3.2.3. EIS Measurements

Electrochemical impedance spectroscopy is an effective method in the field of corrosion electrochemistry [34]. Figure 4 shows the Nyquist diagrams of EH47 steel corroded in seawater for different immersion durations (0, 14, 28, and 49 days). It can be seen from Figure 4 that the obvious capacitive arc is an incomplete semicircle. The radius of the semicircle arc increases gradually. With the extension of full immersion time, the radius of the semicircle arc decreases, which may be caused by the thickening of the rust layer affecting the charging and discharging process. Among them, the arc radius of the day 28 immersion sample is the largest, which indicates that the impedance is the largest, the corrosion rate is the smallest, and the corrosion resistance is good. The arc radius of the day 0 immersion sample is the smallest, which indicates that its impedance is the smallest and the corrosion rate is the largest, which is consistent with the results of the polarization curve analysis above.

Figure 5 shows the Bode diagrams of the samples immersed in seawater for different durations. Figure 5a shows the phase angle versus frequency. It can be seen that the phase angle of 28 days immersion is close to the maximum value. Figure 5b shows that the impedance modulus value increases with the decrease in frequency in the frequency range from 10−<sup>1</sup> Hz to 10 Hz, and the maximum impedance is obtained at day 28 of immersion. Figure 5 further demonstrates that the corrosion resistance of the day 28 immersion sample is the best.

**Figure 5.** Bode diagrams of EH47 samples immersed in seawater for different durations. (**a**) Bode diagram of phase angle versus frequency. (**b**) Bode diagram of Z versus frequency.

The equivalent circuit diagram used for fitting the impedance data is shown in Figure 6. In this circuit, *R*s represents the solution resistance, *R*ct is the charge transfer resistance of the corroded samples and *R*rust is the resistance of the corrosion products on the sample surface.

**Figure 6.** The equivalent circuit used for fitting the impedance data.

*CPE* (constant phase element) reflects the capacitance behavior, which is a frequencydependent capacitance expressed the following equation:

$$\text{CPE} = [\text{Q} \ (\text{J}\omega)^{\text{n}}\text{I}]^{-1} \tag{1}$$

where *Q* is a frequency independent constant, *J* is the imaginary unit, *ω* is the angular frequency, n is an exponential term. If *n* = 0, the impedance is ideal resistance, while it is ideal capacitance if *n* = 1, and 0 < *n* < 1 represents the deviation from the ideal capacitance, which is correlated with the surface roughness and defect [35,36].

The *n* values corresponding to the four groups of different days of immersion in this experiment are all greater than 0.6, which indicates that the diffused impedance characteristics with limited retention layer [26], and the corrosion layer generated on the sample surface after corrosion reaction has produced a barrier effect on the solution, meanwhile, the diffusion reaction of ions contained in the solution to the matrix is limited. *Y*0 is a parameter with dimensions of <sup>Ω</sup>−1cm−2·s<sup>−</sup>n, *CPE*1 represents the capacitance in parallel with *R*ct and *CPE*2 represents the capacitance in parallel with *R*rust. Meanwhile, *n*1 is the exponential term correlated with *R*ct, and *n*2 is the exponential term correlated with *R*rust.

The inhibition efficiencies of the inhibitor from EIS are calculated using the following equation:

$$\text{IE\%} = \frac{\text{Rct} - R\_{\text{ct}}^0}{R\_{\text{ct}}} \times 100 \tag{2}$$

where *R*ct and *R*0ct are the charge transfer resistance in the presence and absence of inhibitor, respectively.

The data was fitted by Zview (Zview3.1, San Francisco, CA, USA). The fitting results are listed in Table 2.


**Table 2.** Fitting parameters of EIS of different EH47 samples.

#### *3.3. The Corrosion Mechanism of EH47*

3.3.1. The Phase Composition of the Corrosion Products for EH47

In order to study the phase composition of the corrosion products of EH47, the X'Pert Powder X-ray diffractometer was used to analyze the corrosion layer of the experimental steel. The X-ray diffraction analysis results of the corrosion products of EH47 after immersion in seawater for 49 days are shown in Figure 7. It can be seen that the corrosion products formed on the surface of EH47 steel are mainly composed of FeOOH, Fe3O4 and Fe2O3 phases.

