respectively. The results showed that duty cycle and frequency were the most effective and the least important variables on the corrosion current density of coatings, respectively. Also, the optimal conditions for forming a coating with the lowest corrosion current density (0.0018 μA/cm2), included the concentration of 7.5 and 3 g/l of calcium acetate and potassium hydroxide salts in the electrolyte, respectively, frequency of 2000 Hz, current density of 5 A/dm2, duty cycle of 50%, treatment time of 15 minutes, and the distance between anode and cathode of 3 cm. In general, the corrosion resistance of the coatings decreased with the increase in their surface roughness and wettability.

Plasma electrolytic oxidation coating has emerged as a relatively new method to increase the corrosion resistance of metals. This research aims to improve the corrosion resistance of coatings applied on pure titanium substrate by optimizing the parameters of the coating process. For this purpose, the experiments were designed using the fractional factorial design method to determine the determinants of a response. The main factors of the process include electrolyte concentration, electrical parameters (current density, frequency and duty cycle) and treatment time at two different levels. The microstructural and phase analysis of the coating were carried out using Scanning Electron Microscopy and X-Ray Diffraction (XRD). Corrosion behavior of coatings in 3.5 wt% NaCl solution was studied by electrochemical impedance spectroscopy analyze. Statistical analysis showed that the optimal conditions can be obtained in the concentration of 5 g/L sodium phosphate, 4 g/L potassium hydroxide, current density 6 amps/square meter, duty cycle 80%, frequency 1000 Hz, and treatment time of 10 min.

The use of an environmentally friendly method called plasma electrolytic oxidation (PEO) on aluminum and its alloys to create a coating with desirable properties is very important. One of the parameters affecting of PEO process as well as the performance of the prepared coatings is the composition of the substrate. In this study, the effect of increasing the percentage silicon of substrate on the plasma electrolytic oxidation process with bipolar pulse current in a silicate-based electrolytic bath was investigated. The results showed that at constant current, the rate of increase of positive voltage decreases with increasing the amount of silicon and the values of positive voltage approach each other with the onset of oxidation. Scanning electron microscopy was used to evaluate the morphology and structure of the coating and X-ray diffraction test was used for phase detection. With increasing the percentage silicon of substrate from 1 to 13% by weight, in addition to reducing the thickness and roughness of the coatings, the average coating speed also decreased from 1 to 0.43μm / min. As the Si content of the substrate increases, volcano-shaped morphology dominated over pancake morphology in the growth of the coating and a porous surface with scattered micro-pores in its structure was observed. The results showed coatings mainly containing mixtures of γ-Al2O3, η-Al2O3, δ-Al2O3, SiO2 and a small amount of mullite and some amorphous phases.

In this study, the effect of coating time on the microstructure and corrosion behavior of AZ31 Mg alloy coated by plasma electrolytic oxidation (PEO) method has been investigated. For this purpose, phosphate-based electrolyte containing hydroxyapatite nanoparticles was used at different times of 5, 10 and 15 minutes. The surface properties and chemical composition of the coatings were investigated using scanning electron microscopy (SEM) and X-ray diffraction (XRD) pattern. The corrosion properties of coatings in the simulated body fluid have been studied by potentiodynamic polarization and electrochemical impedance spectroscopy tests. The results of this study showed that with increasing the coating time to 15 minutes, the size of the pores and the thickness of the coatings increased. The coating created in 10 minutes has the lowest percentage of porosity among the samples. The results also showed that the coating created in 10 minutes had the lowest corrosion current density (2.33 × 10-8 A/cm2) among the samples.

In the present study, the effect of different concentrations of calcium acetate (10 and 20 g/l) and potassium hydroxide (1 and 3 g/l) salts in electrolyte of the plasma electrolytic oxidation process on corrosion resistance of coatings formed on pure titanium has been investigated. After observation the surface structure of the coatings, it was found that the lowest porosity (about 4.86%) was related to the coating formed in the electrolyte containing 20 g/l calcium acetate and 1 g/l potassium hydroxide. This coating also showed the highest contact angle with water droplets (67.89 degrees) or in other words, the lowest wettability. The results of evaluation of corrosion behavior of coatings using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests after 48 hours of immersion in Ringer's solution indicated that the mentioned coating had the lowest corrosion current density (3.06 × 10-8 Ω/cm2 ) and the highest corrosion resistance (448272 Ω cm2 ) among all coatings. The high resistance of this coating against corrosion was due to low porosity percentage and thus reducing the penetration of corrosive ions into the substrate and also low wettability.

The purpose of the present work is to provide a suitable substrate for the deposition of titanium dioxide as a photocatalyst on aluminum. Due to the weak adhesion of photocatalysts to aluminum, there is no possibility of adhering photocatalysts to its surface or the lifetime is very short. For this reason, in this study, the adhesion between TiO2 and aluminum substrate is enhanced by the creation of micron pores on the substrate surface using two methods of anodizing and Plasma Electrolytic Oxidation (PEO) and the results of the two methods are compared. Experiments were carried out to investigate the dye removal over time using Rhodamine 6G aqueous solution in the presence of UV light. SEM images were also prepared from the surface of TiO2-coated aluminum substrates. The results of the study showed that the density of pores on the aluminum surface, and consequently, the dye removal in the PEO method were higher than those of conventional anodizing. In addition, although the dye removal in the PEO method is increased compared to the anodizing method, the overall process time and energy consumption in this method, especially in the pulse mode, were less than those of the anodizing method.

