Studies on Group IV and v Valve Metal Corrosion in Acidic Fluoride Media

Dissolution rates of Ti, Zr, Nb and Ta in acidic fluoride media of different concentrations at various temperatures were measured by weight loss method, and were complemented by electrochemical measurements at room temperatures. A mathematical model to estimate the corrosion rate of these four valve metals based on the overall reaction mechanism for metal dissolution as well as H2 evolution is developed from the weight loss experiments. Valve metal dissolution process occurs through two independent pathways assisted by (HF2)- and equilibrium HF species. Except Ti, the dissolution of other valve metals studied are not significantly influenced by the H2 evolution reaction. Among the four metals evaluated, Nb offers the best corrosion resistance at high temperature and high HF concentration. The corrosion resistance of all four valve metals estimated from linear polarization, electrochemical spectroscopy and Tafel extrapolation in various concentrations of solutions match well. For Ti, dissolution rates calculated from Tafel extrapolation were lower than those based on weight loss experiments, while for Zr, Nb and Ta, they were mostly higher than those based on weight loss experiments.