The behaviour of titanium in nitric-hydrofluoric acid solutions

The behaviour of titanium in HNO3-HF pickling baths has been studied and understood. In a nitric-hydrofluoric acid solution, a polished titanium sample constitutes a passivatable or a passive system depending on the initial concentrations of HNO3 and HF. The titanium dissolution process in such solutions has been discussed using the passivation models described in the literature. During the pickling experiment, passivation of Ti occurs and is related to the concentration of dissolved titanium. XPS analysis of the pickled titanium surface and Raman spectrometry of the depleted pickling solutions show that the decrease of the pickling efficiency is due to the consumption of HF to form titanium fluoro or oxofluoro complexes. As expected, the quantitative addition of new fractions of hydrofluoric acid restores the pickling efficiency of the solution.