Effect of Potassium Hydroxide on Corrosion Behavior of Coatings Formed by Plasma Electrolytic Oxidation Method on the Titanium Substrate

In this study, ceramic coatings were formed successfully by plasma electrolytic oxidation process in aluminate-based electrolyte containing different concentrations of potassium hydroxide (3, 4 and 5 g/l) on pure titanium substrate. All processes were carried out at constant voltage of 420 v and for 180 s. The effects of the chemical composition of electrolyte on the sparking voltage, microstructure and corrosion behavior were studied. The surface and cross-sectional scanning electron microscopic (SEM) images and corrosion tests of coatings in a 3.5 wt. % NaCl solution showed that the increasing of potassium hydroxide concentration from 3 to 4 g/l resulted in increasing the compactness and uniformity, decreasing the average size of surface micro-pores despite increasing the porosity, increasing the thickness and increasing the corrosion resistance. However, increasing of potassium hydroxide concentration in the electrolyte to 5 g/l due to the excessive electrolyte conductivity caused to formation of bigger and stronger sparks and thus increasing the average micro-pores size despite decreasing the porosity and increasing the thickness and decreasing the corrosion resistance of coating. Therefore, the coating formed in the electrolyte containing 4 g/l potassium hydroxide showed the most uniform and dense structure with the
smallest micro-pores size and the best corrosion behavior. In fact, this sample with corrosion resistance about 22.88×105 (Ω cm2 ) increased the resistance of uncoated titanium, 36 times. Using X-ray diffraction pattern, it was determined that thecoating was composed of rutile, anatase and TiAl2O5 phases