Time Dependence of Corrosion Behavior on Ta in NaOH Solutions

Abstract

The time dependence of corrosion behavior on tantalum used in nuclear fuel reprocessing equipment in sodium hydroxide solution was investigated by immersion corrosion tests, and the mechanism of aging change was discussed from surface observations and electrochemical measurements. The immersion tests were carried out at room temperature with NaOH concentrations ranging from 1 to 7 mol･L^－1 and immersion periods ranging from 24 to 168 h, respectively. The corrosion rate increased with NaOH concentration, but peaked with immersion time and then decreased. The time to peak of corrosion rate was shorter with higher NaOH concentration. The XRD patterns and Raman spectra at the surface of the specimens immersed in 7 mol･L^－1 NaOH solution 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.