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Adhesion strength is one of the important properties that reflects the quality of a plasma electrolytic oxidation (PEO) coating. Scratch testing can be considered as an appropriate technique to evaluate the adhesion strength of PEO coatings on magnesium, titanium, and aluminum substrates. The scratch test is usually performed either under a constant or a progressively increasing normal load, where the critical load is used as a measure of adhesion strength of the coatings. In this review paper, the effect of different factors such as duration of coating processing, electrolyte composition, and processing current density, as well as different additives to the electrolyte bath, was studied on the adhesion strength of PEO coatings formed on magnesium, titanium, and aluminum substrates. It is understood that an optimum increase in process time and input energy leads to a corresponding increase in thickness of the PEO dense oxide layer and, consequently, an increase in critical load and adhesion strength. Moreover, the electrolyte composition and additives were found to affect the coating microstructure and composition and, subsequently, the coating adhesion strength. [ABSTRACT FROM AUTHOR]

The article focuses on the use of plasma electrolytic oxidation (PEO) for surface engineering of magnesium alloys so that they can be used in various corrosion, tribological and other surface modified applications. As stated, the demand for magnesium has increased in the recent years due to several factors which include, low density, high oil prices compelling and more production capacity. The PEO process involves the use of high voltages than in anodising and is environmentally friendly.

Abstract: 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. [Copyright &y& Elsevier]

Abstract: Magnesium alloys are highly reactive and require protection for their successful applications. In this work, coatings of 10μm and 25μm 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 10μm 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 10μm thickness showed better corrosion resistance than the unsealed PEO coatings of 25μm 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. [ABSTRACT FROM AUTHOR]

Abstract: A ceramic coating on AA6082 aluminum alloy prepared by plasma electrolytic oxidation (PEO) has been studied and compared against a sulphuric acid hard-anodized coating on the same alloy. Surface morphology and microstructures of the coatings have been examined by scanning electron microscopy. X-ray diffraction is used to determine the phase composition of the coatings. The adhesion strength of the coatings has been evaluated using a scratch test method. The coating''s mechanical properties such elastic modulus and hardness data have been generated using a dynamic ultra-microhardness tester. Sliding wear tests with different loading rates are performed on the coatings in order to assess their wear resistance. Test results show that the PEO treated samples exhibit significantly better mechanical properties compared to hard anodized samples. The elastic modulus and hardness of the PEO coating are 2–3 times greater than of the hard anodized coating and subsequently, an improved wear resistance of the PEO coating has been achieved. The mechanical properties of the coatings and their relations to their tribological performance are discussed. [Copyright &y& Elsevier]

Abstract: Plasma electrolytic oxidation (PEO) is a unique surface treatment technology which is based on anodic oxidation forming ceramic oxide coatings on the surface of light alloys such as Mg, Al and Ti. In the present study, PEO coatings prepared on AZ91D, AZ31B, AM60B and AM50B Mg alloys have been investigated. Surface morphology and elemental composition of coatings were determined using scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS). SEM results showed that the coating exhibited a porous top surface layer and a subsequent dense layer with micro-pores and shrinkage cracks. Phase analysis of coatings was carried out by X-ray diffraction (XRD). XRD analyses indicated that PEO coatings on AZ alloys had higher amount of Periclase (MgO) followed by the presence of Spinel (MgAl2O4) e.g. on the AZ91D alloy compared to that on AM series alloys. In order to examine the effect of substrate composition on adhesion strength of PEO coating scratch tests were carried out. Electrochemical corrosion tests were undertaken by means of potentiodynamic polarization technique in 3.5% NaCl solution at room temperature (20±2°C). Corrosion test results indicated that the corrosion rates of coated Mg alloys decreased by nearly two orders of magnitude as compared to bare Mg alloys. PEO coatings on AZ series alloys showed better corrosion resistance and higher adhesion properties than AM series alloys. In addition to the PEO processing parameters, such are mainly attributes of the compositional variations of the substrate alloys which are responsible for the formation, phase contents and structural properties of the PEO coatings. [Copyright &y& Elsevier]