Time dependence of corrosion behavior on Ta in NaOH solutions

Ishijima, Yasuhiro ; Ueno, Fumiyoshi  ; Abe, Hitoshi 

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 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 NaOH solution for more than 48 h showed NaTaO formation. The polarization resistance decreased with immersion time for all NaOH concentrations up to about 24 h after immersion. Thereafter, the polarization resistance increased with immersion time in 7 mol/L NaOH solution and remained almost constant in the other NaOH concentrations. Findings suggested that the change in the corrosion rate was affected by the film formation during immersion, since the time dependence of the polarization resistance and the sum of film resistance and charge transfer resistance had the same tendencies. The precipitation film was mainly NaTaO formed by the dissolution of the passivity film on Ta.