3.1.2. Corrosion Morphology

To investigate the corrosion process of the ductile iron pipe in the real service environment of thermal pipeline, an immersion corrosion test was conducted in the autoclave. The corrosion morphology of the two different specimens after different periods of immersion is shown in Figure 2. As the immersion time increases, there was a marked increase in the corrosion product (Figure 2a,b,e,f). As shown in Figure 2c,d, the surface of the ductile iron without surface mechanical treatment featured large pits after the rust removal, indicating that serious localized corrosion was initiated on the specimen's surface. As the acceleration time increases, the number of large-sized holes increases, and local corrosion intensifies [23,24]. The whole interface between the graphite spheres and the matrix was dissolved on the ductile iron specimen that had not received surface mechanical treatment. This is primarily the result of the holes and shrinkages formed around the graphite spheres. Aggressive ions such as Cl− would be preferentially enriched at these defect sites, resulting in the local acidification of the solution environment. The interface with high electrochemical activity would be dissolved easily. The mechanically treated ductile iron sample shows slight corrosion morphology (Figure 2g). Serval shallow pits were randomly distributed on the specimen's surface after 120 h of immersion. The corrosion is relatively mild, mainly comprising the pits left by the dislodging of graphite balls after the spread of corrosion. As the immersion time increased to 240 h, larger sized pits could be seen on the surface of the mechanically treated ductile iron specimen (Figure 2h). This is mainly due to the integration of the pits formed after the dislodging of the graphite balls. However, the number of pits was significantly smaller than that of the ductile iron samples that had not received surface mechanical treatment (Figure 2d). As the upper graphite sphere is detached, the graphite sphere buried at the bottom is also exposed.

**Figure 1.** The microscopic morphology of the two ductile iron samples before the experiment. (**<sup>a</sup>**–**f**) ductile iron pipe with no surface mechanical treatment, (**g**–**k**) ductile iron pipe with surface mechanical treatment.

**Figure 2.** Surface corrosion morphology of different specimens after the immersion test. (**<sup>a</sup>**,**b**,**e**,**f**) Before and (**<sup>c</sup>**,**d**,**g**,**h**) after the corrosion rust removal. (**<sup>a</sup>**,**c**,**e**,**g**) Immersion at 120 h, and (**b**,**d**,**f**,**h**) immersion at 240 h. (**<sup>a</sup>**–**d**) Ductile iron with no surface mechanical treatment, (**<sup>e</sup>**–**h**) Ductile iron with surface mechanical treatment.

The SEM observation results show that there are many shrinkage holes and tail-like crevices on the surface of the ductile iron. After mechanical polishing to remove these obvious defects and applying a high-pressure immersion corrosion test, the degree of corrosion on the specimen's surface is significantly reduced, as is the number of local corrosion holes. It can initially be concluded that the corrosion resistance of ductile iron is significantly improved by the use of suitable surface mechanical treatments.

In order to analyze the development of the different pits in a quantitative manner, the 3D corrosion pit morphology of the specimen after rust removal was observed using confocal laser scanning microscopy (CLSM). The results are shown in Figure 3. Due to the presence of more tail-like defects with the shrinkage and loosening of the tissue, the corrosion on the surface of the non-mechanically treated ductile iron samples increased after the immersion test (Figure 3a,c). There was also an increase in the depth of the localized corrosion pits and an increase in the corrosion area. In contrast, the surface-treated ductile iron samples corroded slightly (Figure 3b,d); this is consistent with the results observed using SEM.

The pits on the surfaces of both samples were analyzed quantitatively and the results are shown in Figures 4 and 5. After 120 h of immersion corrosion, the number of pits (124/15 μm) and the maximum depth of the pits in the surface of the mechanically treated ductile iron samples were significantly lower than in the untreated specimens (220/25 μm). Equations (5) and (6) are used to describe the effect of the surface treatment on improving the pitting resistance of the ductile iron material:

$$\text{pren}(\text{number}) = \frac{n - n\_{st}}{n} \times 100\% \tag{5}$$

$$\text{pren}(\text{depth}) = \frac{d - d\_{st}}{d} \times 100\% \tag{6}$$

where *n* is the number of etch pits on the ductile iron without surface treatment, and *nst* is the number of etch pits for the ductile iron with surface treatment. Additionally, *d* is the maximum pit depth for the ductile iron without surface treatment, and *dst* is the maximum pit depth for the ductile iron with surface treatment. The results show that the number

