# Pore Solution Chemistry of Calcium Sulfoaluminate Cement and Its Effects on Steel Passivation
## 3 Results and Discussion
### Factors Influencing Steel Passivation
#### 3.3.3 Effect of Sulfate Ion  
Figure 5 shows the effect of SO4\({}^{2-}\) on the corrosion potential under different levels of alkalinity. It can be seen in Figure 5a that although the potentials of all samples stabilize within the range of passivation after four days, the potentials decrease after the addition of Na2 SO4. However, there is no clear correlation between the extent of potential reduction and the SO4\({}^{2-}\) concentration. When the OH\({}^{-}\) concentration drops to 0.01 M, the samples will depassivate as long as the SPSs contain a thimbleful of sulfate, and the time necessary for depassivation negatively correlates to the concentrations of sulfate. Once NaOH reduces to 0.001 M, as shown in Figure 5c, all samples are not able to passivate, despite of the SO4\({}^{2-}\) concentration. Several papers have been published on the effects of SO4\({}^{2-}\), but they neglect the influence of hydroxyl concentrations [10; 11; 12; 13; 14]. From the evolution of the corrosion potential in Figure 5, it is evident that the depassivation caused by SO4\({}^{2-}\) is sensitive to alkalinity. When the concentration of hydroxyl is lower than 0.01 M, the destruction of passive film will be obvious. The underlying mechanism is probably related to the way that the sulfate ion damages passive film. Regarding the depassivation triggered by chloride ions, an adsorption mechanism is applied, i.e., Cl\({}^{-}\) competes with OH\({}^{-}\) at the steel surface, thereby gradually destroying the passive film [36]. The potential generally decreases with the increase of chloride concentrations, while the sulfate ion plays a totally different role. It directly participates in the passivation process to generate a film with a structure of inferior quality [10; 37]. This theory agrees with the experimental results. The decline of the potentials displayed in Figure 5a after adding sulfate implies that the tendency of depassivation increases, which means the stability of the passive film weakens. However, since the alkalinity is high enough, passivation can still be maintained, and the drop in potential is not affected by the increase of sulfate concentrations. When the concentration of hydroxyl decreases, the unstable passive film can no longer be maintained. Therefore, depassivation happens to all the steel samples in the manner of the potential development depicted in in Figure 5b,c. Figure 4: Effect of OH\({}^{-}\) concentration on the OCP. 3.4 Effect of Al(OH)\({}_{4}\)\({}^{-}\) on the corrosion potentials of steel samples in SPSs containing 0.05 M Na\({}_{2}\)SO\({}_{4}\) is shown in Figure 6. In Figure 6a, the high concentration of OH\({}^{-}\) can keep all samples well passivated, so the association between Al(OH)\({}_{4}\)\({}^{-}\) and passivation is not obvious. When the concentration of hydroxyl reduced to 0.01 M, the potential developments change significantly. In cases of SPSs containing NaAlO\({}_{2}\) concentrations lower than 0.015 M, all the steel electrodes corrode, while the other two samples mixed with higher amounts of aluminate are able to passivate, exhibiting potentials higher than \(-\)140 mv. However, if the alkalinity further decreases to 0.001 M as shown in Figure 6c, the potentials of all samples drop under \(-\)670 mv. By sorting the potentials according to their values, an interesting point is noted that the tendency to corrode, expressed by potential values, negatively correlates to the concentration of aluminate. Hence, it can be inferred that Al(OH)\({}_{4}\)\({}^{-}\) is beneficial to the passive film's stability. The protection provided by Al(OH)\({}_{4}\)\({}^{-}\) is attributed to its ability to release hydroxyl. In the corrosion process, localized acidification occurs in the active point because of the hydrolysis of metal ion [38; 39; 40], as explained by Equation (6). If there is no immediate additional supplement of OH\({}^{-}\) from the adjoining solution, this active point will not be repaired (repassivation) and corrosion occurs.