3.2. Evaluation of the Efficiency of the Intelligent OH−-Regulated Microcapsules
The corrosion morphology of the sample obtained using the XCT measurement is shown in
Figure 6. The original slice, shown in
Figure 6a, was processed with image segmentation to distinguish different materials, including the reinforcing bar, corrosion products, the cement matrix, and cracks (
Figure 6b). Thus, two-dimensional (2D) and three-dimensional (3D) information (the corrosion morphology of the reinforcing bar, development of corrosion products, and information on cracks) in the different samples could be vividly characterized.
Based on the XCT images, the corrosion process is visualized in
Figure 7. The 2D morphological changes of the maximum corroded section in the control sample (Sample A) are exhibited in
Figure 7a. Therefore, the corrosion initiation and cracking time can be determined. Prior to the first wet-dry cycle test (the initial state), the overall surface of the reinforcing bar was smooth and without rust. After 72 h of the wet-dry cycles, corrosion initiated on the surface of the reinforcing bar, and corrosion products appeared around the bar. After 288 h of continuous cycles of the wet-dry test, the expansive stress finally resulted in the cracking of the mortar matrix due to the accumulation of the corrosion product. A small amount of the corrosion products penetrated into the cracks (crack a1) in the mortar matrix. After 648 h of accelerated corrosion, as the cracks (cracks a1 and a2) developed, the corrosion process accelerated. The corrosion process is depicted in
Figure 7b in corresponding 3D images.
In the samples with the microcapsule (Samples B and C), the corrosion initiation time was delayed to 144 h (
Figure 7a), which is 72 h later than that of the control sample. After 1,008 h wet-dry cycles, the corrosion process of Samples B and C developed much more slowly than that of Sample A, and no obvious cracks were observed in the 2D images of Samples B and C. The corresponding 3D corrosion morphology of the reinforcing bar is also exhibited in
Figure 7b.
To further access the protection efficacy of the intelligent OH
−-regulated microcapsule, the XCT images were quantitatively analyzed using a method that has been verified using the gravimetric method derived in previous research [
29,
33]. The area of the reinforcing bar in different slices is represented as Equation (3):
where
Ai is the area of the reinforcing bar in different slices (
i represents the different XCT testing times,
i = 0 h, 72 h, 144 h, and so on),
nA is the number of the pixels of the reinforcing bar in each slice, and
ZA represents the real area of one voxel (15.0173 × 15.0173 μm
2).
The changes in the cross-sectional area,
Ai, of the reinforcing bar along the height of the steel bar at different testing times are shown in
Figure 8.
The area loss of the corroded reinforcing bar can be calculated as Equation (4):
where ∆
A is the area loss of the corroded reinforcing bar in each slice,
A0 is the area of the reinforcing bar in each slice at 0 h (the cross-sectional area of the reinforcing bar at 0 h in
Figure 8), and
Ai is the area of the reinforcing bar of each slice at different XCT testing times during the acceleration test.
The area loss of the reinforcing bar for each section was calculated through Equations (3) and (4). The maximum corroded cross-section for each sample was then obtained, and these are shown in
Figure 9. With the progress of wet-dry cycles, these curves fluctuated in the height direction. This means that some pitting corrosion occurred and expanded during the corrosion process. The maximum area loss of the reinforcing bar for Sample A was approximately 0.7 mm
2 at 648 h, as shown in
Figure 8A. In a range of heights from 2500–4000 µm, dramatic loss occurred in the cross-sectional area due to corrosion. In Samples B and C at 1008 h, the cross-sectional area loss of the maximum corroded section was only 0.20 mm
2 and 0.25 mm
2, respectively (
Figure 8B,C). During the entire experiment, in the control sample, the corrosion developed more locally. While in the samples with microcapsules, the corrosion occurred relatively uniformly along the height of the reinforcing bars. These results were in agreement with the 3D visualized results in
Figure 7.
The total areas of the reinforcing bar were then multiplied by the voxel height (15.0173 µm) to obtain the volume of the reinforcing bar,
Vi. The volumetric loss of the corroded reinforcing bar, ∆
V, was obtained using the following Equation (5):
where
V0 is the volume of the reinforcing bar at 0 h and
Vi is the volume of the reinforcing bar at different times during the acceleration test.
The obtained volume and volumetric loss of the reinforcing bar for the samples are listed in
Table 7. The time-dependent volumetric loss of the corroded reinforcing bar is plotted in
Figure 10, which shows that the corrosion process in the samples with microcapsules developed much slower than that in the control sample. These results correspond with the results shown in
Figure 7 and
Figure 8. In Samples B and C, no crack was observed during the entire 1008 h of the accelerated corrosion experiment, while Sample A reached the same volumetric loss at 288 h and cracks were obvious (
Figure 10b).
Additionally,
Figure 11 presents the
Rp and
icorr of different kinds of samples through the wet-dry cycles. From the linear polarization results, all samples with OH
−-regulated microcapsules showed higher linear polarization resistance (
Rp) than that of the control samples. The
Rp of samples fluctuated during a specific period, mainly due to the OH
− regulation, which contributed to a decrease in the [Cl
−]/[OH
−] value and then alleviated the corrosion development. In contrast, without the help of OH
−-regulated microcapsules, corrosion initiation and propagation of the control samples occurred more easily. The adverse trend of
icorr was shown in
Figure 11b. These electrochemical results were basically in accordance with the results from the XCT measurements.