Hyun Sam Ryu

Antibacterial properties of Ag (or Pt)-containing calcium phosphate coatings formed by micro-arc oxidation

Silver (or platinum)-containing calcium phosphate (hydroxyapatite (HA) and tricalcium phosphate (alpha-TCP)) coatings on titanium substrates were formed by micro-arc oxidation (MAO) and their in vitro antibacterial activity and in vitro cytotoxicity were evaluated. MAO was performed in an electrolytic solution containing beta-glycerophosphate disodium salt pentahydrate (beta-GP) and calcium acetate monohydrate (CA), and Ag and Pt were introduced in the form of AgNO(3) (or CH(3)COOAg) and H(2)PtCl(6), respectively. The MG63 and human osteosarcoma (HOS) cell lines were used to investigate the proliferation and differentiation behavior of the cells, respectively, whereas two strains of bacteria, Staphylococcus aureus and Escherichia coli, were used to evaluate the antibacterial activity of the coatings. The phase, morphology, and Ag content of the coatings were strongly dependent on the applied voltage and Ag precursor concentration. HA and alpha-TCP phases were detected in the coatings oxidized above 400 V and the presence of Ag was confirmed by EDS. While the coatings with a high content of Ag were cytotoxic and those obtained in the Pt-containing electrolyte had no apparent antibacterial activity, the calcium phosphate coatings obtained in the low Ag concentration electrolyte exhibited in vitro antibacterial activity but no cytotoxicity. Thus, biocompatible calcium phosphate coatings on Ti implants with antibacterial activity can be achieved by one-step MAO.

Corrosion Resistance and Antibacterial Properties of Ag-Containing MAO Coatings on AZ31 Magnesium Alloy Formed by Microarc Oxidation

Silver-containing oxide coatings on AZ31 Mg alloys were fabricated by microarc oxidation (MAO) in AgNO 3 -containing sodium silicate (Na 2 SiO 3 )-based electrolyte, and their physical and chemical properties were investigated, particularly focusing on corrosion resistance and antibacterial activity. The porous oxide coatings consisting of Mg 2 SiO 4 and MgO formed in both AgNO 3 -containing and AgNO 3 -free electrolytes and the MAO coatings were composed of a porous outer layer and a dense inner layer. MAO in AgN0 3 -containing electrolyte resulted in a thicker oxide coating, especially a thicker fluorine (F)-rich inner layer. Fluorine (F) was rich in the dense inner layer, and Ag was preferentially located close to the coating surface. The potentiodynamic test indicated that Ag-containing MAO coating had a more positive corrosion potential (-1.42 V), lower corrosion current density (0.02 μA/cm 2 ), and thus higher corrosion resistance (1824 kΩ cm 2 ) compared to Ag-free MAO coatings (-1.53 V, 0.32 μA/cm 2 , and 131 kΩ cm 2 , respectively). The electrochemical impedance spectroscopy results revealed that the higher corrosion resistance of Ag-containing MAO coating was due to an order of magnitude higher resistance of the dense inner layer. Ag-containing MAO coating showed an excellent antibacterial activity over 99.9% against two strains of bacteria, Staphylococcus aureus and Escherichia coli.

Effects of KF, NaOH, and KOH Electrolytes on Properties of Microarc-Oxidized Coatings on AZ91D Magnesium Alloy

Oxide coatings were formed on AZ91D Mg alloy by microarc oxidation (MAO) in a sodium aluminate (NaAlO 2 )-based electrolyte, and the effects of additive electrolytes, KF, NaOH, and KOH, on the physical and chemical properties of the MAO coatings were investigated, particularly focusing on their corrosion resistance determined by potentiodynamic and potentiostatic tests. The anodizing characteristics (breakdown voltage and ignition time) and the phase, surface morphology, and thickness of the MAO coatings were strongly dependent on the additive electrolytes. The intense and continuous sparking in the KF-NaAlO 2 electrolyte resulted in a thick MgAl 2 O 4 coating containing F - ions, whereas the MAO coating obtained in either NaOH or KOH-NaAlO 2 electrolyte was thin and poorly crystalline, resulting from the unstable and discontinuous sparking. The MAO coating obtained in the KF-NaAlO 2 electrolyte exhibited the highest corrosion potential, the lowest corrosion current density, and the highest polarization resistance, which were closely related to the crystalline MgAl 2 O 4 phase and the relatively thick dense inner barrier layer. The pitting corrosion occurred in AZ91D Mg alloy, and the pitting was significantly protected by the MAO coating obtained in the KF-NaAlO 2 electrolyte. The pitting potentials for the MAO coatings have been determined by potentiostatic tests.

