Corrosion behaviour of tantalum in sodium hydroxide solutions

The corrosion behaviour of tantalum has been investigated in sodium hydroxide solutions at different temperatures, using open-circuit potential measurements, potentiodynamic polarization, polarization resistance method and electrochemical impedance spectroscopy. Tantalum showed a passive behaviour in 5 and 10 wt % NaOH at 25, 50 and 75 degreesC, and in 15 wt % NaOH at 25 and 50 degreesC. In 15 wt % NaOH at 75 degreesC and in 30 wt % NaOH at all temperatures, tantalum presented a passive-active transition (self-activation) due to the spontaneous dissolution of its superficial air-formed oxide, and afterwards remained in the active state for long times, forming a polytantalate compound. In all cases, the corrosion rates increase with increasing NaOH concentration and temperature.