Sodium Aluminate Concentration Effects on Microstructure and Corrosion Behavior of the Plasma Electrolytic Oxidation Coa
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INTRODUCTION
THE major features of titanium and its alloys including high strength-to-weight ratio, relatively low density, high corrosion resistance, high melting point, good mechanical behavior, good fatigue strength and toughness, low elastic modulus, good biocompatibility, and antimagnetic properties have made them significant engineering materials in many industries such as aerospace, chemical, biomedical, automotive, energy, military, and marine.[3,4] Moreover, titanium and its alloys show certain protective abilities due to the spontaneous formation of very stable and dense oxide films on their surface in contact with air. However, the low thickness of the oxide film (5 to 70 A˚) results in its easy destruction under certain conditions.[5,6] Despite the mentioned issues, the low wear resistance, low hardness, high friction coefficient, and poor corrosion resistance in aggressive media such as sulfuric acid, hydrochloric acid, and phosphoric acid are among the disadvantages limiting the applications of these alloys.[2–4] Producing a hard layer on the surface by alloying or deposition of hard coatings has led to the improvement of the [1,2]
MARYAM MOLAEI, ARASH FATTAH-ALHOSSEINI, and SEYED OMID GASHTI are with the Department of Materials Engineering, Bu-Ali Sina University, 65178-38695 Hamedan, Iran. Contact e-mail: [email protected] Manuscript submitted December 15, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
properties of titanium and its alloys; this is gained by numerous surface treatment technologies, including physical and chemical vapor deposition, diffusion processes, plasma spray, plasma immersion implantation, laser beam treatments, and nitriding.[7] Most of these techniques are expensive and time consuming and may not provide satisfactory adhesion. The PEO process is derived from conventional oxidation and it is known as a solution to these limitations. Using this method, ceramic coatings can rapidly be formed on the surface of light metals such as titanium, aluminum, magnesium, zirconium, and their alloys, with the thickness of typically about 10–100 lm, leading to an improvement in their mechanical properties and corrosion and wear resistance.[7–10] This technique is based on the electrical discharges (numerous transient, fine, short-lived discharges) on the surface of the sample (anode) by applying high voltages (above the dielectric breakdown voltage) to the material accompanied by gas evolution in an aqueous solution[11,12] mostly consisting of silicate, phosphate, and aluminate components.[13] PEO coatings are composed of predominant substrate oxides and complex oxides of the electrolyte components. These oxides sinter into the metal surface at the breakdown sites.[14,15] PEO coatings are created under different processes of film forming, breakdown, melting, sintering, and film forming again.[16] Moreover, operational convenience, high efficiency, low pollution, adjustment of the process parameters, deposition of uniform coatings on the sample with complex geometries, and the possibility of t
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