Removal of Surface Scale from Titanium Metal by Etching with HF-HNO3 Mixed Acid

摘要

Etching of commercial pure titanium (CP-Ti) covered with metal oxide scale transferred to the surface from roller mills like those used for steel refining was investigated based on potentiodynamic polarization measurements and quantitative analysis of metal dissolved in HF or HF-HNO3. CP-Ti prepared by an industrial titanium supplier, titanium plate with scale (S-Ti), and annealed and pickled titanium (AP-Ti) were examined. The titanium substrate under the scale layer immediately dissolved in HF solution of concentration 1.0 mol.L-1. The etching behavior was examined in detail by electrochemical analysis in dilute HF. At HF concentrations less than 0.1 mol.L-1, the oxidized layer of S-Ti remained. X-ray photoelectron spectroscopy was used to identify the major components of the S-Ti surface, such as copper and iron impurities. In HF-HNO3 solution, the scale was removed slowly, even at high HF concentrations. The amounts of dissolved titanium indicated that calcium on the titanium metal surface increased the etching rate, and the minimum apparent activation energies,.Ea, of the etching reactions were observed at a concentration of 0.0316 mol.L-1 HF aq. for S-Ti and AP-Ti because of the trade-off between the HF activity and ionic dissociation. Etching of CP-Ti began with dissolution of the passive layer of titanium; corrosion of S-Ti was the result of destruction of the titanium oxide layer by F-and dissolution of the pure titanium substrate. The etching behavior of S-Ti at high HF concentrations suggested that scale peeling by substrate etching is a promising method for efficient scale removal. The HNO3 concentration had little effect on the anodic polarization curves of CP-Ti. This is attributed to the presence of a stable oxide layer on the titanium metal. We investigated the details of the S-Ti etching mechanism in HF-HNO3 for efficient scale removal.