**Figure 7.** XRD pattern of the corrosion product of EH47 after full immersion in seawater for 49 days.

After the EH47 steel sample is immersed in seawater for 49 days, the corrosion product layer extends from the outer layer to the inner part, and the content of O element gradually decreases, while the content of Fe element gradually increases, corresponding to the results of the previous energy spectrum analysis, which reflects the characteristics of comprehensive corrosion [37]. In the early stage of corrosion, due to the existence of inclusions in EH47 steel and the inhomogeneous composition of the sample surface, numerous microcells will occur on the surface of the sample immersed in seawater. The equation of this reaction is as follows:

Anodic reaction:

$$\text{Fe} \rightarrow \text{Fe}^{2+} + 2\text{e}^- \tag{3}$$

Cathodic reaction:

$$\rm O\_2 + 2H\_2O + 4e^- \rightarrow 4OH^- \tag{4}$$

Dissolved Fe2+ is deposited on the metal surface and is hydrolyzed and oxidized to Fe3+ and finally γ-FeOOH is formed. The corrosion reaction is as follows:

$$\text{Fe}^{2+} + \text{H}\_2\text{O} \rightarrow \text{FeOH}^\* + \text{H}^\* \tag{5}$$

$$2\text{FeOH}^+ + \text{O}\_2 + 2\text{e}^- \rightarrow 2\text{y-FeOOH} \tag{6}$$

With the progress of the reaction, the dissolved iron will continue to be oxidized to the unstable intermediate after deposition, and the intermediate will be further oxidized after dehydration to form β-FeOOH. With further corrosion, the corrosion layer will thicken and the dissolved oxygen will be difficult to diffuse to the surface of the steel substrate. A part of γ-FeOOH is transformed into α-FeOOH by amorphous iron hydroxide, and parts of β-FeOOH and γ-FeOOH are reduced to Fe3O4 due to the cathodic reaction.

Cathodic reaction:

$$2\text{ }6\text{FeOOH} + 2\text{e}^- \rightarrow 2\text{Fe}\_3\text{O}\_4 + 2\text{H}\_2\text{O} + \text{OH}^-\tag{7}$$

Therefore, the corrosion products of EH47 steel after immersion in seawater for 49 days are mainly composed of FeOOH, Fe3O4 and Fe2O3.

3.3.2. The Microstructure of Corrosion Products of EH47 Formed on the Sample Surface under Different Full Immersion Corrosion Times

It can be seen that corrosion products formed on the sample surface are light yellow and loose granular clusters in a network distribution, and a small part of the corrosion products are aggregated into blocks, while a small part of the dark brown substrate is exposed after 20 days of immersion in seawater (Figure 8a). The corrosion products on the surface of the sample become large lumps, and some of the corrosion products fall off, and the exposed part of the black matrix increases as shown in Figure 8b after 40 days of immersion. As shown in Figure 8c, the color of corrosion products on the surface of the sample becomes darker after 60 days of immersion. The newly generated light yellow corrosion products appear on the original dark yellow corrosion products in irregular network blocks. The surface of the sample is almost completely covered by the newly pale yellow corrosion products, with obvious thickness differences between the upper and lower layers, as shown in Figure 8d, after 80 days of immersion.

**Figure 8.** The OM microstructure of corrosion products of EH47 formed on the sample surface under different full immersion corrosion times: (**a**) 20 days, (**b**) 40 days, (**c**) 60 days, and (**d**) 80 days.

#### 3.3.3. The Mechanism of Immersion Corrosion of EH47

In order to investigate the mechanism of immersion corrosion of EH47, the full immersion corrosion tests were performed in natural seawater for different total immersion durations. Furthermore, the rust layer on the surface of the sample for different total immersion durations was removed and the cross section of the sample matrix after the full immersion corrosion tests in natural seawater were measured by SEM and EDS to study the mechanism of immersion corrosion of EH47.

Figure 9 shows the SEM and EDS diagrams of the sample matrix. In order to investigate the corrosion mechanism, the sample matrix was obtained after the rust layer on the samplesurfacefordifferentdurationswas removed.

**Figure 9.** SEM and EDS diagram of the sample surface in which the rust layer was removed after different seawater immersion durations: (**a**) 20 day (500×); (**b**) 40 day (500×); (**c**) 20 days; (**d**) EDS (20 days); (**e**) 40 days; (**f**) EDS (40 days); (**g**) 80 days; (**h**) EDS (80 days).

Figure 9a,b show low magnification SEM images of the sample matrix. It can be seen from that the corrosion products on the surface of the substrate after 20 days of immersion show massive accumulation. Figure 9b shows the rust layer on the surface of the sample after 40days of immersion. It can be seen from the figure that the corrosion products on the surface of the matrix are of poor continuity and there are many black cracks and black holes on the corrosion product film.