Titanium oxide coatings were prepared on commercially pure titanium substrates using plasma electrolytic oxidation process under current density of 0.15 A/cm2 in electrolyte containing 5-15 g/L potassium phosphate for period of 120-600 s. The effects of oxidation time and electrolyte concentration on surface morphology, thickness, roughness, relative porosity, pores size distribution, microstructure and corrosion behavior were evaluated. Investigation of voltage-time variations showed that increasing the electrolyte concentration up to 15 g/l caused an acceleration of microdischarge events and reduced the breakdown voltage from 593 V to 516 V. Morphological observations revealed that the pores were located at long distances and protuberances around the pores were more prominent. By increasing the oxidation time to 600 s, the thickness and roughness of the coatings increased, and due to the presence of 11% porosity of the coating surface, the total number of pores with diameters greater than 1 μm to 18% of the total surface pores reached. Investigation of the corrosion behavior revealed that the thickness of 8.2 ± 0.3 μm and low porosity, increased the corrosion resistance of the sample coated in the concentration of 15 g/L of potassium phosphate up to 1670 kΩ.cm2.

In this paper, zirconium oxide ceramics coating (ZrO2) were produced on Zircaloy-4 alloy using plasma electrolytic oxidation (PEO). Sodium silicate and Sodium aluminate based electrolyte was selected in PEO process and the effects of the concentration of Sodium aluminate (0, 2.5, 5, 7.5, and 10 g/L) on the microstructure, phase structure and the behavior of corrosion of formed coatings. In order to examine the morphology and phase structure of coatings, scanning electron microscopy (SEM) and x-ray diffraction (XRD) were used respectively. The corrosion behavior of the coated samples was evaluated by potentiodynamic polarization in 0.5 M LiOH solution. All oxide ceramics showed improved corrosion resistance. The results show that it's the NaAlO2 leads to inhibition of monoclinic ZrO2 phase in PEO coatings and stabilization of the t-ZrO2 phase in the electrolyte of 10g/L NaAlO2 (A10) leads to decrease corrosion current density its up to 1.10*10-7A/Cm2 . After the addition of 10 g/L Sodium aluminate (A10) in the electrolyte %20 tetragonal zirconia was formed on the surface. The improvement in corrosion properties mainly depends on the phase composition of the produced coatings, so that in the electrolyte of 10g/L NaAlO2 (A10) showed the best corrosion resistance in comparison to other coatings.

In this study, ceramic coatings were formed successfully by plasma electrolytic oxidation process in aluminate-based electrolyte containing different concentrations of potassium hydroxide (3, 4 and 5 g/l) on pure titanium substrate. All processes were carried out at constant voltage of 420 v and for 180 s. The effects of the chemical composition of electrolyte on the sparking voltage, microstructure and corrosion behavior were studied. The surface and cross-sectional scanning electron microscopic (SEM) images and corrosion tests of coatings in a 3.5 wt. % NaCl solution showed that the increasing of potassium hydroxide concentration from 3 to 4 g/l resulted in increasing the compactness and uniformity, decreasing the average size of surface micro-pores despite increasing the porosity, increasing the thickness and increasing the corrosion resistance. However, increasing of potassium hydroxide concentration in the electrolyte to 5 g/l due to the excessive electrolyte conductivity caused to formation of bigger and stronger sparks and thus increasing the average micro-pores size despite decreasing the porosity and increasing the thickness and decreasing the corrosion resistance of coating. Therefore, the coating formed in the electrolyte containing 4 g/l potassium hydroxide showed the most uniform and dense structure with the
smallest micro-pores size and the best corrosion behavior. In fact, this sample with corrosion resistance about 22.88×105 (Ω cm2 ) increased the resistance of uncoated titanium, 36 times. Using X-ray diffraction pattern, it was determined that thecoating was composed of rutile, anatase and TiAl2O5 phases

In this paper, zirconium oxide coating was produced on Zircaloy-4 alloy using plasma electrolytic oxidation (PEO). Sodium silicate and sodium pyrophosphate based electrolyte was selected in PEO process and the effects of concentration of sodium pyrophosphate (0, 2.5, 5, 7.5, and 10 g/L) on the microstructure and the behavior of electrochemical corrosion of formed coatings was studied. In order to examine the morphology and phase structure of coatings, scanning electron microscopy (SEM) and x-ray diffraction (XRD) were used respectively. The corrosion behavior of the coated samples was evaluated by electrochemical impedance spectroscopy (EIS) in 0.5 M LiOH solution. The results show that, ZrO2 coating causes an increase in corrosion resistance inner layer from 0.61 MΩ.cm-2 for the bare sample to 126 MΩ.cm-2 for the sample which was coated in an electrolyte with the combination of 10g/L Na2SiO3 and 5g/L Na4P2O7 (sample S5). This corrosion behavior was related to the coating characteristics. So that in the electrolyte of 5g/L Na4P2O7 (S5) has compact/uniform structure in comparison to other coatings. Also, this sample contains the monoclinic ZrO2 phase of 94% and the tetragonal ZrO2 phase of 6% respectively.

