The corrosion behavior of multi-pass friction stir processed (FSP) pure titanium was studied in 0.5 M H2SO4 solutions. Microstructures of treated and untreated samples were characterized using scanning electron microscopy. It was found that the grain size decreased with increasing the number of applied passes of FSP. Electrochemical tests including potentiodynamic polarization measurements and electrochemical impedance spectroscopy showed that three passes of FSP treatments resulted in a Ti sample which exhibited the best passive behavior and had the highest corrosion resistance among all samples in strongly acidic solutions of 0.5 M H2SO4. These improvements can be attributed to the emergence of diverse structural defects and grain refinement induced by FSP treatments. Moreover, Mott–Schottky analysis was performed to investigate the semiconducting properties of passive films. It was found that the semiconducting behavior remained the same after FSP treatments but it reduced donor densities and surprisingly introduced an additional donor level.
