*2.1. Materials*

The chemical composition of the ductile iron castings used in this work is 3.75% C, 1.8% Si, 0.45% Mn, 0.07% P, 0.012% S, 0.06% Mg, and Bal. Fe. Defects including shrinkage and holes can be easily formed on the material's surface in the manufacturing process. Two different specimens were produced for this research. One type of specimen was produced by being slightly mechanically ground to 2000#, with surface defects maintained on the specimen surface. Another type of specimen was mechanically ground to 2000#

until the defect layer disappeared. All of the specimens were polished to a mirror finish with diamond abrasives with a particle size of 1.0 μm.

#### *2.2. Environmentally Simulated Solution*

Before the immersion experiment, nitrogen was introduced into the environment; this required deoxygenation to simulate an anoxic environment, in which the simulated liquid was deoxygenated at a constant speed of every 250 mL for 3 h. The concentration was then measured and controlled at 0.3 mg∗L−1, and then start the immersion experiment. A total of 8 different environments were configured for the experiments, as shown in Table 1. The water hardness was adjusted with CaCO3, and the Cl− concentration was adjusted with NaCl. The same batch was immersed in a large 8-hole water bath during the immersion process. A small water bath was used for the electrochemical measurements to ensure a constant temperature during the test.

**Table 1.** Characteristic parameters of the simulated solution.


*2.3. Autoclave Immersion Experiment*

Solution environment no. 8 was selected for the experiment. Ductile iron samples of 50 mm × 25 mm × 4 mm, both with and without surface mechanical treatment, were polished with silicon carbide paper (2000#) and then cleaned with ethanol and dried using cold air. After determining the original weight of the specimen, the specimen was suspended from the inner wall of the beaker using a nylon fishing line. During the immersion process, the autoclave was pressurized to 1 MPa and the water temperature was controlled at 90 ◦C; the test cycles were 1220 h and 2440 h. Four parallel specimens were prepared for each material, and three of them were used to determine the loss in thickness. The last sample was used for scanning electron microscopy (SEM) and laser scanning confocal microscopy (CLSM) characterization. The rust on the specimen was ultrasonically cleaned in a descaling solution that consisted of 98%HCl (500 mL), H2O (500 mL), and hexamethylenetetramine.
