Papers by Keyword: Plasma Electrolytic Oxidation (PEO)

Abstract: The corrosion mechanisms of plasma electrolytic oxidation (PEO) films formed on AZ31 magnesium alloys were discussed in alkaline silicate solution. The structure and element contents of the films after immersed in 3.5 wt.% NaCl solution were investigated by SEM and EDS. The results indicated that the corrosion occurred on AZ31 magnesium alloy and the treated PEO samples during five days immersion. The corrosion procedure of the samples was depicted.

Abstract: The hybrid method of plasma electrolytic oxidation (PEO) and hot-dipping aluminum (HDA) was employed to deposit composite ceramic coatings on the surface of Q235 steel. The cross-section microstructure and surface morphology of the treated specimens were investigated with scanning electron microscopy (SEM). Especially, the composition of the composite coatings was investigated with transmission electron microscopy (TEM). The corrosion resistance was analyzed by immersing and corrosion polarizing experiments. The results indicate that metallurgical bonding can be observed between the ceramic coatings and the steel substrate. There are many micro-pores which act as discharge channels in the PEO coatings. The phase composition of the ceramic coatings is mainly composed of amorphous phase and crystal Al2O3 oxides. The crystal Al2O3 phase includes κ- Al2O3, θ- Al2O3 and β- Al2O3.The corrosion resistance of the PEO samples is much better than that of the HDA samples whether immersed in the NaCl solution or in the sea water.

Abstract: In this study, zirconium oxide coatings were formed on pure zirconium by microarc oxidation technique with the electrolytes containing KOH and different amounts of sodium silicate (0-40 gr/lt) for the same coating duration of 2 hours. The microstructure, surface roughness, phase content and chemical composition of the coatings were characterized by using scanning electron microscopy, profilometery and X-ray diffractometry. It was found that the coatings on surface of zirconium consist of monoclinic ZrO₂ (m-ZrO₂) and tetragonal ZrO₂ (t-ZrO₂) phases and the addition and increasing sodium silicate concentration in the electrolyte increases amount of t-ZrO₂ phase. The coatings were well adhered to Zr substrate with some cracks and porosities in the coating for all concentrations of sodium silicate. The coating thickness and surface roughness increased with sodium silicate concentration in the electrolyte. A glaze like Si rich structure and its increase with Si rate was evident in the outermost region of the coating.

Abstract: The present study emphasizes the effect of addition of sodium citrate (C₆H₅Na₃O₇.2H₂O) and sodium tungstate (Na₂WO₄.2H₂O) to a silicate based electrolyte system on the corrosion behavior of PEO treated ZM21 magnesium alloy. The phase composition of the as-developed coating was evaluated by X-ray diffraction (XRD) analysis, while its surface morphology, thickness and elemental distribution were observed using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). Potentiodynamic polarization tests were done in 3.5 wt% NaCl solution to analyze the corrosion behavior of the ceramic coatings in simulated marine environment. The results of XRD showed that the phase composition of all coatings comprised of Mg₂SiO₄ and MgO irrespective of the additive used. In addition to Mg, Si and O, the presence of W, C in EDS spectrum indicated that these elements were incorporated into the coating from the electrolyte system containing tungstate and citrate. The corrosion test results revealed that the PEO coatings obtained in tungstate containing electrolyte solution showed higher corrosion resistance than those formed in citrate containing electrolyte solution.

