Characterization of the products obtained from the reactions of Zircaloy-4 with an acid mixture of concentrated HNO3 and dilute HF with the aim of understanding pickle salts of zirconium alloys

Abstract

Pickling of zirconium alloys in nitric-hydrofluoric acid has been widely used in the nuclear materials industry. However, the pickle salts still remain a curiosity and a thorough knowledge of the products of the system Zr–(HNO₃–HF) still needs to be well established. The present study investigated the products obtained from the reactions of Zircaloy-4 with an acid mixture of HNO₃ (65 wt%) and HF (40 wt%) in a volume ratio of 94 : 6, similar to the one used under nuclear industry conditions. It shows that after the reactions, a gel-like solution was present. When drying it at a high relative humidity (at 25 °C and 70 ± 5% RH), an amorphous solid product formed, while in the case of at a low relative humidity (at 15/25 °C and 25 ± 5% RH), besides the amorphous solid, crystalline zirconium hydroxide nitrates Zr(OH)₂(NO₃)₂·4.71H₂O and Zr(OH)₂(NO₃)₂·1.65H₂O (possible) with low degree of crystallinity formed. These compounds were identified with the help of successfully synthesizing completely crystalline Zr(OH)₂(NO₃)₂·4.71H₂O from the gel-like solution by ultrasonication, which in itself was a breakthrough. Such a study can be expected to provide a deep understanding of pickle salts of zirconium alloys in practical use.