Fig xA Dependence of the critical fluoride concentration on pH
Table 1 lists the determined critical fluoride concentrations for titanium at various dapt secretase using the methods above. The critical values at pH = 3.0∼5.0 are higher than those obtained by Nakagawa et al., which may be due to the higher dissolved oxygen concentrations and the absence of chloride ions in the present study . Fig. 3 shows the dependence of the critical value on pH. The black solid and red hollow symbols represent the tested lower and upper limit values of the theoretical critical fluoride concentrations, respectively. The former is regarded as the critical value. Based on Fig. 3 and Table 1, it is clear that the determined critical values are quite close to the theoretical ones, which indicates that the conclusions made from these values are reliable. It can be seen from Fig. 3 that the dependence of the critical fluoride concentration on pH exhibits two obvious linear regions: Region I at pH = 1.0∼2.5 and Region II at pH = 3.0∼5.0. At Region I, the slope (∂pF/∂pH) is −0.17. Although there is no experimental reference data, it is comparable to the results obtained by Fovet et al. using thermodynamic model at similar pH range . At Region II, the slope is −0.78, which is consistent with that of −0.7 obtained from the experiments at pH > 3 carried out by Nakagawa et al.  and . This suggests that the dependence of the critical fluoride concentration on pH is mainly affected by pH rather than the dissolved oxygen or chloride ions. Meanwhile, it should be emphasized that the ∂pF/∂pH value of −0.17 at Region I is larger than that of −0.78 numerically, which is in line with the results of Fovet et al. based on the thermodynamic model. Unfortunately, Fovet et al. did not discuss this phenomenon further. Based on Eqs. (1) and (2), it can be deduced that different ∂pF/∂pH values may result from different cathodic and anodic processes between Region I and Region II. Therefore potentiodynamic polarizations are used to investigate cathodic and anodic processes and obtain relevant electrochemical parameters to explain the phenomenon shown in Fig. 3. The corresponding results will be shown and discussed in Section 3.2.