**3. Results**

#### *3.1. Energy Spectrum Analysis of Steel*

Figure 1 is the surface EDS analysis and optical micrograph of the original EH47. It can be seen from Figure 1b that the surface of the original steel EH47 is composed of three main elements: Fe, Mn and C, indicating that Fe, Mn and C are the primary elements. The

optical micrograph of the original EH47 is presented in Figure 1c. The microstructure is mainly composed of ferrite and bainite.

**Figure 1.** (**a**) The surface of the original steel EH47; (**b**) surface EDS analysis of EH47 steel; (**c**) the optical micrograph before the immersion test of EH47 steel.

#### *3.2. Electrochemical Characteristics of EH47 in Seawater*

EH47 samples were immersed in seawater for 0, 14, 28 and 49 days for electrochemical experiments. The open circuit potential, polarization curve and Nyquist diagram of electrochemical impedance spectroscopy were measured.

#### 3.2.1. Open Circuit Potential of EH47

The change in open circuit potential can indicate the corrosion state and corrosion behavior of the material surface [27]. Figure 2 shows the open circuit potential curves of EH47 steel samples immersed in seawater for different durations (0, 14, 28, and 49 days) at room temperature. It can be seen from Figure 2 that the corrosion potential changes significantly after total immersion in seawater for 14 days, and the minimum value is obtained after 28 days of immersion. The variation in the open circuit potential from 14 days to 28 days is lower than that from 0 days to 14 days, which indicates that the corrosion rate of EH47 steel is relatively slow with the increase in corrosion time from 14 days to 28 days. With further extension of the corrosion time, the open circuit potential moves in the positive direction and the corrosion rate decreases gradually.

**Figure 2.** Open circuit potential of the EH47 steel sample immersed in seawater for different durations.

#### 3.2.2. Potentiodynamic Polarization Tests

Figure 3 shows the potentiodynamic polarization curves of the EH47 samples immersed in seawater for different immersion durations (0, 14, 28, and 49 days) at room temperature. It can be seen from Figure 3 that the cathodic branch at day 0 immersion indicates the limiting diffusion control characteristics of the reduction reaction of dissolved oxygen. The diffusion of dissolved oxygen cannot be more effectively inhibited because there is no rust layer on the surface of the sample under this condition. With the increase in immersion time, the cathodic branch at day 0 immersion is not significantly different from 14, 28 and 49 days. The relatively smooth curves indicate that oxygen reduction can occur but it is not diffusion-controlled. The limit control characteristics of dissolved oxygen reduction on the surface of the steel with a rust layer have disappeared and changed to the charge transfer control transformation dominated by the reduction of corrosion products in the whole process of corrosion. The anode branch has a certain passivation characteristic, and the passivation range is about 0.6 V (from −0.9 V to −0.2 V). The experimental steel underwent a cathodic electrode reaction under the limit diffusion control of dissolved oxygen; meanwhile, the anode electrode reaction underwent electrochemically active dissolution under the control of charge transfer (current). After forming a certain corrosion layer thickness at the early stage of corrosion, the reductive corrosion product γ-FeOOH appears in the corrosion layer [28]. As the corrosion reaction continues, the reduction reaction (Fe3+ + e<sup>−</sup> → Fe2+) of the corrosion layer mainly occurs in the cathode region. With the continuous reaction, Fe3O4 and β-FeOOH were formed with high stability in the corrosion layer, which played a certain protective role, resulting in a relatively slow corrosion rate and a relatively low corrosion current [29].

A comparison of the corrosion current density (*I*corr) of samples at different corrosion durations in seawater, according to the data in Table 1, shows that the maximum value of *I*corr is 1.444 × 10−<sup>4</sup> A/cm<sup>2</sup> on day 0, because there is no rust layer on the surface of the day 0 immersion sample, and that the oxygen on the surface of the sample is sufficient, the anodic dissolved Fe2+ can diffuse rapidly, and the corrosion rate is the fastest [30]. With the prolongation of immersion time, the *I*corr of the EH47 steel sample first decreases and then increases, and reaches the minimum value of 8.093 × 10−5A/cm2 at day 28 immersion. Because of the dense and tight corrosion layer formed on the surface of the sample under this condition, which plays a prominent role in protecting the matrix and hinders the diffusion of Fe2+ generated by the anodic reaction on the steel surface in seawater, the corrosion resistance of the sample is relatively good at day 28 immersion. With a further increase in immersion time, the rust layer formed on the surface of the sample is easily removed after long-term immersion in seawater, which provides a channel for the diffusion of iron ions into seawater under the condition of sufficient O2 [31]. Furthermore, the

corrosion products become loose and porous, and the protective effect of the corrosion layer on the matrix is weakened, which makes the *I*corr of the EH47 steel sample increase after 49 days of immersion [5,32,33].

**Figure 3.** Potentiodynamic polarization curves of the EH47 samples immersed in seawater for different durations.

**Table 1.** Results of fitting polarization curves of the EH47 sample immersed in seawater for different corrosion durations.

