*2.1. Material*

The material in this work is SA106B carbon steel, and its chemical composition is given in Table 1. The specimens were cut from the Jiangsu Chengde steel tube share Co., Ltd in China. Mainly, its metallographic structure consists of ferrite and pearlite, as shown in Figure 1, and its mean grain size is 30 μm. The projectile used for the shot peening was made of GCr15 balls with a diameter of 0.8 mm, and the shot velocity was 80 m/s. The process was carried out for 5, 10, 15 and 20 min, periods. The samples were rotated during the shot peening process. For the RASP-processed for 20 min SA106B carbon steel specimen, the average hardness at the top surface is 260 HV (Figure 2). The hardness decreased gradually from the surface to the inner, and finally approached a stable value (175 HV) at a depth of 120 μm. Furthermore, according to the well-known Hall–Petch equation, the changes in hardness are inversely proportional to the variation in the size of grains formed by RASP [18]. Therefore, the smaller the grain size, the higher the hardness will be.


**Table 1.** Chemical composition of the SA106B carbon steel (wt. %).

**Figure 1.** Optical microscope of SA106B low-carbon steel before the rotationally accelerated shot peening (RASP).

**Figure 2.** Hardness gradient as a function of depth for the RASP-processed for 20 min sample.

## *2.2. Electrochemical Measures*

The specimens with a size of 10 mm × 10 mm × 5 mm were used in the electrochemical tests. They were spot-welded to the back of the samples with copper lines for the electrical contact. Prior to each electrochemical experiment, the exposed surface (with an area of 1 cm2) of the specimens were wet ground with a series of SiC papers ranging from #600 to #3000, cleaned thoroughly with alcohol, acetone, and deionized water in sequence, and then dried quickly with flowing compressed hot air. The samples were polished first and then immersed in the 0.05 M H2SO4 + 0.05 M Na2SO4 and 0.2 M NaCl + 0.05 M Na2SO4 solutions, respectively. The electrochemical measurements were performed at room temperature. The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) experiments were conducted by using a three-electrode system. A platinum sheet was used as the auxiliary electrode. A saturated calomel electrode (SCE) as the reference electrode, and a specimen as the work electrode. The samples were washed with distilled water and immediately dried to analyze the corrosion morphology after the electrochemical measurements. To confirm the data reproducibility, the polarization tests were performed at least three times. The EIS tests were carried out at the open circuit potential (OCP) with a sinusoidal potential amplitude of 10 mV, running from 100 kHz to 10 MHz
