Effects of the solution temperature and the pH on the electrochemical properties of the surface oxide films formed on Alloy 600

The surface oxide films on Alloy 600 have been investigated as a function of the solution temperature and the pH by using a cyclic voltammetry, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and a depth profiling by Auger electron spectroscopy (AES). H3BO3, Na2SO4 and NaOH aqueous solutions with temperatures in the range of 30–300 °C were used as the test solutions. As the solution temperature of the 0.5 M H3BO3 increased, the thickness of the passive film increased but the resistance of the passive film was diminished, which is coincident with a solution temperature dependency of the passive current in the potentiodynamic curve. The inner oxide film on Alloy 600 was distinguishable from the Cr-rich outer oxide film above 100 °C. From the Mott–Schottky relation, the oxide formed in 0.5 M H3BO3 at 300 °C showed a p-type semiconductor property, accompanied by a Cr-rich oxide film throughout the whole oxide film unlike the n-type oxide films up to 250 °C. The oxide resistance of the passive film decreased in the order of 0.5 M H3BO3, 0.1 M NaOH and 0.5 M Na2SO4, which is consistent with the pH dependency of the passive current. Ni-rich oxide films of a p-type were formed in the 0.5 M Na2SO4 or 0.1 M NaOH.