Figure 9c,e,g show high magnification SEM images of the sample matrix. Figure 9c presents that the continuity of the corrosion products is poor, and the black pitting holes in the corrosion product film indicate that pitting corrosion has occurred. The EDS test results on the surface of the substrate are shown in Figure 9d. After 20 days of immersion, the surface of the substrate is mainly Fe, O, C, Mn and Cu, with the highest content being that of Fe, and a small amount of Cr, Co and Ni are also detected. It can be seen from Figure 9e, which indicates the corrosion mechanism here is mainly crevice corrosion, that there are many black cracks on the matrix of corrosion products after 40 days of immersion. Crevice corrosion is a strong localized corrosion of a metal surface immersed in seawater (or other corrosive media), often occurring in crevices. The principle of crevice corrosion is similar to pitting corrosion. The anode is in the crevice, and the cathode is in the large area outside the crevice, thus forming the corrosion battery. Cl− can be enriched and H+ can be formed in the crevices, reducing the pH value. The crevice corrosion is mainly due to the existence of gaps, leading to dielectric inhomogeneity caused by the media. The EDS test results on the surface of the substrate are shown in Figure 9f. After 40 days of immersion, the surface of the substrate is mainly Fe, O, C, Mn and Cu, and a small amount of Cr, Co, Ni and Na can also be detected. Figure 9g shows the SEM of the sample substrate after 80 days of immersion. It can be seen that the corrosion products on the surface of the matrix are of poor continuity, where black holes and fine cracks appear in the corrosion products. This shows that pitting and crevice corrosion occur with the increase in corrosion time. The EDS test results on the surface of the substrate are shown in Figure 9h. After 80 days of immersion, Fe, O, C and Mn are the main parts of the surface of the substrate. A small amount of Cr, Co and Ni are detected because the EDS energy spectrum just hits the matrix, and is not affected by the corrosion products, and the seawater composition is not detected.

Figure 10 shows the SEM and EDS of the cross section of the sample matrix after immersion in natural seawater for different total immersion durations, which removed the rust layer on the surface of the sample. As shown in Figure 10a, a continuous inner rust layer was formed after 20 days of corrosion. The uniformity of the rust layer is poor, and some areas show pitting morphology. The EDS test is carried out on the cross section of the matrix shown in Figure 10b. The test results show that little Cl− is detected in the inner area of the rust layer, which indicates that the content of Cl− in the rust layer near the substrate is very low, the rust layer has a protective effect, and the content of Ca2+ is high. Figure 10c shows the SEM of the cross section of the sample matrix after 40 corrosion days. It can be seen that the continuous inner rust layer has been formed on the sample surface and the uniformity of the rust layer is poor. Meanwhile, the flake corrosion products and obvious pitting pits can be observed in some areas. The EDS test results on the cross section of the matrix are shown in Figure 10d. The EDS results show that Ca2+ content detected inside the rust layer is high, which may be caused by the deposition of salt in seawater on the surface of the corrosion products. The content of Cl− compared with that of 20 corrosion days is increased. Figure 10e shows that the rust layer on the surface of the sample is still discontinuous after 80 days of corrosion, and there are obvious cracks separating the rust layer, which is caused by crevice corrosion. The EDS test results are shown in Figure 10f. Ca2+ and O2+ are detected in the inner area of rust layer after 80 days of corrosion, and Cl− is also detected, which is lower than that of 40 days of immersion and higher than that of 20 days of immersion.

**Figure 10.** SEM and EDS diagram of a cross-section of sample substrate after different seawater immersion durations: (**a**) 20 days, (**b**) EDS (20 days), (**c**) 40 days, (**d**) EDS (40 days), (**e**) 80 days, (**f**) EDS (80 days).

SEM and EDS were used to observe and analyze the rust layer on the surface of the sample after 20 days of total immersion corrosion in seawater. it is found that the corrosion products are mainly laminated with poor continuity. There are black holes in the corrosion product film, which indicates that pitting corrosion is the main corrosion mechanism. After 40 full days of immersion, the corrosion products are laminated and reticulated, and there are still black holes in the corrosion products. The samples show slight cracks, which indicates that the corrosion mechanism is mainly pitting corrosion, accompanied by a small amount of crevice corrosion. After 80 days of corrosion, obvious cracks appear on the surface of the sample, which is the starting point of crevice corrosion. Local corrosion, such as pitting and crevice corrosion, occurs easily in seawater due to the local failure of passivation. Through the EDS analysis of the cross section of the matrix, it can be seen that Ca2+ appears on the surface of the matrix, and the content of Ca2+ is high, which may be caused by salt deposition on the surface of the corrosion products in seawater. At the same time, a very small amount of Cl− appears, indicating that the rust layer still has a protective effect, preventing Cl− penetration, and the corrosion layer is relatively stable.
