Hydrogen absorption of commercial pure titanium in acidulated phosphate fluoride (APF) solutions of various concentrations has been examined by hydrogen thermal-desorption analysis. In immersion in the 2.0% APF solution of pH 5.0 at 25 °C, the amount of absorbed hydrogen increased with immersion time and then saturated at approximately 800-900 mass ppm. During the early stage of the 24 h immersion, the amount of hydrogen absorbed in the 0.2% APF solution was almost the same as that in the 2.0% APF solution, i.e., 200 mass ppm, whereas it saturated at 300 mass ppm with longer immersion time. For the specimen immersed in the 2.0% APF solution for 24 h, hydrogen desorption was observed with a peak at approximately 550 °C. As the immersion time increased, the second desorption appeared in the range from 150 to 400 °C. In contrast, for the specimens immersed in the 0.2% APF solution, the desorption peak temperature tended to shift from 550 to 450 °C with immersion time. Hydrogen absorption was hardly observed in the specimen immersed in the 0.02% APF solution. The concentration of the APF solutions also affected the type of corrosion product and the surface topography of the immersed specimens. The results of the present study clearly indicate that the degradation of the mechanical properties or fracture of titanium caused by hydrogen absorption occurs in acidic fluoride solutions.
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