Abstract: A permanent mould cast creep resistant MRI 230D Mg alloy was laser surface alloyed (LSA) with Al and Al₂O₃ in order to improve its wear and corrosion resistance. However, this treatment was successful only in improving wear resistance but not corrosion resistance due to the presence of micro−cracks in the coated layer, which has been discussed in an earlier paper. The LSA coated Mg alloy has been further subjected to plasma electrolytic oxidation (PEO) treatment in alkaline silicate electrolyte in order to cover those micro−cracks and improve corrosion resistance, which is discussed in the present manuscript. For comparison, the PEO coating has also been applied on the as−cast MRI 230D Mg alloy. The microstructural characterization of coatings and corroded surfaces was carried out by scanning electron microscope and X−ray diffraction. Electrochemical corrosion tests were conducted in 3.5 wt% NaCl solution having neutral pH to investigate the corrosion behavior. The LSA coatings consisted mainly of β (Mg₁₇Al₁₂) phase, the coatings produced by PEO treatment on MRI 230D Mg alloy consisted mainly of Mg₂SiO₄ phase, and hybrid coatings of PEO on LSA consisted of Mg₂SiO₄ and MgAl₂O₄ phases in the PEO layer. Scanning electron micrographs of the cross−section revealed that the PEO treatment covered the micro−cracks present in the LSA and corrosion tests revealed that it improved the corrosion resistance, though not to the extent of the corrosion resistance of the PEO coated MRI 230D Mg alloy. All the samples exhibited localized form of corrosion.

Abstract: The surface topographic of ceramic coating on Ti6Al4V alloy, which was fabricated by Plasma Electrolytic Oxidation, was investigated by SEM and 3D profile. The chemical compositions and the phase structures were detected by EDS, XRD, FT-IR. The PEO coating shows a rough porous morphology and the chemical phases are anatase and rutile TiO₂. In addition, both of calcium-phosphorus ratios of the two samples were above 1.67. The bioactivity was focused on after soaking in SBF (simulated body fluid). It can be concluded by the experimental data that the surface roughness and the composition are corelated to form hydroxyapatite (HA), and the coatings containing Ca and P can induce bioapatite quickly.

Abstract: A ZrO₂-Y₂O₃-containing composite ceramic coating was firstly in situ prepared on AZ91D magnesium alloy by plasma electrolytic oxidation (PEO) technique in an alkaline silicate-containing electrolyte. The morphology, chemical composition and corrosion resistance of the PEO coating were investigated by environmental scanning electron microscopy (ESEM), X-ray diffractometer (XRD), energy dispersive X-ray (EDX) spectrometer, dropping corrosion and electrochemical corrosion test. The results showed that the ceramic coating consisted of two distinct structural layers: an outer loose layer and an inner dense layer; it was composed of t-ZrO₂, Y₂O₃, SiO₂ and some magnesium compounds, such as MgO, MgF₂ and Mg₂SiO₄. In addition, the ceramic coating also showed excellent dropping and electrochemical corrosion resistance, which was mainly attributed to its special phase composition and microstructure.

Abstract: Plasma electrolytic oxidation (PEO) of an 6061 Al alloy was accomplished in an electrolyte with sodium hexametaphosphate (10 g/L) and sodium silicate (10 g/L) Coatings were produced at four different current densities (4, 6, 8 and 10 A/dm²). Phase composition, morphology and tribological characteristics of the coatings were studied by X-ray diffraction (XRD), scanning electron microscope (SEM) and ball-on-disk friction testing. The results show that among the four ceramic coatings which were produced under four current densities, the coating produced under the current density of 8 A/dm² got the lowest wear rate with the value of 4.5 × 10^-5 g/m.

Abstract: The effect of discharge parameters (the electrode distance, the electrolyte volume) of the plasma electrolytic oxidation on the magnesium alloy films’ surface morphology were investigated. The results show that the appropriate electrode distance is 30 mm, which exhibits a better PEO surface morphologies. The electrolyte volume should not be too large or too small, in the paper, the appropriate electrolyte volume is between 250-350 mL.

Abstract: In order to get a clear picture for describing the growth process of plasma electrolytic oxidation coatings under escalating voltage waveform, the characteristics of PEO coatings formed at different reaction stages were systemically investigated. The morphology and corrosion resistance of the films were studied by scanning electron microscope and potentiodynamic polarization curves. The uniform, semi-transparent and better corrosion resistance of the oxide films on the magnesium electrode surface were formed owe to the extended anodizing time at the initial stage under escalating voltage mode. After sparking occurred, generated ceramic coatings were brokedown, melted, cooled and solidified continuously, so the ceramic coatings were uniform and dense. It also exhibited superior corrosion resistance.