of pits decreased by 43.6%, and the maximum pit depth reduction rate reached 40%. The K-value distribution results show that, after the 120 h immersion corrosion test, the volume of etch pits on the surface of the ductile iron samples without surface mechanical treatment reached 2500–10,000 μm3. The K value distribution is not completely concentrated in the small corrosion pit area; it is evenly distributed between the medium and large corrosion pits. This indicates that the large pits observed by SEM are not coincidental. As shown in Figure 4b, the volume of corrosion pits on the surface of the sample is distributed below 2500 μm<sup>3</sup> after the surface mechanical treatment.

**Figure 3.** 3D corrosion morphology of different specimens after the immersion test. (**<sup>a</sup>**,**<sup>c</sup>**) represent the ductile iron that had not received surface mechanical treatment; (**b**,**d**) represent the ductile iron that had received surface mechanical treatment.

**Figure 4.** The number of etching pits and the K-value statistics of the CLSM results after the 120 h accelerated test. (**<sup>a</sup>**,**b**) Ductile iron with no surface mechanical treatment, (**<sup>c</sup>**,**d**) ductile iron with surface mechanical treatment.

**Figure 5.** Distribution of the number of etch holes as a function of their size and the statistics of the K value of the CLSM results after the 240 h accelerated test. (**<sup>a</sup>**,**b**) Ductile iron with no surface mechanical treatment, (**<sup>c</sup>**,**d**) ductile iron with surface mechanical treatment.

After continuing the immersion in the autoclave up to 240 h, the corrosion of both ductile iron samples increased to different degrees. Specifically, the number, volume, and depth of pits increased, especially for specimens that had not received surface mechanical treatment. Although the surface corrosion pits on the surface of the mechanically treated ductile iron samples remained small, medium and large volume corrosion pits with volumes greater than 2500 μm<sup>3</sup> began to appear, and reached a maximum volume of 8000 μm3. These occasional large pits are formed by the fusion of several small pits during the corrosion evolution process. Compared to the state of the surface at 120 h, the number of corrosion pits with a volume distribution of 2500 to 10,000 μm<sup>3</sup> was, predictably, increased for the samples without surface mechanical treatment. Calculations show that, in the surface-treated ductile iron material, the number of corrosion pits can be reduced by 51% and the maximum pit depth is slowed by 50% after 240 h of immersion.

#### 3.1.3. Mechanisms of the Localized Corrosion Initiation

The CLSM results showed that the corrosion of the ductile iron was effectively slowed down after the mechanical treatment was used to eliminate the obvious defects on the surface of the ductile iron. Combined with the SEM observations, these finding sugges<sup>t</sup> that the main reason that the increased corrosion of the ductile iron occurs in the simulated water quality is due to localized corrosion [23,25,26]. After the ductile iron that had not had its surface defects removed was soaked in the autoclave, there were many large corrosion pits on the surface of the sample. These local corrosion pits are large, deep, and numerous. Additionally, when these surface defects are removed, the surface of the ductile iron is basically flat, except for a very small number of small holes. However, as the corrosion proceeds, the surface will continue to exhibit shrinkage holes and defects, and the rate of corrosion will gradually increase.

Observation results by SEM, the localized corrosion process can be described as follows: (1) With the aggressive ions accumulated in the defects at the matrix and the graphite nodule, the initiation of localized corrosion is triggered (Figure 6a). (2) With the evolution of the

localized corrosion, the galvanic effect [27–29] between the matrix and graphite promotes the development of localized corrosion (Figure 6b). (3) With the growth of smaller pits, larger pits would be formed, resulting from the consolidation of these smaller pits (Figure 6c). The depth of the pits increases as the small graphite spheres continue to fall off during the corrosion process.

**Figure 6.** The localized corrosion initiation process in ductile iron (**<sup>a</sup>**–**<sup>c</sup>**).

3.1.4. Simulation of the Corrosion Process

In order to further confirm the conclusion offered in 3.1.3, and to describe clearly the dynamic process of the increased corrosion of ductile iron by shrinkage holes, a finite element simulation [30–33] was used to assess the ductile iron's surface at the shrinkage gap. The model defines the kinetic characteristics of the actual electrolyte environment and the cast iron substrate. The planar geometry and mesh division of the model are shown in Figure 7. The whole model consists of spherical graphite, an electrolyte, and a cast iron electrode. Marking points 1,2,3 facilitates the observation of shape changes due to corrosion in the later simulation studies.

