• Journal of the Korean institute of surface engineering
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• v.54 no.5
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• pp.238-247
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• 2021

PEO (plasma electrolytic oxidation) was applied to modify the surface of AZ31 magnesium alloy in this study. The mixed solution of sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) was used as the electrolyte, and 0 - 0.05 g/L of Ca-GP (Glycerol Phosphate Calcium salt) was added in the electrolyte as an additive. PEO treatment was conducted at a current density of 30mA/cm² for 5 minutes using a DC power supply. The surface properties were identified by SEM, XRD and surface roughness analyses, and the corrosion resistance was evaluated by potentiodynamic polarization and immersion tests. In addition, the biocompatibility was evaluated by immersion test in SBF solution. As the concentration of Ca-GP was increased, the surface morphology was denser and more uniform, and the amount of Ca and the thickness of oxide layer increased. Only Mg peak was observed in XRD analysis due to very thin oxide layer. The corrosion resistance of PEO-treated samples increased with the concentration of Ca-GP in comparision with the untreated sample. In particular, the highest corrosion resistance was identified at the group of 0.04g Ca-GP through potentiodynamic polarization and immersion tests in saline solution (0.9 wt.%NaCl). During the immersion in saline solution, pH rapidly increased at the beginning of immersion period due to rapid corrosion, and then increase rate of pH decreased. However, the pH value in the SBF temporarily increased from 7.4 to 8.5 during the day, then decreased due to the inhibition of corrosion with HA(hydroxyapatite) formation.

• Journal of the Korean institute of surface engineering
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• v.52 no.3
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• pp.138-144
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• 2019

In this work, PEO (Plasma Electrolytic Oxidation) film formation behavior of Al6061 alloy was investigated as a function of applied current density of AC at 310 Hz in the range from $120mA/cm^2$ to $300mA/cm^2$ in 0.5 M $Na_2SiO_3$ solution. When applied current density is lower than a critical voltage of about $132mA/cm^2$, voltage reaches a steady-state values less than 120 V without generation of arcs and metallic color of the alloy surface remains. On the other hand, when applied current density exceeds about $132mA/cm^2$, voltage increases continuously with time and arcs are generated at more than 175 V, resulting in the formation of PEO films with grey colors. Two different types of arcs, large size and small number of arcs with orange color, and small size and large number of arcs with white color, were generated at the same time when the PEO film thickness exceeds about $50{\mu}m$, irrespective of applied current density. Formation efficiency of the PEO films was found to increase with increasing applied current density and the growth rate was obtained to be about $5{\mu}m/min$ at $300mA/cm^2$. It was also found that surface roughness of the PEO films with $70{\mu}m$ thickness is not dependent on the applied current density.

• Journal of the Korean institute of surface engineering
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• v.53 no.6
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• pp.351-359
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• 2020

Formation behavior and properties of PEO (Plasma Electrolytic Oxidation) film on AA2024 were investigated under application of pulsed current as a function of Na3PO4 concentration in 0.05 M Na2SiO3 solution by analyzing voltage-time behavior, in-situ observation of arc generation, observation of surface morphology and measurements of thickness and surface roughness. Arc generation voltage decreased with increasing Na3PO4 concentration. Color difference of PEO films between edge and inner part disappeared by addition of Na3PO4. It was also observed that size of nodules on PEO film decreased with increasing Na3PO4 concentration. Thickness of PEO films formed on AA2024 increased with increasing Na3PO4 concentration. Whereas, surface roughness of PEO films decreased with increasing Na3PO4 concentration up to 0.05 M of Na3PO4 which is attributed to the deceased size of nodules on the PEO films. However, the surface roughness increased with increasing Na3PO4 concentration more than 0.07 M of Na3PO4 which seems to be due to the formation of non-uniform PEO films with smooth surface and large size pores formed by orange-colored big arcs. The experimental results suggest that added sodium phosphate less than 0.2 M in an alkaline silicate solution can contribute to the formation of relatively thick and uniform thickness of PEO films under arc generation voltage lower than 300 V.

• Journal of the Korean institute of surface engineering
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• v.54 no.4
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• pp.184-193
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• 2021

This study demonstrates formation behavior and morphological changes of PEO (Plasma Electrolytic Oxidation) films on AZ91 Mg alloy as a function of pre-treatment time in 1 M HF solution at 25 ± 1 ℃. The electrochemical behavior and morphological changes of AZ91 Mg alloy in the pre-treatment solution were also investigated with pre-treatment time. The PEO films were formed on the pre-treated AZ91 Mg alloy specimen by the application of anodic current 100 mA/cm² of 300 Hz AC in 0.1 M NaOH + 0.4 M Na₂SiO₃ solution. Vigorous generation of hydrogen bubbles were observed upon immersion in the pre-treatment solution and its generation rate decreased with immersion time. It was also found that 𝛽-Mg₁₇Al₁₂ in AZ91 Mg alloy was dissolved and a protective thin film of MgF₂ was formed on the AZ91 Mg alloy surface during the pre-treatment process in the 1 M HF solution. PEO film did not grow on the AZ91 Mg alloy specimen when the surface was not pre-treated and irregular PEO films with nodular defects were formed for the specimens pre-treated up to 1 min. Uniform PEO films were formed when the AZ91 Mg alloy specimen was pre-treated more than 3 min. The growth rate of PEO films on AZ91 Mg alloy increased significantly with increasing pre-treatment time.

