3.2.2. Electrochemical Test

The polarization curve can accurately reflect the corrosion kinetics of the electrode. It is able to express both the cathodic oxygen consumption and the anodic electrode dissolution. It is widely used in metal corrosion studies [23,39–42]. The results are shown in Figure 11. The figure shows that the two ductile iron samples exhibit the same electrochemical reaction mechanism in different solution environments. The cathodic process primarily concerns the consumption of oxygen, and the anodic process concerns the dissolution of the metal. Under low oxygen dissolution conditions, the cathodic process is limited. It is worth noting that the surface-treated ductile iron samples in solution no. 4 are always passivated after a certain anodic potential polarization. The two materials also exhibit significant differences in their corrosion kinetics at different immersion cycles when the water environment changes.

**Figure 11.** Dynamic potential polarization curves of two types of ductile iron during different periods of immersion in eight simulated water conditions, (**<sup>a</sup>**–**<sup>c</sup>**) are ductile iron without mechanical treatment; (**d**–**f**) are ductile iron with mechanical treatment.

The polarization curve data in the range of ±(100 to 125) mV, relative to the open circuit potential, were intercepted to fit the icorr, and the results of the fit are shown in Table 2. At 0 d, the icorr of the unsurfaced samples was higher than that of the surface-treated samples in all simulated environments (Figure 12a). However, the icorr of the surfacetreated samples started to improve as the immersion time increased. By 10 d of immersion, the icorr of the surface-treated samples in the No. 3, 5, 7, and 8 solutions had surpassed that of the unsurfaced samples (Figure 12c). This verifies the hypothesis previously made in the autoclave immersion experiment. The mechanical treatment eliminates the obvious defects on the ductile iron surface; however, as the corrosion proceeds, uniform corrosion will cause defects to gradually be exposed on the sample surface, accelerating the occurrence of local corrosion. Overall, compared to the icorr average, the surface-treated ductile iron samples showed significantly higher corrosion resistance in harsh water conditions than the non-surface-treated samples.

To confirm the findings of the kinetic potential test, an electrochemical workstation was used to perform EIS tests on both materials under the same conditions in solutions no. 2, 4, 5, and 8. The nyquist plot (Figure 13a) shows that the EIS in solutions no. 8 and 7 appears to be characterized by high frequency capacitive arcs and low frequency Warburg impedance. It shows that there is a diffusion impedance for the electrochemical reaction under this condition. The equivalent circuit in Figure 13h was used to fit the EIS data. Additionally, only one capacitive arc exists for the EIS in solutions no. 3 and 5. The equivalent circuit in Figure 13g was used to fit the EIS data [43]. R1 is the solution resistance, and R2 is the equivalent resistance of the charge transfer impedance in the interface region. CPE1 is the equivalent capacitance of the bilayer in the interface region, and W1 is the Warburg impedance [44,45].


**Table 2.** Fitting results of icorr by potentiodynamic linear polarization scanning.

**Figure 12.** Fitting results of the icorr values of different ductile iron samples (**<sup>a</sup>**–**<sup>c</sup>**).

**Figure 13.** EIS test results. (**<sup>a</sup>**–**<sup>c</sup>**) are nyquist plots, (**d**–**f**) are bode plots and phase angles, (**g**,**h**) are the equivalent circuits, (**i**,**j**) are the comparison fitting results of RP at 0 d and 10 d.

The fitted results are shown in Table 3. It can be seen, the charge transfer resistance of the surface-treated ductile iron samples is generally higher at the beginning of the immersion process (Figure 13i). As the immersion time increases, the Rp of the surfacetreated ductile iron material starts to decrease at 10 d of corrosion. A weaker corrosion inhibition effect was exhibited. This also explains the sudden increase in the icorr of the surface-treated ductile iron at 10 d (Figure 12c).


**Table 3.** Fitting results of Rs, Rp, and WR by EIS.

3.2.3. The Influence of Environmental Factors

In order to further quantify the influence of different water quality environmental factors on the corrosion process of ductile iron with different treatments, the xgboost algorithm was used to compare the influence of environmental factors (pH, Cl− concentration, hardness, oxygen content, and temperature) on the corrosion ductile iron with different surface states. The calculation results are shown in Figure 14.

As can be seen from Figure 14a, for the ductile iron material without surface mechanical treatment, the presence of dissolved oxygen in the solution plays a major controlling role among all environmental factors, contributing to 68.5% of the increase in icorr. The remaining environmental factors were evenly distributed below 15% in terms of the degree of influence on icorr [46,47]. It is noteworthy that the contribution of Cl− concentration to icorr was only 2%. This is because the restricted oxygen levels caused a significant slowing of local corrosion. At the same time, temperature also reduces the corrosion acceleration effect of Cl− [39]. The contribution of each environmental factor to the elevated icorr under fully oxygenated conditions is shown in Figure 14b. At this point, the Cl− [25] concentration played a control role and contributed up to 85% of the increase of icorr. This indicates that the acceleration effect brought about by Cl− can only occur under conditions of sufficient oxygen. The study in 3.1 found that the corrosion of ductile iron intensified because of the severe localized corrosion caused by the concentration cell effect, and the results of the kinetic law analysis here also confirmed this. The continued dissolution of the anode under conditions of sufficient oxygen causes Cl− to continue to diffuse into the slit between the graphite and the cast iron, exacerbating the corrosion of the ductile iron. this is why the icorr contribution is so high.

Figure 14c shows the contribution of environmental factors to the icorr of ductile iron after eliminating obvious defects on the surface. It can be deduced that the influence effect of each environmental factor becomes uniform. This means that, under this condition, the effect of the deterioration of the water quality environment on the corrosion acceleration of ductile iron is obviously weakened; moreover, the oxygen concentration is not a highly influential environmental factor, but the contribution rate is still high. This may be the

reason for the weakening of the local corrosion effect. Under fully oxygenated conditions, the contribution of Cl− concentration to icorr suddenly decreases. This indicates that, as the degree of local corrosion decreases, Cl− concentration is no longer the most important environmental factor affecting icorr. However, pH and immersion time contributed to the increase in icorr, which may be due to the fact, that the originally flat surface of the mechanically treated ductile iron samples gradually exhibited more defects as the corrosion progressed, leading to increased localized corrosion. However, it still corrodes less than ductile iron without any surface treatment. For temperature and water hardness, the icorr contribution did not change significantly before and after mechanical treatment under fully oxygenated conditions. This indicates that an increase in temperature from 60 ◦C to 90 ◦C does not bring about a significant increase in localized corrosion.

**Figure 14.** Analysis of the contribution of environmental factors to icorr. (**<sup>a</sup>**,**<sup>c</sup>**) show the contribution of non-surface-treated ductile iron in all environments and in fully oxygenated conditions, respectively; (**b**,**d**) show the contribution of the surface-treated ductile iron in all environments and in fully oxygenated conditions, respectively.
