A Passive Film Formed on Alloy 600 in High Temperature Aqueous Solution

Alloy 600(74wt% Ni, 15wt% Cr, 9wt% Fe) is used as a material for steam generator tubing in pressurized water reactors(PWR) due to its high corrosion resistance under a PWR environment whose primary side and secondary side are operated under 340oC, 150bar and 290oC, 50bar, respectively. However, in spite of its outstanding corrosion resistance, a stress corrosion cracking(SCC) which has been one of the most important degradation issues, has occurred occasionally owing to the severe high temperature and pressure condition of a PWR. It is expected that a SCC is deeply related to an electrochemical property of a passive film formed under a high temperature and pressure condition. Therefore an understanding of the basic electrochemical behaviors regarding an anodic dissolution and a passivation of the bare surface of metals and alloys provides important information about a SCC mechanism and control. In the present work, the passive oxide films on Alloy 600 were investigated as a function of the dissolved oxygen content and the pH by using a potentiodynamic polarization, electrochemical impedance spectroscopy(EIS) and a depth profiling by Auger electron spectroscopy(AES). For this purpose, 0.5M H3BO3, 0.5M Na2SO4 and 0.1M NaOH aqueous solutions with/without dissolved oxygen at 300oC were used as the test solutions. The resistance of an oxide formed in an aerated solution was smaller than that in a deaerated solution while the oxide film in the aerated solution was thicker than the oxide film in the deaerated solution.