**Figure 7.** Schematic diagram of the geometry and meshing of the simulation model.

In the early stages of corrosion, when uniform electrochemical corrosion occurs over the entire area, the crescent-shaped gaps around the graphite were filled with the electrolyte solution (Figure 7b). This is because the shape of the gap was not extended when corrosion occurred for 1d (Figure 8a,e). When corrosion occurs for 3 d (Figure 8b,f), the shape of the gap changes due to corrosion. Additionally, because of charge conservation, as Fe2+ dissolves and oxygen are consumed, higher amounts of H+ and Cl− in the solution diffuse into the gap. The pH and Cl− concentrations also changed. The highest Cl− concentration was found at the bottom of the gap (point 3), reaching 190 ppm, and the pH decreased to 9. This also resulted in a concentration difference within the gap from the overall solution environment [25,34].The gap kept expanding. After 7 d of corrosion (Figure 8c,g), the gap gradually evolved into a teardrop-shaped etch pit [35]. The Cl− concentration in the gap reached a maximum of 260 ppm and the pH dropped to a minimum of 8.9. At this

point, the gap expanded at an increasingly rapid rate. This continuous anodic dissolution also continuously produced Fe2+ and consumed oxygen, prompting more migration of Cl ions and transport of H into the crescentic gap. further acidifying the solution in the interstitial space. Because of this vicious cycle, the initial gap gradually expanded into a teardrop-shaped corrosion pit at 10 d (Figure 8d,h). This is consistent with the statistics provided by the CLSM. The pH of the corrosion pits (point 3) was only 8.32 at this time, and the Cl− concentration increased to 700 ppm.

**Figure 8.** The pH and Cl− diffusion at different times. (**<sup>a</sup>**–**d**) culated results of pH at 24 h, 72 h, 168 h, and 240 h, (**<sup>e</sup>**–**h**) alculated results of Cl− diffusion at 24 h, 72 h, 168 h, and 240 h. On this scale, 1 represents 100 ppm concentration.

According to the guidelines of the physical model adopted in this work, as corrosion occurs, the variation in the anode's surface potential in the crescent-shaped gap is shown in Figure 9a~d. Inside the gap, the local potential difference in the interface iron/near solution gradually becomes more negative from the top to the bottom of the gap. The electrode potential at the top of the gap (point 1) was −0.62 V at 10 h and 90 h. The anode's metal corrosion potential at the bottom of the slit (point 3) decreased to −0.65 V. A more negative electrode surface potential also leads to a higher corrosion rate and more severe corrosion at the bottom of the crevice (Figure 9c). This is because, with the negative shift of the electrode potential, a higher overpotential is generated in the interface region between the electrode and the electrolyte (Equation 4), which results in an enhanced corrosion kinetic process. At 240 h, the surface corrosion potential of the electrodes inside the gap continues to shift negatively. The potential at point 1 decreases to −0.65 V and the surface potential at point 3 decreases to −0.73 V. The corrosion process still gradually increases.

#### *3.2. The Effect of the Water Environment on the Corrosion Kinetics of Ductile Iron* 3.2.1. Corrosion Rate Analysis

Figure 10 shows the corrosion rate of the two ductile iron materials after the autoclave immersion experiment. After 120 h of immersion, the corrosion rate of the ductile iron samples without surface treatment was much higher than that of the surface-treated samples. With the immersion time increased to 240 h, the corrosion rate of the surface-treated samples was still lower than the specimen without surface treatment. This corrosion kinetic law indicates that the corrosion rate of ductile iron can be substantially slowed down after surface treatment to eliminate obvious defects on the surface. It also shows that the corrosion damage of ductile iron in a harsh water quality environment is caused by very severe local corrosion [36,37] due to the formation of dense differential cells [38] by shrinkage holes.

**Figure 9.** Calculated relative changes of local metal potential at different times. (**<sup>a</sup>**–**<sup>c</sup>**) potential distribution clouds at 10 h, 90 h, and 240 h, respectively, (**d**) relative changes of local potential of the metal surface along the y-direction, and (**e**) the corrosion rate distribution along the y-direction.

**Figure 10.** Corrosion rates of different ductile iron samples after different periods of immersion.
