Time Dependence of Corrosion Behavior on Ta in NaOH Solutions

The time dependence of the corrosion behavior of tantalum (Ta), which is used in nuclear fuel reprocessing equipment, in sodium hydroxide (NaOH) solutions was investigated by immersion tests, and the mechanism of the time dependence was examined via surface observations and electrochemical measurements. The immersion tests were conducted at room temperature with NaOH concentrations ranging from 1 to 7 mol.L-1 for immersion periods of 24 to 168 h. The corrosion rate increased with the NaOH concentration but peaked with the immersion period and then decreased. The time to peak of the corrosion rate was shorter with higher NaOH concentration. The X-ray diffraction (XRD) patterns and Raman spectra of the surfaces of the specimens immersed in the 7 mol.L-1 NaOH solution for more than 48 h showed Na8Ta6O19 formation. The polarization resistance showed a constant value or an increase after a decrease immediately after immersion. It was suggested that the change in corrosion rate is affected by the formation of film during immersion, since the time dependence of polarization resistance and the sum of film resistance and charge transfer resistance have same tendencies. The film was considered to be mainly Na8Ta6O19 formed by the dissolution of Ta.