The test surface of each specimen worked as the working electrode, the saturated calomel electrode (SCE) worked as the reference electrode, and a platinum foil worked as the counter electrode. After the working electrode was immersed in a test solution for 30 min to attain a stable open circuit potential, the potentiodynamic polarization studies were carried out in the potential range of -1500 to 700 mV and at a scan rate of 5 mV/s.