Electrochemical Corrosion Properties of Nanostructured YSZ Coated AZ31 Magnesium Alloy Prepared by Aerosol Deposition

Yttria stabilized zirconia (YSZ) coating was formed on an AZ31 Mg alloy by aerosol deposition (AD) at room temperature, and its electrochemical corrosion properties were investigated in 3.5 wt % NaCl solution by potentiodynamic polarization test, potentiostatic test, and electrochemical impedance spectroscopy (EIS). The crack-free, dense, and ~4 μm thick YSZ coating was successfully obtained by the AD method. The YSZ coating was polycrystalline and composed of ~10 nm nanocrystallites. The potentiodynamic test indicated that the YSZ coated AZ31 Mg alloy had a more positive corrosion potential (-1.39 V vs SCE), much lower corrosion current density (0.02 μA/cm 2 ), and much higher polarization resistance (2500 kΩ cm 2 ) in comparison to an uncoated Mg alloy. The pitting corrosion was observed in the YSZ coated Mg alloy, and the determined pitting potential (E b ) was close to -0.8 V vs SCE. EIS results demonstrated that the impedance of the YSZ coated Mg alloy was ~ 10 7 Ω cm 2 at low frequencies, which was ~5 orders of magnitude higher than that of the uncoated Mg alloy. Furthermore, the high impedance ( > 10 6 Ω cm 2 ) of the YSZ coated Mg alloy was still maintained after 120 h immersion in 3.5 wt % NaCl solution. Consequently, the corrosion resistance of the AZ31 Mg alloy was significantly improved by the AD YSZ coating.

Effect of Electrolyte Compositions on the Corrosion Behavior of Oxide Films on AZ91D Magnesium Alloy Prepared by Micro-Arc Oxidation

Mg and its alloys have attracted an extensive attention in the field of structural materials such as automobile components, aerospace components, and computer parts due to high strength/weight ratio, good electromagnetic shielding, high thermal conductivity and good recycling ability (1). Furthermore, the use of magnesium alloys with a low density in the automotive industry is predicted to be continuously expanded because of the demands for reducing environmental burden and improving energy efficiency (2). However, magnesium and its alloys are extremely susceptible to corrosion resulting in decreased mechanical stability and surface contamination, so this point greatly restricts their applications (3). To enhance the poor corrosion resistance, various physical and chemical surface treatments have been performed. Among the various techniques, chemical-conversion coating is widely used, but conventional conversion coatings are gradually restricted because it is based on chromium compounds that have been shown to be highly toxic carcinogens (1,4). Thus, micro-arc oxidation (MAO), which is an environmental friendly process and is capable of protecting the magnesium alloys from corrosion and wear condition, has been attracting a renewed interest over the last few years(5). The extensive researches have been carried out in evaluating the effect of electrolyte composition and applied potential on the properties of MAO films. However, the effect of electrolyte compositions on the corrosion behavior of MAO films has not yet been discussed in detail. Thus, the aim of this work is to develop a proper MAO films on AZ91D magnesium alloy in various electrolyte solution. Corrosion behavior of these oxide films was evaluated in terms of electrolyte composition and concentration, and we examine relation between electrolyte condition and corrosion behavior. In this study, the protective oxide coatings were formed on AZ91D magnesium alloys by micro-arc oxidation (MAO) in several aluminate electrolytes containing potassium hydroxide, potassium fluoride, and sodium hydroxide. MAO was conducted in the range of current density between 13 – 21 A/dm using a pulse power supply for 10 – 60 min. The corrosion resistance of MAO films was performed by EG&G potentiostat/galvanostat model 273 with model 352 corrosion software in 3.5 % NaCl solution at pH 7.0. The crystalline MgO and MgAl2O4 spinel phases along with amorphous phase were detected by XRD (Fig. 1). With increase of electrolyte concentrations, the content of crystalline MgO and MgAl2O4 phases decreased and coating layers became more porous (Fig. 2). While using a less conductive electrolyte raises the breakdown voltage, it also increases the arc duration. In addition, all MAO specimens exhibited superior corrosion resistance compared to untreated AZ91D. The results indicate that the phase, morphology, thickness, and corrosion resistance of MAO films were found to be strongly dependent on electrolyte concentration and surface current density. Surprisingly, MAO films composed of amorphous phase showed much better corrosion resistance than MAO films with crystalline MgO and MgAl2O4 (Fig. 3).

Gas Sensing Property of Porous SnO2 Layer Synthesized through Anodic Oxidation

SnO2 porous structure was fabricated by anodic oxidation. The Sn coated SiO2/Si wafer was used as an anode and it was anodized in 0.3 M oxalic acid aqueous solution using DC power supply. The phase of formed oxide was rutile and the morphology showed a nano porous structure. The porous SnO2 with nano-channel was obtained at 6V in 0.3 M oxalic acid and the pore diameter was approximately 20 nm. A post annealing was performed at 500 ° for 3 h to enhance the crystallinity. To evaluate the gas sensing properties, a comb like Pt pattern was deposited on the anodized and annealed specimen and its gas sensing properties were measured as a function of gas concentration and working temperature