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Malayoglu U.,Dokuz Eylul University | Tekin K.C.,Dokuz Eylul University | Shrestha S.,Keronite International Ltd
Surface and Coatings Technology | Year: 2010

Magnesium alloys are highly reactive and require protection for their successful applications. In this work, coatings of 10 μm and 25 μm nominal thicknesses were prepared by Plasma Electrolytic Oxidation on AM50B and AM60B alloys. The effect of subsequent post-PEO treatments such as alkaline phosphate, alkaline silicate and sol-gel sealing was studied on coatings with 10 μm thickness. The microstructure and composition of the PEO coatings and post-treated coatings were analysed by Scanning Electron Microscope and Energy Dispersive X-ray Spectroscopy. Phase analysis of the coatings was carried out using X-ray Diffraction method. Potentiodynamic polarization method was used to evaluate the corrosion behaviour of these coatings in 3.5% NaCl solution. Electrochemical corrosion results indicated that the sealed PEO coatings of 10 μm thickness showed better corrosion resistance than the unsealed PEO coatings of 25 μm thickness. The structural surface defects of the PEO coatings were sealed by post-treatments resulting in decreases in surface roughness and open pores. Sol-gel post-treatment provided the most effective sealing to the PEO coatings. © 2010 Elsevier B.V.

Curran J.A.,Keronite International Ltd
Transactions of the Institute of Metal Finishing | Year: 2011

Plasma electrolytic oxidation can harden the surface of aluminium to the extent that it enables steel replacement in many applications. On magnesium, the technology offers a chromium-free corrosion protection superior to that offered by hexavalent chromium. Finally, on titanium, the process offers wear protection, an anti-galling surface, and a bio-compatible surface. In all cases, the process offers coating structures and compositions which cannot be achieved by any other techniques. © 2011 Institute of Metal Finishing.

Dunleavy C.S.,University of Cambridge | Curran J.A.,University of Cambridge | Curran J.A.,Keronite International Ltd | Clyne T.W.,University of Cambridge
Surface and Coatings Technology | Year: 2011

Plasma electrolytic oxidation (PEO) typically involves hundreds to thousands of discharges per square millimetre per second. In the present work, the characteristics of individual discharges are measured and investigated through an experimental survey of over 3.7. million individual events. The parameters considered include the voltage at which a discharge event commences, the peak current to which it develops, the duration of the discharge, and the current-time profile of individual discharges. It is shown that there is a strong correlation between the voltage at which a discharge event commences and the peak current to which it subsequently develops. There are also strong correlations between these characteristics and the duration of the discharges. These correlations show no significant influence of the coating thickness. The current-time profile is similar for most discharges, scaling according to the initiation voltage, suggesting that all PEO discharge events, whether weak or powerful, all develop according to the same physical mechanisms. © 2011 Elsevier B.V.

Tekin K.C.,Dokuz Eylul University | Malayoglu U.,Dokuz Eylul University | Shrestha S.,Keronite International Ltd
Surface and Coatings Technology | Year: 2013

In this work, ceramic-like oxide coatings were produced on rare earth element containing Elektron21 (E21) and WE43 Mg alloys using plasma electrolytic oxidation (PEO) process, and in addition to these alloys, AZ31B Mg alloy was used for comparison studies. Surface morphology and chemical composition of coatings were determined using scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometer (EDS). Phase analysis of coatings was carried out by X-ray diffraction (XRD). Electrochemical corrosion tests were undertaken by means of potentiodynamic polarization and electrochemical impedance spectroscopy techniques in 3.5% NaCl solution at ambient temperature. SEM results showed that the coatings have two layered microstructure with a porous top layer and a dense barrier layer. The SEM images of E21 and WE43 indicated that intermetallic compounds contributed into the coating structure during PEO processing. Electrochemical corrosion test results showed that the corrosion rates of PEO-coated substrates greatly decreased when compared to that of bare Mg alloys. However, bare rare-earth containing alloys showed increased corrosion resistance compared to bare AZ31B alloy. © 2013 Elsevier B.V.

Wheeler J.M.,University of Cambridge | Collier C.A.,University of Cambridge | Paillard J.M.,University of Cambridge | Curran J.A.,Keronite International Ltd
Surface and Coatings Technology | Year: 2010

This paper investigates the micromechanical behaviour and wear properties of coatings on a Ti-6Al-4V alloy generated using the plasma electrolytic oxidation (PEO) technique. Four different compositions of electrolyte were used: aluminate, phosphate, silicate, and mixed phosphate and silicate. The coatings' composition was characterised using X-ray diffraction and energy dispersive spectroscopy, and their morphologies were examined using SEM and optical interference profilometry. Following this, the micromechanical properties of the different coatings and the substrate alloy were examined using nanoindentation, nanoindentation scratch, nanoindentation impact, and modified grit blasting equipment. Correlations between these mechanical performance measures and observed structures are discussed. The aluminate-based coating, which contained a hard Al2TiO5 phase, was found to outperform other candidate systems and gave a performance enhancement over the bare substrate. However, it appears to be prone to delamination. © 2010 Elsevier B.V.

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