An investigation on KOH concentration effect on morphology, phase composition and corrosion behavior of nanocomposite coatings on Ti6Al4V via plasma electrolytic oxidation

Titanium and its alloys are used in a wide range of industries such as biomedical, aerospace and marine fields. In such applications, Ti components are often in tribological contact with different metals and media under stationary or dynamic loading [1]. These contact loads can cause damage of the thin native oxide film which passivates the titanium surface, and titanium with a strongly negative standard electrode potential, undergoes intensive interactions with other materials and/or media [2]. These interactions can generate various adverse effects on titanium components, such as high friction, galvanic and crevice corrosion as well as corrosion embrittlement [3]. Therefore, different surface treatment processes have been developed in order to enhance the tribological behavior of this group of alloys. Plasma electrolytic oxidation (PEO) is a relatively new surface engineering process to apply ceramic-like coatings on light alloys such as titanium, magnesium, aluminum and zirconium in order to improve the corrosion and wear resistance properties of the substrate [4]. PEO is based on conventional anodic oxidation in aqueous electrolyte solutions, but operated above the breakdown voltage, which results in the formation of plasma micro-discharge events [5]. In the present work, Ti6Al4V substrates were used as the anodes. Four electrolyte systems with the same concentration of sodium aluminate (NaAlO2) (10 g/L) and different concentrations of potassium hydroxide (KOH) (0, 2, 4 and 6 g/L), were prepared. PEO test was conducted in constant voltage mode with the voltage of 400 V. Frequency, duty cycle and process time were set to be 1000 Hz, 40% and 10 min, respectively. Morphology and chemical composition 14th Annual Electrochemistry Seminar of Iran Materials and Energy Research Center (MERC), 12- 13 Dec, 2018 234 of the coatings were evaluated by field emission scanning electron microscope (FESEM) and Xray diffraction (XRD). Electrochemical corrosion behavior of the coatings was determined by electrochemical impedance spectroscopy (EIS). The XRD patterns of the coated samples are shown in Figure 1 (a). All the coatings consisted of Al2TiO5 and Al2O3 phases as well as strong Ti peaks from the substrate and by increasing the concentration of KOH, oxide peaks were intensified. FESEM graphs of all the coatings showed a porous structure which is a characteristic of PEO coatings. The EIS results are shown in Figure 1 (b, c and d) and the equivalent circuits used for fitting the PEO-coated samples and the bare specimen are illustrated in Figure 1, e and f. The coating which was formed in the electrolyte with 4 g/L of KOH had the highest corrosion resistance which was 51 times higher than that of the substrate.