Comparative corrosion behaviour of different alloys for nitric acid service in reprocessing plants

In nuclear spent fuel reprocessing plants in India, austenitic stainless steel (SS) 304L serves as the structural material for critical components that handle boiling nitric acid. SS 304L is prone to intergranular corrosion (IGC) in the transpassive region of the polarisation curve. As a result, there is a need to replace SS 304L with corrosion-resistant alloys of Ti or Zr. In this regard, the corrosion behaviour of two candidate alloys, Ti–Al–Zr and Zircaloy-4, was assessed and compared with SS 304L. The corrosion evaluation was performed using electrochemical tests, corrosion immersion tests in three different phases (liquid, vapour and condensate) of nitric acid, and metallographic examination. Both Ti–Al–Zr and Zircaloy-4 exhibited a broad passivity range of up to 3 V Ag/AgCl in various nitric acid solutions. Based on these investigations, it was concluded that both Ti–Al–Zr and Zircaloy-4 are suitable for use in critical components handling boiling nitric acid with a high concentration of strongly oxidising ions. The corrosion resistance of the three alloys tested was ranked as: Zircaloy-4 > Ti–Al–Zr > SS 304L.