• Korean Journal of Materials Research
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• v.16 no.6
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• pp.386-393
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• 2006

Increasing environmental concerns require to solve the problem produced due to the use of heavy metals in coating formulations. Therefore, it is necessary to develop new coating strategy employing inherently conducting polymers such as polyaniline. Polyaniline is a conductive polymer that is synthesized by oxidation polymerization, and the electrochemical and chemical polymerization are possible for the oxidation of aniline. Electrochemical oxidation polymerization produces a fine surface and although voltage control is more convenient, it require electrolytic cells, and elaborate thin film can be acquired with the polymerization. Polyaniline films were electro-polymerized on cold rolled sheets using the galvanostat mode in the oxalic acidaniline-sodium molybdate electrolyte. The structure and properties of polyaniline film were studied using Potentiostat/Galvanostat 263A, FE-SEM,, AFM, SST, Colorimetry. A high corrosion resistance of polyaniline film was observed with an increase of corrosion potential by $500{\sim}600$ mV for the substrate covered with polyaniline.

• Journal of the Korean Society of Industry Convergence
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• v.25 no.2_2
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• pp.309-314
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• 2022

The bonding strength [ISO22674] test was conducted by firing a dedicated ceramic powder on the surface of the dental titanium material treated with micro-arc oxidation. In the test group, an average result value of 34.34 MPa was obtained, and in the control group, a result value of 21.53 MPa was obtained. The bonding strength of the test group was higher than that of the control group by 12.81 MPa, resulting in a 37% improvement in durability of the dental artificial tooth ceramic restoration.

• Journal of the Korean Ceramic Society
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• v.49 no.6
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• pp.498-504
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• 2012

Oxide layers were prepared by an environmentally friendly plasma electrolytic oxidation (PEO) process on an Al-1050 substrate. The electrolyte for PEO was an alkali-based solution with $Na_2SiO_3$ (8 g/L) and NaOH (3 g/L). The influence of the electrical parameters on the phase composition, microstructure and properties of the oxide layers formed by PEO were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The voltage-time responses were recorded during various PEO processes. The oxides are composed of two layers and are mainly made of ${\alpha}$-alumina, ${\gamma}$-alumina and mullite phases. The proportion of each phase depends on various electrical parameters. It was found that the surface of the oxides produced at a higher current density and Ia/Ic ratio shows a more homogeneous morphology than those produced with the electrical parameters of a lower current density and lower Ia/Ic ratio. Also, the oxide layers formed at a higher current density and higher Ia/Ic ratio show high micro-hardness levels.

• The Transactions of the Korean Institute of Power Electronics
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• v.26 no.2
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• pp.90-95
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• 2021

In contrast to AC grounding systems, the ground electrode in DC systems continuously maintains positive or negative polarity. Ground electrodes with (+) polarity proceeds by oxidation reaction. Thus, the DC current should flow opposite to the polarity of the leakage current flowing through the (+) ground electrode by using a compensation electrode, and the current flowing through the (+) ground electrode can be 0A. However, according to protecting the (+) ground electrode, the compensation electrode corrodes and gets damaged. Thus, the (+) ground electrode must be protected from corrosion, and the service life of the compensation electrode must be extended. As an alternative, the average value of the current flowing through the compensation electrode should be equal with the value of the leakage current flowing through the (+) ground electrode by using the square waveform. Throughout the experiment, the degree of corrosion on the compensation electrode is analyzed by the frequency of the compensation electrode for a certain time. In the experiment, the frequencies of the square waveform are considered for 0.1, 1, 10, 20, 50, 100 Hz, and 1 kHz. Through experiments and analysis, the optimal frequency for reducing the electrolytic damage of the (+) electrode and compensation electrode in an LVDC grounding environment is determined.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.32 no.1B
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• pp.79-84
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• 2012

Nitrate is a common contaminant in groundwater aquifer. The present study investigates the performance of the bipolar zero valent iron (ZVI, $Fe^0$) packed bed electrolytic cell in removing nitrate in different operating conditions. The packing mixture consists of ZVI as electronically conducting material and silica sand as non-conducting material between main cathode and anode electrodes. In the continuous experiments for the simulated wastewater (contaminated groundwater, initial nitrate about 30 mg/L as N and electrical conductivity about 300 ${\mu}S/cm$), over 99% removal of nitrate was achieved in the applied voltage 600 V and at the flow rate of 20 mL/min. The optimum packing ratio (v/v) and flow rate were determined to be 1:1~2:1 (silica sand to ZVI), 30 mL/ min respectively. Effluent pH was proportional to nitrate influx concentration, and ammonia which is the final product of nitrate reduction was about 60% of nitrate influx. Magnetite was observed on the surface of the used ZVI as major oxidation product.

• Applied Chemistry for Engineering
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• v.9 no.4
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• pp.475-480
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• 1998

A study on the electrochemical reduction of uranium (VI) to uranium (IV) was carried out in the mixed phases of an organic phase with uranium (VI) and aqueous phase of nitric acid by use of a highly packed glassy carbon (GC) fiber column electrode system, and a model for in-situ electrolytic stripping of uranium (VI) was suggested. The electrochemical reduction of uranium (VI) occurred faster in organic phase than in aqueous phase of the mixed phases. The uranium stripping yield increased and then became constant with the increase of organic flow rate of the electrolytic system due to the increase of diffusion resistance of uranium ions in the organic phase into the aqueous phase. Aqueous flow rate, on the other hand, didn't affect the total uranium (VI) reduction current in the system. The system combined with electrochemical reduction was confirmed to be much more effective than the simple system without it in stripping uranium.