In this study, the microstructure and corrosion properties of titanium oxide ceramic coatings formed in a carbonate electrolyte under the influence of the key factor of the density of the coating process were investigated. Plasma electrolytic oxidation coating made at current densities of 65, 82.5 and 100 mA/cm2, 1500 Hz and 11% duty cycles over a 10 minute and on a commercial titanium surface as the substrate. The results showed that the surface morphology and corrosion properties of the coatings depend on changes in the current density. The increase in the applied current density leads to changes in morphology, from pancreatic to pancake with volcanic spans, and the average diameter of the cavities increases from about 1 micrometer to 4 micrometers, which also increases surface roughness. Changes in the current density also cause variations in the voltage and intensity of the plasma around the samples, which directly affects the intensity of the microcirculation and the thermal gradient of the oxide film and causes the anatase oxidation phase to become more rutile. The corrosion resistance of the coatings increased with increasing current density, so that the corrosion current density decreased 10 times as compared to the uncoated sample, which caused a 2.5 times increase in the thickness of the coating and was more compact. The inner layer of the coating, reducing open path to the surface of the metal and the presence of the main rutile phase were attributed.

In this study, the influence of the sodium phosphate electrolyte concentration on the coating microstructure, phase composition and corrosion resistance of Ti-6Al-4V alloy coated by plasma electrolytic oxidation (PEO) was investigated. For this purpose, alkaline-phosphate electrolytes with different concentration were applied as follows: 8, 12, 16 g/L. The morphology and phase composition of these coatings were analyzed by a scanning electron microscope (SEM), and X-ray diffraction (XRD) technique. Corrosion properties of the coating were studied under simulated body fluid (Hanks) by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. Through the PEO process, an increase in the concentration of sodium phosphate led to increment in the electrolyte electrical conductivity, average pore size and thickness of the coatings. As a consequence of increasing the electrical conductivity, the breakdown voltage in V-t curves decreased. The XRD results showed that with increase in the concentration of sodium phosphate, the metastable anatase phase transformed to the stable rutile phase. The results of the electrochemical tests indicated that the corrosion resistance of the coating produced by PEO process, in the electrolyte with 12 g/L sodium phosphate, was significantly better in comparison to the other specimens. The maximum measured corrosion potential of the coating (391 mV) together with the minimum corrosion current density (6.18×10-8 A cm-2) in the electrolyte with 12 g/L sodium phosphate leads to the highest corrosion resistance of twofold layer, that is, 8.8 MΩ. Corrosion resistance of other samples is 2 to 6 orders of magnitude smaller than sample with 12 g/L sodium phosphate.

In this paper, the effect of sodium silicate concentration on microstructure and corrosion behavior of ceramic coating on 5052 Al alloy formed by plasma electrolytic oxidation (PEO) was studied. For this purpose, the electrolytes with 6, 12 and 18 g/l sodium silicate were used. The scanning electron microscope (SEM) was used for investigation of surface and cross section of coating. The phase analyzing of substrate and coated sample were conducted by X-ray diffraction pattern (XRD). Corrosion behavior of coated sample and substrate were investigated with electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization test in 3.5 %wt. sodium chloride corrosive solution. With survey of SEM images was confirmed that, the coated sample with 12 g/l sodium silicate has most uniform and compact structure with finest pores size. Study of XRD pattern illustrated that there are α- Al2O3, γ- Al2O3, mullite (3Al2O3.2SiO2) and Mg2SiO4 phases in coating. Also, result of corrosion tests proved that the coated sample in 12 g/l sodium silicate has highest corrosion resistance. The corrosion resistance of this coated sample was 1588 KΩ.cm2 that was 188 times more than corrosion resistance of 5052 Al alloy substrate.

Plasma electrolytic oxidation (PEO) process using a pulsed current regime was performed on 7075 Al alloy in silicate based electrolytic bath with and without potassium permanganate as an additive. The results showed that with adding potassium permanganate to the electrolyte, an oxide coating containing Mn as the major element, Si and Al with a higher thickness was formed, which showed different microstructure and surface morphology than that produced in bath containing no additive. According to the observations, oxygen reduction became intense, the sparking quality on the surface was improved and the occurrence of destructive discharges were prevented and thus, a uniform and compact coating with lower defects was achieved. The corrosion behavior of the PEO coatings was evaluated using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization technique in 3.5 % NaCl solution at pH 4. The results showed that the coating produced in the presence of potassium permanganate had the higher corrosion resistance due to its higher thickness and compactness of the inner layer.